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Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine... The characteristics of petroliferous plays in subtle traps within a sequence stratigraphic framework in the Dongying Depres- sion are investigated in this study. Sand bodies within lowstand systems tracts (LSTs) of sequences, comprising incised- channel fills, sublacustrine fans, deltas in LSTs, controlled by syndepositional normal faults, and sand bodies within trans- gressive systems tracts (TSTs) to early highstand systems tracts (HSTs), consisting of beach bars, and turbidites, controlled by the prodelta slope, paleorelief, and syndepositional normal faults, are good subtle reservoirs. Mudstones and shale of deep lake subfacies in TSTs to early HSTs of sequences are source and cap rocks. Abnormal overpressure is the dominant dynamic factor for hydrocarbon migration from source rock to the subtle traps. Normal faults, sand bodies, and unconformities function as conduit systems. Sand bodies distributed in the abnormal overpressure source rocks within LSTs to early HSTs are petroliferous plays in lithologic traps. The petroliferous plays in stratigraphic traps are controlled by unconformities at margins of the Depression. Keywords Subtle traps · Sand bodies within lowstand systems tract · Syndepositional normal fault · Abnormal overpressure · Dongying Depression 1 Introduction targets for hydrocarbon exploration in these mature basins. For example, in the Mid-continent and Rocky Mountain Within the mature basins that have been explored, most areas, the flanks of anticlines and synclines for combination of the traditional and easy-to-find hydrocarbon traps have subtle traps are recommended for hydrocarbon exploration, already been located (Halbouty 1969). Subtle traps, such as even though when tested anticlines on the crests proved dry lithologic, stratigraphic, and structural and lithologic com- (Levorsen 1969). In the North Sea basin, subtle traps in the bination traps, which can hardly be found using the seismic Permian Rotliegend controlled by paleotopography have the data to recognize structural traps, are now the dominant potential for hydrocarbon exploration (Maynard and Gib- son 2001). Stratigraphic traps created by unconformities and preserved on the unconfined slope of the ancestral Missis- Edited by Jie Hao and Xiu-Qiu Peng sippi submarine fans in the northeastern Gulf of Mexico have also been found to contain substantial and profitable * You-Liang Feng fyouliang@petrochina.com.cn hydrocarbon reserves (Godo 2006). In offshore South Afri- can divergent basins, Cretaceous traps within lowstand sys- State Key Laboratory of Lithospheric Evolution, Institute tems tracts (LSTs) of sequences have been recommended of Geology and Geophysics, Chinese Academy of Sciences, for hydrocarbon exploration (Brown et al. 1995). In China, Beijing 100029, China 2 the conventional structural traps have already been found, University of Chinese Academy of Sciences, Beijing 100049, whereas subtle traps are becoming a greater contributor to China 3 the new additions to annual petroleum reserves in the Jiyang Research Institute of Petroleum Exploration Subbasin of the Bohai Bay Basin (Pang et al. 2005). and Development, PetroChina, Beijing 100083, China 4 New theories and methods, including paleotectonic and Energy and Geoscience Institute (EGI), University of Utah, paleogeomorphic reconstructions (Halbouty 1969), sequence Salt Lake City, UT 84108, USA Vol.:(0123456789) 1 3 2 Petroleum Science (2020) 17:1–22 stratigraphy (Posamentier and Vail 1988; Van Wagoner et al. 2 Geological setting and stratigraphy 1990; Brown et  al. 1995), seismic sedimentology (Zeng and Hentz 2004), and analysis of transfer zones in rift basin 2.1 Geological setting (Rosendahl 1987; Morley et al. 1990; Fossen et al. 2010; Paul and Mitra 2013), are used to predict subtle traps. Sequence The Bohai Bay Basin is a large Cenozoic petroliferous stratigraphy has been applied successfully to non-marine strata continental rift basin in Eastern China (Fig. 1a). The basin for constructing a sequence stratigraphic framework, interpret- is bounded by Taihang Mountains to the west, Yanshan ing sequence architecture, and forecasting reservoirs in subtle Mountains to the north, Jiaoliao Uplift to the east, and traps (Shanley and McCabe 1994; Lin et al. 2001; Martino Luxi Uplift to the south (Allen et  al. 1997; Feng et  al. 2004; Zecchin et al. 2006; Feng et al. 2013; Jiang et al. 2013; 2016). During the Cenozoic era, the Bohai Bay Basin Feng et al. 2016). Abnormal overpressure has been studied in underwent Paleogene rifting and Neogene subsiding stages petroliferous basins as a dynamic force for hydrocarbon migra- (Allen et al. 1997; Yang et al. 2016), resulting in thick tion and accumulation in subtle and structural traps (Caillet Paleogene, Neogene, and Quaternary lacustrine deposits. et al. 1997; Lee and Williams 2000; Zhang et al. 2009a; Guo The Bohai Bay Basin consists of several subbasins sepa- et al. 2010). rated by uplifts (Fig. 1a). Some studies also have attempted to interpret the distribu- The  NE-S W  tr ending  Dongying  Depr es - tion and controls of subtle reservoirs by using (1) source rocks sion is located in the southeastern part of the Jiyang Subba- (Hu 2010; Jiang et al. 2014), (2) sedimentary facies (Zou et al. sin. The area of the depression is about 5700 km . Paleogene 2005), (3) the relationship between porosity and fluid potential synrift strata of the depression rest unconformably on pre- (Li and Pang 2004), (4) normal faults and microfractures as Paleogene strata. The Paleogene synrift strata are typically conduit systems for hydrocarbon reservoirs of conventional 4000–7000 m (Yao et al. 1994; Zong et al. 1999; Feng et al. and subtle traps (Losh 1998; Losh et al. 1999; Li et al. 2010; 2013). Lampe et al. 2012), and (5) faults controlling hydrocarbon Raised Precambrian basement blocks are bounded by the migration and accumulation in a continental rift basin (Mu Chenjiazhuang Rise to its north, the Luxi Uplift and Guan- 2012; Wei and Su 2015). Some progress has been made in grao Rise to its south, and the Binxian and Qingcheng Rises understanding the conditions of hydrocarbon migration and to its west (Fig. 1b) (Feng et al. 2013; Pang et al. 2015). accumulation in subtle traps, and the distribution and controls NE-trending extensional structures such as normal faults, of subtle reservoirs within the sequence stratigraphic frame- northern faulted margin, southern hinged margin, deep work in continental rift basins, such as Dongying Depression troughs/sags, intra-depression fault blocks and transfer (Li and Pang 2004; Feng et al. 2005; Pang et al. 2005; Guo zones, anticlines genetically associated with normal faults, et al. 2010, 2012, 2014). In order to perfect the distribution and and negative flower-shaped structures are developed in an controls of subtle reservoirs within the sequence stratigraphic intense dextral transtensional stress field (Allen et al. 1997; framework, this study focuses on four aspects: Ren et al. 2002; Feng et al. 2010, 2013, 2016) during the First, the sequence stratigraphic framework of Paleogene Paleogene (Fig. 1b, c). strata was reconstructed based on 2D and 3D seismic pro- Besides northern faulted margin and southern hinged files, well logs, and drill cores. margin, the depression can be divided into four sags or sub- Second, sand bodies in lowstand systems tracts, con- depressions, such as Minfeng, Lijin, Niuzhuang, and Boxing trolled by sys-depositional normal faults, were identified as sags, by two negative flower-shaped structures—one striking reservoirs in subtle traps and described within the sequence N–E and one E–W—and one NNW-striking Shicun normal- stratigraphic framework of Paleogene strata. fault belt (Fig. 1b, c). Third, the authors describe the types and distributions of The structure of the Dongying Depression is a complex hydrocarbon reservoirs in subtle traps as well as investigate half-graben defined by a faulted margin in the north, sags, conditions of hydrocarbon migration and accumulation. negative flower-shaped structures at the middle, and a hinged Finally, the authors discussed the factors controlling margin in the south (Fig. 1c). petroliferous plays of subtle traps in the sequence strati- Similar to the Bohai Bay rift basin as a whole, the graphic framework. Depression is complicated by episodic rifting events, includ- ing block faulting associated with rapid tectonic subsidence and volcanism (Hsiao et al. 2004, Lin et al. 2004). A two- stage evolution model is accepted, with Paleogene synrift and differential subsidence, and Neogene post-rift and ther - mal subsidence. Paleogene synrift strata formed the major hydrocarbon source rocks, reservoir rocks, and cap rocks in 1 3 Petroleum Science (2020) 17:1–22 3 AC Fig. 1 Tectonic location of Dongying Depression, Bohai Bay Basin, eastern China (a), schematic map of structural units of the Dongying Depression (b), and tectonostratigraphy section (c). The locations of well-tied seismic sections in Fig. 3 a well-tied sequence stratigraphic profile in Fig. 4, well-tied hydrocarbon reservoir sections in Fig. 7 (section ①), 8 (section②), 9 (section ③), 10 (section ④), and well-tied EFP profile in Fig. 13 (section ⑤), 14 (section ⑥), location of hydrocarbon reservoirs forming pattern of subtle reservoirs (AA′) in Fig. 17 are indicated in the map (b). Some important wells are also shown in the map (b) (modified from Feng et al. 2013) the depression (Fig. 2) (Allen et al. 1997; Wan 2004; Feng 2.2 Stratigraphy et al. 2013). The Paleogene synrifting stage consists of four rifting The Paleogene strata in the Dongying Depression consist of episodes (Fig. 2): (1) early-initial rifting beginning in the the Kongdian Formation (E1-2 k) overlain by the Shahejie Paleocene and ending in the Early Eocene (65–50.4 Ma); Formation (E2 s), which is itself overlain by the Dongying (2) late-initial rifting in the Middle Eocene (50.4–42.5 Ma); Formation (E3d) (Fig. 2). (3) rift climax in the Late Eocene (42.5–38 Ma); and (4) The lowest part of the second member of the E1-2 k weakened rifting in the Oligocene (38–24.6 Ma) (Feng et al. comprises conglomerate, sandstone, and coarse sandstone 2013; Yin et al. 2018). interbedded with purple to red mudstones. The deposi- The Dongying Depression is a typical Paleogene rift basin tional environments of the interval are interpreted as allu- (Allen et al. 1997; Zong et al. 1999; Wan 2004). It is one of vial fan and braided river (Li et al. 1992). The uppermost the most petroliferous depressions in the Bohai Bay Basin. or youngest part of the second member of the E1-2 k is Episodic extensional tectonic events and their associated composed of alternating layers of gray mudstone, oil shale, extensional structures and depositional fills have resulted in and medium- to fine-grained sandstones (Fig.  2). The sedi- multiple source rocks (Li et al. 2003; Han et al. 2018), com- mentary environments of the interval are interpreted as binations of reservoirs and cap rocks, and hydrocarbon reser- deep lake, fluvial delta. The first member of the E1-2 k voirs in structural and subtle traps (Li and Pang 2004). High comprises alternating layers of red sandstone and gray density of exploration wells has provided a good chance to mudstone; the sedimentary environment of the interval study the distribution and controls of the hydrocarbon reser- is interpreted as shallow lake (Li et al. 1992; Wang et al. voirs in subtle traps and to examine the conditions of hydro- 2016). carbon migration and accumulation in subtle traps in rift basins within the sequence stratigraphic framework. 1 3 4 Petroleum Science (2020) 17:1–22 Fig. 2 General sequence stratigraphic charts, and the factors for hydrocarbon accumulation of Paleogene strata in the Dongying Depression (modified from Feng 1999; Feng et al. 2013). The classification of different order sequence stratigraphy is based on the changes in lake level, episodic tectonic activities, assemblage biozones, climate, and detailed work of this paper. The ages of sequence boundaries are determined from micropaleotologic data, e.g., ostracoda and palynologic data, paleomagnetic dating, and volcanic rock dating (Chen and Peng 1985; Li et  al. 1992; Yao et al. 1994; Feng 1994). The S is source rock; R is reservoir; C is caprock. FS: first-order sequence; SSq: second-order sequence; Sq: third-order sequence. Sq3-1 means first third-order sequence within second-order sequence 3 (modified from Feng et al. 2013) The lowest part of the fourth member of Shahejie Forma- conglomerate as well as sandstone interbedded with purple tion (E2s4) consists of alternating layers of red sandstone to red mudstones. The sedimentary environments of the and mudstone interbedded with saline deposits. The sedi- interval are interpreted as a meandering river and delta mentary environment is interpreted as a salt lake (Wang plain (Wang 1992; Feng et  al. 2013). The top strata of 1992). The top strata of E2s4 are composed of gray mud- E2-3s2 are composed of conglomerates and coarse sand- stone, oil shale intercalated with sandstone, and thin layers stones interbedded with red mudstone. The sedimentary of limestone. The sedimentary environment is interpreted as environment is interpreted as a braided river (Feng et al. a shallow lake (Song et al. 2012; Lu et al. 2017). 2013; Wang et al. 2019) (Fig. 2). The third member of the Shahejie Formation (E2s3) is The lowest part of the first member of the Shahejie Upper Eocene. The lowest strata of the third member (E2s3) Formation (E3s1) is composed of sandstones interbedded comprise gray to dark mudstones and oil shale. The middle with green and gray mudstones. The middle part of E3s1 and upper parts of the E2s3 are composed of fine sandstone comprises gray mudstone and shale interbedded with thin interbedded with gray mudstone, and coarse sandstone and limestone layers. The uppermost part of E3S1 is composed sandy gravels intercalated with green mudstone. The sedi- of sandstone interbedded with gray mudstone. The deposi- mentary environments of E2s3 are interpreted as a deep lake, tional environment of E3S1 is interpreted as a shallow lake prodelta, and delta front (Feng et al. 1991, 2013; Feng 1999). (Zhang et al. 2014; Wang 1992; Yan et al. 2019). The lowest strata of the second member of the Shahejie The Dongying Formation (E3d) consists of coarse sand- Formation (E2-3s2) consist of conglomerate and sandy stone, medium to fine sandstones interbedded with gray 1 3 Petroleum Science (2020) 17:1–22 5 mudstone, and gray to greenish mudstone and red mud- is about 4.5–10.0 Ma, such as T8/SSb2, T6/SSb3/Sb3-1, stone (Fig. 2). The sedimentary environment of the For- and T2′/SSb4/Sb4-1 (Figs.  2 and 3a, b); (3) third-order mation is interpreted as a meandering river (Wang 1992). unconformities are localized and laterally change to their correlative conformities (Embry 1995, 2002), and the time duration of the two third-order unconformities is about 1.0–2.0 Ma. These unconformities or sequence boundaries 3 Datasets and methods can be identified on seismic profiles, logging curves, and drill cores (Feng et al. 2013). This study was based primarily on petroleum geologi- Third-order sequence boundaries appear on the seismic cal data from 400 exploration boreholes and regional reflection profile as a truncation below the boundary, and 3D seismic data covering approximately 5600 km in the onlap above it. Down-dip toward the center of the depres- Dongying Depression (Figs.  1 and 2). The seismic data sion, such boundaries become correlative conformities were extracted from a series of surveys acquired during (Fig. 3a, b; Sb3-1–Sb3-4) (Feng et al. 2013). 1997–2010 and were reprocessed and assembled into a In well-tied profiles such as those in Fig.  4, a third-order single dataset for this study. Data with a vertical resolution sequence boundary indicates a scoured base at the bottom of of 15–25 m in Paleogene strata (derived from a 35–40 Hz a fining-upward succession from conglomerate at the base dominant-frequency seismic dataset with a 3600  m/s to sandstones and mudstones at the top, the fining-upward average velocity) are sufficient for the reconstruction of succession only existed in lowstand systems tracts (LSTs), a stratigraphic sequence framework within the region of and the SP curve for such LSTs is bell-shaped. Examples seismic coverage. Exploration borehole data, including log include the base of sublacustrine fans or braided channels curves, cores, drill stem test (DST), organic geochemis- (Fig. 4) (Feng et al. 2013). The third-order sequence bound- try data, and results of oil well tests, were also used for ary is also the boundary between the fluvial and deltaic front this research. The research method of this paper has been deposits in lacustrine basins, or subaerial exposure surfaces described as follows: (Fig. 4) (Feng et al. 2013). Finger-shaped SP curves indicate fluvial deposits; serrated-funnel shapes indicate a delta front 1. Interpretation of spontaneous potential curves and seis- deposit within the deltaic succession. mic profiles was used to identify sequences and systems tracts. 2. The sonic logging data, DST, and results of oil well tests 4.2 Sequence hierarchy were used to predict abnormal overpressure in strata. 3. Well logs and 3D seismic data were also used to inter- The stratigraphic architecture of a succession accumulated pret sand bodies as reservoirs of subtle traps on the basis in a rifted lacustrine basin is different from that in a post- of the shapes of the spontaneous potential (SP) curves rift lacustrine succession (Hubbard 1988; Williams 1993; within sequences. Carroll and Bohacs 1999; Feng et al. 2000, 2010, 2016). 4. Source rocks were evaluated and correlated with oil Hubbard (1988) and Williams (1993) classified synrift using organic geochemistry data. and post-rift strata as two kinds of first-order sequences through their studies of rifted basins on continental margins. 4 Stratigraphic sequence The synrift (Paleogene) succession in Dongying Depression could be accordingly classified as a first-order 4.1 Sequence stratigraphy boundaries sequences 1 (FS1,) confined by first-order unconformities. and hierarchy FS1 was subdivided into four second-order sequences (SSq) 1 to 4 from bottom to top (SSq1 to SSq4) recognized The three orders of unconformities are identified in the by second-order unconformities. Paleogene synrift strata in the Dongying Depression and SSq1 corresponds to rifting Episode 1 and roughly con- represent three levels of sequence boundaries: (1) First- sists of the Kongdian Formation (E1-2 k), and two third-order order unconformities are angular unconformities at the top sequences (Sqs) are identified in SSq1 based on their charac- (24.6 Ma) and bottom (65.0 Ma) of the synrift strata, and teristic third-order unconformities and correlative conformi- the time duration of the two unconformities is 40.4 Ma, ties (Van Wagoner et al. 1990). SSq2 corresponds to Episode such as angular unconformities FSb1/Tr (Fig.  3a; Feng 2 and roughly consists of the fourth member of the Shahejie et al. 2013) and FSb2/T1 (Fig. 3a); (2) second-order angu- Formation (E2s4), and two Sqs are recognized in SSq2. SSq3 lar unconformities occur between two rifting episodes, and corresponds to Episode 3 and roughly consists of the third the time duration of the two second-order unconformities member (E2s3) and the lower part of the second member of 1 3 Two-way-time(ms) Two-way-time(ms) 6 Petroleum Science (2020) 17:1–22 Xinzhen negative flower CDP North South shaped structure 140 220 Feng9Yan4 460 500 100 180 260 300 380 420 540580 Ng T1/FSb2 T4/Sb3-3 T2’/SSb4/Sb4-1 Sq3-4 Sq3-3 Sb3-4 T6’/Sb3-2 T4/Sb3-3 Sq3-2 Chennan Sq3-1 T6/SSb3/Sb3-1 fault T6/SSb3/Sb3-1 T7 Tr/FSb1 T8/SSb2 T7/SSb2 A North to south well-tied seismic section on line 625.7 East west Lai43 Shi128 Niu11 Niu25 Niu43Wang68Wang53Wang70Wang11Wang103 T2’/SSb45/Sb4-1 Sq3-4 Sb3-4 Sq3-3 Sq3-1 2 2 2 1 3 6 4 Sq3-2 T4/Sb3-3 5 T6’/Sb3-2 1 T6/SSb3/Sb3-1 97.5 B East to west well-tied seismic section on line 97.5 Fig. 3 Well-tied seismic sections of interpreted structures and sequence stratigraphic framework: a north to south well-tied seismic section on line 625.7(the location is shown in Fig. 1b); b east to west well-tied seismic section on line 97.5 (the location is shown in Fig. 1b); two-way time is used to indicate buried depth of interfaces of strata. Sq3-1 to Sq3-4 stand for four third-order sequences from bottom to top. The numbers ① to ⑥ stand for fourth-order sequences on the seismic profile. T and Sb/SSb/FSb stand for seismic reflection horizon and sequence boundaries, respectively (Feng et al. 2013) the Shahejie Formation (E2s2  ), and four Sqs are identified 5 Reservoirs for subtle traps and their in SSq3. SSq4 corresponds to rifting Episode 4 and roughly controls comprises the upper part of the second member of the Shahejie Formation (E3s2 ) to the Dongying Formation (E3d), and four Based on the seismic data, well logs and cores, and pub- Sqs are recognized in SSq4 (Feng et al. 2013). lished results (Feng et al. 2013, 2016), the authors identi- Based on the presence of initial (Ifs) and maximum flood- fied the sand bodies which are reservoirs for the subtle ing surfaces (Mfs) within third-order sequences as well as traps within the sequence stratigraphic framework of the stratal terminations and stacking patterns, a series of systems Paleogene strata (Fig. 5; Table 1). tracts (i.e., lowstand systems tracts (LSTs), transgressive sys- tems tracts, (TSTs), and highstand systems tracts (HSTs)) can 1. Fluvial delta sand bodies with 5%–18% porosity and −3 2 be distinguished (Fig. 4). 1–20 × 10  μm permeability within LSTs. 2. Fan-delta sand bodies with 8%–15% porosity and −3 2 10–32 × 10  μm permeability adjacent to the northern faulted margin (Fig. 4; Feng et al. 2013) within LSTs. 1 3 Petroleum Science (2020) 17:1–22 7 Fig. 4 Well-tied sequences stratigraphic framework from north to south in the Dongying Depression. The location BB′ of this cross section is shown in Fig.  1b. The well-tied profile shows the stratigraphic stacking pattern of the third-order sequences in second-order sequence 3 (third member to lower part of second member of Shahejie Formation) at faulted and hinged margins. The curve at the left side of the well is spontane- ous potential (SP) log and the curve at the right side of it is the resistivity (RT) log (Feng et al. 2013) 3. Incised-channel fills with 15%–20% porosity and southern hinged margins of the depression, LST sand bodies −3 2 10–20 × 10  μm permeability within LSTs, and sub- of turbidite or sublacustrine fans and small deltas, were also lacustrine fan sand bodies with 15%–20% porosity and deposited on the down-dip (lakeward) side of these faults, −3 2 1.4–650 × 10  μm permeability at the southern hinged and were found along their length too (Fig. 5; Feng et al. margin in LSTs (Fig. 5; Feng et al. 2013; Table 1). 2013). Incised-channel fills within LSTs were deposited on 4. Sand bodies of turbidite fans with 5%–13% porosity and the up-dip (landward) side of the faults. −3 2 1.0–10.0 × 10  μm permeability within LSTS, TSTs, The sand bodies of beach bars within TSTs and early and early HSTs (Figs. 4 and 5; Feng et al. 2013). HSTs were distributed on the ramp or subaqueous high. For 5. Shallow-lake beach bar sand bodies with 6%–15% poros- example, beach bar sand bodies within TSTs were distrib- −3 2 ity and 1.0–5 × 10  μm permeability along the western uted on the ramp of a hinged margin in Sq2-2 and on the and southern ramp slopes of the Depression within TSTs ramp of a faulted margin in Sq3-1, Sq3-2, and Sq3-3. They to early HSTs (Fig. 4; Feng et al. 2013). were controlled by subaqueous highs, such as low buried hills, subaqueous volcanoes, anticlines, and horsts, and were The distribution of sand bodies in the LSTs of the distributed along slopes of subaqueous highs (Song et al. sequences was controlled by syndepositional faults or 2012; Feng et al. 2013). syndepositional fault slope-break zones (Feng et al. 2013, Sand bodies of sublacustrine or turbidite fans within 2016). The sand bodies are good reservoirs for subtle traps TSTs and early HSTs, including sandy debrites and turbid- (Feng et al. 2013). For example, steeply dipping parallel and ites, were developed in deep lake and on prodelta slopes and cross-shaped syndepositional faults were developed mostly were controlled by the slopes and normal faults, respectively at the faulted margins of Sq3-1 and Sq3-2 (Fig. 5; Feng et al. (Feng et al. 1991; Zou et al. 2012; Liu et al. 2017). 2013), the sand bodies of fan deltas, turbidite, or sublacus- trine fans in LSTs were deposited on the down-dip (lake- ward) side of the faults and were found along their length. Only small fluvial deltas in LSTs, controlled by gently dip- ping parallel syndepositional faults, occurred. In Sq3-3, gen- tly dipping parallel syndepositional faults occurred at the 1 3 8 Petroleum Science (2020) 17:1–22 Fig. 5 Distribution maps of sand bodies within LSTs of Sq3-1 to Sq3-3 in Dongying Depression (Feng et al. 2016) 1 3 Petroleum Science (2020) 17:1–22 9 Table 1 Reservoir physical property of sand bodies in lowstand systems tracts (the reservoir physical property data based on Li and Pang 2004, and Petroleum Geological Institute of Shengli Oil Field Corporation Ltd., Sinopec Corp.) −3 2 Type of sand body Porosity (%) Permeability × 10  μm Area/well Systems tracts Sublacustrine fan 15–20 1.4–650 Lijin sag LST Incised-channel fill 15–20 10–20 S103 LST Fan delta 8.0–15 10–32 Well T71 LST Fluvial delta 5–18 1–20 F104 LST Turbidite fan 5–13 1–10 L911 TST Brach bar 6–15 1–5 Linfanjia TST up-dip sand bodies were greater than those of the lenticular 6 Types of subtle traps and subtle reservoirs sand bodies (Table 1; Li and Pang 2004). Hydrocarbon reservoirs of lenticular sand bodies of tur- The Paleogene subtle traps and subtle reservoirs in the bidite fans did not have unified oil–water contacts, and each Dongying Depression occurred in two petroleum plays, one hydrocarbon reservoir had an oil–water contact (Figs. 7 and in the source rocks of the upper part of E2s4 (Sq2-2) and 8). They were distributed in the Sq3-2 (the middle of E2s3) E2s3 (including Sq3-1 and Sq3-2, Sq3-3) and the other in source rock of Lijin and Niuzhuang sags, for example, in the E2s2 to E2d, which are far away from source rocks (includ- hydrocarbon reservoir of the lenticular sand body traps of ing Sq3-4 and Sq4-1 to Sq4-4) (Fig. 2) (Li et al. 2003, 2010; turbidite fan drilled through by the Well Ying 11 in the Lijin Zhang et al. 2009b; Guo et al. 2012; Chen et al. 2016). Sub- sag (Fig. 7). Hydrocarbon reservoirs of up-dip sandstone tle traps and subtle reservoirs in the Depression include (1) pinch-out occurred mainly on the southern hinged margin. lithologic traps and lithologic reservoirs; (2) stratigraphic Oil was found only near the top of the individual up-dip and traps and stratigraphic hydrocarbon reservoirs; and (3) struc- pinch-out sand body (Figs. 7 and 8). tural and lithologic combination traps and their reservoirs (Fig. 6). 6.2 Stratigraphic traps and stratigraphic hydrocarbon reservoirs 6.1 Lithologic traps and lithologic reservoirs Stratigraphic traps, including stratigraphic unconformity The lithologic traps formed by lithologic variation, such and stratigraphic overlap traps, were formed by stratigraphic as lenticular sand bodies and up-dip sandstone pinch-out, truncation below unconformities or sequence boundaries, are widespread in the strata from Sq2-2 (the upper part of with an onlap above the unconformities, which were wide- E2s4) to Sq3-2 (the middle part of E2s3) (Figs. 7 and 8). spread in the Paleogene strata at the southern hinged mar- They were developed in source rocks and in sags. Based gin (Figs. 6, 7, and 8). Beach bar and deltaic front sand- on a sedimentary study, they consist of sublacustrine fan stones, with good physical properties, constitute reservoirs or turbidity deposits that developed in LSTs to TSTs of the in stratigraphic traps (Guo et  al. 2014). The traps were Sq3-1 (the lower part of E2s3) and Sq3-2 (the middle part of capped by the unconformities and mudstones near them. In E2s3) source rocks (Feng et al. 2013) and beach bar deposits the Depression, the hydrocarbon from Sq2-2, Sq3-1, and that developed in Sq2-2 (the upper part of E2s4) (Song et al. Sq3-2 mature source rocks migrated up faults, sand bodies, 2012). and unconformities to form stratigraphic hydrocarbon res- The lateral extent and quality of the reservoirs vary sub- ervoirs including stratigraphic unconformity reservoirs and stantially because of diagenesis and facies variation. The stratigraphic overlap reservoirs (Li et al. 2010). The Jingjia porosity of the lenticular sand bodies of turbidite fans and oilfield (Fig.  8) at the southern hinged margin is a typical beach bars was approximately 5%–13% and 6%–15%, respec- −3 2 example. The first-order unconformity between Neogene and tively, with permeability of approximately 1–10 × 10  μm −3 2 Paleogene and the second unconformity between Sq4-1 and and 1–5 × 10  μm , respectively (Table 1). The up-dip sand Sq3-4 control the distribution of stratigraphic hydrocarbon body pinch-out was developed along flanks of paleostructure reservoirs. highs on the western margin in the Depression, as well as on gentle southern slopes (Figs. 4, 7, and 8). The sand bod- ies forming up-dip and pinch-out were the dominant sand bodies of the sublacustrine fans and deltaic fronts in LSTs (Figs.  4, 7, and 8). The porosity and permeability of the 1 3 10 Petroleum Science (2020) 17:1–22 Fig. 6 Classification maps of subtle traps and hydrocarbon reservoirs in Dongying Depression based on explored examples reserve can exceed 50 million tons (360 million bbl), such 6.3 Lithologic and structural combination traps as the Liangjialou oilfield (Hao et al. 2005), which is a typi- and hydrocarbon reservoirs cal example in subtle reservoirs in a lithologic structural combination trap (Fig. 9). Their reservoirs are sand bodies Based on features of hydrocarbon reservoir profiles (Figs.  7, in LST3-3 within Sq3-3 (Figs. 5 and 9). These sand bodies 8, 9, and 10), lithologic and structural combination traps are belong to deposits in incised-channel fills and sublacustrine a kind of the subtle traps in the Depression too (Figs. 6, 7, fans (Figs. 5 and 9; Feng et al. 2013). They were capped 9, and 10). According to depositional facies maps (Fig. 5) by TST3-3 deep lake mudstone and cut up-dip by normal and logging data, the trap reservoirs were made of sand- faults (Fig. 9). The geochemical properties of the oils have stones of sublacustrine fans with 15%–20% porosity and −3 2 intermediate values between those observed in extracts from 1.4–650 × 10  μm permeability, delta with 5%–18% poros- −3 2 the TST3-1 (the lower part of E2s3) and Sq2-2 (the upper ity and 1–20 × 10  μm permeability, and incised-channel −3 2 part of E2s4) source rocks (Zhang et al. 2004; Hao et al. fills with 15%–20% porosity and 10–20 × 10 μm perme- 2005; Li et al. 2010). As a consequence, the oils in the sub- ability in LSTs (Fig. 5, Table 1). They are capped by deep tle traps were most likely derived from mixed Sq3-1 and lacustrine mudstone and oil shale in TSTs (Figs. 7, 9, and Sq2-2 source rocks, with a predominant contribution from 10). The traps were lateral pinch-out or facies change and the Sq2-2 source rocks (Hao et al. 2005; Li et al. 2010). were cut up-dip by normal faults (Figs. 7, 9, and 10). They Hydrocarbon may have migrated up faults from Sq2-2 and have the potential to form petroleum plays whose proven 1 3 Petroleum Science (2020) 17:1–22 11 Fig. 7 Well-tied profile of hydrocarbon reservoirs in Dongying Depression from north to south. The location of the profile (①) is indicated in Fig. 1b. Symbols of sequences are shown on the profile same as Fig. 2 Fig. 8 Well-tied profile of hydrocarbon reservoirs at Jingjia oilfield within the sequence stratigraphic framework from north to south at south- ern hinged margin. The location of the profile (②) is indicated in Fig.  1b. RT is resistivity curve, SP is spontaneous potential curve. Symbols of sequences are shown on the profile same as Fig. 2 −3 3 Sq3-1 source rocks to these subtle traps (Fig. 9). Another and 10–32 × 10  μm permeability in the LST3-2 of Sq3-2, example is the Tou71 subtle reservoirs at the northern were capped by deep TST2 lake mudstone and were lateral faulted margin (Fig. 10). The reservoirs, ranged from fan pinch-out or facies change and cut by faults. Hydrocarbon deltaic sandstone to conglomerate with 8%–15% porosity from Sq2-2 and Sq3-1 migrated to the subtle traps up faults. 1 3 12 Petroleum Science (2020) 17:1–22 Fig. 9 Well-tied profile of hydrocarbon reservoirs of structural/fault and lithologic combination traps within systems tracts of sequence Sq3-3 from north to south at Liangjialou oilfield. The location of the profile (③) is indicated in Fig. 1b The source rocks in TST3-2 within Sq3-2, such as shale and these parameters, the characteristics of the overpressure mudstone, capped on the traps, could also migrate to the were studied. subtle traps directly (Fig. 9). Based on the results of fluid pressure calculated by Feng et al. (2006) using sonic logging of 300 wells, the features of the fluid pressure of wells in the depression can be divided 7 Distribution features and genesis into two types: normal fluid pressure of wells at margins of abnormal overpressure and shallow wells, and abnormal high fluid pressure of deep wells at sags (Fig. 11). Within abnormal overpressure areas, 7.1 Distribution features of abnormal overpressure such as Well Li101 in the Lijin sag (Fig. 11), fluid pressure can be divided into further three parts. When the depth of DST data and sonic loggings show that abnormal overpres- the well is less than 2200 m, the strata fluid pressure is equal sure develops in Sq3-1 and Sq3-2 strata (Xie 2001; Zhang to the hydrostatic pressure. When the well depth is between et  al. 2009a, b; Guo et  al. 2010; Hao 2013). Abnormal 2200 and 3000 m, the fluid pressure coefficient (PC) of the overpressure is a dominant dynamic factor for hydrocarbon strata is between 1.0 and 1.2. The depth between 2200 m and accumulation in subtle traps in abnormal overpressure strata 3000 m is called the transition zone of fluid pressure. When (Losh 1998; Losh et al. 1999; Li et al. 2004; Guo et al. 2010, the well depth exceeds 3000 m, the PC is greater than that Lampe et al. 2012). To distinguish abnormal overpressure, of 1.2, and the fluid overpressure occurs (Fig.  12). pressure coefficient (PC) and excess fluid pressure (EFP) The plane view and profile distribution of EFP of mud- are used in this study. The pressure coefficient is a ratio of stone can be drawn using EFP data calculated from sonic the actual pore pressure versus the normal hydrostatic pres- logging by Feng et al. (2006) (Figs.  12, 13, and 14). On sure at the same depth. The EFP is a value of the actual the EFP map of the bottom of Sq3-2, high EFP (> 16 Mpa) pore pressure of strata subtracted from the normal hydro- was distributed at Lijin, Boxing, Niuzhuang, and Minfeng static pressure. Because of the limited amount of DTS data, sags. The mean value of EFP occurred in the structural belts sonic logging of 300 boreholes was used to predict the fluid between sags. At the southern margin, EFP was equal to 0, pressure, the fluid pressure coefficient, and the EFP. Using and fluid pressure was hydrostatic (Fig.  13). On the well- tied sections of EFP, the top surface was shallower at the 1 3 Petroleum Science (2020) 17:1–22 13 Fig. 10 Well-tied hydrocarbon reservoir profile of Tou71 structural/fault and lithologic combination hydrocarbon reservoirs within systems tracts of sequence Sq3-2 at faulted margin from north to south. The location of the profile (④) is indicated in Fig. 1b central negative flower shape structural belt, fault belts, and development are compaction disequilibrium and hydrocar- the margins, and deepened at sags, for example, at the Lijin bon generation in the Depression. and Minfeng sags. The highest fluid overpressure occurred Some researchers think compaction disequilibrium is in strata from Sq2-2 to Sq3-1 and at Sq3-2 (Fig. 12). the dominant mechanism for overpressure generation in the Dongying Depression (Xie et al. 1998; Xie 2001). The rea- 7.2 T he genesis and significance of abnormal son is if fluids cannot be expelled sufficiently when a for - overpressure mation is increasing rapidly, the disequilibrium compac- tion occurs rapidly (Osborne and Swarbrick 1997; Hooker In general, mechanism for overpressure includes com- et al. 2017). However, in Dongyin Depression, there are paction disequilibrium (Rubey and Hubert 1959; Magara no apparent characteristics of compaction disequilibrium, 1975; Mara et al. 2009; Ramdhan and Goulty 2011; Lahann such as anomalously high porosities or low density in the 2017), hydrocarbon generation (Bredehoeft et al. 1994; Guo overpressured mudstones (Guo et al. 2010). Therefore, the et al. 2010), and fluid release during dehydration reactions overpressure of mudstones was not generated by compac- (Magara 1975; Hooker et al. 2017). However, clay dehydra- tion disequilibrium, for they are under normal compaction. tion cannot generate significant overpressure unless there is a Other data, such as the sedimentary rate of strata and the perfect seal (Luo and Vasseur 1992; Osborne and Swarbrick hydrocarbon potential of source rocks, show that overpres- 1997). Therefore, the major mechanisms for overpressure sure correlated with hydrocarbon generation other than 1 3 14 Petroleum Science (2020) 17:1–22 Fig. 11 Section of fluid pressure of well Li101 at Lijin sag compaction disequilibrium is caused by quick deposition. from the transformation of high-density organic matter to For example, in well Niu38 the sedimentation rate of the low-density fluids (oil or gas) exceeded the rate of volume upper part of Sq3-2 is approximately 800 mm/1000 years, loss caused by fluid migration and expulsion (Berg et al. and approximately 400–200 mm/1000 years for the lower 1999; Lee and Williams 2000; Fossen et al. 2010; Das- part of Sq3-2 to Sq3-1 (Fig.  15). However, the higher gupta et al. 2016; Hooker et al. 2017). This overpressure overpressure in the Depression is present in the strata of mechanism relies on the kerogen type, density of organic the lower part of Sq3-2 to Sq3-1 and Sq2-2 (Figs. 13 and matter, rock permeability, and thermal history (Osborne 14), which are deep lake deposits with slow sedimenta- and Swarbrick 1997; Tingay et al. 2009; Dixit et al. 2017). tion rate and are excellent source rocks for hydrocarbon These excellent source rocks of the Depression, located in (Fig.  15). Higher overpressure was not present in strata Sq3-1 and Sq2-2, have dominant kerogen types I to II, an of the upper part of Sq3-2, which are prodeltaic deposits oil window at 2700, and contain tight reservoirs (Li and with higher sedimentation rates and are not a good source Pang 2004). These conditions favor overpressure caused rock (Fig. 15). Based on the fluid pressure, PC, and EFP by hydrocarbon generation. predicted using sonic logging, the top surface of the abnor- The overpressure is a dynamic factor overcoming capil- mal overpressure zone (PC < 1.2 or EFP > 0) was located lary resistance (Li and Pang 2004). The relationship between at a depth of 2700–3000 m (Figs. 13 and 14). This is in porosity and the pressure of sand bodies with oil-bearing agreement with the oil window (2700 m) of the Sq3-2 to saturation exceeding 80% in lithologic traps was studied by Sq3-1 and Sq2-2 source rocks. This evidence shows that Li and Pang (2004). They found that porosity was the reverse overpressure in the depression was likely caused by hydro- ratio to fluid pressure in oil-bearing sandstone. Higher over - carbon generation, if the rate of volume increase resulted pressure is favorable for hydrocarbon accumulation in the 1 3 0 Petroleum Science (2020) 17:1–22 15 Fig. 12 Distribution of hydrocarbon reservoirs and EFP contours of mudstone on Sq3-2 top surface (middle part of third member of Shahejie Formation). EFP is excessive fluid pressure Bin408 Li101Niu1 SHi119 He6 Ying68 Ying65 Ying34 Ying11 Yong38 Ying90 West East SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP 2000 2000 Sq3-3 3000 Sq3-2 Sq3-1 Fault Sq2-2 Fault Fault 0 1000m Vertical scale: 0 8000m SP curve Sequence boundary Contour of EFP calculated EFPcurve Horizontal scale: SP: Spontaneous potential EFP: Excess fluid pressure curve Fig. 13 Profile of well-tied EFP in the depression from East to West. The location of the profile (⑤) is indicated in Fig.  1b. EFP is excessive fluid pressure 1 3 Depth/m Depth/m 16 Petroleum Science (2020) 17:1–22 Fig. 14 Profile of well-tied EFP in the depression from north to south. The location of the profile (⑥) is indicated in Fig.  1b. EFP is excessive fluid pressure Fig. 15 Column of lithology, Paleomagnetic dating, sedimentary rate, and organic geochemistry in Well Niu 38 1 3 Petroleum Science (2020) 17:1–22 17 tight sandstones of lenticular lithologic traps in particular lithologic reservoirs in Nuizhang, Boxing, and Lijin sags (Li and Pang 2004). For example, in the Liangjialou and (Figs. 7 and 8). They were distributed in Sq2-2, Sq3-1, and Niuzhuang oilfields, based on the DST data, lithologic res- Sq3-2 source rocks at fluid overpressure zones of the center ervoirs in Sq3-1 and Sq3-2 and Sq2-2 have overpressure and of sags; the overpressure is a dynamic factor overcoming high oil-bearing saturation (Li and Pang 2004; Guo et al. capillary resistance of tight reservoirs to form lithologic 2010; Hao 2013). reservoirs in source rocks (Li and Pang 2004), for example, the Ying 11 hydrocarbon reservoir in the lithologic trap at fluid overpressure zone of the Lijin sag, and the Niuzhuang 8 Distribution of subtle reservoirs oilfield of the lithologic reservoir in the fluid overpressure zone of the Niuzhuang sag (Figs. 7, 12, and 16). Subtle reservoirs are important exploration targets in the Depression (Li et al. 2004) and have special distribution 8.2 Distribution of lithologic and structural characteristics. combination reservoirs The hydrocarbon reservoirs in lithologic and structural 8.1 Distribution of lithologic reservoirs combination traps were distributed in syndepositional faults or slope-break zones and near anticlines and fluid The lithologic reservoirs were distributed at overpressure overpressure to transitional pressure zones of the slopes zones of the depression, which are located at the centers and in the sags (Figs. 7, 9, 10, and 16). In the sequence strati- slopes of sags in Sq2-1, Sq3-1, and Sq3-2, e.g., Niuzhuang, graphic framework, the hydrocarbon reservoirs in litho- Minfeng, Lijin, and Boxing sags (Figs. 7, 12, and 16). The logic and structural combination traps were located in lithologic traps are composed of lenticular turbidite fan and LSTs. For example, (1) the Liangjialou oilfield is a typi- beach bar sand bodies or upward pitchout of sand bodies cal hydrocarbon reservoir in a structural and lithologic within source rocks of Sq2-2, Sq3-1, and Sq3-2, such as combination trap located in LST3-3 within Sq3-3 on Fig. 16 Relationship between distribution of subtle reservoirs and fluid pressure field in Dongying Depression 1 3 18 Petroleum Science (2020) 17:1–22 the southern hinged margin (Fig. 9). The sand bodies of pressure zone of the margins of the Lijin sag source sublacustrine fans and channels controlled by syndeposi- rocks (Fig.  16). Because the Sq3-1, Sq3-2, and Sq2-2 tional faults or slope breaks developed in LST3-3 (Feng source rocks are in the overpressure zone, the oil-bearing et al. 2013); they were capped by deep lake mudstone in fluid should be discharging episodically up the faults or TST3-3. Based on the correlation between source rocks microfractures from the overpressure zone to form sub- and oil, the oil migrated from the Sq2-2 and Sq3-1 source tle reservoirs. Normal faults play important roles in the rocks because of the higher gammacerane content and migration of the hydrocarbon from the source rocks of lower Pr/Ph ratio (Li et al. 2010). In the Sq2-2 and Sq3-1 deep intervals to the traps of shallow intervals. They are source rocks in the f luid overpressure zone (Figs. 11 and conduit systems for hydrocarbon migration (Losh 1998; 12), the hydrocarbon, driven by overpressure, migrated Losh et al. 1999). up faults and accumulated in the structural and lithologic combination traps. The pressure coefficients of the hydro- 8.3 Distribution of stratigraphic hydrocarbon carbon reservoirs obtained using DST ranged from 1.2 to reservoirs 1.48, showing that the hydrocarbon reservoirs had over- pressure (Feng et al. 2006). (2) Another example is the The stratigraphic hydrocarbon reservoirs occur near uncon- Tou71 hydrocarbon reservoirs in LST3-2 of Sq3-2 at the formities or sequence boundaries, which are normal fluid faulted margin (Fig.  10). The structural and lithologic pressure areas in the Depression. First- and second-order combination traps consisted of fan deltaic sand bodies and unconformities or sequence boundaries are favorable for normal faults in LST3-2, which were sealed by oil shale stratigraphic hydrocarbon reservoirs (Figs.  7, 8, 16, and and deep gray mudstone or source rock. Hydrocarbon 17). There are three reasons: (1) the stratigraphic traps were from overlain TST3-2 source rock and the deep interval predominantly controlled by unconformities, specially first- Sq3-1 and Sq2-2 source rocks in overpressure migrated and second-order unconformities, and in the up-dip direc- directly or up faults into the structural and lithologic com- tion stratigraphic traps are sealed by the unconformities; (2) bination traps. the slope areas of the Depression, where a lot of conformi- The Liangjialou oilfield and Tuo71 hydrocarbon res- ties were developed, were the normal fluid pressure areas ervoirs are distributed at overpressure to transitional and the main direction for the migration of hydrocarbon; Fig. 17 Hydrocarbon reservoirs forming pattern of subtle traps within Shahejie Formation in Dongying Depression. The location BB’ of the pat- tern is shown in Figs. 1b and 16 1 3 Petroleum Science (2020) 17:1–22 19 (3) the unconformities, normal faults, and sand bodies are overpressure overcoming capillary resistance (Li and Pang conduits up and along which hydrocarbon can migrate from 2004) favors hydrocarbon accumulation in the lithologic source rocks to the stratigraphic traps (Fig. 17). traps (Losh 1998; Losh et al. 1999; Li and Pang 2004). Based on the described distribution features of subtle Correlation between the oil and source rocks shows that reservoirs above, the distribution of the subtle reservoirs hydrocarbon in Sq3-1 and Sq3-2 lenticular sand bodies shows cyclic features around the source rocks, such as the originated mainly from Sq2-2 and Sq3-1 source rocks. The Lijin, Boxing, Niuzhuang, and Minfeng sags (Fig. 16). The faults and microfractures may act as conduit systems (Li lithologic reservoirs were distributed at fluid overpressure et al. 2010; Guo et al. 2014). The microfractures may be zones of the centers of sags, while structural and lithologic related by hydrofracturing associated with episodic hydro- combination reservoirs were distributed on overpressure to carbon expulsion in the overpressure zone of the depres- transition pressure zones of the slope zones of sags. The sion (Xie et al. 1998). hydrocarbon reservoirs in stratigraphic traps were distrib- The hydrocarbon reservoirs associated with uncon- uted at the margins of source rock kitchens or sages in nor- formities or sequence surfaces were petroliferous plays mal fluid pressure systems containing high-viscosity oil of stratigraphic traps. First- and second-order uncon- (Figs. 16 and 17). In addition, the hydrocarbon reservoirs formities/sequence boundaries were laterally continuous in structural traps were distributed in normal fluid pressure throughout the whole depression and dominant strati- systems too, for example, the hydrocarbon reservoirs in anti- graphic traps were associated with them. The stratigraphic cline and faulted-block traps in Sq3-4 and SS4. traps developed at the margins of the depression, which were the predominant destinations for hydrocarbon migra- tion. The traps were charged by hydrocarbon migrating 9 Controls of petroliferous plays of subtle from the overpressure source rocks up faults, along sand traps in sequence stratigraphic framework bodies, and unconformities (Guo et al. 2014; Figs. 7 and 8). The first- and second-order unconformities controlled Petroliferous plays of subtle traps are composed of sub- the distribution of the petroliferous plays of stratigraphic tle reservoirs. They are developed in special regions traps in particular (Fig. 17). controlled by source rocks, sand bodies/reservoirs, unconformities/sequence boundaries, conduits, and fluid overpressure (Fig. 17). 10 Conclusion Based on exploration data and the above analysis, the sand bodies within LSTs of sequences Sq2-2 to Sq3-3, 1. The sand bodies in the LSTs are controlled by syndepo- controlled by syndepositional faults and incised channels, sitional normal faults or slope breaks. The sand bodies capped by mudstone and shale in TSTs of the sequences, within the TSTs to early HSTs are controlled by slopes constitute petroliferous plays of structural and lithologic of prodelta and paleorelief. These sand bodies are good combination traps in the fluid overpressure zone and tran - reservoirs in lithologic and structural combination and sition pressure zone (Fig. 16). Syndepositional faults not lithologic traps. only controlled the sand bodies in LSTs, but also were 2. Abnormal overpressure, developed mainly in Sq2-2 and conduits up which the hydrocarbon in the deep intervals Sq3-1 and Sq3-2, is a dynamic mechanism for hydrocar- migrated to the subtle traps. Fluid-bearing oil forced by bon migration through conduit systems, such as faults, overpressure in Sq2-2 to Sq3-1, and Sq3-2 migrated up unconformities, and sand bodies. the faults and microfractures to the subtle traps. The rela- 3. Sand bodies within LSTs to early HSTs, covered by tively high paleogeothermal gradient anomalies in these mudstone and shale in TSTs and early HSTs, are petrolif- areas, established by fluid inclusions, are strong evidence erous plays in lithologic and structural combination traps for the presence of overpressure hydrocarbon fluid migra- and lithologic traps. tion (Feng et al. 2013). 4. The petroliferous plays in stratigraphic traps are con- Lenticular sand bodies (beach bars, small sublacustrine trolled by unconformities at margins of the depression. fans, gravity flow deposits of prodelta controlled by faults, slopes of prodelta, and slopes of paleorelief) developed in early HSTs to TSTs in sequences Sq2-2 to Sq3-2. There Acknowledgements The authors thank Petroleum Geological Institute were also lithologic traps encased in mudstone and shale. of Shengli Oil Field Corporation Ltd., Sinopec Corp., for their support Based on our research, hydrocarbon reservoirs in over 85% and permission to use industry data for this research. The three review- of the lenticular sand bodies have overpressure (Li and ers are thanked for their constructive comments. Pang 2004). None of the oil-bearing lenticular sand bodies had near-normal fluid pressure (Zhang et al. 2004). 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Distribution and controls of petroliferous plays in subtle traps within a Paleogene lacustrine sequence stratigraphic framework, Dongying Depression, Bohai Bay Basin, Eastern China

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Springer Journals
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Copyright © The Author(s) 2019
Subject
Earth Sciences; Mineral Resources; Industrial Chemistry/Chemical Engineering; Industrial and Production Engineering; Energy Policy, Economics and Management
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1672-5107
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1995-8226
DOI
10.1007/s12182-019-00387-z
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Abstract

The characteristics of petroliferous plays in subtle traps within a sequence stratigraphic framework in the Dongying Depres- sion are investigated in this study. Sand bodies within lowstand systems tracts (LSTs) of sequences, comprising incised- channel fills, sublacustrine fans, deltas in LSTs, controlled by syndepositional normal faults, and sand bodies within trans- gressive systems tracts (TSTs) to early highstand systems tracts (HSTs), consisting of beach bars, and turbidites, controlled by the prodelta slope, paleorelief, and syndepositional normal faults, are good subtle reservoirs. Mudstones and shale of deep lake subfacies in TSTs to early HSTs of sequences are source and cap rocks. Abnormal overpressure is the dominant dynamic factor for hydrocarbon migration from source rock to the subtle traps. Normal faults, sand bodies, and unconformities function as conduit systems. Sand bodies distributed in the abnormal overpressure source rocks within LSTs to early HSTs are petroliferous plays in lithologic traps. The petroliferous plays in stratigraphic traps are controlled by unconformities at margins of the Depression. Keywords Subtle traps · Sand bodies within lowstand systems tract · Syndepositional normal fault · Abnormal overpressure · Dongying Depression 1 Introduction targets for hydrocarbon exploration in these mature basins. For example, in the Mid-continent and Rocky Mountain Within the mature basins that have been explored, most areas, the flanks of anticlines and synclines for combination of the traditional and easy-to-find hydrocarbon traps have subtle traps are recommended for hydrocarbon exploration, already been located (Halbouty 1969). Subtle traps, such as even though when tested anticlines on the crests proved dry lithologic, stratigraphic, and structural and lithologic com- (Levorsen 1969). In the North Sea basin, subtle traps in the bination traps, which can hardly be found using the seismic Permian Rotliegend controlled by paleotopography have the data to recognize structural traps, are now the dominant potential for hydrocarbon exploration (Maynard and Gib- son 2001). Stratigraphic traps created by unconformities and preserved on the unconfined slope of the ancestral Missis- Edited by Jie Hao and Xiu-Qiu Peng sippi submarine fans in the northeastern Gulf of Mexico have also been found to contain substantial and profitable * You-Liang Feng fyouliang@petrochina.com.cn hydrocarbon reserves (Godo 2006). In offshore South Afri- can divergent basins, Cretaceous traps within lowstand sys- State Key Laboratory of Lithospheric Evolution, Institute tems tracts (LSTs) of sequences have been recommended of Geology and Geophysics, Chinese Academy of Sciences, for hydrocarbon exploration (Brown et al. 1995). In China, Beijing 100029, China 2 the conventional structural traps have already been found, University of Chinese Academy of Sciences, Beijing 100049, whereas subtle traps are becoming a greater contributor to China 3 the new additions to annual petroleum reserves in the Jiyang Research Institute of Petroleum Exploration Subbasin of the Bohai Bay Basin (Pang et al. 2005). and Development, PetroChina, Beijing 100083, China 4 New theories and methods, including paleotectonic and Energy and Geoscience Institute (EGI), University of Utah, paleogeomorphic reconstructions (Halbouty 1969), sequence Salt Lake City, UT 84108, USA Vol.:(0123456789) 1 3 2 Petroleum Science (2020) 17:1–22 stratigraphy (Posamentier and Vail 1988; Van Wagoner et al. 2 Geological setting and stratigraphy 1990; Brown et  al. 1995), seismic sedimentology (Zeng and Hentz 2004), and analysis of transfer zones in rift basin 2.1 Geological setting (Rosendahl 1987; Morley et al. 1990; Fossen et al. 2010; Paul and Mitra 2013), are used to predict subtle traps. Sequence The Bohai Bay Basin is a large Cenozoic petroliferous stratigraphy has been applied successfully to non-marine strata continental rift basin in Eastern China (Fig. 1a). The basin for constructing a sequence stratigraphic framework, interpret- is bounded by Taihang Mountains to the west, Yanshan ing sequence architecture, and forecasting reservoirs in subtle Mountains to the north, Jiaoliao Uplift to the east, and traps (Shanley and McCabe 1994; Lin et al. 2001; Martino Luxi Uplift to the south (Allen et  al. 1997; Feng et  al. 2004; Zecchin et al. 2006; Feng et al. 2013; Jiang et al. 2013; 2016). During the Cenozoic era, the Bohai Bay Basin Feng et al. 2016). Abnormal overpressure has been studied in underwent Paleogene rifting and Neogene subsiding stages petroliferous basins as a dynamic force for hydrocarbon migra- (Allen et al. 1997; Yang et al. 2016), resulting in thick tion and accumulation in subtle and structural traps (Caillet Paleogene, Neogene, and Quaternary lacustrine deposits. et al. 1997; Lee and Williams 2000; Zhang et al. 2009a; Guo The Bohai Bay Basin consists of several subbasins sepa- et al. 2010). rated by uplifts (Fig. 1a). Some studies also have attempted to interpret the distribu- The  NE-S W  tr ending  Dongying  Depr es - tion and controls of subtle reservoirs by using (1) source rocks sion is located in the southeastern part of the Jiyang Subba- (Hu 2010; Jiang et al. 2014), (2) sedimentary facies (Zou et al. sin. The area of the depression is about 5700 km . Paleogene 2005), (3) the relationship between porosity and fluid potential synrift strata of the depression rest unconformably on pre- (Li and Pang 2004), (4) normal faults and microfractures as Paleogene strata. The Paleogene synrift strata are typically conduit systems for hydrocarbon reservoirs of conventional 4000–7000 m (Yao et al. 1994; Zong et al. 1999; Feng et al. and subtle traps (Losh 1998; Losh et al. 1999; Li et al. 2010; 2013). Lampe et al. 2012), and (5) faults controlling hydrocarbon Raised Precambrian basement blocks are bounded by the migration and accumulation in a continental rift basin (Mu Chenjiazhuang Rise to its north, the Luxi Uplift and Guan- 2012; Wei and Su 2015). Some progress has been made in grao Rise to its south, and the Binxian and Qingcheng Rises understanding the conditions of hydrocarbon migration and to its west (Fig. 1b) (Feng et al. 2013; Pang et al. 2015). accumulation in subtle traps, and the distribution and controls NE-trending extensional structures such as normal faults, of subtle reservoirs within the sequence stratigraphic frame- northern faulted margin, southern hinged margin, deep work in continental rift basins, such as Dongying Depression troughs/sags, intra-depression fault blocks and transfer (Li and Pang 2004; Feng et al. 2005; Pang et al. 2005; Guo zones, anticlines genetically associated with normal faults, et al. 2010, 2012, 2014). In order to perfect the distribution and and negative flower-shaped structures are developed in an controls of subtle reservoirs within the sequence stratigraphic intense dextral transtensional stress field (Allen et al. 1997; framework, this study focuses on four aspects: Ren et al. 2002; Feng et al. 2010, 2013, 2016) during the First, the sequence stratigraphic framework of Paleogene Paleogene (Fig. 1b, c). strata was reconstructed based on 2D and 3D seismic pro- Besides northern faulted margin and southern hinged files, well logs, and drill cores. margin, the depression can be divided into four sags or sub- Second, sand bodies in lowstand systems tracts, con- depressions, such as Minfeng, Lijin, Niuzhuang, and Boxing trolled by sys-depositional normal faults, were identified as sags, by two negative flower-shaped structures—one striking reservoirs in subtle traps and described within the sequence N–E and one E–W—and one NNW-striking Shicun normal- stratigraphic framework of Paleogene strata. fault belt (Fig. 1b, c). Third, the authors describe the types and distributions of The structure of the Dongying Depression is a complex hydrocarbon reservoirs in subtle traps as well as investigate half-graben defined by a faulted margin in the north, sags, conditions of hydrocarbon migration and accumulation. negative flower-shaped structures at the middle, and a hinged Finally, the authors discussed the factors controlling margin in the south (Fig. 1c). petroliferous plays of subtle traps in the sequence strati- Similar to the Bohai Bay rift basin as a whole, the graphic framework. Depression is complicated by episodic rifting events, includ- ing block faulting associated with rapid tectonic subsidence and volcanism (Hsiao et al. 2004, Lin et al. 2004). A two- stage evolution model is accepted, with Paleogene synrift and differential subsidence, and Neogene post-rift and ther - mal subsidence. Paleogene synrift strata formed the major hydrocarbon source rocks, reservoir rocks, and cap rocks in 1 3 Petroleum Science (2020) 17:1–22 3 AC Fig. 1 Tectonic location of Dongying Depression, Bohai Bay Basin, eastern China (a), schematic map of structural units of the Dongying Depression (b), and tectonostratigraphy section (c). The locations of well-tied seismic sections in Fig. 3 a well-tied sequence stratigraphic profile in Fig. 4, well-tied hydrocarbon reservoir sections in Fig. 7 (section ①), 8 (section②), 9 (section ③), 10 (section ④), and well-tied EFP profile in Fig. 13 (section ⑤), 14 (section ⑥), location of hydrocarbon reservoirs forming pattern of subtle reservoirs (AA′) in Fig. 17 are indicated in the map (b). Some important wells are also shown in the map (b) (modified from Feng et al. 2013) the depression (Fig. 2) (Allen et al. 1997; Wan 2004; Feng 2.2 Stratigraphy et al. 2013). The Paleogene synrifting stage consists of four rifting The Paleogene strata in the Dongying Depression consist of episodes (Fig. 2): (1) early-initial rifting beginning in the the Kongdian Formation (E1-2 k) overlain by the Shahejie Paleocene and ending in the Early Eocene (65–50.4 Ma); Formation (E2 s), which is itself overlain by the Dongying (2) late-initial rifting in the Middle Eocene (50.4–42.5 Ma); Formation (E3d) (Fig. 2). (3) rift climax in the Late Eocene (42.5–38 Ma); and (4) The lowest part of the second member of the E1-2 k weakened rifting in the Oligocene (38–24.6 Ma) (Feng et al. comprises conglomerate, sandstone, and coarse sandstone 2013; Yin et al. 2018). interbedded with purple to red mudstones. The deposi- The Dongying Depression is a typical Paleogene rift basin tional environments of the interval are interpreted as allu- (Allen et al. 1997; Zong et al. 1999; Wan 2004). It is one of vial fan and braided river (Li et al. 1992). The uppermost the most petroliferous depressions in the Bohai Bay Basin. or youngest part of the second member of the E1-2 k is Episodic extensional tectonic events and their associated composed of alternating layers of gray mudstone, oil shale, extensional structures and depositional fills have resulted in and medium- to fine-grained sandstones (Fig.  2). The sedi- multiple source rocks (Li et al. 2003; Han et al. 2018), com- mentary environments of the interval are interpreted as binations of reservoirs and cap rocks, and hydrocarbon reser- deep lake, fluvial delta. The first member of the E1-2 k voirs in structural and subtle traps (Li and Pang 2004). High comprises alternating layers of red sandstone and gray density of exploration wells has provided a good chance to mudstone; the sedimentary environment of the interval study the distribution and controls of the hydrocarbon reser- is interpreted as shallow lake (Li et al. 1992; Wang et al. voirs in subtle traps and to examine the conditions of hydro- 2016). carbon migration and accumulation in subtle traps in rift basins within the sequence stratigraphic framework. 1 3 4 Petroleum Science (2020) 17:1–22 Fig. 2 General sequence stratigraphic charts, and the factors for hydrocarbon accumulation of Paleogene strata in the Dongying Depression (modified from Feng 1999; Feng et al. 2013). The classification of different order sequence stratigraphy is based on the changes in lake level, episodic tectonic activities, assemblage biozones, climate, and detailed work of this paper. The ages of sequence boundaries are determined from micropaleotologic data, e.g., ostracoda and palynologic data, paleomagnetic dating, and volcanic rock dating (Chen and Peng 1985; Li et  al. 1992; Yao et al. 1994; Feng 1994). The S is source rock; R is reservoir; C is caprock. FS: first-order sequence; SSq: second-order sequence; Sq: third-order sequence. Sq3-1 means first third-order sequence within second-order sequence 3 (modified from Feng et al. 2013) The lowest part of the fourth member of Shahejie Forma- conglomerate as well as sandstone interbedded with purple tion (E2s4) consists of alternating layers of red sandstone to red mudstones. The sedimentary environments of the and mudstone interbedded with saline deposits. The sedi- interval are interpreted as a meandering river and delta mentary environment is interpreted as a salt lake (Wang plain (Wang 1992; Feng et  al. 2013). The top strata of 1992). The top strata of E2s4 are composed of gray mud- E2-3s2 are composed of conglomerates and coarse sand- stone, oil shale intercalated with sandstone, and thin layers stones interbedded with red mudstone. The sedimentary of limestone. The sedimentary environment is interpreted as environment is interpreted as a braided river (Feng et al. a shallow lake (Song et al. 2012; Lu et al. 2017). 2013; Wang et al. 2019) (Fig. 2). The third member of the Shahejie Formation (E2s3) is The lowest part of the first member of the Shahejie Upper Eocene. The lowest strata of the third member (E2s3) Formation (E3s1) is composed of sandstones interbedded comprise gray to dark mudstones and oil shale. The middle with green and gray mudstones. The middle part of E3s1 and upper parts of the E2s3 are composed of fine sandstone comprises gray mudstone and shale interbedded with thin interbedded with gray mudstone, and coarse sandstone and limestone layers. The uppermost part of E3S1 is composed sandy gravels intercalated with green mudstone. The sedi- of sandstone interbedded with gray mudstone. The deposi- mentary environments of E2s3 are interpreted as a deep lake, tional environment of E3S1 is interpreted as a shallow lake prodelta, and delta front (Feng et al. 1991, 2013; Feng 1999). (Zhang et al. 2014; Wang 1992; Yan et al. 2019). The lowest strata of the second member of the Shahejie The Dongying Formation (E3d) consists of coarse sand- Formation (E2-3s2) consist of conglomerate and sandy stone, medium to fine sandstones interbedded with gray 1 3 Petroleum Science (2020) 17:1–22 5 mudstone, and gray to greenish mudstone and red mud- is about 4.5–10.0 Ma, such as T8/SSb2, T6/SSb3/Sb3-1, stone (Fig. 2). The sedimentary environment of the For- and T2′/SSb4/Sb4-1 (Figs.  2 and 3a, b); (3) third-order mation is interpreted as a meandering river (Wang 1992). unconformities are localized and laterally change to their correlative conformities (Embry 1995, 2002), and the time duration of the two third-order unconformities is about 1.0–2.0 Ma. These unconformities or sequence boundaries 3 Datasets and methods can be identified on seismic profiles, logging curves, and drill cores (Feng et al. 2013). This study was based primarily on petroleum geologi- Third-order sequence boundaries appear on the seismic cal data from 400 exploration boreholes and regional reflection profile as a truncation below the boundary, and 3D seismic data covering approximately 5600 km in the onlap above it. Down-dip toward the center of the depres- Dongying Depression (Figs.  1 and 2). The seismic data sion, such boundaries become correlative conformities were extracted from a series of surveys acquired during (Fig. 3a, b; Sb3-1–Sb3-4) (Feng et al. 2013). 1997–2010 and were reprocessed and assembled into a In well-tied profiles such as those in Fig.  4, a third-order single dataset for this study. Data with a vertical resolution sequence boundary indicates a scoured base at the bottom of of 15–25 m in Paleogene strata (derived from a 35–40 Hz a fining-upward succession from conglomerate at the base dominant-frequency seismic dataset with a 3600  m/s to sandstones and mudstones at the top, the fining-upward average velocity) are sufficient for the reconstruction of succession only existed in lowstand systems tracts (LSTs), a stratigraphic sequence framework within the region of and the SP curve for such LSTs is bell-shaped. Examples seismic coverage. Exploration borehole data, including log include the base of sublacustrine fans or braided channels curves, cores, drill stem test (DST), organic geochemis- (Fig. 4) (Feng et al. 2013). The third-order sequence bound- try data, and results of oil well tests, were also used for ary is also the boundary between the fluvial and deltaic front this research. The research method of this paper has been deposits in lacustrine basins, or subaerial exposure surfaces described as follows: (Fig. 4) (Feng et al. 2013). Finger-shaped SP curves indicate fluvial deposits; serrated-funnel shapes indicate a delta front 1. Interpretation of spontaneous potential curves and seis- deposit within the deltaic succession. mic profiles was used to identify sequences and systems tracts. 2. The sonic logging data, DST, and results of oil well tests 4.2 Sequence hierarchy were used to predict abnormal overpressure in strata. 3. Well logs and 3D seismic data were also used to inter- The stratigraphic architecture of a succession accumulated pret sand bodies as reservoirs of subtle traps on the basis in a rifted lacustrine basin is different from that in a post- of the shapes of the spontaneous potential (SP) curves rift lacustrine succession (Hubbard 1988; Williams 1993; within sequences. Carroll and Bohacs 1999; Feng et al. 2000, 2010, 2016). 4. Source rocks were evaluated and correlated with oil Hubbard (1988) and Williams (1993) classified synrift using organic geochemistry data. and post-rift strata as two kinds of first-order sequences through their studies of rifted basins on continental margins. 4 Stratigraphic sequence The synrift (Paleogene) succession in Dongying Depression could be accordingly classified as a first-order 4.1 Sequence stratigraphy boundaries sequences 1 (FS1,) confined by first-order unconformities. and hierarchy FS1 was subdivided into four second-order sequences (SSq) 1 to 4 from bottom to top (SSq1 to SSq4) recognized The three orders of unconformities are identified in the by second-order unconformities. Paleogene synrift strata in the Dongying Depression and SSq1 corresponds to rifting Episode 1 and roughly con- represent three levels of sequence boundaries: (1) First- sists of the Kongdian Formation (E1-2 k), and two third-order order unconformities are angular unconformities at the top sequences (Sqs) are identified in SSq1 based on their charac- (24.6 Ma) and bottom (65.0 Ma) of the synrift strata, and teristic third-order unconformities and correlative conformi- the time duration of the two unconformities is 40.4 Ma, ties (Van Wagoner et al. 1990). SSq2 corresponds to Episode such as angular unconformities FSb1/Tr (Fig.  3a; Feng 2 and roughly consists of the fourth member of the Shahejie et al. 2013) and FSb2/T1 (Fig. 3a); (2) second-order angu- Formation (E2s4), and two Sqs are recognized in SSq2. SSq3 lar unconformities occur between two rifting episodes, and corresponds to Episode 3 and roughly consists of the third the time duration of the two second-order unconformities member (E2s3) and the lower part of the second member of 1 3 Two-way-time(ms) Two-way-time(ms) 6 Petroleum Science (2020) 17:1–22 Xinzhen negative flower CDP North South shaped structure 140 220 Feng9Yan4 460 500 100 180 260 300 380 420 540580 Ng T1/FSb2 T4/Sb3-3 T2’/SSb4/Sb4-1 Sq3-4 Sq3-3 Sb3-4 T6’/Sb3-2 T4/Sb3-3 Sq3-2 Chennan Sq3-1 T6/SSb3/Sb3-1 fault T6/SSb3/Sb3-1 T7 Tr/FSb1 T8/SSb2 T7/SSb2 A North to south well-tied seismic section on line 625.7 East west Lai43 Shi128 Niu11 Niu25 Niu43Wang68Wang53Wang70Wang11Wang103 T2’/SSb45/Sb4-1 Sq3-4 Sb3-4 Sq3-3 Sq3-1 2 2 2 1 3 6 4 Sq3-2 T4/Sb3-3 5 T6’/Sb3-2 1 T6/SSb3/Sb3-1 97.5 B East to west well-tied seismic section on line 97.5 Fig. 3 Well-tied seismic sections of interpreted structures and sequence stratigraphic framework: a north to south well-tied seismic section on line 625.7(the location is shown in Fig. 1b); b east to west well-tied seismic section on line 97.5 (the location is shown in Fig. 1b); two-way time is used to indicate buried depth of interfaces of strata. Sq3-1 to Sq3-4 stand for four third-order sequences from bottom to top. The numbers ① to ⑥ stand for fourth-order sequences on the seismic profile. T and Sb/SSb/FSb stand for seismic reflection horizon and sequence boundaries, respectively (Feng et al. 2013) the Shahejie Formation (E2s2  ), and four Sqs are identified 5 Reservoirs for subtle traps and their in SSq3. SSq4 corresponds to rifting Episode 4 and roughly controls comprises the upper part of the second member of the Shahejie Formation (E3s2 ) to the Dongying Formation (E3d), and four Based on the seismic data, well logs and cores, and pub- Sqs are recognized in SSq4 (Feng et al. 2013). lished results (Feng et al. 2013, 2016), the authors identi- Based on the presence of initial (Ifs) and maximum flood- fied the sand bodies which are reservoirs for the subtle ing surfaces (Mfs) within third-order sequences as well as traps within the sequence stratigraphic framework of the stratal terminations and stacking patterns, a series of systems Paleogene strata (Fig. 5; Table 1). tracts (i.e., lowstand systems tracts (LSTs), transgressive sys- tems tracts, (TSTs), and highstand systems tracts (HSTs)) can 1. Fluvial delta sand bodies with 5%–18% porosity and −3 2 be distinguished (Fig. 4). 1–20 × 10  μm permeability within LSTs. 2. Fan-delta sand bodies with 8%–15% porosity and −3 2 10–32 × 10  μm permeability adjacent to the northern faulted margin (Fig. 4; Feng et al. 2013) within LSTs. 1 3 Petroleum Science (2020) 17:1–22 7 Fig. 4 Well-tied sequences stratigraphic framework from north to south in the Dongying Depression. The location BB′ of this cross section is shown in Fig.  1b. The well-tied profile shows the stratigraphic stacking pattern of the third-order sequences in second-order sequence 3 (third member to lower part of second member of Shahejie Formation) at faulted and hinged margins. The curve at the left side of the well is spontane- ous potential (SP) log and the curve at the right side of it is the resistivity (RT) log (Feng et al. 2013) 3. Incised-channel fills with 15%–20% porosity and southern hinged margins of the depression, LST sand bodies −3 2 10–20 × 10  μm permeability within LSTs, and sub- of turbidite or sublacustrine fans and small deltas, were also lacustrine fan sand bodies with 15%–20% porosity and deposited on the down-dip (lakeward) side of these faults, −3 2 1.4–650 × 10  μm permeability at the southern hinged and were found along their length too (Fig. 5; Feng et al. margin in LSTs (Fig. 5; Feng et al. 2013; Table 1). 2013). Incised-channel fills within LSTs were deposited on 4. Sand bodies of turbidite fans with 5%–13% porosity and the up-dip (landward) side of the faults. −3 2 1.0–10.0 × 10  μm permeability within LSTS, TSTs, The sand bodies of beach bars within TSTs and early and early HSTs (Figs. 4 and 5; Feng et al. 2013). HSTs were distributed on the ramp or subaqueous high. For 5. Shallow-lake beach bar sand bodies with 6%–15% poros- example, beach bar sand bodies within TSTs were distrib- −3 2 ity and 1.0–5 × 10  μm permeability along the western uted on the ramp of a hinged margin in Sq2-2 and on the and southern ramp slopes of the Depression within TSTs ramp of a faulted margin in Sq3-1, Sq3-2, and Sq3-3. They to early HSTs (Fig. 4; Feng et al. 2013). were controlled by subaqueous highs, such as low buried hills, subaqueous volcanoes, anticlines, and horsts, and were The distribution of sand bodies in the LSTs of the distributed along slopes of subaqueous highs (Song et al. sequences was controlled by syndepositional faults or 2012; Feng et al. 2013). syndepositional fault slope-break zones (Feng et al. 2013, Sand bodies of sublacustrine or turbidite fans within 2016). The sand bodies are good reservoirs for subtle traps TSTs and early HSTs, including sandy debrites and turbid- (Feng et al. 2013). For example, steeply dipping parallel and ites, were developed in deep lake and on prodelta slopes and cross-shaped syndepositional faults were developed mostly were controlled by the slopes and normal faults, respectively at the faulted margins of Sq3-1 and Sq3-2 (Fig. 5; Feng et al. (Feng et al. 1991; Zou et al. 2012; Liu et al. 2017). 2013), the sand bodies of fan deltas, turbidite, or sublacus- trine fans in LSTs were deposited on the down-dip (lake- ward) side of the faults and were found along their length. Only small fluvial deltas in LSTs, controlled by gently dip- ping parallel syndepositional faults, occurred. In Sq3-3, gen- tly dipping parallel syndepositional faults occurred at the 1 3 8 Petroleum Science (2020) 17:1–22 Fig. 5 Distribution maps of sand bodies within LSTs of Sq3-1 to Sq3-3 in Dongying Depression (Feng et al. 2016) 1 3 Petroleum Science (2020) 17:1–22 9 Table 1 Reservoir physical property of sand bodies in lowstand systems tracts (the reservoir physical property data based on Li and Pang 2004, and Petroleum Geological Institute of Shengli Oil Field Corporation Ltd., Sinopec Corp.) −3 2 Type of sand body Porosity (%) Permeability × 10  μm Area/well Systems tracts Sublacustrine fan 15–20 1.4–650 Lijin sag LST Incised-channel fill 15–20 10–20 S103 LST Fan delta 8.0–15 10–32 Well T71 LST Fluvial delta 5–18 1–20 F104 LST Turbidite fan 5–13 1–10 L911 TST Brach bar 6–15 1–5 Linfanjia TST up-dip sand bodies were greater than those of the lenticular 6 Types of subtle traps and subtle reservoirs sand bodies (Table 1; Li and Pang 2004). Hydrocarbon reservoirs of lenticular sand bodies of tur- The Paleogene subtle traps and subtle reservoirs in the bidite fans did not have unified oil–water contacts, and each Dongying Depression occurred in two petroleum plays, one hydrocarbon reservoir had an oil–water contact (Figs. 7 and in the source rocks of the upper part of E2s4 (Sq2-2) and 8). They were distributed in the Sq3-2 (the middle of E2s3) E2s3 (including Sq3-1 and Sq3-2, Sq3-3) and the other in source rock of Lijin and Niuzhuang sags, for example, in the E2s2 to E2d, which are far away from source rocks (includ- hydrocarbon reservoir of the lenticular sand body traps of ing Sq3-4 and Sq4-1 to Sq4-4) (Fig. 2) (Li et al. 2003, 2010; turbidite fan drilled through by the Well Ying 11 in the Lijin Zhang et al. 2009b; Guo et al. 2012; Chen et al. 2016). Sub- sag (Fig. 7). Hydrocarbon reservoirs of up-dip sandstone tle traps and subtle reservoirs in the Depression include (1) pinch-out occurred mainly on the southern hinged margin. lithologic traps and lithologic reservoirs; (2) stratigraphic Oil was found only near the top of the individual up-dip and traps and stratigraphic hydrocarbon reservoirs; and (3) struc- pinch-out sand body (Figs. 7 and 8). tural and lithologic combination traps and their reservoirs (Fig. 6). 6.2 Stratigraphic traps and stratigraphic hydrocarbon reservoirs 6.1 Lithologic traps and lithologic reservoirs Stratigraphic traps, including stratigraphic unconformity The lithologic traps formed by lithologic variation, such and stratigraphic overlap traps, were formed by stratigraphic as lenticular sand bodies and up-dip sandstone pinch-out, truncation below unconformities or sequence boundaries, are widespread in the strata from Sq2-2 (the upper part of with an onlap above the unconformities, which were wide- E2s4) to Sq3-2 (the middle part of E2s3) (Figs. 7 and 8). spread in the Paleogene strata at the southern hinged mar- They were developed in source rocks and in sags. Based gin (Figs. 6, 7, and 8). Beach bar and deltaic front sand- on a sedimentary study, they consist of sublacustrine fan stones, with good physical properties, constitute reservoirs or turbidity deposits that developed in LSTs to TSTs of the in stratigraphic traps (Guo et  al. 2014). The traps were Sq3-1 (the lower part of E2s3) and Sq3-2 (the middle part of capped by the unconformities and mudstones near them. In E2s3) source rocks (Feng et al. 2013) and beach bar deposits the Depression, the hydrocarbon from Sq2-2, Sq3-1, and that developed in Sq2-2 (the upper part of E2s4) (Song et al. Sq3-2 mature source rocks migrated up faults, sand bodies, 2012). and unconformities to form stratigraphic hydrocarbon res- The lateral extent and quality of the reservoirs vary sub- ervoirs including stratigraphic unconformity reservoirs and stantially because of diagenesis and facies variation. The stratigraphic overlap reservoirs (Li et al. 2010). The Jingjia porosity of the lenticular sand bodies of turbidite fans and oilfield (Fig.  8) at the southern hinged margin is a typical beach bars was approximately 5%–13% and 6%–15%, respec- −3 2 example. The first-order unconformity between Neogene and tively, with permeability of approximately 1–10 × 10  μm −3 2 Paleogene and the second unconformity between Sq4-1 and and 1–5 × 10  μm , respectively (Table 1). The up-dip sand Sq3-4 control the distribution of stratigraphic hydrocarbon body pinch-out was developed along flanks of paleostructure reservoirs. highs on the western margin in the Depression, as well as on gentle southern slopes (Figs. 4, 7, and 8). The sand bod- ies forming up-dip and pinch-out were the dominant sand bodies of the sublacustrine fans and deltaic fronts in LSTs (Figs.  4, 7, and 8). The porosity and permeability of the 1 3 10 Petroleum Science (2020) 17:1–22 Fig. 6 Classification maps of subtle traps and hydrocarbon reservoirs in Dongying Depression based on explored examples reserve can exceed 50 million tons (360 million bbl), such 6.3 Lithologic and structural combination traps as the Liangjialou oilfield (Hao et al. 2005), which is a typi- and hydrocarbon reservoirs cal example in subtle reservoirs in a lithologic structural combination trap (Fig. 9). Their reservoirs are sand bodies Based on features of hydrocarbon reservoir profiles (Figs.  7, in LST3-3 within Sq3-3 (Figs. 5 and 9). These sand bodies 8, 9, and 10), lithologic and structural combination traps are belong to deposits in incised-channel fills and sublacustrine a kind of the subtle traps in the Depression too (Figs. 6, 7, fans (Figs. 5 and 9; Feng et al. 2013). They were capped 9, and 10). According to depositional facies maps (Fig. 5) by TST3-3 deep lake mudstone and cut up-dip by normal and logging data, the trap reservoirs were made of sand- faults (Fig. 9). The geochemical properties of the oils have stones of sublacustrine fans with 15%–20% porosity and −3 2 intermediate values between those observed in extracts from 1.4–650 × 10  μm permeability, delta with 5%–18% poros- −3 2 the TST3-1 (the lower part of E2s3) and Sq2-2 (the upper ity and 1–20 × 10  μm permeability, and incised-channel −3 2 part of E2s4) source rocks (Zhang et al. 2004; Hao et al. fills with 15%–20% porosity and 10–20 × 10 μm perme- 2005; Li et al. 2010). As a consequence, the oils in the sub- ability in LSTs (Fig. 5, Table 1). They are capped by deep tle traps were most likely derived from mixed Sq3-1 and lacustrine mudstone and oil shale in TSTs (Figs. 7, 9, and Sq2-2 source rocks, with a predominant contribution from 10). The traps were lateral pinch-out or facies change and the Sq2-2 source rocks (Hao et al. 2005; Li et al. 2010). were cut up-dip by normal faults (Figs. 7, 9, and 10). They Hydrocarbon may have migrated up faults from Sq2-2 and have the potential to form petroleum plays whose proven 1 3 Petroleum Science (2020) 17:1–22 11 Fig. 7 Well-tied profile of hydrocarbon reservoirs in Dongying Depression from north to south. The location of the profile (①) is indicated in Fig. 1b. Symbols of sequences are shown on the profile same as Fig. 2 Fig. 8 Well-tied profile of hydrocarbon reservoirs at Jingjia oilfield within the sequence stratigraphic framework from north to south at south- ern hinged margin. The location of the profile (②) is indicated in Fig.  1b. RT is resistivity curve, SP is spontaneous potential curve. Symbols of sequences are shown on the profile same as Fig. 2 −3 3 Sq3-1 source rocks to these subtle traps (Fig. 9). Another and 10–32 × 10  μm permeability in the LST3-2 of Sq3-2, example is the Tou71 subtle reservoirs at the northern were capped by deep TST2 lake mudstone and were lateral faulted margin (Fig. 10). The reservoirs, ranged from fan pinch-out or facies change and cut by faults. Hydrocarbon deltaic sandstone to conglomerate with 8%–15% porosity from Sq2-2 and Sq3-1 migrated to the subtle traps up faults. 1 3 12 Petroleum Science (2020) 17:1–22 Fig. 9 Well-tied profile of hydrocarbon reservoirs of structural/fault and lithologic combination traps within systems tracts of sequence Sq3-3 from north to south at Liangjialou oilfield. The location of the profile (③) is indicated in Fig. 1b The source rocks in TST3-2 within Sq3-2, such as shale and these parameters, the characteristics of the overpressure mudstone, capped on the traps, could also migrate to the were studied. subtle traps directly (Fig. 9). Based on the results of fluid pressure calculated by Feng et al. (2006) using sonic logging of 300 wells, the features of the fluid pressure of wells in the depression can be divided 7 Distribution features and genesis into two types: normal fluid pressure of wells at margins of abnormal overpressure and shallow wells, and abnormal high fluid pressure of deep wells at sags (Fig. 11). Within abnormal overpressure areas, 7.1 Distribution features of abnormal overpressure such as Well Li101 in the Lijin sag (Fig. 11), fluid pressure can be divided into further three parts. When the depth of DST data and sonic loggings show that abnormal overpres- the well is less than 2200 m, the strata fluid pressure is equal sure develops in Sq3-1 and Sq3-2 strata (Xie 2001; Zhang to the hydrostatic pressure. When the well depth is between et  al. 2009a, b; Guo et  al. 2010; Hao 2013). Abnormal 2200 and 3000 m, the fluid pressure coefficient (PC) of the overpressure is a dominant dynamic factor for hydrocarbon strata is between 1.0 and 1.2. The depth between 2200 m and accumulation in subtle traps in abnormal overpressure strata 3000 m is called the transition zone of fluid pressure. When (Losh 1998; Losh et al. 1999; Li et al. 2004; Guo et al. 2010, the well depth exceeds 3000 m, the PC is greater than that Lampe et al. 2012). To distinguish abnormal overpressure, of 1.2, and the fluid overpressure occurs (Fig.  12). pressure coefficient (PC) and excess fluid pressure (EFP) The plane view and profile distribution of EFP of mud- are used in this study. The pressure coefficient is a ratio of stone can be drawn using EFP data calculated from sonic the actual pore pressure versus the normal hydrostatic pres- logging by Feng et al. (2006) (Figs.  12, 13, and 14). On sure at the same depth. The EFP is a value of the actual the EFP map of the bottom of Sq3-2, high EFP (> 16 Mpa) pore pressure of strata subtracted from the normal hydro- was distributed at Lijin, Boxing, Niuzhuang, and Minfeng static pressure. Because of the limited amount of DTS data, sags. The mean value of EFP occurred in the structural belts sonic logging of 300 boreholes was used to predict the fluid between sags. At the southern margin, EFP was equal to 0, pressure, the fluid pressure coefficient, and the EFP. Using and fluid pressure was hydrostatic (Fig.  13). On the well- tied sections of EFP, the top surface was shallower at the 1 3 Petroleum Science (2020) 17:1–22 13 Fig. 10 Well-tied hydrocarbon reservoir profile of Tou71 structural/fault and lithologic combination hydrocarbon reservoirs within systems tracts of sequence Sq3-2 at faulted margin from north to south. The location of the profile (④) is indicated in Fig. 1b central negative flower shape structural belt, fault belts, and development are compaction disequilibrium and hydrocar- the margins, and deepened at sags, for example, at the Lijin bon generation in the Depression. and Minfeng sags. The highest fluid overpressure occurred Some researchers think compaction disequilibrium is in strata from Sq2-2 to Sq3-1 and at Sq3-2 (Fig. 12). the dominant mechanism for overpressure generation in the Dongying Depression (Xie et al. 1998; Xie 2001). The rea- 7.2 T he genesis and significance of abnormal son is if fluids cannot be expelled sufficiently when a for - overpressure mation is increasing rapidly, the disequilibrium compac- tion occurs rapidly (Osborne and Swarbrick 1997; Hooker In general, mechanism for overpressure includes com- et al. 2017). However, in Dongyin Depression, there are paction disequilibrium (Rubey and Hubert 1959; Magara no apparent characteristics of compaction disequilibrium, 1975; Mara et al. 2009; Ramdhan and Goulty 2011; Lahann such as anomalously high porosities or low density in the 2017), hydrocarbon generation (Bredehoeft et al. 1994; Guo overpressured mudstones (Guo et al. 2010). Therefore, the et al. 2010), and fluid release during dehydration reactions overpressure of mudstones was not generated by compac- (Magara 1975; Hooker et al. 2017). However, clay dehydra- tion disequilibrium, for they are under normal compaction. tion cannot generate significant overpressure unless there is a Other data, such as the sedimentary rate of strata and the perfect seal (Luo and Vasseur 1992; Osborne and Swarbrick hydrocarbon potential of source rocks, show that overpres- 1997). Therefore, the major mechanisms for overpressure sure correlated with hydrocarbon generation other than 1 3 14 Petroleum Science (2020) 17:1–22 Fig. 11 Section of fluid pressure of well Li101 at Lijin sag compaction disequilibrium is caused by quick deposition. from the transformation of high-density organic matter to For example, in well Niu38 the sedimentation rate of the low-density fluids (oil or gas) exceeded the rate of volume upper part of Sq3-2 is approximately 800 mm/1000 years, loss caused by fluid migration and expulsion (Berg et al. and approximately 400–200 mm/1000 years for the lower 1999; Lee and Williams 2000; Fossen et al. 2010; Das- part of Sq3-2 to Sq3-1 (Fig.  15). However, the higher gupta et al. 2016; Hooker et al. 2017). This overpressure overpressure in the Depression is present in the strata of mechanism relies on the kerogen type, density of organic the lower part of Sq3-2 to Sq3-1 and Sq2-2 (Figs. 13 and matter, rock permeability, and thermal history (Osborne 14), which are deep lake deposits with slow sedimenta- and Swarbrick 1997; Tingay et al. 2009; Dixit et al. 2017). tion rate and are excellent source rocks for hydrocarbon These excellent source rocks of the Depression, located in (Fig.  15). Higher overpressure was not present in strata Sq3-1 and Sq2-2, have dominant kerogen types I to II, an of the upper part of Sq3-2, which are prodeltaic deposits oil window at 2700, and contain tight reservoirs (Li and with higher sedimentation rates and are not a good source Pang 2004). These conditions favor overpressure caused rock (Fig. 15). Based on the fluid pressure, PC, and EFP by hydrocarbon generation. predicted using sonic logging, the top surface of the abnor- The overpressure is a dynamic factor overcoming capil- mal overpressure zone (PC < 1.2 or EFP > 0) was located lary resistance (Li and Pang 2004). The relationship between at a depth of 2700–3000 m (Figs. 13 and 14). This is in porosity and the pressure of sand bodies with oil-bearing agreement with the oil window (2700 m) of the Sq3-2 to saturation exceeding 80% in lithologic traps was studied by Sq3-1 and Sq2-2 source rocks. This evidence shows that Li and Pang (2004). They found that porosity was the reverse overpressure in the depression was likely caused by hydro- ratio to fluid pressure in oil-bearing sandstone. Higher over - carbon generation, if the rate of volume increase resulted pressure is favorable for hydrocarbon accumulation in the 1 3 0 Petroleum Science (2020) 17:1–22 15 Fig. 12 Distribution of hydrocarbon reservoirs and EFP contours of mudstone on Sq3-2 top surface (middle part of third member of Shahejie Formation). EFP is excessive fluid pressure Bin408 Li101Niu1 SHi119 He6 Ying68 Ying65 Ying34 Ying11 Yong38 Ying90 West East SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP SP EFP 2000 2000 Sq3-3 3000 Sq3-2 Sq3-1 Fault Sq2-2 Fault Fault 0 1000m Vertical scale: 0 8000m SP curve Sequence boundary Contour of EFP calculated EFPcurve Horizontal scale: SP: Spontaneous potential EFP: Excess fluid pressure curve Fig. 13 Profile of well-tied EFP in the depression from East to West. The location of the profile (⑤) is indicated in Fig.  1b. EFP is excessive fluid pressure 1 3 Depth/m Depth/m 16 Petroleum Science (2020) 17:1–22 Fig. 14 Profile of well-tied EFP in the depression from north to south. The location of the profile (⑥) is indicated in Fig.  1b. EFP is excessive fluid pressure Fig. 15 Column of lithology, Paleomagnetic dating, sedimentary rate, and organic geochemistry in Well Niu 38 1 3 Petroleum Science (2020) 17:1–22 17 tight sandstones of lenticular lithologic traps in particular lithologic reservoirs in Nuizhang, Boxing, and Lijin sags (Li and Pang 2004). For example, in the Liangjialou and (Figs. 7 and 8). They were distributed in Sq2-2, Sq3-1, and Niuzhuang oilfields, based on the DST data, lithologic res- Sq3-2 source rocks at fluid overpressure zones of the center ervoirs in Sq3-1 and Sq3-2 and Sq2-2 have overpressure and of sags; the overpressure is a dynamic factor overcoming high oil-bearing saturation (Li and Pang 2004; Guo et al. capillary resistance of tight reservoirs to form lithologic 2010; Hao 2013). reservoirs in source rocks (Li and Pang 2004), for example, the Ying 11 hydrocarbon reservoir in the lithologic trap at fluid overpressure zone of the Lijin sag, and the Niuzhuang 8 Distribution of subtle reservoirs oilfield of the lithologic reservoir in the fluid overpressure zone of the Niuzhuang sag (Figs. 7, 12, and 16). Subtle reservoirs are important exploration targets in the Depression (Li et al. 2004) and have special distribution 8.2 Distribution of lithologic and structural characteristics. combination reservoirs The hydrocarbon reservoirs in lithologic and structural 8.1 Distribution of lithologic reservoirs combination traps were distributed in syndepositional faults or slope-break zones and near anticlines and fluid The lithologic reservoirs were distributed at overpressure overpressure to transitional pressure zones of the slopes zones of the depression, which are located at the centers and in the sags (Figs. 7, 9, 10, and 16). In the sequence strati- slopes of sags in Sq2-1, Sq3-1, and Sq3-2, e.g., Niuzhuang, graphic framework, the hydrocarbon reservoirs in litho- Minfeng, Lijin, and Boxing sags (Figs. 7, 12, and 16). The logic and structural combination traps were located in lithologic traps are composed of lenticular turbidite fan and LSTs. For example, (1) the Liangjialou oilfield is a typi- beach bar sand bodies or upward pitchout of sand bodies cal hydrocarbon reservoir in a structural and lithologic within source rocks of Sq2-2, Sq3-1, and Sq3-2, such as combination trap located in LST3-3 within Sq3-3 on Fig. 16 Relationship between distribution of subtle reservoirs and fluid pressure field in Dongying Depression 1 3 18 Petroleum Science (2020) 17:1–22 the southern hinged margin (Fig. 9). The sand bodies of pressure zone of the margins of the Lijin sag source sublacustrine fans and channels controlled by syndeposi- rocks (Fig.  16). Because the Sq3-1, Sq3-2, and Sq2-2 tional faults or slope breaks developed in LST3-3 (Feng source rocks are in the overpressure zone, the oil-bearing et al. 2013); they were capped by deep lake mudstone in fluid should be discharging episodically up the faults or TST3-3. Based on the correlation between source rocks microfractures from the overpressure zone to form sub- and oil, the oil migrated from the Sq2-2 and Sq3-1 source tle reservoirs. Normal faults play important roles in the rocks because of the higher gammacerane content and migration of the hydrocarbon from the source rocks of lower Pr/Ph ratio (Li et al. 2010). In the Sq2-2 and Sq3-1 deep intervals to the traps of shallow intervals. They are source rocks in the f luid overpressure zone (Figs. 11 and conduit systems for hydrocarbon migration (Losh 1998; 12), the hydrocarbon, driven by overpressure, migrated Losh et al. 1999). up faults and accumulated in the structural and lithologic combination traps. The pressure coefficients of the hydro- 8.3 Distribution of stratigraphic hydrocarbon carbon reservoirs obtained using DST ranged from 1.2 to reservoirs 1.48, showing that the hydrocarbon reservoirs had over- pressure (Feng et al. 2006). (2) Another example is the The stratigraphic hydrocarbon reservoirs occur near uncon- Tou71 hydrocarbon reservoirs in LST3-2 of Sq3-2 at the formities or sequence boundaries, which are normal fluid faulted margin (Fig.  10). The structural and lithologic pressure areas in the Depression. First- and second-order combination traps consisted of fan deltaic sand bodies and unconformities or sequence boundaries are favorable for normal faults in LST3-2, which were sealed by oil shale stratigraphic hydrocarbon reservoirs (Figs.  7, 8, 16, and and deep gray mudstone or source rock. Hydrocarbon 17). There are three reasons: (1) the stratigraphic traps were from overlain TST3-2 source rock and the deep interval predominantly controlled by unconformities, specially first- Sq3-1 and Sq2-2 source rocks in overpressure migrated and second-order unconformities, and in the up-dip direc- directly or up faults into the structural and lithologic com- tion stratigraphic traps are sealed by the unconformities; (2) bination traps. the slope areas of the Depression, where a lot of conformi- The Liangjialou oilfield and Tuo71 hydrocarbon res- ties were developed, were the normal fluid pressure areas ervoirs are distributed at overpressure to transitional and the main direction for the migration of hydrocarbon; Fig. 17 Hydrocarbon reservoirs forming pattern of subtle traps within Shahejie Formation in Dongying Depression. The location BB’ of the pat- tern is shown in Figs. 1b and 16 1 3 Petroleum Science (2020) 17:1–22 19 (3) the unconformities, normal faults, and sand bodies are overpressure overcoming capillary resistance (Li and Pang conduits up and along which hydrocarbon can migrate from 2004) favors hydrocarbon accumulation in the lithologic source rocks to the stratigraphic traps (Fig. 17). traps (Losh 1998; Losh et al. 1999; Li and Pang 2004). Based on the described distribution features of subtle Correlation between the oil and source rocks shows that reservoirs above, the distribution of the subtle reservoirs hydrocarbon in Sq3-1 and Sq3-2 lenticular sand bodies shows cyclic features around the source rocks, such as the originated mainly from Sq2-2 and Sq3-1 source rocks. The Lijin, Boxing, Niuzhuang, and Minfeng sags (Fig. 16). The faults and microfractures may act as conduit systems (Li lithologic reservoirs were distributed at fluid overpressure et al. 2010; Guo et al. 2014). The microfractures may be zones of the centers of sags, while structural and lithologic related by hydrofracturing associated with episodic hydro- combination reservoirs were distributed on overpressure to carbon expulsion in the overpressure zone of the depres- transition pressure zones of the slope zones of sags. The sion (Xie et al. 1998). hydrocarbon reservoirs in stratigraphic traps were distrib- The hydrocarbon reservoirs associated with uncon- uted at the margins of source rock kitchens or sages in nor- formities or sequence surfaces were petroliferous plays mal fluid pressure systems containing high-viscosity oil of stratigraphic traps. First- and second-order uncon- (Figs. 16 and 17). In addition, the hydrocarbon reservoirs formities/sequence boundaries were laterally continuous in structural traps were distributed in normal fluid pressure throughout the whole depression and dominant strati- systems too, for example, the hydrocarbon reservoirs in anti- graphic traps were associated with them. The stratigraphic cline and faulted-block traps in Sq3-4 and SS4. traps developed at the margins of the depression, which were the predominant destinations for hydrocarbon migra- tion. The traps were charged by hydrocarbon migrating 9 Controls of petroliferous plays of subtle from the overpressure source rocks up faults, along sand traps in sequence stratigraphic framework bodies, and unconformities (Guo et al. 2014; Figs. 7 and 8). The first- and second-order unconformities controlled Petroliferous plays of subtle traps are composed of sub- the distribution of the petroliferous plays of stratigraphic tle reservoirs. They are developed in special regions traps in particular (Fig. 17). controlled by source rocks, sand bodies/reservoirs, unconformities/sequence boundaries, conduits, and fluid overpressure (Fig. 17). 10 Conclusion Based on exploration data and the above analysis, the sand bodies within LSTs of sequences Sq2-2 to Sq3-3, 1. The sand bodies in the LSTs are controlled by syndepo- controlled by syndepositional faults and incised channels, sitional normal faults or slope breaks. The sand bodies capped by mudstone and shale in TSTs of the sequences, within the TSTs to early HSTs are controlled by slopes constitute petroliferous plays of structural and lithologic of prodelta and paleorelief. These sand bodies are good combination traps in the fluid overpressure zone and tran - reservoirs in lithologic and structural combination and sition pressure zone (Fig. 16). Syndepositional faults not lithologic traps. only controlled the sand bodies in LSTs, but also were 2. Abnormal overpressure, developed mainly in Sq2-2 and conduits up which the hydrocarbon in the deep intervals Sq3-1 and Sq3-2, is a dynamic mechanism for hydrocar- migrated to the subtle traps. Fluid-bearing oil forced by bon migration through conduit systems, such as faults, overpressure in Sq2-2 to Sq3-1, and Sq3-2 migrated up unconformities, and sand bodies. the faults and microfractures to the subtle traps. The rela- 3. Sand bodies within LSTs to early HSTs, covered by tively high paleogeothermal gradient anomalies in these mudstone and shale in TSTs and early HSTs, are petrolif- areas, established by fluid inclusions, are strong evidence erous plays in lithologic and structural combination traps for the presence of overpressure hydrocarbon fluid migra- and lithologic traps. tion (Feng et al. 2013). 4. The petroliferous plays in stratigraphic traps are con- Lenticular sand bodies (beach bars, small sublacustrine trolled by unconformities at margins of the depression. fans, gravity flow deposits of prodelta controlled by faults, slopes of prodelta, and slopes of paleorelief) developed in early HSTs to TSTs in sequences Sq2-2 to Sq3-2. There Acknowledgements The authors thank Petroleum Geological Institute were also lithologic traps encased in mudstone and shale. of Shengli Oil Field Corporation Ltd., Sinopec Corp., for their support Based on our research, hydrocarbon reservoirs in over 85% and permission to use industry data for this research. The three review- of the lenticular sand bodies have overpressure (Li and ers are thanked for their constructive comments. Pang 2004). None of the oil-bearing lenticular sand bodies had near-normal fluid pressure (Zhang et al. 2004). 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