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Tertiary hydrothermal activity and its effect on reservoir properties in the Xihu Depression, East China Sea

Tertiary hydrothermal activity and its effect on reservoir properties in the Xihu Depression,... Three large-scale episodes of volcanic activity occurred during the Tertiary in the Xihu Depression, located in the East China Sea. Intermediate-felsic magmas intruded along faults and the associated hydrothermal fluids resulted in the hydrothermal alteration of the clastic country rock. To better describe reservoir characteristics, reservoir samples were subjected to the following investigations: thin section examination, scanning electron microscope-energy dispersive spectrometer analysis (SEM–EDS), fluid inclusion homogenization temperature tests, vitrinite reflectance measurements, and X-ray diffraction. The results of this study provide evidence of the following hydrothermal alteration phenomena: brittle fracturing, clastic particle alteration, precipitation of unique hydrothermal minerals (celestite, zircon, apatite, barite, and cerous phosphate). The pres- ence of abnormally high temperatures is indicated by fluid inclusion analysis, the precipitation of high-temperature authigenic minerals such as quartz, illite alteration, and anomalous vitrinite reflectance. Two aspects related to hydrothermal effects on reservoir properties have been investigated in this study: (1) Deep magmatic hydrothermal fluids carry large amounts of dissolved carbon dioxide and sulfur dioxide gas. These fluids percolate into the country rocks along fault zones, resulting in dissolution within the sandstone reservoirs and the development of significant secondary porosity. (2) Magma intrusions increase the temperature of the surrounding rocks and accelerate the thermal evolution of hydrocarbon source rocks. This results in the release of large amounts of organic acids and carbon dioxide, leading the dissolution of the aluminosilicate minerals and volcanic fragments in the reservoirs, and the generation of significant secondary porosity. Keywords Hydrothermal activities · Erosion effects · Clastic rock reservoir · Secondary porosity · Xihu Depression 1 Introduction which are highly reactive (Tao and Xu 1994). Hydrothermal fluids can originate from magmatic fluids, metamorphic flu - The term “hydrothermal fluids” refers to all high-tempera - ids, hot brine and/or formation water in sedimentary basins, ture aqueous fluids (temperature range from 50 to 400 °C) and fluids from primary mantle fluids (Chen et al. 2007). In that contain many chemical materials in solution (e.g., H S, this paper, the term “hydrothermal fluids” refers to magmatic HCl, HF, SO , CO, CO, H, N , KCl, and NaCl), some of hydrothermal fluids. 2 2 2 2 Magmatism occurs in many sedimentary basins around the world and has a significant impact on the generation, migration, and accumulation of oil and gas as well as the Edited by Jie Hao formation of hydrocarbon reservoirs due to the two follow- * Si-Ding Jin ing mechanisms (Ye et al. 2005; Agusto et al. 2013). (1) jinsiding@cdut.edu.cn Thermal baking caused by the magmatic intrusion heats the surrounding rocks and results in mineral transformations. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, The thermal baking effect on hydrocarbon source rocks Chengdu 610059, Sichuan, China accelerates the generation of alkanes, organic acids, and car- College of Energy and Resources, Chengdu University bon dioxide (Guo 2002). Girard and Nahon (1989) proposed of Technology, Chengdu 610059, Sichuan, China the concept of “contact diagenesis,” the “abnormally high Sichuan Key Laboratory of Shale Gas Evaluation temperatures” from the magmatic intrusion result in changes and Exploitation, Sichuan Keyuan Testing Center, in the authigenic mineral assemblage in the contact zone. (2) Chengdu 610091, Sichuan, China Vol:.(1234567890) 1 3 Petroleum Science (2019) 16:14–31 15 A series of water–rock reactions within the country rocks Depression (with an average thermal flow of 71 mW/m ), takes place as hydrothermal fluids move along migration Wei et al. (1994) suggested that regional thermal anomalies pathways. This has a vital impact on reservoir performance. were related to late-stage magmatic activity, fault develop- Hydrothermal activity may result in the pores of reservoir ment, hydrothermal activity, basement uplift and topogra- rocks being filled with zeolite, calcite, chlorite, and siliceous phy. The relatively frequent episodes of Tertiary magmatism minerals, which reduce the porosity and permeability of the in the central and southern parts of this area can be deline- country rocks to a considerable degree and therefore nega- ated in seismic profiles (Hu and Tao 1997), and most wells tively impact the reservoir quality (Wang and Zhang 2001). have encountered magmatic rocks (Zhou and Song 2014). In addition, researchers have suggested that hydrothermal Through analysis of the abnormally high porosity zones fluids related to magmatic-volcanic activity are rich in CO , in the Paleogene of the Xihu Depression, Su et al. (2016) SO , and H S. The migration of these hydrothermal fluids suggested that feldspar and other minerals had been eroded 2 2 along fault and fracture systems accelerates the thermal evo- due to the action of organic acids and/or fluids, resulting lution of the hydrocarbon source rocks and released organic in secondary porosity. Cao et al. (2017) indicated that the acids and CO form a dissolution alteration zone (Heden- presence of abnormal overpressures inhibited compaction by quist and Henley 1985; Yu et al. 2012; Liu et al. 2017; Wang overlying strata and increased the intensity of the dissolution et al. 2017a, b). For example, cyclic thermal convection in effect, therefore resulting in the abnormally high porosities sedimentary basins on the western coast of Canada gener- observed in the deep sandstone reservoirs. ated a dissolution zone of carbonate minerals and improved To date, the effect of magmatic thermal fluids on res- reservoir quality (Charlou et al. 2010; Schmidt et al. 2011). ervoirs in the Xihu Depression has not been studied in In the Pearl River Mouth Basin in China, magmatism not detail. In the case of the Xihu depression, which is diffi- only caused relatively strong deformation and contact meta- cult to explore and develop and therefore has a high eco- morphism but also provided a significant amount of heat, nomic threshold, it is very important to clarify the control- which resulted in abnormally high geothermal gradients ling factors on high quality reservoirs in the context of the accelerating the maturation of hydrocarbon source rocks low porosity and low permeability observed in these deep and leading to the release of more organic acids, resulting reservoirs (> 3200 m). This research has led to a theoreti- in the development of secondary porosity (Nie et al. 2005; cal basis for the evaluation of deep, high quality reservoirs Zhu et al. 1994). Sugisaki and Mimura (1994) proposed in the study area. In this paper, the authors investigated in that hydrothermal fluids rich in CO underwent a chemical detail evidence for deep hydrothermal processes and their reaction with the reservoir rocks, causing the dissolution of effects on surrounding country rocks, documenting for the quartz and feldspar particles which improves the reservoir first time the influence of magmatic hydrothermal fluids on properties of these rocks. Evidence for intrusive magmatic the reservoir properties of the country rocks. activity has been identified in different regions across the The study focuses on the reservoirs of the Paleo- world during exploration for oil and gas, such as in the Mich- gene–Eocene Pinghu Formation and the Oligocene Hua- igan Basin in the United States (Wierzbicki et al. 2006), the gang Formation in the Xihu Depression of the East China Songliao Basin and the Jiyang Depression in eastern China Sea. Cores from 14 wells in the southern part of the depres- (Wang et al. 1990), the offshore Bohai Bay Basin in Eastern sion have been studied. A range of methods, including core China (Wang and Zhang 2001), the Dongpu Depression in and thin section examination, electron microprobe analysis, eastern China (Zhu et al. 1994) and the offshore Yinggehai fluid inclusion analysis, and vitrinite reflectance, have been Basin in Southern China (Wang et al. 2010). In these basins, employed to investigate the samples’ petrography and min- magmatic hydrothermal fluids have altered the reservoirs, eralogy to identify the presence and impact of magmatic and most of these altered reservoirs form a group of key hydrothermal fluids. hydrocarbon play pathways. Consequently, the impact of magmatic activity and associated magmatic hydrothermal fluids on petroleum systems has drawn a significant amount 2 Geological setting of research interest in the field of oil and gas exploration and development (Shu et al. 2003). The Xihu Depression is within the continental shelf basin Exploration in the Xihu Depression, part of the conti- of the East China Sea located on China’s eastern continental nental shelf basin of the East China Sea has resulted in the margin (Fig. 1). It is a tertiary oil and gas bearing depres- discovery of considerable oil and gas reserves. The amount sion in the northern part of the East Zhejiang Depression. of oil and gas resources in the Xihu Depression is esti- Classed as a continental margin rift-depression basin it cov- mated to be more than 4.67 billion tonnes (Cao et al. 2017; ers an area of 59,000 km . The boundary to the west is the Huang et al. 2010). Based on the analysis of temperature Diaoyu Islands fold zone, and to the east are the Hupijiao, and thermal conductivity measurements taken in the Xihu Haijiao, and Yushan Uplifts (Fig. 1). (Liu 1992; Xu et al. 1 3 Qiantang-Tianwaitian great fault Geling great fault 16 Petroleum Science (2019) 16:14–31 (a) (b) Fujiang depression Shanghai Q2 Ningbo Haijiao uplift W10 W7 W8 J1 J2 W9 G2 G1 Magmatic Sag and uplift boundary rock Structural belt Normal Fault boundary W6 Region Major Fault boundary W5 Study area Study well W1 W1 W4 W2 Contrast well 0 10 20 30 km Q1 W3 Fig. 1 Regional tectonic map and distribution of magmatic rocks of the Xihu Depression 1997; Liu et  al. 2003). The structural framework shows NNE-trending normal faults. The Oujiang Movement was NNE–NE, NW, and approximately E–W-trending fault zones the main period of extensional rifting during the early that have undergone intense activity. The Xihu Depression Eocene (E ). (2) The Local Subsidence stage comprises three can be divided into five tectonic units from west to east: The tectonic episodes including: (a) The Yuquan Movement, Baochu slope zone, the San-tan deep sag, the Central anti- the first reversal period during the early Oligocene, with cline, the Bai-di deep sag and the Eastern fault zone (Fig. 1). compression, folding and uplift accompanied by magmatic Cenozoic clastic sediments developed in the Xihu activity, and the development of the boundary interface (T ) Depression include (from bottom to top), the Paleocene (E ), between the faulting stage and the Local Subsidence stage. Eocene Baoshi Formation (E b), Pinghu Formation (E p), (b) The Huagang Movement, the second inversion period 2 2 Oligocene Huagang Formation (E h), Miocene Longjing during the late Oligocene, resulting in folding related to 1 2 Formation (N l), Yuquan Formation (N y), Liulang Forma- reverse faulting, uplift and erosion. This inversion led to 1 1 tion (N l), Pliocene Santan Formation (N s), and Quater- folding, uplift and exposure of the formations proximal to 1 2 nary East China Sea Formation (Qd) (Fig. 2). The Pinghu the fault zone. (c) The Longjing Movement; the third phase Formation depositional environment was a deltaic sedi- of inversion at the end of the Miocene, resulted in tectonic mentary system propagating into a restricted bay that was inversion of the Central Depression Belt characterized by affected by tidal action. The Huagang Formation contains folding, uplifting and erosion. (3) Regional Subsidence three sedimentary systems, namely lacustrine facies, braided stage: The main period of subsidence began Pliocene (N ). river delta facies and fluvial facies. The reservoir lithology During the subsidence of the basin, the Neogene-Quaternary mainly consists of well-sorted feldspathic, lithic sandstones strata were deformed by E–W-trending tensile-shear faults, and lithic feldspathic sandstones with good sorting, and mainly located in the central tectonic zone and the slope of with rarer occurrence of lithic quartz sandstones and felds- the Xihu Depression (Zhang 2013). pathic quartz sandstones. On the basis of the analysis of the The main faults are strike-slip extensional normal faults sedimentary environment, the strata can be characterized as and extensional faults with a dip angle of 60°–80° in the layers of mudstone layers, silty mudstones and sandstones Xihu Depression. The main active time of the faulting was separated by andesites and basalts. late Cretaceous–Oligocene. The distribution of magmatic The Xihu Depression underwent a faulting stage, a local rocks in the research area is clearly controlled by faults (Cai subsidence stage and a regional subsidence stage. (1) Fault- et al. 2014) (Fig. 1). The magmatic rocks are mostly dis- ing stage: The Keelung Movement resulted in initial rift- tributed along deep NE–NNE-trending faults (Zhang et al. ing during the Late Cretaceous. Paleocene-Eocene sedi- 2014). The age of the magmatism decreases gradually from ments distributed across the entire Xihu Depression were west to east; this is consistent with the fault activity time that cut by large basement faults, most of which were NE- and shows a decreasing trend from west to east, associated with 1 3 Diaobei Diaoyudao uplift-fold zone depression Haijiao uplift Donghai basin Bai-di deep sa Bai-di deep sag g Eastern fault zone Lingyin main fault Diaoyu uplift-fold zone Xihu depression Central anticline Tianwaitian main fault Chunxiao main fault Baochu slop zone San-tan deep sag Pinghu main fault Yuquan main fault Lingbei main fault Baoshi main fault Petroleum Science (2019) 16:14–31 17 Fig. 2 The distribution of Formations formation lithology in Xihu Boundary, Tectonic Basin Lithology Depression age movement tectonic stage System Series Group Quat- Donghai Group Regional ernary (Qpdh) subsidence Trough movement T 1.75 Ma Santan Group (N s) Longjing T 5.30 Ma 2 movement Liulang Group (N l) T 10.2 Ma Yuquan Group (N y) Depression stage T 16.2 Ma Longjing Group (N l) T 23.5 Ma Huagang Group T 30.0 Ma (E h) Yuquan T 33.7 Ma movement T 36.5 Ma Pinghu Group (E p) T 38.1 Ma Faulted 4 stage T 41.2 Ma Baoshi Group (E b) Oujiang movement T 53.0 Ma Keelung T 65.0 Ma movement Cretac- eous Mudstone Siltstone Silty mudstone Andesite Legend Basalt Fine sandstones Andesite lava breccia depositional center migration from west to east either (Hu intermediate-mafic igneous rocks, Late Oligocene emplace- and Tao 1997). Faulted regions are ideal pathways for the ment of intermediate-felsic magmatic rocks, and emplace- upward migration of deep fluids (Meng et al. 2008). ment of Miocene magmatic rocks (Shen et al. 2001) (Fig. 2, Based on seismic, gravity and magmatic data, geochemi- Table 1). In particular, the Late Oligocene intermediate-fel- cal data, and isotope chronology data (K–Ar and U–Pb sic magmatic rocks penetrate through the T reflection layer methods), three episodes of Cenozoic volcanic activity (33.7 Ma) and intrude into individual seismic sequences in have been identified by previous researchers. The activi- the form of dykes. The intense magmatic episode in the Mio- ties include two kinds of intrusions and eruptions, forming cene resulted in intrusions and magma flows along the T intrusive rocks and volcanic rocks dominated by interme- group bedding surfaces. diate acid rocks, including Early Eocene emplacement of 1 3 Neogene Paleogene Plio- Palaeo- Miocene Oligocene Eocene cene cene 18 Petroleum Science (2019) 16:14–31 Table 1 The distribution of magmatic rocks in wells Well Magmatic rock section, m Horizon Thickness, m Rock types W5 2809–3104 Huagang Formation (E h) 195 Andesite-dacites, basalt-andesite 3104–3204.7 Pinghu Formation (E p) 100 Meta andesitic volcanic breccia W6 4774–4847 Pinghu Formation (E p) 73 Andesite, diorite, tuff 4895–4983 Pinghu Formation (E p) 22 Altered dacite W7 3566–3685.5 Pinghu Formation (E p) 1.07 Andesite, basalt, andesitic tuff lava, tuff 3840.5–4053.5 Pinghu Formation (E p) 265 Tuff breccia, granite, granodiorite 4190–4240, 4262–4275 Pinghu Formation (E p) 63 Tuff W8 1995–2012.5 Longjing Formation (N l) 17.5 Dacites cutting crystal tuff, dacites tuff breccia diorite, andesite, tuff W9 2381–2560 Longjing Formation (N l) 47 Altered tuff W10 2162.5–2167.9 Huagang Formation (E h) 5.4 Crystal tuff G1 1995–2012.5 Yuquan Formation (N y) 17.5 Tuff Sample data from Wei et al. (1994), Hu and Tao (1997), Zhou and Song (2014) Based on observations of the successive precipitation of acidic fluids linked to the influence of magmatic hydro- minerals in response to pressure and temperature changes, thermal fluids on the source rock thermal evolution has the diagenetic evolution sequence is determined as fol- been discussed in this paper. lows: mechanical compaction, early carbonate cementa- tion, early dissolution (caused by water-soluble corro- sion resulting in feldspar erosion to kaolinite), secondary 3 Methodology enlargement of quartz, late carbonate cementation, late dissolution, iron and calcite and dolomite precipitation. Seventy-four core samples were collected from 10 wells in The formation of secondary pores is mainly controlled the Xihu Depression, well W1 in the Baoshi Fault Zone, by late cementation and dissolution. This late dissolution wells W2–W6 in the Chunxiao Fault Zone, well W7 in the occurs in the middle diagenetic stages A-B that occurred Pinghu Fault Zone, wells W8, W9 in the Tianwaitian Fault in the Late Oligocene and Miocene, thus providing the link Zone and well W10 in the Yuquan main fault (Fig. 1). The between burial history and hydrocarbon generation. Once samples were analyzed at the State Key Laboratory for Oil the source rock passed through the hydrocarbon generation and Gas Reservoir Geology and Exploration at the Chengdu window, three subsequent pulses of hydrocarbon migra- University of Technology. The following is an overview of tion took place during the Early Miocene (25 Ma), Late the methods used in this investigation: Miocene (10.4–6.1 Ma) and Early Pliocene (2.2–0 Ma), (Fig. 3) (Cao 2016; Su et al. 2016). The late dissolution (1) A Nikon E600 microscope and a Wisesoft microscopic occurred prior to hydrocarbon charging. image analyzer were used to examine and study lithol- Previous studies on the burial and thermal history of ogy and pore structures, magnification is 25–400 times. the Xihu Depression suggested that the thermal effects of (2) A CL8200 MK5 cathode luminescence microscope was magmatic intrusives were detectable in the southern part used to determine the texture of quartz and feldspar of the central uplift zone and the Sudi Structural Zone debris, the growth zone of calcite cement, and the clay (Zhou 2003; Su et al. 2016). Correlating with pulses of minerals. The voltage of the beam is 15 kV, and the magmatism in the Xihu Depression, three phases of mag- beam current is 300 μA. matic hydrothermal activity could be identified: an Early (3) A field-emission environmental scanning electron Paleocene-Eocene event, a Late Oligocene event, and a microscope (Quanta 250 FEG, manufactured by FEI, Miocene event. Early and mid-term hydrothermal activity United States) with an energy-dispersive spectrometer has had the greatest impact on hydrocarbon generation (EDS) was used to conduct high-resolution morpholog- and expulsion from source rocks. The regional heat from ical observations and analyses of rock samples in the the Early Paleocene-Eocene magmatism accelerated the ESEM™ vacuum environment. The EDS was used to thermal evolution of the source rocks. By the end of the characterize the structure and composition of the sam- Oligocene, large amounts of hydrocarbons and associated ples from their surface images and component images. gas had been generated (Gu et al. 2001). Therefore, the The resolution of the electronic image is 1.0 nm @ mechanism of dissolution of alkaline minerals by C O -rich 30 kV, 3.0 nm @1 kV, and the resolution of the back- 1 3 Petroleum Science (2019) 16:14–31 19 Early diagenetic stage Middle diagenetic stage Diagenetic Contempor- event aneous stage Stage A Stage B Stage A Stage B Ro 0.35 0.52 1.3 .0 Paleotemperature 65 85 140 175 Maturation stage Immature stage Semi-mature stage Low mature stage High mature stage during diagenesis Compaction and pressure-solution Calcite cementation Cementation of ferroan-calcite Siliceous cementation Cementation of chlorite Cementation of illite Cementation of kaolinite Dissolution Metasomatism Hydrothermal activities Buried depth <1000-2000 m 2000-4000 m>4000 m EO MP E h E3 h E2 p E p E2 p E2b 1 2 3 Stratigraphic ages, Ma 40 20 0 (a) (b) (c) (d) (e) (f) Fig. 3 The coupling diagram of diagenetic evolution, hydrothermal secondary enlargement—Late calcite cementation—The feldspar was activity and oil and gas charging in research in the Xihu Depres- metasomatized by calcite, d W5, 3725.32 m, × 100, Ferroan calcite, e sion (1) the first oil and gas charging stage, (2) the second oil and J1, 3296.12 m, × 20, Feldspar dissolution and kaolinite precipitation, gas charging stage, (3) the third oil and gas charging stage. a W10, f W10, 3444.2  m, × 100, Chlorite rims—Quartz secondary enlarge- 3148.6  m, × 100, Mechanical-chemical compaction, b W8, 3410  m, ment—Late calcite cementation × 100, Early calcite cementation, c W5, 3725.79  m, × 100, Quartz 1 3 Diagenetic evolution sequence Depth, m 20 Petroleum Science (2019) 16:14–31 scattered electronic image is 2.5 nm. The experimental with an average value of 77%. The feldspar content ranges temperature is 21 ± 4 °C; the humidity is ≤ 65% RH. between 10% and 27% with an average value of 11.1%. The (4) A Rigaku D/Max-2500PC fully automatic powder feldspars are primarily potassium feldspars with some acidic X-ray diffractometer was utilized to measure the com- plagioclase. The lithic grain content varies between 5% and positions (total rock + clay) and mineral contents of 40% with an average of 11.6%. Lithic grains consist mainly sample powders. The samples were ground to less of argillaceous, calcareous, and siliceous sedimentary lithic than 40 μm, and then pressed to make specimens for grains or low-grade metamorphic lithic grains with minor testing. The diffraction peak intensity of different min- amounts of volcanic and intrusive lithic grains. Three main eral components is generally expressed by the integral petrological characteristics were identified related to high strength following subtraction of the background. The pressures and hydrothermal fluids associated with magma working voltage of the diffractometer was 40 kV, the intrusion. These were: brittle fractures and alteration of electric current is 40 mA, and the angular accuracy of skeleton particles, precipitation of associated hydrothermal the equipment is better than 0.02 degrees. THMS600G minerals, and the thermal alteration of the country rocks. automatic hot and cold stations (Linkam company) Brittle fracturing and the alteration of skeleton particles and A Nikon E600 microscope was used to measure were observed in the clastic rocks near the magmatic body temperature of fluid inclusions, the determination of in all 10 wells. For example, well-log analysis in wells W1, temperature ranges from − 196 °C to 600 °C, and the W6, W7 and W9 led to the identification of magmatic bodies 2 1 temperature precision is ± 0.01 °C. in the Yuquan Formation (N y), Longjing Formation (N l), 1 1 (5) A J&M microspectrophotometer (Germany) and a Zeiss Huagang Formation (E h), Pinghu Formation (E p), and 3 2 polarizing microscope (Germany) were used to meas- Baoshi Formation (E b). Brittle fractures in clastic parti- ure the vitrinite reflectance, R , of the samples. During cles located near magmatic bodies were seen in thin section the study, the characteristics of the microcomponents of (Fig. 4). the organic matter in the samples were examined under Celestite (SrSO ), apatite (Ca (PO ) (OH) ), barite 4 10 4 6 2 50 times magnification and with a reflectance range of (BaSO ), and cerous phosphate (CePO ·H O) were identified 4 4 2 0.59%–10%. from environmental scanning electron microscope observa- (6) A Zeiss polarized fluorescence microscope A1-HBO tions and energy spectrum analysis of samples in wells W6, 100 (Germany) and a Cooling–Heating Stage W7, and W9 (Fig. 5). Linkam—TH600 were used to analyze the homog- The examination of thin sections and scanning electron enization temperatures of the inclusions in rock sam- microscope analyses showed that large amounts of miner- ples. After observing the microscopic characteristics als in the reservoirs were related to hydrothermal activity, of the fluid inclusions under the polarizing fluores- including the formation of authigenic quartz and transforma- cence microscope, the inclusions were placed in the tion of clay minerals. Cooling–Heating Stage apparatus. The temperature at Authigenic quartz cement was well developed in the which the gas phase or liquid phase of the fluid inclu- study area and was mostly in the form of secondary over- sions disappears is the homogenization temperature. growths and microcrystalline quartz (Fig.  6a, d). Intra- The temperature of the experimental environment is particle fractures in the quartz grains were found in Well 20–25 °C, the humidity is 30%, and the temperature W7 at 3718.32 m using cathodoluminescence (CL). During precision is ± 1 °C. CL examination, quartz generally shows a brown, bluish- (7) An Autopore 9500 made by the Mike Murray Feldman purple, and/or non-luminescent character (Fig.  6b). The Instrument Co. Ltd. (Shanghai) was used to measure quartz at a depth of 3719.12 m was bluish-purple, or light pore-throat radius. The maximum external pressure brown and non-luminescent; the secondary quartz at a depth applicable was 60,000 psia. of 2258.58 m was bluish-purple under CL (Fig.  6c). The characteristics of quartz under CL are known to be closely related to diagenesis temperature, with the color changes 4 Results gradually from blue to bluish-purple as the temperature increases. 4.1 Petrological evidence for the influence Sixty-seven sandstone samples from 1113 to 4500 m in of magmatic hydrothermal fluids Well W7 were used to identify the different clay minerals contents using X-ray diffraction. The results showed that Analysis of 74 thin sections and SEM analyses of 22 sam- the content of kaolinite decreased with increasing depth, ples from the 14 wells from the southern Xihu Depression while illite content generally increased with increasing showed that the quartz content of the deep sandstone reser- depth. Illite was identified in Well W7 at a depth of approx- voirs in the Xihu Depression ranges between 60% and 95% imately 1000 m and accounted for 40%–60% of the total 1 3 Petroleum Science (2019) 16:14–31 21 Fig. 4 Brittle fractures inside quartz grains. a W1, 2215.58 m, b W7, 3443.97 m, c W6, 4052.62 m, d W9, 4198.34 m clay minerals (Fig. 6). Based on SEM observations, illite 4.2 Fluid inclusion temperature indicators occurs in several locations, including pore wall linings, solid inclusions in clastic grains, and pore-l fi ling authigenic illite. Fluid inclusions are relatively well developed in the healed The pore lining authigenic illite crystal morphology is fine fractures in quartz samples from the study area. For example, needles perpendicular to the surfaces of the clastic parti- gas–liquid, two-phase aqueous fluid inclusions were identi- cles (Fig. 6e–h). Where illite occurs as solid inclusions in fied along healed fractures in the quartz grains in well W9 clastic grains, the crystals form parallel to the surface of the located in the Tianwaitian main fault (Fig. 7a). At a depth grains and were not present in contacts between particles. of 2824.6 m, the homogenization temperatures of the inclu- The pore-filling illite was found at the exterior margins of sions range from 131.5 to 139.1 °C with an average value the pore wall lining. Illite is an authigenic clay mineral that of 134.2 °C, which is much higher than the regional burial is known to occur within a temperature range of 120–300 °C temperature of 118 °C at this depth (geothermal gradient of (Yu et al. 2012). 3.7–3.8 °C/100 m in the southern of Central anticline). Simi- Under the normal geothermal gradient, 3.4–3.5 °C/100 m larly, the highest measured homogenization temperature was and considering the burial history of the Xihu Depression, 156.6 °C at a depth of 3294.8 m (Fig. 7b), which was much significant amounts of illite should begin to appear at depths higher than the regional burial temperature of 131.1 °C at exceeding 3200 m. At this depth, formation temperature this depth, assuming a normal geothermal gradient. is approximately 120  °C. Kaolinite becomes unstable at Thirty-five samples from six wells in the Sudi tectonic temperatures between 120 and 150 °C and transforms into belt, taken between 3294.8 and 3365.8 m, were analyzed illite under potassium-rich conditions (Huang et al. 2009) for fluid inclusions. The results showed that the homog - (Eq. (1)), resulting in increased illite content. Substantial enization temperatures of the fluid inclusions were in amounts of illite were identified at shallow depths (approxi- the ranges of 130–140 °C and 150–160 °C at depths of mately 1000 m) in well W7. Therefore, there is a negative 2748.5–3423.7  m (Fig.  8a). Twenty-five samples from correlation between kaolinite and illite at temperatures two wells in the Xiling tectonic belt taken between 3248.5 between 120 and 150 °C in well W7 (Fig. 6). and 3423.7 m were analyzed for fluid inclusions, and the homogenization temperatures of the fluid inclusions varied 3Al Si O (OH) (kaolinite) + 2K from 120 to 130 °C and 150 to 160 °C with sample depths 2 2 5 4 (1) between 3294.8 and 3365.8 m (Fig. 8b). With reference to → 2KAl Si O (OH) (illite)+ 2H + 2H O 3 3 10 2 2 1 3 22 Petroleum Science (2019) 16:14–31 Fig. 5 Hydrothermal minerals identified by FEG-SEM backscatter electron imaging and EDS in the Xihu Depression. a Celestite (SrSO ), W6, 3964.78 m; b Apatite (Ca (PO ) (OH) ), W6, 3964.78 m; c Barite (BaSO ), W9, 3743.72 m; d Cerous phosphate (CePO ·H O), W7, 2215.58 m 10 4 6 2 4 4 2 the Paleogene geothermal gradient of 3.4–3.8 °C/100 m for R values at various depths from three wells (W1, W7, this region (Zhou 2003), and considering the burial his- W9) close to fault zones and affected by magmatic bod- tory, the normal burial temperature at the sampled depths ies and hydrothermal fluids were measured using a J & M should only be 108–126 °C in the Sudi Structural Zone and microspectrophotometer. R values showed thermal anoma- 120–127 °C in the Xiling Tectonic Zone. The abnormally lies at the following depth ranges. In well W1, the highest high fluid inclusion paleotemperatures were observed in R value (0.79%) was obtained at depths between 3060 and the study area. 3200 m (Fig. 9a). In well W7, the highest R values were identified at 2408–2574 m (the range of R is 0.58%–0.65%), 4.3 Fault control on vitrinite reflectance 3292–3354  m (the range of R is 1.22%–1.39%), and and intrusive related hydrothermal fluids 4084–4175 m (the range of R is 0.92%–1.39%) (Fig. 9b). In well W9, the R values were 1.07%–1.12% at depths of Vitrinite reflectance (R ) is an effective index to determine 3427–3600 m (Fig. 9c). At these intervals, the measured R o o organic matter maturity and is related to kerogen type, values were higher than the expected values (the range of R temperature, and pressure (Yu et al. 2012). In general, R is 0.89%–0.96%) at corresponding depths in normal condi- increases gradually with burial depth at a relatively constant tions. Moreover, magmatic rocks were identified based on rate (Su et al. 2016; Liu et al. 2017). well-log data at similar depths to where the high R values were obtained. 1 3 Petroleum Science (2019) 16:14–31 23 Volcanic The distribution Content, % Formation rock of authigenic minerals 20 40 60 80 100 (a) (e) Yuquan Formation (N y) Longjing Formation (N l) (b) (f) Huagang Formation (E h) (c) (g) Pinghu Formation (E p) (d) (h) Baoshi Formation (E b) Fracture zone Illite content Kaolinite content Dacite tuff Legend Tuff Altered basalt Volcanic breccia Andesite Fig. 6 The petrological characteristics of minerals affected by hydro- 3718.32 m, c 3719.12 m); authigenic quartz (d 3968.75 m); kaolinite thermal alteration in well W7, Xihu Depression. Secondary quartz converted into illite (e 2215.58 m, h 4198.13 m); the silk-thread vari- development (a 2215.58  m); the CL characteristics of quartz (b ant of illite (f 3743.72 m, g 3968.75 m) Fig. 7 Micrographs of fluid inclusions from the Xihu Depression a W9, 2824.6 m, 20×, gas liquid hydrocarbon inclusions distributed along quartz healing cracks; b W9, 3294.8 m, 50×, brine inclusions distributed along quartz healing cracks The comparison of 117 R values between different wells consistently higher R values than samples from Q2, J1 that o o shows a strong relationship with the observed distribution were not affected by magmatic intrusions (Fig.  10). Spikes of structural features. The samples from W1, W6, W7, W8, in R values were identified at depths of 2400–2500  m, W9 were located closely to fault zones and affected by 3400–3500  m, and 4000–4300  m. For example, 0.65% magmatic intrusion and hydrothermal fluids. These showed at 2408  m, 1.22% at 3292  m, and 1.39% at 4175  m are 1 3 Depth, m 24 Petroleum Science (2019) 16:14–31 (a) (b) 50 50 Sudi tectonic belt (2748.5 m - 3423.7 m) / N = 35 Xiling tectonic belt (3294.8 m - 3365.8 m) / N = 25 30 30 20 20 <110 110-120 120-130 130-140 140-150 150-160 >160 <110 110-120120-130130-140140-150150-160>160 Temperature interval, °C Temperature interval, °C Fig. 8 Graphical representation of the statistical distribution of fluid inclusions and temperature in the Xiling and Sudi tectonic belts, Xihu Depression (a) R , % (b) R , % (c) R , % o o o 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 1.5 1.5 1.5 2000 1000 1000 (c) (a) (b) 3000 3000 4000 5000 4000 Fig. 9 The relationship curve between R values and depths. a well W1, b well W7, c well W9 considerably higher than the R values at neighboring depths with pore water and generated large amounts of sulfate ranges. In contrast, R values from 109 samples that were not anions. These results demonstrate the effect of fractures affected by fault activity increased approximately linearly on hydrothermal f luid migration. from 0.3% to 1.0% as depth increased. The CO content of the natural gas sampled from wells 4.4 The reservoir porosity characteristics located near fault zones (such as W1, W3, W7, W7, W8, and W9) is relatively high, exceeding 7.35% (W7), (the As depth increases, the porosity and permeability of the highest value is 12.7% (W9). By contrast, the C O con- 403 samples from the five wells (G1, G2, J1, J2, Q2), tent in wells far from fault zones was 0.58%–2.37%. This which were not affected by the hydrothermal alteration can be interpreted as being the result of vertical migra- and lie distal to any faults decreases. The 442 samples tion of deep gas along the fault systems (Fig.  11a). The from wells W1, W4, W5, W7, which were affected 2− SO content of the formation water in wells W1, W3, by hydrothermal activity, showed abnormally high and W8 that intersected fault zones, exceeded 2000 mg/L, porosity at intervals of approximately 2400–2500  m, 2− whereas SO content in wells far from fault zones was 3400–3500 m, and 4000–4300 m depth (Fig.  12), with less than 1000  mg/L (Fig.  11b). The higher amounts average values reaching 23.1%, 19.6%, and 17.5%, respec- of SO near the fault zones were the result of upward tively. As an example, the porosity at 3441 m in well W1 migration of deep hydrothermal fluids, which reacted was as high as 20% and showed pores with an average 1 3 Depth, m Frequency, % Depth, m Frequency, % Depth, m Petroleum Science (2019) 16:14–31 25 R , % were provided by Sinopec Shanghai Oil and Gas Branch). 00.5 1.01.5 The mercury injection capillary pressure (MICP) anal- ysis revealed that the pore-throat radius ranged from 1 to 40  μm. The pore-throat assemblages were mainly medium pores with medium throats, small numbers of large pores with large throats and small pores with thin throats (Fig. 12). Based on thin section examination, the main types of pores present have been categorized and include combinations of interparticle dissolution pores, interparticle pores, intraparticle dissolution pores, inter- particle micropores, moldic pores and dissolution vugs. Well W1 located in the Baoshi Fault Zone and affected by hydrothermal fluids, encountered tuffaceous rocks at 3200 m. At this depth, quartz particles in the reservoirs were subjected to brittle fracturing and alteration, and vitrinite ref lectance was abnormally high. Andesite, dior- ite, tuffaceous rocks, and altered dacitic rock were found at approximately 4800 m in well W6 which was located in the Chunxiao Fault Zone. The vitrinite reflectance in mudstones at this depth reached 1.35%. At a depth of 3840–4275  m, well W7 located in the Tianwaitian Fault Zone and affected by hydrothermal f luids, encoun- tered tuff breccia, tuff, granite, granodiorite rocks. At these depths, the maximum porosity was 25.9% and the minimum was 1.5%, and the average value was 14.5% (Table 2). 44% of the sand thickness (29.5 m) of the Pin- ghu Formation was seen to have porosities greater than 14% (3516–4645 m) (Fig. 12). For the wells close to fault zones affected by hydrother - W1 W6 mal activity, thin section and SEM observations revealed W7 W8 that the dissolution of potassium feldspar, volcanic detritus, W9 J1 Q2 metamorphic lithics and phyllites by CO -rich acidic fluids was extensive, resulting in the formation of intergranular dissolved pores and moldic pores. Based on the compari- Fig. 10 The relationship curve between R values and depth from dif- son of porosity and pore-throat characteristic of the samples ferent wells from these wells, large differences in dissolution distribution are believed to arise from the influx of magmatic hydro - pore-throat radius of 13.2 μm. The maximum porosity of thermal fluids in fracture zones. A multi-well correlation well W6 between 3964 and 4175 m was as high as 14.6% exercise was carried out using the porosity logs and seismic with an average pore-throat radius of 4.12 μm (these data and volcanic detritus interpretation results. It is noted that (a) (b) 14 4000 0 0 W7 W1 W8 W3 W9 Q2 G2 J1 W7 W8 W3 W1 G1 G2 J1 J2 Q2 2− Fig. 11 The measured CO content in gas component analysis (a) and the content of SO in formation water, Xihu Depression (b) 2 4 1 3 Depth, m The content of CO , % 2- The content of SO , mg/L 4 Frequency, % Frequency, % Frequency, % Frequency, % Permeability Permeability Permeability Permeability contribution contribution contribution contribution value, % value, % value, % value, % Depth, m P , MPa c P , MPa c P , MPa c P , MPa 26 Petroleum Science (2019) 16:14–31 1 3 1000 60 100 Porosity, % (a) 0 10 20 30 40 10 40 0.1 20 0.001 0 0 920.0 μm 450.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 J1, 3932.5 m, 10x2(-), Primary pores and intragranular dissolution pores S , % R, μm Hg and the histogram of pore radius 1000 60 100 (b) 10 40 0.1 20 0.001 0 0 470.0 μm 470.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 Q2, 4320.46 m, 10x4(-), Primary pores and intragranular dissolution pores, S , % R, μm Hg with quartz secondary enlargement, and the histogram of pore radius 1000 60 100 (c) (c) 10 40 0.1 20 0.001 0 0 (a) 190.0 μm 450.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 (d) W1, 3441.2 m, 10x4(-), Intergranular dissolution pores, casting pores, S , % R, μm Hg and the histogram of pore radius 1000 60 100 (d) (b) 10 40 0.1 20 G2 J1 J2 Q2 0.001 0 0 450.0 μm 190.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 W1 W4 W7, 4052.62 m, 10x4(-), Intergranular dissolution pores, casting pores, 5000 W5 W7 S , % R, μm Hg and the histogram of pore radius Fig. 12 The relationship curves for porosity and depth, pore types and the histogram of pores radius in wells affected by hydrothermal fluids (particle data from the Shanghai Branch of CNOOC, China) Petroleum Science (2019) 16:14–31 27 Table 2 The porosity values of wells of the central uplift zone and the Sudi Structural Zone (Zhou 2003; Su et al. 2016). District Well Depth, m Porosity, % The impact of intruded magma and hydrothermal flu - Φ Φ Φ max min avg ids on the surrounding rocks has been shown significantly in this paper. The following phenomena are described in Fault zone W1 3440–3447 18.9 15.1 17.0 the study: brittle fracturing and clastic particle alteration. W4 2810–2897 18.8 16.6 17.4 When magma intruded into consolidated country rock, the W5 3792–3840 17.8 10.7 12.7 pressure from this sudden influx resulted in brittle frac- W7 3775–4200 25.6 1.5 14.5 turing of the country rocks, clastic particles expanded, Far from fault G2 2000–3000 32.8 13.9 20.8 cracked and subsequently healed during the upwelling of 3000–4000 19.5 0.7 10.7 deep hydrothermal fluids (Fig.  4). Other observed effects 4000–4072 13.1 1.2 8.7 including: the precipitation of unique hydrothermal min- J1 3724–3984 11.1 3 8.3 erals (Fig. 5) and illite alteration, and kaolinite becomes J2 3420.2–4531 14.6 3.1 10.4 unstable at temperatures between 120 and 150  °C and Q2 3633–4646 16.5 7.1 12.2 transforms into illite under potassium-rich conditions The sample location is marked on the well profile in Fig. 12 (Huang et al. 2009), resulting in increased illite content. The influence of hydrothermal fluids and the local baking the distribution of the volcanic detritus is associated with the effects of magmatic rocks on the surrounding rock strata existence of faults. For example, the net thickness of high (Fig. 6) results in the presence of abnormally high temper- porosity sands (over 14% porosity) is much higher in Wells atures in fluid inclusions (Fig.  7) and anomalous vitrinite W1, W3, W6 and W7, sited in the Baoshi, Chunxiao and reflectance (Fig.  9). These phenomena in the study area Pinghu fault complexes than those from Wells G2, Q2 and J2 are interpreted as being related to the two magmatic ther- where no faults or volcanic detritus are present. According mal events that occurred at the end of the Early Miocene to the porosity logs, higher porosity is observed adjacent to and the Late Miocene (Yang et al. 2001). The deep faults the faults, with porosity values falling in areas away from that propagate from the basement act as a regional conduit the faults (Fig. 13, Table 2). system for upward migration of hydrothermal fluids to the reservoirs, and this results in significant modification and alteration. The high-temperature magma consisting of a 5 Discussion volatile silicate melt released significant amounts of heat and volatiles when migrating upwards and cooling (Tao Previous studies of the burial and thermal history of the and Xu 1994). This process resulted in abnormally high Xihu Depression suggested that the thermal effects of geothermal gradients and affected the prospectivity of the magmatic intrusives were detectable in the southern part surrounding rocks, especially the migration of alkanes and Altitude, m -1800 W1 W3 W6 E h 3 2 W7 -2200 G2 E h 3 1 -2600 E2p3 J2 E2p2 Q2 -3000 E p 2 1 -3400 E b -3800 -4200 Q2 W7 -4600 J2 G2 -5000 W6 Measured porosity Porosity<10% Fault Well Porosity>18% W1 positions W1 Legend W3 E p 10%<Porosity<14% Volcanic rock Magma 2 2 Formation 14%<Porosity<18% Fig. 13 The difference in pore characteristics in samples from wells affected by hydrothermal fluids near faults and from samples not affected by hydrothermal fluids, far from faults (Pie Chart: Ratio of sand body thickness in different porosity intervals to total sand thickness) 1 3 Baoshi main fault Tianwaitian main fault Chunxiao main fault Chunxiao fracture Chuyang fracture 28 Petroleum Science (2019) 16:14–31 Dissolution Cementation W1 W2 W3 W6 W7 W9 Depth, m Hydrocarbon mig- Diagenesis -ration & filling 1000 N2S 3 2 3000 N1 l + N1 y Fluid flowing activity N1 l E3 h Thermal event 1 E3 h 1+2 Fault activity E p Vertical E2 p injection E2 b Tectonic activity Hydrothermal (riched in CO2, SO2) Legend Fault Formation Well Volcanic rock Magma Direction of fluid flowing W1 Fig. 14 The dissolution model of hydrothermal fluids in the Xihu Depression reservoir diagenesis of the formations underlying or over- 2CaAlSi O (calcium feldspar)+ 2H + H O 3 8 2 lying the intrusives. + = Al Si O (OH) (kaolinite)+ 4SiO + 2Ca 2 2 5 4 2 (8) Based on the results above, the depths where abnormally high porosity zones were encountered were consistent with 2+ m − 0 − the depths where the magmatic intrusions and thermal fluid 2Ca + C O + C O = 2CaCO (9) 3 3 activity were noted. 2+ m − − m 2Ca + C O + 2RCOO = CaC O + Ca(RCOOH) The mechanism of dissolution of alkaline minerals by 3 2 late CO -rich acidic fluids is closely linked to the influence (10) of magmatic hydrothermal fluids on the thermal evolution The incursion of high-temperature hydrothermal fluids of source rocks discussed in this paper. The hydrothermal from the crystalline basement is thought to have increased fluids related to the intrusives could carry organic acids and/ formation temperatures in the study area, as average geother- or fluids rich in CO and SO , decreasing the pH of the host mal gradients in the region are as high as 4.22 °C/100 m as 2 2 rock diagenetic environment (Eqs. (2)–(4)) resulting in dis- interpreted from anomalously high vitrinite reflectance val- solution of feldspar grains and carbonate cements (the dis- ues in the surrounding rocks. These hydrothermal incursions solution reaction is shown in Eqs. (5)–(10)) (Wang et al. would accelerate the thermal maturation of organic matter, 2017a, b), leading to secondary dissolution porosity in the releasing large amounts of short-chain organic acids. These host rocks. As illustrated in Eqs. (6)–(8), the dissolution of relatively low molecular weight organic acids can form feldspar leads to the precipitation of quartz as overgrowth organic acid anions with aluminum complexes, enhancing cements. the dissolving capacity of Al and Si in aqueous solution, and increasing some of rock element migration ability in − + RCOOH = RCOO + H (2) groundwater. Thus, the transformation of the silicate mineral + 2− − characteristics and connecting throat pore surface properties 2CO + 2H O = 2H CO = 3H + CO + HCO (3) 2 2 2 3 3 3 cause the variation of oil reservoir porosity and permeability + 2− SO + H O = H SO H SO + O = 2H + SO of rock mass (Oelkers and Schott 1995; Guo et al. 2003; (4) 2 2 2 3 2 3 2 Zhang et al. 2009). + 2+ The high temperatures also enhance the dissolution of CaCO + H = Ca + HCO 3 3 (5) feldspar in the acidic conditions and produce secondary + 2+ 2+ − Ca ⋅ Mg CO + 2H = Ca + Mg + 2HCO porosity (Zhao 2005). Based on these observations, the research team developed a preliminary model to account for reservoir development in the region considering the action 2KAlSi O (potash feldspar)+ 2H + H O 3 8 2 + of hydrothermal fluids associated with igneous intrusives in = Al Si O (OH) (kaolinite)+ 4SiO + 2K (6) 2 2 5 4 2 the Xihu Depression (Fig. 14). The pores generated during the dissolution process improved reservoir quality, but dis- 2+ 2+ 2NaAlSi O (soda feldspar)+ 2H + H O solution products, such as Ca and Mg , could be carried 3 8 2 by the hydrothermal fluids to overlying strata above or to = Al Si O (OH) (kaolinite)+ 4SiO + 2Na (7) 2 2 5 4 2 the peripheral regions where they could be precipitated as 1 3 Petroleum Science (2019) 16:14–31 29 carbonate cements, reducing the reservoir quality in those Furthermore, the hydrothermal fluids associated with areas. Subsequent research should concentrate on how this intrusives had two main impacts on reservoir properties: precipitation of mobile ions affects pore development and (a) formation of secondary dissolution pores improving the porosity in peripheral areas. reservoir quality and (b) precipitation of authigenic minerals 3+ 2+ 2+ from the dissolution products, such as Al, Ca and Mg , precipitated as cement in the overlying strata or the periph- 6 Conclusions eral regions when temperature, pressure or pH conditions changed. Finally, the porosity in the reservoirs decreased and The following conclusions can be drawn from the present the anisotropy increased. So, upcoming research will focus study: on the quantitative characterization of the dissolution and cementation of the deep hydrothermal fluids on the clastic (1) Three Cenozoic episodes of volcanic activity accompa- reservoirs and establish a corresponding model. nied by magmatic intrusions that caused brittle fractur- Acknowledgements This work was funded by the National Natural ing of the surrounding sandstones have been identified. Science Foundation of China (Grant No. 41502135). The authors are Quartz and feldspar particles show cracking on their grateful to the State Key Laboratory of Oil and Gas Reservoir Geology margins related to thermal expansion due to heating. and Exploitation and the anonymous reviewers for their extraordinary Because of the high temperatures, and the CO and SO advice. 2 2 which were carried by the hydrothermal fluids associ- Open Access This article is distributed under the terms of the Crea- ated with the magmatic intrusions, the host sandstones tive Commons Attribution 4.0 International License (http://creat iveco were subjected to thermal alteration, which led to the mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- precipitation of an assemblage of hydrothermal miner- tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the als such as celestite, zircon, apatite, barite, and cerous Creative Commons license, and indicate if changes were made. phosphate. 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Tertiary hydrothermal activity and its effect on reservoir properties in the Xihu Depression, East China Sea

Liu, Yan; Jin, Si-Ding; Cao, Qian; Zhou, Wen
Petroleum Science , Volume 16 (1) – Jan 3, 2019
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Springer Journals
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Copyright © 2019 by The Author(s)
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/s12182-018-0292-4
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Abstract

Three large-scale episodes of volcanic activity occurred during the Tertiary in the Xihu Depression, located in the East China Sea. Intermediate-felsic magmas intruded along faults and the associated hydrothermal fluids resulted in the hydrothermal alteration of the clastic country rock. To better describe reservoir characteristics, reservoir samples were subjected to the following investigations: thin section examination, scanning electron microscope-energy dispersive spectrometer analysis (SEM–EDS), fluid inclusion homogenization temperature tests, vitrinite reflectance measurements, and X-ray diffraction. The results of this study provide evidence of the following hydrothermal alteration phenomena: brittle fracturing, clastic particle alteration, precipitation of unique hydrothermal minerals (celestite, zircon, apatite, barite, and cerous phosphate). The pres- ence of abnormally high temperatures is indicated by fluid inclusion analysis, the precipitation of high-temperature authigenic minerals such as quartz, illite alteration, and anomalous vitrinite reflectance. Two aspects related to hydrothermal effects on reservoir properties have been investigated in this study: (1) Deep magmatic hydrothermal fluids carry large amounts of dissolved carbon dioxide and sulfur dioxide gas. These fluids percolate into the country rocks along fault zones, resulting in dissolution within the sandstone reservoirs and the development of significant secondary porosity. (2) Magma intrusions increase the temperature of the surrounding rocks and accelerate the thermal evolution of hydrocarbon source rocks. This results in the release of large amounts of organic acids and carbon dioxide, leading the dissolution of the aluminosilicate minerals and volcanic fragments in the reservoirs, and the generation of significant secondary porosity. Keywords Hydrothermal activities · Erosion effects · Clastic rock reservoir · Secondary porosity · Xihu Depression 1 Introduction which are highly reactive (Tao and Xu 1994). Hydrothermal fluids can originate from magmatic fluids, metamorphic flu - The term “hydrothermal fluids” refers to all high-tempera - ids, hot brine and/or formation water in sedimentary basins, ture aqueous fluids (temperature range from 50 to 400 °C) and fluids from primary mantle fluids (Chen et al. 2007). In that contain many chemical materials in solution (e.g., H S, this paper, the term “hydrothermal fluids” refers to magmatic HCl, HF, SO , CO, CO, H, N , KCl, and NaCl), some of hydrothermal fluids. 2 2 2 2 Magmatism occurs in many sedimentary basins around the world and has a significant impact on the generation, migration, and accumulation of oil and gas as well as the Edited by Jie Hao formation of hydrocarbon reservoirs due to the two follow- * Si-Ding Jin ing mechanisms (Ye et al. 2005; Agusto et al. 2013). (1) jinsiding@cdut.edu.cn Thermal baking caused by the magmatic intrusion heats the surrounding rocks and results in mineral transformations. State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology, The thermal baking effect on hydrocarbon source rocks Chengdu 610059, Sichuan, China accelerates the generation of alkanes, organic acids, and car- College of Energy and Resources, Chengdu University bon dioxide (Guo 2002). Girard and Nahon (1989) proposed of Technology, Chengdu 610059, Sichuan, China the concept of “contact diagenesis,” the “abnormally high Sichuan Key Laboratory of Shale Gas Evaluation temperatures” from the magmatic intrusion result in changes and Exploitation, Sichuan Keyuan Testing Center, in the authigenic mineral assemblage in the contact zone. (2) Chengdu 610091, Sichuan, China Vol:.(1234567890) 1 3 Petroleum Science (2019) 16:14–31 15 A series of water–rock reactions within the country rocks Depression (with an average thermal flow of 71 mW/m ), takes place as hydrothermal fluids move along migration Wei et al. (1994) suggested that regional thermal anomalies pathways. This has a vital impact on reservoir performance. were related to late-stage magmatic activity, fault develop- Hydrothermal activity may result in the pores of reservoir ment, hydrothermal activity, basement uplift and topogra- rocks being filled with zeolite, calcite, chlorite, and siliceous phy. The relatively frequent episodes of Tertiary magmatism minerals, which reduce the porosity and permeability of the in the central and southern parts of this area can be deline- country rocks to a considerable degree and therefore nega- ated in seismic profiles (Hu and Tao 1997), and most wells tively impact the reservoir quality (Wang and Zhang 2001). have encountered magmatic rocks (Zhou and Song 2014). In addition, researchers have suggested that hydrothermal Through analysis of the abnormally high porosity zones fluids related to magmatic-volcanic activity are rich in CO , in the Paleogene of the Xihu Depression, Su et al. (2016) SO , and H S. The migration of these hydrothermal fluids suggested that feldspar and other minerals had been eroded 2 2 along fault and fracture systems accelerates the thermal evo- due to the action of organic acids and/or fluids, resulting lution of the hydrocarbon source rocks and released organic in secondary porosity. Cao et al. (2017) indicated that the acids and CO form a dissolution alteration zone (Heden- presence of abnormal overpressures inhibited compaction by quist and Henley 1985; Yu et al. 2012; Liu et al. 2017; Wang overlying strata and increased the intensity of the dissolution et al. 2017a, b). For example, cyclic thermal convection in effect, therefore resulting in the abnormally high porosities sedimentary basins on the western coast of Canada gener- observed in the deep sandstone reservoirs. ated a dissolution zone of carbonate minerals and improved To date, the effect of magmatic thermal fluids on res- reservoir quality (Charlou et al. 2010; Schmidt et al. 2011). ervoirs in the Xihu Depression has not been studied in In the Pearl River Mouth Basin in China, magmatism not detail. In the case of the Xihu depression, which is diffi- only caused relatively strong deformation and contact meta- cult to explore and develop and therefore has a high eco- morphism but also provided a significant amount of heat, nomic threshold, it is very important to clarify the control- which resulted in abnormally high geothermal gradients ling factors on high quality reservoirs in the context of the accelerating the maturation of hydrocarbon source rocks low porosity and low permeability observed in these deep and leading to the release of more organic acids, resulting reservoirs (> 3200 m). This research has led to a theoreti- in the development of secondary porosity (Nie et al. 2005; cal basis for the evaluation of deep, high quality reservoirs Zhu et al. 1994). Sugisaki and Mimura (1994) proposed in the study area. In this paper, the authors investigated in that hydrothermal fluids rich in CO underwent a chemical detail evidence for deep hydrothermal processes and their reaction with the reservoir rocks, causing the dissolution of effects on surrounding country rocks, documenting for the quartz and feldspar particles which improves the reservoir first time the influence of magmatic hydrothermal fluids on properties of these rocks. Evidence for intrusive magmatic the reservoir properties of the country rocks. activity has been identified in different regions across the The study focuses on the reservoirs of the Paleo- world during exploration for oil and gas, such as in the Mich- gene–Eocene Pinghu Formation and the Oligocene Hua- igan Basin in the United States (Wierzbicki et al. 2006), the gang Formation in the Xihu Depression of the East China Songliao Basin and the Jiyang Depression in eastern China Sea. Cores from 14 wells in the southern part of the depres- (Wang et al. 1990), the offshore Bohai Bay Basin in Eastern sion have been studied. A range of methods, including core China (Wang and Zhang 2001), the Dongpu Depression in and thin section examination, electron microprobe analysis, eastern China (Zhu et al. 1994) and the offshore Yinggehai fluid inclusion analysis, and vitrinite reflectance, have been Basin in Southern China (Wang et al. 2010). In these basins, employed to investigate the samples’ petrography and min- magmatic hydrothermal fluids have altered the reservoirs, eralogy to identify the presence and impact of magmatic and most of these altered reservoirs form a group of key hydrothermal fluids. hydrocarbon play pathways. Consequently, the impact of magmatic activity and associated magmatic hydrothermal fluids on petroleum systems has drawn a significant amount 2 Geological setting of research interest in the field of oil and gas exploration and development (Shu et al. 2003). The Xihu Depression is within the continental shelf basin Exploration in the Xihu Depression, part of the conti- of the East China Sea located on China’s eastern continental nental shelf basin of the East China Sea has resulted in the margin (Fig. 1). It is a tertiary oil and gas bearing depres- discovery of considerable oil and gas reserves. The amount sion in the northern part of the East Zhejiang Depression. of oil and gas resources in the Xihu Depression is esti- Classed as a continental margin rift-depression basin it cov- mated to be more than 4.67 billion tonnes (Cao et al. 2017; ers an area of 59,000 km . The boundary to the west is the Huang et al. 2010). Based on the analysis of temperature Diaoyu Islands fold zone, and to the east are the Hupijiao, and thermal conductivity measurements taken in the Xihu Haijiao, and Yushan Uplifts (Fig. 1). (Liu 1992; Xu et al. 1 3 Qiantang-Tianwaitian great fault Geling great fault 16 Petroleum Science (2019) 16:14–31 (a) (b) Fujiang depression Shanghai Q2 Ningbo Haijiao uplift W10 W7 W8 J1 J2 W9 G2 G1 Magmatic Sag and uplift boundary rock Structural belt Normal Fault boundary W6 Region Major Fault boundary W5 Study area Study well W1 W1 W4 W2 Contrast well 0 10 20 30 km Q1 W3 Fig. 1 Regional tectonic map and distribution of magmatic rocks of the Xihu Depression 1997; Liu et  al. 2003). The structural framework shows NNE-trending normal faults. The Oujiang Movement was NNE–NE, NW, and approximately E–W-trending fault zones the main period of extensional rifting during the early that have undergone intense activity. The Xihu Depression Eocene (E ). (2) The Local Subsidence stage comprises three can be divided into five tectonic units from west to east: The tectonic episodes including: (a) The Yuquan Movement, Baochu slope zone, the San-tan deep sag, the Central anti- the first reversal period during the early Oligocene, with cline, the Bai-di deep sag and the Eastern fault zone (Fig. 1). compression, folding and uplift accompanied by magmatic Cenozoic clastic sediments developed in the Xihu activity, and the development of the boundary interface (T ) Depression include (from bottom to top), the Paleocene (E ), between the faulting stage and the Local Subsidence stage. Eocene Baoshi Formation (E b), Pinghu Formation (E p), (b) The Huagang Movement, the second inversion period 2 2 Oligocene Huagang Formation (E h), Miocene Longjing during the late Oligocene, resulting in folding related to 1 2 Formation (N l), Yuquan Formation (N y), Liulang Forma- reverse faulting, uplift and erosion. This inversion led to 1 1 tion (N l), Pliocene Santan Formation (N s), and Quater- folding, uplift and exposure of the formations proximal to 1 2 nary East China Sea Formation (Qd) (Fig. 2). The Pinghu the fault zone. (c) The Longjing Movement; the third phase Formation depositional environment was a deltaic sedi- of inversion at the end of the Miocene, resulted in tectonic mentary system propagating into a restricted bay that was inversion of the Central Depression Belt characterized by affected by tidal action. The Huagang Formation contains folding, uplifting and erosion. (3) Regional Subsidence three sedimentary systems, namely lacustrine facies, braided stage: The main period of subsidence began Pliocene (N ). river delta facies and fluvial facies. The reservoir lithology During the subsidence of the basin, the Neogene-Quaternary mainly consists of well-sorted feldspathic, lithic sandstones strata were deformed by E–W-trending tensile-shear faults, and lithic feldspathic sandstones with good sorting, and mainly located in the central tectonic zone and the slope of with rarer occurrence of lithic quartz sandstones and felds- the Xihu Depression (Zhang 2013). pathic quartz sandstones. On the basis of the analysis of the The main faults are strike-slip extensional normal faults sedimentary environment, the strata can be characterized as and extensional faults with a dip angle of 60°–80° in the layers of mudstone layers, silty mudstones and sandstones Xihu Depression. The main active time of the faulting was separated by andesites and basalts. late Cretaceous–Oligocene. The distribution of magmatic The Xihu Depression underwent a faulting stage, a local rocks in the research area is clearly controlled by faults (Cai subsidence stage and a regional subsidence stage. (1) Fault- et al. 2014) (Fig. 1). The magmatic rocks are mostly dis- ing stage: The Keelung Movement resulted in initial rift- tributed along deep NE–NNE-trending faults (Zhang et al. ing during the Late Cretaceous. Paleocene-Eocene sedi- 2014). The age of the magmatism decreases gradually from ments distributed across the entire Xihu Depression were west to east; this is consistent with the fault activity time that cut by large basement faults, most of which were NE- and shows a decreasing trend from west to east, associated with 1 3 Diaobei Diaoyudao uplift-fold zone depression Haijiao uplift Donghai basin Bai-di deep sa Bai-di deep sag g Eastern fault zone Lingyin main fault Diaoyu uplift-fold zone Xihu depression Central anticline Tianwaitian main fault Chunxiao main fault Baochu slop zone San-tan deep sag Pinghu main fault Yuquan main fault Lingbei main fault Baoshi main fault Petroleum Science (2019) 16:14–31 17 Fig. 2 The distribution of Formations formation lithology in Xihu Boundary, Tectonic Basin Lithology Depression age movement tectonic stage System Series Group Quat- Donghai Group Regional ernary (Qpdh) subsidence Trough movement T 1.75 Ma Santan Group (N s) Longjing T 5.30 Ma 2 movement Liulang Group (N l) T 10.2 Ma Yuquan Group (N y) Depression stage T 16.2 Ma Longjing Group (N l) T 23.5 Ma Huagang Group T 30.0 Ma (E h) Yuquan T 33.7 Ma movement T 36.5 Ma Pinghu Group (E p) T 38.1 Ma Faulted 4 stage T 41.2 Ma Baoshi Group (E b) Oujiang movement T 53.0 Ma Keelung T 65.0 Ma movement Cretac- eous Mudstone Siltstone Silty mudstone Andesite Legend Basalt Fine sandstones Andesite lava breccia depositional center migration from west to east either (Hu intermediate-mafic igneous rocks, Late Oligocene emplace- and Tao 1997). Faulted regions are ideal pathways for the ment of intermediate-felsic magmatic rocks, and emplace- upward migration of deep fluids (Meng et al. 2008). ment of Miocene magmatic rocks (Shen et al. 2001) (Fig. 2, Based on seismic, gravity and magmatic data, geochemi- Table 1). In particular, the Late Oligocene intermediate-fel- cal data, and isotope chronology data (K–Ar and U–Pb sic magmatic rocks penetrate through the T reflection layer methods), three episodes of Cenozoic volcanic activity (33.7 Ma) and intrude into individual seismic sequences in have been identified by previous researchers. The activi- the form of dykes. The intense magmatic episode in the Mio- ties include two kinds of intrusions and eruptions, forming cene resulted in intrusions and magma flows along the T intrusive rocks and volcanic rocks dominated by interme- group bedding surfaces. diate acid rocks, including Early Eocene emplacement of 1 3 Neogene Paleogene Plio- Palaeo- Miocene Oligocene Eocene cene cene 18 Petroleum Science (2019) 16:14–31 Table 1 The distribution of magmatic rocks in wells Well Magmatic rock section, m Horizon Thickness, m Rock types W5 2809–3104 Huagang Formation (E h) 195 Andesite-dacites, basalt-andesite 3104–3204.7 Pinghu Formation (E p) 100 Meta andesitic volcanic breccia W6 4774–4847 Pinghu Formation (E p) 73 Andesite, diorite, tuff 4895–4983 Pinghu Formation (E p) 22 Altered dacite W7 3566–3685.5 Pinghu Formation (E p) 1.07 Andesite, basalt, andesitic tuff lava, tuff 3840.5–4053.5 Pinghu Formation (E p) 265 Tuff breccia, granite, granodiorite 4190–4240, 4262–4275 Pinghu Formation (E p) 63 Tuff W8 1995–2012.5 Longjing Formation (N l) 17.5 Dacites cutting crystal tuff, dacites tuff breccia diorite, andesite, tuff W9 2381–2560 Longjing Formation (N l) 47 Altered tuff W10 2162.5–2167.9 Huagang Formation (E h) 5.4 Crystal tuff G1 1995–2012.5 Yuquan Formation (N y) 17.5 Tuff Sample data from Wei et al. (1994), Hu and Tao (1997), Zhou and Song (2014) Based on observations of the successive precipitation of acidic fluids linked to the influence of magmatic hydro- minerals in response to pressure and temperature changes, thermal fluids on the source rock thermal evolution has the diagenetic evolution sequence is determined as fol- been discussed in this paper. lows: mechanical compaction, early carbonate cementa- tion, early dissolution (caused by water-soluble corro- sion resulting in feldspar erosion to kaolinite), secondary 3 Methodology enlargement of quartz, late carbonate cementation, late dissolution, iron and calcite and dolomite precipitation. Seventy-four core samples were collected from 10 wells in The formation of secondary pores is mainly controlled the Xihu Depression, well W1 in the Baoshi Fault Zone, by late cementation and dissolution. This late dissolution wells W2–W6 in the Chunxiao Fault Zone, well W7 in the occurs in the middle diagenetic stages A-B that occurred Pinghu Fault Zone, wells W8, W9 in the Tianwaitian Fault in the Late Oligocene and Miocene, thus providing the link Zone and well W10 in the Yuquan main fault (Fig. 1). The between burial history and hydrocarbon generation. Once samples were analyzed at the State Key Laboratory for Oil the source rock passed through the hydrocarbon generation and Gas Reservoir Geology and Exploration at the Chengdu window, three subsequent pulses of hydrocarbon migra- University of Technology. The following is an overview of tion took place during the Early Miocene (25 Ma), Late the methods used in this investigation: Miocene (10.4–6.1 Ma) and Early Pliocene (2.2–0 Ma), (Fig. 3) (Cao 2016; Su et al. 2016). The late dissolution (1) A Nikon E600 microscope and a Wisesoft microscopic occurred prior to hydrocarbon charging. image analyzer were used to examine and study lithol- Previous studies on the burial and thermal history of ogy and pore structures, magnification is 25–400 times. the Xihu Depression suggested that the thermal effects of (2) A CL8200 MK5 cathode luminescence microscope was magmatic intrusives were detectable in the southern part used to determine the texture of quartz and feldspar of the central uplift zone and the Sudi Structural Zone debris, the growth zone of calcite cement, and the clay (Zhou 2003; Su et al. 2016). Correlating with pulses of minerals. The voltage of the beam is 15 kV, and the magmatism in the Xihu Depression, three phases of mag- beam current is 300 μA. matic hydrothermal activity could be identified: an Early (3) A field-emission environmental scanning electron Paleocene-Eocene event, a Late Oligocene event, and a microscope (Quanta 250 FEG, manufactured by FEI, Miocene event. Early and mid-term hydrothermal activity United States) with an energy-dispersive spectrometer has had the greatest impact on hydrocarbon generation (EDS) was used to conduct high-resolution morpholog- and expulsion from source rocks. The regional heat from ical observations and analyses of rock samples in the the Early Paleocene-Eocene magmatism accelerated the ESEM™ vacuum environment. The EDS was used to thermal evolution of the source rocks. By the end of the characterize the structure and composition of the sam- Oligocene, large amounts of hydrocarbons and associated ples from their surface images and component images. gas had been generated (Gu et al. 2001). Therefore, the The resolution of the electronic image is 1.0 nm @ mechanism of dissolution of alkaline minerals by C O -rich 30 kV, 3.0 nm @1 kV, and the resolution of the back- 1 3 Petroleum Science (2019) 16:14–31 19 Early diagenetic stage Middle diagenetic stage Diagenetic Contempor- event aneous stage Stage A Stage B Stage A Stage B Ro 0.35 0.52 1.3 .0 Paleotemperature 65 85 140 175 Maturation stage Immature stage Semi-mature stage Low mature stage High mature stage during diagenesis Compaction and pressure-solution Calcite cementation Cementation of ferroan-calcite Siliceous cementation Cementation of chlorite Cementation of illite Cementation of kaolinite Dissolution Metasomatism Hydrothermal activities Buried depth <1000-2000 m 2000-4000 m>4000 m EO MP E h E3 h E2 p E p E2 p E2b 1 2 3 Stratigraphic ages, Ma 40 20 0 (a) (b) (c) (d) (e) (f) Fig. 3 The coupling diagram of diagenetic evolution, hydrothermal secondary enlargement—Late calcite cementation—The feldspar was activity and oil and gas charging in research in the Xihu Depres- metasomatized by calcite, d W5, 3725.32 m, × 100, Ferroan calcite, e sion (1) the first oil and gas charging stage, (2) the second oil and J1, 3296.12 m, × 20, Feldspar dissolution and kaolinite precipitation, gas charging stage, (3) the third oil and gas charging stage. a W10, f W10, 3444.2  m, × 100, Chlorite rims—Quartz secondary enlarge- 3148.6  m, × 100, Mechanical-chemical compaction, b W8, 3410  m, ment—Late calcite cementation × 100, Early calcite cementation, c W5, 3725.79  m, × 100, Quartz 1 3 Diagenetic evolution sequence Depth, m 20 Petroleum Science (2019) 16:14–31 scattered electronic image is 2.5 nm. The experimental with an average value of 77%. The feldspar content ranges temperature is 21 ± 4 °C; the humidity is ≤ 65% RH. between 10% and 27% with an average value of 11.1%. The (4) A Rigaku D/Max-2500PC fully automatic powder feldspars are primarily potassium feldspars with some acidic X-ray diffractometer was utilized to measure the com- plagioclase. The lithic grain content varies between 5% and positions (total rock + clay) and mineral contents of 40% with an average of 11.6%. Lithic grains consist mainly sample powders. The samples were ground to less of argillaceous, calcareous, and siliceous sedimentary lithic than 40 μm, and then pressed to make specimens for grains or low-grade metamorphic lithic grains with minor testing. The diffraction peak intensity of different min- amounts of volcanic and intrusive lithic grains. Three main eral components is generally expressed by the integral petrological characteristics were identified related to high strength following subtraction of the background. The pressures and hydrothermal fluids associated with magma working voltage of the diffractometer was 40 kV, the intrusion. These were: brittle fractures and alteration of electric current is 40 mA, and the angular accuracy of skeleton particles, precipitation of associated hydrothermal the equipment is better than 0.02 degrees. THMS600G minerals, and the thermal alteration of the country rocks. automatic hot and cold stations (Linkam company) Brittle fracturing and the alteration of skeleton particles and A Nikon E600 microscope was used to measure were observed in the clastic rocks near the magmatic body temperature of fluid inclusions, the determination of in all 10 wells. For example, well-log analysis in wells W1, temperature ranges from − 196 °C to 600 °C, and the W6, W7 and W9 led to the identification of magmatic bodies 2 1 temperature precision is ± 0.01 °C. in the Yuquan Formation (N y), Longjing Formation (N l), 1 1 (5) A J&M microspectrophotometer (Germany) and a Zeiss Huagang Formation (E h), Pinghu Formation (E p), and 3 2 polarizing microscope (Germany) were used to meas- Baoshi Formation (E b). Brittle fractures in clastic parti- ure the vitrinite reflectance, R , of the samples. During cles located near magmatic bodies were seen in thin section the study, the characteristics of the microcomponents of (Fig. 4). the organic matter in the samples were examined under Celestite (SrSO ), apatite (Ca (PO ) (OH) ), barite 4 10 4 6 2 50 times magnification and with a reflectance range of (BaSO ), and cerous phosphate (CePO ·H O) were identified 4 4 2 0.59%–10%. from environmental scanning electron microscope observa- (6) A Zeiss polarized fluorescence microscope A1-HBO tions and energy spectrum analysis of samples in wells W6, 100 (Germany) and a Cooling–Heating Stage W7, and W9 (Fig. 5). Linkam—TH600 were used to analyze the homog- The examination of thin sections and scanning electron enization temperatures of the inclusions in rock sam- microscope analyses showed that large amounts of miner- ples. After observing the microscopic characteristics als in the reservoirs were related to hydrothermal activity, of the fluid inclusions under the polarizing fluores- including the formation of authigenic quartz and transforma- cence microscope, the inclusions were placed in the tion of clay minerals. Cooling–Heating Stage apparatus. The temperature at Authigenic quartz cement was well developed in the which the gas phase or liquid phase of the fluid inclu- study area and was mostly in the form of secondary over- sions disappears is the homogenization temperature. growths and microcrystalline quartz (Fig.  6a, d). Intra- The temperature of the experimental environment is particle fractures in the quartz grains were found in Well 20–25 °C, the humidity is 30%, and the temperature W7 at 3718.32 m using cathodoluminescence (CL). During precision is ± 1 °C. CL examination, quartz generally shows a brown, bluish- (7) An Autopore 9500 made by the Mike Murray Feldman purple, and/or non-luminescent character (Fig.  6b). The Instrument Co. Ltd. (Shanghai) was used to measure quartz at a depth of 3719.12 m was bluish-purple, or light pore-throat radius. The maximum external pressure brown and non-luminescent; the secondary quartz at a depth applicable was 60,000 psia. of 2258.58 m was bluish-purple under CL (Fig.  6c). The characteristics of quartz under CL are known to be closely related to diagenesis temperature, with the color changes 4 Results gradually from blue to bluish-purple as the temperature increases. 4.1 Petrological evidence for the influence Sixty-seven sandstone samples from 1113 to 4500 m in of magmatic hydrothermal fluids Well W7 were used to identify the different clay minerals contents using X-ray diffraction. The results showed that Analysis of 74 thin sections and SEM analyses of 22 sam- the content of kaolinite decreased with increasing depth, ples from the 14 wells from the southern Xihu Depression while illite content generally increased with increasing showed that the quartz content of the deep sandstone reser- depth. Illite was identified in Well W7 at a depth of approx- voirs in the Xihu Depression ranges between 60% and 95% imately 1000 m and accounted for 40%–60% of the total 1 3 Petroleum Science (2019) 16:14–31 21 Fig. 4 Brittle fractures inside quartz grains. a W1, 2215.58 m, b W7, 3443.97 m, c W6, 4052.62 m, d W9, 4198.34 m clay minerals (Fig. 6). Based on SEM observations, illite 4.2 Fluid inclusion temperature indicators occurs in several locations, including pore wall linings, solid inclusions in clastic grains, and pore-l fi ling authigenic illite. Fluid inclusions are relatively well developed in the healed The pore lining authigenic illite crystal morphology is fine fractures in quartz samples from the study area. For example, needles perpendicular to the surfaces of the clastic parti- gas–liquid, two-phase aqueous fluid inclusions were identi- cles (Fig. 6e–h). Where illite occurs as solid inclusions in fied along healed fractures in the quartz grains in well W9 clastic grains, the crystals form parallel to the surface of the located in the Tianwaitian main fault (Fig. 7a). At a depth grains and were not present in contacts between particles. of 2824.6 m, the homogenization temperatures of the inclu- The pore-filling illite was found at the exterior margins of sions range from 131.5 to 139.1 °C with an average value the pore wall lining. Illite is an authigenic clay mineral that of 134.2 °C, which is much higher than the regional burial is known to occur within a temperature range of 120–300 °C temperature of 118 °C at this depth (geothermal gradient of (Yu et al. 2012). 3.7–3.8 °C/100 m in the southern of Central anticline). Simi- Under the normal geothermal gradient, 3.4–3.5 °C/100 m larly, the highest measured homogenization temperature was and considering the burial history of the Xihu Depression, 156.6 °C at a depth of 3294.8 m (Fig. 7b), which was much significant amounts of illite should begin to appear at depths higher than the regional burial temperature of 131.1 °C at exceeding 3200 m. At this depth, formation temperature this depth, assuming a normal geothermal gradient. is approximately 120  °C. Kaolinite becomes unstable at Thirty-five samples from six wells in the Sudi tectonic temperatures between 120 and 150 °C and transforms into belt, taken between 3294.8 and 3365.8 m, were analyzed illite under potassium-rich conditions (Huang et al. 2009) for fluid inclusions. The results showed that the homog - (Eq. (1)), resulting in increased illite content. Substantial enization temperatures of the fluid inclusions were in amounts of illite were identified at shallow depths (approxi- the ranges of 130–140 °C and 150–160 °C at depths of mately 1000 m) in well W7. Therefore, there is a negative 2748.5–3423.7  m (Fig.  8a). Twenty-five samples from correlation between kaolinite and illite at temperatures two wells in the Xiling tectonic belt taken between 3248.5 between 120 and 150 °C in well W7 (Fig. 6). and 3423.7 m were analyzed for fluid inclusions, and the homogenization temperatures of the fluid inclusions varied 3Al Si O (OH) (kaolinite) + 2K from 120 to 130 °C and 150 to 160 °C with sample depths 2 2 5 4 (1) between 3294.8 and 3365.8 m (Fig. 8b). With reference to → 2KAl Si O (OH) (illite)+ 2H + 2H O 3 3 10 2 2 1 3 22 Petroleum Science (2019) 16:14–31 Fig. 5 Hydrothermal minerals identified by FEG-SEM backscatter electron imaging and EDS in the Xihu Depression. a Celestite (SrSO ), W6, 3964.78 m; b Apatite (Ca (PO ) (OH) ), W6, 3964.78 m; c Barite (BaSO ), W9, 3743.72 m; d Cerous phosphate (CePO ·H O), W7, 2215.58 m 10 4 6 2 4 4 2 the Paleogene geothermal gradient of 3.4–3.8 °C/100 m for R values at various depths from three wells (W1, W7, this region (Zhou 2003), and considering the burial his- W9) close to fault zones and affected by magmatic bod- tory, the normal burial temperature at the sampled depths ies and hydrothermal fluids were measured using a J & M should only be 108–126 °C in the Sudi Structural Zone and microspectrophotometer. R values showed thermal anoma- 120–127 °C in the Xiling Tectonic Zone. The abnormally lies at the following depth ranges. In well W1, the highest high fluid inclusion paleotemperatures were observed in R value (0.79%) was obtained at depths between 3060 and the study area. 3200 m (Fig. 9a). In well W7, the highest R values were identified at 2408–2574 m (the range of R is 0.58%–0.65%), 4.3 Fault control on vitrinite reflectance 3292–3354  m (the range of R is 1.22%–1.39%), and and intrusive related hydrothermal fluids 4084–4175 m (the range of R is 0.92%–1.39%) (Fig. 9b). In well W9, the R values were 1.07%–1.12% at depths of Vitrinite reflectance (R ) is an effective index to determine 3427–3600 m (Fig. 9c). At these intervals, the measured R o o organic matter maturity and is related to kerogen type, values were higher than the expected values (the range of R temperature, and pressure (Yu et al. 2012). In general, R is 0.89%–0.96%) at corresponding depths in normal condi- increases gradually with burial depth at a relatively constant tions. Moreover, magmatic rocks were identified based on rate (Su et al. 2016; Liu et al. 2017). well-log data at similar depths to where the high R values were obtained. 1 3 Petroleum Science (2019) 16:14–31 23 Volcanic The distribution Content, % Formation rock of authigenic minerals 20 40 60 80 100 (a) (e) Yuquan Formation (N y) Longjing Formation (N l) (b) (f) Huagang Formation (E h) (c) (g) Pinghu Formation (E p) (d) (h) Baoshi Formation (E b) Fracture zone Illite content Kaolinite content Dacite tuff Legend Tuff Altered basalt Volcanic breccia Andesite Fig. 6 The petrological characteristics of minerals affected by hydro- 3718.32 m, c 3719.12 m); authigenic quartz (d 3968.75 m); kaolinite thermal alteration in well W7, Xihu Depression. Secondary quartz converted into illite (e 2215.58 m, h 4198.13 m); the silk-thread vari- development (a 2215.58  m); the CL characteristics of quartz (b ant of illite (f 3743.72 m, g 3968.75 m) Fig. 7 Micrographs of fluid inclusions from the Xihu Depression a W9, 2824.6 m, 20×, gas liquid hydrocarbon inclusions distributed along quartz healing cracks; b W9, 3294.8 m, 50×, brine inclusions distributed along quartz healing cracks The comparison of 117 R values between different wells consistently higher R values than samples from Q2, J1 that o o shows a strong relationship with the observed distribution were not affected by magmatic intrusions (Fig.  10). Spikes of structural features. The samples from W1, W6, W7, W8, in R values were identified at depths of 2400–2500  m, W9 were located closely to fault zones and affected by 3400–3500  m, and 4000–4300  m. For example, 0.65% magmatic intrusion and hydrothermal fluids. These showed at 2408  m, 1.22% at 3292  m, and 1.39% at 4175  m are 1 3 Depth, m 24 Petroleum Science (2019) 16:14–31 (a) (b) 50 50 Sudi tectonic belt (2748.5 m - 3423.7 m) / N = 35 Xiling tectonic belt (3294.8 m - 3365.8 m) / N = 25 30 30 20 20 <110 110-120 120-130 130-140 140-150 150-160 >160 <110 110-120120-130130-140140-150150-160>160 Temperature interval, °C Temperature interval, °C Fig. 8 Graphical representation of the statistical distribution of fluid inclusions and temperature in the Xiling and Sudi tectonic belts, Xihu Depression (a) R , % (b) R , % (c) R , % o o o 0 0.5 1.0 0 0.5 1.0 0 0.5 1.0 1.5 1.5 1.5 2000 1000 1000 (c) (a) (b) 3000 3000 4000 5000 4000 Fig. 9 The relationship curve between R values and depths. a well W1, b well W7, c well W9 considerably higher than the R values at neighboring depths with pore water and generated large amounts of sulfate ranges. In contrast, R values from 109 samples that were not anions. These results demonstrate the effect of fractures affected by fault activity increased approximately linearly on hydrothermal f luid migration. from 0.3% to 1.0% as depth increased. The CO content of the natural gas sampled from wells 4.4 The reservoir porosity characteristics located near fault zones (such as W1, W3, W7, W7, W8, and W9) is relatively high, exceeding 7.35% (W7), (the As depth increases, the porosity and permeability of the highest value is 12.7% (W9). By contrast, the C O con- 403 samples from the five wells (G1, G2, J1, J2, Q2), tent in wells far from fault zones was 0.58%–2.37%. This which were not affected by the hydrothermal alteration can be interpreted as being the result of vertical migra- and lie distal to any faults decreases. The 442 samples tion of deep gas along the fault systems (Fig.  11a). The from wells W1, W4, W5, W7, which were affected 2− SO content of the formation water in wells W1, W3, by hydrothermal activity, showed abnormally high and W8 that intersected fault zones, exceeded 2000 mg/L, porosity at intervals of approximately 2400–2500  m, 2− whereas SO content in wells far from fault zones was 3400–3500 m, and 4000–4300 m depth (Fig.  12), with less than 1000  mg/L (Fig.  11b). The higher amounts average values reaching 23.1%, 19.6%, and 17.5%, respec- of SO near the fault zones were the result of upward tively. As an example, the porosity at 3441 m in well W1 migration of deep hydrothermal fluids, which reacted was as high as 20% and showed pores with an average 1 3 Depth, m Frequency, % Depth, m Frequency, % Depth, m Petroleum Science (2019) 16:14–31 25 R , % were provided by Sinopec Shanghai Oil and Gas Branch). 00.5 1.01.5 The mercury injection capillary pressure (MICP) anal- ysis revealed that the pore-throat radius ranged from 1 to 40  μm. The pore-throat assemblages were mainly medium pores with medium throats, small numbers of large pores with large throats and small pores with thin throats (Fig. 12). Based on thin section examination, the main types of pores present have been categorized and include combinations of interparticle dissolution pores, interparticle pores, intraparticle dissolution pores, inter- particle micropores, moldic pores and dissolution vugs. Well W1 located in the Baoshi Fault Zone and affected by hydrothermal fluids, encountered tuffaceous rocks at 3200 m. At this depth, quartz particles in the reservoirs were subjected to brittle fracturing and alteration, and vitrinite ref lectance was abnormally high. Andesite, dior- ite, tuffaceous rocks, and altered dacitic rock were found at approximately 4800 m in well W6 which was located in the Chunxiao Fault Zone. The vitrinite reflectance in mudstones at this depth reached 1.35%. At a depth of 3840–4275  m, well W7 located in the Tianwaitian Fault Zone and affected by hydrothermal f luids, encoun- tered tuff breccia, tuff, granite, granodiorite rocks. At these depths, the maximum porosity was 25.9% and the minimum was 1.5%, and the average value was 14.5% (Table 2). 44% of the sand thickness (29.5 m) of the Pin- ghu Formation was seen to have porosities greater than 14% (3516–4645 m) (Fig. 12). For the wells close to fault zones affected by hydrother - W1 W6 mal activity, thin section and SEM observations revealed W7 W8 that the dissolution of potassium feldspar, volcanic detritus, W9 J1 Q2 metamorphic lithics and phyllites by CO -rich acidic fluids was extensive, resulting in the formation of intergranular dissolved pores and moldic pores. Based on the compari- Fig. 10 The relationship curve between R values and depth from dif- son of porosity and pore-throat characteristic of the samples ferent wells from these wells, large differences in dissolution distribution are believed to arise from the influx of magmatic hydro - pore-throat radius of 13.2 μm. The maximum porosity of thermal fluids in fracture zones. A multi-well correlation well W6 between 3964 and 4175 m was as high as 14.6% exercise was carried out using the porosity logs and seismic with an average pore-throat radius of 4.12 μm (these data and volcanic detritus interpretation results. It is noted that (a) (b) 14 4000 0 0 W7 W1 W8 W3 W9 Q2 G2 J1 W7 W8 W3 W1 G1 G2 J1 J2 Q2 2− Fig. 11 The measured CO content in gas component analysis (a) and the content of SO in formation water, Xihu Depression (b) 2 4 1 3 Depth, m The content of CO , % 2- The content of SO , mg/L 4 Frequency, % Frequency, % Frequency, % Frequency, % Permeability Permeability Permeability Permeability contribution contribution contribution contribution value, % value, % value, % value, % Depth, m P , MPa c P , MPa c P , MPa c P , MPa 26 Petroleum Science (2019) 16:14–31 1 3 1000 60 100 Porosity, % (a) 0 10 20 30 40 10 40 0.1 20 0.001 0 0 920.0 μm 450.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 J1, 3932.5 m, 10x2(-), Primary pores and intragranular dissolution pores S , % R, μm Hg and the histogram of pore radius 1000 60 100 (b) 10 40 0.1 20 0.001 0 0 470.0 μm 470.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 Q2, 4320.46 m, 10x4(-), Primary pores and intragranular dissolution pores, S , % R, μm Hg with quartz secondary enlargement, and the histogram of pore radius 1000 60 100 (c) (c) 10 40 0.1 20 0.001 0 0 (a) 190.0 μm 450.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 (d) W1, 3441.2 m, 10x4(-), Intergranular dissolution pores, casting pores, S , % R, μm Hg and the histogram of pore radius 1000 60 100 (d) (b) 10 40 0.1 20 G2 J1 J2 Q2 0.001 0 0 450.0 μm 190.0 μm 100 80 60 40 20 0 0.0183 0.293 4.6875 75 W1 W4 W7, 4052.62 m, 10x4(-), Intergranular dissolution pores, casting pores, 5000 W5 W7 S , % R, μm Hg and the histogram of pore radius Fig. 12 The relationship curves for porosity and depth, pore types and the histogram of pores radius in wells affected by hydrothermal fluids (particle data from the Shanghai Branch of CNOOC, China) Petroleum Science (2019) 16:14–31 27 Table 2 The porosity values of wells of the central uplift zone and the Sudi Structural Zone (Zhou 2003; Su et al. 2016). District Well Depth, m Porosity, % The impact of intruded magma and hydrothermal flu - Φ Φ Φ max min avg ids on the surrounding rocks has been shown significantly in this paper. The following phenomena are described in Fault zone W1 3440–3447 18.9 15.1 17.0 the study: brittle fracturing and clastic particle alteration. W4 2810–2897 18.8 16.6 17.4 When magma intruded into consolidated country rock, the W5 3792–3840 17.8 10.7 12.7 pressure from this sudden influx resulted in brittle frac- W7 3775–4200 25.6 1.5 14.5 turing of the country rocks, clastic particles expanded, Far from fault G2 2000–3000 32.8 13.9 20.8 cracked and subsequently healed during the upwelling of 3000–4000 19.5 0.7 10.7 deep hydrothermal fluids (Fig.  4). Other observed effects 4000–4072 13.1 1.2 8.7 including: the precipitation of unique hydrothermal min- J1 3724–3984 11.1 3 8.3 erals (Fig. 5) and illite alteration, and kaolinite becomes J2 3420.2–4531 14.6 3.1 10.4 unstable at temperatures between 120 and 150  °C and Q2 3633–4646 16.5 7.1 12.2 transforms into illite under potassium-rich conditions The sample location is marked on the well profile in Fig. 12 (Huang et al. 2009), resulting in increased illite content. The influence of hydrothermal fluids and the local baking the distribution of the volcanic detritus is associated with the effects of magmatic rocks on the surrounding rock strata existence of faults. For example, the net thickness of high (Fig. 6) results in the presence of abnormally high temper- porosity sands (over 14% porosity) is much higher in Wells atures in fluid inclusions (Fig.  7) and anomalous vitrinite W1, W3, W6 and W7, sited in the Baoshi, Chunxiao and reflectance (Fig.  9). These phenomena in the study area Pinghu fault complexes than those from Wells G2, Q2 and J2 are interpreted as being related to the two magmatic ther- where no faults or volcanic detritus are present. According mal events that occurred at the end of the Early Miocene to the porosity logs, higher porosity is observed adjacent to and the Late Miocene (Yang et al. 2001). The deep faults the faults, with porosity values falling in areas away from that propagate from the basement act as a regional conduit the faults (Fig. 13, Table 2). system for upward migration of hydrothermal fluids to the reservoirs, and this results in significant modification and alteration. The high-temperature magma consisting of a 5 Discussion volatile silicate melt released significant amounts of heat and volatiles when migrating upwards and cooling (Tao Previous studies of the burial and thermal history of the and Xu 1994). This process resulted in abnormally high Xihu Depression suggested that the thermal effects of geothermal gradients and affected the prospectivity of the magmatic intrusives were detectable in the southern part surrounding rocks, especially the migration of alkanes and Altitude, m -1800 W1 W3 W6 E h 3 2 W7 -2200 G2 E h 3 1 -2600 E2p3 J2 E2p2 Q2 -3000 E p 2 1 -3400 E b -3800 -4200 Q2 W7 -4600 J2 G2 -5000 W6 Measured porosity Porosity<10% Fault Well Porosity>18% W1 positions W1 Legend W3 E p 10%<Porosity<14% Volcanic rock Magma 2 2 Formation 14%<Porosity<18% Fig. 13 The difference in pore characteristics in samples from wells affected by hydrothermal fluids near faults and from samples not affected by hydrothermal fluids, far from faults (Pie Chart: Ratio of sand body thickness in different porosity intervals to total sand thickness) 1 3 Baoshi main fault Tianwaitian main fault Chunxiao main fault Chunxiao fracture Chuyang fracture 28 Petroleum Science (2019) 16:14–31 Dissolution Cementation W1 W2 W3 W6 W7 W9 Depth, m Hydrocarbon mig- Diagenesis -ration & filling 1000 N2S 3 2 3000 N1 l + N1 y Fluid flowing activity N1 l E3 h Thermal event 1 E3 h 1+2 Fault activity E p Vertical E2 p injection E2 b Tectonic activity Hydrothermal (riched in CO2, SO2) Legend Fault Formation Well Volcanic rock Magma Direction of fluid flowing W1 Fig. 14 The dissolution model of hydrothermal fluids in the Xihu Depression reservoir diagenesis of the formations underlying or over- 2CaAlSi O (calcium feldspar)+ 2H + H O 3 8 2 lying the intrusives. + = Al Si O (OH) (kaolinite)+ 4SiO + 2Ca 2 2 5 4 2 (8) Based on the results above, the depths where abnormally high porosity zones were encountered were consistent with 2+ m − 0 − the depths where the magmatic intrusions and thermal fluid 2Ca + C O + C O = 2CaCO (9) 3 3 activity were noted. 2+ m − − m 2Ca + C O + 2RCOO = CaC O + Ca(RCOOH) The mechanism of dissolution of alkaline minerals by 3 2 late CO -rich acidic fluids is closely linked to the influence (10) of magmatic hydrothermal fluids on the thermal evolution The incursion of high-temperature hydrothermal fluids of source rocks discussed in this paper. The hydrothermal from the crystalline basement is thought to have increased fluids related to the intrusives could carry organic acids and/ formation temperatures in the study area, as average geother- or fluids rich in CO and SO , decreasing the pH of the host mal gradients in the region are as high as 4.22 °C/100 m as 2 2 rock diagenetic environment (Eqs. (2)–(4)) resulting in dis- interpreted from anomalously high vitrinite reflectance val- solution of feldspar grains and carbonate cements (the dis- ues in the surrounding rocks. These hydrothermal incursions solution reaction is shown in Eqs. (5)–(10)) (Wang et al. would accelerate the thermal maturation of organic matter, 2017a, b), leading to secondary dissolution porosity in the releasing large amounts of short-chain organic acids. These host rocks. As illustrated in Eqs. (6)–(8), the dissolution of relatively low molecular weight organic acids can form feldspar leads to the precipitation of quartz as overgrowth organic acid anions with aluminum complexes, enhancing cements. the dissolving capacity of Al and Si in aqueous solution, and increasing some of rock element migration ability in − + RCOOH = RCOO + H (2) groundwater. Thus, the transformation of the silicate mineral + 2− − characteristics and connecting throat pore surface properties 2CO + 2H O = 2H CO = 3H + CO + HCO (3) 2 2 2 3 3 3 cause the variation of oil reservoir porosity and permeability + 2− SO + H O = H SO H SO + O = 2H + SO of rock mass (Oelkers and Schott 1995; Guo et al. 2003; (4) 2 2 2 3 2 3 2 Zhang et al. 2009). + 2+ The high temperatures also enhance the dissolution of CaCO + H = Ca + HCO 3 3 (5) feldspar in the acidic conditions and produce secondary + 2+ 2+ − Ca ⋅ Mg CO + 2H = Ca + Mg + 2HCO porosity (Zhao 2005). Based on these observations, the research team developed a preliminary model to account for reservoir development in the region considering the action 2KAlSi O (potash feldspar)+ 2H + H O 3 8 2 + of hydrothermal fluids associated with igneous intrusives in = Al Si O (OH) (kaolinite)+ 4SiO + 2K (6) 2 2 5 4 2 the Xihu Depression (Fig. 14). The pores generated during the dissolution process improved reservoir quality, but dis- 2+ 2+ 2NaAlSi O (soda feldspar)+ 2H + H O solution products, such as Ca and Mg , could be carried 3 8 2 by the hydrothermal fluids to overlying strata above or to = Al Si O (OH) (kaolinite)+ 4SiO + 2Na (7) 2 2 5 4 2 the peripheral regions where they could be precipitated as 1 3 Petroleum Science (2019) 16:14–31 29 carbonate cements, reducing the reservoir quality in those Furthermore, the hydrothermal fluids associated with areas. Subsequent research should concentrate on how this intrusives had two main impacts on reservoir properties: precipitation of mobile ions affects pore development and (a) formation of secondary dissolution pores improving the porosity in peripheral areas. reservoir quality and (b) precipitation of authigenic minerals 3+ 2+ 2+ from the dissolution products, such as Al, Ca and Mg , precipitated as cement in the overlying strata or the periph- 6 Conclusions eral regions when temperature, pressure or pH conditions changed. Finally, the porosity in the reservoirs decreased and The following conclusions can be drawn from the present the anisotropy increased. So, upcoming research will focus study: on the quantitative characterization of the dissolution and cementation of the deep hydrothermal fluids on the clastic (1) Three Cenozoic episodes of volcanic activity accompa- reservoirs and establish a corresponding model. nied by magmatic intrusions that caused brittle fractur- Acknowledgements This work was funded by the National Natural ing of the surrounding sandstones have been identified. Science Foundation of China (Grant No. 41502135). The authors are Quartz and feldspar particles show cracking on their grateful to the State Key Laboratory of Oil and Gas Reservoir Geology margins related to thermal expansion due to heating. and Exploitation and the anonymous reviewers for their extraordinary Because of the high temperatures, and the CO and SO advice. 2 2 which were carried by the hydrothermal fluids associ- Open Access This article is distributed under the terms of the Crea- ated with the magmatic intrusions, the host sandstones tive Commons Attribution 4.0 International License (http://creat iveco were subjected to thermal alteration, which led to the mmons.or g/licenses/b y/4.0/), which permits unrestricted use, distribu- precipitation of an assemblage of hydrothermal miner- tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the als such as celestite, zircon, apatite, barite, and cerous Creative Commons license, and indicate if changes were made. phosphate. 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