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Hydrocarbon generation from Carboniferous-Permian coaly source rocks in the Huanghua depression under different geological processes

Hydrocarbon generation from Carboniferous-Permian coaly source rocks in the Huanghua depression... Natural gas and condensate derived from Carboniferous-Permian (C-P) coaly source rocks discovered in the Dagang Oilfield in the Bohai Bay Basin (east China) have important implications for the potential exploration of C-P coaly source rocks. This study analyzed the secondary, tertiary, and dynamic characteristics of hydrocarbon generation in order to predict the hydrocarbon potentials of different exploration areas in the Dagang Oilfield. The results indicated that C-P oil and gas were generated from coaly source rocks by secondary or tertiary hydrocarbon generation and characterized by notably different hydrocarbon products and generation dynamics. Secondary hydrocarbon generation was completed when the maturity reached vitrinite reflectance (R ) of 0.7%–0.9% before uplift prior to the Eocene. Tertiary hydrocarbon generation from the source rocks was limited in deep buried sags in the Oligocene, where the products consisted of light oil and gas. The activation energies for secondary and tertiary hydrocarbon generation were 260–280 kJ/mol and 300–330 kJ/mol, respectively, indicat- ing that each instance of hydrocarbon generation required higher temperature or deeper burial than the previous instance. Locations with secondary or tertiary hydrocarbon generation from C-P coaly source rocks were interpreted as potential oil and gas exploration regions. Keywords Hydrocarbon generation · Thermal simulation · Coaly source rocks · Carboniferous-Permian · Huanghua depression 1 Introduction C-P period, leading to the discovery of gas and condensate in drilling wells within the C-P sandstone reservoirs and Huanghua Depression is one of the most petroliferous buried-hill reservoirs in deep parts of the basin (Liu et al. depressions in the Bohai Bay Basin of east China, and the 2017; Zhao et al. 2018). For example, gas and condensate main source rocks for accumulations are Paleogene lacus- produced from well Gbg1 in the Zhanhua sag and well Su20 trine source rocks that generate large amounts of oil and gas on the Suqiao-Wenan slope of the Jizhong sag both originate (Hao et al. 2007; Li et al. 2015; Liang et al. 2018). Coaly from C-P coaly source rocks (Dai and Xia 1990; Liu et al. source rocks were deposited under the basin during the 2017; Gong et al. 2018). In 2017–2018, large quantities of 3 4 3 crude oil (30.2 m /d) and gas (8 × 10  m /d) were produced at depths of 4956.8–4984.9 m in sandstones from well Yg1, Edited by Jie Hao and at depths of 3836–3841 m in Ordovician buried-hill carbonate reservoirs from well Qg8 in Dagang Oilfield of * Jin-Jun Xu the China National Petroleum Corporation (CNPC), which xujj2015@upc.edu.cn revealed great potential for C-P oil and gas exploration Key Laboratory of Deep Oil and Gas, China University (Zhao et al. 2018). of Petroleum (East China), Qingdao 266580, Shandong, During the Paleozoic, there was a huge platform over China north China (Liu 1990; He et al. 1991), onto which coal- Pilot National Laboratory for Marine Science bearing sediments were deposited during the C-P period and Technology, China University of Petroleum (East China), (Lv et al. 2011; Chang et al. 2016; He et al. 2016; Fig. 1). Aoshanwei Wenhai Road 1, Qingdao 266237, Shandong, Previous studies have indicated three instances of tectonic China Vol:.(1234567890) 1 3 Yanshan Uplift Petroleum Science (2020) 17:1540–1555 1541 0 100 km Beijing Su20 Dalian Gg16102 Qg1601 Qg8 Ws1 Gbg1 Yg1 Jianan Source rock Country Study area boundary boundary Fault Well City Fig. 1 Distribution of residual coal source rocks in eastern China subsidence and two uplift events since the Triassic (T) generation) (Qi and Yang 2010; Li et al. 2012; Zhao et al. (Allen et al. 1998; Chang et al. 2018). The coaly source 2015). During the late K to Paleocene (E), secondary rocks were then buried and reached a mature stage during hydrocarbon generation was terminated at the end of rift the mid-T, generating small quantities of hydrocarbons; development (Zhou et al. 2012). Coaly source rocks that this was the first instance of hydrocarbon generation (pri- were not buried to sufficient depths in the J-K period did mary hydrocarbon generation) (Zhang et al. 2014; Chang not generate oil or gas during this period; however, suffi- et al. 2018). The end of primary hydrocarbon generation cient burial for oil and gas generation did occur during the was caused by uplift from the late T (Jin et al. 2009). Dur- tertiary rifting stage (late-secondary hydrocarbon genera- ing the Jurassic (J) to Cretaceous (K), a rift basin formed tion from E). Since the Eocene, a new rift basin developed in east China (Allen et al. 1998; Zhou et al. 2012). Some on residual J-K strata, with some of the coaly source rocks C-P strata were buried to great depths, generating hydro- buried deep beneath the sag areas of the rift generating oil carbons for the second time (early-secondary hydrocarbon 1 3 Bohai Bay basin Dagang Oilfield Shandong Fold Uplift Uplift Eastern Liaoning uplift Eastern Shandong uplift Southwestern Taihang 1542 Petroleum Science (2020) 17:1540–1555 and gas for a third time (tertiary hydrocarbon generation) environment and amount of erosion affect the thickness of (Zheng et al. 2007; Belaid et al. 2010; Chang et al. 2018). coal, carbonaceous mudstone, and dark mudstone layers. As the C-P strata are buried deep below the Bohai Bay Basin, few drilling wells have penetrated these layers; therefore, the distribution of C-P strata beneath the basin 3 Samples and methods has become a key area of geological research (Huang et al. 2010; Chang et al. 2018; Zhao et al. 2018). However, the 3.1 Samples and data locations of secondary or tertiary hydrocarbon generation of coaly source rocks in the basin, as well as the quantity of A total of 23 coal samples were collected from well oil and gas generation, should be determined prior to fur- Gg16102 and Longkou mine, three of which were selected ther exploration. The distribution of residual C-P strata in as thermal experimental samples (Fig. 1; Table 1). The mac- the Huanghua Depression, including that of coaly source eral composition, R , total organic carbon content (TOC), rocks, has previously been determined (Tian et al. 1996; He and Rock–eval pyrolysis were determined for Dagang oil et al. 2016; Zhao et al. 2018). Therefore, the purpose of this field source rock samples and Longkou mine samples col- study is to investigate the quantity of oil and gas generated lected from wells Gg16102, Kg4, Qg1601, Xu14, Ws1, and from coaly source rocks under the Huanghua Depression Ts1. of Bohai Bay Basin by analyzing the secondary, tertiary, and dynamic characteristics of hydrocarbon generation with 3.2 Methods the aim of predicting the hydrocarbon potentials of different exploration areas. 3.2.1 Thermal history recovery Many previous investigations speculated that the geother- 2 Geological setting mal gradient reached 3.5 °C/100 m from late P to early T and then increased to approximately 5.0 °C/100 m in the Dagang Oilfield is located at 37°30′–39°50′N and Mesozoic due to strong magmatic activity before decreasing 116°10′–119°30′E in the Huanghua Depression of the Bohai again to 3.5 °C/100 m during the Cenozoic (Li et al. 2007a, Bay Basin (Fig. 1), which is a typical Mesozoic and Ceno- b; Zhang et al. 2014). Based on the burial history recovery zoic extensional basin in the center of the eastern part of the of coaly source rocks and the paleogeothermal gradient, the North China Craton Block (Su et al. 2014; Li et al. 2015). hydrocarbon generation history of coaly source rocks in typi- Dagang Oilfield is oriented in a north-northeast direction cal wells was recovered using the Easy%R method (Schenk 4 2 covering an area of 1.7 × 10  km . Many studies have ana- et al. 2017). The initial maturities of primary, secondary, and lyzed the distribution of residual C-P coaly source rocks in tertiary hydrocarbon generation processes were also identi- eastern China, which are found throughout almost the entire fied according to the thermal evolution history curve. Dagang Oilfield except for Qikou sag, where they have been eroded (Huang et al. 2010; Chang et al. 2018; Zhao et al. 3.2.2 Thermal simulation experiments and kinetic 2018; Fig. 1). The residual C-P source rocks are also found parameter calculation in Dagang Oile fi ld. Thus, widespread C-P coaly source rocks in the study area provide abundant materials for hydrocarbon The thermal simulator can separately resist high tem- generation. peratures and high pressures up to 800 °C and 120 MPa, Three types of coaly source rocks found in the study area, respectively (Fig. 3). Samples are closed in the autoclave including coal, carbonaceous mudstone, and dark shale, and accompanied by nitrogen or distilled water (to represent were deposited in a paralic environment during the C-P 25% of the sample mass) and heated by a heater strip with period. The C-P succession contains the Benxi Formation a set heating rate. Based on the results of previous experi- (C b), Taiyuan Formation (C t), Shanxi Formation (P s), ments, the relationship between R and temperature was fit- 2 2 1 o and Shihezi Formation (P sh), which is comprised of thick ted (Fig. 4). R values serve as a “bridge” connecting the 1 o (418.0–1768.5 m), unconformable, siliciclastic-dominated, actual degree of thermal evolution and the thermal simula- shallow-marine and paralic deposits overlying Middle Ordo- tion degree. With a heating rate of 60 °C/h, all coal samples vician carbonates (Fig. 2; Kim et al. 2001). C-P coaly source were heated at 50 °C intervals to simulate the actual geologic rocks in the Shanxi Formation (67.0–195.5 m) were depos- conditions of source rocks within different areas during their ited in a continental sedimentary environment, whereas thermal history. source rocks of the Taiyuan Formation (87.5–229.0  m) Among the coaly source rocks, the coal exhibited greater were formed in a paralic environment (Li et al. 2001; Lv and hydrocarbon generation potential with a mean of greater than Chen 2014; Zhao et al. 2017; Fig. 2). Both the depositional 11 mg/g of S + S dominating the generation of coal-related 1 2 1 3 Petroleum Science (2020) 17:1540–1555 1543 Lithostratigraphy Chronostratigraphy Lithology Thickness, m Formation Symbol Quaternary Pingyuan Qp 200-490 Pliocene Minghuazhen Nm 450-1725 Miocene Guantao Ng 170-590 Dongying Ed 439-1500 Oligocene Shahejie Es 1932-6378 Eocene Kongdian Ek 22-1279 Paleocene Wuji K w 150-485 Fengtai K f 205-214 Cretaceous Lugouqiao K l 484-826 Xinzhuang J x 0-429 Jurassic Yaopo J y 260-437 1-2 Shiqianfeng P sh 62-446 Shangshihezi P s 119-472.5 Permian Xiashihezi P x 76-342.5 Shanxi P s 67-195.5 Taiyuan C t 87.5-229 Carboniferous Benxi C b 6.5-83 Fengfeng O f 30.5-254 Shangmajiagou O s 43-308 Ordovician Xiamajiagou O x 88-247 Liangjiashan O l 49.5-248 O y Yeli 25-125 Shale Sandstone Limestone Unconformity Siltstone Conglomerate Argillaceous Crystalline Dolomite Breccia Coal Source rock sandstone rock Fig. 2 Stratigraphic profile and distribution of main source rocks in Dagang Oilfield (modified from Zhao et al. 2018) Table 1 Organic geochemical characteristics and maceral composition of different coals Samples R , % T , °C S , mg/g TOC, % Hydrogen index, Maceral content, % o max 2 mg/g TOC Vitrinite Exinite Inertinite Torbanite Longkou 0.41 407 250.90 57.54 435.9 60.6 39.4 0.0 Humic coal Longkou 0.41 407 92.16 61.33 150.3 93.8 6.0 0.2 C-P Coal Well Gg16102 0.67 432 124.63 65.56 190.1 87.1 11.5 1.2 1 3 Lower Upper paleozoic Mesozoic Cenozoic paleozoic Paleogene Neogene Cooled autoclave 1544 Petroleum Science (2020) 17:1540–1555 (a) Gas Airmanometer cylinder 00 00 00 00 Vent valve 00 00 00 00 Vacuum pump Vacuum meter (b) Pressure sensor (c) Pressure controller Air extraction metering device Computer Vacuum data pump acquisition Autoclave system Vacuum meter Temperature controller Samples Temperature sensor Fig. 3 Schematic showing the thermal simulation of hydrocarbon generation. a The thermal simulator consists of eight autoclaves connected to a gas cylinder and vacuum pump; b each autoclave is monitored by temperature and pressure controllers. The liquid hydrocarbon production in the autoclave was collected at each temperature point after cooling and opening; c the gas hydrocarbon production was collected using the saturated saltwater displacement method crude and condensate; therefore, coal was selected as the coal in the Bohai Bay Basin (Fig. 1). During the thermal experimental sample for thermal simulation (Ahmed et al. simulation of primary hydrocarbon generation, the tempera- 2009; Zhao et al. 2018). According to the thermal evolution ture was increased from 300 to 650 °C with 5–7 g of sample history, three instances of hydrocarbon generation occurred, collected at every temperature point (total of eight points). with regional subsidence developing during the early T and The coal sample for thermal simulation of secondary hydro- all coaly source rocks reaching maturity at approximately carbon generation was collected from Gg16102 and exhib- 0.65% R (Jin et al. 2009), which is represented by the coaly ited lower initial maturity of secondary hydrocarbon gen- source rocks in the uplift area or outside of the Bohai Bay eration (0.67% R ). In the thermal simulation of secondary Basin (e.g., southwestern Shandong). Therefore, no imma- hydrocarbon generation, the sample was also heated from ture samples of C-P coaly source rocks were available in 350 to 650 °C. The simulation was ended at a temperature of the present-day sedimentary strata. To prepare an immature 375 °C, when the sample reached a maturity value of 0.76% sample for the thermal simulation of primary hydrocarbon R . The autoclave was cooled to room temperature and then generation, an experimental sample was prepared from E heated again to perform thermal simulation of secondary coal collected from the Longkou mine of Shandong Province hydrocarbon generation with higher initial maturity (0.76% according to the proportion of vitrinite and exinite of C-P R ) from 400 to 650 °C. An experimental sample with a 1 3 Petroleum Science (2020) 17:1540–1555 1545 5.0 where D is the heating rate (K/min); T is the absolute tem- perature (K); R is the gas constant (8.31447 kJ/(mol K)); 4.5 and XG is the value of XG at the onset of coal thermal i0 i 0.0051x 4.0 y = 0.128e degradation. In this study, the calculated activation energy R = 0.9918 was the average of all hydrocarbon generation. 3.5 3.0 3.2.3 Gas chromatography analysis 2.5 Subsequently, the samples were extracted by a Soxhlet 2.0 apparatus with dichloromethane for 72 h. The extracts were then fractionated by open silica gel column chromatography 1.5 using n-hexane. The resulting saturated hydrocarbons and all 1.0 products were analyzed by gas chromatography (GC) with an Agilent 5890 N GC analyzer for quantitative analysis. 0.5 200300 400 500 600 700 800 4 Results and discussion Temperature, °C 4.1 Multi‑part thermal history of coaly source rocks Fig. 4 Relationship between simulation temperature points and matu- rity 4.1.1 Characteristics of the three hydrocarbon generation events maturity of 0.9% R was prepared at a temperature of 400 °C According to their timing and order, the hydrocarbon gen- from a coal sample of well Gg16102 and then cooled. All eration events were divided into three types: primary hydro- hydrocarbon generation processes were heated to 650 °C carbon generation, secondary hydrocarbon generation, and with 5–7 g of sample collected at every temperature point. tertiary hydrocarbon generation. Occurring in the early P The materials produced at each 50 °C interval during the to middle T, the primary hydrocarbon generation area was thermal simulation were then collected. The generated liquid characterized by shallow burial, with maturity of less than hydrocarbon of each temperature point was extracted from 0.80% R (Zhang et al. 2014), and classified as the initial oil the heated residual using dichloromethane. The weight of oil generation stage (Fig. 5a). Part of the primary hydrocarbon production was weighed after the volatilization of the extrac- generation area remained buried at a shallow depth, with tion. Gas hydrocarbons were collected using the method of some locations uplifted and eroded from the late T to the saturated salt solution displacement (Dai et al. 2013). present. Owing to deep burial during either the Mesozoic or The kinetic parameters of coal cracking were calculated Cenozoic, secondary hydrocarbon generation occurred in the according to the method of Wang et al. (2015) based on Xuhei area during the late J to early K with structural sub- parallel first-order reaction models, which are kinetic models sidence (Fig. 5d). The coaly source rocks of the Qibei slope with multiple frequency factors (MFF models) and a range and Qikou sag were deeply buried and underwent thermal of activation energies. Hydrocarbon generation is assumed evolution, generating petroleum for a second time from the to involve a series of parallel first-order reactions, including Neogene (Fig. 5c, e). The tertiary hydrocarbon generation the activation energy of reaction i (EG ), the pre-exponential area developed in the early to middle T, the late J to early factor of reaction i (AG ), and the generation potential of K, and during the E period. As a result of its burial history, the reaction or reaction fraction i (XG ) (i = 1, 2, …, NKH). tertiary hydrocarbon generation occurred during a wet or dry The formula, derived from the first-order reaction rate equa- gas phase (Fig. 5f; Zhao et al. 2018). tion and the Arrhenius formula, calculates the total mass of hydrocarbon generation of the NG parallel reaction: NKH NG ⎛ ⎛ ⎛ ⎞⎞⎞ � � � � � � ⎜ ⎜ ⎜ ⎟⎟⎟ (1) XKH = XG = XG 1 − exp − AG ∕D ⋅ exp −EG ∕R ⋅ T dT i i0 i i ⎜ ⎜ ⎜ ⎟⎟⎟ i=1 i=1 ⎝ ⎝ ⎝ ⎠⎠⎠ 1 3 R , % o N-Q N-Q N-Q N-Q 1546 Petroleum Science (2020) 17:1540–1555 Primary generation Secondary generation Tertiary generation Evolution Early to middle Triassic Late Jurassic to CretaceousNeogene Neogene stages CP TJ KE CP TJ KE CP TJ KE CP TJ KE Unit: km (a) R = 0.5% Beidagang uplift (well Gg16102) Effectiveness (b) (c) R = 0.5% Ro = 0.5% Ro = 0.5% Ro = 0.5% o Ro = 0.7% Ro = 0.7% R = 0.7% Ro = 0.7% o Kongdian uplift Qibei slope (well Kg4) (well Qg1601) (d) (f) R = 0.5% o R = 0.5% R = 0.7% Ro = 0.7% R = 1.3% Ro = 1.3% Xuhei uplift Wumaying buried hill (well Xu4) (well Ws1) (e) Ro = 0.5% R = 0.7% 0.7 2.0 o 0 0.51.3 2.5 R = 1.3% R , % R = 2.0% Qikou sag Unit: Ma 300 200 100 300 200 100 300200 100 300200 100 Fig. 5 Stages of hydrocarbon generation within the Huanghua Depression. a and b primary hydrocarbon generation during the early to mid-T with maturity of 0.67% R and 0.76% R , respectively; c Neogene late-secondary hydrocarbon generation with low maturity of 1.1% R ; d early- o o o secondary hydrocarbon generation during the late J to early K; e Neogene late-secondary hydrocarbon generation with maturity above 2.0% R ; f Neogene tertiary hydrocarbon generation reached a maturity of 1.6% R 4.1.2 Geological conditions of multiple hydrocarbon different areas. The models provide accurate experimen- generation events tal reference conditions for setting parameters based on the relationship between maturity and heating tempera- According to our reconstruction of the hydrocarbon gen- ture (Table  2; Fig.  4). Based on the influences of tem- eration history, geological models were established for perature and depth, the initial and ceasing R values of 1 3 Dry gas stage Wet gas stage Mature stage Low mature stage R > 2.5% R = 1.3-2.5% R = 0.7-1.3% R = 0.5-0.7% o o o o 340 340 310 310 290 290 280 280 260 260 240 240 230 230 210 210 200 200 190 190 180 180 160 160 Petroleum Science (2020) 17:1540–1555 1547 Table 2 Geological model of multiple hydrocarbon generation events in Huanghua Depression Types Hydrocarbon Presentative wells Generation processes evolution (R /%) Current burial depth generation C-T J-K E to now event Initial point Final point Initial point Final point Initial point Final point A Primary Gg16102 0.5 0.67 – – – – < 3500 B Secondary Ts1 0.5 0.67 – – 0.67 1.1 3500–5500 B Qg1601 0.5 0.76 – – 0.76 1.1 3500–5500 C Tertiary Ws1 0.5 0.67 0.67 0.9 0.9 1.6 >5500 different hydrocarbon generation events were identified 4.2 Variability of hydrocarbon generation processes from thermal history diagrams, in which temperature is and kinetic parameters the dominant factor. The R values of primary hydrocarbon generation ended at 0.67% for wells Gg16102 and Ts1 4.2.1 Peak decrease and delay in multi‑part oil generation and 0.76% for well Qg1601. R values of early-secondary hydrocarbon generation and late-secondary hydrocarbon The complete primary hydrocarbon generation event rep- generation reached 0.9% and 1.1%, respectively. Tertiary resenting immature to mature phases of coaly source rock hydrocarbon generation of well Ws1 finally attained a R (R < 0.5%) is represented by a normal curve in Fig. 6. Liq- value of 1.6% (Table 2). uid hydrocarbon yield reached a minimum of 19.48 mg/g TOC at 300 °C (R = 0.53%), whereas the oil peak appeared 70 0.20 (a) (b) Primary generation Primary generation (R < 0.50%) (Ro < 0.50%) Secondary generation (Ro = 0.67%) Secondary generation (R = 0.76%) Secondary generation o 0.16 (Ro = 0.67%) Tertiary generation (Ro = 0.90%) Secondary generation (R = 0.76%) o 0.12 Tertiary generation (R = 0.90%) 0.08 0.04 0 0 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 Maturity R , % Activation energy generation, KJ/mol 400 0.28 (c) (d) Primary generation (R < 0.50%) Primary generation Secondary generation (Ro = 0.67%) 350 (R < 0.50%) 0.24 Secondary generation (R = 0.76%) Secondary generation 300 Tertiary generation (R = 0.90%) (R = 0.67%) o 0.20 Secondary generation (Ro = 0.76%) 0.16 Tertiary generation (Ro = 0.90%) 0.12 0.08 0.04 0 0 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 Maturity R , % Activation energy generation, KJ/mol Fig. 6 Hydrocarbon generation yield and activation generation distributions of coal samples during the three hydrocarbon generation events. The primary hydrocarbon generation samples are a matched-immature sample from Longkou mine coal. The early-secondary hydrocarbon generation sample is coal from well Gg16102. The late-secondary hydrocarbon generation and tertiary hydrocarbon generation samples are coal from well Gg16102 prepared and heated at 375 °C and 400 °C, respectively 1 3 Gas yield, mg/g TOC Oil yield, mg/g TOC Frequency Frequency 1548 Petroleum Science (2020) 17:1540–1555 at 450 °C (R = 1.06%) with a yield of 60.12 mg/g TOC. 4.2.3 Increase in activation energy generation with time Subsequently, the oil yield began to decrease after 650 °C (R = 1.84%) (Fig. 6a). Compared to primary hydrocarbon The distribution of activation energy reflects the order of generation, the maximum yield of secondary hydrocarbon chemical compounds generated during hydrocarbon gen- generation with the early “oil window” phase (0.67% R ) eration events (Dieckmann 2005). Laboratory experiments appeared at approximately 450 °C (R = 1.06%). The highest have determined the kinetics describing the rate of thermal oil yield of 41.52 mg/g TOC was substantially lower than decomposition of kerogen to petroleum (Pepper and Corvi that of primary hydrocarbon generation (Fig. 6a). A higher 1995; Dieckmann 2005; He et al. 2018a, b). The activation initial maturity of 0.76% reduced the amount of oil produced energy varied considerably among the three hydrocarbon by secondary hydrocarbon generation; the maximum yield generation events with different initial maturities (Fig.  6b, of 39.01 mg/g TOC was less than that of primary and early- d). All events exhibited a broad distribution of activation secondary hydrocarbon generation events. The lowest quan- energies; however, with increasing initial maturity, the tity of oil compounds was obtained at 650 °C (R = 1.84%) activation energy distribution shifted toward higher values with a yield of only 2.38 mg/g TOC. However, with a higher from primary hydrocarbon generation to tertiary hydrocar- initial maturity point (R value of 0.90%), an oil yield of bon generation events, accompanied by an increase in the 10.67  mg/g TOC was obtained. The oil peak of tertiary frequency factor (A ). hydrocarbon generation occurred after 500 °C (R = 1.27%) Primary oil generation exhibited a broader distribution, with a lower yield of 14.40 mg/g TOC. At the end of heating centered around 180–220 kJ/mol with a maximum value of (650 °C, R = 1.84%), the remaining oil potential declined to 190 kJ/mol and an A value of 5.87–9.3 × 10 . Due to the o f nearly zero (Fig. 6a), with the curve of tertiary hydrocarbon low thermostability, heteroatomic compounds, small mol- generation shown as a straight line, representing a low oil ecule benzenes, and alkyl benzenes preferentially cracked yield. and generated light hydrocarbons during primary hydro- The characteristics of the different generation events sug- carbon generation (Li et al. 2007a, b; Yu et al. 2012). At gest that the production quantity and generation curves are an initial maturity of 0.67% R , secondary oil generation controlled by the initial maturity. With increasing maturity, exhibited an almost symmetrical distribution centered on the yield peaks gradually decreased and the curve trans- approximately 230 kJ/mol (A = 8.64 × 10 ). A higher acti- formed from a normal to low-amplitude shape (Fig.  6a). vation energy distribution reflects the formation of short Moreover, the decrease in oil potential was controlled by chains of aliphatic functional groups and oxygen contain- the difference in initial maturity between the two genera- ing groups with higher thermostability, such as isopar- tion events. affinic, naphthenic, and normal alkane hydrocarbons (Li et al. 2007a, b; Yu et al. 2012). For a similar initial matu- rity, a small increase in activation energy of late-secondary 4.2.2 Total decrease in gas generation hydrocarbon generation was observed from 260 to 280 kJ/ mol (A = 1.01–6.34 × 10 ) compared to previous oil genera- The differences in gas generation among the three hydro - tion (Fig. 6b). Compared to the generation event of 0.67% carbon generation events are clearly observed in Fig. 6c, R , more normal alkanes and short chains of aromatic and in which the generation curves increase continuously with methyl benzol would form, corresponding to a higher activa- temperature. With increasing maturity, the total gas produc- tion energy (Walker et al. 2007; Petersen et al. 2009; Chen tion decreased substantially at all temperature points. The et al. 2012; He et al. 2018a, b). The activation energy of maximum gas yield was 387.29 mL/g TOC, obtained dur- 300–330 kJ/mol (A = 4.86–5.94 × 10 ) for tertiary hydro- ing primary hydrocarbon generation at 650 °C (R = 1.84%). carbon generation was substantially higher than that for pri- Correspondingly, the minimum yield of 165.23 mL/g TOC mary and secondary oil generation events. With increasing was obtained at 650 °C during tertiary hydrocarbon genera- maturity, a higher activation energy indicates the cracking of tion. Due to the higher initial maturity (R = 0.9%) of tertiary long chains of aliphatic hydrocarbons and aromatic hydro- hydrocarbon generation, gas production was clearly lower carbons with long side chains. The kerogen may originate than that during primary and secondary hydrocarbon gen- from angiosperm and gymnosperm pollens containing cuti- eration events. Moreover, the gas yield exhibited a greater nite and sporinite that generate hydrocarbons at considerably decrease between early-secondary and late-secondary hydro- higher maturities of 0.85% R or higher (Petersen and Nytoft carbon generation at higher temperature (Fig. 6c). Again, 2006; Petersen et al. 2009; Yu et al. 2012; Chen et al. 2012; the initial maturity was the most important factor affecting He et al. 2018a, b). petroleum production; the higher the initial maturity, the The activation energy distributions of the multiple gas lower the gas production from primary hydrocarbon genera- generation events are similar to those of the multiple oil tion to tertiary hydrocarbon generation events. generation events. The shift from lower to higher activation 1 3 Petroleum Science (2020) 17:1540–1555 1549 energy for both oil and gas generation indicates that petro- yield was larger and occurred earlier than the gas yield leum generation increased from primary to secondary to (Fig. 6a, c). Conversely, due to the high initial maturity, tertiary hydrocarbon generation events. The differences tertiary gas generation exhibited the opposite trend, with in yield among the multiple petroleum generation events gas generation exhibiting lower activation energies of agreed closely with the variations of activation energy. The 310–320 kJ/mol (A = 3.78–4.53 × 10 ) than oil genera- primary hydrocarbon generation event with the lowest acti- tion (310–330 kJ/mol). This result suggests that gas com- vation energy more easily produced petroleum, as shown by pounds began to be the main product instead of oil com- the higher hydrocarbon yield and earlier peak. Oil and gas pounds, as confirmed by the rising curve of gas yield and yield notably decreased with increasing activation energy. falling slope of oil yield in Fig. 6a, c. Thus, the generation Thus, the reduction in petroleum yield depended on the of oil and gas were fully illustrated by the distribution of different activation energies of different generation events activation energy. (Fig. 6b). Even within the same generation event, oil and gas 4.3 Multiple hydrocarbon generation models generation was dominated by the activation energy. That for petroleum generation history is, during primary and secondary hydrocarbon genera- tion events, the main activation energies of gas genera- It was assumed that hydrocarbon generation from coaly tion were all approximately 10–20 kJ/mol greater than source rocks was predominantly affected by the burial his- those of oil generation, indicating that oil was more eas- tory and thermal evolution, without considering differences ily produced than gas compounds. Consequently, the oil in the abundance of organic matter and abnormal heating Ro = 0.5% 0.67% CP TJ KE N+Q 400 70 0 (a) (b) Oil Gas R = 0.5% o Beidagang uplift 200 3000 (well Gg16102) 0 0.7 2.0 0.51.3 R , % 0 0 6000 300250 200150 100500 300200 1000 Geological time, Ma Geological time, Ma Fig. 7 Model of primary hydrocarbon generation Ro = 0.5% 0.76% 0.76% 1.1% N+Q CP TJ KE 400 0 (a) (b) Oil 350 Gas Ro = 0.5% Ro = 0.5% 200 3000 R = 0.7% R = 0.7% 30 o 0 0.7 2.0 Qibei slope 5000 0.51.3 (well Qg1601) R , % 0 6000 300 250 200 150 100 50 0 300200 1000 Geological time, Ma Geological time, Ma Fig. 8 Model of secondary hydrocarbon generation 1 3 Gas yield, mL/g TOC Gas yield, mL/g TOC Oil yield, mg/g TOC Oil yield, mg/g TOC Depth, m Depth, m 1550 Petroleum Science (2020) 17:1540–1555 events (e.g., magmatic intrusion). By combining the ther- 15.77–24.83  mg/g TOC and 77.08–101.71  mL/g TOC, mal simulation experiment results and the thermal evolution respectively (Fig. 8a). history, three models of hydrocarbon generation were estab- lished to predict the oil and gas potential of coaly source 4.3.3 Third generation event rocks within different structural units. Under the influence of Indosinian, Yanshan, and Himalayan 4.3.1 First generation event tectonic movement, C-P coals were buried to great depths, resulting in a third generation event during the late T and The first generation event only generated petroleum from early to middle K and E. The onset and duration of the sec- the late P to early T. The quantity of generated oil and gas ondary and tertiary hydrocarbon generation events differed was dominated by the thermal evolution degree and oil was depending on their location, leading to variations in the the main product (Figs. 7b, 8b). Buried between 2000 m and petroleum yield. In the regional depression of Huabei craton, 2600 m, the source rocks underwent minimal maturation, primary hydrocarbon generation in the Permian was similar varying from 0.5% to 0.7% R in the Beidagang uplift area to that of the first generation event model. Early-secondary and reaching 0.7%–0.8% R in the Qibei slope area. Hydro- hydrocarbon generation occurred during the Mesozoic from carbon generation began at 250 Ma and ceased at 225 Ma 175 to 98 Ma (Fig. 9b) with oil yields ranging from 13.87 (Fig. 7). Taking well Gg16102 as an example, the source to 28.83 mg/g TOC (0.67%–0.90% R ; Fig. 9a). However, rock ended with R values of 0.67%, and the oil yield was the gas yield only increased from 30.59 to 77.09 mL/g TOC. 14.99 mg/g TOC. A minor amount of gas was generated, The last generation event lasted from 24.6 Ma to present approximately 31.28 mL/g TOC (Fig. 7a). and generated large amounts of condensate and natural gas (Fig. 9b). The tertiary hydrocarbon generation event was 4.3.2 Second generation event characterized by lower oil yields and higher gas yields. With increasing R values, the oil yield decreased substantially The second generation event includes early- and late-sec- from that of primary and secondary hydrocarbon genera- ondary hydrocarbon generation, characterized by the decom- tion, from 8.25 mg/g TOC to 16.42 mg/g TOC. In contrast, position of kerogen from the Paleogene to the present day, gas was generated more rapidly and at a higher level, with which may have accumulated in petroleum reservoirs. Due current yields of 165.12 mL/g TOC, which is favorable for to deeper burial, primary hydrocarbon generation of well the formation of petroleum pools (Fig. 9a). Qg1601 ended at a mature stage with R values of 0.76% (Fig. 8). Compared to the low initial maturity of the first gen- 4.4 Factors controlling multiple hydrocarbon eration event, the oil and gas yields increased to 19.48 mg/g generation events TOC and 95.93 mL/g TOC, respectively, in the second gen- eration event (Fig. 8a). Secondary hydrocarbon generation Multi-stage tectonic movements and variable geother- occurred during the Neogene from 98 Ma to present and mal gradients from the Paleozoic to the Cenozoic may be remained in a mature stage with R values of 0.76%–1.1% responsible for the multiple hydrocarbon generation events (Fig. 8b). Higher oil and gas yields were calculated, from of coaly source rocks. Regional subsidence occurred during R = 0.5% 0.67% 0.67%0.9% 0.9% 1.6% CP TJ KE N+Q 400 70 0 (a) (b) Oil Gas Ro = 0.5% 200 3000 Ro = 0.7% R = 1.3% 4000 o 0 0.7 2.0 0.51.3 Wumaying (well Ws1) R , % 0 0 6000 300 250 200 150 100 50 0 300200 1000 Geological time, Ma Geological time, Ma Fig. 9 Model of tertiary hydrocarbon generation 1 3 Gas yield, mL/g TOC Oil yield, mg/g TOC Depth, m Petroleum Science (2020) 17:1540–1555 1551 the late P to early T, and the geothermal gradient reached by the initial maturity of tertiary hydrocarbon generation, 3.5  °C/100  m. Whole source rocks experienced primary with low initial maturity resulting in adequate oil and gas hydrocarbon generation, with R values ranging from 0.5% generation. Observations of condensate oil and gas in well to 0.7% (Zhang et  al. 2014; Chang et  al. 2018). Subse- Yg1 from C-P coaly source rocks confirm the hypothesis quently, regionally integrated rapid uplift occurred within of reservoir formation from tertiary hydrocarbon generation the Huabei platform. Diverse tectonic activity began in the (Zhao et al. 2018). The low initial maturity of late-secondary late T, which controlled various trends of the hydrocarbon hydrocarbon generation suggests considerable condensate generation events. Furthermore, due to strong magmatic oil and gas potential from coaly source rocks. activity with local magmatic intrusions, the geothermal In order to accurately predict the petroleum resource gradient increased to approximately 5.0 °C/100 m in the potential of coaly source rocks, the hydrocarbon genera- Mesozoic (Zhang et al. 2014). Deeply buried areas exhib- tion parameters of oil and gas yields are required (Jin et al. ited highly mature early-secondary hydrocarbon generation, 2009; Zhang et al. 2009; Cheng et al. 2018), which were with a higher heating rate than that of primary hydrocarbon obtained in this study. The oil and gas quantities generated generation. However, the uplifted and eroded area ceased from different periods of the generation events are critical to generate petroleum. During the Cenozoic, the different functions for evaluating the petroleum resource potential of generation processes were enhanced in a complex manner, coaly source rocks, which is the aim of this study (Boreham with geothermal gradients decreasing again to 3.5 °C/100 m et al. 1999; McCarthy et al. 2011). This study indicates the (Li et al. 2007a, b). The current burial depth favors hydrocar- resource potential of coaly source rocks within late-second- bon generation and is also significant for forming petroleum ary and tertiary hydrocarbon generation events during the pools. In some areas, the coaly source rocks only exhibited a Neogene, and reveals the Bohai Bay Basin as an important primary hydrocarbon generation event as they have remained field for deep exploration. Based on the thermal history, the buried at shallow depths since the late J. Some source rocks Qibei slope, Qinan sag, Kongxi slope, and Wangguantun were once again buried to form petroleum during the late- sag are all favorable areas for late-secondary hydrocarbon secondary hydrocarbon generation event from the Paleogene generation (Fig. 10). The Wumaying buried hill and Xin- after uplift during the Mesozoic. The crucial tertiary hydro- gang area are favorable for tertiary hydrocarbon generation carbon generation event also occurred in the Paleogene after (Fig. 10). As a result of the multiple events of hydrocarbon primary and early-secondary hydrocarbon generation events generation, the mid-north area of Dagang oil field has crude during the Paleozoic and Mesozoic, respectively (Jin et al. oil and natural gas potential, whereas the mid-southern and 2009; Zhang et al. 2009; Cheng et al. 2018). Thus, tectonic northern areas of Dagang oil field are prone to generate more evolution and geothermal gradients were key factors control- condensate and abundant natural gas (Fig. 10). Therefore, ling the multiple hydrocarbon generation events. these areas, which are currently buried at depth, have sig- Compared to continuous hydrocarbon generation, the nificant potential resources of petroleum and natural gas. total hydrocarbon generated during the three events exhib- Additionally, this study provides insights into hydro- ited a decreasing trend. Due to hydrocarbon generation ceas- carbon generation of coaly source rocks deposited in other ing multiple times, the hydrocarbon generation peak and depressions of the Bohai Bay Basin. Compared to the Huan- yield was delayed and reduced with increasing activation ghua Depression, the coaly source rocks deposited in the energy. Although petroleum yields varied from primary to Jiyang and Linqing depressions experienced the same three tertiary hydrocarbon generation events according to the ini- hydrocarbon generation events; thus, they are also prone tial and final maturity values, the initial maturity value had to generating abundant coaly-derived gas (Jin et al. 2009; a dominant effect on the petroleum generation potential of Zhang et al. 2009; Huang et al. 2010; Lv et al. 2011). The coaly source rocks. With increasing initial maturity through- coaly source rocks in the Jizhong depression and Dongpu out the different generation events, the oil generation peak sag experienced similar hydrocarbon generation events to was delayed and reduced (Fig. 6). A greater difference in the the Qikou sag in the Huanghua Depression (Zheng et al. initial maturities of different generation events would result 2007; Zhao et al. 2018). Moreover, deeper burial depths are in higher hysteresis. typically associated with significant coaly-derived gas poten- tial (Li et al. 2007a, b; Lv et al. 2011; Gong et al. 2018). 4.5 Eec ff tive hydrocarbon generation forming Therefore, the models established in this study can provide petroleum pools useful information for the exploration of C-P coaly source rocks in the Bohai Bay Basin. Analysis of the different generation models suggests that late-secondary hydrocarbon generation and tertiary hydro- carbon generation were the most useful for forming oil–gas pools. The scale of the paleo-reservoir was likely controlled 1 3 1552 Petroleum Science (2020) 17:1540–1555 010 km Beitang-xingang subsag Kg6 J3 Gg1601 Gg16102 Qikou sag Qibei slope Qg8 Chh24 Qg1 Chh1 Kg4 Zhg1 Kg6 K8 Chg2 Wg1 Xu1 Yg1 Xu14 Ws1 Primary Erosion generation boundary Xu13 Cl1 Cc1 Early-secondary Fault generation Dg1 XU14 Late-secondary Well/ generation name Tertiary Study generation area Fig. 10 Distribution of favorable areas under different hydrocarbon generation models 1 3 Chenghai slope Beidagang buried hill Kongdian subuplift Fault Fault Jianhai subuplift Southern Cangxian uplift Wumaying buried hill Yanshan subsag Xuhei subuplift Heidong Cangdong sag Fault Xuxi Cang Xi Fault Cheng subuplift Qinan subsag Dong Kongxi slope Zengfutai Petroleum Science (2020) 17:1540–1555 1553 the article’s Creative Commons licence and your intended use is not 5 Conclusions permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. 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Hydrocarbon generation from Carboniferous-Permian coaly source rocks in the Huanghua depression under different geological processes

Petroleum Science , Volume 17 (6) – Nov 4, 2020

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Springer Journals
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Copyright © The Author(s) 2020
ISSN
1672-5107
eISSN
1995-8226
DOI
10.1007/s12182-020-00513-2
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Abstract

Natural gas and condensate derived from Carboniferous-Permian (C-P) coaly source rocks discovered in the Dagang Oilfield in the Bohai Bay Basin (east China) have important implications for the potential exploration of C-P coaly source rocks. This study analyzed the secondary, tertiary, and dynamic characteristics of hydrocarbon generation in order to predict the hydrocarbon potentials of different exploration areas in the Dagang Oilfield. The results indicated that C-P oil and gas were generated from coaly source rocks by secondary or tertiary hydrocarbon generation and characterized by notably different hydrocarbon products and generation dynamics. Secondary hydrocarbon generation was completed when the maturity reached vitrinite reflectance (R ) of 0.7%–0.9% before uplift prior to the Eocene. Tertiary hydrocarbon generation from the source rocks was limited in deep buried sags in the Oligocene, where the products consisted of light oil and gas. The activation energies for secondary and tertiary hydrocarbon generation were 260–280 kJ/mol and 300–330 kJ/mol, respectively, indicat- ing that each instance of hydrocarbon generation required higher temperature or deeper burial than the previous instance. Locations with secondary or tertiary hydrocarbon generation from C-P coaly source rocks were interpreted as potential oil and gas exploration regions. Keywords Hydrocarbon generation · Thermal simulation · Coaly source rocks · Carboniferous-Permian · Huanghua depression 1 Introduction C-P period, leading to the discovery of gas and condensate in drilling wells within the C-P sandstone reservoirs and Huanghua Depression is one of the most petroliferous buried-hill reservoirs in deep parts of the basin (Liu et al. depressions in the Bohai Bay Basin of east China, and the 2017; Zhao et al. 2018). For example, gas and condensate main source rocks for accumulations are Paleogene lacus- produced from well Gbg1 in the Zhanhua sag and well Su20 trine source rocks that generate large amounts of oil and gas on the Suqiao-Wenan slope of the Jizhong sag both originate (Hao et al. 2007; Li et al. 2015; Liang et al. 2018). Coaly from C-P coaly source rocks (Dai and Xia 1990; Liu et al. source rocks were deposited under the basin during the 2017; Gong et al. 2018). In 2017–2018, large quantities of 3 4 3 crude oil (30.2 m /d) and gas (8 × 10  m /d) were produced at depths of 4956.8–4984.9 m in sandstones from well Yg1, Edited by Jie Hao and at depths of 3836–3841 m in Ordovician buried-hill carbonate reservoirs from well Qg8 in Dagang Oilfield of * Jin-Jun Xu the China National Petroleum Corporation (CNPC), which xujj2015@upc.edu.cn revealed great potential for C-P oil and gas exploration Key Laboratory of Deep Oil and Gas, China University (Zhao et al. 2018). of Petroleum (East China), Qingdao 266580, Shandong, During the Paleozoic, there was a huge platform over China north China (Liu 1990; He et al. 1991), onto which coal- Pilot National Laboratory for Marine Science bearing sediments were deposited during the C-P period and Technology, China University of Petroleum (East China), (Lv et al. 2011; Chang et al. 2016; He et al. 2016; Fig. 1). Aoshanwei Wenhai Road 1, Qingdao 266237, Shandong, Previous studies have indicated three instances of tectonic China Vol:.(1234567890) 1 3 Yanshan Uplift Petroleum Science (2020) 17:1540–1555 1541 0 100 km Beijing Su20 Dalian Gg16102 Qg1601 Qg8 Ws1 Gbg1 Yg1 Jianan Source rock Country Study area boundary boundary Fault Well City Fig. 1 Distribution of residual coal source rocks in eastern China subsidence and two uplift events since the Triassic (T) generation) (Qi and Yang 2010; Li et al. 2012; Zhao et al. (Allen et al. 1998; Chang et al. 2018). The coaly source 2015). During the late K to Paleocene (E), secondary rocks were then buried and reached a mature stage during hydrocarbon generation was terminated at the end of rift the mid-T, generating small quantities of hydrocarbons; development (Zhou et al. 2012). Coaly source rocks that this was the first instance of hydrocarbon generation (pri- were not buried to sufficient depths in the J-K period did mary hydrocarbon generation) (Zhang et al. 2014; Chang not generate oil or gas during this period; however, suffi- et al. 2018). The end of primary hydrocarbon generation cient burial for oil and gas generation did occur during the was caused by uplift from the late T (Jin et al. 2009). Dur- tertiary rifting stage (late-secondary hydrocarbon genera- ing the Jurassic (J) to Cretaceous (K), a rift basin formed tion from E). Since the Eocene, a new rift basin developed in east China (Allen et al. 1998; Zhou et al. 2012). Some on residual J-K strata, with some of the coaly source rocks C-P strata were buried to great depths, generating hydro- buried deep beneath the sag areas of the rift generating oil carbons for the second time (early-secondary hydrocarbon 1 3 Bohai Bay basin Dagang Oilfield Shandong Fold Uplift Uplift Eastern Liaoning uplift Eastern Shandong uplift Southwestern Taihang 1542 Petroleum Science (2020) 17:1540–1555 and gas for a third time (tertiary hydrocarbon generation) environment and amount of erosion affect the thickness of (Zheng et al. 2007; Belaid et al. 2010; Chang et al. 2018). coal, carbonaceous mudstone, and dark mudstone layers. As the C-P strata are buried deep below the Bohai Bay Basin, few drilling wells have penetrated these layers; therefore, the distribution of C-P strata beneath the basin 3 Samples and methods has become a key area of geological research (Huang et al. 2010; Chang et al. 2018; Zhao et al. 2018). However, the 3.1 Samples and data locations of secondary or tertiary hydrocarbon generation of coaly source rocks in the basin, as well as the quantity of A total of 23 coal samples were collected from well oil and gas generation, should be determined prior to fur- Gg16102 and Longkou mine, three of which were selected ther exploration. The distribution of residual C-P strata in as thermal experimental samples (Fig. 1; Table 1). The mac- the Huanghua Depression, including that of coaly source eral composition, R , total organic carbon content (TOC), rocks, has previously been determined (Tian et al. 1996; He and Rock–eval pyrolysis were determined for Dagang oil et al. 2016; Zhao et al. 2018). Therefore, the purpose of this field source rock samples and Longkou mine samples col- study is to investigate the quantity of oil and gas generated lected from wells Gg16102, Kg4, Qg1601, Xu14, Ws1, and from coaly source rocks under the Huanghua Depression Ts1. of Bohai Bay Basin by analyzing the secondary, tertiary, and dynamic characteristics of hydrocarbon generation with 3.2 Methods the aim of predicting the hydrocarbon potentials of different exploration areas. 3.2.1 Thermal history recovery Many previous investigations speculated that the geother- 2 Geological setting mal gradient reached 3.5 °C/100 m from late P to early T and then increased to approximately 5.0 °C/100 m in the Dagang Oilfield is located at 37°30′–39°50′N and Mesozoic due to strong magmatic activity before decreasing 116°10′–119°30′E in the Huanghua Depression of the Bohai again to 3.5 °C/100 m during the Cenozoic (Li et al. 2007a, Bay Basin (Fig. 1), which is a typical Mesozoic and Ceno- b; Zhang et al. 2014). Based on the burial history recovery zoic extensional basin in the center of the eastern part of the of coaly source rocks and the paleogeothermal gradient, the North China Craton Block (Su et al. 2014; Li et al. 2015). hydrocarbon generation history of coaly source rocks in typi- Dagang Oilfield is oriented in a north-northeast direction cal wells was recovered using the Easy%R method (Schenk 4 2 covering an area of 1.7 × 10  km . Many studies have ana- et al. 2017). The initial maturities of primary, secondary, and lyzed the distribution of residual C-P coaly source rocks in tertiary hydrocarbon generation processes were also identi- eastern China, which are found throughout almost the entire fied according to the thermal evolution history curve. Dagang Oilfield except for Qikou sag, where they have been eroded (Huang et al. 2010; Chang et al. 2018; Zhao et al. 3.2.2 Thermal simulation experiments and kinetic 2018; Fig. 1). The residual C-P source rocks are also found parameter calculation in Dagang Oile fi ld. Thus, widespread C-P coaly source rocks in the study area provide abundant materials for hydrocarbon The thermal simulator can separately resist high tem- generation. peratures and high pressures up to 800 °C and 120 MPa, Three types of coaly source rocks found in the study area, respectively (Fig. 3). Samples are closed in the autoclave including coal, carbonaceous mudstone, and dark shale, and accompanied by nitrogen or distilled water (to represent were deposited in a paralic environment during the C-P 25% of the sample mass) and heated by a heater strip with period. The C-P succession contains the Benxi Formation a set heating rate. Based on the results of previous experi- (C b), Taiyuan Formation (C t), Shanxi Formation (P s), ments, the relationship between R and temperature was fit- 2 2 1 o and Shihezi Formation (P sh), which is comprised of thick ted (Fig. 4). R values serve as a “bridge” connecting the 1 o (418.0–1768.5 m), unconformable, siliciclastic-dominated, actual degree of thermal evolution and the thermal simula- shallow-marine and paralic deposits overlying Middle Ordo- tion degree. With a heating rate of 60 °C/h, all coal samples vician carbonates (Fig. 2; Kim et al. 2001). C-P coaly source were heated at 50 °C intervals to simulate the actual geologic rocks in the Shanxi Formation (67.0–195.5 m) were depos- conditions of source rocks within different areas during their ited in a continental sedimentary environment, whereas thermal history. source rocks of the Taiyuan Formation (87.5–229.0  m) Among the coaly source rocks, the coal exhibited greater were formed in a paralic environment (Li et al. 2001; Lv and hydrocarbon generation potential with a mean of greater than Chen 2014; Zhao et al. 2017; Fig. 2). Both the depositional 11 mg/g of S + S dominating the generation of coal-related 1 2 1 3 Petroleum Science (2020) 17:1540–1555 1543 Lithostratigraphy Chronostratigraphy Lithology Thickness, m Formation Symbol Quaternary Pingyuan Qp 200-490 Pliocene Minghuazhen Nm 450-1725 Miocene Guantao Ng 170-590 Dongying Ed 439-1500 Oligocene Shahejie Es 1932-6378 Eocene Kongdian Ek 22-1279 Paleocene Wuji K w 150-485 Fengtai K f 205-214 Cretaceous Lugouqiao K l 484-826 Xinzhuang J x 0-429 Jurassic Yaopo J y 260-437 1-2 Shiqianfeng P sh 62-446 Shangshihezi P s 119-472.5 Permian Xiashihezi P x 76-342.5 Shanxi P s 67-195.5 Taiyuan C t 87.5-229 Carboniferous Benxi C b 6.5-83 Fengfeng O f 30.5-254 Shangmajiagou O s 43-308 Ordovician Xiamajiagou O x 88-247 Liangjiashan O l 49.5-248 O y Yeli 25-125 Shale Sandstone Limestone Unconformity Siltstone Conglomerate Argillaceous Crystalline Dolomite Breccia Coal Source rock sandstone rock Fig. 2 Stratigraphic profile and distribution of main source rocks in Dagang Oilfield (modified from Zhao et al. 2018) Table 1 Organic geochemical characteristics and maceral composition of different coals Samples R , % T , °C S , mg/g TOC, % Hydrogen index, Maceral content, % o max 2 mg/g TOC Vitrinite Exinite Inertinite Torbanite Longkou 0.41 407 250.90 57.54 435.9 60.6 39.4 0.0 Humic coal Longkou 0.41 407 92.16 61.33 150.3 93.8 6.0 0.2 C-P Coal Well Gg16102 0.67 432 124.63 65.56 190.1 87.1 11.5 1.2 1 3 Lower Upper paleozoic Mesozoic Cenozoic paleozoic Paleogene Neogene Cooled autoclave 1544 Petroleum Science (2020) 17:1540–1555 (a) Gas Airmanometer cylinder 00 00 00 00 Vent valve 00 00 00 00 Vacuum pump Vacuum meter (b) Pressure sensor (c) Pressure controller Air extraction metering device Computer Vacuum data pump acquisition Autoclave system Vacuum meter Temperature controller Samples Temperature sensor Fig. 3 Schematic showing the thermal simulation of hydrocarbon generation. a The thermal simulator consists of eight autoclaves connected to a gas cylinder and vacuum pump; b each autoclave is monitored by temperature and pressure controllers. The liquid hydrocarbon production in the autoclave was collected at each temperature point after cooling and opening; c the gas hydrocarbon production was collected using the saturated saltwater displacement method crude and condensate; therefore, coal was selected as the coal in the Bohai Bay Basin (Fig. 1). During the thermal experimental sample for thermal simulation (Ahmed et al. simulation of primary hydrocarbon generation, the tempera- 2009; Zhao et al. 2018). According to the thermal evolution ture was increased from 300 to 650 °C with 5–7 g of sample history, three instances of hydrocarbon generation occurred, collected at every temperature point (total of eight points). with regional subsidence developing during the early T and The coal sample for thermal simulation of secondary hydro- all coaly source rocks reaching maturity at approximately carbon generation was collected from Gg16102 and exhib- 0.65% R (Jin et al. 2009), which is represented by the coaly ited lower initial maturity of secondary hydrocarbon gen- source rocks in the uplift area or outside of the Bohai Bay eration (0.67% R ). In the thermal simulation of secondary Basin (e.g., southwestern Shandong). Therefore, no imma- hydrocarbon generation, the sample was also heated from ture samples of C-P coaly source rocks were available in 350 to 650 °C. The simulation was ended at a temperature of the present-day sedimentary strata. To prepare an immature 375 °C, when the sample reached a maturity value of 0.76% sample for the thermal simulation of primary hydrocarbon R . The autoclave was cooled to room temperature and then generation, an experimental sample was prepared from E heated again to perform thermal simulation of secondary coal collected from the Longkou mine of Shandong Province hydrocarbon generation with higher initial maturity (0.76% according to the proportion of vitrinite and exinite of C-P R ) from 400 to 650 °C. An experimental sample with a 1 3 Petroleum Science (2020) 17:1540–1555 1545 5.0 where D is the heating rate (K/min); T is the absolute tem- perature (K); R is the gas constant (8.31447 kJ/(mol K)); 4.5 and XG is the value of XG at the onset of coal thermal i0 i 0.0051x 4.0 y = 0.128e degradation. In this study, the calculated activation energy R = 0.9918 was the average of all hydrocarbon generation. 3.5 3.0 3.2.3 Gas chromatography analysis 2.5 Subsequently, the samples were extracted by a Soxhlet 2.0 apparatus with dichloromethane for 72 h. The extracts were then fractionated by open silica gel column chromatography 1.5 using n-hexane. The resulting saturated hydrocarbons and all 1.0 products were analyzed by gas chromatography (GC) with an Agilent 5890 N GC analyzer for quantitative analysis. 0.5 200300 400 500 600 700 800 4 Results and discussion Temperature, °C 4.1 Multi‑part thermal history of coaly source rocks Fig. 4 Relationship between simulation temperature points and matu- rity 4.1.1 Characteristics of the three hydrocarbon generation events maturity of 0.9% R was prepared at a temperature of 400 °C According to their timing and order, the hydrocarbon gen- from a coal sample of well Gg16102 and then cooled. All eration events were divided into three types: primary hydro- hydrocarbon generation processes were heated to 650 °C carbon generation, secondary hydrocarbon generation, and with 5–7 g of sample collected at every temperature point. tertiary hydrocarbon generation. Occurring in the early P The materials produced at each 50 °C interval during the to middle T, the primary hydrocarbon generation area was thermal simulation were then collected. The generated liquid characterized by shallow burial, with maturity of less than hydrocarbon of each temperature point was extracted from 0.80% R (Zhang et al. 2014), and classified as the initial oil the heated residual using dichloromethane. The weight of oil generation stage (Fig. 5a). Part of the primary hydrocarbon production was weighed after the volatilization of the extrac- generation area remained buried at a shallow depth, with tion. Gas hydrocarbons were collected using the method of some locations uplifted and eroded from the late T to the saturated salt solution displacement (Dai et al. 2013). present. Owing to deep burial during either the Mesozoic or The kinetic parameters of coal cracking were calculated Cenozoic, secondary hydrocarbon generation occurred in the according to the method of Wang et al. (2015) based on Xuhei area during the late J to early K with structural sub- parallel first-order reaction models, which are kinetic models sidence (Fig. 5d). The coaly source rocks of the Qibei slope with multiple frequency factors (MFF models) and a range and Qikou sag were deeply buried and underwent thermal of activation energies. Hydrocarbon generation is assumed evolution, generating petroleum for a second time from the to involve a series of parallel first-order reactions, including Neogene (Fig. 5c, e). The tertiary hydrocarbon generation the activation energy of reaction i (EG ), the pre-exponential area developed in the early to middle T, the late J to early factor of reaction i (AG ), and the generation potential of K, and during the E period. As a result of its burial history, the reaction or reaction fraction i (XG ) (i = 1, 2, …, NKH). tertiary hydrocarbon generation occurred during a wet or dry The formula, derived from the first-order reaction rate equa- gas phase (Fig. 5f; Zhao et al. 2018). tion and the Arrhenius formula, calculates the total mass of hydrocarbon generation of the NG parallel reaction: NKH NG ⎛ ⎛ ⎛ ⎞⎞⎞ � � � � � � ⎜ ⎜ ⎜ ⎟⎟⎟ (1) XKH = XG = XG 1 − exp − AG ∕D ⋅ exp −EG ∕R ⋅ T dT i i0 i i ⎜ ⎜ ⎜ ⎟⎟⎟ i=1 i=1 ⎝ ⎝ ⎝ ⎠⎠⎠ 1 3 R , % o N-Q N-Q N-Q N-Q 1546 Petroleum Science (2020) 17:1540–1555 Primary generation Secondary generation Tertiary generation Evolution Early to middle Triassic Late Jurassic to CretaceousNeogene Neogene stages CP TJ KE CP TJ KE CP TJ KE CP TJ KE Unit: km (a) R = 0.5% Beidagang uplift (well Gg16102) Effectiveness (b) (c) R = 0.5% Ro = 0.5% Ro = 0.5% Ro = 0.5% o Ro = 0.7% Ro = 0.7% R = 0.7% Ro = 0.7% o Kongdian uplift Qibei slope (well Kg4) (well Qg1601) (d) (f) R = 0.5% o R = 0.5% R = 0.7% Ro = 0.7% R = 1.3% Ro = 1.3% Xuhei uplift Wumaying buried hill (well Xu4) (well Ws1) (e) Ro = 0.5% R = 0.7% 0.7 2.0 o 0 0.51.3 2.5 R = 1.3% R , % R = 2.0% Qikou sag Unit: Ma 300 200 100 300 200 100 300200 100 300200 100 Fig. 5 Stages of hydrocarbon generation within the Huanghua Depression. a and b primary hydrocarbon generation during the early to mid-T with maturity of 0.67% R and 0.76% R , respectively; c Neogene late-secondary hydrocarbon generation with low maturity of 1.1% R ; d early- o o o secondary hydrocarbon generation during the late J to early K; e Neogene late-secondary hydrocarbon generation with maturity above 2.0% R ; f Neogene tertiary hydrocarbon generation reached a maturity of 1.6% R 4.1.2 Geological conditions of multiple hydrocarbon different areas. The models provide accurate experimen- generation events tal reference conditions for setting parameters based on the relationship between maturity and heating tempera- According to our reconstruction of the hydrocarbon gen- ture (Table  2; Fig.  4). Based on the influences of tem- eration history, geological models were established for perature and depth, the initial and ceasing R values of 1 3 Dry gas stage Wet gas stage Mature stage Low mature stage R > 2.5% R = 1.3-2.5% R = 0.7-1.3% R = 0.5-0.7% o o o o 340 340 310 310 290 290 280 280 260 260 240 240 230 230 210 210 200 200 190 190 180 180 160 160 Petroleum Science (2020) 17:1540–1555 1547 Table 2 Geological model of multiple hydrocarbon generation events in Huanghua Depression Types Hydrocarbon Presentative wells Generation processes evolution (R /%) Current burial depth generation C-T J-K E to now event Initial point Final point Initial point Final point Initial point Final point A Primary Gg16102 0.5 0.67 – – – – < 3500 B Secondary Ts1 0.5 0.67 – – 0.67 1.1 3500–5500 B Qg1601 0.5 0.76 – – 0.76 1.1 3500–5500 C Tertiary Ws1 0.5 0.67 0.67 0.9 0.9 1.6 >5500 different hydrocarbon generation events were identified 4.2 Variability of hydrocarbon generation processes from thermal history diagrams, in which temperature is and kinetic parameters the dominant factor. The R values of primary hydrocarbon generation ended at 0.67% for wells Gg16102 and Ts1 4.2.1 Peak decrease and delay in multi‑part oil generation and 0.76% for well Qg1601. R values of early-secondary hydrocarbon generation and late-secondary hydrocarbon The complete primary hydrocarbon generation event rep- generation reached 0.9% and 1.1%, respectively. Tertiary resenting immature to mature phases of coaly source rock hydrocarbon generation of well Ws1 finally attained a R (R < 0.5%) is represented by a normal curve in Fig. 6. Liq- value of 1.6% (Table 2). uid hydrocarbon yield reached a minimum of 19.48 mg/g TOC at 300 °C (R = 0.53%), whereas the oil peak appeared 70 0.20 (a) (b) Primary generation Primary generation (R < 0.50%) (Ro < 0.50%) Secondary generation (Ro = 0.67%) Secondary generation (R = 0.76%) Secondary generation o 0.16 (Ro = 0.67%) Tertiary generation (Ro = 0.90%) Secondary generation (R = 0.76%) o 0.12 Tertiary generation (R = 0.90%) 0.08 0.04 0 0 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 Maturity R , % Activation energy generation, KJ/mol 400 0.28 (c) (d) Primary generation (R < 0.50%) Primary generation Secondary generation (Ro = 0.67%) 350 (R < 0.50%) 0.24 Secondary generation (R = 0.76%) Secondary generation 300 Tertiary generation (R = 0.90%) (R = 0.67%) o 0.20 Secondary generation (Ro = 0.76%) 0.16 Tertiary generation (Ro = 0.90%) 0.12 0.08 0.04 0 0 0.5 0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 Maturity R , % Activation energy generation, KJ/mol Fig. 6 Hydrocarbon generation yield and activation generation distributions of coal samples during the three hydrocarbon generation events. The primary hydrocarbon generation samples are a matched-immature sample from Longkou mine coal. The early-secondary hydrocarbon generation sample is coal from well Gg16102. The late-secondary hydrocarbon generation and tertiary hydrocarbon generation samples are coal from well Gg16102 prepared and heated at 375 °C and 400 °C, respectively 1 3 Gas yield, mg/g TOC Oil yield, mg/g TOC Frequency Frequency 1548 Petroleum Science (2020) 17:1540–1555 at 450 °C (R = 1.06%) with a yield of 60.12 mg/g TOC. 4.2.3 Increase in activation energy generation with time Subsequently, the oil yield began to decrease after 650 °C (R = 1.84%) (Fig. 6a). Compared to primary hydrocarbon The distribution of activation energy reflects the order of generation, the maximum yield of secondary hydrocarbon chemical compounds generated during hydrocarbon gen- generation with the early “oil window” phase (0.67% R ) eration events (Dieckmann 2005). Laboratory experiments appeared at approximately 450 °C (R = 1.06%). The highest have determined the kinetics describing the rate of thermal oil yield of 41.52 mg/g TOC was substantially lower than decomposition of kerogen to petroleum (Pepper and Corvi that of primary hydrocarbon generation (Fig. 6a). A higher 1995; Dieckmann 2005; He et al. 2018a, b). The activation initial maturity of 0.76% reduced the amount of oil produced energy varied considerably among the three hydrocarbon by secondary hydrocarbon generation; the maximum yield generation events with different initial maturities (Fig.  6b, of 39.01 mg/g TOC was less than that of primary and early- d). All events exhibited a broad distribution of activation secondary hydrocarbon generation events. The lowest quan- energies; however, with increasing initial maturity, the tity of oil compounds was obtained at 650 °C (R = 1.84%) activation energy distribution shifted toward higher values with a yield of only 2.38 mg/g TOC. However, with a higher from primary hydrocarbon generation to tertiary hydrocar- initial maturity point (R value of 0.90%), an oil yield of bon generation events, accompanied by an increase in the 10.67  mg/g TOC was obtained. The oil peak of tertiary frequency factor (A ). hydrocarbon generation occurred after 500 °C (R = 1.27%) Primary oil generation exhibited a broader distribution, with a lower yield of 14.40 mg/g TOC. At the end of heating centered around 180–220 kJ/mol with a maximum value of (650 °C, R = 1.84%), the remaining oil potential declined to 190 kJ/mol and an A value of 5.87–9.3 × 10 . Due to the o f nearly zero (Fig. 6a), with the curve of tertiary hydrocarbon low thermostability, heteroatomic compounds, small mol- generation shown as a straight line, representing a low oil ecule benzenes, and alkyl benzenes preferentially cracked yield. and generated light hydrocarbons during primary hydro- The characteristics of the different generation events sug- carbon generation (Li et al. 2007a, b; Yu et al. 2012). At gest that the production quantity and generation curves are an initial maturity of 0.67% R , secondary oil generation controlled by the initial maturity. With increasing maturity, exhibited an almost symmetrical distribution centered on the yield peaks gradually decreased and the curve trans- approximately 230 kJ/mol (A = 8.64 × 10 ). A higher acti- formed from a normal to low-amplitude shape (Fig.  6a). vation energy distribution reflects the formation of short Moreover, the decrease in oil potential was controlled by chains of aliphatic functional groups and oxygen contain- the difference in initial maturity between the two genera- ing groups with higher thermostability, such as isopar- tion events. affinic, naphthenic, and normal alkane hydrocarbons (Li et al. 2007a, b; Yu et al. 2012). For a similar initial matu- rity, a small increase in activation energy of late-secondary 4.2.2 Total decrease in gas generation hydrocarbon generation was observed from 260 to 280 kJ/ mol (A = 1.01–6.34 × 10 ) compared to previous oil genera- The differences in gas generation among the three hydro - tion (Fig. 6b). Compared to the generation event of 0.67% carbon generation events are clearly observed in Fig. 6c, R , more normal alkanes and short chains of aromatic and in which the generation curves increase continuously with methyl benzol would form, corresponding to a higher activa- temperature. With increasing maturity, the total gas produc- tion energy (Walker et al. 2007; Petersen et al. 2009; Chen tion decreased substantially at all temperature points. The et al. 2012; He et al. 2018a, b). The activation energy of maximum gas yield was 387.29 mL/g TOC, obtained dur- 300–330 kJ/mol (A = 4.86–5.94 × 10 ) for tertiary hydro- ing primary hydrocarbon generation at 650 °C (R = 1.84%). carbon generation was substantially higher than that for pri- Correspondingly, the minimum yield of 165.23 mL/g TOC mary and secondary oil generation events. With increasing was obtained at 650 °C during tertiary hydrocarbon genera- maturity, a higher activation energy indicates the cracking of tion. Due to the higher initial maturity (R = 0.9%) of tertiary long chains of aliphatic hydrocarbons and aromatic hydro- hydrocarbon generation, gas production was clearly lower carbons with long side chains. The kerogen may originate than that during primary and secondary hydrocarbon gen- from angiosperm and gymnosperm pollens containing cuti- eration events. Moreover, the gas yield exhibited a greater nite and sporinite that generate hydrocarbons at considerably decrease between early-secondary and late-secondary hydro- higher maturities of 0.85% R or higher (Petersen and Nytoft carbon generation at higher temperature (Fig. 6c). Again, 2006; Petersen et al. 2009; Yu et al. 2012; Chen et al. 2012; the initial maturity was the most important factor affecting He et al. 2018a, b). petroleum production; the higher the initial maturity, the The activation energy distributions of the multiple gas lower the gas production from primary hydrocarbon genera- generation events are similar to those of the multiple oil tion to tertiary hydrocarbon generation events. generation events. The shift from lower to higher activation 1 3 Petroleum Science (2020) 17:1540–1555 1549 energy for both oil and gas generation indicates that petro- yield was larger and occurred earlier than the gas yield leum generation increased from primary to secondary to (Fig. 6a, c). Conversely, due to the high initial maturity, tertiary hydrocarbon generation events. The differences tertiary gas generation exhibited the opposite trend, with in yield among the multiple petroleum generation events gas generation exhibiting lower activation energies of agreed closely with the variations of activation energy. The 310–320 kJ/mol (A = 3.78–4.53 × 10 ) than oil genera- primary hydrocarbon generation event with the lowest acti- tion (310–330 kJ/mol). This result suggests that gas com- vation energy more easily produced petroleum, as shown by pounds began to be the main product instead of oil com- the higher hydrocarbon yield and earlier peak. Oil and gas pounds, as confirmed by the rising curve of gas yield and yield notably decreased with increasing activation energy. falling slope of oil yield in Fig. 6a, c. Thus, the generation Thus, the reduction in petroleum yield depended on the of oil and gas were fully illustrated by the distribution of different activation energies of different generation events activation energy. (Fig. 6b). Even within the same generation event, oil and gas 4.3 Multiple hydrocarbon generation models generation was dominated by the activation energy. That for petroleum generation history is, during primary and secondary hydrocarbon genera- tion events, the main activation energies of gas genera- It was assumed that hydrocarbon generation from coaly tion were all approximately 10–20 kJ/mol greater than source rocks was predominantly affected by the burial his- those of oil generation, indicating that oil was more eas- tory and thermal evolution, without considering differences ily produced than gas compounds. Consequently, the oil in the abundance of organic matter and abnormal heating Ro = 0.5% 0.67% CP TJ KE N+Q 400 70 0 (a) (b) Oil Gas R = 0.5% o Beidagang uplift 200 3000 (well Gg16102) 0 0.7 2.0 0.51.3 R , % 0 0 6000 300250 200150 100500 300200 1000 Geological time, Ma Geological time, Ma Fig. 7 Model of primary hydrocarbon generation Ro = 0.5% 0.76% 0.76% 1.1% N+Q CP TJ KE 400 0 (a) (b) Oil 350 Gas Ro = 0.5% Ro = 0.5% 200 3000 R = 0.7% R = 0.7% 30 o 0 0.7 2.0 Qibei slope 5000 0.51.3 (well Qg1601) R , % 0 6000 300 250 200 150 100 50 0 300200 1000 Geological time, Ma Geological time, Ma Fig. 8 Model of secondary hydrocarbon generation 1 3 Gas yield, mL/g TOC Gas yield, mL/g TOC Oil yield, mg/g TOC Oil yield, mg/g TOC Depth, m Depth, m 1550 Petroleum Science (2020) 17:1540–1555 events (e.g., magmatic intrusion). By combining the ther- 15.77–24.83  mg/g TOC and 77.08–101.71  mL/g TOC, mal simulation experiment results and the thermal evolution respectively (Fig. 8a). history, three models of hydrocarbon generation were estab- lished to predict the oil and gas potential of coaly source 4.3.3 Third generation event rocks within different structural units. Under the influence of Indosinian, Yanshan, and Himalayan 4.3.1 First generation event tectonic movement, C-P coals were buried to great depths, resulting in a third generation event during the late T and The first generation event only generated petroleum from early to middle K and E. The onset and duration of the sec- the late P to early T. The quantity of generated oil and gas ondary and tertiary hydrocarbon generation events differed was dominated by the thermal evolution degree and oil was depending on their location, leading to variations in the the main product (Figs. 7b, 8b). Buried between 2000 m and petroleum yield. In the regional depression of Huabei craton, 2600 m, the source rocks underwent minimal maturation, primary hydrocarbon generation in the Permian was similar varying from 0.5% to 0.7% R in the Beidagang uplift area to that of the first generation event model. Early-secondary and reaching 0.7%–0.8% R in the Qibei slope area. Hydro- hydrocarbon generation occurred during the Mesozoic from carbon generation began at 250 Ma and ceased at 225 Ma 175 to 98 Ma (Fig. 9b) with oil yields ranging from 13.87 (Fig. 7). Taking well Gg16102 as an example, the source to 28.83 mg/g TOC (0.67%–0.90% R ; Fig. 9a). However, rock ended with R values of 0.67%, and the oil yield was the gas yield only increased from 30.59 to 77.09 mL/g TOC. 14.99 mg/g TOC. A minor amount of gas was generated, The last generation event lasted from 24.6 Ma to present approximately 31.28 mL/g TOC (Fig. 7a). and generated large amounts of condensate and natural gas (Fig. 9b). The tertiary hydrocarbon generation event was 4.3.2 Second generation event characterized by lower oil yields and higher gas yields. With increasing R values, the oil yield decreased substantially The second generation event includes early- and late-sec- from that of primary and secondary hydrocarbon genera- ondary hydrocarbon generation, characterized by the decom- tion, from 8.25 mg/g TOC to 16.42 mg/g TOC. In contrast, position of kerogen from the Paleogene to the present day, gas was generated more rapidly and at a higher level, with which may have accumulated in petroleum reservoirs. Due current yields of 165.12 mL/g TOC, which is favorable for to deeper burial, primary hydrocarbon generation of well the formation of petroleum pools (Fig. 9a). Qg1601 ended at a mature stage with R values of 0.76% (Fig. 8). Compared to the low initial maturity of the first gen- 4.4 Factors controlling multiple hydrocarbon eration event, the oil and gas yields increased to 19.48 mg/g generation events TOC and 95.93 mL/g TOC, respectively, in the second gen- eration event (Fig. 8a). Secondary hydrocarbon generation Multi-stage tectonic movements and variable geother- occurred during the Neogene from 98 Ma to present and mal gradients from the Paleozoic to the Cenozoic may be remained in a mature stage with R values of 0.76%–1.1% responsible for the multiple hydrocarbon generation events (Fig. 8b). Higher oil and gas yields were calculated, from of coaly source rocks. Regional subsidence occurred during R = 0.5% 0.67% 0.67%0.9% 0.9% 1.6% CP TJ KE N+Q 400 70 0 (a) (b) Oil Gas Ro = 0.5% 200 3000 Ro = 0.7% R = 1.3% 4000 o 0 0.7 2.0 0.51.3 Wumaying (well Ws1) R , % 0 0 6000 300 250 200 150 100 50 0 300200 1000 Geological time, Ma Geological time, Ma Fig. 9 Model of tertiary hydrocarbon generation 1 3 Gas yield, mL/g TOC Oil yield, mg/g TOC Depth, m Petroleum Science (2020) 17:1540–1555 1551 the late P to early T, and the geothermal gradient reached by the initial maturity of tertiary hydrocarbon generation, 3.5  °C/100  m. Whole source rocks experienced primary with low initial maturity resulting in adequate oil and gas hydrocarbon generation, with R values ranging from 0.5% generation. Observations of condensate oil and gas in well to 0.7% (Zhang et  al. 2014; Chang et  al. 2018). Subse- Yg1 from C-P coaly source rocks confirm the hypothesis quently, regionally integrated rapid uplift occurred within of reservoir formation from tertiary hydrocarbon generation the Huabei platform. Diverse tectonic activity began in the (Zhao et al. 2018). The low initial maturity of late-secondary late T, which controlled various trends of the hydrocarbon hydrocarbon generation suggests considerable condensate generation events. Furthermore, due to strong magmatic oil and gas potential from coaly source rocks. activity with local magmatic intrusions, the geothermal In order to accurately predict the petroleum resource gradient increased to approximately 5.0 °C/100 m in the potential of coaly source rocks, the hydrocarbon genera- Mesozoic (Zhang et al. 2014). Deeply buried areas exhib- tion parameters of oil and gas yields are required (Jin et al. ited highly mature early-secondary hydrocarbon generation, 2009; Zhang et al. 2009; Cheng et al. 2018), which were with a higher heating rate than that of primary hydrocarbon obtained in this study. The oil and gas quantities generated generation. However, the uplifted and eroded area ceased from different periods of the generation events are critical to generate petroleum. During the Cenozoic, the different functions for evaluating the petroleum resource potential of generation processes were enhanced in a complex manner, coaly source rocks, which is the aim of this study (Boreham with geothermal gradients decreasing again to 3.5 °C/100 m et al. 1999; McCarthy et al. 2011). This study indicates the (Li et al. 2007a, b). The current burial depth favors hydrocar- resource potential of coaly source rocks within late-second- bon generation and is also significant for forming petroleum ary and tertiary hydrocarbon generation events during the pools. In some areas, the coaly source rocks only exhibited a Neogene, and reveals the Bohai Bay Basin as an important primary hydrocarbon generation event as they have remained field for deep exploration. Based on the thermal history, the buried at shallow depths since the late J. Some source rocks Qibei slope, Qinan sag, Kongxi slope, and Wangguantun were once again buried to form petroleum during the late- sag are all favorable areas for late-secondary hydrocarbon secondary hydrocarbon generation event from the Paleogene generation (Fig. 10). The Wumaying buried hill and Xin- after uplift during the Mesozoic. The crucial tertiary hydro- gang area are favorable for tertiary hydrocarbon generation carbon generation event also occurred in the Paleogene after (Fig. 10). As a result of the multiple events of hydrocarbon primary and early-secondary hydrocarbon generation events generation, the mid-north area of Dagang oil field has crude during the Paleozoic and Mesozoic, respectively (Jin et al. oil and natural gas potential, whereas the mid-southern and 2009; Zhang et al. 2009; Cheng et al. 2018). Thus, tectonic northern areas of Dagang oil field are prone to generate more evolution and geothermal gradients were key factors control- condensate and abundant natural gas (Fig. 10). Therefore, ling the multiple hydrocarbon generation events. these areas, which are currently buried at depth, have sig- Compared to continuous hydrocarbon generation, the nificant potential resources of petroleum and natural gas. total hydrocarbon generated during the three events exhib- Additionally, this study provides insights into hydro- ited a decreasing trend. Due to hydrocarbon generation ceas- carbon generation of coaly source rocks deposited in other ing multiple times, the hydrocarbon generation peak and depressions of the Bohai Bay Basin. Compared to the Huan- yield was delayed and reduced with increasing activation ghua Depression, the coaly source rocks deposited in the energy. Although petroleum yields varied from primary to Jiyang and Linqing depressions experienced the same three tertiary hydrocarbon generation events according to the ini- hydrocarbon generation events; thus, they are also prone tial and final maturity values, the initial maturity value had to generating abundant coaly-derived gas (Jin et al. 2009; a dominant effect on the petroleum generation potential of Zhang et al. 2009; Huang et al. 2010; Lv et al. 2011). The coaly source rocks. With increasing initial maturity through- coaly source rocks in the Jizhong depression and Dongpu out the different generation events, the oil generation peak sag experienced similar hydrocarbon generation events to was delayed and reduced (Fig. 6). A greater difference in the the Qikou sag in the Huanghua Depression (Zheng et al. initial maturities of different generation events would result 2007; Zhao et al. 2018). Moreover, deeper burial depths are in higher hysteresis. typically associated with significant coaly-derived gas poten- tial (Li et al. 2007a, b; Lv et al. 2011; Gong et al. 2018). 4.5 Eec ff tive hydrocarbon generation forming Therefore, the models established in this study can provide petroleum pools useful information for the exploration of C-P coaly source rocks in the Bohai Bay Basin. Analysis of the different generation models suggests that late-secondary hydrocarbon generation and tertiary hydro- carbon generation were the most useful for forming oil–gas pools. The scale of the paleo-reservoir was likely controlled 1 3 1552 Petroleum Science (2020) 17:1540–1555 010 km Beitang-xingang subsag Kg6 J3 Gg1601 Gg16102 Qikou sag Qibei slope Qg8 Chh24 Qg1 Chh1 Kg4 Zhg1 Kg6 K8 Chg2 Wg1 Xu1 Yg1 Xu14 Ws1 Primary Erosion generation boundary Xu13 Cl1 Cc1 Early-secondary Fault generation Dg1 XU14 Late-secondary Well/ generation name Tertiary Study generation area Fig. 10 Distribution of favorable areas under different hydrocarbon generation models 1 3 Chenghai slope Beidagang buried hill Kongdian subuplift Fault Fault Jianhai subuplift Southern Cangxian uplift Wumaying buried hill Yanshan subsag Xuhei subuplift Heidong Cangdong sag Fault Xuxi Cang Xi Fault Cheng subuplift Qinan subsag Dong Kongxi slope Zengfutai Petroleum Science (2020) 17:1540–1555 1553 the article’s Creative Commons licence and your intended use is not 5 Conclusions permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. 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