1. Introduction
The Ordos Basin, located in central–western China, is the second largest basin in China. The basin covers an area of 37 km2 and is one of China's typical large-sized inland depression basins (Fig. 1). Within this basin, the Mesozoic strata contain abundant petroleum resources. The Yanchang Formation of the Triassic age and the Yan'an Formation of the Jurassic age are the main oil-bearing series, and their reservoirs are relatively compact. This basin is a typical area for developing low- / ultra-low-permeability reservoirs. A set of black and highly abundant organic shale deposited in the Late Triassic Chang 7 period provides hydrocarbon source conditions favourable for petroleum accumulation. Recently, large-scale highly profitable reservoirs of tight oil and gas, shale oil and gas, and other non-conventional oil and gas resources have been discovered in the sandstone and shale reservoirs that are immediately adjacent to Chang 7 source rocks, and their source and supply conditions have attracted considerable attention from the geological research community (Gao et al. Reference Gao, Liu and Wang2013; Guo et al. Reference Guo, Chen and Song2013; Fu, Yu, & Xu, Reference Fu, Yu and Xu2015; Yang, Niu & Xu, Reference Yang, Niu and Xu2016).
Figure 1. (a) Geological map of the Ordos Basin. (b) Chang 7 shale from the field outcrop section in the Yao county. and (c, d) drill core of Chang 7 shale in well Z153.
Previous studies on Chang 7 hydrocarbon source rocks in the Triassic comprehensively investigated oil and gas exploration and production from the Mesozoic in the Ordos Basin (Yang & Zhang Reference Yang and Zhang2005, 2006; Zhang, Yang & Yang, Reference Zhang, Yang and Yang2008a,b; Zhang, Yang & Xie, Reference Zhang, Yang and Xie2010, 2011; Zhao, Liu & Zhang, Reference Zhao, Liu and Zhang2011; Zhao, Luo & Sun, Reference Zhao, Luo and Sun2012; Zhao, Yao & Sun, Reference Zhao, Yao and Sun2014). To date, however, the geological and geochemical characteristics and formation of Chang 7 shale as well as systematic studies and associated discussion for assessing the resource potential have been researched less. Furthermore, there has been insufficient research that has systematically and comprehensively evaluated the spatial distribution, capability of hydrocarbon generation and expulsion, and resource potential. In this study, field outcrop profile observations and core sampling were performed to select samples for systematic geochemical tests, including analysis of the hydrocarbon potential, organic microscopic components and gas chromatography – mass spectrometry (GC-MS) of mature hydrocarbon and aromatic hydrocarbon as well as trace elements. The hydrocarbon potential and the formation environment of hydrocarbon in Chang 7 shale is clarified, revealing its high generation capacity and expulsion efficiency. This paper details the spatial distribution characteristics of shale and provides a concrete research basis for the evaluation of Mesozoic petroleum resources in the Ordos Basin. Additionally, this study provides detailed and accurate research data for studying the formation and accumulation of non-conventional oil and gas resources, such as shale oil, tight oil and shale gas, in the Chang 7 reservoir in the Ordos Basin.
2. Samplings and analyses
In total, 87 shale samples were collected from the drill cores of the Chang 7 section of the Triassic Yanchang Formation in the Ordos Basin. Of these, 41 samples were selected for organic abundance tests, such as analysis of total organic carbon (TOC) and S1+S2, while 15 samples were used for trace elements analysis, 15 samples were used for GC-MS analysis of the saturated and aromatic hydrocarbons, and 6 samples were used to study the organic microscopic components.
TOC was measured using a LECO CS230-4009 carbon–sulphur analyser at the State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing. Trace elemental analyses were conducted using an inductively coupled plasma mass spectrometer (ICP-MS), an AA-6800 atomic absorption spectrometer and an Axios X-ray spectrometer at No. 203 Research Institute of the Sino Shanxi Nuclear Industry Group. Organic geochemical analysis was undertaken at the State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, and the GC-MS analyses of the saturated and aromatic hydrocarbons were performed using an Agilent 7890-5975C (test execution standard, GB/T 18606-2001) gas chromatograph mass spectrometer. The carrier gas for chromatography spectrometry analysis was 99.999% helium, and the injection port and transmission line were maintained at 300°C. Note that the chromatographic column was an HP-5MS elastic silica capillary column (60 m × 0.25 mm × 0.25 m). Moreover, the GC oven column temperature was initially maintained at 50°C for 1 min, increased to 120°C at 20°C min−1, then increased to 250°C at 4°C min−1 and finally increased to 310°C at 3°C min−1 before being isothermally maintained for 30 min. Chromatography spectrometry analysis was conducted using an HP-5MS elastic capillary column (60 m × 0.25 mm × 0.25 mm), and helium served as the carrier gas at a flow rate of 1 mL min−1. MS analysis was performed at an ionization energy of 70 eV on an electron impact resource. The filament current was maintained at 100 A, and the multiplier voltage was maintained at 1200 V. Hydrocarbon generation was simulated in the High-Temperature, High-Pressure Stimulation Room at the Key Laboratory for Oil and Gas Resources, Chinese Academy of Sciences. A WYMN-3-type high-temperature, high-pressure stimulator was used to perform a thermal simulation experiment of hydrocarbon generation.
3. Results and discussion
3.a. Geological characteristics of black shale
The tectonic uplift related to the Indosinian movement in the Late Triassic period occurred across the entire Ordos Basin, a large intracontinental basin, causing it to enter a formation and development stage as a differential sedimentary basin. The sedimentary environment during the Triassic period changed from marine facies and transitional facies to terrestrial facies, resulting in the formation of a complete and typical continental clastic sedimentary system. Thus, the substantial petroleum resources of the Mesozoic in the Ordos Basin are closely related to the large-scale development of lacustrine organic-rich hydrocarbon source rocks in the Chang 7 member in the Upper Triassic.
The regional tectonic activities that occurred in the Late Triassic were accompanied by clear enhancement of roughly northwest-trending and northeast-trending basement faults in the basin interior, resulting in drastic sinking of the entire basement that caused the basin to evolve into a lacustrine environment. With the deepening of the water (the maximum depth reached 60 m), the lake expanded rapidly and a semi-deep / deep lake environment was formed, which provided important conditions for the blooming and breeding of planktonic algae, benthic algae and aquatic animals.
The Chang 7 shale within the Seventh Member in the Yanchang Formation is commonly black, ash black and crineous. Figure 1b shows shale from an outcrop of the Chang 7 profile in the Yao county. The specimen contains pyrite (up to 40 m thick) and is black and crineous. It has developed horizontal bedding and identified lamellation. Figure 1c, d shows an image of a drilling core in which the Chang 7 shale is black and ash black with developed bedding, and based on the observations of the drilling core, the maximum drill core thickness can reach 102 m.
Logging curves show that Chang 7 black shale is characterized by electrical characteristics, including high resistivity, high concentration of natural gamma rays and high acoustic time difference (Fig. 1). According to the analysis of drill cores in well X17, Chang 7 shale is abundant in organic matter (OM) and has high hydrocarbon potential, high hydrogen index and high maturity (Fig. 2).
Figure 2. Single-well geochemical profile (X17 l) of Chang 7 black shale in the Ordos Basin.
The dispersed OM in sedimentary rocks commonly originates from terrestrial OM that is derived from higher plants and from aquatic OM that is derived from lower organisms, fragments of animal shells and strongly decomposed and destroyed amorphous OM (Cheng, Wang & Zhong, Reference Cheng, Wang and Zhong1995).
Herein, Chang 7 shale was systematically studied using the organic petrologic method. Exinite, bituminite, vitrinite and inertinite were detected in Chang 7 black shale, and hydrocarbon-generating components (such as alginite, bituminite and liptodetrinite) were found to be enriched. The average contents of these components were as follows: exinite (31.18%), bituminite (48.35%), vitrinite (18.24%) and inertinite (2.23%). Exinite was found to contain complicated components that were distributed in dispersed states, including sporophore, resinite, liptodetrinite, and alginate (derived from algae organisms). Sporophore, appearing in yellow–golden colour with yellow fluorescence, was the dominant component, and alginate was found to appear in yellow with green fluorescence under a microscope (Figs 3a, b). Bituminite was amorphous, scattered and concentrated and had an algal texture. The hydrocarbon generation capacity of bituminite was higher than that of exinite. Moreover, bituminite mainly occurred as aluminous yellow bituminite groundmass in which plenty of scattered bituminite groundmass could be seen under the microscope (Fig. 3c). In addition, cutinite was thin-walled and strip-shaped and occurred with yellow fluorescence (Zhao, Luo & Sun, Reference Zhao, Luo and Sun2012; Fig. 3d).
Figure 3. Organic micro-components of the Triassic Chang 7 shale at the southwest edge of the Ordos Basin. (a) Clumpy alginite, well G252, 2558 m, and fluorescence. (b) Alginite, well G252, 2558 m, and fluorescence. (c) Scattered bituminite, well L32, 2793 m, and fluorescence. (d) Cutinite, well Z74, 2298.6 m, and fluorescence.
3.b. Characteristics of trace elements in black shale
Using the geological statistics, the enrichment coefficient of trace elements was determined by the ratio of the average contents of trace elements in samples to the average content of shale and clay rocks (Liu, Meng & Liu, Reference Liu, Meng and Liu2009). Trace elements are thought to be relatively enriched when the enrichment coefficient value is greater than 1, and relatively depleted when this value is less than 1 (Liu, Wu & Yu, Reference Liu, Wu and Yu2005). The trace elements in the Chang 7 black shale collected from the Ordos Basin are Ti, V, Mn, Cu, Zn, Ga, Sr, Ba, Pb, La, Ce, Pr, Nd, Gd, Rb, Cs, Th, U, Li, Be, Cr, Co, Ni, Cd, Sn, W, B and S. The elements with enrichment coefficients greater than 1 are Cu, Zn, Ba, Pb, C and Co, whereas the remaining elements have enrichment coefficients less than 1 and were thus considered to have depleted to different degrees (Table 1).
Table 1. The trace element data of Chang 7 black shale of Ordos Basin
3.c. Evaluation of black shale hydrocarbon potential
The TOC content in sediments comprehensively reflects the depositional environmental factors, including the abundance of original OM, water depth, deposition rate, and physical and chemical conditions of the media (Miao, Zhou & Deng, Reference Miao, Zhou and Deng2004). In this study, 200 samples of Chang 7 black shale were subjected to geochemical tests and analyses. The results revealed that the TOC content of Chang 7 black shale ranged between 3.43% and 24.36%, with an average of 12.43% (Fig. 4). Moreover, S1+S2 ranged between 14.32 mg g−1 and 98.65 mg g−1, with an average of 31.65 mg g−1. Additionally, the content of bitumen ‘A’ ranged between 0.22% and 1.67%, with an average of 0.70%. The parameters of OM abundance indicate that Chang 7 black shale is rich in OM. The hydrogen index ranged between 234.07 mg g−1 and 607.84 mg g−1, with an average of 369.52 mg g−1, indicating that Chang 7 shale comprised a good OM type. Ro ranged between 0.65% and 1.1%, with an average of 0.85%, indicating that the shale was in a mature stage (Fig. 5). The extraction group composition from this shale exhibited signatures of highly saturated hydrocarbons, low aromatic hydrocarbons, nonhydrocarbons and asphaltene. Among these, the content of saturated hydrocarbons was high (40.88–65.54%, with an average of above 57%), the ratio of saturated hydrocarbons to aromatic hydrocarbons was 2.03–4.32 and the average saturation ratio was 3.01. In summary, the results revealed that Chang 7 shale was characterized by high organic abundance, good organic type and high maturity, which are promising indicators of high hydrocarbon potential from this typically high-quality source rock.
Figure 4. Total organic carbon (TOC) histogram of Chang 7 shale in the Ordos Basin.
Figure 5. Ro histogram of Chang 7 shale in the Ordos Basin.
3.d. Molecular geochemical characteristics of black shale
3.d.1. Molecular geochemical characteristics of saturated hydrocarbons in the extract of Chang 7 black shale
The distribution characteristics of n-alkanes reflect the thermal maturity and OM source to some extent. N-alkanes with low carbon numbers were mainly derived from the fatty acids of plankton and algae, while n-alkanes with high carbon numbers were derived from the cuticular wax layer of terrestrial plants. The Chang 7 black shale samples revealed n-alkanes ranging from n-C15 to n-C21, with a unimodal n-alkane distribution maximized at n-C15 or n-C18 and a low carbon-numbered preference, thus indicating algal sources and sapropelic type of OM (Fig. 6a). The Chang 7 shale samples also revealed an obvious carbon preference index (CPI) of 1.2, while the odd–even predominance index (OEP) was 1.32. The ratios of nC21−/nC22+ and C21+C22/C27+C28 were 1.62 and 3.21, respectively, indicating that the source input was dominated by low aquatic organisms, such as algae and microorganisms.
Figure 6. Biomarker compound distribution characteristics of Chang 7 black shale in the Ordos Basin.
The relative abundances of pristine (Pr) and phytane (Ph) are the most important parameters for indicating acyclic isoprenoid alkanes, and the Pr/Ph ratio is commonly used to indicate the redox degree of the OM formation environment. Various values of this ratio imply the following: a value less than 1 represents a strong reducing environment, a value between 0.5 and 1.0 represents a reducing environment, a value between 1 and 2 indicates a weak oxidizing–reducing environment, and a value exceeding 2 indicates an oxidizing environment. The Pr/Ph ratios of the Chang 7 shale ranged between 0.53 and 1.98, with an average of 0.96, indicating a reducing environment in the Chang 7 depositional stage.
In the case of Chang 7 shale samples, the average ratio of tricyclic terpanes to hopanes was 0.29 and that of C19+C20 tricyclic terpanes/hopanes was 0.04. As shown in Figure 6b, the relative abundance of Ts was significantly greater than that of Tm and the average value of Ts/(Ts+Tm) was 0.72. In addition, C29 Ts and C30-diahopane were found to be highly abundant in the shale samples. Gammacerane is usually used as an indicator of a strong reducing and hypersaline environment, and it is closely related to the delamination of a water body. A relatively lower-than-average gammacerane index (0.13) and a gammacerane index range of 0.03–0.18 suggested that the Chang 7 shale was formed in a lake environment containing fresh–brackish water. In addition, the ratio of C31(22S)/(22S+22R) was 0.56, which indicated that a thermal equilibrium point was attained.
High content of steranes, such as pregnanes and homopregnanes, was detected in the Chang 7 shale samples. Among regular steranes, C27, C28 and C29 had an L-shaped distribution, as shown in Figure 6c. C27 was larger than C29 in ααα20R steranes, indicating that the Chang 7 hydrocarbon source was dominated by a rich content of lacustrine lower aquatic organisms. The ratios of 20S/(20S+20R) in αααC29 steranes and ββ/(αα+ββ) in C29 were 0.51 and 0.55, respectively, indicating the entry of Chang 7 shale into a mature stage.
3.d.2. Molecular geochemical characteristics of aromatic hydrocarbons in the extract of Chang 7 shale
Of the polycyclic aromatic hydrocarbons, phenanthrene was found to exist in abundance in the soluble extracts of Chang 7 shale samples obtained from within the Yanchang Formation in the Ordos Basin. Figure 7 shows that the chrysene content of these samples is also relatively high, but naphthalene, anthracene, anthrene, benzanthracene and cadalene are present in minimal amounts. The relative abundance of dibenzothiophene (sulphur fluorene) was low (0.21%) in the trifluorene series compounds of soluble extracts. The relative abundance of dibenzofuran (oxygen fluorene) was high (0.36%). The fluorene content was high, with a relative abundance of 0.43%. The dibenzothiophene:dibenzofuran ratio was 0.6, indicating that the Chang 7 depositional environment was dominated by a reducing condition.
Figure 7. Distribution characteristics of Chang 7 mudstone aromatic compound in the Ordos Basin. (A: Phenanthrene, B: fluoranthene, C: pyrene, D: chrysene, E: benzofluoroanthene, F: benzopyrene, G: biphenyl, H: fluorene, K: dibenzofuran, L: dibenzothiophene and I: 1,2,5-trimethyl-naphthalene).
The aromatized steroids include monoaromatic, diaromatic and triaromatic steroids as well as de-A-monoaromatic steranes. Aromatic steranes were formed via the aromatization of sterols, phytosterones and steroidal biogenic plants, such as planktonic algae, in the early stage of diagenesis. It was determined that the relative abundance of triaromatic steroids in the aromatic fractions of terrestrial crude oil were positively correlated to the salinity of sedimentation water (Meng, Zhang & Cui, Reference Meng, Zhang and Cui1998). The ratio of C26-triaromatic steroids 20S to C28-triaromatic steroids 20S is an effective indicator of the sedimentary source and depositional environment. C28-triaromatic steroids dominate OM formed in freshwater environments, whereas C26-triaromatic–aromatic steroids dominate the OM formed in saltwater and brackish water environments (Meng, Liu & Zhang, Reference Meng, Liu and Zhang2011; Liu, Zhao & Wang, Reference Liu, Zhao and Wang2015). The abundance of triaromatic steroids was relatively high in Chang 7 shale, but the content of methyl-triaromatic steroids was relatively low. C28-triaromatic steroids of soluble extracts were in higher abundance (Fig. 8). The ratio of C26-triaromatic steroids 20S to C28-triaromatic steroids 20S was 0.35, indicating that the oil source rock was formed in a fresh–brackish water deposition environment. In addition, a (C28-triarylene 20S)/(20S+20R) ratio of 0.58 shows that the Chang 7 source rock has entered a mature stage.
Figure 8. Distribution characteristics of triaromatic steroids and methyl-triaromatic aromatic steroids in well G252 for Chang 7 shale in the Ordos Basin.
3.e. Factors controlling black shale formation
3.e.1. High bioproductivity
Based on the observations of drilling cores and outcrop profiles and analyses of the organic petrology samples from Chang 7 shale within the Yanchang Formation in the Ordos Basin, we found that algae bloomed widely within the shale layer (a considerable amount of algae was detected in many shale samples). The organic-rich laminated shale comprised a large amount of algae, with the organic carbon content mostly exceeding 10% (reaching over 30% in some cases). Thus, it is evident that the development and sedimentation of massive algae were the important factors governing shale formation in this area.
During the deposition period of the Chang 7 member, plankton flourished because of the low temperature, low oxygen level and abundant mineral nutriments, resulting in an oxygen-deficient environment in some local areas of the lake basin. The relatively high reproduction rate of plankton led to the consumption of the majority of oxygen within the water and thus increased the reducibility of the aquatic environment, enabling the preservation of a large amount of OM and the final formation of an organic enrichment zone in the Ordos lake basin. The discovery of siliceous rocks, iron dolomite deposits and a marcasite–pyrite–anhydrite mutualism system as well as the filling of self-generated sodium feldspars in early diagenetic fractures was considered to be related to the hydrothermal activity occurring at the bottom of the lake. This indicated that the hydrothermal activity on the lake bed of the Ordos Basin played an important role in promoting the high bioproductivity rates in Chang 7 shale (Zhang, Yang & Xie, Reference Zhang, Yang and Xie2010).
3.e.2. Anaerobic environment
Microelement ratio is an effective indicator of a sedimentary environment (Liu, Qin & Che, Reference Liu, Qin and Che2005; Liu, Wu & Yu, Reference Liu, Wu and Yu2005; Song, 2005; Zhao, Liu & Zhang, Reference Zhao, Liu and Zhang2011). The abundance ratios of a few trace elements, such as V/(V+Ni), V/Cr and Ni/Co, can be used as the geochemical indicators of the redox conditions in water, where the ratio V/(V+Ni)>0.5 commonly represents an anaerobic environment (Tenger et al., Reference Tenger Liu, Xu and Chen2005). In Chang 7 shale, the trace element ratio ω(V)/ω(V+Ni) ranged between 0.64 and 0.83, with an average of 0.83. The ratio of ω(V)/ω(Cr) ranged between 1.26 and 3.39, with an average of 1.29. The ratio of ω(Ni)/ω(Co) ranged between 1.77 and 3.88, with an average of 2.30. These results reveal that Chang 7 shale was deposited in a relatively strong anoxic environment.
Relatively low amounts of gammacerane were detected with saturated chromatography in soluble extracts. The average value of the gammacerane index was 0.13, indicating a lower-salinity and fresh–brackish water deposition environment. Furthermore, clumpy pyrites were visible to the naked eye and widely distributed in the Chang 7 shale. From the observations of the form, texture, and degree of enrichment of pyrites, it can be concluded that the formation of hydrocarbon source rocks occurred in an anaerobic environment.
3.e.3. Palaeo-water depth
The depth of sedimentary water was found to affect and control the abundance of OM in hydrocarbon source rocks. When the water was deep, the environmental reducibility of the bottom medium was enhanced for sediment preservation, and water body stratification and OM preservation were improved. In addition, the water depth was found to control the range of influence of terrestrial components. With increasing water depth and distance to the coastline, the influence of terrigenous constituents on the redox conditions of the water column was found to have weakened. During the sedimentation period of Chang 7, the Sr/Ba (trace elements) ratio was 0.19–0.75 and the palaeo-water depth could have been up to 60–70 m in the Ordos Basin.
3.f. Distribution characteristics and resource potential of shale
The sedimentary evolution history of the Triassic in the Ordos Basin revealed that the relative thickness of the source rocks within the basin was controlled by the sedimentary facies and sedimentary environment. The shale in the deep and semi-deep lake environments was found to have developed with large continuous thickness. In the deposition stage of Chang 7, the sedimentary range rapidly expanded because of the rapid subsidence of the lake basin. Additionally, the lake basin extended to the whole depression to form multi-sedimentary centres, with two sedimentary centres from northwest to southeast (Fig. 9). Shale thickness in the deep lacustrine sedimentary environments in Huachi and Gucheng was generally greater than 20 m, with the maximum thickness reaching 60 m. However, the shale thickness in the semi-deep lake and delta facies was generally less than 5 m. This is distributed in most parts of the basin. It is evident that the distribution of Chang 7 shale in the Ordos Basin was significantly controlled by the sedimentary environment. In summary, the shale in the deep lacustrine depositional environment was characterized by large thickness and good quality. In contrast, the distribution range of shale formed in the shallow lake and the delta facies was wide, but the thickness of the shale was low and its hydrocarbon potential was relatively poor.
Figure 9. Map of Chang 7 shale in the Ordos Basin.
The conversion rate of bitumen ‘A’ decreased with increasing OM abundance, and it showed an increase in the amount of oil extracted from Chang 7 shale, thereby indicating the strong hydrocarbon expulsion capability of the hydrocarbon source rocks with high OM abundance. The thermal simulation results revealed that the Chang 7 shale had an obvious ‘hydrocarbon generation window’ and that the main hydrocarbon generation temperature ranged from 400°C to 500°C. The hydrocarbon generation rate under a consistent temperature and slow heating condition was greater than that under a rapid temperature rise condition. In this respect, the hydrocarbon generation capacity of Chang 7 shale was found to be strong; it has large cumulative hydrocarbon accumulation but a high threshold temperature. The conversion rate of the Chang 7 shale in well G252 with type I kerogen could reach 88% at 450°C, and the accumulated amount of hydrocarbons generated could be up to 483.9 kg t−1 (maximum amount of 164.7×104 t km−2, as shown in Fig. 10). In summary, Chang 7 shale is widely distributed and has a high hydrocarbon generation intensity, high hydrocarbon generation potential, an adequate supply of hydrocarbons and huge resource potential.
Figure 10. Cumulative evolution of the hydrocarbon generation intensity of well G252 in the Ordos Basin.
4. Conclusions
(1) The Chang 7 shale in the Yanchang Formation was black and had obvious bedding. The electrical characteristics exhibited a straight spontaneous potential curve, high resistivity, high concentration of natural gamma rays and high acoustic time difference.
(2) The Chang 7 black shale was rich in OM, with an average TOC value of 12.43% (generally greater than 5%) and S1+S2 value of 31.65 mg g−1. The organic type is good and is dominated by type I or type II1, with a large amount of alginite and a small amount of higher plants. The organic micro-components were rich in terms of hydrocarbons, including bright yellow spherical alginite, bright green and yellow laminar distributed sporophore and bright yellow scattered bituminite. The elemental abundance characteristics and the gas chromatography and mass spectrometry results of saturated hydrocarbons indicated that Chang 7 shale was generally formed in a semi-deep – deep lake environment containing fresh–brackish water. This environment was anoxic and reductive.
(3) In the deposition stage of Chang 7, a large amount of lower aquatic organisms (such as algae) provided abundant organic sources for shale formation. The stable anaerobic environment at the bottom of the lake basin was conducive to the accumulation and preservation of OM. In addition, the hydrothermal activities at the bottom of the lake promoted the bioproductivity rate of Chang 7 shale. Therefore, Chang 7 shale was formed as a source rock with high TOC content, good OM type and high maturity.
(4) The Chang 7 shale was mainly distributed in places such as Jiyuan, Huachi and Gucheng. The general thickness was greater than 20 m (peaking at 60 m). The Chang 7 shale exhibited an obvious ‘hydrocarbon generation window’, and the main hydrocarbon generation temperature ranged between 400°C and 500°C. The conversion rate of type I kerogen of the Chang 7 shale could reach 88% at 450°C, and the accumulated amount of the generated hydrocarbons could reach as high as 483.9 kg t−1.
