Introduction
Since the first major oilfield in China, Karamay Oilfield, was discovered in the Junggar Basin in 1956, the annual crude oil production had reached 1 442×104 t in the basin by 2022, making it one of the key basins for increasing oil reserves and production in China. In contrast to oil exploration, the progress in natural gas exploration in the basin has been relatively slow. By the end of 2022, the proven geological reserves of natural gas and the annual natural gas production in the basin were only 1 933×108 m3 and 38.4×108 m3, respectively. Among the 18 gas fields (reservoirs) with proven reserves in the basin, coal-derived gas accounts for 83%, mainly distributed in the eastern part and southern margins of the basin [1]. In particular, the primary gas source for the gas fields (reservoirs) in the eastern Junggar Basin is from the marine-to-terrigenous transitional source rocks of the Carboniferous system, and the Kelameili Gasfield and Wucaiwan Gasfield are the typical representatives [2-3]. The primary gas source for the gas fields (reservoirs) in the southern margin of the basin is from the Middle-Lower Jurassic coaly source rocks deposited in a swamp environment, such as Hutubi Gasfield and Mahe Gasfield [4-5].
Among the discovered gas fields/reservoirs, less than 3% are oil-type gas fields, primarily located in the northwest margin of the basin, such as the Xiazijie Gasfield [1]. These natural gases are mostly oil-associated gas or gas in gas cap, primarily from the Lower Permian Fengcheng source rocks [6-7]. The traditional view considers them to be oil-prone source rocks, and they are alkaline lacustrine deposits influenced by volcanic activities [8]. Crude oil found in the Karamay and Mahu oilfields is mainly from these source rocks [6,9]. Initially, the Fengcheng Formation source rocks were thought to be primarily oil-prone, and fewer research was performed on their gas-generating potential. In recent years, industrial gas flows have been obtained from the Upper Triassic Baijiantan Formation, the Lower Permian Fengcheng Formation, and the Carboniferous interval from wells such as Chetan 1, Shatan 2, Shixi 18, Penbei 1, Hefeng 1, and Fengyun 1, located in the western Central Depression. The natural gases have a significantly lighter carbon isotope composition than typical coal-derived gases, suggesting they may be highly mature oil-type gases from the Fengcheng source rocks [10⇓-12].
Based on molecular and stable carbon isotopic compositions, this paper identifies the natural gas sourced from the Fengcheng Formation in the western Central Depression of the Junggar Basin. Utilizing methods such as rock-eval pyrolysis, chloroform bitumen "A" extraction, hydrocarbon generation kinetics simulation, basin modeling, and organic petrology, this paper systematically analyzes the gas-generating potential of the Fengcheng source rocks and compares with another set of gas source rocks, carbonaceous rocks. Based on the latest drilling and seismic data, this paper defines the Fengcheng Formation distribution of gas source kitchens and proposes four types of favorable exploration areas (i.e., oil-type gas) in the western Central Depression in the Junggar Basin.
1. Geological setting
Located in the northern part of the Xinjiang Uygur Autonomous Region in northwest China, the Junggar Basin covers an area of approximately 13×104 km2. Structurally, it is situated at the convergence of three tectonic domains, i.e., Kazakhstan, Siberia, and Tarim, and represents a typical superimposed basin from the Upper Paleozoic to the Mesozoic-Cenozoic [13-14]. Based on the structural pattern at the Carboniferous top, the basin can be divided into six primary structural units (Fig. 1a-1d ). This study focuses on the western Central Depression covering four secondary structural units, i.e., the Mahu Sag, the Penyijingxi Sag, the Shawan Sag, and the Zhongguai Uplift (Fig. 1b ).
Fig. 1. Comprehensive assessment map of petroleum geology conditions in the study area. (a) Comprehensive stratigraphic column of the western Central Depression in the Junggar Basin; (b) Ro contours on the bottom; (c) thickness contours; (d) contours of the gas-generating intensity of Fengcheng source rocks in the Junggar Basin. |
The stratigraphy in the study area is complete. From the top down, they are the Cretaceous, the Jurassic, the Triassic Baijiantan Formation, Karamay Formation and Baikouquan Formation, the upper Wuerhe Formation, lower Wuerhe Formation, Xiazijie Formation, Fengcheng Formation and Jiamuhe Formation of the Permian, and the Carboniferous units (Fig. 1a ). Regional unconformities are found between the Carboniferous and the Permian, the Permian and the Triassic, the Triassic and the Jurassic, and the Jurassic and the Cretaceous (Fig. 1a ) [15].
Three sets of source rocks, i.e., the Middle-Lower Jurassic, the Lower Permian Fengcheng Formation, and the Carboniferous, are found in the western Central Depression. Influenced by the southward tilting of the Junggar Basin happening within the Himalayan Movement, the whole Jurassic interval has not entered the oil-generating window in the study area [6], and as such, it is not discussed in this paper. This paper focuses on the hydrocarbon-generating potential of the Fengcheng source rocks, and it is compared with the Carboniferous source rocks.
During the Permian, the sedimentary environment in the Junggar Basin started to transfer from marine to lacustrine [16-17]. When the Early Permian Fengcheng Formation was deposited, the basin was a semi-closed to closed hydrogeological environment, resulting in a complex lacustrine sedimentary system that includes volcanic clastics, terrigenous clastics, carbonates, evaporites, etc. [17-18] Based on the content of evaporites and their response characteristics on well logs, the Fengcheng Formation can be divided into three members from the bottom to the top, referred to as Member 1, Member 2, and Member 3 (Fig. 1a ). Member 1 that followed the sedimentary and volcanic activity background from the underlying Jiamuhe Formation, where the water salinity was lower, mainly includes conglomerate, large-scale volcanic rock, and ash-rich sandstone (Fig. 1a ) [17,19]. During the deposition of Member 2, volcanic activities diminished, and the water salinity reached the highest, resulting in abundant accumulation of natrocarbonatite in the central basin [17,20]. Member 2 is characterized by layered evaporite which is primarily natrocarbonatite (Fig. 1a ), and interbedded dolomite, natrocarbonatite and dark shale are common. The evaporite is easily identified by the high resistivity values on well logs [17,19]. When Member 3 was deposited, volcanic activities became weaker and weaker, the lake water salinity decreased, natrocarbonatite deposition ceased, and dolomitic rock gradually transferred to argillaceous rock with increasing input of terrigenous clastics (Fig. 1a ) [18,21].
2. Hydrocarbon-generating potential of source rock
2.1. Organic matter types
In the western Central Depression, the Fengcheng Formation source rocks exhibit a light carbon isotopic composition (δ13C) for kerogen, ranging from -30.3‰ to -25.0‰ (-27.2‰ on average), indicating types I to II1 kerogen, which is prone to oil generation. In contrast, the Carboniferous source rock has a heavier δ13C value, ranging from -27.1‰ to -20.3‰ (-22.4‰ on average), indicating types II2 to III kerogen, which is prone to gas generation.
The Fengcheng source rocks have abundant sapropelite macerals, such as alginite and mineral bitumen matrix, constituting 10.7% to 54.4%, and 40.8% on average (Fig. 2a-2c ). In contrast, the Carboniferous source rocks have less sapropelinite macerals, ranging from 3.1% to 37.1%, and 12.5% on average (Fig. 2a-2c ). Additionally, the Fengcheng source rocks contain rich exinite macerals such as sporinite, resinite, and debris exinite (Fig. 2a-2c ), constituting 10.3% to 75.8%, with an average of 37.1%, significantly higher than the Carboniferous source rock, whose content of exinite macerals ranges from 1.4% to 30.4%, with an average of 11.7%. The Fengcheng source rocks also contain relatively abundant pyrite, ranging from 0.3% to 6.8% (3.5% on average), markedly higher than the Carboniferous source rocks (0.3% to 1.2%, and 0.6% on average). This reflects that the former deposited in a stronger reducing environment that provided better conditions for organic matter preservation. The Carboniferous source rocks are characterized by a high vitrinite content, ranging from 25.1% to 85.3%, and 60.0% on average. Abundant structured vitrinite, such as clay minerals, is visible in the mixed matrix. They are short stripes with fragmented structures, along with numerous debris vitrinite and debris inertinite (Fig. 2d-2f ). In contrast, the Fengcheng source rocks have a vitrinite content ranging from 12.6% to 36.6% (23.2% on average), significantly lower than the Carboniferous source rock. In summary, the Fengcheng source rocks in the western Central Depression are rich in sapropelinite and exinite macerals, indicating oil-prone microfacies, while a high vitrinite content, indicative of gas-prone microfacies, characterizes the Carboniferous source rocks. This finding is consistent with the characteristics reflected by the carbon isotope compositions of kerogen.
Fig. 2. Microphotographs of organic macerals of the Lower Permian Fengcheng and Carboniferous source rocks in the western Central Depression of the Junggar Basin. (a) Calcareous tuff, TOC=2.3%, Fengcheng Formation, 4 562.5 m, Well Xia-72, blue light; (b) Tuffaceous dolomitic mudstone, Fengcheng Formation, TOC=1.8%, 3 423.0 m, Well Wu-35, blue light; (c) Silicified dolomitic shale, Fengcheng Formation, TOC=1.6%, 4 038.5 m, Well Fengnan-2, blue light; (d) Argillaceous tuff, Carboniferous System, TOC = 1.05%, 2 322.7 m, Well Che-372, oil immersion, plane-polarized light; (e) Tuffaceous shale, TOC=0.67%, Carboniferous System, 1 557.9 m, Well Bai-55, oil immersion, plane-polarized light; (f) Tuffaceous carbonaceous mudstone, TOC=5.90%, Carboniferous System, 2 246.3 m, Well Pai-66, oil immersion, plane-polarized light. Td-Structured vitrinite; Cd-Vitrinite debits; Id-Inertinite debits; Ld-Lipnite debits; Al-Alginite; MB-Mineral bitumen matrix; MiS-Micro sporinite; Py-Pyrite; Tu-Tuff; Tuf-Tuff debits; Re-Resinite; Cl-Clay mineral. |
2.2. Organic matter maturity and hydrocarbon evolution
Statistical analysis of measured data from 37 core samples of Fengcheng Formation source rocks in the western Central Depression reveals a wide distribution of vitrinite reflectance (Ro), ranging from 0.53% to 1.52%, and 0.82% on average. This distribution spans immature to overmature stages, with most samples situated in the early stages of the primary oil-generating window (Ro of 0.7%-1.0%). The maximum pyrolysis temperature (Tmax) of the Fengcheng source rocks ranges from 403 °C to 451 °C (429 °C on average), indicating that the source rock is in the peak oil-generating stage. Based on the Ro and Tmax values, the overall maturity of the Fengcheng source rocks is relatively low, with only a few samples entering the gas condensate/wet gas-generating stage. This is primarily attributed to the majority of these samples being from the Mahu Sag, where the burial depth of the Fengcheng Formation is much shallower than that of the Shawan Sag and the Penyijingxi Sag. Additionally, the cored wells are mainly situated at structural highs. Therefore, these data alone cannot reflect the degree of thermal evolution of the Fengcheng source rocks in the present study area. This study drew the present plane distribution of Ro of the Fengcheng Formation source rocks based on the structural map of the bottom of the Fengcheng Formation in the western Central Depression, the depth-Ro relationship of the source rock samples, and the elevation of the cored points in the wells (Fig. 1b ). The results indicate that the Fengcheng source rocks in the Mahu Sag are in the primary oil-generating window at present with its southern part entering the gas condensate/ wet gas-generating stage. The Fengcheng source rocks in the Penyijingxi Sag have generally entered the gas condensate/ wet gas-generating stage. Those in the Shawan Sag have the highest thermal maturity, being in the gas condensate/wet gas-generating stage to the dry gas-generating stage (Fig. 1b ). The statistics show that the area where the bottom of the Fengcheng Formation has entered the dry gas-generating stage (Ro>2.0%) is approximately 13 000 km2. The area where it has entered the gas condensate/wet gas-generating stage (Ro>1.4%) is approximately 21 400 km2 (Fig. 1b ).
To reflect the hydrocarbon generation and evolution process of the Fengcheng source rocks and the difference in this process in different hydrocarbon-generating sags, three virtual wells were designed at the deepest points, one in the Mahu Sag, one in the Penyijingxi Sag, and the last in the Shawan Sag, based on seismic and drilling data (Fig. 1b ). The burial and thermal evolution histories of the Fengcheng source rocks were reconstructed using Petromod software (Fig. 3 ), with key parameters such as regional heat flow, erosion amount, and geothermal gradient obtained from references [22⇓⇓⇓⇓⇓-28]. Simulation results show that the Fengcheng source rocks in the Shawan Sag reached the oil-generating threshold at the earliest time (Middle Permian), entered the primary oil-generating window in the early Late Permian, started gas condensate/wet gas generation in the Early Triassic, and changed to generate dry gas by the Early Cretaceous, and now they are still generating dry gas (Fig. 3a ). During the whole history, three important tectonic uplifting events occurred at the end of the Triassic, the end of the Jurassic, and the end of the Cretaceous, respectively, leading to a temporary halt in hydrocarbon generation (Fig. 3a ). In the Penyijingxi Sag, the Fengcheng source rocks reached the oil-generating threshold in the Middle Permian, entered the primary oil-generating window in the late Late Permian, started the gas condensate/wet gas-generating stage in the late Triassic, and entered the dry gas-generating stage by the late Early Cretaceous, but they have not yet wholly entered the main dry gas-generating stage as of now (Fig. 3b ). In the Mahu Sag, the Fengcheng Formation source rocks reached the oil-generating threshold in the middle Late Permian, entered the primary oil-generating window in the late Late Permian, entered the gas condensate/wet gas-generating stage in the early Jurassic, but they have not yet entered the main dry gas-generating stage as of now (Fig. 3c ).
Fig. 3. Burial and thermal history of hydrocarbon-generating sags in the western Junggar Basin (see virtual well locations in |
2.3. Organic matter abundance and gas-generating potential
In the western Central Depression, the Fengcheng source rocks have TOC values ranging from 0.52% to 3.19%, and 1.22% on average. As indicated by S2, the hydrocarbon-generating potential is 0.81-17.70 mg/g, and 3.70 mg/g on average. The chloroform bitumen “A” content ranges from 0.094 3% to 2.675 4%, and 0.491 0% on average. Most of the source rocks are medium to good, and some are excellent (Fig. 4 ). The Carboniferous source rocks in the western Central Depression have TOC ranging from 0.10% to 2.29%, and 0.90% on average, S2 ranging from 0.05 mg/g to 1.64 mg/g, and averaged 0.33 mg/g, and chloroform bitumen “A” from 0.000 9% to 0.184 4%, and averaged 0.031 7%. The Carboniferous source rocks are poor to medium (Fig. 4 ). In conclusion, the organic matter abundance of the Fengcheng source rocks is significantly superior to that of the Carboniferous source rocks, indicating a better hydrocarbon-generating potential.
Fig. 4. Hydrocarbon-generating potential plots of Lower Permian Fengcheng and Carboniferous source rocks in the western Central Depression of the Junggar Basin. |
The TOC and pyrolysis parameters are significantly influenced by maturity. Considering that some samples exhibit high maturity, this study employed a statistical model proposed by Banerjee [29] to restore the original hydrogen index (HIo, Eq. (1)) of the Fengcheng and Carboniferous source rocks and obtain the corresponding conversion ratios (TR, Eq. (2)). Based on the method proposed by Peters et al. [30], the original TOC (TOCo) of the source rocks was then restored (Eq. (3)). The results indicate that the HIo values of the Fengcheng and Carboniferous source rocks are 577 mg/g and 280 mg/g, respectively. The TOCo values of the Fengcheng and Carboniferous source rocks range from 0.80% to 3.30% (1.69% on average) and from 0.37% to 2.41% (1.13% on average), respectively. It can be observed that maturity has a limited impact on the organic matter abundance of the two sets of source rocks in the study area.
Comparing the pyrolysis parameters of the Fengcheng and Carboniferous source rocks, it is evident that the Fengcheng source rocks have higher chloroform bitumen “A” and free hydrocarbon content (S1), indicating better retention of liquid hydrocarbons (Fig. 4 ). While the Fengcheng source rocks thermally cracks into natural gas, the retained oil will also undergo secondary cracking, making a significant contribution to gas generation. On the other hand, the Carboniferous source rocks are gas-prone. With lower chloroform bitumen “A” and S1, their gas generation almost depends on the cracking of kerogen. Taking into account the contribution of secondary cracking of the retained oil in the Fengcheng source rocks and the similarity in hydrocarbon generation and evolution processes in different systems for the Carboniferous source rocks, gold tube pyrolysis experiments in a closed-system were conducted to compare the gas-generating capabilities and processes of these two sets of source rocks. The samples of the Fengcheng source rocks were taken from Well Fengnan 2, characterized by silicified dolomitic shale with a TOC value of 1.56% and a measured Ro value of 0.60%. The samples of the Carboniferous source rock were taken from Well Shaqiu 12, consisting of siliceous mudstone with a TOC value of 2.17% and a measured Ro value of 0.54%. The results show that the maximum gas (C1-5) yield from the Fengcheng source rocks can be 651 mL/g, while that from the Carboniferous source rock is only 381 mL/g, which is significantly lower than the former (Fig. 5 ). Some scholars have pointed out that there is a significant difference in the gas-generating potential of different types of source rocks in a closed system, mainly influenced by the quantity of liquid hydrocarbons generated from the source rocks [31]. The Fengcheng source rocks, dominated by types I-II1 kerogen (Fig. 2 ), have a high potential for oil generation. In addition to being expelled from the source rock through primary migration, the liquid hydrocarbons generated also remain to a considerable extent within the source rock, which may crack into gas later, making a significant contribution to gas generation from the source rock [31]. The hydrogen index (HI) of the samples of the Fengcheng source rocks used in the hydrocarbon-generating simulation experiment is 683 mg/g, with a maximum oil-generating potential of 725 mg/g. In a closed system, the liquid hydrocarbons initially generated can crack again into natural gas, significantly enhancing the late-stage gas-generating potential of the Fengcheng source rocks. In contrast, the Carboniferous source rocks, dominated by types II2-III kerogen (Fig. 2 ), have a hydrogen index of 274 mg/g according to the samples used in the simulation experiment, so their oil generation potential (only 186 mg/g at most) is low, and accordingly, their gas-generating potential is significantly lower than the Fengcheng source rocks (Fig. 5 ).
Fig. 5. Total (C1-5) yield of Lower Permian Fengcheng and Carboniferous source rocks in a closed system. |
Based on the gas production data at different heating rates (20 °C/h and 2 °C/h), the kinetic parameters of hydrocarbon generation for the Fengcheng and Carboniferous source rocks in the western Central Depression can be further calculated. The hydrocarbon-generating process for both sets of source rocks can be analyzed by comparing their kinetic parameters. The sample of the Fengcheng source rock in this study has an activation energy for methane generation ranging from 197.4 to 298.2 kJ/mol, with a dominant frequency of 256.2 kJ/mol, and a frequency factor of 2.54×1012 s-1. The sample of the Carboniferous source rock has an activation energy for methane generation ranging from 193.2 to 281.4 kJ/mol, with a dominant frequency of 247.8 kJ/mol, and a frequency factor of 7.03×1011 s-1. Applying the above kinetic parameters to the thermal history conditions of the Shawan Sag (Fig. 6a ), hydrocarbon generation and evolution curves for the Fengcheng and Carboniferous source rocks can be obtained. It can be observed that the hydrocarbon generation for the Fengcheng source rocks was relatively slow before the Jurassic, with a methane conversion rate of only 21%. Since the Cretaceous, the methane conversion rate has increased rapidly, reaching 57% today (Fig. 6a ). In contrast, the Carboniferous source rocks achieved a methane conversion rate of 42% before the Jurassic, and the methane conversion rate has steadily increased since the Cretaceous, reaching a cumulative methane conversion rate of 81% today (Fig. 6b ). These results indicate that the Carboniferous source rocks started generating gas earlier, while significant methane generation by the Fengcheng source rocks occurred after the Jurassic. Comparing the methane production rates at different stages, it is evident that the Fengcheng source rocks produced only 132 mL/g of methane before the Jurassic, which is lower than the Carboniferous source rocks (159 mL/g). Since the Cretaceous, the Fengcheng source rocks have generated 243 mL/g of methane, significantly higher than the Carboniferous source rock (152 mL/g). Comparing the thermal evolution histories and the gas production processes of the Fengcheng source rocks in three sags (Fig. 3 ), the main gas-generating stages in the Penyijingxi and Mahu sags should be later than those in the Shawan Sag. Considering that the western Central Depression has undergone multiple tectonic movements, and early traps are prone to destruction, the higher gas-generating potential and late-stage gas generation of the Fengcheng source rocks are more conducive to the accumulation of natural gas resources.
Fig. 6. Gas conversion rate of the Lower Permian Fengcheng and Carboniferous source rocks in the Penyijingxi Sag in the Junggar Basin. |
3. Distribution of scale-effective gas source kitchens
The concept of hydrocarbon source kitchens integrates information such as the thickness of source rocks, organic matter abundance, organic matter type, spatial distribution and maturity, ultimately manifesting in the hydrocarbon-generating intensity of source rocks [32]. In this study, the term “scale-effective gas source kitchens” refers to the gas source kitchens capable of forming large to medium-sized gas fields.
During the late Early Permian (when the Fengcheng Formation was deposited), the western and southern margins of the Junggar Basin underwent post-collisional extension, leading to the widespread development of fault-controlled basins and collage island arc terrains. The former includes the Mahu Sag controlled by the Wuxia Fault Zone, and the Shawan Sag controlled by the North Tianshan Fault Zone and the Hongche Fault Zone (Fig. 1c ). The latter are like the Penyijingxi Sag situated between the colliding Mosuowan Island Arc Terrain and the Luliang-Xiayan Island Arc Terrain. These sags/depressions provided space for sediment accommodation and favorable zones for Fengcheng source rocks deposited during the interglacial period at the Late Paleozoic ice age [33-34]. Owing to extensive lake transgression (or influenced by marine transgression), the lake or marine-terrigenous transitional area where the Fengcheng source rocks were deposited was larger than when the Jiamuhe Formation was deposited. Despite extensive lake transgression, the Fengcheng Formation in the western Central Depression was in a saline lake environment because of the semi-arid climatic conditions. This is particularly evident in the second member of the Fengcheng Formation, where interbedded siliceous rock, natrocarbonate rock and dark shale are widely developed. Alkaline lakes typically exhibit distinct water stratification, favoring the preservation of organic matter and providing an environment conducive to the proliferation of specific bacterial and algal species. This ecological setting contributes to the formation of high-quality source rocks in the Fengcheng Formation.
Based on the logging characteristics of the Fengcheng source rocks in the study area and using 3D and 2D seismic framework profiles in the western Central Depression, the spatial distribution of the Fengcheng was predicted (Fig. 1c ). The results indicate that the remaining Fengcheng source rocks are mainly distributed along the basin margin faults and sags between the island arc terrains within the basin. These source rocks gradually thin out in the direction of the uplift formed by the island arc terrains (Fig. 1c ). In the Mahu Sag, the Fengcheng source rocks are mainly distributed along the Wuxia Fault Zone. They have been almost eroded in the west but became as thick as up to 700 m in the east and thinner toward the Luliang-Xiayan Uplift in the east. In the Shawan Sag and its periphery, the Fengcheng source rocks are mainly distributed along the North Tianshan Fault Zone and the Hongche Fault Zone. Most of the source rocks have been eroded in the west and south but may reach 900 m in the northeast, then gradually thinner toward the northern Mosuowan Uplift and the eastern Monan Uplift. Active later than the deposition of the Fengcheng Formation, the Hongche Fault Zone caused destruction to the Fengcheng Formation, making the latter almost completely eroded on the western hanging wall. In the Penyijingxi Sag, the Fengcheng source rocks are mainly distributed around sags between the island arc terrains. They connect to the Shawan Sag in the southwest, thin out and pinch out to the northwest toward the Xiayan Uplift, and gradually pinch out to the east toward the Monan Uplift and the Mosuowan Uplift. The thickness is relatively small, ranging from 400 m to 600 m overall (Fig. 1c ). The distribution of the Fengcheng source rocks developed in these three sags is broad, and among which the total area where the thickness is greater than 100 m may be 2.21×104 km2 (Fig. 1c ).
Previous studies have shown that large and medium-sized gas fields in China are mainly distributed within or near gas source kitchens with hydrocarbon-generating intensity greater than 20×108 m3/km2 [35⇓-37]. Recent research indicates [38] that hydrocarbon-generating intensity greater than 10×108 m3/km2 can form large gas fields (with proven geological reserves exceeding 300×108 m3) in deep and ultra-deep tight gas reservoirs. To determine the gas-generating potential of the Fengcheng source rocks in the western Central Depression, we studied the spatial distribution of the gas-generating intensity using the following formula:
Ek = 1 000-TOC×ρ×H×HIo×TR×k
The results show that the overall gas-generating intensity of the Fengcheng source rocks is greater than 10×108 m3/km2 (Fig. 1d ). In the Penyijingxi Sag and the Shawan Sag, there are two gas-generating centers where the gas-generating intensity is greater than 20×108 m3/km2, 1 920 km2 and 4 520 km2, respectively. The maximum gas- generating intensity in the two gas source kitchens can reach 400×108 m3/km2 (Fig. 1d ), which is comparable to the gas-generating intensity of the Kuche Depression in the Tarim Basin, the Deyang-Anyue Depression in the Sichuan Basin, and the Xujiaweizi Depression in the Songliao Basin [39]. Additionally, this study discovered a gas-generating center in the foreland thrust belt on the southern margin, covering an extensive area of 5 900 km2 where the gas-generating intensity is greater than 20×108 m3/km2 (Fig. 1d ). Recently, numerous oil and gas shows originated from the Permian source rocks have been reported in the foreland thrust belt [40]. This indicates that, besides the western Central Depression, the gas-generating potential of the Fengcheng source rocks in the foreland thrust belt is also worthy of further investigation. In summary, the Fengcheng Formation in the southern region to the Mahu Sag in the Junggar Basin can form large to medium-sized gas fields. Exploration in the vicinity of scale-effective gas source kitchens (with gas-generating intensity greater than 10×108 m3/km2) has already discovered a significant amount of oil-type gas originated from the Fengcheng source rocks, confirming the above observations (Fig. 1d ).
4. Gas source correlation
For original thermogenic gases, as thermal maturity increases, the δ13C values become higher as enrichment in 13C [41⇓-43]. Numerous field examples and thermal simulation experiments confirmed that natural gases from different sources exhibit distinct isotope evolution paths, often represented by the correlation between δ13C1 and δ13C2 ratios[43⇓⇓-46]. Studies indicated that at the same or similar thermal evolution stages, coal-derived gas tends to have a more enriched 13C carbon isotope composition than oil-type gas [40⇓⇓-43].
Milkov, based on geochemical data from over 30 600 natural gas samples worldwide, provided a new chart to differentiate coal-derived gas from oil-type gas, optimizing their distribution ranges [47]. In this study, we utilized the aforementioned tool to analyze the origin of 158 natural gas samples from 129 gas wells around the western Central Depression. The samples cover seven secondary structural units, including the Wuxia Fault Zone, Kebai Fault Zone, Hongche Fault Zone, Mahu Sag, Zhongguai Uplift, Dabasong Uplift, and Mosuowan Uplift, and twelve stratigraphic units, including the Cretaceous Qingshuihe Formation, Jurassic Badaowan Formation and Sangonghe Formation, Triassic Baikouquan Formation, Karamay Formation, and Baijiantan Formation, Permian Upper Wuerhe Formation, Xiazijie Formation, Lower Wuerhe Formation, Fengcheng Formation, Jiamuhe Formation, and the Carboniferous units. The natural gas samples are relatively few from the southern part of the study area, primarily due to current exploration limitations. The analysis results indicate that natural gas in the study area can be classified into two types.
The first type of natural gas has relatively lighter δ13C1 and δ13C2 values, ranging from -54.4‰ to -29.8‰ (-37.3‰ on average) and -40.9‰ to -27.1‰ (-30.5‰ on average), falling within the distribution range of oil-type gas. The second type of natural gas has relatively enriched 13C, with δ13C1 and δ13C2 values ranging from -40.8‰ to -25.8‰ (-32.8‰ on average) and -27.6‰ to -22.9‰ (-25.4‰ on average), falling within the distribution range of coal-derived gas (Fig. 7a ). As mentioned earlier, the Fengcheng Formation and the Carboniferous units are the two primary source rocks in the western Central Depression, with the former being sapropel-type source rocks and the latter humic type source rocks, corresponding to the first and the second types of natural gas, respectively. According to the principles of carbon isotope fractionation [42,48], the carbon isotopic compositions of the kerogen in the two hydrocarbon source rocks also have a close genetic relationship with the two types of natural gas.
Fig. 7. Genesis identification charts of natural gas in the western Central Depression of the Junggar Basin (see the spatial distribution of natural gas samples from Fengcheng Formation in |
The C2-4 carbon isotopic compositions exhibit substantial parent material inheritance. Although the thermal maturity of source rocks influences them, the impact is much smaller than that of δ13C1. Thus, C2-4 carbon isotopic compositions are effective indicators to distinguish coal-derived gas from oil-type gas [49⇓-51]. Due to factors such as sample quantity and heterogeneity of organic matter in source rocks, different scholars have proposed standards with some differences, but the overall boundary for the C2-4 carbon isotopic composition of the two types of natural gas is around (-28.0±2)‰ [49⇓-51]. The first type of natural gas, except for the lighter δ13C2 values (Fig. 7a ), also exhibits relatively lighter δ13C3 and δ13C4 values, ranging from -37.7‰ to -26.7‰ (-29.8‰ on average) and -36.8‰ to -26.6‰ (-30.1‰ on average), respectively, showing distinct characteristics of oil-type gas, corresponding to the Fengcheng source rocks (Fig. 7b ). The second type of natural gas shows enriched 13C in δ13C3 and δ13C4 ratios, ranging from -27.3‰ to -19.9‰ (-23.5‰ on average) and -26.5‰ to -17.4‰ (-23.7‰ on average), respectively, exhibiting characteristics of coal-derived gas, corresponding to the Carboniferous source rocks (Fig. 7b ).
In the western Central Depression, oil-type gases can be further classified into two subtypes based on maturity. The first subtype of natural gas has a low dryness coefficient (C1/∑C1—4), with a main range of 0.8 to 0.9 and an average value of 0.84. Calculated using the conversion formula proposed by Zhao Wenzhi et al. [52] for equivalent vitrinite reflectance (Roeq) from δ13C1, the average Roeq value is 0.76%, indicating predominantly wet gas (the global dryness coefficients are less than 0.95), reflecting a lower maturity. This portion of natural gas is mainly oil-associated gas, distributed in the Mahu Sag and its surrounding areas, suggesting a relatively small resource scale. The second subtype of natural gas has an average dryness coefficient of 0.95, with dry gas (C1/∑C1—4 values greater than 0.95) accounting for 59.3%. The calculated Roeq values range from 1.07% to 2.69% (1.89% on average), indicating that this portion of natural gas is primarily derived from the Fengcheng source rocks during the condensate/dry gas-generating stages. This highly mature oil-type gas is mainly distributed in the Zhongguai Uplift and to its south, especially in the Zhongguai Uplift itself. It represents a significant natural gas type for future exploration of large-scale natural gas reserves.
5. Favorable areas for natural gas exploration
The formation and distribution of oil and gas in the western Central Depression, specifically in the Fengcheng Formation, exhibit distinct source-controlled characteristics. Both conventional and unconventional oil and gas discoveries are influenced by source kitchens. By now, natural gas discoveries are more pronounced “near- source accumulations”. Based on the analysis of factors such as the distribution of effective gas source kitchens, source-reservoir connectivity, and potential trap types, four types of favorable exploration fields for oil-type gas derived from the Fengcheng source rocks are proposed in the western Central Depression.
(1) Structural traps neighboring to source kitchen: The Carboniferous structural traps, closely neighboring to the Fengcheng source kitchens, display a large-span, positive structural development conducive to forming large gas reservoirs. According to statistics, within the domain controlled by the gas source kitchen of the Fengcheng Formation, the Carboniferous anticline traps cover an area of 672 km2, and the fault nose and fault block traps encompass an area of 2 116 km2. The estimated gas resource is projected to be (5 000-8 000)×108 m3. Well Xishi 16 has made a breakthrough in the Carboniferous anticline trap, confirming the presence of condensate oil and gas derived from the Fengcheng source rocks. This substantiates the promising prospects for natural gas exploration in this domain (Fig. 8 ).
Fig. 8. Comprehensive evaluation map of favorable areas for Fengcheng-sourced gas exploration in the western Central Depression of the Junggar Basin. |
(2) Stratigraphic traps neighboring to the source kitchen: Overall, the Fengcheng Formation exhibits distinct characteristics of layer-by-layer northward thinning and pinching out, laying the geological foundation for developing extensive stratigraphic traps. Exploration has confirmed that the fan delta fronts depositing in the Fengcheng Formation contain high-quality reservoirs. Meanwhile, the near-shore lacustrine mudstone can provide effective sealing, facilitating the widespread accumulation and storage of natural gas (Fig. 8 ). Preliminary assessments suggest that the favorable exploration area of the stratigraphic overburden belt of the Fengcheng Formation neighboring to the source kitchen covers an area of approximately 1 400 km2, with estimated gas resource surpassing 5 000×108 m3. A breakthrough has been made to the Fengcheng Formation in Well Shatan 2, where the natural gas exhibits lighter carbon isotopic compositions, predominantly originated from the Fengcheng source rocks (Fig. 8 ).
(3) Shale gas reserves in source kitchen: Located in the highly mature gas source kitchen of the Fengcheng Formation, the argillaceous rock is extensively distributed and characterized by high brittleness, highly developed fractures, and overpressure. These geological conditions provide a stable and high-yield environment for gas production. The favorable area for this type of shale gas is shallower than 5 500 m and covers 5 000 km2. Once a breakthrough is made, it is expected to discover a large gas field. Well Fengyun 1 which is being drilled in the Fengcheng Formation, has encountered self-source and self-reservoir shale gas zones in the southern part of the Mahu Sag (Fig. 8 ). The Fengcheng source rocks in this area are still at the early stage of gas generation and they are situated outside the primary gas source kitchen. It is speculated that the Penyijingxi and the Shawan sags may be favorable fields for exploring lacustrine shale gas in the Fengcheng Formation since the Fengcheng source rocks in these sags exhibit relatively high maturity.
(4) Structural traps in source kitchen: In the western Central Depression of the Junggar Basin, a favorable structural background known as Uplift in Depression is widely developed. Located at gas-generating centers and with large-span hydrocarbon supply windows, these structures have abundant gas sources, and even the positive structures provide favorable conditions for forming large-scale reserves. In the structural highs, recent studies suggested the possible development of high-energy oolitic beach reservoirs and the presence of tight sandstone and gravel reservoirs in the Fengcheng Formation. Wells Mahu 26 and Mahu 39 in the Mahu Sag have discovered oolitic sandstone and sandstone containing ooids, where concentrated brackish water and a relatively high-energy environment near the wave base are crucial for developing such reservoirs. Well-seismic tie data indicate that these reservoirs were mainly developed in ancient uplifts. Water salinity increases as the lake level drops, causing carbonate oversaturation and precipitation. During this period, the sand bodies in the ancient salients are influenced by wave action, experiencing continuous oscillations. Fragments become ooids through rolling movements, and during the diagenetic period, ooid cores (terrigenous detritus) and some intervals undergo organic acid dissolution, forming storage space. Current evaluation suggests that these traps cover an advantageous overlapping area of approximately 800 km2, making them worthy of exploration attention (Fig. 8 ).
6. Conclusions
In the western Central Depression of the Junggar Basin, the Fengcheng Formation serves as a set of significant source rocks characterized predominantly by kerogen Type I to Type II1 and moderate to good quality. Early liquid hydrocarbons left in the source rocks can generate substantial gas in the late stage. The Fengcheng source rocks in the Penyijingxi Sag and the Shawan Sag have started peak gas generation. The natural gas exploration prospects in these areas are promising.
The Fengcheng source rocks are distributed widely in the western Central Depression, and the accumulative area with a thickness greater than 100 m reaches 2.21×104 km2. The major gas source kitchens are located in the Penyijingxi and Sag the Shawan Sag, with gas intensity exceeding 20×108 m3/km2, and 1 920 km2 and 4 520 km2, respectively, indicating a potential for the formation of large to medium-sized gas fields.
The oil-type gas originated from the Fengcheng source rocks, with light δ13C1 and δ13C2 values ranging from -54.4‰ to -29.8‰ (-37.3‰ on average) and -40.9‰ to -27.1‰ (-30.5‰ on average), respectively, constitutes the main component of natural gas in the western Central Depression, promising a realistic area for natural gas exploration.
In the western Central Depression, four favorable exploration areas have been identified, i.e., structural traps neighboring to the source kitchen, stratigraphic traps neighboring to the source kitchen, shale gas reserves in the source kitchen, and structural traps in the source kitchen. Breakthroughs have been made in the first three areas, indicating an accelerating exploration potential for oil-type gas sourced from the Fengcheng source rocks in the study area.
Nomenclature
a, b—constants reflecting the rate of change of hydrogen index at maximum pyrolysis temperature, dimensionless;
Ek—hydrocarbon-generating intensity of source rocks, 108 m3/km2;
H—thickness of source rocks, m;
HI—hydrogen index, mg/g;
HIo—original hydrogen index, mg/g;
k—empirical constant, 103 m3/t;
Ro—vitrinite reflectance, %;
Roeq—equivalent vitrinite reflectance, %;
S2—hydrocarbon retention amount, mg/g;
Tmax—maximum pyrolysis temperature, °C;
TR—kerogen conversion rate for hydrocarbon generation, %;
TOC—total organic carbon content, %;
TOCo—original TOC, %;
ρ—density of source rocks, kg/m3.