Introduction
The Sichuan Basin is a large multicycle superimposed basin in western China, with multiple sets of source rocks developed in marine strata and abundant oil and gas resources. In recent years, significant progresses have been made in oil and gas exploration in the basin [1⇓⇓⇓-5]. The exploration process of marine oil and gas in this basin can be traced back to the 1940s, but the large-scale oil and gas discoveries were truly achieved in the 21st century [6]. In 2003, the conventional marine natural gas explora-tion in the Sichuan Basin made the first breakthrough in the Kaijiang-Liangping continental shelf in northeastern Sichuan Basin, and large Permian-Triassic biogenic reef flat gas reservoirs were discovered [7]. Since then, the exploration for large-scale reef flat reservoirs has rapidly extended, and five large gas fields with natural gas reserves of 100 billion cubic meters (Puguang, Yuanba, Western Sichuan, Anyue, and Penglai) have been discovered [8-9]. Multiple sets of gas-bearing strata have been discovered, including the Sinian Dengying Formation, Cambrian Longwangmiao Formation, Carboniferous Huanglong Formation, Middle Permian Qixia Formation and Maokou Formation, Upper Permian Changxing Formation, Lower Triassic Feixianguan Formation and Jialingjiang Formation, and Middle Triassic Leikoupo Formation, forming four gas-bearing provinces in eastern, western, southern and central Sichuan Basin. Well JY1 achieved shale gas breakthrough in the Wufeng Formation-Longmaxi Formation in 2012, confirming the discovery of the Fuling shale gas field [10⇓-12], which was built into the first commercially developed shale gas field in 2014 outside of North America. Henceforth, the basin has witnessed a rapid extension of shale gas exploration, with average annual proven reserves of 2.0-6.5 times that of conventional gas. Seven large gas fields with reserves of over 100 billion cubic meters (Fuling, Changning, Zhaotong, Weirong, Weiyuan, Luzhou and Qijiang) have been discovered successively [13-14]. As of the end of 2022, the cumulative proven natural gas reserves in the marine strata of the Sichuan Basin were 6.29×1012 m3, including 3.33×1012 m3 marine conventional gas and 2.96×1012 m3 unconventional gas, accounting for 53% and 47%, respectively. Over the past 10 years, shale gas production grew from zero to a proportion of 42.8% by 2022, recording a qualitative leap in shale gas exploration and development.
From the perspective of exploration mode and progress, the exploration of marine oil and gas in the Sichuan Basin has broken through the traditional idea of "from source rocks to traps". It has transformed in four aspects: from outer-source to inner-source, from conventional reservoir to unconventional reservoir, from shallow strata to deep strata, and from single trap to continuous geological body. The current exploration pattern is centered on source rocks, with multiple types of reservoirs and multiple types of extensive gas-bearing traps and continuous breakthroughs. Therefore, it is necessary to study the hydrocarbon accumulation laws and main controlling factors from the perspective of the whole petroleum system (WPS) according to the idea of "integrated conventional and unconventional hydrocarbon accumulation" to guide future exploration.
Focusing on the development and distribution of the main source rocks, this paper divides the marine whole petroleum systems in the Sichuan Basin according to the combination relationship of hydrocarbon accumulation elements, and analyzes the hydrocarbon accumulation characteristics of each whole petroleum system. Moreover, this paper summarizes the laws of integrated conventional and unconventional hydrocarbon accumulation, and points out the beneficial directions for future exploration. The results are expected to provide a basis for exploration decision-making and deployment.
1. Division of marine whole petroleum systems and hydrocarbon accumulation in the Sichuan Basin
1.1. Division of marine whole petroleum system
The theory of the whole petroleum system studies the hydrocarbon accumulation elements, processes, and models of all types of reservoirs and resources originated from a set of source rocks, in order to guide petroleum exploration [15⇓⇓-18]. The marine strata in the Sichuan Basin underwent two extensional-convergent tectonic cycles, with three sets of regional high-quality source rocks (Lower Cambrian, Lower Silurian, and Upper Permian) and three sets of regional cap rocks (Middle-Lower Cambrian, Upper Permian, and Middle-Lower Triassic), which form three sets of source-seal combinations vertically to control the hydrocarbon accumulation in multiple reservoirs within the combinations (Fig. 1 ). Through comparative analysis of gas sources, it is found that within each source-seal combination, most gas reservoirs have affinity (Fig. 2 ). Based on the comparison of source-seal combinations and gas reservoir affinity, the marine strata in the Sichuan Basin can be divided into three WPSs: Cambrian, Silurian and Permian (Table 1 ). Each WPS is mainly characterized by independent hydrocarbon accumulation; however, due to the uneven distribution of regional cap rocks and the development of local large faults, natural gas can accumulate cross systems in some areas, forming fault-controlled complex petroleum systems.
Fig. 1. Division of marine whole petroleum systems in the Sichuan Basin. |
Fig. 2. Origin and gas-source correlation of marine natural gas in the Sichuan Basin. |
Table 1. Hydrocarbon accumulation types of marine whole petroleum systems in the Sichuan Basin |
| Name | Type | Source rock | Reservoir rock | Cap rock |
|---|---|---|---|---|
| Permian whole petroleum system | Conventional | Dalong Fm., Longtan- Wujiaping Fms. | Changxing-Feixianguan, Jialingjiang and Leikoupo Fms. | Middle-Lower Triassic regional cap rock |
| Unconventional | Shale in Dalong and Wujiaping Fms., Coal beds in Longtan Fm. | |||
| Silurian whole petroleum system | Conventional | Lower Silurian, Lower Permian | Huanglong Fm., Qixia-Maokou Fms. | Upper Permian regional cap rock |
| Unconventional | Shale in Wufeng-Longmaxi Fms., shale and marl in Maokou Fms. | |||
| Cambrian whole petroleum system | Conventional | Qiongzhusi Fm., Deng-3 Mbr. | Dengying, Canglangpu and Longwangmiao Fm. | Middle-Lower Cambrian regional cap rock |
| Unconventional | Shale in Lower Cambrian Qiongzhusi Fm. | |||
1.2. Cambrian whole petroleum system
1.2.1. Sedimentary filling process and hydrocarbon accumulation elements
The Cambrian whole petroleum system is a set of hydrocarbon accumulation combination with the main source rock in the Cambrian and the regional cap rock in the Middle-Upper Cambrian-Ordovician. It was controlled by a regional extensional-convergent tectonic cycle of the Yangtze Plate from the Neoproterozoic to the Early Paleozoic. During the Late Sinian, the Yangtze Plate as a whole was in a stage of extensional splitting and accompanied by multiple cycles of transgression-regression. Multiple intracontinental rifts (e.g. Mianyang-Changning and Chengkou-Western Hubei) were developed on the northern edge of the plate, forming a platform-shelf differentiation pattern. The platform margin developed thick sediments of mound-shoal facies, while there are siliceous shale, mudstone, and micritic dolomite within continental shelf. There was great controversy over the origin of the Mianyang-Changning rift trough, which was believed to be an "eroded trough" by some scholars [19-20]. With the deepening of exploration research, more and more evidences have shown the presence of deep-water sediments in rift trough, which are contemporaneous with the high-energy facies zones at the platform margin, but with different facies [21]. During the Early Cambrian, extensive black shale sediments were formed within the rift trough under the action of early transgression. In the late stage, the southwestern edge of the Yangtze Plate underwent enhanced tectonic compression, with sufficient supply of terrigenous debris which rapidly filled the rift trough. During the Middle-Late Cambrian, a unified carbonate platform was formed in the Yangtze Plate, which developed a set of carbonate and gypsum-salt sediments. Since the Ordovician, influenced by transgression and tectonic subsidence, the sedimentary water bodies of the Yangtze Plate continued to deepen, receiving a set of limestone, marl and clastic sediments. The Cambrian whole petroleum system began to form paleo-oil reservoirs during the Early Carboniferous, and these oil reservoirs were transformed into gas reservoirs during the Late Triassic and finally adjusted and finalized during the Himalayan period (Fig. 3 ).
Fig. 3. Event diagram of Cambrian whole petroleum system. |
During the platform-shelf differentiation from the Late Sinian to Early Cambrian, a set of continental shelf source rocks and marginal-platform mound-shoal reservoirs was developed. During the Middle-Late Cambrian to Early Ordovician when platforms were unified, a set of grain shoal dolomite karst reservoirs existed around the central Sichuan paleo-uplift. During the Middle-Late Cambrian to Ordovician, thick layers of gypsum- salt rocks and marl with clastic cap rocks were developed.
These constitute the basic hydrocarbon accumulation elements of the Cambrian whole petroleum system (Fig. 4 ).
Fig. 4. Sedimentary filling sequence of hydrocarbon accumulation elements in Cambrian whole petroleum system. |
1.2.2. Hydrocarbon accumulation sequence and controlling factors
The Cambrian WPS presents a hydrocarbon accumulation sequence of continental shelf facies shale gas reservoirs with the shale serving as both source rock and reservoir rock (self-generation and self-storage), marginal platform facies-controlled conventional gas reservoirs with gas generated by shelf facies and stored in marginal facies (shelf-generation and margin-storage), and intra-platform fault- and facies-controlled gas reservoirs with gas stored in grain shoals facies far from the source rock (lower-generation and upper-storage) (Fig. 5 ).
Fig. 5. Distribution pattern of hydrocarbon accumulation sequence of Cambrian whole petroleum system. |
The Cambrian Qiongzhusi Formation shales of deep- water continental shelf facies contain reservoirs with organic pores (diameters of 100-300 nm) as the main storage space. These shales have a relatively high TOC value (2.0%-5.9%) and the gas generation intensity of (80-200)×108 m3/km2. The mechanism and main controlling factors of shale gas accumulation in deep-water continental shelf facies are similar to those in the Silurian, with the only difference being that the former has a higher maturity. It is only in regions with relatively low degree of thermal evolution in the periphery of the early paleouplifts that shales exhibit higher gas content. All wells that have revealed commercial flow of shale gas are distributed around the Huangling, Central Sichuan and Hannan paleo-uplifts. The Cambrian Qiongzhusi Formation silty shales of shallow-water continental shelf facies develop gas reservoir with inorganic pores as the main storage space (Fig. 6 ), where the gas is also classified as a type of unconventional shale gas, since the rock-forming minerals have grain size smaller than 0.062 5 mm. These shales have a low TOC value (less than 0.5%) and the gas generation intensity of (5-10)×108 m3/km2. They serve as both source rock and reservoir rock, with gas extensively accumulated after a near-source micro-migration [22]. The hydrocarbon accumulation process features a hydrocarbon charging into the reservoir space near the source rock before densification.
Fig. 6. Reservoir space characteristics of silty shale in Qiongzhusi Formation of Well JS103. (a) FIB-SEM 3D rebuilt maps, 3 337.04 m, porosity of 3.13%, matrix+pore; (b) FIB-SEM 3D rebuilt maps, 3 337.04 m, porosity of 3.13%, pore; (c) and (d) SEM images, 3 350.00 m. |
The source rocks of continental shelf facies and the marginal-platform in the Sinian Dengying Formation can form a good source-reservoir assemblage. Early studies believed that the source rock for the marginal-platform gas reservoirs was the Lower Cambrian Qiongzhusi Formation, but a comparison of the natural gas geochemical characteristics revealed significant differences of gas sources between the Sinian Dengying Formation at the platform margin and the Cambrian Longwangmiao Formation within the platform [23]. It is speculated that deep-water shale deposits are developed in the Dengying Formation within the Mianyang-Changning rift. The Dengying Formation exhibits a similar reservoir-forming model with hydrocarbon generated in continental shelf and stored in platform margin to the Upper Permian Changxing Formation, with the main controlling factor also being the favorable source-reservoir assemblage.
The Cambrian-Ordovician intra-platform shoal dolomite reservoirs are widely distributed along the Central Sichuan paleouplift and have good reservoir properties owing to the prolonged karstification during the Caledonian period. However, their formation is controlled jointly by faults and high-energy facies belts, since they are far away from the source rocks. Multiple periods (Tongwan, Caledonian, Hercynian, and Himalayan) of strike-slip faults are developed in central Sichuan Basin. These faults connect the Lower Cambrian source rocks with the intra-platform shoal reservoirs, forming fault-controlled lithological gas reservoirs, such as the Longwangmiao Formation gas reservoir in Moxi. The wells closer to the source-connecting faults exhibit distinctly high production [24].
1.3. Silurian whole petroleum system
1.3.1. Sedimentary filling process and hydrocarbon accumulation elements
The Silurian whole petroleum system consists of the Lower Silurian source rock and the Middle-Lower Permian regional seal. It was formed under the control of the tectonic compression at the end of the Early Paleozoic within the Yangtze Plate and the subsequent extension. During the Late Ordovician to Early Silurian, the Yangtze Plate was under a tectonic setting of compression in the south and extension in the north, and enclosed by three paleo-uplifts (Xuefengshan to the east, Central Guizhou to the south and Central Sichuan to the west). This resulted in a semi-enclosed large cratonic depression in the southeastern-eastern Sichuan region, with a set of deep-water continental shelf shale deposited in the early stage and a set of silty mudstone and siltstone deposited in the late stage [25]. During the Late Silurian to Devonian, the Sichuan Basin was generally in a state of uplift and erosion. During the Late Carboniferous, with the intensifying extension at the periphery of the Yangtze Plate, the eastern Sichuan region experienced intermittent transgression, depositing a set of carbonate rocks in tidal flat facies. During the Early-Middle Permian, the Yangtze Plate experienced a large-scale transgression, and the Sichuan Basin developed a set of limestone and marl deposits dominated by open platform facies. The Silurian WPS began to accumulate oil since the Middle Permian, and became a gas system cracked from crude oil during the Early Jurassic to Late Cretaceous. It was finalized into a gas reservoir after adjustment during the Himalayan (Fig. 7 ).
Fig. 7. Event diagram of Silurian whole petroleum system. |
During the Early Silurian, high-quality source rocks of deep-water continental shelf facies were formed in the stagnant environment within the cratonic depression. During the Late Carboniferous, unconformity-related karst dolomite reservoirs were formed within the restricted platform of the craton. During the Early-Middle Permian, shale and marl cap rocks were formed in the open platform within the craton (Fig. 8 ). They constituted the basic hydrocarbon accumulation elements of the Silurian whole petroleum system.
Fig. 8. Sedimentary filling sequence of hydrocarbon accumulation elements in Silurian whole petroleum system. |
1.3.2. Hydrocarbon accumulation sequence and controlling factors
The Silurian whole petroleum system mainly contains three types of natural gas reservoirs: the shale gas reservoir with self-generation and self-storage, the near-source and intra-source tight sandstone gas reservoir, and the fault- and facies-controlled gas reservoir with lower-generation and upper-storage (Fig. 9 ).
Fig. 9. Distribution pattern of hydrocarbon accumulation sequence of Silurian whole petroleum system. |
The accumulation of the shale gas featured by "self-generation and self-storage" in the Ordovician Wufeng Formation to the Silurian Longmaxi Formation follows a two-factor enrichment pattern, that is, development of high-quality deep-water continental shelf shale is the basis for generation and storage of shale gas, and good preservation condition is critical for accumulation and production of shale gas [26]. The shale of deep-water continental shelf facies is silicon-rich with a high carbon content, and the high-quality shale in the Wufeng to Longmaxi formations (TOC values greater than 2.0%) has a large thickness (usually 20-50 m). In deep water environment, siliceous organisms thrived, leading to abundant siliceous sponge spicules and radiolarian fossils. The abundant siliceous organisms provided a good material basis for organic matter enrichment and improved the reservoir properties and brittleness of the shale. For the high-quality shale interval of the Longmaxi Formation in Well JY1, the measured TOC ranges from 1.06% to 6.28% (averaging 3.50%), and the porosity ranges from 2.78% to 7.08% (averaging 4.80%). Due to the support of siliceous minerals, the porosity and gas-bearing properties of the shale do not decrease with the depth. A series of shale gas wells with depths exceeding 4 000 m have achieved high production. For example, the shale in Well QYS1 with a burial depth of nearly 5 000 m has an average porosity of 7.0% (Fig. 10 ) and a high total gas content (averaging 8.17 m3/t); the shale in Well DYS2 with a burial depth of 4 200 m has an average porosity of 7.38% and a total gas content of 6.69 m3/t. These findings indicate that deep shale has the characteristic of "overpressure controlling enrichment" [27], which demonstrates that the quality of the original source rock is the primary factor controlling the enrichment of shale gas. Therefore, the reservoir with self-generation and self-storage characteristics has a natural advantage of resources, as long as there are good preservation conditions and shale generally contains high-pressure gas.
Fig. 10. Images of organic pores in Ordovician Wufeng- Longmaxi shales of Well QYS1. |
The Silurian Longmaxi Formation and Xiaoheba Formation sandstone reservoirs and Shiniulan Formation limestone reservoirs are generally low-energy deposits with low original porosity, large later burial depth and high densification degree. Although they are near the source rock, it is difficult to form large-scale reservoirs. Natural gas enrichment in these reservoirs is mainly controlled by late faults and fractures. Thus, the reservoirs are mainly developed in the core of anticline structure and near faults. For instance, in Well PQ2 in eastern Sichuan Basin, the porosity of siltstone reservoir in the Xiaoheba Formation is less than 2%, and the gas-producing layer is controlled by fracture development degree.
The Middle Carboniferous Huanglong Formation in eastern Sichuan Basin is a set of large-scale unconformity karst reservoir, far away from the Silurian high-quality source rock. Its hydrocarbon accumulation was mainly controlled by Indosinian paleo-uplifts and Late Yanshanian-Himalayan structural deformation. Source-connecting faults played an important role in the adjustment of ancient oil and gas reservoirs. During the Indosinian, under the control of Kaijiang paleo-uplift in eastern Sichuan Basin, large paleo-oil reservoirs were formed in the Carboniferous. Along with the burial heating, the paleo-oil reservoirs were transformed into paleo-gas reservoirs during the Early Yanshanian. During the Late Yanshanian-Himalayan, due to severe tectonic deformation in eastern Sichuan Basin, the Carboniferous paleo-gas reservoirs were fragmented. Specifically, the gas reservoirs in the faulted high-steep anticlines were generally destroyed, while the stratigraphic-structural gas reservoirs in the low buried anticlines and synclines were preserved [28].
1.4. Permian whole petroleum system
1.4.1. Sedimentary filling process and hydrocarbon accumulation elements
The Permian whole petroleum system comprises the Middle-Upper Permian source rock and the Middle-Lower Triassic as regional seal. Its formation was controlled by the regional extension-convergence tectonic cycle of the Yangtze Plate during the Late Paleozoic. During the Early-Middle Permian, the Yangtze Plate as a whole was in a stage of extension and splitting, accompanied by multiple cycles of transgression and regression [5]. The shoreland-tidal flat environment during the Early Permian gradually changed into the carbonate platform sedimentary environment during the Middle Permian [29]. During the Middle Permian, marginal-platform shoals and intra-platform shoals were universally developed, and high-quality carbonate reservoirs of shoal facies could be formed in the rift shoulder at platform margin and the high part of local micro-amplitude structures within the platform [30-31]. In the faulted depression, argillaceous limestone and deep-water siliceous rock of slope-shelf facies were developed. During the Late Permian, strong tension formed a passive continental margin at the northern margin of the Yangtze Plate, further expanding the faulted depression within the plate, and forming a "platform-shelf" sedimentary differentiation pattern in the basin. The carbonaceous siliceous shales of deep-water shelf facies deposited in the Kaijiang-Liangping rift in the north of the basin are a set of high-quality source rocks. In the Deyang-Wusheng platform subsag in the south, the tension was relatively weak, and muddy shale, coal seam and siltstone were deposited in tidal flat-shoreland environment. Marginal-platform reef and shoal deposits were developed along both sides of rift and platform subsag. During the Early Triassic, as the tension around the Yangtze Plate weakened, the early intraplate rift was gradually filled and leveled up, forming a stable intra-cratonic depression basin with thick gypsum salt rocks and carbonate rocks. In the early stage of the Middle Triassic, with the intensifying Indosinian Movement, terrigenous clastic deposits began to appear on the southern and eastern margins of the Sichuan Basin [32]. Till the Late Triassic, with the closing of the Paleotethys and the formation of the Longmenshan orogenic belt, the marine sedimentation of the Sichuan Basin ended and the evolution of the continental sedimentary basin started. Later, the paleo-oil reservoirs began to form since the Middle Jurassic, and the oil reservoirs were transformed into gas reservoirs as the result of phase change during the Early Cretaceous. The gas reservoirs were finalized after adjustment during the Himalayan (Fig. 11 ).
Fig. 11. Event diagram of Permian whole petroleum system. |
The sedimentary filling and evolution of the basin control the hydrocarbon accumulation elements of the Permian WPS. During the strong tectonic differentiation period in the Middle-Late Permian, large reef-bank reservoirs and shelf high-quality source rocks were formed in the platform-edge zones. During the pan platform period in the Early-Middle Triassic, intra-platform shoal reservoirs and gypsum salt cap rocks were formed, constituting multiple types of source-reservoir-cap rock assemblages (Fig. 12 ).
Fig. 12. Sedimentary filling sequence of hydrocarbon accumulation elements in Permian whole petroleum system. |
1.4.2. Hydrocarbon accumulation sequence and controlling factors
The Permian whole petroleum system mainly develops three types of natural gas reservoirs: the shale gas reservoir with self-generation and self-storage, the conventional reservoir with shelf-generation and margin-storage, and the fault- and facies-controlled gas reservoir with lower-generation and upper-storage (Fig. 13 ).
Fig. 13. Distribution pattern of hydrocarbon accumulation sequence of Permian whole petroleum system. |
The accumulation and enrichment of shale gas in the reservoirs with self-generation and self-storage in the Gufeng Member of the Maokou Formation/Wujiaping Formation/Dalong Formation were mainly controlled by the shelf sedimentary environment and later structural modification. The sedimentary environment controlled the types, abundance, and quantity of organic matter, and the closed to semi-closed low-energy environment in deep-water shelf is conducive to the formation of high-abundance source rocks and siliceous rocks. All three sets of shale have the characteristics of high TOC, high silicon content, high porosity, and low clay content. Taking Well HY1HF in the Shizhu synclinorium as an example, the Wujiaping Formation shale has average TOC of 8.0%, average silicon content of 44.6%, average porosity of 5.4%, average gas content of 3.4 m3/t, and average clay mineral content of 21.0%; the Gufeng Member of the Maokou Formation has average TOC of 14.5%, average silicon content of 40.4%, average porosity of 4.9%, average gas content of 3.1 m3/t, and average clay mineral content of 11.6%. In Well LY1 in the Puguang region, the Dalong Formation shale has average measured TOC of 6.7%, silicon content of 39.6%, porosity of 3.5%, gas content of 6.0 m3/t, and clay mineral content of 4.2%. The three sets of deep-water continental shelf shales in the Permian are mainly composed of organic pores, with intragranular dissolution pores, particle edge pores, and microfractures (Fig. 14 ). Micropores and mesopores are dominant, with larger pore specific surface area and pore volume, providing a good storage space for shale gas enrichment. The style and degree of tectonic transformation determine the final enrichment degree of shale gas. The high-steep anticline zone in the Puguang region was formed late and only uplifted slightly, with faults not cutting through the gypsum salt caprock. Therefore, its overall preservation conditions are good. Well LY1, where the pressure coefficient of the Permian Dalong Formation shale gas reservoir is 1.97, obtained a high production gas flow (42.66×104 m3/d) in testing. The Shizhu synclinorium structure was formed early, uplifted greatly, and adjacent to the Qiyueshan Fault, which affected the preservation of shale gas. The shale gas reservoir of the Wujiaping Formation in Hongxing region exists in a system of normal pressure-weak overpressure, with a pressure coefficient of 1.15-1.30.
Fig. 14. Images of Permian shale reservoir spaces in wells LY1 and HY1. |
The formation of conventional reservoirs with shelf-generation and margin-storage in the Changxing-Feixianguan formations was controlled by the lateral connection between high-quality reservoir rocks of reef-bank facies and source rocks of continental shelf facies, with the characteristics of near-source enrichment, phase transformation and dynamic adjustment. The high-energy facies zones of the platform margin formed by the large-scale uplift-sag pattern are conducive to early dolomitization and exposed dissolution, controlling the distribution of large-scale high-quality reservoirs. Later fracture transformation is beneficial for improving permeability, and early hydrocarbon charging is also favorable for reservoir pressure maintenance and improvement. In the Yuanba gas field, with the buried depth of 6 000-6 500 m, crude oil was cracked to gas during the Late Jurassic-Early Cretaceous, leading to an ancient fluid pressure of 142 MPa (pressure coefficient greater than 2.0). A large number of overpressure fractures were formed to improve the heterogeneity and overall permeability of the reservoirs [33].
The intra-platform gas reservoirs with lower-generation and upper-storage were formed under the control of source-connecting faults. The intra-platform shoals of the Middle-Lower Triassic are widely distributed, but far from the source rocks, so hydrocarbon accumulation relies on a good transport system. However, the Sichuan Basin witnessed a large-scale fault development in the Late Yanshanian and Himalayan periods, which are later than the oil generation period of the Permian source rocks. Therefore, fault-controlled gas reservoirs usually did not evolve as paleo-oil reservoirs. In the Hebachang gas field in northeastern Sichuan Basin, for example, the structural traps and source-connecting faults were formed during the late Yanshanian, later than the extensive oil generation period of the Permian source rocks, and in a good consistency with the kerogen cracking period, so dry gas reservoirs were directly formed [34].
1.5. Fault-controlled compound petroleum system
Under the control of the Tethys and Pacific tectonic domains, the Sichuan Basin underwent multiple periods and directions of tension and compression [35-36]. During the multi-period tectonic evolution, basin-margin thrust strike-slip faults and large intra-basin basement strike- slip faults were formed. In some areas, large faults penetrated multiple sets of petroleum systems, forming fault- controlled compound petroleum systems (Figs. 15 and 16 ).
Fig. 15. Plane distribution of multiple periods of faults in the Sichuan Basin. |
Fig. 16. Cross-system accumulation pattern of typical fault-controlled oil and gas reservoirs in Sichuan Basin (see the section location in |
NE-trending thrust-strike-slip faults are developed in the piedmont zone of Longmen Mountain, western Sichuan. The piedmont nappe zone is dominated by thrust faults, and the piedmont depression zone is dominated by high-steep strike-slip faults. According to the cutting patterns of faults and the deformed layers of fault-related folds, the NE-trending faults in western Sichuan Basin are believed to be initially formed in the Indosinian [37-38]. The thrust-strike-slip faults penetrate upwards to directly connect the Cambrian Qiongzhusi Formation and Permian Longtan Formation source rocks with the overlying Permian and Triassic dolomite reservoirs vertically, which is conducive to the cross-strata migration of natural gas, thus forming a fault-controlled complex petroleum system with lower-generation and upper-storage (Fig. 16a )
Several groups of large strike-slip faults trending in SN, EW and NW directions are developed in central and eastern Sichuan regions, and were active in multiple periods in successive stages during the Late Caledonian, Hercynian and Indosinian [36], creating favorable channels for vertical migration of natural gas upwards from deep layers. The basement strike-slip faults in central Sichuan can effectively connect the Cambrian and Silurian source rocks with the Permian reservoirs to form hydrocarbon pool across petroleum systems (Fig. 16b ). In addition to connecting source rocks, some drilled wells revealed that the migration of deep hydrothermal fluids along the strike-slip faults can effectively improve the carbonate reservoirs of the Middle Permian Maokou Formation by enhancing the porosity and permeability, and control the distribution of high-quality reservoirs of the Maokou Formation [39]. The high-yield gas wells in the Middle Permian Maokou Formation are mainly distributed along NW-trending strike-slip fault zones of the Hercynian [40].
In the Micang Mountain-Daba Mountain piedmont zones in northern Sichuan, there are two groups of thrust-strike-slip faults (NE and NW), which were mainly formed during the Yanshanian-Himalayan periods. The thrust-strike-slip faults in continental strata are controlled by the gypsum salt detachment layer and smaller in scale. They mainly cut from the Middle Triassic to the Lower Jurassic, and connect the coal-measure source rocks and sandstone reservoirs of the Xujiahe Formation, forming near-source fault-controlled oil and gas reservoirs. The marine faults are large and cut the Upper Permian to the Middle Jurassic, thus they can effectively connect the Upper Permian Dalong Formation and Wujiaping Formation source rocks and Xujiahe Formation reservoirs, allowing the natural gas generated from the cracking of the underlying source rocks to efficiently transport to the Xujiahe Formation, where distal fault- controlled oil and gas reservoirs are finally formed (Fig. 16c ). Two types of faults are developed in the Xujiahe Formation gas field in the Tongnanba region, with the characteristics of dual-source hydrocarbon supply, three- dimensional hydrocarbon transportation, and fractured reservoirs controlling production [37]. High-yield wells are located within 300 m from faults, such as Well Ma6 (with a production capacity of 36.67×104 m3/d) and Well Ma8 (with a production capacity of 20.63×104 m3/d).
2. Accumulation and enrichment rules and main control factors of marine whole petroleum systems
Compared with continental monocycle basins, the marine multicycle basins represented by the Sichuan Basin have the WPSs with orderly coexistence of multi-type gas reservoirs horizontally and multicycle superimposition vertically. Around the hydrocarbon generation centers of continental basins, there are usually shale oil and gas reservoirs, tight gas reservoirs and conventional gas reservoirs in coexistence. In contrast, the gas reservoirs in the same petroleum system in a marine basin are characterized by self-generated and self-stored high-pressure shale gas reservoirs, platform-edge reef flat near-source normal-pressure gas reservoirs, and intra-platform shoal lower-generated and upper-stored high-pressure gas reservoirs. Moreover, since the marine superimposed basins in China are geologically old and have experienced many tectonic evolution stages, they have generally experienced a dynamic hydrocarbon accumulation process with multi-source hydrocarbon generation, oil-gas transformation, and late adjustment. Overall, the formation and distribution of large gas fields in the Sichuan Basin are jointly controlled by high-quality source rocks, large-scale reservoirs and good preservation conditions.
2.1. High-quality source rock is fundamental for large gas fields
Controlled by two stages of intracontinental extension during the Late Sinian-Early Cambrian and Middle-Late Permian, and intracontinental compression during the Late Ordovician-Early Silurian, the Sichuan Basin developed three sets of deep-water continental shelf source rocks in the Lower Cambrian, Lower Silurian and Upper Permian. The three sets of source rocks have high organic matter abundance, good organic matter type, high hydrocarbon generation intensity and wide distribution [27,41⇓ -43]. This provides a good material basis for extensive conventional and unconventional oil and gas accumulation, and also controls the distribution of large gas fields. The discovered medium- to large-sized gas fields are all located near the hydrocarbon generation centers (Fig. 17 ). Three large gas fields (Puguang, Yuanba and Luojiazhai) have been proved around the Permian hydrocarbon generation center, with proved gas reserves of 8 234×108 m3. Eight large gas fields, Fuling, Luzhou, Weiyuan, Changning, Zhaotong, Qijiang, Weirong and Dachigan, have been proved around the Silurian hydrocarbon generation center, with proved gas reserves of 30 415×108 m3. Two large gas fields, Anyue and Penglai, have been proved around the Cambrian hydrocarbon generation center, with proved gas reserves of 16 270×108 m3.
Fig. 17. Distribution of three marine hydrocarbon generation centers and gas reservoirs. |
2.2. Large-scale reservoirs are the guarantee of near-source and intra-source gas enrichment
For conventional gas reservoirs, high-quality carbonate reservoirs are the main targets for gas exploration. During the marine sedimentary evolution of 4×108 years, the Sichuan Basin developed several sets of carbonate reservoirs, covering almost all known types of carbonate reservoirs [9,44 -45]. Widely-distributed carbonate reservoir rocks in the basin include algal dolomite of the Sinian Dengying Formation, granular dolomite of the Cambrian Longwangmiao Formation and Canglangpu Formation, dolomitic karst breccia and granular dolomite of the Carboniferous Huanglong Formation, dolomite and karst limestone of the Middle Permian Qixia-Maokou formations, bioclastic dolomite of the Upper Permian Changxing Formation-Lower Triassic Feixianguan Formation, granular dolomite of the Lower Triassic Jialingjiang Formation, and algal dolomite of the Middle Triassic Leikoupo Formation. The reservoirs are mainly distributed in bands or plane style along the paleo-uplift, platform-margin and intra-platform rift margin belts, and are controlled by high-energy sedimentary facies belt, early karstification and early dolomitization. The marine strata in the Sichuan Basin are geologically old, deeply buried, and diagenetically complex, making it impossible to maintain the storage space of high-quality reservoirs. Therefore, hydrocarbon charging in the early diagenetic stage had an important impact on reservoir maintenance. Most of the proven reservoirs of large gas fields in the Sichuan Basin are dolomites in reef flat facies. In addition to good original physical properties, these reservoirs also have advantages of near-source hydrocarbon accumulation. The early hydrocarbon charging inhibited the occurrence of destructive diagenesis, thus keeping the pores well-preserved [46] for extensive enrichment of natural gas.
For unconventional reservoirs, deep-water continental shelf shale acts as high-quality source rock and also high-quality reservoir rock [47]. As silicon in seawater is an essential nutrient element for marine organisms, its content directly affects the primary productivity. Therefore, siliceous matter and organic matter in deep-water continental shelf marine shale coexist with the same source. High silicon content and carbon content are common characteristics of high-quality source rocks, which are shared by marine shales of Lower Cambrian, Lower Silurian and Upper Permian [26,48]. Biogenic silicon in shale is conducive to reservoir development and maintenance. Biogenic amorphous SiO2 spheres are tightly packed, and the pores between spheres are filled with silica, which is easy to dissolve during diagenesis, possibly resulting in certain micro-pores [49⇓-51]. In addition, primary pores are developed in siliceous organisms and have better connectivity. For example, after the organic matters filled in siliceous organisms such as radiolarians and sponge spicules are decomposed, although they will be partially or completely filled again by secondary quartz or organic particles, they can still be partially or even fully opened, forming effective shale gas reservoir spaces. During diagenesis, biogenic siliceous shale experienced mechanical compaction and chemical compaction simultaneously during the early diagenetic stage, allowing amorphous opal-A to transform into rigid crystalline quartz. While the shale porosity decreased, a rigid framework composed of quartz particles was formed, increasing the hardness of siliceous shale, improving the support and anti-compaction capabilities, which is conducive to the preservation of various pores in shale [52]. In summary, high-silicon and high-carbon source rocks are also high-porosity shale reservoirs, which are the guarantee for shale gas enrichment.
2.3. Good preservation conditions are the key to natural gas accumulation
Before the Yanshan Movement, the Sichuan Basin experienced multiple tectonic movements, which were dominantly uplift and subsidence movements, with no extensive folding occurred. The long-lasting sedimentation and subsidence effectively promoted and maintained the early hydrocarbon accumulation in marine strata. Before the Late Jurassic, most of the marine source rocks had entered the peak period of hydrocarbon generation. In the large-scale reservoirs adjacent to the source rocks, extensive oil charging occurred to form the paleo-oil reservoirs, and most of the paleo-oil reservoirs transformed to paleo-gas reservoirs from crude oil cracking [53⇓-55]. After the Yanshan Movement, with the gradual intensification of peripheral orogeny, folding deformation and intense uplift began to occur in the periphery and interior of the Sichuan Basin. Apatite fission track low-temperature thermal chronological data show that after the Late Cretaceous, the Sichuan Basin experienced two periods of extensive uplift denudation, with the maximum denuded thickness of 4 000 m [46]. Under the influence of large-scale folding deformation and tectonic uplift, marine oil and gas reservoirs have generally undergone structural adjustment. In the structurally stable areas which were weakly reworked, the gas reservoirs maintain the original structural appearance, but become further enriched toward the high part of the structure, such as the Anyue gas field in central Sichuan Basin. In areas with severe structural deformation, the original gas reservoirs are fractured to form new structural gas reservoirs, such as the Carboniferous gas fields in eastern Sichuan Basin. In areas with more severe structural reworking at the basin margin and outside the basin, conventional gas reservoirs have been completely destroyed, and only areas with relatively stable structures have preserved shale gas reservoirs, such as Jiaoshiba and Pengshui gas reservoirs. The roof and floor of Silurian shale gas reservoirs within the basin are characterized by tight lithology, large thickness, low porosity and low permeability, which is conducive to the preservation of shale gas. Since the crystalline basement of the Sichuan Basin is stable, and the surrounding orogenic belts do not form extensive fold and thrust belts, most of the marine strata are intact in the basin. Coupling with the dual-protection by the Middle-Lower Triassic gypsum salt cap rock and the Upper Triassic-Jurassic continental cap rock, marine oil and gas reservoirs are well preserved, which is critical to the progress of natural gas exploration in the basin.
3. Exploration targets of marine oil and gas
3.1. Reef flat facies zones
The reef flat facies zones at large platform edges are the key exploration region of large-scale natural gas fields. At the edges of the Kaijiang-Liangping continental shelf and Mianyang-Changning rift in the Sichuan Basin, the proven conventional gas reserves are 2.3×1012 m3, accounting for 91% of the proved marine conventional gas reserves. Recently, new exploration progresses have been made in the platform-margin zones on the east side of the Kaijiang-Liangping continental shelf and the west side of the Mianyang-Changning rift, which suggests a large exploration potential in the platform-margin zones in deep layers and complex structural zones. As more research insights are obtained, the high-energy grain shoal facies zone of the Qixia Formation in western Sichuan Basin and the high-energy grain shoal facies zone of the second member of the Maokou Formation running transversely in central Sichuan Basin have been confirmed, with high-yield oil and gas flows with a daily output of more than one million cubic meters, becoming a new direction for increasing reserves.
3.2. Deep shale gas in Silurian Longmaxi Formation
Deep shale gas is an important domain for increasing reserves and production in the future. The area of the Lower Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding bottom boundary with burial depth less than 3 500 m is about 6.3×104 km2, and the area with burial depth greater than 3 500 m is about 12.8× 104 km2. The area of the deep layers is about twice that of the middle and shallow layers [56]. The deep layers have rich shale gas resources. The favorable exploration area of deep Silurian shale gas (at buried depth of 4 000-5 000 m) in the Sichuan Basin within the Sinopec mining right area is 1 502 km2, corresponding to the geological resources of 1.27×1012 m3; the favorable exploration area of ultra-deep shale gas (at buried depth of 5 000-6 000 m) is 3 687 km2, corresponding to the geological resources of 3.4×1012 m3. Both are important domains for increasing reserves and production of shale gas in the Sichuan Basin. Several deep wells with a vertical depth of more than 3 500 m, such as DY2HF, WY1HF and YY1HF, have revealed high initial production, which confirms the good exploration prospect of deep shale gas in the basin.
3.3. Large lithological complexes related to source-connecting faults
The latest research shows that the Sichuan Basin develops multi-phase and multi-directional basement strike- slip faults [36]. These faults connect the source rocks and the high-energy facies zone in the platform, forming overpressure lithologic gas reservoirs with local high yield. For instance, several wells have obtained high-yield gas flows in the Cambrian Longwangmiao Formation and Permian Maokou Formation in central Sichuan Basin [57-58]. The intra-platform grain shoal reservoirs are widely distributed in the Cambrian, Ordovician, Permian and Triassic of the Sichuan Basin, with large exploration space. Therefore, the large lithological complexes related to source-connecting faults may be an important domain for increasing large-scale reserves in the future.
3.4. Other types of unconventional gas
In addition to shale gas reservoirs of deep-water continental shelf facies, near-source and intra-source unconventional oil and gas reservoirs of shallow-water continental shelf facies and transitional facies are worth exploring. The Cambrian shale gas reservoirs of shallow-water continental shelf facies, Silurian tight sandstone gas reservoirs, Permian Maokou Formation tight marl gas reservoirs, and Longtan Formation coalbed gas reservoirs, etc. have the characteristics of hydrocarbon accumulation in large area and with low abundance. Exploration of these reservoirs has made certain breakthroughs in some areas [59-60], but large-scale development has not yet begun. The hydrocarbon accumulation mechanism and exploration potential of these gas reservoirs need to be further studied. Moreover, the economic development mode of these tight unconventional gas reservoirs with low abundance needs further discussion.
4. Conclusions
Three sets of regional source rocks and three sets of regional cap rocks are developed in the marine strata of the Sichuan Basin. Three whole petroleum systems Cambrian, Silurian and Permian, are recognized and are featured by isolated hydrocarbon accumulation, and the natural gas in the systems has affinity. In a single whole petroleum system, the general accumulation sequence includes shale gas reservoirs of continental shelf facies, facies-controlled gas reservoirs of marginal-platform facies, and intra-platform fault- and facies-controlled gas reservoirs, which is characterized by 3D hydrocarbon accumulation and orderly distribution. Conventional and unconventional oil and gas reservoirs follow the hydrocarbon accumulation pattern with multi-source hydrocarbon generation, oil-gas transformation and dynamic adjustment. High-quality source rock is fundamental for the formation of large gas fields. The natural gas in the systems is enriched near and inside the source. Development and maintenance of large-scale reservoirs are the guarantee of gas enrichment. Good preservation conditions are the key to natural gas accumulation. The main targets of future exploration are large-scale marginal-platform reef-bank facies zone, deep shale gas, and large-scale lithological complexes related to source-connecting faults.