Geological Characteristics of Deep Shale Gas in the Silurian Longmaxi Formation in the Southern Sichuan Basin, China

Abstract

To investigate the geological characteristics and exploration potential of shale gas in the southern Sichuan Basin, we analyze the coupling relationship between the hydrocarbon generation and storage conditions of the Longmaxi Formation and discuss the preservation conditions from the lateral and vertical migration mechanisms of shale gas. According to the results, the organic-rich shale at the bottom was formed in a strongly euxinic and anoxic reducing environment, the oxygen content increased in the upper water body in which the Longmaxi Formation was deposited, and the water body became oxidized. The organic matter type in the shale is dominantly type I kerogen and a small amount of type II1 kerogen. The organic matter content is more than 3.0% and is in the high-to postmature stage. The enrichment of siliceous organisms increases the organic matter and enhances the brittleness of shale, resulting in “superior hydrocarbons and a favorable reservoir”. Pyrolysis of organic matter promotes the formation of organic matter pores and dissolution pores, resulting in a coupled “source-reservoir” accumulation control system. The high vertical formation pressure guarantees the sealing of shale and restrains the lateral escape of shale gas. The high-angle intersection of the highly filled fractures and the current crustal stress can effectively enhance the fracture sealing and inhibit the vertical escape of shale gas, forming a three-dimensional effective closure. Hence, the area featuring a short tectonic uplift time, small amplitudes, large-scale underdeveloped fractures, and a high formation pressure coefficient is a favorable area for shale gas exploration, according to the analysis of three-dimensional preservation conditions.

Densification mechanism of deep low-permeability sandstone reservoir in deltaic depositional setting and its implications for resource development: A case study of the Paleogene reservoirs in Gaoshangpu area of Nanpu sag, China

“…After strong mechanical compaction and calcareous cementation above 3000 m, a few intergranular pores with weak siliceous cementation and dissolution remain. Dissolution mostly occurs in feldspar and lithic minerals along fractures or bedding (Li et al, 2019b;Fan et al, 2020a;He et al, 2020;He et al, 2021;He et al, 2022a;Li et al, 2022a;Li et al, 2022b;Fan et al, 2022).…”

Natural Fracture Development Characteristics and Their Relationship with Gas Contents─A Case Study of Wufeng–Longmaxi Formation in Luzhou Area, Southern Sichuan Basin, China

“…Sichuan Basin located in the northwestern region of the Upper Yangtze Craton is a large-scale superimposed basin developed from sedimentation on the craton and also one of the most important gas-producing basins in China. , Surrounded by Qiyue Mountain, Daba Mountain, Longmen Mountain, and Dalou Mountain, its tectonic pattern is characterized by uplift in the middle, depression in the south, and intensive folds in the east and west (Figure a,b). ,, The 203 well area of the Luzhou block is located in Luzhou city, Sichuan Province, and structurally located in the south of the Yongchuan broom-like tectonic belt, which is the junction areas of the paleo-uplift-middle-slope gentle belt in central Sichuan, the paleo-middle-slope low-fold belt in southwestern Sichuan, and the ancient depression-middle-uplift low-steep belt in southern Sichuan, China . It is composed of some areas of Yundingchang structure, Gufoshan structure, Tiziya structure, tide structure, Longdongping structure, and Xindianzi structure.…”

“…The gas-bearing property of shale is controlled by multiple factors, and a synergistic relationship can be observed among fracture development, organic matter content, and brittleness indices. On the one hand, a large amount of biogenic silicon provided by the sedimentation after the death of low-grade siliceous organisms increases the brittleness of the reservoir and facilitates the development of fractures in the later tectonic movement . Based on the actual shale gas exploration in the Sichuan Basin, the concentrated development member of fractures also witnesses high shale gas-bearing properties, with high TOC contents. , Moreover, the brittle mineral content, fracture density, and gas-bearing data at different depths in Wells L205 and L208 were statistically analyzed to study the relationship between the degree of fracture development and the gas-bearing property of shale in the Lu203 well area.…”

“…27−29 The 203 well area in the Luzhou block is located at a complex tectonic environment in the Yongchuan broom-like tectonic belt of the southern Sichuan Basin. The Wufeng−Longmaxi Formation shale is buried mostly below 3500 m. 30 Specifically, the sedimentary environment of shale of the Wufeng−Longmaxi Formation (O 3 w-S 1 l), TOC content, thermal evolution degree, plane distribution, and paleo-tectonic stress field were analyzed in the preliminary study, proving that there are abundant shale gas resources in the Wufeng−Longmaxi Formation of the Luzhou block, presenting a great exploration prospect. 31 The formed shale deposition has a complex tectonic environment as it was affected by multistage tectonic movements such as the late Indosinian movement, the late Yanshan movement, and the…”

Pore structure and fractal characteristics of the marine shale of the longmaxi formation in the changning area, Southern Sichuan Basin, China

“…Currently, three national shale gas demonstration areas, Fuling, Weiyuan-Changning, and Zhaotong, have been built for the shale gas of the Wufeng-Longmaxi Formation in the Sichuan Basin. These areas mentioned above mainly contain mediumshallow shale gas reservoirs with a buried depth of less than 3,500 m. Organic rich shale generally has the characteristics of low porosity and permeability (porosity less than 8% and permeability less than 10 nD) and mainly develops micron to nanometre pores (He et al, 2021(He et al, , 2022aLi et al, 2022aLi et al, , 2022bFan et al, 2022). The pore structure is complex, and the specific surface area is large.…”