Abstract
Sichuan Basin is the only successful basin for shale gas exploration in China. In addition to the main shale in the Lower Silurian Longmaxi formation, the lower Cambrian Qiongzhusi shale is an important potential formation. However, it was once considered that shale gas is difficult to enrich because of its poor sealing conditions and hydrocarbon migration to adjacent reservoirs. With the increasing research on hydrocarbon generation and reservoir in shale of Qiongzhusi Formation in recent years, it has become an important exploration target in Sichuan Basin. The enrichment of oil and gas is closely related to fluid activities. Limited by the degree of exploration, there is little analysis of fluid activities in Qiongzhusi Formation, and there is little analysis of shale gas enrichment potential from the perspective of fluid. The hydrocarbon generated from Qiongzhusi shale in the rift could migrate laterally to the uplift area and form a reservoir in Dengying Formation. The fluid activities from source rock to reservoir are basically the same. Therefore, this paper reconstructed the history of hydrocarbon activities in Dengying reservoirs based on fluid inclusion analysis. Then the fluid activity process in Qiongzhusi shale was studied, and its enrichment conditions of shale gas was discussed. The results show that the hydrocarbon activities of Dengying Formation can be divided into three stages: 1) oil charging stage, 2) oil cracking gas generation stage and 3) gas reservoir adjustment stage. The first stage is under normal pressure, and the second and third stages developed overpressure with pressure coefficients of 1.3 and 1.2, respectively. High pressure coefficient is an important indicator of shale gas enrichment. Because the source rock of Qiongzhusi Formation has always been the main source rock of Dengying Formation, it can supply hydrocarbon to Dengying Formation only with overpressure in gas generation stage. Therefore, overpressure in the last two stages of gas generation indeed existed. As long as the sealing condition of shale itself is not particularly poor, shale gas “sweet points” would be formed. Therefore, the thick shale in Deyang-Anyue rift is the focus of shale gas exploration in Qiongzhusi Formation.
Highlights
Shale gas is a kind of unconventional natural gas and has been successfully developed commercially abroad
The cavities are usually filled with bitumen and minerals, such as recrystallized dolomite, FIGURE 3 | Different kinds of fluid inclusions in quartz filled in the cavity of dolomite in Dengying formation. (A) A perfect crystal of quartz; Raman spectra respectively show (B) methane gas inclusion, (C) methane gas inclusion with bitumen, and (D) methane-bearing two-phase aqueous inclusion
Microscopic photos show that bitumen is widely developed among dolomite grains (Figure 2C), but the degree of filling is different, indicating that the carbonate reservoir has been filled with oil to varying degrees, and the oil has been fully cracked in a high degree of thermal evolution process
Summary
Shale gas is a kind of unconventional natural gas and has been successfully developed commercially abroad. It has become an important new energy (Han, et al, 2016). China started late and made a breakthrough in Sichuan Basin until 2010, when shale gas was drilled in Wufeng Formation of Upper Ordovician and Longmaxi formation of Lower Silurian (Huang et al, 2012; Jin et al, 2016). China has started to exploit the shale gas in marine strata with the depth shallower than 3500 m in the southern Sichuan Basin (Yang et al, 2021). The history of hydrocarbon evolution is still not clear, which restricts the further exploration
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