Shale gas enrichment conditions and favorable exploration areas of Upper Permian Dalong Formation in the western Hubei and eastern Chongqing region

In recent years, multiple wells have achieved great discoveries and industrial flow of shale gas in the Permian Dalong Formation in the western Hubei and eastern Chongqing region, which reveals the promising exploration and development prospects of shale gas in the new series of strata. In this situation, strengthening geological understanding further is of great significance to guiding shale gas exploration in the next step. Based on core observation, thin section identification, scanning electron microscopy (SEM) observation, and wireline/mud logging and organic geochemical data analysis, this paper systematically studies the sedimentary characteristics, reservoir characteristics, organic geochemical characteristics, gas content and preservation conditions of the shale in the western Hubei and eastern Chongqing region, and ascertains the favorable exploration areas of Dalong Formation marine shale gas. And the following research results are obtained. First, during the Dalong period of Late Permian, the deposit of deep-water platform basin facies was developed in the western Hubei and eastern Chongqing region, with the development thickness of organic rich shale ranging from 10 m to 60 m. At the current burial depth, it is characterized by a large depth inside the basin and a small depth outside the basin. Second, the Dalong Formation is divided into three sections from bottom to top. The first section is mainly composed of siliceous shale, the second section is lithologically complex, and the third section is dominated by mixed shale. The logging response is overall characterized by three highs, two lows and one decrease, i.e., high GR, high CNL, high AC, low LLD (LLS), low DEN, and a gradual decrease in LLD (LLS) towards the bottom. Third, the brittle minerals in the Dalong Formation shale are mainly quartz and calcite, with an average content of 40.58% and 23.76%, respectively. The reservoir spaces are classified into two categories, i.e., pore and fracture. The pores are further classified into organic matter pores, mineral intergranular pores, intergranular dissolution pores, and intragranular dissolution pores. The fractures are further classified into interlayer fractures in minerals, fractures between organic matters and minerals, and fractures in organic matters. Organic matter pores are dominant, whose pore size distribution presents a four peak feature and pore volume is mainly concentrated in the range of 1 to 10 nm and 500 to 5 000 nm. Fourth, the Dalong Formation has high organic abundance, strong reservoir heterogeneity, high degree of thermal evolution and developed organic pores, with type II1 as the primary organic matter type and type II2 as the auxiliary. Fifth, the Dalong Formation shale has high gas content, and good roof and floor conditions play an important role in sealing. The preservation conditions are overall good inside the basin where tectonic uplift and denudation and faulting have little effect, and the preservation conditions in the core of the residual syncline outside the basin are good as well. In conclusion, the Dalong Formation shale is a new field of shale gas exploration in the Sichuan Basin. There are 5 favorable exploration areas inside the basin, with a total area of 5 500 km2, thus it is suggested to speed up the pace of their exploration evaluation. What's more, there are 7 favorable exploration areas outside the basin, with a total area of 6 700 km2, thus it is recommended to select favorable blocks with large residual areas for risk exploration to search for new breakthroughs and discoveries. © 2024 Natural Gas Industry Journal Agency. All rights reserved.