Abstract
Compared with the overpressure conditions of shale reservoirs in the Jiaoshiba area, exploring the controlling factors of changes in shale reservoir physical properties under normal-pressure accumulation is of great significance to shale gas exploration. To achieve this, X-ray powder diffraction, nitrogen adsorption, and scanning electron microscopy were carried out to determine mineral content, permeability, porosity, and pore structure for well core and outcrop samples of the Wufeng–Longmaxi Formation in southeastern Chongqing, China. Field observations and drilling cores revealed abundant structural fractures in southeastern Chongqing, offering potential main storage space for shale gas. Findings also determined that high-quality source rocks are located in the lower part of the Wufeng–Longmaxi Formation, which features low porosity and low permeability, yet existing structural fractures could potentially improve permeability to a certain extent. The main factors controlling the shale reservoir with the above characteristics include the depositional environment, mineral components, total organic carbon content, and tectonic movement. The deep-water anoxic depositional environment was conducive to the complete preservation of organic matter and sedimentation of the biogenic siliceous minerals, which had a strong effect on improving the porosity of organic matter and brittleness of the rocks. The high content of quartz was found to improve rock compression resistance and brittleness, and increasing clay proportion was found to enhance the interlayer pores of clay minerals. Total organic carbon content, specific surface area, pore volume, and fracture development were all found to have direct contributions to shale gas entrapment. Overall, the most important factor resulting in normal-pressure accumulation in southeastern Chongqing was the intense tectonic movements since the Late Jurassic period which destroyed the original overpressure accumulation conditions.
Highlights
As an unconventional gas, shale gas is of great significance to the global energy supply
The analysis of clay minerals, pore volume, and specific surface area in the Longmaxi Formation shale of the Yushen 6 and Yushen 8 wells in southeastern Chongqing in this study shows a definite positive correlation between these characteristics, indicating that the pore-specific surface area and pore volume of organic-rich mud shale vary with the increase in clay mineral content (Figures 12A,B)
The Wufeng–Longmaxi Formation mud shales in the northwestern part of the study area have a higher content of brittle minerals and organic carbon than those in the southeastern part in the southeastern Chongqing
Summary
Shale gas is of great significance to the global energy supply. Yuan et al (2020) proposed that the Pengshui– Longmaxi Formation shale gas reservoir experienced overpressure during its geological formation period, yet during uplift, the overpressure caused ruptures and fractures, resulting in the loss of shale gas and release of overpressure, transitioning the pressure environment to atmospheric. This is because the tectonic deformation experienced in the periphery of the Sichuan Basin was earlier and more intense than that in the interior of the basin, which resulted in the formation of distinctly contrasting accumulation environments between Jiaoshiba and Pengshui (Fan et al, 2019). This study uses petrographic observations, low-temperature nitrogen adsorption, and scanning electron microscopy to better evaluate shale gas reservoir characteristics and guide the division of shale gas-favorable blocks in southeastern Chongqing
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