Characteristics and Genetic Mechanisms of Normal-Pressure Fractured Shale Reservoirs: A Case Study From the Wufeng–Longmaxi Formation in Southeastern Chongqing, China

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.

Resercoir space characteristics and pore structure of Jurassic Lianggaoshan Formation lacustrine shale reservoir in Sichuan Basin, China: Insights into controlling factors

“…Organic pores are generally considered to be the main pore type of shale reservoirs and one of the key factors for shale gas enrichment (Xi et al, 2018;Cai et al, 2022;Wang et al, 2022;Xi et al, 2022). Organic matters in Lianggaoshan shale is mainly woody organic matter, followed by solid bitumen.…”

Disturbance and Control of National Strategic Gas Storage Induced by Adjacent Tunnel Blasting

“…Because of their excellent confinement safety, cost-effectiveness, and stability, underground gas storages (Xiong et al, 2021) have received widespread attention as national energy reserves that can be used for nuclear waste disposal (Li et al, 2014;Mahlia et al, 2014). currently, there are three main types of underground energy reservoirs: 1) underground salt cavern gas reservoirs (Deyi et al, 2016;Fan et al, 2019;Fan et al, 2020;Peng et al, 2020; with crystalline structures (Jiang et al, 2021), high denseness (Kang et al, 2021), high ductility (strain can reach almost 30-40%) (Liu et al, 2020a), low permeability (<10 −20 m 2 ) (Liu et al, 2020b), ultra-low porosity (<1%) and self-healing characteristics (Urai et al, 1986), which comprise salt rock as the constitutive medium; 2) depleted oil and gas reservoirs formed by the transformation of non-operational wells (Wang et al, 2021); 3) underground water-sealed gas/oil reservoirs wherein oil and liquid gas are sealed in large underground caverns that have been excavated (Chung et al, 2009;Lee and Lim, 2010).…”