Differences in Pore Type and Pore Structure between Silurian Longmaxi Marine Shale and Jurassic Dongyuemiao Lacustrine Shale and Their Influence on Shale-Gas Enrichment

The Silurian Longmaxi (S(1)l) marine shale and Jurassic Dongyuemiao (J(1)d) lacustrine shale in the Sichuan Basin, West China have attracted considerable attention from the oil-and-gas industry in China. Currently, the differences in pore types and pore structures between them are poorly understood, which has limited shale-resource exploration in the Sichuan Basin. This paper systemically compares the pore characteristics of Longmaxi shale and Dongyuemiao shale and investigates their impact on shale-gas enrichment by integrating field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), low-pressure gas (CO2 and N-2) adsorption and mercury-intrusion porosimetry, high-pressure sorption isotherms, gas-saturation measurement, molecular-dynamics simulation, etc. The results show that the S(1)l organic-rich marine shale and the J(1)d lacustrine shale have different pore types and pore structures. The S(1)l shale is dominated by organic pores, mainly micropores and mesopores with ink-bottle-like pore shapes, while the J(1)d shale is primarily composed of clay-mineral pores, mainly mesopores and macropores with slit- or plate-like pore shapes. Organic pores can provide considerable storage space for shale-gas enrichment in S(1)l marine shale, which also determines the adsorption capacity of shale reservoirs. Although organic pores are not the most prevalent in the Dongyuemiao lacustrine shale, they also play an important role in enhancing reservoir quality and absorbed-gas enrichment. Clay-mineral pores contribute weakly to the storage space of J(1)d-lacustrine-shale reservoirs. Mesopores are the most important form of storage space in both S(1)l shale and J(1)d shale, contributing significantly to shale-gas enrichment. Micropores are secondary in importance in S(1)l marine shale, while macropores are secondary contributors to pore volume in J(1)d lacustrine shale.