Continental shale pore structure characteristics and their controlling factors: A case study from the lower third member of the Shahejie Formation, Zhanhua Sag, Eastern China

Abstract

Since shale oil and gas primarily occur in micro-nano pores and fractures, research on pore structure characteristics is important to understand shale oil and gas accumulation mechanisms. Studies on the pore structure of the lower third member of the Shahejie (Es3l) Formation in Zhanhua Sag, Jiyang Depression, Bohai Bay Basin, Eastern China, are quite limited, thereby, restricting further investment on shale oil and gas exploration. We studied continental shale pore structure characteristics and their controlling factors by using a series of laboratory experiments on core samples taken from the Luo-69 Well. The results demonstrate that the Es3l continental shale has a high proportion of calcareous minerals, low siliceous minerals, and low clay minerals. It is characterized by low total organic carbon (TOC) content, low thermal maturity, and Type I and II organic matter. Five different kinds of pore spaces are developed in the shale samples: organic matter-hosted pores, intergranular pores, intercrystalline pores, dissolution pores, and micro-fractures, with the majority of slit-shaped and ink-bottle-shaped pores. Macropores make the greatest contribution to the total pore volume, whereas micropores provide the major surface area. As the organic-rich laminated calcareous shale (ORLCS) presents a larger pore volume, surface area, and mercury withdrawal efficiency, it is a favourable shale lithofacies for shale oil and gas storage and flow. Compared with the Longmaxi marine shale, macropores well-developed in the Es3l shale make great contribution for free hydrocarbon accumulation. Samples with different TOC contents, mineral compositions, thermal maturities, and lamellar layers exhibit distinct differences in pore structure. Mineral composition and thermal maturity primarily affect micropores and mesopores. Macropores are influenced by mineral composition, TOC content, thermal maturity, and lamellar layers.