Characteristics of organic pores in Middle and Upper Permian shale in the Lower Yangtze region

A set of transitional shale reservoirs developed in the Middle and Upper Permian strata in the Lower Yangtze region, with a maceral composition significantly different from that of marine shale. Organic petrology, scanning electron microscopy, field emission scanning electron microscopy in combination with argon ion polishing, gas-filled porosity and mercury intrusion experiments were conducted in order to reveal the characteristics of organic pores and their influencing factors. The results showed that organic pores were well developed overall in the Middle and Upper Permian shale, but have significantly different characteristics in different maceral grains. Vitrinite generally has no or rare pores developed, solid bitumen could produce a small quantity of large-scale isolated pores, and sapropelinite usually contains a large number of small pores, which is the main contributor to organic porosity. Pyrite and/or clay minerals could be mixed with organic matter and then enhance the development of pores in organic matter grains, which may be related to the effect of pyrite and clay minerals on organic matter generation and decomposition. Although there is a positive relationship between specific surface area and TOC content, the relationship of porosity with TOC content is rather complex. The TOC content has a positive relationship with porosity for the Middle and Upper Permian shale when w(TOC)<6.16%, whereas porosityis generally low and has a slight decreasing trend with increasing TOC content when w(TOC)>6.16%. The pore size distribution also revealed that the high TOC content shale has lower meso-macropore volume than the shale with a low TOC content. The characteristics of pore development in the Middle and Upper Permian shale illustrated that solid bitumen and hydrogen-depleted components increased with increasing TOC content, and these macerals would fill in the mineral intergranular pores and reduce the overall space of shale. Meanwhile, with higher TOC content, the shale is more likely to be compacted, resulting in the collapse of mesopores and macropores, further reducing the porosity of shale.