Influences of organic matter and kaolinite on pore structures of transitional organic-rich mudstone with an emphasis on S2 controlling specific surface area

Nanopore networks in organic-rich mudstone play a key role in providing pore space, surface area and channels for gas storage and migration. Controls of nanopore development are still poorly understood, especially for transitional mudstone. Differing from marine and lacustrine mudstone, transitional mudstone is characterized by high kaolinite and low illite contents, and complex organic input. Six transitional mudstones in Linxing Area, eastern Ordos Basin were selected to figure out the influence of kaolinite and organic matter (OM) on pore structures. Sample preparation (OM-removal and kerogen isolation), mineralogical, organic petrographic and geochemical analyses and textural analysis (low-pressure Ar and CO2 adsorption) were performed. Data in other publications were used to test and perfect our conclusion of specific surface area changing with S2 (second peak in Rock-Eval pyrolysis). Our study demonstrates the pores within 5–50 nm are mainly controlled by quartz contents and are negatively correlated with kaolinite or clay mineral contents. Kaolinite content has little influence on specific surface area, but organic pores, most of which are within 0.3 to ∼5 nm, contribute much to specific surface area. Higher proportion of marine OM has larger organic pores. “Quasi-parabolic +” was used to describe the firstly increasing, secondly decreasing and then uncertain relationship between specific surface area and S2 values. The relationship was found in both transitional and marine mudstones. Surface area changing with S2 is because TOC, thermal maturity and organic input affect organic pore development and also control S2 values. Micropore specific area of kerogen has a firstly increasing and secondly decreasing trend with hydrogen index (HI) values while S2 affects micropore surface area of mudstones. Our findings can be used to estimate pore parameters of organic-rich mudstone.