Organic matter pore structure evolution from early-mature to over-mature stages

Organic matter (OM)-hosted pores are critical to controlling shale oil and gas adsorption in black shales. However, the specifics of pore structure evolution in OM during thermal maturation are not yet well understood. In this study, the shales of the early-mature and middle-mature Triassic Yanchang Formation of the Ordos Basin, late-mature Ordovician–Silurian Renheqiao Formation of the Baoshan Block, and over-mature Ordovician–Silurian Wufeng-Longmaxi Shale of the Sichuan Basin were investigated. Multiple techniques were utilized to investigate the evolution of OM type and pore structure characteristics during thermal maturation. OM in the early-mature Yanchang Formation shales is dominated by the oil-prone macerals bituminite and alginite. In comparison, OM in the late-mature Renheqiao Formation and over-mature Wufeng-Longmaxi Shale is dominated by solid bitumen and pyrobitumen, respectively. Organic pores are hosted by secondary OM, solid bitumen and pyrobitumen, and are best developed in the over-mature stage. The loss on ignition-corrected specific surface area (SSA) and pore volume of isolated OM increase with increasing thermal maturity, with the SSA and pore volume ranging from 5–10 m 2 /g and 0.02–0.05 cm 3 /g at vitrinite reflectance < 0.7% to 200–250 m 2 /g and 0.3–0.4 cm 3 /g at equivalent vitrinite reflectance 2.5–3.0%, respectively. OM in the oil window-maturity Yanchang Formation shales has a shale oil adsorption pore volume (pore size < 12 nm) of 0.0417 to 0.0784 cm 3 /g and can adsorb 46–87 mg oil/g TOC. This study underscores the necessity of a good understanding of OM type and thermal maturity in order to properly assess the oil and gas adsorption by OM in black shale strata.