Development of Organic Pores in the Gufeng and Longmaxi Formation Shale, Sichuan Basin: Combined Effects of Bioprecursor, Pore Filling, and Mineral-Organic Interactions.

The Gufeng Formation of the Permian in the northern Sichuan Basin hosts a high-carbonate shale reservoir, representing a key emerging target for unconventional gas exploration in the basin. However, the mechanisms governing organic pore development in this unit remain poorly understood. In this study, samples from the Gufeng and Longmaxi formations were analyzed using organic petrology, scanning electron microscopy (SEM), and nitrogen adsorption experiments. Three groups of samples were prepared for comparative treatment: original, chloroform-extracted, and acid-treated. This approach enabled a comparative assessment of the integrated effects of bioprecursor types, early cementation, bitumen filling, and mineral-organic interactions on organic pore development. The main findings are as follows: (1) bioprecursor types control the characteristics of kerogen-hosted pore development. Benthic algae retain a relatively complete biological structure, with well-developed internal pores (>100 nm). In contrast, pores developed in planktonic algae are concentrated in the 10-50 nm range. The Gufeng Formation contains a higher proportion of planktonic algae (>30%), resulting in lower kerogen pore-generation potential compared to the Longmaxi Formation. (2) Cementation and bitumen infilling cause significant damage to organic pores. In the Gufeng Formation, microcrystalline calcite infilling occurs within various kerogen types, leading to over 60% loss of mesopores and macropores. Additionally, during the oil window stage, bitumen primarily fills pores <10 nm, causing a further 7-12% pore loss. Although bitumen-hosted pores in the Gufeng Formation increase with maturity (Ro 0.7-2.0%), pore volume shows no significant correlation with Ro, reflecting the combined effects of early cementation and bitumen infilling. (3) Mineral-organic interactions induce differentiation in organic pore development. In the Longmaxi Formation, clay minerals promote hydrocarbon generation and pore formation in mineral-associated organic matter through an "adsorption-catalysis" mechanism, resulting in well-developed organic pores. In the Gufeng Formation, Lewis acid sites on carbonate mineral surfaces may promote early hydrocarbon generation, but they cannot sustain continuous pore development during mid-to-late hydrocarbon generation stages. Surface complexation enhances the thermal stability of organic matter, thereby delaying hydrocarbon generation and organic pore development. This study reveals a multifactor synergistic mechanism responsible for limited organic pore development in carbonate-rich shales, providing a new perspective for reservoir evaluation and favorable target selection in high-to-overmature carbonate-rich shale systems.