Influence of Sedimentary Environment on Organic Matter Enrichment in Shallow Marine Shelf Organic-Rich Shales: Study of the Wujiaping Formation in Eastern Sichuan Basin

Abstract

The Upper Permian Wujiaping Formation in the Sichuan Basin exhibits significant exploration potential, yet systematic investigations into organic matter (OM) accumulation processes in black shales deposited in shallow-marine shelf environments remain scarce. This study focuses on shales from the Wujiaping Formation in the East Sichuan Basin, employing sedimentological, mineralogical, and geochemical analyses to investigate shale mineral composition, OM characteristics, and depositional environments, thereby establishing an OM enrichment model for transitional facies in the Wujiaping Formation. Results indicate that Wujiaping shales contain type II–III kerogen, with OM predominantly composed of vitrinite and inertinite derived from terrestrial plants, collectively reflecting a high-maturity wet-gas generation stage. Mineralogically, the shales are dominated by clay, exhibiting strong gas adsorption capacity and abundant pore systems, including interparticle pores (interP), intraparticle pores (intraP), OM-hosted pores, and microfractures, all contributing to gas storage and migration. The shales were deposited under predominantly oxic conditions, with intermittent dyoxic episodes in relatively shallow water. Terrigenous input and sedimentation rate are identified as primary controls of OM accumulation in the Wujiaping Formation. This study enriches the data set for shale gas exploration in the Sichuan Basin and, combined with prior research on deep marine and continental facies of the Wujiaping Formation, provides a comprehensive characterization of natural depositional environment sequences.