Characteristics and controlling factors of unconventional reservoirs: a study of the Middle Triassic Leikoupo Formation in the Sichuan Basin

In 2020, a novel unconventional reservoir exhibiting coexistence of oil and gas was discovered within the Leikoupo Formation in the central Sichuan Basin. Characterized by extra-low porosity and low permeability, the genesis of reservoir pores lacks systematic investigation. This study integrates petrography, XRD, rock-pyrolysis, biomarker, fluorescence microscopy, gas adsorption and CT scanning to elucidate the characteristics of this reservoir and decipher the relationship between reservoir formation and original sediments. Experimental results indicate that the reservoir space is primarily controlled by the primary mineral framework and hydrocarbon generation overpressure, developing clay intergranular pores, organic matter (OM) pores, and lamination fractures. Mutual adsorption between clay and organic matter facilitates OM preservation to a certain extent. However, their heterogeneous distribution contributes to reservoir heterogeneity and induces multi-point hydrocarbon generation. The findings underscore the critical roles of sedimentary differentiation, organic-inorganic interactions, and internal hydrocarbon generation dynamics within a closed system in the formation of this unconventional shale reservoir in the Leikoupo Formation. They further emphasize the sedimentologically constrained evolutionary characteristics of the shale reservoir. This study delivers sweet spot evaluation insights and a conceptual framework for unconventional petroleum research.