Lithofacies paleogeography of Middle Triassic Leikoupo Formation in the southern Sichuan Basin and its implications for conventional and unconventional dual-domain exploration

The Leikoupo Formation of the Middle Triassic is one of the earliest strata for oil and gas exploration in the Sichuan Basin, but its exploration has long been limited to scattered small gas fields. Recent studies reveal that the Leikoupo Formation holds dual prospects for marine conventional and unconventional oil and gas reservoirs, but the existing paleogeographic pattern is inadequate for the new exploration needs. This study focused on the Leikoupo Formation in the southern Sichuan Basin. Based on the detailed analysis of sedimentary facies in outcrops and drill cores, the lithofacies paleogeography was reconstructed and the sediment and reservoir development patterns were studied. The sedimentary environment types in the Middle Triassic Leikoupo Formation of the southern Sichuan Basin included platform tidal flats, tidal shoals, shelf bays, and saline lagoons. During the transgression period (members 1 and 3 of the Leikoupo Formation), relatively high-energy marine conditions favored the development of high-energy shoal facies and organic-rich mudstone or marl. During the regressive period (members 2 and 4 of the Leikoupo Formation), the energy was relatively lower, favoring the development of dolomite and evaporite successions. The development of conventional carbonate rock reservoirs in the Leikoupo Formation of the southern Sichuan Basin was primarily controlled by the paleogeomorphic pattern. The combined effects of wave and tidal actions led to the extensive distribution of thin shoal-facies dolomite reservoirs. The transgression during the depositional period of member 3 of the Leikoupo Formation played a significant role in the formation of unconventional reservoirs of organic-rich mudstone or marl. It not only altered the original paleogeomorphic pattern but also transported organic-rich clastic materials from the Badong Formation into the interior of the Upper Yangtze Craton. Under intense evaporation during the later stages, intersalt hydrocarbon reservoirs with a "salt rock and marl" cycle were formed. The paleogeographic reconstruction in both conventional and unconventional domains of the Leikoupo Formation in the southern Sichuan Basin not only reveals a new exploration area for marine intersalt unconventional resources, but also confirms the favorable conditions for the development of high-quality source rocks within the formation.