Paleogeomorphological reconstruction and geological implications of the weathered-crust karst on the top of the Middle Triassic Leikoupo Formation in the Longgang area, Sichuan Basin, China

Abstract

Weathered-crust karst carbonate is developed in the third submember of the fourth member of the Middle Triassic Leikoupo Formation (Lei43 submember) in the Longgang area of Sichuan Basin, China, and acts as an important oil and gas reservoir. To reconstruct the paleogeomorphology of the weathered-crust karst, we analyzed the seismic thickness of the interval from the erosional unconformity surface on the top of the Leikoupo Formation to the karst at the bottom of the third member of the Xujiahe Formation (Xu3 member), based on existing three-dimensional seismic and drilling data, and the sedimentary characteristics of the Leikoupo and Xujiahe formations. There are three secondary types of geomorphological unit, namely the karst highlands, the karst transitional zone, and the karst basin, and these have distinct karst water development patterns and hydrological conditions that determine the intensity of karstification and reservoir quality. Among them, the karst highlands have a poor reservoir capacity due to their long-term exposure above the water table, which resulted in severe denudation. The karst transitional zone features the superimposition of multiple periods of strong karstification, and this has resulted in high-quality reservoir conditions in the karst monadnocks but poor reservoir conditions in the karren. The karst basin represents a drainage area that experienced weak karstification, and the reservoir capacity is generally poor, although some good reservoirs were developed in shoals. Paleogeomorphological maps provide excellent guides to finding karst monadnocks in karst transitional zones, and monadnocks should serve as the main targets of exploration in the weathered-crust karst reservoir on top of the Lei43 submember.