An inland facies-controlled eogenetic karst of the carbonate reservoir in the Middle Permian Maokou Formation, southern Sichuan Basin, SW China

Abstract

Carbonate karst is one of the research highlights in the field of carbonate reservoir geology. Here, we report on a new type of karst formed in the Middle Permian Maokou Formation, southern Sichuan Basin, SW China, i.e., inland facies-controlled eogenetic karst, which is different from the previously defined telogenetic karst. This karst is eogenetic as the formation was buried at shallow depths prior to being subaerially exposed for a period of 7–8 Myr, in the paleo-continental region of the Upper Yangtze Uplift. Subaerial exposure may have been caused by a sea level regression during the Tungwu Orogeny, which gave rise to a depositional hiatus over a broad area. The top of the Maokou Formation is commonly marked by a weathered crust and an unconformable relationship with overlying layers. Below the surface, the Maokou Formation contains sediments deposited by an underground drainage. The geological setting can be inferred from an inland karst far from coastline. The subsurface karst interval consists mainly of coarse-grained limestone and micrite, with the former occurring in shoal facies deposited in a high-energy depositional environment, and the latter in non-shoal facies deposited in a low-energy environment. Both of them were interbedded with in variable thicknesses. The coarse-grained limestone layers with high porosity and permeability acted as inception horizons, more favourable for the development of karst than the micritic layers with low porosity and permeability. Therefore, in places where both coarse-grained limestone and micrite are present, the karst is considered to be facies-controlled. The primary permeability of the coarse-grained limestone, combined with the permeability provided by faults and fractures, provides sufficient channels for karst water. Formation of the karst system was characterized by contemporaneous development at multiple levels, as controlled by the stratigraphic position of coarse-grained shoal facies. The karst reservoir therefore developed in both karst highland and karst transitional zone (area between the karst highland and karst basin). According to this model of karstification, hydrocarbon exploration should focus on karst highlands located on palaeohighs and in synclines located far from fault zones.