The Middle Permian Maokou Reservoir (Southern Sichuan Basin, China): Controls on Karst Development and Distribution

Abstract The Middle Permian Maokou limestone is a major natural gas producing reservoir in the southern Sichuan Basin, China. While matrix porosity is low, bit drops, losses while drilling, blowouts, and extremely high well testing flow rates indicate the presence of a dual permeability system in the form of paleocaves, which were formed by karstification in the late Maokouan Age. A combination of geological, paleogeomorphological and karstic analyses reveal that the development and distribution of the karst features within the reservoir are primarily regulated by shoal facies, paleotopography and syndepositional faults. In our conceptual paleogeomorphological model, the potential cave-bearing zones are located along the rim and outer edge of the Luzhou Paleohigh, which was the highland during the karst. Outside the highland were the karst slopes, and the poljes were on the flanks of the study area. Reservoir performance data suggest, there are two types of sweet spots in the study area. The first corresponds to the syndepositional fault zones along the rim of the karst highland and on the karst slope, where the caves appear to distribute along the faults, impacting the reservoir’s vertical permeability. The second corresponds to the shoals of the Mao 2 and Mao 3 members on the lower western karst slope and the upper eastern karst slope, where the caves are widespread, enhancing the horizontal permeability and connectivity of the reservoir. However, the caves are rare in the central karst highland, lower karst slopes, and poljes, which is consistent with the limited gas discoveries.