Differences between the platform-margin shoal reservoirs and the platform-interior shoal reservoirs of the Middle Triassic Leikoupo Formation, Sichuan Basin, China

The carbonate shoal grainstone reservoirs of the Middle Triassic Leikoupo Formation in the Sichuan Basin provide a typical example of gas exploration in shoal carbonate reservoirs. The carbonate shoal reservoirs in Leikoupo Formation are of two types: platform-margin shoals and platform-interior shoals. Based on a comprehensive data analysis, and through analysis and correlation of grain shoal reservoir characteristics, it has been shown that there are significant differences between platform-margin and platform-interior shoal reservoirs in terms of the types of reservoir space, the relationship between porosity and permeability, and the pore throat structures. The pores in the platform-margin shoal reservoirs are mainly relict intergranular pores and intergranular dissolution pores, and their porosity and permeability show a good positive correlation. The throats are thick and the pore throats have a good distribution. Isolated intragranular solution pores and moldic pores are mainly developed in the platform-interior shoal reservoirs. There is a weak positive correlation between porosity and permeability, pore throats are less developed, and the pore throats have poor configurations. Differences in sedimentary environment and diagenetic evolution are the primary causes of the different reservoir characteristics of the two shoal facies. The water body in the platform-margin shoal deposits had a higher energy than that in the platform-interior shoal deposits, and its wave base was deeper that of the platform-interior shoals. The shoal bodies at the platform margin are much thicker than those in the platform interior. The grainstones of platform-margin shoals are much better sorted. The rocks have undergone compaction during most post-depositional stages. The thicker grainstones of the platform-marginal shoals preserved their original intergranular pores better than the lower-thickness grainstones in the platform-interior shoals. This was favorable for formation of intergranular dissolution pores during later burial. In contrast, the grainstones of the platform-interior shoals are less thick and early intergranular cements are well developed, resulting in large-scale loss of the original intergranular pores. However, as a result of selective dissolution by early fresh water in the meteoric diagenetic environment, intragranular solution pores and moldic pores were readily formed. Thus, the platform-margin shoal reservoirs show preservation of the original intergranular pores enhanced by burial dissolution, but the platform-interior shoal reservoirs are mainly the result of early selective dissolution by meteoric fresh water.