Perhydrous organic carbon logging evaluation method in shore and shallow lacustrine shale formations: A case study of the Mesozoic lacustrine shale in the western Sichuan Basin, China

Abstract

The types of organic carbon in shore and shallow lacustrine shales are more diverse than those in marine formations. It is of great importance to establish log interpretation models of the separate types of organic carbon content for hydrocarbon generation potential estimation. First, we summarize the main rock types and their logging characteristics in the shore and shallow lacustrine shale reservoirs and analyze the types of organic carbon in shore and shallow lacustrine shales using core pyrolysis analysis and kerogen maceral microscopic identification. Second, we establish the calculation method of the total organic carbon (TOC) content in the lacustrine shale formation by regression analysis. Third, we divide the organic matter in shore and shallow lacustrine shales into two parts: perhydrous organic carbon (perhydrous macerals) with a higher hydrogen index, higher H/C atomic ratio, and stronger hydrocarbon generation potential and subhydrous organic carbon (subhydrous macerals) with a lower hydrogen index, lower H/C atomic ratio, and poorer hydrocarbon generation potential. Furthermore, we introduce the concept of the mixed skeleton, combined with the formation component triangular chart, and establish calculation methods for perhydrous and subhydrous organic carbon contents. Finally, we apply this method to the Mesozoic shore and lacustrine shale in the western Sichuan Basin, China. The application indicates that the kerogen types of the Mesozoic shore and shallow lacustrine formations in the western Sichuan Basin are mainly type III and type IIB kerogen with a low content of perhydrous macerals. The perhydrous organic carbon content calculated by the logging evaluation model is positively correlated with the perhydrous maceral content obtained by kerogen maceral microscopic identification and the oil generation potential S2 obtained by core pyrolysis analysis. Logging analysts should use the perhydrous organic carbon content to estimate the hydrocarbon generation potential of shore and shallow lacustrine shale formations.