Effects of Organic Matter Graphitization on Shale Gas Accumulation in the Lower Paleozoic Longmaxi Formation from the Changning Area, Southern Sichuan Basin

Abstract

Abstract To investigate the characteristics of graphitized shale and the influence of organic matter graphitization on shale gas accumulation, Raman spectroscopy analysis, hydrocarbon generation simulation technique, reservoir physical property test, rock mechanics experiment, and field emission scanning electron microscopy (FE-SEM) were carried out on graphitized and nongraphitized shales of Longmaxi Formation in Changning area, southern Sichuan Basin. The results indicate that organic matter graphitization is a result of the thermal metamorphism of organic matter, and the resistivity of graphitized shale is generally lower than 10 Ω·m. The logging resistivity curves of the low-resistivity wells in the Wufeng Formation (O3w), Long11 sublayer (S1l11(1))-Long12 sublayer (S1l11(2)), Long13 sublayer (S1l11(3)), and Long14 sublayer (S1l11(4)) show box-shaped characteristics, and the logging resistivity curves at the boundary of the S1l11(3) and S1l11(4) exhibit a sharp drop. The graphitization of organic matter has great damage to hydrocarbon generation and reservoir capacity of shale, which is mainly manifested as the following: (1) the reduction of residual hydrocarbon caused by excessive thermal evolution weakens the gas generation capacity of shale, and the thermal evolution rate of graphitized shale is faster than that of nongraphitized shale; (2) the brittleness of graphitized shale increases due to the evolution of organic matter from a viscoelastic state to a glassy state; (3) organic pores and clay intercrystalline pores are destroyed or even disappeared during graphitization, which results in the deterioration of reservoir physical properties. Graphitized organic matter has good electrical conductivity, which is the reason for the formation of low-resistivity shale in the study area; (4) early hydrocarbon generation, poor reservoir physical properties, and unfavorable preservation are the main factors for the low gas content of graphitized shale. Therefore, it is of great significance to strengthen the study on organic matter graphitization and identify the graphitization area of organic matter for clarifying the next exploration direction and avoiding the risks of Lower Paleozoic shale gas exploration.