Difference in pore structure characteristics between condensate and dry shale gas reservoirs: Insights from the pore contribution of different matrix components

Abstract

A breakthrough in shale gas exploration was made in the lacustrine shale of Qianfoya Formation located in the Sichuan Basin; a gas analysis has shown that the reservoir therein is a condensate gas reservoir. To enhance the production of shale gas in the Sichuan Basin, reduce the exploration risk, and facilitate development, the marine dry gas shale of the Longmaxi Formation was selected as a comparison to analyze the pore structure characteristics of condensate and dry gas reservoirs. The results showed that the dry gas shale reservoir is mainly developed as organic matter (OM) pores, followed by clay mineral (CM) pores. In condensate gas shale reservoirs, the OM pores, CM pores, intragranular dissolution (ID) pores, and microfractures are common. In the dry gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 10%–65%, 0%–23%, 0%–22%, and 0%–13%, respectively. Micropores were mainly provided by OM pores; mesopores were mainly provided by OM and CM pores; macropores were mainly provided by OM, CM, and ID pores. In the condensate gas shale reservoir, the surface porosities of the organic matter, clay minerals, brittle minerals, and microfractures were 0%–52%, 6%–34%, 0%–18%, and 0%–21%, respectively. Micro-mesopores were mainly provided by OM and CM pores, while macropores were mainly provided by CM pores and microfractures. Because of the influences of material composition, diagenetic evolution, and hydrocarbon generation at the burial depth, there were significant differences in the surface porosities of the matrix components and the distribution and proportions of the different pores between the dry and condensate gas shale reservoirs. This essentially led to a difference in the reservoir space.