Pyrite morphological evidence for sedimentary conditions of the Wufeng formation–Longmaxi formation in south-east Chongqing area–insights for high-quality shale gas reservoir formation mechanism

As one of the important minerals in marine shale reservoirs, the development characteristics of pyrite can provide guidance for the exploration of deep shale gas resources and the study of high-quality reservoir development mechanisms. In this study, samples of the Wufeng and Longmaxi Formation in the southeast Chongqing area were selected, and the development characteristics of framboidal pyrite were revealed through a combination of qualitative and quantitative method. High-quality reservoirs of the upper Wufeng Formation and the bottom of the Longmaxi Formation are developed with smaller sizes, weaker variation and a narrower size distribution of pyrite framboids compared with other units of the strata, suggesting relatively stable euxinic conditions during deposition. The development characteristics of framboidal pyrite and some of the key reservoir evaluation parameters such as organic matter content and brittle mineral content, etc., are controlled by similar factors. Therefore, pyrite morphological evidence can be used as a potential indicator of high-quality shale reservoirs.