Pyrite type and its effect on shale gas accumulation: a case study of Wufeng-Longmaxi shale in Sichuan Basin and its periphery

Abstract

Pyrite is one of the most common minerals in shale, but its type, genesis and effect on shale gas accumulation are still in the initial study stage. The shale in the Upper Ordovician Wufeng Formation and the Lower Silurian Longmaxi Formation in the Sichuan Basin and its periphery was studied. Strawberry-like, stratified, tuberculosis, euhedral and anhedral pyrites were identified through thin section observation, scanning electronic microscopy analyses. The pyrites in the Wufeng-Longmaxi formations are mainly developed during the syngenetic and early diagenetic stages. The strawberry-like pyrites are formed in the syngenetic stage, indicating a reducing environment, which is conducive for organic matter enrichment and preservation. The pyrite content has a positive correlation with the organic carbon content. The layers with a high content of pyrites are usually rich in organic matter. Strawberry-like pyrites, together with biogenic quartz and terrestrial detrital quartz, formed a rigid particle grid supporting the original intergranular pores, which was beneficial to hydrocarbon charging, preservation and organic matter-hosted pore development. The bottom of the Wufeng -Longmaxi shale with well developed strawberry-like pyrite in the Sichuan Basin and its periphery, indicate it not only has a good condition for organic matter enrichment, but also is conducive to the formation of high-quality shale gas reservoirs. The strawberry-like pyrite content can be used as a clue for identifying shale gas "sweet spots".

Keywords

• pyrite
• shale gas
• wufeng formation
• longmaxi formation
• sichuan basin