Morphological characteristics of framboidal pyrite and their paleo-environmental significance in Longmaxi Formation of Weirong area, southern Sichuan Basin

The morphological characteristics of framboidal pyrite have important indicative significance for the shale sedimentary environments and shale gas enrichment. Taking the shale in the first member of the Longmaxi Formation from well WY1 in the Weirong area of southern Sichuan Basin as the study subject, argon ion scanning electron microscopic analyses and ImageJ image processing were conducted to obtain the morphological characteristics and particle sizes of framboidal pyrite and its microcrystals. By analyzing parameters such as trace elements, organic matter abundance, and gas content, the study explored the indicative role of framboidal pyrite morphology in paleo-water redox environments and shale gas enrichment. The results demonstrated that parameters such as the average particle size of framboidal pyrite, the average particle size of microcrystals, and the crystalline structures showed significant variations vertically. The average particle sizes of pyrite and its microcrystals were 4.02 μm and 0.45 μm, respectively. The particle sizes of both pyrite and its microcrystal gradually increased from bottom to top. The crystalline structure of microcrystals from the lower part was mainly truncated octahedrons, and the upper part featured truncated octahedrons, supplemented by octahedrons and truncated tetrahedrons. The roundness of microcrystals gradually decreased from bottom to top. The morphological characteristics of framboidal pyrite and its microcrystals indicated that the reducibility of the sedimentary water body in the first member of the Longmaxi Formation gradually weakened, transitioning from an euxinic and anoxic environment to a suboxic environment. This aligned well with the redox environment reflected by trace elements. Furthermore, contents of organic matter and desorbed gas showed a significant positive correlation with pyrite content, but a clear negative correlation with the particle sizes of framboidal pyrite and its microcrystals. The findings show that an euxinic and anoxic environment is favorable for the enrichment of organic matter and the generation, accumulation, and preservation of shale gas.