Statistical analysis and significance of pyrite in the Wufeng-Lower Longmaxi Shale Formation in South China

Abstract

Pyrite from outcrops of Late Ordovician–Early Silurian marine shales of the Sichuan Basin was studied in the laboratory setting. We characterized various occurrences of pyrite using optical microscopy, reflected light microscopy, X-ray analysis, scanning electron microscopy, and energy-dispersive analysis, and image analysis to aid visual estimates of pyrite in polished sections. We observed two classes and four subclasses of pyrite in this marine shale depositional environment. A total of 2964 pyrite framboids and 16,393 pyrite monocrystal particles were measured, with an average particle size value from 2.57 to 6.24 μm in framboidal pyrite and from 0.32 to 0.61 μm in component monocrystals. Over 88% of framboidal pyrite particle size ranges from 1.5 to 9 μm, and the diameter between 0.2 and 0.8 μm is greater than 89% of total pyrite monocrystal particles. The cubic form of pyrite has particle sizes that range from 2.56 to 101.93 μm, more than 83% ranging from 5 to 30 μm. Image analysis of pyrite particle reveals dimensions between 1.0 and 7.5 μm and shape factors from 0.9 to 1.0 (nearly round), indicating that the primary pyrite type in marine shale formation is framboidal pyrite. The framboidal pyrite has important geological significance for marine shale formations. Size analysis, organic matter content, and stratigraphic distribution of framboidal pyrite reveal that the average particle size of framboidal pyrite varies according to deposition in either a dysoxic or a euxinic sedimentary environment. The Fe and S atomic content in framboidal pyrite is relatively low and correlates with high C atomic content, which suggests a close relationship between framboidal pyrite and organic matter. We identified six pore types in this study. Pores are developed in organic matter that occurs between framboidal monocrystals, as well as in organic matter that surrounds framboidal pyrite. Despite their small size, organic porosity remains conducive to the natural gas adsorption, and porosity associated with pyrite may also contain methane in the subsurface.