Influence of sedimentary environment on the brittleness of coal-bearing shale: Evidence from geochemistry and micropetrology

Abstract

The brittleness of shale is an important index to evaluate the fracture system and reformability of a shale reservoir. This study provides geochemical and micropetrological parameters as proxies for the paleo-hydrodynamics, redox condition, paleosalinity, and paleoclimate of transitional coal-bearing shale in the Huainan coalfield, eastern China. Thus, combining these data with organic geochemistry parameters, this study aims to investigate the most important factors controlling shale brittleness during the sedimentary process. The results indicate that various important physical and chemical factors (sedimentary facies, hydrodynamics, redox condition, salinity, and climate) of the paleoenvironment have exerted an intense influence on shale brittleness involving complex mechanisms. Rare earth element indexes suggest that marine facies are conducive to the enrichment of carbonate minerals and quartz, thereby improving the brittleness of shale. Micropetrologyical evidence indicates that with an increase of hydrodynamic force, a large number of terrestrial materials are introduced, leading to increased clay mineral content and significantly reducing the brittleness of shale. U/Th, V/Cr, and Ni/Co ratios and Pyrite indexes suggest that reducing water condition accounts for the enrichment of pyrite and the preservation of biogenic quartz. Thus, this increases the content of brittle minerals, while an oxidative environment significantly decreases the brittleness of shale. B/Ga ratios indicate that higher salinity promotes the dissolution of CO2, leading to precipitation of carbonate minerals and thus increasing the content of brittle minerals in shale, which is supported by the salinity characteristics obtained by the B content. In addition, Ba content indicates that a warm climate (humid) is the best condition for the growth of marine life and the development of carbonate minerals, which promotes the enrichment of brittle minerals obviously. While a hot climate (dry) results in the import of more terrestrial clay minerals in the provenance, thereby inhibiting shale brittleness.