247 / 2019-06-30 09:36:19

Sedimentary controls on shale composition: a case study of Wufeng–Longmaxi shales in South Sichuan Basin, China

black shale; sedimentary condition; mineral composition; TOC content

This study investigates the sedimentary controls on lateral heterogeneity of Wufeng-Longmaxi marine shales in the southern Sichuan Basin, China, based on core description, thin sections observation, geological chemistry and XRD data analysis. Shale thickness, lithofacies, mineral composition and TOC content changes noticeably throughout the studied marine shales within each of the graptolite biozones. Supply of terrestrial detrital material and bottom-water redox condition are considered to be the major controls on the shale property. Greater stagnation and more anoxic condition indicate a limited supply of terrestrial material, thus favoring the preservation of organic matter. Effective dilution of organic matter only occurred in Cystograptus vesiculosus graptolite biozone (LM4) when the apparent sedimentation rate increased over 15.0 m/Ma. Sulfate and iron reduction during the early diagenesis produce a diagnostic cement assemblage including framboidal pyrite and non-ferroan carbonate mineral. In more anoxic condition, limited detrital iron supply from the source region may lead to more acidic pore water conditions during early diagenesis, thus restricting early carbonate cementation and resulting in more clay-rich lithofacies. Therefore, the highest-quality shale gas reservoirs do not necessarily occur in the deepest and most restricted part of the basin, but rather in an environment that preserves sufficient organic matter and produces sufficient brittle minerals such as carbonate and quartz.