Heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin of China

Abstract

This study characterizes the vertical heterogeneity of the Lower Silurian Longmaxi marine shale in the southeast Sichuan Basin based on core description, thin sections observation, geological chemistry data interrogation, and QEMSCAN image analysis. Lithofacies associations, mineralogy, graptolite species occurrence, rock fabrics, as well as TOC (Total Organic Carbon) and gas content, change noticeably and regularly in the Lower Silurian Longmaxi marine shale within sequence stratigraphic framework. Lithofacies associations mainly include in-situ graptolite rich and siliceous shale in the transgressive systems tract (TST), micro-offsite graptolite rich shale and shale interbedded with siltstone laminaes in the early highstand systems tract (EHST), and micro-offsite graptolite rich and carbonate rich shale in the late highstand systems tract (LHST). The TST has a greater abundance of Diplogratus than EHST and LHST, and has a lesser level of Monograptus, which is conversely higher in the EHST and LHST. The TST shale is silica rich, while the LHST shale is generally clay rich. The EHST shale is more balanced in its silica and clay content than the TST and LHST are. The sedimentary structure and fabric of shale shows that shale is homogenous in the TST, with laminaes well developed in the EHST but decreasing in the LHST. TOC and gas content are highest in the TST and decrease upward. The TST, with high TOC, high gas content, high brittle minerals (silica rich) and anoxic-suboxic to dysoxic sedimentary environment, is the most potential shale gas target interval in the study area. Our research also shows that the TOC mainly controls shale gas accumulation and that the redox conditions mainly control the preservation of organic matter. The silica also plays a significant role in shale gas content due to its contribution to hydraulic fracturing. Finally, the study proposes a depositional and heterogeneity model for the Lower Silurian Longmaxi marine shale in the study area.