Sequence stratigraphy and its application in marine shale gas exploration: A case study of the Lower Silurian Longmaxi Formation in the Jiaoshiba shale gas field and its adjacent area in southeast Sichuan Basin, SW China

Abstract

Based on outcrops, cores, well logs data, and mineralogical, geochemical and petrophysical analysis, we studied the sequence stratigraphy of marine shale dominated the Lower Silurian Longmaxi Formation, favorable commercial shale gas interval within the sequence stratigraphic framework. We interpret the Lower Member of the Longmaxi Formation as a third order sequence which is composed of transgressive systems tract (TST), early highstand systems tract (EHST) and late highstand systems tract (LHST). The TST mainly consists of mixed siliceous mudstone and clay-rich siliceous mudstone with high total organic carbon (TOC) content, high content of brittle minerals and the highest gas content which is the most favorable shale gas reservoir interval; the EHST is mainly composed of mixed siliceous mudstone and clay-rich siliceous mudstone in the lower part and mixed argillaceous mudstone in the upper part with well-developed laminaes, moderate TOC content and gas content; finally, the LHST is mainly composed of organic-lean and clay-rich mudstone (e.g. silica-rich argillaceous mudstone and mixed argillaceous mudstone) with the lowest gas content. Sequence stratigraphic framework controls the distribution of TOC, mineralogy in time and space, which further controls the sweet spot areally and pay zone vertically for shale gas accumulation in the southeast Sichuan Basin. The data collected from the commercial shale gas wells confirmed that the sequence stratigraphy is valid for the shale gas exploration and can be applied to the shale gas exploration in the other area.