The coupling mechanism between geomorphology of shale sedimentary and differential enrichment of shale gas in Longmaxi Formation in Changning area

There are spatial heterogeneities in the development process of shale in Longmaxi Formation of Lower Silurian in Changning area, limiting high-productivity areas to specific structural units. Based on the newly obtained drilling data of Changning gas field (area), combined with the sedimentary paleogeomorphology of Longmaxi Formation, this paper explores the paleoproductivity and oxidation-reduction environment under different geomorphic units, and expounds the connection mechanism between geomorphic units and shale gas enrichment. The results show that :(1) The sedimentary palaeogeomorphology of Longmaxi Formation in Changning area presents a multi-level pattern of inter-uplift and depression, and two low amplitude uplifts are formed in the southwest and northeast. The provenance direction indicated by the spatial change of mineral composition is highly consistent with the palaeogeomorphology. (2) Compared with the ancient depression, the ancient uplift has more terrestrial nutrients and higher paleomarine productivity, which is conducive to the advantageous enrichment of organic matter. The near-source characteristic renders the ancient uplift a brittle area with relatively high quartz content. However, with the reduction of sea level, the oxidation of ancient uplift is stronger than that of ancient depression, which is not conducive to the preservation of organic matter. (3)The porosity, TOC (Total Organic Carbon content), and gas content of the Longmaxi Formation in Changning area are closely correlated, indicating that organic pores are important components of high-quality shale pores. (4) Compared to the ancient depression, the ancient uplift is more conducive to hydrocarbon generation and storage of high-quality shale organic matter, and it also facilitates later production fracturing.