Geochemical Characteristics of the Upper Permian Longtan Formation from Northeastern Sichuan Basin: Implications for the Depositional Environment and Organic Matter Enrichment

A set of organic-rich shales of the upper Permian Longtan Formation, which is widely developed in the northeastern part of the Sichuan Basin, is a key formation for the next step of exploration and development. At present, most studies on this set of formations have focused on the reservoir characteristics and reservoir formation mechanism of the shales, and basic studies on the palaeoenvironment and organic matter(OM) enrichment mechanism have not been fully carried out. In this paper, we recovered the sedimentary palaeoenvironment by mineralogical, elemental geochemical and organic geochemical analyses, and explored the enrichment mechanism of OM under the constraints of palaeoenvironmental evolution. The shales can be divided into two stages of sedimentary evolution: compared with the shales of the Lower Longtan Formation, the shales of the Upper Longtan Formation are relatively rich in quartz, poor in clay and carbonate minerals, and the OM type changes from type Ⅲ to type Ⅱ2. The depositional environment has undergone a change from sea level rise, from warm and wet climate to dry and cold climate, and from oxygen-poor condition restricted to open reduction environment; the land source input has decreased, the siliceous mineral content has increased, the biological productivity has improved, and the deposition rate has changed from high to low. A depositional model was established for the shales of the Longtan Formation, reflecting the differential reservoir formation pattern of organic matter. For the Lower Longtan Formation shales, the most important factors controlling OM content are terrestrial source input and deposition rate, followed by paleoclimate and paleooxygen conditions. For the Upper Longtan Formation shales, the most important controlling factor is paleo-productivity, followed by sedimentation rate. The depositional model constructed for the Upper and Lower Longtan Formation shales can reproduce the enrichment of organic matter and provide a basis for later exploration and development.