Characteristics of the shale sedimentary environment and organic matter enrichment mechanism in the Jurassic Lianggaoshan Formation in the East Sichuan Basin

Abstract

Objective The good oil test results of Taiye 1 and Xingye L1 wells in the Jurassic Lianggaoshan Formation shale indicate the breakthrough of shale oil/gas exploration in the Jurassic continental shale in the Fuxing area of the eastern Sichuan Basin. To further clarify the oil/gas exploration potential and the organic matter enrichment mechanism of the Jurassic Lianggaoshan Formation shale, utilizing logging and core data of two typical wells Xingye X and Xingye Y, the whole-rock X-ray diffraction, organic carbon, major and trace elements were analyzed. Methods The Liang-2 lower submember was deposited in the environment of warm-wet (moderate weathering conditions) and freshwater, and there was no obvious difference in palaeoclimates of different unit formations. The La-Co data suggests a semi-deep to deep lake environment with a palaeowater depth ranging from 10.9 to 56.1 m; the anoxic reducing environment persisted on the lake floor despite the lake level fluctuation, but the palaeoproductivity was vertically increased. The shale interval is rich in sandy laminae, but the deposition rate of the shale layer is lower than that of the sandstone layer. Results According to the correlation analysis between sedimentary environment parameters and organic carbon content (TOC), the organic matter enrichment in the Liang-2 lower submember shale was mainly controlled by factors of palaeowater depth, palaeoredox environment, deposition rate, and palaeoproductivity, and slightly affected by the palaeosalinity and palaeoclimate conditions. The variation in the deposition environment resulted in the vertical heterogeneity of shale. Conclusion The shale facies of Unit 6, formed in the good conditions for the organic matter enrichment referring to an anoxic environment in deep water, with a low deposition rate, a high palaeoproductivity, and the slight influence of terrigenous sources, was a favorable target layer for exploration.

Keywords

• jurassic
• lianggaoshan formation
• shale
• sedimentary environment
• organic matter enrichment mechanism