Occurrence types and enrichment model of organic matter in the sepiolite-containing successions: A case study of the first member of Mid-Permian Maokou Formation, Eastern Sichuan Basin, SW China

The occurrence types and controlling factors of organic matter in the sepiolite-containing successions of the first member of Mid-Permian Maokou Formation (Mao-1 Member for short) in the Eastern Sichuan Basin, SW China, have been investigated through outcrop section measurement, core observation, thin section identification, argon ion polishing-field scanning electron microscopy, energy spectrum analysis, X-ray diffraction, total organic carbon content (TOC), major and trace element analysis. Finally the symbiotic adsorption model of sepiolite for organic matter enrichment has been established. The results show that the sepiolite-containing successions of the Mao-1 Member are composed of the rhythmite of mudstone, argillaceous limestone and limestone, with five depositional intervals vertically and the organic matter mostly developed in the mudstone and argillaceous limestone layers within the lower three intervals. The organic matter occurrence types are mostly layered or nodular in macro to meso-scale, blocky-vein-like under a microscope, but scattered, interstitial or adsorbed at a mesoscopic scale. It underwent transition processes from lower to higher salinity, from oxygen-poor and anoxic reduction to oxygen-poor and localized oxygen enrichment on the palaeo-environment of the Mao-1 Member. The first two intervals of the early depositional phase of Mao-1 Member constitute the cyclothems of mudstone, argillaceous limestone and limestone and quantities of fibrous-feathered sepiolite settle down within the Tongjiang-Changshou sag with continuous patchy organic matter from adsorption of alginate by sepiolite in intercrystalline, bedding surfaces and interlayer pores. The third and fourth intervals in the mid-depositional phase are mostly composed of the mudstone and argillaceous limestone alternations with the continuous patchy or banded organic matter in the surface and inter-crystalline pores of fibrous, feathered and flaky sepiolite. And the fifth interval in the late depositional phase of the Mao-1 Member comprises the cyclothems of extremely thin layered argillaceous limestone and thick-layered limestone with the fibrous sepiolite depositing in the argillaceous limestone and irregular organic matter dispersing around the sepiolite. Therefore, the symbiotic adsorption between organic matter and sepiolite effectively enhances the preservation efficiency of organic matter and improves the source rock quality of the Mao-1 Member, which enhances our understanding on the enrichment model of the depositional organic matter.