Enrichment law of organic matter in marine shale in sichuan basin

Key words: 
• shale gas /
• marine shale /
• lagoon /
• deep water shelf /
• graptolite

Abstract: In order to deeply analyze the sedimentary environment of organic matter enrichment in the shale of Wufeng-Longmaxi Formation in Sichuan Basin, this paper studies the spatial and temporal distribution characteristics of shale and its organic matter by comparing the metallogenic model of coal, combining the spatial distribution of primary productivity and carbon fixation rate of different water bodies, as well as the characteristics of sedimentary facies, paleogeomorphology, fossil development, etc. The study found that the shallower and more closed the water body, the higher the primary productivity and carbon fixation rate; Oil-bearing shale is developed in the restricted desalinated shallow water environment, which has low energy, stagnant water and strong reduction environment, and can develop type I organic matter; Shale oil and gas have a similar pattern with coal seams. The Mawei pattern, the closer to the land, the richer the organic matter is, and is associated with coal, evaporite, etc; The carbon and nitrogen sources in the water body are mainly exogenous. The development of sapropelic shale and stone coal from Proterozoic to Early Paleozoic is suitable for the lack of terrestrial higher plants that produce humus at this time; The open shelf or bay environment has low primary productivity, and the seabed is a high hydrodynamic environment affected by ocean currents, which is not conducive to the development of shale and preservation of organic matter; Marine transgression or high water deposition periods are not conducive to organic matter enrichment. It is concluded that the shale of the Wufeng Formation - Longmaxi Formation is developed in a regressive environment, which is a shallow sea-land transitional environment such as lagoon and relatively closed bay restricted by ancient land and underwater low uplift, and has the characteristics of near land and far water; Finally, a two-dimensional table of water depth and closure is established to describe the enrichment law of organic matter in detail. It is proposed that the sufficient condition for the enrichment of organic matter in marine shale is a closed water body, and the necessary condition is a shallow water body (<40 m).