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The Co-Evolution Process and Mechanism of Shale Hydrocarbon Generation-Reservoir-Preservation: A Case Study of Upper Ordovician Wufeng Formation (O3w) - Lower Silurian Longmaxi Formation (S1l) in the Sichuan Basin and Its Periphery

Abstract

Shale gas displays the self-source, self-reservoir, and self-accumulation, and was mixed by secondary oil cracking and kerogen cracking gases at high maturity. The reservoir space consists of mineral and organic matter pores, and the shale gas was mainly trapped by a unified high pressure system formed by the roof and floor of the shale itself. More attention needs to be paid to the co-evolution process of hydrocarbon generation history, development process of shale gas reservoir, and preservation condition. The Fuling O3w- S1l shale gas in the Sichuan Basin was taken as an example to discuss the co-evolution process of hydrocarbon generation-reservoir-preservation, reveal the mechanism of shale gas enrichment and provide the basis for effective prediction of favorable enrichment areas of shale gas. The O3w-S1l shale is mainly marine planktonic algae-rich siliceous shale, and displays an average TOC content of 3.89%. The average thickness of black shale with TOC>2.0% is 38 m in the lower part, and shale in the upper part has lower TOC content (< 1.0%) with a larger thickness. The average Ro value is 2.79%. The shale in the lower part entered the peak oil generation stage in the Early Jurassic-Middle Cretaceous and reached the maximum depth in the Middle Cretaceous with Ro value over 2.5%, and the kerogen and residual oil generated a large amount of gas and hosted in the shale pores. It had promoted the development of organic matter pores by hydrocarbon expulsion and internal pressure increased during the process of shale hydrocarbon generation. The organic-matter porosity is 3%-4%, which formed a three-dimensional interconnected organic-matter pore network and provided good reservoir spaces and migration channels for shale gas. The upper part of the 1st Member of the S1l became a favorable caprock as a result of porosity decrease by limited increasing of organic pores caused by low TOC content and a relatively small amount of generated hydrocarbon. Meanwhile, the depth of the upper part of the S1l is 2326m-2378m, the shale falls within the brittle-ductile transitional zone and provides effective seal for gas preservation. Therefore, if the sealing capacity of the caprock was formed earlier than the period of peak gas generation and reservoir development, and it had not suffered destruction in later stage, the co-evolution process of shale hydrocarbon generation-reservoir-preservation is consistent with a high extent of shale gas enrichment.