Paleoenvironment evolution and organic matter accumulation of the lacustrine organic−rich shales in fault depression: Example from the Lishu Fault Depression, Songliao Basin, China

This paper is the result of oil and gas exploration engineering.

Objective The paleoenvironmental evolution of shale and its organic matter enrichment mechanisms is one of the key challenges in continental shale oil and gas exploration. The formation mechanisms and controlling factors of organic-rich shales within the K₁sh₂^L Submember of the Lishu Fault Depression urgently require elucidation.

Methods Experiments of bulk mineralogy, major and trace elements, rare earth elements were conducted. Analyses of paleoenvironmental proxies including paleoclimate, paleosalinity, redox conditions, provenance, and paleoproductivity, were studied.

Results The paleoenvironmental evolution of the K₁sh₂^L Submember controlled the characteristics of shales in different layers. The Layer III developed organic-rich massive felsic mudstones, controlled by a semi−humid paleoclimate with high terrigenous influx under deep−water reducing conditions. The Layer II developed organic−rich laminated fine−grained mixed shale, controlled by an arid paleoclimate which promoted high−salinity, stratified reducing waters, coupled with enhanced paleoproductivity due to the input of volcanic nutrients. The uppermost Layer I lacks developed organic−rich shale, correlating with low paleoproductivity levels resulting from reduced terrigenous input.

Conclusions  The combination of relatively arid paleoclimate, tuff development, and anoxic water conditions controlled the enrichment of OM in the calcareous shale of the Shahezi Formation. This integrated paleoenvironmental reconstruction provides critical insights for predicting high-quality shale reservoirs and optimizing exploration strata in faulted basins.