Formation, evolution, reconstruction of black shales and their influence on shale oil and gas resource

Abstract

Black shales are important products of material cycling and energy exchange among the lithosphere, atmosphere, hydrosphere, and biosphere. They are widely distributed throughout geological history and provide essential energy and mineral resources for the development of human society. They also record the evolution process of the earth and improve the understanding of the earth. This review focuses on the diagenesis and formation mechanisms of black shales sedimentation, composition, evolution, and reconstruction, which have had a significant impact on the formation and enrichment of shale oil and gas. In terms of sedimentary environment, black shales can be classified into three types: Marine, terrestrial, and marine-terrestrial transitional facies. The formation processes include mechanisms such as eolian input, hypopycnal flow, gravity-driven and offshore bottom currents. From a geological perspective, the formation of black shales is often closely related to global or regional major geological events. The enrichment of organic matter is generally the result of the interaction and coupling of several factors such as primary productivity, water redox condition, and sedimentation rate. In terms of evolution, black shales have undergone diagenetic evolution of inorganic minerals, thermal evolution of organic matter and hydrocarbon generation, interactions between organic matter and inorganic minerals, and pore evolution. In terms of reconstruction, the effects of fold deformation, uplift and erosion, and fracturing have changed the stress state of black shale reservoirs, thereby having a significant impact on the pore structure. Fluid activity promotes the formation of veins, and have changed the material composition, stress structure, and reservoir properties of black shales. Regarding resource effects, the deposition of black shales is fundamental for shale oil and gas resources, the evolution of black shales promotes the shale oil and gas formation and storage, and the reconstruction of black shales would have caused the heterogeneous distribution of oil and gas in shales. Exploring the formation mechanisms and interactions of black shales at different scales is a key to in-depth research on shale formation and evolution, as well as the key to revealing the mechanism controlling shale oil and gas accumulation. The present records can reveal how these processes worked in geological history, and improve our understanding of the coupling mechanisms among regional geological events, black shales evolution, and shale oil and gas formation and enrichment.