关键词: 陆相煤系地质体系, 烃源岩类型划分, 煤源岩, 成烃-成储演化, 多类型储集空间, 煤源岩油气系统

Abstract: The development of shallow coalbed methane (CBM) has long been constrained by the traditional perception of coal as a solid mineral resource, resulting in its resource assessment confined to depths shallower than 1 500 m and global production stagnated by 2017. Since 2019, China National Petroleum Corporation has redefined coal-rock as a source-reservoir integrated reservoir rock and achieved an exploration breakthrough at a depth of over 1 500 m in Ordos Basin. The development of deep CBM in the basin has shown the characteristics of “gas production immediately upon well opening and then reaching peak rates rapidly”, which has promoted a strategic shift from shallow to deep exploration targets. However, the fundamental geological theory of coal-rock as both source rock and reservoir remains to be systematically advanced. Based on surveys of modern wetland swamps in Sanjiang Plain, Yellow River Delta, and Liaohe River Delta, etc., and by introducing sedimentological and petroleum geological perspectives, this study conducts research on the definition of the coal-bearing geological system, the controls of coal-forming environments, the classification of source rock types, the hydrocarbon generation and reservoir-forming evolution of coal-rocks, and the accumulation models of coal source rock petroleum system. The research results show as follows. (1) The continental coal-bearing geological system is innovatively defined as a composite system of terrestrial biological and geological construction, which initiates with the large-scale growth and preservation of lignin-rich plants and terminates with the completion of extensive swamping. It is controlled by five factors of tectonism, sedimentation, drainage network, soil, and climate, and constructed by an assemblage of coal seat, coal-rock and carbonaceous mudstone. (2) Coal source rock is the third type of high-quality source rock after marine and lacustrine source rocks, with a total organic carbon (TOC) content mostly exceeding 80% and a hydrocarbon generation capacity of up to 300 m³ per ton of coal. Based on the origin of hydrocarbon-generating parent material, source rocks are classified into aquatic organism-, terrestrial plant-, and mixed-type. The coal-rock belongs to a humic (Type Ⅲ kerogen) source rock dominated by woody plant input. (3) A four-stage thermal evolution scheme is proposed based on qualitative changes in hydrocarbon generation behavior [the low maturity, maturity, high maturity and over-maturity stages were divided respectively with the vitrinite reflectance (R_o) of 0.4%, 1.3% and 4.0% as nodes]. Hydrocarbon-generation evolution of coal source rocks can be classified into the biogas stage and the thermal evolution stage. In the mature stage, the contribution rate of the specific surface area of micropores is over 40%. Cleat density and gas yielding intensity peak at R_o=1.3%. The coal-rock reservoir is identified as a multi-scale pore-fracture-surface composite reservoir space system. (4) Coal source rocks constitute an independent petroleum system. Guided by “source-reservoir relationship, accumulation dynamics, occurrence state, and development response”, coal source rock petroleum system can be classified into CBM reservoir, coal seam oil-gas reservoir, and conventional oil-gas reservoir. They possess dual accumulation pathways of “self-generation and self-storage with pressure sealing” and “separate generation and storage driven by buoyancy”. (5) Volume development and three-dimensional development are the most favorable development methods for coal-rock gas fields in China. Drilling and segmented volume fracturing technologies of horizontal wells are the core for the development of coal-rock gas reservoirs. These findings deepen the geological connotation of source-reservoir integrated coal-rock reservoirs, improve the theories of hydrocarbon generation, reservoir formation, and hydrocarbon accumulation in coal-rocks, and provide scientific supports for the assessment, efficient exploration and development of coal-bearing oil and gas resources.

Key words: continental coal-bearing geological system, classification of source rock types, coal source rock, hydrocarbon generation and reservoir-forming evolution, multi-type reservoir space, coal source rock petroleum system