Coal-rock gas accumulation mechanism and the whole petroleum system of coal measures

Abstract

Coal measures are significant hydrocarbon source rocks and reservoirs in petroliferous basins. Many large gas fields and coalbed methane fields globally are originated from coal-measure source rocks or accumulated in coal rocks. Inspired by the discovery of shale oil and gas, and guided by “the overall exploration concept of considering coal rock as reservoir”, breakthroughs in the exploration and development of coal-rock gas have been achieved in deep coal seams with favorable preservation conditions, thereby opening up a new development frontier for the unconventional gas in coal-rock reservoirs. Based on the data from exploration and development practices, a systematic study on the accumulation mechanism of coal-rock gas has been conducted. The mechanisms of “three fields” controlling coal-rock gas accumulation are revealed. It is confirmed that the coal-rock gas is different from CBM in accumulation process. The whole petroleum systems in the Carboniferous–Permian transitional facies coal measures of the eastern margin of the Ordos Basin and in the Jurassic continental facies coal measures of the Junggar Basin are characterized, and the key research directions for further developing the whole petroleum system theory of coal measures are proposed. Coal rocks, compared to shale, possess intense hydrocarbon generation potential, strong adsorption capacity, dual-medium reservoir properties, and partial or weak oil and gas self-sealing capacity. Additionally, unlike other unconventional gas such as shale gas and tight gas, coal-rock gas exhibits more complex accumulation characteristics, and its accumulation requires a certain coal-rock play form lithological and structural traps. Coal-rock gas also has the characteristics of conventional fractured gas reservoirs. Compared with the basic theory and model of the whole petroleum system established based on detrital rock formations, coal measures have distinct characteristics and differences in coal-rock reservoirs and source-reservoir coupling. The whole petroleum system of coal measures is composed of various types of coal-measure hydrocarbon plays with coal (and dark shale) in coal measures as source rock and reservoir, and with adjacent tight layers as reservoirs or cap or transport layers. Under the action of source-reservoir coupling, coal-rock gas is accumulated in coal-rock reservoirs with good preservation conditions, tight oil/gas is accumulated in tight layers, or conventional oil/gas is accumulated in traps far away from sources, and coalbed methane is accumulated in coal-rock reservoirs damaged by later geological processes. The proposed whole petroleum system of coal measures represents a novel type of whole petroleum system.