Geological characteristics and accumulation mode of coalbed methane in Surat Basin of Australia

Large gas reserves are trapped in the coals of the Middle Jurassic Walloon Subgroup in the Surat Basin. In order to correctly understand the accumulation characteristics, the research means such as sedimentology, geostatistics and natural gas geochemistry are used synthetically to study the sedimentary environment, structural characteristics, coal petrology and quality, origin of gas and models of gas accumulation. The research results show that the coal seams in Walloon subgroup are deposited in the river floodplain of meandering river and the marsh of meandering river delta plain, the coal has the characteristics of low degree of evolution, high ash content and small change in macerals. High permeability and high Langmuir pressure are the prominent characteristics of coal reservoir. Structure transfer zone has an important influence on reservoir permeability. The gas content with the Walloon Subgroup coals is low, and it is most likely to be dominantly secondary biologic gas，thermogenic gas also act to increase gas contents. Coalbed methane accumulation type varies from regions and formations that the Juandah Coal Measures in the northern syncline area is hydrodynamic seal type, the Taroom Coal Measures in Undulla nose zone and some part of the eastern slope is an anticline-water drive sealing type.