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Sedimentary Architecture in Relation to Gas Migration; Late Permian Blackwater Group, Bowen Basin, Australia
Coalseam gas in the Bowen Basin is a major resource and potential energy source. Total methane generation from coalseams is around 150 m3/t of coal at bituminous ranks. Methane adsorption isotherms for Bowen Basin coals indicate a maximum sorption capacity of up to 45 m3/t. Excess gas generated of 50 m3/t is not capable of being stored within the coal structure and has migrated upsection. To advance understanding of sedimentary architecture in relation to gas migration we have investigated the lithofacies mosaic of the regionally developed Rangal Coal Measures (RCM), based upon extensive highwall exposures at the Newlands mine. Additional intensive drilling adjacent to the highwalls has allowed reconstruction of the lithofacies patterns and relationships to be mapped in detail over a 50 km2 area. In addition, the regional distribution of the RCM has been mapped by seismic profiling across the entire northern Bowen Basin with stratigraphic control provided by a number of deep stratigraphic drillcores. Facies analysis at Newlands has identified six terrestrial depositional environments: fluvial channels; levee bank to crevasse splay; floodplain; marsh; peat-mire and lacustrine. Temporal control afforded by two tuff beds and three thin coal seams, shows that siliciclastic deposition developed as a succession of rapidly aggrading, overlapping lobe systems. Similar facies mosaics apply to other units of the coal-bearing Blackwater Group. Only the lacustrine facies has the potential of providing stratigraphic seals. Lack of large gas reservoirs in the Bowen Basin is probably due to rapid basinal aggradation by immature volcanolithic sands and silts and laterally impersistent caprock assemblages, thereby permitting extensive leakage. Gas leakage is estimated at 300-500 trillion cubic feet (Tcf) compared to total gas-in-place of c. 150 Tcf.
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