--> Coal Seam Gas in the Galilee Basin, Australia: Insights From Burial History and Exhumation Analysis
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Coal Previous HitSeamNext Hit Gas in the Galilee Basin, Australia: Insights From Burial History and Exhumation Analysis

Abstract

The late Carboniferous-Middle Triassic Galilee Basin in central Queensland covers an area of ~ 250,000 km2 and is widely considered a frontier for coal Previous HitseamTop gas (CSG) exploration. However, relatively little is known about the coal reservoirs, in terms of both their CSG prospectivity and the burial/thermal histories of the host sequences. In this study, we have used a holistic, mixed-method approach on a basin-wide scale that includes the use of seismic interpretation, stratigraphic mapping, reservoir and gas data analysis including carbon isotopes, multivariate statistical analysis of core data, outcrop investigation, 1D basin modelling and a quantification of regional apparent exhumation from well logs. Our geohistory analysis indicates that relatively slow subsidence rates prevailed in the Galilee Basin from the Carboniferous to the Late Cretaceous, punctuated by two significant exhumation events. The Permian coal measures generated up to 40m3/t of gas when maximum burial (1.5 to 2 km) was reached in the Late Cretaceous. Rapid subsidence (50 m/Myr compared to a background 5 m/Myr) followed by rebound at 95 Ma, resulting in a kilometer scale uplift and erosion phase. Geohistory models and exhumation analyses provide evidence for a slab detachment event, post dating the cessation of subduction along eastern Gondwanaland, indicating that subduction dynamics and upper mantle processes had a profound effect on basin evolution and the resource potential of the Galilee Basin. Isotopic evidence suggests the remaining gas is biogenically derived, with a remnant mixed thermogenic signature. The bulk of this thermogenic gas was likely lost as a consequence of the Late Cretaceous exhumation event. It is therefore concluded that burial and exhumation history has had an overriding influence on variations in coal properties on present day gas contents. Low gas saturations and distance from market, despite high permeabilities and thick coals are the major challenges to developing the CSG resources of the Galilee Basin.