Abstract
Coals in the Sydney Basin contain large amounts of gas ranging in composition from pure methane (CH4) to pure carbon dioxide (CO2). These gases are derived from thermogenic, magmatic and biogenic sources and their present-day distribution is mainly related to geological structure, depth and proximity to igneous intrusions.A coal bed methane (CBM) study of the Camden area of the Sydney Basin has been jointly conducted by Sydney Gas Company NL (SGC) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO). The delineation of high production fairways is vital for any CBM project development to be commercially successful. An integrated research project employing various methods of reservoir characterisation, including geological, geochemical, geomechanical and gas storage analyses contribute to this delineation for the Camden area, where SGC is currently developing the 300-well Camden Gas Project.In particular, accurate determinations of gas content, saturation levels, composition and origin, as well as interpretations about distribution, are essential for identifying sweet spots for CBM production optimisation. The extent of gas saturation is a function of numerous factors, including amounts of gas generated between the Permian and Late Cretaceous, amounts expelled from the system during Late Cretaceous-Tertiary uplift and amounts of subsequent secondary biogenic methane generated and absorbed in the coals. The extent of this secondary biogenic gas generation appears to be greatest in coals proximal to the basin margins, where meteoric waters carrying bacteria and nutrients had ready access. Significant enhancement of methane content also occurs, however, in deeper parts of the basin where permeable structures exist.The integrated study shows that high production CBM wells drilled to date by SGC are located in zones of enhanced permeability. In these locations original thermogenic wet gases have been removed and additional secondary biogenic methane has been generated due to microbial alteration of coal, hydrocarbons and carbon dioxide. This process has replenished the coals by enhancing the methane contents of the respective seams and this phenomenon can be termed ‘bio-enhancement’ in the context of CBM production.
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