Search

Gas content and gas composition trends with depth have been investigated for the Sydney Basin. Four distinct zones have been identified, which can be classified according to depth below ground surface. An upper low gas zone (Zone 1, 0–100 m), dominated by CO2and with very low gas contents (<0.7 m3/t), is underlain by a biogenic methane-rich zone (Zone 2, 100 to ∼250 m), with a rapid rate of increase in gas content with depth, followed by a mixed gas zone (Zone 3, ∼250 to ∼600 m), comprising biogenic and thermogenic methane and magmatic CO2, and having a lower rate of gas content increase with depth relative to Zone 2, and Zone 4 (∼600 m+), which contains thermogenic methane and other ‘wet gases.’ A model is proposed that provides a rationale for the origin and timing of emplacement of the various coal seam gases in the Sydney Basin. [ABSTRACT FROM PUBLISHER]