Emerging Continuous Gas Plays in the Cooper Basin, South Australia
The Late Carboniferous-Late Triassic Cooper Basin is an intracratonic, non-marine basin consisting of a basal glaciogenic succession overlain by fluvio-deltaic, floodplain and lacustrine deposits with extensive and thick coal measures.
The presence of a basin centred gas accumulation (BCGA) in the Nappamerri Trough, Cooper Basin has been suspected for over two decades. Resistivity of the Permian succession exceeds 20Ωm over large intervals, tests have recovered gas with no water, and gas is located within overpressured compartments indicative of hydraulic isolation. The Permian succession in the Nappamerri Trough is up to 1000m thick, comprising very thermally mature, gas-prone source rocks with interbedded sands, ideal for the creation of a BCGA. Excluding the Murteree and Roseneath shales, the succession comprises up to 45% carbonaceous and silty shales and thin coals deposited in flood plain, lacustrine and coal swamp environments.
The Early Permian Murteree and Roseneath shales are thick, generally flat lying, and laterally extensive, comprising siltstones and mudstones deposited in large and relatively deep freshwater lakes. Total organic carbon values average 3.9% in the Roseneath Shale and 2.4% in the Murteree Shale. The shales lie in the wet gas window (0.95 - 1.7% Ro) or dry gas window (>1.7% Ro) over much of the Cooper Basin.
Thick Permian coals in the deepest parts of the Patchawarra Trough and over the Moomba high on the margin of the Nappamerri Trough are targets for deep coal seam gas. Gas desorption analysis of a thick (18 metre) Patchawarra coal seam returned excellent total raw gas results ranging 15.3 - 27.8 scc/g and averaging 21.2 scc/g (680 scf/ton) over 10 metres. This equates to 15 Bcf gas in place in a 15m seam in a one square kilometre area. Scanning electron microscopy has shown that the coals contain significant microporosity in variably preserved cell lumen within telinite and semifusinite bands.
Recent off-structure drilling in the Nappamerri Trough has confirmed the presence of gas saturation through most of the Permian succession, including the Roseneath and Murteree shales. Basin centred gas, shale gas and deep coal seam gas plays in the Cooper Basin are now the focus of an escalating drilling and evaluation campaign.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California