The Structural Evolution and Coal-Bed Methane Potential Of The Ween Trough, Cooper Basin, South Australia.
Honours Degree, 2000
University of Adelaide
Coal-bed methane represents a new hydrocarbon play in the Cooper Basin, and its potential is being investigated by Strike Oil NL in the Weena Trough of the Southern Cooper Basin. This study presents a model for the structural evolution of the Weena Trough, and discusses coal-bed methane potential of the area. The study involved interpretation of regional seismic reflection profiles and well logs from the area.
The Weena Trough underwent sedimentation from Late Carboniferous to Early Triassic times. The east-west orientation of the Weena Trough is postulated to be due to the glacial reactivation of underlying east-west lineaments in the Cooper Basin, resulting in the northern and southern bounding faults. The structural evolution comprised an early extensional regime and three subsequent compressional stages, each separated by gentle subsidence. The extensional regime was active during the deposition of the Merrimelia Formation and Lower Patchawarra Formation and is witnessed by growth mapped on the northern and southern bounding faults of the Weena Trough. The first compressional stage occurred after the deposition of the Daralingie Formation and resulted in the Tinga Tingana reverse fault and Daralingie Unconformity. The final compressional stage occurred at the end of sedimentation in the Cooper Basin in Mid Triassic times, as indicated by the unconformity at the top of the Cooper Basin.
Coal-bed methane is most prospective where the coals are thickest. The coals in the Weena Trough were interpreted on the seismic reflection profiles, and variations in amplitude were interpreted to relate to the thickness of the coals. A seismic amplitude map of the Patchawarra Formation coal was produced and interpreted to infer locations of thick coals. The coals of the Patchawarra Formation are thickest on the Weena-1 structure, north of the Weena-1 structure, on the Tinga Tingana-1 structure, and west of Cherri-1.
Structural permeability of coal cleats is a critical factor in the economics of coal-bed methane production. Hence, coal permeability is directly linked to in situ stress magnitudes. The optimal orientation of coal cleats is parallel to the maximum horizontal stress (ie the minimum horizontal stress is normal to the cleats). The east-west structural trend in the Weena Trough is likely to induce similar trending face cleats in the coal. If the regional east-west orientation of the maximum horizontal stress seen in the Cooper Basin applies to the Weena Trough, the cleats are optimally oriented to be open and hydraulically conductive. This greatly enhances the prospectivity of coal-bed methane in the Weena Trough. In situ stress is lower at shallower depths, therefore coal beds on structural highs will tend to be more hydraulically conductive, and hence more prospective. Combining all of the above factors, the areas of maximum coal-bed methane potential are on the structures penetrated by Weena-1 and Tinga Tingana-1.