Geological Controls on Production of Coal Bed Methane and Development of the Argyle Block, Surat Basin.
Dharmarajah, Vistoria L.
Honours Degree 2008
University of Adelaide
The aim of this project was to analyse controls on coal bed methane production in the Argyle Block, Surat Basin, Queensland. The Argyle Block is operated by the Queensland Gas Company (QGC) who sponsored this project. In particular, the project aimed to investigate the influence of pre-existing geological structure and present-day stress on production, and thereby their implications for field development in the area.
The tops of the Hutton Sandstone, Taroom Coal Measures, Tangalooma Sandstone and the Walloon Subgroup (all of Jurassic age) were interpreted on nine seismic reflection profiles from three different surveys in the Argyle Block and surrounding areas. The interpreted structures and structural history are consistent with that previously proposed for the region, with the key structural events being:
• Late Permian-Early Triassic normal faulting, and;
• Strike-slip and reverse movements in Cretaceous reactivating older normal faults.
The Argyle Block are displays relatively minor faulting and gentle folding. Nine faults have been mapped across the block and surrounding area and these trend north-south to northeast-southwest. No correlation between proximity to mapped structures and coal bed methane production (or whether wells are pumped or non-pumped) can be observed when comparing the location of mapped structures to production figures from wells. However, there is an insufficient sampling of wells at varying distances from faults to be confident that no such relationship exists.
Both pre-existing natural fractures and present-day stress-induced tensile fractures and borehole breakouts were interpreted on image logs. Three image logs (two resistivity images and one acoustic image) were interpreted by the author and the results were combined with interpretations of image logs from additional wells by JRS Petroleum Research (JRS), Weatherford Geoscience and Technology and QGC. Relatively few present-day stress-induced features (seven drilling-induced tensile fractures and breakouts) were confidently interpreted by the author, with the majority being breakouts on the acoustic image log from Argyle #6. These breakouts indicate a north-south oriented present-day maximum horizontal stress direction. This direction is consistent with the regional trend of present-day stress observed in southeast Queensland. Nonetheless, the QGC data suggest slight rotation of present-day stress (by ~20°-25°) around the major structure of the region, the Undulla Nose. The author interpreted 24 natural fractures from the image logs, and as also shown by the other available interpretations, the orientations of these pre-existing natural fractures are scattered. The scattering of pre-existing natural fracture orientations reflects the complex structural history of the region with normal, strike-slip and reverse faulting, each of which would have generated associated fracture sets of different orientations. Given there are a variety of pre-existing natural fracture orientations, the present-day stress regime is likely to be a key control on directional permeability, which is likely to be best developed in the north-south direction.