Studies of normal fault systems in modern extensional regimes (e.g. Basin and Range), and in exhumed, ancient rift basins (e.g. Gulf of Suez Rift) have shown a link between the evolution of fault-related footwall topography and associated erosional drainage systems. In this study, we use 3D seismic reflection data to image the footwall crest of a gravity-driven fault system developed during late Middle Jurassic to Early Cretaceous rifting on the Halten Terrace, offshore Mid-Norway. This 22-km-long fault system lacks significant footwall uplift, with hangingwall subsidence accommodating throw accumulation on the fault system. Significant erosion has occurred along the length of the footwall crest and is defined by 96 catchments characterized by erosional channels. These erosional channels consist of small, linear systems up to 750 m long located along the front of the fault footwall. Larger, dendritic channel systems extend further back (up to 3 km normal to fault strike) into the footwall. These channels are up to 7 km long, up to 50 m deep and up to 1 km wide. Fault throw varies along strike, with greatest throw in the centre of the fault decreasing towards the fault tips; localized throw minima are interpreted to represent segment linkage points, which were breached as the fault grew. Comparison of the catchment location to the throw distribution shows that the largest catchments are in the centre of the fault and decrease in size to the fault tips. There is no link between the location of the breached segment linkage points and the location and size of the footwall catchments, suggesting that the first-order control on footwall erosion patterns is the overall faultthrow distribution.
Extensive drilling of the Walloon Subgroup for coal seam gas (CSG) during the last decade has revealed a world-class CSG play on the northern flank of the Surat Basin. Resources discovered in the Walloon Subgroup exceed 30 TCF; this gas now underpins four CSG-to-liquefied natural gas (LNG) projects.
Results to date have revealed the highly heterogeneous nature of the Walloon Subgroup and its associated coal properties. The Walloon Subgroup is typically 350 m thick and contains an average of 30 m of net coal that is interbedded with a range of clay-rich, fluvio-lacustrine lithologies. The most prospective area of the play occurs down-dip and adjacent to the Walloon subcrop edge, where high permeability exists combined with a thick section of net pay. Coals in the Walloon Subgroup are low rank (0.35–0.65% Ro) with gas contents ranging between 1–15 m3/tonne (dry ash-free). Average coal ply thickness is 30 cm, making correlation and prediction of reservoir properties difficult. Reservoir properties—including permeability, gas content and saturation—differ as a result of compositional variability of the coal seams and also the tectonic history.
Mapping of sparse 2D seismic data has highlighted the distribution of major structural features throughout the basin. Coal fracture permeability ranges from less than 0.1 mD to more than 2,000 mD, and mapping has identified areas where permeability appears to be enhanced on structures that have undergone mid Cretaceous–Eocene deformation.
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