Seismic stratigraphic relationships within a lowstand reservoir system: examples from the Barrow Group, Southern Exmouth Sub-Basin, NW Australia

Abstract

The Late Jurassic–Early Cretaceous Eskdale and Macedon members of the lower Barrow Group comprise some of the main oil-bearing reservoirs in the Exmouth Sub-basin. These high quality sandstones form the reservoirs in the Stybarrow and Eskdale oil fields. Understanding the architecture of these deepwater successions is important in both exploration and development projects. This paper documents detailed stratigraphic relationships and depositional geometries as defined on high quality seismic data sets and associated well data. An initial phase of lowstand deposition (Eskdale Member) is recorded by the development of two main canyon systems; the Eskdale and slightly younger Laverda canyons. These systems are remarkably well imaged on 3D seismic data, allowing for detailed definition of channel morphology and associated fill and spill facies. Channel complexes are up to 1 km-wide and 100 m-deep, and display evidence for multiple phases of erosion and in-channel aggradation. Overbank/spill facies are also identifiable, including crevasse lateral lobes and ‘chute’ channels. These canyon systems fed contemporaneous downdip basin floor fans that display a variety of classical fan morphologies and depositional elements including terminal lobes, fan pinchout edges, distributary channel systems and localised outflow facies. The distribution and morphology of the Eskdale and Laverda canyons and associated fan intervals can be related to topographic gradient changes within the basin (i.e. from shelf to slope to basin floor). These topographic changes are in turn a response to regional tectonism, in particular active rifting along basin margins. An ensuing phase of less confined, shelf-slope turbidite deposition (Macedon Member) records late-stage lowstand processes. Detailed well and seismic control from the Stybarrow Field and surrounding areas has identified multicyclic sands recording deposition of stacked turbidite lobes. These lobe complexes are more laterally continuous than the canyon facies and are comprised of amalgamated sheet sands and lower-relief channel sands, and are generally between 15–25 m thick. In the greater Stybarrow area the original lobate geometries have been subsequently modified by a phase of late-stage erosion. Outcrop analogues for the Macedon Member can be seen in the lobe complexes from the Tanqua Fan intervals of the Karoo Basin, which are similar in both scale and morphology. These lobe complexes extend laterally for tens of kilometres with constituent individual lobes often displaying evidence for compensational depositional processes. This paper was originally published in the Proceedings of the West Australian Basins Symposium 2013, which was held from 18–21 August 2013 in Perth, Australia.