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Approaches to the recognition of Fluvial Sub-Environments in Subsurface Successions: Case Study From the Fluvio-Deltaic Triassic Mungaroo Formation, Exmouth Plateau, Australia


The fluvial and fluvio-deltaic Triassic Mungaroo Formation was deposited within a transgressive system tract. It forms the principal reservoir for a major gas play offshore northwest Australia, whose key reservoir characterization challenge is to better understand the style and distribution of channelized depositional elements. This study addresses this challenge by mapping a high-resolution 3D seismic volume from the Exmouth Plateau of the Northern Carnarvon Basin, supported by lithofacies analysis of core and wireline data. Specific objectives are to: (i) catalogue sub-seismic scale fluvial to deltaic architectural elements in core and wireline logs; (ii) map the plan-form morphology of seismic-scale fluvio-deltaic elements, (iii) classify key stratigraphic intervals according to their accommodation setting, and (iv) match the intervals to likely modern analogues. Seven sub-seismic-scale architectural elements are identified in core: primary (high energy) channel, low energy channel, proximal crevasse splay, distal crevasse splay, gleysol (swamp), lake, and inter-distributary bay. Flattening the seismic cube has enabled visualization of stratally-aligned slices, within which such architectural element types can be mapped; attribute analysis highlights fluvial deposits. The following seismic elements were identified and mapped in GIS: valley/channel belt, valley margin, floodplain, and gleysol (mire). Well-log data confirm that valley-margin elements contain sub-seismic scale sands of probable crevasse-splay and accessory channel origin. Additionally, spectral decomposition allows a higher-resolution visualization of channel geobodies. The dimensions and sub-areal proportions of seismic elements are used assess likely accommodation conditions within different intervals. Apparently higher-accommodation settings led to the progressive fill of multi-lateral channel and valley elements (<7 km width), as well as the establishment of distributary channel networks and widespread gleysol development. Low accommodation resulted in laterally constrained (<1 km width) channel elements, potentially within incised valleys, with associated valley-margin elements. Negligible accommodation settings resulted in overprinting of channel elements and minimal conservation of overbank elements. Modern analogues for the system including the Kobuk River, Alaska and Cumberland Marshes, Saskatchewan are proposed, along with depositional models for the studied intervals.