Refining the Mungaroo Formation and Lower Barrow Group depositional models, Exmouth Plateau, North Carnarvon Basin: WA-390-P phase II exploration

Abstract

Permit WA-390-P, in Australia’s Exmouth Plateau, has been the subject of an extensive exploration drilling campaign with gas discovered in the Late Triassic Mungaroo Formation and the Cretaceous Lower Barrow Group. Characterisation of these reservoirs with 3D seismic data, well logs, core and biostratigraphic information has allowed insight and refinement to the previously established depositional models. The Mungaroo Formation comprises a thick succession (more than 2 km) of delta plain deposits characterised on 3D seismic by channel morphologies of differing sizes and orientations. Well penetrations in the channels reveal sand-bodies that can be classified as either single-storey or multistorey. Single-storey sand-bodies are thin (less than 15 metres), narrow in planform (less than one kilometer), lack evidence of lateral accretion and occasionally exhibit a funnel geometry. Multistorey channels are characterised by relatively thick, vertically and laterally amalgamated sand-bodies (more than 15 metres), in a broad channel morphology (more than one kilometer) bounded at its base by a composite surface of erosion. Single-storey channels have been interpreted as distributary channels and multistorey channels as incised valleys. In contrast, the Lower Barrow Group is a contemporaneous wave-dominated delta and slope-to-basin-floor sediment gravity flow system. The depositional environments formed progradational clinoform seismic stratigraphic units that filled accommodation generated during rifting. The delta is organised into arcuate to cuspate lobes that show changes in the shelf-slope trajectory with variations in accommodation and sediment supply. During falling trajectories of the shelf-slope break, the slope is demarcated by gullies forming a line of feeder systems that transport sediment from the delta shoreface into the deep-water. The sediment gravity flows formed coalescing fans that blanket the toe-of-slope and basin floor.