Honours Thesis Abstract

LATEST JURASSIC-EARLY CRETACEOUS DEPOSITIONAL HISTORY OF THE EASTERN MARGIN OF THE EXMOUTH SUB-BASIN

Matthew Strika

UWA, WA

2006 Bonwick - AIG Honours Bursary Winner

The Exmouth Sub-basin, in the offshore Northern Carnarvon Basin, hosts a number of hydrocarbon reservoirs in the Early Cretaceous Barrow Group. The depositional history of the Barrow Group along with the underlying Dingo and Dupuy Formations, and overlying Mardie Greensand Member of the Muderong Shale, was examined via logging of cored intervals from several wells situated on the eastern margin of the Exmouth Sub-basin. The Dupuy Formation sandstones are quartzarenites, subfeldsarenites and sublitharenites. The Barrow Group sandstones are predominantly quartzarenites. Provenance analysis of the modal composition of these sandstones indicates a cratonic source dominated by alkali igneous rocks. In addition subrounded to rounded populations of quartz and heavy mineral grains suggest recycling of a quartz-rich sedimentary source. Prior to the deposition of the Barrow Group in the Early Cretaceous faulting at the margins of the Exmouth Sub-basin resulted in the uplift of Triassic material. It is likely that the Triassic Mungaroo Formation was uplifted and eroded to form the source of the Barrow Group sediments.

The stratigraphic succession in the five wells of the study area has been divided into twenty facies and five facies associations. FA1, FA2, FA3 and FA5 are characterised by highly bioturbated fine sandstones and sandy siltstones with Helminthopsis, Palaeophycus, Thalassinoides and Planolites. These facies associations represent deposition on lower shoreface to inner shelf settings. FA4 is characterised by unbioturbated, medium to coarse sandstones with scoured pebbly bases, minor planar cross bedding arranged in fining upwards trends and represents deposition of fluvial channel fills. Interpreted depositional environments are distributary channels (FA4) on a delta plain and lower shoreface to inner shelf settings (FA1, FA2, FA3, FA5) on a delta front. Sandstones in the Macedon-Pyrenees field located to the west are compositionally similar to the Barrow Group sandstones and a similar mixed provenance is interpreted. Integration of seismic and biostratigraphic data with the sedimentological data is used to interpret deposition of the Macedon sandstones on a more distal portion of the lower shoreface to inner shelf of the delta front.