THE M. AUSTRALIS SANDSTONES, DAMPIER SUB-BASIN, AUSTRALIA

Abstract

Three independent Barremian sandstone units that are characterised by the M. australis palynozone have been identified in the Lewis Trough and Enderby Terrace of the southeastern Dampier Sub-Basin, offshore Western Australia. They constitute a basin-floor fan unit and shelfal transgressive unit that are characterised by the lower Af. australis sub-zone, a shelfal marine to fluvial unit that is characterised by the middle M. australis sub- zone and a shelfal marine unit that is characterised by the upper M. australis sub-zone. The M. australis sandstones are characterised by their excellent reservoir quality, generally common to abundant glauconite content and common provenance.

Core, wireline log and seismic data from wells in the Lewis Trough indicate the sediments characterised by the lower M. australis sub-zone form a mass-flow deposit on the regionally extensive Intra-Muderong Hiatus. Transgressive shelfal greensands, interpreted to lie within the latter part of the lower M. australis sub-zone overlie the In tra-Muderong Hiatus on the Enderby Terrace. The glauconitic sandstones characterised by the middle M. australis sub-zone were deposited during a relative highstand and overlie a maximum flooding surface identified in wells on the southern Enderby Terrace. These deposits form the reservoir section for the Wandoo and Stag oil fields. At Wandoo they form a series of seismically definable progrades, whereas at Stag they are the distal toe-sets that lie sub-parallel to the underlying surface. The subsequent sequence boundary is identified in wells and on seismic data as an erosional surface cutting the underlying sediments. Glauconite-rich, transgressive deposits form a fining-up sequence overlie the sequence boundary. Glauconitic sandstones characterised by the upper M. australis sub-zone were deposited at the palaeo- shelf break during a minor regression.