TRAPPING MECHANISMS AND THE HYDROCARBON POTENTIAL OF THE EXMOUTH PLATEAU, WESTERN AUSTRALIA

Abstract

An interpretation of the structural configuration and hydrocarbon potential of the Exmouth Plateau has been made utilizing the results of G.S.I's 1976 Scientific Investigation (No. 10 S.L.).

Seismic sections suggest sediments down to at least four seconds (two way time) below the sea floor. A prominent regional unconformity separates faulted and dipping sediments from generally flat overlying beds.

Eight horizons of apparent regional importance were picked and correlated to existing well control.

Structure maps were prepared for the regional unconformity and three overlying horizons, including the interpreted top of the Barrow Group. Mapping showed the main potential hydrocarbon traps to be large fault block features associated with the regional culmination of the unconformity event, and broad anticlinal drapes at top Barrow Group level. Geological interpretation suggests that the fault blocks consist of late Middle to Upper Triassic sediments, probably sandstone and shale. These sandstones could be potential reservoirs for hydrocarbons generated at depth. Seismic evidence of hydrocarbon generation includes a probable gas chimney associated with the western bounding fault of the highest fault block.

The present sea floor topographic high of the Exmouth Plateau is coincident with the large drape structure embracing a foresetting package within the Barrow Group. Velocity studies over the Plateau suggest a probable lack of sand in the upper part of the Barrow Group section. Sourcing of sands in the lower part is envisaged as originating from sediments in older, underlying fault blocks. The Triassic fault blocks are considered to have the higher potential for oil reserves, but recoverable volumes are not likely to be in the 'giant' category (500 million barrels).