THE GWYDION DISCOVERY: A NEW PLAY FAIRWAY IN THE BROWSE BASIN

Abstract

The Gwydion-1 oil and gas discovery well is located in exploration permit WA-239-P on the sparsely explored Yampi Shelf area of the Browse Basin. The Gwydion feature was first recognised as a series of stacked seismic amplitude anomalies, which were interpreted to represent hydrocarbon-bearing Barremian to Albian age shallow marine sandstones draped over a prominent basement high. Amplitude versus offset analysis and modelling supported this interpretation.

Gwydion-1 was spudded on 4th June, 1995, and discovered three gas bearing zones and one oil and gas bearing zone. The lowermost zone is Barremian to Hauterivian in age and consists of 12.6 m of net gas-filled glauconitic sand overlying a 9.5 m net oil-filled quartz sand. The three overlying hydrocarbon zones consist of glauconitic reservoirs of Barremian to Albian age.

The play fairway for Gwydion-style traps has been named as the Echuca/Swan-Bathurst Island Group/Shelfal Play Fairway. It comprises mature Swan Group and Echuca Shoals Formation source rocks, and Bathurst Island Group reservoirs and seals. The limits of the play fairway on the shelf are controlled by the existence of topographic relief in the underlying basement metasediments. Migration pathway analysis suggests that the eastern margin of the Browse Basin is favourably situated to receive charge from the mature source rocks within the basin.

The dominant northwesterly dip of the strata on the Yampi Shelf limits the potential for structural traps. Accordingly, a thorough understanding of the sequence stratigraphic architecture of the succession is necessary in order to generate the stratigraphic play concepts which hold the bulk of the prospectivity in the area.

Gwydion-1 was plugged and abandoned as an uneconomic oil and gas discovery. It was, however, significant as it validated a new play type and generated renewed interest in the eastern margin of the Browse Basin for the first time since the mid 1970s; an area previously thought to be too shallow, too far from mature source and lacking reliable seal.