THE SOUTHERN BONAPARTE GULF BASIN - NEW PLAYS

Abstract

Integration of regional exploration data with a new basin model involving progressive basinward salt withdrawal has generated new exploration plays in WA-128-P and WA-211-P, which forms part of the offshore Southern Bonaparte Gulf. This area provides a hydrocarbon habitat that is unique to this part of the Bonaparte Gulf Basin.

Three major Palaeozoic megasequences, MS-I, MS-II (A & B) and MS-III, are identified on seismic data and correspond to major stages in the structural and depositional development of the basin, from Silurian through to Triassic times.

Early exploration, targeted on structural highs, encountered numerous hydrocarbon shows. Of most recent significance are the Turtle-1 and Turtle-2 wells. Turtle-1 (1984) targeted a midbasin, MS-I high and recovered oil in MS-III. Turtle-2 (1989) tested an additional 510 m-thick, MSII onlap sequence and encountered, within fractured intervals, significant oil and gas influx accompanied by massive lost circulation. Significant live oil was produced on test despite huge damage inflicted to the fracture porosity and permeability during the fourteen-day well control period.

Recent geochemical work indicates that the oils recovered from MS-II and MS-III have a common marine source. Oils from MS-III are associated with incompetent seals and meteoric waters and are variably degraded and exhibit low GOR. In contrast, oils of MS-II, associated with competent seals, exhibit high GOR. In consequence, a diversity of new exploratory plays are indicated:

Fractured reservoirs in MS-II, stratigraphic onlaps flanking MS-I structures.

Stacked turbidites and basin floor fans deposited in the MS-II salt-withdrawal sub-basins.

Carbonate banks within the MS-II sub-basin's marginal carbonate complexes.

MS-I carbonate structures localised over early salt structures and onlapped by MS-II sediments.

Structural and stratigraphic traps of MS-II and III associated with diapiric salt.

By virtue of the stratigraphic and structural relationship of MS-II sediments which onlap the flanks of structural highs, these plays have not been previously drilled in optimal locations. Consequently, a new phase of exploration specifically targeted at these plays is now planned to proceed in parallel with exploration of MS-III targets in structural traps. The application of the basin model concepts to these MS-III plays has revealed subtle crestal faulting associated with structural highs. The structural and stratigraphic implications of these observations indicate new plays on the flanks of previously drilled structures.