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Sedimetology, Diagenesis And Controls On Reservoir Quality In Cored Intervals Of The Browse Basin, North West Shelf, Western Australia.

Tianen, Sharon Ann

Honours Degree, 1994

University of Adelaide

Abstract

The Browse Basin is one of several major Mesozoic depocentres located beneath the remote North West Shelf of Western Australia. It has an aerial extent of some 140 000 square kilometres, the majority of which lies in water depths exceeding 200 metres.

Despite the discovery of a number of significant gas fields and encouraging oil shows the basin remains one of the least explored of all the offshore basins of the North West Shelf. This is due in part to economic difficulties in recovering hydrocarbons from such remote and deep wells but is more commonly attributed to poor reservoir development within the basin. In response to this problem, macroscopic and microscopic examination of all available conventional core was undertaken, supplemented with geophysical evidence and special analysis investigations and combined to provide an understanding of the major controls on reservoir quality and development.

Brewster #1A indicated fair reservoir development within a shallow marine shoreface deposit. Reservoir quality within these quartz arenites is controlled largely by the development of authigenic quartz overgrowths and compaction. Facies influence is limited to the presence of labile framework grains. Permeability is particularly poor and has been attributed to the growth of pore lining authigenic clays.

Potential reservoirs have developed in the fluvio-deltaic quartz arenites of Rob Roy #1. Reservoir quality is generally good with secondary porosity developed after the dissolution of diagentically early anhydrite.

Yampi #1 indicates very poor reservoir development. Deposited in a fluvially dominated deltaic setting, diagenetic modifcation did little to enhance reservoir quality due largely to the presence of significant amounts of ductile matrix.

The three Scott Reef wells have developed what is essentially a fluvio-deltaic system although some intervals show a marked shallow marine influence. Reservoir quality varies between very good at Scott Reef #1, fair at North Scott Reef #1 and generally poor at Scott Reef #2A. The dominant influence on reservoir quality is diagenetic adjustment, but facies type bas a fundamentally important role. Two major lithologies; quartz arenites and bioclastic grainstones, were identified and although both promoted reservoir development, the latter displayed the most potential to develop into a viable deep reservoir target. The best developed of these were seen at Scott Reef #1 where mixing zone/reflux type dolomitisation of uncemented skeletal fragments created significant shrinkage porosity. Later dissolution temporally associated with hydrocarbon introduction and late stage dolomites further enhanced secondary porosity. A similar facies was indicated at North Scott Reef #1 but the introduction of an early phreatic cement reduced permeabilities such that the flow of dolomitising fluids was curtailed. Other less developed reservoirs are present in the Scott Reef area and occur in the clean arenaceous intervals. Reservoir quality is controlled in a similar manner to Brewster #1A and results in secondary porosity being the dominant porosity type.

Hydrocarbon migration has occurred in the Scott Reef area. Its introduction postdates the bulk of the authigenic cements and its temporal association with burial type dolomites suggests the introduction of late stage dolomitising fluids is associated with fluids generated during hydrocarbon maturation and migration. This association is best illustrated in Scott Reef #l.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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