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Seismic And Stratigraphic Interpretation Of The Candace Terrace And Preston Shelf, Northern Carnarvon Basin, Western Australia

Mack, Daniel Aaron

Degree of Master of Science, 1993

University of Adelaide

Abstract

The Hydrocarbon Prospects of the Triassic sequence, spectacularly displayed on the Candace Terrace and southwestern Preston Shelf (the Eastern margin of the Barrow Sub-basin of the North West Shelf - W.A.), were investigated chiefly by seismic interpretation. The potential of the Triassic has been established in the Carnarvon Basin, as demonstrated by giant gas/condensate reservoirs on the Rankin Platform. There is incentive for further exploration of shelf areas following recent discoveries (eg. Nickol Bay #1) in Triassic reservoirs on the Lambert Shelf, to the north of the study area.

The Triassic sequence was deposited in response to the evolving northwest margin of Australia following crustal separation and subsidence at the close of the Permian, and contains obvious elements of a transgressive-regressive cycle. Uplift and erosion, associated with the main Callovian break-up event stripped the overlying Jurassic and eroded the top of the Triassic section in the study area. Left lateral wrenching along the Flinders Fault Zone is interred from the spatial distribution of compressive and extensional zones in the study area.

Basal transgressive, transgressive, and progradational deposits are identified on the basis of seismic character. Each genetic unit possesses a characteristic seismic signature clearly illustrated on the seismic data. Well log analysis supports the proposed depositional environments. The upper Locker Shale to lower Mungaroo Formation in Arabella #1 shows a marine to fluvial transition dominated by deltaic plain inter-distributary bay deposits. Such units are recognised by their mineralogical assemblages of calcite, pyrite and lignite and by both fining upward and coarsening upward cycles on the Gamma Ray log. Seismic character allowed sandstone packages to be identified in the hanging wall adjacent to the Mermaid Fault, a major detachment surface. These packages were mapped from section to section and appear to offer potential shoe-string style reservoir targets with their distribution controlled by the adjacent normal growth fault.

Analogue sandbox modelling demonstrated that evolution of hanging wall structure is controlled by the ramped listric geometry displayed by the Mermaid Fault. The high degrees of match between the sandbox analogues and the seismic sections suggests the northwest-southeast sections are true dip lines, parallel to the direction of extension.

Most of the Triassic plays are high risk within the study area, and appear to have been adequately tested by wells already drilled. Access to mature source rocks seems to be the major risk on the Candace Terrace and southwestern Preston Shelf. Good quality source rocks are present within the Triassic sequence, and areas where deeper burial has occurred, allowing thermal maturity to be reached, or where migration pathways to the Jurassic Dingo Claystone are less complex, may prove prospective.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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