The Angel Gas Field, North West Shelf, Australia: An Integrated 3d Seismic And Petrophysical Study
Doctor of Philosophy, 1996
University of Adelaide
The Angel Gas Field on the North West Shelf, offshore Western Australia was studied by integrating a number of geological and geophysical disciplines. The aim of the study was to use all available information, primarily 3D seismic data, to describe and predict the geometry, petrophysical characteristics, and fluid content of the reservoir unit of the Angel Field. This integrated approach has also generated new ideas and methods such as recognition and mapping of massive dolomite-cemented zones in the sandstone from the 3D seismic data, and development of a model for the age and mode of formation of the dolomite cement in the sandstone from integration of 3D seismic mapping and carbon isotope and other petrographic data. It has focussed on the use of 3D seismic data, specifically analysis of the pre-stack gathers and the stacked data, to delineate the reservoir quality.
AVO analysis, using intercept-gradient crossplots, was a useful and robust method, especially when empirically calibrated to the well control via Biot-Gassman fluid replacement modelling, a new approach developed in this study. AVO analysis gave a good gas indicator, and a semi-quantitative porosity and Vclay indicator. The AVO gas response, as calculated by the R-R. method, shows anomalies that are in accordance with the relative change predicted by theory. These anomalies coincide with the time closures at top reservoir level. It appears that in the study area the method provides a useful hydrocarbon indicator, except when the reservoir sandstone contains dolomite-cemented layers. The presence of dolomite-cemented zones severely reduces the usefulness of AVO analysis, by overwhelming any potential gas effect. The properties of the overlying shale also have a significant effect on the AVO response, with the potential to cause false gas anomalies. The extent of the dolomite-cemented zones of the Angel Formation was mapped from the 3D seismic data. The dolomite cement is interpreted to occur only in the northern part of the field, in a roughly oval-shaped area along the crest of the structural trend. The dolomite cement formed in the Miocene, synchronous with hydrocarbon migration into the structure.
The Dampier Sub-basin appears to be underlain by a low-angle detachment fault which dips towards the west, and has a ramp-flat-ramp geometry. Extension and subsidence during the Triassic and Early Jurassic caused rotation and uplift of the Madeleine Trend, the narrow linear intra-basin high which underlies the Angel Field. Miocene-to-Present E-W compression caused right-lateral strike-slip movement along the Madeleine Trend, and the formation of the faults that cut the reservoir interval. Only the NW-SE normal faults are likely to be open to fluid flow, while the three other fault sets recognised are likely to be closed. The main E-W fault zone in the Angel Field appears to be acting as a seal, as different hydrocarbon-water contacts are interpreted to the north and the south of the fault zone.
The Angel Formation was sub-divided into three units by integrating 3D seismic and palynological data. The seismic horizons that separate these layers are very close to time-synchronous in the study area. Each of the three units is interpreted to have been sourced from a different area, in contrast to the usual assumption that the Legendre Trend in the east was the major sediment source, by combining seismic reflection patterns with heavy mineral data. The Lower Unit was sourced from the south-west, probably from the Rankin Trend high via the Kendrew Terrace. The Middle Unit was sourced mostly from the north, with a minor contribution from the south. The northern source was probably local areas of uplift such as the DeGrey Nose and the uplifted fault blocks of the Beagle Sub-basin. The Upper Unit was sourced mostly from the southeast, probably from the Legendre Trend, with a small amount from the Rankin Trend.
The best reservoir quality (cleanest sandstone and highest porosity) in the Upper Unit is interpreted to occur along the Madeleine Trend, and the lowest quality within the Kendrew Terrace, to the north-west of the Angel Field. The dolomite cement in the Angel Formation drastically reduces porosity, and therefore also reservoir quality, in the northern portion of the field. Eleven time closures at Upper Unit level were studied using both the pre- and post-stack 3D seismic data. Ten of these showed an AVO gas anomaly, and the other was within the dolomite-cemented zone. The one gas anomaly that did not occur within a time closure, on the northern flank of the Angel Field, can be explained by offset-dependent tuning from an intra-formation reflector.
This example demonstrates the potential for integrated studies of known oil and gas fields, now that 3D seismic surveys of increasing quality and resolution are being routinely acquired over old acreage as basins become mature for petroleum exploration, and emphasis shifts to efficiency of development of known fields.