Gas was discovered in intra-Mt Goodwin Sub-group sandstones (Ascalon Formation) of the southeastern Bonaparte Basin in Blacktip–1 in 2001 from a zone characterised by a discrete seismic amplitude anomaly. This integrated study uses wireline logs, cores, cuttings, palynology, micropaleontology and geochemical analyses to determine the depositional environment of the Mt Goodwin Sub-group reservoirs and the source rock potential of this large, latest Permian (Changhsingian) to Early Triassic (Induan Olenekian) section of the Bonaparte Basin in northern Australia. Specific outcomes include a better understanding of the Early Triassic reservoir sandstone depositional environment and recognition of marker horizons on electric logs and seismic profiles, resulting in a more consistent regional interpretive framework for the uppermost Permian (Changhsingian) and Early Triassic (Induan Olenekian), in the Bonaparte Basin.
The Woollybutt Oil Field was discovered in 1997 and further appraisal drilling delineated two separate oil accumulations in the Early Cretaceous Barrow Group Sandstone; the Woollybutt North and South Fields. The Woollybutt North Field came on production in April 2003 with an expected field life of three years. Subsequent review and development drilling in 2005 extended the Woollybutt North Field life to 2008. The tie back of the Woollybutt South Field in 2008, previously thought to be an uneconomic area, has again increased total field life expectancy and consequently increased the expected ultimate recovery for the field.
Issues with the unknown level of productivity of the poorer quality basal Mardie Greensand reservoir and the seismic velocity gradients across the field, generated a large uncertainty in both well productivity and structural closure of the field.
A review was undertaken which integrated geophysical, geological and reservoir engineering expertise. This review resulted in the revision of the Woollybutt geological model and the development of the Woollybutt South Field. An additional benefit was to also extend the production life of the Woollybutt North Field.
This paper describes the original geophysical and geological model and demonstrates how simulation history match feedback can influence the mapping of the structural closure of the field. This result was integrated with a wireline mini drill-stem test within the poorer quality reservoir sandstone and led to the development of a new geological model. Subsequently the decision was made to develop the Woollybutt South Field, which is dominated by this poorer reservoir; the basal Mardie Greensand.
Woollybutt is operated by Eni Australia (65%) with partners Exxon Mobil (20%) and Tap Oil (15%).
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