Cornea Case Study: Lessons Forthe Future

Abstract

The Cornea Field was discovered in January 1997. The Cornea structure represents a relatively simple trap configuration comprising a large, elongated, four-way dip closure, formed by the drape of Albian age sediments over a northeast to southwest trending positive basement feature. It is located on an active hydrocarbon migration fairway with proven charge and is capped by a regionally extensive Albian seal. An associated Direct Hydrocarbon Indicator extends over an area of 50 km2 and this feature coincides with gas-fluid contacts throughout the field.Subsequent to the initial Cornea discovery wells in permit WA–241-P, further exploration drilling took place within WA–241-P, and also within two, newly-acquired, adjacent permits, WA–265-P and WA–266-P, during 1998 and 1999. Detailed field assessment during and subsequent to these wells, involving geological, geophysical and reservoir engineering disciplines, indicated that the field consisted of an extensive gas cap over a thin, laterally discontinuous oil rim. The poor quality of the Lower Cretaceous marine reservoir succession and the extremely low expected oil recovery indicated that recoverable volumes would be low.This paper contains a review of the data and studies that led to a full understanding of the field. Key learning points from the case study are introduced here and may help future exploration campaigns in other areas.

Yampi Shelf, Browse Basin, North-West Shelf, Australia: a test-bed for constraining hydrocarbon migration and seepage rates using combinations of 2D and 3D seismic data and multiple, independent remote sensing technologies

“…This discovery, whilst sub-commercial, demonstrated long-range migration (50-80 km) of liquid hydrocarbons onto the ramp margin, and entrapment around a small, Proterozoic basement high. Similarly, the discovery in 1997 of the Cornea field (Stein et al, 1998;Ingram et al, 2000) on the northern Yampi Shelf (Fig. 1) by Shell Development Australia and its partners, Chevron and Cultus Petroleum, further boosted interest in the area.…”

“…1) by Shell Development Australia and its partners, Chevron and Cultus Petroleum, further boosted interest in the area. The oil in both the Gwydion and Cornea fields appears to have been generated within Early Cretaceous (Valanginian to Barremian) source rocks (Spry and Ward, 1997;Blevin et al, 1998;Ingram et al, 2000). Dry gas (!2% wet) is also reservoired in these discoveries, suggesting that the source rocks for the oil and the gas are probably quite different.…”

“…A general overview of the regional seal thickness in the northern Browse Basin was produced by image processing the derived thickness of the key sealing interval intersected in all available exploration wells (wlate Aptian-Albian; Ingram et al, 2000). The interval used was the 95-115 Ma sequence.…”

“…An assessment of the exploration implications of these seismic and sniffer results was made by O'Brien et al (1996a), several months prior to the discovery by Shell in 1997 of the Cornea oil and gas accumulation (Ingram et al, 2000). The Cornea appraisal wells established the presence of a minimum 25 m gas column and a minimum 18 m oil column in the Albian reservoir sequence (Ingram et al, 2000), with in-place reserves reported to be hundreds of millions of barrels. The trap was interpreted to be filled to structural spill-point.…”

“…Key parts of the present study were undertaken while the Yampi Shelf was still a largely undrilled, exploration frontier, and hence a comparison between the pre-drill predictions (based upon the seismic-sniffer-SAR-ALF remote sensing study), and the post-drill results around the Cornea field, as discussed by Ingram et al (2000), is possible.…”