London-based Borders & Southern (B&S) completed a comprehensive seismic reservoir characterization study to investigate the Early Cretaceous shallow marine play fairway of its Darwin discovery offshore the Falkland Islands. The objective was to improve understanding of the Darwin reservoir interval, both within and surrounding the discovery itself and its projected facies trend along the Aptian shelf. The analysis was based on 2500sq km of high-quality prestack depth migration 3D seismic data. Results from the study have allowed B&S to revise resource estimates for Darwin and close-by prospects.
The initial interpretation of the new 3D data confirmed the basic structural mapping and extended B&S knowledge northward of the discovery. The mapping also highlighted a number of interesting seismic amplitude anomalies, some of which were coincident with previously mapped leads. One of the key insights from this first phase of work was the influence of stratigraphic thinning of reservoirs on regional prospectivity. Potential reservoir intervals in the Early Cretaceous section, including the Darwin reservoir, thin toward the west and northwest. As a consequence, the shelf portfolio is now enhanced to include combined structural and stratigraphic traps.
The next phase of the evaluation comprised a seismic inversion study which utilized the well-to-seismic calibration to help characterize the reservoir and further aid with prospect definition. The inverted seismic data describes the internal rock properties of the reservoir such as lithology type, porosity or fluid type, rather than just the boundaries as mapped from conventional seismic reflection data. The processed data incorporates the well log data into a quantitative rock-property description of potential reservoir intervals. This study has identified new potential reservoir intervals and helped better define prospects and their resource assessments.
The results have highlighted a previously unrecognized reservoir interval extending over part of the Darwin East and Darwin West structures, just above the main reservoir. The interval is poorly developed in the discovery well, but can be seen to expand to the east and south on the inverted seismic data. This porous interval is considered to be charged and to have a common contact with the main Darwin reservoir. Consequently, the Darwin resource assessment has now been revised to include this interval.
Two lower intervals on Darwin West, previously identified and described but not included in the resource assessment, have also been included. The revised best estimate (P50) un-risked estimate for the combined Darwin East and Darwin West structures, incorporating all proven and potential reservoirs, is 360 million bbl of recoverable condensate. The main near-field prospects targeted by the seismic inversion study are Covington, Morgan, Sulivan, Stokes and Wickham.
The principal target for Covington and Morgan is the main Darwin shallow marine sandstone reservoir. This north-northwest trending interval appears laterally continuous and represents a series of eastwards prograding sand bars on the Aptian shelf. Based on the seismic amplitude response, B&S reports that these prospects are more likely to be oil charged. Best estimate (P50) un-risked recoverable resource estimates for the Covington and Morgan prospects are 216 and 230 million bbl of oil respectively.
The target intervals for the Sulivan and Stokes prospects are older and deeper than the main proven Darwin reservoir. This older stratigraphy was not penetrated in the discovery well and some uncertainty remains as to the age and depositional environment of the potential reservoir. Based on the amplitude anomalies, the hydrocarbon phase is believed to more likely to be gas condensate, similar to Darwin. Best estimate (P50) un-risked resource estimates for Sulivan are 473 million bbl of recoverable condensate and for Stokes 134 million bbl of recoverable condensate.
The Wickham prospect has a target stratigraphically younger than the main Darwin reservoir. This shallow marine sand interval was not encountered in the discovery well. The prospect is on the downthrown side of the main Darwin fault. Based on the amplitude response, this prospect is believed to be more likely oil charged. The best estimate (P50) un-risked recoverable resource is estimated at 119 million bbl.
Future subsurface technical work will continue to refine the B&S assessment of this area and will develop a greater understanding of the Early Cretaceous deepwater channel and fan play, which includes the Burgess and Bute prospects.
Image courtesy of Borders & Southern
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