Seismic Imaging of Play Types in Cretaceous Sandstone Reservoirs, Offshore Sierra Leone, West Africa

Anadarko (S.L.) Co. and its partners Repsol Exploracion Sierra Leone Co., Tullow Sierra Leone B.V. and Woodside Energy (Sierra Leone) Pty, Ltd., drilled the Venus B1 exploration well in Sierra Leone in late 2009. This was the first deepwater well in the greater Liberia Basin, which includes southern Sierra Leone, and the first well drilled in its territorial waters in twenty-four years. The well was designed to penetrate an Upper Cretaceous, stratigraphic play type that is similar to that which yielded the discovery of the Jubilee field in western, deepwater Ghana. The interpretation of 3D seismic data allows for the recognition of a variety of slope targets off the coast of Sierra Leone that resemble those on the Ghana slope.

The Venus B1 well penetrated approximately 15 meters of net hydrocarbon pay and confirmed the viability of the Upper Cretaceous stratigraphic play type in the basin. Two Upper Cretaceous zones of reservoir sandstone were penetrated: a shallow, well-imaged turbidite complex, and a deeper, poorly-imaged turbidite complex that was suspected, but not easily recognized, prior to Venus B1. Post-well models of the wireline log signature show that Class II or IIP Amplitude vs. Offset (AvO) responses should be visible in the 3D seismic trace data, but were not always observed in the seismic gathers.

These reservoir sandstones are buried deeply enough that water bottom (seafloor) multiples may contribute to the difficulty in resolving the reservoirs. The seismic data was reprocessed to better remove multiples and better flatten the seismic data gathers. Following the reprocessing effort, the seismic imaging of the reservoir packages was improved substantially.

The seismic reprocessing also allowed the identification of a series of previously unidentified turbidite systems present between the shallow, well-imaged turbidite reservoirs and the deeper, more difficult to image turbidite reservoirs. This intervening play, composed of slope channel complexes, is now readily imaged on the reprocessed 3D seismic data. This third play type is the first recognition in Sierra Leone of the analogous Tweneboa and Enyenra fields play type discovered in Ghana.

Three subsequent wells: Mercury-1, Mercury-2, and Jupiter-1, have been positioned to test and delineate all three of these play types. Mercury-1 (2010) encountered approximately 45 meters of net hydrocarbon pay. The other two wells are scheduled to be drilled in late 2011 and early 2012.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California