Reservoir Description of South Tano Field, Offshore Ghana: A Case History

Richard P. Lockwood, Adrian M. Hassall, Adzei-Akpor

South Tano Field, offshore Ghana, is a fault and dip closure at the Albian/Cenomanian level, discovered by Phillips Petroleum Company in 1978. The Cenomanian Limestone seal produces a high amplitude reflector on seismic that helps define the top surface of the Upper Albian reservoir. This paper describes probable sedimentological and structural processes that resulted in the present day heterogeneity and distribution of the reservoir sandstones.

The stratigraphic section is related to a three-part tectonic history: 1) Early Cretaceous pre-rift continental sediments, overlain by 2) Aptian/Albian syn-rift shallow marine sediments, succeeded by 3) Late Cretaceous and Tertiary post-rift marine sediments. The Aptian/Albian rifting was caused by transtensional tectonics related to the separation of the South American and African plates and resulted in the formation of rotated fault blocks. Preservation of the reservoir section is significantly related to its position on the fault block.

The Upper Albian sandstones have an average porosity of 15%. Individual sandstone bodies are 5 to 48 feet thick and consist mainly of fine to medium-grained immature sandstones with carbonaceous matter and rare shale clasts. Small scale cross-bedding with intervening shale laminae are present and grain size sequences are usually upward coarsening. Biostratigraphic data and fossil fragments such as bryozoans, crinoids and echinoids indicate inner shelf water depths. The sandstones are interpreted as tidal ridge sandstones deposited with an orientation of approximately NW-SE, parallel to an Early Cretaceous seaway between Africa and South America.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995