Abstract: Reservoir Potential of the Carbonate Forereef-Slope Setting

Harris, Paul M. (Mitch) - Chevron Petroleum Technology Co. and Robert P. Wright - Chevron North America Exploration and Production Co.

Regardless of whether the shelf-to-basin profile is a ramp, a low-relief rim, or a high-relief rim with zones of bypass, depositional processes can produce reservoir quality facies in the forereef-slope setting. Carbonate production generates grain-rich facies tracts on the slope during lowstands of relative sea-level, and sediment from the platform top carbonate factory is transported downslope as sediment gravity flows and debris flows during transgressions and relative highstands.

Producing field analogs for each forereef-slope type confirm that primary reservoir quality can indeed be preserved within these depositional settings. In some cases, secondary porosity is created by dissolution and dolomitization. The analogs are Bu Hasa and Fateh fields from the Middle East, Poza Rica field from Mexico, and Mescalero Escarpe field from the U. S. Permian Basin.

- Bu Hasa field is a giant onshore field in Abu Dhabi producing from the Lower Cretaceous (Aptian) Shuaiba formation. Packstones and grainstones on the low-relief seaward side of a rudist reef complex account for a component of the production.

- Fateh field is a giant offshore field in Dubai producing from the Middle Cretaceous (Cenomanian) Mishrif formation. Forereef grainstones and packstones produce out of a thick, laterally-extensive wedge that thins away from a salt-cored high.

- Poza Rica field is a giant offshore field in the Tampico embayment producing from the Middle Cretaceous (Albian-Cenomanian) Tamabra formation. Forereef grainstones and packstones and intraclast breccia of in-part shelf-derived skeletal material occur as a thick wedge at the basin margin.

- Mescalero Escarpe field in the northern Delaware Basin produces from the Lower Permian (Leonardian) Bone Spring formation. Dolomitized megabreccia and packstones occur at the lower portions of a high-relief slope.

Seismic profiles illustrate stratal geometries within a sequence stratigraphic framework which are suggestive of the depositional settings shown by the producing field analogs. Although reservoir quality and trap are risks, the analogs demonstrate that the forereef-slope is a viable, and probably under-explored, play type. The seismic example illustrates the multiple opportunities that this play type can generate during the evaluation of a carbonate margin.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil