Complex Structural/Stratigraphic Combination Traps with a Diagenetic Overprint and the Recognition of Bypassed Pay in Pennsylvanian Rocks of the Anadarko Basin, U.S.A.

Diego I. Sanabria, German D. Molina, Robert B. Lieber, and John R. Adams
BP America Inc, Houston, TX

The Anadarko Basin constitutes one of the most prolific Paleozoic basins of North America. Current BP exploration efforts in this basin focus on Pennsylvanian rocks along the Mountain Front area in Wheeler, Beckham, and Roger Mills. Most large hydrocarbon accumulations associated with prominent structural features have already been tested in this mature basin. Consequently, remaining potential is generally in the form of more complex stratigraphic/structural traps with a diagenetic overprint. Additional potential is related to the recognition of bypassed pay in wells originally drilled for deeper targets. Refining the search for such commercial opportunities requires a multidisciplinary approach and a solid understanding of the depositional and diagenetic history of the stratigraphic units involved.

Pennsylvanian rocks of the Anadarko basin were accumulated in a variety of depositional settings including alluvial fans, braid plains, fan deltas, prograding shorelines, deltas, and incised valley fills. Deposition was followed in most cases by complex diagenetic histories including multiple phases of cementation and dissolution. Almost invariably primary porosity has been completely destroyed by cementation. Thus, most commercial hydrocarbon accumulations are related to secondary porosity created by dissolution of earlier cements and/or framework grains. An exception to this generalization is the rare occurrence of primary porosity preserved by early hydrocarbon migration in some portions of the basin.

Correlation between depositional facies and preferential generation of secondary porosity is not always straightforward suggesting that a subjacent depositional control on diagenesis is not always present. Morrow sands are a good example of both, good depositional control on porosity and lack of it, depending on their position along the depositional transect.