Abstract: Stratigraphic Sequences and Reservoir Facies of the Wapanucka and Spiro Formations
Darrell L. Mauldin
An understanding of the stratigraphic boundaries and lithofacies of the Spiro, middle shale (sub-Spiro shale in the subsurface), and the Wapanucka, will lead to a better prediction of the location of petroleum reservoirs in southeastern Oklahoma. Although the Spiro rests unconformably on the Wapanucka in some parts of the Arkoma basin; the base of the Spiro and the top of the Wapanucka are conformable surfaces throughout the frontal Ouachita out crops. Small, discontinuous deposits of cross-bedded sandy calcarenites and bioclastic sandstones commonly occur within the middle shale sequence. These are commonly referred to as the `sub-Spiro sand' in the subsurface, and can make it difficult to distinguish the base of the Spiro and top of the Wapanucka sequences.
At the Wapanucka shelf margin, subtidal cycles are capped by shoaling carbonate sand bodies (bioclastic and oolitic grainstones). In areas of paleotopographic highs, these shoal deposits can build up above sea level and form island beaches. Subaerial exposure is evidenced by the abundance of trees that grew on these islands.
Sponge boundstones and phylloid algal bioherms grew adjacent to the shelf margin, while tubular algal (Donezella) boundstones occasionally formed small banks in slightly deeper water. The Spiro contains identical lithofacies to the Wapanucka; however, in certain areas, significant amounts of sand were transported to form shallow marine bars.
In addition to the sandstones of the Spiro, several limestone facies of the Wapanucka and Spiro are potential hydrocarbon reservoir rocks. Fracture styles, related to certain lithofacies in outcrop, can aid in predicting fractured reservoirs in the subsurface.
AAPG Search and Discovery Article #90957©1995 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma