ABSTRACT: Middle Pennsylvanian Strawn Phylloid Algal Mound Reservoirs in a Third-Order Transgressive Systems Tract, Delaware Basin, New Mexico

PULS, DAVID D., and ADAM J. FOX, Exxon Company, U.S.A., Houston, TX

Middle Pennsylvanian Strawn carbonates were deposited as thin (10-50 ft), fourth-order cycles within the framework of a third-order sequence. In some areas, the Strawn contains a lowstand systems tract composed of sandy carbonate at its base; however, the Strawn is more commonly found as a transgressive systems tract (TST) followed by a highstand systems tract (HST). Detailed seismic geometries show transgressive onlap of the TST and progradational downlap of the HST, and thickening of the TST with corresponding thinning of the HST basinward (confirmed by well log correlation). The most important parameter that controlled deposition during the TST was a high rate of relative sea level rise and increasing accommodation. This resulted in retrogradational to aggradational, upward-thicken ng fourth-order cycles of laterally restricted (600-2400 ac), phylloid algal mounds (100-400 ft thick) and associated facies where they formed on a subsiding shelf. In the highstand, a lower rate of relative sea level rise and progressively decreasing accommodation produced progradational, upward-thinning cycles with limited vertical mound growth. Broad grain-prone shoals and small bioherms characterize HST facies. Because the HST does not allow the extensive growth or "keep-up" character that is found in the TST, the best reservoirs are confined to the lower Strawn. Thick mound-prone trends in the TST are coincident with excellent oil and gas production in the Strawn. Identification of reservoir trends within a genetically related stratigraphic bundle (in this case the TST) promotes a f cused exploration effort for the most prospective reservoirs.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)