Outcrop and Subsurface Models of Permian Slope and Basin Floor Carbonates

Charlie Kerans¹, Ted Playton¹, and Xavier Janson^2
1Department of Geosciences, University of Texas at Austin, Austin, TX
²Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Exploration for carbonate slope and basin floor reservoirs represents an underdeveloped play type. Significant risks include reservoir quality prediction and top and lateral seal development. Reduction of risk can come from improved conceptual models developed from detailed outcrop studies and careful examination of core, log, and 3D seismic data within a sequence framework. The Permian of the Permian Basin is an area of known deep water carbonate production that can serve as a test case. Although hydrocarbons occur in Wolfcampian through latest Guadalupian shelf strata, production from Permian slopes is restricted to Wolfcampian through middle Leonardian strata. Guadalupian shelf strata dominate Permian Basin production but Guadalupian slope and basin-floor reservoirs are not known.

Outcrop studies of Wolfcampian, Leonardian, and Guadalupian carbonate slopes rimming the Permian Basin, and subsurface studies of Wolfcampian through Leonardian slope reservoirs form the basic data for this study. Permian slope systems range from retrograding distally steepened and prograding mud-rich ramps with minimal sediment-gravity flow input to subvertical reef-rimmed margins with massive amalgamated talus aprons and bioclastic turbidites. Slope systems with proved hydrocarbon production were restricted to examples with channelization of the mid to lower slope and basin-floor-fan deposition decoupled from the grain-rich part of the upper slope, preferably in topographic lows. Best-developed channels were found where older backstepped margins or earlier fan complexes that formed terraces on the slope that were areas of erosion and canyon cutting. Large shifts in eustatic amplitude associated with icehouse to transitional settings appear to favor development of these terraced slopes, creating backstepped margins and lowstand wedges that are later incised.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas