Geologic Evolution of the Late Permian Capitan Shelf Margin, Northern Delaware Basin
GROVER, GEORGE A., Arabian Chevron, Inc., Dhahran, Saudi Arabia
A two-phase model, based on outcrop and subsurface data rimming the northern half of the Delaware basin, characterizes the evolution of the late Guadalupian Capitan shelf margin, a margin that prograded up to 19 km basinward while an interval of over 700 m accumulated. Phase 1, during Seven Rivers shelf (early Capitan) deposition, accounts for 70-80% of the total progradation, over 50% of the total aggradation, and corresponds with shelf facies devoid of siliciclastics, emplacement of abundant carbonate debris on the slope and basin margin, and deposition of 50-70% of the Bell Canyon siliciclastic interval in the basin. The clastics bypassed the growing Capitan margin and were equally important to that of the allochthonous carbonate debris in filling accommodation space to facilitate rogradation of the margin. The second phase, during Yates-Tansill (middle-upper Capitan) time, was dominated by aggradation, steepening of the shelf margin, deepening of the basin, and deposition of abundant siliciclastics on the shelf. This model differs from previous reconstructions that show uniform growth of the Capitan reef, and it contradicts the long-standing dogma of reciprocal sedimentation.
This two-phase growth model adds insight into deposition of the two principle Guadalupian reservoir facies that account for nearly 50% of the Permian basin in-place oil reserves. Offlapping sheets of inner shelf carbonates (e.g., San Andres Formation, McElroy field) relate to periods of shelf progradation whereas widespread sheets of shelf clastics (e.g., Yates Formation, N. Ward Estes field) reflect periods of shelf aggradation. The model should be useful in evaluating the evolution of other shelves, particularly mixed shelves.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)