Differential Compaction at Pre-Existing Shelf Margins as a Control on Stratigraphic Traps in Permian mid- To outer Shelf Successions
Charles Kerans¹, Samuel F. Hiebert¹, and J. Alex Parker¹
¹The Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin
Permian carbonate and mixed siliciclastic-carbonate reservoirs provide some of the world's best examples of stratigraphic trap development. The Abo trend in fields like Kingdom, the Lower Clear Fork in the Sunterra-Prentice-Wasson trend, the Grayburg along the North Cowden trend in front of San Andres margins like Goldsmith, and the minor Yates-Tansill production in front of the time-equivalent shelf-crest complexes in front of North Ward Estes are examples of this subtle accommodation control on reservoir quality.
Grayburg mixed siliciclastic-carbonate strata form an important element of the conventional secondary and tertiary hydrocarbon production from the Permian Basin, with Means, North Cowden, Foster, South Cowden, McElroy, North McElroy, Dune, Mabee, and others representing a prolific east-side Central Basin Platform trend. In 2011-2013 Hiebert, Parker, and Kerans undertook a significant revision of the Grayburg outcrop along the Shattuck Escarpment, southern Plowman Ridge, and for the first time, collected sections from Manzanita Ridge. Combined with reconnaissance data from the Bush Mt exposures, these 30 new stratigraphic sections, supported by aerial and gigapan photography, extensive petrography, isotope chemostratigraphy, and detailed cycle-scale geobody mapping of key grainstones, has permitted an updated sequence model for the Grayburg Fm. including 60 high-frequency cycles contained within 3 composite sequences. One of the more significant observations from this work was that the main ooid-grainstone-bearing cycles formerly placed in the G11 HST can now be shown clearly to form a retrogradationally stacked succession that records increasing accommodation and tidally dominated deposition near the end of Grayburg time.
A clear outcome of this stratigraphic framework was the recognition of syndepositional differential compaction of lower Grayburg strata on the seaward side of the terminal San Andres shelf margin. This compaction controls a 20% expansion of the Grayburg section, and development of thicker and more extensive grainstone geobodies. A 25 % increase in ooid grainstones marks this transition, and corroborates observations from subsurface reservoirs that show the importance of either pre-existing shelf margins or shelf-crest topographic anomalies in controlling reservoir quality.
Regardless of whether the dominant pay facies is ooid grainstone such as in the Grayburg examples, or fusulinid-skeletal grain-dominated packstone as seen in the Lower Clear Fork Sunterra trend, a subtle increase in high-energy subtidal carbonates in front of a compactional hinge that transitions landward into anhydrite-cemented low-porosity platform interior facies is sufficient to allow formation of stratigraphic traps with tens of millions of barrels of reserves.
AAPG Datapages/Search and Discovery Article #90190©AAPG Southwest Section Annual Convention, Midland, Texas, May 11-14, 2014