AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Controls on the Distribution and Geometries of Sandstone Bodies in Platform Carbonate Systems: Examples from the Middle Permian (Guadalupian), Permian Basin, Texas
(1) Bureau of Economic Geology, University of Texas, Austin, TX.
Although mixed shallow water platform systems containing both carbonate and siliciclastic facies are common in the geological record, the processes that lead to this admixture and the geometries of the resulting facies are not well understood. Detailed, core-based study of two producing oil fields in the Permian Basin (Permian Grayburg Fm) has shed important light on these questions.
We examined more than 16,000 ft of core from 54 wells in two fields along the eastern side of the Central Basin Platform. Cores were used to define facies, stacking patterns, and cyclicity and pore types. Core data were also used to calibrate wireline logs as a basis for defining field-wide correlations and sequence architecture.
Both fields display similar assemblages of carbonate and siliciclastic facies and record a similar accommodation history. By contrast, the abundance, distribution, and reservoir quality of siliciclastic facies (sandstone, siltstone, and siliciclastic-rich carbonate) in the two reservoirs varies widely. In South Cowden reservoir, siliciclastics are limited to a few intervals associated with cycle-scale flooding surfaces and transgression and are non-porous. In North Cowden field, 20 mi (32 km) to the north, siliciclastics are locally thicker, are found in both transgressive (TST) and highstand (HST) systems tracts, and contain significant porosity and permeability.
Essentially all of these siliciclastics can be tied to low accommodation sedimentation associated with early TST or late HST. This association is consistent with enhanced flux of siliciclastics onto carbonate platforms during sealevel fall and lowstand and is supported by both outcrop and subsurface studies of other Permian successions.
Data from North Cowden field suggest two distinctly different patterns of siliciclastic bed geometries. Siliciclastics associated with major flooding events (e.g., composite or third-order sequences) display greater continuity (along both strike and dip), although they are commonly thinner and of lower reservoir quality. Siliciclastics associated with high-frequency sequence (HFS) flooding events, by contrast, display limited dip continuity but are thicker and of higher reservoir quality. In many cases they are developed as thick strike-elongate successions immediately distal to backstepping tidal flat complexes. These geometries may be the result of more-pronounced topographic relief produced by high rates of aggradation during HFS sedimentation.