Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
2004 West Texas Geological Society Fall Symposium, Midland, Texas, October 2729.
Lower Leonardian Sequence Stratigraphy and Reservoir Development: Fullerton Clear Fork Field, Permian Basin
Stephen C. Ruppel and Rebecca H. Jones
Fullerton field is the largest and most productive Leonardian reservoir in the Permian Basin. With an OOIP of more than 1.2 billion barrels and a cumulative production of nearly 310 million barrels, this shallow-water platform carbonate reservoir is truly a giant. Like most Clear Fork reservoirs, however, recovery efficiency is low relative to other platform carbonate reservoirs. A critical issue to understanding the distribution of original and remaining oil resources, and thus unlocking the key to improving the recovery of the remaining oil, is a detailed understanding of the reservoir architecture. Here we report recent investigations into the facies, sequence stratigraphy, and diagenetic controls on reservoir development and heterogeneity in Fullerton Clear Fork field.
The Clear Fork reservoir at Fullerton, which is productive from parts of the Lower Clear Fork, Wichita, and Abo Formations, also contains one of the most extensive reservoir data sets in the Permian Basin. Our interpretations of the stratigraphy, facies, and diagenesis of these units comes from the description of more than 14,000 feet of core and 1,700 thin sections, correlation of 45-70 stratigraphic markers in nearly 900 wells, interpretation of more than 26 square miles of 3D seismic and 222 miles of 2D seismic, and integration of all data with analogous outcrop models.
Important conclusions of our study include the following:
• The Lower Clear Fork (which represents the upper part of Leonardian sequence L2) consists of a succession of three high frequency sequences (HFS), each of which records sea level rise (transgression) and fall (regression). Reservoir development is largely restricted to subtidal facies (late transgression and early highstand).
• Highest porosity and permeability in the Lower Clear Fork is associated with incompletely dolomitized grain-rich packstones and grainstones.
• Lower Clear Fork facies are discontinuous at the cycle scale but facies tracts are widely correlative at the HFS scale.
• The Wichita, a thick (250 to 350 ft), succession of aggradational restricted tidal flat facies, includes both highstand system tract equivalents of the Abo (Leonardian sequence L1) and transgressive systems tract equivalents of the Lower Clear Fork (Leonardian sequence L2).
• Although the Wichita reservoir architecture is generally time parallel and flat, it is more controlled by diagenesis than by depositional facies.
• Wichita rocks locally contain high porosity but usually contain relatively low permeability and display poor small scale continuity.
• Limestone intervals in the dominantly dolomitic Wichita display extremely low porosity and permeability and act as local fluid flow baffles.
• The Wichita and Abo are time-equivalent facies of one other. The facies contact is expressed on seismic by a prominent apparent top lap surface that is not a time line.
• The Abo consists largely of porous outer ramp fusulinid/crinoid facies whose clinoformal architecture is clearly expressed on seismic. Lateral facies continuity is poor.
• Karst features (including inclined beds and monomict and polymict cave fill breccia) are common in the Wichita and at the Abo Wichita contact. These features are primarily the result of karsting during platform emergence at the end of L1 deposition.
• Gamma ray logs are useful for general correlations only. Like other Leonardian reservoirs, high gamma ray is usually an indication of the presence of clays and silt in tidal flat facies; low gamma ray response indicates subtidal facies.
• A high resolution cycle stratigraphic framework can be defined in Lower Clear Fork rocks using combined core, log, and outcrop relationships. High resolution correlations are not possible in the Wichita and Abo.
• 3D seismic provides excellent resolution of both the reservoir architecture and the distribution of reservoir porosity at the HFS scale.
• Combined seismic, core, and log data demonstrate that depositonal and diagenetic processes responsible for reservoir development were strongly affected by tectonic movement.
These findings provide important insights not only for more effective further development of Fullerton field but for Clear Fork reservoirs throughout the Permian Basin.