From Bureau of Economic Geology, The University of Texas at Austin (
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Permian Basin Section-SEPM Monthly Luncheon Presentation, Midland, Texas, October 2003

Reservoir Architecture of the Giant Fullerton Clear Fork Field; Insights for Reservoir Characterization in the Permian Basin

Stephen C. Ruppel

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In terms of both cumulative oil production (more than 300 million bbl) and original oil-in-place (approximately 1.5 billion bbl), Fullerton Clear Fork field is the largest oil reservoir productive from Leonardian carbonates in the Permian Basin. Like many Leonardian reservoirs, however, recovery efficiency at Fullerton is low (20 percent). Among the major causes of low recovery efficiency in Fullerton and other Clear Fork reservoirs is an incomplete understanding of reservoir architecture. Integration of outcrop studies with core, log, and seismic data from Fullerton makes it possible to construct an accurate model of the reservoir architecture that serves as the key basis for reservoir modeling of the Fullerton field and as a basis for developing a better understanding of other Leonardian reservoirs in the Permian Basin.

The "Clear Fork" reservoir at Fullerton field actually consists of Abo, Wichita and Lower Clear Fork (LCF) facies that together define two depositional sequences. The Abo facies, which forms the base of the Fullerton reservoir, is a succession of clinoformal fusulinid wackestones and packstones that represent distal, outer ramp, deposition during early Leonardian sequence L 1. Key operational challenges in the Abo are difficulty of correlation and poor lateral continuity caused by the clinoforms, and the presence of local overprinting karst.

The overlying Wichita facies is a tidal flat succession whose lower part represents the proximal equivalent of the subtidal Abo facies (sequence L 1 highstand systems tract) and whose upper part represents the basal transgressive leg of the LCF (sequence L 2 transgressive systems tract). Fundamental characteristics of the Wichita include (1) the dominance of mud-dominated tidal flat facies with high porosity but generally very low permeability and oil saturation, (2) poor facies continuity, (3) vertical, low-permeability flow baffles formed by relatively continuous beds of impermeable limestone, and (4) local development of karst solution collapse and cave infill.

The dominantly subtidal LCF at Fullerton comprises three high frequency sequences each having an upper (highstand) and a lower (transgressive) leg made up of tidal flat deposits, and a middle (maximum flooding) leg dominated by subtidal deposits. Critical aspects of LCF reservoir development are: (1) reservoir porosity and permeability are restricted to subtidal sequence legs, (2) subtidal cycles are relatively continuous, (3) cycle-capping grain-rich facies contain the best porosity and permeability, (4) limestone-rich subtidal intervals are highly porous but generally moldic, and (5) most reservoir permeability is concentrated in the LCF.