--> ABSTRACT: Depositional Controls on Permeability Within a Dolomitized Carbonate Ramp: The Cornell Unit, Wasson San Andres Field, Yoakum County, Texas, by Keith E. Winfree; #91020 (1995).

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Depositional Controls on Permeability Within a Dolomitized Carbonate Ramp: The Cornell Unit, Wasson San Andres Field, Yoakum County, Texas

Keith E. Winfree

The Cornell Unit produces from the Permian San Andres carbonate ramp on the Northwestern Shelf of the Midland Basin. The reservoir is a dolomite with variable amounts of anhydrite and chert. The primary controls on permeability are the original depositional textures and the stacking of parasequences.

Lithofacies can be classified by texture, grain type and sedimentary structures into five groups that have unique porosity and permeability characteristics. These groups correspond to depositional environments:

Table

Lateral variations in horizontal permeability are minor except in the deep ramp facies. The fining-upward grainstones of the deep ramp form lenses of permeable rock encased in mudstones. Vertical permeability is low in all facies except the barrier bank and lagoon rocks.

In the Cornell Unit, hydrocarbons are trapped within three depositional sequences which stack in a progradational pattern. Laterally-continuous permeable zones occur in the upper parts of the lower two sequences where barrier-bank rocks dominate. The lower parts are dominated by low-permeability outer ramp rocks. A transgressive systems tract that separates the lower two sequences from the upper forms a field-wide layer of low permeability. The upper sequence has laterally-continuous permeable layers of barrier bank rocks capped by a thick section of impermeable tidal flat rocks.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995