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Reservoir Geology of Denver Unit, Wasson San Andres Field, Gaines and Yoakum Counties, Texas

Robert L. Mathis

Core studies were undertaken to determine the facies relationships, continuity, and areal distribution of pay zones in the Denver unit, San Andres reservoir (operated by Shell Western Exploration and Production Inc.). Examination of slabs and thin sections from more than 4,000 ft of core from 19 wells provided the basis for detailed log correlation and zonation of the reservoir over the entire Denver unit. Core examination revealed a classic example of a shoaling-upward sequence where the original sedimentary environments clearly influenced the development and distribution of porosity. The resultant, highly stratified reservoir model evolved because of an interdisciplinary engineering effort to describe the reservoir geometry.

The Wasson reservoir is divided into two parts. The lower part, known as the "Main Pay," consists of dolomitized open-marine packstones and wackestones, which constitute the main part of the oil reservoir. This section is overlain by progressively poorer quality, shallower water restricted marine and intertidal rocks known as the "First Porosity." These rocks grade upward into the nonproductive dense mudstone and anhydrite of the overlying supratidal section, which forms the reservoir seal.

The principal contribution to reservoir development is made by a mixture of two end-member rock types: pelletal packstones and moldic (skeletal) wackestones. The pelletal packstones are usually well burrowed, contain varying amounts of skeletal debris, and are usually less than 10 ft thick. The moldic wackestones are slightly burrowed and occur as patchy accumulations. Fluid-flow properties reflect the relative proportions of pelletal packstone and moldic wackestone. The pelletal packstones have abundant interparticle porosity with varying amounts of moldic pores; their porosity commonly ranges between 15 and 20%, but occasionally is as high as 25%. Permeability is usually between 10 and 50 md, but rarely exceeds 100 md. The most abundant pore type in the wackestones is moldic. This r ck type has a permeability of less than 1 md, although moldic porosity may range as high as 10%. In the CO2 pilot test with numerous logging observation wells, the velocity of flood fronts correlated closely with the relationship between rock type, porosity, and permeability.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.