Abstract

Through the collaborative efforts of a multidisciplinary team, various subsurface data types have been integrated to produce a conceptual geologic model for reservoir prediction within the upper Clear Fork and Glorieta formations of the Robertson field area, West Texas. Detailed description of 1434 m of core, 109 thin sections, and 241 line-kilometers of 2-D (two-dimensional) seismic indicates the stratigraphic interval accumulated as a progradational succession of platform-interior subtidal and intertidal facies. Reservoir intervals preferentially occur within subtidal facies having intercrystalline porosity associated with replacement by coarsely crystalline dolomite. Intertidal facies were replaced by a finely crystalline phase of fabric-preserving dolomite and are dominated by an ineffective fenestral pore system. Facies and reservoir distribution is largely controlled by antecedent topography. Structural highs generated during the terminal phase of Ouachita-Marathon compression were the preferred site of intertidal facies deposition. Adjacent structural lows have a higher proportion of reservoir-prone subtidal facies. Analysis of compensated-neutron log porosity indicates that subtidal-prone intervals may be broadly characterized as having less than 11% porosity, whereas intertidal-prone intervals generally exceed 11% porosity. From this observation, a simple computer algorithm allows facies interpretation within wells lacking core. Intertidal-subtidal facies ratio maps generated from this algorithm closely match the distribution of pre-Wolfcampian structural elements and present-day structure. Intertidal facies are more common across the crest of structural highs. Future drilling in the North Jenkins area should pursue structural-flank positions where subtidal facies are volumetrically more abundant. Care should be taken, however, to avoid completions below the composite oil-water contact. Subtidal facies are compartmentalized within individual sequences of a progradational sequence set. Progradational stacking and compactional drape over deeper structures produces discrete flow units with potentially independent oil-water contacts.

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