Modeling dolomitized carbonate-ramp reservoirs: A case study of the Seminole San Andres unit - Part I, Petrophysical and geologic characterizations

Major issues in characterizing carbonate-ramp reservoirs include geologic f ramework, seismic stratigraphy, interwell heterogeneity including rock fabr ic facies and permeability structure, and factors affecting petrophysical p roperties and reservoir simulation. The Seminole San Andres unit, Gaines Co unty, West Texas, and the San Andres outcrop of Permian age in the Guadalup e Mountains, New Mexico, were selected for an integrated reservoir characte rization to address these issues. The paper is divided into two parts. Part I covers petrophysical and geologic characterization, and part II describe s seismic modeling, reservoir geostatistics, stochastic modeling, and reser voir simulation. In dolomitic carbonates, two major pore types are interparticle (includes i ntergranular and intercrystalline) and vuggy. For nonvuggy carbonates the t hree important petrophysical/rock fabric classes are (I) grainstone, (II) g rain-dominated packstone and medium crystalline dolostone, and (III) mud-do minated packstone, wacke-stone, mudstone, and fine crystalline dolostone. C ore data from Seminole showed that rock fabric and pore type have strong po sitive correlations with absolute and relative permeabilities, residual oil saturation, waterflood recovery, acoustic velocity, and Archie cementation exponent. Petrophysical models were developed to estimate total porosity, separate-vug porosity, permeability, and Archie cementation exponent from w ireline logs to account for effects of rock fabric and separate-vug porosit y. The detailed and regional stratigraphic models were established from outcro p analogs and applied to seismic interpretation and wireline logs and cores . The aggradational seismic character of the San Andres Formation at Semino le is consistent with the cycle stacking pattern within the reservoir. In p articular, the frequent preservation of cycle-based mudstone units in the S eminole San Andres unit is taken to indicate high accommodation associated with greater subsidence rates in this region. A model for the style of high-frequency cyclicity and the distribution of r ock-fabric facies within cycles was developed using continuous outcrop expo sures at Lawyer Canyon. This outcrop model was applied during detailed core descriptions. These, together with detailed analysis of wireline log signa tures, allowed construction of the reservoir framework based on genetically and petrophysically significant high-frequency cycles. Petrophysical prope rties of total and separate-vug porosities, permeability, water saturation, and rock fabrics were calculated from wireline log data. High-frequency cycles and rock-fabric units are the two critical scales for modeling carbonate-ramp reservoirs. Descriptions of rock-fabric facies sta cked within high-frequency cycles provide the most accurate framework for c onstructing geologic and reservoir models. This is because petrophysical pr operties can be better grouped by rock fabrics than depositional facies. Th e permeability-thickness ratios among these rock fabric units can then be u sed to approximate fluid flow and recovery efficiency.