Geologic Investigation of Cross-Well Seismic Response in a Carbonate Reservoir, McElroy Field, West Texas

Abstract

Cross-well seismic data from McElroy field, a Permian dolomite reservoir in west Texas, demonstrate that high-resolution velocity and reflection images are obtainable in this carbonate reservoir. Our geologic ground-truthing results suggest that cross-well data, when integrated with porosity models based on log facies, add value to reservoir characterization. The cross-well data added information at the interwell scale that we could get no other way. Reservoir quality in the portion of McElroy field that we investigated does not obviously relate to core-based lithofacies due to a complex diagenetic overprint, primarily cementation by gypsum and anhydrite. The coincidence of S-wave reflections on the cross-well data with decreases in porosity or gypsum cement from whole-core analysis suggests that total porosity and mineralogy dominantly influence velocity. Overall, the vertical location of layers generated by reflection imaging correlates fairly well with major log variations. In particular, positive events on the S-wave images correspond almost exactly with increases in sonic velocity, increases in resistivity, increases in bulk density, and decreases on the neutron porosity log from high porosity (or gypsum) to low porosity (or gypsum). Both the log and cross-well data respond to the same diagenetic overprint and its resulting petrophysical characteristics; therefore, we group log data into log facies using multivariate statistical techniques, such as cluster analysis, rather than using core data for correlating reservoir flow units and relating them to the cross-well images. Many of the positive-amplitude events on the S-wave profiles correspond to transitions, in a vertical sense, between the best reservoir cluster and less porous reservoir clusters, which indicates the strong relationship between velocity and porosity. In addition, lateral variations in many of the positive-amplitude events can be tied to changes in porosity and differences in the clusters between the wells. Comparing geostatistical porosity models directly to the S-wave images suggests that the S-wave reflection images appear to be resolving lateral changes in porosity of less than 56 m (185 ft) but more than 15 m (50 ft).