GEOLOGIC INVESTIGATION OF CROSS-WELL SEISMIC RESPONSE IN A CARBONATE RESERVOIR, MCELROY FIELD, WEST TEXAS

Cross-well seismic data from McElroy field, a Permian dolomite reservo ir in west Texas, demonstrate that high-resolution velocity and reflec tion images are obtainable in this carbonate reservoir. Our geologic ' 'ground-truthing'' results suggest that cross-well data, when integrat ed with porosity models based on log facies, add value to reservoir ch aracterization. The cross-well data added information at the interwell scale that we could get no other way. Reservoir quality in the portio n of McElroy field that we investigated does not obviously relate to c ore-based lithofacies due to a complex diagenetic overprint, primarily cementation by gypsum and anhydrite, The coincidence of S-wave reflec tions on the cross-well data with decreases in porosity or gypsum ceme nt from whole-core analysis suggests that total porosity and mineralog y dominantly influence velocity. Overall, the vertical location of lay ers generated by reflection imaging correlates fairly well with major log variations. In particular, positive events on the S-wave images co rrespond 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 ove rprint and its resulting petrophysical characteristics; therefore, we group log data into log facies using multivariate statistical techniqu es, such as cluster analysis, rather than using core data for correlat ing reservoir flow units and relating them to the cross-well images. M any of the positive-amplitude events on the S-wave profiles correspond to transitions, in a vertical sense, between the ''best'' reservoir c luster and less porous reservoir clusters, which indicates the strong relationship between velocity and porosity. in addition, lateral varia tions in many of the positive-amplitude events can be tied to changes in porosity and differences in the clusters between the wells. Compari ng geostatistical porosity models directly to the S-wave images sugges ts that the S-wave reflection images appear to be resolving lateral ch anges in porosity of less than 56 m (185 ft) but more than 15 m (50 ft ).