Carbonate Facies Identification Using Borehole Image Logs: An Example from Fullerton Field, Andrews County, Texas

FULLMER, SHAWN and STEPHEN C. RUPPEL, Bureau of Economic Geology. The Jackson School of Geosciences, The University of Texas at Austin, Austin, TX 78713

Borehole image logs are an undervalued and underutilized technology for delineating facies and cyclicity in carbonate reservoirs. Most wireline logging devices provide relatively poor resolution of carbonate facies and cyclicity – key elements of carbonate reservoir architecture and heterogeneity. Microresistivity borehole image logs, by contrast, provide high resolution images of major facies variations encountered in the borehole. Successful use of image logs for facies identification depends upon and must start with the identification of facies and facies stacking patterns in cores. Many carbonate facies have distinctive characteristics that allow them to be confidently identified on resistivity image logs. Facies stacking patterns observed in cores provide the framework necessary to guide interpretations of cyclicity on image logs. This kind of core-based calibration ensures accurate interpretation of resistivity images and consistency between interpretations made from cores and those made from image logs. In this study, we first identified facies and styles of cyclicity in the Tubb, Lower Clear Fork, and Wichita from cores in Fullerton field. We compared facies in a nearby core with resistivity images on an FMI log to identify six key facies: tidal flat, nodular subtidal, subtidal wackestone/packstone, fusilinid wackestone/packstone, grainstone, and collapse breccia/karst. Cycles were defined on the image log based on stacking patterns observed in core. The resulting log of facies and cycles provides nearly as much information as a core. The utility of evaluating boreholes using this approach is that high resolution data regarding facies and cyclicity can be extrapolated from areas with core control to areas with poor to no core recovery.