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.