RUPPEL, STEPHEN C.
Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

Abstract: Application of Borehole Imaging Logs for Definition of Facies, Cyclicity, and Petrophysics in Shallow-Platform Carbonate Reservoirs

Although the utility of borehole imaging logs for characterizing reservoir fractures is well known, their potential for providing critical stratigraphic and petrophysical data in matrix-dominated reservoirs is virtually untapped. In carbonate reservoirs, in particular, efficient recovery of existing hydrocarbons requires accurate definition of facies architecture and rock fabric variability. Cores are fundamental for this task but because of cost are too few in most reservoirs to provide sufficient control. Study of borehole imaging logs in Permian Basin carbonate reservoirs shows that when properly calibrated, imaging logs can equal or exceed cores in identifying facies and cycle boundaries and thus can be an effective lower cost alternative. In dolomitized middle Permian (Guadalupian and Leonardian) reservoirs, distinction of outer ramp fusulinid facies, middle ramp subtidal facies, and inner ramp tidal-flat facies is crucial because their petrophysical attributes are significantly different and their stacking patterns are fundamental to establishing a field-wide correlation framework. Cycletop tidal-flat rocks, for example, commonly contain similar porosity to subtidal rocks but virtually no permeability. Imaging logs accurately distinguish among these facies and thus permit definition of vertical facies successions, cycle boundaries, and flow unit architecture. Log resolution in many cases exceeds that of cores. Dissolution associated with karsting at cycle boundaries is common in limestone reservoirs of the Lower Permian (Wolfcampian). Borehole imaging logs delineate facies successions and cycle tops and also define karst profiles and associated variations in rock fabric attributes. Because poor core recovery is common in shale-filled karst cavities, in many cases only imaging logs permit identification of karst processes.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas