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