Kansas Geological Survey, Open-file Report 2008-5
Susan E. Nissen, John H. Doveton, and W. Lynn Watney
KGS Open File Report 2008-5
A significant number of carbonate reservoirs worldwide have been modified by karst. Karst can impact reservoir performance either by enhancing or destroying porosity/permeability.
We integrated core, log, 3D seismic, and production data to identify the nature, distribution, and impact of karst in a Permian San Andres carbonate reservoir in Waddell Field, west Texas. The reservoir in our study area is highly compartmentalized and fluid production is extremely variable. Reservoir heterogeneity appears to be related to stratigraphy and diagenesis, as well as karst features associated with a subaerial exposure surface at the top of the San Andres Formation. Core data indicate that the uppermost San Andres consists of sporadically porous "macro" karst, characterized by intense chaotic brecciation and anhydrite replacement, followed by isolated, late stage anhydrite dissolution. The karst overprints high frequency sequences composed of gypsiferous oolitic, fusulinid, and skeletal packstone-grainstone reservoir rock with moldic, vug, and fracture porosity. We used wireline log petrophysical solutions to quantitatively discriminate the anhydritic karst from the underlying packstone-grainstone strata in uncored wells. The base of the karst zone corresponds to a sharp decrease in seismic impedance, and 3D seismic data are used to map the base of karst between wells. The karst zone exhibits high variability in thickness; however, the zone is generally thicker on higher portions of a SE-trending anticline that runs through the study area, suggesting a structural control on karst development. Seismic data show that the karst zone truncates the base of the porous reservoir in some areas and seismic attributes reveal potential reservoir compartment boundaries. Better understanding of local karst control on fluid flow in this reservoir can improve reservoir management decisions.
Kansas Geological Survey, Energy Research
Updated June 2, 2008
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