Abstract: Discrimination Between Karst and Tectonic Fractures in the Ellenburger Formation, West Texas: Implications for Exploration Models
HOAK, T.E., SAIC and Kestrel Geoscience, LLC, Littleton CO; K.R. SUNDBERG, Phillips Petroleum Co., Bartlesville OK; P. ORTOLEVA and M. Shebl, Laboratory for Computational Geodynamics, Indiana University, Bloomington, IN.
A cooperative analysis of the Central Basin Platform and eastern Midland Basin is being conducted by the Indiana University-Laboratory for Computational Geodynamics (IU-LCG), Phillips Petroleum Company (PPC), and Science Applications International Corporation (SAIC). The U.S. Department of Energy is funding the project to numerically model the evolution of a fractured carbonate oil reservoir. As part of this modeling effort, we have characterized fracture genesis and fracture intensity for fifteen cored wells throughout the study area.
In the Ellenburger Formation, it is extremely difficult to distinguish between multiple phases of karst-related fracturing and overprinting tectonic fractures. From our analyses of cored wells, we have found agreement with earlier workers that many tectonic fractures crosscut the late-stage baroque dolomite cements. Caution must be applied when using this criteria, however, because we have found several cores in which there appear to be several phases of baroque dolomite cement. In addition, many late “tectonic” fractures are unmineralized and cannot be readily distinguished from coring-induced fractures, especially in areas of incomplete core recovery, and widespread rubble zones.
In our analyses, we are evaluating the relationships between fracture intensity, morphology, host lithology, fracture plane mineralization, and position on a tectonic structure. As part of this analysis, we have identified objective criteria that permit the accurate delineation of fracture genesis in the Ellenburger Formation. We have applied these criteria to the fifteen cores studied in this project. From this analysis, we have been able to characterize variations in Ellenburger tectonic fracture intensity by separating these fractures from karst-related features. In general, the majority of fracturing in the Ellenburger is caused by karst-related fracturing. Numerous examples where tectonic fractures crosscut karat fracture breccias allow the chronology to be well-defined. More massive, tidal flat facies show the greatest fracturing. The orientation of these fractures, combined with their relationship to adjacent karat breccia zones, suggest that many of these tidal that facies fractures represent incipient karst-related fractures. These relationships appear to be valid for nearly all cores studied to date. We have been able to more precisely define the spatial significance of the fracture data sets by use of oriented core and fracture imaging logs in Andector Field. In this area, we have access to several hundred feet of oriented core (that also penetrates basement) along with a fracture imaging log run over the same interval. The data from these two sources will be used to calibrate the fracture results obtained from other adjacent fields where detailed orientation data are lacking but quantitative core fracture data have been collected.
Given the historic interest in the large hydrocarbon reserves in West Texas carbonate reservoirs, results of this integrated modeling project will have tremendous implications for exploration and production strategies targeting vuggy, fractured carbonate systems not only in West Texas, but throughout the globe.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah