Geostatistical Characterization and Comparison of Outcrop and Subsurface Facies: Shannon Shelf Sand Ridges

ABSTRACT Permeability and porosity data from outcrop measurements and subsurface cores have been analyzed for geostatistical similarities and differences. The formation is the Shannon in the Powder River basin, and we compare an outcrop of the Salt Creek Anticline with Teapot Dome reservoir (5 mi away and at 300 ft depth) and with Hartzog Draw field (40 mi away and at 9000 ft depth). The emphasis is on quantitative geological comparisons, and how the results can improve reservoir characterization. The tools used here to characterize and compare two separate facies from three different occurences of shelf sand ridge deposits are: (1) distribution functions, (2) variograms (correlation length), and (3) log permeability versus porosity plots. Results include: first, a remarkable agreement in permeability distributions for the High Energy Ridge Margin (HERM) facies in the outcrop and Teapot Dome reservoir, even though the porosities are lower in the subsurface by 4 percentage points on average. The permeability agreement is remarkable because the outcrop and subsurface correspond to different depositional events (upper and lower Shannon, respectively). Second, the distribution functions for permeability and porosity in Hartzog Draw are shifted to substantially lower values than those in the outcrop. Third, correlation lengths which characterize the vertical variability are short (usually a few feet). In the lateral direction the correlation lengths could be longer, but are still probably less than 10 ft. Fourth, four separate zones are found to comprise the 9 foot thick HERM facies in the outcrop, and each has a characteristic permeability distribution. Furthermore, large lateral permeability fluctuations (factor of 100) in a 1-ft thick zone are attributed to the variability of glauconite-rich layers. The overall permeability distribution agreement between outcrop and Teapot Dome mentioned above suggests using permeability distributions obtained from outcrop measurements to improve the reservoir modeling in Teapot Dome.