The study of outcrops which are reservoir analogs helps to improve the characterization of internal heterogeneities in reservoirs, both from a sedimentological and reservoir perspective. Geostatistical parameters can be computed from outcrop data which can then be integrated into subsurface data bases to better constrain geostatistical reservoir simulations.

The Castlegate Sandstone (Campanian-Maastrichtian) is used as a case study to illustrate a method of data collection and analysis. The Castlegate was selected as a reservoir analog of a braid-plain to braid-delta system. The outcrop exposures in the Book Cliffs offered the possibility to better understand the internal architecture and heterogeneities of such a depositional system.

The Castlegate comprises an aggradational succession of fluvial sand sheets with a variable erosional, cut and fill inter-relationship. Although predominantly fluvial, there are several brackish marine incursions of variable extent interspersed through the section.

Three localities were selected to represent roughly proximal to distal positions within the braid-plain/braid-delta. Outcrop data consist of measured vertical sections and continuous photo-mosaics. Three principal ‘photo-facies' were identified following a sedimentological interpretation of the outcrop and photo-mosaics: clean sandstone, heterolithic sandstone and shale.

The photo-mosaics were scanned and transformed into digital images for geostatistical analysis, with each photo-facies represented by a different color code. Vertical and horizontal proportion curves were constructed from the photo-mosaics. From these, variograms were computed. Each vertical proportion curve characterizes the vertical distribution and heterogeneity for each vertical section. Each of the three localities studied display different characteristics of heterogeneity, and vertical and horizontal length scales which portray real differences in the proximal-distal architecture of the braid-plain/braid-delta.

The geostatistical parameters derived from the Castlegate Sandstone characterize the internal facies architecture of one example of a braid-plain/braid-delta. Some of the results from this study will be used to constrain geostatistical simulations of the reservoir units in the Terra Nova field (Upper Kimmeridgian, Jeanne d'Arc Formation) on the Grand Banks (Newfoundland, Canada).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah