Spatial variations in petrophysical properties have been described for interwell scale outcrop analogs to subsurface reservoir facies in the marginal marine upper Almond formation near Rock Springs, Wyoming. The objective of this study was to determine interwell scale lateral trends in heterogeneity that can be translated to nearby analogous reservoirs.
More than 1,600 one-inch diameter plugs were collected for petrophysical analysis from outcropping tidal delta, tidal channel and foreshore facies. These facies comprise reservoir quality sandstones in the Greater Green River Basin east of the Rock Springs Uplift. Modal analysis of 78 thin sections provided compositional and packing parameters.
Porosity loss in outcrop and near-outcrop tidal channel sandstones was due to both compaction (COPL) and cementation (CEPL), whereas porosity loss in tidal delta and shoreface sands was dominated by compaction. For all three facies a correlation can be found between increased intergranular volume (IGV), carbonate cements and CEPL if petrographic and petrophysical properties are first sorted according to IGV. In contrast, increased IGV correlates negatively with porosity, COPL, grain contact index, compaction index, and relative amount of lithics.
Geostatistical analysis of the petrophysical data was undertaken to determine its distribution structure, characterize lateral continuity and predictability within this system. Samples from equivalent facies at different geographic locations show geostatistical continuity although they were often found to be statistically dissimilar. Results of geostatistical analysis, therefore, are representative of the internal architecture of the facies studied. Although the statistics of reservoir properties are geographically variable, the data can be used in a predictive sense because it is geostatistically consistent in terms of variance and continuity of parameters. Geostatistical trends interpreted from the variogram analysis were imposed by ordered internal facies architecture. For our data set, oscillations beyond the variogram range could be related to the distribution of bed set boundaries.
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