Calcite cement distribution and its effect on fluid flow in a deltaic sandstone, Frontier Formation, Wyoming

Abstract

Precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production. To predict subsurface flow through cemented reservoirs, permeability distributions used in fluid-flow models must reflect this diagenetic overprint. Calcite cements in sandstones commonly occur as irregularly distributed concretions, which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells. Outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements. The size and distribution of calcite concretions were mapped in outcrops of the Frewens sandstone, Frontier Formation, in central Wyoming. Large, tabular calcite concretions in this deltaic sandstone generally follow basinward-inclined bedding. Median thickness of the concretions is 0.6 m, length is 4.2 m, and width is 5.3 m. The highest cement fraction is in the high-permeability facies at the top of the sandstone body. Concretion centers are approximately Poisson distributed within the sandstone. The upward-increasing cement fraction is caused by upward-increasing concretion size. Lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution, with a mean of 12% (s = 5%). Spatial distribution of calcite cement in the Frewens sandstone was modeled using indicator geostatistics. Variograms were inferred from outcrop maps of cement. Indicator semivariograms of cement have a range of 30 m horizontally and 2.5 m vertically, dimensions that correspond approximately to the size of the largest concretions. Stochastic images of cement were created using indicator simulation with vertically varying cement proportion. Flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone. The fastest path through the sandstone is in the lightly cemented, high net-to-gross center of the sandstone body. Because the cement mainly occurs within the highest permeability facies in the sandstone body, a model based on depositional facies alone would overestimate upscaled permeability of the Frewens sandstone.