Implications of Cell Size on Modeling of Facies Proportions: A Modern Carbonate Platform Example

Gregory S. Benson¹ and Steve Bachtel^2
1 ExxonMobil Upstream Research Company, Houston, TX
² ConocoPhillips Co, Houston, TX

In shallow carbonate platform interiors, wide variations in microenvironments and associated depofacies are found in close proximity. When depofacies are larger than the geologic model cell size, each cell can reasonably be assigned a discrete rock type. However, if the cells need to be made so large that they encompass a mix of depofacies, the modeler must choose either to assign the cell to its most populous facies (ignoring minority rock types that may contribute significantly to flow) or to characterize the cells as a mixture using multiple facies proportions.

This research investigates how modeling parameters for facies proportions need to change as a function of cell size. As cells get larger the variance of facies proportions is progressively reduced because cells encompass more diversity, eventually becoming almost homogeneous in their degree of diversity. For example, 50-meter patch reefs would account for 100% of four contiguous 25-meter cells, and the surrounding cells would contain 0% of patch reef depofacies. However, within 100-meter cells, that same 50-meter patch reef would account for only 25% of the cell. For 25 meter cells, the range of expected patch reef proportions would vary from 0 to 100%. For 100 meter cells, the range of patch reef proportions would depend on the changing density of these relatively small patch reefs over the platform.

Also, experimental variogram ranges are observed to become larger as cell dimensions increase. At larger cell sizes the exact locations of individual patch reefs are lost, but larger patterns of clustering begin to emerge.

These insights are useful whether creating facies probability models for use as conditioning volumes for small, discrete cell assignments (OBM or SISIM) or as large, common-scale facies proportions for purposes of assigning flow characteristics for input to reservoir simulation (whole-cell effective property estimation).