The Importance of Layering in Reservoir Characterization:
South Wasson Clear Fork Reservoir, Gaines County, Texas

F. Jerry Lucia, James W. Jennings, Jr., and Stephen C. Ruppel

ABSTRACT
The number and location of layers have a major impact on reservoir simulation results. In the past, computer capacity has limited the number of layers that could be used and scale-up from numerous layers to a few layers was a major issue. With the improvement in computer technology, however, more layers can be accommodated and the number and location of the layers have become a major issue. Two layering methods used to describe the South Wasson Clear Fork reservoir (SWCF) are compared, rock-fabric flow layers and proportional layering. The rock-fabric method divides the high-frequency cycles (HFC) into two flow layers, an upper grain-dominated layer and a lower mud-dominated layer. HFC's are based on upward-shallowing successions observed in core and outcrop and correlated using established sequence boundaries as a guide and porosity as an indication of grain content. The proportional method is based on dividing the intervals between five major correlation markers into a fixed number of layers on the basis of a combination of vertical porosity variograms and cycle thickness.

Maintaining high and low permeability is a major goal when defining layers; high permeability controls the flow rate, and low permeability controls cross flow. In the SWCF reservoir, permeability is a simple function of porosity because the effect of grain/crystal size and sorting has been reduced by a diagenetic overprint of poikilotopic anhydrite. Fractures (a type of touching vug) probably provide a small improvement in permeability over the matrix, although many fractures observed in cores are filled with anhydrite. Therefore, vertical porosity profiles from wireline logs can be converted to permeability profiles using a single porosity-permeability transform. A comparison of permeability values by layer shows that the proportional layers tend to group high and low permeability, whereas the rock-fabric flow layers tend to maintain high and low permeability. This difference results in significantly different predictions of reservoir performance.

F. Jerry Lucia, Senior Research Fellow
Bureau of Economic Geology
The University of Texas at Austin
University Station, Box X, Austin, TX 78713-8972
Telephone 512-471-7367; Fax 512-471-0140
E-mail: jerry.lucia@beg.utexas.edu