South Wasson Clear Fork Reservoir Modeling: Outcrop and Subsurface Geology – the Critical Basis for Defining Reservoir Framework

Stephen C. Ruppel, James W. Jennings, Jr, and F. Jerry Lucia
The Bureau of Economic Geology
John A. and Katherine G. Jackson School of Geosciences
The University of Texas at Austin
Austin, Texas

A high resolution framework is essential for meaningful distribution of petrophysical properties in a reservoir model. Such a framework can only be developed from a well-integrated geological data set that includes subsurface cores, fully calibrated wireline logs, and an appropriate outcrop model.

To develop a framework for the South Wasson Clear Fork reservoir, we first collected and integrated descriptive geological data and petrophysical measurements from Clear Fork reservoir-equivalent outcrops in the Sierra Diablo of West Texas. These data provided key insights into the sequence and cycle stratigraphic architecture of the succession and geostatically significant data on the spatial distribution of petrophysical properties within this architectural framework. The outcrop geological model was then used to help interpret subsurface and cores and log data to produce a cycle-based, architectural model for the reservoir.

Key defining data from outcrop include (1) sequence stratigraphic architecture, (2) distribution and geometry of component facies tracts, (3) diagnostic facies-stacking patterns within cycles, (4) lateral continuity of cycles and sequences, and (5) vertical and lateral distribution of petrophysical properties. Subsurface cores provide reservoir specific information on facies-stacking patterns and vertical and lateral distribution of petrophysical properties within cycles. Wireline logs, selected for their ability to identify cycle facies and boundaries, are then used to extrapolate core-based relationships throughout the reservoir. The resulting model provides the basis for constructing a 3D reservoir framework into which porosity, permeability, and water saturation can be distributed.

This procedure creates the most geologically realistic model by honoring original depositional architecture, which in most shallow water carbonate platform reservoirs is the primary controlling factor in the distribution of reservoir properties.