Abstract: The DAK Formation Evaluation Model for the Permian Basin Clearfork

By

E. A. Clerke¹, K. W. Williams², and L. A. Pearce^3
1Pennzoil Exploration and Production Co., Houston
²Shell Development Co., Bellaire
³Shell Western E&P, Houston

All of the authors were employed by Shell Western E&P when this work was completed in 1989.

The Clearfork DAK [dolomite, anhydrite, potassium] Mineralogy model is a new formation evaluation procedure developed specifically for the Permian Basin Clearfork formation. Using the physical, geochemical, and mineralogic properties of the Clearfork, a natural log response analysis frame has been developed. The result is a computation of the formation mineralogy and porosity both of which are accurate and valuable for describing reservoirs with this variable lithology matrix.

The Clearfork formation with low permeability and porosity is an economic target for infill drilling and secondary recovery because of its thickness (1,300') and discontinuity. The thick Glorieta-Clearfork interval is actually a series of stacked shallowing-upward carbonate shelf sequences and intraformational clastic rich seals.

Silt was periodically deposited along with the anhydritic dolomite. This mixed lithology created a long standing problem in formation evaluation. The clastic-rich intervals are important because they are locally a reservoir seal and are a time stratigraphic marker. A regional geologic model is used to relate silt mineralogy and log response to the large scale clastic sediment transport process. Our model was tested using core data from the TXL, Central Robertson, and South Wasson Clearfork Units.

Applying the DAK formation model results in improved infill assessment and waterflood development. The computed mineralogy and porosity are in excellent agreement with core data. Further, the log derived mineralogy-facies is a major input to reservoir description: identifying chronostratigraphic supratidal clastics for sequence and flow unit correlations. The application of this model to Clearfork reservoir description results in:

• Log-derived porosity and mineralogy for reservoir description and pay delineation
• Volumetrics and stratigraphic correlations
• Improved completions and producer-injector coupling
• Improved assessment of development drilling opportunities.

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