Reservoir Characterization: A Case Study from the Clearfork Formation of the Central Basin Platform, West Texas

S. C. Atchley, M. G. Kozar, D. L. Cantrell, A. J. Coker, M. G. Gorski, P. A. Larabee, M. W. Longtine, P. A. Merkley

Differential paleotopography along the northern portion of the Central Basin Platform at the time of Clearfork and Glorieta deposition controlled local trends of accommodation, and therefore, facies distribution. Prior to Permian deposition, the Ouachita-Marathon orogeny generated, or possibly reactivated, a number of fault-bound blocks. During Clearfork and Glorieta deposition, these blocks served as topographic highs. Reservoir quality and heterogeneity is strongly influenced by the interplay of differential paleotopography with short- and long-term changes in relative sea level. An integrated data set consisting of seismic, core, thin sections, and well logs was used to establish a sequence stratigraphic reservoir zonation for the Clearfork and Glorieta formations. Seq ences (10's to 100+ feet thick) and composite sequences (100's of feet thick) each consist of a repetitive stack of intertidal- and subtidal-prone parasequences (1 to 10's of feet thick). Intertidal-prone successions are most common over paleo-highs, with an increasing subtidal dominance into the troughs of adjacent paleo-lows.

Although porosity is highest in intertidal facies, pores consist of primary, fabric-selective fenestrae which are poorly connected. The highest permeabilities are preferentially associated with subtidal facies which were subjected to an early phase of "coarse dolomitization." Because reservoir quality is facies dependent, development of an accurate reservoir model for simulation and exploitation requires facies identification in wells lacking core control. Using statistical analysis, we are able to predict the intertidal- and subtidal-prone facies distributions in wells lacking core control by applying an 11 percent neutron porosity cutoff, i.e., greater than 11 percent, intertidal-prone; less than or equal to 11 percent, subtidal-prone. Mapped output from this algorithm indicates tha subtidal facies become increasingly abundant down the flanks of paleo-highs. A progressive increase in total fluid production down the flanks of paleo-highs supports this interpretation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995