Geologic Description of San Andres Reservoir, Mallet Lease, Slaughter Field, Hockley County, Texas: Implications for Reservoir Engineering Projects

W. J. Ebanks, Jr.

The lower San Andres dolomite reservoir in the Mallet lease area of Slaughter field, a giant stratigraphic trap in Hockley County, Texas, has been the subject of integrated geologic and engineering reservoir analyses. The geologic study provided a good physical model from which engineering studies could account successfully for past production history and predict the response to future operations.

The Mallet reservoir, which is about 200 ft thick, is stratified into a repeating sequence of permeable and porous, laterally extensive units that dip southward at a rate of 50-60 ft/mi. This stratification results from the vertical and orderly repetition of the facies in sedimentary sequences. Each sequence begins with a minor disconformity, above which are subtidal sediments, then intertidal, and finally supratidal sediments, representing repeated progradation of mud-flat and adjacent nearshore marine sediments over deposits of the shallow marine shelf. Only the subtidal facies is sufficiently permeable to be considered net pay. Anhydrite cement strongly affects porosity and permeability because it preferentially occludes larger pores and causes fluid flow to be controlled by the very fine matrix pore system.

The various geologic factors that influence reservoir quality in numerical models may be incorporated by defining model layers on the basis of "flow units." Flow units are subdivisions of the reservoir that are expected to behave similarly to the oil recovery process. Their geometries are defined on the basis of geologic facies and patterns of diagenesis, and they are assigned properties determined from petrophysical measurements of porosity, permeability, fluid saturations, and other critical aspects of reservoir quality.

AAPG Search and Discovery Article #91037©1987 AAPG Southwest Section, Dallas, Texas, March 22-24, 1987.