Petrological and Petrophysical Mapping of How Units for an Enhanced Recovery Project: Happy Spraberry Field, Garza County, Texas

Edward C. Roy III, Bradley S. Hammel

The Spraberry Formation (Permian) has long been recognized as a prolific hydrocarbon producer in the Midland Basin of West Texas where it consists of interbedded, deep-water sandstones and shales, many of which are interpreted to be turbidites. Spraberry reservoirs commonly produce from fracture porosity, as depositional porosity is low owing to high matrix content and pore reducing burial diagenesis. However, Spraberry lithologies are quite different in the shallower settings around the basin margin. The Happy Spraberry Field in Garza County, for example, produces from a carbonate reservoir. There the Spraberry Formation consists of interbedded siltstones and shales overlain by a carbonate sequence that attains thicknesses of up to 100 feet. The carbonate rocks include b ndstone and rudstone patch reefs along with oolitic and peloidal grainstones. Reservoir porosity and permeability are confined primarily to grainstones that have undergone early diagenetic leaching with concurrent formation of grain-moldic and vuggy porosity. The Happy Spraberry Field is a candidate for enhanced recovery. A fundamental problem in any development, infill drilling, waterflood, and enhanced recovery is determining the spatial distribution of effective (connected) porosity. In this patch reef-grainstone complex, zones of maximum effective porosity (termed flow units) are initially identified by plotting log k vs. ^phgr, from core analysis, to locate zones with greatest correspondence. Thin sectioned samples from representative intervals are examined to categorize pores by ge esis (depositional, diagenetic, fracture and combination) and geometry (size, shape and visible connections). Finally, flow units are identified on the basis of grouped data from core descriptions, thin section study and plots of core analyses. They are then compared with borehole log traces to seek identifying log signatures to aid in correlation from well-to-well. Flow units, in general, are found to have common genetic characteristics so that identification of their origin and diagenetic history can be used effectively in conjunction with structure and paleostructure maps, along with stratigraphic cross sections to create a 3-dimensional picture of flow units within the broader reservoir unit.

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