ABSTRACT: Quantification of Geologic Descriptions for Reservoir Characterization in Carbonate Reservoirs

F. Jerry Lucia, Gary W. Vander Stoep

Recognition that a large volume of oil remains in carbonate reservoirs at the end of primary depletion and waterflooding has prompted the reevaluation of the reserve-growth potential of many existing carbonate reservoirs. Reevaluation typically involves interdisciplinary teams of engineers and geologists. For these interdisciplinary teams to be effective, observational geologic data must be converted into numerical engineering data for input into production-performance and economic computer models. Types of numerical data required include porosity, absolute permeability, relative permeability, fluid saturation, and capillary pressure, all of which are related to the size and distribution of pore space. Rock fabrics control the size and distribution of pore space and defin facies that best characterize carbonate reservoirs. Thus, the link between facies descriptions and numerical engineering data is the relationship between pore-size distribution and present carbonate rock fabric.

The most effective way to convert facies descriptions into engineering parameters is by considering three basic rock-fabric categories. The first category is interparticle pore space (both intergranular and intercrystalline pore types) with pore-size distribution controlled primarily by the size and shape of grains or crystals. Grain or crystal size is the key geologic measurement and, along with porosity, provides the basis for converting geologic descriptions into values for permeability, saturation, and capillarity. The second category is separate-vug pore space, such as moldic or intraparticle pore space. Separate-vug pore space adds porosity but little permeability to the reservoir rock. The contribution to saturation and capillarity depends upon the size of the separate-vug pore space. For example, moldic separate vugs will be saturated with oil, whereas microporous grains will be saturated with water. The third category is touching-vug pore space, which is vuggy pore space that is interconnected on a reservoir scale. The engineering parameters for this category are related to three diagenetic and tectonic factors.

Facies descriptions of carbonate reservoirs are characteristically geometric and interpretive in nature, whereas engineering descriptions are characteristically site-specific and numerical in nature. Reservoir characterization for improved recovery from existing reservoirs requires the integration of geometric with numerical information in order to show the three-dimensional distribution of remaining hydrocarbon saturation. Rock fabric studies are the key link to accomplish this integration.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990