Abstract: Characterization and Reservoir Quality Assessment of a Mixed Carbonate-Siliciclastic Reservoir: Cretaceous Escandalosa Formation, "O" Member, Barinas-Apure Basin, Venezuela

Kupecz, Julie A.; Luisa Figueroa; Rosa Aquino; Elizabeth Hernández; Mercedes Pérez; Maria Prietó; Rodolfo Salazar - PDVSA; and Dan J. Hartmann - DJH Energy Consulting

The "O" Member of the Cenomanian-Turonian Escandalosa Formation is a significant hydrocarbon producer in the Barinas-Apure Basin of Venezuela. Facies include sandstone, mixed sandstone-dolostone, dolostone, pelecypod limestone, foraminiferal limestone-siltstone, and shale. The facies are interpreted to represent deposition in a shallow-marine carbonate platform environment having variable input of siliciclastics, which changes abruptly in the northern part of the study area to a deeper-water, middle-neritic setting. Dolomite replaces platform carbonates but does not replace the basinal foraminiferal carbonates.

The "O" member can be subdivided into nine depositional cycles, or para-sequences, that are correlatable within the study area, with hydrocarbon reduction from the dolomitic intervals. Exposure after deposition of each cycle has caused dissolution of allochems and has generally resulted in an increase in porosity at cycle tops. Due to both facies and diagenetic control on reservoir quality, potentially productive intervals are compartmentalized and represent individual flow units (Fig. 1). The location of productive dolomitized intervals depends on the areal extent of transgression and progradation, and has been mapped for each cycle.

Dolomites within the "O" member display a complex diagenetic history. Cathodoluminescence (CL) petrography illustrates extensive modification of early-generation dolomites by late-stage fluids, including several generations of recrystallization, dissolution and cementation. Dolomite neomorphism has, in general, improved reservoir quality by increasing crystal size and pore-throat radii (Fig. 2, Group III) and, therefore, increasing permeability.

Based on petrography, the "O" member can be subdivided into nine different pore types: (1) dolomite with intercrystalline porosity; (2) dolomite with variably cemented intercrystalline porosity; (3) dolomite with tight matrix; (4) calcite with tight matrix, with vugs and molds; (5) calcite with tight matrix; (6) sandstone with carbonate matrix; (7) sandstone with intergranluar porosity; (8) sandstone with variably cemented intergranular porosity; (9) siltstones with poorly-developed porosity. By integrating mercury injection capillary pressure data, these pore types can be combined into three groups having similar characteristics of fluid flow. The three major pore-type groups, in order of decreasing reservoir quality (Fig. 2) are: Group I (pore types 1, 7); Group II (pore types 2, 4, and some 6, 8); and Group III (pore types 3, 5, 9, and some 6, 8). Calibration of porosity and permeability data, capillary pressure data, and pore type characterization thus allows the estimation of the size and distribution of pore throat radii and the estimation of pressure (or equivalent hydrocarbon height) necessary for fluid flow. As a result, the distribution of reservoir quality can be mapped, areas of potentially high reservoir quality can be predicted, and reserves calculations can be refined.

Three-dimensional mapping of reservoir parameters of the "O" Member (e.g., distribution of dolomite; porosity; permeability; R₃₅) has improved our understanding of the distribution of the reservoir, and is useful in reservoir simulation studies as well as for the prediction of exploration trends. Figure 2 illustrates the distribution of dolomite in one individual cycle. This type of visualization has allowed us to predict the vertical and lateral distribution of reservoir-quality rock, the presence of fluid-flow intervals and the compartmentalization of each productive cycle.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil