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Capturing Geological Heterogeneity in Reservoir Architecture Through Quantitative Characterization and Modeling of an Analogous Outcrop, Fan 3 of the Skoorsteenberg Formation, Tanqua Karoo, Sub-Basin, South Africa


Depositionally analogous outcrop and subsurface examples are commonly utilized to provide insights into reservoir architecture, especially during appraisal and early field development stages where well control is sparse and seismic data are not well constrained. Information from these analogues is often employed though the application of measurements within quantitative databases that can aid in constraining geobody dimensions. However, the databases are often incomplete or of a form too general for detailed stratigraphic modeling. High resolution field mapping of well exposed outcrop analogues can provide the level of stratigraphic detail necessary to capture geological heterogeneity as well as reservoir scale architecture for a range of depositional environments. Numerous data collection techniques are used in the mapping of key stratigraphic horizons in these field studies, which are then integrated within geomodeling software packages to develop 3D models of the relevant reservoir architecture of the outcrops. The focus of this study is “Fan 3” of the Permian-age Skoorsteenberg Formation, within a well exposed and relatively structurally undeformed outcrop belt in the Tanqua Karoo Sub-basin in South Africa. The depositional architecture of proximal to distal lobes and lobe complexes of Fan 3 was identified as a potential analogue to unconfined deepwater Gulf of Mexico and Alaskan deepwater sandstone reservoirs. Measured sections were tied together with detailed mapping of key stratigraphic surfaces bounding lobe and interlobe units using differential GPS, with accuracy on the order of decimeters. The section measurements, gamma ray profiles and dGPS surfaces were brought together in 3D modeling software along with supplemental data from other published studies of Fan 3. These data were then used to build a 3D reconstruction of the facies architecture of the lobes comprising Fan 3. The result of this effort is a three-dimensional facies model constrained by detailed spatial and descriptive outcrop data. This geomodel can be populated with specific reservoir properties (such as porosity and permeability) in order to simulate a possible, or geologically realistic, scenario. Here, the methodology is presented, from data collection to geomodel creation, in a timeframe applicable to deepwater exploration and development.