Predicting Interwell Heterogeneity in Fluvial-Deltaic Reservoirs: Effects of Progressive Architecture Variation Through a Depositional Cycle from Outcrop and Subsurface Observations
Paul R. Knox, Mark D. Barton, 1999. "Predicting Interwell Heterogeneity in Fluvial-Deltaic Reservoirs: Effects of Progressive Architecture Variation Through a Depositional Cycle from Outcrop and Subsurface Observations", Reservoir Characterization—Recent Advances, Richard A. Schatzinger, John F. Jordan
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Early recognition of stratigraphic heterogeneity can lead to improved esti-mates of hydrocarbon reserves and more efficient development strategies. Although good models based on studies of modern and ancient fluvialdeltaic deposits exist for predicting interwell heterogeneity, a single model applies only to genetically related deposits of a single pulse of progradation and retreat, which describes only one, or a part of one, reservoir. Recent studies document significant changes in depositional style in adjacent stratigraphic intervals, and a model for predicting this variability is needed; therefore, we have carried out studies of outcrop reservoir analogs and of reservoirs that suggest that variability is predictable if assessed within the framework of depositional cycles and that differences between adjacent high-frequency cycles can substantially impact production behavior and reserve-growth potential.
Outcrop studies of the Cretaceous Ferron sandstone of Utah have demonstrated that (1) incised fluvial deposits in progradational parts of low-frequency depositional cycles tend to be narrow, deep, and internally homogeneous, whereas those in retrogradational parts of such cycles tend to be wider, internally heterogeneous, and display lateral channel migration, and (2) river-dominated delta-front styles are more common in progradational parts of intermediate-frequency cycles, whereas wave-dominated delta-front styles are more common in retrogradational parts. Detailed investigations of two fluvial-influenced upper delta-plain reservoirs in the Oligocene Frio Formation of Tijerina-Canales-Blucher (T-C-B) field, south Texas, demonstrate that (1) a fluvial reservoir deposited early in an intermediate frequency cycle contains several narrow laterally isolated channel belts, whereas an adjacent reservoir deposited late in that cycle contains a single broad channel belt that is internally heterogeneous, and (2) this difference in styles is paralleled by a strong difference in production behavior, with a gas completion in the former reservoir draining 40 ac (16 ha), whereas the typical oil completion in the latter reservoir is approximately 1.5 ac (0.61 ha), and less than 10% of the original oil in place in the latter reservoir has been recovered.
The application of these findings to other reservoir intervals and other basins should be made with appropriate caution, but they have implications for project economics throughout field life, from the exploration phase through mature-field redevelopment. Benefits range from improved prospect ranking to improved development efficiency and better prioritization of mature fields for acquisition or characterization. Application of these techniques to U.S. fluvial-deltaic reservoirs could improve near-term reserve-growth potential, preventing permanent loss of strategic resources by curtailing premature field abandonments.