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ABSTRACT

Large volumes of mobile oil remain in mature Gulf Coast fluvial-deltaic reservoirs, in some cases more than has been produced through five decades. Detailed characterization studies that identify the location of remaining oil are needed to recover this large resource. Such studies rely on the confident identification of reservoir architecture and heterogeneity, which are difficult to determine with the limited subsurface data typically available in mature fields. Observations from the Cretaceous Ferron Sandstone of Utah indicate that architecture and heterogeneity are related to position within a high-frequency depositional cycle and are consequently predictable. To test the application of these observations to Gulf Coast reservoirs, the architecture and heterogeneity of upper delta plain channel belt reservoirs within a flooding surface-bounded interval in Tijerina-Canales-Blucher (T-C-B) field, located within the Frio Formation Fluvial-Deltaic Sandstone (Vicksburg Fault Zone) play in South Texas, were examined.

Channel belt sandstone reservoirs deposited during the progradational/aggradational portion of a depositional cycle are narrow, ranging from 0.4 to 1.5 mi in width, and internally homogeneous, with single gas completions draining comparatively large reservoir volumes of approximately 40 ac. In contrast, channel belt deposits of the retrogradational portion of a cycle are broad, more than 3.5 mi in width, and internally heterogeneous, with individual completions typically draining oil reservoir volumes of 1.5 ac. These patterns parallel observations of incised valley channel architecture in the Ferron and are probably strongly influenced by progressive increases in baselevel rise through a cycle. However, climate may also influence patterns, as indicated by similarities with Pleistocene Gulf Coast fluvial systems that are interpreted to vary in architecture as a consequence of changing river discharge volumes.

Application of Ferron and Scott/Whitehill observations to other Gulf Coast reservoirs should be made cautiously, with appropriate considerations of position along depositional dip and age-specific eustatic amplitudes being incorporated. Predictions of reservoir architecture and heterogeneity made on the basis of position within a depositional cycle can assist in the prioritization of reservoirs for detailed characterization studies, in overall estimations of reserve-growth potential, and in focusing of mature field revitalization strategies.

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