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Facies Architectural Elements and Sandstone-Body Continuity in Highstand, Fluvial-Dominated, Deltaic Reservoirs in the Upper Cretaceous Woodbine Group, East Texas Field

Ambrose, William A.1; Loucks, Robert 1; Hentz, Tucker F.1; Bonnaffe, Florence 1
1 Bureau of Economic Geology, Austin, TX.

A variety of facies architectural elements, observed in >1,500 ft (>450 m) of 30 conventional cores, are inferred to control reservoir quality and sandstone-body continuity in fluvial-dominated deltaic reservoirs in the lower section of the Woodbine Group in East Texas field. Architectural elements in each facies in the deltaic succession are composed of one or more sandstone beds with unique characteristics that include grain-size profile, stratification, internal bounding surfaces, and accessory features such as burrows, clay clasts, shell fragments, and root mottling. For example, Woodbine distributary-channel deposits consist of three primary architectural elements: (1) fine-grained, crossbedded, and plane-bedded sandstone with an erosional base, (2) fine-grained sandstone with subvertical and contorted fabrics, and
3 very fine to fine-grained sandstone with plane beds and asymmetric ripples, which correspond to channel-floor dunes, channel-margin slumps and/or dunes with oversteepened foreset bedding with dewatering structures, and late-stage, waning-current flow deposits, respectively. Porosity and permeability values, differing in each of these distributary-channel architectural elements, are inferred to reflect primary depositional fabrics. Similar variations in porosity and permeability occur in architectural elements of channel-mouth-bar, delta-front, and splay facies, which, together with distributary-channel facies, define the sandy depositional framework. Larger-scale sandbody heterogeneity in the field is controlled by distribution of narrow distributary-channel sandstones that pinch out over short distances (typically <500 ft [<150 m]) into delta-plain and interdistributary-bay siltstones and mudstones. A full understanding of reservoir compartmentalization, fluid flow, and unswept mobile oil in the field should include spatial distribution of principal architectural elements.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009