Shelf Sandstones in the Woodbine--Eagle Ford Interval, East Texas: A Review of Depositional Models
Sandra Phillips, Donald J. P. Swift, 1985. "Shelf Sandstones in the Woodbine--Eagle Ford Interval, East Texas: A Review of Depositional Models", Shelf Sands and Sandstone Reservoirs, R. W. Tillman, D. J. P. Swift, R. G. Walker
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This paper reviews studies of Woodbine--Eagle Ford reservoir sandstones from the subsurface of East Texas and evaluates shelf sand depositional models in the light of recent studies of fluid and sediment dynamics on modern shelves. The application of fluid and sediment dynamical principles has reaffirmed some shelf depositional models, traditionally applied to the East Texas basin, but modifies or discredits others; in these cases, new models are proposed. Three distinct types of reservoir-quality shelf sandstones can be recognized in these studies; (1) sand ridge deposits, (2) tabular or sheet sandstones, and, (3) lenticular (topographically controlled) sandstones. This preliminary classification is based on external sand body geometries, facies associations and facies distributions.
Sand ridge deposits occur at Kurten Field as stacked, en echelon, linear sandstone bodies deposited on the muddy shelf of the east side of the Cretaceous Interior Seaway. Sandstone bodies are asymmetric in cross-section with steeper eastern flanks and are elongated in a north-south direction. Sand ridge deposits at Kurten Field occur stratigraphically adjacent to deposits of the Harris Delta. Sand ridge deposition probably occurred in an inner to middle shelf environment during small scale transgressive episodes,} possibly associated with the abandonment of delta lobes (autocyclic transgression). Intermittent, alongshelf, geostrophic flows appear to be the most likely mechanism of sand transport and deposition.
Tabular shelf sandstones occur in the lower Woodbine at Damascus Field as a complex of single to multistory thin beds within a dominantly shale section. Cores display stacked, massive to laminated, fining-upward sandstone sequences, with abundant soft-sediment deformation and primary structures indicating rapid sedimentation. Sandstones form a series of thin sheet-like deposits elongated across the strike of the paleoshoreline. Sanddeposition took place in an inner to middle shelf environment, during a general period of shoreline regression. Deposition is suggested to have occurred in localized zones of alongshelf flow deceleration and expansion during storms. Bouma-like vertical sequences of primary structures in Damascus sandstones indicate that these beds are tempestites (i.e. suspension deposits produced by storm flows).
Lenticular shelf sandstones are present in the uppermost Eagle Ford (Sub-Clarksville) section in Grimes County, Texas. Fining-upward sandstone sequences consist of amalgamated, massive to cross-stratified beds with erosional bases, overlain by bioturbated shaley sandstones. Individual sandstone bodies have restricted areal extents and deposition appears to have been controlled by local, salt-related topographic lows. These Sub-Clarksville sands were apparently deposited during a regional transgression which succeeded a phase of sea level stillstand. Remobilization of the substrate by wind-forced storm currents during transgression appears to have formed broad erosional surfaces, accompanied by deposition of sands swept into zones of local flow deceleration. Transgressive sand ridges may have formed contemporaneously on other parts of the late Eagle Ford shelf.
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Shelf sandstone reservoirs are becoming a more and more common exploration target. What they are, how they may be characterized, and how they differ from shoreline and deep-water deposits in the subject of this publication. Shelf sands and sandstone reservoirs are among the more poorly understood types of sandstones. Continental, shoreline and deep water sandstones have all been studied in much more depth than have shelf sands and sandstones. However, during the last fifteen years significant progress has been made in understanding shelf sands and sandstones. Studies of modern sediments have allowed us to understand many of the depositional processes active on the shelf. This book is intended to be an up-to-date summary of shelf processes and products. The papers are intended for those new to shelf sands and sandstones as well as the shelf specialist.