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Deeper-water deposition in intrashelf basins: Example from the Lower Cretaceous (Albian) upper Glen Rose Formation in the Houston trough, eastern Texas
1Greenhill and Co., Houston, Texas; [email protected]
2Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas; [email protected]
3Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas; [email protected]
In eastern Texas, a thick (150–200 ft) interval of Lower Cretaceous upper Glen Rose D strata was deposited in the Houston trough intrashelf basin landward of the Stuart City reef margin and seaward of the main eastern Texas intrashelf basin. The stratigraphic section in this intrashelf basin forms a primarily oil-bearing hydrocarbon system where porous grainstones in the Alabama Ferry and Fort Trinidad fields were deposited coevally with organic-rich calcareous argillaceous mudstones. Here, we present evidence to suggest that the depositional setting within the Houston trough and the contained ooid grainstone beds were of deeper-water origin (400–500 ft), below storm wave base. Furthermore, the water column was stratified, allowing dysaerobic to anaerobic conditions to develop, fostering accumulation and preservation of organic matter. Grain-rich gravity flows consisting of ooids, intraclasts, and thin-walled mollusks, derived from the shallower shelf to the northwest, were deposited in the basin, forming the Alabama Ferry and Fort Trinidad fields.
The upper Glen Rose interval is composed of lithofacies that vary laterally within the Houston trough. Five lithofacies are recognized on the basis of sedimentary features, texture, fabric, biota, and organic-matter content: (1) laminated–nonlaminated argillaceous lime mudstone to lime wackestone, (2) laminated–nonlaminated calcareous siliciclastic mudstone to siltstone, (3) intraclast–ooid skeletal lime packstone, (4) intraclast–bivalve–ooid lime grainstone, and (5) oyster lime floatstone and rudstone.
Grain-rich lithofacies laterally adjacent to and interbedded with organic-rich argillaceous lime mudstones and calcareous siliciclastic mudstones were previously interpreted as being deposited in a tidally influenced, high-energy, shallow-water shoal environment; however, reconstruction of the depositional setting and interpretation of the vertical and lateral distribution of lithofacies suggest that these lithofacies were deposited as grain-rich gravity flows into the deeper-water Houston trough. This interpretation suggests a new conceptual framework integrating facies models from submarine channel and fan complexes to explore for grainstone reservoirs similar to Alabama Ferry and Fort Trinidad fields.
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