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Thin sandstones in the Canyon Group produce oil from stratigraphic traps in the Roundtop area, Fisher County, Texas. Cores from Tolar Canyon field show that the sandstones are turbidites of channel and overbank origin. Channel sandstones are composites of stacked AE and ABE turbidite sequences. Overbank sandstones represent more complete sequences of the ABCE and ACE types. Interbedded sections of basinal shale contain thin ripple lenses of sandstones representing CE sequences.
The sandstones are fine grained (0.18 mm) and consist of 72% quartz, 2% feldspar, 19% rock fragments, and 6% clay matrix. Cement is present in a total amount of 11% and is mostly calcite with some silica overgrowths. Thicker channel sandstones have the best average permeability, about 12 md, and thinner channels have lower permeabilities of 2 to 4 md, whereas rippled sandstones have very low permeability because of clay matrix and interbedded shale.
Massive channel sandstones are closely bounded laterally by thinly bedded levee deposits which change abruptly to basinal shale. Channel reservoirs are 10 to 30 ft (3 to 10 m) thick, only about 500 ft (150 m) wide, and form narrow, sinuous, dip-trending bodies at several stratigraphic levels within the Canyon interval of about 300 ft (92 m). The abrupt lateral change from channel to overbank and then to basinal deposits suggests that channels were of the "constructional" type and composed of successive flow units, each of which consisted of contemporaneous channel-fill and levee facies.
This depositional system is distinctly different from submarine-fan deposits which are also composed of channel and overbank sediments but form thicker, composite sandstone bodies of greater areal extent and broadly lobate morphology. In contrast, the "constructional" channel sandstones are thin units isolated in a dominant basin-shale sequence. Recognition of these reservoir types may be important in exploration and field development in other turbidite sections.
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