Abstract
Fluvial sheet sandstone provides an ideal reservoir for hydrocarbons. Such a sandstone body is large enough to contain giant hydrocarbon reserves as well as an aquifer that could flush the hydrocarbons.
The outcrops of the Westwater Canyon Member in the Morrison Formation (Jurassic), northwestern New Mexico, show representative attributes of a fluvial sheet sandstone. This sandstone body consists of coalescing fluvial-channel systems that in turn are composed of still smaller coalescing individual channels. The sheet is more than 63 mi (100 km) wide transverse to the paleocurrent direction and averages 200 ft (61 km) in thickness; the downcurrent dimension is not preserved but may have been as much as 100 mi (160 km). Channel systems internal to the sheet average 7 mi (11 km) wide and 50 ft (15 m) deep. The individual channel deposits that form the systems average 600 ft (183 m) wide by 12 ft (4 m) thick.
Any one vertical column through the sheet sandstone is usually more than 90 percent sandstone. Further, each vertical column consists of repetitive incomplete (top-missing) fluvial sequences that show upward-decreasing grain sizes and scales of sedimentary structures.
Fluvial sheet sandstone probably is deposited by aggrading and coalescing braided-stream systems.
