Abstract
Sussex sandstone produces oil in a stratigraphic trap at House Creek field in the south-central Powder River basin. The total thickness of the Sussex is 40 ft (12 m) and reservoir sandstone is as much as 30 ft (9 m) thick, about 1 mi (1.6 km) wide, and 25 mi (40 km) long. The sandstone is probably of marine origin but has an unexpected bedding sequence: (1) an upper bioturbated mudstone that contains scattered granules of chert; (2) thin, coarse-grained, pebbly sandstone; (3) thin-bedded, cross-laminated sandstone with deformed shale clasts; (4) ripple-bedded sandstone with shale laminae; and (5) a basal shale that contains ripple lenses of sandstone and rare bioturbation. Mean size of quartz is 0.22 mm (fine grained), and grain size increases upward from 0.13 mm at the base to 0.28 mm in the pebbly sandstone. The sandstone contains monocrystalline quartz, 48 percent; feldspar, 11; polycrystolline quartz, 10; chert, 13; and clay matrix, glauconite, and chlorite, 15 percent. The Sussex represents a prograding sequence of sand that was transported southeastward by low-flow-regime currents along a relatively shallow, marine shelf. Bioturbation, strotigraphic position, and similarity to other sandstones in the some sequence suggest a neritic environment of water depths in the range of 100 to 200 ft (31 to 62 m). The thin, pebbly sandstone is not fully explained because its coarse texture suggests stronger, perhaps channelized, currents. Nevertheless, the narrow, prograding sequence probably represents a major transport route for sand across the shelf toward a deeper basin beyond.
