WITTON, BETH M., ROGER M. SLATT, and NEIL F. FURLEY, Colorado School of Mines, Golden, CO, W. BRUCE WARD, WILLIAM MURPHY, JOHN ORANGE, and ANDY REISCHER,Theoretical and Applied Geology, and HERMAN HOMANN, Baker Atlas

Abstract: Confined, Unconfined and Thin-Bedded Turbidites and Sandy Debris Flows, Lewis Shale, Wyoming

Overall geometries of confined, unconfined and thin-bedded turbidites may be resolved using 3-D seismic data; however, smaller-scale geometries that are important in controlling lateral and vertical reservoir connectivity and that may compartmentalize reservoirs are below seismic resolution. Smaller-scale 3-D geometries are exposed within outcrops of confined, unconfined and thin-bedded turbidites and sandy debris flows in the Upper Cretaceous (Maastrichtian) Lewis Shale, Sierra Madre Uplift,Wyoming.

Facies were identified, 3-D geometries measured, and an environmental model was developed for surface exposures of the Dad Sandstone and the Upper Shale Members of the Lewis Shale.The outcrop data were compared to STAR acoustic and electrical borehole images of correlative strata in a nearby well.The study area included eight hundred feet (244 m) of vertical section and extended one mile (1.6 km) laterally. Sections were measured and sandstone beds were walked out laterally.To aid visualization, permit accurate measurements of sandstone bodies and determination of 3-D geometries, the outcrop data were rendered in 3-D on a computer.The framework of this computer visualization consists of (a) topographic measurements, (b) detailed measurements of sandstone outcrops and (c) aerial and land-based photographic images.The photographic images were accurately mapped onto the topography.

The studied sections of the Dad Sandstone and the Upper Shale members consist predominantly of (1) thin interbeds of very-fine sandstone, mudstone and siltstone and (2) thick, very fine to medium-grained sandstones found at different stratigraphic levels throughout the formation. Field measurements indicate that the massive sandstones range from laterally continuous to discontinuous. Laterally discontinuous sandstones are flanked and overlain by thin-bedded deposits. Observations of sedimentary features in the outcrops indicate that the Dad Sandstone is a deep-water (below storm- wave base) turbidity current and sandy debris flow deposit.

Individual facies types with specific vertical and lateral features and geometries seen in outcrop can also be recognized in a correlative Lewis Shale interval through the interpretation of STAR acoustic and electrical borehole images. In the subject well, located approximately seven mi (11 km) west of the outcrop exposure, four unique types have been identified. Each of the four facies types contains a specific bedding style, sedimentary structure assemblage and lithology response on borehole images.

Electrofacies "A" consists of massive to parallel-laminated sandstones that exhibit (a) Bouma sequences (Ta-Tb), (b) sandstone slump structures (due to rapid deliquification), (c) convolute bedding and (d) loaded lamination.

Electrofacies "B" consists of graded beds with (a) Bouma intervals Ta-Tc present, (b) rapidly deposited non-graded beds, (c) concentrated sandy debris-flow beds, (d) abundant climbing ripple marks, (e) injection structures and (f) oriented flame structures.

Electrofacies "C" consists of thin sandstone beds separated by siltstone and mudstone.This monotonous facies type is repetitive over hundreds of stratigraphic feet.

Electrofacies "D" consists of laminated mudstone, siltstone and fine-grained sandstone.

Electrofacies "E" occurs near the base to upper-middle portions of the Dad Sandstone. Gross interval thickness is 1000 ft (304 m).

Electrofacies "D" separates individual packages of Electrofacies "A." Electrofacies "B" which is 80 ft (24 m) thick, occurs above Electrofacies "A." Electrofacies "C" which is the 400 ft (122 m) thick Upper Shale Member of the Lewis, occurs at the top of the section, overlying Electrofacies "B".

We are attempting to establish the lateral continuity of these differing facies by walking out surface outcrop exposures and by correlating with the subsurface data. Facies "A" in outcrop, equivalent to Electrofacies "A" in the subsurface, consists of individual bedsets ranging from 10 to 16 ft (3 to 5 m) thick. Lateral continuity may range from 0.5 to 1 mi (0.8 to 1.6 m) long although in the subsurface where the system is thicker, lateral continuity may be much greater. These deposits are interpreted to be laterally continuous sheet sands deposited close to the toe of slope on the basin floor. Facies "B" in outcrop, which is equivalent to Electrofacies "B", ranges in thickness from 5 to 40 ft (1.5 to 12 m) and lateral continuity of these discontinuous lenticular sandstone bodies may range from 75 to 420 ft (23 to 128 m). Facies "B" is interpreted to be laterally confined channelized deposits. Facies "C", equivalent to Electrofacies "C" is variable in both thickness and lateral continuity.Thickness ranges from a few inches to 4 ft (1.2m). Lateral continuity may range from a few feet to several hundred feet. These deposits are interpreted to be prodelta shingled turbidites and gravity flows. Facies "D" in outcrop, correlative to Electrofacies "D" in the subsurface, consists of laminated mudstone and siltstone with small laminations to 6 in (15 cm) thick sandstone beds.

In order to measure true sandstone geometry from weathered exposures, the data are interpreted in 3D. Virtual 3D representations of the data offer a superior framework for determining 3D continuity and connectivity.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England