Geometry, Distribution, and Infill Character of Erosional Scours in a Thin-bedded, Distal Lower Shoreface Sandstone Reservoir Analog: Grassy Member, Blackhawk Formation, Book Cliffs, East-Central Utah, USA

Abstract

Many wave-dominated deltaic reservoirs contain significant hydrocarbon volumes within intervals of distal lower shoreface deposits that consist of thinly interbedded sandstones and shales. Recovery from these reservoir intervals is expected to depend in part on the proportion of sandstone beds that are connected by erosional scours, and the lateral extent and continuity of shale interbeds. Cliff-face exposures of the “G2” parasequence, Grassy Member, Blackhawk Formation in the Book Cliffs of central Utah allow detailed analysis of the geometries, spatial distribution, and infill character of large (metre-scale), steep-sided erosional scours within such thin-bedded sandstones and shales. The study utilises a high-resolution 3D digital outcrop model (DOM) that measures 148 m (length) × 24 m (width) × 15 m (height), equivalent in size to part of an inter-well volume. The DOM is integrated with 12 measured sections that record lithology, grain size, sedimentary structures and palaeocurrent directions. 120 observed erosional scours exhibit three types of cross-sectional geometry in cliff faces of all orientations: simple symmetric, simple asymmetric, and complex “stepped” morphologies. There is no consistent sense of scour asymmetry. The erosional scours are up to 1.1 m in thickness and 14.4 m in apparent width, and have steep sides that strike N155 ± 61o, perpendicular-to-oblique to the local paleoshoreline trend. Thus the steep-sided erosional scours appear to have limited lateral continuity along strike and down dip, implying that they are similar to non-channelized flute casts and /or pot casts of variable azimuthal orientation in their 3D geometry. Scours occur at multiple stratigraphic levels and are not confined to any particular geographic location within the study area. Although erosional scours show some degree of localised clustering, implying that they were re-occupied during multiple erosional events, analysis of scour density per bed suggests that they tend to be more common in association with thick (>1 m) sandstone beds that contain hummocky cross-stratification of large wavelength, amplitude and laminaset thickness. Characterization of these large, steep-sided erosional scours provides a template for their 3D geometry and spatial distribution, which is linked to bed and laminaset thickness that can be measured in core. The results will aid prediction of sandbody connectivity in thin-bedded distal lower shoreface sandstone reservoirs.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017