Effects of Erosional Scours on Reservoir Properties of Thinly Bedded, Distal Lower Shoreface Sandstone Reservoirs

Abstract

The reservoir properties of thinly interbedded sandstone and mudstone intervals of shallow marine reservoirs are difficult to predict. Important controls on hydrocarbon recovery from such intervals are expected to include the sandstone-to-mudstone ratio, the proportion of sandstone beds that are connected by sandstone-filled erosional scours, and the lateral extent and continuity of mudstone interbeds. These heterogeneities all lie below seismic resolution and are sparsely sampled by wells. We address the characterisation of thinly interbedded sandstone and mudstone intervals of wave-dominated deltaic reservoirs using a high-resolution digital outcrop model of 148 m length, 94 m width and 15 m height, constructed from an outcrop analog (distal lower shoreface deposits of the “G2” parasequence, Blackhawk Formation, Book Cliffs, east-central Utah). These analog data are used to characterize the geometries, dimensions and spatial distributions of large (metre-scale) erosional scours observed within distal lower shoreface deposits. The erosional scours range from 0.9 m to 15.1m in apparent width, are up to 1.1 m in thickness, and have steep sides (up to 35 degree) that strike approximately perpendicular to the local paleoshoreline trend. Scour density is greater in thick (>0.5 m) amalgamated sandstone beds and increases upwards within vertical successions of upward-thickening amalgamated sandstone beds.

The relative influence of different heterogeneities on sandstone bed connectivity and sandstone-to-mudstone ratio is determined using reservoir models that explore geologic scenarios based on the outcrop analog, in combination with experimental design and analysis of variance. The interplay between the thickness of mudstone interbeds and sandstone-filled erosional scours controls the fraction of connected sandstone. For a given thickness and spacing of sandstone beds, scour density and scour geometry influence the sandstone-to-mudstone ratio. Pot casts of semi-ovoid geometry and small volume are associated with lower values of sandstone-to-mudstone ratio than gutter casts of channelized geometry and large volume. There are also distinct differences in the dimensions, lateral continuity, and spatial distribution of remnant mudstone “patches” that result from erosion by pot and gutter casts. This suite of results aids prediction of effective vertical permeability in thinly interbedded sandstone and mudstone intervals of wave-dominated deltaic reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018