Geometry, distribution and fill of erosional scours in a heterolithic, distal lower shoreface sandstone reservoir analogue: Grassy Member, Blackhawk Formation, Book Cliffs, Utah,USA

Abstract

Many shoreface sandstone reservoirs host significant hydrocarbon volumes within distal intervals of interbedded sandstones and mudstones. Hydrocarbon production from these reservoir intervals depends on the abundance and proportion of sandstone beds that are connected by erosional scours, and on the lateral extent and continuity of interbedded mudstones. Cliff-face exposures of the Campanian 'G2' parasequence, Grassy Member, Blackhawk Formation in the Book Cliffs of east-central Utah, USA, allow detailed characterization of 128 erosional scours within such interbedded sandstones and mudstones in a volume of 148 m length, 94 m width and 15 m height. The erosional scours have depths of up to 1Á1 m, apparent widths of up to 15Á1 m and steep sides (up to 35°) that strike approximately perpendicular (N099 AE 36°) to the local north-south palaeoshoreline trend. The scours have limited lateral continuity along strike and down dip, and a relatively narrow range of apparent aspect ratio (apparent width/depth), implying that their three-dimensional geometry is similar to non-channelized pot casts. There is no systematic variation in scour dimensions, but 'scour density' is greater in amalgamated (conjoined) sandstone beds over 0Á5 m thick, and increases upward within vertical successions of upward-thickening conjoined sandstone beds. There is no apparent organization of the overall lateral distribution of scours, although localized clustering implies that some scours were re-occupied during multiple erosional events. Scour occurrence is also associated with locally increased amplitude and laminaset thickness of hummocky cross-stratification in sandstone beds. The geometry, distribution and infill character of the scours imply that they were formed by stormgenerated currents coincident with riverine sediment influx ('storm floods'). The erosional scours increase the vertical and lateral connectivity of conjoined sandstone beds in the upper part of upward-thickening sandstone bed successions, resulting in increased effective vertical and horizontal permeability of such intervals.

Characterization of effective permeability in heterolithic, distal lower-shoreface sandstone reservoirs: Rannoch Formation, Brent Group, UK North Sea

“…We used data from an outcrop analogue: the G2 parasequence of the Grassy Member, Blackhawk Formation (Late Cretaceous), exposed in the Book Cliffs of east-central Utah, USA (see Onyenanu et al, 2018 for details) and related reservoir modelling experiments (see Onyenanu et al, in press for details) to characterise the geometry and distribution of sandstone-filled erosional scours, and their facies architectural context. Sedimentological facies analysis of measured sections from outcrop and from the Rannoch Formation cores enables comparison between the two datasets.…”

“…The thickness of individual sandstone beds and sets of amalgamated sandstone beds ('conjoined sandstone beds' sensu Onyenanu et al, 2018) varies from 0.1 to 3.0 m, while mudstone-interbed thickness varies from 0.1 to 1.5 m thick. Despite such variation in sandstone-bed and mudstoneinterbed thickness, the interval can be subdivided into several smaller successions in which sandstone-bed thickness and amalgamation increase upwards (denoted by unfilled triangles in Fig.…”

“…Kjønsvik et al, 1994;Sech et al, 2009;Onyenanu et al, 2018;Onyenanu et al, in press). In locations offshore of deltaic distributary mouths, sandstone-filled erosional scours at the base of sandstone beds have the potential to erode through mudstone interbeds, thereby increasing the vertical connectivity of sandstone beds that may otherwise be isolated (Eide et al, 2015;Onyenanu et al, 2018;Onyenanu et al, in press). The bed-scale architecture and distribution of sandstone-filled erosional scours lies below seismic resolution and is sparsely sampled by wells, but relevant information can be provided by outcrop analogues of interwell volumes.…”

“…Previous work has shown that hydrocarbon recovery from heterolithic, distal lower shoreface and distal wave-dominated delta-front deposits is controlled by the proportion of sandstone, the connectivity of sandstone beds, and the lateral extent and continuity of mudstone interbeds (e.g. Kjønsvik et al, 1994;Sech et al, 2009;Onyenanu et al, 2018;Onyenanu et al, in press). In locations offshore of deltaic distributary mouths, sandstone-filled erosional scours at the base of sandstone beds have the potential to erode through mudstone interbeds, thereby increasing the vertical connectivity of sandstone beds that may otherwise be isolated (Eide et al, 2015;Onyenanu et al, 2018;Onyenanu et al, in press).…”

“…This paper has two aims: (1) to document the character and vertical distribution of sandstone beds in heterolithic, distal lower shoreface deposits of the Rannoch Formation, Brent Group using core from reservoirs near the northern limit of Brent Delta progradation, and (2) to assess the impact of sandstone-filled erosional scours on sandstone-bed connectivity and resulting estimates of effective kv/kh using data from an outcrop analogue and associated reservoir modelling experiments (Onyenanu et al, 2018;Onyenanu et al, in press).…”