Sedimentology and reservoir properties of tabular and erosive offshore transition deposits in wave-dominated, shallow-marine strata: Book Cliffs, USA

Abstract

This is a postprint, published version here: http://dx.doi.org/10.1144/petgeo2014-015 AbstractFacies models for wave-dominated shorelines include an "offshore transition zone" between shelfal mudstones and nearshore shoreface sandstones. Offshore transition zone deposits are commonly tabular sandstone beds interbedded with continuous mudstone beds. However, observations from the Blackhawk Formation show that the offshore transition zone locally consists of erosive-based sandstone beds with "pinch-and-swell" geometries containing steep-walled gutter-casts, in areas larger than 6 x 2 km along strike and dip. This increases the amount of sand-on-sand-contacts, and leads to improved vertical permeability. Predicting the distribution of erosive offshore transition within the subsurface is therefore desirable.In this study, offshore transition zone deposits have been studied using virtual outcrops. Tabular offshore transition zone deposits have continuous sandstone and mudstone beds much longer than 500 m, and erosive offshore transition zone deposits have discontinuous shales on average 60 m long. Reservoir modelling shows a 10-3 times increase in vertical permeability in erosive compared to tabular offshore transition deposits, the magnitude decreasing with increasing fraction of shale.Erosive offshore transition deposits occur near distributary channels, subaqueous channels and abrupt bathymetric breaks. A regional study shows that erosive offshore transition zone deposits are mainly developed where parasequences prograde into deeper water offshore the platform break of the preceding parasequence, are commonly associated with basinal turbidites, and may be related to erosion by bypassing turbidity currents.2

Effects of erosional scours on reservoir properties of heterolithic, distal lower-shoreface sandstones

“…Distal intervals of interbedded sandstones and mudstones in wave-dominated shoreface and deltaic environments that are associated with erosional scours are interpreted to occur in regions directly offshore of river mouths (Eide et al, 2015;Collins et al, 2017;Onyenanu et al, 2018). In these settings, offshore-directed downwelling storm flows coincided with peak riverine sediment delivery due to storm-driven precipitation ("storm floods" sensu Collins et al, 2017), resulting in deep, localised erosional scour at the base of storm-event sandstone beds.…”

“…Outcrop analogues indicate that heterolithic distal lower shoreface and wave-dominated delta front deposits can contain sandstone-filled erosional scours with the potential to erode through mudstone interbeds and thereby enhance vertical connectivity between sandstone beds and reduce mudstone continuity (e.g. Eide et al 2015;Collins et al 2017;Onyenanu et al 2018). Such scours are commonly inferred to have a near-linear channelised geometry (gutter cast) or a semi-ovoid geometry (pot cast) in three dimensions (e.g.…”

“…Whitaker 1973;Myrow 1992). Scours also vary in their distribution, being more densely spaced in locations offshore of coeval river mouths, where a combination of stormgenerated currents and riverine sediment influx increased erosional potential during storm events (Eide et al 2015;Collins et al 2017;Onyenanu et al 2018).…”

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

“…Intervals of interbedded sandstones and mudstones associated with erosional scours are common in distal lower shoreface and distal wave-dominated delta front environments that lie directly offshore of river mouths (e.g. Eide et al, 2015;Collins et al, 2017;Onyenanu et al, 2018). In these regions, erosional scours are interpreted to have formed by storm-generated flows at or near their peak velocity, during periods of net offshore sediment bypass and transport that coincided with peak riverine sediment supply ('storm flood' of Collins et al, 2017).…”

“…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.…”