Christian Haug Eide, John Howell, Simon Buckley

Facies 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.