Tidal Signatures in the Lower Brent Group of the Northern North Sea

Abstract

Conventionally, the lower Middle Jurassic of the Brent Group in the northern North Sea, accumulating Rannoch, Etive and parts of Ness formations, has always been classed as a northward prograding delta system. Recently, it is shown to include thick (15–40 m) sandy deposits with tidal signatures, are also high quality reservoirs. Tidal signatures in these three formations are various but may be of similar origin and controlled by syn-depositional system. The Rannoch formation has generally been assumed as wave-dominated delta front that expressed by repeated HCS and SCS storm events. However, in this case, 20% of the total thickness of the cored part, consists of tidally influenced deposits as capping to storm-event beds. Evidence of tidal currents are manifested as tidal bundle, tidal rhythmites, double mud drapes and reactivation surface. The whole packages are dominated by alternation of storm-event beds and fair-weather tidal intervals. The overlain Etive formation is genetically-related with the Rannoch Formation, but there is a marked erosional surface suggesting the onset of transgression and causing development of small scale estuary in inter-deltaic lobe. Tidal signatures within Etive formation are manifested as fluid mud in distributary channels. The basal Ness formation is composed of nearly 40 m thick, marine sandstone units, showing compound cross-bedding, and Ophimorpha traces, they are likely to be stacked tidal bars composed of single or compound dunes. The overall upward-finning succession, suggests that the bars fill channels in an open-estuary setting that was transgressive. The clean nature, good sorting, and coarse grain size suggests that these estuarine tidal bars received sediment supply at the estuary mouth by strong tidal currents. Tidal signatures repeated in the lower Brent Group is regionally occurred and may be related to tectonic rifting, where tidal influence is amplified by reshaping of shoreline geometry causing by variations in the subsidence/uplift rate.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015