--> Reservoir Architecture of Deep Marine Deposits in a Rift Basin: An Outcrop Study From East Greenland

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Reservoir Architecture of Deep Marine Deposits in a Rift Basin: An Outcrop Study From East Greenland

Abstract

Footwall-sourced, subaqueous clastic fans deposited during rift climax form important targets for hydrocarbon exploration in rift basins. Current understanding of these types of reservoirs is mostly based on well- and seismic reflection data. We have studied extensive outcrops of marine syn-rift deposits of the Wollaston Forland Basin in East Greenland and present a dataset that consists of photographic coverage and sedimentological logs. The studied coarse-grained gravity flow deposits formed on the slope and at the base of slope of a fault-bounding clastic wedge. We observed a variety of complex geometrical relationships between different grainsize populations. These relationships are interpreted to represent possible scouring, mud entrainment, partial bypass and/or liquefaction. In a reservoir these features are expected to have a considerable effect on reservoir quality and connectivity. The scale of osberved architectural complexities, ranging from one meter to tens of meters, typically falls entirely below the resolution of seismic reflection data while their geometry and lateral continuity is also unlikely to be captured by well-data. Many of the observed fabrics, bed geometries and erosional features can be explained as a consequence of changes in support mechanism of gravity flows, changing from plastic behaviour in dense, laminar flows to Newtonian behaviour in diluted turbulent flows, and back. Also, turbulent flows may change flow regime between super- and sub-critical flows at a hydraulic jump. Such flow transformations are interpreted to be a function of initial sediment composition, travel distance, potential mud entrainment and not least the specific physiography of the basin into which the gravity flows were released which, being a half graben, was characterised by strong contrasts in slope angles. We intend to use our field observations to recreate the depositional environment and gravel-rich gravity flows of the Wollaston Forland Basin by using a numerical model. This way we can further constrain the implications of hydraulic aspects of footwall-sourced gravity flows on grainsize distribution, reservoir connectivity and run-out distance. Our work thus forms a contribution to the understanding of syn-rift reservoirs in marine or lacustrine rift basins, with direct implications for reservoir quality, size and connectivity.