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

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Reservoir Architecture of Deep Marine Rift Climax Deposits: An Outcrop Study From East Greenland

Abstract

Numerous studies have pointed at the significance of relay ramps delivering sand to active rift basins during the climax stage. In marine rift systems, relay ramps can provide a conduit between a subaerially exposed highland in the footwall down into a deep-water hanging wall basin. Due to their isolated nature, deep subaqueous rift basins are prone to anoxia and deposition of organic-rich rocks that may generate oil and gas. A potential reservoir close to the hydrocarbon kitchen minimizes migration risk. Outcrop examples of deep marine rift deposits are rare, and interference between sediment influx through relay ramps and from fault scarps in a rift setting as commonly can be inferred from seismic studies is scarcely documented in outcrop. In the Wollaston Forland extensional basin (East Greenland), coarse clastic submarine gravity flows were delivered from the subaerially exposed footwall and deposited in a c. 1 km deep, 20 km wide and 100 km long depocentre in the hanging wall during the rift climax phase. Proximally, the stratigraphy is characterized by thick, uniform but disorganized successions of gravels and breccias containing huge rock-fall slabs. These deposits signify slope aprons and gravel-rich fans sourced from the immediate footwall. Locally, the apron and fan beds interfinger with more sand-prone facies that consist of fine- to medium-grained turbidites and matrix-rich event beds. These facies reflect deposition in a sand-rich submarine fan environment; the abundance of reworked mud and carbonaceous material suggests that it was sourced from a more mature delta system. The interfingering of coarse gravity flow deposits and the sandy fan deposits is only observed south-east of a prominent south-facing relay ramp. The invoked presence of a delta delivering sand to the basin at a relay ramp is consistent with conceptual models. The studied bedforms display a complex architecture in outcrop, comprising scours, heaps and lobe pinch-outs. Most of these features occur on a scale of meters to tens of meters, and are thus below seismic resolution. On seismic data from similar basins such wedges are typically characterized as poorly reflective and chaotic. Given the range of grain sizes that occur together, the complex stratal architecture has a profound influence on reservoir performance. This study provides an overview of the heterogeneity that can be expected in a siliciclastic reservoir deposited in a deep subaqueous setting during rift climax.