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Controls on the Evolution of Ancient, Deep Lacustrine, Sand-Rich, Fan Systems: Early Cretaceous Hydrocarbon Reservoirs of the North Falkland Basin, Falkland Islands


Ancient, deep-lacustrine fans are rarely described in academic literature. Here we document the various styles of, and controls on, sub-aqueous, sand-rich fan systems. These systems are intercalated with organic-rich source rocks in the lacustrine petroleum system of the early Cretaceous North Falkland Basin. The North Falkland Basin is an asymmetric failed-rift basin, with lacustrine conditions developing towards the end of the syn-rift phase, becoming fully established during the early post-rift. The youngest examples of syn-rift fan deposition were sourced by river systems draining extensive hangingwall platform areas comprising mixed sedimentary and volcanoclastic terranes. Although spectacularly developed as classic fan-shaped systems, as imaged on 3D reflection seismic data, they have so far been shown to contain poor reservoir quality, with siliciclastic deposition marred by a volcanogenic component. These fans were shut-off once the lacustrine system became fully established, leading to balanced or over-balanced lake fill conditions. The ensuing early post-rift phase was dominated by sediment input from the east, with a series of deep-water fan emplacement across the basin bounding faults, draining the sedimentary-dominated footwall regions. These fans tend to be turbiditic in origin and are typically narrower, more linear and partially confined by a combination of pre-existing basin topography. They entered the basin initially down major relay ramp systems, but subsequently down steep canyons associated with only small relay jogs along the basin margin. Concomitantly, a major delta system drained into the basin from the north with five, seismically-distinguishable, episodes of delta progradation clearly evident. The relative lowstand conditions of the lake, separating these five progradational packages, demonstrate a clear temporal relationship with easterly derived fan deposition. To understand the sediment input story in the early post-rift fill of the North Falkland Basin it is essential to examine all possible controls on sediment input, including: variations in sediment availability, lake base-level, climatic variability and tectonic re-organisation. This is vital as they form the primary hydrocarbon reservoirs in the North Falkland Basin. Furthering our understanding of these systems will aid in improving future exploration success in the North Falkland Basin and, crucially, within other frontier areas world-wide.