Sand-Rich, Lacustrine Turbidite Fans of the North Falkland Basin, South Atlantic: An Emerging Hydrocarbon System

Abstract

Throughout the last decade, hydrocarbon exploration focus has shifted to lacustrine basins, with a number of major discoveries being made; the Sea Lion Fan in the North Falkland Basin (NFB) being a prime example. The NFB is a half-graben that formed as a result of east-west extension, associated with the opening of the South Atlantic in the early Cretaceous. The early post-rift basin fill records deposition within a large lake system, which occupied the centre of the asymmetric rift. The lacustrine sediments are~1.5 km thick and comprise: organic-rich, hemi-limnic mudstones; intercalated turbidite fan deposits and progradational fluvial-deltaic successions.

The earliest, pioneering fans were deposited during the late syn-rift and transitional tectono-stratigraphical packages. Some of these systems (Liz and Beth fans) entered the lake from the western side of the basin, and were fed by river systems draining extensive hangingwall platform areas, which were comprised of mixed sedimentary and volcanoclastic terranes. Consequently, they display less-favourable reservoir attributes, with the volcanoclastic component providing clay-material that reduced porosity. Sediment delivery to these early fans eventually shut-down, most-likely as the lacustrine system became fully established, creating balanced or over-balanced lake-fill conditions.

During the early post-rift, fans initially entered the basin down major relay ramp systems, but subsequently down steep canyons associated with small relay jogs in the basin bounding faults. These fans tend to be narrow, linear and confined by a combination of pre-existing basin topography and prior fan systems. The sediment source is considered to be fringing littoral deposits, which were shed into the lake during times of either footwall regeneration of lake level lowering, likely controlled by climatic variability. The fans that entered the basin along the eastern margin during this time have been the main focus of hydrocarbon exploration in the NFB. They act as excellent hydrocarbon reservoirs (Sea Lion, Zebedee, and Isobel Deep), which are aerially extensive and, in some places, approach 60 m in thickness. The intercalation of source rocks and reservoirs rock in the basin centre allows for a relatively simple source to reservoir migration story.

This study characterises the various types, styles and geometries of these sand-rich, lacustrine turbidite fans.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018