P. C. Richards* and B. V. Hillier**
*British Geological Survey, West Mains Road, Edinburgh, EH9 3LA.
**Shell Production and Development Ltd (Falklands Branch), Shell Centre, London.
Correspondence to pcr@bgs.ac.uk
Six wells were drilled in the extensional North Falkland Basin in 1998. The wells encountered a Devonian to Cenozoic stratigraphy dominated by thick Mesozoic syn- and post-rift successions. Although most previously published models predicted that the succession would most likely be of marine origin, it is in fact predominantly terrestrial; marine conditions did not become established in the basin until the Late Cretaceous. The oldest rocks recorded are Devonian and these were penetrated in only one well. The overlying succession comprises: a fluvio-lacustrine, early syn-rift interval of ?mid-Jurassic to Tithonian age; a late syn-rift fluvio-lacustrine interval of Tithonian to Berriasian age; a rift-sag transitional unit of Berriasian to Valanginian age; an early post-rift lacustrine unit of Valanginian to early Aptian age; a middle post-rift, transgressive unit of Aptian to Albian age; a late post-rift, terrestrial to marine unit of Albian to early Palaeocene age; and a post-uplift thermal subsidence unit of Palaeocene to Recent age.
Much of the sediment appears to have been derived from volcanic and/or metamorphic terranes, probably located to the north or NW of the basin. As well as the volcanic material which occurs in the ground mass and as lithoclasts in many of the units, some volcaniclastic rocks and minor amounts of ashfall tuffs are observed, particularly within the late syn-rift succession.
P. C. Richards* and B. V. Hillier**
*British Geological Survey, West Mains Road, Edinburgh, EH9 3LA.
**Shell Production and Development Ltd (Falklands Branch), Shell Centre, London.
Correspondence to pcr@bgs.ac.uk
Six wells were drilled in the North Falkland Basin in 1998. Five of these wells recorded oil shows, and up to 32% gas was also recorded in mud returns to the rig floor. However, none of the wells encountered commercially viable petroleum accumulations.
The syn-rift and early post-rift intervals contain thick, lacustrine claystones with oil source potential as indicated by TOC values up to 7.5% and Rock-Eval S2 values of up to 102 kg HC per tonne of rock. These source rocks were immature or only marginally mature in five of the wells but had attained maturity in one of them. Modelling suggests that the main source interval may well be within the peak oil generation window in deeper, undrilled parts of the basin. Calculations of the amount of oil expelled range up to 60 billion barrels.
Most of the wells tested a closely-related set of plays in large structures associated with a sandstone interval near the top of the late syn-rift to early post-rift source-rock succession. Post-drilling geological modelling of the basin suggests that oil is unlikely to have migrated into this sandstone play at the localities tested, and that the wells consequently failed largely due to a lack of charge. However, the play maintains exploration potential elsewhere. Other plays, particularly those stratigraphically associated with the base rather than the top of the source rock, may have a higher chance of exploration success.