DURHAM, MARTIN J., LASMO; PETER C. BURGES, LASMO; and ROBERT BOTTINGA, REPSOL

Abstract: Geology of the North Falkland Graben and Implications for Future Hydrocarbon Exploration

The North Falkland Graben is a north to south trending basin located approximately 100 kilometres north of the Falkland Islands.The basin is some 250km long and 50km wide, and together with the Western Rift Complex and the Southern Half Graben comprise the North Falkland Basin (Fig. 1).

Seismic data indicates that the graben is strongly asymmetric, and is controlled by a major down-to-the-west fault on its eastern flank. The basin floor rises to the west to form a major intra-basinal high named the Orca Ridge, and then falls off to form the Western Terrace/Minke sub basin along the western margins of Tranche "C".

The basin fill of the North Falkland Graben comprises both pre-Atlantic Rift sediments of ?Middle Jurassic age, (Megasequence 1), and Atlantic syn-rift and post-rift sequences (Megasequence 2) (Fig. 2). By contrast, the sedimentary fill of the Western Rift Complex and the Southern Half Graben is thought to include only pre-Atlantic Rift sediments.

Rifting is believed to have commenced during the Late Jurassic and continued through until the Early Cretaceous. Prior to drilling, the basin model prognosed the basin fill to be characterised by a non-marine syn-rift sequence followed by a marine post-rift succession akin to that seen in the Outeniqua Basin Complex of offshore South Africa, and the Austral/Malvinas Basins of offshore Argentina. However, the results of the first four wells suggest that marine conditions were not established until the late post-rift phase of basin development. The North Falkland Graben is now thought to show significant stratigraphic similarity to the petroliferous San Jorge Basin of Argentina.

Rifting ceased by the Valanginian, and the early post-rift development of the basin took place in a lacustrine environment, the sediments being predominantly organic claystone thought to be equivalent to the D 129 Claystone of the San Jorge Basin. Post-well geochemistry indicates that this claystone has excellent oil source rock potential, with TOC values ranging between 4 and 8 percent with the organic matter consisting mostly of Type 1 amorphous kerogen. However it is immature at the 14/24-1 location.The lacustrine environment gave way to a fluvio-deltaic regime in the Barremian, which persisted until mid Cretaceous times. This sequence comprises of a series of stacked sandstones, siltstones and claystones. The reservoir potential of these sandstones, like those encountered in the syn-rift sequences is variable, with the quality locally affected by pore filling kaolinite. Marine conditions were finally established by Late Albian to Cenomanian times, and persisted until the present day.

Six wells have now been drilled in the basin.Amerada Hess operated the first well in the basin, 14/9-1, which was located on the Orca Ridge on Tranche A. The well encountered live oil shows in Cretaceous aged post-rift sequences, and reached a total depth in Palaeozoic basement. The second well (14/13-1), was drilled by LASMO on the Minke prospect in Tranche C. The well tested a major structural high up-dip and some 14 kilometres to the southwest of 14/9-1. The well was plugged and abandoned in ?Paleozoic metasediments without encountering hydrocarbon shows. Shell drilled the third offshore well, 14/5-1A located on Tranche B which tested the Sebald prospect, an inversion feature located in the more axial part of the North Falkland Graben.The well encountered oil and gas shows in syn and post rift sequences. IPC's well, 14/24-1, was located on the Braela prospect. The well attained total depth in syn-rift reworked volcaniclastics without recording hydrocarbon shows whilst drilling.

The four operators (Amerada Hess, IPC, LASMO and Shell) formed an alliance (FOSA) to share the cost of establishing and conducting exploration operations in this remote region of the world. Significant savings were realised. The operational success of the alliance can be used as a template to explore in other remote regions of the world.

Fig. 1. Map showing the North Falkland Basin and its components (North Falkland Graben, Western Rift Complex, and Southern Half Graben).

Fig. 2. Cross- section of North Falkland Graben showing (1) pre- Atlantic Rift sediments of ?Middle Jurassic age (Megasequence 1), and (2) Atlantic syn- rift and post- rift sequences (Megasequence 2).

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England