North Falkland Basin

In 1998, a significant lower Cretaceous lacustrine source rock was discovered, with a Tithonian-Aptian lacustrine section over 1000m thick. It forms part of a source-seal couplet, and a deeper petroleum system with a mid Jurassic to Tithonian source rock has also been identified. The mature layer is below 2700m below sea level, while the immature layer above serves as a regional seal.

The North Falkland Basin has a geothermal gradient of 44°C/km. The timing of oil generation is uncertain but is estimated to have occurred between 70 and 100 million years ago. A model based on a peak heat flow of around 80 mW/m2 at 90 MA closely matches the observed VR, temperature, and geochemical data observed in the released 1998 wells. At a depth of around 3000m below sea level, over 50% of the organic material will have been converted to oil.

The exploration wells drilled in the basin found reservoir rocks of varying quality, and oilfields on the eastern flank of the basin have high-quality reservoir rocks.

During the early and late syn-rift period (mid-Jurassic to Tithonian/Berriasian), the North Falkland Basin received fluvial and fluvio-lacustrine deposition with some tuffaceous input. The basin was isolated from the South Atlantic Ocean during the Jurassic. The early syn-rift was fluvially dominated in the basin's northernmost part, while the late syn-rift was lacustrine dominated with minor fluvial input. Lateral fans shed clastic detritus of reservoir quality into the basin from the surrounding Devonian platform during both episodes of the syn-rift phase. The thickest late syn-rift reservoir interval to date was found in well 14/5-1A, with porosities up to 30%. In 2010, the Liz field on the western flank of the basin was discovered to have gas condensate and dry gas at two levels within the syn-rift succession. Wet gas/condensate was found in sandstones up to 70m thick, with porosities of about 12%, while dry gas was in early syn-rift volcaniclastic sandstones up to 112m thick, with porosities of about 22%.

During the early post-rift period, deltaic sediments prograded into the lake basin from the north, while fans prograded into the basin from the eastern and western margins of the basin. A significant basinwards shift of facies occurred due to a lowering of lake levels, resulting in an attached lowstand fan and several sand-rich fans that form the Sea Lion and its associated satellite fields. Oil and gas discoveries made in the basin margin-derived fan sands during the 2010-12 drilling campaign are discussed. Many early post-rift sandstone targets developed encased within the source-seal couplet of the thick lacustrine claystone are still to be tested in the basin.

During the middle post-rift period, deposition was fluvially dominated. Cores from the Hess 14/9-1 well suggest an initial marginal lake setting which became more dominantly fluvial over time. The reservoir rocks lie above the main source interval and were the main exploration target in six wells drilled in 1998. They were present in all wells and reached over 133 m thickness in Well 14/24-1. In the 2010-12 drilling campaign, the sandstones were targeted in just two wells but were dry.

Oil and gas discoveries have been made on both sides of the basin with oil and gas shows recorded from reservoirs drilled in the center. The Sea Lion field on the eastern side of the basin has an API of about 28°, low gas-oil ratios, and a wax content of about 18%. Two wells were tested in 2010, one flowing at a rate of 1,800 barrels of oil per day and the other at 5,500 barrels per day with the aid of vacuum-insulated tubing and electro-submersible pumps. The Sea Lion field has different fluid characteristics on either side of the structural, water-bearing low that divides the field into eastern and western sections.

Two petroleum systems have been identified in the North Falkland Basin: one for oil and one for wet and dry gas. The oil is derived from lacustrine source rocks, while the gas is from a deeper fluvio-lacustrine source rock. Further analysis is needed to understand the deeper, gas-prone source rock. A deep source has also generated oil slicks observed north of the wells.

The Zebedee discovery in 2015 revealed a new fan system located south of Sea Lion and its satellites. Wireline logging indicated 25.3 meters of net oil pay in Zebedee, with 18.5 meters of net gas pay in Hector and an additional 2.6 meters of net oil pay in the F2 oil-bearing sand. Pressure data suggests that Hector may be oil-bearing in a downdip location due to an offset gas gradient.