NORTH SEA OIL

North Sea oil

North Sea oil

North Sea Oil Province

The North Sea Oil Province is one of the world's major oil-producing regions. The geological history of the oil province was dominated by an episode of late Jurassic to earliest Cretaceous crustal extension, which developed the Viking Graben, Moray Firth and Central Graben rift systems. Syn-rift, organic-rich marine mudstones (Kimmeridge Clay Formation) are the source rocks for virtually all of the region's hydrocarbons. Post-rift thermal subsidence enabled these source rocks to become mature for hydrocarbon generation along the rift axes from Paleogene times onwards. Hydrocarbon migration has been mainly vertical. Consequently, most of the producing oil and gas fields lie within the geographical boundary of the mature source rocks. (DECC 2008c)

Pre-rift

Pre-rift producing fields can be subdivided into three categories: Palaeozoic, Triassic to Lower Jurassic, and Middle Jurassic. Those fields having reservoirs of Palaeozoic (Devonian, Carboniferous or Permian) age (Figure) are concentrated on tilted footwall blocks, are adjacent to the major graben-bounding faults, or are on intrabasinal highs (e.g. Buchan, Auk and Argyll fields). Reservoir quality in these areas is difficult to predict, but may be enhanced by syn-rift fracturing, or by leaching beneath a base Cretaceous unconformity. (DECC 2008b)

Pre-rift producing fields with reservoirs of Triassic to Lower Jurassic age have traps either in tilted footwall blocks adjacent to the major graben, as subcrop closures beneath syn-rift or post-rift strata, (e.g. Marnock-Skua Field) or in stacked plays beneath producing Middle or Upper Jurassic sandstones (e.g. Statfjord Field). These reservoirs are typically thick, highly feldspathic, fluvial channel and sheetflood sandstones. (DECC 2008a)

The pre-rift, Middle Jurassic tilted fault-block play is one of the most productive in the North Sea (e.g. Brent and Ninian fields) (Figure 6). The Middle Jurassic reservoirs were deposited within a diachronous clastic wedge, mainly in coastal or delta-plain environments in the south, and as coastal barrier and wave-dominated deltaic facies in the north.

Pre-rift, Middle Devonian lacustrine sediments, a partial source for oil in the Beatrice Field in the Inner Moray Firth (Figure 6), are now thought to be widespread beneath the northern half of the oil province. This distribution provides a possibility that the geographic range of Palaeozoic hydrocarbon resources may extend far beyond the graben margins. (Bain 2003:5)

Syn-rift

The Upper Jurassic syn-rift play is currently, and will continue to be, one of the most active in the North Sea. The play is mainly confined ...