Courtesy of ukooa



Oil and gas in the northern North Sea

F27: Anatomy of the North sea

F28: Effects of salt movement

Concealed beneath the blanket-like sag of the North Sea basin is a complex of older basins and rift valleys (grabens) between elevated 'highs' and platforms. In the southern and central North Sea, the thickest sediments are in the Permian basins and in the deep Central Graben. The northern North Sea is dominated by the sediment-filled Viking Graben. Were the graben empty, Mount Everest would just about fit into it - upside-down. F27 shows the Permian basins and buried grabens as they would appear if all rocks younger than 285 million years were stripped away. The early rifting stage of basin formation lasted until about 125 million years ago, and was followed by the main sagging stage. The Earth's crust is thinned symmetrically across the entire width of the northern North Sea. Two further important influences on the structure in the central and southern North Sea are deformation produced by mobile salt masses (F28) and 'inversion' of basins accompanied by erosion (see F15).

Brent Field, discovered in the far north of the area in 1971, contains oil and gas within tilted layers of sandy rock. 170 million years ago, these layers were part of a river delta. Since then, the tilting movements, associated with the rifting Viking Graben have been followed by a long period of sagging. Muddy sediments - including Kimmeridge Clay, the source of the oil - have draped across the titled blocks (F33), filling the subsiding troughs between them, and sealing the eroded upper edges of the sandstone layers (F41) to form traps. Much later, oil was expelled downwards into the sandstones from the thick mudrock, now deeply buried within the troughs. The oil migrated up the tilted sandstone layers to collect in the crests. Some of the gas came from coal within the delta sediments. Oil is still migrating through the area. The sandstone layers, each more than 200 metres thick, have held over 500 billion litres of oil, for millions of years, within an area of 17 by 5 km.

F42: Piper field

Piper Field, discovered in 1973, lies at the edge of an arm of the buried rift valley. Oil in this field is trapped within a tilted sandstone layer cut by faults. The sandstone was deposited 145 million years ago, during late Jurassic times, as sand bars around a series of river deltas. Kimmeridge Clay source rock overlies the oil-filled sandstone, acting as part of the seal. However, the trapped oil is mostly derived from within the rift valley, on the south side of the field, where source rock is thicker and hotter. The oil has migrated to the field area at some time after a mudrock seal was laid down 70 million years after the sand. Oil is prevented from leaking out at the faults, or from the eroded edges of the sandstone, by this seal. About one cubic kilometre of the sand is filled with 150 billion litres of oil over an area of 30 square kilometres.

F43: South brae field

South Brae Field contains oil and gas in the sandy debris which accumulated at the foot of a steep submarine slope. This coarse sediment was deposited along the western edge of the buried rift valley, in the southern part of the Viking Graben. At this time, 140 million years ago, the organic mud of the Kimmeridge Clay source rock was being deposited across the area. During episodes of instability, fan deposits of rock fragments and sand spread out from the submerged, rifting edge of the graben, while organic mud deposition was confined to the floor of the undersea rift valley away from the steep margins. Thus the reservoir rocks are now found as sand sheets and wedges of conglomerate - pebbles and boulders in sand - interlayered with black, oily mudrock. Here, therefore, the reservoir rock is the same age as the source rock. South Brae Field was discovered in 1977, its oil being found deeper down than in most North Sea fields, within a maximum of thickness of over 500 metres of reservoir rock. The oil is hot, gassy and corrosive.