|BRITAIN'S OFFSHORE OIL & GAS
Oil and Gas from the Buried Rift Valley
The Britain's Offshore Oil and Gas in the northern and central areas of the North Sea is dominated by the geographical history of the buried rift valley, or graben. The rift is seen in the map on the right (F32), which shows the present-day shape of the surface of all rock that is more than 285 million years old. During this last 285 million years, since the start of Permian times, subsidence along the line of the rift valley created a changing pattern of land, lake and sea environments, and influenced the thickness and type of sediments that accumulated, the depth to which they are now buried, and the trap structures that formed. Consequently, hydrocarbon deposits have been trapped in a much greater variety of rocks and structures than in the southern North Sea. Much of the oil and gas is found in sandstones that are less than 200 million years old. The main sandstone and limestone reservoir rocks are shown, in green ornament in the column on the right in F32.
Rifting movements affected this area for more than a hundred years. They were most intense during the Jurassic Period, and Jurassic rocks provide the most important oil source and reservoirs beneath the North Sea. The main source of oil and gas in the area is the 140 million year-old Kimmeridge Clay. The most prolific oil-bearing reservoirs beneath the northern North Sea are the Jurassic 'Brent Delta' sands. Brent Delta sediments also contain coal seams derived from vegetation on the swamps.
These are the sources of some of the gas now trapped in the area. The sediments that built up the delta were transported northwards by rivers draining volcanic uplands which had risen up at the junction of three 'arms' of the rift valley (F36). As large as the Nile Delta, the Brent Delta is now buried and broken into a series of tilted blocks (F33) which act as traps where overlying rocks seal down oil and gas.
The Kimmeridge Clay is particularly rich in hydrocarbons along the line of the rift valley. This is because the slow subsidence of the rift helped to set up the right environment for a rapid build-up of thick mud layers, rich in planktonic algal remains, on the deepest parts of the seabed (F34). Climate and sea conditions were ideal for the massive growth of 'blooms' of plankton. Dead plankton sank in vast numbers, and the seabed bacteria feeding on their remains made the mud stagnant, so that particles from the plankton cells were preserved in it and slowly buried. The buried mud became compressed to form the Kimmeridge Clay. The thickest mud layers were deposited over the rift and have since subsided deep within the rift heating up slowly as they became more deeply buried. The Kimmeridge Clay has been mature for millions of years, generating first oil and then gas (see F4, page 6). F35 shows the areas where it is mature and generating oil and gas right now.
Most of the sandstone reservoirs in the northern North Sea were originally parts of river deltas and 'submarine fans' - lobes and sheets of sediment which were re-deposited from slumping and flowing masses of unstable sea-bed (F38). Many important oil and gas occurrences are in Jurassic rocks of this type but some of the largest are in submarine fan sandstones which were deposited more recently, 50 million years ago or so (F37).
Around 150 million years ago, in Jurassic and on into early Cretaceous times, parts of the sea floor repeatedly sank. The great rift valley system, or graben, was rapidly subsiding. Beneath the sea, the Earth's crust continued to fracture along huge faults, and large blocks dropped down and tilted to form long ridges along the sea floor.
These movements continually triggered the slumping of soft sediments into the deeper troughs. Unstable areas of seabed would start to shift until rock fragments and particles were carried away across the sea floor as fast-flowing currents of watery sediment. These settled out as fans or more widespread sheet-like deposits. Coarse, sandy rubble was dropped near steeply sloping sea floors. Channels and fans of sand and silt spread our further across the sea floor, building thick layers out from the submarine ridges. Some of the sandy rocks which were laid down in this way are permeable enough for oil and gas to flow through them with ease. These rocks now hold oil and gas pools in trap structures, such as those of the Brae, Galley, Claymore and Magnus fields. The traps formed well before the oil and gas migrated in from the Kimmeridge Clay above or around them.
The North Atlantic Ocean was opening rapidly around 50 million years ago and through this time of great crustal activity the area from the Scottish Highlands to the Shetland Islands was uplifted (F37) causing rivers to erode and move huge amounts of sediment. Unstable masses of sand and silt built out across the surrounding shelves, while reactivation of older faults continually triggered great flows of sediment from the edges of the submarine shelves, out across the deeper sea floor underlain by the rift valley (F37). Submarine channel deposits and fans, built up into widespread layers of sandy sediment. These have hardened to form beds of silt and sandstone with shale layers. Where they have become parts of suitable trap structures, as in the Forties, Montrose, Frigg and Cod fields, they may hold considerable quantities of oil and gas which have migrated upwards from the deeply-buried source rock. Under some parts of the North Sea area, the oil and gas have then migrated almost horizontally some tens of kilometres along sandy layers until they have either escaped or become trapped.
Natural storage of oil and gas beneath some parts of the North Sea depends upon the presence of thick layers of salt, especially those laid down in the tropical sea during the Permian Period, around 250 million years ago. In the arid climate, rapid evaporation of the continually inflowing seawater resulted in the build-up of more than 2000 metres of salt.
In the central North Sea area, trap structures have been created where low-density salt layers have risen through overlying rock (F39). Some of these structures have trapped oil and gas, particularly within the Central Graben, where the chalk in the Norwegian and Danish sectors has been fractured and domed by rising salt.
In southern areas of the North Sea, however, salt layers of the same age act as hydrocarbon seals. Here, the source rock and the main reservoir rock lie beneath the salt and are not affected by its movement. Fractures in the salt heal by salt-flow, so the rock makes an excellent seal.
Chalk acts as an oil seal in some areas and as a reservoir rock in others. Normally, its permeability is low - oil will not flow through it. Chalk mainly consists of tiny mineral crystals formed by algae, which drifted as plankton in the seas about 100 to 65 million years ago. The crystals, made of calcite, collected on the seabed as white, limy mud which hardened to form chalk rock. Where deeply buried, its minute pore spaces become naturally cemented and the rock hardens. Deep within the Central Graben, however, some chalk is much more permeable than normal and contains oil and gas. Besides fracturing, a crucial factor was that of sediment-slumping on the seabed. Movements across the Central Graben rift-edge caused the sediment to flow and re-deposit as a very porous, watery slurry. In places, the pores were filled with oil at high pressure before the crystals could become cemented into a tight mass.