|BRITAIN'S OFFSHORE OIL & GAS
The North Sea - the Southern Gasfields
Almost all of the hydrocarbon fields in the southern part of the North Sea are gasfields. From 285 million years ago, this basin area was set amid the vast Pangaea continent. During its slow drift northwards across the equator (see F14), the environment in the basin gradually changed as the climate altered. Seas and lakes came and went, surrounding hills were worn down and the basin continued to deepen. During this continual change, a series of rock layers and structures have followed each other in a way which, by chance, allowed the creation and storage of natural gas beneath the area presently occupied by the southern North Sea (F31).
F30a shows the environment 300 million years ago when the area lay over the equator. Lush, swampy rain forest covered the flat lands across the area of Britain and the North Sea. These were the Carboniferous coal forests, the remains of which now provide Britain's coal and natural gas resources. Layers of vegetation were periodically submerged as the land sank and sea level fluctuated. As they became more deeply buried the plant layers were converted to coal seams in beds of shale and sandstone; these rocks are the Coal Measures, which underlie the area shown in F29a. Continued sinking, particularly in the area of the North Sea, has caused the generation of large quantities of gas; parts of the Coal Measures were at the right depth for gas generation to take place over 140 million years ago. The process is continuing still in some places. Because gas permeates upwards, it would all be lost at the land surface or sea-bed but for the existence, in certain areas, of overlying rocks which contain and seal in gas, efficiently trapping it. These rocks are the products of varying desert environments which affected the mid-continental regions of Pangaea during the Permian and Triassic Periods. The trap structures formed later, many around 70 million years ago.
By 270 million years ago, a desert lake in the south was bordered by massive sand dunes cut by wadies (F30b). These sands built up to a 300 metre-thick sandstone formation while the area subsided. Some of the dune sands form the most permeable parts of the Rotliegend Sandstone Group (F29b) and hold much of our natural gas. 250 million years ago, salt layers were deposited in the inland Zechstein Sea over the dune sands (F29c and 30c). These layers are now thick beds of salt which, in places, act as a gas seal.
F29 shows how part of the southern area contains Permian Zechstein salt above Rotliegend Sandstones which, in turn, lies over Coal Measures. Wherever these three layers lie one above the other there is a chance that gas may be held, so long as the rocks are in the form of a trap.