|BRITAIN'S OFFSHORE OIL & GAS
Forces That Shape the Earth's Crust
The white-hot, partially molten interior of the Earth is in constant motion. This transmits itself to the more rigid outer layer, the lithosphere, which is also constantly on the move (F12). New lithosphere is created along mid-ocean ridges where molten rock is injected, cooling to form new ocean crust, the top layer of this young lithosphere (F13). The lithosphere moves away from the ridges in the process of sea-floor spreading, and is destroyed wherever it slides back into the Earth, along subduction zones. Since it is thicker and lighter than the oceanic crust, continental crust is not subducted and so is mostly much older than oceanic crust. The great slabs of lithosphere between mid-ocean ridges and subduction zones are called plates.
The complex interactions of oceanic and continental lithosphere, powered by plate movements are called plate tectonics. In addition to the opening out of ocean basins, the main effects of plate tectonics are the growth and break-up of continents.
Continents grow by formation of new continental crust along volcanic belts and by the addition of terranes, which are pieces of continental material and ocean island arcs formed elsewhere and rafted into older continents by sea-floor spreading. These collisions telescope the continental crust and produce mountain ranges. Conversely, where the spreading process locates itself under a continent, the continent may eventually split apart. A new ocean will form between the rifted parts which may then travel long distances as parts of the moving plates. The rate of growth and horizontal movement of plates is anything from about 2cm to 10cm per year, about the same as one's fingernails. The drifting of rifted continents may carry them through several climatic zones, for example, from equatorial humid through tropical arid to temperate and arctic, over tens or hundreds of million years (F14). This is of great importance to the generation and trapping of oil and gas, as are the structural disruptions brought about by plate tectonics.
Large areas of the continental crust are covered by layers of sedimentary rock which are thickest in the middle of basins. Nearly all oil and gas is found in such basins, which are formed over many millions of years by stretching of the crust combined with sagging. The North Sea is a classic example. Most basins have a two-tier structure; the lower tier is faulted into blocks while the upper-tier is a simple sag (F16b). There are different theories to explain basin formation. The lithosphere may stretch uniformly like toffee, fracturing the upper brittle layers into tilted blocks, then sag as the underlying, partly-molten layer (asthenosphere) cools down. Alternatively the entire lithosphere may be detached along a huge low-angle fault (F16a) to which curved listric block-faults are linked. The reality may be a combination or stretching at depth with detachment high up in the lithosphere.
Compression of the upper continental crust by plate tectonic mechanisms results in buckling and telescoping of rock layers to form fold and thrust belts (F15). The telescoping is often related to a deep detachment, above which a stack of thrust sheets pile up. Large masses of lightweight granite give buoyancy to the crust. The highs that result may be marked by reduced deposition of sediments or actual emergence and erosion. Beyond the thrust belt, rock strata may undergo compression. This tends to expel the contents of basins upwards and outwards in a process termed inversion. The expulsion often takes place along the same listric faults that guided the basin's development. Basin inversion is a very important mechanism in gas and oil field formation. It may create good structural traps for oil and gas, and may prevent "over-cooking" of the source rock. However, it may also permit the escape of hydrocarbons or cause erosion of source rocks or reservoir rocks.