8 Unit summary
The main points covered in this unit are summarised below.
- Hydrocarbons are compounds composed mainly of carbon and hydrogen. In addition to hydrocarbons, petroleum contains significant quantities of oxygen, nitrogen, sulphur, nickel and vanadium, plus minor quantities of a host of other elements. Petroleum may be liquid (crude oil), gaseous (natural gas) or solid (bitumen).
- The necessary ingredients for any accumulation of petroleum – petroleum charge – are source, maturation, migration pathway and trap. As knowledge about these factors develops during the exploration and development of a petroleum-rich basin they help define petroleum plays, which are realistic strategies for directing further development.
- An effective source rock contains sufficient organic matter, in the form of kerogen, to produce fluid hydrocarbons when heated (matured) to temperatures above the generation threshold of about 50°C.
- Fluid petroleum migrates for two reasons. During burial and compaction, fine-grained, clay-rich sediments lose pore space between their grains. Water and petroleum thus expelled from source rocks start primary migration. Petroleum moves buoyantly in response to the pressure gradient within permeable strata as secondary migration begins.
- For petroleum to become concentrated in an economic oil or gas field, suitable rocks must be charged with petroleum fluids. These reservoir rocks must not only be porous enough to contain substantial amounts of petroleum but must also be permeable enough so that fluids can flow in to accumulate, and out during extraction. Almost all reservoir rocks are sandstones or limestones.
- Permeable reservoirs must be capped by impermeable seals, usually mudstones, or evaporates in one of several kinds of trap.
- Structural traps are formed by the deformation of sedimentary rocks and include anticlinal domes and fault traps. Other examples of structural traps include those associated with salt masses that have moved upwards because of their low density, sometimes to ‘intrude’ other sediments. Stratigraphic traps occur where there is a barrier caused by lateral permeability variation in sedimentary rocks.
- The main method of determining whether an area has potential traps for petroleum is seismic exploration. Seismic sections provide images of the subsurface. Once detected, a potential trap can be mapped in detail using 3-D seismic data to define its shape and the thickness of petroleum-bearing parts of the reservoir. Porosity and permeability of the reservoir rock determined by direct measurements of exploration-well samples then allow the volume of oil and gas that can be recovered to be estimated.
- The most significant local environmental risks from petroleum production come from accidental spillages during transportation.
- Primary recovery methods produce at best only 30% of the oil present. Secondary recovery techniques that pump pressurised water and gas into the reservoir can boost the amount to ∼65%, and more still can be recovered by injecting steam or chemical and biological additives into the reservoir to reduce viscosity. The feasible recovery has an important bearing on the estimates of a field's reserves.
- In 2004, global petroleum reserves were 162 billion toe of oil (1189 billion barrels) and 180 trillion cubic metres of gas.
- Petroleum reserves in the British North Sea are minute (0.5%) compared with those of the Middle East, which controlled 62% of the world proved reserves of crude oil in 2004.
- Large amounts of oil and natural gas are locked into oil sands and gas hydrates, but, apart from the Canadian oil sands, they do not yet constitute a globally significant economic resource.