There are two key steps in determining whether the geological structures of an area contain sufficient hydrocarbon deposits to be economically exploited. These steps are summarised below and further detail can be found in "The Story of Oil and Gas" at the Oil & Gas UK web site.
1. Seismic surveys
The first step is a seismic survey where sound waves are used to develop an image of subsurface strata and structures — features where hydrocarbons could accumulate and be retained (see figure below). The sound waves are typically generated by airguns with reflections from the underlying rocks recorded by hydrophones towed behind the survey vessel. These reflections are analysed by computer to allow 3D visualisation of the subsurface geology. The possible sources of environmental effects from seismic survey are illustrated in the figure below. Of these, the prime concern is generally regarded as the short and longer term effects of noise on whales, dolphins and other marine mammals.
2. Exploration drilling
The second step is to drill into the geological structure to ascertain what it contains. Drilling is conducted from a jack-up rig in shallow waters of less than about 100m, or from a semi-submersible rig or a drill ship in deeper waters. The legs of a jack up rig rest on the seabed while the semi-submersible rig/drill ship floats and is anchored in location. The well is drilled in a series of steps of decreasing size with increasing depth. As each step is drilled the hole is lined with steel pipe which is cemented in place to prevent collapse and the flow of liquids into or out of the well. As the drill bit penetrates the rock, a fluid termed "drilling mud" is pumped around it to lubricate, cool and carry away the rock cuttings. The drilling mud normally contains a variety of minerals and chemicals designed to give it required properties such as viscosity to aid the removal of rock chippings, and weight to balance the pressures being encountered in the well.
The possible sources of environmental effects from exploration drilling are illustrated in the figure below. Of these, the prime concerns are generally regarded as the effects of discharges of drilling mud and rock cuttings on seabed fauna and if the well contains hydrocarbons, a possible accidental oil spillage.
On completion of an exploration well, the well is either removed or left in a safe condition for potential future use. If hydrocarbons were encountered the next step would be to assess the size of the reserves and extent of the reservoir. This is carried out through the drilling of additional (appraisal) wells and sometimes more detailed seismic surveying.
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