Off Shore Hydrocarbon

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What is hydrocarbon?
A compound of hydrogen and carbon, such as any of those that are the chief components of petroleum and natural gas OR
Organic compound (such as benzene, methane, paraffin) made of two elements carbon and hydrogen and found in coal, crude oil, natural gas, and plant life. Hydrocarbons are used as fuels solvents, and as raw materials for numerous products such as dyes, pesticides, and plastics; petroleum is a mixture of several hydrocarbons. Offshore hydrocarbons

Natural oil and gas found in rocks beneath the seabed give us the fuel we need for cooking and heating in our homes, for power stations, motor vehicles and aeroplanes. Oil is also used to make all sorts of plastic products from bottles to mobile telephones, and for chemicals used in factories and farming. In short, our modern-day society relies heavily on a steady supply of oil and gas, generally known as hydrocarbons or fossil fuels (together with coal).

Oil and gas fields on the UK continental shelf. From Wagner, K. (Dec 2010), AAPG European Region Newsletter, Vol. 5, p4.

Oil and gas formation
Hydrocarbons are found in sedimentary rocks beneath the seabed, usually along the edges of continents where sediments (clay, silt, sand and gravel particles) transported by rivers were laid down in thick sequences in the geological past. These sediments also trap dead organic matter from plants and animals. Over millions of years, the sediments are progressively buried and turn into rocks (sandstones and shales). The organic matter is turned into oil and methane gas (CH4),  through the action of elevated temperatures and pressures. The oil and gas usually form in organic-rich shales. They then migrate through fractures and pool in highly porous and permeable rock formations, such as sandstones and limestones, creating a hydrocarbon reservoir. Reservoir characterization

Seismic reflection amplitude (grey scale) image showing the Luva gas field offshore Norway. The reservoir interval is indicated by the white (upper boundary) and green (lower boundary) lines. Image courtesy of OHM Ltd. Reservoir characterization is the culmination of advanced geophysical techniques, including seismic, electromagnetic and borehole measurements, and rock physics knowledge. The aim is to produce a model of the reservoir in terms of its size, oil and gas content, and flow properties. This information is used by reservoir engineers to predict how much oil and gas the reservoir will yield over the life of the oil field. How we obtain this information is described in more detail below. Off shore hydrocarbon exploration

Seismic exploration

Cut away view of a 3D seismic cube beneath the seabed showing geological reflectors. The data were obtained using the NOC P-cable system. Image courtesy of Christian Berndt. Hydrocarbon exploration requires detailed imaging of the rocks beneath the seabed where reservoirs can be found, usually at depths between 1 and 4 km. The only way to image the rocks is to use geophysical techniques such as controlled-source reflection seismology. Seismic waves transmitted from an array of airguns towed by a ship are reflected off geological interfaces beneath the seabed. The reflected waves are detected by long (up to 6 km) hydrophone streamers towed behind the ship. The results are presented as seismic cross-sections through the Earth. These cross-sections can reveal geological structures where oil may be trapped. They can also reveal the actual presence of hydrocarbons, through, for example, flat spots (seismic reflections from the interface between oil and gas, or water and oil), bright spots (reflections from gas-charged rocks which have a larger contrast than those from liquid-saturated rocks) and AVO (amplitude variation with offset) anomalies.

Seismic reflection survey geometry for imaging sub-seabed geology Electromagnetic exploration
Another geophysical technique increasing used for...
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