Figure 6.3: Different types <strong>of</strong> oil and gastraps: A - an upfold (anticlinal) trap; B- fault traps; C - traps caused by a risingsalt dome; D - a trap caused by a body <strong>of</strong>coarser porous sediment in finer impermeablesediment; E - an unc<strong>on</strong>formity trap.Figure 6.4: A seismic cross secti<strong>on</strong>, showingsedimentary beds that dip to <strong>the</strong> left cutby an unc<strong>on</strong>formity surface; if <strong>the</strong> dippingsedimentary rocks have alternating reservoirand cap rocks, and if <strong>the</strong> rock above<strong>the</strong> unc<strong>on</strong>formity is a cap rock, oil/gas maybe trapped in <strong>the</strong> reservoir rocks.First, geophysicists will run a series <strong>of</strong> geophysical surveys, such as gravity and magneticsurveys which might suggest areas where a trap may have been formed, as in Figure6.3. In likely areas <strong>the</strong>y <strong>the</strong>n run seismic surveys where shock waves are reflected from<strong>the</strong> rock layers below <strong>the</strong> surface, showing a cross secti<strong>on</strong> <strong>of</strong> <strong>the</strong> geology, as in Figure6.4. Modern geophysical surveys using <strong>the</strong> latest technology can reveal <strong>the</strong> shape andcharacter <strong>of</strong> <strong>the</strong> geology beneath <strong>the</strong> surface very clearly, but <strong>the</strong>y cannot show if a trapc<strong>on</strong>tains hydrocarb<strong>on</strong>s. <str<strong>on</strong>g>The</str<strong>on</strong>g> <strong>on</strong>ly way to test whe<strong>the</strong>r a potential trap c<strong>on</strong>tains oil/gas isto drill a <str<strong>on</strong>g>we</str<strong>on</strong>g>ll (Figure 6.2).During <strong>the</strong> drilling <strong>of</strong> an explorati<strong>on</strong> <str<strong>on</strong>g>we</str<strong>on</strong>g>ll, <strong>the</strong> job <strong>of</strong> <strong>the</strong> geologist is crucial, sincedrilling is vastly expensive and needs to be run as efficiently as possible. Most drilling isd<strong>on</strong>e using drill bits that ‘chew up’ <strong>the</strong> rock into tiny chips, so that <strong>the</strong> geologist has tointerpret <strong>the</strong> geology from <strong>the</strong>se chips. Ho<str<strong>on</strong>g>we</str<strong>on</strong>g>ver, if <strong>the</strong> geologist needs more informati<strong>on</strong>about <strong>the</strong> rock sequence, a hollow drill bit is used, producing cylindrical cores <strong>of</strong> rock(although this is even more expensive). <str<strong>on</strong>g>The</str<strong>on</strong>g> geologist will examine <strong>the</strong> micr<strong>of</strong>ossils from<strong>the</strong> chips and cores to find out where in <strong>the</strong> geological sequence <strong>the</strong> drill bit is drilling. If<strong>the</strong> borehole has not yet reached, or is drilling in, <strong>the</strong> reservoir rock sequence, it shouldkeep drilling. If it is below <strong>the</strong> likely sequence, <strong>the</strong> borehole should be aband<strong>on</strong>ed as <strong>the</strong><str<strong>on</strong>g>we</str<strong>on</strong>g>ll is ‘dry’ and oil/gas has not been found. <str<strong>on</strong>g>The</str<strong>on</strong>g> rock chips are used to give o<strong>the</strong>r details<strong>of</strong> <strong>the</strong> rock as <str<strong>on</strong>g>we</str<strong>on</strong>g>ll, such as its permeability, whilst o<strong>the</strong>r sensors test for oil or gas.When <strong>the</strong> borehole has been completed, it is geophysically surveyed by lo<str<strong>on</strong>g>we</str<strong>on</strong>g>ring sensorsdown <strong>the</strong> hole in ‘downhole logging’. Even if <strong>the</strong> hole is a dry <strong>on</strong>e, it may give detailshelpful to interpreting <strong>the</strong> geology in <strong>the</strong> next hole to be drilled. Since geologists decidewhere holes are to be drilled and when <strong>the</strong>y should be aband<strong>on</strong>ed, <strong>the</strong>ir decisi<strong>on</strong>s are vitalto successful hydrocarb<strong>on</strong> prospecting.139
When an oil/gas field has been found, a series <strong>of</strong> producti<strong>on</strong> <str<strong>on</strong>g>we</str<strong>on</strong>g>lls is drilled to extract<strong>the</strong> hydrocarb<strong>on</strong>s. <str<strong>on</strong>g>The</str<strong>on</strong>g> hydrocarb<strong>on</strong>s are <strong>the</strong>n pumped to oil terminals to be refined,before being sold to provide po<str<strong>on</strong>g>we</str<strong>on</strong>g>r or for use in <strong>the</strong> chemical industry.During <strong>the</strong> explorati<strong>on</strong> and producti<strong>on</strong> processes, great care must be taken to preventleaks. While leaks <strong>of</strong> oil can devastate <strong>the</strong> envir<strong>on</strong>ment, gas leaks can cause highlydangerous explosi<strong>on</strong>s. If <strong>the</strong> oil leaking from an oil <str<strong>on</strong>g>we</str<strong>on</strong>g>ll catches fire, it can cause enormousenvir<strong>on</strong>mental problems and is very difficult, dangerous and expensive to put out. Thismeans that <strong>the</strong>re has to be very careful m<strong>on</strong>itoring during borehole drilling, which has toc<strong>on</strong>tinue right through <strong>the</strong> producti<strong>on</strong> life <strong>of</strong> a <str<strong>on</strong>g>we</str<strong>on</strong>g>ll.6.3 Mineral prospecting and miningAll <strong>the</strong> mineral deposits that <str<strong>on</strong>g>we</str<strong>on</strong>g>re easy to find have already been found, so geologistshunting for minerals today have to use a wide range <strong>of</strong> prospecting techniques in <strong>the</strong>irsearch for anomalies. A mineral anomaly is something that is different from <strong>the</strong> backgrounddata, so if <strong>the</strong> background copper c<strong>on</strong>tent <strong>of</strong> rocks and soils in a regi<strong>on</strong> is less thanten parts per milli<strong>on</strong> copper, and a soil anomaly <strong>of</strong> 35 parts per milli<strong>on</strong> copper is found,<strong>the</strong>re may be a copper deposit nearby.Different techniques are used in searching for different minerals, but <strong>the</strong> principles aresimilar. First a survey is carried out across a large area. When an anomaly is found,much more detailed surveys are carried out to pinpoint <strong>the</strong> origin <strong>of</strong> <strong>the</strong> anomaly andoutline its size and shape. <str<strong>on</strong>g>The</str<strong>on</strong>g>n a series <strong>of</strong> trenches or boreholes is used to find <strong>the</strong> source<strong>of</strong> <strong>the</strong> minerals producing <strong>the</strong> anomaly. At this stage, <strong>the</strong> prospecting geologist willprobably hand over to a mining geologist, who will <strong>the</strong>n excavate pits for evaluating <strong>the</strong>ore while carrying out a programme <strong>of</strong> borehole-drilling to outline <strong>the</strong> three-dimensi<strong>on</strong>alsize, shape and richness <strong>of</strong> <strong>the</strong> ore body. If <strong>the</strong> deposit is ec<strong>on</strong>omically viable, and socould give a pr<strong>of</strong>it to <strong>the</strong> mining company, a commercial mine will be excavated to exploit<strong>the</strong> ore (an ore being an ec<strong>on</strong>omic c<strong>on</strong>centrati<strong>on</strong> <strong>of</strong> metal minerals).Geologists working for mining companies prospect ei<strong>the</strong>r greenfield regi<strong>on</strong>s, where no oredeposits have previously been found, or brownfield areas, near known deposits, whereo<strong>the</strong>r similar deposits may be found. If you <str<strong>on</strong>g>we</str<strong>on</strong>g>re prospecting a greenfield area <strong>of</strong> countrywith unknown geology, you might first examine any remote sensing data for <strong>the</strong> regi<strong>on</strong> forunusual features, including satellite data using visible light or o<strong>the</strong>r parts <strong>of</strong> <strong>the</strong> spectrum,and aerial photographs. <str<strong>on</strong>g>The</str<strong>on</strong>g> next stage might be to fly a geophysical survey, duringwhich variati<strong>on</strong>s in gravity and geomagnetism are mapped. This would be follo<str<strong>on</strong>g>we</str<strong>on</strong>g>d upby work <strong>on</strong> <strong>the</strong> ground, where more detailed geophysical surveys may be carried out.Geomagnetic ground surveys would provide more detail <strong>on</strong> magnetic anomalies, whilstelectrical surveying might find highly c<strong>on</strong>ducting (low resistivity) ore deposits. Geigercounter surveys would be used to find radioactive uranium-bearing deposits.Geochemical surveying would be carried out to find, for example, copper, lead, zinc anduranium anomalies but also anomalies <strong>of</strong> ‘pathfinder’ elements that are <strong>of</strong>ten associatedwith o<strong>the</strong>r ore deposits, such as molybdenum for copper, mercury for lead and zinc,140
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Basic Books in ScienceBook 6<strong
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BASIC BOOKS IN SCIENCE- a Series of
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Looking ahead - If you came across
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3.2 Plate tectonics (20th Century)
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1.30 Coal seams in an opencast coal
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3.4 Alfred Wegener, the polar explo
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Hematite, Fe 2 O 3 - earthy red, me
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proper geological maps. Fossils hav
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Chapter 2Reading landscapes: how la
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