The planet we live on: The beginnings of the Earth Sciences

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Figure 6.3: Different types of 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 ofcoarser porous sediment in finer impermeablesediment; E - an unconformity trap.Figure 6.4: A seismic cross section, showingsedimentary beds that dip to the left cutby an unconformity surface; if the dippingsedimentary rocks have alternating reservoirand cap rocks, and if the rock abovethe unconformity is a cap rock, oil/gas maybe trapped in the reservoir rocks.First, geophysicists will run a series of 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 they then run seismic surveys where shock waves are reflected fromthe rock layers below the surface, showing a cross section of the geology, as in Figure6.4. Modern geophysical surveys using the latest technology can reveal the shape andcharacter of the geology beneath the surface very clearly, but they cannot show if a trapcontains hydrocarbons. <strong>The</strong> only way to test whether a potential trap contains oil/gas isto drill a <strong>we</strong>ll (Figure 6.2).During the drilling of an exploration <strong>we</strong>ll, the job of the geologist is crucial, sincedrilling is vastly expensive and needs to be run as efficiently as possible. Most drilling isdone using drill bits that ‘chew up’ the rock into tiny chips, so that the geologist has tointerpret the geology from these chips. Ho<strong>we</strong>ver, if the geologist needs more informationabout the rock sequence, a hollow drill bit is used, producing cylindrical cores of rock(although this is even more expensive). <strong>The</strong> geologist will examine the microfossils fromthe chips and cores to find out where in the geological sequence the drill bit is drilling. Ifthe borehole has not yet reached, or is drilling in, the reservoir rock sequence, it shouldkeep drilling. If it is below the likely sequence, the borehole should be abandoned as the<strong>we</strong>ll is ‘dry’ and oil/gas has not been found. <strong>The</strong> rock chips are used to give other detailsof the rock as <strong>we</strong>ll, such as its permeability, whilst other sensors test for oil or gas.When the borehole has been completed, it is geophysically surveyed by lo<strong>we</strong>ring sensorsdown the hole in ‘downhole logging’. Even if the hole is a dry one, it may give detailshelpful to interpreting the geology in the next hole to be drilled. Since geologists decidewhere holes are to be drilled and when they should be abandoned, their decisions are vitalto successful hydrocarbon prospecting.139
When an oil/gas field has been found, a series of production <strong>we</strong>lls is drilled to extractthe hydrocarbons. <strong>The</strong> hydrocarbons are then pumped to oil terminals to be refined,before being sold to provide po<strong>we</strong>r or for use in the chemical industry.During the exploration and production processes, great care must be taken to preventleaks. While leaks of oil can devastate the environment, gas leaks can cause highlydangerous explosions. If the oil leaking from an oil <strong>we</strong>ll catches fire, it can cause enormousenvironmental problems and is very difficult, dangerous and expensive to put out. Thismeans that there has to be very careful monitoring during borehole drilling, which has tocontinue right through the production life of a <strong>we</strong>ll.6.3 Mineral prospecting and miningAll the mineral deposits that <strong>we</strong>re easy to find have already been found, so geologistshunting for minerals today have to use a wide range of prospecting techniques in theirsearch for anomalies. A mineral anomaly is something that is different from the backgrounddata, so if the background copper content of rocks and soils in a region is less thanten parts per million copper, and a soil anomaly of 35 parts per million copper is found,there may be a copper deposit nearby.Different techniques are used in searching for different minerals, but the 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 the origin of the anomaly andoutline its size and shape. <strong>The</strong>n a series of trenches or boreholes is used to find the sourceof the minerals producing the anomaly. At this stage, the prospecting geologist willprobably hand over to a mining geologist, who will then excavate pits for evaluating theore while carrying out a programme of borehole-drilling to outline the three-dimensionalsize, shape and richness of the ore body. If the deposit is economically viable, and socould give a profit to the mining company, a commercial mine will be excavated to exploitthe ore (an ore being an economic concentration of metal minerals).Geologists working for mining companies prospect either greenfield regions, where no oredeposits have previously been found, or brownfield areas, near known deposits, whereother similar deposits may be found. If you <strong>we</strong>re prospecting a greenfield area of countrywith unknown geology, you might first examine any remote sensing data for the region forunusual features, including satellite data using visible light or other parts of the spectrum,and aerial photographs. <strong>The</strong> next stage might be to fly a geophysical survey, duringwhich variations in gravity and geomagnetism are mapped. This would be follo<strong>we</strong>d upby work on the ground, where more detailed geophysical surveys may be carried out.Geomagnetic ground surveys would provide more detail on magnetic anomalies, whilstelectrical surveying might find highly conducting (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 of ‘pathfinder’ elements that are often associatedwith other 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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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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5.14 A GPS remote volcano monitorin
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Figure 1.2:minerals.A sandstone roc
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Even diamond can have different col
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Figure 1.8: A red garnet crystal in
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Figure 1.12: Crystals of minerals i
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Hematite, Fe 2 O 3 - earthy red, me
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Figure 1.17: Sedimentary rocks show
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Figure 1.20: A close up view of a p
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Figure 1.26: Ancient wave ripple ma
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Figure 1.30: Coal seams in an openc
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Figure 1.32: Close up view of a pie
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Figure 1.35: A fossil colonial cora
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Figure 1.38: Fossil ammonites, indi
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Figure 1.39: ‘Massive’ igneous
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Figure 1.41: A close up view of a p
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Figure 1.44: A basalt flow that coo
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Figure 1.47: Deposit of volcanic as
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Figure 1.49: An old slate quarry, s
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Figure 1.51: Slate, a low-grade met
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Figure 1.53: Schist, a medium-grade
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Figure 1.56: Metaquartzite, produce
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Figure 1.58: A region of folded roc
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Figure 1.61: A normal fault. This s
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Figure 1.64: An unconformity. Older
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proper geological maps. Fossils hav
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Cephalopods are not extinct, but th
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Figure 1.68: A shelled cephalopod f
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You can try the rock sequencing pri
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Chapter 2Reading landscapes: how la
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Figure 2.3: Rock fragments loosened
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Figure 2.6: The jo
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Figure 2.10: The m
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are forming them. Similarly the sha
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Figure 2.19: This photo was taken f
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Figure 2.25: Dinosaur tracks conser
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• On coastal sections, whenever p
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found evidence that the Earth was a
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Figure 3.3: James Hutton, the ‘Fo
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Figure 3.5: A page of Wegener’s 1
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Figure 3.7: The st
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Figure 3.10: The r
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Figure 3.13: The m
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Figure 3.15: The S
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Figure 3.19: An ocean versus contin
Page 103 and 104: Figure 3.23: Map of the major tecto
Page 105 and 106: Figure 3.26: Global temperature cha
Page 107 and 108: Figure 3.29: A computer generated p
Page 109 and 110: Figure 3.31: The
Page 111 and 112: planet extended th
Page 113 and 114: Figure 4.1: William Smith’s geolo
Page 115 and 116: Millionsof yearsago (Ma)01000Some M
Page 117 and 118: ocks whilst the first definite plan
Page 119 and 120: Figure 4.9: The ch
Page 121 and 122: Figure 4.12: The 4
Page 123 and 124: Millionsof years Some Major Earth E
Page 125 and 126: Figure 5.2: Strike-slip movement (r
Page 127 and 128: Figure 5.4: The So
Page 129 and 130: Figure 5.6: An ash eruption rising
Page 131 and 132: Figure 5.10: A hazard zone map of t
Page 133 and 134: isk to humans. The
Page 135 and 136: Figure 5.14: A GPS (global satellit
Page 137 and 138: None of these methods has yet prove
Page 139 and 140: Figure 5.17: A windfarm in Ireland.
Page 141 and 142: Figure 5.19: A beautifully preserve
Page 143 and 144: Another ‘missing link’ find has
Page 145 and 146: Figure 5.22: A dinosaur reconstruct
Page 147 and 148: Figure 5.25: A working aggregate-pr
Page 149 and 150: Figure 5.26: The E
Page 151 and 152: Chapter 6Understanding what geologi
Page 153: Figure 6.2: A drilling rig used for
Page 157 and 158: Figure 6.6: Groundwater flowing out
Page 159 and 160: Dam disaster in Italy, when the wav
Page 161 and 162: Figure 6.10: A slab foundation, bui
Page 163 and 164: An example of this is investigation
Page 165 and 166: GlossaryAbsolute age The</s
Page 167 and 168: Carbon capture The
Page 169 and 170: Crustal shortening This results of
Page 171 and 172: Evaporite deposits (or evaporites)
Page 173 and 174: Geophysical survey Using the method
Page 175 and 176: “Integrated waste management” <
Page 177 and 178: Metamorphism The r
Page 179 and 180: Pore spaces (or pores) Gaps bet<str
Page 181 and 182: Saltation Sediment movement by flui
Page 183 and 184: Suspension Sediment movement by flu
Page 185 and 186: AcknowledgementsPermission to repri
Page 187 and 188: Figure 2.3 A scree slope. Photo ID:
Page 189 and 190: Figure 1.15a Hematite.Figure 1.15b
Page 191 and 192: Figure 1.28 Dune cross bedding in s
Page 193 and 194: Figure 3.18 An island arc volcano,
Page 195 and 196: Figure 5.21 Excavations at the dino
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