Seismic Reflection Surveys (geophysical exploratio
Can be land based or ship based
- Artificial seismic waves are generate by explosions or vibrations on land and by air guns in water
- The seisic waves travel into the Earth and are reflected at layer boundariers within sedimentary sequences
- The reflected waves travel back up to the surface where they are detected by an array or recievers (geophones on land and hydrophones at sea). Their location is accurately pinpointed using GPS. Offsure seismic surveys are particulary efficent as as a large number of hydrophones can be towed behind a ship (a ships path is also not restricted like vibrotrucks are on land)
- The time taken for the reflected waves to arrive back at the recievers is called the travel time. This can be used to calcute the depth at the reflective layer. The data is used to plot a seismic profile whihc show the subsurface layering and can be interpreted to identify potential traps
Gravity Surveys (geophysical exploration technique
Can be land or airborne - are useful for exploration for oil, natural gas and metals
- A gravimeter measures small variations in the Earth's gravitational field strength. The units of measurment are milligals (mGal)
- Gravimeters can be mounted in road vehicles, helicopters or planes, allowing rapid coverage of large areas. Survey points are located using GPS
- The gravity data is corrected for the affects of latidude, altitude and topography leaving the variations resulting from the underlying rock types
- Maps are plotted with lines joining points of equal gravitational field strenght and anomalies can then be identifed. A gravity anomaly is a departure from the normal value
- A positive gravity anomaly result from excess mass - this may be due to the presence of a n anticline or an uplifted block bounded by faults, which could be potential trap structures
- A negative gravity anomaly results from a defecit of mass. This may be due to the presence of a low-density salt dome. In this case the exploration target would be around the edge of the saltdome
- When potential oil and natural gas traps have been located, exploration drilling is then carried out.
- Holes are drilled with cylindrical drill bits studded with diamonds.
- The rotating drill bit is cooled and lubricated by drilling mud containing the mineral barite to make it dense enough to reach the bottom of the hole.
- Millimetre sized rock chips or a continous core can be recovered from teh hole. Recovering continous rock core is very expensive so this is only done at critical depths.
Mud Logging (Exploration drilling)
- Rock Chips are brought up to the surface in the drilling mud
- These are sieved from mud, washed and examined under a microscope by geologists called mud loggers
- Mud loggers identify the rock types and microfossils present at different depths down the hole
- This allows them to build up a picture of the changing rock types down the hole and to correlate the geology between the boreholes
Down-hole Logging (Exploration logging)
It is possible to mount geophysical instruments on a sonde. The sonde is passed down the drill hole on a cable called wireline. The sonde then records data as it is slowly pulled up from the bottom of the hole.
- Porosity (the higher the porosity the higher the possible oil and natural gas content of a reservoir rock)
- Gamma ray spectroscopy - this counts the gama rays emitted from rocks as a result of natural radioacttive decay (potential source rocks such as black oil shales and musdtones give a high gamma ray count)
- Resistivity - this measures the resistance of the rock to the flow of electricity. Water is the main conductor present in rocks and gives a low resistance (the presence of hydrocarbons results in a very high resistance)