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.
Measurements include:
- 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)
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