Week 3
- Oct 20, 2015
- 2 min read
In petroleum geology, source rock refers to rocks from which hydrocarbons have been generated. Once it has been generated, it moves to other rocks where it gets trapped and then gets accumulated.
In geophysical techniques, we try to measure physical properties of the subsurface using different kinds of surveys. Once some of these properties are known it is easy to predict whether a particular area may contain hydrocarbons or not.
The properties measured are:
Seismic velocity
Density
Magnetic susceptibility
Electric conductivity
For each of these properties, a particular technique is used.
For seismic velocity, seismic surveys are used. They make use of the earth’s gravitational field to find anomalies. Many times, these anomalies are due to the presence of hydrocarbons.
The devices used are gravimeters. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and other geophysical research, and for metrology.
As for density, it is detected by means of magnetic surveys. They employ the use of magnetometers, which can be borne on aircraft to cover a large horizontal area.
A low density being detected means that there is a space lacking in mass underground, which means anticline hydrocarbon content, salt domes or sedimentary rocks.
High density on the other hand points to mass excess, as it means that there is more mass available to exert more force. It predicts the presence of metal ores, metamorphic rocks and igneous rocks.
Magnetic susceptibility is thus property important for predicting oil and gas.
Electromagnetic surveys detect the electric conductivity of the formations. A special type of electromagnetic surveys is sea bed logging, which can differentiate between water-filled and oil-filled reservoirs. It can be undertaken using both horizontal and vertical transmitters/receivers. It is a great method as it is highly local and is harmless to the environment.
Finally, seismic surveying is a vital part of exploring for oil and gas. The method requires a controlled seismic source of energy (such as dynamite) and a receiver. The waves are generated and reflected, and the reflected waves are then picked up by sensors called geophones.
The results obtained are then studied.
The main application of these technologies is reducing, rather than eliminating, the risk of drilling dry wells.
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