THe first step is usually collating all the available information. This often results in some fairly sophisticated industrial espionage and law suits. The following list is not the exact order all the time but is a what is used generally.
1. A detailed topographical survey. A detailed survey showing upward bends in sedimentary beds can indicate that petroleum and gas are pushing up the beds. In shallow deposits this can be enough to start drilling, especially when the area in known to produce oil.
2. Detailed geological mapping of outcrops containing the beds of interest. Often these susurveys are done far away from the area of interest because because the rocks are exposed there but not in the possible drilling zone. Many small and micro fossilss are good indicators and so are tabulated and their colors detailed (they often show how much geological 'cooking' has been done).
3. Seismic studies can indicate the presence of fluids below surface.
The speed of return or lack of a return of sound wave can be very strong indicators.
4. SOme companies test for certain gases being leaked from the surface, some put less importance on this.
5. First drill holes. This are very carefully logged for many known factors including rock type, dates (relative and absolute) conductivity, seeping of fluids, fossils, radioactivity and so on. ANd anything odd or interesting is logged carefully.
6 Pressure test.
7 Mapping of all results in diferernt forms and statisical work to help place the postion of the drill for the next hole.
8. Geological mapping is updated vertically and horozontally continually in an attempt to find the limits of the deposit and the potential of production within it. What can be compared with known deposits is also done constatnly.
There are a lot of details in all these steps but this serves as a broad outine.
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