How can the oil companies detect the presence of oil underneath the soil or even the seabed?

As i know that they have been seeking for the petroleum since 100 years ago. How can the oil companies detect the presence of oil underneath the soil or even the seabed?
The factors that lead to a petroleum resource are:


1. Source material -- kerogen is the organic precursor to petroleum


2. Transport mechanism -- some physical feature that shows geologists how broadly dispersed kerogen can have necessary pathways to ';collect'; a larger concentration


3. Trapping mechanism -- a physical feature that prevents the petroleum from escaping; anticlines (e.g. the top of folded strata), salt domes, faults leading to impermeable sealing strata, and other so-called subtle traps


5. evidence that the kerogen has been subjected to the right window of temperatures to convert the kerogen





techniques for identifying these features include all kinds of geophysical and geochemical tools; the most typical is seismic. Occasionally, a seismic survey will reveal a ';bright spot'; which is a unique feature of natural gas reservoirs. Geologists use all the data available to identify the most likely targets to drill. The most expensive tool is drilling. All the drill information is collected and added to give a ';picture'; of what the subsurface layers are composed of. How can the oil companies detect the presence of oil underneath the soil or even the seabed?
You cant really ';detect the presence of oil'; underground or under the sea floor unless there's an obvious pocket of liquid and even then you can't be completely sure its oil, it could also be natural gas, water or salt. (Salt isn't a liquid but it can act like one when it's under pressure)


The way they decide where to drill is by seismic studies and stratigraphic analysis.





With a seismic study they look at data gathered from thumper trucks (really big trucks that hit the ground with a big heavy foot), explosions, or natural earthquakes. By looking at how seismic waves travel through certain materials you can make a pretty good guess about what is there. They know what oil shale looks like in these data and can create models about where the best places to look will be.





Stratigraphic analysis is where scientists look at drill cores from the surrounding areas and tell you what kind of rocks are down there. By comparing where these rocks are in sequence with places they have found oil before and with modern deposits they can make a good guess at where the best places to look will be.


You can't really KNOW there is oil until you drill but you can get it down to about a 99% chance.