How do they know oil is below before they drill

Question

How do they know oil is below before they drill

Answer 1

IN the days before technology provided a window to see below the surface, old oilmen found places where oil seeped to the surface and then went and drilled on a nearby hill.  And found oil.  

THe oil being less dense than surrounding rock, will seep through porous rock until it's trapped.  Layers of earth that have experienced pressure and have uplifted into a dome are called an anticline. (Take an onion, cut it veritcally from top to root, turn it on it's side cut side down and you have anticline of layers)  Anticlines often form low hills.  Now imagine that one of those layers is really hard, and the oil can't seep through.  The oil will get trapped against that layer.  

Now a days they use sound waves driven into the earth and microphones to "listen" to the waves as they bounce off changes in the sub surface rock structure to given them an idea of what the geological structure is.  Gravity data, magnetic data, electrical data an seismic data all help find oil and oil trapping structures.

Answer 2

The previous answers are correct but not complete. Even with geophysics before drilling the oil company has to understand the geology and production history of the area.

First. Is there a source rock that could generate petroleum? (ie a black shale  containing un-oxidized plankton - generally deposited in deep water). This stuff is called kerogen

Second: Has the source rock been heated enough to generate oil from the shale source. This is called the "Hydrocarbon Kitchen" - generally the deeper rocks are buried the hotter they get - deep magma and hydrothermal fluids can also cook the source rocks

Third: If oil is generated it will tend to rise upward IF there are pathways - often fractures are the route.

Forth: Is there a trap? This has to be a porous rock (ie sandstone) that is capped by a impermeable  cap or seal (often shale). This "compartment" is known as the reservoir.

Last: The reservoir has to be permeable - the small spaces in the rock (filled with oil) have to be connected so oil will flow into the well.

Most of these factors cannot be predicted by using just geophysics - geological study of nearby wells (cores and well logs) and areas where the rocks outcrop are needed.  Also important is the production history of an area unless the proposed well is in a new area.

Successful oil exploration and production requires a TEAM of geophysicists, geologists and engineers (and economists). Understanding the geology is critical.

Drilling a well is still an educated guess. About 1 in 10 wells are successful on average. This ratio hasn't changed over time - better technology but fewer remaining reservoirs have keep it constant.

Hope this helps and my apology to my geophysical colleague.  

Answer 3

there is no absolute certainty of oil, but the necessary geologic structures for finding oil are present before drilling. otherwise it is known as wild catting.

Answer 4

Wallace Pratt, a famous petroleum geologist once said when asked where is oil found he said.  "Oil is found in the minds of men".

Of course he meant that oil was found first by a smart professional geologist visulizing the source, reservoir, and trap (3 basic criteria for oil) in their minds before they drill.  To do that one needs to know the surface and subsurface geology the geologic history and the structureal historty of the earth beneath your feet.