To find oil...

Where does oil come from? Is it off the coast of NC? This is going to be my new blog topic series... I've seen so many politicians here lately discussing whether or not they support drilling off the coast of NC. I think they are just trying to say the "right thing" so you will vote for them: Let me inform you about where our oil comes from! DISCLAIMER: I may be a geologist, but I am no petroleum expert. My expertise lies in something far more important than oil... water!!! For oilfields to be present requires a particular set of geological coincidences. You need good quality, widespread reservoir rocks, such as porous sandstones or limestones, with an excellent cap rock or seal (marine shales or evaporites) to prevent the oil escaping.

Oil collects in folded and faulted rocks. Oil reservoirs in the Middle east and in Alaska are primarily this type of accumulation. Evaporite deposits can form salt domes beneath the surface. Since oil is less dense than water, the oil will accumulate in the top of a salt dome. Oil also gets trapped along the outside of the dome where the existing sediments deform/fault. Most of the oil in the Gulf of Mexico comes from or adjacent to salt domes.

You also need an excellent-quality, widespread and voluminous area of source rock (typically organic-rich shales), from which to generate and expel the oil into the reservoirs. These source rocks need to be at the correct pressure and temperature (in other words, depth, depending on the local geothermal gradient) before oil and gas can be generated and expelled. Also, the source rock needs to be close to or juxtaposed against the reservoir so that the oil can easily migrate from the source to the reservoir. Lastly, the geometry of the reservoir needs to be such that it is capable of trapping the oil. This can either involve the original depositional "architecture" of the reservoirs (wedge-shaped units can make good traps), or involve folding and faulting of the rocks to provide a suitable structure. Or a combination of both. In the case of faulted and folded structures, they have to have been formed in geological time before the source rocks became mature enough to expel oil into them. When you combine all this together, it's a wonder that you find oil anywhere! In fact you can find oil in most places in the world, concentrated along continental margins or rifted basins where the above geological conditions are most likely to be found. The reason that there is so much oil is found in Alaska, the Middle East, and the Gulf of Mexico is then easily explained by geological chance. In the Middle East, for example, there are big Jurassic reef carbonate reservoir rocks with excellent porosity, juxtaposed directly against very organic-rich marine shales, and sealed by marine shales or carbonates. All of these were formed in the ancient "Tethys" ocean before the Arabian and Asian tectonic plates collided. The collision of those plates, leaving behind the remnant "Gulf" (not the Gulf of Mexico, I'm talking about the Gulf of Arabia), has created vast numbers of suitable geological traps. Take a look at Southern Iran on Google Earth, where huge numbers of folds are beautifully exposed:

Can you see those folds?