Where Do Oil Companies Find Oil? Basic Petroleum Geology, Part II

The Essentials for an Oil Field: Source, Reservoir, Trap, and Seal

Last time, we learned how oil (petroleum) forms: it’s a slow process by which an oil source – a rock full of ancient plant and animal life - transforms deep underground, over hundreds of thousands or millions of years. The long time this takes explains why, on a human scale anyway, oil is a non-renewable resource. Geologists call this “cooking,” and even call the deeply-buried area a “kitchen” or a cooking pot. To extend that metaphor, after we cook dinner, it has to be brought to the dining table so we can eat. In the world of oil and gas, our dining table is what is called a reservoir.

Remember that one of the four things needed to turn organic matter into oil is pressure? Well, you can imagine that with thousands of feet of rock on top of it, our source layer has been subjected to a lot of pressure. At about the same temperature, pressure, and time that our oil finishes cooking; water trapped in the buried layers starts moving, pushing the newly-generated oil ahead of it. That happens because oil and water don’t mix, just like an oil and vinegar salad dressing never quite mixes. The water and oil move from the source bed into adjacent rock layers with tiny cracks, or tiny spaces between the grains. From there, all that oil and water has one purpose: to move to the surface, a movement called migration (did you know that petroleum migrates, just like birds and butterflies?). This step, like the cooking process, is very long and slow.

At this point, we need to talk about rocks for a minute. Oil-bearing rocks are almost always the kind of rocks called sedimentary, which form huge layers like some Texas-sized wedding cake. To our moving oil and water, each layer is one of two kinds of rock: it’s a reservoir or a seal. The difference is that a seal is impermeable to fluids like oil and water, meaning that it won’t allow them to move through it, forming a barrier. A reservoir is permeable, however, and fluids can move through it, either when migrating out of the kitchen or someday moving into a well. Our big slug of oil and water starts out in the deep parts of sedimentary basins, which are shaped much like what they’re called: gigantic shallow bowls. All those layers of rock are slightly tilted, following the curve of that bowl’s sides from rim to bottom. This allows our oil and water to keep moving toward the surface even when it can’t go straight up. Sometimes it makes it all the way to sunlight, forming an oil seep like the world-famous La Brea Tar Pits near Los Angeles in the USA.

Most of the time, however, our mix of oil and water runs into some sort of physical barrier. That barrier brings the migration to a stop. The permeable rock through which the oil and water have been migrating might run into a geological wall, in the form of a fault such as the San Andreas Fault (also near Los Angeles). This is quite literally a dead end, and those fluids get trapped because they can’t reverse course and go back downhill. The rock layer through which our fluids are migrating might end for other geological reasons, one of which is that the conditions for the layer’s creation or deposition did not exist everywhere. Again, the moving fluids become trapped because they can’t go backwards. A common occurrence is that all those sedimentary layers become crumpled or folded near the edges of our basin, forming sort of a three-dimensional roller coaster shape. The fluids migrate uphill into high spots, but then they can’t move down. All of these situations are common forms of traps – the third component needed for an oil field. If you think about it, though, the oil could just move sideways or across whatever is in the way – that is, it could move, unless there are layers of impermeable rocks or seals, surrounding it.

So to recap, to make petroleum you need a source of organic carbon that can be cooked in a huge geological pressure cooker. Once our dry bits of organic carbon are transformed by heat, pressure, and time into liquid petroleum; it (along with a lot of water) starts searching for a route to the surface. This is migration, when fluids move out of the source layer and into a layer that lets natural uphill movement happen. This rock layer, which is permeable to oil and water, is reservoir rock. If that reservoir rock ends or gets bent back downhill at some point, then the moving fluids are caught in a trap. They can only stay trapped, though, if there are non-reservoir rocks surrounding the trap that prevent them from moving straight up or going out the sides of the trap. These are the seals. Those are the four essential parts of an oil deposit: reservoir, source, trap and seal.

One more point to consider: sometimes people think of oilfields as underground lakes and rivers, but this is not the case. Oil is found in reservoir rocks, not in puddles and pools. A reservoir rock looks just as solid as any other rock, but under a microscope you’d see that it’s really made of tiny grains, and between those grains are even tinier empty spaces. You can make a model of a reservoir rock by dumping a handful of marbles into a water glass – there are lots of odd-shaped spaces left over because the marbles are spheres and don’t fit together like puzzle pieces. If you pour water into that marble-filled glass it can fill those void spaces, which geologists call pores. In good reservoirs, pores occupy 25% or more of the rock’s total volume; all filled with oil and water. That might not seem like much when you hold a single rock in your hand, but an oil field is a lot bigger than your hand. A good-sized oil field is miles across and the reservoir layers can be hundreds of feet thick. If you don’t think that’s a lot, let’s do the math: 25% of the volume of a cylinder one mile in diameter and 100 feet thick is still almost 180 million cubic feet! So zillions of spaces so tiny you can’t see them still add up to a whole lot of volume – and that’s where oil fields come from.

Now that we know how an oil reservoir is found, next time we’ll look at how oil companies find reservoirs.


This is the second of a series of minilectures on the petroleum industry from the ground up

1) Where does Oil Come From?
2) Where do Oil Companies Find Oil?  <== You are here. Future installments include:
3) How do Oil Companies Find Oil?
4) The Economics of Petroleum Exploration and Production
5) Refining
6) The Economics of Big Oil
7) The Future of Oil