How Do Oil Companies Find Oil? Basic Petroleum Geology, Part III

The Hunt for Hydrocarbons

Last time, we learned that four things are necessary to create and capture oil. Those four are, an organic carbon-rich source that gets “cooked” deep underground; a reservoir rock layer with bazillions of tiny empty spaces to hold liquid petroleum and water; a shape to the underground rocks that will trap the fluids in a confined space; and non-reservoir (impermeable) rocks to seal the hydrocarbons inside the trap. Now that we know which puzzle pieces have to join to create an oil field, to find oil we have to look for places where all four pieces are present.

Scientists who work for oil companies will tell you, “There is no direct hydrocarbon indicator.” That’s a fancy way of saying that there is nothing we can see or measure that lets us just point to a spot and say, “Drill here!” and know that we’ll strike oil. Instead, we have to study blurry images of the rocks deep beneath the surface and use training and experience to interpret them. That’s how we hunt for places where source, reservoir, trap, and seal all come together in the right relationships.

The first people to use oil found puddles where it had leaked out on the ground’s surface, like at La Brea Tar Pits in California, USA. Oil found at the surface, however, is usually gummy and thick because it’s been exposed to air and water; so early use was often as salves and medicines (not a good idea, really, since petroleum is an organic poison). Not only that, but the amount available in these leaks, or "seeps," is small. Early entrepreneurs dug wells by hand near seeps looking for larger deposits and, when they were successful, also noticed that the oil was higher quality – it was lighter and thinner, and could be burned in a lamp, for instance. The first successful drilled oil well in North America, the Drake #1 in Pennsylvania, was located near a surface seep. The presence of a seep is the closest thing there is to a direct hydrocarbon indicator, but we still have to figure out which direction to go if we want to find the good stuff!

As we entered the age of oil, demand grew faster than wells drilled or dug near a few surface seeps could supply it. Short supply means higher prices, and this bigger payback for the work ushered in an era of surface mapping to hunt for oil. By studying the shapes and order of rocks exposed at the surface, early oil-company geologists (scientists who study the earth) could identify possible traps in the crumpled layers around basin edges. Drilling holes in geological structures – bent or broken sedimentary rocks - caused a boom in oil exploration in the early twentieth century. This method also introduced the oil-seekers to risk: even though a trap is visible, that doesn’t mean that the other three pieces of the puzzle are present. If there’s no reservoir rock, there can be no oil accumulation. If there’s no source, there is nothing to put in a reservoir. And if there is no seal, anything that does enter a trap simply leaks out. Early “wildcatters,” as oil-drillers were called, either learned how to identify which structures had the best chance of all four components being present, or they went broke drilling “dry holes.”

Where there’s money to be made, technology soon comes along to make it easier (or not as hard) to earn it. This has happened in the “oil patch” many times. The first big leap in exploration came soon after World War I began, when French scientists developed a way to record the order and thickness of rock layers encountered in a well, and well logging was born. Well log measurements were soon invented that helped scientists figure out which deeply buried rocks are sources, reservoirs, or seals. Almost a century later, well logs are still used to help geologists understand the rocks in the subsurface, though the sorts of information collected today are much more complex than the first logs.

Just before World War II, there was second forward leap in exploration. Research scientists devised a way to bounce sound waves off underground layers, and record the waves that return to the surface. By carefully measuring the time it takes for that sound to return, scientists can create a sort of “sound image” similar to the layering of the rocks underground. This method, called seismic exploration, created a new field for scientists who called themselves geophysicists. The new tool came along just in time: geologists doing surface mapping had found most of the fields visible on the ground. With seismic tools, the two groups of scientists – geologists and geophysicists – could work as a team to identify traps that aren’t visible from the surface.

Today, geologists and geophysicists work together exploring for oil fields using powerful computers and special software. Exploration geologists and geophysicists are like detectives: they spend their days following subtle clues, putting together complex puzzles for which there are never enough pieces – and when they’re done, the ultimate test of their puzzle-solving skill is an exploration well. As a famous geologist said more than fifty years ago, “All the easy oil has been found.” These days, the hunt for oil involves months and years of painstaking work by highly-trained professionals using state-of-the-art technology. Even with all that power brought to bear on the problem, only about one of every eight exploration wells drilled finds enough oil to pay the cost of the well.

And here you thought all they had to do was stick a pipe in the ground…


This is the third 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?
3) How Do Oil Companies Find Oil? <== You are here.  Future installments include:
4) The Economics of Petroleum Exploration and Production
5) Refining
6) The Economics of Big Oil
7) The Future of Oil

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