Welcome back! Did you know that you and I are related? Yes, we’re both based on carbon. I’m a hydrocarbon and you’re a carbon based life form. That’s what gave scientists the clue that petroleum actually derives from living plants and animals.
Way back in Paleozoic and Mesozoic time, like 70 to 400 million of years ago, there were thick jungles and the seas teemed with plant and animal life. Whenever, some of these plants and animals, dead or alive, got covered with sand or mud so they didn’t decay. The process of becoming petroleum had begun. Sometimes this was due to mud slides, or shifting sand dunes, or even volcano blasts or meteors colliding with earth and throwing up big clouds of dust. The deposited layer prevented further decay and as layers were added on top, the pressure increased.
Sometimes these layers were thrust deeper and deeper as the earth’s crust shifted. If they got down very far, not only did the pressure increase but so did the temperature. Of course you know that the centre of the earth is so hot, rocks are liquid. That’s pretty hot, eh! Along with all of this, there were bacteria in action and chemical changes and over thousands and thousands of years, all of this produced natural gas and crude oil.
Let me tell you more about how this happened. You had all this organic material buried under the sand and mud. As the pressure increased, the petroleum was driven into nearby rocks. Now a rock may look solid but actually it has many pores in. Some rocks have more pores and bigger pores than others. There are three basic kinds of rock: igneous, metamorphic and sedimentary. Petroleum is usually found around sedimentary rocks. [These were formed by the same laying down of sand and silt and mud that was helping to make petroleum.] However, not all sedimentary rocks contain petroleum. In fact, the odds of me becoming a drop of heavy oil were pretty slim. Only about 2 % of organic material ever becomes petroleum and only about 0.5% accumulates in sufficient quantities to be recoverable.
The secret to this happening is that hydrocarbons are lighter than water. As an experiment, put a drop of oil in a pan of water. You will see it migrate upward. So oil within porous rocks will migrate upward until it is trapped by a layer of non-porous rock. Now at this time I’m going to start sounding like a geologist for a bit. Geologists are one of many scientists that have to know all sorts of things in order for us to have the petroleum industry we have today. One of the principal geologists in the early days of the Lloydminster field was Professor Edmunds of the University of Saskatchewan, pictured below.
O.K. The non-porous rock that traps and holds the rising petroleum comes in various shapes, often folds. An upfold or arch is called an anticline. A downfold or trough is called a syncline. A very short anticline whose crest plunges in all directions from a high point is called a dome. These structures are commonly associated with trapped oil and gas. When layers of rock fracture, rather than fold, it is called a fault. Faults can also trap gas and oil and often salt water from the ancient seas which once covered this area. Heavy Oil is usually associated with sandstone, and oil sands. In the Lloydminster area these sands are often channel sands, which were once part of river beds. Like sand bars in major rivers, these sand deposits come in various sweeping shapes. Sometimes they are like oxbows (channels that were cut off from the main stream flow). Also, they can be a varying depths shapes and designs, all of which makes it harder to predict where I might be. Next time you see a river with sand bars, think that in a few million years, humans several hundred meters above might be trying to find these sand bars and the oil reserves they contain.
So, to review: Three things are required for oil to accumulate
1. source of oil and gas
2. reservoir rock, porous enough to let oil flow through
3. a trap or barrier (like a sand bar) to stop fluid flow so accumulation can occur
As the reservoir fills over the years, segregation occurs in the reservoirs. This just means that oil rises above the salt water, and gas rises above oil. Here’s a diagram of how what that looks like:
So there I waited, for thousands and thousands of years; waiting for someone to "discover" me for my present career.
For a more detailed explanation - click here