The Origin of Oil
by Jonathan Sowers, CLS
Senior Diagnostician

Most of the readers of these articles deal with lubricants and lubrication on a daily basis, and all of us use oil and gas products in our cars, at home, school, and work. But despite the ubiquitous presence of petroleum products, few really know where and how oil and gas came into being. In order understand this, some basic rock fundamentals must first be explained because most oil and gas is found in rocks deep under the earth’s surface. In fact, the word petroleum comes from the Greek word petra – rock, and the Latin word oleum – oil, hence petroleum literally means “rock oil”. 

The earth’s crust is made up of three types of rock: igneous, sedimentary, and metamorphic. Igneous rock is formed when molten rock, magma, erupts on the surface and cools and solidifies, such as granite and basalt. Sedimentary rock is initially formed in horizontal layers as particles of older rock, silt, sand, and mud are eroded from the surface and are washed down streams and rivers into lakes and oceans. Over millennia, the layers become thick and the weight of the overlying sediments compacts earlier deposits. Minerals in the water cement these deposits together into sedimentary rocks, such as limestone, sandstone and clay. Metamorphic rocks (either igneous, sedimentary or other metamorphic rocks that change in to something else) come from deep in the earth where they are subjected to very high temperatures and pressures causing a change in structure, such as limestone metamorphosed into marble. 

You may be asking yourself just how do we get oil out of a solid rock? The answer is that some rocks are not quite as solid as they may appear. Under magnification certain rocks can be seen to contain many tiny individual openings called pores. The oil and gas will be located within the pores and pore spaces throughout the rock. Rocks with high porosity (many pores) will contain more oil than a rock with low porosity (few pores). Porous rocks may also be formed with the pores being connected enabling the oil and gas to flow through the rock; these are known as permeable rock. It is this permeability which enables oil and gas to readily flow to the surface if a well is drilled into the rock. However, in the western United States are located vast deposits of oil and gas trapped in porous rock that is not permeable. This rock is referred to as shale, and the oil - shale oil or tight oil. In western Canada there are similar deposits found in tar sands with low permeability. These oil deposits must be collected through advanced mining, drilling and stimulating technologies.

So, how did the oil and gas form and get trapped in the rock? The most popular theory is that petroleum came from the remains of plants and animals, called the organic theory (organic because plants and animals are living organisms). Most geologists think that very small (many microscopic) plants and animals lived in the rivers and oceans millions of years ago, as they do today. The rivers carried these plants and animals down to the seas along with river silts and muds. Also the oceans were full of microscopic plant and animal life. As they died, these once living small organisms sank and were deposited on the ocean floor mixing with the silt, sand and mud. This rich organic compound was layer by layer cut off from the oxygen dissolved in the water. Without oxygen the normal rate of organic decomposition was altered. Over many thousands of years a thick body of organic sediments accumulated at the bottom of the seas. As more time passed the tremendous weight of ongoing layers of sediment pushed the lower sediments deep into the earth causing changes to occur. The lowest layers of sediment became rock. It is theorized that high temperatures and pressures, along with bacteria and various chemical reactions transformed the trapped organic remains into oil and gas.

Chemically, oil and gas is formed from hydrogen and carbon atoms. The deeper the rock, the higher its temperature and pressure. When the temperature reaches approximately 150°F, carbon and hydrogen atoms in the rock begin to combine chemically to form hundreds of different kinds of hydrocarbon molecules, i.e., a chain of carbon atoms with hydrogen atoms attached. Oil and natural gas are mixtures of different kinds of hydrocarbon molecules. This chemical process continues until the temperatures are between 225°F and 350°F, above which the longer chain molecules break up into smaller, lighter molecules such as methane gas. Once temperatures reach above 500°F the organic materials are destroyed as a source for petroleum. 

Now, while the hydrogen and carbon atoms are combining in the pores of sedimentary rocks, there are other tremendous forces at work. Earthquakes are moving the earth’s crust up, down and sideways. Layers of rock are folded into arches and troughs. Mountains are formed and erosion from wind, rain and ice are wearing away the earth’s surface. The intense pressures by the overlying rock beds cause the newly formed petroleum to flow through cracks and fissures, from pore to pore, migrating from deep down towards the earth’s surface. The oil and gas continuously move toward the surface until they are blocked by an impermeable layer. A petroleum trap is an arrangement of rock layers that contains an accumulation of hydrocarbons, consisting of an impermeable caprock overlaying porous and permeable rock. The hydrocarbons accumulate and are stored in the porous layer and the caprock prevents their escape. The porous rock containing the accumulated hydrocarbons is called a reservoir. Usually the reservoir contains layers in this order: gas, oil and salt water (the salt water is the ancient seawater that existed when the formation was originally laid down). It is the oil and gas located in these traps that we drill for today, both on land and off-shore.

References: Kate Van Dyke, Fundamentals of Petroleum, University of Texas at Austin