The Pennsylvania State University Petroleum and Natural Gas Engineering Program
By Kyle Barnoff
When an acquaintance learns about my study of Petroleum Engineering I am usually showered with questions, compliments or jeers. More often I experience the latter, but one topic comes up most frequently. Where does oil come from? Seems like a simple question, but it has numerous correct answers. I will answer all of them in this essay, including where oil is stored underground, how it gets there and where scientists believe it all came from.
Use your imagination to construct a large oil reservoir thousands of feet underneath the ground. You probably imagine a big lake of oil trapped in a single void in bedrock. Thanks for playing along, but this idealistic image is wrong.
When I talk about oil reservoirs with intelligent professors outside of Petroleum Engineering, this is the picture they paint for me. Realistically the earth is similar to a multi-layered hamburger from your favorite fast food chain. Each layer, the bun, the cheese and the hamburger, are
Image 1. A sponge shares the structural properties of a reservoir rock.
different layers of rock with their own distinct properties. None of these layers can hold oil. But if we throw a small sponge in the middle of the sandwich we can create a potential reservoir. This sponge best represents the type of rock that holds oil underground.
A sponge holds liquid like a “reservoir rock” holds oil. Little holes and cavities inside rocks allow oil to be stored. These rocks are usually sandstones, limestone or dolomites. Oil can be extracted because like a sponge, the rock’s small cavities have interconnecting tunnels allowing oil to flow from thousands of feet away to a small cylindrically shaped hole drilled into the rock.
Many underground rocks have small cavities, but only a minority become reservoir rocks. A reservoir rock always lies underneath a cap rock. Like the cloth of a hot air balloon prevents air from escaping, a cap rock keeps oil in the underlying reservoir rock. Within the reservoir rock gas may exist above the oil, and water below the oil. The layering is similar to the separation seen in oil based salad dressings, and it occurs for the same reason. Finding new cap rocks and new reservoir rocks in hopes of discovering new oil fields is the job of a Petroleum Geologist.
Image 2. A simple drawing of an oil and gas reservoir. Gas and oil, being lighter than water, rise above the water but are prevented from escaping by a cap rock. Like a sponge, the reservoir rock contains each liquid in very small cavities.
Now that we know oil is stored in sponge-like rocks below the earths surface, and held in place by cap rocks, we can think about how it got there. According to Norman Hyne, Ph.D of The University of Tulsa, the oil and gas come from a rock called a Black Shale, which is composed of old organic matter that once collected on the sea floor.
Image 3. Methane escapes from the La Brea Tar Pit in Los Angeles, California.
Over time the bottom of the sea floor is compressed and heated by additional layers of organic matter. The organic material is heated and pressurized. Temperatures and pressures increase over millions of years until the organic carbon molecules break down into much smaller particles that begin to rise out of the black shale. These particles rise because they are lighter than water. On some occasions these smaller organic molecules are trapped by a cap rock and an oil reservoir forms. Otherwise the oil makes its way to the surface of the earth and forms tar pits and/or releases methane gas into the atmosphere. Thus from creation to storage, the story of petroleum is complete.
The nature of oil storage sheds light on the challenges and inefficiencies of oil extraction. Even though billions of barrels of oil have been extracted from reservoir rocks, billions of barrels remain. From decomposed sea creatures to broken down macromolecules in ancient rocks, the history of petroleum is old.
Works Referenced
Hyne, N., (2001). Nontechnical Guide to Petroleum Geology, Exploration, Drilling, and Production. Tulsa: Penn Well Corp.
http://laist.com/attachments/la_henry/labrea_tarbubble.jpg