Pondering the Process of Oil Formation
Amazing fluid, oil. It powers our vehicles, heats our homes, wraps our sandwiches, and inspires an occasional war. Oil touches all of our lives, especially in Alaska, but what is it?
Some oil, particularly that from Alaska's North Slope, is the remains of prehistoric creatures and plants from the sea, according to Michael Whalen, an assistant professor of geology at the University of Alaska Fairbanks.
The critters and algae that are now crude oil lived hundreds of millions of years ago when a shallow ocean covered what is now Alaska's North Slope. The gradual rise of the Brooks Range, caused by the Pacific plate shoving over the top of the North American plate, pushed out the ocean and eventually buried enormous amounts of ocean plants and animals.
As this organic mix was forced farther downward and was subjected to pressure from the rocks above and heat from the inner earth, it cooked for a few million years or so. This unsavory stew of former life eventually became fluid.
Because oil is a relatively light liquid, it will migrate above denser fluids, such as water, unless confined in what geologists call a hydrocarbon trap. A hydrocarbon trap consists of porous rock that acts as sort of an oil-holding sponge and a roof of non-permeable rock to prevent oil from moving upward.
Oil trapped in underground pockets of the North Slope comes in many flavors, some of it light-colored and easily pourable and some of it blackish and tar-like. Oil's density and viscosity depends on exactly what types of animal and plant life were cooked to form the oil. Prehistoric plants, for example, turn into light oils and gas. Oil's character is further determined by the pressure and heat it forms under.
Oil is comprised of hydrocarbons, chains of hydrogen and carbon molecules of varying lengths. The length of the molecule chains determines how gooey the oil is. Cathy Hanks, a research assistant professor of geology at UAF, came up with a simple analogy to relate hydrocarbon length to stickiness. Imagine short hydrocarbon chains as blades of grass and long hydrocarbon chains as branches. Where grass is easy to pour into a garbage bag (as short-hydrocarbon oil is), branches are more resistant, tending to stick to each other and other things (as do the molecules of long-hydrocarbon oil).
Prudhoe Bay has several underlying oil deposits in layers that look something like a colossal layer cake tipped on its side, Hanks said. The different deposits, among them the Sadlerochit, Kuparuk, and Lisburne formations, are trapped in different types of rocks. In the Sadlerochit formation, for example, sandstone holds the oil. The Sadlerochit formation is currently the major source of Alaskans' Permanent Fund dividend checks, Hanks said. Oil from the Sadlerochit formation is about 8,500-feet deep. Oil extracted there is about 200 degrees Fahrenheit, a bit hotter than the 150-degree oil from the 6,000-foot deep Kuparuk formation.
Straight from the earth, crude oil contains hydrocarbons plus small amounts of oxygen, nitrogen, sulfur, salt, water, and trace amounts of certain metals, according to the textbook Fundamentals of Petroleum. Oil refineries, using heating and distillation processes, break up long-chain hydrocarbons and separate crude oil into gasoline, diesel fuel, kerosene, lubricating oils, waxes and asphalt. After further processing at petrochemical plants, crude oil can be converted to fertilizer and plastics.
This sticky black tar extracted from deep within the earth is incredibly useful, valuable, and fought over. As I was scanning other texts for oil information, I found an unexpected but insightful paragraph at the end of an essay on oil formation in The Dynamic Earth, written by Brian Skinner and Stephen Porter: "Iraq invaded Kuwait in 1990 in order to control the gigantic Kuwaiti oil pools; the United Nations went to war in 1991 to expel the Iraqi invaders. How sad it is that we humans find it necessary to kill each other because of tectonic events that happened millions of years ago."
