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. 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. 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.