Earth Parts #32 - Petroleum Origins & Geology

Continuing in my episode series about sedimentary rocks, this episode focuses on the geologic origins of petroleum as a sedimentary material. Petroleum, also called crude oil, is one of the big three fossil fuels that are currently exploited by our civilization, the other two being coal and natural gas. Fossil fuels are called that because they are literally the fossilized remains of dead life, compressed & heated in the Earth's crust over geologic time and rendered into a mixture of hydrocarbon compounds.

Petroleum forms as a result of geologic burial of algae, plankton & bits of dead land plants in usually marine settings of abundant biological productivity. Over time, sediment mixed with organic remains accumulate into layers that become progressively buried by later sediment.

After geologic burial, algal & plant remains in sediment begin to break down in the absence of oxygen by anaerobic bacteria, which progressively convert dead biomass into C-H-rich kerogen. Kerogen is a waxy residue that usually occurs as finely-disseminated material among compacted grains of sedimentary rock. Kerogen isn't useful as a fuel, but kerogen can convert into petroleum eventually at depth in the crust.

Diagenesis is the process of sediment changing into sedimentary rock, and that includes compaction of grains alongside the rendering of dead biomass into kerogen. Diagenesis begins just after burial, and kerogen formation typically occurs at depths of 1 or 2 km and temperatures of less than 60º C.

Katagenesis follows diagenesis, at depths of 2-4 km (roughly) and temperatures from 60ºC up to about 120º C. During katagenesis the kerogen content of sedimentary rock converts into petroleum, a hydrocarbon mix. At greater temperatures, deeper within the crust, petroleum is unstable and breaks down eventually to natural gas. The range of conditions in which petroleum can form is called the "oil window", and at temperatures hotter than the oil window petroleum is geologically destroyed.

Exploration for petroleum is problematic because oil is a liquid and doesn't remain (often) in rock where it forms originally, which geologists call the "source rock". Oil can move upward through porous and permeable rock because it is lighter than that material, and oil can become trapped in geologic structures that impose an impermeable layer across which oil cannot move. Examples of geologic structural traps include anticlinal traps, fault traps and salt dome traps.

The utility value of petroleum comes from the fact that different hydrocarbons in oil have different boiling points and can be separated from each other by temperature. During petroleum distillation raw crude oil is heated until it is mostly vapor, other than dense, heavy hydrocarbons that separate off to form asphalt, tar and pitch. The remaining vapor, as it rises through a distillation tower, cools and condenses out into a series of substances ranging from heavy hydrocarbon gels like industrial greases and lubricants, down through liquid fuels and fuel oils, kerosene, diesel fuel, gasoline and finally compressed gasses like butane, propane and methane.