365 Days of Climate Awareness 86 – Basics of Petroleum

Petroleum is the liquid remainder of organic life from tens to hundreds of millions of years ago, stored in the pores and crevices within crustal rock. It forms from buried organic matter—frequently marine life, covered in sediment in a largely anoxic environment so the organic components are not oxidized—and buried deeply under overlying layers of sedimentary rock. With time and heat, the organic materials which become liquids are known to us as petroleum. Some become natural gas (which might remain in solution within the petroleum), and others become solid rock, which we know as coal. It is very true to say that petroleum is the liquid form of stored solar energy, since the organic remains which compose it derive from ancient food webs powered ultimately by the sun.

Regions where major deposits of oil have been found tended to be shallow seas many millions of years ago. The oil deposits of the Midwest and western United States are the products of the marine deposits in the Western Interior Seaway, which existed in the late Cretaceous age (145-65 mya). The huge reserves of the Mideast are the result of the ancient ocean Tethys which existed between modern-day Asia and Africa between 255 and 65 mya.

Tethys Sea (between Australia, AFrica and southeast Asia), ca. 94 mya.

Petroleum is commonly referred to by its raw extracted form, crude oil. It is a brownish-black, gluey fluid, but is hardly uniform around the world, or even within one field. The chemical composition can vary widely, from thinner, less dense mixtures with smaller molecules and lower viscosity, to thicker, denser crudes with larger molecules and high viscosity. Minor components such as sulfur and metals in the oil can have major effects on its behavior. Less dense crude is called “light”, and denser crude is called “heavy”. Crude without sulfur is called “sweet”, and crude with a significant amount of sulfur is called “sour”.

Western Interior Seaway, ca. 100 mya.

Early oil wells were shallow (less than 100 feet below the surface) by modern standards, where drilling a mile below ground is not uncommon. The practice has become far more complicated, as the depth of wells, and the complexity of the geological formations along  the way, have grown. The field has grown from, relatively speaking, sticking a straw in the ground to advanced recovery methods such as “fracking”, where steam and chemicals are injected at high pressure into rock formations, fracturing the rock and allowing trapped hydrocarbons to flow. In the mid-century the hunt for oil moved offshore, on Lake Macaraibo in Venezuela, the Caspian Sea, and the Gulf of Mexico. Offshore oil is now a significant component of global production.

Tomorrow: the upstream oil industry.