Passage III

Petroleum (crude oil) is brought to Earth's surface from large underground deposits that are mostly 0.5—0.7 km below Earth's surface. Two scientists discuss the origin of petroleum.

Scientist 1

All of Earth's current supply of petroleum formed within the last 600 million years from once-living matter. Petroleum formation began when the remains of microscopic marine organisms accumulated on the seafloor over a long period. Fine-grained ocean sediments buried the remains. Beneath the ocean floor, at depths of less than 10 km below Earth's surface, heat and pressure over millions of years changed the remains into petroleum.

After forming, the petroleum migrated upward to depths where it accumulated in large deposits at specific locations in Earth's crust. Petroleum contains biomarkers, compounds that can be produced only through the breakdown of once-living matter. Fossils of the marine organisms that provided the raw material for petroleum are found in rocks brought to the surface from locations where petroleum is known to have formed.

Because of the unique conditions and time necessary for petroleum formation, there is a limited supply available in Earth's crust.

Scientist 2

All of Earth's petroleum has formed from simple, inorganic carbon compounds at depths from 100 km to 200 km below Earth's surface. Petroleum formation has been occurring constantly since shortly after Earth's formation. The process begins as water reacts with simple, inorganic carbon compounds to form simple hydrocarbons such as methane. Those simple hydrocarbons then combine under pressure to produce petroleum.

After forming, the petroleum migrates upward to much shallower depths where it accumulates in large deposits at specific locations in Earth's crust. Most petroleum deposits exist along tectonic plate boundaries, where the petroleum can rise from depths greater than 100 km through deep fractures in Earth's crust. A helium isotope, commonly found associated with petroleum, originates only at depths greater than 100 km and is brought up from those depths by the rising petroleum.

Since petroleum is constantly forming and rising toward the surface, there is an unlimited supply available.