How did crude oil and natural gas form? What forces of nature created reservoir and source rocks?
Crude oil and natural gas are called fossil fuels because they were created from biotic materials: tiny terrestrial marine plants and animals. Millions of years ago, these microscopic plants and animals absorbed energy from the sun, which was stored as carbon molecules in their bodies. They died and were buried by layers of sediment, water and sand. Microscopic organisms fed on the decomposing organic material in a process called biogenesis. Specific high temperatures and pressure conditions are required to induce formation of hydrocarbons, such as crude oil, wet gas, dry gas, etc.
We can create methane gas from decaying organic matter through bacterial fermentation: a chemical transformation by bacteria that chemically alters different substances. A Winogradsky column (see 1.2) demonstrates how reducing bacteria fermenting carbon can produce methane and sulfur. It has been shown in the laboratory that reducing bacteria tend to convert organic matter into a petroleum-like substance. Geologic time and the rock cycle play an important role in the formation of these hydrocarbons.
There are two types of rocks that are important in crude oil and natural gas formation: reservoir rocks and source rocks. Reservoir rocks are where the hydrocarbons migrate (from historic production) and become trapped. Source rocks (i.e. shale) are not typically porous or permeable. These rocks are where the biotic materials are deposited and the hydrocarbons are created. (There is more about porosity and permeability in our next set of activities.)