Origin of Hydrocarbons

Origin of Hydrocarbons

Abraham Pineo Gesner, a Canadian geologist developed a process to refine a liquid fuel from coal, bitumen and oil shale. His new discovery, which he named kerosene, burned more cleanly and was less expensive than competing products, such as whale oil. In 1850, Gesner created the Kerosene Gaslight Company and began installing lighting in the streets in Halifax and other cities. By 1854, he had expanded to the United States where he created the North American Kerosene Gas Light Company at Long Island, New York.

Rapid burial of the remains of the single-celled planktonic plants and animals within fine-grained sediments effectively preserved them. This provided the organic materials, the so-called protopetroleum, for later diagenesis (a series of processes involving biological, chemical, and physical changes) into true petroleum.

The first, or immature, stage of hydrocarbon formation is dominated by biological activity and chemical rearrangement, which convert organic matter to kerogen. This dark-coloured insoluble product of bacterially altered plant and animal detritus is the source of most hydrocarbons generated in the later stages.

Deeper burial by continuing sedimentation, increasing temperatures, and advancing geologic age result in the mature stage of hydrocarbon formation, during which the full range of petroleum compounds is produced from kerogen and other precursors by thermal degradation and cracking (in which heavy hydrocarbon molecules are broken up into lighter molecules). Depending on the amount and type of organic matter, hydrocarbon generation occurs during the mature stage at depths of about 760 to 4,880 metres (2,500 to 16,000 feet) at temperatures between 65 °C and 150 °C (150 °F and 300 °F). This special environment is called the “oil window.” In areas of higher than normal geothermal gradient (increase in temperature with depth), the oil window exists at shallower depths in younger sediments but is narrower. Maximum hydrocarbon generation occurs from depths of 2,000 to 2,900 metres (6,600 to 9,500 feet). Below 2,900 metres, primarily wet gas, a type of gas containing liquid hydrocarbons known as natural gas liquids, is formed.

At the end of the mature stage, below about 4,800 metres (16,000 feet), depending on the geothermal gradient, kerogen becomes condensed in structure and chemically stable. In this environment, crude oil is no longer stable, and the main hydrocarbon product is dry thermal methane gas.

Oil and natural gas is believed to have been generated in significant volumes only in fine-grained sedimentary rocks (usually clays, shales, or clastic carbonates) by geothermal action on kerogen, leaving an insoluble organic residue in the source rock. The release of oil from the solid particles of kerogen and its movement in the narrow pores and capillaries of the source rock is termed primary migration.

The hydrocarbons expelled from a source bed next move through the wider pores of carrier beds (e.g., sandstones or carbonates) that are coarser-grained and more permeable. This movement is termed secondary migration and may be the result of rocks folding or raising from changes associated with plate tectonics.

The distinction between primary and secondary migration is based on pore size and rock type. In some cases, oil may migrate through such permeable carrier beds until it is trapped by a nonporous barrier and forms an oil accumulation.

Since nearly all pores in subsurface sedimentary formations are water-saturated, the migration of oil takes place in an aqueous environment.

The porosity (volume of pore spaces) and permeability (capacity for transmitting fluids) of carrier and reservoir beds are important factors in the migration and accumulation of oil. Most conventional petroleum accumulations have been found in clastic reservoirs (sandstones and siltstones).