Hydrocarbon Generation

 

 

Origins of Oil and Gas

We are all aware that oil and gas are recovered from deep below the earth's surface, but how did it get there in the first place?

The most popular theory is known as the Organic Theory. This theory states that oil and gas have biological origins. Small sea creatures from the days when the earth was mostly covered in water died and settled to the bottom of the ocean floor. Layer upon layer of silt, sand and clay built up on top of them over time. Through the process of decay, as well as ever increasing heat and pressure, the former sea creatures were converted to oil and gas. Over millions of years, continuous pressure actually compressed those layers of silt and clay into layers of rock. This is known as "reservoir rock".

The temperature under the earth's surface increases the deeper you go underground. At about 150 F, oil begins to form. Oil formation ceases at about 300 F. Oil formed at lower temperatures (i.e. closer to the surface) is called immature and is heavy. Oil formed deeper under the surface is called mature and is light. At temperatures above 300 F, oil is thermally cracked to produce light gases (i.e. natural gas). Since temperature increases with depth, natural gas wells are typically drilled much deeper than oil wells. Of interest in Canada is the formation of the tar sands in Northern Alberta. They are thought to have formed from the same oil that is recovered conventionally in other areas of the Western Canada Sedimentary Basin, but at one period in the Earth's geological history, the oil seeped to the surface where it degraded into tar.

This is probably the story of the origin of oil and gas that we are all familiar with. However, there is an interesting alternative theory. It is known as the Inorganic Theory and was developed by Mendeleev (he of the periodic table) in the early 1800's. He believed that petroleum came from deep within the earth, noting how petroleum seemed to be associated with large features of the earth like mountains and valleys rather than the finer scale sedimentary deposits. There are many today who believe in this theory, and there is some evidence to back them up. The following observations are taken from the text "Modern Petroleum: A Basic Primer of the Industry", 3rd Edition, by Berger and Andersen, Pennwell Publishers (1992):
Petroleum and natural gas are frequently found in geographic patterns of long lines or arcs which are related more to deep-seated, large-scale structures of the earth's crust, rather than to smaller-scale sedimentary deposits.
Hydrocarbon-rich areas tend to be rich at many levels and extend down to the crystalline basement that underlies the sediment.
Some petroleum from deeper and hotter levels almost completely lacks biological evidence.
Methane is found in many areas where biological origin is improbable.
Hydrocarbon deposits of large area often show common chemical features independent of the varied composition or geological ages in which they are found.
The regional association of hydrocarbons with the inert gas helium, and a higher level of helium seepage in petroleum-bearing regions has no explanation in the biological theory.

Many simply say that the vast amount of petroleum present deep within the earth could not possibly be explained through formation by organic debris.

 

Oil seepage

 

How Oil and Gas were Formed

 Oil has formed throughout much of the Earth's history, in fact, oil is being formed in some parts of the Earth today. Almost all oil and gas comes from tiny decayed plants, algae, and bacteria. At certain times in the Earth's history conditions for oil formation have been particularly favourable. Oil from the North Sea is mainly found in rocks that formed during the Jurassic period - about 150 million years ago, long before people appeared on Earth.

During this time the seas and swampy areas were rich in microscopic plants and animals.

When these died they slowly sank to the bottom forming thick layers of organic material. This in turn became covered in layers of mud that trapped the organic material.

Oil and gas were formed by the anaerobic decay of organic material in conditions of increased temperature and pressure.

The layers of mud prevented air from reaching the organic material. Without air, the organic material couldn't rot in the same way as organic material rots away in a compost heap. As the layers of mud grew in thickness, they pushed down on the organic material with increasing pressure. The temperature of the organic material was also increased as it was heated by other processes going on inside the Earth.

Very slowly, increasing temperature, pressure and anaerobic bacteria - micro-organisms that can live without oxygen - started acting on the organic material. As this happened the material was slowly cooked and altered, like food in a pressure cooker. In this was the energy first given to the plants by the sun is transferred and the organic matter is changed into crude oil and gas.

Oil forms first, then as the temperature and pressure increase at greater depth gas begins to form.

Temperatures within the Earth's crust increase with depth so that the sediments, and any plant materials they contain, warm up as they become buried under more sediment. Increasing heat and pressure first cause the buried algae, bacteria, spores and cuticles (leaf skin) to join their wax, fat and oil to form dark specks called kerogen.

The cellulose and woody part of plants are converted to coal and woody kerogen. Rocks containing sufficient organic substances to generate oil and gas in this way are known as source rocks. When the source rock starts to generate oil or gas it is said to be mature.

As the source rock gets hotter, chains of hydrocarbon chemicals use this heat energy to break away from the kerogen to form waxy and viscous heavy oil. At greater depth, the temperature rises. At higher temperatures the chains of hydrocarbons become shorter and break away to give light oil and gas. Most North Sea Oil is the valuable light oil. Gas from the Southern North Sea is methane.

Oil and gas are called 'hydrocarbons' because they mostly contain molecules of the elements hydrogen and carbon.

Crude oil is a complex mixture of hydrocarbons with small amounts of other chemical compounds that contain sulphur, nitrogen and oxygen.

Traces of other elements, such as sulphur and nitrogen, were also present in the decaying organic material, giving rise to small quantities of other compounds in crude oil.

Hydrocarbon molecules come in a variety of shapes and sizes, (straight-chain, branched chain or cyclic), this is one of the things that makes them so valuable because it allows them to be used in so many different ways.

Oil and gas form as the result of a precise sequence of environmental conditions:
The presence of organic material
Organic remains being trapped and preserved in sediment
The material is buried deeply and then slowly "cooked" by increased temperature and pressure.

 

The organic carbon produced in the water column varies from ~0.1% to 5%, depending on various factors such as the following:
Oxygen depletion in bottom waters or in sediment as a result of high organic input,
Adsorption of certain compounds to mineral particles,
Preservation of organic compounds as shell constituents,
Changes in the rate of deposition of sediment organic matter,
High input of terrigenous organic compounds, which are more stable than organic matter, and
Dominant input of argillaceous sediments where oxygenation of pore water is restricted.

 

Organic matter undergoes changes in composition with increasing burial depth and temperature. The three steps in the transformation of organic matter to petroleum hydrocarbons are termed diagenesis, catagenesis, and metagenesis. (Tissot and Welte, 1984). Petroleum hydrocarbons exist as gaseous, liquid, and solid phases, depending on temperature, pressure, burial time, and composition of the system.

 

General scheme of the evolution of the organic fraction and the hydrocarbon produced.

 

C1-C4 Hydrocarbons

C1-C4 hydrocarbons (methane, ethane, propane, and butane) are found predominantly in the gaseous phase at surface conditions. These hydrocarbon gases, largely methane (C1), may be generated in significant quantities in sediment, either under near-surface conditions by bacterial action (Claypool and Kaplan, 1974) or at greater depths by thermochemical action (Schoell, 1988).

Biogenic gas (microbial methane) is produced in sulfate-depleted marine sediment where accumulation rates exceed ~50 m/m.y. and organic matter is preserved. Microbial methane production occurs by reduction of dissolved bicarbonate (CO2), and the process competes with sulfate reduction for electrons (hydrogen) generated by the anaerobic oxidation of organic matter. At sedimentation rates slower than ~50 m/m.y., sulfate is continually replenished by diffusion from overlying seawater until metabolizable organic matter is completely consumed, leaving none for methane generation.

Thermogenic gases (C1-C4) are produced in sediments at rates that are proportional to temperature. In most ODP holes with normal geothermal gradients (20°-50°C/km), sediment temperatures are insufficient to produce more than trace amounts of thermogenic gases. High concentrations of thermogenic gases in sediments at shallow depths and low temperatures generally indicate the existence of a hydrocarbon migration pathway. However, it is becoming increasingly recognized that the C2-C4 gases can be produced bacterially along with C1, although not in high concentrations (Vogel et al., 1982; Wiesenburg et al., 1985; Oremland et al., 1988; Feary, Hine, Malone et al., 2000). Either biogenic or thermogenic gas can be hazardous. Either can cause a blowout and catch fire. Biogenic and thermogenic gases usually (but not always) can be distinguished on the basis of chemical and carbon isotopic composition (not available on the JOIDES Resolution). However, it is amount of the gas and the possibility of high-pressure accumulation that poses the hazard, not the mechanism of origin.
C5 and Heavier Hydrocarbons

C5 and heavier hydrocarbons (oil), predominantly liquid, are almost exclusively the product of thermal generation from hydrogen-rich organic matter in deeply buried sediments (oil of microbial origin is unknown). This generation occurs at rates that become quantitatively important only as temperatures reach 90°-150°C (typically at burial depths of 2500-5000 m for average geothermal gradients). Hydrocarbon gases are generated with the oil, and although they consist largely of methane, they usually also include heavier hydrocarbons. Thermogenic conversion of organic matter to hydrocarbons continues at accelerating rates with increasing depth and temperature until all organic matter, including the oil itself, has been converted largely to methane and carbon-rich residues (Ocean Drilling Program, 1992).