Transformation

We have examined the type of raw material needed and how it must accumulate in the natural environment. The next link in the process is to examine what happens to this organic matter (OM) when buried and subjected to increased temperature and pressure. One thing to remember is that not all of the organic carbon (OC) in sedimentary rocks is converted into petroleum hydrocarbons. A portion of the Total Organic Carbon (TOC) consists of Kerogen. We will look at the transformation of OM first to kerogen, then to petroleum hydrocarbons.

The only elements essential to the transformation of organic matter (OM) into petroleum are hydrogen and carbon. Thus the nitrogen and oxygen contained in the OM must somehow be removed while at the same time preserving the hydrogen-rich organic residue. The formation of petroleum at this point must occur in an oxygen-deficient environment, not be subjected to prolonged exposure to the atmosphere or to aerated surface or subsurface waters containing acids or bases, come into contact with elemental sulfur, vulcanicity, or other igneous activity, and have a short transportation time from the time of death to that of burial. All of these conditions must be met in order to avoid decomposition of the OM. All of this implies that as dead organic matter falls to the sea floor (organic rain), the hydrocarbon constituents needed for creating the end product will be preserved only if the water column through which they are falling is anoxic - lacking living organisms, fall is rapid - the particle size must not entirely be microscopic, bottom dwelling predators are lacking, and there is a rapid sedimentation rate - rapid deposition buries the OM below the reach of mud-feeding scavengers.

Once the organic material is buried within the sea floor, transformation begins. It is a slow process that occurs to the OM. The general process can be illustrated by the following formulas:

OM + Transformation = Kerogen + Bitumen (by product)
Kerogen + Bitumen + more Transformation = Petroleum

There are three phases in the transformation of OM into hydrocarbons: Diagenesis, Catagenesis, and Metagenesis (Tissot, 1997). Diagenesis occurs in the shallow subsurface and begins during initial deposition and burial. It takes place at depths from shallow to perhaps as deep as 1,000 meters and at

Click to to view a chart showing the liquid window -- the transformation that takes place to create oil.
temperatures ranging from near normal to less than 60^oC. Biogenic decay aided by bacteria (such asThiobacillus) and non-biogenic reactions are the principal processes at work producing primarily CH₄ (Methane), CO₂ (Carbon Dioxide), H₂O (Water), kerogen, a precursor to the creation of the petroleum, and bitumen. Temperature plays an important role in the process. Ambient temperatures increase with depth of burial which decreases the role of bacteria in the biogenic reactions because they die out. However, much of the initial methane production begins to decline because it is the bacteria that produces the methane as a by-product during diagenesis. Simultaneous to the death of the bacteria however, the increased temperatures accelerate organic reactions.

Kerogen: the name given to insoluble, disseminated organic (carbonaceous) matter in sediments
Bitumen: the name given to soluble, disseminated organic (carbonaceous) matter in sediments

The Catagenesis (meaning thermodynamic, nonbiogenic process) phase becomes dominant in the deeper subsurface as burial (1,000 - 6,000 m), heating (60 - 175^oC), and deposition continues. The transformation of kerogen into petroleum is brought about by a rate controlled, thermocatalytic process where the dominant agents are temperature and pressure. The critical temperature is about 60^o C which is called the critical jump temperature; this is the beginning of oil formation which is referred to as the liquid window. The temperatures are of non-biological origin; heat is derived from the burial process and the geothermal gradient that exists within the earth's crust. The catalysts are various surfactant materials in clays and sulfur. Above 200^o C, the catagenesis process is destructive and all hydrocarbons are converted to methane and graphite. And at 300^o C, hydrocarbon molecules become unstable. Thus thermal energy (temperature) is a critical factor, but it is not the only factor The time factor is also critical because it provides stable conditions over long periods of time that allows the kerogen sufficient cooking time - exposure time of kerogen to catagenesis. Thus the Catagenesis phase involves the maturation of the kerogen; petroleum is the first to be released from the kerogen followed by gas, CO₂ and H₂O.

The third phase is referred to as Metagensis. It occurs at very high temperatures and pressures which border on low grade metamorphism. The last hydrocarbons released from the kerogen is generally only methane. The H:C ratio declines until the residue remaining is comprised mostly of C (carbon) in the form of graphite.

Sources:

"Geology of Oil," Steven Cooperman, Ph.D.

"Understanding Petroleum Exploration and Production," National Energy Foundation, Student Activity Guide

"The Upstream: A Guide to Petroleum Exploration and Production," Exxon Corporation Informational Brochure

NORTH, F. K., 1985, Petroleum Geology: Allen & Unwin, Inc., Winchester, MA.