Lets discuss the quality of source rock organic matter. Namely, how different types of organic matter have different hydrocarbon generating potential or quality.
In terms of source rock evaluation, there are three main factors to consider:
The more oil prone the kerogen is in a source rock, the higher its quality is. Oil prone organic material contains more hydrogen and therefore has higher quality.
And, when we're talking about organic matter or kerogen, kerogen is a solvent-insoluble portion of the organic matter or “TOC” that is converted to hydrocarbons when exposed to elevated temperatures as the source rock undergoes burial.
So, what we're looking for is hydrogen rich organic matter, and you can see in these two chemical structures, the normal alkanes and the aromatics, that both these types of organic matter are hydrogen rich. They have a high ratio of hydrogen to carbon.
There are two different ways of classifying quality. One is a chemical classification that's used primarily by geochemists, and the second is a microscopy or optical classification that's used by organic petrographers.
Looking at the chemical classification of organic matter first, we see that:
- Type one is hydrogen rich and very oil prone.
- Type two has medium amounts of hydrogen and oxygen and will generate oil and some gas.
- Type three is oxygen rich and hydrogen poor and at best will generate gas at the proper levels of maturity with very little liquid hydrocarbons.
- Type four is the oxidized residue that has no significant hydrocarbon generating potential.
If you look at these different structures of the various types of kerogen, you can see that the amount of hydrogen to carbon varies. See examples below:
The thermal maturity
Maturity is very important, because it affects both the quantity and the quality of organic matter. As maturity affects the interpretation of all different types of geochemical data, you can't accurately interpret geochemical data without knowing its maturity level.
But first, what do we mean by maturity? Maturity is the extent of heat driven reactions that convert sedimentary organic matter into petroleum and, ultimately, into gas and graphite. When we discuss maturity, we want to know whether the source rocks have been heated sufficiently to generate oil and gas
And you can't accurately interpret the quantity and quality of organic matter without first knowing its maturity. We start with two questions:
- Are the source rocks are mature and,
- Have already generated hydrocarbons.
If the source rocks are mature and have generated hydrocarbons, are those hydrocarbons more likely to be oil, condensate, or gas? For a mature source rock, you need a Total Organic Carbon value greater than one percent to have generated an expel hydrocarbons. What is Organic Carbon? Organic carbon is derived from biologic material and contrast to inorganic carbon, which is derived from mineral matter.
So to summarize, when we're doing source rock evaluation, we want to look at the quantity, quality, and maturity of the organic matter in the source rock, and all these factors need to be combined into an integrated source rock evaluation program.