- [Instructor] In this Knowledgette presentation, which represents Part one of a series of lectures on Basic Petroleum Source Rock Evaluation, I'm going to first discuss a broad overview of Source Rock Evaluation followed by a more detailed discussion of how to measure the quantity of Source Rock Organic Matter. When you're evaluating in area, there are several questions you might want to ask regarding source rocks. First of all, are there any source rocks present? Secondly, how you identify the source rocks? You may also want to know how rich the source rocks are. Another important question is whether or not 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? In terms of basic Source Rock Organic Matter Evaluation there are three parameters you want to consider. First of all the Quantity of Organic Matter. Secondly, the Quality of the Organic Matter. And finally, the thermal Maturity of the Organic Matter. When talking about the Quantity of Organic Matter, the basic question is whether or not the rocks contain sufficient organic matter to have generated oil and gas. And, Total Organic Carbon or TLC measures the quantity of organic matter in a rock in weight percent. When we talk about weight percent, for example, if we have 100 grams of Sedimentary Rock, and and one percent TOC, that means that only one gram of the 100 grams of Sedimentary Rock is Total Organic Carbon. For a Type II Kerogen, you need an excess of two percent TOC in an immature source rock to have generated and expelled hydrocarbons. 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. The depositional environment extorts a large control over the amount of organic matter contained in a source rock. By depositional environment in this content, I mean depositional environment in its broadest terms. For example, we're concerned about the type of organic matter that's deposited in the depositional environment, whether it's algae, bacteria, terrestrial organic matter, that sort of thing. We're also concerned about the productivity of organic matter in the depositional environment, and the preservation of organic matter in the depositional environment. Preferable, we have an anoxic depositinal environment where you preserve the organic matter, as opposed to anoxic depositional environment where the organic matter is oxidized and destroyed. The Total Organic Carbon is comprised of two major components. First of all, the Solvent Extractable Organic Matter the EOM or Bitumen, and secondly, the Kerogen which is Solvent Insoluble in contrast to the Bitumen which is Solvent Soluble. The Kerogen can be further subdivided into Reactive Carbon which is the carbon that's convertible to hydorcarbons and Inert Carbon which is the carbon that is dead or Residual and cannot be converted to hydrocarbons. You can think of Inert Carbon as charcoal. However, TOC is not a reliable measure of source potential. And, this is because only the reactive carbon generates hydrocarbons. If we look at the vertical axis, where we have TOC and weight percent, the sample on the left has a substantial portion of reactive carbon so it's a good source rock. In contrast, the sample in the right has a large amount of inert carbon in it making is a poor source rock. What's the difference between reactive and inert carbon? Hydrogen. What we want is a source rock that has a high ratio of hydrogen to carbon. That is we want hydrogen rich organic matter. And, these two chemical structures show normal Alkanes or Paraffins and Aromatics where you have a high hydrogen to carbon ratio. Sampling can also affect TOC. As with most sedimentary rocks, you have both vertical and lateral variability in the source rock. And, sampling needs to take this into consideration so that you have samples representing both the vertical and the lateral variability. Also, if the source rock is 100 meters thick, the average TOC is going to vary depending upon your sampling scheme. For example, if you take only one sample in that 100 meters you're going to have a different average TOC then if you take 10 samples. That is one sample every 10 meters or 100 samples where you have one sample every meter. In addition, you need to find out whether the individual doing the sampling high-graded the samples. That is did they only take the richest intervals ignoring the later organic intervals. Finally, what type of sample was used? Were the sample taken from cuttings, whole core, or sidewall cores? With respect to cuttings, dilution by overlying cave cutting material can dilute the TOC by as much as 50%. To summarize we want to look at Quantity, Quality, and Maturity when doing Source Rock Organic Evaluation. And, all of these factors need to be combined into an integrated Source Rock Evaluation program, as will be shown in future Knowledgette presentations.