- [Instructor] In part two of this series of lectures on Basic Petroleum Source Rock Evaluation, I'm gonna be discussing the quality of source rock organic matter. That is, whether the source rock organic matter is oil prone or gas prone, and that is that different types of organic matter have different hydrocarbon generating potential or quality. Some types are oil prone where others are more gas prone. In the first series of lectures, we identified five questions that are of use when evaluating source rocks for exploration purposes. Of these five questions, in this lecture, I'm going to focus in on how rich the source rocks are. That is, what is the quality of these source rocks? In terms of source rock evaluation, there are three main factors to consider, the quantity, quality, and maturity. The more oil prone the kerogen is in a source rock, the higher its quality is, and that's what we're going to be focusing in on this lecture. Oil prone organic material contains more hydrogen and therefore has higher quality. And if you'll recall from the first lecture, 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. And one of the questions we want to ask is will the source rock organic matter generate mostly oil, gas, or both? And when we're looking at this question, we're talking about immature kerogens because the type of hydrocarbon that will be generated varies with the maturity of the kerogen. 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. And you'll notice as I go through this talk that there is some lack of consistency and terminology between the various slides and references, and this is because of the differences in chemical versus optical classifications. So if we look at A, the algal kerogen, we see that the atomic hydrogen to carbon ratio is 1.65, and the primary hydrocarbons that are generated will be oil. If we look at the liptinitic kerogen, type B, we have a hydrogen to carbon ratio of 1.28, and this kerogen will generate oil and gas. If we look at C, the humic kerogen, which is generally of terrestrial origin, the hydrogen to carbon ratio is only 0.84, and the primary generated product will be gas. This is another way of looking at the chemical classification of organic matter. It's a modified Van Krevelen diagram in which we have hydrogen index as a proxy for elemental hydrogen to carbon on the vertical axis and oxygen index acting as a proxy for the oxygen to carbon ratio on the horizontal axis. And again, the kerogen is divided into types based on elemental composition and potential maturation pathway. So we're getting the composition by the vertical and horizontal axes. And then these lines that run from the top of the oil prone kerogen down to the lower left corner and the gas prone kerogen from right to left, those are the maturation pathways. So you look at both the composition and the maturation pathways to type the kerogen. And you'll notice that in this particular modified Van Krevelen diagram, the Eocene kerogen from the Green River plots primarily along the line for the type one oil prone kerogen. And for the type two oil prone kerogen, we have Jurassic source rocks from Saudi Arabia and Toarcian source rocks from France. And then along the horizontal gas prone type three maturation pathway, we have Tertiary rocks from Greenland. In terms of the petrographic or optical classification, optical kerogen classifications can be based on either transmitted or reflected light microscopy. In the next series of slides, we're going to have different kerogen types shown by reflected light microscopy. Here is an example of type one kerogen from Devonian shales in the Appalachian region featuring Tasmanites algae, and again, this is reflected light. In the unpolarized white light on the left, you can see the shale background material with the brown Tasmanites algae embedded in the shale matrix. On the right, we have an ultraviolet light version in which the hydrocarbons in the Tasmanites algae are fluorescing. Looking at a type two kerogen, again this is in reflected light, and it's from the Bakken Formation, and it features abundant, amorphous to very fine-grained organic material. You can see the liptinite rich shale in the unpolarized, white light on the left, but it's much easier to see organic material in the ultraviolet light slide on the right where you can see both liptinite and alginite fluorescing in this ultraviolet slide. In terms of type three kerogen, this reflected light shows vitrinite and inertinite in a shale matrix. This table shows a comparison of optical and chemical kerogen types, and again, you'll see a little bit of discrepancy between the terminology for optical and chemical kerogen types from one source or reference to another. Looking at some of the kerogen types, an optical kerogen type that's amorphous can be a type one, two, or three chemical kerogen type. So you need to be very careful when you're looking at amorphous kerogen in terms of how you classify it because it can be either type one, two, or three. If we look at woody, coaly, and vitrinite optical kerogen types, you can see that in terms of the chemical kerogen type, they're all type three. We also have inertinite, which is the residual leftover carbon that's a type four kerogen type. This display is another way of comparing chemical and microscopy classifications, and this type of diagram can be drawn in a number of different ways. What we're looking at is from a chemical perspective, the atomic hydrogen to carbon on the vertical axis and the atomic oxygen to carbon ratio on the horizontal axis. We also have types one and two kerogens, type three and type four, and these kerogens in terms of the chemical classification can be corresponded to different types of macerals in the microscopy classification. So alginite, sporinite, cutinite, and resinite are all macerals. And macerals are to kerogen as minerals are to rocks. That is, just as you would have a rock made up of different minerals, kerogens are made up of different maceral types. And type one is predominantly alginite, and cutinite and resinite are type two. Type three is generally derived from woody land plants while type four is detrital organic material oxidized by thermal maturation including fire or by biological or sedimentary recycling. However, instead of the pure kerogen types as we've seen on previous diagrams, in reality, most organic matter actually contains a mixture of different kerogen types. 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 as will be shown in future Knowledgette presentations.