- [Janell] In this continuing series of talks on Basic Petroleum Source Rock Evaluation, the first maturity lecture covered the introduction and the effect of thermal maturity on total organic carbon. This second maturity lecture covers three different ways to determine thermal maturity. The three different ways that I'll be discussing in this part two are vitrinite reflectance, thermal alteration index, and hydrocarbon composition. Looking first at vitrinite reflectance, which is probably the most common way of determining thermal maturity. In measuring vitrinite reflectance, you take a sample and this sample can either be a whole rock sample or an isolated kerogen sample, and either way the sample was polished and the light is reflected off of the sample and the reflected light is measured by a sensor and with increasing maturity, the amount of light reflected from the sample, increases. And it's very important to get the right polish on the sample. Insufficient polish of samples is common and results in artificially lowered maturity measurements. This next diagram shows a complete reflectogram, giving the reflectance of all macerals in a kerogen sample. As you can see, there are a number of different macerals in this sample. Recall, that just as a rock is made up of many different minerals, kerogen is made up of different macerals. Determining which maceral is true vitrinite when doing vitrinite reflectance is subjective, but if the true vitrinite is not correctly identified and measured by the organic petrographer doing the work, then you get an erroneous maturity measurement. And in this sample, telocollinite is the true vitrinite and has a vitrinite reflectance measurement of 0.61, which is the early oil window. However, if the organic petrographer erroneously picks desmocollinite as true vitrinite, you're going to get a measurement that's too low. Similarly, if the organic petrographer looks at the oxidized vitrinite, you're going to get a vitrinite reflectance measurement that's too high. A more accurate way of determining vitrinite reflectance is a semi-quantitative technique called Vitrinite Inertinite Reflectance Fluorescence, or VIRF, which was developed by Doctor J. Newman and this technique does a much better job of separating the various macerals and is not nearly as subjective as the traditional technique. In this technique, fluorescence is on the vertical axis and reflectance is on the horizontal axis. Normal vitrinite, which is the vitrinite population on which the reflectance measurement needs to be made is shown by the open squares. The closed squares give a suppressed vitrinite population. The solid triangles give the caved vitrinite population. And as you would expect from caved cutting samples, the maturity is lower than the true vitrinite in the open squares. Reworked vitrinite is shown by the solid circles and reworked vitrinite is vitrinite that is recycled from one sedimentary rock that's older to a younger sedimentary rock and as you might expect, this recycled or reworked vitrinite has a higher measurement than true vitrinite. Lastly, the solid diamonds are inertinite, and they have a higher reflectance and lower fluorescence than the true vitrinite population in the open squares. This slide shows Thermal Alteration Index or TAI at different vitrinite reflectance levels and Thermal Alteration Index uses transmitted light microscopy on pollen and spores to determine the thermal maturity. It used to be commonly done in conjunction with vitrinite reflectance in labs, but it's actually not used very much anymore. But I think this slide is helpful in showing you the types of the changes that occur in organic material with increasing thermal maturity. Starting with the least mature pollen and spores, the TAI at a vitrinite reflectance of .55 shows that the pollen and spores are yellow in color. At a vitrinite reflectance of .70 which is in the early oil window, color of pollen and spores becomes an orange color. At a vitrinite reflectance of .90 which is a peak oil generation, the pollen and spores begin to turn brown. When you get to a vitrinite reflectance of 1.10 which is in the late oil window, the pollen and spores become dark brown. And when you enter the wet gas window, at a vitrinite reflectance of 1.40, the pollen and spores become black. Another way of determining maturity is by looking at changes in hydrocarbon composition and if we look at this diagram, the different maturity zones are on the far left and the generated products, corresponding to the maturity zones on the left, are to the right of these maturity zones. So if we start at the top of the immature zone, which it corresponds to the zone of diagenesis, you can see that the hydrocarbons are inherited biomarkers and these are biomarkers that are inherited from the organic matter; they're not actually generated products. The common generated product is biogenic gas, which is methane. Going next to the oil window, or the zone of catagenesis, you can see that the predominant product that's generated is oil with some gas which is probably mostly wet gas. At a higher maturity, at the wet gas window, the amount of oil generated becomes less and you get more gas generated. Which again is probably wet gas. Going into the zone of metagenesis, that's where you get dry gas generated, and you can see that it's all red here, indicating dry gas generation. This next diagram is a Van Krevelen Diagram and it basically shows that as vitrinite reflectance and maturity increase, the different types of kerogen converge to a common endpoint. And this diagram shows composition on the axes, the different kerogen types in Roman numerals one, two, and three, the maturation pathways for these different kerogen types in black lines with arrows on them, and then the circles show the samples at different maturity levels. And then there are different zones shown diagrammatically to the upper right, where the dots are further apart. This is the zone of diagenesis and moving towards the lower left, where the dots in the background show the zone of catagenesis, and finally, in the lower left corner you have the zone of metagenesis. So on the vertical axis we have the atomic ratio of hydrogen to carbon and the horizontal axis we have the atomic ratio of oxygen to carbon. So you can see how the composition changes with increasing maturity and as you move with increasing maturity this yellow line is vitrinite reflectance of .5, which is immature. This green line is a vitrinite reflectance of one which is at peak oil generation, which occurs in the zone of catagenesis, and finally, this red line is a vitrinite reflectance of two which puts you into the zone of metagenesis. And note, that even though the starting compositions of kerogen types one, two, and three are different, by the time they follow the appropriate maturation pathways, when they end up in the zone of metagenesis, they're basically the same, chemically. So in the zone of metagenesis, you can't chemically distinguish the original kerogen type when it reaches this endpoint of thermal maturity. This next slide shows schematic changes in kerogen structure, atomic hydrogen to carbon ratio, and generated hydrocarbons with increasing thermal maturity. If you look at the diagram in the top, this shows the structure of the initial starting kerogen. The zig-zag lines are normal alkanes and the hexagons are aromatic compounds. The hydrogen to carbon ratio of this initial starting kerogen is 1.43. As you move down to the second box, which is more mature than the top box, it shows on the left the residual kerogen left after generation has started and the generated hydrocarbon products on the right. As you can see, the residual kerogen on the left after generation has started has more aromatic compounds in it, the hexagons, and fewer normal alkanes. The generated hydrocarbons are smaller fragments that have broken off from the initial kerogen. The hydrogen to carbon ratio of the residual kerogen is 1.29. Looking at the next figure, again the residual kerogen is on the left and you can see that there are even more aromatic compounds and fewer normal alkanes and that the generated hydrocarbons are smaller fragments, mostly wet gases. The hydrogen to carbon ratio of the residual kerogen in this case is 0.96. The bottom diagram which is the most mature, again shows residual kerogen on the left, which in this case is graphite, and the generated hydrocarbons on the right, which are methane. The hydrogen to carbon ratio for this last figure, for this last residual kerogen is 0.47. So to summarize, in these two talks we've seen how maturity affects both the quantity and the quality of organic material and all these factors, quantity, quality, and maturity, will be combined into a future integrated source rock interpretation in an upcoming Knowledgette Presentation.