- [Instructor] Now that we have covered the importance of determining the quantity, quality, and maturity of organic matter in basic source rock evaluation, it is time to discuss how we obtain this information. Although there are many different geochemical analyses that can be performed, running total organic carbon and pyrolysis analyses upfront can provide much of the basic information needed to help estimate the quantity, quality, and maturity of organic matter in source rocks. With respect to the quantity of organic matter in the source rocks, Total Organic Carbon Measurement by the LECO Carbon Analyzer is one of the more commonly used analyses. For this analysis, about one gram of sample is needed. The samples are powdered, weighed, and treated with acid to remove the inorganic carbon, that is the carbonate minerals, from the sedimentary rock. The TOC values are then determined by combusting the organic carbon and measuring the resulting carbon dioxide produced. The amount of carbon dioxide produced is directly proportional to the total organic carbon in the rock. However, total organic carbon values can be inflated by the presence of sulfur compounds, water, and carbonate minerals if they have not been removed prior to the LECO analysis. The TOC measured by the LECO method also does not include the free hydrocarbons in the sample. Most of the free hydrocarbons are volatilized when samples are dried after acid treatment. Although a number of pyrolysis instruments have been developed that do total organic carbon as well as pyrolysis, many people still request LECO TOC. Why are people interested in TOC? As shown in this diagram, it's because TOC is often related to the amount of hydrocarbons generated. If we look at this diagram which is for the Chattanooga shale, where gas yield is on the vertical axis, and total organic carbon is on the horizontal axis, you can see that there is an R squared of 0.7642 showing a relationship between gas yields and total organic carbon values. In terms of getting quality and maturity estimates, a commonly used analysis is Rock-Eval Pyrolysis. In rock-eval pyrolysis, about 100 milligrams of ground sample is put in a crucible and inserted into an oven in the pyrolysis instrument. If we look at the axes on this diagram, we see that on the vertical axis is the oven temperature, and on the horizontal axis is the time of the analysis. Each analysis takes approximately 20 minutes. The dark line shows the temperature program that is used in each analysis. Starting at 300 degrees C, S1, which is the volatile hydrocarbons, that is the hydrocarbons already generated, come off. Then as the temperature increases, S2 which is the hydrocarbons derived from kerogen pyrolysis, that is the remaining hydrocarbon generation potential, come off, and finally S3 is the peak that shows the total CO2 derived from kerogen pyrolysis peaks. Tmax is the oven temperature at which the maximum S2 yield is obtained. You can also get hydrogen index, which is S2 divided by TOC, times 100. And oxygen index, which is S3 divided by TOC, times 100, from this analysis. In this particular pyrogram, the S2 peak is well defined in this example. But you can have bimodal S2 peaks, and as the sample gets more mature, the S2 peak becomes broader, and flatter. You always want to request the pyrograms when having pyrolysis analysis done. It is especially important to examine the shape of the S2 peak when analysis evaluating the S2 measurement. Also examine the pyrogram to determine if contaminants might be present. Bimodal S2 can result in lowered Tmax values, and may also indicate the presence of the heavy ends of the hydrocarbons that have been generated. And by heavy ends we mean compounds such as asphaltenes. To obtain maturity by rock-eval pyrolysis, there are a number of things you can look at. In the diagram on the far left, it shows how the S1 and S2 peaks change as maturity changes, from immature at the top of the diagram, through the oil zone, and then to the top of the wet gas zone, at the bottom window. The S1 peaks, that is the hydrocarbons already generated, increase with increasing maturity. And with increasing maturity, S2, that is the remaining generative potential, decreases. And the shape of the S2 peak changes. The S2 peak also shifts to the right with increasing maturity, resulting in an increase in Tmax with increasing thermal maturity. Looking at the diagrams on the right, it shows different pyrolysis parameters that can be used to indicate maturity. In the top diagram, the production index, which is S1 divided by the sum of S1 plus S2, increases with increasing maturity. The oil window extends roughly from a production index of 0.1 to a production index of 0.4. On the bottom diagram, it is shown how Tmax increases with increasing maturity. Remember, Tmax is an analytical parameter, it is not the maximum temperature to which the sample was exposed to in the subsurface. The oil window starts approximately at a Tmax of 435 degrees C, and ends at a Tmax of about 455 degrees C. You have to be careful with this though, because the oil window as defined by Tmax varies with kerogen time. This is another reason for an integrated interpretation. Almost nothing in geochemistry is hard and fast. We're just looking at approximate guidelines. You need to be especially careful to compare various maturity parameters and remember you can get yourself in a lot of trouble by trying to cookbook an interpretation. This next slide shows a comparison between a Van Krevelen diagram in the top, which is generated by elemental analyses, and a modified Van Krevelen diagram on the bottom which is obtained by pyrolysis and TOC analyses. On the Van Krevelen diagram, the atomic hydrogen to carbon ratio is on the vertical axis, and on the horizontal axis is the atomic oxygen to carbon ratio. On the modified Van Krevelen diagram, hydrogen index is on the vertical axis, and oxygen index is on the horizontal axis. And hydrogen index and oxygen index as shown by the equations on the left, are generated from rock-eval pyrolysis and TOC analyses. For these two diagrams the same samples were used in each one, and in general the four rock samples in these two diagrams each contain only one kerogen type. For example, the Eocene, Green River formation contains only type I kerogen, and the Tertiary samples from Greenland contain only type III kerogen. In reality though, most rocks contain a mixture of kerogen types. In both diagrams, the maturation pathways for the three kerogen types are shown in black lines. At the end of maturation, all three kerogen types end up in the lower left hand corner of the diagrams. Hydrogen index versus oxygen index data can be generated more rapidly and at less expense than the elemental analyses needed for atomic hydrogen to carbon versus oxygen to carbon, yet as you can see the results on both diagrams are roughly comparable. Vitrinite reflectance, percent R0, and TAI, the thermal alteration index, are shown on the Van Krevelen diagram at the top. You have to be careful in comparing vitrinite reflectance and TAI though, because they may vary from one lab to another. This next slide shows how at high maturities, the pyrolysis S2 peak becomes flat. This sample has an S2 of 2.02, and a flat S2 peak. Thus, this sample has little if any remaining potential left to generate hydrocarbons. Examination of the pyrogram shows a flat S2 peak, making the Tmax value of 491 unreliable. That is, where along the flat S2 peak should Tmax actually be measured. So, by using the combination of total organic carbon and pyrolysis data, it is possible to rapidly and inexpensively obtain an initial estimate of the quantity, quality, and maturity of the organic matter in a source rock. However, additional analyses such as vitrinite reflectance are needed to confirm the maturity of the organic matter and in turn the total organic carbon value and kerogen type. Now that the basics of source rock evaluation have been presented, the next lecture will get to the bottom line, that is, an example of how to interpret and apply TOC and pyrolysis analyses to a specific source rock, and how that interpretation affects the economics of a play.