- [Janell] Now that the basics of source rock evaluation and analysis have been covered it is time to see how this information can be applied to an actual example of source rock evaluation. For this case study samples from the Eagle Ford formation will be used. All of these questions will be addressed in this Eagle Ford example. Combine TOC and pyrolysis analyses will be used to initially evaluate the quantity, quality, and maturity of Eagle Ford samples from three different wells. First shot field where the three wells of interest to this study are located are found in Southeast Texas. The three wells of interest are the Bell Sample number one, the Estrada et al number one, and the Robinson-Troell number one. These three wells are on the southwest flank of the San Marcos Arch and are updip from the Karnes Fault Zone. Note that the Bell Sample number one and Robinson-Troell number one are only about 10 miles apart. Using a TOC of two as the minimum value needed for a source rock to both generate and expel hydrocarbons, these TOC values show there are source rocks present in the Eagle Ford in all three wells under consideration. Using a T max of 435 degrees C as the onset of oil generation and a T max of 455 degrees C as the end of the oil window these T max values show that the Eagle Ford maturity in these three wells ranges from the early oil window in the Bell sample number one to peak oil generation in the Estrada et al number one to the late oil window in the Robinson-Troell number one. Cuttings from the Bell Sample number one generally have lower T max values than do the Bell Sample number one Eagle Ford core samples even though there are from about the same depths. This is something to watch and keep in mind as we proceed with the evaluation. Finally, although the Bell Sample number one and Robinson-Troell number one are only about 10 miles apart they have notable differences in thermal maturity. The difference in thermal maturity between these two wells cannot be explained by differences in depth to the Eagle Ford alone. There must be another factor that is also contributing to the differences in thermal maturity. This is a log-log cross plot of Eagle Ford S2 versus TOC data. Pyrolysis S2 the remaining hydrocarbon-generating potential is on the vertical axis and TOC is on the horizontal axis. Thus, this plot gives a quick overview of the quantity and generative potential of the organic matter in a source rock. For the Bell Sample number one core samples the TOC values generally range from good to very good and the S2 values typically range from fair to very good. The Estrada cuttings have TOC values extending from fair to very good and S2 values extending from poor to good. Interestingly, although more mature, the core samples from the Robinson-Troell number one well tend to have higher TOC values than the other two wells. Although, the S2 values just range from fair to good. In this plot 0.5 is used as the cutoff for fair TOC. However, this value is probably too low even though it is from Peters and Casa 1994 which is one of the few publications that gives classifications for TOC ranges. The average TOC for a marine shale is probably between 0.8 and 1%. For an immature source rock a TOC of 2% is probably needed to both generate and expel hydrocarbons while for a mature source rock a TOC of 1% is probably needed to generate and expel hydrocarbons. Thus, the value of 0.5 as the lower end of fair TOC is probably not accurate. Even factoring in TOC loss with maturation a higher minimum TOC than 0.5 is likely needed to generate and expel hydrocarbons. This is a linear-linear cross plot of the Eagle Ford S2 versus TOC data. The R squared values for all but the Bell Sample number one cutting samples are very good. R squared for the Bell Sample number one cuttings was not plotted because of the scatter on these data. Therefore, there continue to be a number of indications that there may be problems with the Bell Sample number one cuttings. Note that the Bell Sample number one core samples tend to plot in the mix two, three oil gas prone field even though an average hydrogen index of 402 indicates oil prone kerogen. Hydrogen index is 100 times the slope of the least squares regression line through these data sets. Similarly, an HI of 196 for the Robinson-Troell number one data points indicates these samples are just below an HI of 200 which is the lower cutoff for oil prone kerogen. However, the apparent variations in kerogen quality between the three wells is probably due to changes in maturity rather than to actual changes in kerogen type. This pseudo-Van Krevelen diagram for the Eagle Ford in these three wells tends to confirm the previous data. That is, the Eagle Ford core in the Bell Sample number one has the highest hydrogen index and contains the most oil prone kerogen while the Eagle Ford core on the Robinson-Troell number one has the lowest average hydrogen index value just below 200. Also, the Bell Sample number one cuttings have a lower average hydrogen index value than the Bell Sample number one core and there appears to be more scatter in the cuttings data. Another parameter that can be considered in the evaluation of the Eagle Ford data is the variation in the percent reactive carbon for the Eagle Ford in these three wells. This plot of percent reactive carbon versus depth shows the Bell Sample number one core samples have the most reactive carbon in them followed by the Estrada cuttings and then the Robinson number one core. These difference in percent reactive carbon, as with the differences in TOC and kerogen quality, are probably largely due to changes in maturity between the Eagle Ford in these three wells. That is, with increasing thermal maturity the reactive carbon is progressively converted to hydrocarbons leaving behind the larger residue of inner carbon. Again, notice how the Bell Sample number one cuttings seem to have lower percent reactive carbon than do the Bell Sample number one core samples. Thereby further confirming the hypothesis that there may be problems with the Bell Sample number one cuttings. To summarize, when interpreting TOC and pyrolysis data for source rocks you need to consider the type of samples being used. Are they core or cuttings? Also, look at the raw data especially the pyrograms. This is particularly true once the samples enter the oil window where bimodal S2 peaks due to the presence of generated hydrocarbons may be the case. Plot the data by stratographic units. Many service company labs do not do this, making it difficult to identify the true source rocks. Compare various plots of the data that present the data from different perspectives. How does maturity effect the data or plots. As can be seen from this example, maturity has an affect on TOC, kerogen quality, and the percent of reactive carbon. Finally, integrate all of the data and plots into an integrated interpretation. In terms of a source rock evaluation summary using TOC and pyrolysis data it is possible to obtain a quick overview of source rock quantity, quality, and maturity. Maturity especially needs to be taken into consideration because it can have an effect on both the quantity of organic matter present as well as the apparent quality of the organic matter. That is, with increasing maturity the kerogen quality can change from oil prone to gas prone. Finally, as shown in this example all these factors need to be combined into an integrated source rock evaluation program.