- [Instructor] The next lectures in the series on basic petroleum source rock evaluation will address maturity. And there are going to be two parts to the discussion of maturity. First of all, part a, which will introduce maturity and the effect of maturity on TOC. And then part b, which will talk about the different ways of measuring maturity. In our previous talks on source rock evaluation, we've gone over quantity and quality of organic matter. As I said, in this talk we will discuss maturity. And maturity is very important, because it affects both the quantity and the quality of organic matter. And you can't accurately interpret the quantity and quality of organic matter without first knowing its maturity. Looking at the series of questions on source rock evaluation that we began considering in the first lecture, in this lecture we're going to emphasize two of them, which are, whether or not the source rocks are mature and have they already generated hydrocarbons? And then, if the source rocks have generated hydrocarbons, are those hydrocarbons more likely to be oil, condensate, or gas? What do we mean by maturity? Maturity is the extent of heat driven reactions that convert sedimentary organic matter into petroleum and, ultimately, into gas and graphite. When we discuss maturity, we want to know whether the source rocks have been heated sufficiently to generate oil and gas. And we can measure the extent to which hydrocarbons have been generated to oil and gas by both kerogen and bitumen maturity parameters. And recall that kerogen is the insoluble part of organic matter, while bitumen is the soluble part of organic matter. And, as I was saying earlier, maturity is very important because it affects the interpretation of all different types of geochemical data, and you can't accurately interpret geochemical data without knowing its maturity level. Looking first at how TOC varies with increasing maturity. With increasing maturity, as the kerogen generates and expels hydrocarbons, TOC decreases. And the TOC loss varies with kerogen type. And on this diagram, we have remaining hydrocarbon generation potential on the vertical axis and total organic carbon on the horizontal axis. And with increasing maturity, we lose total organic carbon and move towards this lower left corner. So, for Type I kerogen, which is generally lacustrine in origin, the loss may be as high as 70% as we move towards the lower-left corner and higher maturity levels. An example of a formation containing Type I kerogen is the Green River Formation. For a Type II oil prone kerogen, which is usually marine in origin, the loss in TOC can be as much as 50% as maturity increases and we move towards this lower left hand corner. The Mancos is an example of a formation that contains Type II kerogen. And then looking at Type III kerogen, which is generally of terrestrial origin and gas prone, the loss in TOC with increasing maturity can vary anywhere from 12 to 20%. So, once we get to this lower left hand corner, we can't tell whether we started off with Type I, Type II, or Type III organic matter. If we want to look at an example of the hydrocarbon generation potential of a source rock and how it decreases with thermal maturation, again on the vertical axis, we have remaining hydrocarbon generation potential and on the horizontal axis, we have total organic carbon in weight percent. And this is the maturation trend for the Barnett Shale. And you can see that we lose TOC, as well as remaining hydrocarbon generation potential, with increasing maturity of the Barnett Shale. And in the next part of our discussion on thermal maturity, we will look at three different ways of measuring thermal maturity. Vitrinite reflectance, thermal alteration index, or TAI, and hydrocarbon composition.