Thermal Indicators

The broad category called “Thermal indicators” includes a variety of measured parameters that can provide information about the thermal history of rocks. Geological events that can affect thermal history include burial, erosion, changes in surface temperature, igneous and hydrothermal events, groundwater flow, radiogenic heat production from sediments, and changes in basal heat flow through time. Although thermal indicators can be very useful, all of them have limitations and potentially serious weaknesses. Therefore, any application of thermal indicators in reconstructing geological and thermal history must begin with a critical evaluation of the validity of the indicators being used. Some thermal indicators, for example, require the presence of organic matter derived from terrestrial plants (vitrinite reflectance [Ro], TAI), which can severely limit their application in non-clastic sediments or in rocks older than Devonian. Another example is that evolutionary changes in plants in parts of Southeast Asia, Australia, and New Zealand during the Tertiary yield a special type of “perhydrous” vitrinite that does not increase in reflectance as quickly as normal vitrinite, thus leading to “suppressed” Ro values and underestimation of the degree of cooking the rocks have experienced.

Caution is essential in choosing the most appropriate thermal indicators for each specific case, and careful quality control of thermal indicator data should always be performed. We are particularly cautious about routine and universal usage of Ro data without the requisite QC, and thus recommend FAMM (e.g., Wilkins et al., 1992, 1995, 1997, 1998) and VIRF (Newman, 1997; Newman et al., 1997, 2000) as useful alternatives in cases where Ro is suspect.

We are also concerned about the misuse of Tmax derived from Rock-Eval pyrolysis. Tmax is calibrated to Ro only for Type III kerogens, but is often applied erroneously to kerogens of all types. The widely used conversion equation can lead to substantial errors because there is no exact universal relationship between Ro and Tmax. In addition, Tmax is susceptible to relatively large measurement uncertainty.

As an alternative or supplement to existing techniques, we recommend the use of the Mean Ea value derived from the source-rock kinetic analysis (Waples et al., 2002, 2010). This parameter must be calibrated individually for each kerogen or organofacies, but once calibrated it can provide accurate assessments of Transformation Ratio (TR) from 0 to about 0.95 (that is, 95% of hydrocarbons have been generated). Moreover, the TR values derived in this way can be easily correlated with Ro values. We derive the Mean Ea using data obtained from pyrolysis using our Source Rock Analyzer (SRA) and kinetic analysis using our proprietary ORFA software.

Much current unconventional exploration in the US focuses on Paleozoic rocks, where measured Ro values are often unreliable. In cases where the maturity is high because of deep burial or high heat flow, Mean Ea may not be fully satisfactory. In such cases we recommend that Conodont Alteration Index (CAI) be considered. Recent advances in linking CAI values to thermal history (Voldman et al., 2008) indicate that this technique may become much more useful than it has been in the past. We plan to include calculation of CAI values in our Novva software in the future.

StratoChem Services offers several types of thermal-indicator analysis in its own laboratory: Ro, TAI, and Mean Ea. VIRF can be obtained only through Jane Newman in New Zealand, while FAMM analysis is only performed by CSIRO in Sydney, Australia.

In addition, some biomarker parameters have been used as thermal indicators, although their history is spotty and most of them have a severely limited range of maturity over which they are applicable. Our favorite is the Ts/Tm ratio in the triterpanes, as determined by GC-MS analysis. However, this parameter, which in our experience is reliable and valid over a very wide range of maturity, must be calibrated for each kerogen type or organofacies. StratoChem provides Ts/Tm ratios routinely for all GC-MS analyses.

Rather than relying on a single molecular indicator of maturity, we prefer to use a “basket” of several indicators. We used this approach very successfully in our Three Forks Stain Study. The basket approach allows one to compensate for errors in any one indicator and to emphasize those indicators that function best at the level of maturity of each sample.

Other thermal indicators of some interest are apatite or zircon fission-track analysis (AFT, ZFT), fluid inclusion homogenization temperatures (FIHT), and gas-isotope data. We are happy to use those data in our interpretations if they are available.