Print this pageA New Tool for Quantifying Thermal Maturity: Total Reflectance Spectrophotometry of Fossils
DEATON, BOBBY C., WILLIAM L. BALSAM, and PETER R. THOMPSON
Assessing thermal alteration of sediments and rocks is one of the more perplexing problems of diagenesis, especially in the temperature range of oil, gas, and dry gas generation. Commonly, thermal alteration in this temperature range is determined by changes in color of organic material in fossils such as pollen, conodonts, or ostracodes, or as free organic matter such as vitrinite. Relating these color changes to temperature leads to a subjective scale such as the conodont alteration index (CAI). Such scales are imprecise because color determination by the human eye is subjective. By using a total reflectance spectrophotometer we have devised a simple, non-destructive method for objectively determining thermal maturity, which was first applied to conodonts. By analyzing the reflectance of about 30 large conodont fragments we were able to relate quantitatively the average slope of the reflectance curve from 550-800 nm to CAI. This relationship is most accurate for CAI=1 to CAI=4 where the spectral slopes change rapidly. Using two conodont samples with known thermal maturities a relationship between spectral slope and in situ alteration temperature was also demonstrated. We have also analyzed foraminifera, permeated with organic material and subjected to low temperature thermal alteration. In these samples the slope of the spectral curve also changes as a function of the degree of thermal alteration. Using a second order regression equation we quantified this change and related down hole temperature to spectral slope. These studies suggest that total reflectance spectrophotometry provides a non-destructive, quantitative technique to estimate thermal maturity.