Temperature and Time in the Thermal Maturation of Sedimentary Organic Matter

Abstract

Models that consider temperature alone or a combination of temperature and heating duration have been defined for the thermal maturation of sedimentary organic matter (OM). Both types of models appear to give adequate maximum paleotemperature estimates, but these estimates are imprecise due to problems in measuring thermal maturity (rank), heating duration, and maximum temperature (T max). Must temperature and functional heating duration (t) be considered or is T max alone sufficient to characterize thermal maturation to the precision level now possible in sedimentary environments? This question is addressed by developing a temperature-heating duration model for OM thermal maturation based on a broad geological data base and comparing it to other empirical models that consider T max alone or T max and t. If a first-order reaction can be assumed for OM thermal maturation, then its reaction rate constant, k, is equal to -ln(f)·(1/t), where t is the functional heating duration and f is the fraction of reactable OM. The fraction of reactable OM (f) is the complement of transformation ratio (r) estimated from Tissot and Espitalié’s 1975 model of vitrinite reflectance (R m ) evolution. The functional heating duration for burial diagenesis is calculated by determining the elapsed time as temperature increases within 15°C of T max without exceeding the time necessary for the controlling reactions to approach completion. Geologic field data and OM thermal maturation experiments extrapolated to a geologic time and temperature range suggest OM thermal maturation reactions approach completion in about 106 to 107 years during burial diagenesis. The elapsed time near T max is used as the reaction time in geothermal systems and contact metamorphism by intrusive sheets. The calculated reaction rate when plotted on an Arrhenius diagram [ln k versus absolute temperature (1/T)] falls along three subparallel straight-line segments. These segments correspond to OM thermal maturation in three different environments: burial diagenesis, geothermal systems, and contact metamor-phism by intrusive sheets. These environments were individually analyzed by linear regression of ln(k) and 1/T data which for a single reaction mechanism conform to a straight line, ln(k) = (−E a /R)1/T + ln A, where E a is the activation energy for the reaction, R is the gas constant, and A is the Arrhenius factor. OM thermal maturation in burial diagenesis shows a strong linear relationship (correlation coefficient, r = −0.84) with an E a of 9 kcal/mol (38 kJ/mol) and an A of 7 × 10−11/s. OM thermal maturation in geothermal systems shows a moderate linear relationship (r = −0.74), an E a of 11 kcal/mol (46 kJ/mol), and an A of 2.5 × 10−7/s. OM thermal maturation from contact metamorphism by intrusive sheets is modeled by a linear relationship (r = −0.64), with an E a of 12 kcal/mol (50 kJ/mol), and an A of 1.5 × 10−3/s. In summary, these data conform to lines with a uniform and similar slope, indicating that OM thermal maturation consists of a pseudoreaction that has a similar E a in each environment. OM thermal maturation differs in burial diagenesis, geothermal systems, and contact metamorphism by intrusive sheets primarily in the reaction rate at a given temperature. This may be due to differences in pressure between these environments. The different reaction rate in each environment is also expressed in the time required for OM thermal maturation to stabilize. The regression line calculated for geothermal systems overlies a T max-R m model that is based on geologic evidence that indicates t was not important after 104 years. The regression line from contact metamorphism by igneous sheets is in fair agreement with data from laboratory experiments that indicate stabilization after 10−1 to 10° years. Comparison of these calculated regression curves to published models of OM thermal maturation indicates that precision is not increased by considering heating duration. The data can be adequately modeled by considering T max alone.