INAN, SEDAT, and ALAN DAVIS, Pennsylvania State University, University Park, PA
Accurate determination of burial path, position of a sedimentary layer in time and space, and paleo-temperatures along that path are essential in modeling the thermal maturity of the sedimentary layer and thus, in estimating the timing and nature of generated hydrocarbons.
A set of confined pyrolysis experiments that simulate various burial paths in sedimentary basins has been carried out on a subbituminous coal (PSOC-1546, Dietz Seam, MT) to investigate the evolution of kerogen maturation. Mean max reflectivity of vitrinite in oil immersion [%R(0)] has been used as the thermal maturity indicator.
Results indicate that under continuous heating (progressive burial in natural systems), the role of temperature and time are, as expected, exponential and linear, respectively. At constant temperature (nondepositional hiatus), maturity increases linearly with time. However, at negative heating rate (cooling, uplift, and erosion in natural
system), increase in %R(0) is negligible (only 6% of what was measured at max temperature).
The significance of this observation is that, in natural systems, thermal maturity of kerogen is recorded at maximum temperature (generally maximum burial depth) and essentially ceases at this point.
A new technique that ties thermal maturity modeling with observed vitrinite reflectance is suggested for identifying the onset of the development of such an unconformity and estimating the thickness of eroded section at an unconformity (or unconformities). The basic hypothesis underlying this technique, which has been confirmed by pyrolysis experiments, is that the level of thermal maturity observed in sedimentary layers underlying an unconformity was reached prior to uplift and erosion. Simple maturity of sedimentary layer older than unconformity depends mainly on the temperature and time effective at the onset of uplift and erosion.
AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)