OPTIMAL METHODS FOR ASSESSING MATURITY OF ORGANIC MATTER IN THE SUBSURFACE - WHY IT MATTERS

Thermal maturity estimates of organic matter (OM) distributed in shales have a direct impact on the geochemical and geomechanical modifications predicted in subsurface modeling. Temperature and burial profiles of OM bearing strata affect whether the OM has generated hydrocarbons, developed an organic pore network, established a microfracture network, and altered its geochemical framework.

Numerous techniques have been utilized to assess the thermal evolution of OM as it is subjected to burial and potential exhumation through geologic time. Optical microscopy of OM macerals using vitrinite reflectance measurements (R_o) were initially analyzed to determination maturation levels. Results are subjective to analyst interpretation and experimental technique. A more quantitative method involves pyrolysis of small samples using ramped temperature increases. One parameter measured after pyrolysis is the Hydrogen Index (HI), an estimate of remaining OM that may be converted into hydrocarbons. This information along with the total organic carbon (TOC), maximum temperature of the OM peak (TMAX), and an estimate of the original HI prior to burial can provide a more robust estimate of thermal maturity using a transformation ratio (TR) calculation.

This research involves comparing vitrinite reflectance measurements with geochemical pyrolysis results analyzed from the same sample. The New Albany Shale Group in the Illinois Basin contains significant amounts of OM and samples were collected from locations in both Illinois and Indiana. With exceptions for extremely low or elevated maturity levels, TR maturity calculations systematically exceed measured R_o values by a range between 15 – 25%. Predicted R_o calculations based upon the kinetic chemical transformation of vitrinite for a specific OM type exceed TR results by 20 – 35%. The disconnection between R_o and TR values is due to the kinetic thermal evolution of vitrinite compared with the whole kerogen. Additional methods utilizing aromatic markers, biomarkers, and NMR data are needed to constrain maturity estimates. Reasonable assessments of OM maturity will allow better prediction of sealing capacity of hydrocarbon-bearing intervals, aquifers, methane storage pools, and potential CO₂ injection and storage sites.