Calibrating Stable Carbon Isotopes of Reservoir Fluids as a Thermal Maturity Indicator

Dolan, Michael¹, Anne Grau², Kevin Ferworn³, Stephen W. Brown<sup>3

1</sup>Dolan Integration Group, Louisville, CO
²EOG Resources Inc, Denver, CO
³GeoMark Research, Ltd, Houston, TX

     Organic thermal maturity indicators are used to determine the maximum level of maturity for a given rock unit. Thermal indicators such as measured vitrinite reflectance, thermal alteration index (TAI), and the Rock-Eval ™ parameter Tmax measure the rock unit directly and indirectly and require core or cuttings for analysis. Stable carbon isotopic analysis of mud gas, production gas and headspace gases provides the opportunity to measure the maturity of reservoir fluids. Measuring the maturity of reservoir fluids in conventional and unconventional plays allows for a more complete interpretation of the petroleum system elements such as source maturity, migration of hydrocarbons, and charge history and timing. In the case of shale gas plays understanding the maturity of the fluids can be a proxy for the maturity of the shale itself if there is no migration of the gas out of the rock, into the rock, or the gas within the source rock is a risiduum. Mud Gas Isotope Analysis (MGIA) is a technique that allows sampling of the mud stream gases while drilling to measure d¹³C isotopic concentrations of C1-5 components. d¹³C ethane and d¹³C propane are good thermal maturity indicators and can be derived from MGIA. These maturity parameters can be used qualitatively to understand relative maturity of the fluid/rock or, if calibrated to shale rocks, can be used as a robust quantitative thermal maturity parameter. Calibrations can be achieved using analogous systems from a global thermal maturity database or from basin and formation specific data. The objective of this talk is to provide information regarding limitations of global rock maturity data when calibrating basin and formation specific fluid maturity parameters. Also, the use of the gas maturity parameter in assessing conventional and unconventional hydrocarbon systems in the Rocky Mountains will be discussed.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah