An Inorganic Thermal Maturity Indicator in the Gas-Condensate to Dry Gas Preservation Window: ⁴⁰Ar/³⁹Ar Feasibility Study

Peter Copeland, Christine Skirius, and Brenda Claxton

Core samples of Devonian Gilwood Sandstone and Shale from the Alberta Basin, North Central Alberta, Canada were used to investigate the feasibility of using ⁴⁰Ar/³⁹Ar age spectrum results as an inorganic indicator of thermal maturity. The core samples are from several wells in the Pine Creek and Kaybob South field areas and were chosen because (1) the samples are presently in the gas-condensate to dry gas preservation windows, (2) the sandstones contain appropriate feldspars for Ar analysis, and (3) the section contains interbedded shales for assessment of thermal maturity. ⁴⁰Ar/³⁹Ar age spectrum results within the thermal range (~130 degrees to ~250 C) from sandstone detrital feldspar grains are compared to vitrinite reflectance measurements from interbedded shales.

The temperature history of the samples as indicated by ⁴⁰Ar/³⁹Ar age spectrum results is correlated to vitrinite reflectance measurements in order to arrive at an inorganic thermal calibration. Such an inorganic thermal calibration can then be tested in areas or basins that do not have shales to derive organic data such as vitrinite reflectance measurements or in areas of very old sediments (~Silurian or older) where organic thermal maturity indicators are not well calibrated.

Unique advantages of the ⁴⁰Ar/³⁹Ar method for application to petroleum exploration within sedimentary basins are that it can simultaneously provide information about (1) the time of basin heating, (2) the temperature of a heating episode, (3) the duration of a heating episode, and (4) the age of the sediment source or provenance.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California