AN  INTEGRATED PETROLEUM  EVALUATION OF NORTHEASTERN  NEVADA


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40Ar/39Ar Age Spectrum
Interpretation
Summary
 

 

 

 

 

 

 

MICROCLINE PALEOTHERMOMETRY

Standard geochemical thermometry methods such as pyrolysis, thermal alteration index, and vitrinite reflectance do not accommodate the complexity of thermally reactivated basins or provide the temporal information necessary to reconstruct the timing of hydrocarbon generation events.

For this reason we have employed a 40Ar/39Ar age spectrum technique which provides low temperature paleothermometry data by detailing the radiogenic decay of potassium to argon in detrital microcline feldspar. By collecting and analyzing detrital microclines an assessment of the thermal evolution of a basin or area can be performed (Harrison and Be, 1983).

This technique relies on the closure temperature, the temperature below which radiogenic 40Ar is not lost from the lattice, of a K-Feldspar. This closure temperature has been experimentally determined to lie between 100 and 200 degrees centigrade (Harrison and McDougall, 1982; Harrison and Be, 1983).

In essence, potassium feldspars such as microcline lose substantial amounts of 40Ar at relatively low temperatures near 150 degrees centigrade over geological periods of time. Since these feldspars show diffusion gradients with known diffusion parameters, the age, duration and maximum temperature of a thermal event greater than the closure temperature can be assessed (Harrison and Be, 1983). Since these temperatures overlap the petroleum generation window from about 90 to 150 degrees centigrade, the age and duration of thermal events capable of hydrocarbon generation can be determined.

The following section discusses the general procedure and interpretation of results from the microcline paleothermometry data collected for this evaluation. The microcline sample descriptions, age spectrum plots, analytical data plots, inverse isochrons and arrenhius plots are provided in Appendix VIII. The reader is encouraged to refer to this appendix while reading this section.


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