THERMAL MATURITY TRENDS IN WAXY LACUSTRINE CRUDE OILS DETERMINED FROM BIOMARKER ANALYSIS

Highly paraffinic or waxy oils are often generated in and produced from lacustrine petroleum systems. Such oils present unique analytical challenges for characterization by commonly used gas chromatographic methods, particularly when paraffinic components dilute non-alkane compounds of interest like molecular biomarkers. In this study, we examined the bulk geochemical properties, normal alkanes and isoprenoids, and other molecular biomarkers present in waxy lacustrine crude oils produced from the Eocene Green River Formation in the Uinta Basin of Utah. Most of the samples (13/14) were produced from the informal Uteland Butte member, which represents a transgressive freshwater lacustrine stage of Lake Uinta. One sample was produced from the Mahogany zone in the upper, more saline lacustrine interval of the Green River Formation. The Uteland Butte ranges in thickness from 50 to 300+ ft and contains limestone, dolostone, and organic-rich mudstone and siltstone, with sandstone and ostracodal limestone in marginal areas. The oils produced from the Uteland Butte contain high concentrations of long chain normal alkanes (up to C40) causing them to solidify under ambient conditions. The waxes vary from black to yellow based on thermal maturity and present a range of production, transportation, and refining difficulties, along with analytical challenges. In this study, thermal maturity was determined based on bulk geochemical properties, as well as analysis of whole oils and saturate fractions by gas chromatography coupled with flame ionization detection. These analyses indicate that Uteland oils represent the early to mid-late oil window. To remove interferences and facilitate biomarker analysis by GC coupled with mass spectrometry (GC-MS), a dewaxing procedure using boiling methanol was applied to the samples to separate the paraffin material and concentrate biomarkers. Several biomarker ratios determined for the dewaxed maltenes, including those derived from pristane and phytane, trisnorhopanes, and various sterane parameters, show trends that correspond to results from bulk geochemistry. This combination of traditional bulk geochemistry and GC-MS data collected after paraffin removal provides a thorough and consistent assessment of thermal maturity in highly paraffinic lacustrine oils.