Abstract
Rhenium–osmium (Re–Os) geochronology of marine petroleum systems has allowed the determination of the depositional age of source rocks as well as the timing of petroleum generation. In addition, Os isotopes have been applied as a fingerprinting tool to correlate oil to its source unit. To date, only classic marine petroleum systems have been studied. Here we present Re–Os geochronology and Os isotope fingerprinting of different petroleum phases (oils, tar sands and gilsonite) derived from the lacustrine Green River petroleum system in the Uinta Basin, USA. In addition we use an experimental approach, hydrous pyrolysis experiments, to compare to the Re–Os data of naturally generated petroleum in order to further understand the mechanisms of Re and Os transfer to petroleum.The Re–Os geochronology of petroleum from the lacustrine Green River petroleum system (19±14Ma – all petroleum phases) broadly agrees with previous petroleum generation basin models (∼25Ma) suggesting that Re–Os geochronology of variable petroleum phases derived from lacustrine Type I kerogen has similar systematics to Type II kerogen (e.g., Selby and Creaser, 2005a,b; Finlay et al., 2010). However, the large uncertainties (over 100% in some cases) produced for the petroleum Re–Os geochronology are a result of multiple generation events occurring through a ∼3000-m thick source unit that creates a mixture of initial Os isotope compositions in the produced petroleum phases. The 187Os/188Os values for the petroleum and source rocks at the time of oil generation vary from 1.4 to 1.9, with the mode at ∼1.6. Oil-to-source correlation using Os isotopes is consistent with previous correlation studies in the Green River petroleum system, and illustrates the potential utility of Os isotopes to characterize the spatial variations within a petroleum system.Hydrous pyrolysis experiments on the Green River Formation source rocks show that Re and Os transfer are mimicking the natural system. This transfer from source to bitumen to oil does not affect source rock Re–Os systematics or Os isotopic compositions. This confirms that Os isotope compositions are transferred intact from source to petroleum during petroleum generation and can be used as a powerful correlation tool. These experiments further confirm that Re–Os systematics in source rocks are not adversely affected by petroleum maturation. Overall this study illustrates that the Re–Os petroleum geochronometer and Os isotope fingerprinting tools can be used on a wide range of petroleum types sourced from variable kerogen types.
Highlights
The majority of large oil discoveries are sourced from marine shales associated with major ocean anoxic events of the Jurassic and Cretaceous (Klemme and Ulmishek, 1991)
This study presents Re–Os geochronology of various petroleum phases derived from the Green River petroleum system that broadly agrees with previous petroleum generation basin models (Ruble et al, 2001)
The results provide valuable insight into Re– Os petroleum generation geochronology of variable petroleum phases derived from lacustrine Type I kerogen, illustrating that the Re–Os systematics are similar to those in marine petroleum systems (Selby et al, 2005, 2007; Selby and Creaser, 2005a; Selby et al, 2007; Finlay et al, 2011)
Summary
The majority of large oil discoveries are sourced from marine shales associated with major ocean anoxic events of the Jurassic and Cretaceous (Klemme and Ulmishek, 1991) Such large discoveries are becoming increasingly rare and there is a need to understand the mechanisms of smaller petroleum systems and more accurately trace oils to their source in order to improve migration pathway models and reserve calculations (Hindle, 1997). The Os isotope fingerprinting tool uses the initial 187Os/188Os composition (Osi) of petroleum which reflects the source rock at the time of petroleum generation (Osg) providing a novel inorganic oil-source correlation technique (Selby et al, 2007; Finlay et al, 2011, 2012). Inorganic geochemistry provides a correlation tool where traditional organic methods may be hampered by biodegradation (Finlay et al, 2012)
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