--> Lead Isotope Analyses of Crude Oils and Their Source Shale Beds Could Prove Useful in Determining the Time of Formation of Oil

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Lead Isotope Analyses of Crude Oils and Their Source Shale Beds Could Prove Useful in Determining the Time of Formation of Oil

Abstract

Crude oils produced from the Upper Devonian-Early Mississippian Woodford Shale and organic fractions of the source shale beds were analyzed for their lead isotopic compositions. The samples analyzed for this study all came from production fields in Payne County, north-central part of Oklahoma. The 207Pb/204Pb and the 206Pb/204Pb ratios of the organic components of the source shale samples yielded a well-defined linear trend (R2= 0.98), the slope of which gives a date of about 340 million years, corresponding to Late Mississippian time. The crude oil samples were found with a very a narrow range of the lead isotope values, unlike the range of values that were found for the organic fractions of the shale samples. Remarkably, the lead isotopic values of the crude oil samples lie directly on the linear trend defined by the lead isotopic compositions of the organic fractions of the shale samples. The 340 million years date represents a close approximation to the time of formation of crude oils in the Devonian-Early Mississippian Woodford Shale source rocks in the Payne County, Oklahoma. The evidence produced by this study is the very first of the kind pointing to a direct evidence on the time of formation of oil in a source shale bed. This study potentially opens a new window to a deeper and further understanding of the origin of oil.