Abstract
Although Pb, U, and Th may be fractionated between crude oil and formation waters, Pb isotopes are not. This unique property makes Pb isotopes a particularly useful marker of hydrocarbon generation and migration. Here we show that Pb isotopes offer a new vision of long-range (secondary) oil migration relevant to the formation of oil fields. North Sea oils are largely generated from Jurassic black shales, yet their Pb isotopes are mixtures of Cenozoic to Proterozoic end-members. The same observation is made for crude oils from the Paris Basin, the Barents Sea, Libya, Kuwait, Kazakhstan, and Australia. Bulk Pb in crude oil therefore, for the most part, is foreign to its source rock(s). Our high-precision Pb isotope data on 195 crude oils worldwide, the first such data set in the published literature, and 17 Northern European black shales indicate that deep-seated Pb components originating beneath the source rocks are ubiquitous in crude oil. This implies that oil fields are embedded in basinal convective systems of hydrous fluids heated from below. Plumes of hot fluids rise from the lower thermal boundary layer, which Pb isotopes require douse the basement, into the core of the porous-flow convective cell where they dissolve the newly formed hydrocarbons sequestered in the source rocks. The fluids finally unload unmixed formation waters and crude oil at the base of the upper (conductive) boundary layer where they can be trapped in favorable sites. Based on these new insights we argue that Pb isotopes in crude oil constitute a good tracer of oil migration.
Highlights
Petroleum formation requires at least two distinct stages: hydrocarbon formation from organic compounds present in the source rock and migration from the source rock into reservoirs
Our high-precision Pb isotope data on 195 crude oils worldwide, the first such data set in the published literature, and 17 Northern European black shales, as well as U, Th, and Pb concentration data on 36 of the crude oils and all 17 black shales, shed new light on oil migration processes
The Pb isotope data require that Pb in crude oils systematically is a mixture of components of different ages ranging from Cenozoic to Proterozoic, and that ingrowth of radiogenic 206Pb, 207Pb, and 208Pb is significant since the parent-daughter ratios U/Pb and Th/Pb of the analyzed oils are much higher than anticipated
Summary
Petroleum formation requires at least two distinct stages: hydrocarbon formation from organic compounds present in the source rock and migration from the source rock into reservoirs. The limited amount of published data on trace elements and radiogenic and heavy metal isotopes in crude oil bear out both the analytical challenge and lack of information and first-order patterns that could improve the understanding of oil generation. Crude oil is notoriously difficult to mineralize and represents a daunting matrix problem for mass spectrometric techniques that require a high degree of element purity. Water-hydrocarbon unmixing at low temperature (Maczynski et al, 2004) is the most likely cause of the large range observed for metal abundances (e.g., Ventura et al, 2015).
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