Precise timing of petroleum migration

In their slack moments, petroleum geologists ponder on when oil and gas got into a particular reservoir and became trapped.  One aspect of the conundrum is easy to answer: after the reservoir rock and trap formed.  But timing is not so trivial, for an important consideration in exploration for new oilfields concerns the actual rock that sourced hydrocarbons in known fields, almost always a highly reduced, black mudrock in which unoxidised dead organic matter accumulated and matured. Repeated anoxic events, both regional and global, provide several alternatives in many petroleum provinces.

Hydrocarbons, having formed under highly reducing conditions, contain several metals and other elements well above normal crustal concentrations.  Among these are rhenium and osmium, which allow radiometric dating through the decay of 187Re to 187Os.  In principle, therefore, it is possible to date oil and relate it to a particular source rock. Interestingly, it is easier to date the actual time at which oil has accumulated in a trap.  In an analogous way to the equilibration of parent and daughter isotopes in magmas, which is halted by crystallization so that the system evolves and dating can be done, once oil settles in a trap after migration the timing can be dated sing the Re-Os method.  David Selby and Robert Creaser of the University of Alberta, Canada applied this approach for the first time, using the vast reservoirs of oil sand in Alberta as a test (Selby, D. & Creaser, R.A. 2005.  Direct radiometric dating of hydrocarbon deposits using rhenium-osmium isotopes.  Science, v. 308, p. 1293-1295). The oil in the sands were emplaced around 112 ±5 Ma ago, during the Early Cretaceous, not long after the host sandstones had been deposited.  Previous work using ideas on oil maturation suggested that migration had taken place during the Early Palaeocene, around 60 Ma ago, when potential source rocks were heated by tectonic burial during the Laramide orogeny.  The Re-Os results point to migration from the west while the Cretaceous sedimentary basin was filling.  This may explain the high viscosity of the oils as a result of near-surface biodegradation.

Another product of isotopic dating is establishing the initial 187Os/188Os ratio of the petroleum system, which relates to that of the original source and its isotopic evolution.  In the case of the oil sands this value points to source rocks of earlier Mesozoic and even Palaeozoic age, rather than a Cretaceous source that had been suggested previously.

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