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Petroleum is not considered a rock, but it is formed by a process, so it has an age.

What methods do geologists employ to determine the age of a petroleum deposit?

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Crude oil is a product of partial degradation of organic matter, e.g. of plants that lived long time ago. During photosynthesis, they constantly incorporate carbon (from $\ce{CO2}$) to accumulate biomass. But carbon is not uniform, it consists of different isotopes, and a tiny fraction of radioactive $\ce{^{14}C}$. Slowly, over time (with a half-life of $\pu{5730 y}$, ref.), $\ce{^{14}C}$ decays and yields $\ce{^{14}N}$.

While alive, decaying $\ce{^{14}C}$ is replenished constantly, but not so for dead plants buried in the mud. So radiocarbon dating, the determination of the relative amount of still present $\ce{^{14}C}$ compared to the isotopes $\ce{^{12}C}$ and $\ce{^{13}C}$ allows to compute the time interval since the plants died. It may be by recording remaining radiation, or by mass spectroscopy; the two methods complement each other.

Because other elements' isotopes equally undergo nuclear reactions with other half-life times, the analysis of isotopic composition for the sake of geochronology need not be restrained to carbon.


As a side note, the isotopic fingerprint of a sample, not necessarily (only) on the radioactive isotopes, equally allows to assign the geographic origin of (in geological time scale) contemporary samples, too. With applications in food science, forensics, jewelry (provenance of natural vs. artificial gemstones [an elder example]), etc.

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  • $\begingroup$ Isn't crude oil too old for carbon-14 dating to work? Radiocarbon dating is only good for 50,000 years or so, while oil is generally accepted to be far older than that. $\endgroup$
    – Mark
    Jul 2 at 1:09
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They microscopically examine drill debris to determine if they are in the target formation , typically coral/marine shells. This is not done for all wells but for exploratory and step-out wells ( expanding a field). Primary interests are gas/oil ratio, hydrocarbon/water ratio, sulfur level. Age is only of interest as it gives information of the other properties. They have a good idea of the formations and ages before they start drilling. Exploratory and in-fill wells would be handled very differently. Drillers may refer to infill wells as "post holes". I have been retired for awhile but a lot of progress was/is being made in "measurement while drilling " so they know where the bits are and where it is going in real time so microscopic exam may no longer be as important

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