Oil preserved in fluid inclusions in Archaean sandstones

Oil is generally thought to be geologically young, as it is thermodynamically unstable when subjected to elevated temperatures over long periods in open systems1,2. Indeed, almost all petroleum production comes from rocks younger than 400 million years (ref. 3). Although the oldest known oil occurs in rocks 1,650 Myr old4, suitable source rocks were abundant in older geological successions5 and circumstantial evidence suggests that some of these generated hydrocarbons early in their history6. Here, we report the discovery of oil preserved in fluid inclusions in sandstones dating back ∼3,000 Myr. Most inclusions lie within healed microfractures confined to individual detrital quartz grains, indicating that their oil was emplaced before Archaean or Palaeoproterozoic metamorphism sealed all voids and thus came from older (in some cases Archaean) sources. The fluid inclusions apparently acted as inert pressure vessels that protected the oil from subsequent degradation by circulating fluids and mineral catalysts. Because of its great age, this oil can potentially yield valuable information about the size and diversity of the early biosphere, particularly if it contains molecular fossils (biomarkers) of the primordial organisms from which it was derived.