OIL PRESERVED IN FLUID INCLUSIONS IN ARCHEAN SANDSTONES

Oil is generally thought to be geologically young, as it is thermodyna mically unstable when subjected to elevated temperatures over long per iods in open systems(1,2). Indeed, almost all petroleum production com es from rocks younger than 400 million years (ref. 3). Although the ol dest known oil. occurs in rocks 1,650 Myr old(4), suitable source rock s were abundant in older geological successions(5) and circumstantial evidence suggests that some of these generated hydrocarbons early in t heir history(6). Here, we report the discovery of oil preserved in flu id inclusions in sandstones dating back similar to 3,000 Myr. Most inc lusions lie within healed microfractures confined to individual detrit al quartz grains, indicating that their oil was emplaced before Archae an or Palaeoproterozoic metamorphism sealed all voids and thus came fr om older (in some cases Archaean) sources. The fluid inclusions appare ntly acted as inert pressure vessels that protected the oil from subse quent 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.