Did oil and gas fields form during the Precambrian?

Since the origin of life it is certain that a proportion of biological materials would have been preserved in sediments after organisms died. As today, such material would have evolved or matured as the host sediments were buried and heated. There is plenty of evidence that such maturation did occur as far back as 3250 Ma ago, but signs that oil-fields formed by migration and trapping have proved elusive. Several lines of evidence, such as carbon-isotope anomalies in Precambrian limestones, point to periods when enormous amounts of organic material were buried, much as happens in the formation of Phanerozoic petroleum source rocks during periods of ocean anoxia. Before about 2400 Ma, when evidence for an oxidising surface environment first appears in the rock record, such conditions would have been pervasive. The first hints of large-scale petroleum formation and migration have been found in the low-grade Pilbara craton (3500-2850 Ma) of Western Australia and 2770-2450 Ma sediments that overlie the older Archaean complex (Rasmussen, B. 2005. Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales. Geology, v. 33, p. 497-500). Black shales in the Pilbara contain not only lots of fine-grained carbonaceous matter, but some in forms that clearly suggest that they had been thermally matured (‘cracked’) to low-viscosity fluids that could migrate. There are blobs of bitumen contained within iron sulfide layers that seem to have formed later, to engulf petroleum liquids. Molecules within the bitumens resemble those formed by photosynthesising blue-green bacteria, methanogen and sulfate-reducing bacteria and arguably perhaps primitive eukaryotes. It appears that the bitumens probably formed as residues as lighter and more fluid hydrocarbons migrated out of these substantial source rocks. What has yet to be demonstrated are Archaean and Palaeoproterozoic reservoir rocks where such migrating petroleum accumulated. Another question is whether or not the source rocks, which are extremely widespread and thick, might have retained some potential for sourcing petroleum much later in the geological history of Western Australia and similar cratons elsewhere.


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