ARCHEAN OIL - EVIDENCE FOR EXTENSIVE HYDROCARBON GENERATION AND MIGRATION 2.5-3.5 GA

Archean sedimentary rocks from the Pilbara Craton, Australia, contain evidence for petroleum generation and migration in the form of bitumen nodules produced by radiogenic immobilization of fluid hydrocarbons a round detrital uraninite, thorite, and monazite grains. The nodules ar e preserved in sandstones at several stratigraphic levels in the Forte scue Group (similar to 2.75 Ga) and Lalla Rookh Formation (similar to 3.0 Ga), both nonmarine successions, and in deltaic sediments of the M osquito Creek Formation (similar to 3.25 Ga). The most ancient evidenc e comes from the Warrawoona Group (>3.46 Ga), where hydrocarbon drople ts were apparently formed in situ from kerogenous sediments by flash m aturation during early hydrothermal silicification. Bituminous relies of petroleum are also commonly preserved in shallow-marine sandstones of the Black Reef Formation (similar to 2.59 Ga) and the Witwatersrand Supergroup (similar to 2.85 Ga) from the Kaapvaal Craton, South Afric a, along with subeconomic methane accumulations. In all cases, the pet roleum was apparently sourced from Archean shales, generated during th e Archean, and migrated before the late Archean or early Early Protero zoic metamorphism occluded fluid pathways. Given this widespread and a bundant evidence for hydrocarbon generation and migration in Archean d epositional basins, it seems that primordial bacterial biomass, produc ing labile type I kerogen, was often buried in sufficient quantities t o successfully generate and expel petroleum. Depositional basins on an cient cratons clearly contained permeable rocks amenable to the migrat ion, and probably to the accumulation, of petroleum. Thus, the main fa ctors precluding the discovery of economically exploitable hydrocarbon accumulations in Archean basins are the subsequent destructive effect s of deformation and metamorphism, which causes trap breaching, imperf ect sealing, or thermal obliteration. However, there are ancient stabl e cratons where such disruption may not have occurred, and so petroleu m explorers may wish to reassess the possibility of finding valuable h ydrocarbon resources in Archean rocks.