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Evidence for pervasive petroleum generation and migration in 3.2 and 2.63 Ga shales

¹ School of Earth and Geographical Sciences, University of Western Australia, Crawley, WA 6009, Australia

Petroleum generation largely occurs through the thermal decomposition of organic matter. The presence of oil-bearing fluid inclusions and pyrobitumen in Archean rocks suggests that similar processes operated as early as ca. 3.25 Ga. However, direct evidence of petroleum generation from potential source rocks is lacking, and an abiogenic origin has been proposed for some Archean carbonaceous residues. Pilbara craton ca. 3.2 Ga and ca. 2.63 Ga black shales were found to contain abundant kerogenous streaks and laminae, as well as bitumen nodules (comprising a radioactive mineral core surrounded by a carbonaceous rim) and pyrobitumen (formerly petroleum) globules, films, and aggregates. The bitumen nodules formed around detrital radioactive grains via polymerization of fluid hydrocarbons generated within the shale and represent diagnostic indicators of oil generation in ancient shales. The bitumen globules, films, and masses are preserved within authigenic pyrite and demonstrate that a separate hydrocarbon phase had developed in the shale matrix during burial, providing compelling evidence for in situ petroleum generation and expulsion. The abundance of bitumen nodules and residual pyrobitumen in black shales across the Pilbara craton suggests that hydrocarbon generation from kerogenous shales was a common phenomenon during the Middle to Late Archean. The petroleum was generated from organic matter that accumulated in marine environments, most probably comprising the remains of photosynthetic and chemosynthetic organisms, pointing to a sizeable biomass as early as 3.2 Ga.

Key Words: petroleum generation • organic matter • black shale • pyrobitumen • Archean biosphere • Pilbara craton

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