AAPG Bulletin, V. 82 (1998), No. 1 (January 1998), P. 50-69.

Archean Oil: Evidence for Extensive Hydrocarbon Generation and Migration 2.5-3.5 Ga¹

Roger Buick,² Birger Rasmussen,³ and Bryan Krapez³
 

© Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved
 

¹Manuscript received April 24, 1996; revised manuscript received December 9, 1996; final acceptance June 16, 1997.
²Department of Geology and Geophysics, University of Sydney, Sydney, New South Wales 2006, Australia.
³Key Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia.

We thank G. Ellis, P. Hunter, Marathon Petroleum, Mt. Leyshon Gold Mines, D. I. Groves, and the E. deC. Clarke Museum for samples; M. J. Lemos de Sousa for photographs; T. S. Blake and K. R. Barnes for stratigraphic advice and field support; R. D. Müller for mathematical modeling; I. J. O’Brien for technical assistance; D. I. Groves, A. H. Knoll, R. A. Alexander, and P. A. DiBona for criticism; and J. A. Curiale, W. A. Young, and K. T. Biddle for reviews. This work was supported by an Australian Research Fellowship and ARC Grant A39600061 (RB), MERIWA Grant M244 (BR) and a UWA Research Fellowship and AMIRA-ARC Research Grant (BK). 

ABSTRACT

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 around detrital uraninite, thorite, and monazite grains. The nodules are preserved in sandstones at several stratigraphic levels in the Fortescue Group (~2.75 Ga) and Lalla Rookh Formation (~3.0 Ga), both nonmarine successions, and in deltaic sediments of the Mosquito Creek Formation (~3.25 Ga). The most ancient evidence comes from the Warrawoona Group (>3.46 Ga), where hydrocarbon droplets were apparently formed in situ from kerogenous sediments by flash maturation during early hydrothermal silicification. Bituminous relics of petroleum are also commonly preserved in shallow-marine sandstones of the Black Reef Formation (~2.59 Ga) and the Witwatersrand Supergroup (~2.85 Ga) from the Kaapvaal Craton, South Africa, along with subeconomic methane accumulations. In all cases, the petroleum was apparently sourced from Archean shales, generated during the Archean, and migrated before the late Archean or early Early Proterozoic metamorphism occluded fluid pathways.

Given this widespread and abundant evidence for hydrocarbon generation and migration in Archean depositional basins, it seems that primordial bacterial biomass, producing labile type I kerogen, was often buried in sufficient quantities to successfully generate and expel petroleum. Depositional basins on ancient cratons clearly contained permeable rocks amenable to the migration, and probably to the accumulation, of petroleum. Thus, the main factors precluding the discovery of economically exploitable hydrocarbon accumulations in Archean basins are the subsequent destructive effects of deformation and metamorphism, which causes trap breaching, imperfect sealing, or thermal obliteration. However, there are ancient stable cratons where such disruption may not have occurred, and so petroleum explorers may wish to reassess the possibility of finding valuable hydrocarbon resources in Archean rocks.