PETROLEUM IN GLACIALLY-RELATED SANDSTONES OF GONDWANA: A REVIEW

The supercontinent of Gondwana (Antarctica, Africa, Australia, South America, India, the Arabian Peninsula, Madagascar and Sri Lanka) covered more than six times the combined area of the United States and Brazil. In this paper, the petroleum occurrences and potential of glacially-related rocks of Carboniferous-Permian age in this vast area are reviewed in terms of basin types, reservoirs, seals, traps and source rocks. Significant amounts of heavy oil, sourced by Precambrian algal-rich shales, are produced from glacially-related sandstones in some 20 fields in Oman. Bolivia has 27 oil- and gasfields sourced from underlying Devonian shales, and oil production occurs in two fields in nearby Argentina. In Australia, there is production from two basins: the Cooper Basin has 24 productive fields (mostly gas), and the Canning Basin has six small fields. In Brazil, four non-commercial shows have been found in the Paraná Basin and one in the Amazonas Basin. In Paraguay, southern Africa and India, there is no production from glacially-related sandstones. Production and shows occur in cratonic basins either in the interior or along the margins of Gondwana, except for those in Bolivia, Argentina and South Africa, which occur in foreland basins. Sandstone reservoirs were deposited mostly as glacial outwash, as small deltas, either in lakes or marine basins, or as turbidites in marine basins bordered by continental ice sheets. Seals are diamictites, mudstones, shales and possibly diabases in the Paraná Basin. Traps include tectonic and salt-related anticlines as well as pinch-outs against glacial channels. Glacial outwash and glacio-marine sandstones are thus important petroleum exploration targets. Commercial production to date is sourced from both older and younger non-glacial beds. The fields in Oman were sourced by Precambrian algal-rich shales, and those in Bolivia and adjacent Argentina by Devonian and possibly Silurian organic-rich shales. The source of the gas in the Cooper Basin is from overlying coal beds. Neither modern marine, high-latitude muds nor ancient glacially-influenced marine shales contain sufficient organic material to be considered adequate source rocks. This review indicates that petroleum production in Gondwana's glaciogenic deposits mostly occurs in cratonic and passive margin basins; it occurs wholly in sandstone reservoirs, chiefly in structural traps, and nearly everywhere, reservoirs are sourced by hydrocarbons from non-glaciogenic shales (either older or younger). The only “glacially unique” features of petroleum in Gondwana deposits are its seals of diamictites, which surely must also be true when considering the petroleum possibilities of older ancient glacial and glacially-influenced deposits.