Upwelling and Petroleum Source Beds, With Reference to Paleozoic

Abstract

Upwelling zones are areas of persistent high organic productivity in the oceans and represent one type of setting for the deposition of petroleum source beds. Upwelling currents are driven by winds associated with the major features of atmospheric circulation, and the locations of ancient upwelling zones can be predicted from global atmospheric circulation models. Qualitative circulation models for the past are now made possible by the availability of good reconstructions of past continental positions and paleogeography. Atmospheric circulation was modeled on paleogeographic reconstructions for the following Paleozoic stages: Franconian (Late Cambrian), Llandeilo-Caradoc (Late Ordovician), Wenlock (Late Silurian), Emsian (Early Devonian), Visean (Early Carboniferous), Wes phalian (Late Carboniferous), and Kazanian (Late Permian). Four types of persistent upwelling currents are recognized. Coastal upwelling currents (e.g., the Peru Current) are the most familiar and are the types usually cited as petroleum source-bed settings. However, three additional types of upwelling have also been important in the past, especially during times of high sea-level stands. They are associated with divergence under stable atmospheric low-pressure systems and comprise: (1) symmetrical divergence at the equator, (2) symmetrical divergence in wide oceans at about 60° lat. (e.g., around Antarctica), and (3) radial divergence in more restricted oceans at about 60° lat. (e.g., around the southern tip of Greenland). To the extent that the data allow reliable conclusions, the distribution of Paleozoic petroleum source beds appears to correspond closely to the distribution of predicted upwelling zones. Coupled with sea level models, the upwelling model can be particularly powerful, because while sea-level models predict likely times of source-bed deposition, the upwelling model contributes information on the sites of source-bed deposition.