Oil Shale Stratigraphy in Shallow Marine Vs. Lacustrine Settings — A Global Perspective

Dag Nummedal¹, Yuval Bartov¹, Rick Sarg¹, Jeremy Boak¹, and Alan R. Carroll^2
1Colorado Energy Research Institute, Colorado School of Mines, Golden, CO
²Department of Geology and Geophysics, University of Wisconsin, Madison, WI

Depositional controls on large oil shale deposits of the world
Four large oil shale deposits of the world (the largest?) were deposited in Eocene lakes of the U.S. Rocky Mountains (Green River Formation), Cretaceous shelves across the southern margin of the Tethys Ocean (Morocco to Egypt, Israel, Jordan and Iraq), Upper Permian lakes in NW China’s Junggar-Turpan-Hami basin (Lucaoguo Formation), and also Upper Permian lakes of South America and South Africa (Gondwana; Irati-Whitehill Formations). Together, these contain vastly greater oil resources than all the oil the world has consumed since the industrial era began.

These oil shale deposits (like hydrocarbon source rocks in general) share conditions of high productivity of organic material (mostly algae, with or without admixture of terrigenous organics) and anoxic bottom conditions for good preservation. Organic carbon concentrations are very high in all these four deposits, with common values of 15% to 20% TOC by weight, reaching a maximum of 35%. For the Cretaceous marine oil shales, these optimal conditions were attained in the very late Cretaceous (mostly in the very early Maastrichtian stage) during periods of rapid sea level rise which shifted the ocean anoxic zone far landward into fold-related minibasins with stagnant bottom conditions. Interbedded phosphorites probably represent the regressive and lowstand conditions.

The three large lacustrine oil shale deposits all occupy lakes in major orogenic zones (where most large lakes tend to form), and may - like the rich sapropels of the Black Sea today - record those particular conditions when near-surface organic productivity coincided with anoxic lake floors. These conditions appear to have occurred when marine waters started spilling over the Bosporus into the Black Sea to mix nutrients and increase productivity of organic-rich sapropel.

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