A New Look at Large-scale Evaporite Deposits: A Worldscale Exploration Paradigm

John K. Warren
Sultan Qaboos University, Muscat, Oman

Petroleum systems in Phanerozoic evaporite-filled basins are common in the Middle East, West Africa, South America, the Gulf of Mexico, the North Sea, Siberia, Russia and Canada. All these systems contain salt seals and evaporitic source rocks that were deposited at scales and in marine-associated tectonic-climatic settings that are poorly developed to non-existent in Quaternary depositional systems. In fact, study of evaporite systems of the last 2 million years leads to a conclusion that large evaporite systems are responses to continental hydrologies in dominantly supra-sealevel continent-continent collision belts. In any of these Quaternary-age examples there is no more than a poor potential for the future development of large-scale evaporite-associated petroleum systems.
In contrast, a worldwide compilation of ancient evaporite-associated petroleum systems shows they were dominantly subsealevel basins subject to hydrographic isolation from the marine realm at the time that thick salt beds were accumulating. Evaporitic source rocks tend to best developed in the restricted mesohaline parts of foreland basins or in the bottoms of density-stratified intraplatform sags. The largest of these evaporitic petroleum systems tend to be depositional responses to continent-continent collisions or to intracratonic sag phases following the rifting of a supercontinent. Stratiform salt seals of many such supergiant systems tend to be marine-fed sub-sealevel platform evaporites where greenhouse eustacy facilitated deposition. Regional draping of the salt seal atop earlier basement structures, along with gentle gravity glide subsidence, then created the most voluminous petroleum systems in the world. Somewhat smaller in terms of recoverable hydrocarbon volumes, but still giant systems, typify basinwide (aka basin-centre and megahalite) tectonic settings. There km-scale subsealevel drawdown drove marine-fed precipitation thick salt seals independent of greenhouse or icehouse eustacy. Subsequent loading and stretching lead to structuration that allowed hydrocarbons to accumulated in both subsalt and suprasalt settings.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece