Zumberge, John¹, Roger Summons² 
(1) GeoMark Research, Ltd, Houston, TX 
(2) MIT, Cambridge, 

ABSTRACT: Crude Oils Provide Molecular and Isotopic Clues about OAEs

Understanding the carbon isotopic composition and molecular fossil distribution in crude oils generated from marine carbonate, marl, and shale petroleum source rocks is an expedient method for investigating the nature and distribution of ‘ocean-wide’ anoxic events (OAE) throughout the Phanerozoic. The evolution of microbial biosynthetic pathways is reflected in the changing composition of crude oil through geological time. Oils are widely available, accessible, abundant and carry comparable evolutionary information to that buried in sedimentary rocks. Also, oils integrate the natural variability in source rock organofacies. 
The oils examined were not only generated from source rocks deposited during the well-known Cenomanian/Turonian (CT) event, but also from possible older OAEs. Oil samples analyzed from the CT event are from the west coast of central to southern Africa, as well as oils from the Sub-Andean trend of northern South America and the Western Interior Seaway of North America. Surrounding the proto-North Atlantic Ocean, Kimmeridgian-aged oils in the North Sea, Porcupine Basin, and Grand Banks were generated from siliciclastic sediments while equivalent-aged oils from Portugal’s Lusitanian Basin, offshore Morocco, and southern Yucatan are from marl/carbonate facies. 
Oils generated during the Frasnian, basal Silurian, and Middle Ordovician are also compared. ‘Infracambrian’ oils from Oman, India, and East Siberia are highly depleted in ¹³C and have unique sterane distributions and may represent rocks deposited during the final ‘anoxic event’ of the Proterozoic.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.