Vladimir E. Andrusevich¹, Michael H. Engel², John E. Zumberge³
(1) The University of Oklahoma, Norman, OK
(2) The University of Oklahoma, Norman
(3) GeoMark Research, Inc, Houston, TX

Abstract: Constraining the Geologic Age of Crude Oils Using Their Stable Carbon Isotopes: New Endeavor at the Old Problem

After two decades of research, the general consensus amongst geochemists is that the stable isotopic compositions of bitumens, kerogens and crude oils are useful for intrabasinal correlations. However, attempts at using stable carbon isotopes for determining the geologic ages of crude oils and for constraining depositional environments of their respective source rocks on a global time scale have been largely unsuccessful. This may in part reflect a lack of paleogeographic constraints of the basins investigated. In a study of oils from Upper Jurassic petroliferous basins, we observe a strong correlation between the stable isotope compositions of the oils and the paleolatitudes of the basins from which they are derived. The stable isotope compositions of these oils are primarily controlled by differences in carbon isotope fractionation of the marine biomass (phytoplankton) associated with differences in paleotemperatures of surface water layers in the basins. For relatively small, restricted hypersaline basins, the influence of paleolatitude needs to be corrected by taking into account a greater warming of surface waters that results in lower solubility of carbon dioxide and a depressed fractionation effect during photosynthesis. The results of this interbasinal approach as a function of paleolatitude helps to explain the large range (5 per mil) of carbon isotope values for Upper Jurassic age oils. By correcting for paleolatitude, it may be possible to establish the geologic ages of crude oils based on their stable carbon isotope compositions.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana