--> ABSTRACT: Use of the Hydrogen and Carbon Stable Isotopes in the Characterization of Oils from the Potiguar Basin (Onshore), Northeastern Brazil, by E. V. Santos Neto, J. M. Hayes, and L. A. Trindade; #91019 (1996)

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Use of the Hydrogen and Carbon Stable Isotopes in the Characterization of Oils from the Potiguar Basin (Onshore), Northeastern Brazil

E. V. Santos Neto, J. M. Hayes, and L. A. Trindade

Hydrogen and carbon isotopic compositions of marine-evaporitic, lacustrine, and mixed oils were measured in selected samples from the onshore Potiguar Basin, Brazil. The isotopic data, together with GC and GC/MS results and geological information, were used to investigate the relationship between ^dgrD and ^dgr13C in the assessment of source rock facies, thermal maturity, biodegradation, and distance of secondary migration. The marine-evaporitic oils are depleted in D (-117^pmil to -136^pmil), and enriched in 13C (-25.6^pmil to -26.4^pmil), whereas the lacustrine oils are enriched in D (-88^pmil to -101^pmil) and depleted in 13C (-30.1^pmil to -33.6^pmil). Values of ^dgrD for mixed oils are intermediate between those of marine-evaporitic a d lacustrine oils. The results suggest that ^dgrD is a reliable tool for identification of oils that differ genetically, and the hydrogen isotopic compositions of the oils can be related to the ^dgrD of the water in the paleoenvironments where the organic matter was produced. Therefore, ^dgrD can be a useful tool to estimate, at least roughly, the paleo-^dgrD of the waters of the depositional environments and indirectly may give clues about paleoclimatic conditions. There was no detectable influence of thermal maturation, biodegradation, or distance of secondary migration on the ^dgrD of the oils. Marine-evaporitic and lacustrine oils can easily be distinguished using ^dgr13C, but resolution of marine-evaporitic and mixedoils is not always possible. This indicates that ^dgr13C can be a good tool for geochemical correlation, but is not diagnostic per se regarding the origins of all different types of oils. However, coupling of ^dgr13C and ^dgrD provides more powerful results.

The inverse relationship observed between ^dgrD and ^dgr13C in the Potiguar oils seems to reflect ultimately the tectonic and stratigraphic evolution of the basin and the paleoclimate at the time of deposition of the organic matter. The same relationship may be found in other basins that experienced a similar tectonic and stratigraphic evolution.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California