AAPG Bulletin, V. 83 (1999), No. 3 (March 1999), P. 496-518
.

Use of Hydrogen and Carbon Stable Isotopes Characterizing Oils from the Potiguar Basin (Onshore), Northeastern Brazil¹

Eugenio Vaz dos Santos Neto² and John M. Hayes³
 

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved
 

¹Manuscript received April 1, 1997; revised manuscript received June 24, 1998; final acceptance August 25, 1998.
²Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405. Permanent address: Center of Excellence in Geochemistry of Petrobrás/Cenpes, Cidade Universitária, Quadra 7, Ilha do Fundão, Rio de Janeiro, RJ, 21949-900, Brazil; e-mail: eugenio@cenpes.petrobras.com.br
³Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, Indiana 47405. Permanent address: Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543-1539.
 

We are grateful to Petrobrás for the sponsorship of this project and the authorization to publish this paper. Our thanks to Márcio R. Mello and Luiz Antonio F. Trindade for the suggestions, to Neil Hurley, Alan R. Carroll, Barry Katz, and Philip O. Meyers for their careful revision and comments that improved this paper, and to Julie R. Primack for her support lab work. 
 

ABSTRACT

Hydrogen and carbon isotopic abundances were measured in representative samples of marine- evaporitic, lacustrine, and mixed oils from the onshore Potiguar basin. The isotopic data, together with chemical compositions and geological information, were used to investigate relationships among dD_oil and d¹³C_oil and source, thermal maturity, biodegradation, mixing, and the distance of secondary migration of these oils. The marine-evaporitic oils are depleted in deuterium (-135 to -114‰), and the lacustrine oils are enriched (-101 to -88‰). Values of dD for mixed oils are intermediate (-113 to -104‰) between those of marine-evaporitic and lacustrine oils, showing that dD_oil is a reliable tool to identify oils that differ genetically. Values of dD_oil were not directly related to thermal maturation, biodegradation, or distance of the secondary migration. Hydrogen isotopic compositions of the oils can be related mainly to the dD_water in the paleoenvironments in which the primary producers lived; therefore, dD_oil values can provide rough estimates of dD_water in the depositional environments of the source rocks, and consequently may provide evidence about paleoclimatic conditions at the site.

Marine-evaporitic oils are enriched in ¹³C (-26.5 to -25.7‰), and lacustrine oils are depleted (-33.6 to -30.2‰) in ¹³C. Relative contributions of marine-evaporitic and lacustrine sources to mixed oils thus can be calculated independently from carbon or hydrogen isotopic mixing models. When these calculations are done independently, the mixed oils separate into two families, and a previously unrecognized component possibly associated with environments that formed during the lacustrine-to-marine transition is revealed. The investigation thus demonstrates the superior resolving power of two-element mixing systematics.