Summary

Geochemical, stratigraphic, and data for source rocks in the Potiguar Basin indicate that conditions in depositional environments were variable. Three oil families can be recognized in the basin, e.g., lacustrine, marine-evaporitic, and mixed. However, minor variations within each oil family are not always conspicuous in GC, GC-MS, d¹³C_oil, and dD_oil results. In order to improve the characterization of the Potiguar oils, the carbon-isotopic compositions of n-alkanes from oils sampled in three different areas of the basin were investigated. Previous classifications of the oil types followed the parameters already published (Table I, Mello et al., 1988; Trindade et al., 1992, Santos Neto and Hayes, 1997). It should be mentioned that the compositions of the mixed oils can vary between these of the lacustrine and marine-evaporitic end-members.

Based on the d¹³C values of n-alkanes in the C₁₇ to C₃₁ range together with other geochemical and isotopic data available, the Potiguar Basin oils examined here fall into two major groups sourced, respectively, by the Neocomian lacustrine shales of the Pendancia Formation and by the Aptian, lacustrine-to-restricted marine shales and marls of the Alagamar Formation.

The n-alkanes in the lacustrine oils are depleted in ¹³C over the entire range of carbon numbers (C₁₇ to C₃₁), but three subgroups can be distinguished. The first has a trend of decreasing values of d¹³C in the range n-C₁₇ to n-C₂₃ and a trend of enrichment in ¹³C from n-C₂₄ to n-C₃₁. The two other subgroups have a “flat” shape in their respective isotopic distributions, but with detectable differences in the overall enrichments in ¹³C.

The second major group is represented by mixed oils, and by one marine-evaporitic oil. Both have similar distributions of d¹³C among the n-alkanes: an open and asymmetric “U” shape. That similarity suggests that n-alkanes in mixed oils are mostly derived from marine-evaporitic sources. Although ranges of isotopic variation in the oils are similar, average values of I vary and are related to the geographic position of each oil field in the basin. This pattern provides evidence of varying inputs of lacustrine (isotopically lighter) and marine (isotopically heavier) sources through time, with these related in turn to distances of secondary migration and thermal evolution of the sedimentary pile (Trindade et al., 1992).