MOLDOWAN, J. M., J. E. DAHL, B. J. HUIZINGA, and S. R. JACOBSON, Chevron Oil Field Research Company and Chevron Overseas Petroleum, Inc., CA, and D. W. TAYLOR, Indiana University, S.E., IN
ABSTRACT: The Relationship of Angiosperms and Oleanane in Petroleum through Geologic Time
The biological marker oleanane has been suggested as an indicator of angiosperm (flowering plant) input into source rocks and their derived oils. Parallels should therefore be evident between the angiosperm fossil record and oleanane occurrence and abundance. A global selection of more than 50 core samples from marine rocks of different ages and from different locations was quantitatively analyzed for oleanane to determine its abundance over geologic time relative to the bacterial marker hopane. Oleanane was recognized using Metastable Reaction Monitoring (MRM) GC-MS.
A parallel was observed between the oleanane/hopane ratio and angiosperm diversity in the fossil record through time. The first fossil evidence of angiosperms is during the Early Cretaceous with radiation during the Late Cretaceous and Tertiary. Occurrences of oleanane are confirmed throughout the Cretaceous system. Early-to-middle Cretaceous (Berriasian to Cenomanian) occurrences are sporadic and oleanane/hopane ratios are typically less than 0.07. Late Cretaceous (Turonian to Maastrichtian) oleanane/hopane ratios range up to 0.15 with higher ratios in many Tertiary samples. Thus, it appears that oleanane/hopane ratios of oils can restrict the age of their unavailable or unknown source rocks. High ratios indicate Tertiary age, and lower ratios can indicate Cretaceous or Tertiary age, depending on depositional environment. While these data do not rule out pre-Cretaceous
oleanane, preliminary data show that the oleanane/hopane ratios for Jurassic and older rock extracts are typically below our detection limits (< 0.03).
While oleanane precursors are abundant in angiosperms, they also occur, rarely, in other modern plant groups. We identified oleanane in low abundances in three Early Cretaceous fossil bennettitaleans, an extinct plant group (Late Triassic to Late Cretaceous) thought to be related to angiosperms. These findings suggest that oleanane could be present in low abundance in some pre-Cretaceous rocks and oils.
AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.