Trace metals and organic geochemistry of the Machiques Member (Aptian-Albian) and La Luna Formation (Cenomanian-Campanian), Venezuela

Two proposed source-rock sequences in the Lower (Aptian-Albian) and the Upper (Cenomanian-Campanian) Cretaceous sediments of the Perija Range, Maracaibo Basin, were geochemically analyzed in order to find environmentally controlled characteristic fingerprints. Trace metals and sulfur concentrations were found to be closely related with the physicochemical conditions during deposition of the racks. Based on trace metal analyses and sulfur relationships with TOC, the Aptian-Albian sequence (Machiques Member) shows alternate oxic and anoxic episodes close to the bottom-water surface where mixed marine and terrestrial organic matter were deposited. The Cenomanian-Campanian sequence (La Luna Formation) persistently shows anoxic-euxinic conditions that allowed the preservation of marine organic matter. Ratios of trace metals (V, Mo and Zn) characterize both Cretaceous sedimentary columns, however, some limitations are Visualized in using trace metals as correlation tools in further geochemical studies. Biomarker ratios are related to the organic matter type and lithology. Some specific ratios and individual compounds allow a clear differentiation between these two sequences and offer, in the context of delta(13)C, an excellent tool for further oil-source rock correlation studies. (C) 1999 Elsevier Science B.V. All rights reserved.