Geochemical characterization of lithofacies and organic facies in Cretaceous organic-rich rocks from Trinidad, East Venezuela Basin

Organic-rich Cretaceous sediments from southern Trinidad show differences in hydrocarbon source potential which can be geochemically related to lithofacies and organic facies. Sediments which exhibit good to excellent potential for oil were deposited during the Campanian through Cenomanian in clastic-starved environments, as evidenced by the inverse relationship between Hydrogen Index and the parameters Al2O3/TOC and Th/U. Principal Components Analysis (PCA) of combined TOC, Rock-Eval, major oxide and trace element data shows three lithofacies—carbonate, siliceous and clastic—within samples exhibiting hydrocarbon source potential. The highest petroleum potential is associated with the siliceous and carbonate lithofacies, which consist of hemi-pelagic sediments deposited under low oxygen conditions. The clastic lithofacies,have lower petroleum potential and represent nearshore sediments which have been transported to the continental slope via submarine canyons. Extractable saturated and aromatic hydrocarbon compositions vary with lithology. PCA of conventional biomarker and 2- and 3-ring aromatic hydrocarbon distributions shows that the carbonate lithofacies are characterized by an enrichment in αββ steranes and dibenzothiophenes. Two organic facies can be recognized within the carbonate lithofacies that differ in their relative bundance of tricyclic terpanes gammacerane and bisnorhopane. The siliceous lithofacies are characterized by an abundance of bisnorhopane, ααα (20R) steranes and a slight enrichment in napthalenes relative to phenanthrenes. The clastic lithofacies exhibit high hopane contents relative to steranes, an enrichment in moretanes and 18α (H)-oleanane, and an enrichment in phenanthrenes relative to napthalenes.