Print this pageBiostratigraphic Definition of Depositional Cycles and Environments, Plio-Pleistocene, Offshore Southern Trinidad
ARMENTROUT, JOHN M., RICHARD C. BECKER, and LARRY B. FEARN
The Neogene depositional history of the Eastern Venezuela Tertiary Basin reflects dominance of north to northeast transport of deltaic sedimentation overprinted by complex tectonics. Exploration within this setting requires precise correlations and carefully constrained depositional models. The assessment of risk of exploration plays is facilitated by an understanding of depositional environments based on integration of biostratigraphy, seismic sequence stratigraphy, log-motif analysis and seismic facies mapping. Correlation of the offshore Trinidad Columbus and Carambola wells, and their use in calibrating seismic reflection profile analysis, illustrates this process of integrated stratigraphic interpretation.
The seismic reflection profiles are dominated by parallel continuous reflections and the tectonic overprint of phases of extension and compression. Locally developed unconformities and clinoforms of basin-slope scale identify what may be several sequence boundaries. Relative abundance plots of climatically controlled littoral and terrigenous derived palynomorph taxa show well developed arid-to-wet cycles. Biofacies analysis of benthic forminiferal assemblages identifies patterns of shallowing and deepening environments. Integration of palynological and foraminiferal analyses defines two distinct associations/trends interpreted as environmentally significant:
a. Low sealevel association -- Arid palynomorph assemblage and shallowing foraminiferal biofacies.
b. High sealevel association: Wet palynomorph assemblage and deepening foraminiferal biofacies.
Within the context of major marine incursions, these biofacies cycles, when integrated with seismic facies and wireline log stacking patterns, provide a framework for defining and correlating sequences, identifying depositional environments and risking reservoir and seal potential.
A late Pliocene prograding complex encased in deep marine shales was initially interpreted as a highstand delta. Further analysis of regional seismic and detailed biostratigraphy demonstrated that the sands were part of a lowstand delta. This change in systems tract analysis has significant implications for development strategy.