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The AAPG/Datapages Combined Publications Database
AAPG Special Volumes
Abstract
Zavala, C., J. Marcano, J. Carvajal, and M. Delgado,
DOI:10.1306/13271350St613438
2011 by The American Association of Petroleum Geologists.
Genetic Indices in Hyperpycnal Systems: A Case Study in the Late Oligocene–Early Miocene Merecure Formation, Maturin Subbasin, Venezuela
Carlos Zavala,1 Jose Marcano,2 Jair Carvajal,3 Manuel Delgado4
1GCS Argentina, Universidad Nacional del Sur, Bahia Blanca, Argentina
2Petroleos de Venezuela Sociedad Anonima (PDVSA), Laboratorio Geologico El Chaure, Puerto La Cruz, Venezuela
3Petroleos de Venezuela Sociedad Anonima (PDVSA), Laboratorio Geologico El Chaure, Puerto La Cruz, Venezuela
4Petroleos de Venezuela Sociedad Anonima (PDVSA), Laboratorio Geologico El Chaure, Puerto La Cruz, Venezuela
ACKNOWLEDGEMENTS
We thank PDVSA for permitting publication of the methodology used for the analysis
of the Merecure Formation in the subsurface. Roger Slatt and an anonymous reviewer made important suggestions that substantially improved the contents of this chapter.
ABSTRACT
The mapping and analysis
of ancient turbidite deposits using geographically dependent sedimentologic indices (or proximality) determined by genetic facies relationships and grain-size variations were mostly applied during the early years of turbidite research. Nevertheless, the understanding that fine-grained deposits (thin-bedded turbidites) can also occur in proximal (interchannel) areas, and the impossibility of differentiating them from real distal deposits, revealed many inconsistencies in this approach to the development of turbidite fan facies models. More recently, a genetic facies tract has been proposed for hyperpycnal systems, which distinguishes between thin-bedded deposits accumulated in interchannel areas from those deposited on distal areas.
The genetic facies tract proposed for hyperpycnal systems in marine settings comprises three main facies categories related to bed-load, suspended-load, and lofting processes, respectively. Bed-load-related facies are coarse grained and related to drag forces provided by the overpassing turbulent flow. Suspended-load-related facies are composed of fine-grained sandstones and originate from the gravitational collapse of the suspended load as the long-lived flow progressively wanes. Lofting-related facies are the result of the fallout of very fine-grained sands, silts, plant debris, and micas from lofting plumes mainly in flow-margin areas. Proximal (Pt) and lateral (Lt) facies indices have been defined based on the relative abundance of B, S, and L facies within a stratigraphic interval. The Pt and Lt indices are dimensionless numbers that range from 100 to 0. The Pt index measures how proximal a given stratigraphic section is with respect to the entire hyperpycnal system. The Lt index provides an indication of the stratigraphic lateral distance of a given section with respect to the flow axis. These genetic indices were applied to hyperpycnal deposits of the late Oligocene–early Miocene Merecure Formation, Maturin Subbasin, Venezuela.
Results suggest that the Merecure Formation may be more extensive than previously considered with a main sediment source from cratonic areas located in the south and southeast. Index mapping also suggests a syntectonic accumulation during the Oligocene, which controlled the subaqueous topography and, thus, the distribution of sand bodies.
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