Figure 1. Comparison between hypopycnal (A, inflow density < reservoir) and hyperpycnal (B, inflow density > reservoir) flows (original concept by Bates, 1953). Note that in the case of the hyperpycnal flow the fluvial discharge sinks below the water body, continuing its travel basinward as a quasi-steady underflow. Figure 1B was redrawn from a pioneer work after Knapp (1943).
	Figure 2: Main characteristics of long-lived hyperpycnal flows and their typical deposits. The complexity of these flows results in the accumulation of composite beds (Zavala et al., 2007a).
	Figure 3. Basic conceptual diagram for facies analysis of hyperpycnal deposits with associated bedload in marine settings. Facies B includes all those facies related to bedload processes developed at the base of an overpassing, long-lived turbulent flow. Facies S corresponds to the gravitational collapse of sand-sized materials transported in suspension by the turbulent flow (suspended load facies). Facies L is related to the fallout of fine-grained materials lifted-up by the interstitial freshwater contained in the flow once it lost part of the sand-sized suspended load. After Zavala et al. (2006b).
	Figure 4: The use of ternary diagrams allows one to plot the relationships between bedload, suspended load, and lofting facies and to show the position of the studied locality with respect to the system as a whole. In a single well, ternary analysis can be useful to determine sedimentary trends (progradation/retrogradation) between different depositional sequences.

Facies Analysis: A Genetic Approach

The basic classification scheme used in this study is shown in Figure 3 (Zavala et al., 2006b) and is based in the distinction of three main facies categories related to bedload (Facies B), suspended load (Facies S), and lofting (Facies L). Facies B comprises the coarsest materials present in the tract transported by drag and shear forces provided by the overpassing turbulent hyperpycnal flow. Consequently, bedload facies is characteristic of proximal positions. Three main categories are recognized, termed B1 (massive fine-grained conglomerates), B2 (pebbly sandstones with asymptotic low-angle cross-stratification) and B3 (pebbly sandstones with diffuse planar lamination). Facies S is almost fine-grained and relates to the gravitational collapse of suspended load transported in turbulence in the main body of the hyperpycnal flow. Four facies types are recognized within this category: denominated S1 (massive sandstones), S2 (laminated sandstones), S3 (sandstones with climbing ripples) and S4 (massive siltstones and mudstones). Facies L (lofting) relates to the buoyancy reversal provoked by the lift-up of a less dense fluid (in the case of freshwater) in marine environments. Finest suspended materials are also lifted from the substrate and settle down, forming silt/sand couplets of great lateral extent (lofting rhythmites, Zavala et al., 2006c). Facies analysis based on a genetic classification provides new perspectives to the paleoenvironmental understanding and the prediction of reservoir quality.

The genetic-oriented analysis applied to the study of hyperpycnal systems permits the facies mapping and the recognition of bypass, depositional, and lateral areas in the subsurface. With the scope of better management of the facies dataset, two main indexes were considered in this study--termed proximity (Pt) and laterality (Lt) indices (Zavala et al., 2007b). These indices should be calculated individually for each studied locality.

The proximity index (Pt): The Pt index is a dimensionless number that measures how proximal the locality is in respect to the system considered as a whole. It is based on the relative dominance of bedload facies in proximal positions and the basinward increasing of suspended load facies as the long-lived hyperpycnal flow progressively wanes with the subsequent collapse of suspended materials. The proximity index can be calculated as follow:

Pt=100B/(B+S)

Where Pt is the proximity index, B is the total thickness of bedload facies, and S is the total thickness of suspended load facies in the analyzed core. Note that only hyperpycnal facies are considered.

The Pt index varies between 0 and 100. The greater the Pt index is, the more proximal the considered location will be within the hyperpycnal system. In fact, Pt index between 100 and 50 characterizes proximal system areas, while Pt index between 50 and 0 suggests intermediate positions in the system. When Pt reaches 0, it marks the channel-lobe transition and the beginning of the distal system area. Additionally, the decay rate of the proximity index can be used as a proxy to estimate the dimensions of the hyperpycnal system under study.

The laterality index (Lt): Because of the gravity nature of the hyperpycnal flow, coarse-grained facies is very sensitive to any subaqueous topography. Facies B and S tend to infill the lowermost positions of the submarine landscape. In contrast, lofting facies mostly characterizes relatively elevated areas located laterally with respect to the main axis of the hyperpycnal flows. Consequently, the Lt index is a dimensionless number that will measure the relative location of the analyzed well with respect to the main depocenters. The Lt index is useful to delineate the location of synsedimentary-growing tectonic structures in the subsurface. The laterality index can be obtained as follows:

Lt=100L/(L+B+S)

Where Lt is the laterality index, L is the total thickness of lofting facies, B is the total thickness of bedload facies, and S is the total thickness of suspended load facies in the analyzed core. Note that only the hyperpycnal facies are considered.

In the main depocenters affected by coarse-grained hyperpycnal sedimentation, the laterality index tends to be low, typically less than 15, while lateral uplifted areas have laterality index that exceeds 35.

Ternary indices: In addition to the proximity and laterality indices, ternary indices and diagrams are useful to depict the different proportions between the three main facies categories used in the genetic analysis (B, S, and L facies, Figure 4).

B, S and L indices are calculated, comparing the total thickness of each category with respect to the total thickness of hyperpycnal facies, in the form that B+S+L=100. The ternary diagrams allow one to define several “fields” (uplifted areas, flow margin, flow axis, proximal channels, and distal lobes, Figure 3) which are useful in analyzing the position of the well with respect to the hyperpycnal system considered as a whole.

The use of genetic indices in hyperpycnal systems allows one to determine source areas and to map and predict the distribution of coarse-grained clastic facies (Marcano et al., 2008). Nevertheless, the analysis of genetic indices must be done within a sequence stratigraphic framework in order to analyze data related to the same stratigraphic (coeval) interval. In the case of core studies, the analyzed interval should be representative of the sequence under consideration.

The writers acknowledge the permission of PDVSA for publishing the methodology used for the analysis of the Merecure Formation in the subsurface.

Bates, C., 1953, Rational theory of delta formation: AAPG Bulletin, v. 37, p. 2119-2162.

Knapp, R.T., 1943, Density currents: Their mixing characteristics and their effect on the turbulence structure of the associated flow: Proceedings 2nd Hydrology Conference, University of Iowa, Studies in Engineering Bulletin 27.

Marcano, J., Carvajal, J., Delgado, M., and Zavala, C., 2008, Facies prediction in hyperpycnal systems. The Oligocene Merecure Formation, Venezuela; oral presentation at Hedberg Conference.

Mutti, E., Mavilla, N., Angella, S., and Fava L.L., 1999. An introduction to the analysis of ancient turbidite basins from an outcrop perspective: AAPG Continuing Education Course Note 39, p. 1-98. Tulsa.

Plink-Björklund, P., and Steel, R.J., 2004, Initiation of turbidite currents: Outcrop evidence for Eocene hyperpycnal flow turbidites: Sedimentary Geology, v. 165, p. 29-52.

Zavala, C., Ponce J., Arcuri, M., Drittanti, D., Freije, H., and Asensio, M., 2006a. Ancient Lacustrine hyperpycnites: A depositional model from a case study in the Rayoso Formation (Cretaceous) of west-central Argentina: Journal of Sedimentary Research, v. 76, p. 40-58.

Zavala, C., Arcuri, M., and Gamero H., 2006b, Towards a genetic model for the analysis of hyperpycnal systems: 2006 GSA Annual Meeting, 22-25 October, Philadelphia, PA., USA. Topical session T136: River Generated Hyperpycnal Events and Resulted Deposits in Modern and Ancient Environments.

Zavala, C., Gamero H., and Arcuri, M., 2006c, Lofting rhythmites: A diagnostic feature for the recognition of hyperpycnal deposits: 2006 GSA Annual Meeting, 22-25 October, Philadelphia, PA., USA. Topical session T136: River Generated Hyperpycnal Events and Resulted Deposits in Modern and Ancient Environments.

Zavala, C., Arcuri, M., Gamero Díaz, H., and Contreras, C., 2007a, The composite bed: A new distinctive feature of hyperpycnal deposition: 2007 AAPG Annual Convention and Exhibition (April 1 - 4, 2007). Long Beach, California USA.

Zavala, C., Marcano J., and Carvajal J., 2007b. Proximity and laterality indexes: A new tool for the analysis of ancient hyperpycnal deposits in the subsurface. GSTT 4th Geological Conference, “Caribbean Exploration – Planning for the Next Century,” June 17-22, 2007 – Port of Spain, Trinidad.

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