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Extensional Folding in the Eastern Venezuela Basin: Examples from Fields of Oritupano-Leona Block*
By
Jesús S. Porras1, Eduardo L. Vallejo1, Denis Marchal1, and Carlos Selva1
Search and Discovery Article #50003 (2003)
*Adapted from “extended abstract” for presentation at the AAPG Annual Meeting, Houston, Texas, March 10-13, 2002.
1Pérez Companc, Caracas, Venezuela
Introduction
A large variety of folds have been identified in the Oritupano-Leona area. The Oritupano-Leona Block is located at the Greater Oficina Trend (Figure 1). More than 12 isolated oil fields constitute the block. Structurally, the fields are located in the southern flank of the Eastern Venezuela Basin (EVB), in the foredeep platform zone (Parnaud et al., 1995) (Figure 1). The field is affected by normal faulting mainly trending N60°E (Figure 2). Productive section is represented by the Oficina Formation (Early Miocene) (Figure 2), although uppermost reservoirs of the underlying Merecure Formation (Oligocene) have produced an acceptable volume of oil. Cumulative oil production of the block is over 340 MMbbls since its discovery in the1940’s. Reservoirs are composed of sandstones of estuarine to shallow-marine environments.
Detailed 3D
seismic interpretation distinguished various types of folds in the area:
normal- and reverse-drag 
fold
, composite fold structure and folds related to
plastic flow. Longitudinal folds that include normal- and reverse-drag folds
are common features in several fields and reported by different workers (Mencher
et al., 1953; González de Juana et al., 1980; Azalgara et al., 2000; Porras et
al., 2001). These folds are responsible for major oil accumulations. Normal-drag
folds form the largest, with extended reservoirs in the footwall of master
normal faults (Azalgara et al., 2000), whereas reverse-drag folds provide the
structural closure for trapping in the hanging wall, especially in Leona Field (Porras
et al., 2001). Composite fold structures and folds related to plastic flow are
also described. The Junta field trap is a complex structure formed by a
combination of folds related to normal fault growth, linkage, and interaction.
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Click to view sequence of time map and line drawing.
Structural FrameworkThe southern flank of the Eastern Venezuelan Basin represents a relative stable region characterized by extensional tectonics. The study area shows a regional dip about 4° North. The fault planes display a general planar geometry and dip around 45°. Two main fault systems are identified; they follow a NE-SW and a W-E trend (Figure 2). Both main fault systems control trapping. The first fault system is parallel to the southern hinge of the basin. The NE-SW fault system has synthetic dips, with dipping planes to NW. The fault planes affect the whole section, including the basement. In general, this system corresponds to regional lineaments and shows syn-sedimentary characters. The Guara-Leona and Junta-Merey trends correspond to this system (Figure 2). Fault throws can reach 1000 feet. Their origin is associated to the flexural uplift which affected the basin during the Oligocene-Miocene times (Elrich and Barrett, 1992). The second fault system, trending W-E, display both antithetic and synthetic dips. The fault traces show a dogleg pattern. It derives from linkage of segmented fault planes (Laubscher, 1956, Cartwright et al., 1995; Marchal et al., 1998). Longitudinal folds are mainly related to this fault system and constitute the most important traps in the Oritupano-Leona area; the production comes mainly from reservoirs located in the upper and middle section of the Oficina Formation. This fault system seems to be in relation with the extensional strain resulting from dextral strike-slip movements along the boundary of South-America and Caribbean plates (Elrich and Barrett, 1992).
Normal-Drag FoldsNormal-drag folds are recognized in most of the fields of the Oritupano-Leona Block (Figure 3a). They form anticlines in the footwall of main antithetic faults and synclines in the hanging wall. The hinges of these normal-drag folds are parallel to the fault strike. Even though the largest reservoirs of the Oritupano-Leona Block are located in the footwall (Figure 3a), significant hydrocarbon production has come from traps located in the hanging wall (e.g., 41 MMBbls of oil in Oritupano B and 400 MBbls of oil in Oritupano C). In the Leona Field these folds reach their maximum expression.
Reverse-drag folds are well expressed in Leona Field (Figure 3b). They
form anticlines in the hanging wall of main faults. A narrow, 10-km long
discontinuous belt of
Composite
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Figure 1. Oritupano-Leona location map. The
study area is located on the southern flank of the Eastern Venezuela
Basin, in the foredeep platform zone.
Figure 2. Map showing normal-fault
trends and major oil fields in the Oritupano-Leona Block, along with a highly reduced log
of the Oficina Formation and overlying and underlying units.
Figure 3. Different features of folding in
Oritupano-Leona Block. a) Normal-drag folds. b) Reverse-drag folds. c)
and d) Composite 
Figure 4. Junta field structure. a) Time map
of Base Oficina. b) Line drawing with the interpretation of the
structural elements.
