Deeply buried, Early
Cretaceous paleokarst
terrane, southern Maracaibo
Basin, Venezuela
Marı́a Verónica Castillo and Paul Mann
ABSTRACT
Cretaceous carbonate rocks formed an extensive passive-margin
section along the northern margin of the South American plate and
are now found in outcrops in elevated and deformed ranges like the
Mérida Andes and Sierra de Perijá. Regional seismic profiles correlated with well data show that a 300-m (984-ft)-thick Cretaceous
carbonate platform underlies all of the Maracaibo Basin of western Venezuela. We examined the Cretaceous carbonate section beneath the southern Maracaibo Basin using a 1600-km2 (617-mi2)
area of three-dimensional (3-D) seismic reflection data provided
by Petróleos de Venezuela, S. A., along with wells to constrain the
age and environments of seismic reflectors. Well data allow the
identification and correlation of the major lithologic subsurface
formations with formations described from outcrop studies around
the basin edges.
Seismic reflection time slices at a depth range of 3.7 – 4.5 s (5 –
7 km; 3.1 – 4.3 mi) reveal the presence of a prominent, irregular
reflection surface across the entire 3-D study area that is characterized by subcircular depressions up to about 600 m (1968 ft)
wide and about 100 m (328 ft) deep. We interpret the subcircular
features as sinkholes formed when the Lower Cretaceous carbonate
platform was subaerially exposed to weathering in a tropical climate. The scale of the observed circular features is consistent with
dimensions of limestone sinkholes described from modern karst
settings. Correlation of the inferred karst horizon with well logs
shows that the paleokarst horizon occurs within the shallow-water
carbonate rocks of the Aptian Apón Formation. We infer that the
karst formed during an Aptian eustatic sea level fall described from
Aptian intervals in other parts of the world, including the Gulf
of Mexico. The Aptian paleokarst zone provides a previously
unrecognized zone of porosity for hydrocarbons to accumulate
Copyright #2006. The American Association of Petroleum Geologists. All rights reserved.
Manuscript received February 18, 2005; provisional acceptance April 6, 2005; revised manuscript
received September 28, 2005; final acceptance October 12, 2005.
DOI:10.1306/10120505034
AAPG Bulletin, v. 90, no. 4 (April 2006), pp. 567 –579
567
AUTHORS
Marı́a Verónica Castillo Department
of Geological Sciences and Institute for
Geophysics, Jackson School of Geosciences,
University of Texas at Austin, 4412 Spicewood
Springs Road, Building 600, Austin, Texas 78759;
present address: ENI, Caracas, Venezuela;
[email protected]
Marı́a Verónica Castillo is an exploration geoscientist at ENI Venezuela in Caracas and lecturer on three-dimensional seismic interpretation at the Universidad Central de Venezuela
in Caracas. She obtained her Ph.D. in geology
at the University of Texas at Austin in 2001,
where she focused on the structural evolution
of the Maracaibo Basin, Venezuela. Her current
interest is using merged 3-D seismic data sets
for regional basin analysis.
Paul Mann Institute for Geophysics, Jackson
School of Geosciences, University of Texas
at Austin, 4412 Spicewood Springs Road,
Building 600, Austin, Texas 78759;
[email protected]
Paul Mann is a senior research scientist at the
Institute for Geophysics, University of Texas
at Austin. He received his Ph.D. in geology at the
State University of New York in 1983 and has
published widely on the tectonics of strike-slip,
rift, and collision-related sedimentary basins.
A current focus area of research is the interplay
of tectonics, sedimentation, and hydrocarbon
occurrence in Venezuela and Trinidad.
ACKNOWLEDGEMENTS
We thank Petróleos de Venezuela, S. A., for providing seismic and well data used in this study
and for supporting M. Castillo as a Ph.D. student
in geology for 4 years at the University of Texas at
Austin. We thank J. Lugo, F. Audemard, A. Bally,
A. Escalona, A. Salvador, I. Azpiritxaga, and B.
Hardage for valuable discussions and reviews.
The authors acknowledge the financial support
for this publication provided by the University of
Texas at Austin’s Geology Foundation and the
Jackson School of Geosciences. University of
Texas, Institute for Geophysics contribution 1773.
Editor’s Note
Color versions of figures may be seen in the
online version of this article.
beneath the Maracaibo and perhaps other basins
formed above the extensive passive margin of northern
South America.
INTRODUCTION
Cretaceous Passive-Margin Section beneath Maracaibo Basin
Significance
A 300-m (984-ft)-thick, mixed carbonate-clastic section of Cretaceous sedimentary rocks was deposited
in the Maracaibo Basin during a tectonically stable period of slow, passive-margin sedimentation along the
northern edge of the South American continent (Mann
et al., 2006). Clastic sedimentation occurred during an
earliest Cretaceous transgressive period following Late
Jurassic rifting (Lugo and Mann, 1995). By the end of
the Early Cretaceous, clastic rocks passed upward into
carbonate lithologies. Mixed carbonate and clastic deposition continued until the lower Turonian (Parnaud
et al., 1995; Villamil and Pindell, 1998) (Figure 1). The
outer (northern) part of the Maracaibo platform was
tectonically deformed by the arrival of the Pacificderived Caribbean plate in the late Paleocene and Eocene (Lugo and Mann, 1995; Mann et al., 2006). Compilation of well data and outcrop data by González de
Juana et al. (1980) and Parnaud et al. (1995) shows that
most of the Lower Cretaceous Maracaibo Basin was
underlain by a carbonate platform of inner- to middleshelf depth. Areas of shallower water and main shelf
areas with mixed carbonate-clastic deposition were located to the south and southeast of the present-day
Maracaibo Basin (Figure 1).
The widespread Cretaceous carbonate platform is
of great economic significance to hydrocarbon exploration in the Maracaibo Basin because the platform
contains Upper Cretaceous black shales of the La Luna
Formation that form the main source rocks for more
than 50 billion bbl of cumulative oil production from
the Maracaibo Basin (Escalona and Mann, 2006b).
Carbonate rocks also act as important reservoirs for oil
production in the basin (Azpiritxaga, 1991). A thick
and widespread shale seal is present above the carbonate platform rocks (Colón Formation in Figure 2).
Stratigraphic Setting
Transgressive, Barremian clastic deposits are overlain
by a Lower Cretaceous carbonate-platform succession,
collectively known as the Cogollo Group (Figure 2A).
568
The platform was well established over the large region shown in Figure 1 by the beginning of the Aptian
(Garner, 1926; Parnaud et al., 1995). The Cogollo Group
is composed of several formations, including, from
oldest to youngest, the Apón (Sutton, 1946), Lisure
(Rod and Maync, 1954), Aguardiente (Notestein, et al.,
1944), and Maraca formations (Rod and Maync, 1954)
(Figure 2A).
Type localities for outcrops of these formations
are found in the incised river valleys draining into the
Maracaibo Basin (González de Juana et al., 1980). Formation names and lithologic descriptions of these
units indicate lithologic similarity over distances of
hundreds of kilometers, although Renz (1981) notes
significant thickness variations in the platform units
across the area of the Sierra Perijá, Maracaibo Basin,
and Mérida Andes. Thickness variations are attributed
to Early Cretaceous arching in the continental crust
and uplift of the proto-Andean mountains (Renz, 1981;
Salvador, 1986).
Structural Setting
Regional mapping by Audemard (1991) showed that
the Paleozoic–Mesozoic acoustic basement of the Maracaibo Basin and its overlying carbonate platform uniformly dip southeastward (Figure 2B). Tilting is related
to the east-west shortening of the Maracaibo Basin and
late Neogene overthrusting of the Mérida Andes over
the southeastern edge of the basin (Duerto et al., 2006).
Acoustic basement reaches a maximum depth of 9 km
(5.5 mi) near the southeastern edge of the basin near
the area described in this article (Figure 2B). Interpretation of three-dimensional (3-D) seismic volumes by
Escalona and Mann (2006a) and Castillo and Mann
(2006) shows that the main structures in the carbonate platform rocks are northeast- to northwest-striking
faults related to either the reactivation of sub-Cretaceous
basement faults or to the Eocene flexural deformation
of the basin.
Objectives of This Paper
This article advances the understanding of the
Maracaibo Basin reservoirs of Lower Cretaceous age
by presenting 3-D seismic reflection and well evidence
of a regionally extensive and deeply buried Cretaceous
paleokarst terrane at a depth of 5 – 7 km (3.1 – 4.3 mi)
beneath the southern Maracaibo Basin. The karst zone
provides a previously unrecognized zone of porosity
for hydrocarbons to accumulate within the deeply buried passive-margin carbonate section of the Maracaibo
Basin.
Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin
Castillo and Mann
569
Figure 1. Paleogeography of the Maracaibo Basin area from the Aptian to Cenomanian modified from Parnaud et al. (1995). A mixed carbonate-clastic section was deposited
under stable conditions along a broad passive margin, fringing the northern edge of the South American continent. (A) Aptian paleogeography. Contours show thickness of the
Aptian interval. (B) Albian – lower Cenomanian. Contours show thickness of the Aptian –lower Cenomanian interval. (C) Top of the lower Cenomanian. Positive land areas by the
top of the lower Cenomanian are interpreted as a sea level drop in the Lake Maracaibo area.
570
Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin
Figure 2. (A) Regional-stratigraphic chart for Cretaceous passive-margin carbonate rocks in the area of the
Maracaibo Basin compiled from González de Juana et al. (1980), Audemard (1991), and Lugo (1991). Numerical
time scale is based on Gradstein et al. (1995). (B) Structural contour map in kilometers showing the top of the
acoustic basement (either Late Jurassic sedimentary rocks or Paleozoic metamorphic rocks) and the overlying
carbonate platform dipping to the southeast (map modified from Audemard, 1991). (C) Log of a well in the northern
part of the study area showing measured thicknesses of formations and gamma-ray response from Cretaceous
carbonate section.
STRATIGRAPHY OF THE MARACAIBO
CRETACEOUS CARBONATE PLATFORM
Sea Level History
Figure 1 summarizes the paleogeographic evolution
of part of the Lower Cretaceous Cogollo Group as
proposed by Parnaud et al. (1995). The area of the
3-D seismic study area is boxed in Figure 1. In general, platform deposits of the Aptian and lower Albian
were dominated by mixed shallow-water facies (carbonate, sandstone, and siltstone) and shelf mudstone
(González de Juana et al., 1980). The lower Albian is
followed by an abrupt transgression (La Luna Formation), which in turn is followed by Cenomanian regression (Colón Formation) (Figure 2A). The Cogollo
Group, which includes the Apón, Lisure, and Aguardiente formations, generally records rising sea level
conditions (Figure 2A).
The Apón Formation of Aptian age consists of
shallow-water carbonates deposited under restricted
platform conditions (Figure 1). This formation is composed of thick-bedded, gray and blue-gray, hard, dense,
and locally fossiliferous limestone interbedded with
subordinate amounts of dark gray calcareous shale and
sandy shale (Sutton, 1946; p. 1642) (Figure 2B). The
thickness of the Apón Formation ranges from about
600 m (1968 ft) measured in continuous exposures in
river valleys of the Sierra de Perijá (Sutton, 1946) to an
average thickness of 100 m (328 ft) beneath the area
of Lake Maracaibo (Lugo and Mann, 1995). Figure 2C
shows the gamma-ray response of the Apón Formation
based on a well in the northern part of the 3-D study
area boxed in Figure 1. The log of the Apón Formation
shows a generally coarsening- or shallowing-upward
trend near its base and a succession of thinner cycles
near its top, suggestive of more rapid sea level fluctuations (Figure 2C).
The Lisure Formation of Albian age consists of gray
or green calcareous and glauconitic sandstone, micaceous sandstone, and glauconitic, gray, fossiliferous
limestone (Figure 2A). These rocks were deposited in a
more circulated, seaward area of the platform during
the middle to upper Albian (Figure 1B). The average
thickness of the Lisure Formation in the Lake Maracaibo area is 120 m (393 ft) (Lugo and Mann, 1995).
To the south-southeast, this formation grades into a
more clastic, shallower water facies known as the
Aguardiente and Peñas Altas formations (Figure 1B).
The Aguardiente Formation of Albian age contains
well-stratified, glauconitic sandstone interbedded with
black or gray shales (González de Juana et al., 1980). A
similar, slightly younger formation (Escandalosa Formation) overlies the Aguardiente Formation.
The Maraca Formation consists of about 10–15 m
(33–49 ft) of brown to gray massive limestone interbedded with black shale (Figure 2). Studies of well
cores from the Lake Maracaibo area by Azpiritxaga
(1991) show that these upper Albian rocks represent
open-water, high-energy deposits that accumulated as
broad subtidal to intertidal shoal complexes.
3-D SEISMIC REFLECTION EVIDENCE FOR
CRETACEOUS PALEOKARST TERRANE
3-D Seismic Data
Seismic reflection time slices derived from the interpretation of 1600 km2 (617 mi2) of 3-D seismic reflection data from the southern Maracaibo Basin at a
depth range of 3.7 – 4.5 s (5– 7 km; 3.1 – 4.3 mi) reveal
the presence of a prominent, irregular reflection surface across the entire study area characterized by subcircular depressions up to 600 m (1968 ft) wide and
about 100 m (328 ft) deep. Figures 3A, B, and 4 include
three interpreted time slices that highlight subcircular features interpreted as sinkholes in a Cretaceous
paleokarst terrane. The geology of these slices and the
methods used in the construction of these maps are
discussed in detail by Castillo and Mann (2006). The
identified subcircular features are more affected by
faults to the north shown in the time slice at 3.8 s twoway traveltime (TWT) than to the south in time slices
at 4.1 and 4.2 s TWT.
Two-Dimensional Seismic Data and Well Correlation
In widely spaced, two-dimensional (2-D) seismic profiles, it is difficult to differentiate the individual formations described above that collectively make up the
Cogollo Group (Figure 2A). This 2-D imaging problem
is likely the reason why the paleokarst terrane has not
been previously described from seismic profiles in the
Maracaibo Basin. On 2-D seismic profiles, rocks of the
Cogollo Group commonly consist of a packet of continuous subparallel reflectors as illustrated from the
2-D line selected from the 3-D data volume (Figure 5B).
Figure 5A shows a synthetic seismogram calibrated to
seismic reflectors at the top of Cretaceous carbonates.
Seismic resolution at this level is about 100 m (328 ft).
Castillo and Mann
571
572
Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin
Figure 4. (A) (1) Uninterpreted
seismic reflection time slice at
4.2 s TWT from the 3-D seismic
reflection study area in the
southern Maracaibo Basin.
(2) Interpreted seismic reflection time slice following the
method of Castillo and Mann
(2006). The Lower Cretaceous
section displays subcircular features interpreted as sinkholes
in a paleokarst terrane. (B) Radar
image indicating the location of
the seismic survey in the southern Maracaibo Basin and the
stratigraphic column with color
code correlated with the colors
used on the interpreted time
slice.
Figure 3. (A) (1) Uninterpreted seismic reflection time slice at 3.8 s TWT from the 3-D seismic reflection study area in the southern
Maracaibo Basin. (2) Interpreted seismic reflection time slice following the method of Castillo and Mann (2006). The Lower Cretaceous
section displays subcircular features interpreted as sinkholes in a paleokarst terrane. (B) (1) Uninterpreted seismic reflection time slice at
4.1 s TWT from the 3-D seismic reflection study area in the southern Maracaibo Basin. (2) Interpreted seismic reflection time slice
following the method of Castillo and Mann (2006). The Lower Cretaceous section displays subcircular features interpreted as sinkholes
in a paleokarst terrane. (C) Radar image indicating the location of the seismic survey in the southern Maracaibo Basin and the
stratigraphic column with color code correlated with the colors used on the interpreted time slice.
Castillo and Mann
573
Figure 5. (A) Sonic log, synthetic seismogram, and seismic data response to Lower Cretaceous lithologic formations from a well
penetrating into the Cretaceous passive-margin section. (B) (1) Uninterpreted seismic line 1050 extracted from the 3-D seismic data
volume. (2) Interpreted seismic line showing Lower Cretaceous units. Seismic reflectors correlated with the Apón Formation are
discontinuous and, in some cases, truncated. We interpret this truncation surface as the expression of a regional paleokarst horizon.
(C) Location map of the well and seismic line.
574
Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin
Table 1. Geometric Characteristics of Scales of Subcircular, Geologic Features from Stewart (1999)
Geologic Feature
Diapir (salt, shale)
Pillow (salt, sand, shale)
Withdrawal basin (salt)
Polygonal fault system
Carbonate dissolution/
collapse pit
Volcanic diatreme/maar
Volcanic calderas
Volcano (igneous)
Gas pockmark
Reef/carbonate mound
Glacial kettle hole
Pull-apart basin
Impact crater
Scale Typical
Diameter (km)
Shape Typical Length/Width
(Horizontal Aspect Ratio)
1– 5
1– 5
2–15
0.3 –2
0.01 –1
1
1+
1+
1
1
1–5
0.01 – 1
0.5 –1
0.5 –1
0–1
No
No
Yes
Yes
–
0.01 –3
2–50
1–50
0.01 –0.3
0.01 –2
0.01 –1
0–40
1–100 +
1
1
1+
1
1+
1+
2– 5
1
2+
0.1 –1
0.1 –0.5
0.01 –0.2
0.1 –0.5
0.01 –0.1
0–1
0.1 –0.2
–
No
–
Yes
No
Yes
Yes
Yes
The highest amplitude reflector is correlated with
carbonates of the Socuy Member and the La Luna and
Maraca formations because of the large impedance
contrast between the overlying shales of the Colón
Formation and the underlying carbonates (Figure 5B).
In vertical seismic reflection profiles, the seismic reflectors correlated with the Apón Formation are discontinuous (Figure 5A). The seismic correlation indicates
that the discontinuous reflectors of the Apón Formation coincide with the proposed paleokarst relief interpreted from the 3-D seismic time slices in Figures 3A, B,
and 4. Overlying the discontinuous reflectors of the
Apón Formation are high-amplitude, subparallel, and
discontinuous reflections of the upper Cogollo Group,
La Luna Formation, and Socuy Member.
DISCUSSION
Paleokarst Interpretation
In previously published studies of surficial, modern
karst areas, the identification of paleokarst terranes is
based on lithostratigraphic characteristics, including
visual identification of hiatuses in carbonate sections,
paleosols, sinkhole fillings, and filled solution cavities
(White and White, 1995). However, paleokarst surfaces are rarely recognized in the rock record (James
and Choquette, 1984).
Shape Typical Depth/Width
(Vertical Aspect Ratio)
Coherent Internal
Structure/Fill(?)
In the south Maracaibo study area, the identification of the paleokarst surface is based on the identification of sinkholes interpreted on 3-D seismic time slices
(Figures 3A, B; 4) that are then correlated with the
vertical seismic profile, and well shown in Figure 5.
The Apón Formation is the thickest (90 m; 295 ft) and
most massive carbonate unit in the Cogollo Group
(Figure 2). For that reason, it would be the most likely
unit of the Cogollo Group (Figure 2B) to exhibit prominent, subcircular karst weathering features if subaerially exposed in a tropical climate.
Stewart (1999) provided an interpretation of geological features that are approximately circular in plan
view but are difficult to interpret from 2-D seismic reflection profiles alone. He provided a summary of the
geological and geometrical criteria that might be used
for identification of circular features interpreted from
3-D seismic reflection data ( Table 1). His compilation
shows that the scale range for carbonate dissolution and
collapse features in karst terranes ranges from 10 m
(33 ft) to 1 km (0.6 mi), which is comparable to the
sizes of circular features observed using the seismic
data (600 m [1968 ft] wide) (Figures 3, 4).
Hardage et al. (1996) showed 3-D seismic reflection evidence of the effects of widespread carbonate
karst collapse of Paleozoic carbonate rocks of the Ellenburger Group on the overlying Pennsylvanian clastic
stratigraphy in the Fort Worth basin of Texas. In their
study, Hardage et al. (1996) used 3-D seismic data
to identify circular to oval-shaped depressions with
Castillo and Mann
575
Figure 6. Seismic reflection
time slice at 820 ms from an
industry 3-D survey in the Gippsland basin, Australia, from Brown
(1999) (used with permission
from AAPG). Circular features are
interpreted by Brown (1999) as
sinkholes in a paleokarst topography of Miocene age. No horizontal scale or north direction
was provided in the original
publication.
diameters ranging from 150 to 915 m (492 to 3001 ft).
A similar scale of karst pits (200– 500 m; 656 – 1640 ft)
was identified from 3-D seismic data shown by Brown
(1999) for Miocene carbonate rocks of the Gippsland
basin, southeastern Australia (Figure 6). The dimensions of the Texas and Gippsland examples are comparable both to the range proposed in the Stewart
(1999) study and to the circular features we observe in
the subsurface of the Maracaibo Basin.
A final reason for our karst interpretation is the
Aptian age of the Apón Formation. The Aptian is a
time of global regression and sea level fall as discussed by Scott et al. (1988) (Figure 7). In the Maracaibo Basin, Azpiritxaga (1991) estimated a sea level
drop at the time of the Apón Formation based on the
cyclical pattern of interpreted fining-upward and
thinning-upward cycles from well logs from the Cogollo
Group (Figure 7). Given the global distribution of the
Aptian sea level fall, we propose that a eustatic change
in sea level was responsible for the subaerial exposure
and karst weathering of the Apón Formation. We can
rule out a sea level lowering caused by tectonic up576
lift. According to previous workers like Audemard
(1991) and Villamil and Pindell (1998), tectonic effects
did not affect the region of the Maracaibo Basin until
70 Ma, long after the deposition of the Apón Formation
(Figure 2A).
Reservoir Implications of the Deeply Buried
Paleokarst Terrane
Previous workers have proposed that carbonate rocks
of the Cogollo Group may have been subaerially exposed in the north-central parts of the Maracaibo Basin,
and that this exposure may have led to a complex diagenetic history that ultimately affected the porosity
distribution of this section (Kummerow and Pérez de
Mejı́a, 1989). These authors indicate that the greatest
reservoir potential in the north-central part of the basin
created by this diagenetic mechanism and lies within
the Apón and Maraca formations (Figure 2). In contrast, Azpiritxaga (1991) proposed that the porosity
of the Cogollo Group was mainly created by fractures
along major faults.
Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin
Figure 7. Comparison of sea level curves constructed from worldwide examples of Cretaceous rocks (modified from Scott et al.,
1988). The gray line indicates a sea level fall during the Aptian that is present in most of the sections compiled. The sea level curve for
the Maracaibo Basin, Venezuela, is taken from Azpiritxaga (1991).
Méndez (1989) also proposed the influence of the
vadose and phreatic zones during the diagenetic evolution of the carbonates of the Cogollo Group in the Perijá,
Urdaneta, and central lake areas of the Maracaibo Basin.
In addition, he proposed that brecciation and collapse
affected the base of the Apón Formation during the
Lower Cretaceous. Méndez (1989) did not consider
Oligocene basin inversion and uplift a significant mechanism for brecciation and collapse of Cretaceous carbonates as proposed by Kummerow and Pérez de Mejı́a
(1989).
Nelson et al. (2000) studied the Cretaceous reservoirs in the La Paz field in the northwestern part of
the Maracaibo Basin. They concluded that the anomalously high oil production in some parts of the field
resulted from the development of secondary porosity
formed during the Eocene inversion and exposure of
the Cretaceous carbonate rocks.
In southern Lake Maracaibo, Castillo and Mann
(2006) recognize no evidence for exposure of Cretaceous carbonate rocks at any time during their Paleogene history. We suggest that the brecciation and collapse of the Apón Formation recognized by Méndez
(1989) in the northern part of the lake may be another
manifestation of the same Cretaceous weathering event
we describe in this article. If the Cretaceous weathering
event is a regional eustatic event as we propose, then
the Apón Formation should contain excellent porosity
and should be targeted as a potentially high-quality
reservoir unit in the Cogollo Group.
CONCLUSIONS
The interpretation of seismic reflection time slices
allowed the identification of subcircular features in the
Lower Cretaceous carbonates of the Cogollo Group.
According to their scale, these features were interpreted as sinkholes in a paleokarst terrane that formed
by subaerially tropical weathering of the shallow-water
carbonate lithologies of the Apón Formation. In addition, this study suggests that the paleokarst surface
formed during the Aptian, during a well-known eustatic drop in the sea level observed in other parts of the
world (Figure 7).
Castillo and Mann
577
Our paleokarst interpretation may allow a better
understanding of reservoir characteristics at this level
in the Cogollo carbonate platform, where porosity is
generally attributed to fracturing along major faults
instead of subaerial weathering during the Aptian. If
eustatic in origin, the paleokarst surface may extends
beneath all of the Maracaibo Basin and perhaps other
hydrocarbon-bearing basins of the northern margin of
South America and therefore may prove to be a useful
play concept.
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