deeply buried, early cretaceous paleokarst terrane, southern maracaibo basin, venezuela

AUTHORS

Mara Veronica Castillo Departmentof Geological Sciences and Institute forGeophysics, Jackson School of Geosciences,University of Texas at Austin, 4412 SpicewoodSprings Road, Building 600, Austin, Texas 78759;present address: ENI, Caracas, Venezuela;[email protected]

Mara Veronica Castillo is an exploration geo-scientist at ENI Venezuela in Caracas and lec-turer on three-dimensional seismic interpreta-tion at the Universidad Central de Venezuelain Caracas. She obtained her Ph.D. in geologyat the University of Texas at Austin in 2001,where she focused on the structural evolutionof the Maracaibo Basin, Venezuela. Her currentinterest is using merged 3-D seismic data setsfor regional basin analysis.

PaulMann Institute for Geophysics, JacksonSchool of Geosciences, University of Texasat Austin, 4412 Spicewood Springs Road,Building 600, Austin, Texas 78759;[email protected]

Paul Mann is a senior research scientist at theInstitute for Geophysics, University of Texasat Austin. He received his Ph.D. in geology at theState University of New York in 1983 and haspublished widely on the tectonics of strike-slip,rift, and collision-related sedimentary basins.A current focus area of research is the interplayof tectonics, sedimentation, and hydrocarbonoccurrence in Venezuela and Trinidad.

ACKNOWLEDGEMENTS

We thank Petroleos de Venezuela, S. A., for pro-viding seismic and well data used in this studyand for supporting M. Castillo as a Ph.D. studentin geology for 4 years at the University of Texas atAustin. 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 supportfor this publication provided by the University ofTexas at Austins Geology Foundation and theJackson School of Geosciences. University ofTexas, Institute for Geophysics contribution 1773.

Editors NoteColor versions of figures may be seen in theonline version of this article.

Deeply buried, EarlyCretaceous paleokarstterrane, southern MaracaiboBasin, VenezuelaMara Veronica 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

Merida Andes and Sierra de Perija. Regional seismic profiles cor-

related with well data show that a 300-m (984-ft)-thick Cretaceous

carbonate platform underlies all of the Maracaibo Basin of west-

ern Venezuela. We examined the Cretaceous carbonate section be-

neath the southern Maracaibo Basin using a 1600-km2 (617-mi2)

area of three-dimensional (3-D) seismic reflection data provided

by Petroleos 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 3.74.5 s (5

7 km; 3.14.3 mi) reveal the presence of a prominent, irregular

reflection surface across the entire 3-D study area that is char-

acterized 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 cli-

mate. 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 Apon 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

AAPG Bulletin, v. 90, no. 4 (April 2006), pp. 567579 567

Copyright #2006. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received February 18, 2005; provisional acceptance April 6, 2005; revised manuscriptreceived September 28, 2005; final acceptance October 12, 2005.

DOI:10.1306/10120505034

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 sec-

tion of Cretaceous sedimentary rocks was deposited

in the Maracaibo Basin during a tectonically stable pe-

riod 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 de-

position 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 Pacific-

derived Caribbean plate in the late Paleocene and Eo-

cene (Lugo and Mann, 1995; Mann et al., 2006). Com-

pilation of well data and outcrop data by Gonzalez 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 middle-

shelf depth. Areas of shallower water and main shelf

areas with mixed carbonate-clastic deposition were lo-

cated 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 explo-

ration 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 carbon-

ate platform rocks (Colon 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).

The platform was well established over the large re-

gion 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 Apon (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 (Gonzalez de Juana et al., 1980). For-

mation 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 Perija, Maracaibo Basin,

and Merida 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 PaleozoicMesozoic acoustic basement of the Mara-

caibo Basin and its overlying carbonate platform uni-

formly dip southeastward (Figure 2B). Tilting is related

to the east-west shortening of the Maracaibo Basin and

late Neogene overthrusting of the Merida 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). Interpre-

tation of three-dimensional (3-D) seismic volumes by

Escalona and Mann (2006a) and Castillo and Mann

(2006) shows that the main structures in the carbon-

ate 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 57 km (3.14.3 mi)

beneath the southern Maracaibo Basin. The karst zone

provides a previously unrecognized zone of porosity

for hydrocarbons to accumulate within the deeply bur-

ied passive-margin carbonate section of the Maracaibo

Basin.

568 Deeply Buried, Early Cretaceous Paleokarst Terrane, Southern Maracaibo Basin

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STRATIGRAPHY OF THE MARACAIBOCRETACEOUS 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 gen-

eral, platform deposits of the Aptian and lower Albian

were dominated by mixed shallow-water facies (car-

bonate, sandstone, and siltstone) and shelf mudstone

(Gonzalez de Juana et al., 1980). The lower Albian is

followed by an abrupt transgression (La Luna Forma-

tion), which in turn is followed by Cenomanian re-

gression (Colon Formation) (Figure 2A). The Cogollo

Group, which includes the Apon, Lisure, and Aguar-

diente formations, generally records rising sea level

conditions (Figure 2A).

The Apon Formation of Aptian age consists of

shallow-water carbonates deposited under restricted

platform conditions (Figure 1). This formation is com-

posed 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 Apon Formation ranges from about

600 m (1968 ft) measured in continuous exposures in

river valleys of the Sierra de Perija (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 Apon Formation

based on a well in the northern part of the 3-D study

area boxed in Figure 1. The log of the Apon 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 fluc-

tuations (Figure 2C).

The Lisure Formation of Albian age consists of gray

or green calcareous and glauconitic sandstone, mica-

ceous 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 Mara-

caibo 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 Penas Altas formations (Figure 1B).

The Aguardiente Formation of Albian age contains

well-stratified, glauconitic sandstone interbedded with

black or gray shales (Gonzalez de Juana et al., 1980). A

similar, slightly younger formation (Escandalosa For-

mation) overlies the Aguardiente Formation.

The Maraca Formation consists of about 1015 m

(3349 ft) of brown to gray massive limestone inter-

bedded 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 FORCRETACEOUS PALEOKARST TERRANE

3-D Seismic Data

Seismic reflection time slices derived from the in-

terpretation of 1600 km2 (617 mi2) of 3-D seismic re-

flection data from the southern Maracaibo Basin at a

depth range of 3.74.5 s (57 km; 3.14.3 mi) reveal

the presence of a prominent, irregular reflection sur-

face across the entire study area characterized by sub-

circular 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 subcircu-

lar 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 two-

way 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 pro-

files, it is difficult to differentiate the individual for-

mations 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 con-

tinuous 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).


Figure 3. (A) (1) Uninterpreted seismic reflection time slice at 3.8 s TWT from the 3-D seismic reflection study area in the southernMaracaibo Basin. (2) Interpreted seismic reflection time slice following the method of Castillo and Mann (2006). The Lower Cretaceoussection displays subcircular features interpreted as sinkholes in a paleokarst terrane. (B) (1) Uninterpreted seismic reflection time slice at4.1 s TWT from the 3-D seismic reflection study area in the southern Maracaibo Basin. (2) Interpreted seismic reflection time slicefollowing the method of Castillo and Mann (2006). The Lower Cretaceous section displays subcircular features interpreted as sinkholesin a paleokarst terrane. (C) Radar image indicating the location of the seismic survey in the southern Maracaibo Basin and thestratigraphic column with color code correlated with the colors used on the interpreted time slice.

Figure 4. (A) (1) Uninterpretedseismic reflection time slice at4.2 s TWT from the 3-D seismicreflection study area in thesouthern Maracaibo Basin.(2) Interpreted seismic reflec-tion time slice following themethod of Castillo and Mann(2006). The Lower Cretaceoussection displays subcircular fea-tures interpreted as sinkholesin a paleokarst terrane. (B) Radarimage indicating the location ofthe seismic survey in the south-ern Maracaibo Basin and thestratigraphic column with colorcode correlated with the colorsused on the interpreted timeslice.


Figure 5. (A) Sonic log, synthetic seismogram, and seismic data response to Lower Cretaceous lithologic formations from a wellpenetrating into the Cretaceous passive-margin section. (B) (1) Uninterpreted seismic line 1050 extracted from the 3-D seismic datavolume. (2) Interpreted seismic line showing Lower Cretaceous units. Seismic reflectors correlated with the Apon Formation arediscontinuous 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.


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 Colon

Formation and the underlying carbonates (Figure 5B).

In vertical seismic reflection profiles, the seismic reflec-

tors correlated with the Apon Formation are discon-

tinuous (Figure 5A). The seismic correlation indicates

that the discontinuous reflectors of the Apon Forma-

tion coincide with the proposed paleokarst relief inter-

preted from the 3-D seismic time slices in Figures 3A, B,

and 4. Overlying the discontinuous reflectors of the

Apon 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 sur-

faces are rarely recognized in the rock record (James

and Choquette, 1984).

In the south Maracaibo study area, the identifica-

tion of the paleokarst surface is based on the identifica-

tion 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 Apon 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 promi-

nent, subcircular karst weathering features if subaeri-

ally exposed in a tropical climate.

Stewart (1999) provided an interpretation of geo-

logical features that are approximately circular in plan

view but are difficult to interpret from 2-D seismic re-

flection 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 reflec-

tion evidence of the effects of widespread carbonate

karst collapse of Paleozoic carbonate rocks of the Ellen-

burger 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

Table 1. Geometric Characteristics of Scales of Subcircular, Geologic Features from Stewart (1999)

Geologic Feature

Scale Typical

Diameter (km)

Shape Typical Length/Width

(Horizontal Aspect Ratio)

Shape Typical Depth/Width

(Vertical Aspect Ratio)

Coherent Internal

Structure/Fill(?)

Diapir (salt, shale) 15 1 15 No

Pillow (salt, sand, shale) 15 1+ 0.011 No

Withdrawal basin (salt) 215 1+ 0.51 Yes

Polygonal fault system 0.32 1 0.51 Yes

Carbonate dissolution/

collapse pit

0.011 1 01

Volcanic diatreme/maar 0.013 1 2+

Volcanic calderas 250 1 0.11 No

Volcano (igneous) 150 1+ 0.10.5

Gas pockmark 0.010.3 1 0.010.2 Yes

Reef/carbonate mound 0.012 1+ 0.10.5 No

Glacial kettle hole 0.011 1+ 0.010.1 Yes

Pull-apart basin 040 25 01 Yes

Impact crater 1100+ 1 0.10.2 Yes


diameters ranging from 150 to 915 m (492 to 3001 ft).

A similar scale of karst pits (200500 m; 6561640 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 dimen-

sions of the Texas and Gippsland examples are com-

parable 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 Apon Formation. The Aptian is a

time of global regression and sea level fall as dis-

cussed by Scott et al. (1988) (Figure 7). In the Mara-

caibo Basin, Azpiritxaga (1991) estimated a sea level

drop at the time of the Apon 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 Apon Formation. We can

rule out a sea level lowering caused by tectonic up-

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 Apon Formation

(Figure 2A).

Reservoir Implications of the Deeply BuriedPaleokarst Terrane

Previous workers have proposed that carbonate rocks

of the Cogollo Group may have been subaerially ex-

posed in the north-central parts of the Maracaibo Basin,

and that this exposure may have led to a complex dia-

genetic history that ultimately affected the porosity

distribution of this section (Kummerow and Perez de

Meja, 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 Apon and Maraca formations (Figure 2). In con-

trast, Azpiritxaga (1991) proposed that the porosity

of the Cogollo Group was mainly created by fractures

along major faults.

Figure 6. Seismic reflectiontime slice at 820 ms from anindustry 3-D survey in the Gipps-land basin, Australia, from Brown(1999) (used with permissionfromAAPG). Circular features areinterpreted by Brown (1999) assinkholes in a paleokarst topog-raphy of Miocene age. No hori-zontal scale or north directionwas provided in the originalpublication.


Mendez (1989) also proposed the influence of the

vadose and phreatic zones during the diagenetic evolu-

tion of the carbonates of the Cogollo Group in the Perija,

Urdaneta, and central lake areas of the Maracaibo Basin.

In addition, he proposed that brecciation and collapse

affected the base of the Apon Formation during the

Lower Cretaceous. Mendez (1989) did not consider

Oligocene basin inversion and uplift a significant mecha-

nism for brecciation and collapse of Cretaceous carbo-

nates as proposed by Kummerow and Perez de Meja

(1989).

Nelson et al. (2000) studied the Cretaceous res-

ervoirs in the La Paz field in the northwestern part of

the Maracaibo Basin. They concluded that the anom-

alously 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 Creta-

ceous carbonate rocks at any time during their Paleo-

gene history. We suggest that the brecciation and col-

lapse of the Apon Formation recognized by Mendez

(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 Apon 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 inter-

preted as sinkholes in a paleokarst terrane that formed

by subaerially tropical weathering of the shallow-water

carbonate lithologies of the Apon Formation. In addi-

tion, this study suggests that the paleokarst surface

formed during the Aptian, during a well-known eu-

static drop in the sea level observed in other parts of the

world (Figure 7).

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 forthe Maracaibo Basin, Venezuela, is taken from Azpiritxaga (1991).


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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deeply buried, early cretaceous paleokarst terrane, southern maracaibo basin, venezuela

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