Petroleum System Mbo Basin

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A U T H O R S

Alejandro Escalona Institute for Geo-physics, Jackson School of Geosciences, Uni-versity of Texas at Austin, 4412 SpicewoodSprings Road, Building 600, Austin, Texas,78759; [email protected]

Alejandro Escalona is a postdoctoral researcherat the Institute for Geophysics, University ofTexas at Austin. He received his Ph.D. in ge-ology at the University of Texas at Austin in2003, where he focused on the stratigraphicand structural evolution of the Maracaibo Basin,Venezuela. He is currently interpreting re-gional seismic and well data from offshoreVenezuela to link offshore and on-land Cenozoicdepocenters.

Paul Mann Institute for Geophysics, Jack-

son School 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 atthe State University of New York in 1983 andhas published widely on the tectonics of strike-slip, rift, and collision-related sedimentary ba-sins. His current focus area of research is theinterplay of tectonics, sedimentation, and hydro-carbon occurrence in Venezuela and Trinidad.

A C K N O W L E D G E M E N T S

We thank Petroleos de Venezuela, S. A., forproviding seismic and well data used in thisstudy. This work was supported by Grant40499-AC8 from the Donors of the PetroleumResearch Fund of the American ChemicalSociety to P. Mann. We thank S. Talukdar,D. Goddard, and R. Erlich for valuable re-

views. The authors acknowledge the financialsupport for publication costs provided by theUniversity of Texas at Austins Geology Founda-tion and the Jackson School of Geosciences.University of Texas, Institute for GeophysicsContribution 1775.

Editors Note

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

An overview of the petroleumsystem of Maracaibo BasinAlejandro Escalona and Paul Mann

A B S T R A C T

The geologically complex Maracaibo Basin in northwestern Vene-

zuela is one of the most prolific hydrocarbon basins in the world.

Having a basinal area of 50,000 km2 (19,300 mi2), the basin has

produced more than 30 billion bbl of oil, with estimated re-

coverable oil reserves of more than 44 billion bbl. The central

elements of the petroleum system of the basin include (1) a world-

class source rock (Upper Cretaceous La Luna Formation), depos-

ited on a shelf-to-slope environment under anoxic conditions and

modified by intermittent oxygenated periods and tectonic events;(2) high-quality clastic reservoir rocks deposited in Eocene and

Miocene fluviodeltaic settings; (3) two main periods of rapid tec-

tonic subsidence responsible for two pulses of voluminous hydro-

carbon generation, first, during Paleogene CaribbeanSouth Ameri-

can oblique plate collision and, second, during the Neogene uplift

of the Sierra de Perija Merida Andes; and (4) lateral and vertical

migration of oil along strike-slip, normal, and inverted faults, as

well as a regional unconformity of late EoceneOligocene age.

The maturation, migration, and trapping of hydrocarbons were

closely controlled by the tectonic evolution of the Maracaibo Basin.

During the Paleogene, the development of a foredeep along thenortheastern margin of the basin and the strike-slip reactivation of

the rift-related Jurassic faults on the Maracaibo platform controlled

the early structural setting of the source and reservoir rocks. Hy-

drocarbons migrated updip from source rocks beneath the north-

northeastern margin of the basin along north-south strike-slip faults

and into overlying Eocene clastic reservoirs in the south-central parts

of the basin. The second period of the Maracaibo Basin petroleum

system developed during subaerial exposure of most of the Mara-

caibo Basin during Oligocene Miocene uplift of the adjacent Sierra

de Perija and Merida Andes. Uplift of mountain ranges surround-

ing the basin folded and depressed the interior of the basin to formthe extensive Maracaibo syncline. Because of the formation of the

Maracaibo syncline, oil generation began in the central and southern

parts of the synclinal basin and migrated northward. Hydrocarbons

migrated up the flanks of the Maracaibo syncline along reactivated

AAPG Bulletin, v. 90, no. 4 (April 2006), pp. 657678 657

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

Manuscript received February 19, 2005; provisional acceptance April 21, 2005; revised manuscript

received September 28, 2005; final acceptance October 14, 2005.

DOI:10.1306/10140505038


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strike-slip faults and into Miocene rocks adjacent to

the uplifted mountain ranges. Escaping oil has formed

numerous surface seeps along the edges of the Mara-

caibo Basin.

INTRODUCTION

The Gulf Caribbean region currently contains 5% of

the total ultimate recoverable reserves of hydrocar-

bons on Earth (Horn, 2003) (Figure 1A). Venezuela

has the largest reserves of hydrocarbons of all the hy-

drocarbon regions of the western hemisphere, with

proved oil reserves of about 70 billion bbl oil and

proved gas reserves of 147 tcf (Figure 1) (U.S. Geo-

logical Survey, 2000; Audemard and Serrano, 2001).

These reserve estimates do not include the immense,

unconventional reserves of the Orinoco heavy oil belt,

with an estimated approximately 1200 billion bbl of

heavy and extra-heavy oil in place (Fiorillo, 1987;

U.S. Geological Survey, 2000).

The active tectonic setting of petroleum in Vene-

zuela is complex. Several tectonic belts that include

volcanic-arc, fore-arc, and back-arc basins are found off-

shore of the Venezuelan margin (Figure 2A). A west-to-

east younging pattern of thrusts and lateral ramp faults

and foreland basins onshore (Babb and Mann, 1999;Mann, 1999) (Figure2A) wereproduced by diachronous

oblique convergence between Caribbean arc terranes

and the South American continental margin from Late

Cretaceous (western area of Colombia) to the present

(eastern area of Trinidad) (Figure 2B). This ideal com-

bination of tectonic and stratigraphic events yielded

one of the most prolific petroleum systems in the world.

The 50,000-km2 (19,300-mi2) area of the Mara-

caibo Basin (Figure 3) is the most productive hydro-

carbon basin in the CaribbeanSouth America region

(Figure 1D). The ultimate total recoverable oil reserves

Figure 1. (A) Distribution of ultimate recoverable oil in the world; (B) distribution of recoverable oil in the world; (C) distribution ofgiant oil fields; and (D) ultimate oil reserves in the Caribbean and Gulf of Mexico (MMBOE). All data are from Horn (2003).

658 An Overview of the Petroleum System of Maracaibo Basin


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Figure 2. (A) Topographic-bathymetric map showing six main tectonic belts observed along the northern margin of South America:1 = Venezuela basin; 2 = Leeward AntillesAves Ridge; 3 = Grenada-Bonaire-Falcon basins; 4 = Lesser Antilles arcCordillera de laCosta; 5 = Tobago-Carupano basins; 6 = Barbados accretionary prism Columbus basin Eastern Venezuela Basin Maracaibo Basin.(B) Inferred position of the leading edge of the Great arc of the Caribbean at 90 Ma = Late Cretaceous; 60 Ma = Paleocene; 50 Ma =Eocene; 35 Ma = Oligocene; 15 Ma = Miocene; 0 Ma = Holocene (modified from Lugo and Mann, 1995).

Escalona and Mann 659


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are 44,188 billion bbl of hydrocarbon (Horn, 2003), and

total cumulative oil production is more than 30 billion

bbl of oil during its last 80 yr of commercial production

history (Talukdar and Marcano, 1994). The Maracaibo

Basin is considered a supergiant oil field because it con-

tains more than 10 giant oil fields, each with ultimately

recoverable hydrocarbons greater than 500 million bbl

(Halbouty, 2001; Mann et al., 2003).

The Maracaibo Basin is located in a triangular in-

termontane depression bounded by the Merida Andes

and Sierra de Perija (Figure 3). Eocene clastic rocks of

the basin are the most prolific reservoirs for light and

medium oil and account for 50% of the basins re-

serves (Talukdar and Marcano, 1994). Miocene clas-

tic rock reservoirs include 44% of known reservoirs,

whereas Paleocene, Cretaceous, and basement rocks

Figure 3. Oil fields, oil seeps, and major faults of the Maracaibo Basin. Most oil fields are located along major subsurface strike-slip faults, including the Icotea and Pueblo Viejo faults (map modified from Zambrano et al., 1971; location of oil and gas seeps are

from Link, 1952).



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include 6% of the known reservoirs (Talukdar and

Marcano, 1994). Eocene reservoirs are characterized by

complex stratigraphic and structural traps formed

during Eocene oblique convergence between the Ca-

ribbean and South American plates (Escalona, 2006;

Escalona and Mann, 2006a, b), Miocene reservoirs are

mainly found along the eastern edge (Bolivar Coast) of

the present-day Maracaibo syncline and are affectedmainly by east-west convergence (Taboada et al., 2000;

Guzman and Fisher, 2006) (Figure 3). Oil seeps fringing

the Maracaibo Basin are indicative of the prolific and

widespread petroleum system underlying the entire ba-

sin (Link, 1952) (Figure 3).

The main objective of this article is to provide an

overview of the petroleum system of the Maracaibo

Basin in the context of its tectonic history. In this article,

we summarize the most important tectonic events that

affected the generation, migration, and trapping of hy-

drocarbons and integrate relevant hydrocarbon and geo-chemical observations presented by previous workers.

GEOLOGIC SETTING

The sedimentary history of the Maracaibo Basin began

during the Late Jurassic, with the deposition of rift-

related rocks (La Quinta Formation) in structural lows

or half grabens controlled by linear, north-northeast

striking normal faults (Maze, 1984; Lugo and Mann,

1995). During the Early Cretaceous Paleocene, amixed clastic-carbonate platform developed across

the area of present-day Maracaibo Basin (Figure 1).

Thermal subsidence and tectonic quiescence of the

passive margin led to sediment accumulation and the

absence of deformation of the basin during this period

(Lugo and Mann, 1995). The few structures present in

the Maracaibo Basin during the Cretaceous formed by

tectonic uplift of the Western and Central Cordilleras

of Colombia (Figure 1). This uplift is responsible for an

increase in subsidence by the end of the Cretaceous

that resulted in deposition of thick marine shale of theColon Formation during the Maastrichtian (Lugo and

Mann, 1995; Parnaud et al., 1995). During the late

TuronianCampanian, the La Luna Formation was de-

posited in a shelf-slope setting under anoxic conditions.

The La Luna Formation became the main source rock

of northwestern South America (Renz, 1981; Bralower

and Lorente, 2003).

Late Paleocene and early to middle Eocene oblique

convergence between the Caribbean plate and the

northwestern margin of South America (Figure 2B)

produced a complex foreland wedge filled by clastic

sediments in the northeastern part of the Maracaibo

Basin (Stephan, 1977; Pindell and Barrett, 1990; Lugo

and Mann, 1995). The foreland basin was characterized

by an approximately 5-km (3.1-mi)-thick Eocene

wedge of fluvial-deltaic sedimentation (Misoa Forma-

tion), where the most prolific hydrocarbon reservoirs

of the Maracaibo Basin are concentrated. Paleogenecollision was characterized by northwest to southeast

migration of the depocenter through time over a lateral

distance of about 150 km (93 mi) (Stephan, 1985;

Lugo and Mann, 1995; Escalona and Mann, 2006a).

Isostatic rebound affected the central and eastern parts

of the Maracaibo Basin and produced the widespread

Eocene unconformity that exposed and subaerially

eroded the central and northeastern parts of the basin

until the end of the Oligocene (Escalona and Mann,

2003a, 2006a). Fluvial and shallow-marine sedimen-

tation continued in the south and southwest areas ofthe Maracaibo Basin (Erlich et al., 1997). The Eocene

unconformity represents the main seal above Eocene

reservoirs, but it is locally breached by faulting, allowing

the upward ascent of hydrocarbons into Miocene res-

ervoirs at the basin edges (Figures 3, 4).

The Miocene Holocene period is characterized by

the uplift and erosion of the Sierra de Perija and the

Merida Andes on the western and southeastern flanks

of the basin (Kohn et al., 1984; Shagam et al., 1984).

The formation of the north-south trending Maracaibo

syncline (Castillo, 2001; Mann et al., 2006) representsthe final stage of this uplift and convergence. The Mar-

acaibo syncline closely controls the present-day geo-

graphic configuration of the basin and the location of its

marginal oil seeps (Figure 3). The convergent structural

styles seen on seismic lines at deeper levels in the basin

are controlled by Oligocene and Miocene inversion of

Eocene rift-related structures in the central part of the

basin (Escalona and Mann, 2003b; Castillo and Mann,

2006; Duerto et al., 2006). Eocene inversion of rift-

related structures also caused faulting of lower Mio-

cene rocks overlying Eocene reservoir rocks. Followinga period of isostatic rebound during the Oligocene

(Escalona, 2003; Escalona and Mann, 2006a), a phase

of rapid MioceneHolocene subsidence began. Subsi-

dence was caused by the uplift of the bounding Sierra

de Perija and Merida Andes mountain ranges that is,

in turn, related to the convergence and subduction of

the Caribbean plate and collision of the Panama arc in

northwestern South America (Kellogg and Bonini,

1982; Taboada et al., 2000; Colmenares and Zoback,

2003; Cortes and Angelier, 2005).



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PETROLEUM SYSTEMS

Figure 3 shows the distribution of hydrocarbon res-

ervoirs in the Maracaibo Basin (Zambrano et al., 1971).

Most Eocene reservoir rocks are spatially aligned with

the north-southstrikingIcotea and Pueblo Viejo faults,

whereas most Miocene reservoirs rocks are clustered

along the eastern and northeastern margin of the present-day Lake Maracaibo (Figure 3). Ninety four percent of

hydrocarbon reservoirs in the Maracaibo Basin are found

within EoceneMiocene clastic rocks (Talukdar and

Marcano, 1994). Only 6% of reservoirs are found within

underlying CretaceousPaleocene carbonate rocks and

basement.

Figure 4 shows an east-west and a north-south

interpreted seismic line in the central Maracaibo Ba-

sin, summarizing the main elements of the Mara-

caibo petroleum system from Cretaceous source

rock to Eocene and Miocene reservoirs. The two in-terpreted seismic lines show the northeast thicken-

ing of the Eocene clastic wedge, the southwest thick-

ening of the MioceneHolocene clastic wedge, and

the main structural and stratigraphic controls of the

basin inherited from the north-northeaststriking fault

family.

Source Rocks

Hydrocarbon source rocks in the Maracaibo Basin areUpper Cretaceous marine carbonate rocks (calcare-

ous shales and argillaceous limestones) that make up

the La Luna Formation of CenomanianCampanian

age. Previous geochemical studies show that the La

Luna Formation is the source of 98% of the total

oil reserves found in the Maracaibo Basin (Zambrano

et al., 1971; Young et al., 1977; Renz, 1981; Talukdar

and Marcano, 1994). An additional 2% of the total oil

reserve was derived from nonmarine coals and shales

of the Paleocene Orocue Formation that are found in

the southwestern part of the basin (Talukdar andMarcano, 1994; Yurewicz et al.1998). Gonzalez de

Juana et al. (1980) proposed that Eocene and Mio-

cene terrestrial source rocks, now deeply buried in

the southern part of the basin, may act as additional

source rock to the La Luna Formation. Geochemical

analysis of Tertiary sedimentary rocks indicates no

significant hydrocarbon potential for Eocene and Mio-

cene shale, nor is there any evidence for oils corre-

lated to this type of source rocks (Talukdar and Mar-

cano, 1994; Tocco and Margarita, 1999).

Depositional Setting of Source Rocks of the La Luna Formation

The La Luna Formation has been the subject of many

previous studies since the beginning of the petroleum

exploration in the Maracaibo Basin in the early 20th

century. Previous studies that describe the deposi-

tional setting and composition of the La Luna Forma-

tion include the pioneering study of Renz (1981) and

more recent works by Perez-Infante et al. (1996), Er-lich et al. (1999a), Erlich et al. (2000), and a source

rock conference convened by SEPM and Petroleos de

Venezuela S.A. (PDVSA) (Bralower andLorente, 2003).

Figure 5 shows a stratigraphic chart with the position

of the La Luna Formation in the Cretaceous sequences

of the Maracaibo Basin, its isopach, and its typical well-

log response.

The La Luna Formation was deposited over a pe-

riod of approximately 20 m.y., extending from the

upper Cenomanian to upper Campanian (Figure 5).

Its thickness ranges from 60 m (196 ft) beneath thesouthern part of the basin to 150 m (492 ft) beneath

the northern part of the basin (Renz, 1981; Lugo and

Mann, 1995; Bralower and Lorente, 2003) (Figure 5B).

The La Luna Formation was deposited in oxygen-

depleted bottom-water conditions in a shelf-to-slope

marine environment (Perez-Infante et al., 1996), in-

fluenced by episodic bottom currents, debris flows,

turbidites, faulting, and intermittent upwelling condi-

tions (Macsotay et al., 2003; Zapata et al., 2003).

Paleowater depth of the La Luna Formation is inter-

preted to have been more than 40 m (131 ft) in a deepshelf setting, ranging from below storm-wave base

(Macsotay et al., 2003) to a depth of several hundred

meters (Boesi and Goddard, 1991; Parra et al., 2003).

Figure 6 shows the paleogeographic reconstruc-

tions for the La Luna Formation in the Maracaibo

Basin during the Cenomanian Campanian (Erlich et al.,

1999a). The structural configuration of the basin dur-

ing the Late Cretaceous was possibly influenced by

uplift of the Central Cordillera of Colombia (Renz, 1981;

Erlich et al., 1999a; Macsotay et al., 2003) (Figure 6).

Renz (1981), using cross sections from outcrops alongthe mountain range bounding the Maracaibo Basin,

identified basement paleohighs (e.g., Merida arch)

and basins (e.g., Machiques, Uribante, and Barquisi-

meto) in the areas surrounding the Maracaibo Basin

(Figure 6A). These paleohighs produced the thickness

variations in Cretaceous passive-margin sediments, in-

cluding the La Luna Formation in the south and south-

western areas of the basin (Renz, 1981).

The most controversial of these geological features

is the Merida arch in the southern and central regions



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Figure

4.

(A)Interpretedeast-west

seismiclineintheMaracaiboBasin.

Th

esectionshowsthemainstructuralandstratigraphicfeaturesoftheMaracaib

oBasinanditspetroleum

systems(seeFigure3forlocation).Migrationpathsfrom

sourcetoreservo

irarelocalizedalongmajorfaultsinthebasin(e.g.,

Icoteafault,

PuebloViejo,andA,

B,andEfaults).

HydrocarbonreservoirsareconcentratedinstructuralhighsbeneaththeEoceneunconformityandintheMiocen

ealongthenorthandeasternflankso

ftheMaracaibosyncline.

(B)Interpretednorth-southseismiclineintheMaracaiboBasin(seeFigure3

forlocation).HydrocarbonreservoirsintheMioceneareconcentratedintheupdippartoftheMiocene

clasticwedge.

Eoceneturbiditesprov

idegoodexplorationtargetsnorthoft

heBurroNegrofault.



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of the present-day Maracaibo Basin (Dewey and

Pindell, 1986; Salvador, 1986). The existence and ori-

entation of the Merida arch is significant for petroleum

systems because its existence likely controlled the dis-

tribution and thickness of Cretaceous source rock be-

neath the Maracaibo Basin. Two proposed orientations

for the Merida arch follow:

1. An arch perpendicular to the trend of the present-

day Merida Andes: This postulated arch would

be parallel to other arches in the region like the

northwest-southeast striking Baul arch that out-

crops east of the Andes and separates the Barinas ba-

sin from the Guarico subbasin to the east (Figure 2).

Cross sections along the Merida Andes based on

outcrop mapping by Renz (1981) and Salvador

(1986) show thinning or absence of Lower Cre-

taceous rocks (Ro Negro, Apon, and Aguardiente

formations), overlain by a thin section of UpperCretaceous rocks (Maraca, La Luna, and Colon for-

mations). Isopach maps of Cretaceous rocks beneath

the central Maracaibo Basin show Cretaceous rocks

thinning 1020 m (3366 ft) in the south and cen-

tral areas of Lake Maracaibo (Gonzalez de Juana

et al., 1980; Lugo and Mann, 1995). Figure 5B shows

an isopach of the La Luna Formation from Lugo and

Mann (1995). The La Luna Formation thins approxi-

mately 10 m (33 ft) in the south-central part of

Lake Maracaibo (dashed in Figure 5B). This subtle

change in thickness is interpreted by Lugo andMann (1995) as the continuation of the Merida

arch in the south and central areas of the Maracaibo

Basin.

2. An arch parallel to the trend of the present-day Me-

rida Andes and not affecting the area of the Mara-

caibo Basin: This proposed arch formed the Turo-

nian uplift of the Cordillera Central of Colombia

(Macsotay et al., 2003). This tectonic event might

have produced partial tectonic inversion along pre-

Cretaceous rift-related faults, which followed the

present-day strike of the Merida Andes (Macsotayet al., 2003) and the trend of the Neogene right-

lateral Bocono strike-slip fault zone (Schubert, 1982;

Kellogg, 1984; Stephan, 1985; Dewey and Pindell,

1986; Audemard et al., 1999).

Small changes in thickness of passive-margin rocks

between 10 and 20 m (33 and 66 ft) in the south-

central areas of Maracaibo Lake might be attributed

to facies changes or depositional processes instead

of paleostructural relief above a northwest-southeast

striking arch. Integration of outcrop and subsurface

data in both flanks of the Merida Andes foothills and

in southern Lake Maracaibo is required to solve theextent and orientation of the Merida arch in the Mar-

acaibo Basin. Geologic data used for interpreting the

Merida arch have been limited to outcrops in the

Merida Andes (Renz, 1981; Salvador, 1986) or using

sparse wells and two-dimensional seismic lines in the

southern Maracaibo Basin (Audemard, 1991; Lugo,

1991).

A Santonian change in depositional environment to

more oxygenated and cooler waters in the La Luna

Formation (Tres Esquinas Member) suggests the ad-

vent of tectonic activity (Erlich et al., 2000; Bralowerand Lorente; 2003; Parra et al., 2003; Zapata et al.,

2003). Late Cretaceous tectonic activity was possibly

related to the reactivation of faults beneath the basin

or regional plate convergence in western Colombia

that caused abrupt changes in the paleotopography

and paleoclimate and ended passive-margin conditions.

An increase in upwelling and more oxygenation of

shelf waters of northern South America may be related

to (1) the migration of the South American plate to-

ward the Cretaceous intertropical convergence zone

(Villamil et al., 1999); (2) an increase in freshwater run-off produced by the emergent Central Cordillera of Co-

lombia (Erlich et al., 2003); and (3) the establishment

of wet-dry cycles and submersion of paleobathymetric

barriers for ocean circulation (Erlich et al., 2003).

La Luna Source Rocks and Hydrocarbon Characteristics

The La Luna Formation is considered a good to ex-

cellent, oil-prone source rock (Talukdar et al., 1986;

Talukdar and Marcano, 1994; Yurewicz et al., 1998).

Comparison of gas-chromatographic and biomarker

characteristics of oils and La Luna source rock extractsshows that the La Luna Formation is the source rock

for more than 98% of the oil accumulations in the Mara-

caibo Basin (Talukdar et al., 1986; Talukdar and Mar-

cano,1994; Yurewicz et al., 1998; Erlich et al., 1999b;

Figure 5. (A) Regional-stratigraphic chart of the Albian to Maastrichtian stages in four different areas of the Maracaibo Basin (I toIV) (modified from Erlich et al., 1999a; Castillo, 2001). (B) Location map showing locations I to IV of the stratigraphic chart and thetotal thickness in meters of the La Luna Formation from well logs (modified from Lugo and Mann, 1995). (C) Gamma-ray log of awell in the south Lake Maracaibo area showing a typical response from Albian to Maastrichtian (modified from Castillo, 2001).



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Figure 6. Paleogeographic maps from Albian to Campanian (modified from Erlich et al., 1999a).The Cenomanian to Turonian period represents a mixed carbonate-clastic platform in the Mara-caibo Basin areas. The middle to outer shelf depositional environment characterized the centralMaracaibo Basin from the Albian to Campanian.



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Tocco and Margarita, 1999; Gallango et al., 2002). The

La Luna source rocks contain oil-prone type II kerogen

and are rich in hydrogen content, with the bulk of the

organic matter derived from algae and bacteria (Perez-

Infante et al., 1996). The average original total organic

carbon (TOC) of La Luna source rocks in the Mara-

caibo Basin is 5.6% (Talukdar and Marcano, 1994).

Maximum TOC values are locally as high as 16.7%(Erlich et al., 1999b). In the southwestern area of the

basin, the average TOC is 4.3% (Catatumbo; Yurewicz

et al., 1998; Llanos et al., 2000). In the Sierra de Perija

area, TOC values range from 3.7 to 5.7% (Gallango

et al., 2002) (Figure 7). In the Merida Andes, TOC val-

ues range between 1.7 and 2% (Erlich et al., 1999b)

(Figure 7).

Oil quality variations in oils derived from La Luna

source rocks are controlled by thermal maturity and

in-reservoir alteration (Talukdar and Marcano, 1994).

Unaltered oils vary in oil quality (API) according totheir maturity: marginally mature oils range from

11 to 16j API; mature oils range from 20 to 39j API;

and highly mature oils range from 37 to 55jAPI. With

increasing maturity, API gravity and saturated hydro-

carbon content increase, whereas vanadium, sulfur, and

polar compounds decrease (Talukdar et al., 1986;

Talukdar and Marcano, 1994). Unaltered oils are wide-

ly distributed in the Maracaibo Basin. The oils mi-

grated into reservoirs during the Eocene and later

during the Miocene Holocene ( Talukdar and Mar-

cano, 1994).Oil alteration in reservoirs occurred mainly as a

result of biodegradation and oil-oil mixing (Talukdar

et al., 1986; Talukdar and Marcano, 1994). Altered

oils mostly occur in the central and northeastern res-

ervoirs of the Maracaibo Basin (Figure 2). Biodegra-

dation of oils in shallow Eocene reservoirs occurred

during the Oligocene and in shallow Miocene reser-

voirs during the late Miocene Holocene. Biodegraded

oils have low API (


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Figure 7. Distribution in percentages of hydrocarbon generated by La Luna Formation source rocks in the Maracaibo Basin basedon calculations by Horn (2002). Total organic carbon (TOC) values were taken from the following sources: Llanos et al. (2000), Erlichet al. (1999b), Yurewicz et al. (1998), and Gallango et al. (2002). Distribution of oil seeps from Cretaceous and Paleocene sourcerocks is taken from Link (1952).



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Reservoir Rocks

Reservoir rocks in the Maracaibo Basin are found

throughout the stratigraphic section and range from

fractured basement metamorphic rocks to shallow, un-

consolidated, Miocene rocks. Structural traps are con-

trolled by a variety of features, including normal faults,

inverted faults on the flexed continental plate (Hardingand Tuminas, 1989; Escalona and Mann, 2003b), folds

in the foreland basin, and subsurface strike-slip faults

forming north-south anticlines (Escalona and Mann,

2003b). All trap types were charged with hydrocarbons

from underlying Cretaceous source rocks of the La

Luna Formation (Zambrano et al., 1971; Gonzalez de

Juana et al., 1980; Talukdar and Marcano, 1994; Erlich

et al., 1999a). Stratigraphic traps are found in hetero-

geneous, mixed fluvial, and tidal-dominated deltaic

systems defining regressive-transgressive cycles on the

Eocene Maracaibo shelf and nearshore to fluvial Mio-cene sandstone rocks (Guzman and Fisher, 2006).

Major reservoir facies are stacked distributary channels

and tidal bars (Maguregui, 1990; Ambrose et al., 1995;

Escalona, 2003). Hydrocarbon reservoirs can be clas-

sified in three main types:

1. Sub-Eocene reservoirs (Figures 9, 10): These reser-

voirs are located in deeply buried Cretaceous lime-

stone and Paleocene sandstone in central and south-

ern Maracaibo Basin (Figure 9A, D) andin less deeply

buried Cretaceous limestone and basement rocksin northwestern areas of the basin (Figure 10H).

Reservoirs include fractured rocks (basement and

Cretaceous limestone) associated with the reactiva-

tion of north-south strike-slip, northwest-southeast

striking normal faults (Figure 10B, D) and thrusts

(Figure 9A, C) related to the uplift of the Merida

Andes (Castillo and Mann, 2006).

2. Eocene reservoirs (Figures 10, 11): These are the

most prolific reservoir rocks in the Maracaibo Basin.

They are characterized by structural traps associated

with anticlines formed by strike-slip reactivationof north-northeaststriking faults (e.g., Icotea and

Pueblo Viejo faults and their related northwest-

southeast normal faults; Escalona and Mann, 2003b).

Traps also formed in fluvial-deltaic (tide-influenced)

sandstone facies traps truncated by the Eocene un-

conformity (cf. Figures 10E, H; 11K, L). The most

productive Eocene reservoirs are located in the cen-

tral and northeastern regions of the Maracaibo Basin.

3. Miocene reservoirs (Figure 11I, L): These form the

second most prolific reservoirs in the Maracaibo

Basin. The reservoirs are mainly fluvial sandstone

facies located in anticlines of early Miocene age (re-

activation of Eocene structures, Figure 11J, L) and

stratigraphic wedges beneath the Eocene unconfor-

mity (Figure 11I).These productions occur along the

northeastern shore of the Maracaibo Lake, near the

trace of the Burro Negro fault (Figures 3, 8). Where

no structural or stratigraphic traps existed, oil es-caped to the surface and formed seeps that outline

the edges of the Maracaibo Basin (Figures 3, 8).

Migration and Trapping

The petroleum system evolution of the Maracaibo Ba-

sin is summarized in four schematic cross sections in

Figure 12. Hydrocarbon migration and trapping oc-

curred in two main, tectonically controlled phases as

previously proposed by Zambrano et al. (1971), Gon-

zalez de Juana et al. (1980), Talukdar et al. (1986),and Talukdar and Marcano (1994).

1. Carbonate platform phase (Late Cretaceous

Paleocene) (Figure 12A): During this phase, the La

Luna Formation source rock was deposited on a

shallow, passive-margin, shelf-to-slope environment.

It thickness ranges from 40 to 150 m (131 to 492 ft)

(Figure 5B). Carbonate thickness variations were

controlled by minor basement relief of underlying

pre-Cretaceous structures like the Merida arch.

2. Foreland phase (early Eocene) (Figure 12B): Obliquecollision between the Caribbean and South Ameri-

can plates formed an asymmetric wedge of fluvial-

deltaic Eocene rocks that were deposited in a foreland

basin (Lugo and Mann, 1995; Escalona and Mann,

2006a). Cretaceous source rocks were buried to

depths of 5 km (3.1 mi) in the north-northeastern

part of the Maracaibo Basin and reached the oil win-

dow. A pull-apart basin controlled by reactivated Ju-

rassic north-northeaststriking faults formed in the

central Maracaibo Basin (Icotea subbasin; Escalona

and Mann, 2003b). Strike-slip faults provided ver-tical pathways for hydrocarbon migration from Cre-

taceous source rocks (La Luna Formation) to Eocene

reservoir sands.

The deeply buried Icotea pull-apart basin provides an

alternative setting for hydrocarbon generation above

deeply buried Cretaceous rocks (Figures 4; 12B, C)

(Escalona and Mann, 2003b). Vertical displacement

of major strike-slip faults bounding pull-aparts al-

lowed juxtaposition of Cretaceous source rocks and

Eocene reservoir rocks (Figure 4). Anticlinal traps



14/22



15/22

formed during creation of the pull-apart basin are

sealed by the Eocene unconformity (Escalona and

Mann, 2003b). The regional north-northeast dip of

the basin contributed to updip oil migration toward

the central areas of the Maracaibo Basin, where

higher quality fluvial and deltaic reservoir facies

are present (Escalona, 2003; Escalona and Mann,

2006). Trapping beneath the Eocene unconformityin the south-central Maracaibo Basin also occurs in

fluvial-dominatedreservoirs of Eoceneage (Escalona

and Mann, 2006b) (Figures 4B, 10).

3. Isostatic rebound phase (late Eocene Oligocene)

(Figure 12C): During the Oligocene, most of the

Maracaibo Basin was subaerially exposed and eroded

by isostatic rebound that followed the end of the

convergence foreland basin phase. This period of re-

bound and erosion lasted approximately 20 m.y. in

the central parts of the basin and is characterized by

the loss of hydrocarbons to the surface (Talukdarand Marcano, 1994). Furthermore, biodegradation

of oils occurred because of the invasion of meteoric

waters into shallowly buried Eocene reservoirs

(Bockmeulen et al., 1983; Talukdar and Marcano,

1994) (Figure 12C).

4. Maracaibo syncline phase (Miocene Holocene)

(Figure 12D): This phase of basin development was

characterized by uplift of the Sierra de Perija and the

Merida Andes, the formation of the north-south

trending Maracaibo syncline (Castillo and Mann,

2006), and early Miocene inversion of Eocene struc-tures in the central part of the basin. In contrast to

the Eocene, the Neogene depocenter was located in

the southern Maracaibo Basin, where continental

facies pinch out to the east-northeast to form major

stratigraphic traps (Figures 3, 4, 11).

The migration of depocenters from the northeast-

ern basin during the Eocene to the south-southeastern

basin in the MioceneHolocene contributed to a sec-

ond pulse of maturation of Cretaceous source rocks of

theLa Luna Formation in thecentral and southern partsof the Maracaibo Basin (Figure 8). This new period of

oil generation charged reservoirs of Eocene and Mio-

cene age. For reservoir rocks younger than Eocene, hy-

drocarbon migration occurred along fault zones that

breached the Eocene unconformity (Figure 12). These

diverse migration paths allowed east-northeast up-

dip migration from the deep part of the basin to Mio-

cene reservoirs (Figures 4, 12D). In Miocene reservoir

rocks, hydrocarbons are mainly trapped by (1) inverted

structures (Figures 4, 11I, L; 12D); (2) stratigraphic

wedges to the northeast (Guzman and Fisher, 2006)

(Figures 4, 11); and(3) seeps to the east, west, and southof the Maracaibo syncline (Zambrano et al., 1971;

Gonzalez de Juana et al., 1980) (Figures 3, 8).

CONCLUSIONS

The complex interplay of deformation, burial, and

sedimentation in the Maracaibo Basin during the Cre-

taceous and Tertiary combined to make the basin one

of the most effective and prolific petroleum systems on

Earth. Deposition and distribution of ideal source andreservoir rocks were stratigraphically and structurally

controlled by multiple tectonic events that led to hydro-

carbon generation, migration, and accumulation. The

main conclusions of this review include the following:

1. Geochemical analysis reveals that more than 98% of

the oil accumulation of the Maracaibo Basin was

sourced by the CenomanianCampanian La Luna

Formation. The La Luna Formation was deposited

under anoxic conditions with intermittent tectonic

and depositional events, including reworking by bot-tom currents, and entry of turbidites and debris flows

into the basin.

2. Three main tectonic phases of deformation are re-

sponsible for the multiphase evolution of the pe-

troleum system in the Maracaibo Basin: Phase 1: Paleogene oblique collision between the

Caribbean and northwestern South America: The

Maracaibo passive margin during the Paleocene

early Eocene created an ideal mechanism for the

rapid burial and maturation of the source rock,

the La Luna Formation in the northeastern areaof the Maracaibo Basin (Figures 7, 8, 12). The

Paleogene foreland basin and a major right-lateral

ramp fault (Burro Negro fault)controlled the initial

generation and migration event of hydrocarbons

Figure 8. Burial histories of wells in the Maracaibo Basin based on data compiled by Horn (2002) from the following sources:(A) Sanchez (1993), (B) Delgado (1993), (C) Molina (1992), (D) Molina (1993), (E) Ramirez and Marcano (1992), and (FH) Lugo andMann (1995). The percentage of hydrocarbon generation using Lopatins (1971) equations for a type II kerogen source rock basinis based on calculations by Horn (2002) for each well location. Shaded areas represent main periods of tectonic subsidence.



16/22

Figure

9.

Examplesoffoursub-Eoc

enehydrocarbonreservoirsintheMar

acaiboBasin.

Thesereservoirsarelocatedindeeplyburiedandfracturedmetamorphicbasementrocks

andinCretaceousand

Paleocenes

edimentaryrocks(>5-km

[>3.1-mi]depth).Theinsetmap

in

theupperrightcorner(modified

from

PDVSA

pamphlets,

1995and

1996,

unpublisheddata)providestheloca

tionofthesections.



17/22

Figure

10.

ExamplesofEocenehydrocarbonreservoirsintheMaracaiboBasin.

Eoceneclasticrocksarethemos

tprolificreservoirsintheMaracaiboBasinandareconcentrated

mainlyinthecentralandnortheaster

nareasofthebasinalongnorth-northe

aststrikingfaults.

Theinsetmapintheupperrightcorner(modifiedfrom

PD

VSApamphlets,

1995and

1996,unpublisheddata)providesth

elocationofthesections.



18/22

Figure

11.

ExamplesofMiocene

Holocenehydrocarbonreservoirsint

heMaracaiboBasin.

Thesereservoirs

arelocatedmainlyinthenorthandnortheastern

areasofthe

MaracaiboBasin.

Themostprolificre

servoirsarelocatedalongthenortheas

terncoastlineofthepresent-dayLakeMaracaibo(BolivarCoast)andalongthetraceoftheBurroNegro

faultzone.

Theinsetmapintheupperrightcorner(modifiedfrom

PDVSApamphlets,

1995and1996,unpublis

heddata)providesthelocationofthe

sections.



19/22

in the Maracaibo Basin. The source rock entered

the oil window in the northeastern part of the

basin adjacent to the Burro Negro fault zone

(Figures 7, 8). The fault was the approximate

southern boundary of the Paleogene depocenter

and fold-thrust belt located north of the fault.

Hydrocarbons migrated updip and southward

into the platform using strike-slip and normal

Figure 12. Summaryof four main tectonicphases controllingthe petroleum systemof the Maracaibo Basin:(A) carbonate platformphase; (B) foreland basin

phase; (C) isostatic re-bound phase; and (D)Maracaibo synclinephase.



20/22

faults as pathways. Hydrocarbons were trapped

in reservoir facies located within different struc-

tural highs. The La Luna Formation source rock

in the northen part of the basin is presently in

an overmature stage because of its deep (>5 km;

>3.1 mi) burial (Figures 7, 8). Phase 2: Late EoceneOligocene isostatic rebound:

Isostatic rebound was related to the release of con-vergent stresses as the collision progressed east-

ward and southeastward of the Maracaibo Ba-

sin. Hydrocarbons trapped during this period

in near-surface settings may have undergone

biodegradation. Phase 3: Uplift of the Sierra de Perija and Merida

Andes: This regional uplift is the main tectonic

mechanism responsible for the inversion of the

basin depocenter and creation of the second ma-

ture area of the La Luna source rock in the south-

ern part of the basin (Figure 7). The main clasticdepocenter tilted from the northeast to the south-

southwest during the Miocene to Holocene. The

La Luna Formation source rock entered the oil

window across the entire Maracaibo Basin. East-

west convergence formed the Maracaibo syn-

cline, reactivated major strike-slip faults as reverse

faults that breached the Eocene unconformity

(Figure 12D). The hydrocarbons used fault

breaches in the Eocene unconformity to migrate

updip from Eocene to Miocene reservoirs along

the flanks of the basin (Figure 4). The La Lunasource rocks in the south-central areas of the basin

are still in the mature to early mature stage and,

therefore, still have significant remaining hydro-

carbon generation potential (Figures 7, 8).

3. The Maracaibo Basin has a promising hydrocarbon

discovery potential in the mostly undrilled deeper

structural and stratigraphic traps of the central and

eastern basin (e.g., Icotea and Pueblo Viejo sub-

basins) (Figure 4A). More than 14 billion bbl of me-

dium to light oil of ultimate recoverable reserves

are predicted to be produced from these areas (U.S.Geological Survey, 2000).

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