Summa Et Al 2003 Hydrocarbon Systems NE Venezuela

Hydrocarbon systems of Northeastern Venezuela: plate

through molecular scale-analysis of the genesis and

evolution of the Eastern Venezuela Basin

L.L. Summaa,*, E.D. Goodmanb,1, M. Richardsona,2, I.O. Nortonb,3, A.R. Greenb,4

aExxonMobil Upstream Research Co., P.O. Box 2189, Houston, TX 77252-2189, USAbExxonMobil Exploration Co., P.O. Box 4778, Houston, TX 77060, USA

Abstract

The prolific, oil-bearing basins of eastern Venezuela developed through an unusual confluence of Atlantic, Caribbean and Pacific plate

tectonic events. Mesozoic rifting and passive margin development created ideal conditions for the deposition of world-class hydrocarbon

source rocks. In the Cenozoic, transpressive, west-to-east movement of the Caribbean plate along the northern margin of Venezuela led to the

maturation of those source rocks in several extended pulses, directly attributable to regional tectonic events. The combination of these

elements with well-developed structural and stratigraphic fairways resulted in remarkably efficient migration of large volumes of oil and gas,

which accumulated along the flanks of thick sedimentary depocenters.

At least four proven and potential hydrocarbon source rocks contribute to oil and gas accumulations. Cretaceous oil-prone, marine source

rocks, and Miocene oil- and gas-prone, paralic source rocks are well documented. We used reservoired oils, seeps, organic-rich rocks, and

fluid inclusions to identify probable Jurassic hypersaline-lacustrine, and Albian carbonate source rocks. Hydrocarbon maturation began

during the Early Miocene in the present-day Serrania del Interior, as the Caribbean plate moved eastward relative to South America. Large

volumes of hydrocarbons expelled during this period were lost due to lack of effective traps and seals. By the Middle Miocene, however,

when source rocks from the more recent foredeeps began to mature, reservoir, migration pathways, and topseal were in place. Rapid,

tectonically driven burial created the opportunity for unusually efficient migration and trapping of these later-expelled hydrocarbons. The

generally eastward migration of broad depocenters across Venezuela was supplemented by local, tectonically induced subsidence. These

subsidence patterns and later migration resulted in the mixing of hydrocarbons from different source rocks, and in a complex map pattern of

variable oil quality that was further modified by biodegradation, late gas migration, water washing, and subsequent burial.

The integration of plate tectonic reconstructions with the history of source rock deposition and maturation provides significant insights into

the genesis, evolution, alteration, and demise of Eastern Venezuela hydrocarbon systems. We used this analysis to identify additional play

potential associated with probable Jurassic and Albian hydrocarbon source rocks, often overlooked in discussions of Venezuela. The results

suggest that oils associated with likely Jurassic source rocks originated in restricted, rift-controlled depressions lying at high angles to the

eventual margins of the South Atlantic, and that Albian oils are likely related to carbonate deposition along these margins, post-continental

break up. In terms of tectonic history, the inferred Mesozoic rift system is the eastern continuation of the Espino Graben, whose remnant

structures underlie both the Serrania del Interior and the Gulf of Paria, where thick evaporite sections have been penetrated. The pattern of

basin structure and associated Mesozoic deposition as depicted in the model has important implications for the Mesozoic paleogeography of

northern South America and Africa, Cuba and the Yucatan and associated new play potential.

q 2003 Elsevier Ltd. All rights reserved.

Keywords: Venezuela; Hydrocarbon systems; Hydrocarbon exploration; Tectonics; Petroleum geochemistry; Hydrocarbon migration; Caribbean

1. Introduction

The complex tectono-stratigraphic provinces that com-

prise Northern Venezuela and adjacent ocean basins contain

several world-class petroliferous basins. Venezuela’s oil

fields alone have produced over 50 billion barrels of oil to

date, and remaining oil reserves are estimated to be over 70

billion barrels, plus the 250 billion barrels estimated to be

0264-8172/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0264-8172(03)00040-0

Marine and Petroleum Geology xx (2003) xxx–xxx

www.elsevier.com/locate/marpetgeo

1 Tel.: þ1-281-654-7342; fax: þ1-281-654-7726.2 Tel.: þ1-713-431-6014; fax þ1-713-431-6310.3 Tel.: þ1-713-431-4240; fax: þ1-713-431-6193.4 Tel.: þ1-281-654-7529; fax: þ1-281-654-7780.

* Corresponding author. Tel.: þ1-713-431-7102; fax: þ1-713-431-6151.

E-mail addresses: [email protected] (L.L. Summa),

[email protected] (E.D. Goodman), mark.richardson@

exxonmobil.com (M. Richardson), [email protected] (I.O.

Norton), [email protected] (A.R. Green).

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http://www.elsevier.com/locate/marpetgeo

recoverable from the Orinoco Heavy Oil Belt (James,

2000b). The hydrocarbons have accumulated in a unique

tectonic setting with a complex paleotectonic, fluid flow and

stratigraphic history. This complexity poses special chal-

lenges for exploring and developing the resource, and has led

to many years of detailed study and analysis of the region’s

petroleum geology. A recent synthesis of this work, with a

complete list of references can be found in James (2000a,b).

The motivation for our study was Venezuela Exploration

License Round I (1996), in which relatively untested

acreage was tendered for exploration licenses in several

large, disconnected tracts covering the full extent of

onshore, northern Venezuela. Fiscal terms favored the

government, bidding was expected to be competitive, and

selectivity was required. To address this challenge, we used

an integrated lithospheric plate- to molecular-scale

approach to unravel the complex hydrocarbon systems

history of the region, and understand the distribution and

character of the discovered resources. Our approach

included not just the evaluation of existing data, but

emphasized the acquisition of new field-geologic

observations, and collection of primary data deemed critical

to the project. With this integrated set of data and analyses,

we had the technical basis to evaluate the available acreage

from a genetic, geoscience perspective, and take advantage

of exploration opportunities as they became available. In

addition to positioning us for new opportunities, the study

also demonstrated the benefits of this type of approach for

evaluating the distribution and quality of oil and gas in other

complex tectonic settings, and accelerated our development

of new methodologies for hydrocarbon systems analysis.

In this discussion, we use the Eastern Venezuela Basin

portion of our study as an example of our genetic approach

to hydrocarbon systems analysis, beginning with the

tectonic evolution and crustal types underlying the sedi-

mentary basins, and culminating with the molecular

geochemistry of hydrocarbon-bearing fluid inclusions.

Location of the Eastern Venezuela Basin behind an active,

curvilinear belt of subduction, transpression, and tectonic

thickening has led to its comparison with other global

foreland basins. However, at both crustal and lithospheric

scales, its genesis and evolution are unique, making it one of

Fig. 1. Tectono-stratigraphic domains of northern South America and southern Caribbean. Boundaries separate distinct genetic provinces that have been

identified for purpose of communicating the similarities and differences between different geographic areas. Boundaries are color-coded in the legend. Key oil

and gas fields are shown in green and red, respectively. See also Table 1.

L.L. Summa et al. / Marine and Petroleum Geology xx (2003) xxx–xxx2

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the most prolific oil-bearing basins in the world. The

integration of diverse data types led us to understand where

and when key hydrocarbon systems elements came together

to realize its unique oil and gas-generating capability. As a

result of the study, we also recognized new and deeper

potential plays, and improved our global paleogeographic

models. We summarize these findings in Section 2.

2. Tectono-stratigraphic domains of northern South

America and the southern Caribbean

As a first step in developing the hydrocarbon system

history of Venezuela, we subdivided northern South

America and the southern Carribbean into tectono-strati-

graphic domains. The resulting map served both as the basis

for describing similarities and differences between areas, for

micro-plate reconstructions, and as an index map to enhance

communication between specialists working different tech-

nical aspects of the analysis. The area is so complex that a

minimum of 20 distinct, genetic province types is necessary

to realistically capture the distribution of tectono-strati-

graphic styles (Fig. 1). In discussing Fig. 1, we focus mainly

on the onshore, autochthonous provinces. Criteria for

defining the domains are summarized in Table 1. The

major domains that we have broken out for the regional

analysis of onshore, northern Venezuela include the

following:

† Espino Graben. This feature contains remnants of a

failed Jurassic-Early Cretaceous rift system. We postu-

late the eastward extension of the graben beneath the

Serrania del Interior and beyond, based on analysis of

oils in seeps and hydrocarbon-bearing inclusions, as

summarized in the discussion of basin architecture,

below. This postulated extension of the graben-fill

sediments has significant implications for deeper play

potential beneath the Serrania.

† Serrania del Interior mixed-style province. At the

surface, the Serrania del Interior is a thin-skinned fold-

thrust belt involving the Cretaceous and Tertiary

sections. We have interpreted the deep structure to be

composed of Mesozoic basement-involved normal faults,

similar in age to the Espino Graben, some of which have

been inverted. Differences in sediment thicknesses across

these inferred, Mesozoic normal faults may be respon-

sible for differences in the maturity of Cretaceous source

rocks preserved in the Serrania, as summarized in the

maturation discussion that follows. The sedimentary

section underlying the Serrania del Interior is believed to

be composed of Jurassic sedimentary and volcanic rocks,

Paleozoic sediments and metasediments, and evaporites,

which may form a decollement for the shallow thrusts.

† Faulted Platforms. In Eastern Venezuela, these are

dominated by ENE-striking (,N70E) normal faults.

The faults are interpreted as ‘flexural’ normal faults, and

represent something of a paradox, as they are interpreted

to have formed in a foreland setting, coincident with

contraction toward the hinterland (Bradley & Kidd,

1991). In this model, as thrusting and loading occur in the

hinterland, crustal-scale flexure occurs in the foreland,

with broadly distributed tensional faulting. These faults

form important petroleum traps (e.g. Oficina Field),

although there is also a large component of stratigraphic

trapping.

† Contractional domains. In addition to the Serrania

mixed-style province, other areas dominated by contrac-

tion include the North Monagas province, which contains

the well-known El Furrial trend (Aymard et al., 1990).

Although El Furrial is located in a dominantly

thin-skinned contractional province, it appears to have

elements of thick-skinned faulting as well (Roure,

Carnevali, Gou, & Subieta, 1994). Indeed, these thin-

skinned provinces overlie older, seismically imaged

extensional provinces and, in some cases, older normal

faults may be reactivated during compression. At the

southern end of the North Monagas contractional

province, and extending into the Maturin Foreland

province, is a complex zone of shale ridges either

underlain by contractional or wrench-related features.

Table 1

Criteria for defining tectono-stratigraphic domains

Criterion Example

Crustal type and age Guyana Shield is characterized by Precam-

brian crust

Distinct subsidence

mechanism

Gulf of Venezuela, Falcon, Bonaire, Cariaco

Trough, Crupano, Paria, and Caroni sub-basins

have all subsided due to Tertiary strike-slip

tectonics in an unstable transform plate margin

setting

Gravity/magnetics data Leeward Antilles Arc has a prominent gravity

high signature

Deformation processes

and timing

Llanos, Lara, Guarico, Oficina/Temblador/

Orinoco and Delta Amacuro foreland domains

are ‘flexural normal fault domains’, i.e. areas of

basement-involved normal faults formed due to

large-wavelength curvature of basement during

tectonic loading in the hinterland; distinguished

from each other based on timing of deformation

Structural style Serrania del Interior is distinguished by ‘mixed-

style’ compressional versus thick-skinned

domains

Tectonic position/

stratigraphy

Barbados and Curacao Ridge accretionary

wedges are distinguished by tectonic

setting; they contain mainly sedimentary rocks

deposited on oceanic crust, but

decoupled from that crust during subduction,

and often folded and deformed

Present-day tectonic

setting

Merida Foredeep is a depression located in front

of the asymmetrically bivergent

Merida Andes Compressional Fault Domain

Relative lack of

deformation

Guyana Stable Shelf domains, underlain by

Guyana Shield basement

L.L. Summa et al. / Marine and Petroleum Geology xx (2003) xxx–xxx 3

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† Maturin Foredeep. This is a composite domain that

formed due to Neogene contraction to the north. It

consists of Mesozoic normal faults inverted in the

Neogene, compressional structures and detached normal

faults related to dominantly east-directed progradation of

the paleo-Orinoco River system.

3. Tectonic evolution of northern South America

and the surrounding ocean basins

We began the analysis of individual hydrocarbon

systems elements by unraveling the tectonic evolution of

northern South America, using our new plate reconstruc-

tions, and incorporating significant published contributions

(Pindell & Barrett, 1990). All of these studies support

dividing the plate tectonic evolution of northern South

America into two main stages: (1) Mesozoic rifting and

passive margin development, and (2) Cenozoic transpres-

sive west-to-east motion of the Caribbean plate along the

northern margin. In this discussion, we present our own

reconstructions, as they contain significant departures from

those previously published, especially with respect to the

impact of Tertiary Pacific tectonic events on onshore

northern South America.

The plate-scale evolution is illustrated in simplified

format in Fig. 2a–f. In Early Jurassic time (Fig. 2a) North

America, Mexico, Yucatan, parts of Cuba and Africa were

parts of Pangea. The tip of south Florida was about as far

south as northern Venezuela. Along the Pacific margin,

protracted rifting during the Jurassic involved large areas

of Mexico and what is now the northern Andes Mountains

in Colombia and Venezuela. To the east, early rifting led to

seafloor spreading and eventual opening of the North

Atlantic and its southwestern extension, the proto-Car-

ibbean ocean (Fig. 2a–b; 140 Ma). This latter basin formed

along a new plate boundary between North American and

South American continental blocks. Its history has been

largely obscured by motion of the Caribbean plate. Africa

started spreading from northern South America by 112 Ma,

as the South Atlantic opened, stranding numerous failed

rifts at high angles to new plate boundary, including the

Espino Graben. The proto-Caribbean ocean basin termi-

nated to the west against a volcanic arc developed along

the Farallon plate subduction zone (Fig. 2b; 95 Ma). This

geometry persisted into the latest Cretaceous (Fig. 2c)

when the Caribbean plate started moving in from the

Pacific. During this period, northern South America was a

passive margin, with paleogeographic and paleo-oceano-

graphic conditions ideal for deposition of high-quality

source rocks.

The Caribbean plate initially moved to the northeast past

South America, but in the Eocene (Fig. 2d) when this plate

collided with Cuba, the motion changed to easterly. The

change in plate motion had profound effects on the present-

day northern South American continent, which lay several

hundred kilometers south of the inferred plate boundary

between North America and South America. Flexure in

front of the eastward-advancing Caribbean subduction zone

depressed the Maracaibo area of Venezuela, allowing the

deposition and stacking of major deltas. Thrusting in this

area also resulted in obduction of the Lara Nappes and, as

the Caribbean moved to the east, the Villa de Cura Nappes

(Fig. 1). Eastward motion of the Caribbean plate continued

through the Tertiary (Fig. 2e and f), setting up the major

basin-forming and hydrocarbon maturation and yield events

that took place across eastern Venezuela. In response to

ongoing eastward motion of the Caribbean plate, basin

subsidence events generally young toward the east along the

northern margin of Venezuela.

By the Early Miocene (Fig. 2e), northwestern South

America itself started to break apart along major faults.

Before this time, the Caribbean-South America plate

boundary was a relatively narrow zone probably located

close to the South American continent-ocean boundary. By

the Early Miocene, the boundary zone became a diffuse area

involving most of the crust that was rifted during Jurassic

time. We refer to this remobilized area as the ‘Bonaire

block’ (Fig. 2e). In Fig. 2e and f, darker gray areas denote

present-day outlines of the fragments of the Bonaire block,

while lighter gray denotes areas of compression. The

Bonaire block was deformed in a dextral transpressive

sense that continues today, terminating to the east near

Trinidad (Fig. 2e). The northeasterly-directed, dextral,

strike-slip component of motion was largely driven by the

NE-directed motion of the Cocos plate.

Near the end of the Miocene, there was a change in the

Cocos-Nazca plate boundary with respect to NW South

America, so that only the Nazca plate was subducting under

the Bonaire block (Fig. 2f). This resulted in a component of

eastward compression along the formerly strike-slip faults

bounding the Venezuelan Andes, with associated strain

partitioning along these fault zones. Eastward-directed

compression led to the uplift of the present-day Andes,

which sent vast volumes of sediments far eastward, across

the continent to the Orinoco delta and its deepwater

distributary systems. The Maracaibo basin itself underwent

renewed subsidence due to flexure-related uplift of the

surrounding Merida and Perija Andes regions. At present,

the active tectonic boundary in northern Venezuela follows

the southern margins of the Bonaire block, with the east-

southeast motion of the Caribbean driving transpression in

the Eastern Venezuelan Basin (Fig. 2f). To the west the

Bonaire block overrides the Carribean plate, north of Lake

Maracaibo, while northeastern Venezuela is being subducted

under the Caribbean along a transpressive plate boundary.

4. Basin architecture, tectonics, and sedimentary fill

The Eastern Venezuela Basin includes the region of

thinned continental crust bounded by the Guyana Shield


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Fig. 2. (a–f) Plate reconstructions. Each major plate is a different color, as denoted by the legend at the corner of each diagram. The 190 Ma

reconstruction (a) represents the plate configuration just prior to Jurassic rifting. The 95 Ma reconstruction (b) represents the maximum rate of

divergence between North and South America. All of northern Venezuela is a passive margin at this time. The 68 Ma reconstruction (c) represents the

onset of northeastward motion of the Caribbean plate, which resulted in the collision of the Cuban arc system with North America. The northern edge

of Venezuela is still a passive margin, as the North America plate boundary is far offshore. The 40 Ma reconstruction (d) shows the collision between

Cuba and the Caribbean plate, which resulted in a change in plate motion to easterly. North America and South America also began to converge at this

time. The 25 Ma reconstruction (e) illustrates the breakup of northwestern South America along major faults. Northwestern motion of the Cocos plate

drove transpression in the Venezuelan Andes, and began to push the Bonaire Block to the east. The arrival of the Carribean arc initiated compression

in the Paria ranges of eastern Venezuela. The final reconstruction (f) shows the present-day orientation of the plates. The Caribbean plate continues to

move eastward, driving continued transpression in eastern Venezuela. Eastward motion of the Nazca plate drives ongoing compression in the Andes,

and continuing translation of the Bonaire Block.


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Fig. 2 (continued )


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to the south, accreted metamorphic rocks to the north,

the Espino Graben to the west, the Barbados accretionary

complex to the northeast, and Atlantic oceanic crust to the

east (Fig. 1). The basin, with its vast hydrocarbon deposits,

is situated above, and is best defined by one of the largest

gravity lows on Earth (Fig. 3). The lithospheric-scale

gravity anomaly largely reflects the tectonic depression of

continental crust, due to subduction of South America under

the Caribbean in northeastern Venezuela. The gravity

anomaly is part of a continuous arc that begins southeast

of the Cariaco Trough (Fig. 1). This arc extends eastward

along the Maturin subbasin axis, then offshore east of

Trinidad, where there is a major internal tectonic boundary,

and finally, northward along the Barbados accretionary

prism, where Atlantic oceanic crust is being subducted

under the Caribbean Plate. This subduction process has

created the Antilles arc, and formed a gravity high in the

southeast Caribbean. The large magnitude gravity low itself

disappears in the vicinity of Trinidad where continental

crust thins and transitions to oceanic crust. The earthquake

epicenters shown in Fig. 3 reflect strain partitioning in a

scattered distribution around the active surface plate

boundary along the Araya–Paria–Northern Ranges pro-

vince (Fig. 1). To the south, an active fold-thrust belt

propogates southward over the large gravity low. Offshore

to the north, deep earthquakes are associated with the

arcuate subduction pattern described above, in addition to

strike-slip faults in the shallow crust. Onshore, earthquake

density decreases sharply west of the city of Maturin.

Eastern Venezuela’s basement structure and sediment fill

reflect its complex tectonic history and unique gravity

signature. Fig. 4 is an interpretive map that depicts the

distribution of sedimentary thickness across the basin. This

map is based on a compilation of (a) constraints from

petroleum drilling, with interpretations of Paleozoic and

basement penetrations from the literature; (b) estimates of

top crystalline basement based on internal calculations of

several hundred magnetic data points, and (c) constraints

from company seismic and published maps. Basement is

variable, but usually consists of Precambrian or Paleozoic

crystalline rocks. The general reliability of the magnetic

data interpretation was established via borehole calibration

to top basement structure in the Orinoco and Oficina-

Temblador Areas.

The main depocenters depicted in Fig. 4 include the

Espino Graben, the Serrania del Interior, the Maturin

foredeep, and onshore and offshore Trinidad, all fed by

the paleo-Orinoco River system. Most of the discovered

hydrocarbons are located adjacent to present-day structural

deeps, with a notable exception lying near the axis of

sedimentary accumulation (e.g. Furrial trend) discussed

below. Although not apparent on the map, it is notable that

the Serrania del Interior has undergone at least 15,000–

20,000 ft of post-depositional erosion, based on estimates of

vitrinite reflectance, sonic velocities, apatite fission track

analysis, and clay mineralogy. Thus, the great thickness of

sediment present today is actually significantly reduced

from its original thickness as a result of uplift and erosion

Fig. 3. Gravity map of northern Venezuela and southern Caribbean (Isostatic Residual onshore; Free Air offshore). Relative gravity lows are shown in cool

colors, and relative gravity highs are shown in warm colors. The extreme gravity low (blue) is centered directly beneath the Eastern Venezuela Basin. The

gravity low extends to the Barbados Accretionary prism at the leading edge of the Caribbean Plate. Modern seismicity is shown by the pink dots.


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Fig. 4. Top basement structure-total sediment fill. Darker colors represent greater total sediment fill. Locations of the main depocenters (Espino Graben, Eastern Venezuela Basin, and Antilles Forearc) are

labeled. The thickest sediment fill lies within the gravity low shown in Fig. 3. Note the variability in total sediment thickness in the present-day Serrania del Interior. At least some of this thickness variation may

be associated with changes in original depositional thickness across major normal faults. This variation is important for source rock maturation. Arrows on the south side of the basin identify two generally N-

trending, Archean-aged basement highs that focused sediment dispersal from the Guyana shield northward, during the Cretaceous through the Middle Miocene, resulting in extremely effective lateral migration

pathways. Oil and gas fields are shown in the green and red polygons.

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that began in the Miocene in response to the eastward

passing of the Caribbean plate to the north of the present

Serrania range.

An understanding of specific basin subsidence mechan-

isms provided important constraints for models of source

rock maturation discussed later in the paper. In the East

Venezuela Basin, subsidence occurred in response to a

variety of crustal to lithospheric-scale mechanisms:

† Mesozoic rifting and passive margin development:

subsidence occurred during faulting and thermal decay

associated with continental extension and sea floor

spreading.

† Tertiary subduction-related processes: this basin is part

of a curvilinear belt of tectonically-thinned crust that

forms the remarkable arcuate gravity low discussed

above. Neogene subsidence continues mainly because

South America is being pulled downward beneath the

Caribbean Plate.

† Tertiary flexural tectonic loading: tectonic loading

caused by southward-vergent thrust sheets localized the

basin axis in the Maturin foredeep. Southward migration

of this depocenter through time played an important role

in hydrocarbon systems evolution. The thrust-related

flexure is also superimposed on the much larger litho-

spheric-scale low, as imaged by the gravity map.

† Loading of sediments: sediment loading was associated

with extensive clastic deposition in major river systems

and the juxtaposition of high-standing accreted blocks.

During and before the mid-Tertiary, reservoir quality

clastics were funneled northward into the basin and

toward the Caribbean by an ancient river system with

headwaters in the Guyana Shield. During Andean uplift,

and uplift in northern Venezuela, eastward-flowing river

systems such as the modern Orinoco began to dominate.

† Transtension: the opening of the strike-slip bounded

Cariaco and Paria subbasins (Fig. 1) occurred during the

Late Miocene. The presence of extensional structures of

similar orientation in the Serrania del Interior, between the

two rhombochasms, may suggest that this mode of

subsidence is beginning to occur there, and may ultimately

bring about the collapse of this high-standing block.

The contours of total sediment fill as mapped in Fig. 4 also

imply that Mesozoic faults of the Espino graben system

actually continue eastward under the Serrania del Interior and

ultimately offshore. This interpretation is controversial, as

the oldest exposed section in the Serrania is Valanginian

(T.C. Huang, 1996; personal communication). Besides the

potential field data, several other indirect pieces of evidence

point to the eastward extension of the rift system and

associated Jurassic and lower Cretaceous section. First,

analysis of geochemical samples from seeps, fluid inclusions,

and reservoired oils provides evidence for a broad system of

oils derived from hypersaline, elevated salinity source rocks,

mostly likely Jurassic to early-Cretaceous in age, and

discussed in detail in Section 5. In addition to the

geochemical evidence, just west of Trinidad, the Couva-

Marine 2 and Couva-Offshore wells penetrated a thick

section of layered anhydrites (up to 9000 ft, according to well

reports). These evaporites, presumably early Cretaceous or

Jurassic in age, may have been deposited at the transition

between the end of rifting and the onset of sea floor spreading,

as in the Aptian salt basins of West Africa. In this model, the

evaporites would overlie older rift lake deposits, representing

an inferred early marine influx into silled, restricted basins.

Alternatively, the anhydrites are entirely non-marine in

origin and were formed in deep lakes with very high

evaporation/precipitation ratios. Finally, geophysical evi-

dence suggests a deep, older sedimentary section beneath the

thin-skinned fold-thrust belt exposed in the Serrnia del

Interior, a section that could have been preserved in Jurassic

age rift structures, and since inverted as the range was

elevated in the Miocene. This interpretation implies that

potential deep gas plays exist in the Serrania del Interior. As

discussed below, however, large amounts of hydrocarbon

were likely lost during mid-Tertiary subsidence. An alternate

hypothesis has the Serrania Range underlain by a stack of

thin-skinned thrust sheets. We believe that the relatively

uniform level of exposure at the outcrop and near-surface

better supports mixed-style contraction with significant

inversion.

One additional, relatively subtle feature of the total

sediment fill map is also of note in evaluating the

hydrocarbon systems. On the south side of the Eastern

Venezuela Basin, two Archean-aged basement highs (Fig. 4)

divide major zones of sediment fill. A major drainage

entered the basin between these two highs, focusing quartz-

rich sediment towards the northwest from Guyana Shield

source terranes (D. Swanson, 1995; written and oral

communication). This drainage provided sediments for

channel sandstone reservoirs in the Oficina trend, shallow

and deep marine clastics now exposed in the Serrania del

Interior and, probably, deep-water sandstones and siltstones

of Miocene age found on Barbados (Baldwin, Harrison, &

Burke, 1986). With Early to Mid-Miocene uplift of the

Serrania del Interior, these streams were deflected eastward

towards the Atlantic coast. Along with Andean uplift, the

Serrania uplift event helped to create today’s generally

east-directed Orinoco drainage system. We hypothesize,

however, that the long-lived N–S trending depositional

pattern established when the present Serrania area was low-

lying, and focused by the basement highs, helped provide

for exceptionally efficient ‘plumbing’ pathways for lateral

migration of hydrocarbons out of the foredeep.

5. Eastern Venezuela Basin evolution and hydrocarbon

systems development

Fig. 5 shows five cross-sections that depict the evolution

of the Eastern Venezuela Basin, and serve to illustrate


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Fig. 5. Sequential NW–SE, 1:1 geologic cross-sections depicting the tectonic and structural evolution of eastern Venezuela and impact on Querecual source rock maturation. See plate reconstructions for map

scale views. The present-day section shows the location of two basin modeling sites discussed in the text. Although not shown in these figures for purposes of simplicity, the top of the oil window is at ,3.5 km

depth. Restored fault geometries are based in part on quantitative restorations of seismic-based cross-sections. Stratigraphy is highly generalized. The 92 Ma cross-section is poorly constrained.

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the connection between the plate tectonic evolution, local

structuring, subsidence, uplift and hydrocarbon systems.

The geochemical specifics of these hydrocarbons and

associated lines of evidence supporting the timing of key

events are discussed in later sections of this paper. The line

of section runs from the edge of Guyana Shield outcrop,

across the Orinoco Heavy Oil Belt, Oficina Anticlinorium,

Maturin Foredeep, North Monagas Fold Thrust Belt and

Serrania del Inerior, stopping at the surface trace of the El

Pilar Fault. As a first-order observation, it is a testimony to

the forgiving nature of the hydrocarbon system that the

volumes of oil and gas preserved along this line of section

make it arguably one of the most petroliferous cross-

sections in the world in an area of great tectonic complexity.

The main features of each cross-section are summarized

below:

† 92 Ma. This is the defining event for the petroleum

geology of the Eastern Venezuela Basin, though the

basin setting at this time is not well understood. At

around this time, Cretaceous source rocks were deposited

in a passive-margin setting above Jurassic and early

Cretaceous rift fill. The geometries of syn-rift faults are

interpreted to influence the subsequent maturation

patterns of Cretaceous source rocks presently exposed

in the Serrania del Interior. Near the top of the rift-fill

sequence, we have depicted evaporites, as seen in wells

off western Trinidad. These are interpreted to influence

the structural evolution of this region at later times.

Between 92 and 20 Ma, the time interval shown in the

next cross-section, several important disruptions to this

‘passive’ margin occurred (Speed, 1995), including well-

documented early Tertiary uplift and erosion that is

difficult to relate to Caribbean-Atlantic plate interactions.

† 20 Ma. By this time, allochthonous nappes associated

with the encroaching Caribbean Plate have approached

from the northwest, (Fig. 2), and a deep foreland flysch

trough (Carapita Basin) formed where the Serrania del

Interior is now located. As this basin subsided and filled,

significant burial of Cretaceous source rocks drove early

maturation and expulsion of hydrocarbons. It is highly

likely that these early-generated hydrocarbons were lost

due to a lack of effective traps and regional seals.

† 15.5 Ma. By the Middle Miocene, oblique convergence

between the Caribbean and South American plates

continued to drive shortening, as reflected by the

obduction of the south-vergent Caribbean Nappes

(present-day Araya–Paria–Northern Ranges province

in Fig. 1). Both thin- and thick-skinned contractional

features have developed during the past 4.5 million

years. In a relatively short amount of time, significant

section was eroded from the eventually-inverted Carapita

Basin due to (a) tectonic uplift along new and reactivated

faults, and (b) isostatic uplift resulting from a change in

plate motions and associated stresses. In this model,

tectonic uplift occurs during times of significant

convergence between the Caribbean and South American

plates. During times of relatively low convergence along

the transform boundary, the relaxation of transpressive

stresses allows for regional isostatic rebound. We

attribute the major regional unconformities that exist at

approximately 15.5 and 10 Ma, as reflected in the

stratigraphic section, to these isostatic uplift events.

Based on interpretations of seismic reflection data, thrust

faults in this area cease to be active shortly after 15.5 Ma.

As a result of flexure associated with continued

contraction to the north, the Furrial Anticlinorium will

be buried again, with significant implications for source

maturation. Maturation of source rocks during this period

occurs in localized, transpression-related depocenters,

resulting in limited ‘wasting’ of hydrocarbons from the

broadly distributed Querecual source rock system.

† 10 Ma. Fault-related and isostatic processes continue to

drive regional uplift and erosion, and the development of

the regional unconformity at ,10 Ma. The inferred

hinge-line that separates regions of significant uplift and

erosion from those that primarily subsided has shifted

southward from the leading edge of the Caribbean

Allochthon toward the region near the southern limits of

the modern Serrania Range. Significantly, this hinge

zone lies at what is today the complex transition zone that

intersects the Quiriquire Field Area. By 10 Ma, a ‘new’

foreland depocenter develops over and in front of the

Furrial Anticlinorium and drives widespread maturation

and yield from Cretaceous source rocks in that area.

Concurrent marine shale deposition provides a topseal

for the migration and entrapment of large volumes of

fluids that now reside in the Orinoco Heavy Oil Belt.

Normal faults of the Oficina-Temblador area form at this

time due to flexure of the broad anticlinorium in the

Eastern Venezuela Basin foreland, and provide structural

traps for oil migrating from the north.

† Present Day. The Serrania del Interior remains a

significant topographic feature, with the El Pilar Fault

as the tectonic boundary that separates, at the surface,

parautochthonous rocks of the Serrania del Interior

from allochthonous, metamorphic rocks of the Araya–

Paria Peninsula (Fig. 1). The axis of the Maturin basin

has shifted slightly southward, with a thick Pliocene

section deposited there by the eastward-migrating

proto-Orinoco River system south of the deformation

front. Detached normal faults continue to be active. A

series of compressional shale ridges forms the

southern limit of the deformation front, probably

cored by older transpressional features. Active matu-

ration and yield persists in the deeps.

The events described above make up the principle

elements in the evolution of the Cretaceous hydrocarbon

system and help to explain the development of unique,

world-class accumulations, despite this complex tectonic

history. The presence of well-defined structural


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and stratigraphic fairways, combined with rapid,

tectonically-driven burial, created the opportunity for

unusually efficient migration and trapping of hydrocarbons

expelled during the Late Miocene, and resulted in the large

accumulations observed on the basin margins. Although

significant volumes of hydrocarbons were lost during initial

episodes of maturation in the Middle Miocene, sufficient

organic-rich rock still remained by the Late Miocene, when

all of the elements of the hydrocarbon systems were in place

and continued deformation created new kitchens. The

ongoing eastward shift of major depocenters through time,

coupled with lateral migration, caused hydrocarbons from

the different source rocks to be mixed, resulting in a

complex pattern of variable oil quality, which was further

modified by late gas migration, biodegradation, water

washing, and subsequent burial. In Section 6, we examine

the individual hydrocarbon system elements in detail.

6. Hydrocarbon systems

6.1. Source rocks

The depositional setting, stratigraphy and geochemical

characteristics of major eastern Venezuelan hydrocarbon

source rocks are addressed in several published studies and

thus summarized only briefly in this paper (Alberdi &

Lafargue, 1993; Arnstein et al., 1982; Erlich & Barrett,

1992; Krause & James, 1989; Parnaud et al., 1995; Persad,

Talukdar, & Dow, 1993; Talukdar, Gallango, & Ruggeiro,

1987; Tocco, Alberdi, Ruggeiro, & Jordan, 1994). The

major contribution of this study to the understanding of

hydrocarbon source rocks in eastern Venezuela was the

characterization of probable Jurassic-Cretaceous and

Aptian–Albian source rock units using new data from

petroleum seeps, reservoired oils, and fluid inclusions, as

described below. As exploration matures in eastern

Venezuela, these petroleum systems may provide the

opportunity for deeper play potential.

Fig. 6 summarizes the key stratigraphic horizons in

relation to their counterparts in western Venezuela. The two

well-documented groups of hydrocarbon source rocks of

Eastern Venezuela include: (1) Cretaceous, marine Quer-

ecual and San Antonio Formations, and (2) Miocene non-

marine and deltaic facies of the Merecure and Oficina

Formations as well as the partially time-equivalent, marine

Carapita shales. Fig. 7 shows the distribution of effective

source rocks from both of these depositional groups, based

on analysis of organic richness and maturity.

The Querecual and San Antonio Formations (also

known as the Guayuta Group) are thought to have

generated over 90% of the discovered hydrocarbons in

the basin, not including the Orinoco Heavy Oil Belt.

These Cenomanian through Campanian-age marine shales,

calcareous shales and bituminous limestones were depos-

ited under anoxic conditions in a shelfal setting as part of

a depositional system that stretched across northern South

America. The Guayuta Group is over 1000 m thick in the

Serrania del Interior and thins southward onto the South

American craton (Fig. 7). The organic matter in the

Guayuta Group is typically Type II, with measured

hydrogen index (HI) up to 700 mg hydrocarbon/gm

organic carbon and total organic carbon (TOC) up to

8%. Many of the measured samples have already reached

maturity. In-house calculations suggest that original

immature TOC values may have been as high as 12%.

However, there is significant lateral and vertical varia-

bility in source rock characteristics, and not all of the

Guayuta Group can be classified as a source of petroleum.

In general, oil potential decreases southward to the

Cretaceous onlap and upward through the San Antonio

Formation (Fig. 7). This variation in hydrocarbon source

potential is due to increasing contribution of Type III

organic matter, coupled with poorer conditions for the

preservation of organic matter.

Non-marine and deltaic intervals within the Miocene

Merecure and Oficina Formations also contribute to

hydrocarbon accumulations in both the Maturin and

Guarico Sub-Basins. Some shales and coals within these

units contain Type III/I organic matter capable of generating

both oil and gas, with TOC values that exceed 5%. In-house

calculations suggest that these source rocks may have

generated up to 5% of the discovered hydrocarbons in

eastern Venezuelan. Both source rock data and paleogeo-

graphic studies indicate that the oil potential of these source

Fig. 6. Stratigraphic column for Venezuela. Shows comparison between

major units for eastern and western Venezuela.


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Fig. 7. Distribution of effective source rocks. The large filled regions depict the distribution of Cretaceous source rocks. Cretaceous source quality is highest in the northern portion of the study area, and degrades

toward the south due to an increasing contribution of terrigenous organic material and poorer preservation. Cretaceous source rocks are assumed to have been eroded north of the El Pilar fault, and in the Gulf of

Paria. The southern boundary of effective source rocks represents the maximum onlap edge. The much smaller zone of effective Tertiary source rocks is the area inside the tan polygon. The boundary of this zone

is controlled by maturation.

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rocks diminishes eastward as deltaic facies of the Merecure

Group change to open marine shales of the Carapita

Formation. The basal Carapita Fm in the Maturin Sub-

Basin contains potential oil and gas source rocks with mixed

Type III/II organic matter, HI values of 350 and TOC values

of up to 4.5%. However, the remainder of the Carapita

Formation appears to be non-source prone. The eastern-

most limit of effective Miocene source rocks is difficult to

define due to deep burial and lack of samples. These source

rocks appear to be effective at least as far east as Pedernales

Field, based on biomarkers from seeps in the Guanipa area,

which are discussed more fully in Section 6.2. There are also

many areas where these rocks are thermally immature and

hence have not generated hydrocarbons. Integration of this

study’s potential Miocene source rock maps with Tertiary

thermal maturation maps produced the distribution of

effective Miocene source rocks incorporated in Fig. 7.

As noted above, in addition to the two well-known

source rock groups just described, we infer the presence

of two older source rocks, not widely considered in

Venezuelan petroleum geology: (1) Jurassic to early

Cretaceous, lacustrine source rocks with biomarkers that

occur in non-marine, elevated salinity environments, and

(2) Aptian–Albian marine source rocks with geochemical

characteristics similar to lower Cretaceous carbonate source

rocks found elsewhere around the Atlantic margins, and

distinct from Guayuta Group source rocks. These con-

clusions are based on detailed geochemical analysis of

petroleum seeps and reservoired oils described below.

6.2. New hydrocarbon occurrence data:

seeps and inclusions

Multiple publications summarize the oil and gas

occurrences of eastern Venezuela (Aymard et al., 1990;

James, 2000a,b; Parnaud et al., 1995; Talukdar et al., 1987).

At the time of this study, however, very limited hydrocarbon

source, maturation and migration data existed over a large

portion of our study area, from the Temblador trend east

across the Orinoco Delta and north across the Maturin Sub-

basin to Trinidad, and into the Serrania Del Interior. To fill

the gap in our knowledge of this portion of the study area,

we collected oil and gas samples from several seeps located

just onshore in eastern Venezuela, and analyzed hydro-

carbon-bearing fluid inclusions trapped in quartz cements in

Miocene sandstones that crop out in the Serrania del

Interior. The hydrocarbon occurrence map (Fig. 8) shows

the distribution of oil and gas seeps, and fluid inclusion

sample localities. The interpreted sources of the seeps and

reservoired oils are summarized in Fig. 9. When integrated

with regional stratigraphy and structural geology, the

detailed geochemical analyses and interpretation of the

seeps and inclusions allowed us to define the hydrocarbon

systems in the area with little well data and limited

geophysical coverage. This integration further enabled us

to extend the distribution of known, mature hydrocarbon

source rocks and postulate new source intervals.

The seeps vary from 0.5 m to 5 km in diameter and most

are continuously flowing. Guanoco Lago Asphalto, just west

of the Gulf of Paria on the northern flank of the Orinoco

Delta is already well known (Halse, 1932). It shows clearly

on satellite images and is the largest known natural surface

hydrocarbon manifestation in the world. The seep is 5 km

across and contains an estimated 50 million barrels of

heavy, biodegraded crude at the surface. The oils in that

seep correlate well with other Gulf of Paria oils sourced

from the Cretaceous Guayuta Group of Eastern Venezuela

and the equivalent Naparima Hill and Gautier Formations of

Trinidad. Other oil seeps collected from the eastern

Venezuelan coast and tidal inlets south of Trinidad have

also been correlated with the Cretaceous Querecual marine

source rocks and establish the effectiveness of cross-stratal

migration pathways to the surface.

We infer the presence of Jurassic-Cretaceous lacustrine

source rocks from several reservoired oils, as well as oil

samples recovered in two seeps on the northern Paria

Peninsula, and in two populations of hydrocarbon-bearing

fluid inclusions. The oils in the inclusions were analyzed via

a bulk technique, in which several grains containing

inclusions are crushed, and the oils from the inclusions are

extracted into a mass spectrometer. This population of oils

has a significantly different biomarker and isotopic

signature from the Cretaceous-sourced oils and is inter-

preted to be from a lacustrine, perhaps elevated salinity

source rock, possibly of late Jurassic or early Cretaceous

age. Detailed analysis of high-resolution biomarker data,

particularly the regular sterane distributions from the seeps

and inclusions were used to support the interpretation

(Fig. 10). The presence of an additional, older source rock is

also supported by the interpretation that widespread

Querecual source rocks have been eroded from the northern

Gulf of Paria area where the seeps occur. Biomarkers

characteristic of this postulated Jurassic/Early Cretaceous

source also occur in reservoired oils and seeps to the south

of the Paria Peninsula and may indicate a mixing of the

lacustrine, elevated salinity hydrocarbons with Querecual,

marine, calcareous black shale-sourced oils.

The distribution of Jurassic/Cretaceous-sourced oils is

summarized in Fig. 9, as well as on the inset map in Fig. 10.

We found no evidence for the presence of this source facies

to the west, along the southern border of the present-day

Espino Graben. There may be a number of reasons for this,

including sample bias, change in source facies, lack of

preservation due to migration prior to seal development, and

failure to recognize a Jurassic biomarker signature within

largely Tertiary sourced oils. However, further efforts to

quantify the distribution of this facies were beyond the

scope of this study.

In contrast to the northern Gulf of Paria, oil seeps from

the southwest coast of the Gulf of Paria are interpreted to be

a mixture of Cretaceous marine and Tertiary non-marine


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Fig. 8. Hydrocarbon occurrences. Hydrocarbon seeps and fluid inclusion localities superimposed on oil and gas occurrences. Note the abundance of seeps along the eastern Venezuelan margin.

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Fig. 9. Oil families. Interpreted sources of seeps, inclusions, and reservoired oils. Two main families comprise the majority of occurrences: (1) Guyuta group, marine-sourced oils, shown in blue, and (2) Tertiary,

non-marine sourced oils, shown in green. Oils derived from probable Jurassic, elevated-salinity source rocks are shown in pink. A single seep that correlates with a carbonate, bacterial, Type II source occurs on

the far eastern side of the delta, and is denoted by a blue square.

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sourced oils. This extends the distribution of known Tertiary

non-marine, effective source rocks eastward from the

Oficina and El Furrial trends.

On the extreme southeastern edge of the Orinoco Delta

an isolated seep shows evidence of yet another source rock

type (Fig. 9). The oil in this seep is interpreted to have been

derived from an early Cretaceous age, carbonate source rock

with a biomarker signature distinct from the Cretaceous

Guayuta Group. It is more closely correlated to carbonate-

sourced oils found elsewhere around the Atlantic Margin,

and along the coast of West Africa. Its appearance in

Eastern Venezuela is significant, in that previous studies

(James, 2000a) have also described metamorphosed Lower

Cretaceous carbonates and clastics in the Serrania del

Interior. These sediments include graphitic schists and

marbles that suggest at least intermittent accumulation of

organic-rich sediments along the Early Cretaceous passive

margin of Northern Venezuela. Examination of the tectonic

reconstructions for the Cretaceous (Fig. 2) suggests that

unmetamorphosed sediments of similar age could now be

found in Cuba and the Yucatan, with possible implications

for the hydrocarbon systems of those two regions.

The majority of the gases recovered from seeps in the

Orinoco Delta area are either biogenic in origin or derived

from the biodegradation of earlier-generated hydrocarbons.

If there are gases generated by thermally overmature source

rocks in the area, they do not appear to be migrating to the

surface. However, gases collected near the Paria Peninsula,

not far from El Pilar, do have high thermal maturity and are

in part generated from the thermal breakdown of carbonates.

6.3. Oil families

In addition to analyzing oil and gas seeps and fluid

inclusions, we also reviewed the geochemical (bulk and

Fig. 10. Fluid inclusion analysis. Several populations of oil inclusions were observed trapped in quartz cements in outcrops sampled in the Serrania. Biomarker

analysis of the oils in the inclusions is consistent with oils in seeps derived from probable Jurassic, elevated-salinity source rocks. The distribution of oils from

these source rocks is shown superimposed on the modeled eastward extension of the Espino Graben in the lower right corner of the diagram. It is this mapped

distribution of oils that helps validate the eastward extension of the graben.


ARTICLE IN PRESS

molecular) characterizations of 250 reservoired oils and

generated a detailed map of oil families across eastern

Venezuela (Fig. 9). Supplemental bulk and molecular

geochemical analyses were carried out on all available oil

samples. The study took advantage of samples from our

corporate oil library, which includes oils collected from

Venezuela from 1912 until 1976. The resulting distribution

of oil types across Eastern Venezuela was used to define the

various hydrocarbon systems, map migration fairways and

predict oil quality.

As described above, oil families associated with eastern

Venezuela’s two main source rock intervals (described in

Section 6.1) account for nearly all of the hydrocarbons

discovered to date. However, Fig. 9 also shows distinct

variations in the source facies, from dominantly carbonate

over most of eastern Venezuela, to dominantly shale over

central Trinidad. Mixing of later-generated Tertiary-sourced

oils is common, especially in the Oficina reservoirs and

along the edge of the Orinoco Heavy Oil Belt. Tertiary, non-

marine-sourced oils are most common in the Oficina, Anaco

and Las Mercedes Trends. Along the eastern edge of the

basin, Cretaceous marine oils are also mixed with oils

derived from the inferred non-marine, elevated salinity

source of probable Jurassic age. Mixing of oils from these

different oil families has significant implications for oil

quality, as discussed in Section 6.1.

6.4. Regional maturation and yield models

As part of the regional hydrocarbon systems analysis,

we performed thermal history and yield calculations to

infer the timing of hydrocarbon maturation and yield. The

results of our thermal history analyses are summarized in

Figs. 11 and 12, which show modeled present-day thermal

maturities on regional horizons equivalent to the tops of

Cretaceous Guayuta Group, and Miocene Merecure Group

source rocks. Note that these maps greatly simplify the

actual maturity patterns likely to be present in the thrusted

structures of the Serrania del Interior and foothills. The

regional surface was mapped using a hanging wall cut-off,

and thus ignores the maturities in the toes of the

individual thrust sheets. On Fig. 11, burial history insets

show examples of individual sites used to constrain the

maps. Labels also indicate the onset of maturation for the

Guayuta Group, superimposed on the present-day

maturities.

The models were calculated using a 1D, in-house basin-

modeling program, and calibrated using measured present-

day temperatures and thermal maturities. Temperatures

were collected from well logs and tests. Maturities are

inferred from a combination of organic and inorganic

thermal history indicators, including Ro, TAI, Tmax, illite

ages, clay-mineral transformation ratios, apatite fission

tracks, fluid inclusions, and quartz cement abundance.

Uplift timing was constrained by apatite fission track

measurements on outcrop samples from the Serrania del

Interior. The uplift ages are variable, but generally range

from ,30 Ma in the west, to ,5 Ma in the east. Where no

measured temperature or maturity data were available,

thermal models were extrapolated from the nearest control

point by reconstructing regional heat flow and structure,

while honoring local stratigraphy.

Based on these models, Cretaceous source rocks are still

in the oil window over large areas of eastern Venezuela,

despite being buried to great depth. The great depth of the

oil window is controlled mainly by low heat flows

associated with rapid subsidence of thinned continental

crust, as outlined by the large gravity low. Cretaceous

source rocks do enter the gas window in offshore eastern

Venezuela, and also in portions of the Serrania del Interior,

but not across the entire outcrop belt. Tertiary source rocks

are overmature only in the far western portion of the study

area. Although the regional nature of our mapping

simplified the maturity patterns, we were surprised to find

that not all of the source rocks in the Serrania appear to be

overmature, despite its 15,000–20,000 ft of uplift. We

hypothesize that the variation in maturity is controlled by

sediment thickness variations adjacent to reactivated normal

faults that underlie the Serrania. This implies some

remaining potential for the Cretaceous hydrocarbon system

in the Serrania, which might be better defined with more

detailed mapping and integration of structural and thermal

maturity data.

The thermal history models further suggest that

hydrocarbon yield occurred in several pulses, attributable

to regional tectonic events, as shown in Fig. 11. As the

Carribean plate moved eastward (Fig. 2c), southeast-

directed transpression and nappe emplacement drove

maturation of local source rocks episodically along the

northern margin of the craton, with active kitchens

moving south and east through time. Maturation began

in the Early Miocene in the western part of the study area,

and the Middle Miocene in the present-day Serrania. We

hypothesize that significant volumes of hydrocarbons

generated during these initial episodes of maturation

were lost, because effective traps and seals were not yet in

place. The detailed relationships between maturation and

trap/seal timing are described in Section 6.5 on the

evolution of the eastern Venezuela hydrocarbon systems.

From the Upper Miocene to the Present, the hydrocarbon

maturation kitchens continued to move southward and

eastward. Local kitchens ceased generation when the

craton rebounded after passage of the leading edge of the

Caribbean plate and are considered to be ‘fossil’ systems.

The exception is in far eastern Venezuela, where

Caribbean-related tectonism is still active, and both

Cretaceous and Tertiary sources are still yielding today.

Multiple pulses of maturation and migration appear to

have had significant effects on the quality of reservoired

oils in eastern Venezuela, as discussed below.


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Fig. 11. Regional top Cretaceous maturity map. Green fields indicate where top Cretaceous source rocks are presently in the oil window. Red fields indicate where the top Cretaceous is overmature. Calibration

sites and thermal history control points are superimposed. The two insets show 1D burial histories for two sites in the northern part of the study area. The rocks at Site 1 were uplifted in the Middle Miocene,

whereas the rocks at Site 2 were continually buried. Blue labels superimposed on the maturity fields show the onset of maturation and yield.

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Fig. 12. Regional Tertiary maturity map. Green fields indicate where Miocene source rocks are presently in the oil window. Red fields indicate where the Miocene source rocks are overmature. Calibration sites

and thermal history control points are superimposed.

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Fig. 13. Regional secondary migration analysis. Secondary migration vectors are superimposed on top Cretaceous maturity contours. The vectors are color keyed based on whether they tap kitchens dominantly in

the oil window, or dominantly in the gas window. The bold line running through the center of the Maturin sub-basin is a major drainage divide. South of that line, hydrocarbon migration is mainly strata-parallel,

from north to south. North of that line, hydrocarbon migration is vertical along faults, and lateral within individual thrust sheets.

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6.5. Regional migration models

Fig. 13 is a regional synthesis of lateral migration

pathways at the top of the present-day Cretaceous source

horizon. Migration pathways for oil and gas are linked to the

maturation analysis and distinguished by the green (oil) and

red (gas) colors of the migration vectors. Similar to previous

studies (Talukdar et al., 1987), north of the foreland axis, we

model vertical migration along thrust faults, coupled with

strata-parallel migration within reservoir-prone units. South

of the foreland axis, migration is mainly strata-parallel,

from north to south, and aligned with the orientation of the

major depositional fairways. Not shown in this regional

analysis are smaller scale modifications to lateral migration

pathways associated with faults in the Oficina Trend. The

ENE–WSW orientation of normal faults in the Oficina

Trend are likely to have had the effect of diverting

southward-directed migration toward the southwest.

As part of the migration analysis, we performed a formal

verification or ‘audit’ of hydrocarbon type and quality, by

comparing the observed physical properties of reservoired

hydrocarbons to properties predicted from hydrocarbon

yield and migration models. This analysis was largely

qualitative because multiple hydrocarbon sources contribute

to Eastern Venezuela accumulations, and the data available

for the audit were largely from scouting reports. None-

theless, audits were performed for oils in Cretaceous, Upper

Oligocene–Lower Miocene, and Upper Miocene–Pliocene

reservoirs, using over 100 fluid properties observations for

reservoirs in Central Venezuela, the southern portion of the

Oficina Trend, and the Furrial Trend. The maturities of most

of these oils coincide with the present day maturities in the

drainage areas, consistent with the migration vectors shown

in Fig. 13. Although the complexities of the migration

pathways within the thrust belts are not shown on this

regional map, detailed maturation and migration analyses

within individual thrust sheets also support the general

conclusion that the maturities of the reservoired hydro-

carbons coincide with the present-day maturities of the

kitchens. The impact of multiple pulses of hydrocarbon

maturation and yield is most evident in the variable quality

of the oils preserved in different reservoirs. A number of the

oils in both the Oficina and Furrial Trends show evidence

for a complex charge history, with earlier charged oils

biodegraded and subsequently recharged. Details of these

observations are discussed in Section 6.6 on oil quality.

An additional issue for secondary migration analysis in

eastern Venezuela has been to explain the large volumes of

hydrocarbons observed in the Orinoco Heavy Oil belt,

assuming that migration occurred from the north, and that

the only allowable volumes of hydrocarbons were those

generated post-15 Ma (post-seal deposition). George and

Socas (1994) explained this discrepancy by inferring lateral

migration from the east. Our calculations suggest that for a

source rock with original total organic carbon (OTOC)

ranging from 5–12%, and hydrocarbon yield ranging from

200-400 mg hc/g OTOC, over an area of 100–150 km2,

assuming relatively efficient lateral migration, sufficient

hydrocarbon is yielded from the drainages to the north.

However, focusing of migration from the northeast toward

the southwest by flexural normal faults would allow the

‘effective’ kitchen areas to be much larger than inferred

from our simple regional reconstructions.

6.6. Oil quality

As alluded to in the previous sections, variations in

source quality, maturation, migration pathways, and post-

emplacement processes have contributed to significant

variability in the quality of eastern Venezuela crude oils.

East Venezuelan crudes have a wide range of properties

from the well-known heavy (5–208 API), high sulfur

(1–6% S) oils of the Orinoco Heavy Oil Belt to the light

(308 þ API), low sulfur (,0.5% S) oils of the Oficina and

Anaco trends. We have systematically integrated the

detailed oil and rock geochemistry with maturation timing,

migration analysis and geologic framework to identify the

major processes controlling variation of oil quality and to

predict the distribution of oil quality.

Low quality oil is defined as low gravity, high in sulfur,

asphaltenes and metal contents, with high acid numbers,

whereas high quality oil is just the reverse. The primary

controls on oil quality in eastern Venezuela are hydrocarbon

source facies and maturity. Cretaceous, marine carbonate

sources tend to produce low quality oil, and Tertiary, non-

marine clastic sources tend to produce waxy, high quality

crudes. Mixing small amounts of Tertiary-sourced oil with

the dominant Cretaceous oil improves the quality of the

Cretaceous-sourced crudes. We believe this process is

responsible for some of the higher oil quality areas in the

western portion of the Orinoco Heavy Oil Belt. Increasing

the maturity of both oil sources also increases the quality of

the generated oils, as demonstrated by the deeper Oficina

Trend fields. At a first order, one can predict the quality of

reservoired oils by mapping the distribution of source rocks,

maturation, timing, and migration pathways.

In a number of accumulations in eastern Venezuela, oil

quality has been modified by a variety of post-emplacement

processes, here classified as a secondary control. Interpret-

ations and distribution of post-emplacement processes

affecting the oils, based on geochemically analyzed

samples, are summarized in Fig. 14. Post-emplacement

alteration of oil has a significant impact on oil properties

with potential to increase or decrease oil quality. Post-

emplacement alteration processes that generally decrease

oil quality in Venezuela include water washing and

biodegradation. Other post-emplacement alterations can

decrease oil quality in parts of the hydrocarbon system

while improving oil quality in other parts of the system.

These processes include gas deasphalting, gas fractionation,

thermal cracking and remigration. Mixing of primary oil

types and post-emplacement products is also commonly


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Fig. 14. Oil quality distribution. Oil and gas occurrences are color-coded according to the types of post-emplacement processes that have influenced present-day quality. Note the complex number of processes

that have impacted hydrocarbon quality, particularly toward the flanks of the major depocenters.

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observed and has a significant impact on oil quality. An

example of multiple effects and mixing is the biodegrada-

tion of marine, Type II-generated oil, followed by mixing

with younger, non-marine sourced oil, recharging the

reservoir. This phenomenon is linked to the episodic nature

of hydrocarbon yield.

7. Regional tectonic and hydrocarbon systems synthesis

In this paper we have discussed the plate tectonic history,

tectonic domains, basin evolution and the development and

demise of hydrocarbon systems in the Eastern Venezuela

Basin, as reflected in source rock distribution, maturation,

fluid migration and entrapment. Fig. 15 is a diagram that

attempts to synthesize the significant tectonic and hydro-

carbon systems events that have occurred in northern South

America over the past 160 million years. This regional

history provides context for, and is consistent with, the

complex sequence of events depicted in Fig. 5.

The upper part of the diagram is a sketch map whose

lower horizontal boundary parallels the active tectonic

boundary separating the ‘stable’ and ‘mobile’ parts of the

study area (as shown in Fig. 1, this is a non-linear surface

running from the Guyana Shelf to the south of the Bonaire

Block). For display purposes only, this line is rotated so

Fig. 15. Time–distance diagram. The diagram is keyed to the map of northern Venezuela and depicts the relationship of key tectonic phases to source rock

deposition and maturation events. See text for full discussion.


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the active Merida Andes zone is aligned with the active

margins of north central and northeastern Venezuela.

Beneath the map, the lower part of the diagram is keyed

geographically to the blue and yellow basins that lie at the

southern end of the map. The Maracaibo Basin is shown in

blue, and the central and eastern Venezuela basins are

shown in yellow.

The lower part of the diagram is color coded to reflect

four major tectonic phases, with source rock deposition

shown in green and maturation pulses indicated with red

dots. The time-spatial tie of the tectonics to the hydrocarbon

maturation/migration is particularly striking. At a first order,

the major La Luna/Querecual/San Antonio source rocks

become thermally mature diachronously along the northern

south American margin in association with Caribbean

tectonic events. The blue region indicates a time–distance

distribution of Mesozoic rifting beginning at about the same

time as the opening of the Gulf of Mexico and eventually

leading to the opening of the south Atlantic. In Venezuela,

rifting is associated with red beds, lakes, volcanics and the

deposition of the saline-lacustrine organic rich rocks

discussed above. In the eastern part of the study area, we

believe that the potentially important Albian source rock

was deposited near the end of this rifting event. The

uncolored part of the lower diagram represents a general-

ized, long-lived, post-rift ‘passive margin’ setting that

existed before the onset of Caribbean tectonics. Although

we describe this time period as a ‘passive margin’, we do not

imply tectonic quiescence in northern Venezuela during this

entire interval. There was, for example, significant Eocene

uplift in eastern Venezuela, based on the presence of

shallow-water carbonates overlying the Paleocene, deep

water Vidono Shale in a fairly rapid succession (Fig. 6).

The pink region indicates the time-distance distribution

of significant Caribbean and South American plate inter-

actions along a transpressional boundary. The leading edge

of deformation lies at the subduction zone east of the

Barbados accretionary prism. Behind the leading edge,

Neogene rhomb grabens have opened along a diffuse and

evolving plate boundary (yellow basins in upper diagram).

Important Miocene source rocks are deposited in this

tranpressional phase and the bulk of the key maturation

events occur here. The yellow-colored region indicates

areas impacted by the Pacific-centric tectonic events

discussed at the beginning of this article and associated

changes in the Nazca and Cocos plate motions. Onshore,

these events are captured in the creation of the Bonaire

Block as a tectonic flake moving northward with respect to

the Caribbean, and the strike slip systems along the Merida

and Perija Ranges as active tectonic boundaries.

8. Summary

While northern Venezuela has been the subject of a

significant number of studies over the years, our evaluation

confirms that there is still more to be learned, particularly as

new data are acquired and incorporated into an integrated

analysis spanning plate—to molecular-scale elements. Our

analyses suggest that newly described, pre-100 Ma source

intervals have likely generated oil and gas. This knowledge

may drive the development of new play concepts to exploit

hydrocarbon resources where these source rocks are

preserved, particularly as gas becomes more economic in

this region. In addition, coupling tectonic, maturation, and

source analyses has allowed us to characterize deeper play

potential in the Serrania del Interior, based on a pre-

Querecual hydrocarbon system, although high-quality

seismic does not yet exist to test this and explorability

may be difficult. Similarly, our integrated approach allowed

us to recognize the potential for more widespread Lower

Cretaceous source rocks, with implications for potential

hydrocarbon systems around the circum-Atlantic, including

Cuba, northwest Africa and the Yucatan. Finally, our studies

helped us to better characterize the discovered resource in

eastern Venezuela, and allowed us to make predictions

regarding the remaining potential. This information has

given us the technical basis to be selective in reacting to new

opportunities in this extensive and complex region.

Acknowledgements

The authors gratefully acknowledge ExxonMobil

Exploration and Research Companies for permission to

publish this work. GeoMark Research Inc. gave permission

to use petroleum geochemical data from their Venezuelan

Oil Study. Jay Jackson and John Steritz (ExxonMobil)

completed the quantitative structural restorations that

formed the basis for some of the basin evolution cross-

sections. Bob Ferderer and Darcy Vixo (ExxonMobil)

analyzed the gravity and magnetic data, and we thank

GETECH for permission to publish the gravity map. Philip

Koch (ExxonMobil) was instrumental in contributing to the

hydrocarbon systems analysis. Bob Pottorf and Jim

Reynolds performed the fluid inclusion analyses. Our

work also benefited from discussions with numerous

ExxonMobil colleagues, and Francois Roure and David

Ford provided extremely helpful reviews of the manuscript.

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