Summa Et Al 2003 Hydrocarbon Systems NE Venezuela
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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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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.
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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
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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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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.
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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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