Stratigraphic Synthesis of Western Venezuela

Abstract

The sedimentary basins of western Venezuela contain large volumes of oil. However, most of the largestructures have already been produced. Exploration for new reserves of light and medium oil now

depends on integrated studies that lead to a more comprehensive basin evaluation. This paper presents anintegrated account of the Lake Maracaibo and Barinas-Apure basins of western Venezuela. It is a fully inte-grated study but focuses on the genetic and seismic stratigraphy of more than 600 wells, reference outcrops,and 4000 km of reflection seismic data.

Six unconformity-bounded supersequences record the dynamics of Mesozoic–Cenozoic basin evolutionfrom extension to collision. Supersequence A was deposited during an episode of Jurassic rifting, and superse-quence B corresponds to the subsequent Early–Late Cretaceous passive margin. Supersequence C marks thetransition to a compressive regime in the Late Cretaceous and early Paleocene. Compression resulted fromcollision and obduction of the Pacific volcanic arc with the South American plate. Supersequence D records thedevelopment of the late Paleocene–middle Eocene foreland basin in front of the volcanic arc and emplacementof the Lara nappes. Supersequences E and F are attributed to modification of the foreland basin by lateEocene–Pleistocene collision of the Panamá arc. The uplifted Serranía de Perijá, Macizo de Santander, andMérida Andes partitioned the foreland basin, creating the present Lake Maracaibo and Barinas-Apure basins.

Supersequence B contains the Cretaceous La Luna source rock (sequences K3, K4, K5). The Colón andBurgüita formations form the principal supersequence C seals (sequence K6). The principal reservoir unitsoccur in supersequence D, including the prolific Eocene Misoa and Gobernador formations (sequences T1, T2).Reservoirs of the La Rosa and Lagunillas formations occur in supersequence F and in the Betijoque Molasse.

Resumen

Las cuencas sedimentarias del Occidente de Venezuela contienen inmensas acumulaciones de hidrocar-buros. Sin embargo, la mayoría de las estructuras grandes ya han sido explotadas. La exploración que

incorpore más reservas de crudo liviano y mediano debe entonces basarse actualmente en estudios integradosque conduzcan a una evaluación más exhaustiva de estas cuencas. Este artículo presenta una vision integradade las cuencas del Lago de Maracaibo y Barinas-Apure, en el occidente de Venezuela. Es un estudio totalmenteintegrado y enfocado especialmente a la estratigrafía genética y sísmica de más de 600 pozos, afloramientos dereferencia y alrededor de 4000 km de líneas sísmicas.

Seis supersecuencias, limitadas por discordancias, evidencian la dinámica evolución de las cuencas en elMesozoico–Cenozoico, de un proceso de extensión a uno de colisión. La supersecuencia A fue depositadadurante un episodio de apertura de corteza del Jurásico. La supersecuencia B corresponde al margen pasivosubsiguiente, durante el Cretácico Temprano al Tardío. La supersecuencia C marca la transición a un régimencompresivo en el Cretácico Tardío y Paleoceno Temprano. La compresión es el resultado de la colisión yobducción del arco volcánico pacífico al Oeste con la placa suraméricana. La supersecuencia D pone de mani-fiesto el desarrollo de la cuenca de antepaís del Paleoceno Tardío–Eoceno Medio, al frente del arco volcánicopacífico, y el emplazamiento de las napas de Lara. Las supersecuencias E y F se atribuyen a las modificacionesen la cuenca de antepaís debidas a la colisión Eoceno Tardío–Pleistoceno del arco de Panamá. Los levan-tamientos de la Serranía de Perijá, del Macizo de Santander y de los Andes de Mérida particionaron la cuencade antepaís generando así las actuales cuencas del Lago de Maracaibo y Barinas-Apure.

La supersecuencia B contiene la roca madre La Luna de edad Cretácico. Las formaciones Colón y Burgüitacoforman los sellos principales de la supersecuencia C. Las principales unidades reservorio se ubican en lasupersecuencia D, incluyendo a las prolíficas formaciones Misoa y Gobernador del Eoceno. Las formaciones LaRosa y Lagunillas generan reservorios dentro de la supersecuencia F en la cual se ubica la sedimentaciónmolássica de la formación Betijoque.

François Parnaud

Yves Gou

Jean-Claude Pascual

Beicip-FranlabPetroleum Consultants

Rueil-Malmaison, France

Maria Angela Capello

Irene Truskowski

Herminio Passalacqua

Intevep, S.A.Caracas, Venezuela

681Parnaud, F., Y. Gou, J.-C. Pascual, M. A. Capello, I. Truskowski, and H. Passalacqua,1995, Stratigraphic synthesis of western Venezuela, in A. J. Tankard, R. Suárez S., andH. J. Welsink, Petroleum basins of South America: AAPG Memoir 62, p. 681–698.

INTRODUCTION

Western Venezuela can be divided into several struc-tural units (Zambrano et al., 1970) (Figure 1): (1) theGuyana shield or Cuchivero granitic province(Menéndez, 1968); (2) the Mérida Andes mountain range,which separates the Barinas-Apure basin to the southeastfrom the Lake Maracaibo basin in the northwest; (3) theSerrania de Perijá in the west, which separates the LakeMaracaibo basin (east) from the Colombian César-Ranchería basin (west); and (4) the Serrania de Trujillo,which separates the Lake Maracaibo basin from the Laranappes (Stephan, 1985).

These structural units record a long history of basinevolution (Figure 2). The earliest episode involved theprogressive evolution from extension to a Caribbean–Tethyan passive margin at the edge of the SouthAmerican plate. This evolution spanned late Triassic–Cretaceous time. It was followed by collision of thePacific plate with the South American plate andbuilding of a Late Cretaceous-Paleocene mountain rangewith an associated foreland basin. Collision andmigration of the Caribbean plate since the Paleoceneresulted in the Lara thrust belt and the Eocene forelandbasin. The subsequent Andean orogeny is attributed tocollision of the Panamá arc. This collision also parti-tioned the Cretaceous passive margin into the post-middle Miocene Lake Maracaibo and Barinas-Apurebasins.

This geologic history is expressed in a hierarchy ofdepositional sequences. On a large scale, the stratigraphycan be divided into Paleozoic and Mesozoic–Cenozoicsuccessions. Jurassic extension records separation ofNorth and South America (Pindell and Erikson, 1993;Parnaud et al., 1994). The Mesozoic–Cenozoic successioncontains a suite of unconformity-bounded sequences thatdescribe the dynamics of basin evolution along thenorthern part of the South American plate (Figures 2, 3),as follows:

• Supersequence A resulted from an episode ofJurassic rifting.

• Supersequence B corresponds to the Early–LateCretaceous stage of passive margin development.

• Supersequence C was a transitional phase of theLate Cretaceous–early Paleocene passive margindeposition behind the compressive arc.

• Supersequence D was deposited in a foreland basinduring the late Paleocene–middle Eocene, whencollision and obduction of the Pacific volcanic arcoverrode the South American plate and emplacedthe Lara nappes.

• Supersequences E and F were related to the lateEocene–Pleistocene phase of foreland basin subsi-dence caused by collision of the Panamá arc. Thisepisode of deformation was responsible for separa-tion of the Lake Maracaibo and Barinas-Apurebasins.

Internally, these six supersequences comprise a seriesof minor sequences that reflect eustatic processes and

structural–tectonic modifications (Figure 3). Sedimento-logically, the Cretaceous and Paleocene consist of aheterogeneous carbonate-siliciclastic association. Incontrast, the Cenozoic is represented mainly by a silici-clastic sedimentary system.

Dating of the sequence boundaries and the maximumflooding surfaces was established with available strati-graphic and biostratigraphic information and bycomparison with the global sea level charts of Haq et al.(1987). We believe that tectonism due to emplacement ofthe Lara nappes modified the eustatic signature, particu-larly in the flexured zones such as those located in frontof the deformation and along the zone of lateral rampsthat limit the nappes’ extent along the eastern border ofthe lake.

PALEOZOIC SUCCESSION

The sequences deposited during the Paleozoic wereidentified in several areas, in particular, the Guyanashield, Mérida Andes, Lake Maracaibo basin, andSerrania de Perijá. A stratigraphic column for thePaleozoic of western Venezuela has been established(González de Juana, 1980).

682 Parnaud et al.

Figure 1—Location of the study area and stratigraphicsyntheses of Barinas-Apure and Lake Maracaibo basins.Cross sections: A–A' is in Figure 3, B–B' and C–C' inFigure 5, and I–I' and J–J' in Figure 15. Circled numbersrefer to the locations of these numbered figures.

Stratigraphic Synthesis of Western Venezuela 683

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The lower Paleozoic in the southern flank of the MéridaAndes consists of fossiliferous siltstones of the OrdovicianCaparo Formation (Christ, 1927) and the Silurian ElHorno Formation (Martín Bellizia, 1968). The middlePaleozoic Río Cachirí Group of Liddle (1928) occurs in theSerrania de Perijá. This group consists of marine shelfsedimentary rocks that are rich in fauna, such asbrachiopods and pelecypods. The upper Paleozoic in theMérida Andes is stratigraphically diverse. The Mucu-chachí Formation (Christ, 1927) records marine inunda-tions, the Sabaneta Formation (Oppenheim, 1937) showsan episode of continental deposition, and the PalmaritoFormation (Christ, 1927) is evidence of a final marineevent. In the Serrania de Perijá, the upper Paleozoic isrepresented by two sets. A lower set includes the Cañodel Noroeste, Caño Indio, and Río Palmar formationswhich are believed to be equivalent to the SabanetaFormation toward the east in the Mérida Andes. Ayounger set, containing the Palmarito Formation, iscomposed of sandy facies and marine limestones of innershelf origin. In the western Venezuelan basins, no wellhas thus far reached this stratigraphic level.

Nevertheless, in the subsurface of the Barinas-Apurebasin south of Apure (Elorza zone), a sequence has beeninterpreted in seismic sections with a thickness of 2 sec(about 4500 m). This sequence is characterized by parallelreflections that are relatively continuous beneath theCretaceous section (Figure 4A). Its top corresponds to aregional unconformity that is well defined by trunca-tions, while its base is difficult to recognize. A Paleozoicage is inferred on the basis of regional correlations andthe structural style that affects it. This Paleozoic succes-sion was subjected to a strong compressive event thatresulted in thrusts and fault-bend folds. These structureswere subsequently peneplaned and unconformablycovered by Mesozoic strata (Figure 4A). The structuralstyle distinguishes the Paleozoic rocks from the Jurassicsequences.

In the Lake Maracaibo basin, drilling encounteredmetamorphic rocks beneath the Cretaceous. These strati-graphic levels were considered by González de Juana etal. (1980) to be possible equivalents of Paleozoic forma-tions in the central Andean region. Nevertheless, seismicdata do not record a Paleozoic stratigraphy in the LakeMaracaibo basin similar to that interpreted for theBarinas-Apure basin. We believe that this may reflect thelevel of metamorphism.

The hydrocarbon potential of the Paleozoic successionis poorly known. Alberdi et al. (1994) showed that theorganic matter of the Palmarito Formation samples has ahigh maturity level, which precludes an accurate evalua-tion of its real initial hydrocarbon potential.

MESOZOIC–CENOZOIC SUCCESSIONThe Mesozoic–Cenozoic succession results from the

Jurassic rift phase attributed to the fragmentation ofPangea and from the Cretaceous–Tertiary phase ofcollision between Pacific and South American plates(Figure 2).

Jurassic Supersequence A: ExtensionSupersequence A occurs in outcrops of Perijá and

Mérida Andes and in the subsurface of the LakeMaracaibo and Barinas-Apure basins. In Perijá, superse-quence A forms the La Ge Group (Hea and Whitman,1960) and includes the Tinacoa (Liddle et al., 1943),Macoíta (Hedberg and Sass, 1937), and La Quinta(Künding, 1938) formations. These formations reflect sedi-mentation in continental environments that were locallysourced by volcanic material such as volcanic ashes. In theMérida Andes, this Jurassic supersequence is identified asthe La Quinta Formation, which was also deposited in acontinental setting with conspicuous red sandstones.

Stratigraphic Synthesis of Western Venezuela 685

Figure 4—Seismic profiles. (A) Paleozoic succession. (B)Triassic–Jurassic extensional supersequence A. SeeFigure 1 for locations.

In the subsurface of the Barinas-Apure basin, it isdifficult to identify supersequence A in seismic sectionswithout well control. However, southwest of the Barinasbasin, where outcrops show a significant development ofthis Triassic–Jurassic sequence, some seismic linesdisplay a 1-sec-thick (3000-m) sequence beneath theCretaceous (Figure 4B). The base of the supersequence isobscure, but its top is expressed as a strong unconform-ity. Its seismic signature consists of discontinuous reflec-tions of variable amplitudes. The structural style of theseintervals is generally characterized by normal faults thatbound small half-grabens.

In the subsurface of the western part of LakeMaracaibo basin, west of the Icotea structural trend, ahalf-graben is observed in seismic sections and has aneastward-dpping basin-forming fault. The base of super-sequence A is not recognized, but its upper surface ismarked by truncated reflections. Wells indicate that thissequence corresponds to the La Quinta Formation.

Because of an apparent absence of source rocks andpoor reservoir characteristics, supersequence A has littlehydrocarbon potential. There is no production fromJurassic levels in the study area.

Cretaceous Supersequence B: PassiveMargin

At the beginning of the Cretaceous, a marine trans-gression caused inundation of the Guyana shield. Thistransgression is correlated to the eustatic changes thatoccurred worldwide and lasted until Cenomanian–Campanian time (Figure 2). Sporadic volcanic materialwithin the La Luna Formation suggests the presence of avolcanic arc toward the west, implying subduction of thePacific plate. The apparent reduction of fault-controlledsubsidence, the overall transgressive deepening of thebasin, and the stratigraphy suggest that Cretaceoussupersequence B was deposited as a passive marginterrace wedge behind a volcanic arc. The passive marginphase ended with collision of the Pacific arc and theSouth American plate and flexural subsidence offoreland basins.

During this period of passive margin development,several sequences were deposited (Figure 3). Togetherthey formed the terrace wedge of supersequence B. Thestructural style is characterized by steeply dippingreverse faults of post-Cretaceous age. Structuralinversion of older normal faults are interpreted fromseismic data (Figure 5, C–C’).

In general, it is difficult to differentiate these passivemargin sequences seismically, except for local reflectiontruncations (e.g., onlaps and downlaps) in an otherwisecontinuous supersequence B with persistent subparallelreflections. The internal seismic sequence boundariesseparate six depositional sequences, K0 through K5, asdiscussed below.

Neocomian–Barremian Sequence K0

In the Early Cretaceous, a thick sequence of conti-nental sediments was deposited in three troughs: theMachiques trough in Perijá, the Uribante trough in

Táchira, and the Barquisimeto trough in Trujillo. Inaddition to these troughs, this sequence forms a wide-spread cover, except in the southwestern part of Apure.Hedberg (1931) initially described sequence K0 in theNegro River area (Serrania de Perijá), and it was laternamed the Río Negro Formation (Hedberg and Sass,1937). It marks the basal continental component of theCretaceous passive margin basin. Although it is wide-spread, its age is poorly constrained; available evidencesuggests a Neocomian–Aptian age. Seismic resolution isinsufficient to establish the upper bounding surface withconfidence.

This Lower Cretaceous sequence has a low hydro-carbon potential. Source rocks are unknown, andalthough it has a high sandstone content, reservoir char-acteristics are generally believed to be poor.

Aptian Sequence K1

Continental Rio Negro deposition was terminated bya Cenomanian–Campanian marine transgression thatflooded the Guyana cratonic platform (Figure 2). Thistransgression was episodic as evidenced by a back-stepping suite of depositional sequences, the first ofwhich has an Aptian age (Figure 3). The Aptian Apónsequence (K1) (Sutton, 1946) is characterized by shallowmarine shelf sedimentation and displays lateral facieschanges. Toward the east (Mérida Andes), littoral sand-stones form the basal part of the Peñas Altas Formation(Renz, 1959) (Figure 6). This sequence consists of threeparts (Figure 3):

• The lower part is interpreted as a transgressivesystems tract (TST) and comprises several retro-gradational parasequences. This TST correspondsto the Tibú Member and was deposited in an innershelf environment where littoral bioclastic barsdeveloped.

• The middle part forms the maximum floodingsurface (MFS) and include the Machiques Memberand its laterally equivalent, the GuáimarosMember. Both were deposited in middle shelf envi-ronments with several intercalations of shallowerdeposits. Based on previous work, an interval richin Orbitolina texana is equated with the middleAptian event (the 111-Ma MFS of Haq et al., 1987).

• The upper part of this sequence is characterized bya highstand and progradation. This regressive partis the Piché Member of the Lake Maracaibo zone,which was deposited in an inner shelf environ-ment. In the Mérida Andes, the central part of thePeñas Altas Formation was deposited in a littoralsetting in which bars developed.

In the Lake Maracaibo succession, reflection seismicdata does not clearly distinguish this sequence fromother Cretaceous units. Nevertheless, its base does showlocal onlaps and its top shows weak truncations. Theinternal geometry of sequence K1 is marked by threehigh-amplitude reflectors (Figure 7A). The thickness ofthis sequence varies up to 300 m, but it is absent in theBarinas-Apure basin (Figures 5, B–B’, and 6). The

686 Parnaud et al.

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estimated sedimentation rate is about 25 m/m.y.The Aptian depositional sequence has good hydro-

carbon potential in the western part of the LakeMaracaibo basin. The Machiques source rock is rich intype II organic matter (Alberdi et al., 1994). The reservoirintervals are believed to have dissolution and fractureporosity.

Albian–Lower Cenomanian Sequence K2

The second major marine transgression took placeduring the Albian, invading the entire study area fromthe Serrania de Perijá to the southeastern limit of theBarinas-Apure basin and toward the Guyana shield(Figure 2). This depositional sequence (K2) includes theLisure (Rod and Maync, 1954), Maraca (Rod and Maync,1954), La Puya (Renz, 1959), lower Capacho (Sievers,1888), Aguardiente (Notestein et al., 1944), and basalEscandalosa (Renz, 1959, S and R members) formations.This sequence has a threefold subdivision (Figure 3):

• The lower part is a transgressive systems tract thatwas internally built by retrograde parasequences.During this TST, the Lisure Formation wasdeposited in a middle shelf environment. At thesame time, transgressive sands of the Aguardiente

Formation were deposited in an inner shelf settingwith fringing shoreline facies (Figure 8).

• The middle part corresponds to the maximumflooding surface (the 97-Ma MFS of Haq et al.,1987) developed in the S Member of the Escan-dalosa Formation in the Barinas-Apure basin. Inthe Lake Maracaibo basin, this MFS is absent due toerosion (Figure 9). There the shales of the SMember were deposited in an open marine middleshelf environment.

• The upper part of the sequence is a progradationalhighstand systems tract. It is related to the regres-sive sandstones of the R Member in the Escan-dalosa Formation and was deposited in a shallowlittoral environment with shoreline, barrier bar,and coastal lagoon sedimentation. The lateralequivalent in the Lake Maracaibo area is absentdue to erosion.

688 Parnaud et al.

Figure 6—Paleogeography of the Aptian depositionalsequence K1. Legend: 1, Lara nappes, actual position; 2,positive areas; 3, nearshore clastics; 4, inner shelf carbon-ates and shales; 5, middle shelf carbonates and shales; 6,thickness contours in feet.

Figure 7—Seismic profiles. (A) Aptian depositionalsequence K1 showing onlaps and truncations. (B) Eocenesupersequence D showing top and base bounding uncon-formities. Columns: 1, depositional sequences; 2, superse-quences. See Figure 1 for locations.

In Perijá and Lake Maracaibo, Canache et al. (1994)identified a hiatus of early Cenomanian age betweensequence K2 and the overlying upper Cenomanian strataof sequence K3 (Figures 3, 9). Erosion has partiallytruncated the Albian sequence. We attibute this hiatus tocollision of the Pacific volcanic arc with South Americancontinental crust and to flexural deformation in front ofthe applied load. A foreland basin is located west ofSerrania de Perijá, and an associated forebulge occurs inthe Perijá and Lake Maracaibo areas. Forebulge upliftresulted in emergence, restricted deposition, and erosionof the upper part of sequence K2 during the early Ceno-manian. This is reflected in a regressive wedge in theMérida Andes and in deposition of the highstand regres-sive Escandalosa Sandstone. Following this event, a newtrangression flooded the entire area. This transgressionprobably resulted from renewed compression andregional flexural downwarping. Basinal accumulation ofshaly calcareous facies of the La Luna Formationinitiated late Cenomanian sedimentation.

In Barinas seismic sections, identification of thesequence K2 is generally tenuous. However, there aresome onlaps of the transgressive systems tract (Aguardi-ente Formation) and local downlap of the highstandsystems tract (R Member of Escandalosa Formation)associated with a gentle unconformity at its top. Seismic

data from the Lake Maracaibo area also shows basalonlaps (Figure 7A), as well as an irregular upper surfacethat we interpret as a paraconformity.

This sequence, which is regionally persistent (Figure8), has a thickness varying up to 600 m. Estimated sedi-mentation rates are 50 m/m.y. in the southern part of theMérida Andes and 12 m/m.y. in the shelf zone.

The hydrocarbon potential of sequence K2 lies in theLisure reservoirs with their fracture porosity and insandstone reservoirs in the Aguardiente (TST) andEscandalosa (R Member, HST) formations that preserveintergranular porosity.

Upper Cenomanian–Lower CampanianSequences K3, K4, and K5

Foreland downwarping following deposition of EarlyCretaceous sequences resulted in episodic late Ceno-manian–early Campanian transgression and three back-stepping depositional sequences: K3, K4, and K5 (Figure2). These sequences are present in the Serrania de Perijáand Lake Maracaibo basin as the La Luna Formation andthe Tres Esquinas Member (Stainforth, 1962), in theMérida Andes as the Capacho (upper Seboruco andGuayacán members; Sievers, 1888) and La Luna forma-tions, and in the Barinas-Apure basin as the Escandalosa(P and O members; Renz, 1959) and Navay (La Morita and

Stratigraphic Synthesis of Western Venezuela 689

Figure 8—Paleogeography of the Albian–lower Ceno-manian depositional sequence K2. Legend: 1, Lara nappes,actual position; 2, positive areas; 3, nearshore clastics; 4,inner to middle shelf carbonates and shales; 5, thicknesscontours in feet.

Figure 9—Paleogeography at top of Albian–lower Ceno-manian depositional sequence K2. Legend: 1, Lara nappes,actual position; 2, positive areas; 3, nearshore clastics; 4,inner shelf sandstones and carbonates; 5, middle shelfcarbonates and shales.

Quevedo members; Kehrer, 1938; Renz, 1959) formations.Several maximum flooding surfaces are recognized.

The upper Cenomanian interval at the base of the LaLuna Formation is not obvious. Haq et al. (1987) docu-ment their 92-Ma MFS as a prominent event. In contrast,the next MFS at the base of the La Morita Member is welldefined in the Barinas-Apure basin (equivalent to the 88-Ma datum of Haq et al., 1987). The third MFS occursin the Tres Esquinas Member in the Lake Maracaibobasin (79-Ma datum of Haq et al., 1987). The highstandsystems track of each sequence and the correspondingsequence boundaries are not precisely determined.

The characteristics of sequences K3, K4, and K5 are asfollows:

• Volcanic ash beds at the base of the La LunaFormation suggest the presence of the Pacificvolcanic arc west of the study area.

• The basin deepens rapidly from inner shelf tobathyal depths, possibly reflecting eastwardmigration of the forebulge toward the Barinas-Apure basin complex.

• The three transgressive sequences each culminatein maximum flooding surfaces.

• There are two well-defined facies tracts (Figure 10).In the west, the middle shelf–bathyal tract containscalcareous and shaly facies of the La LunaFormation. Eastward detrital littoral sediments ofthe Escandalosa Formation (P member) and NavayFormation (Quevedo Member) were deposited(Figure 11).

These sequences were established seismically on thebasis of onlap relationships. However, no particularinternal geometric characteristics were observed (Figure7A). The thickness of the sequences varies from 150 to>600 m (Figure 10). Sedimentation rates of 30 m/m.y. inthe southern Andes and 8 m/m.y. in the Maracaibo shelfarea are estimated.

This succession has substantial hydrocarbon potential,including the exceptional La Luna source rock (Alberdiet al., 1994) and favorable reservoirs with intergranularporosities in the Escandalosa (TST) and Navay (QuevedoMember, HST) formations.

Late Cretaceous-Paleocene Supersequence C:Passive to Active Margin Transition

In the Late Cretaceous, a new phase in tectonicevolution was marked by collision of the Pacific volcanicarc with the South American plate. This collision trans-formed the passive margin into an active belt, creating aforeland basin with an associated foredeep to the west(Perijá area) and a forebulge in the Barinas area. Never-theless, toward the north and northeast, the passivemargin setting persisted until emplacement of the Larathrust belt and nappes. This history indicates a scissor-type closure of the old passive margin through the LateCretaceous and early Paleocene (Figure 2). This transi-tional phase was also characterized by a regression thatresulted in three depositional sequences, K6, K7, and K8,as discussed below (Figure 3).

Upper Campanian–Maastrichtian Sequence K6

Regression began at the beginning of the Late Creta-ceous. Simultaneously, toward the west, the Pacificvolcanic arc collision formed a foredeep within whichshaly facies of the Colón Formation were deposited(Liddle, 1928). The associated forebulge migrated fromthe Lake Maracaibo depocenter to Barinas-Apure, wherearenaceous shoreline facies of the Burgüita Formationwere deposited (Renz, 1959) (Figure 12). Sedimentationof this sequence (K6) ended in a highstand systems tractthat is expressed in the Mito Juan Formation (Garner,1926). The lower and upper boundaries of this sequenceare assigned to the late Campanian and late Maas-trichtian (Figure 3).

Several smaller scale depositional units build theinternal fabric of sequence K6, with indeterminateflooding surfaces. Generally the shaly units of the ColónFormation are interpreted as transgressive drapes andthe sandier stratigraphy of the Mito Juan Formation ashighstand progradational depositional systems.

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Figure 10—Paleogeography of upper Cenomanian–lowerCampanian depositional sequences K3, K4, and K5.Legend: 1, Lara nappes, actual position; 2, positive areas;3, nearshore clastics; 4, internal to middle shelf sand-stones and carbonates (Guayacan Member); 5, outer shelfto upper bathyal carbonates and shales; 6, thicknesscontours in feet.

In the Barinas-Apure basin, this sequence is poorlydistinguishable on seismic profiles (Figure 13A). It is thinand has no obvious seismic signature apart from onlapgeometries and truncations beneath the upper boundingsurface. In the central areas, the sequence is deeplyeroded and discontinuous. In the Lake Maracaibo basin,the base of sequence K6 corresponds to a continuousstrong reflector (Figure 7B). However, its upper surface isless obvious, except toward the west, where it has trun-cation relationships (Figure 13B).

Internal clinoform geometries indicate local eastwardprogradation of the Mito Juan Formation (Figure 13B).Eroded topsets suggest latest Cretaceous tectonism in thePerijá area. This depositional sequence is generally moreargillaceous than the previous stratigraphy and conse-quently less competent. Because of this, the brittle faultstyles of the lower sequences differ from that in theCampanian–Maastrichtian rocks, where disharmonyoccurs. The thickness of the sequence varies up to 900 m(Figure 12). Sedimentation rates of 65 m/m.y. in the shelfand 150 m/m.y. in the Perijá foredeep are estimated.Sandstones of the Burgüita Formation (TST) in Barinasand the Mito Juan Formation (HST) offer reservoirpotential.

Upper Maastrichtian–Lower PaleoceneSequences K7 and K8

Toward the end of the Cretaceous, the Perijá foredeepwas filled with highstand sediments of the Mito Juansequence (K6), which were sourced from the west. Theentire area was affected by erosion above shallowingbasement. A new transgressive episode from thenortheast deposited two subordinate Paleocenesequences, K7 and K8 (Figure 3). The lower sequence

Stratigraphic Synthesis of Western Venezuela 691

Figure 11—Genetic stratigraphy of Cretaceous succession.See Figure 1 for location.

Figure 12—Paleogeography of upper Campanian–Maas-trichtian depositional sequence K6. Legend: 1, Laranappes, actual position; 2, positive areas; 3, inner tomiddle shelf clastics; 4, outer shelf shales and scarcesandstones; 5, thickness contours in feet.

covered the entire shelf and displays marine characteris-tics, while the upper sequence is essentially deltaic.

The shelf terrace wedge (K7) comprises several forma-tions. The Guasare Formation (Garner, 1926) consists ofshallow marine deposits in the Lake Maracaibo basin.The Trujillo Formation (Hodson, 1926) comprises deepermarine deposits northeast of the lake area, while theCatatumbo Formation (Notestein, 1944) was built bydeltaic sedimentation toward the south (Figure 14). Theoverlying deltaic suite (K8) contains three formations: theBarco and Los Cuervos formations to the south and theMarcelina Formation to the north. Toward the northeast,lowstand facies of the Trujillo Formation (Hodson, 1926)developed.

We have mapped three sedimentary domains in eachsequence (Figure 14). The first is in the southwest andconsists of sandstones and mudstones of the OrocueGroup (Notestein, 1944) and Marcelina Formation. Thesecond occurs in the central area, where shallow marinebioclastic and calcareous sediments of the GuasareFormation were deposited. The third sedimentarydomain, located toward the north, contains bathyalsediments of the Trujillo Formation.

The base of the shelf depositional system in theMaracaibo basin is characterized seismically by anerosional surface (Figure 13B). The succeeding deltaicsystem has discontinuous and strong amplitude reflec-tions; its base is marked by onlaps and downlaps and itstop by local truncations. This deltaic sequence thickenstoward the west where it locally forms delta-front fans.These Paleocene rocks are up to 600 m thick (Figure 14).An average sedimentation rate of 30–80 m/m.y. isestimated.

The hydrocarbon potential of these sequences isrestricted to the terrigenous clastics of the BarcoFormation (TST). The carbonaceous rocks of theMarcelina Formation (HST) are unlikely to offer sourcepotential as expected (Alberdi et al., 1994).

Upper Paleocene–Middle Eocene Supersequence D: Collisional Basins

Emplacement of the Lara nappes began north of theLake Maracaibo basin at the end of the Paleocene (Figure2). These nappes gradually encroached eastward,forming new foreland basins. One of these trendsN 20º W, parallel to the northeastern margin of Lake

692 Parnaud et al.

Figure 13—Seismic profiles. (A) Supersequences B and Cshowing basement topography and Cretaceous onlaps. (B) Paleocene fans in the Marcelina Formation (deposi-tional sequences K7 and K8). Columns: 1, depositionalsequences; 2, supersequences. See Figure 1 for location.

Figure 14—Paleogeography of the upper Maastrichtian–lower Paleocene depositional sequences K7 and K8.Legend: 1, Lara nappes, actual position; 2, positive areas;3, continental to deltaic clastics; 4, inner to outer shelfcarbonates and shales; 5, bathyal with turbidites, shales,and scarce sandstones; 6, thickness contours in feet.

Maracaibo; another is oriented approximately east-westin front of the nappes. This flexural deformation isreflected in a suite of transgressive and regressive cyclesof Eocene age. The base and top of supersequence Dcorrespond to regional unconformities that are expressedseismically by numerous onlaps and truncations (Figure7B). Intermittent subsidence and a possible eustaticoverprint resulted in three depositional sequences, T1,T2, and T3, as discussed below (Figure 3).

Upper Paleocene–Lower Eocene Sequence T1

There are two parts to this upper Paleocene–lowerEocene Sequence (T1) (Figure 15, I–I'). During the earlierphase of base level lowering, erosion was followed bydeposition of continental sediments in the southern partof the Lake Maracaibo basin. Deep marine conditions inthe northern part of the basin resulted in sedimentationof lowstand turbidites of the Trujillo Formation. In thesecond phase, transgression related to shelf flexure infront of the applied nappe load reached the central partof the Lake Maracaibo basin. Toward the south, conti-nental deposition persisted, such as in the MiradorFormation (Garner, 1926) (Figure 15, I–I'). Another trans-gression in the early Eocene deposited the stacked Misoa“C” sandstones (sequences T1-1 to T1-5) (Figure 16).Depositional sequence T1 culminated in a highstandsystems tract and deposition of the deltaic lower Misoa“B”(Sequence T1-6; Figure 16).

Three sedimentary domains are recognized forsequence T1 (Figure 17). The first domain in the south-western and southern parts of the Lake Maracaibo basinis characterized by continental sedimentation of theMirador Formation. An inner shelf to shore zone domainoccurs in the central Lake Maracaibo basin and isreflected in the sandstone-mudstone Misoa Formation.The third sedimentary domain in the north consists ofdeep marine shales of the Trujillo Formation. The entiresuccession is up to 4000 m thick (Figure 17). Sedimenta-tion rates vary from 190 m/m.y. in the shelf areas to 500m/m.y. in the foredeep of the lateral ramp of the Laranappes.

Stratigraphic Synthesis of Western Venezuela 693

Figure 15—(Top) Cross section I–I' showing stratigraphicrelationships among Upper Cretaceous–lower Tertiarysequences K7, K8, and T1. (Bottom) Cross section J–J'showing stratigraphic relationships between middleEocene depositional sequences T2 and T3 in the Barinas-Apure basin. See Figure 1 for locations.

Figure 16—Genetic stratigraphy of the Misoa Formation.See Figure 1 for location.

This upper Paleocene–lower Eocene succession offerssignificant petroleum exploration potential, withnumerous sandstone reservoirs in the transgressive andregressive depositional systems. The deltaic intervalsrequire detailed analysis to characterize reservoir distrib-ution.

Middle Eocene Sequences T2 and T3

During the middle Eocene, two major events changedthe configuration of the basin. First, southward encroach-ment of the Lara nappes resulted in flexural subsidenceof the Barinas-Apure basin shelf and in marine inunda-tion. Basal sandstones of the Gobernador Formationwere followed by sedimentation of deep water shales ofthe Pagüey Formation (Pierce, 1960) (Figure 18). Second,tectonic loading by the Lara nappes produced a hingeline along the Lake Maracaibo shelf in the northeasternsector. There, deposition of the shallow shelf upperMisoa “B” sediments (Figure 16) was followed by deeperwater conditions and shale accumulations of the PaujíFormation (Tobler, 1922). This earlier sequence (T2)terminated in a progradational highstand system thatwas closely linked to the nappes and fed from thenortheast instead of from the southwest (Figure 19).Thus, following sedimentation of the basal sandstones of

the upper Misoa “B” Formation (Figure 16), shelf flexureresulted in turbiditic lowstand sedimentation and accu-mulation of the bathyal Paují Formation. Growth of theLara nappes was reflected in a forced northeastwardprogradation (upper part of the Paují Formation).

After the highstand progradation of deltaic sedimentsof the Cobre Formation in the southern part of the Barinas-Apure basin, a new transgressive cycle developed. These“Guanarito Sandstones” (Figures 15, J–J’, and 18) are illdefined, but may benefit from study of the related PagüeyFormation which crops out in the Mérida Andes.

694 Parnaud et al.

Figure 17—Paleogeography of upper Paleocene–lowerEocene depositional sequence T1. Legend: 1, Lara nappes,actual position; 2, positive areas; 3, continental to deltaicclastics; 4, inner to outer shelf sandstones and shales; 5, bathyal with turbidites, shales, and scarce sandstones;6, thickness contours in feet.

Figure 18—Eocene genetic stratigraphy. See Figure 1 forlocation.

The thickness of these middle Eocene sequences T2and T3 ranges up to 1500 m. Estimated sedimentationrates are 20 m/m.y. in the shelf area and 150 m/m.y. inthe foredeep. The hydrocarbon potential of thesesequences is due to the thick sandstones bodies of theMisoa “C” (TST), Misoa “B” (TST and HST), and Gober-nador (TST) formations.

Upper Eocene–Lower Miocene Supersequence E: Collisional Basins

Toward the end of the Eocene, the entire area changed(Figure 2). Positive relief in the east and northeastseparated the continental Lake Maracaibo basin from themarine basin located in Falcón (Figure 20). Uplifts westand south of the Serrania de Perijá and the easternColombian Cordillera fed a fluviodeltaic depositionalsystem. Marine circulation from the east continued toaffect the Barinas-Apure basin. This marine influenceextended to the Lake Maracaibo basin at the end of lateOligocene–early Miocene time.

Two depositional sequences are recognized (Figures 3,20, 21). The first (T4) was deposited during the lateEocene and early Oligocene in two different sedimentarydomains. A deltaic domain in the western part was fed

from Colombia (Carbonera Formation, Notestein, 1944;and La Sierra Formation, Hedberg et al., 1937). Marinesediments were deposited in the eastern part of the basinwhere it was open to the sea (Arauca Member of theGuafita Formation; Ortega et al., 1987). The base of thislower sequence corresponds seismically to an unconfor-mity (Figure 13A) that represents erosion of the Eocenefrom west to east and erosion of the Paleocene sectionsouth of Lake Maracaibo basin. There is seismic evidencethat the Carbonera Formation pinches out. In theBarinas-Apure basin, the top of this sequence is difficultto map, although there are local onlap and truncationboundaries. The sequence thins toward the north whereit disappears (Kiser, 1989). To the south, near La Victoriafield, the geometry of this sequence is that of a transgres-sive system characterized by onlaps and a highstandofflapping or progradational system.

The second sequence (T5) was deposited in lateOligocene–early Miocene time during widespreadmarine inundation (León Formation in Lake Maracaibobasin, Notestein, 1944; and Guardulio Member ofGuafita Formation in Barinas-Apure basin, Ortega et al.,1987). The base of this sequence is unconformable andmarked by truncations and onlaps. The sequence thinstoward the east of the Lake Maracaibo basin and wedgesout along the Icotea structural trend (Figure 5, C–C’).

Stratigraphic Synthesis of Western Venezuela 695

Figure 19—Paleogeography of middle Eocene depositionalsequences T2 and T3. Legend: 1, Lara nappes, actualposition; 2, positive areas; 3, inner to middle shelf sand-stones and shales; 4, outer shelf to bathyal shales; 5,thickness contours in feet.

Figure 20—Paleogeography of upper Eocene–Oligocenedepositional sequences T4 and T5. Legend: 1, Laranappes, actual position; 2, positive areas; 3, lacustrine tobrackish sandstones, shales, and coal; 4, deltaic withmarine influence, sandstones, and shales.

The thickness of these two sequences varies up to 1100m and has an estimated sedimentation rate of 50 m/m.y.Sandstone horizons in this upper Eocene–lower Miocenesuccession have reservoir potential, including the AraucaMember of the Guafita Formation and the CarboneraFormation. Local source rock intervals have beendescribed (Alberdi et al., 1994).

Middle Miocene–Pleistocene Supersequence F: Collisional Basins

During the middle Miocene, large-scale compres-sional tectonism initiated the Macizo de Santander,Serrania de Perijá, and Mérida Andes ranges. TheMérida Andes orogenesis culminated in the Plio-Pleis-tocene. This mountain building event correlates with twodepositional sequences, T6 and T7 (Figure 3). Deforma-tion also resulted in partition or isolation of the LakeMaracaibo and Barinas-Apure basins (Figure 22). Severalangular unconformities in the foothills of the northernand southern Andes record this tectonic history.

Rapid uplift was accompanied by molasse sedimenta-tion along the margin of the Mérida Andes range.Marine sedimentation persisted in the Lake Maracaibobasin, but it gradually changed to a freshwater paleo-geography as the marine environment shrank towardthe north. In the Lake Maracaibo basin, a new transgres-sive phase began during the middle Miocene andresulted in deposition of the onlapping La RosaFormation (Liddle, 1928). This was followed by regres-sive progradation and contraction of marine influence(Lagunillas Formation; Hedberg et al., 1937). Molassesediments of the Betijoque Formation (Garner, 1926)were deposited along the Andean range at the same time(Figure 22). Freshwater paleoenvironments dominatedthe center of the Lake Maracaibo basin, as reflected in thedeposits of La Puerta (Garner, 1926 ) and Los Ranchos(Liddle, 1928). Contemporaneous deposition of themolasse Parángula and Río Yuca formations occurred inthe Barinas-Apure basin (Mackenzie, 1937).

These sequences locally exceed 5500 m in thickness inthe flexural foredeeps. Sedimentation rates greater than250 m/m.y. have been estimated, but they decreased to150 m/m.y. along the foreland ramp. Several sandstoneintervals have reservoir potential, especially the Lagu-nillas Formation. There are also subordinate reservoirintervals, such as the sandstones of the Santa BárbaraMember in the La Rosa Formation.

696 Parnaud et al.

Figure 21—Genetic stratigraphy showing comparison ofCretaceous–Tertiary depositional sequences K5, T4, T5,and T6. See Figure 1 for location.

Figure 22—Paleogeography of middle Miocene–Pleis-tocene depositional sequence T7 related to Andean oro-genesis. Legend: 1, Lara nappes, actual position; 2,positive areas; 3, molassic depocenter; 4, lacustrine tobrackish sandstones and shales.

CONCLUSIONS

This study of the Lake Maracaibo and Barinas-Apurebasins has integrated several disciplines, includinggenetic stratigraphy from wells and outcrop sections andseismic stratigraphy. Data acquired over many years bydifferent companies have been synthesized; 600 wellswere correleted and 4000 km of seismic lines interpreted.On this basis, stratigraphic and paleogeographic modelshave been developed that relate interaction among thePacific, Caribbean, and South American plates and theway they periodically reorganized.

A new stratigraphic chart has been constructed in asequence stratigraphy framework. This analysis empha-sizes the dynamics of basin subsidence and the tectoniccontrol of sedimentation. Examples include the turbiditefacies of the Trujillo and Paují formations, which weattribute to flexural subsidence of the foreland basin andemplacement of the Lara nappes. Although tectonismappears to have been the reason for the hierarchy ofunconformity-bounded depositional sequences, eustaticprocesses may have been important at times. However, itis not always possible to separate these components.

The Mesozoic–Cenozoic geologic history of the studyarea can be divided into several supersequencesreflecting the history of extension and several episodes ofcollision. Six principal stages of basin formation corre-sponding to six supersequences were recognized anddescribed. Sequence A accumulated in an extensionalsetting. With the cessation of fault-controlled subsidence,passive margin supersequence B was deposited.However, the presence of six depositional sequences(K0–K5) shows that postrift subsidence was episodic.This postrift margin includes the principal source rocks(e.g., La Luna Formation). The transition to a collisionalmargin and a flexural style of foreland basin subsidenceoccurred in supersequence C (three depositionalsequences, K6–K8). Supersequences D, E, and F reflectmultiple stages of compressional subsidence andcollision of the Caribbean and Pacific volcanic arcs. TheEocene Misoa and Gobernador formations (depositionalsequences T1–T3) contain significant reservoir intervals.Supersequence F also contains potential reservoir rocks(e.g., La Rosa and Lagunillas formations). In this super-sequence, the Betijoque Molasse reflects rapid subsidencealong the orogenic front.

Estimated sedimentation rates reflect the dynamics ofthe tectonic and depositional environments. These sedi-mentation rates represent averages for the entireplatform. The changes observed directly relate tochanges in tectonic subsidence rather than environ-mental conditions. The lowest rates correspond topassive margin susidence (10–60 m/m.y.) and thehighest to the compressional phases, especially the timeof encroachment of the Lara nappes (500 m/m.y.).

New biostratigraphy resolves several important strati-graphic hiatuses (e.g., between the Lisure–Maraca andthe La Luna) in an area that includes the central andnorthern Perijá and a large part of the Lake Maracaibobasin. This particular hiatus is attributed to developmentof a forebulge associated with Pacific volcanic arc

collision. This forebulge migrated eastward ahead of theadvancing overthrust load in the Barinas-Apure basinduring the Paleocene.

Integration of outcrop, well, and seismic data hasallowed us to resolve some problems. One such problemwas the distribution of the Guafita Formation and itsconstituent Arauca and Guardulio members in theBarinas-Apure basin. Another example is the relation-ship between sedimentation of the different facies of thePaují Formation and their geodynamic setting. The basalpart of the Paují Formation is composed of an outer shelfto bathyal facies tract that reflects flexure of LakeMaracaibo basement due to the tectonic load of thenappes. The upper part of this formation is composed ofmore detrital facies related to a progradational highstandsystem fed from the nappes in the northeast.

Nevertheless, the stratigraphic columns of theBarinas-Apure and Lake Maracaibo basins require morework to standardize and interpret the stratigraphy. Thereare several important stratigraphic issues that should beaddressed. (1) The systems tracks and bounding uncon-formities of the Peñas Altas Formation are yet to beresolved. (2) The precise age and distribution of thestratigraphic hiatus at the top of the Albian–lower Ceno-manian should be studied. (3) The evolution, diversity,and abundance of the late Cenomanian–early Campan-ian fauna requires attention. (4) Detailed work shouldfocus on the ages of the Eocene sequences. (5) The rela-tionship of sedimentation of the Paují Formation toemplacement of the Lara nappes along its lateral ramp isnot well understood, including definition of the lowertransgressive component of the Lake Maracaibo shelfand the upper progradational stratigraphy. (6) The strati-graphic relationships among the Oligocene sequences ofthe Lake Maracaibo and Barinas-Apure basins have notbeen studied in detail.

Acknowledgments The authors wish to thank the manage-ment of Intevep, S.A., for permission to publish this paper, aswell as the industry sponsors of this project, Corpoven, S.A.,Lagoven, S.A., and Maraven, S.A., the operational affiliates ofPDVSA, for financial support and for providing the subsurfacedata. We also thank geologists François Roure and BernardColletta (Institut Francais du Pétrole, France) for theirconstructive criticisms of the manuscript. This paper has bene-fitted from the comments of Anthony Tankard, AnthonyEdwards, and Ross McLean. We are grateful for the assistanceof drafting personnel at Intevep, S.A.

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Authors’ Mailing Addresses

François ParnaudYves GouJean-Claude PascualBeicip-FranlapPetroleum ConsultantsB.P. 21392500 Rueil-MalmaisonFrance

Maria Angela CapelloIrene TruskowskiHerminio PassalacquaIntevep, S.A.Apartado Postal 76343Caracas 1070AVenezuela

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