Regional geologic and tectonic setting of the Maracaibo supergiant basin, western Venezuela

Transcription

1 Regional geologic and tectonic setting of the Maracaibo supergiant basin, western Venezuela Paul Mann, Alejandro Escalona, and María Verónica Castillo ABSTRACT This special issue contains eight topical studies on the structure, stratigraphy, and petroleum system of the Maracaibo Basin, a supergiant basin in western Venezuela. Most of the work reported in this special issue is the product of thesis-related research by master s and doctoral-level students at the Jackson School of Geosciences of the University of Texas at Austin during a collaborative relationship with the Venezuelan national oil company, Petróleos de Venezuela, S. A., that was initiated in the late 1980s. This introductory article presents a regional overview of the tectonic setting and geology of the Maracaibo Basin. With a cumulative oil production of more than 30 billion bbl, since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl, the Maracaibo Basin is the most prolific hydrocarbon basin in the Western Hemisphere. Unlike the more extensive Gulf of Mexico giant hydrocarbon provinces, the relatively small size (50,000 km 2 ; 19,305 mi 2 ), relative simplicity in its structure and stratigraphy, and wealth of surface and subsurface data make the Maracaibo Basin an attractive target for basinwide synthesis. The objective of this article is to present a regional compilation of two-dimensional (2-D) and three-dimensional (3-D) seismic data, wells, and outcrop data at a basinwide scale to reveal the basin s 3-D structure and stratigraphy. Moreover, we show regional tectonic reconstructions, regional geologic maps, and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges, the distribution of the reservoir rocks, the reactivation of older fault trends, and the timing of maturation for underlying source rocks. Many of these topics are discussed in greater detail in the other eight articles in this special issue. AUTHORS Paul Mann Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Road, Building 600, Austin, Texas 78759; paulm@utig.ig.utexas.edu Paul Mann is a senior research scientist at the Institute for Geophysics, University of Texas at Austin. He received his Ph.D. in geology at the State University of New York in 1983 and has published widely on the tectonics of strike-slip, rift, and collision-related sedimentary basins. A current focus area of research is the interplay of tectonics, sedimentation, and hydrocarbon occurrence in Venezuela and Trinidad. Alejandro Escalona Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Road, Building 600, Austin, Texas Alejandro Escalona is a postdoctoral researcher at the Institute for Geophysics, University of Texas at Austin. He received his Ph.D. in geology at the University of Texas at Austin in 2003, where he focused on stratigraphic and structural evolution of the Maracaibo Basin, Venezuela. He is currently interpreting regional seismic and well data subsurface data from offshore Venezuela to link offshore and on-land Cenozoic depocenters. María Verónica Castillo Department of Geological Sciences and Institute for Geophysics, John A. and Katherine G. Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Road, Building 600, Austin, Texas 78759; present address: Enersis S.A (ENI) Caracas, Venezuela María Verónica Castillo is a geoscientist at ENI Venezuela in Caracas and a lecturer on threedimensional seismic interpretation at the Universidad Central de Venezuela in Caracas. She obtained her Ph.D. in geology at the University of Texas at Austin in 2001, where she focused on the structural evolution of the Maracaibo Basin, Venezuela. Her current interest is using merged 3-D seismic data sets for regional basin analysis. Copyright #2006. The American Association of Petroleum Geologists. All rights reserved. Manuscript received February 17, 2005; provisional acceptance June 10, 2005; revised manuscript received September 28, 2005; final acceptance October 11, DOI: / AAPG Bulletin, v. 90, no. 4 (April 2006), pp

2 ACKNOWLEDGEMENTS The results presented in this overview and in the other articles in this special issue would not have been possible without the long-term cooperation, data contributions, and financial support of Petróleos de Venezuela, S. A. (PDVSA). All seismic, well, and other geologic information used in this issue is used with expressed permission from PDVSA. We give special thanks to William F. Fisher and Amos Salvador of the University of Texas at Austin Department of Geological Sciences for their steadfast encouragement, supervision, and financial support for University of Texas at Austin graduate student research on the Maracaibo Basin. At the University of Texas at Austin Bureau of Economic Geology, Noel Tyler, Edgar Guevara, Bill Ambrose, and H. Zeng began working closely with PDVSA in 1991 on seismic stratigraphy and reservoirs, supervised and supported one University of Texas at Austin graduate student (J. Maguregui, M.S., 1990), and published three technical reports. At Rice University, Albert Bally supervised one Rice Ph.D. student (Felipe Audemard, Ph.D. 1991) and served on the University of Texas at Austin Ph.D. Committee of María Verónica Castillo (Ph.D., 2001). Finally, we acknowledge all those University of Texas at Austin and Rice University graduate students whose work is not presented in this issue but whose research and related publications were essential for creating the foundation for this basinwide synthesis: Jesús Maguregui (University of Texas at Austin, M.S., 1990), Isaskun Azpiritxaga (University of Texas at Austin, M.A., 1991); Jairo Lugo (University of Texas at Austin, Ph.D., 1991), Felipe Audemard (Rice University, Ph.D., 1991), Johnny Pinto (University of Texas at Austin, M.A., 1991), Ramón Gómez (University of Texas at Austin, M.A., 1995), Pedro León (University of Texas at Austin, M.A., 1997), Ronald Oribio (University of Texas at Austin, M.A., 1997), and Felix Díaz (University of Texas at Austin, M.Sc., 1998). We thank A. Bally, J. Blickwede, and D. Goddard for constructive reviews of this article. The authors acknowledge financial support for this publication provided by the University of Texas at Austin s Geology Foundation and Jackson School of Geosciences. University of Texas, Institute for Geophysics contribution Editor s Note Color versions of figures may be seen in the online version of this article. INTRODUCTION Global Significance of Hydrocarbons of the Caribbean and Gulf of Mexico Sedimentary basins of the Gulf of Mexico and northern South America host a discontinuous belt of giant oil and gas fields that collectively contribute 5% of the ultimate hydrocarbon reserves of the world (BP, 2002) (Figure 1A, B). Most of the hydrocarbons of northern South America occur onshore in Venezuela, with less significant deposits in the adjacent countries of Trinidad and Tobago and Colombia (Figure 1A). Shelf and deep-water exploration is advanced in Trinidad and Tobago but much less advanced in the Caribbean Sea north of Venezuela and Colombia. The giant hydrocarbon provinces of the Gulf of Mexico form a geographically distinct province from northern South America that is separated by the hydrocarbon-poor region of the Caribbean Sea, Central America, and the Greater Antilles (Figure 1A). Despite their present geographic separation by km ( mi), both the northern South America and Gulf of Mexico eastern Mexico provinces were once contiguous prior to the breakup of Pangea and therefore share many similarities in their Late Jurassic structure and stratigraphy. The Maracaibo Basin is located on the southwestern edge of the Caribbean Sea in western Venezuela near its border with Colombia (Figure 1A). Venezuela has the fifth largest hydrocarbon reserves in the world, with cumulative oil production of about 60 billion bbl and proven oil reserves of more than 70 billion bbl (Audemard and Serrano, 2001; BP, 2002; Horn, 2003; Escalona and Mann, 2006c). Most of the oil and gas produced in Venezuela is exported to the United States, with a much smaller amount being exported to hydrocarbon-poor nations in the circum-caribbean and South America. In recent years, liquefied natural gas mainly produced in the eastern offshore area of Trinidad has also become a major export to the United States. Objectives of This Issue Unlike the more extensive Gulf of Mexico giant hydrocarbon provinces, the relatively small (50,000 km 2 ;19,305mi 2 )MaracaiboBasin makes an attractive target for the type of basinal synthesis presented in this special issue. We are particularly enthusiastic about the generous contribution of Petróleos de Venezuela, S. A. (PDVSA) to our study of regional, two-dimensional (2-D) seismic lines, along with merged three-dimensional (3-D) seismic data coverage that extends more than 30% of the total basin area (Castillo, 2001; Escalona, 2003). The overall objective of this article and the special issue as a whole is to use a regional compilation of all these data types at a basinwide scale to reveal the basin s 3-D structure and stratigraphy and its key tectonic stages during the Late Cretaceous and Cenozoic. The regional study presented in this article introduces the more detailed articles in the issue that focus on particular areas of the basin. 446 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

3 Figure 1. (A) Location of giant oil and gas fields (red dots) of the Caribbean and Gulf of Mexico region compiled by Mann et al. (2003) and plotted on a bathymetric and topographic basemap from Sandwell and Smith (1997). A giant oil field is considered to be one for which the estimate of ultimately recoverable hydrocarbons is greater than 500 million bbl of oil; a giant gas field contains greater than 3 tcf of gas. As of 2004, the Maracaibo Basin of northwestern South America (boxed area) contains 14 individual giant oil fields. With a cumulative oil production of more than 50 billion bbl since the early 20th century and proven reserves of more than 70 billion bbl, the Maracaibo Basin is the most prolific, single supergiant basin in the Western Hemisphere. (B) Distribution of ultimate hydrocarbon reserves from BP (2002) showing that the Caribbean Gulf of Mexico region contributes 5% of the world s ultimate reserves. (C) Distribution of ultimate reserves in the Caribbean region showing that the relatively small Maracaibo Basin contributes 37% of the ultimate reserves in the Caribbean Gulf of Mexico region. The much larger area of the Maturin foreland basin in eastern Venezuela contributes only about half of this amount to the reserves total. Mann et al. 447

4 Figure 2. (A) Tectonic setting of the Maracaibo Basin in northwestern South America. Major plates in the Caribbean region and compilation of earthquake focal mechanisms are shown on a gravity map of the Caribbean compiled by Sandwell and Smith (1997). Focal mechanisms shown in red are from earthquakes from 0 to 75 km (0 to 46 mi) in depth, blue mechanisms are from 75 to 150 km (46to93mi)indepth,and green mechanisms are greater than 150 km (93 mi) in depth. (B) Compilation of GPS results showing plate motions in the region of northern South America. Sources of GPS vectors include Freymueller et al. (1993); Pérez et al. (2001); Trenkamp et al. (2002). Note the northnortheastward movement of the East Andean block (EAB) that encompasses the smaller, fault-bounded Maracaibo block (MB). Key to other abbreviations: EPFZ = El Pilar fault zone; BFZ = Boconó fault zone; SCDB = South Caribbean deformed belt; SMBFZ = Santa Marta Bucaramanga fault zone; EAFZ = Eastern Andean fault zone. Our goal both in this article and the special issue as a whole is to show linkages between deformation, carbonate and clastic depocenter formation, fault reactivation, reservoir development, and sedimentation patterns and to relate these basinwide events to even larger, platescale tectonic events produced by interactions between the Caribbean and South American plates. We hope that this type of basinwide information will help focus future research and exploration efforts in the Maracaibo Basin. The remaining reserves of the Maracaibo Basin are equivalent in size to more than 100 giant fields. TECTONIC SETTING OF THE MARACAIBO BASIN Plate Boundary Zone Deformation The Maracaibo supergiant basin is situated in a wide and diffuse zone of seismically-active, plate boundary deformation produced by the present-day interaction of three plates: Caribbean, Nazca, and South America (Figure 2A). Global positioning system (GPS)-based geodetic studies carried out since the late 1980s and compiled in Figure 2B have constrained the movement 448 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

5 of the three larger, bounding plates of the region, as well as smaller plates or blocks in their broad, plate boundary zones (Freymueller et al., 1993; Pérez et al., 2001; Trenkamp et al., 2002) (Figure 2B). Geologic mapping, earthquake, and GPS studies show the presence of an elongate, 1400-km (870-mi)-long North Andean block moving to the north-northeast along the right-lateral East Andean-Boconó fault zones at rates of about 6 9 mm/yr ( in./yr) relative to stable South America (Pennington, 1981) (Figure 2B). Previously proposed tectonic mechanisms for northeastward transport of the North Andean block include (1) Panama arc collision with northwestern South America (Pindell and Dewey, 1982); (2) mantle flow from the Pacific into the Caribbean (Russo and Silver, 1992); (3) collision of the Carnegie Ridge with northwestern South America (Pennington, 1981); and (4) oblique subduction along the Ecuador trench (Kellogg and Mohriak, 2000). Regardless of its mechanism, the motion of the North Andean block, which started during the late Miocene, has led to the superposition of strike-slip and convergent tectonics of late Neogene age on rocks previously deformed in the Late Cretaceous and early part of the Cenozoic. Maracaibo Block The northern area of the North Andean block can be further subdivided into the Maracaibo block, a triangular wedge of continental crust that includes the Maracaibo Basin and is bounded to the east by the Boconó right-lateral strike-slip fault, to the west by the leftlateral Santa Marta Bucaramanga fault zone, and to the north by thrusts faults of the South Caribbean deformed belt (Mann and Burke, 1984; Taboada et al., 2000) (Figure 2B). Total right-lateral (northward) displacement of the Maracaibo block relative to South America has been estimated to be in the range of km (31 62 mi) (Stephan, 1977; Escalona and Mann, 2006a). Global positioning system measurements and earthquake studies to date are insufficient to show separate and kinematically distinct North Andean and Maracaibo blocks, although measurements by Trenkamp et al. (2002) support active left-lateral slip on the Santa Marta Bucaramanga fault zone at the western edge of the Maracaibo block (Figure 2B). The 2 6-km ( mi)-thick petroliferous and sedimentary section of the Maracaibo Basin occupies a stable sag area of the interior of the Maracaibo block and is passively rafted northward by strike-slip faults that bound the Maracaibo block. To the east of the Maracaibo block (Figure 2B), the right-lateral Boconó fault zone abruptly curves to the east and transitions into the well-known El Pilar right-lateral strike-slip fault system (Schubert, 1982; Audemard and Audemard, 2002) (Figure 2B). To the west of the Maracaibo block, the Panama arc continues to converge in an east-west direction against the northwest corner of South America at rates of about 20 mm/yr (0.78 in./yr) (Taboada et al., 2000; Trenkamp et al., 2002; Colmenares and Zoback, 2003). North of the Maracaibo Basin, a km (6 11-mi)-thick oceanic plateau and oceanic crust of the Caribbean plate is underthrusting northern South America as shown by a weakly active Benioff zone beneath the Maracaibo Basin (Kellogg and Bonini, 1982; Colmenares and Zoback, 2003). This weakly active Benioff zone passes downdip into an approximately 600-km (372-mi)-long subducted slab imaged using seismic tomography (van der Hilst and Mann, 1994; Taboada et al., 2000). The subducted slab can be traced to the surface at the South Caribbean deformed belt, where it is associated with recent accretion of offscraped sediments at the South Caribbean deformed belt (Ladd et al., 1984). REGIONAL GEOLOGIC SETTING OF NORTHERN SOUTH AMERICA SUPERGIANT BASINS Classification of Basins of Northern South America The Maracaibo Basin forms a segment of an arcuate belt of foreland basins formed during the Cenozoic as a result of collision of a Pacific-derived Caribbean arc with the South American craton (Erlich and Barrett, 1990; Pindell and Barrett, 1990; Lugo and Mann, 1995; Escalona and Mann, 2006a). The belt of foreland basins closest to the South American craton formed entirely on continental rocks of South America and includes, from west to east, the Llanos basin of eastern Colombia, the Barinas basin of western Venezuela, the Maracaibo Basin, and the Eastern Venezuela Basin. Hydrocarbon Distribution in Northern South American Basins As also seen in Figure 1A, giant fields cluster in three of these basins: the Llanos (McCollough and Carver, 1992; Cooper et al., 1995), Maracaibo (Escalona and Mann, 2006a, c), and Eastern Venezuela (Erlich and Barrett, 1992; Di Croce et al., 1999). All three of these foreland basins are highly asymmetric, with thickening Mann et al. 449

6 of clastic fill toward a major thrust fault and with pronounced thinning of clastic sediments toward the craton. These basins are underlain by north- and northwestdipping carbonate rocks deposited on the passive margin of South America prior to the arrival of the Caribbean arc. Mann et al. (2003) classify these basins as continental collision basins related to terrane accretion, arc collision, and/or shallow subduction to distinguish them from more familiar foreland basins that were produced in continent-continent collisional settings. Sources and Traps in Foreland Basins Source rocks in the LLanos, Maracaibo, and Eastern Venezuela basins include Upper Cretaceous black shale deposited during sea level highstands during the precollisional, passive-margin phase (Buitrago, 1994; Talukdar and Marcano, 1994; Erlich et al., 2003; Escalona and Mann, 2006c). Traps include (1) normal or inverted fault traps on the flexed South American plate; (2) deeply buried thrusts and folds of the clastic foreland basin; and (3) younger structures and stratigraphic traps above the deformed interval that have received remigrated hydrocarbons from breached reservoirs derived from folded and thrusted rocks below. Reservoirs include fractured carbonate and sandstone that both predate and accompany the foreland basin history (Erlich and Barrett, 1992). A less hydrocarbon-rich, and less explored belt of hydrocarbon basins is found in the lower, middle, and upper Magdalena basin of Colombia, the Gulf of Venezuela, the Falcón basin of Venezuela, and the Caribbean Sea area north and east of Trinidad and Tobago. These rocks are close to or overlie the abrupt structural contact between arc-related rocks of the Caribbean arc and continental margin rocks of South America. For that reason, their source rocks, structures, petroleum potential, and maturation history should not be assumed to be identical to that of the hydrocarbon-rich belt of foreland basins overlying continental crust (Escalona and Mann, 2006c). Clustering of Giant Fields in Foreland Basins More than 30 individual giant oil fields, each with ultimately recoverable reserves greater than 500 million bbl, are located in onshore Venezuela, with most of those fields in the more cratonward belt of foreland basins that include the Llanos, Maracaibo, and Eastern Venezuela (Figure 1). This belt of giant fields produces predominantly oil with only a few giant gas fields containing greater than 3 tcf of recoverable gas. Clusters of giants are found in those areas with the deepest clastic depocenters, indicating a linkage between deep burial and basin maturity (Escalona and Mann, 2006c). As of 2004, the Maracaibo Basin (boxed area in Figure 1A) contained 14 individual giant oil fields, with most occurring along the eastern coast of the lake and in the central lake areas where lower Cenozoic clastic sedimentary rocks are thickest. With a cumulative oil production of more than 30 billion bbl since the early 20th century and estimated ultimate oil reserves of more than 40 billion bbl, the Maracaibo Basin is the most prolific supergiant basin in the Western Hemisphere as a whole. Only giants of the Persian Gulf region, Alaska (Prudhoe Bay), and the Gulf of Mexico (Cantarell complex) can rival the magnitude of the cumulative production and proven reserves of the Maracaibo Basin (Mann et al., 2003). Hydrocarbon production in the Maracaibo Basin comes from a variety of reservoirs, including Tertiary and Cretaceous clastic and carbonate rocks. Tertiary reservoirs are composed of fluvial-dominated and tidaldominated deltaic systems in the Eocene (Marguregui, 1990; Ambrose et al., 1995; Escalona, 2006) and fluvial systems with associated incised alluvial valleys in the Miocene (Guzman and Fisher, 2006). Cretaceous reservoirs consist of fractured and karstic carbonate reservoirs (Azpiritxaga, 1991; Chacartegui et al., 1995; Nelson et al., 2000; Castillo, 2001). Escalona and Mann (2006c) review the petroleum system of the Maracaibo Basin in detail. FOUR MAJOR TECTONIC STAGES IN THE EVOLUTION OF THE MARACAIBO BASIN Approach to Tectonic Reconstructions In this section, we use plate tectonic reconstructions of northern South America constructed using the software and methods of the University of Texas at Austin PLATES project ( /projects/plates/). Reconstructions are shown at critical times during the evolution of the Maracaibo Basin from 80 to 5 Ma. The positions of larger plates surrounding the Caribbean plate (North and South America and Africa) are reasonably well constrained going back to the Early Cretaceous and Late Jurassic, whereas the position of other plates forming the Pacific margin of the Caribbean are only known for the late Cenozoic (Müller et al., 1999). 450 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

7 For brevity, we illustrate only the Late Cretaceous to Holocene tectonic evolution of the Maracaibo Basin in Figure 3A F because the events most closely linked to its petroleum systems date from this period. Previous tectonic reconstructions that include the Late Jurassic period of rifting between North and South America include Pindell and Barrett (1990), Bartok (1993), and Mann (1999). Tectonic Stages in the Evolution of Northern South America Most previous workers have recognized the importance of tectonic stages in understanding the complex stratigraphic and structural evolution of the northern margin of South America. For example, Pindell and Dewey (1982), Eva et al. (1989), Pindell and Barrett (1990), Lugo and Mann (1995), and Mann (1999) all identified a Late Jurassic rift stage related to the opening of North and South America, a protracted Cretaceous period of passive-margin formation following the rift event, a Paleogene period of oblique collision between a westward-moving Caribbean island arc and the passive margin of South America, and a Neogene period of strike-slip faulting and Andean uplift that is particularly intense and widespread in western Venezuela and Colombia. The timing of these four events, along with the positions of the larger plates known from marine magnetic anomalies in oceanic plates, provide the fundamental constraints on the tectonic models that we show in Figure 3A F. On the left side of each reconstruction are the unornamented basement blocks as taken from the PLATES program. Each basement block has been defined on the basis of its radiometric age, lithologic composition, volcanic geochemistry, and sedimentary facies. Gaps between blocks represent areas of subsequent crustal shortening that have been estimated from outcrop and seismic reflection studies. Mismatched edges of blocks represent subsequent strike-slip displacements. These shortening and strike-slip estimates sometimes vary widely between individual authors, so we indicate the authors we have chosen to follow in these reconstructions. On the right side of each reconstruction are inferred sedimentary cover sequences for the basement blocks that have been compiled from the literature. With the exception of Pindell et al. (1998), most authors display their inferred paleogeography on a basement of present-day geography. Instead, we show paleogeographic interpretations that were made in conjunction with the tectonic reconstructions. Late Cretaceous, Approximately 88 Ma (Coniacian) Prior to the Coniacian, Late Jurassic rifting between North and South America created a 1800-km (1118-mi)- wide seaway between North and South America that is commonly referred to as the Proto-Caribbean seaway (Pindell and Barrett, 1990; Bartok, 1993; Mann, 1999). The passive margin of northern South America is characterized by a broad, mixed carbonate-clastic shelf on which were deposited an extensive area of middle to outer shelf, fine-grained, organic-rich rocks that form the main source rocks for hydrocarbons in northern South America (La Luna Querecual Formation) (Cooper et al., 1995; Escalona and Mann, 2006c). Rocks of this formation are particularly widespread in the area west and southwest of present-day Lake Maracaibo, and for that reason, we have inferred an embayment of the passive margin in that region (Figure 3A). The passive margin narrows in an eastward direction (Erlich and Barrett, 1992) and curves abruptly to the southeast in its eastern area near present-day Trinidad (Di Croce et al., 1999). Middle Paleocene, Approximately 60 Ma By the Paleocene, the eastward-moving Great Arc of the Caribbean had begun to subduct Mesozoic oceanic crust of the Proto-Caribbean seaway and to influence sedimentary facies in the northern part of the Maracaibo Basin (Pindell and Barrett, 1990; Lugo and Mann, 1995; Escalona and Mann, 2006b). The Great Arc sweeps in a diachronous manner from west to east across the passive margin, with its initial flexural subsidence in Venezuela recorded by Paleogene clastic sedimentation in the Maracaibo foreland basin. The Great Arc is a composite structure that includes a back arc, volcanic arc, forearc, and accretionary prism areas that are identified on the key in Figure 3A (Mann, 1999). The South America Great Arc collision marks the end of the passive-margin phase in the Maracaibo Basin and the beginning of the foreland basin phase that is of critical importance for the formation of reservoir rocks and the maturation of the underlying source rocks of the passive margin (Escalona and Mann, 2006c). Prior to this collision, most of the area of the Maracaibo Basin remained a stable, shallow carbonate platform. Collision of the Caribbean arc will bend the northnortheastern part of the platform area downward beneath the encroaching thrust faults and tear faults and form a major foreland basin of late Paleocene early Eocene age (Lugo and Mann, 1995; Escalona and Mann, Mann et al. 451

8 Figure 3. (A) Tectonic reconstruction of basement blocks and paleogeographic map for the northern South America at approximately 88 Ma (Coniacian). White areas in tectonic reconstruction reflect areas of future shortening. Key provides the names of numbered features and paleoenvironments in (B F). (B) Middle Paleocene; (C) middle Eocene, approximately 44 Ma; (D) Oligocene, approximately 30 Ma; (E) middle Miocene, approximately 14 Ma; (F) early Pliocene, approximately 5 Ma. See text for discussion.

9 Figure 3. Continued.

10 Figure 3. Continued.

11 2006b). Sands filling the foreland basin both from the proto-maracaibo River draining the continental area to the south and from the uplifted highlands associated with thrusting to the north will act as high-quality reservoirs for future hydrocarbons in the basin (Escalona et al., 2004). Middle Eocene, Approximately 44 Ma By about 44 Ma, a large area of proto-caribbean oceanic crust had subducted beneath the northwestern corner of South America (indicated in Figure 3C by the blue areas visible between crustal blocks). The present-day lake area had become the site of a coastaldeltaic complex that fed an extensive offshore area of deep-marine sedimentation that filled the forearc and back-arc areas of the passing Great Arc (Figure 3C). In the middle Eocene, parts of the Great Arc began to overthrust the north-sloping passive margin. In the Maracaibo area, collision-related shortening led to thrust emplacement of the Lara nappes (Stephan, 1977, 1985). This shortening culminated in the late Eocene Oligocene uplift and erosion of the present-day lake area and the formation of the prominent Eocene unconformity that is a highly angular contact in some locations (Escalona and Mann, 2006b). Oligocene, Approximately 30 Ma During the Oligocene, the Great Arc continued its collision with the passive margin and began to form the Eastern Venezuelan foreland basin by the same tectonic process that formed the Maracaibo foreland basin in the Paleogene (Pindell and Barrett, 1990; Erlich and Barrett, 1992) (Figure 3D). In the Maracaibo Basin, fluvial sedimentation of the proto-maracaibo River was diverted by the uplift of the Colombian Andes, and the Orinoco River formed to carry most fluvial sediments eastward along the margin (Díaz de Gamero, 1996; Escalona et al., 2004) (Figure 3D). Regional uplift in the Maracaibo Falcón area related to continued convergence and isostatic rebound shifted the position of the shelf edge far to the north (Guzman and Fisher, 2006). Uplift of the Sierra de Perijá west of the Maracaibo Basin occured at this time and is recorded by a large clastic wedge filling the basin from the west. The uplift of the Sierra de Perijá may be related to the shallow subduction of the Caribbean crust and the formation of basement uplifts on the overriding South America plate (Kellogg, 1984; van der Hilst and Mann, 1994; Taboada et al., 2000). Middle Miocene, Approximately 14 Ma By about 14 Ma, the Eastern Venezuela foreland basin was undergoing maximum subsidence as a result of the oblique collision of the Great Arc and the formation of a fold-thrust belt in the Serranía del Interior (Erlich and Barrett, 1992; Roure et al., 1997). Thrust-related deformational effects occurred as far east as Trinidad and produced a major regional unconformity spanning the middle Miocene interval over much of this area (Tyson, 1990). Sedimentation in the Maracaibo Basin shows the beginning of the uplift of the Mérida Andes east of the lake (Castillo and Mann, 2006; Guzman and Fisher, 2006). In this middle Miocene period, the Maracaibo Basin was filled by a fluvial-deltaic system related to the proto- Maracaibo River draining from the Andes to the south of the basin (Escalona et al., 2004). Guzman and Fisher (2006) discuss the narrow strait connecting the proto- Maracaibo River in the Maracaibo Basin to a more openmarine area. Early Pliocene, Approximately 5 Ma By the early Pliocene, the region looked very similar to its present-day appearance (Figure 3F). Deformation was most intense in the far east near Trinidad, where the collision between the leading edge of the Caribbean plate and the passive margin continues to the present day (Babb and Mann, 1999; Boettcher et al., 2003). Strikeslip faulting along the various faults bounding the edges of the Maracaibo block and along the El Pilar fault zone mark the terminal stages of plate convergence (Trenkamp et al., 2002). By the early Pliocene, all fluvial sedimentation was concentrated on the Orinoco River, which rapidly filled in the recently formed Columbus foreland basin east of Trinidad (Di Croce et al., 1999; Wood, 2000). OVERVIEW OF THE GEOLOGY OF THE MARACAIBO BASIN The purpose of this section is to provide a brief overview of the regional geology of the Maracaibo Basin, along with a summary of previous geologic studies that have leduptothebasinsynthesispresentedinthisissue. The physiographic Maracaibo Basin occupies a 50,000-km 2 (19,305-mi 2 ) triangular, intermontane basin in western Venezuela (Figure 4). Lake Maracaibo occupies about 30% of the surface of Maracaibo Basin Mann et al. 455

12 Figure 4. Present-day distribution of outcrops and subcrops of major megasequences in the area of the Maracaibo Basin. Outcrop data are from Maze (1984) and Borges (1984). (A) Outcrop (orange areas) and subcrop (brown stippled areas) of rift-related red beds of the Late Jurassic La Quinta Formation. Faults known or inferred to have been active during the rift phase are indicated. (B) Outcrop (dark green areas) and subcrop (dark green stippled areas) of Cretaceous passive margin-related carbonate rocks of various formations. Faults known or inferred to have been active during the passive margin phase are indicated. The Mérida arch from Salvador (1986) is shown as a dotted red line. (C) Outcrop (blue areas) and subcrop (blue stippled areas) of Paleogene foreland basin rocks of various formations. Faults known or inferred to have been active during the foreland basin phase are indicated. (D) Outcrop (yellow areas) and subcrop (yellow stippled areas) of Neogene basinal rocks of various formations related to Andean uplift and strike-slip motion of the Maracaibo block. Faults known or inferred to have been active during the Andean uplift and Maracaibo block strike-slip displacement are indicated. and forms a shallow topographic depression with a maximum water depth of 30 m (98 ft). The geologic maps in Figure 4A D show the distribution of both outcrops and subcrops of rocks in the Maracaibo Basin region, whose structure and stratigraphy record four major tectonic events described and partly shown in the tectonic reconstructions in Figure 3A F. A major point of these maps is to show that the physiographic Maracaibo Basin acts to preserve an approximately 7-km (4.3-mi)-thick, relatively undeformed section of Jurassic Holocene age that records all four of the major tectonic events described above (Figure 3A F). Therefore, in addition to its importance as a supergiant hydrocarbon basin, the subsurface geology of the Maracaibo Basin provides an important record of the geologic and tectonic history of northwestern South America that would be much more difficult to reconstruct from more fragmentary and more deformed outcrops in the surrounding mountain ranges (Figure 4A D). 456 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

13 Figure 5. Chart of the Mesozoic and Cenozoic formations and their sedimentary facies of the Maracaibo Basin along the line of cross section shown on the inset map (modified from Parnaud et al., 1995). Formations to the left of the chart are found in the Sierra de Perijá, formations in the middle are found in the Maracaibo Basin, and formations to the right are found in the Mérida Andes. We identify six unconformity-bound tectonosequences in the Maracaibo Basin that are numbered on the left side of the chart. Bounding unconformities include pre-cretaceous unconformity, Paleocene unconformity, Eocene unconformity, and upper Miocene unconformity. The six tectonosequences are related to four major tectonic phases identified as I IV on the left side of the chart. These tectonic phases include the following: I = pre-cretaceous (Late Jurassic) rift phase; II = Cretaceous (Neocomian to Maastrichtian passive-margin phase); III = Paleogene foreland basin phase; and IV = late Oligocene Holocene Andean uplift, strike-slip, and shortening phase. Total sediment thickness to the top of Paleozoic acoustic basement (kilometers) of the Maracaibo Basin is shown on the inset map. Modified from Parnaud et al. (1995). Tectonosequences of the Maracaibo Basin Figure 5 shows a regional-stratigraphic chart modified from Parnaud et al. (1995) and Castillo (2001) summarizing the main tectonosequences, formation names, and paleoenvironments across the Maracaibo Basin. These tectonosequences are bounded by major basinwide unconformities that include the sub-cretaceous unconformity, the Paleocene unconformity, the Eocene unconformity, and the lower Miocene unconformity (Figure 5). The unconformities are designated by the stratigraphic age of their hiatus (i.e., Eocene unconformity). The inset map of Figure 5 shows the total sedimentary thickness of the basin above the acoustic Paleozoic basement and the line of section along which the stratigraphic chart was made. These tectonosequences are defined by unconformities in outcrops and by major discontinuities on seismic reflection data (i.e., downlaps, onlaps, and erosional truncations as defined by Mitchum et al., 1977; Vail et al., 1977). The subsurface discontinuities are also defined using well-log correlations tied to seismic data (Lugo and Mann, 1995; Parnaud et al., 1995; Escalona, 2006; Escalona and Mann, 2006b). A general description of the tectonosequences in the Maracaibo Basin is summarized below using both outcrop and subcrop descriptions. More detailed descriptions of the tectonosequences at their type sections in outcrop around Mann et al. 457

14 the basin are provided by Sutton (1946), Gonzálezde Juana et al. (1980), Audemard (1991), and Parnaud et al. (1995). Tectonosequence 1: Late Jurassic Rifting Tectonosequence 1 represents the acoustic basement of the Maracaibo Basin and the lower limit of sesmic imaging and deep exploration drilling in the basin (Lugo and Mann, 1995) (Figure 5, inset). The sequence consists of upper Paleozoic metasedimentary rocks (Mucuchachí Formation) and overlying Upper Jurassic red beds of the La Quinta Formation derived from the erosion of Paleozoic metamorphic blocks rifted and exposed as highlands during the breakup of Pangea (Schubert et al., 1979; Maze, 1984). Rift-related red beds are locally sourced by pyroclastic material (La Gé Group) that was deposited in elongate half grabens (Lugo and Mann, 1995; Parnaud et al., 1995). Rift-related half grabens containing the Jurassic rocks underlying the Maracaibo Basin have a north-northeast trend (Audemard, 1991; Lugo and Mann, 1995) but have also been proposed based on stratigraphic thickness variations in the mountain ranges surrounding the Maracaibo Basin (Schubert et al., 1979; Maze, 1984) (Figure 4A). Tectonosequence 2: Cretaceous Passive Margin Tectonosequence 2 deposited on a broad passive margin (Figure 3A), includes Lower Cretaceous carbonate and clastic units and is bounded by the basal Cretaceous unconformity separating the Cretaceous carbonate platform from the underlying metamorphic basement rift features described above. The structural configuration of the basin during this period was characterized by paleohighs, basins, and tectonic activity west of the Maracaibo Basin, which most workers relate to the uplift of the Central Cordillera of Colombia (Renz, 1981; Erlich et al., 1999; Macsotay et al., 2003). Renz (1981), using cross sections from outcrops along the mountain range bounding the Maracaibo Basin, interpreted a basement paleohigh, the Mérida arch. Lugo and Mann (1995) interpreted the continuation of the Mérida arch into the southern end of Lake Maracaibo, which affected the thickness of the Cretaceous passive-margin sediments (Figure 3A). The top of the tectonosequence is defined by the Socuy Member of the Colón Formation (Figure 5). Along with the Socuy Member, the Cretaceous passivemargin tectonosequence includes the following formations shown on the chart in Figure 5 and described in detail from outcrop studies of the basin edges by the following authors: Río Negro (Hedberg, 1931), Apón (Sutton, 1946); Lisure (Rod and Maync, 1954), Aguardiente (Notestein, et al., 1944), La Luna (Garner, 1926), and the Socuy Member of the Colón Formation (Sutton, 1946; González de Juana et al., 1980). The Apón, Lisure, Aguardiente, and Maraca formations all make up the Cogollo Group (González de Juana et al., 1980). All carbonate rocks of the Cogollo Group were deposited on a shallow carbonate platform and are characterized by two main depositional styles: fining-upward cycles during the Aptian middle Albian and coarsening-upward cycles in the upper Albian (Azpiritxaga, 1991). The Upper Cretaceous La Luna Formation overlying the Cogollo Group forms a world-class source rock that is responsible for more than 98% of the hydrocarbons generated in the Maracaibo Basin (Talukdar and Marcano, 1994; Nelson et al., 2000; Escalona and Mann, 2006c) (Figure 4B). The top of the organic-rich La Luna Formation is defined by carbonate rocks of the Socuy Member. This contact is characterized on seismic data by a prominent, continuous reflector produced by the acoustic impedance between underlying shale of the La Luna Formation and overlying carbonate rocks of the Socuy Member. Tectonosequence 3: Campanian Maastrichtian Foreland Basin Tectonosequence 3 formed by early effects of oblique collision between the Great Caribbean arc and northwestern South America (Figure 3B, C), is bounded at its base by the Socuy Formation and at its top by the Paleocene unconformity. The tectonosequence was deposited in a foreland basin and is composed of clastic sedimentary rocks of the Upper Cretaceous Colón (Liddle, 1928) and Mito Juan (Garner, 1926) formations, along with the Paleocene Guasare Formation (Lugo and Mann, 1995; Parnaud et al., 1995) (Figure 5). Pelagic, clastic rocks of the Colón Formation are inferred to have been deposited in the distal region of a foreland basin that resulted from the Caribbean volcanic arc collision with northwestern South America (Cooper et al., 1995; Parnaud et al., 1995) (Figure 3A). In the Maracaibo Basin, the Colón Formation is transitional into the overlying Mito Juan Formation that was deposited in brackish to marine environments (Sutton, 1946). Paleocene rocks of the Maracaibo Basin consist of a shallow-marine, mixed clastic-carbonate platform section. The top of this section produces an extensive and continuous seismic reflector beneath the Lake Maracaibo area (Lugo and Mann, 1995; Castillo and Mann, 2006). 458 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

15 Sandstone of the Cretaceous Colón Formation exhibits a major change in lithology from underlying Jurassic and Lower Cretaceous continentally derived quartz-rich, and continental stratigraphic units. The appearance of a belt of graywackes and subgraywackes in the Colón Formation in the western and southwestern quadrant of the Maracaibo Basin suggests the accretion of an arc terrane to the west and southwest of the Maracaibo Basin (Van Andel, 1958). Audemard (1991) and Marcha (2004) interpret easterly and northeasterly dipping clinoforms inferred from 2-D and 3-D seismic data in the northwestern parts of the basin to support this accretion event. Marcha (2004) concluded that the overlying Paleocene Guasare Formation was deposited on relatively flat topography and was not influenced by the earlier collision and event to the west. Lugo (1991) suggested that relative sea level drop during the Late Cretaceous Paleocene is responsible for the regressive facies of the Colón Formation observed in the Maracaibo Basin at this time. The existence of an Upper Cretaceous lower Paleocene foreland basin west of the Maracaibo Basin, therefore, remains controversial. Tectonosequence 4: Paleocene Oligocene Foreland Basin Phase Tectonosequence 4 is composed of lacustrine to fluvialdeltaic rocks defined by the Paleocene unconformity at their base and the Oligocene Miocene unconformity at their top (Figure 5). The sedimentary units in this tectonosequence record a sedimentary transition from passive to active margin. This transition coincided with the southward thrust emplacement of the Lara nappes in the middle late Eocene (Stephan, 1985; Audemard, 1991; Lugo, 1991; Parnaud et al., 1995) (Figure 3C, D). Formations included in this tectonosequence include the well-studied, fluviodeltaic Misoa Formation (Marguregui, 1990; Lugo and Mann, 1995; Escalona and Mann, 2006b), the more distal to deep-water sedimentary rocks of the Trujillo Formation (Mathieu, 1989) and the shallow-marine Paují Formation (Sutton, 1946; González de Juana et al., 1980; Mathieu, 1989) (Figure 5). Tectonosequence 4 is characterized by an overall regressive character defined by fluvial facies. The Eocene succession is composed mainly of medium- to fine-grained, subangular to rounded quartz sandstone with subordinate shale (Lugo and Mann, 1995). The Misoa Formation is the reservoir rock for most of the major oil fields of the Maracaibo Basin and is discussed in detail by Escalona and Mann (2006b, c). Tectonosequence 5: Oligocene Uplift of the Sierra de Perijá Tectonosequence 5 is bounded by the Eocene unconformity at its base and the upper Miocene unconformity at its top (Figure 5). Shallow-marine to continental clastic deposits dominate this tectonosequence and include transgressive sands of the Icotea Formation of late Oligocene age. The Oligocene clastic wedge was deposited during the main uplift of the Sierra de Perijá, which controlled subsidence as well as sediment dispersal into its associated depocenter (Audemard, 1991; Castillo, 2001). Tectonosequence 6: Early Miocene to Quaternary Erosion of Adjacent Mountain Ranges Tectonosequence 6 is defined by the lower Miocene unconformity at its base and the present-day floor of Lake Maracaibo at its top and consists of clastic sedimentary rocks produced by erosion of the uplifted Sierra de Perijá and Mérida Andes. Based on fissiontrack age determinations, pulses of uplift of the Sierra de Perijá and Mérida Andes occurred during the late Miocene Pliocene and Pliocene Pleistocene (Kellogg, 1984; Kohn et al., 1984; Shagam et al., 1984; De Toni and Kellogg, 1993). Lower middle Miocene rocks consist of shallow-marine deposits that gradationally pass upward into late Miocene continental deposits (La Rosa and Lagunillas formations; González de Juana et al., 1980; Guzman and Fisher, 2006). The Pliocene Holocene part of the tectonosequence includes the Onia and El Milagro formations that were deposited in fluvial-deltaic and lacustrine environments (González de Juana et al., 1980; Audemard, 1991). PREVIOUS WORK ON THE OUTCROP AND SUBSURFACE GEOLOGY OF THE MARACAIBO BASIN Early Studies The search for hydrocarbons has been the main impetus for geologic studies in the Maracaibo Basin since the beginning of the 20th century and especially in the period after World War I (Sutton, 1946). The presence of hydrocarbons in the Maracaibo Basin has been known for centuries because oil seeps are plentiful along all the surrounding mountain fronts of the Maracaibo Basin (Sutton, 1946; Link, 1952; Escalona and Mann, 2006c) (Figure 6). Early exploration wells, Mann et al. 459

16 460 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

17 including those of the Bolivar Coast at the northeastern edge of Lake Maracaibo, were drilled adjacent to natural oil seeps, including the well-known Mene Grande, or Big Seep field (Link, 1952). Early geologic studies included systematic field mapping and compilation of subsurface data from exploration wells (Hedberg, 1931; Notestein et al., 1944; Van Andel, 1958; Salvador, 1961; Brondijk, 1967; Feo- Codecido, 1970; Van Veen, 1972; Renz, 1981). At the special request of the AAPG, Sutton (1946) compiled all existing knowledge of the first 30 yr of petroleum exploration and development geology in the Maracaibo Basin into a single special issue of AAPG Bulletin. Private and Government Geological and Geophysical Studies ( ) The arrival of 2-D seismic acquisition methods to the Maracaibo Basin in the 1960s ushered in a new and productive phase of geologic studies that led to a greatly improved understanding of the subsurface geology of the basin. As a result of improved exploration and development methods, Venezuela became the world s largest oil exporter in An enormous amount of surface and subsurface geologic studies were performed but were dispersed among different national and international oil companies working in the basin. In 1976, nationalization of the oil industry gave the Venezuelan government ownership of the entire oil infrastructure and database. This led to the creation of PDVSA and the Venezuela Petroleum Corporation. At the request of these organizations, a major compilation of the surface and subsurface petroleum geology and stratigraphy of Venezuela was conducted by González de Juana et al. (1980). Early Academic Studies and Student-Related Work During the 1980s, United States and French academic geologists, including Pindell and Dewey (1982), Burke et al. (1984), Kellogg (1984), Mann and Burke (1984), Stephan (1985), Salvador (1986), Ostos (1990), and Pindell and Barrett (1990) began to synthesize the geologic and tectonic information from the Maracaibo Basin. By the 1990s, petroleum exploration and production activities in the Maracaibo Basin led to the acquisition of thousands of kilometers of 2-D and 3-D seismic reflection data and the drilling of more than 10,000 exploration wells (Figures 7, 8). Parts of this huge data set were made available for thesis studies by Venezuelan geologists obtaining degrees at universities in the United States and France (Mathieu, 1989; Audemard, 1991; De Toni and Kellogg, 1993; Lugo and Mann, 1995; Parnaud et al., 1995; Roure et al., 1997; Duerto, 1998) (Table 1). Student Research Projects at the University of Texas at Austin and Other Universities The three units of the University of Texas, Jackson School of Geosciences (Department of Geological Sciences, Institute for Geophysics, and Bureau of Economic Geology [BEG]), have been active in subsurface seismic-stratigraphic and well research in Venezuela since the late 1980s. Much of this effort has been in the form of student M.S. and Ph.D. projects sponsored by PDVSA and supervised by W. Fisher, A. Salvador, N. Tyler, and P. Mann at the University of Texas at Austin (Table 1). In addition, several major research projects and technical publications have been produced at the BEG (Ambrose et al., 1995, 1998; H. Zeng, 2002, personal communication). These studies have been primarily focused at the scales of individual exploration blocks. Figure 7 and Table 1 compile the location and name of all the master s theses and Ph.D. dissertations completed at the University of Texas at Austin on the Maracaibo Basin over the last 15 yr. Figure 8 shows the location of the most relevant regional studies in the Maracaibo Basin done or published by institutions other than the University of Texas at Austin. As seen in Figures 7 and 8, Figure 6. Surface geologic map of the Maracaibo Basin region (modified from Borges, 1984) combined with a seismic time slice from a merged 3-D seismic data set at 1 s two-way traveltime (TWT) beneath the floor of Lake Maracaibo. Colors for outcrops and subcrops seen on the 3-D seismic time slice indicate the age of rocks and are shown in the figure legend. The present-day topographic and geologic configuration of the Maracaibo Basin is controlled by uplift of the Mérida and Sierra de Perijá mountain ranges and by formation of the Miocene Holocene Maracaibo syncline with a roughly north-south trending axial trace. Global positioning system velocity vectors from Pérez et al. (2001) and Trenkamp et al. (2002) indicate direction and relative rate of displacement of the Maracaibo block to the north-northeast relative to the stable South America plate to the east of the basin. North-northeast striking, pre-oligocene faults characterize the subsurface of central Maracaibo Basin. The Burro Negro fault bounds the present Maracaibo Basin along its northeastern boundary. Mann et al. 461

18 Figure 7. Topographic map of the Maracaibo Basin showing location of PDVSA seismic data used by University of Texas at Austin master s and Ph.D. graduate students during research projects in the period from 1987 to Boxes indicate areas of 3-D seismic data. Note that 2-D and 3-D seismic data almost completely cover the area of Lake Maracaibo. these combined studies cover most of the area of the Maracaibo Basin and include a large number of topics, ranging from reservoir characterization to basin evolution. A common element of all these studies was a reliance on 2-D seismic data and well correlation. Impact of 3-D Seismic Data on Basin Research By the late 1990s, the availability of 3-D seismic data, shown as boxes in Figures 7 and 8, began to impact the level of understanding of the structure and stratigraphy of the Maracaibo Basin. The 3-D seismic data were initially used for reservoir characterization and detail strucutral analysis of complex Eocene reservoirs that are widely distributed across the Maracaibo Basin (e.g., León et al., 1999; Link et al., 1999; Benkovics and Helwig, 2001). Many of these early 3-D seismic studies exist only as internal, unpublished PDVSA reports that concentrate on the more intensively explored north-central and eastern parts of the basin. In contrast to these specific exploration-related efforts, Castillo (2001), Escalona (2003), Escalona and Mann (2003), and Castillo and Mann (2006) made regional interpretations of time slices from merged 3-D seismic data sets provided by PDVSA and covering about 30% of the area of the basin (Figures 6, 7). These 3-D data were augmented by regional 2-D seismic lines. In the following section, we summarize the main results of these more regional studies using both 2-D and 3-D seismic data and relate this information to the four main tectonic stages of the basin described above. OVERVIEW OF THE SUBSURFACE GEOLOGY OF THE MARACAIBO BASIN USING REGIONAL 3-D SEISMIC DATA Merge of Surface Geology with 3-D Subsurface Seismic Time Slices Figure 6 shows the present-day surface geology of the Maracaibo Basin from Borges (1984), merged with an interpreted time slice at 1.0 s two-way traveltime beneath the floor of Lake Maracaibo from Castillo (2001). The present-day topographic and geologic configuration of the basin is controlled by the uplift of the Mérida 462 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

19 Figure 8. Topographic map of the Maracaibo Basin showing tracks of PDVSA seismic data used by graduate students and researchers at other universities during the period of Work by Parnaud et al. (1995) and Roure et al. (1997) was done as part of a collaborative study between the Institut Français du Pétrole and PDVSA. Boxes indicated areas of 3-D seismic data. These studies are available publicly as M.S. theses or dissertations. Some have been summarized in published articles and abstracts. Andes and Sierra de Perijá along the mountain front fault zones described by Duerto et al. (2006). Global positioning system velocity vectors from Pérez et al. (2001) and Trenkamp et al. (2002) indicate the direction and relative rate of displacement of the Maracaibo block to the north and northeast relative to the stable South America plate (Figure 2B). The Maracaibo Basin is a particularly complex sedimentary basin for two reasons. First, Late Jurassic rifting introduced a strong north-south grain to the floor of the basin that was subject to later reactivation (i.e., Icotea, Pueblo Viejo, and Urdaneta faults); second, convergence directions varied from northeast-southwest in the Eocene to more east-west directions in the post- Eocene (Escalona and Mann, 2006a); and third, the basin remained in a zone of active plate boundary deformation between the Caribbean, South American, and Nazca plates for a remarkably long period from Paleocene to Holocene. However, despite this complex tectonic setting and protracted structural history, regional 2-D lines and 3-D seismic time slices reveal that large areas of the central basin have remained remarkably stable and undeformed throughout the basin s history. The protracted history of faulting in the basin requires a stepwise approach to fault mapping because lumping of faults of all ages onto a single map can lead to the misperception of a high degree of structural complexity (Figure 4). In fact, most faulting in the central part of the Maracaibo Basin is confined to Eocene and older rocks and therefore is deeply buried by up to 5 km (3.1 mi) of little or undeformed sedimentary rocks (Figure 4). Regional 3-D seismic data, which can be viewed in horizontal time slices, are particularly useful for showing how most faults are confined to deeper levels of the basin. 1.0-s Time Slice from Regional 3-D Seismic Data Subsurface deformation in the basin at the 1.0-s time slice (Figure 9) intersects the stratigraphic level from upper Miocene to Pleistocene or during the period of tectonosequence 6 shown in Figure 5. These Neogene rocks dip into the north-south trending Maracaibo syncline of Castillo and Mann (2006). The Maracaibo syncline, a previously unrecognized feature of the Maracaibo Basin prior to Castillo (2001), is inferred to Mann et al. 463

20 Table 1. List of the University of Texas at Austin Department of Geological Sciences M.A. M.Sc. Theses and Ph.D. Dissertations in the Maracaibo Basin Completed by Venezuelan Graduate Students and the University of Texas at Austin Bureau of Economic of Geology Publications on the Maracaibo Basin Department of Geological Sciences M.S. and Ph.D. theses and dissertations in the Maracaibo Basin Number Author Degree Year Title Supervisor(s) 1 Marguregui, J. M.A Evolution and reservoir rock properties of middle Eocene tide-dominated deltaic sandstones in eastern Lagunillas field, Maracaibo Basin, Venezuela 2 Azpiritxaga, I. M.A Carbonate depositional styles controlled by siliciclastic influx and relative sea level changes, Lower Cretaceous central Lake Maracaibo, Venezuela 3 Lugo, J. Ph.D Cretaceous to Neogene tectonic control on sedimentation: Maracaibo Basin, Venezuela 4 Pinto, J. M.A Sequence-stratigraphic interpretation of upper Paleocene middle Eocene Rocks: Bloque III, Lake Maracaibo, Venezuela 5 Gómez, R. M.A Depositional system analysis of C-6-X and C-7-X members of Misoa Formation, Bachaquero Suroeste field, Lake Maracaibo, Venezuela 6 León, P. M.A Seismic and geological characterization of the middle Eocene Misoa Formation, centro Lago field, Maracaibo Basin, Venezuela 7 Oribio, R. M.A Reservoir architecture and reserve growth potential of Miocene fluvial-deltaic deposits, Bachaquero field, Maracaibo Basin 8 Díaz, F. M.Sc Architecture of a shore-zone reservoir system in Barua field, Maracaibo Basin, western Venezuela 9 Guzman, J. Ph.D Miocene stratigraphy and depositional framework of northeastern Maracaibo Basin, Venezuela: Implications for reservoir heterogeneity prediction in tectonically active settings 10 Castillo, M. Ph.D Structural analysis of Cenozoic fault systems using 3D seismic data in the southern Maracaibo Basin, Venezuela 11 Escalona, A. Ph.D Regional tectonics, sequence stratigraphy and reservoir properties of Eocene clastic sedimentation, Maracaibo Basin, Venezuela Fisher, W., Tyler, N. Salvador, A., Bebout, D. Salvador, A., Mann, P. Buffler, R. Tyler, N., and Fisher, W. Tyler, N. Tyler, N. Tyler, N. Fisher, W., Tyler, N. Mann, P., Fisher, W. Mann, P., Fisher, W. Bureau of Economic Geology publications on the Maracaibo Basin Number Author Year Title 1 Ambrose, W., et al Production optimization of tide-dominated deltaic reservoirs of the lower Misoa Formation (lower Eocene), LL-652 Area, Lagunillas field 2 Ambrose, W., et al Geologic controls on reservoir architecture and hydrocarbon distribution in Miocene shoreface, fluvial, and deltaic deposits in the Miocene Norte Area, Lake Maracaibo, Venezuela 3 Zeng, H., et al. 2002, unpublished report Seismic sedimentology by stratal slicing: A case history in the Miocene Norte Area, Lake Maracaibo, Venezuela 464 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

21 Figure 9. (A) Uninterpreted regional time slice of the Lake Maracaibo area at 1 s two-way traveltime (TWT). (B) Interpreted seismic time slice at 1 s TWT, with major structural and stratigraphic features indicated. Stratigraphic units on the time slice are keyed to the stratigraphic column in (C) on the right. This time slice intersects Pleistocene Lower Cretaceous rocks. The axial trace of the Maracaibo syncline is gently curved and extends from the southern Maracaibo Basin to the central part of Lake Maracaibo. Mann et al. 465

22 represent post-pleistocene east-west shortening known from earthquake focal mechanisms and fault studies in northwestern South America (Taboada et al., 2000). The axis of the syncline can be easily defined because of prominent reflectors in the Neogene section (Figure 5). Stratigraphic dips defined by Castillo (2001) on the western limb of the syncline in the southern lake range from 1 to 3j. The exact extension of the Maracaibo syncline into older Paleogene rocks in the northern part of the 3-D survey of the basin is not clear, but the same synclinal outcrop pattern is also present in that area (Figure 9). The eastern limb of the Maracaibo syncline is disrupted by young north-south trending folds in the Ceuta and Tomoporo areas (Figure 9) also manifesting widespread late Neogene, east-west shortening across the Maracaibo block. The Icotea and Pueblo Viejo faults are present only at deeper levels in the basin and do not affect the super-eocene section. A surrounding surficial geologic map shown in Figure 6 shows a combination of northeast-trending fault and fold trends that either do not deform in the central basin area or are older features not affecting rocks of this age. 3.4-s Time Slice from Regional 3-D Seismic Data A deeper horizontal slice at 3.4 s reveals the full extent of the Icotea and Pueblo Viejo faults because this level of the basin is mostly beneath the Eocene unconformity (Figure 10). The Icotea fault can be traced as a linear, slightly arcuate, approximately 100-km (62-mi)-long feature with a prominent 10-km (6-mi)-wide pull-apart or stepover basin along its trace in the central lake area (Escalona and Mann, 2003). This pull-apart formed during the Eocene and is filled by a 3-km (1.8-mi)-thick section of Eocene clastic rocks. Escalona and Mann (2003) calculated that km ( mi) of extension occurred to form this small but deep basin. This amount of extension provides a minimum right-lateral offset for the Icotea strike-slip fault zone. To the north, the Icotea fault forms a sharp, linear boundary at the 3.4-s horizon between sub-cretaceous rocks to the east and Cretaceous rocks to the west, suggesting a downto-the-west throw consistent with the 2-D lines discussed by Audemard (1991) and discussed below using these 2-D seismic lines. Vertical throw appears to decrease along the central and southern parts of the Icotea fault (Figure 10). A prominent feature of the southern fault is its arcuate trace and termination in the area of a detailed 3-D study described by Castillo and Mann (2006). The curving part of the trace is associated with an increased down-tothe-east throw associated with the formation of an asymmetrical Eocene-age half graben. The subparallel VLE fault exhibits a strongly curved northern segment that obliquely intersects the central trace of the Icotea fault. The southern trace of the VLE fault abruptly terminates (Castillo and Mann, 2006). The Pueblo Viejo fault and an unnamed parallel fault are also straight and appear to terminate at least on their southern ends. The north-northeast trend of the faults observed on the 3.4-s time slice (Icotea, Pueblo Viejo, VLE, etc.) was likely inherited from normal faults present in the basement and formed during the Late Jurassic Early Cretaceous rifting phase (Maze, 1984; Lugo and Mann, 1995) (Figure 4A). These faults were reactivated and inverted during the Eocene to middle Miocene and then became quiescent. 3.8-s Time Slice from Regional 3-D Seismic Data A deeper horizontal slice at 3.8 s reveals the full extent of the subvertical Icotea and Pueblo Viejo faults, deforming and overlying the basement rocks in the northern part of the basin and sedimentary rocks in the southern part (Figure 11). The Icotea fault can still be traced in basement rocks in the northern part of the basin as a linear to slightly arcuate feature. In the central part of the time slice, moderately continuous seismic reflectors are interpreted along the Icotea fault trace and correspond to locally extended strata in the Eocene pull-apart basin (Escalona and Mann, 2003). Vertical throw appears to progressively decrease from the central to southern parts of the Icotea fault (Castillo and Mann, 2006). OVERVIEW OF THE SUBSURFACE GEOLOGY OF THE MARACAIBO BASIN USING REGIONAL 2-D SEISMIC TRANSECTS Interpretation of three regional seismic transects spanning the Maracaibo Basin (Figures 12 14) allows visualization and interpretation of three distinct clastic wedges formed at different times and correlatable to the major tectonic phases of the basin summarized in Figure 5. Clastic wedge thickness ranges from approximately 2 to 5 km (1.2 to 3.1 mi). The orientation and thickness of each wedge is shown schematically in the bottom right corner of the three regional 2-D seismic transects (Figures 12 14). 466 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin

23 Figure 10. (A) Uninterpreted regional time slice of the Lake Maracaibo area at 3.4 s two-way traveltime (TWT). (B) Interpreted seismic time slice at 3.4 s TWT, with major structural and stratigraphic features indicated. Colors of stratigraphic units on the time slice are keyed to the stratigraphic column in (C) on the right. The 3.4-s TWT time slice intersects Miocene to basement units. Widely spaced reflectors indicate gently dipping horizons, and closely spaced reflectors represent steeply dipping horizons. The most important faults at this level include the north to northeast striking Icotea and Pueblo Viejo faults. The Icotea fault terminates to the south on normal splay faults. (C) Radar image showing area of 3-D coverage and stratigraphic column of the Maracaibo Basin. Mann et al. 467

24 468 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin Figure 11. (A) Uninterpreted regional time slice of the Lake Maracaibo area at 3.8 s two-way traveltime (TWT). (B) Interpreted seismic time slice at 3.8 s TWT with major structural and stratigraphic features indicated. This 3.8-s TWT time slice intersects Miocene to basement units. The most important structural features include the north-northeast striking Icotea and Pueblo Viejo faults, the Icotea pull-apart basin at a left step of the Icotea fault in central Lake Maracaibo, and the structural highs formed in the central-southern region between the Icotea and VLE fault. (C) Radar image showing area of 3-D coverage and stratigraphic column of the Maracaibo Basin.

25 Figure 12. (A) Uninterpreted 2-D regional seismic line extending from the eastern flank of the Sierra de Perijá to the foothills of the Mérida Andes to the east (location map in C). (B) Interpretation of seismic line in (A). The main structures observed in the Maracaibo syncline are rift-related high-angle faults that were reactivated during the Paleogene and early Miocene (Icotea and Pueblo Viejo faults) and a triangle zone related to the uplift of Sierra de Perijá to the west. Three clastic wedges are interpreted and shown schematically in (D): (1) Maracaibo clastic wedge of Eocene age thickens to the north-northeast; (2) Perijá clastic wedge of Oligocene Miocene age thickens to the west; and (3) Mérida clastic wedge of Miocene Holocene age thickens to the south and fills the Maracaibo syncline. The formation of each of these wedges is closely related to a regional tectonic event affecting the Maracaibo Basin. (C) Radar image indicating the relative position of the 2-D seismic transect in the Maracaibo Basin and depth to acoustic, Paleozoic basement in kilometers. (D) Schematic diagram showing the relative thickening direction of the three interpreted clastic wedges in the Maracaibo Basin. Mann et al. 469

26 470 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin Figure 13. (A) Uninterpreted 2-D regional seismic line that extends from the eastern flank of the Sierra de Perija to the center of the Lake Maracaibo (see C for location). (B) Interpretation of regional seismic line. The main structural features include the rift-related high-angle faults that were slightly reactivated during the Paleogene and early Miocene, the Perija triangular zone, and the Rosario anticline. The Rosario anticline is interpreted as a Miocene-age, east-vergent thrust-related fold fault that involves pre-cretaceous basement. Note that on this line, both the Perija and Me rida clastic wedges are present. (C) Radar image indicating the relative position of the 2-D seismic transect in the Maracaibo Basin and depth to acoustic, Paleozoic basement in kilometers. (D) Schematic diagram showing the relative thickening direction of the interpreted clastic wedges in the Maracaibo Basin.

27 Figure 14. (A) Uninterpreted 2-D regional seismic line along the southwestern margin of Lake Maracaibo ending in the Mérida Andes foothills (see C for location). (B) Interpreted 2-D regional seismic transect. The main fault present is the triangular zone produced by uplift of the Mérida Andes and the Mérida clastic wedge. (C) Radar image indicating the relative position of the 2-D seismic transect in the Maracaibo Basin and depth to acoustic, Paleozoic basement in kilometers. (D) Schematic diagram showing the relative thickening direction of the interpreted clastic wedges in the Maracaibo Basin. Mann et al. 471

28 Figure 15. Burial histories of six deep wells and their relationship to the main depocenters in the Maracaibo Basin (modified from Lugo, 1991; Castillo, 1995). Three main burial episodes include (1) Paleocene Eocene foreland phase of subsidence in the north-northeast of the basin and formation of the Maracaibo clastic wedge; (2) late Eocene Oligocene phase of subsidence to the west produced by uplift of the Sierra de Perijá and formation of the Perijá clastic wedge. During this phase, an uplift or rebound is observed in the north-northeastern part of the basin; (3) Miocene Pliocene phase of subsidence to the southeast of the basin produced by the uplift of the Mérida Andes and formation of the Mérida clastic wedge. An uplift or rebound phase is observed in the north-northeastern part of the basin during the late Eocene Oligocene. Each wedge filled a depocenter created by a depression of the Maracaibo Basin located either in front of a thrust belt (Audemard, 1991; Lugo and Mann, 1995) or adjacent to a Laramide-style basement uplift associated with uplift of the northern Andes (Kellogg, 1984; De Toni and Kellogg, 1993; Taboada et al., 2000). Burial histories in the Maracaibo Basin in Figure 15 from Lugo and Mann (1995) and Castillo (2001) show that the main pulses of depocenter formation occured in two main phases: (1) during the Paleocene early Eocene in the northeast part of the basin (Figure 15, locations A C); and (2) starting at the Oligocene Miocene in the north and western parts of the basin (Figure 15, locations A C) and migrating to the southern part of the basin during the Miocene Pliocene (Figure 15, locations D F). An uplift or rebound phase affecting 472 Regional Geologic and Tectonic Setting of the Maracaibo Supergiant Basin