Alejandro Escalona

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

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 masters 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, Petrleos 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 km2; 19,305 mi2), 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 basins 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.

Chronology of Cenozoic tectonic events in western Venezuela and the Leeward Antilles based on integration of offshore seismic reflection data and on-land geology

Newly acquired, deep-penetration Broadband Onshore-Offshore Lithospheric Investigation of Venezuela and the Antilles Arc Region seismic reflection data from offshore western Venezuela (Bonaire Basin) and around the Leeward Antilles are combined with existing geologic and geophysical data sets to examine the chronology of Late Cretaceous–Cenozoic tectonic events in this part of the Caribbean–South American plate boundary zone. These tectonic events have controlled the maturation and structural trapping of known hydrocarbons in the offshore Bonaire Basin and the adjacent onland Falcn Basin. We infer three tectonic phases that are constrained using these combined data sets. (1) The late Eocene–early Oligocene, north-south opening of the 3–6-km (1.8–3.7-mi)-thick Falcn-Bonaire Basin occurred along east-west–striking normal fault systems that have locally been inverted by later tectonic phases. These Paleogene normal faults rifted the Upper Cretaceous arc crust and Paleogene marine depositional sequences within the offshore Bonaire Basin. (2) Northwest-striking normal faults crosscut the older normal faults of the Bonaire Basin and Leeward Antilles and form deep, submarine rifts that contain up to 4 km (2.5 mi) of sedimentary fill and form deep-water channels between islands of the Leeward Antilles. Offshore well data and age of onshore sediments in the Falcn Basin indicate that this second phase of rifting occurred mainly during the late Oligocene to early Miocene and remains active to the present. (3) Inversion of the subaerial Falcn Basin commenced during the middle Miocene. This inversion phase is reflected in the present-day pattern of an east-northeast–trending fold-thrust belt that can be traced over 200 km (124 mi) along strike in the Falcn Basin. A second offshore fold-thrust belt (La Vela) can be traced over a distance of 175 km (108 mi) along strike and parallel to the northeast-trending Falcn Basin coast. Restoration of imbricate thrusts seen on seismic lines perpendicular to the La Vela fold-thrust belt indicates a minimum of 7 km (4.3 mi) of northeast-southwest–directed, thin-skinned shortening. Geochemical work indicates that source rocks for scattered occurrences of hydrocarbons in the Falcn Basin and its coastal zone are Paleogene and Miocene marine shale. Reservoir rocks are Tertiary marine sandstone and shale deposited in Paleogene rifts formed during the first tectonic phase in the late Eocene to early Oligocene. Structural traps were formed by thrusting during the second tectonic phase in the late Oligocene to early Miocene.

Sequence-stratigraphic analysis of Eocene clastic foreland basin deposits in central Lake Maracaibo using high-resolution well correlation and 3-D seismic data

Eocene clastic rocks of the Maracaibo Basin were deposited in an asymmetrical foreland basin formed during the oblique Paleogene collision between the Caribbean and South American plates. In this study, we use more than 300 wells and 2000 km2 (772 mi2) of seismic data in the central Maracaibo Basin to produce a detailed sequence-stratigraphic interpretation of the Eocene Maracaibo foreland basin. The base of the Eocene stratigraphic succession in the central Maracaibo area is characterized by an approximately 250-m (820-ft)-thick, aggradational succession of fluviodeltaic sandstone overlain by an approximately 600-m (1968-ft)-thick retrogradational succession of shallow-marine shale and sandstone containing minor progradational units. The upper part of the foreland basin sequence is marked by an approximately 100-m (328-ft)-thick aggradational succession of fluviodeltaic sandstone. In the approximately 1000-m (3280-ft)-thick Eocene section, we interpreted 17 parasequence sets, 6 genetic sequences, and 1 depositional sequence. Only one classic sequence boundary was interpreted within the Eocene section that marks the boundary between the retrogradational shallow-marine section and the overlying aggradational fluviodeltaic succession. Based on the stratigraphic architecture and thickening trends of several of the parasequence sets, we conclude that the main source of clastic sedimentation was located on the South America craton south of the Maracaibo Basin, instead of along the thrusted, north-northeastern margin of the basin as proposed by previous workers. A lack of recognition of classic sequence boundaries suggests that Eocene clastic rocks of the central Maracaibo foreland basin were not subaerially exposed during most of the Eocene, and that their stratigraphic architecture was controlled by tectonic subsidence related to thrusting along the northeastern edge of the foreland basin. Eustasy was not an important control on the stratigraphic evolution of the foreland basin until its middle Eocene aggradational period that marked the end of foreland basin subsidence. Well logs and three-dimensional seismic data show that depositional environments on the Eocene delta plain and shelf of the central Maracaibo foreland basin were dominated by fluvial and tidal processes that are similar to modern depositional processes of the Orinoco delta in eastern Venezuela.

Deep structure of the Mérida Andes and Sierra de Perijá mountain fronts, Maracaibo Basin, Venezuela

The Maracaibo Basin is a 50,000-km2 (19,305-mi2) topographic depression bounded to the east and south by the Mrida Andes of Venezuela and to the west by the Sierra de Perij of Venezuela and Colombia. Both uplifted mountain blocks expose Paleozoic basement rocks and Mesozoic–Cenozoic carbonate and clastic rocks that were mainly folded and thrusted by regional shortening in the Paleogene and late Neogene. Using geologic maps, seismic reflection data, and wells from the steep mountain front areas, we test different structural models of how late Paleogene–Neogene convergence is accommodated along subsurface faults and folds at the mountain fronts. Seismic imaging of the deep (5-km; 3.1 mi) structure of both Maracaibo mountain fronts shows basinward-dipping monoclines with stratal dips ranging from 20 to overturned and an almost complete lack of faulting in the basin-edge monocline or in adjacent, horizontally bedded strata of the Maracaibo Basin. Seismic data reveal the presence of one or more triangle zones at depth along both the Mrida Andes and Sierra de Perij that exhibit characteristic thrust-wedge geometries. The lower part of the wedge is defined by imbricate thrust faults dipping beneath the mountain block and involving the Paleozoic basement. The upper part of the wedge is defined by a basinward-dipping thrust associated with fault-propagation folds at the surface and an overlain basin-edge monocline. The creation of the steep to overturned dips of the monocline is attributed to the effects of 6–10 km (3.7–6.2 mi) of shortening along the lower zone of imbricated thrust faults. This 6–10 km (3.7–6.2 mi) of shortening, calculated from triangle zones of both the Sierra de Prija and Mrida Andes, is significantly less than regional estimates from 12 to 60 km (7.4 to 37 mi) of shortening inferred by previous workers from regional balanced cross sections that assume low-angle thrust-type geometries. We propose that a pop-up style of deformation related to the inversion of Jurassic rift features may be a more realistic interpretation of the convergent fault systems that have controlled uplift of the Mrida Andes and Sierra de Perij. Inversion of relatively steep, basement normal faults at the edges of and within both ranges may explain lesser amounts of observed shortening.

An overview of the petroleum system of Maracaibo Basin

The geologically complex Maracaibo Basin in northwestern Venezuela is one of the most prolific hydrocarbon basins in the world. Having a basinal area of 50,000 km2 (19,300 mi2), the basin has produced more than 30 billion bbl of oil, with estimated recoverable oil reserves of more than 44 billion bbl. The central elements of the petroleum system of the basin include (1) a world-class source rock (Upper Cretaceous La Luna Formation), deposited on a shelf-to-slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events; (2) high-quality clastic reservoir rocks deposited in Eocene and Miocene fluviodeltaic settings; (3) two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation, first, during Paleogene Caribbean–South American oblique plate collision and, second, during the Neogene uplift of the Sierra de Perij–Mrida Andes; and (4) lateral and vertical migration of oil along strike-slip, normal, and inverted faults, as well as a regional unconformity of late Eocene–Oligocene age. The maturation, migration, and trapping of hydrocarbons were closely controlled by the tectonic evolution of the Maracaibo Basin. During the Paleogene, the development of a foredeep along the northeastern margin of the basin and the strike-slip reactivation of the rift-related Jurassic faults on the Maracaibo platform controlled the early structural setting of the source and reservoir rocks. Hydrocarbons migrated updip from source rocks beneath the north-northeastern margin of the basin along north-south strike-slip faults and into overlying Eocene clastic reservoirs in the south-central parts of the basin. The second period of the Maracaibo Basin petroleum system developed during subaerial exposure of most of the Maracaibo Basin during Oligocene–Miocene uplift of the adjacent Sierra de Perij and Mrida Andes. Uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive Maracaibo syncline. Because of the formation of the Maracaibo syncline, oil generation began in the central and southern parts of the synclinal basin and migrated northward. Hydrocarbons migrated up the flanks of the Maracaibo syncline along reactivated strike-slip faults and into Miocene rocks adjacent to the uplifted mountain ranges. Escaping oil has formed numerous surface seeps along the edges of the Maracaibo Basin.

Tectonic controls of the right-lateral Burro Negro tear fault on Paleogene structure and stratigraphy, northeastern Maracaibo Basin

The northeastern Maracaibo Basin in western Venezuela was deformed by Paleogene thrusting and an associated tear fault (Burro Negro right-lateral strike-slip fault zone), related to Paleogene oblique collision between the Caribbean and South American plates. Two different tectonic models have been previously proposed for the thick Paleogene depocenter located along the northeastern margin of the Maracaibo Basin. The first model proposes that the depocenter is a foreland basin controlled by southwestward-directed overthrusting during a late Paleocene–middle Eocene oblique collision between the Caribbean and South American plates. The second model, supported by data presented in this study, proposes that the asymmetric Paleogene Maracaibo sedimentary wedge was controlled by strike-slip displacement along a right-lateral tear fault, separating southeast-directed thrust sheets to the east (Lara nappes) from a more stable platform area to the west (Maracaibo Basin).Regional seismic lines recorded to 5 s two-way traveltime reveal the structure of the asymmetric Paleogene depocenter in the northeastern part of the Maracaibo Basin. The approximately 100-km (62-mi)-long Burro Negro fault is a right-lateral strike-slip fault separating less deformed, inner- to outer-shelf rocks of the western Maracaibo Basin from highly deformed, deep-marine rocks of the eastern Maracaibo Basin. Seismic lines northeast of the Burro Negro fault zone show elongate, subsurface basins bounded by partially inverted reverse and strike-slip faults filled with about 3 km (1.8 mi) of Oligocene and Miocene clastic marine sedimentary rocks. Structural highs of Eocene rocks are characterized at depth on seismic reflection lines by chaotic seismic reflections that underlie the more coherently stratified Oligocene and Miocene subbasins. We interpret these structural highs as steeply dipping fault zones and shale diapirs activated during Eocene–Oligocene oblique plate convergence.The geology and overall structural configuration of the northeastern Maracaibo Basin and the Burro Negro fault zone support its origin as a right-lateral tear fault. In our model, the Burro Negro fault zone accommodated southeastward migration of the thrust front in the deep-water area east of the fault in the present-day Falcn region. The Paleogene clastic wedge of the Maracaibo Basin exhibits many common features of a classic foreland basin, including onlap onto an arch or forebulge located near the center of the present-day Lake Maracaibo. This paleogeographic coincidence of the Burro Negro fault zone and the Maracaibo shelf edge suggests that the paleotrend of the South American passive margin prior to collision was serrated in map view. West-to-east migration of the Caribbean–South America oblique collision formed progressively younger, parallel tear faults to the east of the Maracaibo Basin that may have formed by the same tectonic process as the Burro Negro fault zone.

Three-dimensional structural architecture and evolution of the Eocene pull-apart basin, central Maracaibo basin, Venezuela

Abstract Three-dimensional seismic mapping of interpretative sub-surface time slice sections, incorporating age data and detailed structural observations places constraints on the structural architecture and stratigraphic evolution of the Icotea pull-apart basin in the central Maracaibo basin, Venezuela. All four fault-bounded sides of the Icotea basin, its ∼3-km-thick Eocene basin fill of clastic sediments and faults pre-dating the opening of the basin can be mapped in three dimensions. The development of the Icotea basin provides an excellent observational basis for understanding the structural history of three sets of regionally extensive faults and testing two models for the formation of pull-apart basins in general. Three main fault trends produced at different periods in the formation of the Cretaceous–Recent Maracaibo basin are mapped using 3D seismic data in the central part of the basin: (1) N–NE-striking normal faults, including the Icotea fault; these faults formed originally during Late Jurassic–Early Cretaceous rifting and reactivated as left-lateral strike–slip faults during late Paleocene–Eocene convergent deformation between the Caribbean and South American plates; (2) NW striking faults formed by late Paleocene–Eocene overthrusting of the Caribbean plate and downward flexure of the South American plate to create a major foreland basin depocenter; and (3) NE-striking normal faults, of pre-Cretaceous age, were reactivated during late Paleocene–Eocene plate convergence. Three-dimensional architecture of the Icotea basin interpreted from five time slices through the basin and its flank areas supports the simple pull-apart model for the Eocene opening of the Icotea basin. The amount of extension across the suite of normal faults ranges between 0.8 and 2.25 km. This range of offset is consistent with some previous estimates of minor left-lateral displacement along the Icotea fault but is inconsistent with either low-angle or high-angle thrusting during Eocene time, as inferred from previous interpretations of widely spaced two-dimensional seismic lines. The normal faults that formed the pull-apart basin reactivated pre-existing faults due to plate flexure.

Late Holocene strike-slip offset of a subsurface channel interpreted from three-dimensional seismic data, eastern offshore Trinidad

A right-lateral strike-slip fault offset of a shallowly buried fluvial paleochannel is interpreted from three-dimensional (3-D) seismic data in the eastern offshore area of Trinidad. The fault represents the eastern offshore continuation of the main South America–Caribbean plate boundary zone in Trinidad, the Central Range fault zone. Previous global positioning system–based geodetic studies and trenching have shown that the 50-km-long on-land segment of the Central Range fault zone accommodates a significant part of the present-day interplate motion. The 3-D seismic data shown here reveal that the 60-km-long offshore continuation of the Central Range fault zone forms a prominent seafloor lineament and dextrally offsets a shallowly buried (83 m below sea level), late Quaternary fluvial channel by 322–506 m. Based on the eustatic sea-level curve, we infer that the channel began to be incised during the beginning of the Last Glacial Maximum ca. 30 ka and was subsequently offset by the Central Range fault zone. Using the offset amounts and our inferred age for the filled channel, we can estimate a long-term slip along the fault of 17–19 mm/yr for the Central Range fault zone. Because no documented earthquake has occurred on the Central Range since A.D. 1800, as much as 3.7 m of elastic strain may have accumulated on the Central Range fault zone that could be released by a future magnitude >M7.5 earthquake.

Geographic information system–based fuzzy-logic analysis for petroleum exploration with a case study of northern South America

The petroleum industry is increasingly using geographic information systems (GISs) for mapping and spatial database needs because they are useful for elucidating and exploiting spatial relationships between geologic and geophysical data. However, the petroleum industry, in general, does not exploit the full potential of GIS as an analysis tool. In particular, GIS offers spatial and analytical support for multicriteria evaluation (MCE) methods, which are used to combine data to show areas best fulfilling specific criteria. Petroleum explorations would benefit from an MCE method that is spatial, is flexible for combining heterogeneous data, considers the interpretive nature of the data, is geologically applicable, and is applicable for frontier areas or where little information exists regarding probabilities of the presence of petroleum. This study proposes a GIS-based MCE method for petroleum exploration based on fuzzy logic, which fulfills the previously stated requirements using 16 subcriteria and one constraint combined in tiers to produce a favorability map of potential exploration areas. A case study applied to northern South America, chosen because of its centrality to petroleum exploration, shows potential new exploration areas in the Cretaceous–Paleogene and Miocene–Holocene. The method was validated by comparing the favorability maps of one non–geologic age–specific and of two geologic age–specific favorability maps to known producing fields. We conclude that the method can be applied in an exploration setting and, as such, is applicable for other regions of the world.

Sequence stratigraphy and lateral variability of Lower Cretaceous clinoforms in the southwestern Barents Sea

An extensive seismic database covering an area of 100,000 km2 (38,610 mi2) and 16 wells are integrated to define a sequence-stratigraphic framework for the Lower Cretaceous succession in the southwestern Barents Sea. Seven sequences (S0–S6) are defined, and the geometry, trajectory, and lateral variability of decompacted seismic clinoforms are described to elucidate the depositional history of the basin and to better understand coarse-grained sediment transport from the shelf to basin. Three different clinoform scales are recognized: (1) clinoform sets with 35–60 m (115–197 ft) height, interpreted as deltaic or shoreline clinoforms; (2) clinoform sets with 60–110 m (197–361 ft) height, interpreted as sediments prograding on a continental shelf; and (3) clinoforms with greater than 150 m (>492 ft) height, which represent shelf-margin clinoforms. Furthermore, clinoforms are grouped into two main progradation directions: (1) clinoforms prograded to the southeast in sequences 2–3, in the Fingerdjupet Subbasin and the western Bjarmeland platform, indicating a source of sediments located in the west-northwestern Barents Sea, and (2) clinoforms prograded to the southwest in sequences 1–6, in the eastern part of the Bjarmeland platform, Nordkapp Basin, and Finnmark platform, indicating a second source of sediments located in the east-northeast. Additionally, in the Hammerfest Basin, clinoforms prograded to the southeast off the Loppa high in sequences 5–6. Low-relief (35–60 m [115–197 ft]), high-gradient, and oblique clinoforms are observed within sequence 2 in the western Bjarmeland platform. The high-gradient foresets are interpreted as potential coarse-grained deposits or as a result of clinoforms prograding to progressive deeper waters, resulting in steeper foresets. Clinoforms located in the eastern part of the study area are interpreted as sourced by a mud-rich system, reflecting a long transportation distance. However, thin, heterolithic patterns in the gamma-ray log possibly reflect thin, sheetlike sands. The height of the clinoforms seems to be a factor controlling the sediment bypass to deep water in the study area. When the height is more than 200 m (656 ft), bottomset deposits are common. This study contributes to a better understanding of the paleogeography and the evolution of the frontier southwestern Barents Sea during the Early Cretaceous and to comprehending the variables increasing the bypass of coarse-grained sediments to deep-water settings.