Abstract A period of structuring, uplift, non-deposition and/or erosion in the Campanian in Western Venezuela, different from the generally known Late Cretaceous event is proposed to explain: (1) a varying time gap (10 to 1 million years) from east to west across the Maracaibo basin between La Luna and Colón formations; (2) a correlating time gap of 11 million years between the Santonian and Upper Campanian sediments in the Barinas basin; (3) structuring at the Top La Luna seismic horizon in the southwestern, west and central parts of the Maracaibo basin which is not reflected in the overlying section; (4) different thickness patterns in the isopach maps for the units underlying the Top La Luna seismic level and the immediately overlying section at least in area of the Colón Unit; (5) an abrupt change in vitrinite reflectance values in the SW of the basin from 0.47–0.60% above to 1.09–1.80% below the top of La Luna Formation; and (6) fission track ages in the range 70–80 Ma in the Circum-Caribbean. The predominant north–south trend of this structuring suggests that it is related to changes on the dynamics of the South American plate boundary during Campanian that may have involved a major igneous and volcanic event registered 70–80 Ma.

Abstract Present-day Venezuela may be divided into the following major structural provinces—Perijá Mountains Goajira-Paraguaná arch, Maracaibo basin, Falcon, Venezuelan Andes, Caribbean ranges, Barinas-Apure basin, Eastern Venezuela basin, and Guayana shield. The pre-Cretaceous history is little known. A Paleozoic geosyncline may have existed in Western Venezuela, but any important regional metamorphism there was pre-Devonian. ? Mid-Ordovician, Late Devonian or early Carboniferous, and Permian to Triassic deformations may be postulated, but their trends cannot be evaluated at present. Toward the end of the Paleozoic widespread uplift, accompanied by faulting and volcanism, raised the whole country above sea level. The Perijá, Maracaibo, Andes, and Barinas-Apure provinces owe their character as distinct units to the Tertiary Andean deformation, with the basins sinking as the mountains arose. The Cretaceous and Eocene oil fields of Western Venezuela owe their existence to the Andean orogeny; the middle to late Tertiary fields are less directly linked to it. There is some indication that movements began first in the northwest (Sierra de Perijá) during the Eocene, progressed southeastward across the more-or-less stable Maracaibo platform, reached the Andes at the close of the Eocene, and culminated in the Mio-Pliocene. The mountains, with dominant trend of N. 35° E. for the Perijá and N. 50° E. for the Andes, are essentially complexly folded and faulted structural arches with high angle reverse, normal, and wrench faults. Mountainward-dipping reverse faults are thought to bound their flanks. Both the Maracaibo and Barinas-Apure basins have asymmetrical cross sections with deepest zones close to the flanks of the Andes. The displacement on a large northeast Bocono fault trend may have to be taken into account in reconstructing the deformation of the Andes. Beginning in the Late Jurassic, the Caribbean sea began an extensive transgression of northern and western Venezuela. An east-west trending Caribbean geosyncline developed in the extreme north. Volcanism and major deformation of the Caribbean geosyncline began about Middle Cretaceous time, leading to the folding, faulting, and metamorphism of the previously deposited Mesozoic rocks. However, sedimentation, volcanism, deformation, and metamorphism, although on decreasing scale, continued throughout Late Cretaceous and Paleocene time and perhaps into the early Eocene. Dominant structural features in the Caribbean ranges trend N. 60°---80° E.; wrench (strike-slip) faulting is common. To many geologists, an outstanding tectonic feature of northern Venezuela is a series of long east-west trending, right-lateral wrench faults, that are located close, and roughly parallel, to the coast. Best known of these are the Oca fault in the west and the Pilar fault in the east. It is possible that these faults are at least as old as Cretaceous, that they are related to the tectonic history of the general Caribbean area, as suggested by Bucher and others, and that they have played a major role in the deformation of all of Venezuela. The importance of these faults as dominant features has yet to be proved and caution is advised in evaluating their significance in the tectonic history of Venezuela.

Abstract The sedimentary basins of western Venezuela contain large volumes of oil. However, most of the large structures have already been produced. Exploration for new reserves of light and medium oil now depends on integrated studies that lead to a more comprehensive basin evaluation. This paper presents an integrated account of the Lake Maracaibo and Barinas-Apure basins of western Venezuela. It is a fully integrated study but focuses on the genetic and seismic stratigraphy of more than 600 wells, reference outcrops, and 4000 km of reflection seismic data. Six unconformity-bounded supersequences record the dynamics of Mesozoic-Cenozoic basin evolution from extension to collision. Supersequence A was deposited during an episode of Jurassic rifting, and supersequence B corresponds to the subsequent Early-Late Cretaceous passive margin. Supersequence C marks the transition to a compressive regime in the Late Cretaceous and early Paleocene. Compression resulted from collision and obduction of the Pacific volcanic arc with the South American plate. Supersequence D records the development of the late Paleocene-middle Eocene foreland basin in front of the volcanic arc and emplacement of the Lara nappes. Supersequences E and F are attributed to modification of the foreland basin by late Eocene-Pleistocene collision of the Panama arc. The uplifted Serranla de Perija, Macizo de Santander, and Mérida Andes partitioned the foreland basin, creating the present Lake Maracaibo and Barinas-Apure basins. Supersequence B contains the Cretaceous La Luna source rock (sequences K3, K4, K5). The Colon and Burgúita formations form the principal supersequence C seals (sequence K6). The principal reservoir units occur in supersequence D, including the prolific Eocene Misoa and Gobemador formations (sequences TI, T2). Reservoirs of the La Rosa and Lagunillas formations occur in supersequence F and in the Betijoque Molasse.

ABSTRACT Well, seismic, and outcrop data were used to constrain the timing and development of unconformities associated with uplift events, and to aid in the definition of lithostratigraphic units of the Eocene of the southern Maracaibo and west-central Barinas/Apure basins. Palynology provided the main control on age dating of the sections, and graphic correlation was used to illustrate the amount of missing time in the lithostratigraphic record. An integration of biostratigraphic and lithostratigraphic data from wells and outcrops shows that deltaic to shallow marine conditions prevailed over most of the Maracaibo and west-central Barinas/Apure basins during the middle Eocene. Fluvial sedimentation dominated in areas of high standing basement features, and was accentuated by the development of incised valleys and their subsequent marine fill sequences along the southern and southeastern margins of the Maracaibo Basin. The main controls governing depositional facies, and the presence or absence of unconformities were differential structural motion and, to a lesser extent, relative sea level fluctuations. A basal Eocene to late Paleocene unconformity, as well as several intra-middle Eocene unconformities appear to have been primarily caused by a drop in base level (sea level + structural subsidence), forming incised valleys over most of the study area. Biostratigraphic (palynological) data, as well as field observations and subsurface mapping indicate that other unconformities within the upper middle Eocene, and between middle and upper Eocene strata are ravinement surfaces caused by transgressive marine re-working of pre-existing units. Sequence boundaries caused by incised valleys often overlie transgressive deposits, and can merge with ravinement surfaces and other sequence boundaries in areas of intense erosion. This is especially true for the multiple intra-middle Eocene unconformities and sequence boundaries observed and inferred for the eastern part of the Maracaibo Basin; these are not reflected in the outcrop belt to the south because the rate of accommodation space was not rapid enough to allow their preservation. Nevertheless, marine transgressive deposits in the southern part of the basin can be clearly recognized (where they are preserved) because of their contrast with otherwise dominant fluvio-deltaic strata; the recognition of specific transgressive events in the eastern part of the basin is less certain because they are less clearly differentiated within the mostly marginal marine strata of the area. The presence of favorable reservoir facies in Eocene rocks in the study area is primarily a function of original sandstone composition, grain size, and burial depth. In the northeastern part of the study area, estuarine channels and transgressive marine/tidal bars have the best reservoir properties in the Misoa Formation. In the southern Maracaibo Basin, fluvial channels and transgressive marine/tidal bars are also the best reservoirs in the Mirador Formation. Transgressive marine/tidal bars within the Carbonera Formation also have favorable reservoir characteristics. Productivity of the best Eocene reservoirs is generally high.