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 This paper synthesizes technical, commercial, and strategic issues regarding the petroleum systems and exploration of the northwest corner of South America in three parts: a technical analysis of the presence and distribution of the hydrocarbon systems; analysis of the areas with the largest remaining potential; and Colombia’s exploration strategy in relation to its geology and potential. The technical portion outlines, with the use of countrywide paleogeographic maps, the main reasons for the presence and distribution of hydrocarbons in the region. Regional petroleum systems are described using a process and genesis methodology. The proposed hypothesis for source-rock quality and distribution combines regional plate tectonics with the formation of a large igneous province, global warming, increased upwelling, widespread anoxia-dysoxia, and sea-level rise. The hypothesis for reservoir distribution and quality comprises details of a regional orogenic event that affected the northwest margin of South America. The distribution of different reservoir qualities is determined by the erosion of an elongated mountain belt formed by this orogeny, the associated synorogenic sedimentation, and the evolution of a closing foreland basin that received sediments from the west from a mountain chain with varied basement stratigraphy, and from the east, sediments from the Guyana Shield. The explanation for regional seal deposition lies in the tectonic extension and foundering of large portions of Colombia and western Venezuela, the regional subsidence associated with diminished plate-tectonic convergence, and the associated increase of accommodation space. The inversion of the Eastern Cordillera and the development of doubly verging thrust belts bounding the mountain belt explain the formation of most traps. Adjacent forelands and foredeeps to the mountain belt and thick molasse packages combined with tectonic burial of the source rock explain generation and migration patterns. This paper also considers where the largest volumes of hydrocarbons will be found in the future, based on technical issues, and on the exploration history of different play areas in Colombia.

Abstract A soil-gas survey was conducted over a number of prospects in the plains (“llanos”) and the Andean foothills regions in the Barinas-Apure Basin of western Venezuela. Initial attempts to numerically distinguish a seep-gas signal from background gases failed. Basic statistics allow only a tenuous differentiation between background gases and possible seep-gas populations having slightly anomalous hydrocarbon-gas concentrations. Further data processing on this specific population using advanced vector analysis facilitated the discrimination within a variety of soil-gas sources and distinguished suites of samples with weak, seepage-related surface-gas patterns that resemble normal condensate/wet-gas sources at depth. An important observation from this surface geochemical program was the presence of a condensate/wet-gas trend in the foothills, confirming subsurface geochemical model predictions of the existence of wet reservoir gases in this area. Plots of the distribution of the various soil-gas sample suites on base maps of identified exploratory prospects show distinct clustering of seepage-related sample suites over areas with structural highs. Further scrutiny of the distribution of these “geochemical exploration leads” (GEL anomalies), in terms of coincidence with oil-productive areas, reveals that the GEL anomalies occur predominantly over now-productive wells. This observation also prompted the reevaluation of an old well recently associated with a surface GEL anomaly but originally abandoned and classified as dry in the 1930s. Reinterpretation of the well-log information now indicates 30 m of net pay in sandy reservoirs. Future drilling should provide more information on the extent and impact of this surface-gas technology on exploration decision making and risk analysis. Overall, the active seepage in the two work areas is relatively weak, although higher levels are observed in the foothills region because of favorable gas/oil ratios (GORs), normal reservoir pressure, and intense tectonic fracturing. These factors provide more favorable vertical or near-vertical gas migration than occurs in the plains region, where low GORs, reservoir underpressure, and less intense tectonic fracturing diminish vertical migration. In addition, initial (presurvey) concerns of intense in-situ biological soil-gas formation in a tropical environment, which would mask or diminish possible seep signals at the surface, could be resolved: Deep soil-gas sampling below a shallow zone of intense soil microbial activity, in combination with numerical surface-gas data processing, permits recognition and evaluation of subtle surface-seep trends in the area.

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.

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.