Abstract Hundreds of outcrop samples were analyzed for source rock evaluation. La Luna, Los Cuervos, and Carbonera Formations are considered excellent potential oil sources. However, La Luna and Los Cuervos were the oil-generating units according to 1-D modeling. Therefore, we define La Luna–K/T(!) and Los Cuervos–K/T(!) systems. The study identifies two genetic oil families related to two distinct oil-source rocks from the Upper Cretaceous La Luna/Navay Formations and the Paleocene(?)–middle Eocene Los Cuervos Formation. The interpretation is based on evaluation of source rocks, oil-source rock correlations, oil maturities, and thermal modeling. The integration of the geochemical and geological data, modeling results, and structural interpretation of the area allowed the definition and outline of active kitchens beneath thrust sheets or in deeply buried areas. The results allowed us to map the active oil kitchens present below certain thrusts or thick molassic sediments, and to describe the petroleum systems in the area. La Luna Formation contains good to excellent oil-prone kerogen Type I-II, while Los Cuervos Formation contains gas-oil-prone Type II–III. Numerous oil seeps contain oil that was expelled at the peak of oil generation stage (0.85–0.90% Ro), and which was derived from marine sources. We interpreted this oil to have been generated by the La Luna Formation. We established a geochemical correlation between some oil seeps with the extracted organic matter from Los Cuervos Formation in the Burgua Depression. The analyses indicate a marine algal-bacterial organic-matter source for the Navay and La Luna Formations and a higher plant source for the Los Cuervos Formation. Modeling suggests that maturation was reached during the Andean orogeny, as a result of deposition of thick molasse, and also by tectonic loading. Clastic and carbonate potential reservoirs of Cretaceous (K) and Tertiary (T) age and regional seals of the Colón and León Formations are widespread in the area. Modeling results indicate that migration started during the middle Miocene and is related to Andean tectonic loading. Subsequent migration took place during the late Pliocene, to be blocked by the Capacho and Boconó fault systems.

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 Oil exploration in Colombia has traditionally taken place in areas with relatively few structural complexities. However, in the last decade, exploration has moved to regions characterized by complex structural deformation, poor seismic resolution, and many stratigraphic problems, such as in the Llanos foothills. In this region, the major reservoirs occur in mostly continental Paleogene sequences, where palynomorphs are usually the only fossil group found. Thus, palynology has become an important tool in controlling the stratigraphic position of a well during drilling, in testing diverse seismic and structural interpretations, and correlating reservoirs. This study provides a biostratigraphic framework for the Cuervos Formation (Late Paleocene to earliest Eocene) of the Llanos and Llanos foothills, making use of graphic correlation. We used 14 sections with palynological information from outcrops, well cores, and well ditch cuttings. Five informal palynological zones are proposed. Based on the biostratigraphic model produced by this analysis, we reinterpreted a recently drilled well in the Llanos foothills upon which several previous interpretations had been made. A sidetrack of that well was subsequently drilled, validating the proposed model. This is a positive test that the biostratigraphic framework developed for the Cuervos Formation is reliable and can be successfully applied to exploration in the Llanos foothills.

Abstract Integrated studies of the hydrocarbon system in frontier areas are important to assess exploration risk. The Rubio block located in the Táchira Depression has been the focus of various geological studies, yet very few studies of its petroleum system have been published. Based on seismic, surface geologic, and geochemical data, a study of the petroleum system has been developed. We have identified, based on quality limited seismic and surface data, at least three major periods of deformation in the study area, ranging from the Upper Cretaceous to the present. The first event is of Late Cretaceous age and is characterized by compression. The second period is characterized by extension and took place at least during the Paleocene. Both events may be related to the development of the Colombian Central Cordillera and the flexure caused by its tectonic load, which affected western Venezuela from the Upper Cretaceous to the Paleocene. The third event is characterized by compression that has occurred quite recently, and it may be associated with the development of the Mérida Andes. This last event is responsible for the development of the major structures in the Rubio block. It is worth noting that western Venezuela also was affected by a Jurassic extensional event that is not illuminated by the seismic data of the study area but has been amply shown in both surface and seismic data in the surrounding areas. According to the 1-D and 2-D geochemical modeling results, we propose that maturity of the source rock (the La Luna Formation) is greater to the west than to the east. Hydrocarbon expulsion from the source rock started in the western portion of the area 25 m.y. This local kitchen stopped expelling because of its uplift, which was caused by the latest compression. On the other hand, the La Luna Formation in the eastern portion started to expel hydrocarbon very recently, which is still ongoing. This active oil kitchen may be feeding the oil seeps in the eastern part of the study area.