Abstract The structural and stratigraphic framework of Maraven's Block I was re-interpreted using 3-D seismic and existing data as part of an evaluation of the remaining oil potential. More than 1800 MMBO have been produced from Block I in the past 40 years, mainly from structural traps. In order to maintain production levels, it has become increasingly important to define the seismic stratigraphic framework for the area and to accurately locate faults and stratigraphic pinchouts. The dominant structures are the Icotea fault, its conjugate fault system, and the Eastern Boundary fault. The most prominent fault is the NE-striking Icotea fault, which subdivides the area into two main structural blocks, a graben in the West Flank and a horst in the East Flank. The Icotea fault is a highly complex fault zone with a long history of deformation. It is a nearly continuous fault zone with both vertical and lateral offsets and is locally inverted. Along the eastern flank of the Icotea, prominent reverse-fault bounded upthrown blocks, called the Attic, have developed. Along the western flank, contraction has re-activated listric faults into reverse and thrust faults. Major northwest-striking normal faults delineate a large paleoarch that occurs in the south-center of the East Flank. This phase of faulting produced small horst and graben blocks bounded by normal faults that dip to the northeast and southwest. The Eastern Boundary fault is subparallel to the Icotea fault and is an east-dipping normal fault that has been locally inverted and occurs in a synclinal area of the block. Two play concepts, utilizing (1) horizontal wells in Attic and (2) vertical wells along the Eastern Boundary fault, were successfully tested during this study. The stratigraphic section includes, from oldest to youngest, pre-Triassic basement rocks; the Jurassic graben-fill Quinta Formation; the Cretaceous Rio Negro, Cogollo Group, La Luna, Colon, and Mito Juan formations; the Paleocene Guasare Formation; the Eocene Misoa Formation; the Miocene La Rosa, Lagunillas, and La Puerta formations; and the Quaternary El Milagro Formation. Only the lower part of the Eocene Misoa Formation (C sands) is preserved in Block I, and most of the Eocene B sands and all of the Pauji were either eroded or not deposited in this area. The main reservoirs occur in the Eocene Misoa Formation and the basal Miocene Santa Barbara member of the Lagunillas Formation. Sedimentation occurred throughout the Eocene and was strongly influenced by tectonism. The Eocene section in the horst block is up to 760 m thick and is bracketed by two major unconformities. The upper angular unconformity places the basal Miocene Santa Barbara member (16–25 Ma) over the Eocene Misoa C sands (45–54 Ma). The lower disconformity (54 Ma) occurs at the top of the Paleocene Guasare Formation. In between, eight seismic sequences occur within the Eocene horst section. The adjacent stratigraphic sections east and west of the horst block are thicker than the East Flank section. The C sands in Block I form a retrogradational clastic sequence deposited as transgressive (70–80%), highstand (10–15%), and lowstand wedge and incised valley fill (10–15%) systems tracts with prominent marine-flooding surfaces separating these systems tracts. The main reservoirs are thick-bedded transgres-sive sandstone deposits.

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 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.

Abstract The Cretaceous interval in the Maracaibo consists mainly of fine-grained clastic and carbonate sediments deposited during a period of passive margin development. Stratigraphically, the early Cretaceous Apon Formation of Aptian age contains interbedded rich organic dolomitic and calcareous shale and black bituminous limestone (Mercedes, Tibu, Guaimaros and Machiques Members). Not only do these constitute important source and reservoir rocks in the southwest region of the Maracaibo Basin, but also represent maximum flooding surfaces within the overall depositional characteristics in the Basin. To the north, the Apon along with the Lisure and Maraca Formations of the Aptian-Albian Cogollo Group, consisting mostly of limestone and fine-grained sandstone, represent a shallow to middle shelf environment transgressive systems tract. In the south, the Cretaceous Capacho, consisting of black shale and the La Luna Formation comprised of interbedded calcareous shale and black cherty limestone, were deposited during a period of relative sea level rise. The La Luna Formation is the primary source rock in the basin as well as a reservoir in places where microfractures produce the necessary permeability for hydrocarbon flow. Numerous geochemical studies performed in the Maracaibo Basin provide evidence for the existence of multiple stratigraphic intervals containing hydrocarbon generating organic-rich shale and calcareous mudstone within the Cretaceous. Integrated reservoir studies and production data indicate that some of the same intervals are excellent oil and gas producers where fracture systems exist. The above-mentioned stratigrahic and depositional characteristics are responsible for making the Maracaibo Basin one of the most prolific producers of hydrocarbons in the world.