Western Venezuela Research Articles

Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela

Other| October 01, 2003 Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela MARIEN PARRA; MARIEN PARRA 1Petróleos de Venezuela, PDVSA, División Oriente, Apdo. 3426, Puerto La Cruz 6023, Venezuela parramo@pdvsa.com Search for other works by this author on: GSW Google Scholar LORENA MOSCARDELLI; LORENA MOSCARDELLI 2University of Texas at Austin, University Station C1000, Austin, TX, 78712 Search for other works by this author on: GSW Google Scholar MARIA ANTONIETA LORENTE MARIA ANTONIETA LORENTE 3GeoAmbio, Apdo 281, Jumilla 30520, Spain Search for other works by this author on: GSW Google Scholar PALAIOS (2003) 18 (4-5): 321–333. https://doi.org/10.1669/0883-1351(2003)018<0321:LCAALM>2.0.CO;2 Article history accepted: 01 Jun 2003 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation MARIEN PARRA, LORENA MOSCARDELLI, MARIA ANTONIETA LORENTE; Late Cretaceous Anoxia and Lateral Microfacies Changes in the Tres Esquinas Member, La Luna Formation, Western Venezuela. PALAIOS 2003;; 18 (4-5): 321–333. doi: https://doi.org/10.1669/0883-1351(2003)018<0321:LCAALM>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The Tres Esquinas Member of the La Luna Formation is a glauconite-rich phosphorite unit associated with changes that took place in depositional environments in the Maracaibo Basin at the end of the Late Cretaceous. The unit marks the end of La Luna Formation sedimentary cycle. This paper presents results of petrographic studies of samples from two sections, one outcrop (Río Guaruries), and one well core (Perijá), and from seismic profiles across the Perijá and Colón areas in the Maracaibo Basin.The Tres Esquinas Member is three meters thick in both locations. In the Río Guaruries outcrop, it contains an abundant foraminiferal fauna in addition to common allochemical material. Perijá core samples contain voluminous carbonate matrix, scarce allochemical materials, rare foraminifera, and poor definition of mineral facies, which may reflect higher-energy conditions on the sea floor.The deposition of the Tres Esquinas Member resulted from altered sea-floor topography during an episode of intense tectonic activity in the eastern part of the Maracaibo Basin. The modified shelf configuration ended anoxic conditions on the sea floor and led to increased erosion characteristic of the Tres Esquinas Member. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Biostratigraphic, Sedimentologic, and Chemostratigraphic Study of the La Luna Formation (Late Turonian-Campanian) in the San Miguel and Las Hernandez Sections, Western Venezuela

Other| October 01, 2003 Biostratigraphic, Sedimentologic, and Chemostratigraphic Study of the La Luna Formation (Late Turonian–Campanian) in the San Miguel and Las Hernández Sections, Western Venezuela EGLEE ZAPATA; EGLEE ZAPATA 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com Search for other works by this author on: GSW Google Scholar VICTOR PADRON; VICTOR PADRON 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com Search for other works by this author on: GSW Google Scholar ISIRIS MADRID; ISIRIS MADRID 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com Search for other works by this author on: GSW Google Scholar VANESSA KERTZNUS; VANESSA KERTZNUS 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com Search for other works by this author on: GSW Google Scholar IRENE TRUSKOWSKI; IRENE TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar MARIA ANTONIETA LORENTE MARIA ANTONIETA LORENTE 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information EGLEE ZAPATA 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com VICTOR PADRON 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com ISIRIS MADRID 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com VANESSA KERTZNUS 1Universidad Central de Venezuela, Faculty of Engineering, Department of Geology, Los Chaguáramos, Caracas, 10, Venezuela, zapata.eglee@excite.com IRENE TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela MARIA ANTONIETA LORENTE 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 367–377. https://doi.org/10.1669/0883-1351(2003)018<0367:BSACSO>2.0.CO;2 Article history Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation EGLEE ZAPATA, VICTOR PADRON, ISIRIS MADRID, VANESSA KERTZNUS, IRENE TRUSKOWSKI, MARIA ANTONIETA LORENTE; Biostratigraphic, Sedimentologic, and Chemostratigraphic Study of the La Luna Formation (Late Turonian–Campanian) in the San Miguel and Las Hernández Sections, Western Venezuela. PALAIOS 2003;; 18 (4-5): 367–377. doi: https://doi.org/10.1669/0883-1351(2003)018<0367:BSACSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract Depositional paleoenvironments of the La Luna Formation were established based on investigation of sedimentology, foraminiferal assemblages, and stable isotope composition of two sections in the Venezuelan Andes. Planktonic foraminiferal biostratigraphy indicates that the age of the San Miguel section spans from the late Turonian to the early Campanian and the Las Hernández section ranges from the early Coniacian to the early Campanian. The base of the La Luna Formation becomes progressively younger towards the south. Early-diagenetic processes have altered stable isotopic composition of carbonates, however stratigraphic changes in carbon isotope values can be used to correlate between the sections.The environment of deposition of the La Luna Formation changed during the Late Cretaceous. Interval I, from the early Coniacian to mid-Santonian, was characterized by anoxic conditions. Alternating anoxic-dysoxic environments in Interval II lasted from the mid-Santonian to the base of the Campanian. Interval III, in the early Campanian, was marked by more oxygenated conditions. These trends can be partially observed in other ocean basins. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela

Other| October 01, 2003 An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela LINDA M. DE ROMERO; LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu Search for other works by this author on: GSW Google Scholar IRENE M. TRUSKOWSKI; IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar TIMOTHY J. BRALOWER; TIMOTHY J. BRALOWER 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. Search for other works by this author on: GSW Google Scholar JAMES A. BERGEN; JAMES A. BERGEN 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. Search for other works by this author on: GSW Google Scholar OSCAR ODREMAN; OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela Search for other works by this author on: GSW Google Scholar JAMES C. ZACHOS; JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 Search for other works by this author on: GSW Google Scholar FRANCIA A. GALEA-ALVAREZ FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information LINDA M. DE ROMERO 1Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599, deromero@email.unc.edu IRENE M. TRUSKOWSKI 2Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela TIMOTHY J. BRALOWER * Current address: Department of Geosciences, Pennsylvania State University, University Park, PA 16802. 3Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC 27599 JAMES A. BERGEN ** Current address: BP America, Inc., 501 Westlake Park Blvd., Houston, TX 77079. 4Department of Geological Sciences, CB #3315, University of North Carolina, Chapel Hill, NC OSCAR ODREMAN 5Escuela de Ingeniería Geológica, Universidad de Los Andes, Mérida, Venezuela JAMES C. ZACHOS 6Earth Sciences Department, University of California, Santa Cruz, CA 95064 FRANCIA A. GALEA-ALVAREZ 7Petróleos de Venezuela, PDVSA, Exploración, Apdo. 829, Caracas 1010A, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Online ISSN: 1938-5323 Print ISSN: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 349–366. https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Article history Accepted: 24 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation LINDA M. DE ROMERO, IRENE M. TRUSKOWSKI, TIMOTHY J. BRALOWER, JAMES A. BERGEN, OSCAR ODREMAN, JAMES C. ZACHOS, FRANCIA A. GALEA-ALVAREZ; An Integrated Calcareous Microfossil Biostratigraphic and Carbon-Isotope Stratigraphic Framework for the La Luna Formation, Western Venezuela. PALAIOS 2003;; 18 (4-5): 349–366. doi: https://doi.org/10.1669/0883-1351(2003)018<0349:AICMBA>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The biostratigraphy of the La Luna Formation has long been in dispute, despite the importance of this unit as the most important source rock for hydrocarbons in Venezuela. In this paper, calcareous microfossil biostratigraphy combined with carbon-isotope stratigraphy provides a stratigraphic framework for the formation, permitting revision of temporal and spatial patterns of deposition of organic-rich sediments. Detailed studies were conducted on a cored borehole and five outcrop sections distributed across the Maracaibo Basin of western Venezuela.Planktic foraminifera have fair to good preservation, and nannofossils are poorly preserved. Many of the Cenomanian to Campanian planktic foraminiferal marker species are present, permitting the application of a traditional zonal scheme. An informal nannofossil biostratigraphic zonal scheme, based primarily on dissolution-resistant species, has been developed. Integration of these zonal schemes has enabled the correlation of changes in carbon-isotope ratios to the global C-isotope stratigraphy. The results have been used to estimate temporal variation in sedimentation rates as well as to reconstruct depositional patterns across the Maracaibo Basin. Deposition began in the eastern and northwestern part of the Maracaibo Basin in the middle Cenomanian and progressed towards the south and west, reaching the southwestern corner by the middle Coniacian. Although the uppermost part of the formation could not be dated, deposition in the eastern basin continued at least into the Coniacian and in the western basin at least until the middle–late Santonian. Sedimentation rates were highly variable with a period of condensed sedimentation at the Cenomanian–Turonian boundary. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela

Other| October 01, 2003 Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela OLIVER MACSOTAY; OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela Search for other works by this author on: GSW Google Scholar ROBERT N. ERLICH; ROBERT N. ERLICH 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com * Corresponding author. Search for other works by this author on: GSW Google Scholar TULIO PERAZA TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Search for other works by this author on: GSW Google Scholar Author and Article Information OLIVER MACSOTAY 1Urbanización El Trigal Norte, Ave. Atlántico, no. 155-61B, Valencia, Venezuela ROBERT N. ERLICH * Corresponding author. 2Burlington Resources International, 400 North Sam Houston Parkway East, Suite 1200, Houston, TX 77060, rerlich@br-inc.com TULIO PERAZA 3Intevep-Laboratorios Generales, Sur 1, nivel 2, oficina 222, Los Teques, Edo. Miranda, Venezuela Publisher: SEPM Society for Sedimentary Geology Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Online Issn: 1938-5323 Print Issn: 0883-1351 Society for Sedimentary Geology PALAIOS (2003) 18 (4-5): 334–348. https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>2.0.CO;2 Article history Accepted: 01 Jun 2003 First Online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation OLIVER MACSOTAY, ROBERT N. ERLICH, TULIO PERAZA; Sedimentary Structures of the La Luna, Navay and Querecual Formations, Upper Cretaceous of Venezuela. PALAIOS 2003;; 18 (4-5): 334–348. doi: https://doi.org/10.1669/0883-1351(2003)018<0334:SSOTLL>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyPALAIOS Search Advanced Search Abstract The Late Cretaceous seas of Venezuela were characterized by deposition of organic carbon-rich limestones, cherts, and shales in the Maracaibo and Barinas/Apure Basins of western Venezuela (La Luna and Navay Formations) and in the Maturín and Guarico Sub-basins of eastern Venezuela (Querecual, Tigre, and San Antonio Formations). Deposition of these units typically occurred under low-energy, low-oxygen conditions that were punctuated by the episodic activity of bottom currents, debris flows, turbidites, syndepositional faulting, and by intermittent oxygenation. These events produced a characteristic set of sedimentary structures that can be used to provide more detailed interpretations of each depositional system. Four main types of sedimentary structures noted in this study are: (1) hydrodynamic—primarily produced through the actions of currents; (2) rheotropic—structures produced during loading and dewatering of unconsolidated sediments; (3) biogenic—formed through the actions of living organisms, such as trace fossils; and (4) diagenetic—structures and textures formed through near surface and burial diagenetic processes.Identification of these structures can provide new insights on the hydrodynamic and biologic settings of Venezuelan Cretaceous strata. Utilizing observations made in this study along with previously published works, bottom-water conditions in the La Luna depositional system are interpreted to have been maintained consistently in a state of anoxia or very low dissolved oxygen content. This contrasts with bottom waters in the Querecual depositional system, during which strong periods of anoxia or low dissolved oxygen content were punctuated by periods of intermittent oxygenation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Wellbore Stability Simulations for Underbalanced Drilling Operations in Highly Depleted Reservoirs

Summary The need to increase productivity and to reduce drilling formation damage favors the use of underbalanced drilling technology. The main idea is to drill with equivalent circulating densities (ECD) that are less than the formation pore pressure and to avoid contact between the drilling fluid and the formation. In highly depleted reservoirs, pore pressures can be very low. Therefore, extremely low-density fluids, such as foams, are used to achieve circulating densities lower than the pore pressure. In such cases, the induced modification of the in-situ stresses has to be supported mainly by the rock, with little contribution from the drilling fluid pressure. In that sense, the application of underbalanced drilling depends on the mechanical stability of the drilled formation, among other factors. In general, poorly consolidated, depleted formations are not suited for that technology. This paper presents the wellbore stability simulation performed to establish the feasibility of using underbalanced drilling in highly depleted reservoirs in western Venezuela. The in-situ stress field and the mechanical properties of the formation were obtained. Pore pressure as low as 800 psi at 5,500 ft (2.7 lb/gal equivalent fluid density) was measured. The finite difference method and an elastoplastic constitutive model was used to obtain the new stress, deformation, and pore pressure distribution. The undrained condition (immediately after the wellbore is drilled) as well as the drained condition were analyzed. The analysis showed that horizontal wells could be drilled in an underbalanced condition with low instability risk. Following the recommendations, four horizontal wells were drilled in underbalanced conditions. Values as low as 2.0 lb/gal ECD were used to drill the wells, and no wellbore instability problems were reported. Production tests showed an enormous increase in the well productivity index in comparison with conventional overbalanced drilling.

Magnetic and mineralogical studies to characterize oil reservoirs in Venezuela

High-resolution aeromagnetic surveys over oil fields and magnetic susceptibility (MS) measurements in soils, sediments, and drill cuttings have been proposed as complementary or alternative means of exploration and assessment of hydrocarbon reservoirs. Some micromagnetic anomalies detected by these studies could be the likely result of the existence, at shallow depths, of authigenic magnetite and/or Fe-sulphides. It has been hypothesized that such magnetic minerals are the chemical byproducts of primary Fe-oxides exposed to a reducing environment induced by the underlying reservoir. Secondary magnetite could appear as spherical aggregates (framboids) of submicronic crystals identifiable by scanning electron microscopy (SEM). The formation of these framboids seems linked either to inorganic chemical processes associated with the presence of hydrocarbons or to oil biodegradation. In the last few years, we have examined the causal relationship between framboids of magnetic minerals and hydrocarbons via rock magnetic and electron paramagnetic resonance (EPR) analyses of drill cuttings. The samples used have been taken at different depths (within the first 1500 m) of producing and nonproducing wells from two oil fields in southwestern Venezuela. We have complemented MS results with S-ratio logs, SEM, X-ray energy dispersion (EDX), IRM acquisition curves, and high-temperature susceptibility measurements. More recently we have also studied soil samples from an oil prospective area in western Venezuela, where rock magnetic and EPR results were compared with ethane gas analyses and remote sensing data. Rock magnetic and EPR studies have been performed in 14 oil wells from La Victoria (3) and Guafita (11) fields (Figure 1). Since our research has focused at the near-surface expression of hydrocarbon migration, the samples analyzed (drill cuttings) were taken from different depth at intervals of about 15 m between 100 and 1500 m. All samples belong to a single group of molasses from strata that lie far above the producing levels …

Taxonomy, Distribution, and Natural History of the Genus Heteromys (Rodentia: Heteromyidae) in Western Venezuela, with the Description of a Dwarf Species from the Península de Paraguaná

Heteromys anomalus is widely distributed along the Caribbean coast of South America and was generally considered the only spiny pocket mouse present in Venezuela until H. australis was reported recently from the Cordillera de Mérida. Here, I revise the spiny pocket mice of western Venezuela and compare specimens from the semi-insular Península de Paraguaná with samples from throughout the distributional area of H. anomalus. Morphological comparisons with all other species of the genus suggest that the Heteromys from Paraguaná are closely related to H. anomalus, but univariate and multivariate analyses show that they are much smaller than samples of H. anomalus. In addition, adults from Paraguaná display differences in cranial shape when compared with adults of H. anomalus. Hence, specimens from Paraguaná are here described as a new species, Heteromys oasicus, and taxonomic treatments of H. anomalus and H. australis in western Venezuela are also provided. Although the original geographic context of this case of dwarfism in body size is unknown, H. oasicus currently appears to be isolated in mesic vegetation on the low Cerro Santa Ana and Fila de Monte Cano, which lie in a matrix of tropical thorn forest. The new species represents one of the few vertebrates considered endemic to Paraguaná, but basic inventories and taxonomic studies are not yet complete in the region.

Geomorphic and geologic evidence of ongoing uplift and deformation in the Mérida Andes, Venezuela

The Mérida Andes is a mountain chain that trends SW–NE in western Venezuela, extending for some 350 km from the Colombian–Venezuelan border to the city of Barquisimeto. Its highest peaks reach 5000 m in elevation in the central portion, near Mérida. This chain appears to be the northeastward topographic prolongation of the Eastern Cordillera of the Colombian Andes. The latter belongs to the main Andes chain that extends along the Pacific coast of South America. However, there is no direct genetic relationship between the Mérida Andes and Eastern Cordillera. The Mérida Andes uplift is not produced by convergence across a conventional type-B subduction, as most of the South American Andes are. In addition, the Mérida Andes and Eastern Cordillera are separated by the southern termination of the left-lateral strike-slip Santa Marta-Bucaramanga fault, and the NW–SE trending Santander Massif. The present Mérida Andes chain build-up results from Pliocene–Quaternary transpression due to oblique convergence between two continental blocks: South America and Maracaibo Triangular Block. The present geodynamic setting is responsible for ongoing strain partitioning along the Mérida Andes where the foothills and the mountain belt have been shortened transversely in a NW–SE direction whereas the Boconó fault, roughly located in the core and along the Mérida Andes axis, accommodates dextral slip. Tectonic inheritance in the Mérida Andes plays a major role, since the chain growth partly results from inversion of Jurassic (half-) grabens, exposing Precambrian and Paleozoic rocks of the South American continental crust along the chain core. Evidence of very different kinds attests to the ongoing growth of the chain. From the geomorphic viewpoint, the axial valleys display well-preserved Quaternary staircase terrace systems with more than 500 m of vertical drop between the oldest terrace and present river beds. Rivers cutting across the structural grain of the chain show very distinct transverse “wine cup” profiles, attesting to the rapid uplift of the chain. In some cases, rivers have downcut over a 1000 m of relief, where the lower 200–300 m of these profiles exhibit subvertical walls. Other features that support the significant uplift of the chain and its youth are synorogenic mollasic deposits along both flanks of the chain, deposited in flexural basins, whose thicknesses reach 8 and 3 km on the northwest and southeast of the Mérida Andes, respectively. These continental deposits of essentially Plio–Quaternary age are arranged in up-dip convergence growth wedges, which are being deformed or destroyed by basin-vergent intracutaneous wedges, triangular zones and/or flat-and-ramp structures rooted under the Mérida Andes. The Mérida Andes core comprises igneous and metamorphic rocks formed at depths of 8–10 km, that are cropping out at its highest summits at about 5000 m elevation, implying a total uplift of the order of 12–15 km in the last 3–5 Ma (average uplift rate of 2–5 mm/a). Activation age and amount of uplift match rather well with those derived from fission-track thermochronology. The entire mountain belt exhibits widespread seismicity, essentially restricted to upper crustal levels (seismogenic layer). As well, focal mechanism solutions confirm the strain partitioning taking place in the Mérida Andes at present: right-lateral strike-slip along the sub-axial Boconó fault and basinward thrust solutions on both flanks. Underfed/under-destruction pull-apart basins (Las González and Apartaderos) along the Boconó fault appear to be complementary evidence of rapid uplift of the Mérida Andes.

Shrew opossums (Paucituberculata: Caenolestes) from the Huancabamba region of east Andean Peru.

Shrew opossums of the genus Caenolestes are restricted to the northern Andes and include four species: C. fuliginosus, C. convelatus, C. caniventer and C. condorensis (Albuja and Patterson 1996). Caenolestes fuliginosus is a highland species of the paramos of central Ecuador, Colombia and western Venezuela, but the remaining three species occur in sub-tropical montane forests at lower elevations (Albuja and Patterson 1996). Caenolestes convelatus and C. caniventer are known from the western slopes of the Andes, the former from Colombia and northern Ecuador and the latter from southern Ecuador and northern Peru (Bublitz 1987), while the recently described Caenolestes condorensis is currently known only from the isolated Cordillera del Condor along the eastern slopes of the Andes in southern Ecuador (Albuja and Patterson 1996). The first records of Caenolestes from Peru were collected in the Departamento de Piura (Barkley and Whitaker 1984) and were originally reported as C. fuliginosus but later were later identified as C. caniventer (Albuja and Patterson 1996). The Departamento de Piura lies just north of the Huancabamba Depression, a region defined by the arid valley of the Rio Chamaya and adjacent low elevation passes (Fig. 1). Many species of Andean bird reach their southernmost distributional limit at the Huancabamba Depression (Parker et al. 1985), and it has been postulated that the region might also serve as a barrier to the southward dispersal of Caenolestes (Vivar et al. 1997). However, these same authors also cautioned that the Huancabamba region remains poorly surveyed for mammals and that what may appear to be a pattern may in fact be the result of inadequate sampling. In this paper we present additional data on the distribution of Caenolestes from localities in Peru north and south of the Huancabamba Depression.

AN ILLUSTRATED KEY FOR THE IDENTIFICATION OF POLLEN FROM PANTEPUI AND THE GRAN SABANA (EASTERN VENEZUELAN GUAYANA)

This work consists of a key, descriptions and illustrations, to identify the most significant angiosperm pollen types (85 genera/species, belonging to 36 families), from a paleoecological point of view, of the Gran Sabana and the ‘tepui’ (table mountains) summits from eastern Venezuelan Guayana. The pollen key and plates contain the most frequent and abundant pollen types found so far by the author in the Holocene sediments, as well as the significant medium and minor elements. This is enough to carry out a standard, successful palynological analysis in the area with a small percentage of unknown taxa, and can be considered a first step to be enhanced with future studies, especially from the ‘tepui’ summits of the western Venezuelan Guayana.

Parasites, fossils and geologic history: Historical biogeography of the South American freshwater croakers, Plagioscion spp. (Teleostei, Sciaenidae)

A hypothesis on the historical biogeography of the freshwater croakers, Plagioscion spp. (Teleostei: Sciaenidae), is developed based on data from the phylogeny of their monogenoidean parasites (Platyhelminthes), the geology of South America and the fossil record. Analyses suggest that the common ancestor of Plagioscion spp. colonized freshwater concomitantly with the common ancestor of their parasites, Euryhaliotrema spp. Colonization probably occurred via a marine transgression through western Venezuela that developed about 20 million years ago (Mya). This transgression with its postulated highly variable temporal and spatial salinity conditions most likely represented the facilitating event associated with freshwater colonization. A less likely alternative, that colonization occurred via the Mar del Plata in southern South America, is not supported by the geographical distributions and putative phylogeny of extant species of Plagioscion.

A Middle-Late Devonian fish fauna from the Sierra de Perijá, western Venezuela, South America

A Middle-Late Devonian fish fauna from the Sierra de Perijá, western Venezuela, South America

Contrasting sylvatic foci of Venezuelan equine encephalitis virus in northern South America.

The ecology of Venezuelan equine encephalitis (VEE) virus transmission was compared at three enzootic foci: two forest sites in the Catatumbo region of western Venezuela that have yielded small numbers of virus isolates since the 1970s, and another focus in the middle Magdalena Valley of Colombia that has consistently yielded many VEE virus isolates. Our results demonstrated dramatic differences in VEE virus isolation rates from sentinel hamsters, as well as differences in mosquito species composition and captured mammals with antibodies to VEE virus, between the Colombian and Venezuelan study sites. The higher isolation rate of enzootic VEE virus in the Colombian site was associated with a more abundant fauna of spiny rats (Proechimys spp.), known reservoir hosts of enzootic VEE virus. Mosquito collections demonstrated that the Colombian forest had a higher mosquito diversity and species evenness than either of the Venezuelan forests. The Colombian focus was especially richer in its Culex (Melanoconion) spp. fauna, a subgenus that includes all proven enzootic vectors for VEE virus. Our results suggest that the greater abundance, diversity, and stability of enzootic vector populations, combined with the greater density of rodent reservoir hosts, explains the higher levels of VEE virus circulation in the Colombian focus compared with the Venezuelan forests.

A New Species of Spiny Pocket Mouse (Heteromyidae: Heteromys) Endemic to Western Ecuador

Whereas previous treatments have considered Heteromys australis the only spiny pocket mouse present in Ecuador, morphological and morphometric analyses of specimens from Ecuador and southwestern Colombia reveal the presence of two species of the genus. Heteromys australis is distributed in evergreen forests from eastern Panamá and western Venezuela through Colombia to extreme northwestern Ecuador, where it inhabits wet, unseasonal areas of the Chocó and adjacent western slopes of the Andes. We here describe a new species, Heteromys teleus, found only in evergreen forests of central-western Ecuador, in areas less mesic and more seasonal than those characteristic of H. australis. Both species possess dark gray dorsal pelage, but H. teleus differs by larger (nonoverlapping) measurements of the hind foot and distinctive cranial proportions. Most notably, the rostrum of the new species is strikingly wide and massive, and the interparietal is narrow and rounded (in contrast to the wide, diamond-shaped interparietal of H. australis). The ranges of the two species together conform to the previously recognized Chocoan evergreen-forest fauna of western Colombia and northwestern Ecuador. However, the restriction of H. teleus to evergreen but seasonal forests of the southern Chocó (transitional between the relatively unseasonal evergreen forests of the central Chocó to the north and highly seasonal xeric regions to the south) is unique within currently recognized species of mammals. Biogeographic overviews hint at similar patterns in other groups, but more alpha-taxonomic research is necessary to evaluate mammalian distributional patterns in the region properly. Most suitable habitat for H. teleus has been converted to agricultural uses, and its current distribution is likely restricted to a handful of small-to-medium-sized forest patches.

Effect of Depletion Rate on Gas Mobility and Solution Gas Drive in Heavy Oil

Summary Some heavy oil reservoirs in western Canada and Venezuela show anomalously high primary recovery under solution gas drive process. The pressure decline rate in these reservoirs is low compared to that expected under solution gas drive in conventional oil reservoirs. There is now increasing evidence that gas mobility is extremely low in these reservoirs. The objective of this study is to conduct solution gas drive experiments in a sandpack saturated with a heavy oil and examine the effect of depletion rate. Depletion rate was varied by more than two orders of magnitude. The results showed that gas mobility was a function of depletion rate and decreased with increasing depletion rate. Other notable observations were that supersaturation increased with depletion rate and that critical gas saturation was 3 to 4%, slightly increasing with increasing depletion rate. Interpretation of the results confirmed that gas mobility is quite low. Representation of the low mobilities using relative permeability required low values of the order of 10−5–10−4, which decreased with increasing depletion rate.

Feeding behavior of Lutzomyia pseudolongipalpis (Diptera: Psychodidae), a putative vector of visceral leishmaniasis in Venezuela.

Lutzomyia pseudolongipalpis Arrivillaga & Feliciangeli is the first new sand fly species in the L. longipalpis species complex that has been formally described since it was separated by genetic as well as by morphological characters. It is the putative vector of the American visceral leishmaniasis in La Rinconada, Curarigua, a restricted focus in central western Venezuela. We investigated the feeding behavior of this species. The blood meals from 210 of 429 (48.9%) engorged females caught by CDC light traps were identified by a dot enzyme-linked immuno-absorbent assay using antisera against humans and common domestic animals. We propose a new index, the host selectivity index, which is the number of sand flies fed on a given host relative to the available biomass of that host, as an indicator of the feeding behavior of this phlebotomine sand fly. The host selectivity index is compared with the forage ratio, which is the percentage of sand flies fed on a given host by the percentage which that host represented in the total census of available animals and humans. The most attractive animal for L. pseudolongipalpis in Curarigua was the dog, whereas humans were shown to be relatively unattractive. However, not only selectivity or biomass, but also the accessibility to this host may have influenced these results. The low population density of dogs and the low accessibility of L. pseudolongipalpis to humans in relation to domestic animals might help to explain the sporadic transmission of visceral leishmaniasis in this focus.

A high-frequency sequence study: a Miocene deltaic and estuarine succession in the Eastern Maracaibo Composite Foreland Basin, Western Venezuela

ABSTRACT The Maracaibo Basin in northwestern Venezuela, one of the most prolific oil-producing areas in the world, is a composite foreland basin limited by foredeeps both to the south and west, as well as by piggy-back basins and half-grabens in the east. The western foredeep is part of a retro-foreland basin associated with the Oligocene-Miocene uplifted Sierra de Perija. The La Rosa and the lower Lagunillas formations (lower-middle Miocene) were deposited in the distal portion of this retro-foreland basin. Cores and well logs allow the identification of tidally influenced fluvial, tidal flat, central basin and embayment, shoreface and offshore, lagoonal and organic rich deposits, as well as zones of subaerial exposure within regressive deltaic or strandplain, and transgressive barrier bar or estuarine environments. Core-log calibrations and the identification of stacking patterns and key stratigraphic surfaces allow two types of high-frequency stratigraphic sequences to be recognized. Most sequences are of a simple transgressive-regressive type, implying continuous but irregular rates of relative rise of sea level. However, some have a lowstand segment, implying a relative sea level fall associated with the regressive-to-transgressive turnaround. The repetition of high-frequency sequences, each one of duration less than 0.5 m.y., is believed to have been controlled by high subsidence rates in the basin. The basinward and landward stepping of sequences is interpreted in terms of milder and strong advance of the western thrust-belt wedge and the accompanying variable migration of a forebulge to the east. End_Page 3--------------------------

Is Rhodnius robustus (Hemiptera: Reduviidae) responsible for Chagas disease transmission in Western Venezuela?

We present evidence for the putative role of Rhodnius robustus as extradomestic vector of Chagas disease in Western Venezuela. First, we assessed the validity of this triatomine species by genetic characterization in relation with some other species of the prolixus group. Random amplified polymorphic DNA data showed a clear separation between this species and R. prolixus and indicated a probable genetic heterogeneity within R. robustus. Faeces and gut contents were microscopically examined in 54 of 137 R. robustus collected in palm trees. According to this morphological examination, 18% were positive for Trypanosoma cruzi, 11% harboured T. rangeli and 11% showed mixed infection. Five of the seven samples examined gave a polymerase chain reaction major band of 270 bp specific of T. cruzi. The hybridization probes showed that R. robustus may transmit clones 20 and 39 (or genetically related ones) in Venezuela. Such a transmission might occur when, in absence of domestic R. prolixus and attracted by artificial light, R. robustus enters houses and feeds on humans, or when people are bitten outdoors. The lack of bugs inside houses could mean that the insects leave houses after feeding, or die without reproducing there.

ABSTRACT: Geochemical Exploration in Northern South America: Recent Successes in Venezuela and Colombia

Detailed geochemical and research studies document that hydrocarbon microseepage from petroleum accumulations is common, is predominantly vertical (with obvious exceptions in some geologic settings), and is dynamic (responds quickly to changes in reservoir conditions).Since microseepage is nearly vertical, the extent of an anomaly at the surface can approximate the productive limits of the reservoir at depth. Furthermore, the detailed pattern of seepage can reflect reservoir heterogeneity, discriminate between charged and uncharged compartments, and identify areas of bypassed pay. Results of recent microbial and soil gas surveys in Venezuela and Colombia establish the value of hydrocarbon microseepage data for high-grading prospects and aiding field development projects. These surveys were conducted by Geo-Microbial Technologies in the Eastern Venezuela basin, the Maracaibo-Catatumba basin in western Venezuela, the Guajira and Cesar Rancheria basins in northern Colombia, and the Middle Magdalena Valley basin in central Colombia.The Guajira survey documented previously unrecognized oil potential in a basin known only for its dry gas.Results from eastern Venezuela and Cesar Rancheria successfully discriminated prospects on basis of probably hydrocarbon charge. Surveys over two old oil fields in western Venezuela and in the Middle Magdalena Valley identified bypassed pay and several new drilling opportunities. High-resolution microseepage surveys offer a flexible, low-risk and low-cost environmentally friendly technology that complements traditional geologic and seismic data. Properly integrated with other exploration data, their use has led to discovery of new reserves and drilling of fewer dry or marginal wells.

ABSTRACT: Biostratigraphic Analysis and Environmental Implications of La Luna Formation, R`o Loro Section, Western Venezuela

ABSTRACT: Biostratigraphic Analysis and Environmental Implications of La Luna Formation, R`o Loro Section, Western Venezuela