Venezuela Basin Research Articles

Combination of extended elastic impedance and rock physics templates for reservoir characterisation in Temblador field, Eastern Venezuela basin

The Temblador Field is located on the southern flank of the Eastern Venezuela Basin. The Oficina Formation, present in the field, represents one of the most important producing Formations in the country. In this project, a detailed characterisation and delimitation of the sands of the Jobo and Morichal Members of the Oficina Formation. The work was outlined in three stages. The first stage consisted of studying the elastic properties of the reservoirs through unconsolidated rock physics modelling through the elaboration of the “Rock Physics Template” (RPT). Likewise, during this phase, different crossed graphs were prepared that allowed lithological discrimination. In the second stage, cubes of elastic properties such as P-wave impedance, S-wave impedance, Mhu-Rho (μρ) and Lambda-Rho (λρ) were obtained through the inversion of the Extended Elastic Impedance. Finally, the third stage consists of the classification of the lithofacies, of the cubes resulting from the inversion, through a Bayesian classification. These results allowed the construction of sand probability maps that allow delimiting the best quality sands within the Jobo and Morichal Members. In this way, the present work provides an additional tool when carrying out reservoir exploration studies, reducing uncertainty when locating a new prospect.

Dynamic downscaling of wind speed over the North Atlantic Ocean using CMIP6 projections: Implications for offshore wind power density

Offshore wind energy is an important agent to fight climate change. However, it is simultaneously very sensitive to climate change. This study analyzes the future changes in wind speed of 10 m above sea surface (V10) in the North Atlantic Ocean and how these variations may affect offshore wind energy resources for three potential subregions (the United States (US) East Coast, western Iberian Peninsula, and the Caribbean Sea). Dynamic downscaling of three different future scenarios of the CESM2 global climate model (CMIP6 project) was performed using the WRF-ARW atmospheric model. V10 is expected to decrease in the winter and spring seasons but increase in summer and autumn, mainly in tropical regions up to 30 °N. Annually, it shows the maximum increase in the tropical region. For the Iberian Peninsula subregion, significant increases in summer are expected for wind power density (WPD) along the 21st century, but there is uncertainty for the other seasons. A WPD decrease in winter and increases in summer and autumn are expected along the 21st century for the US subregion. No significant changes were observed at annual scale. Finally, for the Caribbean Sea, a decrease is projected in the Yucatan Basin and considerable increases are foreseen for the Colombia and Venezuela basins.

SEVERE BIODEGRADATION OF CRUDE OILS FROM THE QUIRIQUIRE FIELD, EASTERN VENEZUELAN BASIN

Five crude oil samples from the Quiriquire field (Maturin sub‐basin, Eastern Venezuelan Basin) were analysed to evaluate their levels of biodegradation. The oils were obtained from coarse sandstones and conglomerates of the Pliocene Quiriquire Formation at depths <1000 m. Analyses of the samples’ bulk physicochemical parameters indicate variations in API gravity and in the content of saturated hydrocarbons and NSO+asphaltenes, and also in the saturate/aromatic ratio which increases in more biodegraded oils. n‐Alkane distributions are characterized by a dominant unresolved complex mixture (UCM) or hump under an envelope of peaks which lack the acyclic isoprenoids pristane and phytane. The alteration of steranes and terpanes together with the presence of 25‐norhopanes and 17‐nor‐tricyclic terpanes, and the alteration of low molecular‐weight (C20‐C21) triaromatic steroids, phenanthrene, methyl‐phenanthrene, dibenzothiophene and methyl‐dibenzothiophene, indicate that the oils have undergone severe biodegradation.The oils contain compounds with different susceptibilities to biodegradation which is probably a consequence of the mixing of different oil charges in the Quiriquire Formation reservoir. The oils were derived from underlying source rocks in the Upper Cretaceous Guayuta Group (Querecual and San Antonio Formations), and migration into the shallow reservoir at Quiriquire field likely occurred continuously through time. Although the oils have undergone severe biodegradation, it was possible to make some inferences about their origin. Thus, the analyzed oils are interpreted to have originated from marine shale or marl source rocks containing mixed organic matter deposited under anoxic‐suboxic conditions and were generated at near peak oil window maturities.

Estimating global oilfield-specific flaring with uncertainty using a detailed geographic database of oil and gas fields

Associated gas flaring during crude oil production is an important contributor to global warming. Satellite technology has made global flaring monitoring possible with high spatial resolution. In this study, we construct a granular database to geographically match global oil and gas fields with remote sensing flaring data from the Visible Infrared Imaging Radiometer Suite from 2012 to 2019. The geographic information system database contains over 50 000 oil and gas fields and around 4700 infrastructure sites (e.g. refineries, terminals) in 51 countries and regions, representing 96% of global oil production and 89% of natural gas production. Over 2900 fields and 140 infrastructure sites in 47 countries contain matching flares. The annual matched flare volume covers 89%–92% of the satellite-estimated flaring volume of these countries and 85%–87% of total worldwide volume detected by the satellite. In 2019, a set of 263 ‘high-flare’ fields (which flare more than 0.1 billion cubic meters per year) account for 67% of the total matched satellite-estimated volume. These fields are mainly concentrated in the Persian Gulf, West and East Siberia, Eastern Venezuela Basin, Permian and Williston Basins in the United States, the Gulf of Mexico, and West and North Africa. Accounting for asymmetric instrument uncertainty suggests that country-level flaring rates are accurate to within −8% to +29%, the global average within 1%.

Relationship between viscosity and biodegradation of oils from the Eastern Venezuelan Basin

Venezuela has great reserves of heavy and extra-heavy oils, product of biodegradation, which requires knowledge of the relationship between the viscosity and biodegradation due their important topic for reservoir evaluation and improved oil recovery. In view of its importance, this work studies the relationship between viscosity and biodegradation level in oils from Junín and Ayacucho and Socororo fields from the Eastern Venezuela Basin. Oils from Ayacucho and Socororo present biodegraded oils mixtures, as consequence their API gravities, SARA composition and viscosity is variable. The logarithm of viscosity shows an acceptable correlation with API gravities, and SARA composition. The level of biodegradation determined by the Manco (Modular Analysis and Numerical Classification of Oils) scale does not allow a clear relationship between the biodegradation level and viscosity. However, the MN2 number can be used in the study of crude oil mixtures. The identification of origin and biodegradation level of oils and their relationship with viscosity should be considered for the implementation of adequate production strategies for reservoirs of complex oil composition, to allow the application of suitable methods for the recovery of crude oil, even in nearby wells whose composition and viscosity of the crude oil may be very different.

Depositional age and provenance of high-grade paragneisses from the Mérida Andes, Venezuela: Implications for the Ediacaran–Cambrian tectonic setting of northwestern Gondwana

Isotopic, geochemical, and geochronological data are provided to constrain sedimentary sources and depositional ages of high-grade paragneisses of the Iglesias Complex in the Mérida Andes (Venezuela). U-Pb geochronology of detrital zircons suggests a maximum depositional age of 540–530 Ma, whereas age spectra reveal sources from Pan-African-Brasiliano belts, the Amazon Craton, and Oaxaquia-Putumayo basement. Reworking of such Gondwanan sources is also reflected in whole-rock Nd and Hf crustal residence ages of ~2.0–1.3 Ga. Post-depositional disturbance of the U-Pb system by metamorphism during the early Paleozoic and Permo–Triassic is assessed through in-situ analysis of zircon rims and discordant analyses. Field evidence, geochemical and isotopic signatures are consistent with pelitic-psammitic and volcanic-volcaniclastic compositions for sedimentary protoliths. The former show provenances from continental intermediate sources, whereas the latter are probably sourced in an immature volcanic arc, suggesting the initiation of subduction of the Iapetus oceanic crust beneath northwestern Gondwana around 530 Ma. Sedimentary precursors were likely deposited during the latest Ediacaran–earliest Cambrian in an extensive continental shelf, fed by detritus draining from the topographic highs of the Pan-African-Brasiliano belts, across the Amazon craton and into the Iapetus Ocean. Thus, the main direction of sediment flow was opposite to the present-day transport from the Andes Cordillera by the Pacific Ocean to the Amazon fan in the Atlantic Ocean. Chemical trends in gneisses suggest fluvial sedimentary protoliths and cold-climate weathering at high latitudes. Metasedimentary rocks in the Mérida Andes show similarities with equivalent units in Peri-Gondwanan crustal fragments such as the Acatlán Complex and the Santander Massif, but no correlation with Ediacaran metasedimentary rocks of the Maya Block. Ediacaran–Cambrian fossiliferous sedimentary rocks in the Eastern Venezuela Basin and the Colombian foreland basin are possible equivalents but these units remained unmetamorphosed, as its paleoposition was far from the subduction trench during the Early Ordovician climax of the Famatinian orogeny.

Structure, age, and origin of the Caribbean Plate unraveled

Our compilation of marine magnetics of the Caribbean Plate allows us to identify magnetic anomalies from the Cretaceous Normal Superchron (CNS) in the Colombia and Venezuela basins, from which we propose a model for the age and origin of the Caribbean Plate and its Large Igneous Province (CLIP). The comparison of selected marine magnetic profiles across these basins with high resolution profiles from other areas across the CNS indicates that the Colombia Basin formed 73-92 million years ago (Ma) and the Venezuela Basin 92-108 Ma. The two basins and the older part of the CLIP formed at a ridge axis in the Pacific Ocean, near the Central American Seaway. These results strongly constrain regional plate tectonic models for America and the Pacific Ocean.

The preserved plume of the Caribbean Large Igneous Plateau revealed by 3D data-integrative models

Abstract. Remnants of the Caribbean Large Igneous Plateau (C-LIP) are found as thicker than normal oceanic crust in the Caribbean Sea that formed during rapid pulses of magmatic activity at ∼91–88 and ∼76 Ma. Strong geochemical evidence supports the hypothesis that the C-LIP formed due to melting of the plume head of the Galápagos hotspot, which interacted with the Farallon (Proto-Caribbean) plate in the eastern Pacific. Considering plate tectonics theory, it is expected that the lithospheric portion of the plume-related material migrated within the Proto-Caribbean plate in a north–north-eastward direction, developing the present-day Caribbean plate. In this research, we used 3D lithospheric-scale, data-integrative models of the current Caribbean plate setting to reveal, for the first time, the presence of positive density anomalies in the uppermost lithospheric mantle. These models are based on the integration of up-to-date geophysical datasets from the Earth's surface down to 200 km depth, which are validated using high-resolution free-air gravity measurements. Based on the gravity residuals (modelled minus observed gravity), we derive density heterogeneities both in the crystalline crust and the uppermost oceanic mantle (<50 km). Our results reveal the presence of two positive mantle density anomalies beneath the Colombian and the Venezuelan basins, interpreted as the preserved fossil plume conduits associated with the C-LIP formation. Such mantle bodies have never been identified before, but a positive density trend is also indicated by S-wave tomography, at least down to 75 km depth. The interpreted plume conduits spatially correlate with the thinner crustal regions present in both basins; therefore, we propose a modification to the commonly accepted tectonic model of the Caribbean, suggesting that the thinner domains correspond to the centres of uplift due to the inflow of the hot, buoyant plume head. Finally, using six different kinematic models, we test the hypothesis that the C-LIP originated above the Galápagos hotspot; however, misfits of up to ∼3000 km are found between the present-day hotspot location and the mantle anomalies, reconstructed back to 90 Ma. Therefore, we shed light on possible sources of error responsible for this offset and discuss two possible interpretations: (1) the Galápagos hotspot migrated (∼1200–3000 km) westward while the Caribbean plate moved to the north, or (2) the C-LIP was formed by a different plume, which – if considered fixed – would be nowadays located below the South American continent.

Multiple reaction monitoring tool applied in the geochemical investigation of a mysterious oil spill in northeast Brazil.

In 2019, much of the northeastern coast of Brazil was impacted by a mysterious oil spill that caused an environmental disaster affecting 1009 beaches. Four samples were collected in the beaches between Sergipe and Pernambuco for geochemical characterization of the spilled oil and to compare with those main produced in Sergipe-Alagoas basin. Our approach in this evaluation was the use of a highly selective technique of sequential mass spectrometry by multiple reaction monitoring, to obtain the diagnostic ratios of hopanes and steranes biomarkers. Using these biomarkers ratios associated with multivariate statistical analysis, we found direct correlation between the spilled oil collected along the northeastern coast and no relationship between Sergipe-Alagoas basin crude oils was found. Furthermore, reported data for oils from Orinoco belt in Venezuelan basins were used for qualitative evaluation considering the indicative aspects suggested by the literature. Presence of highly specific biomarker 18α(H)-oleanane, and five other important diagnostic ratios evidenced correlation between the spilled oil and Naricual formation crude oils. Besides, due to the oleanane index, Ayacucho's crude oil presented the strongest factor of correlation with the spilled oil found on the northeast coast of Brazil.

Moho depth map of northern Venezuela based on wide-angle seismic studies

As part of the lithosphere, the crust represents Earth's rigid outer layer. Some of the tools to study the crust and its thickness are wide-angle seismic studies. To date, a series of seismic studies have been carried out in Venezuela to determine in detail the crustal thickness in the southern Caribbean, in the region of the Caribbean Mountain System in northern Venezuela, as well as along the Mérida Andes and surrounding regions. In this study, a review of the wide-angle seismic data is given, incorporating new data from the GIAME project for western Venezuela, resulting in a map of Moho depth north of the Orinoco River, which serves as the basis for future integrated models. Differences in Moho depths from seismic data and receiver function analysis are discussed. From the Caribbean plate, Moho depth increases from 20 to 25 km in the Venezuela Basin to about 35 km along the coast (except for the Falcón area where a thinning to less than 30 km is observed) and 40–45 km in Barinas - Apure and Guárico Basins, and Guayana Shield, respectively. Values of more than 50 km are observed in the Maturín Basin and in the southern part of the Mérida Andes.

Organic geochemistry of kerogen from La Luna Formation, Western Venezuelan Basin, using diffuse reflectance – Fourier transform infrared spectroscopy (DRFTIR)

Twelve samples of marine type II kerogen from La Luna Formation (Maracaibo sub-Basin, Western Venezuelan Basin) have been characterized via diffuse reflectance – Fourier-transform – infrared spectroscopy (DRFTIR). The samples were selected from different regions in the sub-Basin and correspond to different maturity levels with subtle variations in organic facies. For the analysis, the kerogen samples were mixed with potassium bromide (KBr) at a 5% w/w concentration. Spectra were obtained using a Varian 640-IR spectrometer operating in Fourier Transform mode and coupled with an EasiDiff diffuse reflectance accessory. Each spectrum was generated within the 400–4000 cm−1 frequency interval, at 4 cm−1 resolution, after 52 scans. IR parameters known to distinguish organic facies, such as the A factor and the C factor, separated the kerogen from anoxic environment (samples from the Maraca Creek) versus the kerogen from suboxic/disoxic conditions (samples from La Molina and San Antonio). Organic facies indices by Chen et al. (1998) showed to be influenced by maturity effects. Higher thermal maturity of samples from La Molina creek was supported by increasing aromaticity (indices by Lis et al., 2005), as well as decreasing aliphaticity as expressed by a new parameter termed here in as AII. This parameter compares aromatic signals in the 700–900 cm−1 interval with the aliphatic bands centered at 2953, 2923, and 2856 cm−1. This is the first study that applies DRFTIR compositional parameters to characterize kerogen type II from marine carbonates.

Tectonic terranes underlying the present-day Caribbean plate: their tectonic origin, sedimentary thickness, subsidence histories and regional controls on hydrocarbon resources

Abstract Thick sedimentary cover (≤16 km), vintage seismic and disparate crustal terranes have hindered understanding of the basement underlying the Caribbean plate. The plate formed by Early Cretaceous to Miocene amalgamation of four crustal types: the Caribbean Large Igneous Province oceanic plateau; the Chortis continental block; the related Great Arc of the Caribbean and Siuna/Mesquito Composite Oceanic Terranes island arc blocks; and the Colombian and Venezuelan basin oceanic crust. We characterize each terrane through interpretation of surface geology, 62 000-line-km of 2D seismic reflection data, 366 seismic refraction stations, 47 wells, 74 basement samples, 2D forward modelling, magnetic and gravity anomaly grids, and integration of previous studies. Basins overlying island arc crust are small, fault-bounded and deep, while on continental crust, they are broader and shallower. Strongly flexed oceanic and oceanic plateau crust along amagmatic subduction zones on the southern and northeastern edges of the Caribbean plate produce the largest and deepest sediment-filled basins. Areas of proven hydrocarbon source rocks and mapped seeps are associated with continental and island arc terranes in the western Caribbean plate, while organically-rich, but immature, Late Cretaceous source rocks occur across the more elevated areas of the central and eastern Caribbean plate interior.

A river-dominated to tide-dominated delta transition: A depositional system case study in the Orinoco heavy oil belt, Eastern Venezuelan Basin

The Orinoco heavy oil belt (OHOB), which has the most significant heavy oil reserve in the world, has shown various sedimentary patterns and different evolution processes within its different blocks. We selected two blocks, Junin in the west and Carabobo in the east, to analyze the depositional environment distinctions due to different transgression processes. Based on the interpretation of 3D seismic data and electrical well-logging facies and analysis of depositional phenomena using cores, we clarified diverse depositional associations and constructed a sedimentary evolution model of the depositional system in the study area. We suggest that the depositional system transitioned from a braided river delta to a tide-dominated delta in the Early Miocene and that two factors mainly controlled the transition: paleo-flow and paleo-geomorphology, which are related to tectonic uplift and transgression processes. Transgression increased gradually through tectonic activity, though transgression rates varied area to area, resulting in different depositional environment transitions and sand-body distribution patterns. In the Carabobo block (east), a gentle slope developed, indicating a fast transgression rate. The intense interaction between river and tide flow led to a large inter-distributary channel area with high-sinuosity- or tidal-channels (relative to the Junin block). In the Junin block (west), tectonic activity developed a large accommodation area providing space for the sediment produced, slowing the transgression rate, and contributing to a mainly fluvial delta environment. Due to the gradual intensive transgression, channel stack patterns transitioned from braided-to meandering-channel in the river dominated delta or tidal-channel in the tide-dominated delta.

Gradual Evolution from Fluvial Dominated to Tide Dominated Deltas and Channel Type Transformation: A Case Study of MPE3 Block in the Orinoco Heavy Oil Belt of the Eastern Venezuelan Basin

Based on the data of core description and sporopollen analysis, the gradual evolution of deltas in vertical direction and transition of channel types in the MPE3 block of the eastern Venezuela Basin have been surveyed by seismic phase and well logging facies interpretation. The results show that due to the great sea level rise, the sedimentary system of the Miocene Oficina Formation in the MPE3 block shifted from the distal‐source sandy braided river delta to tide‐affected delta, and eventually to tide‐dominated delta. Vertically, during the early stage of sedimentation of Oficina Formation, the distributary channels of the delta were dominated by braided river channels. While in the later stage, as the tidal effect was gradually intensified, the channel changed from braided channel to meandering channel. On plane, as a result of differential transgression, sedimentary framework and distribution of sand bodies vary across the study area. Compared with the eastern part, the western part has more braided channels, larger channel bars, less developments of distributary bay and higher ratio of sand to mud. Whereas the braided channels in the south are larger than those in the north. It is the first time we pointed out the impact of marine transgression differences on the sedimentary facies distribution and river type transition in the study area. Factors like the structural and paleogeomorphological change, sea level variation, supply of sediments have strong influence on the evolution of sedimentary system and distribution of sandbodies. It is predicted that the major sandbody is more developed in the central south, which can guide the subsequent horizontal well development.

Deep-burial dissolution in an Oligocene-Miocene giant carbonate reservoir (Perla Limestone), Gulf of Venezuela Basin: Implications on microporosity development

Abstract Sedimentary rocks tend to progressively lose porosity with depth due to mechanical and chemical compaction. In carbonates, this trend is hard to predict since many factors can create porosity in the burial setting. The Oligocene-Miocene Perla Limestone, a giant gas reservoir located in the Gulf of Venezuela Basin, shows a complex porosity system marked by a strong diagenetic control. Despite exhaustive depositional facies modelling carried out in the carbonate reservoir, inconsistencies remained when distributing petrophysical properties with depositional facies. These inconsistencies become more important in areas strongly affected by intense diagenetic processes. After the initial investigation, burial dissolution was identified as the main diagenetic process affecting porosity in this succession. To understand the origin of the burial dissolution process and its distribution along the reservoir, detailed petrographic, mineralogical and isotopic studies were carried out on recovered cores from different exploration wells. Results show a series of diagenetic features which were associated with the evolution of the strata including burial dissolution, dolomitization and compaction as the main processes. After testing several hypotheses, the results support an inorganic-CO2 model as responsible for the burial dissolution process in the Perla Limestone. According to this model, deep-burial dissolution created pervasive microporosity, with subordinated moldic and vuggy pores that enhanced the reservoir quality in localized areas of the carbonate succession. These features are located where the ascent of hydrothermal CO2-rich fluids funneled by discontinuity surfaces (faults, fractures, stylolites, etc.) reached the Perla Limestone.

Mineralogía y distribución de arcillas en las formaciones productoras Naricual y Oficina de la Cuenca Oriental de Venezuela

Las formaciones Naricual (Oligoceno Tardío-Mioceno Temprano) y Oficina (Mioceno) pertenecientes a la Cuenca Oriental de Venezuela, son de importancia económica por la cantidad de arenas productoras de hidrocarburos que contienen. Sin embargo, ha tenido lugar una disminución acelerada de la producción en los campos de esta cuenca, ocasionada por varios factores, entre ellos los asociados con la migración de finos en los intervalos productores y a los daños de formación generados por la presencia de diferentes minerales de arcilla. El propósito de este trabajo es determinar la composición y distribución de las fases mineralógicas presentes en las arenas productoras de ambas formaciones, con el fin de establecer el potencial del daño que pueden generar los minerales de arcilla presentes en el flujo de fluidos durante la producción de petróleo. La caracterización mineralógica fue llevada a cabo mediante análisis petrográfico, Difracción de Rayos X (XRD), ensayos de disolución en ácidos y Microscopia Electrónica de Barrido (SEM). La mineralogía de las muestras está gobernada por cuarzo, albita, calcita, dolomita, pirita y muscovita. La arcilla predominante es caolinita, aunque también se detecta en algunas muestras clorita, illita/esmectita y glauconita; el porcentaje de arcillas oscila entre 6 y 12 %, situándose el promedio en 6,8% para las areniscas de Naricual, y 7,2% para Oficina. Los resultados obtenidos constituyen un insumo fundamental al momento de orientar planes de acción orientados al aumento de la producción.

The spatial discrepancy of Miocene transgression and its corresponding channel transformation pattern: A case study of the carabobo region in the northeast Venezuela basin

The Eastern Venezuela Basin serves as a world-class petroliferous basin, with the sedimentary environment controlled by interactions between the Caribbean and South American plates. This complex tectonic movement could have an important impact on the unsynchronized sedimentary evolution process and heterogeneous marine transgression of different regions within the basin. Thus, it is important to clarify the impact of interactions between the fluviatic- and marine-deposits on the development of the sedimentary environment, the transition of different delta types, and sand-body distribution within this complicated tectonic movement. Based on interpretation of 3D seismic data, electrical well-logging facies and analysis of the sedimentary characteristics of the core, we were able to identify the different sedimentary association types and understand the evolutionary process of the sedimentary system within the study area. We found that the differential sedimentary process in the eastern and western Carabobo block in the Eastern Venezuela Basin was caused by differential uplift and the corresponding spatial discrepancy transgression process in the Middle and Early Miocene foreland basins in this region. The flat geomorphology in the eastern Carabobo block contributed to the obvious sea level transgression process from North to South, but the uplift in the center of the western Carabobo block continuously blocked the sea level transgression. The different transgression processes led to the different sedimentary evolution processes in the eastern and western Carabobo blocks. In the eastern Carabobo block, braided river deltas (23.3–20.43 Ma) gradually changed into meandering river deltas (20.43–15.97 Ma) and finally into tide-influenced estuaries (15.97 Ma). However, in the western subsiding Carabobo block, impacted by the uplift in the center of this area, braided river deltas (23.3–15.97 Ma) in front of the uplift (adjacent to the sea) quickly changed into tide-influenced estuaries (15.97 Ma) with the generation of meandering distributary channels. However, the relatively weak transgression impacting the area behind the uplift resulted in braided channels being retained. Along with the evolution of the sedimentary environment, the tide-related deltaic distributary channel began to change from a braided channel to a meandering one. After rebuilding the paleo-hydrodynamic field, ancient geomorphology, and combining the hydrodynamic equations, we were able to quantitatively explain why sausage-shaped mouth bars occurred in the West and isolated mouth bars occurred in the East. Differential sedimentary environment evolution in eastern and western Carabobo is related to the spatial discrepancy of transgression, which resulted in the rate of change in hydrodynamic intensity, the direction of water flow confluence, and paleo-geomorphology.

Ambient Noise Tomography Images Accreted Terranes and Igneous Provinces in Hispaniola and Puerto Rico

AbstractThe North American‐Caribbean plate boundary near the islands of Hispaniola and Puerto Rico is a complex region in which strain is accommodated by different modes of deformation. In 2013, 16 broadband stations were deployed in the Dominican Republic to densify the local permanent network. One goal is to combine their data with those from permanent networks in the Caribbean to develop a better understanding of the crust and upper mantle structure in the Greater Antilles. Here we use the records of 204 stations spanning almost 7 years of data to obtain group velocity maps. We observe at short periods the contacts between accreted terranes and the igneous provinces of Hispaniola and Puerto Rico, at longer periods the collisional boundary between the buoyant Bahamas Carbonate Platform and Hispaniola, and the transition from Hispaniola's accreted terranes to the faster lithosphere of the Venezuelan Basin and Cayman Trough.

Sedimentology, ichnology, and sequence stratigraphy of the Miocene Oficina Formation, Junín and Boyacá areas, Orinoco Oil Belt, Eastern Venezuela Basin

Abstract The Miocene Oficina Formation is widely distributed in the subsurface of the Orinoco Oil Belt of eastern Venezuela. Despite its economic importance as one of the most important hydrocarbon reservoirs worldwide, little is known about its sedimentologic, ichnologic and sequence-stratigraphic aspects. Sedimentary facies, trace fossils, depositional environments, stratal stacking patterns and surfaces of sequence-stratigraphic significance have been analyzed in the Junin and Boyaca areas. The Oficina Formation is subdivided here into three informal members: lower, middle, and upper. The lower member consists mainly of lowstand fluvial braided-channel deposits filling an incised valley. No evidence of marine conditions is present and the only biogenic structures detected are root trace fossils present at the top of a few fining- and thinning-upward channelized units. The middle member is much more heterogeneous, both vertically and laterally, comprising high-sinuosity estuarine-channel, tidal-flat, tidal-creek and tidal-sandbar deposits, stacked to form a retrogradational package and representing the transgressive systems tract. The middle member is characterized by widespread evidence of tidal influence (e.g. inclined heterolithic stratification) and by the presence of the impoverished Cruziana Ichnofacies and the Skolithos Ichnofacies, both reflecting brackish-water conditions. This member is thought to represent deposition in an estuarine system. The upper member consists predominantly of highstand delta-plain deposits, stacked forming a progradational pattern. These deposits locally show intense bioturbation with a predominance of continental trace fossils, illustrating the Scoyenia Ichnofacies. Deltaic deposits display a mixed influence of tides and river processes. Previous studies invariably assumed a deltaic model for the whole Oficina Formation. However, integration of sedimentologic, ichnologic, and sequence-stratigraphic datasets suggests a more complex depositional evolution, comprising a fluvio-estuarine valley incised during a relative sea-level fall that became filled during the lowstand and subsequent transgression, culminating with deltaic progradation during the highstand.

Structural features and oil-and-gas bearing of the Caribbean region

The structure of the Caribbean region testifies to the extremely unstable condition of the terrestrial crust of this intercontinental and simultaneously interoceanic area. In the recent geological epoch, the Caribbean region is represented by a series of structural elements, the main of which are the Venezuelan and Colombian deep-sea suboceanic depressions, the Nicaraguan Rise, and the Greater and Lesser Antilles bordering the Caribbean Sea in the north and east. There are 63 sedimentary basins in the entire Caribbean region. However, only the Venezuelan and Colombian basins, the Miskito Basin in Nicaragua, and the northern and eastern shelves of the Antilles, Paria Bay, Barbodos-Tobago, and Grenada basins are promising in terms of oil-and-gas bearig. In the Colombian Basin, the southwestern part, located in the rift zone of the Gulf of Uraba, is the most promising. In the Venezuelan Basin, possible oil-and-gas-bearing basins showing little promise are assumed to be in the northern and eastern margins. The main potential of the eastern Caribbean region is attributed to the southern margin, at the shelf zone of which are the Tokuyo-Bonaire, Tuy-Cariaco, Margarita, Paria Bay, Barbados–Tobago, and Grenada oil-and-gas-bearing basins. The rest of the deepwater depressions of the Caribbean Sea show little promise for hydrocarbon research due to the small thickness of the deposits, their flat bedding, and probably a lack of fluid seals.