Venezuela Basin Research Articles

The experimental study on H2S generation during thermal recovery process for heavy oil from the Eastern Venezuela Basin

Hydrogen sulfide (H2S) is toxic, corrosive and environmentally damaging. It is not only found in oil and gas development, but is also often found in heavy oil exploitation. In this study, three heavy oils were selected from the Orinoco Heavy Oil Belt in the southern part of the Eastern Venezuela Basin. Thermal cracking experiments in gold sealed tubes were then conducted using the heavy oils. The objective of the experiment is to unravel the H2S generation mechanism and utility in establishing a development program for heavy oil thermal recovery. The results of the oil isothermal cracking experiments show that the H2S yield increases with the increasing cracking temperature and holding time at 150 °C and 250 °C. Carbon dioxide (CO2) is the main component in gaseous products and its concentration is more than 80% in our experiments. The yields of CO2, H2S and total hydrocarbon gas present similar varying trend that increases with increasing isothermal time. The sulfur contents in group compositions of the original oil from the CJS-48 well and that of the residual oils with different cracking time at 250 °C were then measured. The analytical results show that most sulfur (>75%) exists in aromatics both in original oil and in the residual oils cracked at 250 °C, not to mention, no sulfur was measured in saturates. Although the decrease of sulfur in aromatics with the increased cracking time is low, it has great significance to the H2S generation during thermal recovery of heavy oil for more than 75% sulfur existed in aromatics. The decrease of sulfur content in resin and asphaltene of cracking residues with increased cracking time indicates that the sulfur existed in resin and asphaltene has some contribution to H2S generation during the thermal recovery process of heavy oil.

Vanadium, nickel and sulfur in crude oils and source rocks and their relationship with biomarkers: Implications for the origin of crude oils in Venezuelan basins

This work presents a study of vanadium, nickel and sulfur concentrations and biomarkers in a suite of crude oils and source rocks from three Venezuelan basins (Maracaibo and Eastern basins and Barinas sub-basin). Crude oils range from unaltered to altered by biodegradation, and source rocks are characterized by having different kerogen types (Type II, III or IV) and maturity levels (early mature to post-mature). Vanadium, nickel and sulfur concentrations, V/Ni or V/(V+Ni) and saturate biomarkers were used to classify the oils and source rocks based on inferred paleo-redox environment, lithology and maturity of the source rock. Oils are classified into five groups based on V/Ni ratios; they appear to be related to variations in the paleo-redox environment (from suboxic-dysoxic to anoxic-suboxic) of source rocks with variable lithological composition and organic matter input, regardless of their maturity or biodegradation level. These five groups are also classified based on biomarkers related to maturity, organic matter type, paleo-redox environment and source rock lithology. In source rocks, vanadium, nickel and sulfur concentrations, together with V/Ni or V/(V+Ni) ratios, provide information about the paleo-redox environment and are related to lithology, regardless of their maturity. These results also indicate variable paleo-redox conditions during the sedimentation of Venezuelan source rocks. All this suggests that, while the main source rocks in Venezuelan basins are well established, there are still uncertainties regarding their lateral and vertical variations in organic and inorganic facies and paleo-redox conditions. The presence of other source rocks contributing to the accumulation of hydrocarbons in the Venezuela basins is also a possibility.

Curie point depth in the SW Caribbean using the radially averaged spectra of magnetic anomalies

We have estimated the Curie Point Depth (CPD) using the average radial power spectrum in a tectonically complex area located in the SW Caribbean basin. Data analyzed came from the World Digital Magnetic Anomaly Map, and three methods have been used to compare results and evaluate uncertainties: Centroid, Spectral Peak, and Forward Modeling. Results show a match along the three methods, suggesting that the CPD values in the area ranging between 6km and 50km. The results share the following characteristics: A) High values (>30km) are in continental regions; B) There is a trend of maximum CPD values along the SW-NE direction, starting from the Central Cordillera in Colombia to the Maracaibo Lake in Venezuela; C) There is a maximum CPD at the Sierra Nevada de Santa Marta (Colombia) as well as between Costa Rica - Nicaragua and Nicaragua - Honduras borders. The lowest CPD values (<20km) are associated with the coastal regions and offshore. We also tested results by estimating the geothermal gradient and comparing measured observations of the study area. Our results suggest at least five thermal terrains in the SW Caribbean Basin: A) The area that is comprising the Venezuela Basin, the Beata Ridge and the Colombia Basin up to longitude parallel to the Providencia Throat. B) The area that includes zones to the north of the Cocos Ridge and Panam Basin up to the trench. C) The orogenic region of the northern Andes and including areas of the Santa Marta Massif. D) The continental sector that encompasses Nicaragua, northern Costa Rica and eastern of Honduras. E) Corresponds to areas of the northern Venezuela and Colombia, NW of Colombia, the Panamanian territory and the transition zones between the Upper and Lower Nicaragua Rise.

Evidence for mixed and biodegraded crude oils in the Socororo field, Eastern Venezuela Basin

This work presents a geochemical study of crude oils from the Socororo field (Socororo Major Area) in the Maturín Sub-basin of the Eastern Venezuelan Basin. The V+Ni total concentrations and V/(V+Ni) ratios suggest the presence of two oil groups. These oils are heavy aromatic–asphaltic and according to their sulfur content, API gravity and hydrocarbon compositions, fall into the same two groups established from the V and Ni proxies. Although the analyzed crude oils show evidence of biodegradation, it was possible to determine, based on various biomarkers parameters, that the oils originated from marine organic matter in a marine carbonate or marine marl source rock deposited under anoxic to suboxic conditions. The oils were generated near the peak of the oil window. Distributions of n-alkanes, isoprenoids, hopanes and steranes combined with a prominent unresolved complex mixture (UCM) indicate that the oils are biodegraded. Moreover, the presence of pristane and phytane, along with alteration of hopanes and steranes and the presence of 25-norhopanes, reveals that these oils are mixtures showing different levels of biodegradation. Sulfur content, V and Ni concentration and saturate, aromatic, resin and asphaltene (SARA) composition provide additional evidence for oil mixing. Such mixed oils originate from several episodes of charging and biodegradation in the reservoirs. The first charge was severely biodegraded and many biomarkers susceptible to degradation (n-alkanes, acyclic isoprenoids, terpanes, methyl hopanes, methyl steranes and steranes) were extensively altered or completely removed. The second oil charge was subjected to a lower degree of biodegradation and only the n-alkanes show significant biodegradation. Both oil charges seem to come from the same source rock, which was deposited under variable redox conditions. Moreover, due to their similar maturity, the generation and migration of these oils to reservoirs likely occurred close together in time.

Integrated charge and seal assessment in the Monagas fold and thrust belt of Venezuela

Conventional basin and petroleum systems modeling uses the vertical backstripping approach to describe the structural evolution of a basin. In structurally complex regions, this is not sufficient. If lateral rock movement and faulting are inputs, the basin and petroleum systems modeling should be performed using structurally restored models. This requires a specific methodology to simulate rock stress, pore pressure, and compaction, followed by the modeling of the thermal history and the petroleum systems. We demonstrate the strength of this approach in a case study from the Monagas fold and thrust belt (Eastern Venezuela Basin). The different petroleum systems have been evaluated through geologic time within a pressure and temperature framework. Particular emphasis has been given to investigating structural dependencies of the petroleum systems such as the relationship between thrusting and hydrocarbon generation, dynamic structure-related migration pathways, and the general impact of deformation. We also focus on seal integrity through geologic time by using two independent methods: forward rock stress simulation and fault activity analysis. We describe the uncertainty that is introduced by replacing backstripped paleogeometry with structural restoration, and discuss decompaction adequacy. We have built two end-member scenarios using structural restoration, one assuming hydrostatic decompaction, and one neglecting it. We have quantified the impact through geologic time of both scenarios by analyzing important parameters such as rock matrix mass balance, source rock burial depth, temperature, and transformation ratio.

Lower to Middle Miocene Planktonic and Benthic Foraminifera from the Carapita Formation, Eastern Venezuela Basin and Cipero Formation, Southwestern Trinidad

We document the biostratigraphic ranges of 18 taxa of planktonic, 33 calcareous benthic and 11 agglutinated benthic foraminifera from the Lower to lower MiddleMiocene ones (N4/M1 to N9/M6) of three exploration wells in the Eastern Venezuela Basin and an outcrop section from Rio Oregano near the type locality of the Carapita Formation of eastern Venezuela and the correlative Cipero Formation of Trinidad. The generally superior preservation of material in the outcropping Cipero Formation has been used as a standard of reference thereby enhancing identification of specimens in the generally more poorly preserved material in the subsurface Carapita Formation of Venezuela. The biostratigraphic data provided here serve as the basis for a reconstruction of the geohistory of the Eastern Venezuelan Basin including determination of multiple unconformities (and duration of associated hiatuses) and paleobathymetric history of the basin in a companion paper (Sanchez et al. 2014).

Neogene history of the Carapita Formation, Eastern Venezuela Basin

Based on the stratigraphic distribution of planktonic microfossils (mostly planktonic foraminifera) and abundance patterns of benthic foraminifera we determine the temporal completeness of one land section and three wells through the Lower and Middle Miocene Carapita Formation of eastern Venezuela, compare the extent of the hiatuses in the sections and document changes in paleodepth at these localities during the early and Middle Miocene. We determine that changes in paleodepth are associated with hiatuses, but see no relationshipwith the global changes in sea-level inferred from deep sea isotope records. This strongly suggests that there was a strong tectonic forcing on stratigraphic architecture at upper and middle bathyal depths, as to be expected in a tectonically active area. However, similar stratigraphic patterns are also observed elsewhere, implying that a widespread tectonic structuring of the stratigraphic architecturemay have been operative. Recognition of hiatuses (not merely unconformities) as primary stratigraphic components will make possible an uninterrupted documentation of sequences boundaries from subaerial to bathyal environments, and help determine objectively the structural mechanism(s) operating on the genesis of stratigraphic sequences (sensu Catuneanu et al. 2009). Biostratigraphy (and biochronology) are the main tools to understanding this structuring.

Study of the biodegradation levels of oils from the Orinoco Oil Belt (Junin area) using different biodegradation scales

This work presents a study of the molecular composition of the saturated and aromatic hydrocarbon fractions of crude oils from the Orinoco Oil Belt (Junín area) in the Eastern Venezuelan Basin, with the purpose of classifying these samples following two distinct biodegradation assessment schemes: the PM scale [Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, New Jersey, p. 363] and the Manco scale [Larter, S., Huang, H., Adams, J., Bennett, B., Snowdon, L.R., 2012. A practical biodegradation scale for use in reservoir geochemical studies of biodegraded oils. Organic Geochemistry 45, 66–76]. Both scales agree on the presence of different levels of biodegradation for the analyzed oils, although they are based on different groups of compounds. The PM scale uses mainly compounds from the saturated hydrocarbon fractions (e.g., n-alkanes, acyclic isoprenoids, terpanes and steranes) as well as aromatic steroids. On the other hand, the Manco scale considers other compounds (e.g., alkyltoluenes, naphthalene, methylnaphthalene, phenanthrene, alkylphenanthrenes and methyldibenzothiophenes) not included in the PM biodegradation scale. Thus, the combined use of these two scales allows the determination of the level of biodegradation of both saturated and aromatic compound classes. Dibenzothiophene (DBT), which was not included for the Manco score determination, also shows variations in peak intensity when compared to C4-alkylnaphthalenes, presumably associated with the process of biological alteration. The differences in the biodegradation levels observed in the present study may be attributed to variations in parameters that control biodegradation rates laterally across the study area or the existence of varying communities of microorganisms, among other possible factors.

Curie point depth in Venezuela and the Eastern Caribbean

We estimate the Curie point depth (CPD) variations of Venezuela (continental crust, South American plate) and the Eastern Caribbean (oceanic crust, Caribbean Plate) by using spectral analysis of the magnetic anomalies, extracted from the 2010 Enhanced Magnetic Model (EMM2010), available at the National Oceanic and Atmospheric Administration (NOAA). To test the reliability of the spectral content of this model, for a small region, we compare he CPD derived from the EMM2010 against the one from aeromagnetic data. We also compile heat flow data from previous studies to correlate them with the CPD lateral variations. The estimations show that the CPD in Venezuela and the Eastern Caribbean ranges between 54 and 17km. The mean depth value within the continental crust is around 38km. On the Guayana Shield, it has a mean value of 42km and reaches a maximum of 54km. As the Moho depth is at most 50km, the upper mantle beneath the craton is magnetized. Continental lateral variations appear to be linked to the isostatic state and age of the different provinces, and mark the limit between the Precambrian and the Paleozoic provinces. The Maracaibo Basin is revealed as a thermally stable one with a constant CPD, while the Eastern Venezuela Basin is thermally affected. Most of the Eastern Caribbean seems stable, with a large non-perturbed area with a mean CPD value of 23km. As the crustal thickness is at most 20km, the isotherm is located within the upper mantle. A CPD minimum located on the Lesser Antilles arc is concentrated in its northern part, and can be related to the subduction zone that is most active. Finally, a shallow area within the Eastern Caribbean corresponds to the thin crust region in the Venezuela Basin, although it might be linked to mantle dynamics.

Lithospheric magnetic mapping of the northern Caribbean region

A more complete crustal perspective of the northernmost part of the Venezuela Basin is provided by data from an international public database processed with new geomagnetic models (CM4) together with data from aeromagnetic surveys and from a recent marine cruise in areas offshore Puerto Rico and the Dominican Republic. The magnetic anomaly map set off three main domains: the North Atlantic plate, a narrow zone extending from the Dominican Republic to Puerto Rico-Virgin Islands, and the North Caribbean Plate. We focused mainly on the latter, applying the Euler deconvolution algorithm in the Venezuela Basin. Shallow and middle depth sources (located between 2km and 12km) are inferred to be the dominant sources (85%), while deep sources are located mostly in the Venezuela Basin, and scarcely appear in the Beata and Aves Ridges. We performed 2D+1/2 modeling on an eastwest transect, using Bouguer gravity and magnetic anomaly data to derive a more detailed crustal description of the northern part of the Venezuela Basin. The model suggests that in the Caribbean crust, long wavelengths (>200km) are controlled by variations in crustal thickness, whereas the intermediate ones (50-100km) are mainly controlled by variations in their magnetic properties. It also suggests the presence of a local anomalous body located in the east-west transect, between 487km and 560km, displaying a remarkable positive magnetization contrast (0.3 SI) with regard to the rest of the basin. This body shows a slight increase in density (3010kg/m3), and extends from the bottom of the UCL (Underplate Cumulate Layer) to a depth of 7.5km. We correlate it with the volcanic wedge reported by Driscoll and Diebold (1998).

Case Studies Examine Exploitation Strategies for Development of Mature Gas and Oil Fields

This article, written by Editorial Manager Adam Wilson, contains highlights of paper OTC 23373, ’Exploitation Strategy in the Development of Mature Gas and Oil Fields: Case Study,’ by Gina Vega Riveros, Reservoir Consultant, and Emad Yehya, Smoothwell Oil Services, prepared for the 2012 Offshore Technology Conference, Houston, 30 April-3 May. The paper has not been peer reviewed. Copyright 2012 Offshore Technology Conference. Reproduced by permission. Two case studies examine the creation of strategies for the development of mature gas/condensate and oil fields. For the gas/condensate field, the integration of geologic, seismic, production, and reservoir data identified new opportunities for drilling. For the oilfield case study, it is clear that the strategy of exploitation can be improved. According to study results, the oil and gas condensate in place have been increased by 9% and 22%, respectively, compared with the official reserves estimates. Gas/Condensate Case Study Geologic Framework. The gas/condensate reservoirs are in the Roble, San Joaquin, and Guario fields, approximately 11 km southwest of Anaco, Venezuela. The geological model in the Guario 3 reservoir allowed for the integration of four reservoirs as a unique hydraulic unit (Fig. 1). Geologically, the study area is within the greater Oficina area, on the south flank of the Eastern Venezuela basin. The Merecure formation is Oligocene to early Miocene and was deposited in a transitional environment, yielding sandstones with good lateral extension. Petrophysics. In Guario 3, ME-G and ME-H sands have an average effective porosity of 14%. Permeability ranges from 20 md to more than 70 md. The water resistivity of 0.1 Ω·m, cementation exponent of 2.00, saturation exponent of 2.00, and shale volume cutoff of 40% were obtained from field data. Both ME-G and ME-H sands showed similar petrophysical properties. Reservoir Properties. The northwest-ern crest of the Guario 3 reservoir is defined as gas/condensate crude. The eastern part contains a volatile zone of medium gravity with a high concentration of paraffin. The integrated static and dynamic models validated the existence of an areal and vertical communication of four reservoirs into a unique reservoir named Guario 3 (G-3). Geochemical Study. To verify vertical and lateral reservoir communication between ME-G and ME-H sands, gas chromatographic correlations of samples from wells in different reservoirs were used to establish a clear vision about fluid communication. Correlations for Predicting Gas/ Condensate Reservoirs. Many empirical correlations have been reviewed in the literature, and they take into account retrograde condensate. The average ranges for applying the correlations and the equations used can be found in the complete paper. Using the correlations, the original gas condensate in place was 1226.84 billion scf, which was considered satisfactory in comparison with the volumetric method of 1267.37 billion scf.

Seasonal and Interannual Modulation of the Eddy Kinetic Energy in the Caribbean Sea

Abstract Variability of the mesoscale eddy field in the Caribbean Sea is analyzed over the period 1993–2009 using geostrophic anomalies derived from altimeter data and a high-resolution regional model. The Colombia Basin presents the largest values of eddy kinetic energy (EKE) and its semiannual cycle, with a main peak in August–October and a secondary peak in February–March, is the dominant feature in the whole Caribbean EKE cycle. The analysis of energy conversion terms between low-frequency currents and eddies explains these peaks by enhanced baroclinic and barotropic instabilities, in response to seasonally varying currents in the region of the Guajira Peninsula. The semiannual acceleration of the atmospheric Caribbean low-level jet intensifies the southern Caribbean Current (sCC) twice a year in this region, together with its vertical and horizontal velocity shears. The asymmetry of the EKE seasonal cycle in the Colombia Basin is explained by a summer peak in the annual cycle of the whole sCC. Numerical results suggest that the arrival of more energetic North Brazil Current rings during part of the year have almost no impact on the seasonal cycle of EKE in the Colombia Basin. Instead, they are shown to contribute, together with the annual cycle of the Caribbean inflow through the southern passages of the Lesser Antilles, to an annual peak of EKE in the Venezuela Basin between May and August. At the interannual scale the mechanism is similar: interannual variability of the alongshore wind stress controls the speed of the southern Caribbean Current and the energy of the eddies in the Colombia Basin through instability.

Magnetic anomalies in the Eastern Caribbean

Despite the many different studies on the origin and evolution of the Caribbean Plate, no proposal has been widely accepted so far. A key element within this field of research is the characterization of the plate subsurface oceanic crust, as it would clarify the conditions in which it originated, the geological period when it formed and its possible geographical location at this first evolution stage. Based on partial results of this research work, we can say that the conclusions of previous studies are valid to a great extent, namely the NE–SW orientation of the striped magnetic anomalies in the Venezuelan Basin’s western section and the existence of W–E preferentially oriented stripes parallel to the Leeward Antilles. The magnetic response of the triangular section in the southeast of the Venezuelan Basin represents cretaceous magnetic quiet zone (CMQZ) and therefore shows a considerable attenuation of the stripe pattern, indicating that the whole East Caribbean subsurface features oceanic crustal material. As for the period recorded by the Caribbean magnetic stripes, we propose the interval between chrons M23 and M0, and part of CMQZ. The wavelengths of the identified anomalies suggest that the ridge associated with the formation of Caribbean ocean floor was slow-spreading when compared to average currently active ridges.

River-gulf system—the major location of marine source rock formation

Petroleum was generated from sedimentary rocks. The world’s oldest oil source rock so far was found in Proterozoic rocks. Since then, 73% to 81% of the earth’s surface has been covered with sedimentary rocks. However, only quite a limited area is rich in oil and gas. It is found that source rocks have controlled oil and gas distribution, and they are mainly formed in two systems: (1) river-lake systems and (2) river-gulf systems. Phytoplankton is an important source of kerogen, the blooming of which is strongly dependent on nutrients. Rivers are the major nutrient provider for basins. Rivers around lakes and an undercompensation (where the sedimentation rate is less than the rate of basin subsidence) environment provide favorable conditions for phytoplankton blooming in lakes. Gulfs are usually located at the estuary of big rivers, characterized by restricted current circulation and exchange with the open sea, which benefit maintaining the nutrient density, phytoplankton levels and organic matter preservation. The river-gulf system is the most favorable place for marine source rock development. Most of the world famous marine petroleum-rich provinces are developed from river-gulf systems in geological history, such as the Persian Gulf Basin, Siberian Basin, Caspian Basin, North Sea, Sirte Basin, Nigerian Basin, Kwanza Basin, Gulf of Mexico, Maracaibo Basin and the Eastern Venezuelan Basin.

Potential petroleum source rocks in a tertiary sequence of the Eastern Venezuelan Basin

Abstract A core of a Tertiary age sequence from the Eastern Venezuelan Basin was analyzed in order to determine its potential for petroleum generation. Conventional geochemical methods, like Rock-Eval pyrolysis, biomarkers from saturated fractions and aromatic hydrocarbon ratios were used for assessing source-rock quality. The application of such methods indicated predominantly the presence of terrigenous organic matter with marine influence, deposited under suboxic to oxic conditions typical of continental environments. Thermal maturation in the range from beginning to mid oil window and organic matter type indicate that the sequence could have generated mainly gaseous hydrocarbons. Analysis by electron probe microanalyses (EPMA) indicates that sulfur is associated to both organic (bitumen and kerogen) and inorganic (mineral) phases and organic matter is observed filling fractures in the rocks.

Sedimentology and depositional model for the Early Miocene Oficina Formation in the Petrocedeño Field (Orinoco heavy-oil belt, Venezuela)

The Early Miocene Oficina Formation in the Petrocedeño Field (Venezuela) produces extra-heavy oil with an initial gravity of 8.5° API. Reservoir zonation is successfully based on a 3rd-order sequence stratigraphic framework for which a detailed sedimentological understanding is crucial. This paper presents a, primarily core-based, detailed account of the sedimentology and sequence stratigraphy of the Oficina Formation in the Junín (formerly Zuata) Region of the Orinoco heavy-oil belt. In Petrocedeño, the Oficina Formation is typified by the long-term (approximately 7 Ma; 2nd-order sequence) change of a fluvial dominated deltaic system to a tide-dominated lower delta plain and subsequently a tide-dominated, subtidal, upper delta platform. A fluvial braidplain environment with mostly sand-dominated braided and sinuous rivers (Sequences 1 and 2) changed into a fluvially-dominated but notably tidally-influenced delta plain with straight and sinuous channel belts (Sequences 3–6). Subsequently, a mixed-energy (fluvial and tidal) delta front environment with numerous distributaries formed. This delta front drowned and changed into an estuary system with bay-head delta mouth bars (Sequence 7). Maximum transgression was reached in Sequence 8 with the development of fully marine shale. Subsequently, a low-gradient tide-dominated lower delta plain with distributaries and meandering channel belts established (Sequences 9 and 10). These were gradually transgressed to develop into proximal delta platform environments with a limited number of distal distributaries and large protected subtidal areas (Sequence 11). Together, the lower delta plain and upper delta platform facies form one of the few currently known subsurface examples of a tide-dominated delta. The change in depositional style was controlled by a long-term increase of accommodation space in the developing Eastern Venezuelan Basin. The lower part of the formation was primarily controlled by compressional tectonic activity (third-order Sequences 1 through 6). The upper half, however, was primarily controlled by Early Miocene eustatic changes (Sequences 7 through 11).

Sedimentological and ichnological signatures of changes in wave, river and tidal influence along a Neogene tropical deltaic shoreline

AbstractAnalysis of Neogene cores from the Eastern Venezuela Basin along 65 km of a west–east trending shoreline allows characterization of the sedimentological and ichnological signatures of wave, river and tidal processes. The area displays deltas prograding northward from the Guyana Shield. Twenty‐three facies are defined and grouped into four categories (wave‐influenced, river‐influenced, tide‐influenced and basinal). Wave‐dominated deltaic deposits occur mostly in the Tácata Field. The delta plain was characterized by tide‐influenced distributary channels separated by interdistributary bays. Fluvial discharge in the delta front and prodelta was repeatedly interrupted by storm‐wave reworking and suspended sediment fallout. Delta‐front and prodelta deposits contain some ichnotaxa that typically do not occur in brackish water (for example, Chondrites and Phycosiphon). Amalgamated storm deposits are unburrowed or contain vertical Ophiomorpha. Lateral (especially on the updrift side) to the river mouths, waves caused nearly continuous accretion of the associated strandplains. These deposits are the most intensely bioturbated, and are dominated by the estenohaline echinoid‐generated ichnogenus Scolicia. River‐dominated deltaic deposits are present in the Santa Bárbara, Mulata, Carito and El Furrial Fields. Low‐sinuosity rivers characterized the alluvial plain, whereas the subaerial delta plain was occupied by higher‐sinuosity rivers. The subaqueous delta plain includes distributary channels and tide‐influenced interdistributary bays. Further seaward, successions are characterized by terminal distributary‐channel and distributary mouth‐bar deposits, as well as by delta‐front and prodelta deposits showing evidence of sediment gravity‐flow and fluid‐mud emplacement. Delta‐front and prodelta deposits are unbioturbated to sparsely bioturbated, suggesting extreme stress, mostly as a result of high fluvial discharge and generation of sediment gravity flows. Tidal influence is restricted to interdistributary bays, lagoons and some distributary channels. From an ichnological perspective, and in order of decreasing stress levels, four main depositional settings are identified: river‐dominated deltas, tide‐influenced delta plains, wave‐dominated deltas and wave‐dominated strandplain–offshore complexes.

Caribbean Plate margin evolution: constraints and current problems

Oceanic crust was generated at multiple spreading centres during the Jurassic and Early Cretaceous, forming a “proto-Caribbean” oceanic domain. During the Cretaceous, part of that crustal domain thickened into an oceanic plateau, of petrologic Mid-Ocean Ridge (MOR) to Ocean Island Basalt (OIB) affinity. Simultaneously, the South and North American continental plates developed rifting and tholeiitic magmatism in the Middle America region (Venezuela and Cuba). The rifting created space for the proto-Caribbean oceanic domain. Petrological and regional correlations suggest that, beginning in the Cretaceous, the proto-Caribbean domain was involved into two main stages of subduction, referred to as first and second “eo-Caribbean” phases. Each phase is characterized by oblique convergence. The older (mid-Cretaceous) stage, involved in subduction (probably eastward dipping) of thin proto-Caribbean lithosphere, with generation of Island Arc Tholeiitic (IAT) and Calc-Alkaline (CA) magmatism, accompanied by high pressure - low temperature (HP - LT) metamorphic effects, and formation of arc units and ophiolitic melanges (Guatemala, Cuba, Hispaniola and Puerto Rico, in the northern margin; Venezuela in the southern). The Late Cretaceous second stage consisted of westward dipping intra-oceanic subduction; it is recorded by tonalitic arc magmatism related to the onset of the Aves - Lesser Antilles arc system. Since the Late Cretaceous, the inner undeformed portions of the Caribbean oceanic plateau (i.e. the Colombian and Venezuelan Basins) were trapped east of the Pacific subduction of the Chortis, Chorotega and Choco blocks, ultimately building the Central American Isthmus. From Tertiary to Present, continuous eastward movement of the Caribbean Plate with respect to the Americas, gave rise to transpression along both the northern and southern margins, marked by scattered and dismembered ophiolitic terranes.

Late Cretaceous‐Miocene diachronous onset of back thrusting along the South Caribbean deformed belt and its importance for understanding processes of arc collision and crustal growth

The Venezuelan basin is obliquely subducted to the east‐southeast beneath the continental South American plate at the east‐west‐trending South Caribbean deformed belt (SCDB), a 50–100‐km‐wide wedge of accreted sedimentary rocks derived both from offscraping of the 0.5 to 1 km‐thick sedimentary cover of the subducting Venezuelan basin and from incorporation of clastic sediments derived from the continental margin of South America. In this paper we describe the structure and sequence stratigraphy of five Late Cretaceous to recent sedimentary sequences overlying a 230,000 km2 area of the southern Venezuelan basin and Beata Ridge. The data set includes 5900 km of 2D seismic reflection data that are tied to lithologic and age data taken from DSDP sites 146/149 and 150 in the central part of the Venezuelan basin. We use observed patterns of wedging in mapped and dated sedimentary sequences adjacent to the SCDB to determine the age of the diachronous west‐to‐east initiation of thrusting and subduction at the SCDB. The onset of subduction in the SCDB corresponds with the cessation of thrusting from west to east along the zone of collision between the intraoceanic Caribbean arc and the South American continent. This correlation in the age of deformation indicates that collision between the Caribbean arc and the South American continent led to subduction polarity reversal, a process that is commonly observed at various stages of tectonic evolution in other areas of active, arc‐continent collision including Japan and Sunda.

베네주엘라 초중질유 저류층 지질 특성

베네주엘라 중앙에 위치한 오리노코강을 따라 55,000 <TEX>$km^2$</TEX>의 면적에 동서로 길게 자리하는 오리노코 오일벨트에는 원시부존량이 약 1조 3천억 배럴, 가채매장량이 2,500억 배럴에 달하는 초중질유가 매장되어 있다. 베네주엘라 초중질유는 API 비중이 <TEX>$10^{\circ}$</TEX> 이하이고, 점성도가 5,000 cP 정도로 무겁고 점성이 큰 탄화수소 자원이다. 오리노코 지역의 초중질유는 1930년대 최초로 보고되었지만, 이들의 상업적 개발은 1990년대에 이르러 비가열식 일차생산기법을 통해서 본격적으로 이루어지기 시작하였다. 오리노코 오일벨트는 초중질유 분포 양상에 의해 보야카, 주닌, 아야쿠초, 카라보보의 생산광구로 나누어지며, 이들은 모두 31개의 생산블럭으로 소분류된다. 현재 각 생산블럭은 베네주엘라 PDVSA와 외국계 기업의 합작 형태로 개발되고 있으며, 20개국 이상이 초중질유 개발 프로젝트에 참여하고 있다. 오리노코 오일벨트는 베네주엘라의 주요 석유분지 가운데 하나인 동베네주엘라 분지의 남쪽지역에 위치한다. 동베네주엘라 분지는 쥬라기 판 분화에 의해 형성되기 시작한 수동형 대륙 주변부 분지로 그 면적은 약 120,000 <TEX>$km^2$</TEX>이다. 동베네주엘라 분지에서 백악기 말에 형성된 석회질 셰일은 초중질유의 주요 근원암이다. 분지 내 탄화수소는 북쪽에서 남쪽으로 평균 150 km를 이동하면서 생분해작용을 거쳤으며, 이로 인해 점성과 비중이 높은 초중질유를 분지 남쪽 경계부인 오리노코 지역에 형성하였다. 주요 초중질유 저류층인 마이오세 오피시나층은 하성-에스츄어리 퇴적환경에서 발달한 미고결 사질 및 이질이 교호하는 퇴적체이다. 또한 오피시나층은 판의 운동에 의한 압축작용과 분지침강에 의해 형성된 다수의 신생대 단층이 분포하여 복잡한 저류층 지질 특성을 나타낸다. 불균질한 저류층 암상 분포와 복잡한 지질 구조의 저류층에서 경제적인 생산정의 설계와 효율적인 초중질유 회수를 위해서는 초중질유 저류층의 발달 과정과 그로 인한 지질학적 특성에 대한 심도 깊은 이해가 필요하다. 본 연구에서는 오리노코 초중질유 저류층에서 (1) 사질 저류층 두께 및 분포, (2) 이질 퇴적층의 분포, (3) 단층의 기하학적 분포, (4) 저류층 대상 심도 및 지열 특성, (5) 저류층 지중 응력상태, (6) 초중질유의 화학적 조성 등을 초중질유 생산성에 상대적으로 큰 영향을 미치는 주요한 지질학적 특성으로 주목하였다. 이러한 오리노코 지역의 지질학적 특성들은 3차원 탄성파 탐사, 시추간 물리검층과 같은 최신 기술들을 통해 앞으로 보다 빠르고 정확하게 규명되어질 것으로 기대된다. Extra heavy oil reservoirs are distributed over the world but most of them is deposited in the northern part of the Orinoco River in Venezuela, in the area of 5,500 <TEX>$km^2$</TEX>, This region, which has been commonly called "the Orinoco Oil Belt", contains estimated 1.3 trillion barrels of original oil-in-place and 250 billion barrels of established reserves. The Venezuela extra heavy oil has an API gravity of less than 10 degree and in situ viscosity of 5,000 cP at reservoir condition. Although the presence of extra heavy oil in the Orinoco Oil Belt has been initially reported in the 1930's, the commercial development using in situ cold production started in the 1990's. The Orinoco heavy oil deposits are clustered into 4 development areas, Boyaco, Junin, Ayachoco, and Carabobo respectively, and they are subdivided into totally 31 production blocks. Nowadays, PDVSA (Petr<TEX>$\'{o}$</TEX>leos de Venzuela, S.A.) makes a development of each production block with the international oil companies from more than 20 countries forming a international joint-venture company. The Eastern Venezuela Basin, the Orinoco Oil Belt is included in, is one of the major oil-bearing sedimentary basins in Venezuela and is first formed as a passive margin basin by the Jurassic tectonic plate motion. The major source rock of heavy oil is the late Cretaceous calcareous shale in the central Eastern Venezuela Basin. Hydrocarbon materials migrated an average of 150 km up dip to the southern margin of the basin. During the migration, lighter fractions in the hydrocarbon were removed by biodegradation and the oil changed into heavy and/or extra heavy oil. Miocene Oficina Formation, the main extra heavy oil reservoir, is the unconsolidated sand and shale alternation formed in fluvial-estuarine environment and also has irregularly a large number of the Cenozoic faults induced by basin subsidence and tectonics. Because Oficina Formation has not only complex lithology distribution but also irregular geology structure, geological evolution and characteristics of the reservoirs have to be determined for economical production well design and effective oil recovery. This study introduces geological formation and evolution of the Venezuela extra heavy oil reservoirs and suggest their significant geological characteristics which are (1) thickness and geometry of reservoir pay sands, (2) continuity and thickness of mud beds, (3) geometry of faults, (4) depth and geothermal character of reservoir, (5) in-situ stress field of reservoir, and (6) chemical composition of extra heavy oil. Newly developed exploration techniques, such as 3-D seismic survey and LWD (logging while drilling), can be expected as powerful methods to recognize the geological reservoir characteristics in the Orinoco Oil Belt.