clastic domains of sandstones in central eastern venezuela trinidad and barbados heavy mineral and tectonic constraints on provenance and palaeogeography current models for the tectonic evolution of northeastern south america invoke a palaeogene phase of inter american convergence followed by diachronous dextral oblique collision with the caribbean plate becoming strongly transcurrent in the late miocene heavy mineral analysis of cretaceous to pleistocene rocks from eastern venezuela barbados and trinidad allow us to define six primary clastic domains refine our palaeogeographic maps and relate them to distinct stages of tectonic development 1 cretaceous passive margin of northern south america 2 palaeogene clastics related to the dynamics of the proto caribbean inversion zone before collision with the caribbean plate 3 late eocene oligocene southward transgressive clastic sediments fringing the caribbean foredeep during initial collision 4 oligocene middle miocene axial fill of the caribbean foredeep 5 late eocene middle miocene northern proximal sedimentary fringe of the caribbean thrustfront and 6 late miocene recent deltaic sediments flowing parallel to the orogen during its post collisional mainly transcurrent stage domain 1 3 sediments are highly mature comprising primary guayana shield derived sediment or recycled sediment of shield origin eroded from regional palaeogene unconformities in trinidad palinspastic restoration of neogene deformation indicates that facies changes once interpreted as north to south are in fact west to east reflecting progradation from the maturin basin into central trinidad across the nw se trending bohordal marginal offset distorted by about 70 km of dextral shear through trinidad there is no mineralogical indication of a northern or northwestern erosional sediment source until oligocene onset of domain 4 sedimentation paleocene middle eocene rocks of the scotland formation sandstones in barbados do show an immature orogenic signature in contrast to venezuela trinidad domain 2 sediments this requires 1 at least a bathymetric difference if not a tectonic barrier between them and 2 that the barbados deep water depocentre was within turbidite transport distance of the early palaeogene orogenic source areas of western venezuela and or colombia domains 4 6 from late oligocene show a strong direct or recycled influence of caribbean orogen igneous and metamorphic terranes in addition to substantial input from the shield areas to the south the delay in the appearance of common caribbean detritus in the east relative to the paleocene and eocene appearance of caribbean influenced sands in the west reflects the diachronous eastward migration of caribbean foredeep subsidence and sedimentation as a response to eastward younging collision of the caribbean plate and the south american margin

integration of subsurface applications to develop a dynamic stochastic modeling workflow building stochastic models in a preproduction phase of field development is crucial for accurate horizontal well positioning in the sincor field orinoco heavy oil belt venezuela formed by highly permeable unconsolidated fluvial and deltaic sands of the oficina formation and containing low gravity oil average 8 3 api a multidisciplinary approach to drill the initial approximately 250 horizontal production wells has proven useful to field management geomodeling is used to evaluate well pattern development plans calculate full field production percentile profiles and evaluate uncertainties in water production well production potential and cluster performance data from vertical observation wells full well logging suite and deviated wells to investigate the stratigraphy away from the vertical well are used to characterize reservoir architecture three dimensional seismic data including seismic data inverted into acoustic impedance are used to construct structure maps and shale probability maps the integrated and interpreted information is used to position horizontal wells deterministically in a cluster type group of wells radiating out from a central point in a 3 2 1 6 km 2 1 mi production area development pattern and serves as input to stochastic reservoir models that are conditioned to vertical well observations and the shale probability maps ten realizations are used to estimate the uncertainty range of net to gross in the proposed horizontal well trajectories to ensure a stable model the average of these 10 realizations is used as a trend to generate one stochastic petrophysical model that is subsequently used for flow simulation based on the results and in combination with the confidence in the interpreted geological data for example distribution of areas with a net sand thickness above a minimum proposed horizontal wells are accepted or rejected

the lappajarvi meteorite crater finland petrography rb sr major and trace element geochemistry of the impact melt and basement rocks abstract impact melt lithologies of the 77 m y old finnish meteorite crater lappajarvi as well as the precambrian target rocks have been studied in detail to identify and characterize different impact melt types clast poor clast rich suevitic melt and to study their chemical major and trace elements and isotopic rb sr compositions in comparison to the composition of the target rocks the rb sr system of the whole melt body including the suevitic melt is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks chemical interactions exchange of alkali elements 87sr redistribution between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt exchange reactions between clasts and melt are determined by thermal activation but the degree of shock metamorphism in the clasts plays an important role too major and trace element distributions in impact melt and basement rocks indicate that the lappajarvi melt body chemically is extremely homogeneous even volatile elements such as zn and cu were not strongly fractionated comparison of the abundances of siderophile elements in the impact melt e g 118 177 ppm cr 195 340 ppm ni 6 12 ppb ir and calculated target rock mixture 79 mica schist 11 granite pegmatite 10 amphibolite e g 85 6 ppm cr 54 8 ppm ni 0 5 ppb ir revealed the chondritic nature c or h chondritic of the meteoritic projectile less than 2 of the meteorite can be detected in the coherent melt whereas the suevitic melt is uncontaminated by the projectile

strangways crater northern territory australia siderophile element enrichment and lithophile element fractionation the strangways crater northern territory australia 15 12 s 133 35 e has a central core about 10 km in diameter of shocked granitic gneiss and amphibolite and some remnants of a melt rock sheet surrounded by outer rings of quartzite and siltstone to a diameter of 20 25 km seven samples of melt rock six granitic melts one shale melt clast and four samples of country rock granitic gneiss amphibolite shale quartzite were analyzed by neutron activation analysis for sc cr fe co zn rb zr sb cs ba rare earth elements hf ta th and u the samples were analyzed instrumentally and for ni se pd ag cd re os ir and au they were analyzed radiochemically siderophile elements are significantly enriched in the granitic melt rocks relative to country rocks for example the ir enrichments range from 0 6 to 2 8 ppb the low ir ni ratio 0 16 relative to c1 chondrites excludes a chondritic impacting body and cr enrichment argues against impact by an iron meteorite the strangways crater may have been formed by the impact of an olivine rich achondrite and melt rocks appear to contain about 3 wt of projectile material the composition of the granitic melt rocks cannot be reproduced by any simple mixture of analyzed country rock types and chemical fractionation by selective shock melting appears to have taken place

identification of the projectile at the brent crater and further considerations of projectile types at terrestrial craters abstract impact melt samples from drill hole b1 59 at the 3 8 km diameter brent crater ontario have been analysed for siderophile trace elements indicative of meteoritic contamination samples from the basal melt zone at 823 857 m depth are enriched in ir os pd ni co cr and se over basement with the abundance pattern suggesting a chondritic projectile for brent from a ni cr correlation of 10 melt samples an l or ll chondrite is inferred the contribution of an ultramafic country rock alnoite in the melt is too small to significantly influence its ni cr ratio glass rich breccias from the allochthonous breccias filling the crater also contain a meteoritic component interelement ratios e g ni cr are however fractionated relative to the melt zone samples this as well as the low au content of all brent samples is probably a product of alteration additional data on impact melts from the 65 km diameter crater manicouagan still did not reveal a meteoritic component as also for the mistastin crater 28 km diameter where cr analyses set an upper limit of 1 of an achondritic projectile component in the melt irghizites tektite like glasses from the zhamanshin impact structure have been found to contain high ni and co concentrations and our data show that ir is also enriched it is however not possible to define the projectile type enrichment of an ivory coast tektite in ir is confirmed there are large differences in siderophile element concentrations among tektites with otherwise similar chemical composition there are now four known craters formed by chondrites clearwater east lapparjarvi wanapitei and brent with brent being the smallest of these for smaller craters the projectiles appear to be limited to iron or stony iron meteorites because of atmospheric destruction of relatively small stony meteorites it appears however that all major classes of meteorites are represented among the projectiles at terrestrial impact craters

sedimentological and ichnological analyses of the continental to marginal marine centenario formation cretaceous neuquen basin argentina reservoir implications abstract the upper valanginian lower aptian centenario formation is a significant producer of oil and gas in the neuquen basin western argentina this formation is located exclusively in the subsurface of the eastern and northeastern neuquen basin and is 450 1000 m thick the centenario formation laterally interfingers with the agrio formation previous studies addressing the paleogeographic history of the centenario formation are scarce and a comprehensive geological model has yet to be put forward the current study scrutinizes the centenario formation especially its lower member within the northeastern neuquen basin the study area includes the cerro hamaca oeste senal cerro bayo and volcan auca mahuida oilfields operated by yacimientos petroliferos fiscales ypf sedimentological and ichnological core data geophysical well logs and petrographic thin sections have been utilized to construct a geological model eleven sedimentary facies and three facies associations have been identified from the core dataset providing insights into the paleoenvironmental settings and their stresses on infaunal colonization basin margin deposits from the northeastern part of the study region were formed in continental environments comprising ephemeral fluvial channel complexes and floodplains and are ichnologically represented by rare skolithos and common rhizoliths the central part of the study area is interpreted as recording deposition in ephemeral lakes river dominated lake deltas and coastal lagoons and sabkhas and is represented by a combination of stressed expressions of both the skolithos and scoyenia ichnofacies river dominated storm influenced delta deposits are located towards the southwestern limit of the study area and are ichnologically represented by the skolithos and depauperate cruziana ichnofacies deltaic deposits gradually transition into the basinal facies of the agrio formation to the west overall sedimentologic characteristics suggest semi arid to arid climatic conditions during deposition

does ice drive early maastrichtian eustasy a large 30 40 m rapid 1 m y earliest maastrichtian sea level drop inferred from new jersey sequence stratigraphic records correlates with synchronous 18o increases in deep water benthic and low latitude surface dwelling planktonic foraminifera the coincidence of these events argues for the development of a moderate sized ice sheet during the early maastrichtian

influence of co2 on melting of model granulite facies assemblages a model for the genesis of charnockites it is shown that partial melting studies at crustal pressures in sio2 rich portions of the system kalsio4 mg2sio2 h2o co2 can be used in modeling the anatectic origin of charnockites the univariant reaction phlogopite sanidine quartz vapor enstatite liquid yields an sio2 rich melt granite analog at 3 kbar the vapor composition at the solidus is buffered to high h2o contents by virtue of the coexistence of phlogopite with its breakdown products with higher pressures 8 and 15 kbar the fluid phase is buffered to higher co2 contents and the melt composition becomes enriched in k2o and mgo charnockite analog melting relations are controlled through the expansion of the quartz liquidus field relative to the enstatite and sanidine fields with increasing pressure it is noted that partial melts generated at the base of the crust in the presence of co2 rich fluid will be of an alkaline nature and will crystallize enstatite at lower pressures co2 saturated melting of similar sio2 rich bulk compositions phlogopite absent through the reaction enstatite sanidine quartz co2 liquid occurs at temperatures above 1000 c to about 15 kbar

basin dimensions and morphology as controls on amplification of tidal motions the early miocene north hungarian bay following the late aquitanian sea level fall tide influenced deposition started in the north hungarian bay an embayment in the paratethys open to the north east the relatively narrow funnel shape of the bay supported amplification of tidal movements resulting in the generation of strong tidal currents the length and the depth of the north hungarian bay and the connecting seaway through east slovakia fell into the tidal amplification window and thus fulfilled the conditions needed for resonant amplification of semidiurnal m2 tides tide influenced deposits were formed at both sides of the north hungarian bay they reflect dominant currents in opposite directions and of different strengths at either side of the basin this difference was the result of bottom tide interactions cyclonic anticlockwise residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents producing an anticlockwise time and velocity asymmetrical current system the north hungarian bay and other examples show that amplification of tidal motions and formation of tide influenced deposits may occur if basin dimensions pass through the tidal amplification window this window represents ideal conditions for resonant or amphidromic amplification of tidal currents it determines an ideal length depth or width depth ratio relative to the wavelength of the astronomical tides thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions length depth and width of such basins

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

tectonic evolution of petroleum systems within the onshore llanos basin insights on the presence of orinoco heavy oil analogs in colombia and a comparison with other heavy oil provinces worldwide economic accumulations of hydrocarbons in the onshore llanos basin of colombia are characterized by a central zone casanare province with greater than 20 api gravity oils and a southern province with biodegraded less than 15 api gravity oils to date no conceptual model successfully explains this spatial zonation in this paper we employ multiple one dimensional time temperature models to map the kitchens for three different source rocks and compare maturity levels through the cenozoic with the presence or absence of reservoir seal overburden and traps in paleogeographic maps of the llanos basin we find that the llanos basin cenozoic petroleum migration and charge may have been governed by a sedimentary structural evolution tied to the adjacent orogenic belt in which 1 paleogene stratigraphic traps developed in the south as favored by a more segmented basement and potentially transpressional stresses 2 a subsequent neogene phase with more pervasive east dipping low displacement normal fault traps was discovered and 3 a final pliocene present day phase of contractional traps was found in the easternmost foothill areas when compared with the evolution of several potential kitchens we suggest that upper cretaceous rocks from the eastern cordillera are the primary hydrocarbon source in the zone of heavy biodegraded oils to the south whereas lower cretaceous and selected terrigenous upper cretaceous source rocks are largely responsible for the younger neogene contractional traps of the foothills this evolutionary pattern for the llanos basin favors the presence of smaller but numerous hydrocarbon accumulations rather than the broader zones of heavy oils as found in the orinoco belt of venezuela

the strangways impact structure northern territory australia geological setting and laser probe 40ar 39ar geochronology abstract the strangways impact structure occurs within the mcarthur sedimentary basin northern territory australia the target rocks are primarily quartzites and subordinate shale siltstone sequences of the mesoproterozoic roper group these are primarily underlain by granitoid gneiss which is exposed in a 9 11 km diameter central uplift exposure is relatively limited and the upper parts of the impact structure have been removed by erosion based on the extent of the disturbance of the regional geology the diameter of strangways is estimated to be 24 26 km it is thus a complex crater and probably a central peak basin stratigraphic constraints place the time of impact as post roper group 145 ma in order to better define the time of impact infrared laser spot fusion 40 ar 39 ar data have been obtained from two impact melt lithologies and a highly shocked breccia clast these yield a combined age of 646 42 ma biotite dated from the granitoid gneiss central uplift yields 1686 1237 ma with the younger age reflecting the effects of shock induced resetting and or hydrothermal alteration due to hot melt sheet water interaction following impact the new age of 646 42 ma for strangways places it firmly in the neoproterozoic rather than being ordovician or younger as previously proposed

bedding parallel fibrous veins beef and cone in cone worldwide occurrence and possible significance in terms of fluid overpressure hydrocarbon generation and mineralization abstract bedding parallel fibrous veins are common worldwide in sedimentary basins especially within strata of low permeability the term beef refers to bedding parallel veins of fibrous minerals where the fibres are mutually parallel and have formed quasi vertically more complex on a smaller scale are cone in cone structures yet these are also common within bedding parallel veins for both beef and cone in cone we have compiled a worldwide catalogue 157 localities typically the veins consist mainly of white gangue minerals for example calcite gypsum or quartz but may also contain accessory minerals of economic interest for example bitumen sulphides emerald pitchblende or gold fluid inclusions may contain oil or gas calcite beef 110 localities is common in organic rich shale of marine carbonate origin especially of 1 cambrian ordovician 2 devonian carboniferous 3 early jurassic or 4 cretaceous to palaeogene ages gypsum beef 30 localities is common in evaporitic or lacustrine strata of continental origin especially of triassic or neogene ages quartz beef 17 localities is common within meta turbidite sequences especially of ordovician or proterozoic ages because these modal ages seem to reflect climatic controls we infer that the fibre forming mineral species have not travelled far vertically the same conclusion holds for accessory minerals typical temperatures of formation are 1 up to 60 c for gypsum beef 2 70 c to 120 c for calcite beef and 3 200 c to 350 c for quartz beef hydrocarbon bearing calcite beef may be a good indicator of a petroleum system in which oil or gas migrate together with aqueous solutions we argue that beef and cone in cone layers result from tensile fracturing and vertical dilation coeval with fibre growth possible causes are either 1 force of crystallization or 2 seepage forces due to fluid overpressure for layers that form at depths of several km fluid overpressure is the more likely cause

sequence stratigraphic distribution of coaly rocks fundamental controls and paralic examples significant volumes of terrigenous organic matter can be preserved to form coals only when and where the overall increase in accommodation approximately equals the production rate of peat accommodation is a function of subsidence and base level for mires base level is very specifically the groundwater table in paralic settings the groundwater table is strongly controlled by sea level and the precipitation evaporation ratio peat accumulates over a range of rates but always with a definite maximum rate set by original organic productivity and space available below depositional base level groundwater table below a threshold accommodation rate nonzero no continuous peats accumulate due to falling or low groundwater table sedimentary bypass and extensive erosion by fluvial channels this is typical of upper highstand lowstand fan and basal lowstand wedge systems tracts higher accommodation rates provide relatively stable conditions with rising groundwater tables mires initiate and copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received september 7 1995 revised manuscript received november 25 1996 final acceptance may 5 1997 2exxon production research company 3120 buffalo speedway houston texas 77096 3exxon production research company 3120 buffalo speedway houston texas 77096 present address conoco inc p o box 2197 houston texas 77252 we benefited from the input and assistance of many people of special assistance were the teams involved in collaborative studies with esso australia and esso malaysia p moore m sloan j emmett b burns a partridge s creaney hanif hussein r hill r lovell and m feeley we also thank the rock springs team r beauboeuf p mclaughlin w devlin a carroll y y chen g grabowski jr k miskell gerhardt m farley r webster and j schwalbach group members d curry and j yeakel were always helpful we also enjoyed and profited from many discussions of these concepts with our colleagues outside exxon c diessel r boyd k shanley b zaitlin p mccabe m hendricks a cohen and m kirschbaum we thank the reviewers of company reports whose careful comments on several generations of this work improved it s creaney m feeley j van wagoner f wehr j yeakel and a young f weber and j zullig provided extensive management support k linke translated our sketches into the fine figures herein we value all the help

rare earth element distributions among coexisting granulite facies minerals scourian complex nw scotland the major rock forming mineral phases pyroxenes plagioclase garnet hornblende of a suite of granulite facies gneisses from the scourian complex nw scotland have been analyzed for their rare earth element ree content although host rock compositions range from felsic to ultramafic ree abundances and patterns for each mineral group show only limited variation the rees exhibit regular and consistent distribution patterns for each mineral which suggest together with major element and textural considerations that the observed distribution coefficients approach equilibrium total ree content follows the sequence hornblende clinopyroxene garnet plagioclase orthopyroxene and mass balance calculations show that even in the felsic gneisses 60 of the rees reside in the major rock forming minerals comparisons of both relative ree abundances and distribution coefficients with those in other rock types reveal a striking resemblance with patterns observed in mineral liquid pairs of dacitic rocks these similarities may have arisen during a partial melting episode in which granite granodiorite melts were generated and removed from the scourian complex leaving a residuum which is severely depleted in the incompatible elements including the rees

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

architecture and origin of an amalgamated fluvial sheet sand lower castlegate formation book cliffs utah abstract amalgamated fluvial sheet sandstones in the stratigraphic record are often interpreted as the deposits of braided rivers alluvial sequence stratigraphic models show these sheet sandstones overlying sequence boundaries and deposited during low rates of base level rise and fall since sequence stratigraphic models rely on the balance between creation destruction of accommodation and sediment supply is the sheet like nature of these units controlled by channel style by aggradation rates tied to base level changes or a combination of both the lower member of the castlegate formation in the book cliffs of eastern utah u s a was selected to answer these questions regarding the origin and architecture of amalgamated sheet sandstones the lower castlegate is 60 m thick and is underlain by a regional sequence boundary delineation of the alluvial architecture utilized a combination of outcrop photography measured sections and paleochannel dimension estimation at two outcrops in the type area architectural components within the outcrops were defined and include 1 basal thalweg fill units 2 inclined sandy or heterolithic barform deposits and 3 overbank siltstone and mudstone thalweg fill units represent deposition in the topographically lowest part of the channel and are preferentially preserved thalweg units exist as single units or as a composite of multiple thalweg units overlying thalweg units inclined barform deposits are in places preserved and genetically linked to an underlying thalweg fill unit bar accretion directions when combined with paleocurrent indicators show that lateral downstream and upstream accretion all occur in the lower castlegate overbank deposits are rarely preserved but where present they typically overlie bar deposits the architectural components stack to form a threefold cyclic hierarchy of 1 channel bar systems 2 12 m thick 2 channel belts 2 16 m thick and 3 stacked channel belt complexes 17 40 m thick complete channel bar systems are rarely preserved and occur locally as a succession of multiple stacked thalweg fill units the lack of barforms within a composite thalweg unit suggests that channel migration is the dominant process controlling preservation within channel bar systems possibly associated with a component of local avulsion channel belts are composed of stacked channel bar systems capped by a barform the barform is preserved due to a regional avulsion moving the active channel to another location within the floodplain isolating it from potential reworking stacked channel belt complexes are characterized by alternations of thalweg dominated and barform dominated channel belts the mechanism responsible for deposition at the stacked channel belt complex scale may be creation destruction of accommodation associated with base level changes paleochannel reconstructions show that channel flow depths using bar and underlying thalweg thickness were up to 7 6 m channel depth estimates utilizing a cross bed set thickness of 26 cm averaged 4 6 7 7 m these values were used to calculate channel widths which were estimated at 143 365 m up to a maximum of approximately 1 km regional paleoflow within the lower castlegate was dominantly southeast directed the architectural components paleochannel characteristics and paleocurrent data suggest the lower castlegate member was deposited in low to intermediate sinuosity rivers influenced by autocyclic processes at smaller scales and allocyclic processes at the larger outcrop scale

a river dominated to tide dominated delta transition a depositional system case study in the orinoco heavy oil belt eastern venezuelan basin abstract 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

sedimentology and depositional model for the early miocene oficina formation in the petrocedeno field orinoco heavy oil belt venezuela abstract the early miocene oficina formation in the petrocedeno 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 junin formerly zuata region of the orinoco heavy oil belt in petrocedeno 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

hydrocarbon systems of northeastern venezuela plate through molecular scale analysis of the genesis and evolution of the eastern venezuela basin abstract the prolific oil bearing basins of eastern venezuela developed through an unusual confluence of atlantic caribbean and pacific plate tectonic events mesozoic rifting and passive margin development created ideal conditions for the deposition of world class hydrocarbon source rocks in the cenozoic transpressive west to east movement of the caribbean plate along the northern margin of venezuela led to the maturation of those source rocks in several extended pulses directly attributable to regional tectonic events the combination of these elements with well developed structural and stratigraphic fairways resulted in remarkably efficient migration of large volumes of oil and gas which accumulated along the flanks of thick sedimentary depocenters at least four proven and potential hydrocarbon source rocks contribute to oil and gas accumulations cretaceous oil prone marine source rocks and miocene oil and gas prone paralic source rocks are well documented we used reservoired oils seeps organic rich rocks and fluid inclusions to identify probable jurassic hypersaline lacustrine and albian carbonate source rocks hydrocarbon maturation began during the early miocene in the present day serrania del interior as the caribbean plate moved eastward relative to south america large volumes of hydrocarbons expelled during this period were lost due to lack of effective traps and seals by the middle miocene however when source rocks from the more recent foredeeps began to mature reservoir migration pathways and topseal were in place rapid tectonically driven burial created the opportunity for unusually efficient migration and trapping of these later expelled hydrocarbons the generally eastward migration of broad depocenters across venezuela was supplemented by local tectonically induced subsidence these subsidence patterns and later migration resulted in the mixing of hydrocarbons from different source rocks and in a complex map pattern of variable oil quality that was further modified by biodegradation late gas migration water washing and subsequent burial the integration of plate tectonic reconstructions with the history of source rock deposition and maturation provides significant insights into the genesis evolution alteration and demise of eastern venezuela hydrocarbon systems we used this analysis to identify additional play potential associated with probable jurassic and albian hydrocarbon source rocks often overlooked in discussions of venezuela the results suggest that oils associated with likely jurassic source rocks originated in restricted rift controlled depressions lying at high angles to the eventual margins of the south atlantic and that albian oils are likely related to carbonate deposition along these margins post continental break up in terms of tectonic history the inferred mesozoic rift system is the eastern continuation of the espino graben whose remnant structures underlie both the serrania del interior and the gulf of paria where thick evaporite sections have been penetrated the pattern of basin structure and associated mesozoic deposition as depicted in the model has important implications for the mesozoic paleogeography of northern south america and africa cuba and the yucatan and associated new play potential

four phase tectonostratigraphic development of the southern caribbean abstract although the stratigraphy of northern south america and the southern caribbean has been studied in detail in many areas a clear understanding of the sequence of tectonic events and their relation to regional stratigraphy has not been forthcoming since northern south america collided with southern north america in the late palaeozoic we identify four discrete tectonic phases from the stratigraphy of the area that is now the southern caribbean these phases derived from a synthesis of regional stratigraphy were 1 panama arc collistion with colombia in the late miocene early pliocene which drove a complex triangular wedge bounded by strike slip faults northward into the caribbean 2 right lateral strike slip faulting associated with post eocene eastern motion of the caribbean ocean floor into the gap between the separating americas 3 island arc development and collision associated with the entry of the present caribbean ocean floor an area of buoyant ocean crust into the gap between the separating americas and 4 continental rifting and passive margin development in the late jurassic early cretaceous associated with the separation of north and south america approximately along the palaeozoic suture zone overprinting of younger structures on inherited structures from previous phases is responsible for the overall complexity of the region

perspectives on the sequence stratigraphy of continental strata this report is the result of a working group on continental sequence stratigraphy that was set up at the 1991 nuna conference in banff canada to date sequence stratigraphic concepts have been applied mainly to the marine realm but unconformity bounded units have long been recognized in nonmarine strata successful application of sequence stratigraphic concepts to continental strata requires careful consideration of controls on base level and sediment supply as with shallow marine environments relative sea level can be considered as the stratigraphic as well as the geomorphic base level for coastal nonmarine settings farther inland stratigraphic base level which determines accommodation space is more complex and takes various forms such as the graded profile for fluvial strata groundwater tables for some eolian strata and lake level for some intermontane sediments sediment supply is also generally a more complex variable for nonmarine environments than in the marine realm because of the proximity to the source area the influence of climate and tectonism on sediment supply can clearly be seen in many continental sediments the fact that the major controls of climate tectonism and eustasy are somewhat interdependent and that a change in one parameter will most likely be reflected in others is also more readily apparent in continental strata although in its infancy sequence stratigraphic concepts have been applied to a wide variety of continental settings in attempts to explain variations in facies architecture of particular interest are studies that have linked fluvial architecture on coastal plains to variations in relative sea level as indicated by coeval marine strata and studies of lacustrine environments that show marked variation in facies between highstand and lowstand deposits the application of sequence stratigraphy to continental strata will likely result in the development of better correlation techniques and models that better predict the location and nature of fluvial and eolian reservoirs