geochemical modelling of the principal source rocks of the barinas and maracaibo basins western venezuela this study presents a geochemical modelling of the principal source rocks in the western venezuelan basins the area covers more than 100 000 km sup 2 and includes lake maracaibo and barinas basins the geochemical modelling recognizes three source rocks 1 a principal k3 k4 k5 cretaceous sequences represented by la luna capacho and navay formations 2 a secondary corresponding to the t4 oligocene sequence represent by the carbonera formation and 3 an accessory source rock k7 k8 paleocene sequences represented by the carbonaceous shales and coals of the orocue group and marcelina formation three periods of hydrocarbon expulsion were defined for la luna formation early eocene late eocene middle miocene early miocene and early miocene holocene and a principal period of hydrocarbon expulsion for orocue group and carbonera formation plio pleistocene and middle miocene plio pleistocene the 90 of hydrocarbons generated correspond to the principal source rock la luna formation and the 10 to tertiary source rocks carbonera formation and orocue group five petroleum systems were identified lake maracaibo southwest of the lake maracaibo basin the lara nappes the extensive basins of eastern zulia and the barinas subbasin

early and middle miocene depositional history of the maracaibo basin western venezuela the uplift of the sierra de perij and mrida andes and the marine connection with the falcn basin ultimately controlled the distribution of shallow marine coastal and nonmarine sedimentary rocks in the maracaibo basin during the early and middle miocene these rocks contain the most important shale top seal in the basin and nearly three quarters of the produced plus proven reserves of the supergiant bolvar coastal fields the maracaibo basin has been isolated from extrabasinal drainage systems since the late oligocene and sediments derived from the surrounding highlands were either deposited in the basin or delivered into the neighboring falcn basin through a narrow marine passage the westward extension of the falcn channel four unconformity bounded sequences mapped in the northeastern sector of the maracaibo basin help recreate its regional paleogeography as it was flooded from the northeast through this passage in the early miocene part of the basin became a semi enclosed shallow marine gulf and wave and tide modified deltas prograded across the temporarily inactive lama icotea fault system as sea level dropped the shoreline advanced eastward of the falcn channel and valleys were incised and subsequently filled by transgressive estuarine sediments in the next sea level highstand tidal bar complexes of a tide dominated delta system prograded and filled all available accommodation space in the middle miocene relative sea level dropped into the falcn basin and the maracaibo area became a mixed load fluvial drainage basin by late middle miocene the two basins were separated and the maracaibo basin became an intermontane fluvial lacustrine depression

an analytical solution for tidal fluctuations in unconfined aquifers with a vertical beach 1 the perturbation technique has been commonly used to develop analytical solutions for simulating the dynamic response of tidal fluctuations in unconfined aquifers however the solutions obtained from the perturbation method might result in poor accuracy for the case of the perturbation parameter being not small enough in this paper we develop a new analytical model for describing the water table fluctuations in unconfined aquifers based on laplace and fourier transforms in the new approach the mean sea level is used as the initial condition and a free surface equation neglecting the second order slope terms as the upper boundary condition numerical results show that the present solution agrees well with the finite different model with the second order surface terms unlike teo et al s 2003 approximation which restricts on the case of shallow aquifers the present model can be applied to most of the tidal aquifers except for the very shallow one in addition a large time solution in terms of sine function is provided and examined graphically with four different tidal periods

a geochemical appraisal of oil seeps from the east coast basin new zealand abstract oil seeps and stains from the east coast basin new zealand have been investigated using biomarker and stable carbon isotope analyses to determine oil oil correlations oils sampled from the raukumara peninsula northern east coast basin north island and marlborough southern east coast basin south island are derived from late cretaceous paleocene marine source rocks with a minor terrestrial content and are isotopically light in contrast oils sampled from hawke s bay and wairarapa central and southern east coast basin north island are derived from paleocene marine source rocks which contain high abundances of c30 regular steranes and 28 30 bisnorhopane and are isotopically heavier than the other group biomarkers and bulk carbon isotopes show that there are at least two distinct sources of hydrocarbons in the basin

an experimental study of crude oil alteration in reservoir rocks by water washing abstract the objective of this research was to establish the effect of water washing on the physical and chemical properties of crude oils to simulate water washing under reservoir conditions crude oil was washed in sandstone and limestone cores by as much as 240 liters of a degassed bacteria free water over a period of up to 149 days at 90 c 194 f under a confining pressure of 138 bars 2000 psi in a flow through core holder unit the water washed oils were recovered by soxhlet extraction compared to the original oils water washed oils show lower api gravities lower concentrations of light aromatic compounds such as phenanthrene and dibenzothiophene lower metal and sulfur content lower light paraffins with respect to heavy paraffins and isoprenoids more negative stable carbon isotopic ratios for the nso fraction lower diterpane content relative to triterpane content lower c 27 steranes relative to c 30 hopane and 4 methyl sterane and altered bicyclic alkane and aromatic steroid biomarkers oils washed in limestone cores show a more severe reduction in api gravity and loss of aromatics than those in sandstone cores these changes can be used to discern the effects of water washing from those caused by biodegradation deasphalting and maturation of related crude oils diterpane triterpane and sterane biomarkers are little affected by water washing and therefore can be used to correlate water washed oils with related unaltered oils and source rocks and to assess the depositional environments of the source rocks for the water washed oils although these experiments did not produce tarmat tarmat formation by water washing over geological time cannot be ruled out the api gravity sulfur and metal content of oils may be reduced by water injection

relationship between rank and composition of aromatic hydrocarbons for coals of different origins distributions of di and tricyclic aromatics were determined for upper carboniferous to oligocene coals of various origins in addition to upper carboniferous ruhr coals wealden lower creataceous coals from north west germany lower cretaceous coals from western canada and tertiary coals from japan and colombia were studied the coals ranged from sub bituminous to anthracite in rank and showed vitrinite reflectance from 0 4 to 4 9 rm the previously established relationship between the rank of coal in terms of vitrinite reflectance and the distribution patterns of individual aromatic hydrocarbon isomers has been verified and refined it is demonstrated how the above relationships can be used to calculate vitrinite reflectance from the methylphenanthrene index mpi 1 the chemical rank levels of coals can now be defined in terms of the calculated vitrinite reflectance rc

predicted groundwater circulation in fractured and unfractured anisotropic porous media driven by nuclear fuel waste heat generation the concept of nuclear fuel waste disposal underground is drawing increasing attention due to its many advantages against the current storage methods at surface in this paper we employ the galerkin finite element technique to solve the coupled time dependent heat transfer and fluid flow differential equations and to predict the evolving behaviour of groundwater flow and subsurface temperature distribution associated with a proposed disposal system at the whiteshell research area in southeastern manitoba a two dimensional 2 d numerical model is conceptualized from geological constraints in this particular area to investigate the free convection of groundwater flow driven by nuclear fuel waste heat generation we assume the 2 d model has a flat upper boundary so as to eliminate the effect of topography head buoyancy force due to fluid density variations is therefore the sole driving mechanism for fluid migration case studies for both unfractured and fractured porous media confirm that thermal decay of the buried fuel waste can initiate fluid circulation in the presence of discrete fractures the deep and hot fluid nearby the disposed contaminant can discharge to the biosphere thus potentially threatening human health and the natural environment

laboratory scale investigation of saltwater intrusion dynamics 1 experiments were completed in a laboratory scale porous media tank to study the transport patterns of a saltwater wedge in a freshwater aquifer three types of experiments were performed to develop 1 steady state salt wedge data observed under different hydraulic gradient conditions 2 transient salt wedge data observed under intruding wedge conditions and 3 transient salt wedge data observed under receding wedge conditions furthermore flux measurements were made to quantify the flow characteristics of three distinct steady state experiments the saltwater intrusion model seawat was used to simulate these data sets the model results along with the experimental data are presented as benchmark problems for testing density coupled groundwater flow models a worthiness analysis was completed to test the sensitivity of these experimental problems to density coupling effects the results of our analysis show that the proposed benchmark is a more robust alternative to the traditional henry problem these new experimental data sets can be used to assess the performance of saltwater intrusion models under both steady state and transient conditions

25 norhopanes formation during biodegradation of petroleum in the subsurface abstract quantitative data from petroleum systems in china liaohe basin and canada athabasca tar sands support the theory that 25 norhopanes are produced during biodegradation of petroleum in the subsurface within a single oil column both case histories showed increasing severity of degradation indicated by destruction of hopanes and production of 25 norhopanes downward to the oil water contact in the athabasca samples between the 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 scale of biodegradation levels 5 9 concentrations of c 28 20s triaromatic steroids and other biodegradation resistant compounds increased by 35 reflecting a concentration effect as a consequence of removal of more degradable compounds over the same interval the concentrations of c 28 17 25 norhopane and c 29 17 25 norhopane increased by an order of magnitude thus requiring that the balance be met by their net production during degradation a detailed molecular investigation of the athabasca bitumen revealed that c 30 17 hopane degrades faster than c 29 17 hopane whilst the rate of formation of both c 29 17 25 norhopane and c 28 17 25 norhopane are similar complicating a straightforward interpretation of demethylation of hopanes to form 25 norhopanes hopane degradation in the athabasca tar sand may also occur without the production of 25 norhopanes the results show that even within a single petroleum accumulation a number of mechanisms control changes in the abundance and composition of hopanes and 25 norhopanes

paleoreconstruction by biological markers abstract during diagenesis and conversion of the original lipid fraction of biological systems to petroleum hydrocarbons the following four basic events needed for paleoreconstruction can be monitored by biological markets 1 sourcing 2 maturation 3 migration and 4 biodegradation actual cases of applying biological markers to petroleum exploration problems in different parts of the world are demonstrated cretaceous and phosphoria sourced oils in the wyoming thrust belt can clearly be distinguished from one another by high quality source fingerprinting of biomarker terpanes using gas chromatography mass spectrometry identification of recently discovered biological markers head to head isoprenoids allows subtle source differentiation between oils from sumatra the degree of crude oil maturation in basins from california alaska russia wyoming and louisiana can be assessed by specific novel biomarker ratios 20s 20r sterane epimers field evidence from such interpretation is confirmed by laboratory pyrolysis of the rock an example of immature sumatra rock is given extensive migration is documented by biomarkers in several oils the effect of biodegradation on biomarker steranes and terpanes is exemlified in a california basin all biological marker results are consistent with the geological setting and add a new dimension in assisting the petroleum explorationist toward paleo reconstruction

effects of kinetic mass transfer and transient flow conditions on widening mixing zones in coastal aquifers 1 the width of a mixing zone between freshwater and seawater is important primarily because it directly reflects the extent of mixing and the growth and decay of the mixing zone indicates changes of the flow regime and water exchange between freshwater and coastal seawater wide mixing zones have been found in many coastal aquifers all over the world however the mechanisms responsible for wide mixing zones are not well understood this work examines the hypothesis that kinetic mass transfer coupled with transient conditions which create the movement of the mixing zone may widen mixing zones in coastal aquifers the hypothesis is tested by conducting two dimensional numerical simulations based on a variable density groundwater model for a scaled tank model and a field scale model periodic water levels representing periodic tidal motion and freshwater table fluctuations are imposed at the seaward and landward boundary respectively which cause the movement of the mixing zone both the scaled tank model and the field model show that the combination of the moving mixing zone and kinetic mass transfer may significantly enhance the extent of mixing and create a wider mixing zone than the models without kinetic mass transfer in addition sensitivity analyses indicate that a larger capacity ratio immobile porosity mobile porosity of mass transfer leads to a wider mixing zone and the maximum width of the mixing zone may be reached for a given capacity ratio when the mean retention time scale in the immobile domain the reciprocal of mass transfer rate and the period of water level fluctuations are comparable

convection in groundwater below an evaporating salt lake 1 onset of instability convective groundwater motion can be generated in aquifers beneath and adjacent to saline lakes such motions strongly control the subsurface distribution of salts the formation of saline minerals and the distribution of contaminants that may be disposed of in the saline lake in particular schemes to use saline lakes as evaporation basins for irrigation waste waters must consider the groundwater dynamics associated with convection the convection is driven by the evaporative concentration of salts at the land surface leading to an unstable distribution of density however the evaporative groundwater discharge can dynamically stabilize this saline boundary layer and inhibit convection in this work we investigate the nature and onset of convection for a range of boundary conditions typically found in saline lakes processes involved in the accumulation of salt at the surface of a groundwater discharge zone and in the gravitational instability of the near surface groundwater are considered results of theoretical stability analysis are applied through laboratory and numerical experimentation to the more complex geometries typically found in the field these comparisons indicate that for large saline lakes the stability of the saline boundary layer can be parameterized by traditional rayleigh criteria in which the aquifer permeability and the evaporation rate from the lake bed are principal controlling factors for typical saline lake environments convection will dominate in sediments whose permeability exceeds approximately 10 14 m2 below this threshold permeability the boundary layer should be stabilized by the evaporative flux resulting in the accumulation of salts and evaporites at the land surface these results provide insight into the predictive behavior of groundwater dynamics and solute distributions in many natural systems

the influence of facies and depositional environment on the occurrence and distribution of carbazoles and benzocarbazoles in crude oils a case study from the gulf of suez egypt abstract two major hinge zones separate the gulf of suez gos of egypt into three tectonic sub provinces namely the northern central and southern sectors based on bulk and molecular parameters the crude oils from the gos can be subdivided into three groups according to their geographical origin from the three sectors the geochemical characteristics of the oils are consistent with inferred depositional environments of anoxic marine carbonate facies in the central sector compared to a more oxic environment that has received an increased proportion of clastic input in the southern sector while the northern is intermediate in character most of the oils cover a relatively narrow maturity range 0 86 0 98 r c in the peak oil window the biomarker parameter ts ts tm is controlled by variations of facies and depositional environment but not by maturity concentrations of pyrrolic nitrogen compounds in the gos crude oils are in general relatively low but show a large range of variability in crude oils of the northern sector a strong predominance of c 4 and c 5 carbazoles over the lower homologues is observed towards the south the relative abundance of carbazole and the methylcarbazoles increases successively variations within the methylcarbazoles and c 2 carbazoles are rather small and therefore are not useful for a straightforward classification of oil families however atypical patterns in some of the oils particularly from the northern sector give evidence for contributions from yet unidentified local sources which might not be of general importance in the gos petroleum province benzo b carbazole is near or even below detection limit in the gos crude oils the benzocarbazole a a c ratio shows a consistently positive correlation with api gravity pr ph and ts ts tm and a negative correlation with the sulphur content based on these observations it is inferred that facies and depositional environment of the relevant source rocks are the major factors influencing the benzocarbazole distributions in the central sector of the gos in which oils are predominantly sourced from carbonates deposited under strongly reducing conditions a a c values are mainly below 0 5 in the southern sector where source rocks were deposited under less reducing conditions and received significant clastic input as well as in the northern sector the a a c values are mainly above 0 5

experimental saltwater intrusion in coastal aquifers using automated image analysis applications to homogeneous aquifers summary this paper presents the applications of a novel methodology to quantify saltwater intrusion parameters in laboratory scale experiments the methodology uses an automated image analysis procedure minimising manual inputs and the subsequent systematic errors that can be introduced this allowed the quantification of the width of the mixing zone which is difficult to measure in experimental methods that are based on visual observations glass beads of different grain sizes were tested for both steady state and transient conditions the transient results showed good correlation between experimental and numerical intrusion rates the experimental intrusion rates revealed that the saltwater wedge reached a steady state condition sooner while receding than advancing the hydrodynamics of the experimental mixing zone exhibited similar traits a greater increase in the width of the mixing zone was observed in the receding saltwater wedge which indicates faster fluid velocities and higher dispersion the angle of intrusion analysis revealed the formation of a volume of diluted saltwater at the toe position when the saltwater wedge is prompted to recede in addition results of different physical repeats of the experiment produced an average coefficient of variation less than 0 18 of the measured toe length and width of the mixing zone

generation and migration of hydrocarbons in the maracaibo basin venezuela an integrated basin study abstract the history of hydrocarbon generation and migration in the maracaibo basin venezuela is closely related to its stratigraphic structural and tectonic evolution during the cretaceous and cenozoic times geochemical evaluation of the possible source rocks within the cretaceous and the tertiary sequences indicates that the argillaceous limestones and calcareous shales of the la luna formation cenomanian coniacian are the most important source rocks in the basin these rocks contain organic carbon ranging from 1 5 to 9 6 c original toc 2 5 10 8 have fluorescent amorphous marine type ii organic matter oil prone and have in the immature stage a h index of 700 mg hc g toc and a h c ratio of 1 5 the beginning of the oil window in these rocks occurs at a relatively low thermal maturity lopatin tti about 10 and the peak of oil generation corresponds to a tti value of 35 geochemistry of oils from the different reservoirs cretaceous paleocene eocene and miocene beds and oil oil and oil source rock correlation studies based on biomarkers have identified three genetic types of oil in the basin a marine type essentially derived from the la luna source a terrestrial type possibly originating either from the cretaceous lisure shales or the paleocene shales and a third type derived from the mixing of the first two types the marine type oil is distributed extensively throughout the basin whereas the terrestrial as well as the mixed marine and terrestrial types are found only in the southwestern part of the basin the marine type oils have been further subdivided based on their alteration and maturity reconstruction of the hydrocarbon generating areas at different geological times using tti maturity diagrams indicates that the generation of oil and gas in the basin took place during two periods the first phase occured during the middle to upper eocene times in the northeastern part of the basin while the second phase occured during the upper miocene to recent times in the central western and southern parts of the basin based on organic geochemical and sedimentological characteristics it is proposed that primary migration of oil from the la luna source rocks took place by expulsion of hydrocarbons through abundant microfractures developed by the pore fluid overpressure during hydrocarbon generation mass balance considerations suggest that the expulsion probably occurred as a hydrocarbon single phase and the expulsion efficiency for oil in la luna source rocks should have been very high 30 50 taking into consideration the development of oil and gas kitchens through time the paleostructures at the time of hydrocarbon generation and the characteristics of oils the migration history of the oil and gas generated from the la luna source has been outlined mixing of oils derived at different times eocene and miocene recent from the la luna source of different areas appeared to have taken place in block iv block 11 lago lamar motatan and bolivar coastal fields

regional geologic and tectonic setting of the maracaibo supergiant basin western venezuela this special issue contains eight topical studies on the structure stratigraphy and petroleum system of the maracaibo basin a supergiant basin in western venezuela most of the work reported in this special issue is the product of thesis related research by master s and doctoral level students at the jackson school of geosciences of the university of texas at austin during a collaborative relationship with the venezuelan national oil company petrleos de venezuela s a that was initiated in the late 1980s this introductory article presents a regional overview of the tectonic setting and geology of the maracaibo basin with a cumulative oil production of more than 30 billion bbl since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl the maracaibo basin is the most prolific hydrocarbon basin in the western hemisphere unlike the more extensive gulf of mexico giant hydrocarbon provinces the relatively small size 50 000 km2 19 305 mi2 relative simplicity in its structure and stratigraphy and wealth of surface and subsurface data make the maracaibo basin an attractive target for basinwide synthesis the objective of this article is to present a regional compilation of two dimensional 2 d and three dimensional 3 d seismic data wells and outcrop data at a basinwide scale to reveal the basin s 3 d structure and stratigraphy moreover we show regional tectonic reconstructions regional geologic maps and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges the distribution of the reservoir rocks the reactivation of older fault trends and the timing of maturation for underlying source rocks many of these topics are discussed in greater detail in the other eight articles in this special issue

tectonics basin subsidence mechanisms and paleogeography of the caribbean south american plate boundary zone abstract using a mega regional dataset that includes over 20 000 km of on and offshore 2d seismic lines and 12 wells we illustrate three different stages of fault formation and basin evolution in the caribbean arc south american continent collisional zone transpressional deformation associated with oblique collision of the caribbean arc migrates diachronously over a distance of 1500 km from western venezuela in paleogene time 57 ma to a zone of active deformation in the eastern offshore trinidad area each diachronous stage of pre syn and post collisional basin formation is accompanied by distinct patterns of fault families we use subsidence histories from wells to link patterns of long term basinal subsidence to periods of activity of the fault families stage one of arc continent collision initial collision is characterized by overthrusting of the south and southeastward facing caribbean arc and forearc terranes onto the northward subducting mesozoic passive margin of northern south america northward flexure of the south american craton produces a foreland basin between the thrust front and the downward flexed continental crust that is initially filled by clastic sediments shed both from the colliding arc and cratonic areas to the south as the collision extends eastward towards trinidad this same process continues with progressively younger foreland basins formed to the east on the overthrusting caribbean arc and forearc terranes north south rifting adjacent to the collision zone initiates and is controlled by forward momentum of southward thrusting arc terranes combined with slab pull of the underlying and subducting north dipping south american slab uplift of fold thrust belts arc continent suture induces rerouting of large continental drainages parallel to the collisional zone and to the axis of the foreland basins stage two this late stage of arc continent collision is characterized by termination of deformation in one segment of the fold thrust belt as convergent deformation shifts eastward rebound of the collisional belt is produced as the north dipping subducted oceanic crust breaks off from the passive margin inducing inversion of preexisting normal faults as arc continent convergence reaches a maximum strain partitioning also begins to play an important role as oblique convergence continues accommodating deformation by the formation of parallel strike slip fault zones and backthrusting southward subduction of the caribbean plate beneath the south caribbean deformed belt as subsidence slows in the foreland basins sedimentation transitions from a marine underfilled basin to an overfilled continental basin offshore sedimentation is mostly marine sourced by the collided caribbean terranes localized islands and carbonate deposition stage three this final stage of arc continent collision is characterized by 1 complete slab breakoff of the northward dipping south american slab 2 east west extension of the caribbean arc as it elongates parallel to its strike forming oblique normal faults that produce deep rift and half grabens 3 continued strain partitioning strike slip faulting and folding the subsidence pattern in the caribbean basins is more complex than interpreted before showing a succession of extensional and inversion events the three tectonic stages closely control the structural styles and traps source rock distribution and stratigraphic traps for the abundant hydrocarbon resources of the on and offshore areas of venezuela and trinidad

an overview of the petroleum system of maracaibo basin the geologically complex maracaibo basin in northwestern venezuela is one of the most prolific hydrocarbon basins in the world having a basinal area of 50 000 km2 19 300 mi2 the basin has produced more than 30 billion bbl of oil with estimated recoverable oil reserves of more than 44 billion bbl the central elements of the petroleum system of the basin include 1 a world class source rock upper cretaceous la luna formation deposited on a shelf to slope environment under anoxic conditions and modified by intermittent oxygenated periods and tectonic events 2 high quality clastic reservoir rocks deposited in eocene and miocene fluviodeltaic settings 3 two main periods of rapid tectonic subsidence responsible for two pulses of voluminous hydrocarbon generation first during paleogene caribbean south american oblique plate collision and second during the neogene uplift of the sierra de perij mrida andes and 4 lateral and vertical migration of oil along strike slip normal and inverted faults as well as a regional unconformity of late eocene oligocene age the maturation migration and trapping of hydrocarbons were closely controlled by the tectonic evolution of the maracaibo basin during the paleogene the development of a foredeep along the northeastern margin of the basin and the strike slip reactivation of the rift related jurassic faults on the maracaibo platform controlled the early structural setting of the source and reservoir rocks hydrocarbons migrated updip from source rocks beneath the north northeastern margin of the basin along north south strike slip faults and into overlying eocene clastic reservoirs in the south central parts of the basin the second period of the maracaibo basin petroleum system developed during subaerial exposure of most of the maracaibo basin during oligocene miocene uplift of the adjacent sierra de perij and mrida andes uplift of mountain ranges surrounding the basin folded and depressed the interior of the basin to form the extensive maracaibo syncline because of the formation of the maracaibo syncline oil generation began in the central and southern parts of the synclinal basin and migrated northward hydrocarbons migrated up the flanks of the maracaibo syncline along reactivated strike slip faults and into miocene rocks adjacent to the uplifted mountain ranges escaping oil has formed numerous surface seeps along the edges of the maracaibo basin

deep structure of the merida andes and sierra de perija mountain fronts maracaibo basin venezuela the maracaibo basin is a 50 000 km2 19 305 mi2 topographic depression bounded to the east and south by the mrida andes of venezuela and to the west by the sierra de perij of venezuela and colombia both uplifted mountain blocks expose paleozoic basement rocks and mesozoic cenozoic carbonate and clastic rocks that were mainly folded and thrusted by regional shortening in the paleogene and late neogene using geologic maps seismic reflection data and wells from the steep mountain front areas we test different structural models of how late paleogene neogene convergence is accommodated along subsurface faults and folds at the mountain fronts seismic imaging of the deep 5 km 3 1 mi structure of both maracaibo mountain fronts shows basinward dipping monoclines with stratal dips ranging from 20 to overturned and an almost complete lack of faulting in the basin edge monocline or in adjacent horizontally bedded strata of the maracaibo basin seismic data reveal the presence of one or more triangle zones at depth along both the mrida andes and sierra de perij that exhibit characteristic thrust wedge geometries the lower part of the wedge is defined by imbricate thrust faults dipping beneath the mountain block and involving the paleozoic basement the upper part of the wedge is defined by a basinward dipping thrust associated with fault propagation folds at the surface and an overlain basin edge monocline the creation of the steep to overturned dips of the monocline is attributed to the effects of 6 10 km 3 7 6 2 mi of shortening along the lower zone of imbricated thrust faults this 6 10 km 3 7 6 2 mi of shortening calculated from triangle zones of both the sierra de prija and mrida andes is significantly less than regional estimates from 12 to 60 km 7 4 to 37 mi of shortening inferred by previous workers from regional balanced cross sections that assume low angle thrust type geometries we propose that a pop up style of deformation related to the inversion of jurassic rift features may be a more realistic interpretation of the convergent fault systems that have controlled uplift of the mrida andes and sierra de perij inversion of relatively steep basement normal faults at the edges of and within both ranges may explain lesser amounts of observed shortening

3d mapping hydrodynamics and modelling of the freshwater brine mixing zone in salt flats similar to the salar de atacama chile abstract salt flat brines are a major source of minerals and especially lithium moreover valuable wetlands with delicate ecologies are also commonly present at the margins of salt flats therefore the efficient and sustainable exploitation of the brines they contain requires detailed knowledge about the hydrogeology of the system a critical issue is the freshwater brine mixing zone which develops as a result of the mass balance between the recharged freshwater and the evaporating brine the complex processes occurring in salt flats require a three dimensional 3d approach to assess the mixing zone geometry in this study a 3d map of the mixing zone in a salt flat is presented using the salar de atacama as an example this mapping procedure is proposed as the basis of computationally efficient three dimensional numerical models provided that the hydraulic heads of freshwater and mixed waters are corrected based on their density variations to convert them into brine heads after this correction the locations of lagoons and wetlands that are characteristic of the marginal zones of the salt flats coincide with the regional minimum water brine heads the different morphologies of the mixing zone resulting from this 3d mapping have been interpreted using a two dimensional 2d flow and transport numerical model of an idealized cross section of the mixing zone the result of the model shows a slope of the mixing zone that is similar to that obtained by 3d mapping and lower than in previous models to explain this geometry the 2d model was used to evaluate the effects of heterogeneity in the mixing zone geometry the higher the permeability of the upper aquifer is the lower the slope and the shallower the mixing zone become this occurs because most of the freshwater lateral recharge flows through the upper aquifer due to its much higher transmissivity thus reducing the freshwater head the presence of a few meters of highly permeable materials in the upper part of these hydrogeological systems such as alluvial fans or karstified evaporites that are frequently associated with the salt flats is enough to greatly modify the geometry of the saline interface

a novel approach for characterizing the mixing zone of a saltwater wedge 1 the mixing zone between freshwater and seawater in a coastal aquifer is an important transition region that controls regional groundwater flow dynamics and reactive transport processes while experimental procedures have been developed to study the movement of salt wedges in aquifers so far no one has attempted to exclusively map the mixing zone associated with the wedge in a laboratory scale aquifer model we propose a colorimetric experimental method which employs the reaction between alkaline freshwater and acidic saltwater to map the mixing zone the experiment was completed in a two dimensional aquifer model to simulate a steady saltwater wedge we used the density dependent flow model sutra to simulate these experimental results and to estimate the mixing parameters i e dispersivity coefficients the results show that the proposed colorimetric technique is a useful experimental approach to visualize and quantify the level of mixing in a saltwater intrusion experiment

vertical average for modeling seawater intrusion 1 seawater intrusion in coastal aquifers is a 3 d phenomenon however 3 d regional aquifer models are often limited by insufficient geological and hydrological data the large horizontal to vertical scales ratio and by numerical constraints we present an effective formulation for modeling seawater intrusion that relies on a dimensional reduction of the original density dependent flow and transport problem we carry out a vertical integration of the 3 d problem and arrive at a coupled set of 2 d equations for the mean flux and salt concentration which are essentially identical to those of 2 d groundwater flow however two new terms emerge from the integration 1 darcy s law needs not only the buoyancy term reflecting aquifer bottom slope but also another one reflecting variability of aquifer thickness and 2 transport requires a new term reflecting vertical variations of groundwater flux which are essential for density dependent flow and we approximate by means of a fickian dispersion term the proposed equations are verified by direct steady state numerical simulations of confined aquifers the results show that the effective formulation correctly reflects the effective dynamics in the 3 d system

the petroleum system of the central perija fold belt western venezuela the timing of hydrocarbon migration relative to trap formation remains a factor in evaluating the exploration potential of the perija fold belt on the west flank of the lake maracaibo basin based on the data from this article there is evidence of structural traps being filled by two oil migration events at least four major events of deformation have been identified in the area from the late cretaceous to the holocene the first event is late cretaceous in age and characterized by compressional structures the second event is characterized by extensional features and occurred from the late cretaceous to the middle eocene the third and fourth events are characterized by compressional structures that developed during the late eocene and late miocene holocene respectively based on oil oil and oil source rock correlations and one dimensional 1 d basin modeling two phases of oil generation and migration are proposed as responsible for oil accumulation in the central perija fold belt the first phase occurred from a kitchen of late paleocene late miocene age located to the west southwest of the study area the second phase occurred during the middle miocene holocene from a kitchen located to the east of the study area the older kitchen is responsible for oil accumulations found in the upper cretaceous traps the younger kitchen is responsible for possible oil accumulations in traps of both cretaceous and tertiary reservoirs located in the eastern part of the study area

variable density flow in porous media steady state distributions of water potential and salt concentration in coastal aquifers are typically modelled by the henry problem which consists of a fully coupled system of flow and transport equations coupling arises from the dependence of water density on salt concentration the physical behaviour of the system is fully described by two dimensionless groups i the coupling parameter alpha which encapsulates the relative importance of buoyancy and viscous forces and ii the peclet number mbox textit pe which quantifies the relative importance of purely convective and dispersive transport mechanisms we provide a systematic analytical analysis of the henry problem for a full range of the peclet number for moderate mbox textit pe analytical solutions are obtained through perturbation expansions in alpha this allows us to elucidate the onset of density driven vertical flux components and the dependence of the local hydraulic head gradients on the coupling parameter the perturbation solution identifies the regions where salt concentration is most pronounced and relates their spatial extent to the development of a convection cell next we compare our solution to a solution of the pseudo coupled model wherein flow and transport are coupled only via the boundary conditions this enables us to isolate the effects caused by density dependent processes from those induced by external forcings boundary conditions for small mbox textit pe we develop a perturbation expansion around the exact solution corresponding to mbox textit pe 0 which sheds new light on the interpretation of processes observed in diffusion experiments with variable density flows in porous media the limiting case of infinite peclet numbers is solved exactly for the pseudo coupled model and compared to numerical simulations of the fully coupled problem for large mbox textit pe the proposed perturbation approach is applicable to a wide range of variable density flows in porous media including seawater intrusion into coastal aquifers and temperature or pressure driven density flows in deep aquifers

biodegradation scales applications and limitations abstract biodegradation schemes based on the sequential removal of hydrocarbons are commonly applied to indicate the level of degradation that a petroleum accumulation may have experienced but inconsistent patterns of degradation are typically seen if large data sets are examined and oil mixing greatly complicates any attempt at seeking a globally consistent biodegradation scheme in the athabasca oil sands province degradation of steranes diasteranes and hopanes can occur simultaneously along with the formation of 25 norhopanes highlighting the difficulties towards assigning a degradation level to reflect the degree of compositional alteration experienced by a heavy oil or bitumen in addition the aromatic hydrocarbons such as the methyl phenanthrene isomers show differential susceptibility to degradation whilst the 9 methylphenanthrene 9 mp may show selective resistance to degradation compared to 1 methylphenanthrene 1 mp we have also seen that 9 mp may be removed prior to 1 mp in some cases both 9 mp and 1 mp may be strongly removed in comparison with the 3 and 2 methylphenanthrenes the complex interplay of charge history reservoir environment e g presence absence of water legs and extent and mass transport mechanics of the basal biodegradation reaction zone are most likely to have led to the many and varied petroleum compositions that are a feature of the athabasca tar sands and many other degraded oil provinces

the ratios of dibenzothiophene to phenanthrene and pristane to phytane as indicators of depositional environment and lithology of petroleum source rocks abstract the ratio of dibenzothiophene to phenanthrene and the ratio of pristane to phytane when coupled together provide a novel and convenient way to infer crude oil source rock depositional environments and lithologies such knowledge can significantly assist in identifying the source formation s in a basin thereby providing valuable guidance for further exploration the ability to infer this information from analysis of a crude oil is especially valuable as frequently the earliest samples in a new area may be shows and or drill stem test samples from exploratory wells which are characteristically drilled on structural highs stratigraphically remote from the source formation s a cross plot of dibenzothiophene phenanthrene versus the pristane phytane ratios measured on seventy five crude oils from forty one known source rocks ranging in age from ordovician to miocene consistently classified the oils into the following environment lithology groups marine carbonate marine carbonate mixed and lacustrine sulfate rich lacustrine sulfate poor marine and lacustrine shale and fluvial deltaic carbonaceous shale and coal the dibenzothiophene phenanthrene ratio alone is an excellent indicator of source rock lithology with carbonates having ratios 1 and shales having ratios the dibenzothiophene to phenanthrene and the pristane to phytane ratios can also be used to classify source rock paleodepositional environments the classification scheme is based on the premise that these ratios reflect the different eh ph regimes resulting from the significant microbiological and chemical processes occurring during deposition and early diagenesis of sediments the dibenzothiophene phenanthrene ratio assesses the availability of reduced sulfur for incorporation into organic matter and the pristane phytane ratio assesses the redox conditions within the depositional environment interpretation of these ratios has been aided by quantitative biomarker analysis and by carbon isotope data for pristane and phytane obtained by gas chromatography isotope ratio mass spectrometry

metagabbros in the modum complex southern norway an important heat source for sveconorwegian metamorphism abstract the precambrian modum complex in southern norway is characterized by supracrustal rocks penetrated by numerous gabbro intrusions occurring as elongated bodies subparallel with the regional foliation these gabbroic bodies show a partial alteration to amphibolites the best preserved parts of the metagabbros show a subophitic texture with olivine and plagioclase as the major cumulus phases and clinopyroxene orthopyroxene and ilmenite as the major intercumulus phases subsequent alteration of these igneous phases has led to formation of coronas around olivine in contact with plagioclase and recrystallization of clinopyroxene the igneous phases indicate intrusion temperatures in the range of 1100 1000 c and pressures below 5 8 kbar formation of coronas and recrystallization of clinopyroxene occurred in the pressure temperature range of 7 10 kbar and 800 600 c a 1224 15 ma sm nd mineral isochron obtained from igneous phases indicates emplacement at the start of the sveconorwegian metamorphism a complex evolution is suggested for the sveconorwegian metamorphic event involving both magmatism and high grade metamorphism

the controls on the composition of biodegraded oils in the deep subsurface part 1 biodegradation rates in petroleum reservoirs abstract biodegradation rates in oilfields have been assessed conservatively using whole oil column minimum rate estimates diffusion controlled oil column compositional gradient modelling and mixed oil kinetic models biodegradation rate constants first order are around 10 6 10 7 yr 1 for hydrocarbons in the degradation zones these corresponding well with zero order field wide minimum rate estimates of about 10 8 kg hydrocarbons kg oil year for the whole oil column with biodegradation induction times of around 1 2 ma to perturb an entire oil column for light oil reservoirs and 10 20 ma for heavy oil reservoir degradation the results indicate that where we see continuous gradients in the oil columns degradation must have been occurring episodically for many millions of years to remove the n alkanes from an oil i e about 10 of an oil around ca 15 ma is needed for a heavy oil ca 5 ma for a light n sea oil the timescales of oilfield degradation and filling are thus very similar and consequently the degree of biodegradation will be substantially controlled by oilfield charge history assessment of mixed degraded non degraded oil occurrence provides an independent confirmation that these rates are realistic and that timescales of degradation and field charging are similar the maximum effective rate constant of degradation ultimately controlled by the limiting effect of diffusion of alkanes to the oil water contact owc ca 10 4 yr 1 for a 130 m thick oil column first order rate constant is well above the estimated rate constants indicating oil biodegradation rate is not limited by electron donor supply i e hydrocarbons but by supply of nutrients or oxidants this suggests that diffusive transport of nutrients and electron acceptors in the aquifer to the site of biodegradation may be adequate to maintain the low rate biosphere

origin and alteration of oils and oil seeps from the sinu san jacinto basin colombia abstract liquid hydrocarbons have been detected in the subsurface as well as in the surface in the sinu san jacinto basin northwestern colombia the origin of the oils has not been conclusively established especially in the southern part of the basin the most likely source rocks in the basin are the cienaga de oro fm of the oligocene early miocene and the cansona fm of the upper cretaceous in this study oil samples seeps and source rock extracts were analyzed by gc and gc ms and 13 c was determined to identify the source facies the sulphur content and gravity data were also considered two organic facies were identified one constituted by terrestrial organic matter deposited in siliciclastic sediments in marginal marine to deltaic environments and the other made up of marine organic matter deposited in marine costal shelf to pelagic environments the oils from the former organic facies have low sulphur contents whereas the oils from the latter facies have high sulphur levels correlation of the oil seeps from the former facies with the cienaga de oro fm has not been clearly established the oil seeps from the latter facies correlate well with the extracts from the source rocks of the cansona fm deposited along the fold belt of san jacinto east side the oil seeps are affected by moderate to severe biodegradation whereas the oil from the only oil producing well in the sinu basin floresanto 6 well has not undergone biodegradation

the organic geochemistry of the eocene oligocene black shales from the lunpola basin central tibet abstract this paper reports on the depositional paleoenvironment and the potential hydrocarbons of the eocene oligocene black shales from the dingqinghu and niubao formations in the lunpola basin central tibet nineteen samples from two outcrop profiles were analysed the contents of the total organic carbon toc and sulphur were measured other analyses included rock eval pyrolysis solvent extraction and gas chromatography mass spectrometer gc ms the results indicated that the shales from the dingqinghu and niubao formations are thermally immature the pyrolysis data show that the shales contain type i organic matter and that lacustrine algal are the main organic matter sources the low pristane to phytane ratios and the high gammacerane indices indicate that the shales were deposited in a reducing stratified and hypersaline palaeo lake which is consistent with the climate information provided by the development history of palaeo lakes from the eocene to the oligocene epochs

geodynamics of the northern andes subductions and intracontinental deformation colombia new regional seismological data acquired in colombia during 1993 to 1996 and tectonic field data from the eastern cordillera ec permit a reexamination of the complex geodynamics of northwestern south america the effect of the accretion of the baudo panama oceanic arc which began 12 myr ago is highlighted in connection with mountain building in the ec the istmina and ibague faults in the south and the santa marta bucaramanga fault to the northeast limit an e se moving continental wedge progressive indentation of the wedge is absorbed along reverse faults located in the foothills of the cordilleras northward of 5 n and transpressive deformation in the santander massif crustal seismicity in colombia is accurately correlated with active faults showing neotectonic morphological evidences intermediate seismicity allows to identify a n ne trending subduction segment beneath the ec which plunges toward the e se this subduction is interpreted as a remnant of the paleo caribbean plateau pcp as suggested by geological and tomographic profiles the pcp shows a low angle subduction northward of 5 2 n and is limited southward by a major e w transpressive shear zone normal oceanic subduction of the nazca plate np ends abruptly at the southern limit of the baudo range northward the np subducts beneath the choco block overlapping the southern part of the pcp cenozoic shortening in the ec estimated from a balanced section is 120 km stress analysis of fault slip data in the ec northward of 4 n indicates an e se orientation of 1 in agreement with the pcp subduction direction northward near bucaramanga two stress solutions were observed 1 a late andean n80 e compression and 2 an early andean nw se compression

a simulation of large scale groundwater flow and travel time in a fractured rock environment for waste disposal purposes two dimensional regional groundwater flow was simulated based on a conceptual model of low permeability crystalline rocks of the whiteshell research area wra in south eastern manitoba the conceptual model consists of fracture zones that strike in different directions and dip at various angles in the background rock mass the thickness and hydraulic properties of the fracture zones in the conceptual model were varied as were the fluid properties and the boundary conditions of the groundwater flow system the effects of these variations on the groundwater flow pattern and on the convective travel time along pathways from a hypothetical disposal vault at 500 m depth to discharge locations at the ground surface were evaluated the vault was located in the regional discharge area of the groundwater system a homogeneous conceptual model of the wra having only freshwater flow formed a groundwater flow pattern with a regional flow system local flow systems developed increasingly with the introduction of fracture zones 20 m and 3 m thick and depth dependent fluid density this indicates a reduction in groundwater residence time by fracture zones and fluid density flow pathways were analysed using both a stream function and a particle tracking technique the pathways and their lengths from the location of the vault to the surface varied spatially according to the flow patterns the minimum travel time along these pathways was less than 150 000 and greater than 4 000 000 years in models with and without fracture zones respectively indicating that the presence of fracture zones was the major controlling factor a precise knowledge and refinement of conceptual model parameters is necessary during site selection for waste disposal purposes copyright 2004 john wiley sons ltd

the fossil oil seep in mupe bay dorset a myth investigated abstract the story of the mupe bay boulder bed has assumed mythical proportions in the british oil industry it is supposed to demonstrate a seep which was active at the time of deposition of the lowermost wealden sandstones indicating that the lias source rocks offshore to the south had already entered the oil window at that time the boulder bed contains sandstone clasts which are apparently oil cemented in a sandstone matrix which shows lighter oil staining the boulders were obviously soft when deposited as they were deformed during deposition this study suggests that there is no difference in the maturity or source of the oils from the clasts and the host channel sand that there is no significant difference in the diagenetic sequence or in the authigenic mineral assemblage of the two sandstones the conclusion is that the clasts are not contemporaneous oil cemented boulders as previously supposed the only differences noted in this study between the clasts and their host sandstone is in their grain sizes and the degree of biodegradation of the oils this difference in biodegradation can logically be ascribed to the difference in permeability associated with the change in grain size and the resulting penetration of meteoric water the source of the oil in both of the sandstones can be correlated to the lias aromatic biological markers for thermal maturity resistant to biodegradation indicate a maturity equivalent to about 1 vitrinite reflectance or higher for both oils these conclusions remove the constraint of early wealden generation of oil previously imposed on burial history reconstructions for the timing of oil generation and migration in south dorset it is further proposed that osmington bran point lulworth cove mupe bay worbarrow bay area was the site of a large oilfield with reservoirs within the wealden and extending down to the bencliff grits and bridport sandstones of the same order of size as the wytch farm accumulation this oilfield was uplifted and breached during the erosion which followed tertiary compression

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

the organic geochemistry of oil seeps from the sierra de perija eastern foothills lake maracaibo basin venezuela the organic geochemistry of samples from 11 oil seeps was studied the samples were collected from the cachiri area carboniferous region of tule lake maracaibo basin venezuela associated with the tigre fault biomarkers hopanes steranes n alkanes acyclic isoprenoids and aromatic steroids were analyzed using gas chromatography mass spectrometry gc ms these hydrocarbon rich fluids have undergone biodegradation 2 6 on the peters and moldowan scale showing both the partial loss of n alkanes and the microbial degradation of isoprenoids and steranes these oil seeps were generated from a mature calcareous source rock that was deposited in a marine paleoenvironment under reducing conditions moreover these seeps are likely derived from the cretaceous la luna formation that reached a level of maturity near the peak of oil generation in the study area the nature of the studied oil seeps together with the oil generation models reported for this rock unit in the study area suggests that these oils are a mixture of an initially heavy altered oil and a second migrated light crude oil resulting from two generation pulses from the la luna formation evidence for the presence of light oil trapped in the study area should prompt re exploration in the northwestern coast of lake maracaibo in shallow reservoirs previously discarded because they usually demonstrated a lack of light oils

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge