deposition diagenetic and hydrothermal processes in the aptian pre salt lacustrine carbonate reservoirs of the northern campos basin offshore brazil abstract the discovery of large oil accumulations in the rift and sag pre salt sections of the campos and santos basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the brazilian and african marginal basins more than half of brazilian oil production originates from the pre salt reservoirs of these offshore basins a study integrating systematic petrography cathodoluminescence scanning electron microscopy microprobe and x ray diffraction was performed on seven wells in the northern campos basin this study highlights the major primary diagenetic and hydrothermal features of the pre salt succession with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs the pre salt deposits correspond to bioclastic grainstones and rudstones syngenetic crusts of fascicular calcite and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites magnesian silicates are frequently associated with carbonate deposits in the sag phase stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites which displace and replace the syngenetic magnesian clay deposits in the rift section stevensitic ooids are mixed with bioclasts or form ooidal arenites pre salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution eodiagenetic processes include the dissolution neomorphism and cementation of bivalve bioclasts in the rift as well as the dissolution of magnesian silicates and their replacement by calcite spherulites silica and dolomite in the sag section burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems these fluids promote dolomitization silicification and dissolution at varying degrees and intensities eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation redistribution and obliteration of porosity in the pre salt reservoirs

the magmatic plumbing system of the askja central volcano iceland as imaged by seismic tomography the magmatic plumbing system beneath askja a volcano in the central icelandic highlands is imaged using local earthquake tomography we use a catalog of more than 1300 earthquakes widely distributed in location and depth to invert for the p wave velocity vp and the vp vs ratio extensive synthetic tests show that the minimum size of any velocity anomaly recovered by the model is 4 km in the upper crust depth 8 km below sea level bsl increasing to 10 km in the lower crust at a depth of 20 km bsl the plumbing system of askja is revealed as a series of high vp vs ratio bodies situated at discrete depths throughout the crust to depths of over 20 km we interpret these to be regions of the crust which currently store melt with melt fractions of 10 the lower crustal bodies are all seismically active suggesting that melt is being actively transported in these regions the main melt storage regions lie beneath askja volcano concentrated at depths of 5 km bsl with a smaller region at 9 km bsl their total volume is 100 km3 using the recorded waveforms we show that there is also likely to be a small highly attenuating magmatic body at a shallower depth of about 2 km bsl

co 2 degassing in the hartousov mofette area western eger rift imaged by co 2 mapping and geoelectrical and gravity surveys strong subcontinental mantle dominated co2 degassing occurs in the hartousov and bublak mofette fields in the western eger rift the combination of co2 gas flux and soil gas measurements as well as gravity and geoelectric surveys provides insight into the surface and subsurface of this unique mofette area co2 soil gas and gas flux measurements reveal that large amounts of carbon dioxide are released via channels with diameters below 1 m carbon dioxide emissions of several tens and up to more than 100 kg day 1 are ejected via these small seeps measurements with small spacings are necessary to account for the point like focused gas discharge in the lesser degassing surrounding we estimate that between 23 and 97 tons of co2 are released over an area of about 350 000 m2 each day in the hartousov mofette field the application of widely used geostatistical tools leads to estimations of the co2 discharge with very high standard deviations due to the strong positive skewness of the data distribution geophysical investigations via electrical resistivity tomography and gravity measurements were carried out over areas of strong seepage and reveal distinct anomalies in the subsurface below mofettes indicating rock and sediment alterations and or sediment transport by pressurised ascending co2 and water mobilised by it this study reveals that the gas emanations only occur west of a morphological step which is related to a n s oriented fault zone the pocatky plesna fault zone the results of co2 mapping and the geophysical studies can track the course of this fault zone in this area our results fit into a tectonic model in which the mofette fields are in the centres of two independent pull apart basin like structures we hypothesise that the sinistral strike slip movement of the pocatky plesna fault zone leads to a pull apart basin like opening at which the strong mantle derived co2 degassing occurs nowadays since the hartousov and bublak mofette fields only illustrate examples along the n s striking pocatky plesna fault zone its role and other n s striking faults roles of the regensburg leipzig rostock zone for upper mantle degassing might have been underestimated previously

using microearthquakes to track repeated magma intrusions beneath the eyjafjallajokull stratovolcano iceland 1 we have mapped microearthquakes caused by magma migration preceding and during the flank and summit eruptions in march may 2010 of eyjafjallajokull stratovolcano in iceland using a coalescence microseismic mapping technique spatial and temporal clustering of 5 000 microearthquakes under the eastern flank of the volcano illuminates several northeast southwest striking sub vertical dikes at 2 6 km b s l emplaced before the fimmvor uhals flank eruption in march this intense precursory seismicity had a lateral extent of 6 km east west and 3 km north south a sequence of 386 microearthquakes during the summit eruption refined by double difference relative relocation defines a sub linear trend inclined 5 10 from vertical extending from the upper mantle at 30 km depth to the summit crater this sequence includes two major clusters at 19 km and 24 km b s l each containing 100 earthquakes all microearthquakes display characteristics of brittle fracture with several subsets of events exhibiting closely similar waveforms within clusters this suggests similar repetitive source processes the deeper clusters may be caused by fracturing solidified magma plugs that form constrictions in an otherwise aseismic melt conduit or they may occur at exit points from melt pockets in which case they indicate positions of magma storage at depth the seismicity deeper than 10 km only starts three weeks after the onset of the summit eruption after which the largest clusters occur at progressively greater depths this temporal pattern may result from pressure release at shallow levels in the magmatic plumbing system progressively feeding down to mobilize deeper melt pockets

intense seismicity during the 2014 2015 barðarbunga holuhraun rifting event iceland reveals the nature of dike induced earthquakes and caldera collapse mechanisms

pb isotopes in surficial pelagic sediments from the north atlantic we measured pb isotopic composition and concentration in sediment samples close to the sea water interface in 6 box cores from the ne atlantic 2 box cores from the sargasso sea and one from the us continental shelf the anthropogenic pb input to marine sediments due to the increase of pb contamination over the ocean during the last century can be identified in all these cores in the eastern part of the atlantic i e in regions under aeolian influence from europe pb pollution can be recognized using its distinctive unradiogenic composition clearly different from the upper crustal values commonly found in pre holocene sediments in contrast pb pollution in regions influenced by north american sources can be identified only in detailed concentration profiles because the american pb pollution has an isotopic composition much closer to that of the natural detrital pb input coming from weathering of the continental crust pb excess inventories are in good agreement with fluxes estimated from sedimenttrap data and with the time record of pb contamination increase given by analyses in coral growth bands inventories of pb contamination to the sediments of the mud patch american shelf are tenfold higher 84 g cm2 than those to hatteras and bermuda abyssal plains 4 3 and 2 8 g cm2

thermal anomalies associated with forced and free ground water convection in the dead sea rift valley the dead sea rift valley is a left lateral transform along which several rhombshaped grabens were formed at the sea of galilee which is one of these rhomb shaped grabens ambiguous heat fluxes were measured 70 80 mw m 2 at the central part of the lake 36 mw m 2 at the lake s southern coast 10 km apart and most surprising about 135 mw m 2 at the southern golan heights 6 8 km east of the graben margin a detailed geologic cross section traversing the entire sedimentary basin was constructed the hydrodynamics in this cross section were analyzed quantitatively using a two dimensional finite element code that solves the coupled variable density ground water flow and conductive convective heat transfer equations on the basis of numerical simulations different mechanisms of basin scale groundwater convection are suggested for the two sides of the rift that could influence the transport of heat 1 forced convection gravitydriven flow of hot brines from deeper aquifers to the land surface at the western side and 2 large scale free convection buoyancy driven flow of deep ground water at the eastern side the different heat fluxes within the rift valley are attributed to the different lithologies and to the locations of specific conduits through which the hot ground waters ascend from deeper horizons these simulations also explain the different salinities of the hot springs on the two sides of the rift

natural convection in porous permeable media sheets wedges and lenses abstract any fluid saturated porous permeable media subject to a temperature field such that there is a net upward heat flow and the isotherms across it are non horizontal will be subject to convective circulation analytical solutions for the fluid velocity exist only for the restricted case of an inclined layer in which the upper and lower surfaces are maintained at constant temperature this result has been much cited in the geological literature although the conditions under which the analytical solution holds are unrealistic in a geological situation numerical modelling is used to investigate how closely the analytical solution holds under conditions which are more geologically realistic it is found that for an homogeneous aquifer the analytical solution is a reasonable approximation provided the inclunation of the layer is replaced by the inclination of the isotherms in the expression for the fluid velocity in general the magnitude of will be much smaller than that of and depending on the effective thermal conductivity of the inclined layer relative to its surroundings may be of the opposite sign in this instance circulation is in the reverse direction to that obtained from the analytical solution for a vertically heterogeneous system in which the aquifer is divided by a series of thin continuous low permeability layers conditions exist in which the flow may be approximated by that in a homogeneous but anisotropic medium the model is also extended to cover other simple geometrics for which analytical solutions are not available

seismicity of the askja and barðarbunga volcanic systems of iceland 2009 2015 abstract a large seismic network deployed in the icelandic highlands recorded 100 000 earthquakes from 2009 to 2015 we develop a local magnitude scale appropriate for use in central iceland which is similar to the scale used by the iceland meteorological office using this large catalogue of earthquakes we analyze the spatial and temporal changes in seismicity rates and b values in microearthquakes recorded from the usually ductile lower crust we find that b values are high reflecting the presence of high thermal gradients and low stresses driving seismicity associated with the movement of melt in contrast b values in the upper crust are variable low b values indicative of a high stress environment are observed during seismic swarms such as those around mt her ubrei and around bar arbunga caldera a persistently seismically active area around a geothermal area within askja caldera has a b value around 1 but has a strong annual cycle of seismicity we attribute the annual cycle to varying load from the snow cover modulating the seismicity seismicity driven by the intrusion of a large dyke has a b value well above 1 driven by the high pore fluid pressures and thermal gradients around the dyke

nearshore carbonate deposits in lake tanganyika an exceptionally wide variety of carbonate fades dominated by high magnesian calcite occurs along the littoral and shallow sublittoral zones 50 m of lake tanganyika in central africa these facies include exposed and submerged calcite cemented ridges of nearshore terrigenous sand ooid sand shoals and lithified oolite ridges chara meadows of bioturbated calcareous silts gastropod shell blankets and related coquinas and extensive thrombolitic microbial reefs though texturally and compositionally similar to many modern and ancient shallow water marine facies these deposits record carbonate deposition and cementation in a large tropical deep water lake of tectonic origin lithofacies along lake tanganyika represent the broadest spectrum of carbonate deposits yet reported from any modern lake and serve as important analogues for lacustrine carbonate sequences in the stratigraphic record

distribution of natural and anthropogenic lead in mediterranean soils abstract the concentrations and isotopic composition of pb have been combined with a selective sequential dissolution procedure to track the distribution of anthropogenic and natural pb in the soil of semiarid climate and the penetration pathway of petrol pb within the soil profile soil samples were collected from soil profiles adjacent to a major highway 8 23 m and from soil profiles 500 m away from the same highway the selective sequential dissolution procedure was used to determine the distribution of pb between the different soil components soil carbonate organic matter fe oxides and hydrous oxides and aluminosilicates natural pb is associated mainly with aluminosilicates 60 and fe oxides 30 and only a small fraction with the soil carbonate and organic matter 10 in contrast to the distribution of natural pb the distribution of anthropogenic pb which accumulates mainly in the upper part of the soil profile is 40 with the soil carbonate 10 with organic matter 35 with fe oxides and only 15 with aluminosilicates on the basis of concentration and isotopic composition of total pb the deeper horizon of the roadside soil 10 30 cm and the soil sampled 500 m from the highway seem uncontaminated however the isotopic composition of the labile components in the soil pbcarb pborg and pbfe ox indicates that these soils are actually contaminated this contamination implies that over the period of vehicle pollution 40 yr petrol pb has penetrated through the entire roadside soil profile 25 30 cm furthermore pb isotopes in aluminosilicates of the upper part of the roadside soil indicate that this pb too is anthropogenic measurements of pb concentration and isotopic composition in roadside soil profiles sampled in 1997 and archived samples collected at the same location in 1982 provided a unique opportunity to trace petrol pb penetration into the soil

mechanical models for dikes a third school of thought abstract geological and geophysical data from continental volcanic centers and giant radial swarms and from oceanic shield volcanoes and rift zones indicate that dikes propagate laterally for distances that can be 10 to over 100 times their height what traps dikes within the shallow lithosphere and promotes these highly eccentric shapes gravity induced stress gradients in the surrounding rock and pressure gradients in the magma are the primary loading mechanisms pressure gradients due to magma flow are secondary to insignificant because the flow direction is dominantly horizontal this configuration of vertical blade shaped dikes with horizontal dike propagation and magma flow is fundamentally different from the two dike model configurations described in a recent review paper as two schools of thought for mechanical models of dikes in school i a dike is disconnected from its source and ascends under the influence of buoyancy in school ii a dike is connected to a magma reservoir and is driven upward by magma flux from the source we review the geological and geophysical data supporting the vertical dike horizontal flow propagation configuration and suggest the abundance and veracity of these data in many different geological settings and the modeling results that address this physical process warrant adding this as a third school of thought a new analytical solution for the boundary value problem of a homogeneous isotropic and linear elastic solid with a vertical fluid filled crack is used to investigate the effects of gravitationally induced stress and pressure gradients on the aperture distribution dike tip stress intensity and stable height model results indicate that in a homogeneous crust dikes can achieve stable heights greater than a kilometer only if the host rock fracture toughness k ic 100 mpa m 1 2 however density stratification of the crust is an effective mechanism for trapping kilometer scale dikes even if the host rock is very weak k ic 0 this analysis may explain why vertical dikes propagate laterally for great distances but reside within a narrow range of depths in the crust

tracing the transport of anthropogenic lead in the atmosphere and in soils using isotopic ratios the isotopic composition of lead in aerosols and soils in israel is used to characterize the sources of anthropogenic lead in the region to ascertain the isotopic composition of natural rock derived lead in specific areas and to determine rates of anthropogenic lead migration in soils the isotopic composition of lead currently emitted from cars in israel 206pb207pb 1 115 2 is controlled by alkyl lead produced in france and germany in addition to petrol lead two more sources of anthropogenic lead can be detected in sampled aerosols the first one has low concentrations of lead 4 ng m3 and 206pb207pb 1 157 and is most likely lead emitted in turkey that traveled across the eastern mediterranean basin the second type of aerosols contains a mixture of lead emitted in several countries including turkey greece and ukraine 206pb207pb value of 1 155 1 160 pb 20 30 ng m3 anthropogenic lead is more accessible for acid leaching than natural lead therefore it is more labile in the soil the isotopic composition of lead in the acid leached fraction of near road soil profiles record the histor of alkyl lead emission in the country based on changes in the isotopic composition of lead with soil depth it is estimated that anthropogenic lead migrates into the soil at approximately 0 5 cm y a soil profile from a relatively remote area is less contaminated by anthropogenic lead and displays a different distribution of lead isotopic values with depth the isotopic composition of lead suggests that natural lead in soils developed on carbonate bedrock is derived from clays either from the rock esidue the clay fraction in the carbonate bedrock or from airborne clay but not from lead released from the carbonate fraction in the rock

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

origins of elements building travertine and tufa new perspectives provided by isotopic and geochemical tracers fluid rock interaction represents a major process in the formation of calcitic or aragonitic travertine and tufa catt in most cases catt is associated to limestone dissolution somewhere along the hydrogeological pathway however a wide array of other substratum basalts rhyolites carbonatites ultramafics granites dolomites evaporites can act as potential source of elements involved in the formation of catt this study reports on the evaluation of potential geochemical tracers linking catt to their substratum and unravelling the origin of elements a large database was established from available literature data as well as new data acquired in the frame of this study for a set of modern to recent catt ligurian ophiolites italy the chaine des puys limagne graben and paris basin france reunion island indian ocean jebel oust tunisia four most reliable tracing methods are identified 1 13c and 18o cross plot allows distinguishing epigean min 13c 27 2 max 13c 0 9 mean 13c 12 3 for n 314 from hypogean systems min 13c 4 max 13c 11 7 mean 13c 2 87 for n 198 very low 13c values 4 associated to negative 13c values are specifically indicative of an ultramafic source rock 2 barium and strontium cross plot helps to discriminate different groups of source rocks amongst the hypogean catt i source rocks composed of mixed limestones evaporites and dolomites are characterised by low barium 400 ppm contents ii mafic and granitic source rocks are undifferentiated and display similar barium from 15 to 930 ppm and high strontium 200 ppm contents iii the carbonatite group is characterised by its exceptional high barium and strontium values in epigean catt a pure limestone source rock usually relates to very low barium and strontium contents 50 ppm 3 relatively high beryllium content 30 ppm in catt seems to indicate a pure granitoid source 4 high chromium concentrations 20 ppm are systematically documented in modern catt located on an ultramafic substratum the definition of diagnostic compositional fields for actively forming or recently formed catt is influenced by many factors including water composition water temperature dissolved gas composition and concentration biological activity position in the sedimentary body and early diagenesis in addition to substratum lithology however the results of this study illustrate that despite these many factors the combined use of ba sr be cr 13c and 18o may be valuable to discriminate the rock lithology prevailing in the hydrogeological or palaeo hydrogeological reservoir of catt

numerical analysis of seawater circulation in carbonate platforms ii the dynamic interaction between geothermal and brine reflux circulation density driven seawater circulation may occur in carbonate platforms due to geothermal heating and or reflux of water of elevated salinity in geothermal circulation lateral contrasts in temperature between seawater and platform groundwa ters warmed by the geothermal heat flux result in upward convective flow with colder seawater drawn into the platform at depth with reflux circulation platform top waters concentrated by evaporation flow downward displacing less dense underlying ground waters we have used a variable density groundwater flow model to examine the pattern magnitude and interaction of these two different circulation mechanisms for mesosaline platform top waters 50 and brines concentrated up to saturation with respect to gypsum 150 and halite 246 geothermal circulation most active around the platform margin becomes restricted and eventually shut off by reflux of brines from the platform interior towards the margin the persistence of geothermal circulation is dependent on the rate of brine reflux which is proportional to the concentration of platform top brines and also critically dependent on the magnitude and distribution of permeability low permeability evaporites can severely restrict reflux whereas high permeability units in hydraulic continuity enhance brine transport reduction in permeability with depth and anisotropy of permeability kv kh focuses flow laterally in the shallow subsurface 1 km resulting in a horizontally elongated brine plume aquifer porosity and dispersivity are relatively minor controls on reflux platform brines can entrain surficial seawater when brine generating conditions cease but the platform top remains submerged a variant of reflux we term latent reflux brines concentrated up to gypsum saturation have relatively long residence times of at least 100 times the duration of the reflux event they thus represent a long term control on post reflux groundwater circulation and conse quently on the rates and spatial patterns of shallow burial diagenesis such as dolomiti zation

geochemistry and water dynamics ii trace metals and pb sr isotopes as tracers of water movements and erosion processes abstract a dynamic scheme for water movements over a flood in a small watershed was proposed in a previous paper ben othman d luck j m tournoud m g 1997 geochemistry and water dynamics application to short time scale flood phenomena in a small mediterranean catchment i alkalis alkali earths and sr isotopes chem geol 140 9 28 based on major alkali alkali earth trace elements and sr isotopes in the present paper metal contents and pb isotope data are reported for the same samples and compared to natural and anthropogenic local sources analysed for this purpose water treatment plants have stable 206 pb 204 pb but show relatively large 207 pb 204 pb variations presumably related to yet unidentified industrial pb sources some cu zn rich chemicals used extensively on vineyards have unradiogenic 206 pb 204 pb around 17 7 local rains sampled over two years show roughly similar low values pb from rocks is variable and more radiogenic 206 pb 204 pb 18 8 22 8 except for the first hours trace element concentrations in the dissolved load are similar to or slightly higher than those observed over the year and similar to other moderately anthropogenic areas shiller a m boyle e a 1987 variability of dissolved trace metals in the mississippi river geochim cosmochim acta 51 3273 3277 for pb 0 05 0 45 nm zn 10 75 nm and cd 0 03 0 18 nm dissolved u and co show simple variations correlated to carbonate related to local natural sources most trace elements in the particulate load are strongly correlated lead isotopes in the dissolved and particulate loads show ranges over the flood again similar to those observed over the year 206 pb 204 pb 17 9 18 3 implying the same sources the very good alignments observed in 207 pb 204 pb and 208 pb 204 pb vs 206 pb 204 pb diagrams especially for the particulate are interpreted as mixing phenomena generally pb isotopic signature of the dissolved load is less radiogenic than the particulate indicating differences in sources or proportions and absence of isotopic equilibration with respect to the time of transfer pb isotopes shift regularly with time away from the road runoff endmember towards more radiogenic values the strong negative correlation between pb isotopes in the particulate load and sr isotopes in the dissolved load observed for the first time in a small watershed probably reflects the local coupling between mechanical and chemical erosion respectively

reactive transport modeling of brine reflux dolomitization anhydrite precipitation and porosity evolution abstract both geothermal convection and brine reflux drive circulation of sea water derived fluids through carbonate platforms during early burial but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood this paper describes high resolution reactive transport model rtm simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin with flow focused in the more permeable shallow carbonates in a baseline simulation involving an elongate 25 km wide grain dominated packstone platform brine reflux resulted in complete dolomitization beneath the 5 km wide brine pool in 335 ky the dolomite body then extends downward at c 22 m 100 ky into an underlying broad area of partial dolomitization this process enhances porosity at shallow depth but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux in contrast geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time 60 dolomite after 1 my with very minor associated anhydrite cementation reflux diagenesis is sensitive to platform geometry with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline fluid flow across thermal gradients enhances reaction rates and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25 and associated anhydrite by 90 the depth and rate of anhydritization is sensitive to the geothermal heat flux whereas platform top temperatures affect dolomitization rate reflux diagenesis is also sensitive to brine density which affects both fluid flow and reaction rates sediment permeability and reactive surface area rsa are key intrinsic controls on diagenesis where the permeability structure permits sufficient fluid flow diagenesis preferentially affects more reactive fine grained sediments however as flow rates decline reactions become focused in the more permeable but less reactive sediments simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favors grainstones once active reflux ceases brines continue to flow in the subsurface but this latent reflux causes only minor dolomitization due to prior mg2 consumption at shallow depth

seismogenic magma intrusion before the 2010 eruption of eyjafjallajokull volcano iceland we present relatively relocated earthquake hypocentres for 1000 microearthquakes ml 3 that occurred during the 2 weeks immediately prior to the 2010 march 20 fissure eruption at fimmvor uhals on the flank of eyjafjallajokull volcano in iceland our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km2 and approximately 4 km below sea level 5 km below the surface seismic activity in the final 4 d preceding the eruption extended to shallower levels 2 km below sea level and propagated to the surface at the fimmvor uhals eruption site on the day the eruption started we demonstrate using synthetic data that the observed apparent 1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors 0 01 0 02 s in arrival time data where the signal to noise ratio was sufficiently good to make subsample arrival time picks by cross correlation of both p and s wave arrivals the mean depth of 103 events in an individual cluster were constrained to 3 84 0 06 km epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used within clusters of typically 100 recorded earthquakes most of the arrivals exhibit similar waveforms and identical patterns of p wave first motion polarities across the entire monitoring network the clusters of similar events comprise repetitive sources in the same location with the same orientations of failure probably on the same rupture plane the epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex these constrictions may act as a form of valve in the country rock which ruptures when the melt pressure exceeds a critical level then reseals after a pulse of melt has passed through this would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location we infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth with seismogenic pinch points occurring between aseismic compartments of the sills or between adjacent magma lobes as they inflated during the final 4 d preceding the eruption onset between 22 30 and 23 30 utc on 2010 march 20 the seismicity suggests that melt progressed upwards to a depth of 2 km this seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke subsequently on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the fimmvor uhals eruption site

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

petroleum reservoir porosity versus depth influence of geological age average porosity values for the producing zones of oil and gas fields worldwide are examined as a function of the present depth for sandstone and carbonate lithologies divided into 10 groupings by reservoir depositional age precambrian silurian to pliocene pleistocene the wide variations in average reservoir porosity within each depth range reflect the extreme ranges in porosity controlling factors such as depositional facies early

pore networks in holocene carbonate sediments abstract permeabilities of holocene carbonate sediments from florida and the bahamas determined for 74 samples by water flow rates in a variable head permeameter range from 0 6 to 57 000 millidarcies porosities range from 40 to 78 percent these parameters are related to depositional texture as follows table the muddiest sediments i e the finest grained have the highest porosities but lowest permeabilities this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks even as young as pleistocene high porosity and low permeability in sediments show a strong correlation with percentage of fines 62 m from capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 m in diameter at least after drying measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation the calculated rates would require excessive time to produce the degree of cementation seen in late pleistocene rocks of florida and the bahamas the amounts of rainfall and evaporation are the most important factors in the degree of cementation

geothermal convection in the tengiz carbonate platform kazakhstan reactive transport models of diagenesis and reservoir quality a fundamental challenge in carbonate reservoir characterization is predicting the spatial distribution of diagenesis we used a reactive transport model to investigate the viability of geothermal convection and associated patterns of diagenetic porosity modification in the tengiz isolated carbonate platform reservoir before burial forced convection generates significant calcite dissolution locally up to 45 toward the platform center minor calcite cementation up to 0 4 in the slope and moderate calcite dissolution and cementation up to 1 6 in serpukhovian boundstone convective cells the patterns and rates of diagenesis proved critically sensitive to specified vertical permeability after burial with 200 m 660 ft of salt modeled subsurface temperature contrasts drive platform scale free convection flow is hydraulically closed but significant dissolution up to 7 3 after 20 m y occurred in the serpukhovian and visean platform interior and minor cementation up to 0 7 occurred toward the margin a shale filled salt withdrawal basin 500 m 1640 ft deep significantly modifies the subsurface temperature distribution and free convective flow ascending groundwaters beneath the withdrawal basin created a zone of calcite dissolution up to 24 5 in 20 m y with a mushroom geometry and minor cementation up to 2 3 in 20 m y in the distal platform interior and margin rates of diagenesis are dramatically reduced with increasing overburden as compaction retards convective flow from a generic perspective free convection persists if the salt overburden is substituted with shale although flow is reversed resulting in a different distribution of diagenesis simulations of geothermal convection provide a physically viable model for the integration of direct diagenetic observations to augment predictions of reservoir quality in tengiz and other carbonate platform reservoirs

lead and its isotopes in the sediment of three sites on the lebanese coast identification of contamination sources and mobility abstract lead concentrations and isotopic composition of sediment samples collected from three sites within the lebanese coastal zones were measured at akkar dora and selaata akkar is located far from any direct source of contamination while dora and selaata receive urban and industrial wastes respectively low pb concentrations 6 16 g g 1 were detected in the akkar sediments and high concentrations of pb 70 101 g g 1 were detected in the dora sediments measuring stable isotope ratios of pb makes it possible to identify the principal sources of pb in the akkar sediments as pb emitted from gasoline combustion and pb originating from natural sources on the other hand pb stable isotopic ratios in dora sediments indicate that they are more highly influenced by anthropogenic sources isotopic pb ratios in the selaata deposits where pb concentrations range between 5 and 35 g g 1 have an exceptional radiogenic signature for marine sediments 1 25 60 after 24 h of extraction and that the extracted pb is less radiogenic than the residual pb

correction to multiple melt injection along a spreading segment at askja iceland in the paper multiple melt injection along a spreading segment at askja iceland by janet key et al geophysical research letters 38 l05301 doi 10 1029 2010gl046264 2011 we regret that an error was made in the calculation of the v p v s ratios the correct values are presented in this revised version of figure 2 and its caption this change does not affect either the hypocentral locations for which we used the correct mean v p v s value of 1 78 nor the overall conclusions of our paper we still find evidence for relatively elevated v p v s ratios from the lower crustal earthquakes consistent with the presence of melt in the lower crust however the following corrections to the text should be made in light of this discovery

diversity and origin of quartz cements in continental carbonates example from the lower cretaceous rift deposits of the south atlantic margin abstract silica precipitation in continental carbonates is a common process occurring during sedimentation and diagenesis the lower cretaceous rift deposits of the south atlantic equatorial margin which are intensively explored by petroleum companies provide good examples of such silicifications in carbonates exhibiting a wide diversity of petrographic habit of early to late quartz cements in order to understand the palaeoenvironmental and diagenetic conditions leading to this diversity we integrated detailed petrography of diagenetic sequences and quartz habit with 18oquartz measurements by sims of individual cements observed in samples from the offshore and onshore basins of the west african margin the petrographic description highlights the omnipresence of early fibrous microquartz cements exhibiting either length fast or length slow habit in addition to laminated microquartz and micro or mega quartz forms amongst the isotopic analysis the 18oquartz data show that length slow cements are generally strongly enriched in 18o 18oquartz ranging from 31 to 37 smow whereas length fast forms show less elevated values 25 c and

subsurface seepage of seawater across a barrier a source of water and salt to peripheral salt basins many of the great phanerozoic salt basins share a common paleogeography in which a deep lake or sea is separated from the ocean by a narrow barrier examples include the aptian evaporites of the south atlantic rift basins and the messinian salt deposits of the mediterranean sea and the north caspian depression marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins we test an alternate hypothesis that subsurface seepage of seawater through the barrier was a significant source of water influencing both water level and water composition our model permits flux of water and salt into and out of the basin via 1 seepage through the barrier 2 evaporation from the water surface 3 rainfall and or rivers and 4 incorporation into basin sediments our investigation focuses on three settings a modern analog in the gregory rift of northern kenya the messinian mediterranean basin and the early cretaceous aptian salt basins of the south atlantic we test our model through a numerical simulation applying reasonable values for the dimensions of the barrier differences in water level and hydraulic conductivities to calculate flow rates through the barrier modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate size of the peripheral basin and hydraulic conductivity of the barrier the flux of salt is significant over the full range of modeled hydraulic conductivities producing brines in the peripheral lake and or sea within 0 5 m y in most cases this demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions and that marine transgressions across the barrier are not necessary to explain the apparent high salinities

chronology and sources of trace elements accumulation in the rhone pro delta sediments northwestern mediterranean during the last 400years abstract the rhone pro delta sediments receive the particulate inputs from the rhone river the largest freshwater discharge of the western mediterranean sea trace element tes ag cd co cr cu ni pb and zn concentrations and stable pb isotope ratios were determined along a 7 7 m long sediment core collected from the rhone prodeltaic lobe archiving the deposits of the last 400 years trace element mean concentrations during the pre industrialized era before 1850 ce were no different from the average composition of earth s upper crust except for cr principal component analysis performed on tes and tracers suggests three origins for tes that we identified as i marine biogenic material ag cd ii ultramafic rocks li cr ni and iii other clay minerals co cu pb and zn during the pre industrial period several changes in the te al ratios coincided with flood events or and modifications in the channelization of the rhone river mouth some frequencies in the tes temporal variations allow us to hypothesize some influence of climate variation sediments deposited after 1850 ce exhibited concentrations of certain tes exceeding 1 3 3 5 times the concentrations of the pre industrial era ag cd pb cu zn principal component analysis performed on post 1850 ce data allows to distinguish pristine and anthropogenically impacted tes pristine tes such as co cr and ni are linked with clay markers al li and fe co and ni are associated with mn enrichment whereas cr is correlated with fe anthropogenically impacted elements consist of ag cd cu pb and zn vertical profiles show that te contamination has increased dramatically after the iind world war and reached a maximum between 1970 and 1980 consistently with anthropogenic atmospheric emissions changes sediment deposited on the rhone prodelta at the beginning of xviith century have 206 207pb ratios close to values of natural pb 1 200 whereas sediments deposited after 1850 ce are clearly contaminated by european gasoline and industrial pb pools

effects of natural gas acidic components on local porosity generation in a carbonate reservoir insights from reactive transport modeling to investigate the mechanism of local porosity modification in a carbonate reservoir caused by the acidic components associated with natural gas generation a numerical model for the charging of natural gas rich in acidic components co2 was developed the acidic components are originated from maturation of organic matter in the source rock the model includes coupled gas water rock interactions multiphase flow and spatial heterogeneities of the geochemical reactions the simulation results indicate the following 1 when regional groundwater travels through the gas water contact gwc it is acidified by the co2 g in the gas zone and will dissolve carbonate minerals to form local secondary porosity the generated local secondary porosity is approximately 0 04 volume fraction with a maximum value of approximately 0 27 and is located mainly in the vicinity of the gwc 2 under reservoir conditions the solubility of calcite is much higher than dolomite the higher solubility and common ion effect of ca results in calcite being the primary reactive mineral in pure carbonate reservoirs having both calcite and dolomite the presence of anhydrite in the system does not change this situation 3 the groundwater flow field causes gwc evolution to be s shaped the inflection and gentle sloping parts at both ends of the gwc have different angles with the groundwater stream lines resulting in different amounts of mineral dissolution and precipitation and porosity increase 4 the limited amount of co2 from the source rock and groundwater flow in the subsurface seems to be sufficient to support local secondary porosity generation

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

lead isotope signatures of holocene fluvial sediments from the loire river valley abstract the distribution of mn v th pb and isotopes of pb in the labile fraction of sediments from a channel infill in the middle loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10 ka bp the acid extractable matter aem in the sediment samples is variable in amount and in trace element contents iron mn oxyhydroxides are the principle trace element carrying phase in the labile fraction and carbonates are a secondary carrier vanadium and pb originate from the weathering of silicates and are used as a silicate erosion rate index in the fluviatile record most of the aem data plot along a general trend between 3 endmembers basalts cretaceous carbonate rocks and granites in the relationship between 207pb 206pb and 206pb 206pb these endmembers have been mixed in various proportions depending on natural holocene inputs erosion volcanic events or human influences mining and smelting of ore

building icelandic igneous crust by repeated melt injections observations of microseismicity provide a powerful tool for mapping the movement of melt in the crust here we record remarkable sequences of earthquakes 20 km below the surface in the normally ductile crust in the vicinity of askja volcano in northeast iceland the earthquakes occur in swarms consisting of identical waveforms repeating as frequently as every 8 s for up to 3 h we use template waveforms from each swarm to detect and locate earthquakes with an automated cross correlation technique events are located in the lower crust and are inferred to be the result of melt being injected into the crust during melt intrusion high strain rates are produced in conjunction with high pore fluid pressures from the melt or exsolved carbon dioxide these cause brittle failure on high angle fault planes located at the tips of sills moment tensor solutions show that most of the earthquakes are opening cracks accompanied by volumetric increases this is consistent with the failure causing the earthquakes by melt injection opening new tensile cracks analysis of the magnitude distribution of earthquakes within a swarm reveals a complicated relationship between the imposed strain rates and the fluids that cause brittle failure the magnitude of the earthquakes is controlled by the distance fluids can migrate along a fault whereas the frequency of the events is controlled by the strain rate faults at the tips of sills act to focus melt transport between sills and so must be an important method of transporting melt through the lower crust

near bottom suspended sediment variability caused by storms and near inertial internal waves on the ebro mid continental shelf nw mediterranean abstract near bottom currents and suspended sediment concentrations ssc recorded on the ebro continental shelf at 60 m depth during the transition between fair and stormy weather have revealed some of the major processes controlling the sediment re suspension and transport at mid shelf depths in this nw mediterranean area storm waves were the main contributor to the transport of sediment although they only resuspended surface sediments during extreme wave conditions h s 4 5 m and t s 11 s during the recording period the net suspended sediment flux along the shelf was one order of magnitude higher than the net suspended sediment flux across the shelf and the resultant transport was towards the sw and seaward fluctuations of the near bottom ssc during fair weather showed a periodicity around 17 h and were related to the occurrence of near inertial internal waves caused by wind pulses during storm events the energy propagation of these internal waves reached critical angles at the mid and outer shelf which could favor the development and maintenance of the bottom nepheloid layer on the ebro continental shelf as well as on other mediterranean continental shelves

strike slip faulting during the 2014 barðarbunga holuhraun dike intrusion central iceland over a 13 day period magma propagated laterally from the subglacial bar arbunga volcano in the northern rift zone iceland it created 30 000 earthquakes at 5 7 km depth along a 48 km path before erupting on 29 august 2014 the seismicity which tracked the dike propagation advanced in short bursts at 0 3 4 7 km h separated by pauses of up to 81 h during each surge forward seismicity behind the dike tip dropped moment tensor solutions from the leading edge show exclusively left lateral strike slip faulting subparallel to the advancing dike tip releasing accumulated strain deficit in the brittle layer of the rift zone behind the leading edge both left and right lateral strike slip earthquakes are observed the lack of non double couple earthquakes implies that the dike opening was aseismic

changes of lead and barium with time in california off shore basin sediments during ancient times the natural deposition fluxes of lead which can be leached with dilute acid from sediments in santa barbara santa monica and san pedro basins offshore from the los angeles urban complex were about 0 7 0 1 and 0 2 g pb cm 2 yr respectively since there was little difference in biological productivity in surface waters of these basins it is proposed that clay is a major transport vehicle for sequestered soluble lead which then explains why the lead deposition flux within the santa barbara basin was so much larger compared to the other basins the fluxes of silicate mud in the basins in ancient times were about 92 19 and 30 mg cm 2 yr in santa barbara santa monica and san pedro basins respectively today deposition fluxes of acid soluble lead within these three basins are 3 to 9 fold greater being about 2 1 1 1 and 1 8 g pb cm 2 yr respectively partly in the form of directly deposited large sewage particles which account for none 2 3 and 3 4 of the total industrial lead deposition fluxes in the respective basins concentrations of leachable lead in varve dated sediment layers increase with time and isotopic compositions of these leads change in accordance with corresponding known changes of isotopic compositions of industrial lead in the los angeles atmosphere lead remaining in acid leached sediment residues originates from igneous and clay minerals exhibiting no change in concentration or isotopic composition since pre industrial times deposition fluxes of total barium in sediments among the three basins were proportional to mass deposition fluxes before 1950 in the same manner as for lead afterwards there are barium concentration maxima with time in both santa monica and san pedro basin sediments which are attributable to industrial sewage rather than to episodic erosion from barium rich sedimentary evaporite strata exposed locally along the shore an increase of barium concentrations in present day santa barbara basin sediments may reflect dispersal of barium rich drilling mud from local drilling operations

free thermohaline convection beneath allochthonous salt sheets an agent for salt dissolution and fluid flow in gulf coast sediments basinward migration of jurassic salt in the u s gulf of mexico has resulted in the emplacement of large allochthonous salt sheets into shallow miocene to holocene sediments although comparatively little direct information is available on the environment below these salt bodies it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below we derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures from these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport even if the underlying sediments have permeabilities as low as 10 17 m2 0 01 md the calculated maximum darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability when the vertical permeability of the underlying sediment is 10 17 m2 0 01 md salt is dissolved from the base of the sheet at an average rate of 3 5 m m y 1 the corresponding fluid velocities are such that over a 10 m y period the integrated fluid flux in the underlying sediments would be 104 m3 m 2 by comparison integrated fluid flux for compactive expulsion is 103 m3 m 2 thus for the offshore sediments of the gulf of mexico thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport diagenesis and salt tectonics

sedimentary talc in neoproterozoic carbonate successions abstract mineralogical petrographic and sedimentological observations document early diagenetic talc in carbonate dominated successions deposited on two early neoproterozoic 800 700 million years old platform margins in the akademikerbreen group svalbard talc occurs as nodules that pre date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates in the upper fifteenmile group of the ogilvie mountains nw canada the talc is present as nodules coated grains rip up clasts and massive beds that are several meters thick to gain insight into the chemistry required to form early diagenetic talc we conducted precipitation experiments at 25 c with low so4 synthetic seawater solutions at varying ph mg2 and sio2 aq our experiments reveal a sharp and reproducible ph boundary at 8 7 only above which does poorly crystalline mg silicate precipitate increasing mg2 and or sio2 aq alone is insufficient to produce the material the strong ph control can be explained by mg silica complexing activated by the deprotonation of silicic acid above 8 6 8 7 ft ir tem and xrd of the synthetic precipitates reveal a talc like 2 1 trioctahedral structure with short range stacking order hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite hydrated talc and eventually to talc this formation pathway imparts extensive layer stacking disorder to the synthetic talc end product that is identical to neoproterozoic occurrences early diagenetic talc in neoproterozoic carbonate platform successions appears to reflect a unique combination of low al concentrations and by inference low siliciclastic input near modern marine salinity and mg2 elevated sio2 aq and ph 8 7 because the talc occurs in close association with microbially influenced sediments we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry specifically ph and alkalinity increases driven by anaerobic fe respiration

mesogenetic dissolution its role in porosity development in carbonate reservoirs models of porosity formation in carbonate rocks have stressed subaerial exposure and attendant shallow meteoric diagenesis porosity formation also occurs in deep burial or mesogenetic settings as a result of dissolution enlargement of preexisting pores porosity enhancement and creation of new pore systems brines charged with organic acids carbon dioxide and or hydrogen sulfide derived from organic matter diagenesis and thermochemical sulfate reduction are the likely fluids causing significant mesogenetic dissolution enhanced and newly created mesogenetic pore types can mimic pore types formed in shallow meteoric environments and therefore the mesogenetic origin of some porosity may go unrecognized

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

compaction and decompaction algorithms for sedimentary carbonates abstract the mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration backstripping analyses and quantitative models for simulation of carbonate stratigraphy deep water chalks and calcareous oozes are not treated here empirical porosity depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity depth curves for shallow water carbonate muds and sands the curves are exponential in form this review indicates that mud rich shallow marine carbonates behave differently than sand rich grain supported sediments with regard to reduction in porosity and thickness quantitative analysis of compaction behavior in carbonate sediments reveals that barring early cementation carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths e g 50 thickness reduction with about 150 200 m overburden however primary porosities of carbonate muds approximately 70 80 are reduced only to about 42 under that overburden thus the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity this indicates that interparticle cementation with or without chemical compaction is required to transform muddy carbonate sediments into lithified rock in the absence of early lithification carbonate sands compact more slowly than muds primary interparticle porosities of about 40 45 can be maintained in the first 200 300 m of burial with little thickness reduction

geothermal convection in south atlantic subsalt lacustrine carbonates developing diagenesis and reservoir quality predictive concepts with reactive transport models prolific hydrocarbon discoveries in the subsalt commonly known as the presalt section of brazil and the conjugate african margin have created a business imperative to predict reservoir quality in lacustrine carbonates geothermal convection is a style of groundwater flow known to occur in rift settings which is capable of diagenetic modification of reservoir quality we simulated variable density groundwater flow coupled with chemical reactions to evaluate the potential for diagenesis driven by convection in subsalt carbonates rates of calcite diagenesis are critically controlled by temperature gradient and fluid flux following the principles of retrograde solubility simulations predict that convection could operate in rift carbonates prior to salt deposition but with rates of dissolution in the reservoir interval only on the order of 0 01 vol m y which is too low to significantly modify reservoir quality the exception is around permeable fault zones and or unconformities where flow is focused and dissolution rates are amplified to 1 to 10 vol m y and could locally modify reservoir quality after salt deposition simulations also predict convection with a critical function for salt rugosity the greatest potential for dissolution at rates of 0 1 to 1 vol m y occurs where salt welds overlying permeable carbonates thin to 500 m 1640 ft or less with tens of million years residence times feasible convection under these conditions could locally result in reservoir sweet spots with porosity modification of 1 to 10 and potentially an order of magnitude or more in reservoir permeability integrating quantitative model derived predictive diagenetic concepts with traditional subsurface data sets refines exploration to production scale risking of carbonate reservoir presence and quality

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial

trace element and pb isotope variability during rainy events in the nw mediterranean constraints on anthropogenic and natural sources abstract rain samples collected over the period 1995 1999 on the french mediterranean coast have been analyzed for their trace elements and pb isotopic compositions both in the dissolved and particulate loads our results show that metal enrichment factors ef relative to upper erodable crust are high for cu pb zn and cd and comparable to values reported about 10 years ago for north western mediterranean rains overall unradiogenic pb isotopic compositions indicate the still visible influence of french gasoline lead on both dissolved and particulate loads this influence is also seen on aerosols although more variable the examination of air mass trajectories allows us to distinguih the pb isotopic signatures of main sources in particular marseille north italy north africa rains collected and analyzed sequentially show intra rain variations for metal contents metal ef and pb isotopes which may also be related to the main paths and altitudes of the air masses metal efs and pb isotopic data on rains from southern france in the late 1990s are similar to recent data published on french aerosols or sediments which are interpreted as reflecting the phasing out of leaded gasoline in the atmosphere