ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

linear coupling of carbon and strontium isotopes in rotliegend sandstone north sea evidence for cross formational fluid flow isotopic analyses of carbon and strontium in diagenetic dolomite and ankerite present in the eolian lower permian rotliegend sandstone southern north sea show an excellent linear correlation between delta sup 13 c and sup 87 sr sup 86 sr r 0 93 suggesting that the sources of these elements were interlinked the isotopic data indicate that late permian zechstein seas flooded the basin and displaced the interstitial meteoric rotliegent pore fluids producing early dolomite with predominantly marine bicarbonate delta sup 13 c minus 15 per thousand and strontium sup 87 sr sup 86 sr 0 7076 isotopic signatures the isotopic values of this dolomite form a trend to merge with the diagenetically later ankerite which has lower sup 13 c sup 12 c delta sup 13 c minus 4 per thousand and more radiogenic strontium sup 87 sr sup 86 sr 0 7112 the obvious source of low delta sup 13 c carbon was thermal decarboxylation of organic matter in the underlying upper carboniferous mudstones the authors also suggest that the increasing quantities of radiogenic strontium sup 87 sr sup 86 sr 0 720 were released when organic acids dissolved silicates feldspar in the same underlying mudstones the upward movement of fluid or diffusion of ions across formation boundaries therefore progressively became the dominant control on carbonate precipitation in the rotliegend sandstone

dolomitization of shallow water mixed silicilastic carbonate sequences the lower triassic ramp succession of the transdanubian range hungary abstract the lower triassic succession of the transdanubian range in hungary comprises limestones dolomite marl sandstones and siltstones deposited in tidal flat lagoon and ooid shoal environments on the marginal ramp of the western neotethys seven cores were chosen for petrographic and stable isotope investigations aiming to reconstruct the paragenetic sequence with special regard to the dolomitization and hydrothermal events five lithotypes were differentiated i dolomite ii sandy silty clayey dolomite and dolomarl iii dolomitic siltstone and sandstone iv dolomitic limestone and v limestone in these lithotypes three types of dolomites are present non ferroan replacive ferroan replacive and ferroan cement fabric retentive and fabric destructive non ferroan replacive dolomitization are interpreted to have occurred by seepage reflux supporting evidence includes the presence gypsum and anhydrite in the lower triassic beds stable isotope values of the ferroan dolomite 18o of 10 7 to 4 2 and 13c of 4 8 to 4 7 suggest dolomitization by fluids of relatively high temperature the similar stable isotope values 18o of 9 3 to 5 8 and 13c of 1 9 to 2 5 of the non ferroan dolomite phase suggest that the reflux dolomite was overprinted by this second dolomitization event traces of exotic minerals such as barite chalcopyrite galena and sulphosalts were found as fillings of vugs and fractures in the dolomite cemented sandstone the metals could have been sourced from the underlying permian red sandstone beds the heterogeneous sediment composition had profound impact on the diagenesis of these sedimentary successions

episodic formation of refractory inclusions in the solar system and their presolar heritage abstract refractory inclusions ca al rich inclusions cais and amoeboid olivine aggregates aoas in primitive meteorites are the oldest solar system solids they formed in the hot inner protoplanetary disk and as such provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the sun and its planetary system using the short lived 26al to 26mg decay system we show that bulk refractory inclusions in cv vigarano type and cr renazzo type carbonaceous chondrites captured at least two distinct 26al rich 26al 27al ratios of 5 10 5 populations of refractory inclusions characterized by different initial 26mg 24mg isotope compositions 26mg 0 another 26al poor cai records an even larger 26mg 0 deficit this suggests that formation of refractory inclusions was punctuated and recurrent possibly associated with episodic outbursts from the accreting proto sun lasting as short as 10 myr old dust

effect of soil characteristics on relative values of piezocone t bar and ball penetration resistances it has been reported in the literature that net cone penetration resistance is normally higher than the penetration resistance measured using full flow cylindrical t bar or spherical ball penetrometers and also that the ball penetration resistance is generally very close to the t bar penetration resistance in this paper a worldwide high quality database three onshore sites and seven offshore sites was used to identify soil characteristics that contribute to differences in the relative magnitudes of penetration resistance measured with cone t bar and ball penetrometers for soils covered in this database the ratio of t bar to net cone penetration resistance qt bar qnet lay between 0 75 and 1 decreasing with depth from near unity at shallow depths for the four sites where ball penetrometer data were available the ratio of ball to t bar penetration resistance qball qt bar was found to fluctuate around unity with an average just greater than 1 but no distinguishable trend with depth the m

lithology based sequence stratigraphic framework of a mixed carbonate siliciclastic succession lower cretaceous atlantic coastal plain this study presents a lithology based sequence stratigraphic framework and depositional model for lower cretaceous mixed siliciclastic carbonate sediments of the mid atlantic coastal plain eastern united states lithologic data from cores and cuttings were integrated with wireline logs and two dimensional seismic data to document lithofacies variability and stacking patterns across the albemarle basin of eastern north carolina ten facies associations are defined which are variably present within siliciclastic and carbonate dominated depositional profiles interpreted to extend from onshore lowland coastal plain to deep shelf depositional environments three depositional sequences 0 1 2 were identified each with component upward shoaling parasequences seismic reflectors typically coincided with key sequence stratigraphic surfaces which guided correlations between wells parasequences are grouped into parasequence sets with progressive progradational or retrogradational highstand and transgressive systems tracts respectively stacking patterns transgressive parasequences are thinner uniform in thickness and tend to be more dominated by molluskan carbonate facies highstand parasequences have more variable thickness are siliciclastic dominated and tend to be progradational on seismic data late highstand deposits of sequence 1 are dominated by restricted carbonate facies that likely reflect increased aridity lowstand deposits were not recognized from onshore well and seismic data the sequence stratigraphic framework developed documents the complex spatial and temporal facies relationships within a wave dominated mixed carbonate siliciclastic passive margin succession the strata studied document the complex interplay of lithofacies within a transition zone between near shore carbonate dominated strata to the south southeast georgia embayment and siliciclastic dominated marginal marine successions to the north baltimore canyon trough it also provides a useful stratigraphic calibration set for coeval offshore sediments that have been identified as potential areas for hydrocarbon exploration

a multi system geochronology in the ad 3 borehole pannonian basin hungary with implications for dating volcanic rocks by low temperature thermochronology and for interpretation of u th he data independent geochronological and thermal modelling approaches are applied to a biostratigraphically exceptionally well controlled borehole alcs ad 3 in order to constrain the age of cenozoic geodynamic events in the western pannonian basin and to test the efficacy of the methods for dating volcanic rocks apatite fission track and zircon u pb data show two volcanic phases of middle eocene 43 4 39 0 ma and early oligocene 32 72 0 15 ma age respectively apatite u th he ages 23 8 14 8 ma and independent thermal and subsidence history models reveal a brief period of heating to 55 70 ca t 17 ma caused by an increased heat flow related to crustal thinning and mantle upwelling our results demonstrate that contrary to common perception the apatite u th he method is likely to record apparent or mixed ages resulting from subsequent thermal events rather than cooling or eruption ages directly related to distinct geological events it follows that a direct conversion of apparent or mixed u th he ages into cooling exhumation or erosion rates is incorrect

the role of migrating oil field brines in the formation of sediment hosted cu rich deposits oil field brines commonly contain ppm quantities of zn pb and h 2 s but negligible quantities of cu theoretical analysis of the water rock interactions expected during migration through evaporitic and red bed sequences suggests that these reducing oil field brines can become transformed into far more oxidized fluids capable of transporting cu pb zn and sulfate and of forming sediment hosted cu rich deposits calculation of the chemical mass transfer accompanying reaction of a typical reducing brine with anhydrite hematite and quartz followed by the assemblage quartz k feldspar muscovite hematite chlorite and cuprite reveals that the aqueous phase becomes greatly undersaturated with respect to sphalerite galena chalcopyrite bornite pyrite and pyrrhotite as it achieves log f sub o 2 values well above the magnetite hematite buffer consistent with saturation with respect to anhydrite hematite and chalcocite and ore forming concentrations of copper and sulfate consequently evaporites are not only possible sources of salinity and oxidized sulfur for evolving basinal brines but may be crucial to the development of the f sub o 2 conditions required to transport cu in ore forming concentrations reaction of the oxidized brine with organic matter approximated by graphite and diagenetic pyrite results in precipitation of the zonation sequence chalcocite bornite chalcopyrite and pyrite with variable amounts of hematite covellite sphalerite and galena depending on the relative rates of reaction of the graphite and pyrite

petrographic and geochemical characteristics of dolomite types and the origin of ferroan dolomite in the trenton formation ordovician michigan basin u s a three major types of dolomite occur in the trenton formation mid ordovician of the michigan basin these are 1 regional dolomite which is confined to the extreme western edge of the basin 2 cap dolomite which occurs in the upper portion of the trenton and is confined to the basin s southern margin and 3 fracture related dolomite which occurs in association with both large and small scale faults and fractures these three dolomite types can be distinguished from one another by their major element chemistry oxygen isotope ratios and rock texture the regional dolomite is fine grained has 0 34 mol feco3 and mean 18o of 6 8 opbd the cap dolomite is texturally similar to regional dolomite but contains 3 13 0 mol feco3 and has a mean 18o of 7 7 fracture related dolomites are coarse grained low in iron and have the most depleted 18o ratios x 9 0 pdb petrographic relationships imply that the regional dolomite formed prior to the cap dolomite probably during early diagenesis the cap dolomite formed at relatively shallow depths as a result of the interaction of the overlying utica shale and the trenton limestone fracture related dolomites post date the cap dolomite and formed during deeper burial a temperature of precipitation of approximately 80 c was calculated for fracture related dolomites using oxygen isotope data the distribution of the cap dolomite was controlled by the availability of fe2 which was in turn controlled by the availability of s2 in the centre of the basin trenton utica deposition was continuous the upper trenton contained relatively high concentrations of organic matter which was used by sulphate reducing bacteria to produce h2s from seawater sulphate the precipitation of iron sulphides pyrite iron monosulphide followed and used up most of the available fe2 as a result only small amounts of ferroan dolomite formed on the periphery of the basin subaerial exposure resulted in the oxidation of most of the available organic matter sulphate reducing bacteria were therefore limited and produced limited amounts of h2s as a result only a minor amount of iron sulphide iron monosulphide formed the remaining fe2 was then available for the formation of the ferroan cap dolomite this model is supported by the following 1 in the southern margin of the basin the contact between trenton cap dolomite and the overlying utica shale is sharp and probably unconformable in the centre of the basin the contact is gradational 2 in the centre of the basin the total organic carbon content in the upper trenton is an order of magnitude higher than in the cap dolomite 3 the whole rock concentration of iron is high in both the cap dolomite and in slightly dolomitized equivalent beds in the basin centre 4 iron sulphides are abundant in the centre of the basin and mostly in the form of pyrite in the cap dolomite iron sulphide is minor and primarily in the form of iron monosulphide

petrostratigraphic analysis of a late miocene mixed siliciclastic carbonate depositional system calabria southern italy implications for mediterranean paleogeography abstract mixed siliciclastic carbonate sediments are common in shallow water environments in which clastic deposition and carbonate production coexist in the mediterranean basin the tortonian messinian sedimentary successions are commonly comprised of marls alternations of marls with diatomites sands and sandstones limestones especially algal and coral reefs and evaporites most of the studies in the mediterranean basin have been focused on carbonate facies analysis whereas the siliciclastic portions are poorly described similarly to the other marginal settings of the mediterranean basin the capo vaticano area southern calabria southern italy experienced mixed sedimentation at that time petrological analysis was performed on 56 weakly cemented sandstones linking compositional signatures to stratigraphic data with the aim of investigating sedimentary response to the tortonian messinian transgression and its influence on the carbonate production the information obtained is used to refine the regional sediment dispersal pathways and paleogeography by means of bio stratigraphic evidence the studied succession to coeval mixed deposits dislocated in the mediterranean basin is correlated

correlation of carbon dioxide abundance with temperature in clastic hydrocarbon reservoirs relationship to inorganic chemical equilibrium the mol co2 of gas reservoired in sandstones tends to increase with increasing temperature along both the us gulf coast and the norwegian continental shelf each of these provinces displays identical linear correlations between temperature and the log of the partial pressure of co2 i e pco2 in the reservoir fluid that is the product of the mol co2 the reservoir pressure this correlation is the same both in gulf coast reservoirs younger than 17 ma and in norwegian reservoirs of jurassic to triassic age 144 245 ma indicating that it is not dependent on reaction kinetics norwegian shelf data indicate that the same correlation also applies for gas condensate and oil reservoirs we suggest that the log pco2versus temperature correlation is the result of inorganic chemical equilibria between feldspar clay and carbonate minerals a trend of decreasing ph with increasing temperature is fixed by equilibria between aluminosilicate minerals feldspar kaolinite at lower temperatures followed by various reactions between kaolinite illite chlorite and feldspar as temperatures exceed 110 140 c responding to the ph set by the aluminosilicate system co2 activity in the formation water and thereby co2 partial pressure in any gas phase present is controlled by equilibrium with carbonate minerals this model predicts that co2 released by organic maturation will in general result in precipitation of carbonate minerals below 100 120 c and that there should be widespread carbonate dissolution at higher temperatures

δ13c and δ18o values of triassic brachiopods and carbonate rocks as proxies for coeval seawater and palaeotemperature abstract a dataset of 160 isotope 13c and 18o values from anisian ladinian carnian and rhaetian articulate brachiopod shells complemented by 158 carbon and oxygen isotope values from whole rock carbonates define the first continuous stable isotope baseline trends for the triassic seawater the carbon isotope data suggest the existence of several short term high amplitude excursions in the early triassic followed by a predominance of values around 0 5 1 during the middle triassic a rise to 3 5 during the carnian plateau at this level during the late carnian to early norian and a 1 5 decline in the middle norian to values around 2 during the late norian rhaetian interval the causation scenarios for these rapid oscillations are at present equivocal but may in part reflect a biological instability of the carbon cycle following the recovery from the end permian extinction event and or an input of mantle derived co2 from enhanced volcanic activity the 18o values from well preserved brachiopods from the tethyan realm range from 3 9 to 0 6 v pdb these values require open marine triassic seawater 18o values close to 0 v smow for the calculated temperatures to be within the range of tolerance of the coexisting reef building corals a 2 18o increase within the uppermost cordevolian and early julian suggests either a distinct temperature decline in the southern alps during that time interval an increase in seawater salinity or their combination oxygen isotope values for the muschelkalk brachiopods range between 6 2 and 2 0 and likely reflect a strong influx of meteoric waters into the germanic basin that shifted the oxygen isotopes to more negative values

geochemistry and diagenesis of stratabound calcite cement layers within the rannoch formation of the brent group murchison field north viking graben northern north sea comment

tertiary evolution of the intra carpathian area a model abstract the outer carpathian flysch nappes encircle an intra carpathian domain which can be divided into two megatectonic units north pannonian and tisza mostly on the basis of contrasting mesozoic and palaeogene facies development we see two major kinematic problems to be solved 1 1 the present distribution of the mesozoic and palaeogene facies is mosaic like and some belts form exotic bodies within realms of austroalpine affinity 2 2 late eocene palinspastic reconstruction of the outer carpathian flysch nappes suggest that the entire intra carpathian area must have been located several hundreds of kilometres to the south and to the west of its present position neogene extension can account for shortening in the external carpathian nappes but is unable to explain mesozoic facies anomalies and offsets of palaeogene formations we suggest that evolution of the intra carpathian area involved first late palaeogene early miocene juxtaposition of the north pannonian and tisza megatectonic units accompanied by the closure of the external carpathian flysch troughs thereafter extension of this amalgamated unit occurred which was compensated by thrusting of flysch nappes onto the european foreland and formation of molasse foredeeps eastward escape of the north pannonian unit from the alpine collisional belt involved left lateral shear along the pieniny klippen belt and right lateral shear along the mid hungarian zone parts of the late palaeogene basin and an early miocene volcanic edifice were dissected offset and elongated by several 100 kms the driving mechanism of the eastward escape of the intra carpathian area can be related to the collision of apulia and europe and the subduction of the external carpathian crust under the pannonian units the escape ceased gradually in the early miocene when oblique collision between the north pannonian unit and european continent occurred neogene extension of the pannonian region was an areal deformation the extension at locally variable rate resulted in the break up of the heterogenous floor of the neogene basin the driving mechanism of basin extension and contemporaneous compressional deformation of the external carpathians is thought to be related to ongoing subduction involving the marginal part of the attenuated european continental crust tectonic activity in the carpathians and basin subsidence and volcanism shifted in time and in unison from the west toward the east southeast

fluid immiscibility in natural processes use and misuse of fluid inclusion data ii interpretation of fluid inclusion data in terms of immiscibility abstract phase equilibrium analysis of the relevant systems together with the application of the principles of chemical equilibrium put severe constraints on the interpretation of fluid inclusion data in terms of immiscibility in part i following from that point the major limits on the accuracy and even the validity of fluid inclusion quantitative data and their interpretation in terms of fluid composition and density are briefly discussed the practical implications of the general constraints temperature pressure topology of the fluid systems are envisaged emphasis is laid on some important consequences such as the use of isochore intercepts and the possible resulting interpretation of fluid mixing rather than unmixing the case of highly saline inclusions the identification and interpretation of heterogeneous trapping the composition and density constraints on coexisting fluids are presented and illustrated by natural examples taking into account all the measurable parameters in fluid inclusions volume temperature and nature of phase transitions more or less complete individual spectroscopic analyses all the available experimental data and all the theoretical constraints may be long and difficult however it is most generally very informative and productive although part of these data is often sufficient to deny unmixing nevertheless a final example on metamorphic fluids demonstrates how such an approach can prove and also characterize a fluid unmixing during a geologic process

distribution of porosity and permeability in platform dolomites insight from the permian of west texas saller and henderson 1998 presented an article concerning porosity and permeability variations in two west texas oil fields north riley and south cowden fields these fields occur within dolomitized permian platform carbonates near the basinward shelf edge of the central basin platform a paleogeographic element that influenced permian deposition in west texas their carefully documented study outlines several anomalous aspects concerning porosity distribution they noted in particular that porosity distribution is not strictly facies dependent but that porosity tends to increase in a basinward direction they suggested that porosity developed largely during dolomitization by the action of hypersaline evaporative fluids that flowed downdip in a basinward direction during the permian these brines were generated in restricted shelf interior lagoons and moved downdip parallel with bedding in a shallow reflux type of hydrologic system driven by subsurface fluid density differences in the permian clear fork and grayburg formations as shown here in figure 1 see figure 18 in saller and henderson 1998 figure 1 part a shows the scenario proposed by saller and henderson 1998 for porosity creation dolomitization and dolomite cementation of the permian clear fork and grayburg formations of west texas they suggested that macrodissolution of calcium carbonate occurred contemporaneously with dolomitization and dolomite cementation this process of brine backreaction however would cause a net increase of porosity in the shelf interior the opposite of what is observed part b shows a two stage scenario proposed here to circumvent the problem of limestone macrodissolution by shelf interior brines that must have been initially in approximate bulk equilibrium with calcium carbonate the first stage records dolomitization by replacement of limestone with little macrodissolution in stage two evaporative shelf interior brines may have evolved in equilibrium with dolomite only and became undersaturated with calcite and aragonite reflux migration of these brines downdip may

electron microscope analysis of zoned dolomite rhombs in the jet rock formation lower toarcian of the whitby area u k zoned dolomite rhombs which occur in the organic rich jet rock formation toarcian of northeast england have been studied using backscattered scanning electron microscopy transmission electron microscopy and energy dispersive x ray microanalysis the rhombs which are 5 20 m in size are variable in chemical composition but many consist of a dolomite core surrounded by a zone of ferroan dolomite ankerite or ferroan calcite zoned rhombs occur in early diagenetic calcite cemented concretions and layers as well as dispersed throughout the shales distributional and textural evidence suggests that they formed mainly by primary precipitation from pore fluids within the zone of bacterial sulphate reduction the ferroan outer zones probably formed after burial below the sulphate reduction zone when insufficient h 2 s was available to react with all the fe 2 in solution

climatic controls on sedimentary environments in the triassic of the transdanubian range western hungary abstract during the triassic the transdanubian range was a segment of the western neotethys margin where the climate was mostly determined by a strong global monsoonal system as a consequence of the global climatic conditions and its geographic setting warm to hot temperature prevailed in this area however there were significant changes in the precipitation pattern from dry to wet conditions in connection with the intensity of the monsoon system and the actual setting of this area within the monsoon system the aim of our research was to interpret the palaeoclimatic conditions and their changes for the triassic succession of the transdanubian range on the base of geochemical mineralogical sedimentological and palaeontological data with special regard to the climate indicator facies e g different types of dolomites clay mineral assemblages nine evolutionary stages were distinguished during the early triassic mixed siliciclastic carbonate ramp sedimentation took place under arid climate that was interrupted by a humid pulse campil pluvial event in the early anisian a shallow carbonate ramp was developed under arid to semi arid climate the predominatly dry conditions prolonged during the middle anisian the latest anisian and the ladinian carbonate platforms were developed under mostly semi arid climate the semi humid climate of the earliest carnian was followed by a definitely wet interval carnian pluvial event that was characterised by an intense terrigenous input into the julian intraplatform basins arid and humid conditions alternated during the final stage of basin upfilling from the latest julian to early tuvalian then a marked climatic change took place in the latest carnian when the humid climate was changed to semi arid conditions from the middle to late norian a gradual increase of humidity may have taken place that resulted in a decreasing intensity of the early dolomitisation of the carbonate sediments of the broad internal platform at the very end of the norian there was a marked climate change leading to enhanced humidity but with a definite seasonality humid climatic conditions prevailed during the rhaetian kossen event evaluation of the similarities and differences of the palaeoclimatic conditions for the transdanubian range and some selected areas in the western neotethys dolomites northern calcareous alps mecsek and the peri tethyan germanic basin is also presented

mesozoic plate tectonic reconstruction of the carpathian region abstract palaeomagnetic palaeobiogeographic and structural comparisons of different parts of the alpine carpathian region suggest that four terranes comprise this area the alcapa tisza dacia and adria terranes these terranes are composed of different mesozoic continental and oceanic fragments that were each assembled during a complex late jurassic cretaceous palaeogene history palaeomagnetic and tectonic data suggest that the carpathians are built up by two major oroclinal bends the alcapa bend has the meliata oceanic unit correlated with the dinaric vardar ophiolite in its core it is composed of the western carpathians eastern alps and southern alcapa units transdanubian range bukk this terrane finds its continuation in the high karst margin of the dinarides further elements of the alcapa terrane are thought to be derived from collided microcontinents czorsztyn in the n and a carbonate unit tisza in the se the tisza dacia bend has the vardar oceanic unit in its core it is composed of the bihor and getic microcontinents this terrane finds its continuation in the serbo macedonian massif of the balkans the bihor getic microcontinent originally laid east of the western carpathians and filled the present carpathian embayment in the late palaeozoic early mesozoic the vardar ocean occupied an intermediate position between the western carpathian austroalpine transdanubian high karst margin and the bihor getic serbo macedonian microcontinent the vardar and pindos oceans were opened in the heart of the mediterranean adriatic microcontinent in the late permian middle triassic vardar subducted by the end of jurassic causing the bihor getic serbo macedonian microcontinent to collide with the internal dinaric western carpathian margin an external penninic vahic ocean tract began opening in the early jurassic separating the austroalpine western carpathian microcontinent and its fauna from the european shelf further east the severin ceahlau magura also began opening in the early jurassic but final separation of the bihor getic ribbon and its fauna from the european shelf did not take place until the late middle jurassic the alcapa and the tisza dacia were bending during the albian maastrichtian the two oroclinal bends were finally opposed and pushed into the gates of the carpathian embayment during the palaeogene and neogene at that time the main n s shortening in distant alpine and hellenic sectors was linked by a broader right lateral shear zone along the former vardar suture

dolomite diagenesis and porosity preservation in lithic reservoirs carmopolis member sergipe alagoas basin northeastern brazil the lithic sandstones and conglomerates of the carmopolis member of the muribeca formation aptian were deposited bp fan deltas alluvial fans and braid deltas that prograded from low grade metamorphic terrains into the sergipe alagoas rift basin during

variations in petrophysical properties along a mixed siliciclastic carbonate ramp upper jurassic ricla ne spain abstract a multi scale petrophysical study was performed on an upper jurassic siliciclastic carbonate ramp system near the village of ricla spain a combination of textural field observations thin section and grain size analysis chemical mineralogical thermo gravimetric analysis and physical porosity gamma ray and velocity measurements properties was used to capture the heterogeneity of this mixed sedimentary system subsequently synthetic seismic sections with varying resolution were created to show the characteristics of the internal ramp structures the translation from the measured petrophysical properties to a synthetic seismic section helps to better understand subsurface seismic although upscaling problems are a recurring issue the homogeneity of the samples and the continuity of the outcrop and facies reduce the number of uncertainties acoustic properties are generally controlled by the amount and type of pore spaces due to the low porosities the ramp geometry is characterized by an overall lateral homogeneity and vertical heterogeneity textural sedimentological and stratigraphic transitions in the outcrop are related to the defined facies each has its own unique mineralogical signature amplitude variations in the synthetic seismic profile are primarily caused by changes in mineral composition thus facies transitions

subtropical to temperate facies from a transition zone mixed carbonate siliciclastic system palaeogene north carolina usa palaeogene passive margin sediments on the us mid atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate siliciclastic shelves this study utilizes well cuttings outcrop core and seismic data to document temporal and spatial variations in admixed bryozoan rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone this zone was situated between north flowing warm waters of the ancestral gulf stream carbonate dominated settings and south flowing cold waters of the ancestral labrador current siliciclastic dominated settings some degree of mixing of carbonates and siliciclastics occurs in all facies however siliciclastic prone sediments predominate in nearshore settings while carbonate prone sediments are more common in more open marine settings of the inner shelf break and deep shelf a distinctive dual break shelf depositional profile originated following a major late cretaceous to palaeocene transgression that drowned the earlier shallow platform this profile was characterized by prominent mid shelf break dividing the shallow shelf from the deep shelf and a major continental shelf slope break incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin storm wave base also contributed to the preservation of the dual break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid shelf break sediment fines in deep shelf facies were produced in place transported downdip from the shallow shelf by storm ebb currents and boundary currents and reworked from adjacent areas of the deep shelf by strike parallel boundary currents regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf with warmer waters and more humid climates favouring carbonate deposition and cooler more arid conditions favouring glaucony and siliciclastic dominated deposition continuous wave and current sweeping of the shallow shelf favoured deposition of mud lean facies across much of the shallow shelf skeletal components in much of the carbonate rich strata formed in warm nutrient rich subtropical waters as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages these indicators of warm water deposition within the bryozoan mollusk rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage such facies generally are associated with cooler water depositional settings

isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the solimoes basin northern brazil abstract the isotopic composition of diagenetic carbonates sulfates illites and associated diagenetic constituents of the carboniferous jurua sandstones solimoes basin largest hydrocarbon reservoirs of northern brazil was analyzed in conjunction with petrographic characterization and chemical composition eolian jurua sandstones deposited in a coastal sabkha of a wide cratonic sag are the best reservoirs but diagenetic processes introduced strong and complex heterogeneities which affect hydrocarbon production dolomite and anhydrite cementation together with compaction exerts the main control on the quality of the reservoirs early dolomite cements precipitated under strong evaporation while late dolomites are related to thermal decarboxylation late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide authigenic illite k ar ages are related to the voluminous triassic basic magmatism around 200 ma and to the jurassic cretaceous tectonism around 150 ma which may have also affected the late anhydrite and dolomite ankerite cementation this isotopic study revealed the major conditions of the diagenetic processes that strongly affected the jurua reservoirs which are essential for the development of models that will optimize the production as well as decrease the risks in the exploration for new jurua reservoirs

significance of late permian triassic facies zones in terrane reconstructions in the alpine north pannonian domain abstract the present day setting of the tectonic units terranes making up the pre neogene basement of the pannonian basin and the west carpathians is the result of large scale displacements during the alpine collisional phases to explain this setting based on various concepts a number of models have been proposed in the last decade most of them agree in an eastward escape of the north pannonian terrane from the alpine domain towards the pannonian during the palaeogene early miocene period in the present paper a reconstruction of the pre escape setting of the displaced terranes is presented based on facies zones in selected time slices from the late permian to the late triassic according to our reconstruction the inner west carpathian units were located east of the austroalpine units forming the northern shelf of the euhallstatt meliatic basin of the vardar oceanic branch the drauzug should have been located in a position much more external than its present day setting the transdanubian range was situated in the northern neighbourhood of the southern alps whereas the bukk may have been located adjacent to the nw dinarides the mid transdanubian zone is a major shear zone containing elements of the julian alps and sava folds nappe system and the inner dinaric ophiolitic melange although the nappe tectonism may have played an important role in the structural evolution of the north pannonian terrane this mechanism alone can hardly produce hundreds of kilometres of facies offset on the opposite sides of strike slip zones

depositional controls on mixed carbonate siliciclastic cycles and sequences on gently inclined shelf profiles the interplay between carbonate production and siliciclastic input produces mixed systems that typically contain a very high degree of lateral and vertical facies heterogeneity this heterogeneity complicates the sequence stratigraphic analysis of mixed systems outcrop studies facilitate the deciphering of controls and understanding of facies distributions within sedimentary successions the picun leufu anticline in the neuquen basin argentina offers the opportunity to integrate large scale depositional architecture with detailed facies descriptions of the shelf to basin successions of the upper jurassic lower cretaceous quintuco picun leufu vaca muerta system the strata in the system are mixed and range in depositional environments from shallow marine sandstones and limestones to deep basinal shales these environments are arranged in metre scale shallowing upward cycles and cycle sets with increasing carbonate proportions in regressive hemicycles increased input of siliciclastic material from the volcanic arc area occurred during phases of relative sea level rise and was controlled by the intensity of along shelf currents the shelf transport was driven by the available accommodation space on the shelf and therefore was a function of the eustatic sea level fluctuations within the studied section a pure carbonate depositional system developed because siliciclastic input was shut down either due to long lived highstand settings or a sudden climatic change to more arid conditions carbonate siliciclastic mixing in this setting is a function of siliciclastic dilution of the carbonate sedimentation and differs from the classical reciprocal sedimentation model which typically includes shut off of carbonate production during lowstand periods in the regional context the subsurface strata of time equivalent reservoirs in the eastern neuquen embayment display strong similarities of architecture indicating that similar mixing processes occurred along most of the neuquen basin

the origin of authigenic ankerite from the ninian field uk north sea the carbonate minerals ankerite and ferroan dolomite are widely reported in studies of concretions1 2 and of sandstone hydrocarbon reservoirs3 8 these minerals are often thought to have formed relatively late in the sandstones diagenetic history and in the gulf coast their occurrence has been linked to clay mineral diagenesis in the surrounding shales9 i describe here authigenic ankerite from the ninian field10 ankerite formation was probably synchronous with hydrocarbon emplacement and is related to clay mineral diagenesis in the hydrocarbon source rocks of the viking graben it occurs throughout the reservoir sandstones but is particularly abundant below the oil water contact this distribution reflects hydrocarbon emplacement arresting diagenesis at progressively deeper levels of the reservoir the resulting preferential cementation may adversely affect transmissibility between the aquifer and the oil column and consequently could reduce hydrocarbon recovery

origin of late dolomite cement by co2 saturated deep basin brines evidence from the ozark region central united states studies of fluid inclusions in regionally extensive late dolomite cement ldc throughout the ozark region show that co2 effervescence was widespread during dolomite precipitation on the basis of quantitative analyses of inclusion fluids reaction path modeling shows that ldc with trace amounts of sulfides can be deposited by effervescence of a co2 saturated basin brine as it migrates to shallower levels and lower confining pressures this precipitation mechanism best explains occurrences of ldc in the ozark region and may account for ldc found in sedimentary basins worldwide

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral