the mid norwegian margin a discussion of crustal lineaments mafic intrusions and remnants of the caledonian root by 3d density modelling and structural interpretation the high density lower crustal body lcb on the mid norwegian margin is almost universally interpreted to represent magmatically underplated material added to the crust during early tertiary opening of the ne atlantic the thickness of the lcb is uneven and its distribution along the margin is sharply limited by margin perpendicular lineaments three dimensional density modelling constrained by petrophysical and seismic data was performed to investigate the expression of the various lineaments bivrost jan mayen gleipne etc in the crustal architecture with and without the lcb the bivrost lineament separating the voring and lofoten margin segments is clearly expressed and so is arguably a lineament in the sw voring margin notably most of the lineaments can be interpreted as offshore prolongations of major onshore detachments stemming from late caledonian orogenic collapse an alternative interpretation of the lcb is thus that it represents high grade metamorphic rocks remnant from the caledonian root determining the nature of the lcb has profound effects for the volume estimate of magmatic rocks in the north atlantic igneous province and consequently also for the degree of crustal thinning and heat flow

contemporary stress orientations in the faroe shetland region the faroe shetland region of the ne atlantic continental margin contains a number of complexly structured mesozoic palaeogene age rift basins but in comparison with the contiguous british isles and north sea basin the state of crustal stress in the faroe shetland region is poorly understood the orientation of maximum horizontal compressional stress hmax across most of nw europe is roughly nw se which is considered to be controlled by forces acting at the plate boundaries we have determined 16 b d quality hmax orientations based on borehole breakouts interpreted in petroleum wells and define three distinct stress provinces within the faroe shetland region stress orientations in the ne are roughly nw se consistent with the regional pattern of stresses in nw europe and local neotectonic structural trends however contemporary stress orientations in the central and sw of the faroe shetland region exhibit short wavelength distances hmax orientation that is controlled by both plate boundary forces and regional scale sources of stresses

the interaction of fluid inclusions and migrating grain boundaries in a rock analogue deformation and annealing of polycrystalline camphor ethanol mixtures the results of deformation and annealing experiments using a rock analogue containing a liquid phase as observed in transmitted light microscopy are presented samples were made of camphor ethanol mixtures with the polycrystalline solid being a rhombohedral phase and the liquid a saturated solution of camphor in ethanol results show the in situ pore fluid morphology during grain boundary migration recrystallization samples were deformed at high homologous temperatures th 0 7 and strain rates of 1 10 6 to 9 10 4 s 1 for grain boundary migration rates ranging from 10 10 to 10 6 ms 1 zener pinning not only the drag and drop of fluid inclusions by migrating grain boundaries but also the passage of grain boundaries over fluid inclusions without noticeable interaction was observed the drag limiting migration rate is three times lower for post kinematic annealing than for dynamic recrystallization and shows a weak dependence on fluid inclusion size a detailed description is presented of fluid inclusion grain boundary interaction with respect to fluid inclusion sizes and grain boundary migration rates fluid inclusions show up to 39 reduction in diameter after passage of a grain boundary indicating fluid flow along the mobile grain boundary

secular variation in seawater chemistry during the phanerozoic as indicated by brine inclusions in halite analysis of data on the chemical composition of primary fluid inclusions brine inclusions in primary bedded halite from many evaporite formations of northern pangea shows that during the phanerozoic the chemical composition of marine brines was oscillating significantly between the na k mg ca cl type and the na k mg cl so4 type we regard those changes as corresponding to the chemical evolution of the phanerozoic ocean the changes correlate in time with earlier suggested secular changes in the mineralogies of marine nonskeletal limestones and potash evaporites in addition to those secular changes of seawater chemistry the concentrations of k mg ca and so4 ions in marine brines underwent important changes under the influence of local factors including rock water interaction these secondary changes did not influence the principal chemical type of the brine characteristic for a given time interval in the past concentration of the ca ion did not exceed the present concentration in marine water by a

petroleum systems of oman charge timing and risks after 35 years of exploration creaming of the conventional plays in oman is nearly complete and consequently the search has commenced for new less obvious plays many of the new opportunities occur beyond the known hydrocarbon provinces and are considered to have significant charge risks to define these risks extensive basin modeling studies have been conducted in recent years modeling and empirical data show that mesozoic and cenozoic kitchen areas are restricted to western north oman the only areas currently buried at their maximum temperature large parts of north and central oman depend on lateral migration from these kitchens for their charge progressive uplift of the east flank and basin inversion since the middle paleozoic provides favorable conditions for long distance migration in the post carboniferous interval in central oman geochemical tracer molecules benzocarbazoles suggest that a north south trending reactivated basement grain has funneled charge up to 300 km southeastward charge risks increase in the deeper sequence in which eastward migrating hydrocarbons have to traverse the ghaba salt basin a pronounced syncline at depths greater than 3 km the south oman salt basin is currently cool because of shallow depths and hydrodynamic fluid flow activity the shallow post cambrian reservoirs rely on storage of early cambrian ordovician charge by the ara salt cambrian sequence followed by release of hydrocarbons as the salt edge retreats through time basin modeling has outlined the extent of the different petroleum systems and provided us with risk maps to guide our next exploration phase it has revitalized some of the mature plays for instance the gharif formation where oil exploration is now focused along late cretaceous and tertiary migration paths deeper sections are envisaged to have significant scope for gas

deformation mechanisms of deeply buried and surface piercing late pre cambrian to early cambrian ara salt from interior oman we compared microstructures of late pre cambrian to early cambrian ara salt diapirs from the deep subsurface 3 5 5 km of the south oman salt basin and from surface piercing salt domes of the ghaba salt basin laterally these basins are approximately 500 km apart but belong to the same tectono sedimentary system the excellent data situation from both wells and outcrops allows a unique quantification of formation and deformation mechanisms spanning from sedimentation to deep burial and via re activated diapir rise to surface piercement microstructures of gamma irradiated and etched thin sections indicate dislocation creep and fluid assisted grain boundary migration as the main deformation mechanisms operating in the deep subsurface microstructures from the surface are characterised by large old subgrain rich crystals these old grains are partly replaced by new subgrain free and subgrain poor crystals which show gamma irradiation decorated growth bands and fibrous microstructures indicative of pressure solution creep and static recrystallisation most likely due to surface piercement and exposure using subgrain size piezometry the maximum differential stresses for the subsurface salt is 1 7 mpa and those for the surface piercing salt is 3 4 mpa the latter value displaying the high stress conditions in the diapir stem as the salt rises on its way to the surface

controls on the location of compressional deformation on the nw european margin the distribution of cenozoic compressional structures along the nw european margin has been compared with maps of the thickness of the crystalline crust derived from a compilation of seismic refraction interpretations and gravity modelling and with the distribution of high velocity lower crust and or partially serpentinized upper mantle detected by seismic experiments only a subset of the mapped compressional structures coincide with areas susceptible to lithospheric weakening as a result of crustal hyperextension and partial serpentinization of the upper mantle notably partially serpentinized upper mantle is well documented beneath the central part of the southern rockall basin but compressional features are sparse in that area where compressional structures have formed but the upper mantle is not serpentinized simple rheological modelling suggests an alternative weakening mechanism involving ductile lower crust and lithospheric decoupling the presence of pre existing weak zones associated with the properties of the gouge and overpressure in fault zones and local stress magnitude and orientation are important contributing factors

rheology and strength of the lithosphere abstract mechanical properties of lithosphere are of primary importance for interpretation of deformation at all spatial and time scales from local scale to large scale geodynamics and from seismic time scale to billions of years depending on loading conditions and time scale lithosphere exhibits elastic brittle plastic or viscous ductile properties as can be inferred from rock mechanics data a large part of the long term lithospheric strength is supported in the ductile or ductile elastic regime while it also maintains important brittle strength yet at short seismic time scale s the entire lithosphere responds in elastic brittle elastic regime even though rock mechanics experiments provide important insights into the rheological properties of the lithosphere their conditions e g time scales strain rates temperature and loading conditions are too far from those of real earth therefore these data cannot be reliably extended to geological time and spatial scales strain rates 10 17 to 10 13 s 1 without additional parameterization or validation based on geological time scale observations of large scale deformation for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes yses were validated by geodynamic scale observations such as the observations of plate flexure however oceanic lithosphere behavior in subduction zones and passive continental margins is strongly conditioned by the properties of the continental counterpart whose rheology is less well understood for continents and continental margins the uncertainties of available data sources are greater due to the complex structure and history of continental plates for example in a common continental rheology model dubbed jelly sandwich the strength mainly resides in crust and mantle while in some alternative models the mantle is weak and the strength is limited to the upper crust we address the problems related to lithosphere rheology and mechanics by first reviewing the rock mechanics data te flexure and ts earthquake data and long term observations such as folding and subsidence data and then by examining the physical plausibility of various rheological models for the latter we review the results of thermo mechanical numerical experiments aimed at testing the possible tectonic implications of different rheology models in particular it appears that irrespective of the actual crustal strength the models implying weak mantle are unable to explain either the persistence of mountain ranges for long periods of time or the integrity of the subducting slabs although there is certainly no single rheology model for continents the jelly sandwich is a useful first order model with which to parameterize the long term strength of the lithosphere it is concluded that dry olivine rheology laws seem to represent well the long term behavior of mantle lithosphere in oceans margins and continents as to the continent and margin crust rheology analysis of the results of thermo mechanical models and of te data based on the most robust variants of flexural models suggests that continental plates with te 30 50 smaller than their theoretical mechanical thickness hm i e te 20 60 km should be characterized by a weak lower or intermediate crustal rheology enabling mechanical decoupling between crust and mantle older plates such as cratons are strong due to crust mantle coupling and specific properties of the cratonic mantle lithosphere

diagenetic evolution of tortonian temperate carbonates close to evaporites in the granada basin se spain abstract the granada basin se spain is a small basin located in the central part of the betic cordillera structured as such in the late tortonian and initially connected to the atlantic ocean and to the mediterranean sea during the late tortonian normal marine conditions prevailed leading to the deposition of skeletal carbonate sediments on platforms around structural highs the marine connections were later interrupted first to the atlantic ocean and then to the mediterranean sea and a thick evaporite sequence marking the transition from marine to continental conditions was deposited during the latest tortonian in this work the diagenetic evolution of the tortonian temperate carbonates ttc underlying and close to the evaporite bodies is revealed and discussed the diagenetic study includes petrographic analyses conventional petrography cathodoluminescence and fluorescence geochemical analyses major minor and trace elements and 13c and 18o stable isotopes and microthermometry of fluid inclusions in the ttc marine diagenetic processes such as micritization and fibrous calcite cement precipitation and mechanical compaction took place during or just after deposition eogenesis an initial burial event mesogenesis 1 is characterized by 1 stabilization of the temperate water carbonates by freshwater and 2 porosity occlusion via precipitation of low mg bladed and syntaxial mosaic calcite cements the ttc were then subaerially exposed or got close to the surface during evaporite deposition and underwent pedogenesis mg smectite infiltration and pyrite formation telogenesis 1 subsequent brine related diagenetic alterations such as dolomitization and silica halite and sylvite replacements of carbonate grains occurred during a second burial episode mesogenesis 2 concomitant with the messinian lacustrine deposition this being followed by chemical compaction stylolite formation finally the area was uplifted and the ttc exhumed microstalactitic dripstone and fibre whisker calcite cement precipitation and extensive dissolution relate to this pliocene quaternary late event telogenesis 2 in the study case diagenetic history is closely linked to basin evolution as diagenetic pathways of carbonate rocks were related to major geodynamic events including basin restriction leading to evaporite deposition and several episodes of subsidence and uplift up to now only very few diagenetic studies have attempted to demonstrate this correlation between diagenetic history and basin evolution

atmospheric gas in modern and ancient halite fluid inclusions a screening protocol abstract halite possesses great potential for hosting and storing information vital to the reconstruction of earth s ancient climate seawater chemistry and evolving atmosphere here we propose a screening protocol that not only distinguishes between primary and secondary halite but also identifies fluid inclusions that carry original gas trapped during the primary crystallization process an integrated multi analytical protocol is presented for sample preparation petrographic evaluation analytical measurements and distinguishing original gases and contents from contaminants the screening protocol starts with the visual inspection of halite for primary chevron and hopper features and or milky appearance then petrography is used to distinguish between primary and secondary diagenetic crystal fabrics and inclusions the trace element and isotope geochemistry speak directly to the composition of the depositional and or diagenetic fluids and to the formation in marine non marine or diagenetic settings furthermore rare earth elements and redox sensitive elements may address the redox conditions of the salt pan flat brines whereas microthermometry helps characterize the depositional diagenetic environment s temperature finally gas extracted by quadrupole mass spectrometer from gas bubbles in fluid inclusions is screened with concomitant ch4 co2 and ar contents to distinguish and quantify organic matter decomposition contributions and with 40ar 36ar to monitor modern atmospheric gas diffusion leakage issues and thus contamination of the primary gas contents our test case shows that halite collected from cores is well suited in maintaining its original mineralogical texture chemistry and gas content our integrated screening protocol suggests that the 815 myr old halite from the tonian browne formation of the officer basin australia formed under atmospheric conditions of about 10 9 po2 back calculation or 9 6 po2 mean calculation the coeval neoproterozoic gypsum collected in outcrop from minto inlet victoria island canada underwent some mineralogical alteration however experiments suggest that depositional gypsum from outcrop core may have potential of retaining vestiges of original gas in fluid inclusions

deep structure of the ocean continent transition in the southern iberia abyssal plain from seismic refraction profiles ocean drilling program legs 149 and 173 transect we present a wide angle seismic refraction study of an 80 40 km region of the southern iberia abyssal plain south of galicia bank an intersecting grid of two e w and four n s wide angle reflection refraction profiles is used to define variations of the basement velocity structure within this unusually wide ocean continent transition oct these structures can be systematically linked to variations in acoustic basement morphology and to results from ocean drilling program odp boreholes lateral changes in the velocity structure of the basement occur abruptly over distances of 20 km where complex variations may be found thinned upper continental crust 2 5 km thick with velocities of 5 0 6 6 km s is limited to a series of n s fault blocks immediately south of galicia bank this crust is underlain by a high velocity layer 7 3 7 9 km s of weakly serpentinized i e 0 25 peridotite which exists throughout the eastern part of the survey area basement within the oct appears to consist dominantly of a broad region of exposed upper mantle that has been serpentinized heterogeneously both vertically and horizontally in the southeast sector of our survey where basement topography deepens and becomes subdued continental fault blocks are absent instead basement contains an upper layer of more pervasively serpentinized i e 25 45 peridotite that is 2 km thick this layer is characterized by low velocity at the top of basement 4 2 km s that increases rapidly with depth and it probably corresponds to a seismically unreflective layer previously identified in reflection profiles to the south of our survey in the western section of our survey beneath a series of elevated basement ridges velocities are reduced within both the upper basement layer 3 5 6 0 km s and lower layer 6 4 7 5 km s these changes suggest that both upper and lower layers have become more highly serpentinized with values of 60 100 in the upper layer and 25 45 in the lower layer probably during the last stages of rifting and immediately before formation of oceanic crust a normal or slow spreading oceanic crustal structure is not found within the survey region thus it appears that the onset of seafloor spreading occurs in the region west of the peridotite ridge sampled at odp site 897 and east of the j magnetic anomaly

on the arrangement of solid inclusions in fibrous veins and the role of the crack seal mechanism the arrangement of solid inclusions in veins was examined in three different rock types and tectonic settings common to all samples is a new type of sinusoidal inclusions associated with inclusion bands oriented parallel to the vein wall and inclusion trails at high angle to the vein wall crosscutting relationships of inclusion bands and the sinusoidal inclusions imply that inclusion bands do not reflect individual crack seal increments in our veins inclusion bands are not a sufficient criterion to infer the crack seal mechanism further evidence for solid inclusions not formed by the classic crack seal mechanism is given by inclusion bands in gypsum crystals grown in soft clay during incorporation solid inclusions can become folded or boudinaged depending on their orientation with respect to the opening direction the complex displacement field within the vein is recorded by curved crystal fibres which track the opening direction based on the arrangement of solid inclusions within the vein we suggest that our veins grew continuously during syntectonic vein formation solid inclusions formed by a fluctuation of adhesion at the vein wall interface rather than by crack sealing and vein growth was at least partly driven by the force of crystallisation q 2005 elsevier ltd all rights reserved

extinction of cloudina and namacalathus at the precambrian cambrian boundary in oman biostratigraphic carbon isotope and u pb zircon geochronological data from the ara group of oman indicate an abrupt last appearance of cloudina and namacalathus coincident with a large magnitude but short lived negative excursion in the carbon isotope composition of seawater that is globally coincident with the precambrian cambrian boundary u pb zircon age data from an intercalated ash bed directly define this negative excursion to be at 542 0 0 3 ma consistent with previous age constraints from siberia and namibia combined with the global biostratigraphic record these new data strengthen hypotheses invoking mass extinction within terminal proterozoic ecosystems at or near the precambrian cambrian boundary

fluid flow and related diagenetic processes in a rift basin evidence from the fourth member of the eocene shahejie formation interval dongying depression bohai bay basin china the purpose of this paper is to relate diagenetic processes in deeply buried sandstones in the fourth member of the eocene shahejie formation interval bohai bay basin china to pore fluid flow changes with progressive burial based on petrographic mineralogical and geochemical analysis distribution patterns of authigenic minerals are recognized that reflect 1 the sources and patterns of fluid flow and 2 fluid flow in an evolving open to closed system partial to extensive precipitation of calcite and dolomite at or near mudstone sandstone contacts during eogenesis was a result of large scale mass transfer between sandstones and adjacent mudstones this process was driven by steep diffusion gradients from adjacent mudstones in a relatively open geochemical system on the local scale support for this model is provided by large sulfur isotope fractionation between framboidal pyrite and precursor gypsum dissolution of feldspar grains and dissolution of nonferroan carbonate cements during early mesogenesis are spatially associated with quartz and ferroan carbonate cementation respectively this process was related to organic carbon dioxide expelled from adjacent source rocks and indicates a relatively open system during late mesogenesis dissolution of evaporitic cements related to thermochemical sulfate reduction tsr generated ankerite and nodular pyrite cements in adjacent pores a lack of sulfur isotope fractionation between parent anhydrite and late stage nodular pyrite during tsr supports a relatively closed fluid flow system because the velocities of pore fluid flow were low during mesogenesis large scale thermal convection and advection probably did not occur instead diffusion over short distances is inferred as the predominant transport mechanism for dissolved solids that were precipitated as other phases either in situ or in adjacent pores

outcrop scale tomography insights into the 3d structure of multiphase boudins abstract we analyze the 3d geomorphology of a boudinaged amphibolite layer encased in marble using meter scale tomography by serial sectioning high resolution imaging and 3d reconstruction of a 2 m block from a quarry in the high grade core of the naxos core complex local uncertainties in the model are resolved by dissolution of marble in selected slabs yielding a large highly accurate 3d model results show five generations of brittle extension of the amphibolite in ductile marble although examination of the boudinaged layer on the outside of the block only suggested one generation most prominent are the rotated domino boudins where part of the dilatancy between domino boudins can be shown to be due to later reactivation shear fractures between domino boudins have relatively large throw gradients indicating ductile rupture layer parallel stretch as measured by reconstruction of domino boudins decreases with amphibolite layer thickness although the layer has undergone the same bulk stretch everywhere this suggests that with decreasing layer thickness deformation becomes delocalized offering tantalizing glimpses of rheology under metamorphic conditions

polyphase thermal evolution in the infra cambrian ara group south oman salt basin as deduced by maturity of solid reservoir bitumen petrographic and organic geochemical analyses on solid reservoir bitumen from the late precambrian to early cambrian ara group indicate polyphase thermal evolution in the south oman salt basin sosb the ara group constitutes a complex petroleum system in the deep subsurface 3 5 km of interior oman salt diapirs encase isolated partly highly overpressured carbonate bodies intra salt stringers which represent a predominantly self charging hydrocarbon system a large number of stringer cores show that reservoir quality is decreased by the presence of solid reservoir bitumen by using different microscopy techniques this solid reservoir bitumen has been identified within pores pore throats and microfractures of the carbonates solid bitumen reflectance measurements brr were converted to vitrinite reflectance vrr using a new improved calibration vitrinite reflectance geothermometry shows that maximum paleo temperatures have a highly heterogeneous distribution paleo temperatures of 380 c were obtained from coke like solid reservoir bitumen which most likely formed from thermal cracking of oil in our model this process is related to hydrothermal fluids deriving from deeper pre salt strata of the sosb which entered the intra salt carbonate stringers during times of basement tectonic activity via hydrofractures this caused the formation of high maturities pyrobitumens in close vicinity to the fractures and precipitation of migrabitumens with increasing distance to the ht source which is supported by the geochemical composition of the solid reservoir bitumens the influx of these fluids into the carbonate stringers is considered to represent a major contribution to their strong overpressures 2007 elsevier ltd all rights reserved

depositional facies and platform architecture of microbialite dominated carbonate reservoirs ediacaran cambrian ara group sultanate of oman intrasalt carbonates of the ediacaran cambrian ara group constitute a significant reservoir element of the intrasalt stringer play in oman in which dolomitic carbonates are encased in salt at depths of 3 to 7 km 1 9 to 4 3 mi these reservoir carbonates have significant microbial influences although ara group reservoirs are mostly latest precambrian the models developed here may be applicable to younger microbially dominated carbonate reservoirs in basins of higher salinity when higher organisms are excluded in lacustrine settings where calcified invertebrates are not a significant source of carbonate or after periods of mass extinction before faunal recovery a broad range of carbonate facies provides the context in which to understand the origin of the microbialite dominated reservoirs developed across both ramp and rimmed shelf profiles major facies associations include carbonate evaporite transition zone deep ramp and slope subtidal microbialites clastic textured carbonates and restricted peritidal carbonates microbialites are subdivisible into a number of facies that all have significance in terms of understanding environmental history as well as reservoir properties and that help in predicting the location of reservoir fairways microbially influenced facies include shallow subtidal thrombolites with massive clotted textures and very high initial porosities 50 shallow subtidal pustular laminites with cm scale variability of lamina morphology deeper subtidal crinkly laminites that show mm scale variability of lamina morphology and intertidal tufted laminates that show mm to cm scale tufted textures other reservoir facies are more conventional grainy carbonates including ripple cross stratified grainstone packstone hummocky cross stratified grainstone packstone flat pebble conglomerate ooid and intraclast grainstone packstone and cloudina grainstone packstone these facies are almost invariably dolomitized and all have moderate to excellent reservoir quality these facies comprise carbonate platforms broken up during salt tectonics that range up to 160 m 525 ft in thickness and extended laterally prior to halokinesis for tens to over 50 km 31 mi the distribution of reservoir facies follows sequence stratigraphic predictions with microbialites occurring in every accommodation profile late highstand and early transgressive systems tracts favor greater lateral extent of thrombolite build ups whereas later transgressive to early highstand system tracts favor greater lateral discontinuity and compartmentalization of buildup reservoir facies pustular laminites occur in close association with thrombolite buildups but form laterally extensive sheets in late transgressive to late highstand periods crinkly laminites form during late transgressive to early highstand systems tracts and may represent maximum flooding intervals when the flux of carbonate sediment was greatly reduced allowing pelagically derived organics to accumulate

origin and deformation of intra salt sulphate layers an example from the dutch zechstein late permian from salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale the relatively thick zechstein 3 anhydrite carbonate layer in the northern netherlands has been studied previously using 3 d seismic data observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub seismic scale imbedded in the zechstein 2 z2 salt core samples thin sections seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these z2 sulphate layers bromine analyses show that they mark a freshening event in the z2 salt which can be correlated over large distances in the northern netherlands their core calibrated log signature indicates that the z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite the mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows thin anhydrite polyhalite couplets and highs thicker anhydrite layers such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation salt internal geometries can further be used to distinguish between different deformation mechanisms in our study area the distribution of sulphate layers within the z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt related deformation

reactivity of evaporites during burial an example from the jurassic of alabama the jurassic louann salt was the first significant sedimentary unit to accumulate in the gulf of mexico sedimentary basin br cl and sup 87 sr sup 86 sr ratios of halite from a single core into the top of the formation record the evaporation of normal seawater to bittern stage the bittern zone today consists of intergrown halite and sylvite the br and rb contents of the solid phases along with sup 87 sr sup 86 sr ratios and rb sr systematics are inconsistent with precipitation of the existing phases from seawater evaporated in jurassic time rather petrography and fluid inclusion and solid phase chemistry from the bittern zone is consistent with postdepositional water rock interaction which diagenetically modified a marine bittern assemblage to halite sylvite the chemistry of the br and rb rich saline formation waters characteristic of this area today likewise may reflect water evaporite interaction during burial

sequence stratigraphy and cyclic development of basal zechstein carbonate evaporite deposits with emphasis on zechstein 2 off platform carbonates upper permian northeast germany abstract although the basal zechstein is classically subdivided into two cycles z1 and z2 lower part it can alternatively be subdivided into three complete third order depositional sequences zs1 zs3 and the associated lowstand systems tract transgressive systems tract of a fourth sequence zs4 third and higher order sea level fluctuations influenced the depositional and diagenetic history of the carbonate evaporite alternations within the basal zechstein the main subject of the present study the zechstein 2 carbonate stassfurt carbonate or ca2 is interpreted to encompass both transgressive predominantly in slope and basinal settings and highstand systems tracts of zechstein sequence zs3 as well as part of the lowstand systems tract of the fourth zechstein sequence zs4 in northeastern germany hydrocarbon reservoirs of the ca2 developed along both northern and southern margins of the southern permian basin on top of the topography of the zechstein 1 cycle z1 this z1 topography resulted from deposition of the werra anhydrite a1 the highstand systems tract of the second zechstein sequence zs2 the northern ca2 carbonate platform is consistently narrow and fringed by coated grain and composite grain bars and shoals it is characterized by a steep fault controlled slope which precluded the formation of in situ shallow water grain dominated carbonates during relative sea level lowstands displaced shallow water coated grain and composite grain packstones wackestones may however occur as slope fan deposits turbidites in contrast the southern carbonate platform is wide tectonically dissected and fringed by grain dominated shallow water carbonates coated grain and composite grain bars and shoals zechstein 2 shallow water carbonates detectable by basal zechstein seismic anomalies also developed on pre existing topographical highs in off platform positions three depositional cycles ca2 cycles i ii and iii most probably reflecting high amplitude fourth order sea level fluctuations can be identified within the stassfurt carbonate our observations suggest that sea level variations possibly reflecting cyclicity in the milankovitch frequency band 0 1 0 4 ma and the pre zechstein topography pronounced during formation of the werra anhydrite were the most important factors in controlling the depositional as well as the diagenetic history of the zechstein 2 carbonate in off platform positions

the influence of the sabatayn evaporites on the hydrocarbon prospectivity of the eastern shabwa basin onshore yemen the shabwa basin is the northeastern extension of the marib al jawf shabwa system of mesozoic grabens located onshore in the republic of yemen an evaporitic sequence with an estimated maximum depositional thickness of 300 metres was deposited during the tithonian it is designated the sabatayn formation and exerts significant control on most of the play elements in the principal hydrocarbon play systems anticipated in the northeastern part of the basin migration of hydrocarbons from pre salt source rocks into intra and post salt reservoirs is restricted by the evaporites localised heat flow perturbations introduced by the salt increase the maturity of post salt source rocks post and intra salt reservoirs are structured by listric faulting on a salt detachment salt pillowing due to post depositional loading by local salt dissolution and by late folding due to gravity sliding of the post salt section on a salt detachment early dissolution and reprecipitation of salt is responsible for occlusion of porosity in intra salt clastic reservoirs

cementing halite in triassic bunter sandstone tonder southwest denmark as a result of hyperfiltration of brines abstract samples of cores from the bunter sandstone formation of the tonder structure have been carefully prepared without access of fresh water the presence of cementing halite has been proven by chemical analysis and microscopic studies of these samples computer log analysis combined with core analysis show the cementing halite to exist mainly in the sandstone layers of the upper bunter sand unit the depositional environments of the bunter sandstone formation of the tonder structure are outlined it is argued that the cementing halite formed as a result of hyperfiltration of the saturated formation brines through low permeable layers of shale this process took place during the emplacement of nitrogen gas in the upper bunter sand unit late in the diagenetic history of the sandstone

crustal structure of the ne rockall trough from wide angle seismic data modeling two wide angle seismic lines located in the northern rockall trough were acquired in may 2000 one line line e crosses the trough from the continental shelf off lewis to normal oceanic crust west of lousy bank in nw se direction the other line line d intersects with line e crosses the wyville thomson ridge in a sw ne direction and ends in the faeroe shetland basin sonobuoy data and expanding spread profiles acquired in the same area have been remodeled analysis of the seismic data using travel times and amplitudes reveals an up to 5 km thick sedimentary basin including an up to 1 5 km thick basaltic layer which is present in most of the trough further conclusions of this study are that the rockall trough is underlain by highly stretched continental crust of 13 km thickness the crust thickens to 24 km beneath lousy bank which is interpreted to be of continental nature beneath the hebrides continental shelf a three layer continental crust of 26 km is modeled an up to 12 km thick high velocity layer is observed underneath the ocean continent boundary and is interpreted as magmatic underplating resulting from excess volcanism during rifting no evidence for an underplate layer could be distinguished beneath the trough area modeling of the structure of the wyville thomson ridge revealed no existing igneous core of the ridge confirming existing theories that it is a compressional structure

influence of salt structures on reservoir rocks in block l2 dutch continental shelf in the subsurface of the netherlands continental shelf thick layers of zechstein salt have developed into salt domes and ridges that occasionally pierce through the overlying formations to measure the influence of the salt on the laterally adjacent mesozoic sandstone reservoir rocks a cementation model was developed the target area block l2 was chosen for the presence of salt domes reservoir rocks and five representative wells all available well information wire line log test and core data has been used to detect the presence of salt in the lower triassic and upper jurassic reservoir intervals this was done mainly by combining gamma ray sonic resistivity and density data from the well logs into a computer model the cementation model produces displays on which the presence of halite cement is clearly indicated only one well l2 2 located within a few hundred metres of a salt dome showed salt plugging in the main reservoir rock the volpriehausen sandstone lower triassic four other wells located at more than 1 5 km from a salt dome or ridge did not show signs of halite cementation in this reservoir therefore the influence of salt domes on the surrounding reservoir rocks is believed to be restricted to less than 1 5 km at 1 2 km depth the salt plugging of the detfurth sandstone lower triassic can be attributed to early seepage from the evaporitic rot formation because the detfurth sandstone in the l2 2 well is not salt cemented seepage of rot brines into the volpriehausen sandstone reservoirs can be excluded

carbonate deposition and hydrocarbon reservoir development at the precambrian cambrian boundary the ara group in south oman abstract the ediacaran early cambrian ara group in the south oman salt basin consists of six evaporite carbonate cycles a0 a1 to a6 that record tectono eustatic sea level changes the a4 cycle developed at the precambrian cambrian boundary 542 ma it forms a single depositional sequence with evaporites deposited in a lowstand systems tract lst overlying carbonates represent an open marine shallowing upward ramp succession that developed in transgressive tst and highstand systems tracts hst the low energy carbonate ramp occupied a relatively protected site between a large shelf and an exposed paleogeographical high the tst facies include sulfates evaporite carbonate laminites and organic rich carbonate laminites that record initial flooding deepening of the basin and establishment of an outer ramp depositional environment carbonate sediment flux was low and the environment was partly subject to cyclically elevated salinity subsequent hst facies comprise mostly fine grained clastic carbonates and stromatolites that formed in middle and inner ramp settings these facies show evidence of shoaling and establishment of a carbonate factory that probably operated over most of the middle and inner ramp sediment was redistributed in suspension by muddy turbidity currents muddy debris flows storm and shallow water currents during the late hst carbonate facies were affected by elevated salinity and recorded the gradual transition to the overlying lst evaporite unit a combination of strong tectonic subsidence and transient flooding caused significant shallow water evaporite deposition to occur not only down dip but also on top of the former carbonate platform where several hundreds of meters of evaporites accumulated the transgressive carbonate laminites are the main reservoir facies and thus represent a relatively unusual reservoir unit the presence of organic material and the relative scarcity of carbonate mud influenced diagenesis and reservoir properties distribution of organic material and carbonate mud can be linked to specific environmental conditions low physical and biogenic disturbance of sediment and stratified water mass and the sequence stratigraphic position low flux of fine grained carbonate during tst in contrast diagenetic evaporite formation has largely degraded reservoir quality in porous shallow water facies near the top of the a4c

lower crustal composition and crustal lineaments on the voring margin ne atlantic a review abstract a prominent lower crustal high velocity layer observed in the voring basin ne atlantic has been investigated with regard to composition and spatial link to crustal scale lineaments in the southernmost part of the basin the lower crustal layer is interpreted as eclogites based on anomalously high p wave velocities 8 4 km s in the remainder of the basin the layer is preferably interpreted as mafic intrusions emplaced during the last phase of rifting but it cannot be excluded that the body consists of older caledonian mafic rocks direct link with high grade caledonian rocks to the east and westwards to east greenland cannot be made since these rocks are mainly of felsic composition the crustal scale lineaments inferred from wide angle seismic data appear to represent the oceanward extension of post caledonian detachments the spatial variation in the thickness of the layer is within the late rifting magmatic hypothesis partly interpreted in terms of pre magmatic variations in thickness of the lithosphere in addition lateral variations in extension mantle upwelling and decompressional melting during the last phase of rifting may also have contributed

secondary porosity creation of enhanced porosities in the subsurface from the dissolution of carbonate cements as a result of cooling formation waters abstract theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral if the formation water has an initial ph below 6 9 then cooling will result in a water with a considerable potential to leach calcite the temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin where faults cease to act as permeable pathways the escaping formation water may be forced through an aquifer in a partly carbonate cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth

brine mineral reactions in evaporite basins implications for the composition of ancient oceans the chemical evolution of several european mesozoic and tertiary evaporite basins was reconstructed by using mineral associations primary fluid inclusion analyses and numerical simulations of evaporation scenarios the solute proportion recorded in the fluid inclusions can be explained by the evaporation of present day seawater as a major recharge the sulfate depletion in the brines is responsible for the type of potash deposit formed potassium magnessium sulfates or sylvite this sulfate depletion can be due either to dolomitization or to the addition of a cacl 2 rich solution to the basin the sulfate depletion occurred in varying intensity in basins of the same age as well as throughout the evolution of the same basin therefore changes in potash mineralogy and sulfate depletion in fluid inclusions are not conclusive arguments in favor of secular variations in the composition of the ocean as recently proposed by several authors

the internal geometry of salt structures a first look using 3d seismic data from the zechstein of the netherlands abstract we present a first look at the large scale complexly folded and faulted internal structure of zechstein salt bodies in nw europe using 3d reflection seismic reflection data from two surveys on the groningen high and the cleaver bank high we focus on a relatively brittle folded and boudinaged claystone carbonate anhydrite layer the z3 stringer enclosed in ductile salt a first classification of the structures is presented and compared with observations from salt mines and analogue and numerical models z3 stringers not only are reservoirs for hydrocarbons but can also present a serious drilling problem in some areas results of this study could provide the basis for better prediction of zones of drilling problems more generally the techniques presented here can be used to predict the internal structure of salt bodies to estimate the geometry of economic deposits of all kinds and locate zones suitable for storage caverns structures observed include an extensive network of zones with increased thickness of the stringer these we infer to have formed by early diagenesis karstification gravitational sliding and associated local sedimentation later this template was deformed into large scale folds and boudins during salt tectonics salt flow was rarely plane strain producing complex fold and boudin geometries deformation was further complicated by the stronger zones of increased thickness which led to strongly non cylindrical structures we present some indications that the thicker zones also influence the locations of later suprasalt structures suggesting a feedback between the early internal evolution of this salt giant and later salt tectonics this study opens the possibility to study the internal structure of the zechstein and other salt giants in 3d using this technique exposing a previously poorly known structure which is comparable in size and complexity to the internal parts of some orogens

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

microstructural evolution of deformation modified primary halite from the middle triassic rot formation at hengelo the netherlands the microstructure of halite from the subhorizontal bedded main rot evaporite member at hengelo the netherlands akzo well 382 depth interval of 420 460 m was studied by transmitted and reflected light microscopy of gamma irradiation decorated samples primary microstructures compare favourably with those found in recent ephemeral salt pans large blocky fluid inclusion poor halite grains and elongated chevrons are interpreted to have formed in the saline lake stage while void filling clear halite is interpreted to have formed during the desiccation stage of the salt pan in addition in all layers the grains are rich in deformation related substructures such as slip bands and subgrains indicating strains of a few percent the study of gamma irradiation decorated thin sections shows that the main recrystallization mechanism is grain boundary migration grain boundary migration removes primary fluid inclusions and produces clear strain free new grains differential stresses as determined by subgrain size piezometry were 0 45 0 97 mpa the deformation of the salt layers is probably related to cretaceous inversion in the area

diagenesis of saline pan halite comparison of petrographic features of modern quaternary and permian halites abstract petrographic studies of modern saline pan halites saline valley ca salina omotepec baja california mexico and quaternary shallow buried 0 200 m halites saline valley ca bristol dry lake ca searles lake ca qarhan saline pan qaidam basin china lake uyuni bolivia show that the diagenetic modification of halite begins contemporaneously with deposition is most intense within the upper few meters of burial and is essentially complete within the first 45 m of burial halite crusts from modern saline pans that have undergone repeated episodes of flooding evaporative concentration and desiccation contain abundant syndepositional diagenetic features these mature modern halites are dominated by dissolution textures and fabrics formed during flood stages and cementati n textures formed during desiccation stages interlayered mud beds contain varying amounts of displacive halite crystals at shallow burial depths halites retain many textural features of mature modern saline pan halite halites below the first few meters are no longer susceptible to dissolution from floodwaters but continue to be cemented by clear halite within the first 10 m of burial cementation reduces the porosity of halite crusts to less than 10 the remaining pore spaces are completely filled by burial depths of approximately 45 m displacive growth of halite in muds continues at shallow burial depths and is probably limited to the first few tens of meters of burial depth the mechanisms for cementation and displacive growth of halite at shallow burial depths probably include 1 evaporative concentration of groundwater brines and 2 cooling of surface brines when they sink below the sedime t surface undeformed halites from the permian salado and rustler formations of new mexico contain delicate syndepositional textures and abundant clear halite cements that are comparable to those observed in modern saline pan halites and shallow buried halites the permian halites are interpreted to have undergone a depositional and early diagenetic history similar to the modern and quaternary analogs complete cementation of saline pan halites at shallow burial depths has important implications for the origin of saline formation waters in sedimentary basins parent evaporite brines may not be stored in the pores of halite rocks and later expelled during burial compaction if the rocks are cemented early and tightly crystallized halite rocks may also impede the downward migration of dense syndepositional brines

carbon isotopic composition of whewellite cac2o4 h2o from different geological environments and its significance carbon isotopic composition of whewellite cac2o4 h2o varies significantly among individual types of occurrences from the bohemian massif czech republic the 13c values of whewellite found inside pelosiderite concretions in the coal bearing carboniferous kladno basin are unusually high from 3 2 to 14 7 indicating influence of bacterial activity on the oxalate carbon isotopic composition in the tertiary northern bohemian basin where the depth of burial and coalification grades are much lower the 13c values of two whewellite samples are 6 9 and 14 2 whewellites from low temperature hydrothermal veins of the p ibram uranium deposits where bacterial processes are highly unlikely have 13c values of 31 7 to 28 4 identical to carbon isotopic composition of underlying black shales

geology of central libya montes mars aqueous alteration history from mineralogical and morphological mapping abstract we analyze the emplacement chronology and aqueous alteration history of distinctive mineral assemblages and related geomorphic units near hashir and bradbury impact craters located within the libya montes which are part of the southern rim of the isidis basin on mars we derive our results from a spectro morphological mapping project that combines spectral detections from crism near infrared imagery with geomorphology and topography from hrsc ctx and hirise imagery through this combination of data sets we were able to use the morphology associated with specific mineral detections to extrapolate the possible extent of the units hosting these compositions we characterize multiple units consistent with formation through volcanic impact hydrothermal lacustrine and evaporative processes altered pyroxene bearing basement rocks are unconformably overlain by an olivine rich unit which is in turn covered by a pyroxene bearing capping unit aqueously altered outcrops identified here include nontronite saponite beidellite opal and dolomite the diversity of mineral assemblages suggests that the nature of aqueous alteration at libya montes varied in space and time this mineralogy together with geologic features shows a transition from noachian aged impact induced hydrothermal alteration and the alteration of noachian bedrock by neutral to slightly basic waters via hesperian aged volcanic emplacements and evaporative processes in lacustrine environments followed by amazonian resurfacing in the form of aeolian erosion

3d seismic study of complex intra salt deformation an example from the upper permian zechstein 3 stringer western dutch offshore mostoftheinformationonsubsurfaceevaporiticstructurescomesfrom3dseismicdata however this data only provide limited information about the internal structure of the evaporites which is known from salt mines and salt diapir outcrops brittle intra salt layers carbonate anhy drite clay ofatleast 10 m thicknessform goodreflectors inevaporites but the structure and dynam ics of such stringers in the salt movement are poorly understood in this study we investigate the intra salt zechstein 3 z3 stringer from 3d seismic data in an area offshore the netherlands observations show complex deformation including boudinage folding and stacking reflections from thin and steep stringer parts are strongly reduced and we present different structural models andtestsofthese wecompareourobservationstostructuralmodelsfromsaltminesandanalogue numerical models of intra salt deformation a smoothed representation of the upper surface of the stringer fragments follows the shape of top salt but smaller scale stringer geometries strongly differ from this and imply boudinage the imaged disharmonic patterns of constrictional folds pro vide evidence for the complexity of the intra salt in agreement with observations in salt mines this may be explained by interaction of the layered salt rheology complex three dimensional salt flow different phases and styles of basement tectonics and movement of the overburden

hydrothermal dolomite a product of poor definition and imagination abstract the latest dolomite bandwagon is the hydrothermal dolomite model in its present form this bandwagon is doomed or at least very much overstated for at least two reasons 1 there are several definitions of hydrothermal and hardly any author specifies which one s he is using 2 very few of the dolomites hitherto called hydrothermal have been demonstrated to be hydrothermal according to any definition except the worst as presently applied the term hydrothermal dolomite is confusing and or meaningless we suggest to use white s geol soc amer bull 68 1957 1637 time honored definition of hydrothermal as aqueous solutions that are warm or hot relative to its surrounding environment with no genetic implications regarding the fluid source hence a dolomite should be called hydrothermal only if it can be demonstrated to have formed at a higher than ambient temperature regardless of fluid source or drive furthermore this definition does not carry a lower or upper temperature limit even a dolomite formed at 40 c could be hydrothermal by extension dolomites formed at temperatures lower than ambient are not hydrothermal even if they formed at a rather high temperature for example groundwater may penetrate a rock sequence through a highly permeable pathway such that it is heated to 150 c at a depth where the surrounding rock has a temperature of 250 c we suggest to call dolomite formed from this water hydrofrigid dolomite formed in or near thermal equilibrium with the surrounding rocks may be called geothermal furthermore not all saddle dolomite formation requires advection fluid flow to transport mg saddle dolomite can be formed in at least three ways i e from advection local redistribution of older dolomite during stylolitization and as a by product of thermochemical sulfate reduction in a closed or semi closed system only the first and the last of these three possibilities have a chance of being hydrothermal almost all dolomites and dolostones in the western canada sedimentary basin have recently been re interpreted as hydrothermal applying the rationale outlined above reveals however that this basin contains very little hydrothermal dolomite rather most dolomites in this basin and almost all dolostones south of the peace river arch are geothermal and or the proof of a hydrothermal origin has not been made this has important implications beyond the various case studies at hand as attempts to tie dolomitization to orogenic events become moot at least in the southern part of the basin

dedolomitization review and case study of uncommon mesogenetic formation conditions abstract dedolomitization converts a dolomite into a calcite and conventionally is interpreted to form in an eogenetic or telogenetic diagenetic environment where meteoric water dissolves evaporites to supply excess calcium hydrologic connectivity between a dolostone and the surface water may then lead to a dedolomitized interval often times being a diagnostic indicator for an erosional unconformity subaerial exposure and or karst in contrast to the classic models we present a case study for pervasive burial dedolomitization unrelated to meteoric fluids dedolomitization of the zechstein 2 carbonate ca2 gas reservoir in nw germany is strongly altering reservoir quality on a regional scale the ca2 shows a textbook correlation between reservoir quality and mineralogy petrographic analyses show that approximately 80 of all observed calcite reveal a dedolomite microtexture causing a reduction of average matrix porosity by 5 to 10 compared to the dolomite dedolomite follows initial layering and abundantly forms massive concretions which are surrounded by compaction related curvature of the depositional layering in the dolomite host rock such an early burial timing is supported by a slight burial related shift of 3 18o and 1 13c towards lighter values in the dedolomites compared to dolomite an exceptionally large amount of calcium rich fluids must have been mobilized to account for 50 of the ca2 rock volume to be dedolomitized however core fabrics related to meteoric diagenesis such as karst fabrics or typical fresh water stable isotope signatures have been observed neither in the ca2 nor in over and underlying anhydrite beds instead excess calcium likely comes from gypsum to anhydrite conversion and pressure solution within over and underlying anhydrites an influx of strontium rich waters from the anhydrite beds is indicated by up to 5 times higher strontium contents measured in dedolomite fabrics compared to their dolomite host rock homogenization temperatures between 51 and 56 c measured in some early fluid inclusions in dedolomite textures further exclude a pristine meteoric water input and suggest fluid entrapment in a burial depth range of 900 to 1400 m comparing these results with literature data furthermore indicates that mesogenetic dedolomite is either volumetrically underestimated in other carbonate evaporite settings or formed under unique diagenetic conditions across the southern permian basin during zechstein times

a depositional model for the terminal neoproterozoic early cambrian ara group evaporites in south oman six evaporite carbonate sequences are recognized in the terminal neoproterozoic early cambrian ara group in the subsurface of oman individual sequences consist of a lower evaporitic part that formed mainly during a lowstand systems tract overlying platform carbonates contain minor amounts of evaporites and represent transgressive and highstand systems tracts detailed sedimentological and geochemical investigation of the evaporites allowed reconstruction of the depositional environment source of brines and basin evolution at the beginning of the evaporative phase prograding succession a shallow water carbonate ramp gradually evolved into a series of shallow sulphate and halite salinas minor amounts of highly soluble salts locally record the last stage of basin desiccation this gradual increase in salinity contrasts sharply with the ensuing retrograding succession in which two corrosion surfaces separate shallow water halite from shallow water sulphate and shallow water sulphate from relatively deeper water carbonate respectively these surfaces record repeated flooding of the basin dissolution of evaporites and stepwise reduction in salinity final flooding led to submergence of the basin and the establishment of an open water carbonate ramp marine fossils in carbonates and bromine geochemistry of halite indicate a dominantly marine origin for the brines the ara group sequences represent a time of relatively stable arid climate in a tectonically active basin strong subsidence allowed accommodation of evaporites with a cumulative thickness of several kilometres while tectonic barriers simultaneously provided the required restricted conditions subsidence allowed evaporites to blanket basinal and platform areas the study suggests a deep basin shallow water model for the evaporites

the use of environmental markers to distinguish marine vs continental deposition and to quantify the significance of recycling in evaporite basins abstract environmental markers namely the bromine content of halite samples the electrolyte content of primary fluid inclusions in halite and the isotopic composition of sulphates from two tertiary evaporite sequences provide complementary information on the depositional environment marine vs continental the use of these markers together with lithofacies and thicknesses of precipitated evaporites enable the detection and quantification of evaporite recycling within evaporite basins the information provided by the isolated use of each of these geochemical markers should be used with caution as this could lead to erroneous interpretations of the depositional environment or may not detect significant evaporite recycling processes the complementary information provided by geochemical markers enable quantification of solute input from recycling of previous precipitates within the basins themselves or from older evaporites the input of recycled evaporites increases progressively together with evaporite basin restriction to the open ocean

the geochemical evolution of the catalan potash subbasin south pyrenean foreland basin spain over the last decade our studies in ancient evaporitic basins have been based on a detailed study of a single borehole record the detailed findings in medium to large sized evaporitic basins were shadowed with a relevant question can interpretations from a representative evaporitic record in a single borehole be extended to the whole evaporitic basin this paper addresses that question the results obtained are compared with results from another distant point within the basin the general methodology not only proves its reliability in interpreting the evolution of evaporitic basins from a single borehole but reveals its capability to obtain detailed palaeoenvironmental interpretations the chemical evolution of an upper eocene evaporitic sequence from the south pyrenean foreland basin spain has been investigated along the suria 19 borehole record detailed petrographic and mineralogical study x ray microanalysis of frozen primary inclusions trapped in halite cryo sem eds systematic isotopic analysis d 34 s and d 18 o in sulphates and computer based evaporation models have been integrated in a multi proxy methodology this study revealed that a variable amount of ca excess is required throughout different parts of the marine lower halite unit lhu for sylvite instead of k mg sulphates to form this ca excess is in turn different from that required for the western sector of the same evaporitic basin navarrese subbasin quick and variable changes in ca rich brines or equivalent dolomitization required are explained as internal processes within the basin rather than secular variations in seawater chemistry the general hydrological evolution of the catalan subbasin is explained as a restricted subbasin with a first marine stage in which continental input up to 50 of total input had an important control on the geochemistry of the subbasin a second stage was determined during potash precipitation in which the subbasin was cut from any seawater input to end up in its last stage as a purely continental evaporitic basin coupling evaporation models and analytical results we have obtained the proportions of recycling and their sources estimated to change from a 100 total mass of sulphate eocene source to 20 eocene and 80 triassic keuper towards the latest stage of potash precipitation the results obtained have been compared with results from the navarrese subbasin allowing an integrated interpretation of the hydrological evolution of the whole upper eocene south

evaporites marine or non marine division of evaporites into those types that retain information about primary mineralogy textures and structures primary evaporites and those types so extensively altered that few or no primary features remain secondary evaporites only primary evaporites can give unequivocal data bout their marine or non marine origin to help evaluate such primary deposits they have been further divided using the following attributes 1 whether the depositional setting was marine or non marine 2 whether the source water was a seawater b meteoric c hydrothermal d diagenetic e volcanogenic or f mixed specific criteria for making distinctions are 1 kinds of fossils 2 sedimentology of the associated non saline facies 3 kinds of primary saline minerals 4 association of such saline minerals both in qualitative and quantitative terms 5 trace element isotope and fluid inclusion geochemistry of such saline minerals modified journal abstract

discrete element modeling of boudinage insights on rock rheology matrix flow and evolution of geometry 1 we use discrete element model simulations to model the full boudinage process from initial fracturing of intact material to post fracture flow of material into gaps between fragments and to investigate the role which the material properties of the weak and strong layers play in this process the models are deformed in 2d plane strain under a range of confining stresses in coaxial bulk flow results show that the material properties i e mohr coulomb or quasi viscous for the matrix and elastic brittle for the competent layer lead to the development of natural looking boudin morphologies and deformation patterns in the matrix the details of the matrix rheology only have a minor influence on the morphology of the boudins by varying the material properties of the competent layer between fully brittle and semi ductile we obtain a wide range of deformation patterns ranging from pinch and swell structures to a variety of boudin types including drawn shear band and straight sided torn boudins in a number of models we observe rotation of the boudin blocks despite the applied deformation being purely coaxial these rotations are generally related to asymmetrical rhombic boudin shapes some features observed in natural boudins such as concave block faces or the formation of veins between fragments are not modeled because pore fluids are not yet included in our model

interrelation between rifting faulting sedimentation and mantle serpentinization during continental margin formation including examples from the norwegian sea the conditions permitting mantle serpentinization during continental rifting are explored within 2 d thermotectonostratigraphic basin models which track the rheological evolution of the continental crust account for sediment blanketing effects and allow for kinetically controlled mantle serpentinization processes the basic idea is that the entire extending continental crust has to be brittle for crustal scale faulting and mantle serpentinization to occur the isostatic and latent heat effects of the reaction are fully coupled to the structural and thermal solutions a systematic parameter study shows that a critical stretching factor exists for which complete crustal embrittlement and serpentinization occurs increased sedimentation rates shift this critical stretching factor to higher values as sediment blanketing effects result in higher crustal temperatures sediment supply has therefore through the temperature dependence of the viscous flow laws strong control on crustal strength and mantle serpentinization reactions are only likely when sedimentation rates are low and stretching factors high in a case study for the norwegian margin we test whether the inner lower crustal bodies lcb imaged beneath the more and voring margin could be serpentinized mantle multiple 2 d transects have been reconstructed through the 3 d data set by scheck wenderoth and maystrenko 2011 we find that serpentinization reactions are possible and likely during the jurassic rift phase predicted thicknesses and locations of partially serpentinized mantle rocks fit to information on lcbs from seismic and gravity data we conclude that some of the inner lcbs beneath the norwegian margin may be partially serpentinized mantle

formation of celestite in the wenlock oslo region norway evidence for evaporitic depositional environments the steinsfjord formation wenlock of the ringerike district southeastern norway contains marls and carbonates which were deposited in supratidal intertidal and restricted subtidal environments rhythmic lithological variations seen within the unit represent small transgressive and regressive events red arenaceous and argillaceous dolomite reflects minor progradations of the adjacent continental facies from the overlying formation celestite is found in two associations within the strata 1 as aggregates of tiny celestite crystals in dolomicrite of high intertidal facies or 2 as anhedral or euhedral prismatic crystals infilling intrapores and interpores of low intertidal to subtidal facies celestite may have been formed from porewater enriched in so 4 super 2 by solution of calcium sulphate in the high intertidal supratidal environment this sulphate has then reacted with sr super 2 released during aragonite to dolomicrite transformation high intertidal and aragonite to calcite transformation low intertidal subtidal the occurrence of celestite in these deposits is evidence for evaporitic conditions in the uppermost marine sequence of the oslo region s lower paleozoic succession

principles of geochemical prospect appraisal our current understanding of the geochemical processes responsible for the creation of commercial accumulations of oil and gas can be reduced to simple rules and equations to allow the petroleum geologist to use geochemistry to estimate before drilling the likely volumes and compositions of oil and gas delivered to the trap in most sedimentary basins oil is expelled from source rocks between 120 and 150 c whereas most gas and gas condensate are released between 150 and 230 c when the initial potential of source rocks exceeds 10kg mt oil expulsion efficiencies are between 60 and 90 expelled oil and gas migrate as petroleum rich phases driven by fluid potential gradients most flow is laterally updip in beds with effective horizontal permeabilities greater than 1 md in lower permeability rocks the petroleum fluids move vertically up or down along the path of least resistance i e the most permeable or thinnest beds separating them from a high permeability lateral carrier bed the residual saturations of petroleum left behind along the migration pathway are of the order of a few percent of the porosity geochemical analysis of source rocks helps determine the yields and compositions of petroleum fluids expelled in the catchment area of a prospect these fluid masses are converted to volumes from knowledge of the phase relations and densities of petroleum fluids evaluation of the total volume of rock through which the petroleum potentially can migrate and the mean porosity of the rocks that constitute the migration pathway allow migration losses to be assessed these losses constrain both the range of migration of petroleum fluids away from the mature source rock and the volumes of petroleum remaining for entrapment in the prospect

sedimentation cycles and their diagenesis on the slope of a miocene carbonate ramp bahamas odp leg 166 numerous small scale depositional cycles are present in the miocene sediments of seismic sequence m along the margin of great bahama bank odp leg 166 these cycles consist of decimetre to metre scale alternations between light grey and dark grey black wackestones packstones the light grey layers are well cemented and nearly uncompacted they contain planktonic and benthic foraminifera and bioclasts bioturbation in these layers is moderate the dark grey wackestones packstones are uncemented strongly compacted and normally strongly bioturbated the main components are planktonic foraminifera and fine grained bioclasts the dark layers are rich in aragonite and organic carbon and contain around 80 carbonate the light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97 5 light 13c and slightly negative 18o values were observed in the dark uncemented layers while the cemented intervals show heavy 13c values and slightly more positive 18o values the carbon isotope signal between the dark and the light layers shows variations of up to 1 45 both 13c and 18o co vary with carbonate content the formation microscanner images reflect the changes in carbonate mineralogy the natural gamma ray tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy the internal stacking pattern of the cycles is closely tied to sea level the dark layers are deposited during rising sea level while the light layers reflect sediment production and export during highstand and falling sea level these cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat topped platforms most measured parameters suggest that primary sediment composition played an important role in the cementation process the primary differences in composition were enhanced during shallow burial diagenesis the initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite rich layers leading to the formation of the light beds dissolution of metastable high mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite diagenesis was enhanced by the coarse grain size of the primary aragonite rich layers in contrast the more pelagic dark calcite rich layers were only slightly affected by diagenesis and could preserve their aragonite content the uncemented layers subsequently were subject to strong mechanical compaction whereas the cemented beds stayed nearly uncompacted

halite cementation and carbonate diagenesis of intra salt reservoirs from the late neoproterozoic to early cambrian ara group south oman salt basin late neoproterozoic to early cambrian carbonates of the ara group form important intra salt stringer reservoirs in the south oman salt basin differential loading of thick continental clastics above the six carbonate to evaporite cycles of the ara group led to the formation of salt diapirs encasing a predominantly self charging hydrocarbon system within partly highly overpressured carbonate bodies stringers these carbonates underwent a complex diagenetic evolution with one stage of halite cementation in a shallow early and another in a deep late burial environment early and late halite cements are defined by their microstructural relationship with solid bitumen the early phase of halite cementation is post dated by solid reservoir bitumen this phase is most pervasive towards the top of carbonate stringers where it plugs nearly all available porosity in facies with initially favourable poroperm characteristics bromine geochemistry revealed significantly higher bromine contents up to 280 p p m in the early halite compared with the late halite 173 p p m the distribution patterns and the high bromine contents of early halite are consistent with precipitation caused by seepage reflux of highly saturated brines during deposition of the overlying rock salt interval later in burial history relatively small quantities of early halite were dissolved locally and re precipitated as indicated by inclusions of streaky solid bitumen within the late halite cements late halite cement also seals fractures which show evidence for repeated reopening initially these fractures formed during a period of hydrothermal activity and were later reopened by a crack seal mechanism caused by high fluid overpressures porosity plugging by early halite cements affects the poroperm characteristics of the ara carbonates much more than the volumetrically less important late halite cement the formation mechanisms and distribution patterns of halite cementation processes in the south oman salt basin can be generalized to other petroliferous evaporite basins

late precambrian cambrian sediments of huqf group sultanate of oman the huqf group is the oldest known sedimentary sequence overlying crystalline basement in the sultanate of oman it crops out on a broad regional high the huqf axis which forms a dominating structural element on the southeastern edge of the arabian peninsula subsurface and outcrop evidence within and outside of oman suggests that the sediments of the huqf group lie within the age span of late precambrian to early middle cambrian the huqf group is subdivided into five formations corresponding to an alternation of clastics abu mahara and shuram formations and carbonates khufai and buah formations deposited in essentially shallow marine to supratidal or fluviatile conditions and terminated by an evaporitic sequence ara formation evaporites are absent on the huqf axis but they are thickly developed to the west over a large part of southern and central oman where they acted as the major structure former of most of oman s oil fields and even locally pierced up to the surface in the subsurface the huqf group has been reached by the drill mainly in southern oman where its lithostratigraphy resembles that defined in the type area in central oman the oldest sediments exposed in the core of the oman mountains may be correlative with the huqf group regional correlations suggest that the predominantly carbonate evaporitic facies of the huqf group was widely distributed in late precambrian early cambrian time the huqf basin is tentatively considered part of a belt of evaporitic basins and intervening carbonate platforms which stretched across the pangea landmass from the indian subcontinent salt range of pakistan through south yemen oman and saudi arabia into the gulf states and iran hormuz series and carbonate platform north of the zagros

numerical modelling of the displacement and deformation of embedded rock bodies during salt tectonics a case study from the south oman salt basin large rock inclusions are embedded in many salt bodies and these respond to the move ments of the salt in a variety of ways including displacement folding and fracturing one mode of salt tectonics is downbuilding whereby the top of a developing diapir remains in the same vertical position while the surrounding overburden sediments subside we investigate how the differential displacementofthe top saltsurface causedbydownbuilding inducesductilesaltflowand the associ ated deformation of brittle stringers by an iterative procedure to detect and simulate conditions for the onset of localization of deformation in a finite element model in combination with adaptive remeshing the model set up is constrained by observations from the south oman salt basin where large carbonate bodies encased in salt form substantial hydrocarbon plays the model shows that depending on the displacement of the top salt the stringers can break very soon after the onset of salt tectonics and can deform in different ways if extension along the inclusion dom inates stringersarebrokenbytensilefracturesandboudinageatrelativelyshallowdepth spacingof the boudin bounding faults can be as close as 3 4 times the thickness of the stringer in contrast salt shortening along the inclusion may lead to folding or thrusting of stringers

the hatton basin and continental margin crustal structure from wide angle seismic and gravity data results from a wide angle seismic and gravity study between the rockall bank and the iceland basin in the north atlantic are presented crustal and sedimentary structures are resolved in the hatton basin and across the hatton continental margin hcm east of magnetic anomaly 24 the structure of the oceanic crust west of the anomaly is also determined gravity data support the seismic model in areas of good seismic coverage and are used to control the model where the wide angle seismic data are poor a two layer sedimentary sequence is present both in the hatton basin and across the continental margin the lower layer with p wave velocity of about 4 km s is interpreted as pre eocene synrift sediments and is up to 3 5 km thick a younger and thinner 1 2 5 km postrift sequence with a velocity of about 2 km s defines a strong velocity contrast which suggests an erosional unconformity surface the sedimentary structure is distinctly different from that in the rockall trough where a third intermediate layer vp 3 km s occurs the three layer crust characterized by two intracrustal reflections pip1 and pip2 varies from 30 km thick under the rockall bank to about 15 km below the hatton basin where it is stretched by a factor of 2 relative to onshore ireland the crust is thinnest below the hatton bank where the presence of a single intracrustal reflection indicates that the lower crustal layer thins to below the seismic resolution limit below the hcm a region of thick lower crust with anomalously high velocity vp 7 2 km s is resolved by the seismic and gravity data it is connected west of anomaly 24 to a region of oceanic crust which is thicker than in the iceland basin these relationships between the thick lower crust below the hcm and the oceanic crust in the iceland basin are interpreted as evidence for magmatic underplating consistent with previous models for the hcm the inferred unconformity surface between the synrift and postrift layers may be due to regional uplift driven by upwelling of hot asthenosphere before anomaly 24 early eocene time

the paleozoic tectonothermal evolution of the bachu uplift of the tarim basin nw china constraints from u th he ages apatite fission track and vitrinite reflectance data abstract this study uses a new application of the apatite and zircon u th he thermochronometry technique to investigate the thermal history and tectonic uplift of sedimentary basins based on measured apatite and zircon u th he ages apatite fission track data and equivalent vitrinite reflectance measurements of borehole core samples the thermal history of the bachu uplift of the tarim basin was modeled our analysis indicates a complicated thermal history in the bachu uplift region during the paleozoic the thermal gradient initially increased from 28 30 c km in the cambrian period to 30 33 c km in the ordovician period and increased up to 33 34 c km in the silurian period the thermal gradient began to decrease during the devonian period and was estimated to be approximately 30 c km during the late devonian and the early carboniferous periods the low thermal gradient phase was followed by an additional high gradient phase from the late carboniferous to the early permian when the gradient was approximately 32 c km thermal gradients in the paleozoic were substantially different than the present day gradient 21 c km tectonothermal modeling results also revealed uplift events in the bachu uplift region that shifted from south to north e g 150 ma to 100 ma and from west to east e g 250 ma to 150 ma this thermal evolution of the bachu uplift is consistent with its tectonic progression the thermal spike in the late carboniferous and early permian periods could be related to magmatic activities in the early permian period the paleozoic thermal history of the bachu uplift region of the tarim basin could not be reconstructed properly due to a lack of reliable thermal indicators in the lower paleozoic successions and a poor understanding of the thermal evolution of lower paleozoic source rocks during the paleozoic has hindered petroleum exploration in this area this study provides a new paleozoic thermal history and shows that the paleozoic thermal history plays an important role in the maturation process of the lower paleozoic source rocks

deposition and early alteration of evaporites summary the depositional settings for primary and early diagenetic evaporite deposits generally fall into three categories marginal mixed shallow subaqueous and subaerial shallow and deep subaqueous these three environmental groupings hold for both marine and nonmarine settings although the details of continental evaporites may be far more complex than in most marine fed water bodies the primary evaporite morphologies from many continental playa hypersaline marine and marine marginal depositional settings are reasonably well understood because of the numerous detailed studies of recent holocene and cenozoic deposits that serve as models for sedimentary interpretation the sedimentological features that develop in deeper water settings are inferred based on examination of unaltered cenozoic deposits each environmental setting develops characteristic depositional features and patterns and one facies grades into the next because there may be significant physiochemical changes in water composition during deposition as well as sudden change s in both water depth and basinal circulation description and interpretation of evaporative rocks should not be based on mineralogy alone but on the distinct sedimentary characteristics of each part of a deposit in cases where sediments still reflect their primary mineralogy and morphology most of the environments can now be recognized however geochemical studies are commonly required to determine the source s of the original water the determinative geochemical techniques that presently serve as a support to sedimentologic study include studies of fluid inclusions bromine content of chloride salts and the stable isotopes of strontium sulphur carbon and oxygen only after these and perhaps other chemical analyses are considered can the full depositional history of a region be reasonably unravelled

crustal structure and evolution of the outer more margin ne atlantic abstract five regional ocean bottom seismic profiles from the northwestern part of the more margin ne atlantic have been modeled with regard to crustal structure and the interpretations have been supported by the use of multi channel seismic and potential field data the chi squared values uncertainty for the studied profiles are generally in the range 1 3 and the uncertainty in the moho depth is estimated to 1 km the continent ocean transition is found to be relatively sharp 10 40 km wide expressing a change in physical properties corresponding to felsic continental crust along the landward part of the marginal high vp 6 2 6 5 km s and thick mafic oceanic crust further northwestwards the early eocene breakup was associated with excessive magmatism and the models document direct continuity between this magmatism and a lower crustal high velocity layer in the more basin vp 7 1 7 3 km s suggesting a genetic link the jan mayen lineament is observed across the entire study area as northeastward termination of the lower crustal body as well as structuring in the top of the crystalline basement and the moho we present a speculative model involving lower crustal bodies of eclogites vp 8 km s inferred in the easternmost part of the more basin linking the jan mayen lineament to the post caledonian nordfjord sogn detachment

compaction in halite cemented carbonates the dawson bay formation middle devonian of saskatchewan canada halite impregnated carbonates in the dawson bay formation of saskatchewan lie between beds of halite and are buried to a depth of 1 km they exhibit two different diagenetic styles some resisted compaction and had high pre salt porosities others contain compaction broken fossils and pressure solution seams the uncompacted rocks together with the difficulty of explaining how halite cement could enter the dawson bay after overlying bedded halites were deposited suggest that halite cementation occurred early with only a few tens of metres of overburden early diagenetic compaction is suggested by the presence of unbroken displacive skeletal halite crystals which cross cut compaction structures and by the difficulty of explaining how 1 later compaction could occur in halite cemented rocks and 2 how pore fluids could be expelled after surrounding rocks lost their permeability the organic rich nature of many carbonates may explain why compaction was both early and extensive but this explanation fails to explain how similar compaction developed in horizons with lower organic contents chemical compaction may also have been enhanced by aragonite dissolution during seawater evaporation or brine dilution early chemical compaction in dawson bay carbonates indicates that compaction in other carbonates need not signify deep burial diagenesis neither can compaction be used indiscriminately to identify other diagenetic events as being of deep burial origin early halite cementation as in the dawson bay formation preserves carbonates at early diagenetic stages and may thus preserve geochemical information unmodified by later diagenesis

micro and nano pores in intrasalt microbialite dominated carbonate reservoirs ara group south oman salt basin abstract dolomitic microbialites form important exploration targets for the oil and gas industry in many parts of the world it is known that a significant portion of their total porosity and potential storage capacity is formed by micropores despite their economic importance few studies attempted to characterize the micro to nanoporosity in dolomitic microbialites recent advances in the field of microscopy now allow to image and analyze their pore systems on a hitherto unprecedented scale this study evaluates pore systems in neoproterozoic ara group carbonates forming part of the deeply buried intra salt a2c hydrocarbon play of the south oman salt basin sosb micro and nanoporosity there is hosted by aphanocrystalline and to a minor extent microcrystalline dolomite image analysis derived from broad ion beam milling and scanning electron microscopy bib sem of representative samples display a contribution between 50 and 100 from micro and nanopores to total porosities which are up to 17 the main diagenetic controls on microporosity in the a2c are the formation and preservation of primary or early diagenetic aphanocrystalline dolomite and the presence of evaporites calcite and solid bitumen petrographic and petrophysical observations show good preservation of the original textural and geochemical attributes in most dolomite samples even after burial to 5 km depth for the past 500 million years this suggests a structurally and hydrodynamically isolated rockbuffered system with a limited extent of cementation confocal laser scanning microscopy clsm reveals significant amounts of hydrocarbons which manifest as degenerated organic material in the carbonates they originated from precursor bacterial organic matter in microbial laminite thrombolite and peloidal source rocks and might have prevented extensive recrystallization of the aphanocrystalline dolomite matrix and further cementation of the porespace primary dolomite might still be preserved in the a2c microbialites as petrographic observations reveal similarities in crystal sizes and the texture of euhedral aphanocrystalline dolomite as observed in comparable sedimentary systems

bromine geochemistry of salar de uyuni and deeper salt crusts central altiplano bolivia abstract the salar of uyuni in the central bolivian altiplano is probably the largest salt pan in the world 10 000 km 2 a 121 m deep well drilled in the central salar disclosed a complex evaporitic sequence of 12 salt crusts separated by 11 mud layers in the lower half of the profile thick halite beds alternate with thin mud layers the mud layers thicken upwards and show clear lacustrine features the thickness of the salt beds decreases markedly from the base upward the bromine content of the halite ranges from 1 3 to 10 4 mg kg the halite does not originate from the evaporation of the dilute inflow waters of the altiplano which would lead to br content of tens of mg kg the presence of halite of very low br content 2 mg kg in a gypsum diapir strongly suggests that most of the halite deposited at uyuni originated from the leaching of ancient salt formations associated with the numerous gypsum diapirs of the altiplano the deep and thick halite beds were probably deposited in a playa lake as suggested by their very low and fairly constant br content 1 6 2 3 mg kg and by the abundance of detrital minerals thereafter perennial salt lakes of increasing duration flooded the central altiplano increasing amounts of dilute inflow waters of low cl br ratio and the exhaustion of diapiric halite lead to an increase of both the thickness of the lacustrine layers and of the bromine content of the salt crusts 5 10 mg kg deposited as the lakes dried up paleolake levels in the central altiplano rose as those in the northern altiplano the titicaca basin were simultaneously falling the progressive fluvial erosion of the threshold between the northern and the central altiplano lowered the levels of the northern lakes and allowed more dilute waters of low cl br ratio to flood the central altiplano therefore both the thickness of lacustrine sediments and the bromine content of the salt crust increased to the present the three upper crusts show a decrease in bromine content which may reflect a recent modification of the inflow regime to the last lakes the salt crust deposited by the last salt lake 10 000 years ago shows a bromine distribution that unexpectedly increases with depth as the lake dried the salt crust was probably initialy distributed over a wider area than the present flat crust covering the topographic surface up to the level of the lake where halite reached saturation the dissolution of the initial salt of low br content from the margins of the lake to the center is probably responsible for the observed br distribution in the surface salt as in a playa during the holocene a severe drought hit the altiplano and drastically lowered the intracrustal brine table within the salt crust leaving a thin layer of high concentrated residual brine at the bottom of the crust bromine rich halite replaced primary halite by a temperature activated mechanism in the lower crust further increasing the br gradient with depth

structurally controlled hydrothermal dolomite reservoir facies an overview structurally controlled hydrothermal dolomite htd reservoir facies and associated productive leached limestones are major hydrocarbon producers in north america and are receiving increased exploration attention globally they include multiple trends in the ordovician locally silurian and devonian of the michigan appalachian and other basins of eastern canada and the united states and in the devonian and mississippian of the western canada sedimentary basin they also occur in jurassic hosts along rifted atlantic margins in the jurassic cretaceous of the arabian gulf region and elsewhere hydrothermal dolomitization is defined as dolomitization occurring under burial conditions commonly at shallow depths by fluids typically very saline with temperature and pressure t and p higher than the ambient t and p of the host formation the latter commonly is limestone proof of a hydrothermal origin for htd reservoir facies requires integration of burial thermal history plots fluid inclusion temperature data and constraints on timing of emplacement hydrothermal dolomite reservoir facies are part of a spectrum of hydrothermal mineral deposits that include sedimentary exhalative lead zinc ore bodies and htd hosted mississippi valley type sulfide deposits all three hydrothermal deposits show a strong structural control by extensional and or strike slip wrench faults with fluid flow typically focused at transtensional and dilational structural sites and in the hanging wall transtensional sags above negative flower structures on wrench faults are favored drilling sites for htd reservoir facies saddle dolomite in both replacive and void filling modes is characteristic of htd facies for many reservoirs matrix replacive dolomite and saddle dolomite appear to have formed near contemporaneously and from the same fluid and temperature conditions the original host facies exerts a major influence on the lateral extent of dolomitization resultant textures pore type and pore volume breccias zebra fabrics shear microfractures and other rock characteristics record short term shear stress and pore fluid pressure transients particularly proximal to active faults high temperature hydrothermal pulses may alter kerogen in host limestones a process designated forced maturation basement highs underlying sandstone and or carbonate aquifers probably overpressured and overlying and internal shale seals and aquitards also may constrain or influence htd emplacement although many questions and uncertainties remain particularly in terms of mg and brine source and mass balance recognition and active exploration of the htd play continues to expand increasing use of three dimensional seismic imagery and seismic anomaly mapping combined with horizontal drilling oblique to linear trends defined by structural sags helps to reduce risk