modern lacustrine microbialites towards a synthesis of aqueous and carbonate geochemistry and mineralogy abstract lacustrine microbialites record evidence of their depositional environments in their morphology mineralogy and geochemistry this synthesis reviews geochemical data both for microbialites and lake water for 21 modern lacustrine microbialite occurrences younger than 15 000 years although these data are limited several trends and associations are identified that provide useful criteria to aid interpretation of the environmental settings of ancient analogues these microbialites are either thrombolites or thrombolitic stromatolites they form in diverse settings including karst volcanic coastal and inland athalassic lakes surveyed lakes are mostly closed basins subject to evaporative processes lakes vary from stratified to totally mixed examples the major hydrochemical types include na cl ca so4 ca hco3 and soda lakes the salinity and alkalinity of the lakes correlates as expected with the mode of mineralization which in turn is reflected in the microstructure of the microbialites a correlation between the 13ccarbonate and the ca2 alkalinity ratio ca alk is observed generally lakes with ca alk 1 are subject to carbonate precipitation driven by either uptake of co2 that results in a broad range of positive and negative 13ccarbonate or mineralization mediated by sulphate reducing bacteria in saline to hypersaline environments that results in negative 13ccarbonate ca alk 39 are associated with hydromagnesite dolomite high mg calcite hmc and monohydrocalcite are associated with the total aqueous concentration of mg cmg occurring only in lakes with cmg 75 meq l evaporites e g gypsum are related to high lake water salinities the absolute concentration of aqueous si commonly associated with dissolution of diatom tests influences precipitation of crystalline and or amorphous silicates silica to form microbialites with si concentrations above 0 54 mmol l either as amorphous phases or as minerals such as stevensite and kerolite combined the trends observed in this survey demonstrate that modern lacustrine microbialites can preserve information symptomatic of the environments in which they formed however in a limited number of examples this is not the case as subsequent diagenesis may alter the original mineralogy to a point where the geochemical evidence conserved from the depositional environment is lost additionally some geochemical information may be the product of isolated microenvironments within living biofilms and thus not necessarily directly related to the chemistry of ambient lake water further misleading conclusions would result if the hydrochemistry at the time of study had changed from those prevailing when the microbialites were mineralized thus all the correlations developed in this work represent hypotheses to be tested rather than interpretations to be accepted this compilation demonstrates how microbialites respond to different lacustrine hydrochemical environments and is intended to provide a guide for future field studies and laboratory simulations this review also highlights the general scarcity of trace element data for lake waters and microbialites however some recent studies suggest that lacustrine microbialites may fractionate rare earth elements ree during uptake from water whereas marine microbialites appear to record the ree distribution from seawater without modification this possible difference in ree behaviour could have implications for the interpretation of palaeoenvironmental proxies

relationship between biofabric and petrophysics in coquinas insights on brazilian barremian aptian carbonates of the santos and sergipe alagoas basins coquinas are a sedimentary rock composed mainly of shells and their fragments with a carbonate or siliciclastic matrix these rocks exhibit a high range of porosity and permeability values that are related to depositional and diagenetic processes a series of coquina reservoirs were discovered in the campos and santos basins in the eastern margin of brazil and the understanding of the petrophysical properties of these rocks is essential for reservoir production in this work samples from the morro do chaves formation sergipe alagoas basin and the itapema formation santos basin were selected to identify the porosity vs permeability relationship according to the biofabric characteristics and porosity control the biofabric textures where classified into four groups according to taphonomic patterns and diagenetic modification i well sorted and preserved biofabric ii well sorted and altered biofabric iii unsorted and preserved biofabric and iv unsorted and altered biofabric the porosity control varies in all groups from depositional to diagenetic with distinct pore types pore sizes and pore connectivity that are located in different global hydraulic elements the analysis of the pore space was carried out through laboratorial measurements gas injection and by images from high resolution computed tomography from plug samples resulting in a pore network model where values of pore radius pore throat radius and coordination number were obtained the best values of porosity and permeability are related to well sorted altered biofabrics whereas the lowest values either with unsorted biofabrics with low pore connectivity or biofabrics that had the porosity reduced by compaction and cementation

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

oxygen and hydrogen isotopes in the hydrologic cycle changes of the isotopic composition of water within the water cycle provide a recognizable signature relating such water to the different phases of the cycle the isotope fractionations that accompany the evaporation from the ocean and other surface waters and the reverse process of rain formation account for the most notable changes as a result meteoric waters are depleted in the heavy isotopic species of h and o relative to ocean waters whereas waters in evaporative systems such as lakes plants and soilwaters are relatively enriched during the passage through the aquifers the isotope composition of water is essentially a conservative property at ambient temperatures but at elevated temperatures interaction with the rock matrix may perturb the isotope composition these changes of the isotope composition in atmospheric waters surface water soil and groundwaters as well as in the biosphere are applied in the characterization of hydrological system as well as indicators of paleo climatological conditions in proxy materials in climatic archives such as ice lake sediments or organic materials

substrate geology controlling different morphology sedimentology diagenesis and geochemistry of adjacent travertine bodies a case study from the sanandaj sirjan zone western iran abstract a travertine fissure ridge and travertine mound are situated 800 m from each other at the western margin of the active tectono volcanic sanandaj sirjan zone western iran despite their close proximity the two geobodies show a difference in morphology lithofacies lithotypes diagenesis and geochemistry petrographic analysis of the fissure ridge carbonates revealed homogeneous sparitic fabrics with dendritic structures reflecting precipitation under fast flowing conditions from calcite supersaturated spring water the mound carbonates are much more heterogeneous and display dominantly micritic fabrics with mainly volcanic clasts reflecting a lower energy flow regime and lower calcite saturation in contrast to the limited diagenetic overprint of the fissure ridge carbonates the more porous mound carbonates are affected by dissolution cementation and formation of mn fe oxide hydroxides attesting of a strong control on porosity by early diagenetic processes the widespread bright luminescent calcite phases especially in sparitic fabrics within both geobodies display high mn concentrations in calcite indicating suboxic precipitation conditions the overlapping 87sr 86sr signatures and 13c signatures point to a mixture of co2 that dominantly originated from dissolution of the marine carbonate qom formation oligo miocene the different 18o signatures indicate different upwelling systems caused by different substrate settings the fissure ridge formed on a hard and brittle limestone substrate with precipitation from dominantly subsurface sourced fluids the mound travertine formed on top of fairly unconsolidated and fractured volcanic strata the depleted 18o signatures and impure micrite dominated fabrics of the mound travertine reflect precipitation from thermal water that mixed with surrounding groundwater this difference is also reflected in a different calculated precipitation temperature that varies between 4 and 21 c for the mound versus 25 50 c for the fissure ridge our findings show that subsurface geology exerts a major control on precipitation processes resulting in differences in travertine morphology sedimentology diagenesis and geochemistry despite that the travertine bodies occur adjacent to each other

sloping fan travertine belen new mexico usa abstract pliocene to quaternary age travertines are very well exposed in quarries near belen new mexico u s a on the western edge of the rio grande rift system a series of hillside springs produced travertine tongues tens of meters thick and hundreds of meters long the accumulations represent deposits from individual springs as well as the amalgamation of deposits the overall architecture is predominantly composed of sloping fans with a smaller component of terrace mounds the sloping fan deposits commonly have a dip of

carbonate deposition pyramid lake subbasin nevada 1 sequence of formation and elevational distribution of carbonate deposits tufas abstract during the late quarternary the elevation of terrace cutting and carbonate deposition in the pyramid lake subbasin were controlled by constancy of lake level imposed by spill to adjoining subbasins sill elevations are 1177 1183 m mud lake slough sill 1207 m emerson pass sill and 1265 m darwin pass sill carbonate deposition was favored by 1 hydrologic closure 2 proximity to a source of calcium 3 elevated water temperature and 4 a solid substrate the thickness and aspect of tufa are a function oflake level dynamics relatively thin sheets and pendant sheets were deposited during a rising or falling lake the upper parts of thick reef form tufas have a horizontal aspect and were deposited in a lake which was stabilized by spill to the carson desert subbasin the lower parts of the reef form tufas are thinner and their outer surface has a vertical aspect indicating that the lower part formed in a receding lake the thickest and most complete sequences of tufa are mounds that border the pyramid lake shore the tops of the tallest mounds reach the elevation of the darwin pass sill and many mounds have been eroded to the elevations of the mud lake slough sill of the emerson pass sill the sequence of tufa formation from oldest to youngest displayed in these mounds is 1 a beachrock containing carbonate cemented volcanic cobbles 2 broken and eroded old spheroids that contain thinolitic tufa and an outer rind of dense laminated tufa 3 large cylindrical tubular tufas capped by 4 coatings of old dense tufas and 5 several generations of old branching tufa commonly associated with thin platy tufas and coatings of thinolitic tufa 6 young spheroids that contain poorly oriented young thinolitic tufa in the center and several generations of radially oriented young thinolitic tufas near the outer edge 7 a transitional thinolite to branching tufa 8 two or more layers of young branching tufa 9 a 0 5 cm thick layer of fine grained dolomite 10 a 2 cm thick layer of young dense laminated tufa 11 a 0 1 cm thick layer of encrusting tufa that was covered by a beach deposit and 12 a 1 0 cm thick layer of porous encrusting tufa that coated the beach deposit and the sides of tufa mounds the elevational ranges of the principal varieties of tufa are not related to terrace or spill elevations it seems likely that the distribution of tufa varieties is related to variation in the thermal structure or chemistry of lake lahontan it is hypothesized that the thinolitic tufa ikaite formed in the near freezing hypolimnion and the braching tufa calcite formed in the seasonally warmer epiliminion of the lake

the influence of large convective eddies on the surface layer turbulence summary close to the surface large coherent eddies consisting of plumes and downdraughts cause convergent winds blowing towards the plume axes which in turn cause wind shears and generation of turbulence this mechanism strongly enhances the convective heat mass transfer at the surface and in contrast to the classical formulation implies an important role of the surface roughness in this context we introduce the stability dependence of the roughness length the latter is important over very rough surfaces when the height of the roughness elements becomes comparable with the large eddy monin obukhov length a consistent theoretical model covering convective regimes over all types of natural surfaces from the smooth still sea to the very rough city of athens is developed it is also comprehensively validated against data from measurements at different sites and also through the convective boundary layer good correspondence between model results field observations and large eddy simulation is achieved over a wide range of surface roughness lengths and convective boundary layer heights

iron rich melts magmatic magnetite and superheated hydrothermal systems the el laco deposit chile we propose an integrated model that explains the magmatic and hydrothermal features of the unique el laco iron deposit that is located in a pliocene pleistocene volcano of the chilean andes sub volcanic crystallization of an iron rich melt as massive magnetite promoted the exsolution of a small volume of a hydrosaline melt and of large amounts of vapor that led to the formation of an alkali calcic hydrothermal assemblage replacing the host andesite this assemblage is capped and overprinted by a large zone of acid sulfate steam heated alteration forming as a whole a protracted shallow level magmatic hydrothermal system oxygen isotopic data for the massive magnetite 18o 4 3 5 0 and the alkali calcic altered rock diopside 18o 7 2 8 7 magnetite 18o 4 4 6 7 suggest that these rocks are genetically related to the host andesite 18o 7 4 9 6 the estimated temperature of the mineral assemblage 900 c may be the highest recorded in hydrothermal systems is attributed to exsolution of fluids from the crystallizing iron rich melts and is considered unlikely to reflect the convection of surficial fluids

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

facies classification and patterns of lacustrine carbonate deposition of the barra velha formation santos basin brazilian pre salt abstract the lacustrine carbonate rocks of the giant pre salt oil fields in the santos and campos basins differ in a number of important aspects from classic lake deposits prior work has described distinctive and unusual features of deposition early diagenesis and reservoir quality with in these rocks that defy classification using established schemes in addition to their unusual textures debate continues as to how to interpret the vertical stacking pattern of facies a new facies classification scheme is proposed based on the relative abundance of three end member components mud micrite and clay minerals calcite spherulites and fascicular calcite shrubs although focused on the main textural and mineralogical features of the rocks as seen today the proposed scheme takes into account the relative contribution of depositional and early diagenetic components sediment mineralogy and the degree of dissolution application of this scheme offers insight into depositional conditions whilst also linking to reservoir quality basic elementary cycles can be defined using this classification in combination with the variation in the abundance of detrital grains and the chemical composition three possible models are suggested to interpret the origin of the basic cycle within the barra velha formation in the context of variation in climate lake level and water chemistry integration of data from different sources including petrographic core description and chemical data for the basic cycle offers improved process based understanding of metre scale facies patterns and provides a foundation for understanding larger scale vertical variations within and between unconformity bound sequences

thrombolites spherulites and fibrous crusts holkerian purbeckian aptian context fabrics and origins abstract carbonates from the holkerian lower carboniferous concretionary beds of the bristol area the upper jurassic lower cretaceous purbeck limestone group of southern england and the aptian pre salt succession of the south atlantic exhibit spherulites and a range of related crystal shrubs and fibrous crusts some of these fabrics are associated with framework microbialites including thrombolites that developed under varying degrees of non marine influence brackish or fresh waters affected the holkerian and purbeckian microbialites although the holkerian also experienced marine conditions the south atlantic pre salt microbialites were deposited under saline lacustrine conditions the fabrics are a topic of intense discussion and disagreement over the role if any of microbes in spherulite formation whilst the case for microbial influences on the holkerian and purbeckian spherulites is strong the alternative influence of mg si gel or mg clay particularly stevensite is appraised in detail for the aptian spherulites including reference to studies of modern microbial analogues results of published laboratory experiments concerning such gels and studies on contemporaneous aptian facies in the same basins lacking mg clay but including spherulites are considered in an attempt to resolve the abiotic versus biotic spherulite origin that spherulites may be either initiated in suspension or reworked and resedimented provides an added dimension for all three cases the evidence is strongly in favour of spherulite nuclei being directly created by microbial metabolism but the fibro radial cortices may be entirely inorganic or passively controlled by microbial extracellular polymeric substances eps which apparently occur as relatively abundant inclusions within the spherulite crystals the body of evidence suggests that the spherulites and their associated shrubs and crusts are at least partly microbially or biotically influenced

modelling the impact of depositional and diagenetic processes on reservoir properties of the crystal shrub limestones in the pre salt barra velha formation santos basin brazil abstract the cretaceous barra velha formation pre salt of the santos basin brazil includes lacustrine crystal shrub limestones which represent important hydrocarbon reservoirs neither single nor multi phase flow behaviour of these unusual fabrics has been modelled to date here we quantify the impact of depositional and diagenetic processes on selected reservoir properties of the crystal shrub limestones using highly flexible 3d synthetic pore scale models that can explore parameter space rather than producing models validated to actual reservoir properties this seeks to establish trends and identify emergent clustering behaviour shrubby aggregates nucleate from a surface to form upward and outward growing branching fascicular optical calcite crystal fans that are diverse in size and form the model allows exploration of the impact of primary growth parameters number of bifurcations or branching shrub height angle of cone or fan inflation as well as subsequent inter shrub cementation and dissolution on resultant porosity permeability and multiphase flow simulations show that depositional parameters as well as two major diagenetic processes cementation and dissolution control porosity and permeability in some cases these trends diverge considerably from widely referenced kozeny carman kc porosity permeability relationships the angle of shrub inflation produces data clusters that traverse kc bands by contrast the number of bifurcations and the degree of dissolution follows kc trends so providing guidance for potential rock type subdivision of similar lithologies on this basis we propose that the crystal shrub limestones in the barra velha formation can be subdivided into three broad petrophysical reservoir rock types based on the degree of shrubby aggregate bifurcation and extent of dissolution the trends explored here can be used to link conceptual and process based geological models to data so enabling more accurate prediction and upscaling of heterogeneous reservoir properties and their reservoir scale distribution

are spherulitic lacustrine carbonates an expression of large scale mineral carbonation a case study from the east kirkton limestone scotland lacustrine carbonate deposits with spherulitic facies are poorly understood but are key to understanding the economically important pre salt mesozoic strata of the south atlantic a major barrier to research into these unique and spectacular facies is the lack of good lacustrine spherulite dominated deposits which are known in outcrop stratigraphy and petrography suggest one of the best analogue systems is found in the carboniferous of scotland the east kirkton limestone here we propose a hydrogeochemical model that explains why the caco3 sio2 mg si al mineral suite associated with spherular radial calcite facies forms in alkaline lakes above basaltic bedrock demonstrating links between igneous bedrock chemistry lake and spring water chemistry and mineral precipitation this model has implications for studies of lacustrine sediments in rift basins of all ages using empirical and theoretical approaches we analyze the relationship between metal mobilization from sub surface volcaniclastic rocks and the potential for precipitation of carbonate minerals various mg bearing minerals and chalcedony in a lacustrine spherulitic carbonate setting this suite of minerals is most likely formed by in gassing of co2 to a carbon limited alkaline spring water consistent with the reaction of alkali igneous rocks in the subsurface with meteoric groundwater we suggest that an analogous system to that at east kirkton caused development of the pre salt spherulitic carbonate deposits

the hydrochemical evolution of alkaline volcanic lakes a model to understand the south atlantic pre salt mineral assemblages abstract understand the origin of the exotic strontium rich calcite magnesium rich clay silica deposits formed during the early cretaceous in the south atlantic pre salt lakes is challenging our ability to comprehend the chemical evolution of alkaline volcanic lakes here we present a new hydrochemical model based on an open basin concept thermodynamic equilibrium and chemical data from lake baringo east african rift to shed light on the mechanisms facilitating carbonate clay silica precipitation this model explores the effects that the leakage to adjacent waterbodies the combination of lake evaporation and lake recharge with river hydrothermal and marine waters and the bathymetric effects of the partial co2 pressure have in the stationary precipitation of pre salt lacustrine mineral assemblages sediment thickness calculations suggest that the facies cyclicity in the pre salt and elsewhere are likely reflecting temporal fluctuations in the type of waters sourcing the lake and in the degree of leakage to aquifers over multiple evaporative freshening cycles a plausible link between enhanced strontium uptake into pre salt calcite allochems and ca2 co32 lake water stoichiometries allows to infer different scenarios for strontium rich calcite formation with or without magnesium clays our conceptual model allows to quantify the role that hydrology groundwater charge discharge to aquifers evaporation exert on carbonate clay silica precipitation providing with a refined framework to understand the basin scale chemical sedimentation in alkaline lakes furthermore this model represents a robust template upon which critically test the superimposed kinetic factors likely attached to the genesis of specific mineral factories such as those encountered in the south atlantic pre salt alkaline lakes

continental paleotemperatures from an early cretaceous dolomitic lake gansu province china abstract the lower cretaceous strata of asia are renowned for abundant fossil resources these strata can also offer a wealth of quantitative data regarding the paleoclimate and paleoenvironment of the terrestrial cretaceous an important transition in earth history many modern groups of organisms that dominate terrestrial ecosystems today originated or expanded in the early cretaceous in addition the cretaceous period known to have been characterized by a greenhouse climate provides opportunities to understand how the climate system functioned during greenhouse climate states here we utilize the clumped isotope paleothermometer to provide paleotemperature records from a lacustrine sequence in northwestern china previously constrained to the early aptian petrographic and x ray diffraction analyses indicate that the lake sediments produced primary to early diagenetic dolomites likely microbially mediated due to high organic matter input and seasonally warm arid conditions the majority of the dolomitic carbonates formed in pore waters just below the sediment water interface or as primary shallow water precipitates clumped isotope derived temperatures from all carbonates range from 20 6 c for early diagenetic dolomites to 47 2 c for late burial calcite cements accounting for seasonal biases mean annual air temperatures are calculated to have averaged 20 2 c this is broadly consistent with local paleotemperature proxy evidence and climate model paleotemperature estimates the data presented provides further quantitative constraints on the cretaceous greenhouse world as well as insights into depositional settings that produce low temperature dolomites

giant kiruna type deposits form by efficient flotation of magmatic magnetite suspensions kiruna type iron oxide apatite ioa deposits are an important source of fe ore and two radically different processes are being actively investigated for their origin one hypothesis invokes direct crystallization of immiscible fe rich melt that separated from a parent silicate magma while the other hypothesis invokes deposition of fe oxides from hydrothermal fluids of either magmatic or crustal origin here we present a new model based on fe and o stable isotopes and trace and major element geochemistry data of magnetite from the 350 mt fe los colorados ioa deposit in the chilean iron belt that merges these divergent processes into a single sequence of events that explains all characteristic features of these curious deposits we propose that concentration of magnetite takes place by the preferred wetting of magnetite followed by buoyant segregation of these earlyformed magmatic magnetite bubble pairs which become a rising magnetite suspension that deposits massive magnetite in regionalscale transcurrent faults our data demonstrate an unambiguous magmatic origin consistent with the namesake ioa analogue in the kiruna district sweden further our model explains the observed coexisting purely magmatic and hydrothermal magmatic features and allows a genetic connection between kiruna type ioa and iron oxide copper gold deposits contributing to a global understanding valuable to exploration efforts

evaporitic carbonates in the pre salt of santos basin genesis and tectonic implications abstract since the first discoveries of oil accumulations in the carbonate reservoirs of the lower cretaceous barra velha formation of the santos basin their genesis has been the subject of intense discussion different depositional models have been proposed with some authors advocating a microbial origin while others an abiotic origin as travertines or travertine like rocks this search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative an evaporative model supported by our petrographical and geochemical data evaporitic sediment patterns in endorheic basins in a depositional framework of brine drawdown are dominated by ephemeral salt pan hydrology bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment water interface salt grows diagenetically within saline and mud layers beneath the dry pan surface finally laminated sediments are deposited we observe a similar cyclic pattern in the pre salt core we described beds of shrub like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites the model we propose attempts to contribute solving the puzzle of pre salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the santos campos basin were deposited as the initial transitional stage of a closed sub sea level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts carnallite sylvite tachyhydrite basinwide evaporites are singular events in the geological record they form when a giant sub sea level basin becomes a sump of continental marine and hydrothermal fluids producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones the novelty in this paper is that we apply to pre salt carbonate deposition the sedimentology hydrology and geochemistry of ancient evaporitic basins with this concept in mind we propose a geochemical model for the unusual pre salt carbonate precipitation of the santos basin regarding the walvis rio grande volcanic high as a topographic barrier during the aptian we suggest that the main sources of calcium to the barra velha paleolake were hydrothermal brines mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier hyperextension of the lithosphere with a shallow moho increased geothermal gradients to levels favorable for the generation of hydrothermal cacl2 brines a process facilitated by the already elevated ca ion and low sulfate concentration of the cretaceous ocean these cacl2 rich fluids would pass through the volcanic barrier into a sub sea level paleolake in the rift basin the high salinity paleolake would become saturated in calcium bicarbonate carbonate as internally drained nahco3 bearing alkaline waters formed by physical chemical weathering of penecontemporaneous basaltic rocks mix with the cacl2 bearing hot brines percolating through the barrier producing a hybrid brine 2nahco3 cacl2 ca hco3 2 2nacl such a depositional system integrating the geochemical depositional controls of sag phase carbonates with those of the south atlantic evaporites has important implications for unravelling the genesis of these exotic deposits

origins of carbonate spherulites implications for brazilian aptian pre salt reservoir abstract spherulites spherical to elliptical allochems composed of crystals radiating from a common core investigated from a variety of depositional settings e g hot springs ambient water temperature geyser tufa and caliche are all composed of a fine grained nucleus made up of carbonate encrusted bacterial bodies biofilms and or eps and surrounded by a cortex of radiating crystals of either aragonite or calcite the microbes and their by products in the nucleus induced the precipitation of carbonate overcoming the inhibition to initiate crystal formation the enveloping radiating crystals comprising aragonitic cortices tended to grow abiotically producing well formed euhedral crystals with a paucity of included bacterial fossils whereas those cortical crystals made up of calcite commonly contained bacterial fossils indicating that the bacterial colonies contributed to the calcitic cortical crystal precipitation similar spherulites form a thick widespread accumulation in the aptian pre salt lacustrine deposits in the campos basin offshore brazil as with the travertine tufa and caliche spherulites the pre salt spherulites most likely initiated carbonate precipitation around bacterial colonies and or their bioproducts probably while afloat in a lacustrine water column before settling to the water sediment interface absence of inter spherulite sediment and the spherulite to spherulite compaction indicate that cortical crystal growth continued while the spherulites were at the sediment water interface rather than displacively within a sediment

early cretaceous ostracod biostratigraphy of eastern brazil and western africa an overview petroleum exploration in the offshore basins of eastern brazil and western africa has been aided for the last fifty years by an understanding of the early cretaceous ostracod biostratigraphy of the proto south atlantic this review charts the development of the presently accepted zonation from its early development in the reconcavo tucano basin and recognition as palaeontological evidence for continental drift in the 1960s to its application over the following five decades in the brazilian sergipe alagoas araripe and campos basins and the western african congo gabon and kwanza basins the distribution of ostracod species in the lakes which formed during the early stages of the opening of the atlantic during the early cretaceous was often extensive and this has resulted in effectively south atlantic wide application for many species and certainly for the zonation as a whole indeed petroleum companies continue to exploit the predictability of the ostracods in south atlantic operations due to the fact that they are often able to provide accurate constraints on stratigraphic position for a relatively low cost however the ostracod biozonations of the pre salt section for several eastern brazilian basins such as the reconcavo tucano sergipe alagoas and campos have not been thoroughly revised since the 1980s and the taxonomy of these remains somewhat outdated furthermore ostracod biozonations for many basins on both coasts e g the brazilian espirito santo and the western african namibe basins have never been published an updated and revised taxonomy of the ostracods of the basins of eastern brazil and western africa is presented appendix i as well as a revision of the biozonation on both sides of the modern day atlantic ocean it is hoped that this paper will act as a catalyst for further work in the basins surrounding the modern day south atlantic which hold much interest for petroleum companies and micropalaeontologists alike

the mineral diversity of jezero crater evidence for possible lacustrine carbonates on mars abstract noachian aged jezero crater is the only known location on mars where clear orbital detections of carbonates are found in close proximity to clear fluvio lacustrine features indicating the past presence of a paleolake however it is unclear whether or not the carbonates in jezero are related to the lacustrine activity this distinction is critical for evaluating the astrobiological potential of the site as lacustrine carbonates on earth are capable of preserving biosignatures at scales that may be detectable by a landed mission like the mars 2020 rover which is planned to land in jezero in february 2021 in this study we conduct a detailed investigation of the mineralogical and morphological properties of geological units within jezero crater in order to better constrain the origin of carbonates in the basin and their timing relative to fluvio lacustrine activity using orbital visible near infrared hyperspectral images from the compact reconnaissance imaging spectrometer for mars crism along with high resolution imagery and digital elevation models we identify a distinct carbonate bearing unit the marginal carbonates located along the inner margin of the crater near the largest inlet valley and the western delta based on their strong carbonate signatures topographic properties and location in the crater we propose that this unit may preserve authigenic lacustrine carbonates precipitated in the near shore environment of the jezero paleolake comparison to carbonate deposits from terrestrial closed basin lakes suggests that if the marginal carbonates are lacustrine in origin they could preserve macro and microscopic biosignatures in microbialite rocks like stromatolites some of which would likely be detectable by mars 2020 the marginal carbonates may represent just one phase of a complex fluvio lacustrine history in jezero crater as we find that the spectral diversity of the fluvio lacustrine deposits in the crater is consistent with a long lived lake system cataloging the deposition and erosion of regional geologic units thus jezero crater may contain a unique record of the evolution of surface environments climates and habitability on early mars

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

seismic characterisation and origin of clinoforms in lacustrine depositional environments a case study from the cretaceous of the south atlantic

fe o stable isotope pairs elucidate a high temperature origin of chilean iron oxide apatite deposits abstract iron oxide apatite ioa ore deposits occur globally and can host millions to billions of tons of fe in addition to economic reserves of other metals such as rare earth elements which are critical for the expected growth of technology and renewable energy resources in this study we pair the stable fe and o isotope compositions of magnetite samples from several ioa deposits to constrain the source reservoir of these elements in ioas since magnetite constitutes up to 90 modal of many ioas identifying the source of fe and o within the magnetite may elucidate high temperature and or lower temperature processes responsible for their formation here we focus on the world class los colorados ioa in the chilean iron belt cib and present data for magnetite from other fe oxide deposits in the cib el laco mariela we also report fe and o isotopic values for other ioa deposits including mineville new york usa and the type locale kiruna sweden the ranges of fe isotopic composition 56fe 56fe 54fe relative to irmm 14 of magnetite from the chilean deposits are los colorados 56fe 2 0 08 0 03 to 0 24 0 08 el laco 56fe 0 20 0 03 to 0 53 0 03 mariela 56fe 0 13 0 03 the o isotopic composition 18o 18o 16o relative to vsmow of the same chilean magnetite samples are los colorados 18o 2 1 92 0 08 to 3 17 0 03 el laco 18o 4 00 0 10 to 4 34 0 10 mariela 18o 1 48 0 04 the 18o and 56fe values for kiruna magnetite yield an average of 1 76 0 25 and 0 16 0 07 respectively the fe and o isotope data from the chilean ioas fit unequivocally within the range of magnetite formed by high temperature magmatic or magmatic hydrothermal processes i e 56fe 0 06 0 49 and 18o 1 0 4 5 consistent with a high temperature origin for chilean ioa deposits additionally minimum formation temperatures calculated by using the measured 18o values of coexisting los colorados magnetite and actinolite separates 630 c as well as fe numbers of actinolite grains 610 820 c are consistent with this interpretation we also present fe isotope data from magmatic magnetite of the bushveld complex south africa where 56fe ranges from 0 28 0 04 to 0 86 0 07 based on these data and comparison to published fe and o stable isotope values of igneous magnetite we propose extending the magmatic high temperature 56fe range to 0 86 considering that the chilean ioas and kiruna deposit are representative of ioa deposits worldwide the fe and o stable isotope data indicate that ioas are formed by high temperature magmatic processes

pleistocene holocene tectonic reconstruction of the ballik travertine denizli graben sw turkey de formation of large travertine geobodies at intersecting grabens abstract travertine geobodies have been identified as potential reservoir analogues to carbonate build ups in pre salt hydrocarbon systems to investigate travertine geobody deformation faults were mapped in 35 travertine quarries that excavate the ballik travertine i e a c 12 5 km2 large travertine geobody that precipitated at the intersection of the ne margin of the denizli basin and neighbouring baklan graben sw turkey this travertine precipitated from cooling carbonate saturated thermal spring waters that resurfaced along the margin fracture fault network and through neogene unconsolidated underlying sediments from the denizli basin floor to the uplifted graben shoulders fault orientation is dominantly wnw ese oriented with major basin faults showing a left stepping trend along the upper denizli margin travertine is only deformed by extensional normal faults along the lower margin travertine starts with a subhorizontal facies but evolves to a travertine facies formed by a sloping topography with a domal architecture paleostress inversion of fault slip data reveals that an early pleistocene nne ssw extensional transtensional phase initiated the wnw ese oriented graben facing normal fault network in the middle pleistocene the ballik fault network was left lateral strike slip reactivated because it acted as a transfer zone between the nw se extending neighbouring baklan basin and nw se extension along ne sw oriented margin faults of the dghs in this stress configuration travertine precipitated along the sw margin fault of the baklan graben after strike slip reactivation a late pleistocene to current nne ssw extensional stress regime reinstalled during which margin faults widened and active travertine precipitation moved to more central parts of the dghs as different tectonic regimes affect graben intersections reservoir analogues can have a complex deformation history driven by fault reactivation and recurrent stress permutations this study concludes that large travertine geobodies can form at graben intersections because of their susceptibility to enhanced fluid flow through the complex fault fracture network

new field evidence bearing on the origin of the el laco magnetite deposit northern chile a reply the el laco magnetite deposit has been interpreted as lava flows and feeder dikes formed from iron oxide magma but more recently as a product of metasomatic replacement open pit exposure created at laco sur during the 1990s reveals that the massive magnetite contains magnetite veined blocks and smaller fragments of altered andesitic volcanic rock clearly supporting the replacement origin open space growth of magnetite and pyroxene along the walls of chimney structures and veins within magnetite indicates precipitation from aqueous fluid rather than from gas alone the native sulfur bearing cristobalite alunite alteration widespread at el laco and broadly contemporaneous with magnetite formation is recognized as part of a steam heated horizon generated in the vadose zone above a paleowater table consequently the magnetite replacement must have taken place beneath the paleowater table but probably 300 m beneath the paleosurface thus magnetite deposition at or immediately beneath the paleosurface is precluded the main outstanding question is how hot hypersaline brine the probable ore forming fluid at el laco attained such shallow crustal levels within a high volcanic edifice without undergoing the extensive groundwater dilution documented from most shallowly preserved hydrothermal systems

volcanic rock alterations of the kwanza basin offshore angola insights from an integrated petrological geochemical and numerical approach the lower cretaceous presalt section of the kwanza basin angola is in the spotlight following the discoveries of petroleum systems in this basin and more generally in the south atlantic these systems are mostly composed of continental carbonates in close association with volcanic rocks this work is focused on the study of an offshore kwanza presalt volcanic sequence characterized as valanginian trachytic subaerial lava flows a detailed petrological analysis of the altered trachyte in association with fluid inclusion microthermometry was conducted in order to depict the initial mineralogy albite sanidine titanomagnetite and obtain a paragenetic sequence quartz siderite kaolinite calcite thermodynamic equilibrium modelling of the trachytes alteration by meteoric fluids over a range of temperatures 25 c 200 c and co2 partial pressure pco2 0 01 mbar to 100 bar were performed with phreeqc and compared to the observed paragenetic sequence some numerical simulations reflect the observed paragenesis as a result the pco2 is constrained by the occurrence of siderite from 0 1 bar at 50 c to 30 bar at 125 c and kaolinite from 0 2 bar at 50 c to 1 2 bar at 125 c the simulations emphasize the need for a high pco2 in the hydrothermal system to achieve the observed trachyte transformation after reaching equilibrium with the trachytes the simulated fluids highlight a mid alkaline to near neutral ph with high fe hco3 co3 and alkali concentrations the palaeofluids could have evolved from ca and mg rich to ca and mg poor with increasing temperature inversely si concentrations are positively correlated with increasing temperatures this methodology integrating a petrological approach and numerical simulations proves to be a powerful tool leading to better understanding of the proxies pco2 temperature redox conditions controlling paragenesis to push further these simulations are also a step toward improved understanding of palaeofluid evolutions in presalt systems and better prediction of reservoir quality

facies character and depositional architecture of hydrothermal travertine slope aprons pleistocene acquasanta terme central italy abstract hydrothermal travertines develop various depositional geometries from tabular to high relief mounds or aprons with steep slopes under the control of local topography location and geometry of the vents fault activity hydrology water physico chemical properties rates of thermal water flow and carbonate precipitation rates this study focuses on two pleistocene tens of metres thick travertine slope aprons accumulated on fluvial terraces in the tronto river valley acquasanta terme central italy to investigate their facies character geochemical signature porosity and evolution of the depositional geometry through time the two travertine aprons consist of four aggradational progradational units vertically and laterally stacked with onlap and downlap stratal terminations separated by erosional unconformities produced by events of non deposition and erosion due to temporary interruptions of vent activity shifts of the vent location and or deviation of the flow directions the travertine units include various depositional environments 1 smooth slope 2 terraced slope with metre scale sub horizontal pools separated by rounded rims and vertical walls and 3 sub horizontal tens of metres wide ponds smooth slope clinoforms are made of centimetre to decimetre thick layers of crystalline dendrite cementstone laminated boundstone and radial coated grain grainstone precipitated by fast flowing water on inclined substrates rims and walls of terraced slopes are built by crystalline dendrites and laminated boundstone sub horizontal layers of terrace pools and ponds consist of facies precipitated by slow flowing to standing water clotted peloidal micrite dendrite coated bubble boundstone raft rudstone associated with radial coated grains and laminated boundstone carbonate coated reeds occur in distal ponds adjacent to toe of slopes or overlie packstone rudstone with travertine intraclasts and substrate extraclasts marking events of subaerial exposure and erosion travertine facies porosity and permeability range from 4 to 21 and 0 03 to 669 md respectively showing no direct correlation stable isotope values 13 c 5 7 9 3 18 o 9 6 to 12 2 are similar to other travertines precipitated by thermal water in central italy this study identifies the centimetre scale travertine facies variability linking it to the environment of deposition and to the depositional geometry of travertine units affected by the substrate topography and lateral shifts of the active springs despite the different scale and facies composition the geometry of aggradational progradational units of travertine aprons might resemble marine flat topped high relief carbonate platforms travertine units in the subsurface if present with sizes that can be seismically resolved might be wrongly interpreted as carbonate platforms with steep slopes without a detailed facies analysis

discriminating between the origins of remotely sensed circular structures carbonate mounds diapirs or periclinal folds purbeck limestone group weymouth bay uk many sedimentary rock successions contain plan view circular structures such as impacts diapirs and carbonate build ups when remotely sensed it can be difficult to discriminate between their formation mechanisms here we examine this problem by assessing the origins of circular structures imaged in high resolution multibeam bathymetric data from weymouth bay uk the imagery shows 30 150 m across concave down structures within the upper purbeck limestone group on the southern limb of the purbeck anticline similar structures have not been identified in the extensive outcrops around the bay the morphology and geological setting of the structures are consistent with three different interpretations carbonate mounds periclinal folds and evaporite diapirs however none of these structures has been previously recorded in the upper purbeck limestone group outcrops of this internationally renowned geological region we apply a scoring system to 25 features of the circular structures to discriminate between these three alternative interpretations this analysis indicates that evaporite diapirs are the least likely and carbonate mounds the most likely origin of the structures the presence of carbonate mounds revises the upper purbeck palaeofacies distribution in its type area and provides an analogue for the exploration for hydrocarbon reservoirs in lacustrine mounds supplementary material the methods used in this paper and metrics of the circular structures are available at https doi org 10 6084 m9 figshare c 4103840

deep burial hydrothermal alteration of the pre salt carbonate reservoirs from northern campos basin offshore brazil evidence from petrography fluid inclusions sr c and o isotopes abstract petrographic mineralogical elemental isotopic and fluid inclusion analyses were integrated to unravel the diagenetic evolution of brazilian pre salt lacustrine carbonate reservoirs of northern campos basin southeast brazilian margin detailed thin section and cathodoluminescence petrography scanning electron microscopy and electron microprobe analyses established a paragenetic evolution of diagenetic processes and products comprising extensive dolomitization silicification and dissolution a paragenesis including saddle dolomite macrocrystalline calcite mega quartz sr barite celestine fluorite dickite sphalerite galena and other metallic sulfides filling fractures and dissolution porosity and aqueous fluid inclusions with homogenization temperatures of 92 152 c and salinities between 13 and 26 wt eq nacl characterized a hydrothermal system with some analogy to carbonate hosted pb zn mississippi valley mvt and irish type deposits petroleum inclusions and solid bitumen testify atypical oil generation and migration associated with the hydrothermal flow the host pre salt spherulitic and fascicular carbonates present highly radiogenic 87sr 86sr ratios indicating strong interaction with continental crust materials hydrothermal phases show 18o values more negative than syngenetic and diagenetic carbonates the 13c values are interpreted as result of interaction between the hydrothermal fluids and the host rocks the combined data set provides clear evidence of intense hydrothermal alteration of northern campos basin pre salt reservoirs at deep burial conditions 2 km possibly related to late cretaceous or more probably paleogene magmatic activity mixed sourced fluids bearing a basinal signature fed the hydrothermal system and promoted dissolution of the host rocks the hydrothermal alterations had strong impact on the porosity permeability and heterogeneity contributing together with the associated fracturing to the excellent production performance of the pre salt reservoirs

hierarchical approach to define travertine depositional systems 3d conceptual morphological model and possible applications abstract 3d modelling represents a powerful tool to characterize the geobody architecture of depositional systems several examples have been proposed in literature both for marine carbonates and siliciclastics rocks for modelling quantitative data related to orientation shape and dimension are fundamental continental carbonates however are not considered yet likely because they cover a large range of deposits such as speleothems tufas lacustrine carbonates and travertines the interpretation of the depositional environments of the latter is still in fact debated by the scientific community however the interest in continental carbonates and especially travertines increased over the last decades due to the discovery of supergiant unconventional reservoirs in the southern atlantic i e the so called pre salt play aim of this paper is to present an innovative workflow to investigate and describe continental carbonates as geobodies based on a hierarchical approach applied to three large scale travertine settings i e terme di san giovanni italy pamukkale turkey and the lapis tiburtinus travertines italy travertine depo systems are divided in three different depo zones namely proximal intermediate and distal that are characterized by different depo elements depo shapes and depo facies finally a 3d morphological model is created based on numerical and vector approaches and interpolation of both the model can be applied to seismic data of brazilian and angolan pre salt highlighting the similarities between these systems and for the first time the scale independence of these depositional system representing an important advantage in their interpretation the 3d model and the approach proposed facilitates seismic interpretation allowing to define and characterize geometries of inaccessible geological contexts

environmental conditions of deposition of the lower cretaceous lacustrine carbonates of the barra velha formation santos basin brazil based on stable carbon and oxygen isotopes a continental record of pco2 during the onset of the oceanic anoxic event 1a oae 1a interval abstract this study presents new stable carbon and oxygen isotope data from lower cretaceous lacustrine carbonate rock samples recovered from a well drilled in the santos basin offshore southeast brazil these samples represent a record of a continental environment just prior to the opening of the south atlantic ocean and the ultimate break up of gondwanaland the geochemical data along with carbonate mineralogy indicate repeated cycles of lake level variation that could be attributed to climatic oscillations despite the absence of correlations between 13c and 18o values facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here the existence of persisting trends of nearly constant 13c values with a spread in 18o values though suggests long water residence times in the palaeolake equilibrium between atmosphere and lake water co2 as well as significant evaporation of water the overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the gondwanan continent corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the early cretaceous as a result of this recorded trend strongly suggesting equilibrium between lake waters dic dissolved inorganic carbon reservoir and atmospheric co2 the data are most consistent with lacustrine deposition rather than precipitation of travertine contrasting with some suggestions for the genesis of the carbonates of the barra velha formation finally this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric co2 known for this time as the oceanic anoxic event oae 1a if this is correct it also helps to put further time constraints on this studied interval which should not be younger than barremian age and to provide a regional continental perspective on a global event

relationship between spring and geyser activity and the deposition and morphology of high temperature 73 c siliceous sinter yellowstone national park wyoming u s a abstract six classes of alkaline hot springs and geysers are here distinguished in yellowstone national park based on activity and eruptive style 1 non surging non boiling springs 2 gently surging non boiling springs 3 non surging to gently surging boiling springs 4 strongly surging springs 5 fountain geysers and 6 cone geysers each displays morphologically distinctive siliceous sinter deposits in the high temperature 73 c parts of the vent and outflow systems non surging and gently surging non boiling springs that do not overflow their rims develop thin digitate to lilypad like sinter rims that grow horizontally over the pool surfaces similar springs with rim overflow have rims that accrete vertically boiling springs often have nearly circular steep sided pools and rims dominated by splash and rim overflow and composed mainly of spicular and columnar sinter fountain geysers characterized by broad fountain like eruption bursts have a central vent surrounded by a zone of spicular sinter beyond which is a broad flat bottomed moat but lack steep sinter cones cone geysers have narrow vents jet like discharges that cool rapidly and fall close to the vent and steep sided spicule dominated sinter cones sinter deposition takes place within three main zones 1 fully subaqueous sides and bottoms of vent pools outflow basins and outflow channels 2 periodically wetted splash zones surrounding hot springs and geysers and 3 largely subaerial parts of the rim and outflow areas wetted mainly by overflow and surging and that dry quickly fully subaqueous surfaces are covered by fine earthy particulate siliceous sediment pss composed largely of amorphous silica spherules and spherule aggregates that appear to have precipitated around heterogeneous nuclei including microbes splash zones of hot springs and geysers are characterized by spicular sinter largely subaerial zones wetted mainly by rim overflow and surging with little or no splash show complex varieties of terraced columnar and knobby sinter the highest silica deposition rates occur where surfaces are wetted frequently and evaporate to dryness such as the tops of spicules columns and knobs associated lower areas and crevices that remain wet or moist show slow accumulation of pss most continuously wet surfaces in high temperature 73 c hot spring and geyser vent and outflow systems are probably covered by microbial biofilms although microbes may locally influence rates of silica deposition and sinter microstructure the results of this study suggest that the depositional sites morphology and macrostructuring of high temperature sinter are controlled largely by the processes of surface wetting evaporation and drying

a depositional model for spherulitic carbonates associated with alkaline volcanic lakes the south atlantic aptian pre salt reservoirs are formed by a combination of spherulitic carbonates and mg rich clays accumulated in volcanic alkaline lake settings with exotic chemistries so far outcrop analogues characterised by metre thick successions deposited in lacustrine scenarios are elusive so disentangling the genesis of spherulitic carbonates represents a major scientific challenge with business impact in particular the controls on spatial distribution and the environment of spherulitic facies formation remain poorly constrained little studied and hotly debated to shed light on this conundrum a spherulitic carbonate rich alkaline volcanic lacustrine succession has been analysed at outcrop scale the carboniferous east kirkton limestone scotland despite clays being very scarce and limited to layers of amorphous mg si minerals a diverse array of spherulitic calcitic components were formed including coated grains crusts and build ups this setting enables the mechanisms of spherulitic calcite development and the patterns of sediment accumulation to be explored in a geobiological and hydrochemical scenario similar to the pre salt subsurface occurrences but divorced from clay influence the integration of logs borehole data outcrop photomosaics and petrographic observations collectively allowed the reconstruction of a depositional model for the east kirkton lacustrine succession in this model calcite spherule nucleation took place at the sediment water interface in the littoral zone driven by the co occurrence of 1 high alkalinity 2 ca mg rich hydrochemistry and 3 microbial derived colloidal exopolymeric substances these environmental conditions permitted the coeval development of spherulitic cementstone build ups and spherulitic grainstone packstone within the wave agitated zone and the accumulation of floatstones and laminites of spherulitic grains in deeper lake regions by means of downslope reworking this model is consistent with the previously documented microbial bloom occurrences and highlights the need to better understand the complex microbe solution interactions before any reliable facies model is envisaged

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

shrub morpho types as indicator for the water flow energy tivoli travertine case central italy abstract travertines from tivoli area central italy possess abundant shrub like fabrics that are laterally continuous over hundreds of square meters they occur dominantly in horizontal layers with aggradational and progradational stacking patterns their fabrics and morphologies are remarkably similar to the shrubs lithotypes reported in literature for the pre salt reservoirs offshore brazil and angola with huge oil accumulations petrographic and micro computer tomography analyses allowed the identification of six shrub morpho types i e narrow dendriform wide dendriform fili dendriform arborescent arbustiform and pustular dendriform shrubs are the most common lithotype in tivoli area and three different subtypes could be distinguished according to the arrangement of their branches the shrubs consist largely of peloidal micritic aggregates engulfed in spar calcite ranging in average from 1 to 3 cm in height the shrubs are interpreted to have developed in very shallow extensive waterlogged slightly inclined flat areas changing laterally into a slope system with crusts as the main lithotype changes in the hydrodynamic conditions with episodes of stagnancy influenced the shrub morpho types making them very variegated shrub morphologies likely reflect specific micro environments that are controlled by water flow rates evaporation and microbial activity the latter processes influenced shrub fabric and morphology under high flow conditions co 2 degassing is the main process leading to carbonate precipitation consequently denser and tightly packed morphologies will precipitate composing mainly the crust lithotype in this scenario microbes are less dominant dendriform shrubs with narrow wide and fili morphologies are interpreted to occur in moderate to low energy water flows narrow dendriform shrubs reflect faster flowing conditions with decreasing impact of flow on the morphological aspects from wide dendriform shrubs to fili dendriform shrubs slow to very slow flowing waters are more characteristic for the arborescent arbustiform and pustular shrubs that possibly are highly influenced by evaporation a decrease of water flow likely allows higher microbial activity and consequently shrub morphologies become more fragile and enriched in peloidal micritic aggregate fabrics

geobody architecture of continental carbonates gazda travertine quarry suttő gerecse hills hungary abstract the gazda quarry is one of the largest outcrops of the present day sutt travertine complex that is situated close to the town of sutt hungary in the pannonian basin sedimentological and petrographical analyses show that the gazda travertine body is built out of phytohermal wavy laminated massive and flat laminated travertine lithofacies representing reed mound slope lacustrine and palustrine depositional environments respectively based on petrographic analyses the following three main textural features were recognized that allow to describe the most common microscopic features 1 micrite textures 2 micritic coatings and crusts and 3 different types of clasts each major lithofacies is characterized by a particular association of these textural types the geometry of the travertine beds follows the antecedent land surface a ne sw striking pre pleistocene valley it is proposed that in this valley travertine precipitation took place along a gently sloping terrain the gazda travertine system was fed by at least two groundwater springs one of them was located ne of the quarry from which waters were flowing along the axis of the valley the other source was located to the east of the gazda quarry and waters joined the main water course along the valley closest to the confluence a phytohermal mound developed with lateral slope and eventual pond depositional systems onlapping and interfingering with the flat laminated lithofacies indicate alternating activity of the two springs fractures observed in the quarry are mainly related to postdepositional gravitational sliding however neotectonic forces resulting in nw se tension also played a role in the formation of the fractures stable oxygen and carbon isotope analyses show that water temperature was approximately 25 c petrographic studies indicate that only syn diagenetic processes affected the precipitates therefore the original stable oxygen isotope signatures characteristic of the depositional environment were preserved detailed sedimentological and geochemical study of the gazda travertine complex allowed small scale facies characterization providing information about the ancient depositional environment and the heterogeneous nature of these carbonates

anatomy of siliceous hot springs examples from yellowstone national park wyoming usa abstract numerous siliceous hot spring systems in the norris and lower geyser basins of yellowstone national park wyoming provide insights into spring geometries depositional facies and lithofacies associated with modern hot springs analyses of active cistern spring octopus spring deerbone spring and spindle geyser and inactive pork chop geyser siliceous hot springs have facilitated the construction of a facies model for siliceous hot spring deposits at yellowstone yellowstone s siliceous springs tend to group into four broad morphological categories siliceous spires and cones domal mounds terraced mounds and ponds siliceous spires cones are subconical accumulations up to 5 7 m high and about 2 m in diameter and are common deposits in yellowstone lake domal mounds are characterized by siliceous precipitates with a broad lens or shield geometry 2 3 m in vertical relief discharge channels and an areal accumulation of approximately 150 m2 in contrast terraced mounds have a stair step morphology a substantial pool 8 10 m in diameter shrubby precipitates and occupy areas of 2000 m2 siliceous ponds are variable in size have little outflow and exhibit low amounts of silica precipitation of these morphological varieties domal mounds and terraced mounds are thought to have the best long term preservation potential the four spring morphotypes are composed of up to eight cumulative hot spring depositional facies 1 vent 95 c 2 proximal vent precipitation of siliceous sinter is the result of abiotic and biotic processes abiotic precipitational processes are dominant in the vent area whereas biotic influences on the precipitate fabric become progressively more important downflow

shrub and pore type classification petrography of travertine shrubs from the ballik belevi area denizli sw turkey petrographical and petrophysical properties of the turkish ballik belevi travertine shrubs demonstrate the necessity of reservoir oriented classifications for shrub related lithotypes and associated pore types the presented shrub related lithotype classification incorporates morphology size and fabric which are linked to specific pore types and sizes the travertines show highly complex pore networks as observed from computed tomography reconstructions making them of interest for the development and optimization of pore characterization methodologies and techniques following an upscaling approach porosity 3 15 and permeability 0 505 md strongly vary for the ballik belevi travertines the highest porosities and permeabilities are measured for horizontal samples dominated by slightly dissolution enlarged intershrub and interdigit growth framework porosity mostly found for dendritic shrubs the presented shrub and pore type classification can be applied to shrubs worldwide studying outcrop analogues of continental carbonates helps to better understand heterogeneities lateral variability and properties of pre salt cretaceous continental carbonate reservoirs offshore brazil and angola

unsupervised seismic facies classification applied to a presalt carbonate reservoir santos basin offshore brazil mapping of seismic and lithological facies is a very complex process especially in regions with low seismic resolution caused by extensive salt layers even when only an exploratory view of the distribution of the reservoir facies is required the aim of this study was to apply multi attribute analysis using an unsupervised classification algorithm to map the carbonate facies of an exploratory presalt area located in the outer high region of the santos basin the interval of interest is the barra velha formation deposited during the aptian which represents an intercalation of travertines stromatolites grainstones and spherulitic packstones mudstones and authigenic shales which were deposited under hypersaline lacustrine conditions during the sag phase a set of seismic attributes calculated from a poststack seismic amplitude volume was used to characterize geological and structural features of the study area we applied k means clustering in an approach for unsupervised seismic facies classification our results show that at least three seismic facies can be differentiated representing associations of buildup lithologies aggradational or progradational carbonate platforms and debris facies we quantitatively evaluated the seismic facies against petrophysical properties porosity and permeability from available well logs seismic patterns associated with the lithologies helped identify new exploration targets

evolution of diagenetic system and its controls on the reservoir quality of pre salt dolostone the case of the lower ordovician majiagou formation in the central ordos basin china abstract the evolution of diagenetic system and its impact on the carbonate reservoir quality are of paramount importance new investigation of the lower ordovician majiagou formation central ordos basin china has revealed that the diagenesis and quality of pre salt dolostone reservoirs were controlled by the evolution of diagenetic system three different reservoir types grain crystalline and algal dolostone and five kinds of reservoir space intergranular pore intragranular pore intercrystalline pore dissolution pore and vug in karst system and parallel dissolution pore of laminar algae are identified in the majiagou formation pre salt dolostone of the central ordos basin based on the petrographic characteristic the diagenetic process of pre salt dolostone can be divided into four stages the penecontemporaneous diagenesis the early diagenesis the epidiagenesis and the mesodiagenesis late diagenesis petrographic and geochemical analyses of the studied dolostone reservoir indicated that the diagenetic system was open during the penecontemporaneous diagenesis and became closed during the early diagenesis open and closed diagenetic system existed in the west and east of the central ordos basin during the epidiagenesis respectively after that the western and eastern pre salt dolostone reservoirs both evolved into a closed diagenetic system during the mesodiagenesis late diagenesis subaerial meteoric dissolution in an open diagenetic system secondary pore formation and subsequent pore preservation in a closed diagenetic system are the critical factors in the improvement of pre salt dolostone reservoir quality although two cycles from open to closed diagenetic system developed in the western dolostone reservoir the pore space there was filled with overlying weathering mud in the second open diagenetic system during the epidiagenesis which reduced the reservoir quality the eastern dolostone reservoir experienced just one cycle from open to closed diagenetic system the reservoir quality was improved and then preserved thus only a good match among the diagenetic systems during different diagenetic stages will be good for the reservoir formation and preservation therefore the potential high quality reservoirs in the pre salt dolostones mainly located in the eastern part of the central ordos basin

bacterial shrubs crystal shrubs and ray crystal shrubs bacterial vs abiotic precipitation hot water travertine deposits commonly contain shrub like morphologies on the centimeter to meter scale that range from highly irregular forms bacterial shrubs to features that display regular geometric patterns crystal shrubs and ray crystal crusts the bacterial shrubs have been previously recognized as the product of bacterially induced precipitation whereas the ray crystal crusts have been described as due to dominantly abiotic precipitation the bacterial shrubs have a highly irregular morphology similar to woody plants in contrast end member crystal shrubs display distinct crystal habits and regular repeating morphologies i e they commonly have attributes of noncrystallographic as well as crystallographic dendrites there is a complete gradational relationship between the bacterial shrubs and crystal shrubs ray crystal crusts are large fan shaped features composed of subparallel extremely coarse bladed crystals of calcite all three types bacterial shrubs crystal shrubs and ray crystal crusts contain either a dense tangle of bacterial body fossils and or micron sized pores the micropores are bacterial molds the immediately enveloping spar crystals around all three morphologic types are devoid of bacterial body fossils and micropores the bacterial shrubs crystal shrubs and ray crystal crusts are to differing degrees the product of bacterially induced precipitation as well as abiotic mineral precipitation the differences in morphology are due to the relative contribution of bacterially induced precipitation as compared with that of abiotic mineral precipitation bacterially induced precipitates can form in environments too chemically harsh or otherwise inhospitable for other taxa to thrive epiphyton and renalcis commonly formed in occult environments under conditions inhospitable to other taxa consideration of the published descriptions and interpretations concerning epiphyton and renalcis and by analogy the origin of bacterial shrubs have led to the conclusion that epiphyton and renalcis also are bacterially induced precipitates

first record of carbonates with spherulites and cone in cone structures from the precambrian of arctic norway and their palaeoenvironmental significance abstract we report for the first time carbonates from the upper ediacaran sedimentary succession of finnmark arctic norway carbonates occur as calcareous siliciclastic beds lenses and concretions some with calcite spherulites and cone in cone cic calcite in a mudrock to fine grained sandstone succession from approximately 3 m to 26 m above the base of the 2nd cycle of the manndrapselva member of the stahpogieddi formation vestertana group they occur c 40 m below the ediacaran cambrian boundary which is well defined by trace fossils thin section petrography and scanning micro x ray fluorescence elemental mapping reveal a layered composition of the calcareous sedimentary rocks in some of those well developed nested cones of cic calcite form the outer layer thin clay coatings outline individual cones the inner layers are composed of 1 carbonate with calcite spherulites grainstone and 2 thinly laminated fine grained calcareous siliciclastics mudstone and wackestone indicated by elevated concentrations of al si fe and ti the inner siliciclastic layers contain framboidal pyrite and probably organic matter formation of calcite spherulites took place probably at the sediment water interface either in a coastal littoral environment or in situ in the sublittoral zone under high alkaline conditions whereas cic calcite formed during burial diagenesis and clearly in pre caledonian time before metamorphism and cleavage formation this new record of carbonates with calcite spherulites and cic structures from the ediacaran of arctic norway adds to their rare occurrences in the geological record

three dimensional stratigraphic sedimentological forward modeling of an aptian carbonate reservoir deposited during the sag stage in the santos basin brazil abstract stratigraphic sedimentological forward modeling is a useful technique to test the role of parameters that may act in the depositional processes of sedimentary rocks the technique was applied to the santos basin brazil using the software program dionisos to test possible depositional scenarios for an aptian section of a lacustrine carbonate platform developed during the late sag stage uppermost pre salt section of the basin different parameters were tested in a set of numerical simulations with the aim of reproducing the carbonate facies spatial distribution and the overall stratigraphic stacking pattern of the platform as observed in the available well data several models were simulated and four of them are discussed in this paper based on the available descriptive data from 10 wells model 3 is considered to produce the best fit to explain the development of the 100 m thick aptian carbonate platform occurrences of grainstones and stromatolites as proximal and intermediate facies respectively and the overall carbonate stacking pattern thus based on 3d stratigraphic sedimentological forward modeling the best set of environmental conditions able to explain the pattern of carbonate deposition observed in the study area are 1 a slow carbonate depositional rate 0 08 mm y over an interval of 2 4 my which is indicative of a larger microbial contribution and 2 lake level oscillations which were essentially induced by arid climatic conditions these results also enable new perspectives for the application of stratigraphic sedimentological forward modeling as a predictive tool for hydrocarbon exploration in carbonates and for the development of depositional models of other sag carbonate platform settings

presalt stratigraphy and depositional systems in the kwanza basin offshore angola the lower cretaceous presalt section in the kwanza basin contains an excellent petroleum system that includes synrift strata barremian overlain by a sag interval aptian and capped by the loeme salt the upper synrift is generally limestone with widespread mollusk packstones and grainstones coquinas deposited in a fresh to moderately saline alkaline lake the sag interval is characterized by carbonate platforms and silica rich isolated buildups formed in highly evaporated highly alkaline lakes shrubby dendritic microbially influenced boundstones and intraclast spherulite grainstones accumulated in shallow water on platform tops microbial cherts were deposited as organic buildups on large isolated structural highs basinward west of platforms and they apparently formed at low temperatures in very alkaline lake water shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability wackestones and packstones with calcitic grains mainly spherulites in dolomite or argillaceous dolomite were deposited in slightly deeper low energy sag environments in addition clays especially stevensite precipitated out of the silica rich highly alkaline lake waters during sag deposition calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms spherulites probably precipitated just below the sediment water interface spherulites and shrubby calcites are commonly recrystallized spherulites floating in stevensite probably formed in deeper lacustrine environments organic rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals and they are likely the source of most hydrocarbons in this system these interpretations are supported by seismic core petrographic and stable isotope data

sedimentology and petrophysical analysis of pre salt lacustrine carbonate reservoir from the santos basin southeast brazil lacustrine carbonate deposits mark the rift sag sedimentation stage of the so called pre salt section of the campos and santos basins in the southeast brazilian continental shelf these salt sealed carbonate reservoirs form deep water giant and prolific oil fields that are currently responsible for approximately 70 of the brazilian oil and gas production in the santos basin the sag stage carbonates are represented by the aptian barra velha formation characterized by exotic textural and compositional features whose origin development and correlation with petrophysical properties have been broadly discussed in the last decade a geological and petrophysical study has been performed based on core samples plugs and thin sections of a well in the barra velha formation from the eastern santos basin the analyzed core interval was grouped into six depositional facies constituted by the association of five key textural components 1 crystal shrubs 2 calcite spherulites 3 hybrid forms 4 mud grade carbonates and dolomite and 5 intraclasts the relationship between the depositional framework and the poroperm system as evidenced by petrographic image analysis and ct scan data is highly complex the depositional control over porosity is erratic mainly due to the extensive diagenetic and possible hydrothermal overprint dolomite cementation dolomitization pervasive silicification and dissolution processes are the main primary porosity modifiers petrophysical analysis indicates that intraclastic and spherulitic grainstones rudstones affected by matrix dissolution may present the best reservoir properties in certain intervals even surpassing the shrubby framestones which are widely regarded as the best reservoir facies this study seeks to contribute to an emerging body of research about the brazilian pre salt lacustrine carbonates broadening the geological knowledge about these deposit while providing new data to improve hydrocarbon forecast and recovery

origin and variability of a terminal proterozoic primary silica precipitate athel silicilyte south oman salt basin sultanate of oman the precambrian cambrian athel silicilyte is a 400 m thick salt encased siliceous succession in the south oman salt basin it is a self sourcing hydrocarbon reservoir and comprises up to 95 microcrystalline quartz and exhibits wavy discontinuous lamination comprising thin alternating organic rich and silica rich layers textures and geochemical fingerprinting indicate that it is a primary precipitate formed by microbially mediated precipitation of silica from sea water within the water column at the sulphidic oxic interface the unique occurrence of the athel silicilyte in the terminal proterozoic implies that optimal conditions for this style of silica precipitation occurred only briefly basin anoxia coupled with the growth of microbial mats low ph and high silica pore water saturations created optimal chemical conditions for silica precipitation volumes of microcrystalline quartz are highest within the transgressive and early highstand systems tract and towards the centre of the athel basin at the basin margins and within the late highstand systems tract volumes of microcrystalline quartz decreased as the volume of detrital sediment increased mass balance calculations indicate that silica enriched sea water would have been supplied to the basin by infrequent marine incursions that replenished ambient sea water in the upper part of the water column in conclusion precipitation of the athel silicilyte was driven by the coincidence of basin restriction limited clastic input availability of organic matter and water column anoxia the observation that there are few documented examples of chert deposits younger than ca 700 ma prior to the cambrian explosion suggests that although silica budgets within marine basins probably remained high prior to the evolution of silica secreting organisms direct precipitation from sea water was restricted this is tentatively related to the gradual increase in alkalinity of sea water through the palaeo proterozoic and meso proterozoic such that silica precipitation could only occur through the coincidence of basin anoxia and low siliciclastic input

lacustrine carbonate reservoirs from early cretaceous rift lakes of western gondwana pre salt coquinas of brazil and west africa abstract barremian aptian lower cretaceous pre salt lacustrine carbonates host large petroleum reservoirs in south atlantic rift basins yet remain poorly studied in part due to the scarcity of lacustrine carbonate reservoirs throughout the geological record this review provides an integrated discussion of the current state of knowledge on the coquina play having been deposited in extensive rift lakes during the early stages of the opening of the south atlantic thick successions of coquina are recorded from the santos to sergipe alagoas basins and the kwanza to gabon basins on the brazilian and west african continental margins respectively coquinas exhibit similarities in depositional processes across the south atlantic however have distinct diagenetic histories and stratigraphic stacking patterns having been deposited in lakes with different geologies climates and water chemistries west african coquinas exhibit depositional characteristics that are indicative of deposition on distally steepened ramps and rimmed platforms and accordingly facies are somewhat different to those in brazilian basins which were deposited on low angle ramps as petroleum exploration continues into ultra deep waters on the brazilian and west african margins accurate facies models will be important for the predictability of carbonate reservoir intervals to this end onshore outcrop in the sergipe alagoas basin has revealed that the molluscan faunas have marine origins suggesting that facies patterns may be more complex than previously considered it is hoped that this review will provide the impetus for further research on the coquinas especially given the presence of poorly studied easily accessible onshore outcrops

controls on the thermomechanical evolution of hyperextended lithosphere at magma poor rifted margins the example of espirito santo and the kwanza basins high quality long offset seismic data from many distal rifted margins show evidence for hyper extended 10 km thick crust direct observation of such domains is challenging as they lie at great water depth buried beneath thick sedimentary sequences and formed by rock assemblages that are hydrated and geophysically indistinguishable only a few drill holes have penetrated basement at ultradistal rifted margins these observations together with outcrops of preserved analogs exposed in collisional orogens suggest that the complex interaction of detachment faults rooted in a subhorizontal shear zone in the hyperextended crust or in the serpentinized mantle controls the formation of the ocean continent transition while depth dependent thinning controls the early phases of rifting conforming to classical rift models we still have a superficial understanding of how normal faults and subhorizontal shear zones form and evolve during rifting and lithospheric breakup here we develop a rheological parameterization to simulate the formation of and slip on large offset normal faults rooted in growing brittle to ductile shear zones the evolution of these structures leads to the creation of a hyperextended crust and eventually exhumed serpentinized mantle we also propose a simplified formulation to simulate magmatic underplating and seafloor spreading the resulting numerical models provide a self consistent picture for the evolution of magma poor rifted margins from initiation of rifting to seafloor spreading the model results are compared with first order observations of the kwanza and espirito santo conjugate margins in the south atlantic as well as of magma poor margins globally

geophysical evidence for breakup volcanism in the angola and gabon passive margins abstract serial two dimensional reflection seismic profiles spanning the continent ocean transition in the offshore kwanza and southern gabon basins have been interpreted and integrated with 2 5d gravity and magnetic anomaly modelling these continental margins are characterized by the presence of a distal domain in which continental crust has been highly stretched and faulted overlain by thick sequences of early cretaceous syn rift sediments that are in turn overlain by a thick but highly deformed aptian evaporite unit the evaporites are overlain by post breakup sediments cretaceous to recent in age oceanwards syn rift sediments are overlain by a volcanic unit that thickens basinward this volcanic unit forms the base and the lateral equivalent of the distal evaporite basin developing into positive relief volcanic edifices that acted as the oceanward margin of the evaporite basin these volcanic units extend from the distal domain of the passive margin to the oceanic crust and are interpreted to represent volcanism that immediately predates lithospheric breakup and formation of oceanic crust as indicated by the progressive transition into a constant thickness oceanic crust volcanism synchronous with the late stages of passive margin development and related hydrothermalism can account for the high alkalinity that dominated the pre salt lacustrine environments and could also have contributed to the modification of marine waters that led to the deposition of the thick aptian evaporites in the south atlantic ocean breakup volcanism and its associated thermal impact can also help explain the apparently anomalous shallow water depth of these margins preserved during the latest rift and earliest post rift stages

mineralogy and organic geochemistry of the terrestrial lacustrine pre salt sediments in the qaidam basin implications for good source rock development abstract terrestrial pre salt sediments are not only good lithologic reservoirs similar to marine pre salt carbonates but also are supposed to have high hydrocarbon generation potential because the overlying terrestrial thick bedded salts are generally preserved in basin centers however their status as source rock has attracted much less attention than as reservoir in this paper we choose the qaidam basin as an example and study the minerology organic geochemistry and organic petrology of the pre salt sediments in a hydrocarbon rich unit where abundant petroleum resources have been discovered under halite beds lithoscanner logging of six wells and x ray diffraction results of 378 samples show that terrestrial pre salt sediments are characterized by mixed siliciclastic carbonate sulphate rocks and their relative proportions are laterally varied quartz feldspar and clay minerals are of allochthonous origin and classified as argillaceous endmember while dolomite and ankerite are of autochthonous origin as carbonate endmember organic geochemistry analysis of 118 samples show great variability in toc s2 and tmax values and this variability is not related to mineral composition heterogeneity as hydrogen index is not well correlated with detritus carbonate and sulphate minerals contents indicating the interacting controls of source rocks by both exogenetic and endogenetic processes in brackish mesosaline lakes strong bacterial reworking of organic matter results in the dominant macerals of pre salt sediments as bituminite and oil generation at low maturity levels sealed by impermeable halite beds and in close proximity to reservoirs the generating hydrocarbon by pre salt good source rocks is effectively accumulated which entails hydrocarbon exploration under halite beds in other terrestrial salt bearing basins