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

crustal magnetic anomaly in the ordos region and its tectonic implications abstract in this paper the ngdc 720 v3 model is utilized to investigate the distribution of crustal magnetic anomaly and curie surface in the ordos block and its surrounding areas the curie surface is compared with the regional heat flow bouguer anomalies moho and seismicity the interior block has strong wide distributed magnetic anomalies well corresponding to the basement lithofacies the surrounding areas have weak anomalies of various strikes and intensities the magnetic anomaly confirms the geological finding that the yabulai and bayanhot basins are small blocks detached from the northern ordos block the magnetic anomalies at the satellite altitude reveal that deep materials flow northward from the northeast margin of tibet plateau to the qilian fold belt the depth of curie surface ranges from 24 to 42 km and the ordos block is dominated by curie surface depth over 32 km and average heat flow value of 44 mw m2 the distribution of bouguer anomalies is consistent with that of magnetic anomalies the well correlation between the curie surface and the seismicity is also explained

authigenic trioctahedral smectites controlling ph alkalinity silica and magnesium concentrations in alkaline lakes abstract the mineralogical composition of salts and the chemical composition of brines of lake yoa ounianga el kebir northern chad both reveal the neoformation of stevensite a trioctahedral smectite accompanied by aragonite the coprecipitation of these two minerals clearly controls the evolution of ph alkalinity and ca mg and silica concentrations in alkaline brines the geochemical evolution characterized by a strong depletion of ca and mg in solution and an increase in alkalinity and ph is confirmed as the dominant process governing the formation of soda alkaline lakes

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

fluvial landsculpting and the fractal dimension of topography abstract quantitative models of landform development can help us to understand the evolution of mountains and regional topography and the effects of tectonic motions and climate on landscape including its fractal geometry this paper presents a general and powerful three dimensional model of fluvial erosion and deposition at hill to mountain range scale the model works by accumulating the effects of randomly seeded storms or floods precipitons that cause diffusional smoothing then move downslope on digital topography grids that erode portions of elevation differences that transport a slope limited amount of eroded material and that deposit alluvium when their sediment carrying capacity is exceeded the iteration of these simple and almost linear rules produce very complicated simulated landscapes demonstrating that complex landscapes do not require complex laws each process implemented in the model is affected differently by changes in horizontal scale erosion a scale free process roughens topography at all wavelengths this roughening is balanced by diffusive processes scaling as 1 l 2 at short wavelengths and deposition scaling scaling as 1 l 2 at long wavelenghts such a mixture of scale free and scale dependent processes can produce multifractal behavior in the models the fractal dimension of the model topography is much more sensitive to climatic variables than to tectonic uplift landscape evolution may be fractal but it does not seem to be chaotic analysis of topography of areas in southern arizona using variograms shows approximately fractal behavior with mean fractal dimension around 2 2 2 3 departures from an exact fractal relatioship imply that the topography is in detail multifractal the fractal dimension at short wavelengths is less than that at long wavelengths this variation could either be caused by the relative strengths of diffusive and erosional processes shaping the topography or a result of changes in climatic or tectonic conditions still preserved in the landscape

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

seismic history of a bedrock fault scarp using quantitative morphology together with multiple dating methods a case study of the luoyunshan piedmont fault southwestern shanxi rift china abstract the study of bedrock fault scarps is a useful approach to identify paleo earthquakes and acquire the faulting history in bedrock areas where traditional trench techniques are not applicable here we report a study on the luoyunshan piedmont fault lyspf southwestern shanxi rift china although several studies have analyzed the lyspf its faulting behavior is still under debate for example the tectonic geomorphology fails to determine the frequency of past slips and the elapsed time to address these problems we investigated two bedrock fault surfaces luoyuncun northern segment and weicun southern segment using quantitative morphology analyses together with multiple quaternary dating techniques quantitative morphology analysis shows that both bedrock fault surfaces have the characteristics of vertical segmentation this kind of segmentation indicates that the fault surfaces are exhumed intermittently likely owing to periodic faulting earthquakes therefore earthquake events can be identified as morphological segments and the corresponding co seismic displacement can be determined by the height of these segments according to the results from the quantitative morphology and dating techniques four seismic events with a co seismic vertical displacement of 0 9 m 0 82 m 1 64 to 2 38 m and 2 30 to 2 54 m on the northern segment occurred during the late pleistocene whereas two seismic events with a co seismic vertical displacement of 1 33 m and 1 to 1 08 m are identified on the southern segment occurred during the holocene combined with the results of knickpoint series and trench techniques of the lyspf eight paleo earthquake events were identified since 45 ka with an average recurrence interval of 5 8 kyr the more complete paleo earthquake sequence also showed the possibility that the variation of the slip rate along the lyspf depends on the segment and that the displacement produced by the linfen m 7 earthquake of 649 ce has been recorded at the wc fault surface

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

unusual secondary ca mg carbonate kerolite deposits in basaltic caves kauai hawaii abstract aragonite calcite monohydrocalcite dolomite magnesite hydromagnesite and talc like kerolite have been identified in speleothems from basaltic caves on the island of kauai hawaii these secondary minerals have crystallized in the presence of freshwater seeping from the basaltic host rock and the mineralization process is ongoing at some locations the deposits are extremely variable in macroscopic character ranging from millimeter thin powdery coatings to hard crusts several centimeters thick many deposits are associated with actively forming microbial mats and stromatolitic structures are common in the thicker deposits formation of the mineral deposits is a result of a combination of direct precipitation from solution by evaporation and co2 degassing alteration of precursor minerals and or microbial processes and physicochemical conditions within the mats these unusual deposits provide important insights into the mineralogy and geochemistry of low temperature alteration and carb

the evolution of the shiwanghe river valley in response to the yellow river incision in the hukou area shaanxi china abstract tributary response to mainstream incision is an important landscape evolution process the objective of this study is to examine tributary valley evolution in response to mainstream incision the shiwanghe river a tributary of the yellow river in the hukou area was chosen for a case study the terraces and knickpoints of the shiwanghe river were investigated and correlated to those of the mainstream optically stimulated luminescence osl was applied to date fluvial terraces longitudinal profiles of river and terraces were used to analyze valley evolution the terrace sequence of the shiwanghe river near their confluence is almost identical to the yellow river terraces at the hukou area this suggests that terrace formations of the tributary and the mainstream are synchronous and influenced by similar factors but the formation age of the same tributary terrace varies from downstream to the upper reaches of the river valley for such terraces their formation should be controlled by knickpoint migration a sudden drop in base level caused by the yellow river incision would trigger the formation of a knickpoint in the tributary a new terrace would be formed as the knickpoint propagated upstream throughout the tributary valley due to the different erodibility of bedrock a set of interbedded sandstone and shale the major knickpoint would disassemble into a cluster of small ones during its propagation the age of terrace formation with various valley segments depends on knickpoint migration rate and distance from the confluence vertical incision of the yellow river results in knickpoint recession of its tributaries the migration rate of knickpoints was affected by climate lithologic variation and to some extent structural control

mg silica precipitation in fossilization mechanisms of sand tufa endolithic microbial community mono lake california abstract the detection of mg silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in mono lake california has been observed using scanning electron microscopy with backscattered electron imaging sem bse and energy dispersive x ray spectroscopy eds mg accumulated in cell walls of living cyanobacteria is interpreted to promote mg silicification with structural preservation while non cyanobacterial prokaryotes are not fossilized however extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation a mg silicate served to stabilize the biofilm remains further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates this precipitate damaged the microfossil structures but the mg silica fabric was still present the paleontological significance of mg silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed

quantitative morphology of bedrock fault surfaces and identification of paleo earthquakes the quantitative analysis of morphologic characteristics of bedrock fault surfaces may be a useful approach to study faulting history and identify paleo earthquakes it is an effective complement to trenching techniques especially to identify paleo earthquakes in a bedrock area where trenching technique cannot be applied in this paper we calculate the 2d fractal dimension of three bedrock fault surfaces on the huoshan piedmont fault in the shanxi graben china using the isotropic empirical variogram we show that the fractal dimension varies systematically with height above the base of the fault surface exposures indicating a segmentation of the fault surface morphology we interpret this segmentation as being due to different exposure duration of parallel fault surface bands caused by periodical earthquakes and discontinuous weathering we take the average of fractal dimensions of each band as a characteristic value to describe its surface morphology which can be used to estimate the exposure duration of the fault surface band and then the occurrence time of the earthquake that exposed the band combined with previous trenching results we fit an empirical relationship between the exposure duration and the morphological characteristic value on the fault d 0 049 t 2 246 the average width of those fault surface bands can also be regarded as an approximate vertical coseismic displacement of characteristic earthquake similar to the hongdong m8 earthquake of 1303 based on the segmentation of quantitative morphology of the three fault surfaces on the huoshan piedmont fault we identify three earthquake events the coseismic vertical displacement of the characteristic earthquake on the huoshan piedmont fault is estimated to be 3 4 m the average width of these fault surface bands gaps with a width of 0 1 0 3 m between two adjacent bands in which the fractal value increases gradually with fault surface height are inferred to be caused by weathering between two earthquakes or interseismic slip on the fault

are topographic data sets fractal the scale invariant properties of fractal sets make them attractive models for topographic profiles because those profiles are the end product of a complex system of physical processes operating over many spatial scales if topographic data sets are fractal their power spectra will be well represented by lines in log log space with slopess such that 3 s 1 the power spectra from a digital elevation model 30 meter sample spacing of the sierra nevada batholith and from seabeam center beam depths 425 meter sample spacing along a flowline in the south atlantic are curved straight sections in the spectra can be identified but the slopes of those sections are strongly dependent upon the particulars of the data analysis fractal geometry must be used with caution in the discussion of topographic data sets

deep burial dissolution in an oligocene miocene giant carbonate reservoir perla limestone gulf of venezuela basin implications on microporosity development abstract sedimentary rocks tend to progressively lose porosity with depth due to mechanical and chemical compaction in carbonates this trend is hard to predict since many factors can create porosity in the burial setting the oligocene miocene perla limestone a giant gas reservoir located in the gulf of venezuela basin shows a complex porosity system marked by a strong diagenetic control despite exhaustive depositional facies modelling carried out in the carbonate reservoir inconsistencies remained when distributing petrophysical properties with depositional facies these inconsistencies become more important in areas strongly affected by intense diagenetic processes after the initial investigation burial dissolution was identified as the main diagenetic process affecting porosity in this succession to understand the origin of the burial dissolution process and its distribution along the reservoir detailed petrographic mineralogical and isotopic studies were carried out on recovered cores from different exploration wells results show a series of diagenetic features which were associated with the evolution of the strata including burial dissolution dolomitization and compaction as the main processes after testing several hypotheses the results support an inorganic co2 model as responsible for the burial dissolution process in the perla limestone according to this model deep burial dissolution created pervasive microporosity with subordinated moldic and vuggy pores that enhanced the reservoir quality in localized areas of the carbonate succession these features are located where the ascent of hydrothermal co2 rich fluids funneled by discontinuity surfaces faults fractures stylolites etc reached the perla limestone

hydrogeochemistry of lake turkana kenya mass balance and mineral reactions in an alkaline lake abstract lake turkana in northwestern kenya is a closed basin alkaline ph 9 2 lake of moderate salinity tds 2500 ppm principal ions are na hco 3 and ci the lake is essentially polymictic in the northern basin and little compositional variation occurs in surface waters the omo river is the principal influent providing some 80 90 of water input to the lake chloride has an apparent accumulation time of about 2500 years after accounting for burial of interstitial water the bottom sediments are predominantly detrital and fine grained yet mineral water reactions are very important for the geochcmical budget ca 2 is precipitated as calcite na is removed as an exchangeable cation on smectite mg 2 is probably incorporated into a mg silicate phase most likely poorlycrystalline smectite as it enters the lake water k may be used in illite regradation cation exchange is a very important process in the mass balance of this lake over 40 of incoming na is removed as an exchangeable cation after cation exchange and interstitial water burial na has a response time of 2650 years which compares favorably with that of chloride these processes seem to occur rapidly within the water mass of the lake other reactions may be important in regulating interstitial water compositions several changes occur in the upper 3m of sediment interstitial water ph drops to 8 3 and alkalinity increases slightly with depth so 2 4 decreases slightly and amorphous silica saturation is approached these changes are a response to organic matter oxidation and the dissolution of unstable silicates rather than a reversal of reactions occurring in the lake water high rates of sedimentation up to 1 cm per year may minimize the effects of diffusion between the interstitial waters and the lake water although burial of interstitial water assumes considerable importance

fractal structures of river plan forms the basis of existing methods for the quantitative description of the plan geometry of rivers is determination of the characteristics of individual meander bends macroforms this approach is effective when one is working with a few well defined macroforms however the plan pattern of many rivers is really determined at the next higher geometric level the morphologically homogeneous river section which may comprise many tens of macroforms often with several superimposed scales at this level macroform parameters alone do not seem adequate to describe the geometry of the section new indices are required the concept of fractals affords a natural way of defining such indices the presence of hierarchical features superimposed sinuosity on various scales braiding with superimposed bars of a range of sizes etc suggests fractal behavior at least over certain ranges of spatial scale in this paper we demonstrate fractal behavior for 46 river sections in moldavia from measurements made from existing topographic maps we also introduce the notion of internal and external fractal scales that limit the range of fractal behavior here the internal and external scales are the width of the river channel b and of its valley floor b0 respectively using this idea together with elements of fractal geometry we obtain a relation among the sinuosity b b0 and the fractal dimension d of the river bed pattern we propose the use of d as a new and informative parameter for describing the internal structure of the plan pattern of both single thread and multithread rivers

low temperature oolitic talc in upper proterozoic rocks congo abstract a black granule layer in the precambrian west congolian mountain range has been studied situated in a calcareous and dolomitic unit of the schisto calcaire group of the west congolian system this layer called the pseudo oolithe de kisantu is found in all this part of southwest africa it is composed of millimeter sized spheroids or ovoids of pure talc in a siliceous matrix with minor amounts of calcite dolomite and sulfate these oolites have a homogeneous chemical composition and do not contain organic matter their oxygen isotopic composition 18 versus smow is within the range of low temperature clay minerals and outside the composition range of hydrothermal talcs after comparison with other authigenic mg clays found n salt bearing or evaporitic environments the proposed origin for this oolitic talc layer is a two step process 1 direct precipitation of stevensite or sepiolite at low temperature in shallow marine low salinity waters 2 transformation of the stevensite or sepiolite into talc during diagenesis

diagenetic pathways linked to labile mg clays in lacustrine carbonate reservoirs a model for the origin of secondary porosity in the cretaceous pre salt barra velha formation offshore brazil

the cenozoic tectonic evolution of the great north china two types of rifting and crustal necking in the great north china and their tectonic implications abstract the great north china underwent widespread continental rifting during cenozoic time that caused extension and necking of the crust in this area and formed four major rift systems in this paper we provide geological and geophysical evidence that rifting and extension in the western part of the great north china differs from that in its eastern part in age and nature the space time relationships show that the origins of the western rift group of the great north china are probably related to the tibet plateau tectonics whereas those of the eastern rift group are related to the subduction events in the western margin of the pacific plate furthermore the quaternary tectonic regime in the eastern part of the great north china differs from the neogene tectonic pattern and shows relations with western rifts we speculate that during the quaternary the processes that caused rifting in the west have spread further east and now affect the eastern part of the great north china

palaeohydrology of the lower cretaceous pre salt lacustrine system from rift to post rift phase santos basin brazil abstract large oil accumulation discoveries in the last decade in the pre salt rock succession off the brazilian coast aroused renewed interest in carbonate reservoirs prompting several studies aiming at unravelling the conditions of formation of these rocks despite that there are still many gaps in the knowledge concerning environmental conditions of the deposition of the pre salt carbonates of santos basin located at the southeastern brazilian margin in this study we present and discuss a new suite of geochemical and biostratigraphic data of the lacustrine rift to post rift succession broadly correlated with itapema and barra velha formations of the santos basin whose deposition occurred in the aptian the integration of the petrographic biostratigraphic and geochemical data serves the ultimate purpose of the study which is to apply the new radiogenic strontium data as a proxy for the reconstruction of the evolution of the lake hydrology associated with the deposition of the carbonates of the itapema and barra velha formations these data are integrated into numerical models whose results bring new ideas on the general hydrologic conditions of the lake system and on its evolution through time the mass balance calculations indicate that two end members alone can account as potential sources of sr to the palaeolake a mafic and a felsic component respectively the latter likely contributed with significantly higher fluxes roughly one to two and up to four orders of magnitude higher than the former end member for most of the depositional history in order to generate the 87sr 86sr trends observed in the carbonate rocks finally the results obtained from the integration of the observations with the numerical models point to decreasing fluxes of water with time as contributed from both sources suggesting that the formation of the deposits became very likely constrained to progressively more evaporitic and alkaline lake waters these results also open up a new window into the environmental conditions of the interior gondwana continent immediately prior to the definitive opening of the south atlantic ocean

evaluating roughness models of radar backscatter a unique set of data collected for a soil moisture experiment has been used to evaluate three radar backscatter roughness models the data were collected by the nasa space shuttle flight 41g sir b synthetic aperture radar sar in an intensively farmed area near fresno california the sir b data swath included a large number of bare dry fields with a large variety of surface roughnesses the small perturbation model gave the best results particularly when fields with a definite periodic row structure were omitted the standard deviation of surface heights appears to be a good measure of relative roughness conditions but the correlation length did not appear to be a good descriptor of the surface nor did it seem to be related in any way to the measured backscatter

fractal properties of landforms in the ordos block and surrounding areas china abstract this paper investigates the fractal properties of landforms in the ordos block and surrounding areas within china and discusses their geological and geomorphological implications the ordos block is tectonically stable but the surrounding areas are much more active and are affected by shear extensional structures we utilized the variogram method and the cellular fractal model to estimate parameters such as the fractal dimension d ordinate intercept and range r from the srtm3 dem using a moving window operation we suggest that the fractal dimension reflects the frequency of variation in elevation while the ordinate intercept reflects the amplitude of relief the fractal properties range from 90 m to 30 km geomorphological zones can be delineated using the fractal dimension and ordinate intercept these zones are consistent with known qualitative types of topography some basins are characterized by high fractal dimensions and low ordinate intercepts in contrast to mountainous areas with low fractal dimensions and high ordinate intercepts other areas such as the loess plateau and deserts also have unique values of fractal properties the effects of geological and geomorphological processes on the fractal properties are also discussed

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

backscattering behavior and simulation comparison over bare soils using sir c x sar and erasme 1994 data over orgeval abstract during april 1994 the three frequency radar system flew on the space shuttle endeavour known as sir c x sar mission shuttle imaging radar c x synthetic aperture radar over the orgeval watershed france the ground condition stayed very wet throughout the 5 day sar mission the sar imagery allows a data collection over a range of roughness conditions on bare soils three classes were identified very smooth sowings with crusted top layer cloddy surfaces and different ploughed fields for future crops to complement the shuttle radar data three frequencies l c x incidence range from 44 to 57 the helicopter borne scatterometer erasme c and x bands copolorized configurations was used merging of the two databases was possible as a result incidence angles ranging from 25 to 50 are available in c and x bands for the copolarized cross sections then the major objective of the article is over this available data collection to begin the validation of current surface backscattering models to natural surfaces the theoretical integral equation model iem of fung et al 1992 and the empirical model of oh et al 1994 it shows adequacies and limits the iem model reproduces well radar scatter over smooth surfaces but fails over rough surfaces predicting a flatter response with incidence angle than the observed signals in c and x bands difference in backscatter response due to direction angles perpendicular and parallel to the row direction is difficult to reproduce over smooth surfaces by this model integrating anisotropic surface but may be due to the unadequacy of the surface representation the oh algorithm agrees well with the backscatter response over rough surfaces at medium incidence angle but fails with a systematic underestimation over smooth conditions as a conclusion further developments are necessary on derivation of theoritical solutions over rough surfaces and on validation of semiempirical algorithms over data sets of various training sources radar and natural conditions

provenance and depositional environments of quaternary sediments in the southern kalahari basin abstract the vast kalahari basin is characterized by uplifted margins terrestrial sedimentation within semi endorheic sub basins subdued morphology and tectonic quiescence this intracratonic basin has been subjected to a prolonged period of subsidence affecting its sedimentary fill by plate motion and climatic cycles provenance studies of kalahari group sediments mainly focused on the surficial deposits that represent only the last phase of sedimentation leaving unresolved questions related to the rest of the strata the southern kalahari group succession exposed along the walls of the mamatwan mine northern cape south africa reveals three main depositional environments a bottom low energy water body a middle fluvial high energy environment and an upper aeolian sandy unit the entire section which was deposited during the early to middle quaternary records significant environmental and depositional changes suggesting a highly dynamic landscape the fully exposed section 55 m of the kalahari group at mamatwan mine was analysed for its mineralogy elemental composition sr nd and pb isotopic ratios and iron species isotopic fingerprints were used to determine the source rocks of the sediments the basal part of the section was derived from the distant angolan highlands supporting previous suggestions for the existence of a paleo trans kalahari drainage system weathering sensitive indices show that these sediments underwent considerable chemical weathering typical to humid areas mineralogical assemblage and iron phases imply that a brackish and alkaline shallow water body existed during the early middle pleistocene contemporaneous with relative dense hominin occupation of the area during the early middle pleistocene transition the lacustrine environment was rapidly filled with volcano sedimentary archean clasts that were derived mainly from the surrounding hills and experienced limited degree of source rock chemical weathering but underwent subsequent groundwater alteration by iron silica and calcium rich solutions and precipitation of calcrete and silcrete duricrusts these changes in provenance and depositional environments may be related to tectonic adjustments along the southern margins of the kalahari basin ultimately a shift into an aeolian transport dominated landscape occurred in the region this was accompanied by reduced surface water availability and a supply of sediments from paleoproterozoic western and north western source areas following the establishment of the modern wind regime

climatic signals in lacustrine deposits of the upper yacoraite formation western argentina evidence from clay minerals analcime dolomite and fibrous calcite

growing spherulitic calcite grains in saline hyperalkaline lakes experimental evaluation of the effects of mg clays and organic acids the origin of spherical radial calcite bodies spherulites in sublacustrine hyperalkaline and saline systems is unclear and therefore their palaeoenvironmental significance as allochems is disputed here we experimentally investigate two hypotheses concerning the origin of spherulites the first is that spherulites precipitate from solutions super saturated with respect to magnesium silicate clays such as stevensite the second is that spherulite precipitation happens in the presence of dissolved organic acid molecules in both cases experiments were performed under sterile conditions using large batches of a synthetic and cell free solution replicating waters found in hyperalkaline saline lakes such as mono lake california our experimental results show that a highly alkaline and highly saline solution supersaturated with respect to calcite control solution will precipitate euhedral to subhedral rhombic and trigonal bladed calcite crystals the same solution supersaturated with respect to stevensite precipitates sheet like stevensite crystals rather than a gel and calcite precipitation is reduced by 50 compared to the control solution producing a mixture of patchy prismatic subhedral to euhedral and minor needle like calcite crystals enhanced magnesium concentration in solution is the likely the cause of decreased volumes of calcite precipitation as this raised equilibrium ion activity ratio in the solution on the other hand when alginic acid was present then the result was widespread development of micron size calcium carbonate spherulite bodies with further growth time but falling supersaturation these spherules fused into botryoidal topped crusts made of micron size fibro radial calcite crystals we conclude that the simplest tested mechanism to deposit significant spherical radial calcite bodies is to begin with a strongly supersaturated solution that contains specific but environmentally common organic acids furthermore we found that this morphology is not a universal consequence of having organic acids dissolved in the solution but rather spherulite development requires specific binding behaviour finally we found that the location of calcite precipitation was altered from the air water interface to the surface of the glassware when organic acids were present implying that attached calcite precipitates reflect precipitation via metal organic intermediaries rather than direct forcing via gas exchange

self affinity dimensions of topography and its implications in morphotectonics an example from taiwan abstract this paper discusses the self affinity dimensions of landscape surfaces at a short range scale and the link to morphotectonic features of the young orogenic belts of taiwan the variogram method is adopted to estimate such parameters as the fractal dimension d the ordinate intercept and the range r from data subsets of the digital elevation model dem in a moving window operation the fractal morphology expressed by d and is found to be useful in defining geomorphic provinces that are related to tectonic features the mountainous terrain is characterized by high gamma values and low fractal dimensions in contrast to the coastal plains where low gamma values and high fractal dimensions are found a zone defined by the fractal parameters 2 4 d

autogenic microbial genesis of middle miocene palustrine ooids nullarbor plain australia abstract miocene palustrine ooids located in the nullarbor plain southern australia illustrate the importance of microbes and soil processes in the formation of these spherically laminated grains in pedogenic settings these particles are composed of dense minimicrite and include a wide assortment of microbially produced structures such biogenicity includes interpreted extracellular polymers eps algal filaments palygorskite and sepiolite nano fiber mats bacterial spores and meniscate fabrics ooid nuclei are peloids that are interpreted to have formed via microbial binding of lacustrine sediment in a subsequent palustrine environment both at the surface and inside underlying karst cavities the peloids are encapsulated in multigenerational micrite laminations that form the spherical cortex individual cortical laminae are made up of degraded minimicrite crystals and complex encrusting mats of palygorskite and sepiolite nano fibers cortex generation is interpreted to have been an annual process involving alternating soil hydration states in this conceptual model during the wet season ooids were covered in bacterial mucus that bound ions from solution and produced hydrated mg si gels that evenly surrounded the entire ooid gel dehydration during the following dry season resulted in precipitation of fibrous palygorskite and sepiolite succeeding seasonal variations in moisture produced numerous laminae that ultimately resulted in a multigenerational ooid cortex this annual microbial process provides a new mechanism involving an autogenic formation of ooids that may be applied to a wide variety of pedogenic sediments unlike the more common marine ooids that require constant movement to achieve cortical precipitation the process of grainification combined with microbially mediated mineral precipitation under different soil hydration states results in the in place formation of terrestrial ooids

saline lake diagenesis as revealed by coupled mineralogy and geochemistry of multiple ultrafine clay phases pliocene olduvai gorge tanzania mineralogical and geochemical analyses of ultrafine 0 1 m extracts of pliocene clays from olduvai gorge reveal the complexities of clay diagenesis in saline alkaline paleo waters multiple authigenic phases are routinely present these may be distinguished and quantified by decomposition of xrd 060 peaks coupled with geochemical microprobe hrtem aem and crystallographic ir investigation by assigning geochemistry based on xrd results we identify three principal phases by octahedral composition dioctahedral al rich dioctahedral relatively fe rich and mg rich with a trioctahedral component average octahedral formulae are al 1 30 fe 0 57 mg 0 13 al rich al 0 56 fe 0 80 mg 0 62 fe rich and al 0 09 fe 0 23 mg 2 09 mg rich per half formula unit ir analyses support these octahedral structures the mg rich phase either has coexisting dioctahedral and trioctahedral domains within individual sheets or a homogeneous octahedral sheet of intermediate composition this is to our knowledge the first report of an intermediate octahedral occupancy in a 2 1 phyllosilicate hrtem observations indicate both solid state octahedral and illitization reactions and dissolution precipitation layer charge and interlayer cations suggest that during illitization layer charge increase was due to interaction with na rich brines but later k uptake may have been with either saline or fresher fluids with higher k na ratios all three phases occur throughout the basin relative proportions and octahedral compositions vary total mg content of bulk 0 1 m clay fractions a paleochemical indicator in this and other basins is the product of both the relative abundance and the mg content of the mg rich phase in addition to shedding light on the crystal chemistry of authigenic 2 1 phyllosilicates these results demonstrate the paleolimnologic importance of discriminating the presence of multiple authigenic clay phases in lacustrine deposits

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

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

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

simple mathematical model of a rough fracture a simple mathematical model of rough walled fractures in rock is described which requires the specification of only three main parameters the fractal dimension the rms roughness at a reference length scale and a length scale describing the degree of mismatch between the two fracture surfaces fractured samples collected from natural joints and laboratory specimens have been profiled to determine the range of these three parameters in nature it is shown how this surface roughness model can be implemented on a computer allowing future detailed study of the mechanical and transport properties of single fractures and the scale dependence of these properties

stevensite in the modern thrombolites of lake clifton western australia a missing link in microbialite mineralization microbialites form the earliest macroscopic evidence of life and have always been important in particular aquatic ecosystems they demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size microbialites are generally assumed to form by microbial trapping and binding of detrital grains by carbonate organomineralization of microbial biofi lms or by inorganic mineralization around microbial templates here we present a signifi cant discovery that modern thrombolitic microbialites in lake clifton western australia gain their initial structural rigidity from biofi lm mineralization by the trioctahedral smectite mineral stevensite this nucleates in and around microbial fi lament walls when biological processes suppress carbon and ca activities leaving mg to bind with silica and form a microporous framework that replaces and infi lls the fi lament web after microbial materials are entombed local carbon and ca activities rise suffi ciently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely with eradication of biogenic textural features this may explain why many ancient microbialite carbonates lack clear evidence for biogenicity stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcifi cation

the influence of authigenic clay formation on the mineralogy and stable isotopic record of lacustrine carbonates abstract the mineralogical compositional and stable isotopic variability of lacustrine carbonates are frequently used as proxies for ancient paleoenvironmental change in continental settings under the assumption that precipitated carbonates reflect conditions and chemistry of ancient lake waters in some saline and alkaline lake systems however authigenic clay minerals forming at or near the sediment water interface are a major sedimentary component often these clays are rich in mg influencing the geochemical budget of lake waters and are therefore expected to influence the properties of contemporaneous authigenic carbonate precipitates which may also contain mg this paper documents evidence for a systematic feedback between clay mineral and carbonate authigenesis through multiple precessionally driven m scale sedimentary cycles in lacustrine oil shale deposits of the eocene green river formation from the uinta basin ne utah in the studied section authigenic mg rich trioctahedral smectite content varies cyclically between 9 and 39 wt the highest concentrations occur in oil shales and calcareous mudstones deposited during high lake level intervals that favored sedimentary condensation lengthening the time available for clay diagenesis and reducing dilution by other siliciclastic phases an inverse relation between dolomite percentage of carbonate and trioctahedral smectite abundance suggests the mg uptake during clay authigenesis provides a first order control on carbonate mineralogy that better explains carbonate mineralogical trends than the possible alternative controls of 1 variable mg ca ratios in lake water and 2 degree of microbial activity in sediments we also observe that cyclical change in carbonate mineralogy believed to be induced by clay authigenesis also causes isotopic covariation between 13cpdb and 18opdb of bulk sediments because of differences in the equilibrium fractionation factors of dolomite and calcite 2 and 2 6 respectively this provides an alternative mechanism for the common pattern of isotopic covariation which is typically attributed to the effect of simultaneous changes in water balance and biological activity on the carbon and oxygen isotopic composition of lake waters these findings may help improve paleoenvironmental reconstructions based on lacustrine carbonate records by adding to the factors known to influence the mineralogical compositional and stable isotopic signals recorded by lacustrine carbonates

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

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