porosity reduction in sandstone by quartz overgrowth a mathematical model for predicting porosity histories due to quartz overgrowth in quartz rich sandstone reservoirs has been developed the model neglects secondary dissolution porosity and other diagenetic processes such as compaction and precipitation of carbonate and clay minerals nevertheless the model is straightforward and easy to use to calculate porosity the calculated porosity corresponds to measured porosity of simple quartz cemented sandstones in 27 wells in north and south america the model is based on fluid flow through initially porous unlithified sands in which the fluid phase is saturated with si oh 4 and is always in equilibrium with quartz as the continuously circulating fluid migrates updip toward the basin edge it cools and quartz precipitates into the pore spaces causing a loss of porosity basinal fluid velocities may be calculated assuming 1 thermal convection and 2 hydrostatic pressure due to recharge at the edge of the basin porosity diagrams relating porosity to geothermal gradient burial rate and depth of burial are compared to thermal maturation models of source rocks fluid flow history and grain size distribution the rate of porosity reduction depends on the following variables in decreasing order of significance buria rate age initial porosity basin size dip angle fluid dynamics initial permeability and geothermal gradient

diagenesis associated with cooling during burial an example from lower cretaceous reservoir sandstones sirt basin libya fluid inclusions in quartz and dolomite cements were used to determine temperature timing origin and function of fluids in the diagenetic evolution of the lower cretaceous upper sarir sandstone sirt basin libya transmitted light backscattered electron and scanning electron microscope sem cathodoluminescence analyses were carried out to determine the paragenesis in quartz overgrowths fluid inclusions along the grain overgrowth boundary yield homogenization temperatures from 130 to 141 degreesc and salinities between seawater and freshwater aqueous fluid inclusions in later quartz overgrowths have homogenization temperatures from 114 to 129 degreesc and salinities between 18 and 21 wt nacl equivalent fluid inclusions in dolomite have variable homogenization temperatures 120 141 degreesc and high salinities 20 22 wt nacl equivalent this trend in homogenization temperatures of fluid inclusions is interpreted as the result of quartz cement precipitation in the presence of hot low salinity connate fluids connected with an increased heat flow related to regional rifting later quartz overgrowths precipitated during regional cooling concurrent with the end of the rifting and after invasion of saline fluids possibly brought in from adjacent areas or refluxed downward during or after times of evaporite deposition this diagenetic history linked to a decreasing heat flow could be common in other aborted rift basins which are important targets for oil exploration

senonian basin inversion and rejuvenation of rifting in africa and arabia synthesis and implications to plate scale tectonics abstract the late paleozoic to tertiary stratigraphic record of much of the african plate reflects the effects of continental rifting and passive margin development several short lived but widespread and tectonically important compressional or wrench dominated events occurred however during the permian to recent evolution of africa we focus here on the best documented of these events which occurred during the late santonian at that time older sedimentary basins mostly ene wsw trending were folded and inverted including some basins along the tethyan margin from morocco to the syrian arc and the intraplate benue southern chad basins and the lugh mandera basin in oman ophiolites were obducted following the santonian tectonism an extensive phase of rifting occurred in central and north africa and northern arabia spanning from campanian to maastrichtian or paleocene times rejuvenation or acceleration of subsidence occurred in several basins located along the tethyan margin along the atlantic and indian ocean margins as well as within the intraplate domain rifting was sometimes accompanied by magmatic activity especially in offshore northern libya and along the indian ocean margin compressional deformations rejuvenated or developed by latest maastrichtian times along the tethyan margin and locally within the intraplate domain one of the most remarkable attributes of the santonian tectonic event which we consider representative of the major trans african stress field changes is the rapidity with which the change was reflected stratigraphically across north and central africa and its short duration the cause of the santonian compressional event is directly linked to the change in poles of rotation for the opening of the atlantic at 83 85 ma the end of the cretaceous normal magnetic quiet zone this was also synchronous with the obduction of ophiolites along the northeast margin of arabia oman the onset of separation between india and madagascar with formation of the mascarene oceanic basin and the development of the european alpine chain we interpret these phenomena as causally related aspects of a global tectonic event the santonian compressional event as well as the campanian maastrichtian rifting event and the end cretaceous compressional event illustrate the critical connection between intraplate tectonic histories and processes occurring at sometimes very distant plate boundaries

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

dolomites and dolomites in mesozoic platform carbonates of the southern apennines geometric distribution petrography and geochemistry abstract a multidisciplinary study of the dolomitized bodies present among the late triassic to late cretaceous carbonate deposits of the monti lattari belt southern apennines of italy compares the different dolomitization styles present within this succession of these mesozoic platform carbonates and furnishes an overview of their formative genetic models the revealing traces of the diverse modes of diagenesis in this region include both surface and subsurface mechanisms utilizing a geometric reconstruction of the dolomitized bodies an integrated facies and petrographic analysis of the components and a geochemical characterization of the sediments including detailed mineralogy and associated stable isotope values the data presented demonstrate that some intervals are characterized by surface related dolomitization processes middle late triassic and cretaceous while others latest triassic to lower middle jurassic and are mainly dominated by later large scale subsurface dolomitization processes all of the middle late triassic norian successions are pervasively dolomitized exhibit fabric preserving textures and have a distinctly heavy stable isotope signature their origin has been related to diagenesis due to particular climatic and palaeoceanographic conditions leading to the formation of extended blankets of shallow warm saline seawater this setting was terminated by climatic and palaeoceanographic changes during the latest triassic which dramatically reduced the significance of surface related dolomitization on a regional scale the overlying latest triassic rhaetian outcrops consist of a widespread coarse fabric destructive massive dolomite with a lighter isotopic signature these dolomites seem genetically related to the ones of the lower middle jurassic which share the same geometrical petrographic and geochemical characteristics caused by large scale fluid circulation this diagenetic pattern apparently started in the early jurassic during burial driven by thermal convection due to the different temperatures between the oceanic waters and the warmer carbonate platform the middle upper jurassic units feature local episodes of early dolomitization that partially replaced the muddy carbonate facies due to tidal pumping and reflux mechanisms dolomitization is not a pervasive feature during this phase of deposition in the cretaceous portion of the succession dolomites have a totally different and distinct character and are generally related to bedding and show variability in textures and mg ca ratios local association with evaporites geometry of dolomitized bodies and stable isotope geochemistry suggest processes comprising evaporative pumping and reflux all related to the surface environment this evidence points to a renewal of a climatic context more favorable for early dolomitization thus this study permits recognition of the presence of three quite different dolomitization styles within the carbonate platform succession of the southern apennines fold thrust belt one ascribed to the norian one to the rhaetian lower jurassic and one to the cretaceous

geologic co2 storage in arkosic sandstones with cacl2 rich formation water abstract the feasibility of geologic co2 storage in deeply buried arkosic sandstones has been tested using high temperature high pressure short term laboratory experiments and long term numerical simulations with co2 saturated solution rich in cacl2 these conditions mimic the conditions found today in the eocene reservoir sandstones of depleted oilfields in the dongying sag bohai bay basin china experiments at 100 c and 150 c and pco2 of 4 mpa were conducted on sandstones rich in k feldspar and albite without anorthite during the experiments calcite and kaolinite precipitated while albite and k feldspar partly dissolved ca2 in formation water is shown to be critical for mineral trapping of co2 the continuous dissolution of k feldspar and albite at a slow rate for a long time period can prolong duration time of calcite precipitation and increase geologic co2 storage capability by mineral trapping addition of nacl kcl and mgcl2 can prolong the dissolution time of k feldspar and albite and precipitation duration of calcite it also increased the mass of sequestered co2 by mineral trapping the process of geologic co2 storage can be divided into 3 stages in stage i calcite precipitates rapidly and geologic co2 storage is dominated by solubility trapping within 100 years stage ii lasts up to 300 years in the solution without nacl kcl and mgcl2 and 900 years with addition of nacl kcl and mgcl2 into the solution the precipitation rate of calcite decreases abruptly and the mass of mineral trapping of co2 increases with increasing time because dissolution of k feldspar and albite causes decrease of ca activity and prolongs calcite precipitation time the geologic co2 storage is dominated by mineral trapping in stage iii reactions reach equilibrium and the mass of geologic co2 storage reaches the maximum the calculated mass of co2 by mineral trapping in sandstones of the es4x to es3z in the dongying sag is about 3 61gt

sandstone porosity loss a big picture view of the importance of compaction proper assessment of the relative importance of porosity loss by compaction and cementation is important to the prediction of porosity in sandstones studies of mass transport during diagenesis and the modeling of basin fluid flow quantitative estimation of the amounts of porosity loss by compaction and cementation can be made from standard point count data on cement and pore space abundance this method is discussed and its sources of error are evaluated error in the method arises mainly from uncertainty in initial sandstone porosities and the amount of local grain dissolution during burial data on the porosity of modern sands indicate that assumed initial porosities of 40 to 45 will produce small average errors in the calculation of compactional porosity loss for clean sandstones grain dissolution whether resulting in local porosity enhancement by export in solution of former grain mass or local precipitation of authigenic cement will cause an underestimation of the significance of compaction a large database of point count data from diverse sandstones is used to provide a broad perspective on the importance of compaction and cementation to sandstone porosity loss it is shown that the global importance of compactional porosity loss has generally been under appreciated in fact compaction mechanical and chemical is probably the dominant mechanism of porosity loss in most sandstones the extent of compactional porosity loss is related to sand grain composition and basin thermal structure

origin distribution and movement of brine in the permian basin u s a a model for displacement of connate brine na cl halite ca cl and gypsum ca cl brines with salinities from 45 to 300 g l are identified and mapped in four hydrostratigraphic units in the permian basin area beneath western texas and oklahoma and eastern new mexico providing spatial and lithologic constraints on the interpretation of the origin and movement of brine na cl brine is derived from meteoric water as young as 5 10 ma that dissolved anhydrite and halite whereas ca cl brine is interpreted to be ancient modified connate permian brine that now is mixing with and being displaced by the na cl brine displacement fronts appear as broad mixing zones with no significant salinity gradients evolution of ca cl brine composition from ideal evaporated sea water is attributed to dolomitization and syndepositional recycling of halite and bittern salts by intermittent influx of fresh water and sea water halite ca cl brine in the evaporite section in the northern part of the basin differs from gypsum ca cl brine in the south central part in salinity and na cl ratio and reflects segregation between halite and gypsum precipitating lagoons during the permian ca cl brine moved downward through the evaporite section into the underlying lower permian and pennsylvanian marine section that is now the deep basin brine aquifer mixing there withmore pre existing sea water buoyancy driven convection of brine dominated local flow for most of basin history with regional advection governed by topographically related forces dominant only for the past 5 to 10 ma 71 refs 11 figs less

duration of quartz cementation in sandstones north sea and haltenbanken basins abstract evidence from fluid inclusions petrography and burial history modelling is used to estimate the duration of quartz cementation in sandstones the homogenization temperatures of primary fluid inclusions in quartz cements from samples from the brent group north sea and haltenbanken offshore norway show variations 4 of 17 4 and 20 2 c these are interpreted to reflect durations of less than approximately 9 and 17 ma low variances are general features of such data sets and suggest that quartz cementation usually takes place as events with durations of the order of 10 ma or possibly less confining the age of quartz cement growth using a combination of geological petrographical and radiometric dating methods that are independent of fluid inclusions also suggests that quartz cements grow over periods substantially shorter than the age of the sandstone it is not easy to reconcile this deduction with the suggestion that burial itself causes cementation

evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields abstract quartz is a major pore occluding cement in north sea oilfield reservoir sandstones fluid inclusions yield homogenization temperatures t h which have been thought to represent growth temperatures for the host quartz however early diagenetic inclusions from the detrital grain overgrowth boundary yield excessively high palaeotemperatures which do not match the growth temperatures calculated from the depth of cementation the temperatures of these inclusions increase with present day depth of burial and approach present day reservoir temperatures this suggests that the inclusions have begun to reset towards present day pressure temperature conditions the t h of the inclusions hence represent burial temperatures rather than growth temperatures for the quartz experimental studies indicate that the amount of resetting undergone by an inclusion varies depending on its size shape and fluid composition a relationship between inclusion t h and these variables has been observed in inclusions from deeply buried quartz cements in the north sea

diagenetic and thermal history of the jurassic tertiary succession of the zagros mountains in the dezful embayment sw iran constraints from fluid inclusions to constraint the diagenetic and thermal history in the mesozoic tertiary succession of the zagros fold belt a study was performed on fluid inclusions trapped in intergranular vug and fracture filling cements in jurassic to miocene outcrop samples collected along the anneh and fahliyan valleys of khuzestan province southwest iran petrographic observations showed at least two systems of fractures that postdate intergranular cementation two different types of oil filled fluid inclusions occur in the intergranular cements and in the first fracture network but they are absent in the second fracture network microthermometry of fluid inclusions was used to determine the temperature and salinity of fluids responsible for mineral precipitation within intergranular cements and calcite fillings the oldest fractures precipitation occurred from high saline fluids in a temperature range of 60 120 c depending on the stratigraphic depth the strong correlation between fluid inclusions temperature and stratigraphic depth suggests that the intergranular cementation and the first fracture filling event occurred prior to structural deformation furthermore this correlation indicates a relatively constant paleogeothermal gradient at the time of cement precipitation of about 28 c km on the contrary fluid inclusions trapped in cements within the second fracture network lack oil and contain freshwater trapped at low temperatures that do not correlate to the stratigraphic position this suggests second fracturing event occurred after oil migration and during or after deformation of the zagros fold belt when the samples were in a near surface position

high resolution record of the last climatic cycle in the southern carpathian basin surduk vojvodina serbia abstract high resolution study of the surduk loess palaeosols sequence in serbia vojvodina has been performed within a research project eole focusing on the impact of rapid climatic changes during the last climatic cycle in the european loess belt the methodology used for this multidisciplinary approach is based on a continuous sampling column that allows a very accurate correlation between all studied proxies magnetic susceptibility grain size and organic carbon and the dated samples irsl 14 c according to the stratigraphical and sedimentological data the surduk loess sequence appears as a very complete record of the last climatic cycle 19 m and exhibits a similar pattern than other contemporaneous loess sequences from western central and eastern europe the main difference is the evidence of a drier environment all over the last climatic cycle sedimentological and palaeopedological data the high resolution grain size record 5 cm is well correlated with stratigraphical boundaries and highlights a strong variability within the loess deposition especially during the upper pleniglacial between ca 33 and 15 ka during the upper pleniglacial a succession of millennial timescale events characterised by the deposition of coarser loess are particularly well evidenced by grain size data as in some west european records finally an attempt to correlate the variations of grain size parameters at surduk with the greenland grip dust record is proposed according to this study millennial timescale climatic events that characterise the north atlantic area during the last climatic cycle have thus been recorded in the environments located at the southern border of the european loess belt

the application of fluid inclusions to migration of oil and diagenesis in petroleum reservoirs abstract fluid inclusions can be used to interpret thermal history and petroleum maturation and migration relative to burial history temperature pressure and composition data collected from fluid inclusions are used to determine the environment of diagenesis and the timing of cementation and migration cements in petroleum reservoirs contain both oil and aqueous fluid inclusions fluorescence spectroscopy is used to identify oil inclusions and to determine the maturity of entrapped oil the lifetime of fluorescence induced by a pulsed laser is related to the api gravity of entrapped oil interpretation of fluid inclusion data depends on knowing the origin of fluid inclusions and the probability that they survive in the burial environment those aspects of fluid inclusion study are investigated by synthesizing oil and aqueous inclusions in calcite crystals in laboratory experiments examples of how fluid inclusions are used to determine the physico chemical environments of diagenesis in petroleum reservoirs and the timing of cementation and migration are given for the wealden basin england the mishrif formation dubai the smackover formation gulf coast u s a and jurassic sandstones offshore norway in the wealden basin temperature data from fluid inclusions are used to determine that oil migration occurred in the cretaceous and that the reservoir rocks have been uplifted to varying degrees at a time after migration distribution of oil inclusions indicates that generation and migration of oil was principally in the western part of the basin the geochemistry of oil inclusions in calcite cements from the mishrif formation dubai are used to determine the type and maturity of entrapped oil temperature data from oil and water inclusions are used to relate reservoir diagenesis to burial history and the migration of oil in deep smackover reservoirs oil contains h 2 s the origin of the h 2 s is examined by study of fluid inclusions containing h 2 s in jurassic sandstones offshore norway fluid inclusion studies show that silica cementation is related to burial depth whereas a later calcite cementation originated from invasion of a hot fluid

remobilization of deep basin brine during exhumation of the illizi basin algeria abstract understanding the processes that drive fluid flow in sedimentary basins has important implications for models of metallic ore and petroleum migration and accumulation in the subsurface potential drive mechanisms include gravity driven flow compaction driven flow and density driven convection in this study we demonstrate that the brine hosted in ordovician sandstone in the illizi basin in algeria is genetically linked to triassic liassic evaporites deposited 400 km to the north in the berkine basin this observation confirms that long distance lateral brine migration has occurred within the basin in the past we assess the hydrogeologic record preserved in aqueous fluid inclusions within the ordovician sandstone document a marked increase in formation water salinity during cooling and exhumation and evaluate the drive mechanisms for late stage remobilization of deep brines within the basin it is hypothesized that the release of overpressure during exhumation of the illizi basin may have been a critical contributor to updip fluid flux this model could be applicable to other exhumed basins worldwide

melting behavior of fluid inclusions in laboratory grown halite crystals in the systems naclh2o naclkclh2o naclmgcl2h2o and naclcacl2h2o abstract the chemical composition of aqueous fluid inclusions in crystals of halite can be accurately determined from observed melting behaviors of ice hydrohalite and sylvite some fluid inclusion melting behaviors observed in laboratory grown halite crystals systems nacl h 2 o nacl kcl h 2 o nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o differ from predicted stable equilibrium relations in the nacl h 2 o and nacl kcl h 2 o systems observed first melt temperatures are up to 15 c below the equilibrium eutectic temperatures of 21 2 and 22 9 c respectively the final melting temperature of ice in the presence of hydrohalite and the final melting temperature of hydrohalite are reproducible and match predicted melting temperatures the limit of detection of sylvite daughter crystals in the nacl kcl h 2 o system is approximately 5 wt 1 molal kcl final melting temperatures of sylvite match published equilibrium data to within 0 3 c in the nacl kcl h 2 o system at halite saturation m kcl can be determined from the final sylvite dissolution temperature or from the final melting temperature of hydrohalite fluid inclusions in the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems that form stable salt hydrates mgcl 2 12h 2 o and cacl 2 6h 2 o during freezing first melt within 3 c of predicted eutectic temperatures 37 and 52 c however fluid inclusions with mgcl 2 or cacl 2 may also start melting at temperatures as low as 80 c such low first melt temperatures indicate the presence of metastable salt hydrates presumably mgcl 2 8h 2 o mgcl 2 6h 2 o or cacl 2 4h 2 o the formation of metastable phases during freezing of fluid inclusions can lead to misinterpretation of the chemical composition of fluid inclusions in natural samples this is especially true for fluid inclusions with first melt temperatures below 37 c which may be erroneously interpreted as being rich in cacl 2 the final melting of ice in the presence of hydrohalite may vary by more than 15 c in fluid inclusions of different size but identical bulk composition and occurs at lower temperatures than predicted in fluid inclusions from the nacl mgcl 2 h 2 o and nacl cacl 2 h 2 o systems however the final melting temperature of ice in inclusions which fail to nucleate hydrohalite and the final melting temperature of hydrohalite are reproducible to within 0 1 c and can be used to determine mgcl 2 and cacl 2 molalities

from middle jurassic heating to neogene cooling the thermochronological evolution of the southern alps abstract the dolomite region is located in the southern alps which were affected by mesozoic extensional tectonics and by consequent thermal perturbations in this work vitrinite reflectance and apatite fission track analysis are used to estimate the thermal evolution these methodologies have been applied to the permo mesozoic succession which crops out along the transalp seismic profile the regional distribution of the organic matter maturity seems to be mainly controlled by different burials reached during the norian liassic extensional phase in connection with high heat flow values the best solutions obtained from thermal modelling of both vitrinite and fission track data suggest that peak of high heat flow occurred during bajocian bathonian ages when western tethys was characterized by intrusions of gabbros and plagiogranites and extrusion of tholeiite basalts this time coincides with the onset of the drifting phase and related thermal subsidence the following thermal relaxation occurred during continuous sedimentation and the maximum burial does not coincide with peak temperatures cooling history has been carefully analysed through apatite fission track data on samples collected close to the valsugana overthrust which document an important exhumation event at about 10 ma the related erosion has been analysed through the combined use of arenite petrography and fission track analysis on detrital samples of the veneto foredeep succession which represents the storage of detritus during tertiary these data confirm that after serravalian the southalpine domain and related covers were affected by subaerial erosion

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

intraplate stresses and the subsidence history of the sirte basin libya abstract results of quantitative analysis of subsidence data from 20 wells covering six major basement blocks from the sirte basin libya are presented following the late cretaceous onset of extension and associated collapse of the sirte arch the tectonic history post dating this event can be separated into four distinct phases 1 1 minor subsidence during the late cretaceous 2 2 differential fault tectonics and reactivation of basement faults during latest cretaceous to paleocene 3 3 regional uplift and subsidence during the eocene 4 4 minor subsidence during the early oligocene and early miocene a late cretaceous paleocene stretching phase of finite duration can explain the overall patterns of observed tectonic subsidence during the syn and post rift evolution of the sirte basin strong short term deviations from thermally induced long term subsidence observed during the eocene can be explained by intraplate compression and subsequent stress release related to changes in atlantic flowline geometry modelling of the observed tectonic subsidence for the sirte basin yields estimates of extension in excess of estimates inferred from displacements along high angle normal faults the effect of late stage intraplate compression on tectonic subsidence may cause part of the observed discrepancy between estimates of crustal extension