making diagenesis obey thermodynamics and kinetics the case of quartz cementation in sandstones from offshore mid norway calculation of the quantity and distribution of quartz cement as a function of time and temperature depth in quartzose sandstones is performed using a coupled dissolution diffusional transport precipitation model this model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and or clay grains at stylolite interfaces within the sandstones and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible integration of the coupled mass transfer transport equations over geologically relevant time frames is performed using the quasi stationary state approximation results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data are in close agreement with both the overall porosity and the distribution of quartz cement in the middle jurassic garn formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data when quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0 008 cm 1 to 0 34 cm 1 in either case these reactive surface areas are 2 to 4 orders of magnitude lower than that computed using geometric models

relationship between deep diagenetic quartz cementation and sedimentary facies in a late ordovician glacial environment sbaa basin algeria abstract production of hydrocarbons from sandstone reservoirs is complicated by their diagenetic history although many aspects of sandstone diagenesis are well studied the character and processes of glacial associated diagenesis remain unknown to address this problem the origin conditions and timing of quartz cementation in glacial ordovician sandstones of the sbaa basin algeria were constrained on the basis of fluid inclusion microthermometry electron microprobe data and quantification by image analysis combined with optical and cathodoluminescence microscopic studies samples of the cambro ordovician formations in the sbaa basin were investigated from four wells at burial depths between 2 0 and 2 5 km and at ambient temperatures between 110 and 130 c quartz cements range in abundance from 1 to 27 of bulk rock volume in all wells three separate phases of quartz cementation are readily distinguished by cathodoluminescence microscopy the main silica sources are interpreted to be dissolution of feldspar grains intergranular quartz dissolution stylolitization and illitization of smectite fluid inclusion data depths indicate that quartz overgrowths precipitated between 100 to 160 c this range of temperature corresponds to the visean namurian burial phase according to the basin thermal history image analysis demonstrates intergranular quartz dissolution also called pressure dissolution in the presence of thin illite coatings around detrital quartz grains these clay coatings were mechanically emplaced below the ordovician ice sheet by pressurized meltwater circulation and subsequent fluid expulsion within sandy subglacial soft bedrock that was initially porous and permeable therefore the subglacial bedrock lithology directly influenced quartz diagenesis because an increase in the content of clay matrix or the presence of an argillaceous layer can block the circulation of fluids flowing below the ice sheet which prevents the formation of clay coatings quartz cements in this environment are favored in less compacted areas without clay coatings and with high porosity the results of this study clearly illustrate the relationships among sedimentary facies ice dynamics and diagenetic architecture of the formation these types of processes may control the porosity permeability distribution in other glacial sandstone successions through quartz cementation and chemical compaction

reply to a discussion of evidence for resetting of fluid inclusion temperatures from quartz cements in oilfields by osborne and haszeldine 1993

grain boundary contact effects during faulting of quartzite an sem ebsd analysis abstract during low temperature faulting of cambrian quartzite assynt nw scotland stress concentrations develop at grain contacts either at the onset of deformation prior to the establishment of a through going fault plane or within the damage zone remote from the main displacement segment such concentrations contribute to the development of intragranular microfractures cataclastic microstructures and fault rocks this contribution considers the progressive deformation sequence that precedes microfracturing and cataclasis the complexity of this deformation is revealed by scanning electron microscope sem electron backscattered diffraction ebsd dauphine twinning is a widespread feature associated with grain contact stress concentration and forms distinctive ebsd microstructures automatic sem ebsd analysis reveals that whilst initial indentation causes dauphine twinning of many grains continued indentation results in the formation of an arcuate array of subgrains via low temperature plasticity and or microcracking which overprint the dauphine twins these observations are consistent with transmission electron microscopic analysis of quartz crystals used for microhardness indentation tests which reveal that indentation causes an intensely deformed region to develop comprising a high density of microfractures and a submicron scale blocky microstructure that accommodates any plastic deformation deformation mechanisms and associated microstructures develop sequentially with progressive indentation and may provide sites of microfracture nucleation via low temperature ductile fracture the new microstructures assist diffusive mass transfer dmt processes by the formation of a cellular or subgrain array that represents a reduction of several orders of magnitude in apparent grain size and hence in diffusion path length concomitantly associated microfracturing perturbs local thermodynamic equilibrium leading to enhanced dmt crack healing and cementation overgrowths together these processes form the aseismic creep and sealing components of fault zone development

relationship between petroleum generation migration and sandstone diagenesis middle jurassic gifhorn trough n germany abstract the gifhorn trough in north germany contains an interbedded succession of lower to middle jurassic source and reservoir rocks regional variations in the burial history made it possible to investigate different stages in the maturation history of source rocks and the diagenetic history of reservoir rocks this study focuses in particular on the temporal and spatial relation between secondary petroleum migration and diagenetic events in the reservoir sandstones reconstruction of the burial and geothermal history indicate the initiation of petroleum generation at the beginning of the upper cretaceous lower toarcian shales have been identified as the common source rock of all oils based on biomarker and stable isotope studies differences in the composition and maturity of reservoired oils are interpreted to be related to separated oil kitchens of different maximum source rock maturity rather than in reservoir alteration after migration and accumulation major diagenetic events in the shallow marine arenitic reservoir sandstones are linked to various processes operating during burial a fe calcite cement formation isotope analyses indicate that this is facies related from carbonate redistributed at shallow burial depth 800 1200 m b fe dolomite dolomitization of early formed fe calcite is discussed based on pore water and oxygen isotopic composition c late kaolinite two models are discussed to explain the observed simultaneous formation of kaolinite and the creation of minor amounts of secondary porosity d late quartz overgrowths diffusive silica transport from quartz grain dissolution in shales into adjacent sandstones is proposed as a formation mechanism geochemical analyses of carbonate cemented intervals suggest that early generated condensate was trapped during carbonate precipitation prior to the main phase of petroleum migration these early formed carbonate cemented horizons probably compartmentalized the reservoir controlled petroleum migration routes and promoted the formation of diagenetic traps

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

petrographical and petrophysical properties of sandstones statistical analysis as an approach to predict material behaviour and construction suitability most studies dealing with material properties of sandstones are based on a small data set the present study utilizes petrographical and petrophysical data from 22 selected sandstones and 300 sandstones from the literature to estimate predict the material and weathering behaviour of characteristic sandstones composition and fabric properties were determined from detailed thin section analyses statistical methods applied consist of data distributions with whisker plots and linear regression with confidence regions for the petrophysical and weathering properties to identify similarities between individual sandstones and to define groups of specific sandstone types principal component and cluster analyses were applied the results confirm an interaction between the composition depositional environment stratigraphic association and diagenesis which leads to a particular material behaviour of sandstones three different types of pore radii distributions are observed whereby each is derived from different pore space modifications during diagenesis and is associated with specific sandstone types 1 bimodal with a maximum in capillary and micropores 2 unimodal unequal with a maximum in smaller capillary pores and 3 unimodal equable with a maximum in larger capillary pores each distribution shows specific dependencies to water absorption salt loading and hygric dilatation the strength porosity relationship shows dependence on the content of unstable lithic fragments grain contact and type of pore radii distribution cementation and degree of alteration sandstones showing a maximum of capillary pores and micropores bimodal exhibit a distinct hygric dilatation and low salt resistance these sandstones are highly immature sublitharenites litharenites characterized by altered unstable rock fragments which show pointed elongated grain contacts and some pseudomatrix quartz arenites and sublitharenites litharenites which are strongly compacted and cemented show unimodal unequal pore radii distributions low porosity high strength and a high salt resistance the presence of swellable clay minerals in sublitharenites litharenites leads to a medium to high hygric dilatation whereas quartz arenites show little hygric dilatation sandstones with unimodal equal pore radii distribution mostly belong to weakly compacted and cemented mature quartz arenites these are characterized by high water absorption and high porosity low to medium strength and a low salt resistance the data compiled in this study are used to create a sandstone quality catalogue since material properties are dependent on many different parameters of influence the transition between different lithotypes is fluent

soil moisture fluctuations recorded in saharan dust deposits on lanzarote canary islands over the last 180 ka abstract aeolian sediments trapped in volcanically dammed valleys on lanzarote canary islands were investigated in order to reveal environmental changes over the last 180 ka clay content and frequency dependent magnetic susceptibility were used as proxies for pedogenesis and palaeo soil moisture during the last 180 ka these proxies showed a general pattern of enhanced soil moisture during glacials and stadials and more arid conditions during interglacials and interstadials comparisons of these results with proxies from regional palaeoclimate studies identified a positive correlation with proxies of trade wind strength off northwest africa and inverse correlations with both sea surface temperatures in the northeast atlantic and the extent of mediterranean vegetation possible causes for the observed pattern include a glacial enhancement of precipitation from westerly cyclones a change in relative humidity due to fluctuating air temperatures and an occasional influence of the african summer monsoon although it is not yet possible to clearly differentiate among these factors it is clear that the first two factors must have been primarily dominant these results represent the first quasi continuous terrestrial data testifying to environmental changes in the northwest african coastal area for the last 180 ka and complement the abundant data derived from marine cores of the region high latitude dynamics had a major influence in this area and were intermediated by north atlantic sea surface temperatures a possible negative correlation can also be observed with the orbital obliquity cycle with a 10 ka time lag which is similar to the lag recorded from north atlantic sea surface temperatures

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations

stable isotope δ18o δ13c and δd signatures of recent terrestrial communities from a low latitude oceanic setting endemic land snails plants rain and carbonate sediments from the eastern canary islands abstract stable isotopes of oxygen and carbon extracted from fossil land snail shells have been used increasingly to interpret past environments to evaluate the utility of this approach for low latitude oceanic islands populations of the modern helicid land snail theba geminata a species also abundant in the quaternary fossil record of the region were sampled at ten low altitude d and 2 v smow for 18 o ranging from 11 to 2 v smow and from 2 6 to 0 7 v smow respectively the local vegetation is heterogeneous including c 3 c 4 and cam plants 13 c values vary from 13 0 to 29 0 v pdb across plant species of the 24 species five are c 4 15 are c 3 and four are cam plants the 18 o values for shells represent a narrow range of values from 0 3 to 2 5 v pdb which is consistent with the low climate seasonality typifying low latitude oceanic settings hypothetical model of the expected 18 o value for shell aragonite precipitated in equilibrium suggests that the most negative 18 o shell represent the closest estimate for 18 o rain water the 13 c values of shells range from 9 4 to 1 7 v pdb the most positive 13 c values are attributed to a diet based on c 4 plants the comparison of 13 c values of soft tissues and shells suggests that snails ingested notable amounts from 20 up to 40 of foreign carbonates consequently fossil shells with the most negative 13 c values should be selected for radiocarbon dating in future geochronological studies of the region the 13 c values of body tissues vary from 12 0 to 27 2 v pdb indicating that land snails consumed c 3 and c 4 plants indiscriminately the mean carbon isotopic composition averaged across multiple fossil specimens may thus provide a useful tool for reconstructing paleoclimates and paleoenvironments throughout the quaternary history of the canary islands and other comparable low latitude oceanic settings

diagenetic overprinting of the sphaerosiderite palaeoclimate proxy are records of pedogenic groundwater delta18o values preserved meteoric sphaerosiderite lines msls defined by invariant d 18 o and variable d 13 c values are obtained from ancient wetland palaeosol sphaerosiderites millimetre scale feco3 nodules and are a stable isotope proxy record of terrestrial meteoric isotopic compositions the palaeoclimatic utility of sphaerosiderite has been well tested however diagenetically altered horizons that do not yield simple msls have been encountered well preserved sphaerosiderites typically exhibit smooth exteriors spherulitic crystalline microstructures and relatively pure 95 mol feco3 compositions diagenetically altered sphaerosiderites typically exhibit corroded margins replacement textures and increased crystal lattice substitution of ca 2 m g 2 and mn 2 for fe 2 examples of diagenetically altered cretaceous sphaerosideritebearing palaeosols from the dakota formation kansas the swan river formation saskatchewan and the success s2 formation saskatchewan were examined in this study to determine the extent to which original early diagenetic d 18 o and d 13 c values are preserved all three units contain poikilotopic calcite cements with significantly different d 18 o and d 13 c values from the co occurring sphaerosiderites the complete isolation of all carbonate phases is necessary to ensure that inadvertent physical mixing does not affect the isotopic analyses the dakota and swan river samples ultimately yield distinct msls for the sphaerosiderites and mcls meteoric calcite lines for the calcite cements the success s2 sample yields a covariant d 18 o vs d 13 c trend resulting from precipitation in pore fluids that were mixtures between meteoric and modified marine phreatic waters the calcite cements in the success s2 formation yield meteoric d 18 o and d 13 c values a stable isotope mass balance model was used to produce hyperbolic fluid mixing trends between meteoric and modified marine end member compositions modelled hyperbolic fluid mixing curves for the success s2 formation suggest precipitation from fluids that were 25 sea water

porosity prediction prior to drilling in sandstones of the kekiktuk formation mississippian north slope of alaska porosity in sandstones of the kekiktuk formation was successfully estimated prior to drilling of the 1 leffingwell wildcat well north slope of alaska the estimate was based on a calibration dataset used to evaluate the effects of 1 framework grain composition 2 depositional facies and 3 postdepositional processes on porosity of kekiktuk sandstones the sandstones of the kekiktuk formation are chert bearing sublitharenites and quartzarenites characterized by a homogeneous composition of the detrital framework in the study area thus mineral composition is not a major factor responsible for differences in reservoir quality based on outcrop and available core observations the kekiktuk formation was interpreted to include several wet fan deltas the depositional model suggested that the 1 leffingwell well would penetrate the distal fine grained facies of one such system a petrographic study indicated that in fine and very fine grained kekiktuk sandstones such as those predicted in the wildcat porosity was reduced primarily by silica cementation silica cementation in turn is related to burial history because of the relationship among porosity silica cementation and burial history burial history diagrams provided a measure of the effect of burial history on porosity in available calibration wells a synthetic burial history curve was constructed prior to drilling of the 1 leffingwell well from available seismic data this burial history curve was then used to estimate the well s porosity based on the previously established porosity burial history relationship

sources of authigenic quartz in the permian tight sandstones close to gaoqing fault dongying sag bohai bay basin china abstract authigenic quartz is a significant cause of reduced porosity and permeability in the cement of the permian upper shihezi formation p2s sandstones in the gaoqing area dongying sag bohai bay basin china the sandstones are mainly quartzarenite with minor amounts of clay minerals authigenic quartz is generally the most commonly occurring cement and occurs as quartz overgrowth microfracture healing quartz pore filling quartz and microquartz in the sandstones three distinct separate phases of authigenic quartz i ii and iii were distinguished by using cathodoluminescence microscopy cl in the scanned cl images phase i quartz generally appears with light brown luminescence in the 450 670 nm range phase ii quartz is non luminescent in the 420 480 nm range and phase iii quartz appears with dark brown luminescence in the 470 650 nm range phase i quartz originating mainly from pressure solution has a composition similar to that of detrital quartz grains and was formed at approximately 79 3 104 c in the phase ii quartz li na mg2 al3 and ba2 are enriched and the homogenisation temperature of some aqueous inclusions is significantly higher than its maximum burial temperature of 130 c the eu of phase ii quartz is up to 1 42 the silica sources for phase ii quartz are dominated by hydrothermal fluids pressure solution feldspar dissolution and illitisation of smectite also provided a small amount of silica in the phase iii quartz all of the aqueous inclusions are almost higher than its maximum burial temperature al3 mg2 and rb are enriched in most of this quartz which was sourced mainly from hydrothermal fluids pressure solution also provided a very small amount of silica the hydrothermal fluids providing the silica source for phases ii and iii quartz migrated into the p2s sandstones along the adjacent gaoqing fault

inverse procedure for calibrating parameters that control land subsidence caused by subsurface fluid withdrawal 2 field application 1 the inverse procedure invcompac that was developed in part 1 of this two part paper is applied to two aquifer systems at two sites in shanghai the present part 2 illustrates how to calibrate transient land subsidence parameter values on the basis of in situ data the new graphic analytic method yields initial parameter values for use in the inverse procedure for the upper and lower confined aquifer systems at multiple extensometer sites f09 and f16 in shanghai their final parameter values are calibrated further by invcompac a computer based inverse model the simulation error of transient subsidence over a period of 13 and 20 years respectively at sites f09 and f16 is decreased to 11 8 and 13 9 respectively from an initial graphic analytic result of 72 1 and 415 3 this demonstrates that invcompac is applicable to complex field conditions the new graphic analytic method is necessary in order to find a realistic set of initial parameter values calibrated parameter values are used to predict future subsidence in the field predictions indicate that a large amount of ongoing residual nonrecoverable compaction will occur and will not equilibrate with the anticipated decline of groundwater levels at these sites the reason is that the vertical nonrecoverable skeletal specific storage of the aquitards is 2 3 orders of magnitude larger than the vertical recoverable skeletal specific storage of both the aquitards and the aquifer the vertical hydraulic conductivity is too small for the aquitards to drain sufficiently quickly for them to reach a new equilibrium in response to water level declines it is recommended that groundwater levels be kept above the past maximum preconsolidation pressure levels within the aquitards this will prevent continued nonrecoverable compaction from occurring

empirical prediction of porosity and permeability in sandstones current efforts to predict porosity and permeability in sandstones prior to drilling are focused on empirical and process oriented models empirical predictions are based on the correlation between porosity and permeability and a limited number of parameters obtained from calibration data sets or estimated from appropriate geologic models of these parameters the most important are detrital composition grain size sorting and temperature history or pressure history or both despite its limitations the empirical approach provides accurate predrill predictions of reservoir quality in many sandstones containing not only primary but also secondary porosity and permeability predictions of the average porosity and permeability of target sandstones are arbitrarily defined h re as accurate if they fall within 2 porosity of the mean measured porosity and within the same order of magnitude as the mean measured permeability in a sample population representing the interval of interest the effectiveness of the empirical predictive approach is illustrated by case studies from the taranaki basin new zealand and yacheng field people s republic of china these studies indicate that empirical predictions are basin specific or even play specific and require at least some understanding of fundamental processes affecting reservoir quality of a given sandstone target process oriented approaches attempting to model the effect of diagenesis on reservoir quality are hampered by inadequate quantitative understanding of the processes responsible for preserving primary porosity and generating secondary porosity and permeability until adequate quantification of the sandstone diagenesis processes is achieved empirical models have a distinct advantage over process oriented models in providing reliable predictions of reservoir quality in many sandstone intervals

porosity loss fluid flow and mass transfer in limestone reservoirs application to the upper jurassic smackover formation mississippi ooid grainstones of the upper jurassic smack over formation are buried to a depth of over 6 km and exposed to temperatures in excess of 200 c at black creek field mississippi combined effects of mechanical compaction intergranular pressure solution and cementation have reduced intergranular porosity of these ooid grainstones to 0 indicating that porosity reduction has gone to completion modal analysis of 24 samples lacking preburial cements indicates that from the original 40 porosity 13 porosity units range 4 to 21 were lost by mechanical compaction 15 porosity units range 8 to 23 were reduced by intergranular pressure solution and 12 porosity units range 1 to 26 were destroyed by cementation intergranular pressure solution caused an average of 28 range 15 to 51 vertical shortening in smackover ooid grainstones under ideal conditions the 28 vertical shortening will generate enough calcium carbonate to precipitate 10 calcite cement this is close to the measured volume of cements in smackover grainstones 12 suggesting that intergranular pressure solution provided most of the calcite cement present no external sources of calcium carbonate are required fine grained samples that experienced high degrees of intergranular pressure solution and contain only small amounts of cement occur at the top of the reservoir whereas coarse grained samples with abundant cement and low degrees of pressure solution occur in the middle and basal parts of the reservoir suggesting that the fine grained intervals acted as sources and the coarse grained intervals as sinks for calcium carbonate mass transfer of pressure solution generated calcium carbonate from the top of the unit to precipitation sites in the middle and basal parts of the reservoir could have occurred by a non rayleigh type convection cell due to calcite s reverse solubility with respect to temperature the cooling upward moving limb of the convection cell would become progressively more undersaturated and hence able to transport more dissolved calcium carbonate released by intergranular pressure solution in the upper portion of the reservoir pore fluids descending in the downward moving limb of the cell would become progressively more supersaturated and calcium carbonate would tend to precipitate as cement in the middle and basal parts of the reservoir as fluids become progressively hotter

pressure seals their diagenetic banding patterns abstract pressure seals are economically significant geological phenomena since they play an important role in deep natural gas entrapment identified in basins worldwide they offer a new frontier for exploration of natural gas reservoirs below 10 000 ft 3000 m pressure seals are low permeability envelopes which enclose abnormally pressured reservoirs there are three different types of seals basal lateral and top basal seals which define the bottom of abnormal pressure compartments usually follow a stratigraphic horizon lateral seals are generally associated with faults top planar seals may parallel or cut across time stratigraphic boundaries they may be developed in any lithology the southeastern portion of the anadarko basin displays a layered sequence of abnormally pressured fluid compartments at depths ranging from 3 000 to 13 500 ft 900 to 4150 m these compartments are separated from each other by pressure seals in mcclain county a top pressure seal separating two abnormally pressured compartments is located between 10 000 ft 3000 m to 12 000 ft 3650 m within the simpson group ordovician the seal zone is characterized by alternating cemented and porous intervals silica and carbonate cements comprise a diagenetic laminated and banded pattern the compositional and textural heterogeneity within this structure evolved through diagenetic processes active during the seal evolution

melting ice sheets 400 000 yr ago raised sea level by 13 m past analogue for future trends abstract marine isotope stage mis 11 was possibly the longest 423 000 362 000 yr ago and warmest interglacial of the entire quaternary period this resulted from a special arrangement of earth s orbital parameters similar to the present interglacial mis 1 suggesting that mis 11 provides an analogue for future natural climate forcing and sea levels although precise documentation of mis 11 sea level history is therefore crucial especially considering additional impetus from anthropogenic warming the maximum mis 11 sea level remains highly contentious estimates from onshore indicators have ranged from below present sea level to about 20 m resulting from uncertainties in reliability of sea level indicators estimates of their subsequent displacement by vertical crustal motion and in dating we used a clear and unambiguous sea level indicator interface between subtidal and intertidal deposits in well exposed regressive raised shoreline deposits along south africa s southern coast the coast is stable in terms of tectonic setting and falls within a zone of low predicted glacio isostatic adjustment gia and low sensitivity for plausible model parameters the sea level indicators were directly dated using thermally transferred optically stimulated luminescence tt osl which gave a mean age of 390 ka corresponding with mis 11 we determined a precise maximum elevation of 14 m for the mis 11 sea level indicator which corrected for minor crustal uplift and gia yielded a eustatic sea level of 13 m 2 m the complete mis 11 transgressive regressive shoreline succession is preserved and the wave cut platform extends 950 m inland from the present shoreline indicating a prolonged highstand we have also documented sea level fluctuations during mis 11 including an earlier lower highstand close to present sea level which may explain the lower elevation range of some previous estimates at one locality mis 11 deposits form a sea cliff against which mis 5e sediments at an elevation of 6 2 m abut the 13 m mis 11 sea level tallies with total melting of the greenland and west antarctica ice sheets regarded as most susceptible to melting our data predict notably higher sea levels in the future regardless of anthropogenic influence on climate

parameters controlling sonic velocities in a mixed carbonate siliciclastics permian shelf margin upper san andres formation last chance canyon new mexico we have measured the acoustic properties and mineralogic composition of 48 rock specimens from mixed carbonate siliciclastic outcrops of the permian upper san andres formation in last chance canyon new mexico the goals were 1 identify and model the parameters controlling the sonic velocities 2 assess the influence of postburial diagenesis on the acoustic velocities the variation in sonic velocity in the 0 to 25 porosity range is primarily controlled by porosity and secondly by the ratio of carbonate siliciclastic material linear multivariate fitting resulted in a velocity porosity carbonate content transform that accurately predicts sonic velocity at different effective stresses the slope of the velocity porosity transform steepens with increasing carbonate content which may be explained by the higher velocity of carbonate minerals another reason may be the property of carbonate minerals to form more perfect intercrystalline boundaries that improve the transmission properties of acoustic waves and are less sensitive to changes in effective stress the velocity ratio vp vs is an excellent tool to discriminate between predominantly calcitic lithologies ratio between 1 8 and 1 95 and predominantly dolomitic and quartz rich lithologies ratio between 1 65 and 1 8 gardner s experimental curve overestimates and the velocity porosity transforms by wyllie and raymer underestimate the observed sonic velocities probably because they do not account for variations in texture carbonate mineralogy and pore geometry petrographic observations show that postburial diagenesis is minor and does not seem to significantly affect porosity therefore the outcrop data set can be regarded as a proxy for the subsurface analog these findings underline the significantly more complex acoustic behavior in mixed carbonate siliciclastic sedimentary rocks than in pure siliciclastics where mineralogic composition explains most of the observed relationships between porosity and sonic velocity

petrographic constraints on models of intergranular pressure solution in quartzose sandstones abstract petrographic and sem observations on 478 samples of six quartzose sandstones provide a data base that can be used to evaluate the role of intergranular pressure solution ips in sandstone diagenesis and to constrain predictive models of the pressure solution process sem examination of grain contacts that have experienced pressure solution suggests that ips occurs at the interior portions of contacts where the greatest stress is concentrated and that granulation of quartz grains at points of contact may contribute to the process the chemical compaction fabrics that result from ips suggest that the process is most commonly induced by effective lithostatic stress and that the resulting strain is uniaxial numerous geological variables influence ips grain size exerts a fundamental influence with finer grained samples experiencing more ips than coarser grained samples on both local and regional scales ips is inhibited by poor sorting an abundance of ductile grains the presence of shallow cement slow rates of shallow burial and overpressured conditions in contrast ips is enhanced by the presence of illite grain coatings increased maximum burial depth rapid rates of shallow burial longer times spent at great depths higher temperatures and high volumes and rates of fluid flow silica budgets indicate that some of the analyzed sandstones approximate mass balance whereas others have exported silica calculations of fluid flow requirements indicate that advanced stages of ips are favored by high volume high velocity fluid flow such flow can occur as a result of uplift of basin margins which is typical of foreland and intractonic basins

composition porosity and reservoir potential of the middle jurassic kashafrud formation northeast iran abstract in the kopet dagh basin of iran deep sea sandstones and shales of the middle jurassic kashafrud formation are disconformably overlain by hydrocarbon bearing carbonates of upper jurassic and cretaceous age to explore the reservoir potential of the sandstones we studied their burial history using more than 500 thin sections supplemented by heavy mineral analysis microprobe analysis porosity and permeability determination and vitrinite reflectance the sandstones are arkosic and lithic arenites rich in sedimentary and volcanic rock fragments quartz overgrowths and pore filling carbonate cements calcite dolomite siderite and ankerite occluded most of the porosity during early to deep burial assisted by early compaction that improved packing and fractured quartz grains iron oxides are prominent as alteration products of framework grains probably reflecting source area weathering prior to deposition and locally as pore fills minor cements include pore filling clays pyrite authigenic albite and k feldspar and barite existing porosity is secondary resulting largely from dissolution of feldspars micas and rock fragments with some fracture porosity porosity and permeability of six samples averages 3 2 and 0 0023 md respectively and 150 thin section point counts averaged 2 7 porosity reflectance of vitrinite in eight sandstone samples yielded values of 0 64 0 83 in the early mature to mature stage of hydrocarbon generation within the oil window kashafrud formation petrographic trends were compared with trends from first cycle basins elsewhere in the world inferred burial conditions accord with the maturation data suggesting only a moderate thermal regime during burial some fractures iron oxide cements and dissolution may reflect cenozoic tectonism and uplift that created the kopet dagh mountains the low porosity and permeability levels of kashafrud formation sandstones suggest only a modest reservoir potential for such tight sandstones fractures may enhance the reservoir potential

geostatistical inverse modeling for the characterization of aquitard heterogeneity using long term multi extensometer data abstract the classical aquitard drainage model compac was used to simulate the deformation of a one dimensional vertically heterogeneous aquitard i e heter compac by coupling heter compac with the quasi linear geostatistical inversion approach the vertical heterogeneities of hydraulic conductivity k v elastic specific storage s ske and inelastic specific storage s skp fields could be characterized with the availability of long term multi extensometer data the methodology was first tested through a group of ten synthetic cases with randomly generated k v s ske and s skp fields results of the synthetic cases revealed that a geostatistical model could characterize the variability of aquitard hydraulic parameter fields and yields improved fits of simulated versus observed zone deformation than a geological model that treats each zone to be homogeneous subsequently the methodology was applied to a research site situated in changzhou city jiangsu province china where long term deformation behaviors of the strata within the aquifer aquitard system have been monitored vertical heterogeneities of k v s ske and s skp fields of the investigated aquitard at the site were characterized using the quasi linear geostatistical approach the correlation length of the three fields that yielded the best quality fit was assessed to be approximately 50 m and the fitting quality was slightly improved in comparison with that produced from calibrating a geological or zonation model through monte carlo analysis of aquitard deformation utilizing the inversed parameters and corresponding uncertainties the investigated aquitard is estimated to have compacted 914 8 42 6 mm before the rise in groundwater levels in adjoining aquifers

inverse procedure for calibrating parameters that control land subsidence caused by subsurface fluid withdrawal 1 methods 1 land subsidence prediction depends on a selected theoretical model and the calibration of its parameter values an inverse model is developed that calibrates five parameters of a compacting confined aquifer system k v vertical hydraulic conductivity of the aquitards s skv aquitard nonrecoverable skeletal specific storage s ske aquitard recoverable skeletal specific storage ssk skeletal specific storage of the aquifer and p max 0 initial preconsolidation stress within the aquitards this inverse model combines a procedure for finding an initial set of parameter values and an inverse adjustment procedure initial values are estimated by using a new graphic method for the analysis of field data the follow up inverse procedure is a computer algorithm that combines the newton raphson method with helm s one dimensional finite difference subsidence model compac compac offers two options a constant parameter simulation and a stress dependent parameter simulation the classical least squares criterion is employed as the objective function of this inverse procedure three characteristics of the objective function are considered this inverse model is applied to calculations of an idealized compacting confined aquifer system using helm s model this investigation shows that the final values for the five parameters lead to simulated compaction error of less than 1 this contrasts to the initial error of more than 60 these results indicate that the proposed inverse procedure model can be applied to actual field data

cold climate during the closest stage 11 analog to recent millennia abstract the early part of marine isotopic stage 11 near 400 000 years ago provides the closest analog to holocene insolation levels of any interglaciation during the era of strong 100 000 year climatic cycles the ch 4 concentration measured in vostok ice fell to 450 ppb and co 2 values to 250 ppm these natural decreases contrast with the increases in recent millennia and support the early anthropogenic hypothesis of major gas emissions from late holocene farming during the same interval d values fell from typical interglacial to nearly glacial values indicating a major cooling in antarctica early in stage 11 other evidence suggests that new ice was accumulating during the closest insolation analog to the present day a major increase in 18 o atm at vostok a similar increase in marine 18 o values and re initiation of ice rafting in the nordic sea the evidence permits extended 20 000 year intervals of stage 11 interglacial warmth in the antarctic and north atlantic yet it also requires that this warmth ended and a new glacial era began when insolation was most similar to recent millennia the holocene co 2 anomaly was produced only in part by direct anthropogenic emissions over half of the anomaly resulted from the failure of co 2 values to fall as they had during previous interglaciations because of natural responses including a sea ice advance in the antarctic and ice sheet growth in the northern hemisphere

origine eolienne de certains encroutements calcaires sur l ile de fuerteventura canaries orientales abstract on fuerteventura island eastern canaries extensive and diversified calcrete horizons overlie various volcanic rocks which at most include 10 to 12 cac0 3 many data drawn from their geomorphological situation in the field or from analyses in the laboratory show that only a small part of the carbonates have come from the underlying rocks the greater part has been brought in by wind transport after deposition the carbonates have been concentrated by soil forming processes downward migration of carbonate components or calcareous epigenesis when the ocean level was lower than today during the glacial times bioclastic sands were moved inland from the shoreline by the effects of both westerly gales and an arid climate mineralogical isotopical and field data show a good relationship between these eolian sands and the calcretes present on slopes until the tardiglacial period carbonate particles were transported close to the ground or carried in suspension in the air from these old shorelines during holocene time another eolian source supplied calcretes with carbonates at that time the sea level was indeed high but eastern or northeastern winds brought abundant dust from the sahara the methods used in the study of carbonates on fuerteventura island could be applied to study the eolian origin of some carbonates included in other calcretes of north africa or the near east

compaction and porosity evolution of pliocene sandstones ventura basin california sandstones of the pico and repetto formations were studied to quantify the processes involved in compaction and to determine the porosity evolution of the beds during burial the sandstones are mineralogically immature and except for local carbonate cemented beds have insufficient cement to have prevented compaction stratigraphic studies and graphic porosity depth plots suggest that the samples maximum burial depths were approximately 10 000 ft 3 048 m deeper than the present day depths of 850 13 000 ft 259 3 962 m compaction produced closer grain packing as measured by the contact index ci average number of contacts per grain and tight packing index tpi average number of long sutured and embayed contacts per grain for samples with less than 10 cement the ci depth gradient is 0 21 1 000 ft 0 69 1 000 m and tpi depth gradient is 0 27 1 000 ft 0 89 1 000 m assuming sands had 40 initial porosity the sandstones that are at present most deeply buried lost a total absolute porosity of 26 by all compactional processes which includes 14 by grain rearrangement and 6 each by pressure solution and ductile grain deformation porosity loss by nondeformational grain rearrangement was complete by a present day depth of 5 000 ft 1 524 m but in uncemented sandstones pressure solution plus associated grain rearrangement and ductile grain deformation continued to the maximum depths sampled compaction is most abrupt in sands with abundant micas and ductile rock fragments the ci gradient is 0 14 1 000 ft 0 46 1 000 m for samples with ess than 10 ductile grains but 0 22 1 000 ft 0 72 1 000 m for samples with more than 10 ductile grains compaction was almost entirely the result of overburden pressure and was not noticeably retarded in overpressured zones suggesting that overpressures developed after maximum burial was achieved for samples with less than 10 cement the ci is a better depth predictor than the tpi r2 0 58 vs 0 45 but the tpi is a better predictor of pre cement porosity than the ci r2 0 54 vs 0 31 secondary porosity averages 20 40 of the total thin section porosity secondary porosity is present at all depths but makes up the greatest percentage of total porosity between present depths of 6 000 and 9 500 ft 1 829 and 2 896 m except for grain fracturing secondary porosity formed only by dissolution of framework grains fractured grain porosity contributes up to 29 of total porosity 5 absolute porosity in some samples fractures formed during pleistocene tectonic episodes

influence of pressure salinity temperature and grain size on silica diagenesis in quartzose sandstones abstract based on calculated quartz solubilities for near neutral ph conditions in the 1 6 km depth range there is a suggested predictable affect of pressure temperature and salinity on pressure solution in quartzose sandstones the following results were obtained 1 1 quartz solubility related to increasing grain contact pressure with depth is 7 times greater at 4 km than at 1 km m s 1 5 10 3 or 90 ppm vs 2 0 10 4 or 12 ppm respectively 2 2 the difference between solubilities at grain contact and hydrostatic pressures increases with depth excluding overpressuring at 1 km m s 1 0 10 6 or 0 06 ppm at 4 km m s 4 7 10 5 or 2 8 ppm and at 6 km m s 2 2 10 4 or 13 0 ppm 3 3 increasing pore fluid salinity decreases solubility of quartz regardless of depth e g at 3 km and m nacl 3 m s 8 9 10 4 or 54 ppm at 3 km and m nacl 6 m s 7 5 10 4 or 45 ppm 4 4 temperature has the greatest influence on quartz solubility of any parameter studied both high heat flow and high geothermal gradient enhance quartz solubility e g at 1 km and 50 c m s 2 0 10 4 or 12 ppm at 4 km and 140 c m s 1 5 10 3 or 90 ppm these results indicate that pressure solution as well as local and non local migration and reprecipitation of silica and thus reservoir quality destruction in quartzose sandstones should be enhanced under higher temperature conditions where grain contact pressure is near lithostatic and pore fluid salinity is low under conditions of low temperature high pore fluid salinity and grain contact pressure close to hydrostatic pressure solution is discouraged and higher reservoir quality more likely preserved grain size differences in quartzose sandstones also influence pressure solution for very fine fine and medium grained sand laminae the average pressure solution index is about 3 3 2 5 and 2 times that of coarse sand

new evidence for the origin of quartz cements in hydrocarbon reservoirs revealed by oxygen isotope microanalyses quartz cements degrade hydrocarbon reservoir quality by choking off fluid flow paths therefore it is important to understand the sources and timing of quartz cementation we present in situ microanalyses of oxygen isotope ratios in quartz cements from a jurassic hydrocarbon reservoir located offshore norway showing systematic variations that reveal the isotopic history of the pore fluids that precipitated the quartz earliest quartz cements have 18o values of 20 26 subsequent cements drop in 18o value as low as 13 but then increase back to 23 with increasing cement thickness this isotopic trend was observed in multiple overgrowths with temperatures constrained by fluid inclusions and the burial history of the reservoir the isotopic composition of porewaters can be estimated from the measured isotopic ratios in quartz cement results indicate that 97 of the quartz cement precipitated from negative 18o waters originally meteoric that were gradually modified by burial diagenetic reactions as temperatures increased precipitation began in the middle to late cretaceous however the rate of quartz precipitation increased dramatically in the last 3 5 ma as the reservoir was rapidly buried the data show that quartz precipitation is a continuous process during burial and that silica is derived locally from silicate reactions occurring within the sandstone reservoir the isotopic data do not record evidence of episodic cementation from convection driven hot basinal fluids

origin of quartz cement in the lower ordovician dongjeom formation korea abstract this study deals with the origin of quartz cement developed in deeply buried mature sandstones intercalated in a thick carbonate sequence the dongjeom formation early ordovician korea is composed of three coarsening upward sequences and is divided into two facies according to depositional conditions with respect to changes in relative sea level transgressive facies and regressive facies the transgressive facies are characterized by calcite cemented allochem bearing sandstones whereas the regressive facies is characterized by non calcareous quartzose to arkosic sandstones quartz overgrowths occupy significant amounts of pore space average 16 in the regressive facies sandstones fluid inclusions reveal that quartz overgrowths precipitated between 110 and 165 c petrographical study and homogenization temperatures indicate that quartz overgrowths precipitated mostly during and after chemical compaction during the late diagenetic stage considering the 18 o values of detrital quartz grains and the volumes of quartz overgrowth the 18 o values of quartz overgrowth are estimated to range from 14 4 to 19 0 smow on the basis of mass balance silica for quartz overgrowths was principally derived from chemical compaction pressure solution of detrital quartz quartz overgrowths in the dongjeom sandstones precipitated from formation waters 2 1 to 2 5 smow that evolved from contemporary meteoric waters by diffusion

the effects of artificial recharge of groundwater on controlling land subsidence and its influence on groundwater quality and aquifer energy storage in shanghai china long term groundwater overpumping in shanghai has caused serious land subsidence to restore the declined piezometric groundwater levels and to decrease the land subsidence rate artificial recharge ar of groundwater into confined aquifers through injected wells has been proposed as an important measurement since the beginning of the 1960s in the case of shanghai recharge source is tap water whose chemical constituents and temperature are different from groundwater consequently the issues of groundwater quality change and thermal energy are worthy of discussion in this paper we discussed the influences of ar for controlling land subsidence on 1 groundwater level and land subsidence rate 2 groundwater quality 3 aquifer thermal energy storage the results based on the collected long term historical data in the study area show that ar not only is beneficial to groundwater level rising and land rebound but also provides cheap energy sources for industrial production the groundwater quality presented the trend of desalination and a general increase in sulfate iron and manganese contents organic and nitrogenous compounds after the tap water injection however the ar s influence on groundwater quality was limited with the radius 100 m for single well recharge and 1000 m for well groups recharge to control the development of the land subsidence within a safe limit in the study area the injected confined aquifers and the distribution of the ar wells need to be further adjusted due to the limitation of current ar

fossil bee cells from the canary islands ichnotaxonomy palaeobiology and palaeoenvironments of palmiraichnus castellanosi fossil bee cells attributable to the ichnospecies palmiraichnus castellanosi are recorded from the pleistocene and holocene of the easternmost canary islands cells bear a chamber with internal smooth lining spiral closure and an antechamber they have a discrete wall that surrounds both the chamber and antechamber the antechamber shows an internal smooth surface and a structureless filling of palaeosol material these features enable this material to be assigned to the ichnospecies p castellanosi originally described from the palaeogene of uruguay this record constitutes the first documented evidence for bees and hymenoptera in the quaternary of the canaries and the first for this ichnospecies from africa by comparison with extant cells the potential constructors are bees of the genus andrena andrenidae palaeoecological and statistical data suggest that five species may have produced p castellanosi in the canary islands one on lanzarote in the holocene another in south fuerteventura in the upper pleistocene two on gran canaria and another on montana clara islet during the middle pleistocene a high level of breeding success was probably favoured by the lack of predators or parasites and optimal environmental conditions at least in the pleistocene during the pleistocene the abundance of p castellanosi in the lowlands suggests a rich endemic shrubby flora and palaeoclimatic conditions similar to the present but probably with a higher humidity the land snail assemblage associated with low densities of p castellanosi in the holocene mid altitude belt suggests a cool moist palaeoenvironment less favourable for p castellanosi producers

first investigation of quartz and calcite shape fabrics in strained shales by means of x ray tomography abstract we document the evolution of the 3d fabric of shale along a km long strain gradient in the jaca basin southern pyrenees spain with respect to the distance from a thrust samples were collected in the cleavage free domain at the onset of the pencil cleavage domain within the pencil cleavage domain and within the slaty cleavage domain by combining high resolution x ray computed tomography xct and energy dispersive x ray spectroscopy eds the morphology and the shape preferred orientation spo of thousands of quartz grains calcite grains and pores was studied in the least deformed samples quartz and calcite display mean foliation parallel to bedding with comparable dispersion in the pencil cleavage domain quartz foliation still follows the bedding while calcite foliation is mostly governed by cleavage in the slaty cleavage domain calcite shows a much better organization with foliation parallel to cleavage mimicking closely the pore fabric by contrast the quartz shape fabric is much less defined scattered between bedding and cleavage planes this suggests that quartz grain act as rigid marker in the ductile matrix while calcite grain orientation is governed by dissolution precipitation processes

role of pressure solution in the formation of bedding parallel calcite veins in an immature shale cretaceous southern uk bedding parallel fibrous calcite veins in black shales cretaceous southern uk were investigated using a combined field stable isotopic geochemistry petrographic and crystallographic method to examine their formation mechanism calcite veins occur in all shale beds and are most abundant in the bituminous shales of the chief beef beds the calcite fibres in these veins exhibit either an antitaxial fibre growth with curvy stylolites as the median zone or a predominantly syntaxial upwards growth the calcite veins range from 0 49 to 1 78 of 13c values and 6 53 to 0 03 of 18o values which are both similar to those of their host shales our petrographic observations demonstrate that subhorizontal and interconnecting microstylolite networks commonly occur within the calcite veins equant calcite grains in the median zones exhibit indenting truncating and also interpenetrating grain contacts it is interpreted that the fibrous calcite veins were sourced by neomorphic calcite from their host shales with evidence from the 13c signatures pressure solution features stylolites microstylolites and grain contact styles and embedded fossil ghosts within the veins the diagenetic fluids from which calcite was precipitated were a mixing of the original seawaters and 18o depleted meteoric waters development of bedding parallel calcite veins is considered to have been enhanced by pressure solution as a positive feedback mechanism which was facilitated by the overburden pressure as the maximum principal stress calcite fibres with a predominant subvertical c axis orientation exhibit a displacive growth in porous shales and a replacive growth at vein limestone contacts this study highlights the critical role of pressure solution in the formation of bedding parallel calcite veins during burial and diagenesis of immature black shales

influence of grain size and temperature on intergranular pressure solution quartz cementation and porosity in a quartzose sandstone abstract cathodoluminescence petrography and scanning electron microscopy sem reveal that grain size and temperature significantly influenced intergranular pressure solution quartz cementation and porosity evolution in the quartzose hartshorne sandstone middle pennsylvanian of the arkoma basin among samples from each locality a negative linear relationship exists between mean grain size and volume of quartz dissolved by intergranular pressure solution in contrast a generally positive relationship exists between mean grain size and volume of quartz cement although considerable variability exists in this relationship a positive relationship also exists between mean grain size and porosity intergranular pressure solution directly reduced minus cement porosity by causing tighter pack ng of detrital grains it thereby controlled volumes of both cement and primary porosity for this reason little primary porosity is preserved in pressolved finer grained sandstones even though they contain small absolute volumes of quartz cement appreciably more primary porosity may be retained by unpressolved coarser grained sandstones even though they contain larger absolute volumes of quartz cement these relationships also vary with thermal maturity which increases eastward across the basin this thermal maturity trend is the result of elevated temperatures associated with mesozoic rifting and intrusive events in the mississippi embayment among samples of equal grain size more intergranular pressure solution less quartz cement and less porosity are evident in areas of higher thermal maturity as is the case with grain size the influence that temperature exerted on intergranular pressure solution is in turn reflected in volumes of quartz cement and porosity thus primary porosity tends to be preserved preferentially in areas of lower thermal maturity even though the absolute volume of quartz cement is generally higher in areas of higher thermal maturity more quartz has been dissolved by intergranular pressure solution than is present as cement whereas the opposite is true in areas of lower thermal maturity this suggests that intergranular pressure solution was an important agent of mass transfer during diagenesis of these quartzose sandstones

isotopic evidence for the eolian origin of quartz and mica in soils developed on volcanic materials in the canary archipelago quartz and mica contents of soils developed on volcanic materials in the canary archipelago lanzarote tenerife gomera and la palma islands off western africa are often higher in surficial horizons with higher rainfall and older landscape age the soil quartz particles are fine grained less than 53 m in diameter and well sorted the oxygen isotopic abundance 18osmow of fine aerosol sized quartz 1 10 m in diameter showed a range of 16 0 to 18 2 the values are compatible with those of the same sized quartz from two holocene deep sea sediments in cape verde rise 16 0 and 16 1 and saharan dusts 15 8 la laguna tenerife 16 2 barbados west indies a distinct 87sr 86sr ratio of 0 717 for a micaceous horizon from lanzarote is comparable with those of saharan dusts 0 715 to 0 721 but not with reported values 0 7032 from local volcanics these geochemical parameters show that the fine grained quartz together with mica in soils of the canary archipelago are transported from north african sahara deserts by trade winds and negate both possibilities of authigenesis and inheritance from weathering of local volcanics

geometric models of porosity reduction by ductile grain compaction and cementation pore volume reduction of sediments by plastic deformation during compaction and by cementation of grains has been evaluated for different proportions of ductile and hard grains we represent the compaction behavior of grains with a purely geometric model which uses the cooperative rearrangement algorithm to produce dense random packings of partly interpenetrating spheres we varied the fraction of grains assumed to be ductile and the radius of the rigid core of the ductile grains the predicted relationship between the fraction of ductile grains in the sediment and the porosity after compaction agrees well with previously published experimental data in the literature the radius of the rigid core of the ductile grains is an effective way to represent different kinds of ductile material ranging from brittle rigid radius 0 9 to extremely ductile rigid radius 0 7 we simulated quartz cementation in our compacted rock by adding isopachous cement cement thickness was reduced on the smaller grains and increased on the larger grains to account for presumed export of pressure dissolved material from finer grained regions and the import of material into coarser grained regions these simulations yield descriptions of pore scale geometry resulting from processes common in sandstones modeled pore geometry provides insight into transport properties of such rocks for example the models predict to within a factor of five the permeability of samples of tight gas sandstones having little intragranular porosity

predicting lithology and transport properties from acoustic velocities based on petrophysical classification of siliciclastics based on the recently developed petrophysical classification of siliciclastics which takes into account the amount of the volumetric clay content c and textural position of clay it is shown that acoustic velocities can be fairly accurate tools in predicting lithology porosity and ultimately transport properties of these rocks four major petrophysical groups of carbonate and organic poor siliciclastics are distinguished 1 clean arenites c 35 percent the compressional velocity versus porosity relation for consolidated rocks in each of these groups is found to be linear with very high correlation coefficients this allows for remarkably accurate porosity estimates or lithology prediction in consolidated siliciclastics from acoustic velocities compared to the widely used time average wyllie equation or its improved modification raymer equations both of which neglect textural factors

palaeo climate controlled diagenesis of the westphalian c d fluvial sandstones in the campine basin north east belgium the westphalian c and d fluvial sandstones in the campine basin north east belgium are potential reservoirs for the sequestration of co2 and interesting analogues of the hydrocarbon reservoirs in the southern north sea although these sandstones were deposited in a relatively short period of time their reservoir properties and mineralogical compositions are very different a petrographic study complemented with stable isotope analyses fluid inclusion microthermometry and x ray diffraction analyses of the clay fractions of the sandstones which were sampled from deep boreholes 1000 m in the campine basin revealed that these differences are related mainly to the climate at the time of deposition the most important eogenetic processes affecting the westphalian sandstones were the generation of a pseudomatrix by physical compaction of al silicates and lithic fragments that were strongly altered by extensive meteoric leaching kaolinitization of unstable silicates and precipitation of siderite these processes had a detrimental influence on the reservoir properties of westphalian c sandstones but their impact on the westphalian d sandstones was minimal the difference is assumed to be related to the climate at the time of deposition which changed from tropical humid in the westphalian c to semi arid arid during the late westphalian d both the westphalian c and d sandstones were affected by similar mesogenetic processes mesogenetic quartz cementation resulted from chemical compaction and illitization of kaolinite k feldspar and smectitic clays illitization of kaolinite was controlled by the available quantities of co existing kaolinite and k feldspar and mainly affected the westphalian d sandstones illitization of k feldspar was controlled by the k feldspar content it had a much larger impact on the reservoir properties of the westphalian d as in these sandstones k feldspar was less affected by eogenetic alteration the illitization of smectitic clays resulted in illite quartz and ankerite cementation in both reservoirs this process had a more important impact on the westphalian c reservoir since cementation here also resulted from smectite to illite conversion in the interbedded and underlying shales the effect of mesogenetic alterations on the reservoir properties was much less drastic than the impact of eodiagenesis mesogenetic alterations do exert a significant control on the properties of the westphalian d the vast impact of eodiagenesis on the westphalian c sandstones made them less susceptible to mesogenetic alteration the effect of telogenetic processes on the porosity and permeability of the westphalian sandstones was small and restricted to the top reservoir intervals that directly underlie the cimmerian unconformity no significant telogenetic alterations related to the variscan unconformity were observed

review of diagenetic facies in tight sandstones diagenesis diagenetic minerals and prediction via well logs abstract the tight sandstones are characterized by low porosity low permeability complex pore structure and strong heterogeneity due to the extensive diagenetic modifications they experienced understanding of the impact of diagenetic alterations on reservoir quality is crucial to the hydrocarbon exploration and production in tight sandstones diagenetic facies which is the comprehensive description of the diagenesis and diagenetic minerals determines the formation and distribution of sweet spot by correlating the diagenetic facies to well log responses the subsurface distribution of porosity and permeability can be predicted however the prediction of diagenetic facies and reservoir quality via well logs in tight sandstones remains a challenging task this paper critically reviews the impact of diagenesis and diagenetic minerals on reservoir quality in tight sandstones and establishes a model for prediction of diagenetic facies via well logs as assessed from peer reviewed papers in the literature as well as from the authors personal experiences this review begins with reviewing the impacts of compaction cementation dissolution and various types of diagenetic minerals on reservoir quality evolution the definition and classification schemes of diagenetic facies are then discussed and the reservoir quality as well as diagenetic evolution sequence of various diagenetic facies is summarized the same diagenetic facies commonly display similar compositional and textural attributes matrix and cement as well as porosity systems the well log responses gr ac den cnl and rt of various diagenetic facies are summarized by the calibration of log values with cores and related thin sections by translating the diagenetic facies to conventional well logs a predictable model which can be used for subsurface reservoir quality prediction is established then the theory of ecs logs is reviewed and the application of ecs logs in diagenetic facies evaluation is discussed at last the quantitative characterization for various type and degree of diagenesis is reviewed and the subsurface diagenetic facies is predicted by quantitative calculation of the compactional porosity loss cementational porosity loss and dissolution porosity content via well logs correlating the diagenetic facies to well logs provides a powerful tool to predict the distribution of high quality reservoirs in tight sandstones this review will provide insights into the reservoir quality evaluation and sweet spot prediction via well logs in tight sandstones

approaches to predicting reservoir quality in sandstones despite limited understanding of the details of many diagenetic processes empirical techniques can be used effectively to predict reservoir quality prior to drilling the predictive approach depends mainly on the availability of empirical data in the area of interest in frontier basins mean or maximum porosity of a potential sandstone reservoir can be estimated for a given composition and level of thermal exposure or burial depth the reliability of the estimates is constrained by input data approximate values of input parameters can be obtained from seismic data in combination with geological analysis of the area in basins with sufficient information to generate calibration data sets the predictive technique uses regression analysis the applicability of this approach is constrained by the limits imposed by the calibration data set and is generally limited to samples containing less than 10 pore filling cement quartz rich sandstones 85 framework quartz cemented with quartz are an exception to the 10 limit because quartz cementation commonly is related to burial history and rock texture in weakly cemented sandstones the critical variables controlling porosity are detrital composition sorting and burial history permeability can be predicted independently of porosity using the same variables plus grain size these variables can be evaluated from seismic data and facies models t is approach is best suited either for sandstones in which compaction is the main porosity reducing process or for quartz rich sandstones an adequate predictive model for sandstone suites with a wide range of cement content can be obtained by dividing the calibration data set into two or more subsets and developing a predictive model for each for example one subset can be limited to samples with less than 10 cement whereas the second subset will consist of samples with more than 10 nonquartz cement porosity and permeability in the first subset are then expressed by linear regression whereas controls on reservoir quality in the more heavily cemented sandstones are determined independently based on understanding of cement distribution patterns

disproving the presence of paleozoic triassic metamorphic rocks on the island of zannone central italy implications for the early stages of the tyrrhenian apennines tectonic evolution

compaction rock physics and rock properties of sandstones of the sto formation case study of five wells from the south western barents sea norway abstract five wells containing lower middle jurassic sandstones of sto formation from the hammerfest basin 7120 9 1 7121 7 1 the ringvassoy loppa fault complex 7119 12 1 7119 12 4 and the troms finnmark fault complex 7019 1 1 in the barents sea area are considered in this study the sto formation sandstones contain dominantly very fine to medium grained quartz arenites with occasional coarse grained sandstone layers feldspathic and quartz wackes are also present the effect of compaction and exhumation on reservoir properties porosity and permeability and seismic property p wave velocity of these sandstones have been investigated source of quartz cement has also been investigated forty polished thin sections embedded in blue epoxy were studied using optical microscopy scanning electron microscopy and cathodoluminiscence bulk mineralogy was also analysed using x ray diffraction the studied sandstones have experienced cenozoic exhumation ranging between 820 and 1050 m p wave velocity is higher porosities and permeabilities are lower in the western wells 7019 1 1 7119 12 1 and 7119 12 4 compared to the eastern wells 7120 9 1 and 7121 7 1 rock physics models and diagnostics show that the western wells are diagenetically more mature stiffer more compacted and more cemented than the eastern wells these trends are attributed largely to difference in burial history from the east to the west and less to textural variations quartz cement is the most important authigenic mineral in these sandstones quartz cement in the western well 7119 12 1 is predominantly derived from clay induced dissolution at macrostylolites whereas the eastern wells 7120 9 1 and 7121 7 1 are mostly sourced from clay induced dissolution at grain contacts or microstylolites while cementational porosity loss dominates in the western wells compactional porosity loss dominates in the eastern wells compaction can reduce porosities down to 26 and this might be the reason for better porosity preservation and reservoir quality in the eastern wells than in the western wells

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

experimental simulation of plagioclase diagenesis at p t conditions of 3 5 km burial depth dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments the diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a bridgeman type pressure vessel the experiments each of two weeks duration simulated pressure temperature conditions approximating 3 5 km burial depth by using a double acting pore fluid reservoir solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz 90 10 wt respectively

parameter estimation of an overconsolidated aquitard subjected to periodic hydraulic head variations within adjacent aquifers abstract extensometers provide long term records of deformation over a depth interval of interest an overconsolidated aquitard usually undergoes elastic deformation if the hydraulic head within adjacent aquifers remains higher than the preconsolidation head within the aquitard the vertical hydraulic conductivity k v and elastic skeletal specific storage s ske are two critical parameters that control the deformation behavior of an overconsolidated aquitard and they could be interpreted from valuable extensometer data an analytical solution for the elastic aquitard deformation caused by periodic hydraulic head variations phhv within adjacent aquifers is derived using the separation of variables method based on euler s formula the phase difference in phhv between adjacent aquifers is found to reduce the amplitude of periodic aquitard deformation meanwhile the phase difference is found to cause no influence on the growth pattern of the phase shift which is between the periodic aquitard deformation and phhv versus the period a new parameter estimation method is further proposed to interpret aquitard k v and s ske from periodic aquitard deformation data this new method yields robust k v and s ske estimates of a moderately thick aquitard whose deformation history has been recorded by an extensometer located in shanghai china the field study confirms that the phase difference in phhv between the aquifers adjacent to the overconsolidated aquitard of interest cannot be ignored

diagenesis and pore water evolution in permian sandstones gharif formation sultanate of oman abstract shallow marine coastal plain and fluvial sediments of the lower permian gharif formation in the interior oman sedimentary basin presently range in depth from outcrop in the huqf haushi area to almost 5000 m in the northwest this large depth range results from a varied subsidence history with only minor uplift during the late triassic to early jurassic the degree of diagenetic overprint varies depending on burial history sandstone at shallow depth 2600 m is highly compacted and tightly cemented by quartz carbonate and sulfate minerals this varied diagenesis is caused by increasing temperature with burial by surface water infiltration during the late triassic to early jurassic basin inversion and uplift in the late cretaceous along the southeastern basin margin and obduction of the oman mountains in the late cretaceous depositional pore water was initially of low salinity 50 g l with 18osmow values of 0 5 to 1 5o oo in the marine lower gharif and 2o oo in the arid middle and upper gharif during shallow burial late triassic to early jurassic pore water in the three gharif members changed to high salinity brines 180 g l with 18osmow values of 4 to 7o oo highly saline deep burial pore water with a heavy 18o signature 0 5 5o oo is the result of rock water interactions in a rock dominated system present pore water compositions mainly reflect subsurface dissolution of ara group evaporites precipitation of late diagenetic carbonates and sulfates and recharge of surface water along the uplifted southeastern basin margin

quartz cementation and related sedimentary architecture of the triassic solling formation reinhardswald basin germany abstract the olenekian solling formation of the reinhardswald basin was studied with the aim to examine the dependency of the degree of silica diagenesis on the sedimentary architecture the solling formation is composed of stacked sandstone clay siltstone cycles deposited by braided i e subarkoses and meandering i e subarkosic wackes river systems the sand dominated braided rivers of the lower solling formation are characterized by channels and downstream accretion whereas mixed load meandering rivers of the upper solling formation are represented by lateral accretion and laminated sands diagenetic processes which modified the sandstone siltstone successions include compaction feldspar alteration and cementation sandy clay poor successions are highly quartz cemented i e silica importers whereas clay and mica rich successions are more compacted i e silica balanced or silica exporters three quartz cement generations with a total volume of up to 18 indicate that silica from several different sources cemented the sandy fluvial architectural elements of the lower solling formation cementation includes silica supply from internal and external to the sandstone layers which are silica from grain margin dissolution from feldspar alteration silica released from clay siltstones and silica precipitated from basinal brines this cementation history is supported by cathodoluminescence observations and fluid inclusion studies and a consistent burial history was successfully modeled by simulation of the thermal subsidence in contrast to the lower solling formation the fluvial architectural elements of the mixed load systems of the upper solling formation contain only one quartz cementation phase with a total volume up to 8 the reinhardswald basin shows a dependency of the fluvial architecture and the diagenetic features for that we define the term diagenetic architecture

diagenesis and reservoir properties of middle jurassic sandstones traill o east greenland the influence of magmatism and faulting the c 500 m thick middle jurassic sandstones of the fluvial bristol elv and marine pelion formations of the east greenland basin are evaluated here in order to improve the understanding of the processes that influenced the diagenetic evolution the study may help to predict the reservoir properties of sandstones affected by magmatism and faulting both in general and specifically in undrilled areas on and offshore east greenland and in the voring basin on the mid norwegian shelf the study shows a variety of authigenic mineral phases dominated by quartz cement carbonate cement illite and iron oxide one of the clear differences between the two formations is the presence of early carbonate cemented horizons in the marine sandstones these horizons are inferred to reflect a primary concentration of biogenic clasts and fossil shells intense quartz cementation occurs primarily in the fluvial sandstones but the marine sandstones are also highly quartz cemented two episodes of burial and uplift are recorded in the diagenetic sequence and widespread grain crushing in coarse grained intervals is believed to result from overpressure and subsequent compaction due to sudden pressure release along major faults maximum burial depths may only have been around 2000 2500 m cathodoluminescence analyses show that grain crushing was followed by intense quartz cementation the quartz cement is to a great deal believed to have formed due to increased surface area from crushing of detrital quartz grains creating fresh nucleation sites for the quartz cathodoluminescence investigations also show that only minor pressure dissolution has taken place between detrital quartz grains and that the ubiquitous quartz cementation displays several growth zones and was thus in part the result of the introduction of silica rich extra formational fluids related to the flow of hot fluids along reactivated faults and increased heat flow and temperature due to magmatism this interpretation is supported by fluid inclusion homogenization temperatures between 117 and 158 c in quartz cements in one of the two study areas the development of macroscopic stylolites has significantly enhanced quartz cementation probably in connection with thermal convection flow as a result of the magmatic and fault related quartz cementation and illitization the reservoir quality of the sandstone formations deteriorated and changed drastically

quartz pressure solution influence of crystallographic orientation abstract the present contribution examines the relationship between pressure solution and the crystallo graphic orientation of detrital quartz in cleaved sandstones which exhibit a high degree of pressure solution the majority of the detrital grains examined have undergone pressure solution and as a result they have acquired smooth truncated margins along certain cleavage planes the amount of pressure solution varies significantly among the detrital grains this analysis indicates that quartz offers variable resistance to pressure solution along different crystallographic directions quartz grains with a small angle between c axis and the z axis of shortening exhibit the least amount of pressure solution whereas grains with c axes oriented about 50 to z manifest the highest degree of the pressure solution

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

vertical effective stress and temperature as controls of quartz cementation in sandstones evidence from north sea fulmar and gulf of mexico wilcox sandstones abstract we present quantitative petrographic data high spatial resolution oxygen isotope analyses of quartz cement basin modelling and a kinetic model for quartz precipitation for two paleocene eocene wilcox group sandstones from texas and two jurassic fulmar formation sandstones from the central north sea at each location one sandstone has been buried to ca 145 c and one to ca 185 c a key difference between the wilcox and fulmar burial histories is that the wilcox sandstones are currently at higher vertical effective stresses and from basin modelling studies have been subjected to generally higher vertical effective stresses through their burial history the amounts of quartz cement in the wilcox sandstones are between 12 and 18 and between 2 and 6 in the fulmar sandstones high spatial resolution oxygen isotope data obtained from the quartz cements suggest temperature ranges for quartz precipitation from 60 to 80 c to values approaching maximum burial temperature factors such as grain coatings or the timing of petroleum emplacement cannot explain the differences in the amounts of quartz cement petrographic data show that most of the silica for quartz cement can be derived from intergranular pressure dissolution although the sample set is small we interpret the results to suggest that the differences in quartz cementation in fulmar and wilcox sandstones can be explained better by differences in their vertical effective stress history than their temperature history in this case the supply of silica rather than the precipitation of quartz becomes an important control on the rate and extent of cementation

diagenesis of eolian and fluvial feldspathic sandstones norphlet formation upper jurassic rankin county mississippi and mobile county alabama norphlet sandstones in seven cores from mississippi and alabama are arkoses and subarkoses deposited in eolian dune interdune and fluvial environments similar to the deeply buried 5 km tertiary feldspathic sandstones of the gulf basin all detrital plagioclase that survived dissolution has been albitized however some k feldspar instead of dissolving was stabilized by k rich brines derived from adjacent evaporites this k feldspar then developed extensive overgrowths fluvial red sandstone lost all initial porosity by the introduction of preburial pedogenic calcite and compaction initial porosity of eolian sands was reduced by compaction to an average of 29 and later by cementation by quartz carbonates anhydrite halite k feldspar and illite quartz and anhydrite cements precipitated between 90 and 100 c approximately 2 3 km deep carbonates and halite cements formed below 120 c 3 km and late stage illite cement formed between 130 and 150 c 4 5 km deep cements are patchy and some especially quartz and anhydrite are texture selective being more abundant in coarser laminae secondary porosity which makes up approximately half the porosity in thin sections formed by dissolution of detrital grains f ldspar rock fragments and cements anhydrite carbonate halite reservoir bitumen records an early phase of oil entrapment reservoir quality is influenced by the abundance of reservoir bitumen and thread like illite both of which bridge pores the radiometric age of illite approximately 55 ma probably reflects its cementation age but may also record a later recrystallization event depleted oxygen and carbon isotopic values for ferroan dolomite reflect the progressive replacement of earlier more enriched carbonate phases by higher temperature phases isotopic data suggest that during the first 30 to 40 m y of burial subsurface diagenesis of the norphlet formation was dominated by deep circulating hot meteoric water this phenomenon may be characteristic of the early diagene ic history of rifted basins

3d modelling of the effect of thermal elastic stress on grain boundary opening in quartz grain aggregates abstract during the exhumation of rocks and the associated temperature and pressure decrease the anisotropic thermoelastic properties of minerals lead to internal stresses on the grain scale which in turn cause fracturing and opening of grain and phase boundaries to gain deeper insight into the onset of grain boundary cracking and the subsequent evolution of grain boundary fracture networks after crack initiation 3d grain scale numerical modelling has been performed in detail the fracturing and opening of quartz grain boundaries during exhumation of quartzite with millimeter scale grains and randomly oriented crystallographic axes accompanied by cooling from 300 to 25 c and decompression from 300 to 22 mpa have been modelled with contact mechanics and the finite element method model grains have an anisotropic linear elastic rheology and an anisotropic thermal expansion tensor grain boundaries are modelled as contact surfaces with a non linear strain softening rheology in tension and a mohr coulomb rheology in shear grain boundary fractures nucleate at grain vertices and triple lines in agreement with observations in nature and experiment comparison with grain boundary opening data from natural quartzite indicates that the tensile yield strength y of quartz grain boundaries has an upper bound of 75 mpa with best fit results for y 25 mpa moreover the best fit model implies that quartz grain boundary opening initiates after a temperature and pressure decrease of around 80 c and 80 mpa

the effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

modeling visco elastic plastic deformation of soil with modified merchant model eighty four to ninety eight percent of land subsidence in shanghai city is caused by the visco elastic plastic deformation of sediments numerical experiments are done on the sediments with visco elastic plastic deformation in shanghai to verify the modified merchant model mm model and land subsidence model based on the modified merchant ls mm model there are two advantages of the mm model and the ls mm model one is that only a few parameters are involved there are three parameters in the mm model and four parameters in the ls mm model the other one is that both models can describe elastic elastic plastic and visco elastic deformation in addition to visco elastic plastic deformation the corresponding models are developed by setting proper values of the three parameters of 1 and 2 the two advantages make the ls mm model flexible and applicable to the simulation of the large regional land subsidence with visco elastic plastic deformation and other different kinds of deformation the results can be improved by variable parameters especially specific storage

land subsidence due to groundwater withdrawal in the northern beijing plain china abstract beijing is an international metropolis where over exploration of water resource makes land subsidence becoming more and more serious the related problems cannot be avoided in the coming years because of the giant increase of population the aims of this study are to quantify land subsidence over the period 2003 to 2010 grasp the evolution of the process and investigate the relation with the triggering factors in the northern area of the beijing plain various data including deep compaction from vertical multiple borehole extensometers land subsidence from persistent scatterer interferometry and leveling surveys groundwater levels hydrogeological setting from wellbores and landsat tm image were collected and effectively used to detect the spatial and temporal features of land subsidence and its possible relation with groundwater level changes compressible layer thickness and urban development results show that land subsidence is unevenly distributed and continuously increased from 2003 to 2010 the average loss of elevation over the monitoring period amounted to 92 5 mm with rates up to 52 mm y the distribution of the subsidence bowl is only partially consistent with that of the groundwater depression cone because of the variable thickness of the most compressible fine deposits in fact extensometers reveal that silty clay layers account for the larger contribution to land subsidence with the 15 m thick silty clay layer between 102 and 117 m depth accounting for about 25 of the total subsidence finally no clear correlation has been observed between the subsidence rates and the increase of the load on the land surface connected to the impressive urban development this study represents a first step toward the development of a physically based model of the subsidence occurrence to be used for planning remediation strategies in the northern beijing plain

a new type curve method for estimating aquitard hydraulic parameters in a multi layered aquifer system summary we propose a new type curve method for estimating the hydraulic conductivity k and the specific storage s s of an aquitard in a multi layered aquifer system theoretical type curves of rate of aquitard compaction versus time are obtained through drawdown time series in the aquitard under a condition that the drawdown history of the underlying aquifer can be approximated as piecewise linear segments pls in each of which the drawdown shows a linear increase trend with time comparing with previous analytical solutions which are limited by the assumption of a constant head in the overlying unconfined aquifer the superposition principle can be called when water levels fluctuate with time in both upper and lower aquifers adjacent to the aquitard when applying the new pls method to estimate k and s s values of an aquitard the starting point of drawdown history at adjacent aquifers should be determined at first after that the drawdown history is approximated as pls to obtain drawdown rate of change for each linear stage and turning points of drawdown history the new pls method is demonstrated at a field site in changzhou city jiangsu province of china estimated results show that k and s s are respectively 9 80 10 10 m s and 4 46 10 4 m 1 for a thick aquitard and 2 34 3 50 10 10 m s and 2 28 10 4 m 1 for a thin aquitard estimated field k values as well as s s values through the new pls method are generally in accordance with the results obtained from the geologic method by konikow and neuzil 2007 they are also in agreements with the numerically calibrated results of shi et al 2008a at the regional scale and the laboratory experimental values of in situ soil samples collected by geological survey of jiangsu province of china