insolation values for the climate of the last 10 million years new values for the astronomical parameters of the earth s orbit and rotation eccentricity obliquity and precession are proposed for paleoclimatic research related to the late miocene the pliocene and the quaternary they have been obtained from a numerical solution of the lagrangian system of the planetary point masses and from an analytical solution of the poisson equations of the earth moon system the analytical expansion allows the direct determination of the main frequencies with their phase and amplitude numerical and analytical comparisons with the former astronomical solution ber78 are performed so that the accuracy and the interval of time over which the new solution is valid can be estimated the corresponding insolation values have also been computed this analysis leads to the conclusion that the new values are expected to be reliable over the last 5 ma in the time domain and at least over the last 10 ma in the frequency domain

evidence for reduced quartz cementation rates in oil filled sandstones when quartz rich sands are buried and heated pore space is gradually filled by precipitation of quartz cement from aqueous formation fluids here we examine whether the presence of oil in the pore space can retard or halt this loss of porosity by slowing or stopping quartz cementation the effect of oil fill on quartz cementation is examined by using the distribution of quartz cement in the brae formation deep water sandstone reservoir of the miller oil field north sea petrographic data demonstrate that sandstones from the oil zone have much less quartz cement and more porosity than sandstones from the water zone sandstones in both oil and water zones are compositionally and texturally identical and have been affected by a similar burial history kinetic modeling of the cementation process suggests that progressive oil charging has slowed quartz cement growth rates by at least two orders of magnitude halting it completely in the most extreme cases our data demonstrate that early oil charging in the crestal part of an anticline can preserve porosity in deeply buried sandstones this knowledge is especially relevant to porosity prediction for petroleum exploration in deeply buried sandstones

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

sandstone porosity as a function of thermal maturity sandstone porosity decreases in the subsurface as a power function of thermal maturity phi a m sup b where phi is porosity and m is a measure of thermal maturity representing integrated time temperature history a and b are constants for a given sandstone of homogeneous properties but vary between data sets the commonly observed exponential dependence of sandstone porosity upon depth follows as a special case from this power function relation when temperature increases linearly with depth the consideration of sandstone porosity in terms of time temperature exposure offers advantages in the comparison of porosity data from diverse geologic settings the recognition of unusual porosity within a sandstone sequence and the prediction of porosity ahead of the drill and at times in the geologic past

influence of multistage oil emplacement on carbonate cementation in the eocene shahejie formation dongying sag bohai bay basin china abstract the influence of multi stage oil emplacement on carbonate cementation was investigated on the basis of determining the sequence of oil emplacement and carbonate cementation in the clastic reservoirs of the upper part of the fourth member of the eocene shahejie formation es4s in the yanxie 229 area dongying sag the sequence of multistage oil emplacement and carbonate cementation was determined by projecting the homogenization temperatures ths of two phase aqueous inclusions on a burial and thermal history plot the ratio of the number of quartz grains with yellow fluorescent oil inclusions therein to the total number of quartz grains ygoi and the ratio of the number of quartz grains with blue fluorescent oil inclusions therein to the total number of quartz grains bgoi were calculated to analyze the paleo oil saturation of each stage of oil emplacement the ygoi and bgoi values reflect the relative level of the paleo oil saturation of the first stage yellow fluorescent fsyf oil and the second stage blue fluorescent ssbf oil respectively a large number of primary pores were preserved in the oil layers whereas only a small number of dissolution pores and micropores were developed in the oil bearing water layers and dry layers calcite dolomite ferrocalcite and ankerite cementation and two stage oil emplacement occurred in the es4s reservoirs the fsyf oil emplacement occurred earlier than the ankerite cementation and ferrocalcite cementation but later than the calcite cementation and dolomite cementation the ssbf oil emplacement occurred later than all phases of carbonate cementation the dolomite cementation was not influenced by the fsyf oil emplacement because it occurred earlier than the fsyf oil emplacement the ankerite cementation was dominantly inhibited by the fsyf oil emplacement in the oil layers and the degree of inhibition increased with the increase in paleo oil saturation of the fsyf oil the ssbf oil emplacement further increased the paleo oil saturation in the oil layers which enhanced the inhibition of the ankerite cementation and even stopped it the ankerite cementation in the dry layers and oil bearing water layers was slowed down or stopped due to the lack of pores and the low transportation rate of external material which was caused by the intense ankerite cementation prior to the ssbf oil emplacement

diagenetic alterations and reservoir heterogeneity within the depositional facies a case study from distributary channel belt sandstone of upper triassic yanchang formation reservoirs ordos basin china abstract diagenesis is of decisive significance for the reservoir heterogeneity of most clastic reservoirs linking the distribution of diagenetic processes to the depositional facies and sequence stratigraphy has in recent years been discipline for predicting the distribution of diagenetic alterations and reservoir heterogeneity of clastic reservoirs this study constructs a model of distribution of diagenetic alterations and reservoir heterogeneity within the depositional facies by linking diagenesis to lithofacies sandstone architecture and porewater chemistry during burial this would help to promote better understanding of the distribution of reservoir quality evolution and the intense heterogeneity of reservoirs based on an analogue of deltaic distributary channel belt sandstone in upper triassic yanchang formation 83 sandstone plug samples were taken from 13 wells located along this channel belt an integration of scanning electron microscopy thin sections electron microprobe analyses rate controlled porosimetry rcp gas flow measurements of porosity and permeability and nuclear magnetic resonance nmr experiments together with published data were analysed for the distribution mineralogical and geochemical characteristics of detrital and diagenetic components and the distribution of reservoir quality within the distributary channel belt distribution of diagenetic alterations and reservoir heterogeneity within the distributary channel belt sandstones include i formation of high quality chlorite rims in the middle part of thick sandstones with coarser grain sizes and a lower content of ductile components resulted from the greater compaction resistance of these sandstones providing larger pore spaces for chlorite growth leading to formation of the intergranular pore wide sheet like throat and intergranular pore intragranular pore wide sheet like throat 15 k 1md in the middle part of thick sandstones ii formation of thinner chlorite rims in the middle part of thinner sandstones is associated with the intergranular pore intragranular pore narrow sheet like throat 9 18 o vpdb 23 4 to 16 6 13 c vpdb 4 0 to 2 3 favoured in the top and bottom of the channel sandstone which near the sandstone mudstone bouding surface destroying pore space

compaction law for cretaceous sandstones of alaska s north slope lithic arenites and wackes of the national petroleum reserve in alaska contain a significant volume of ductile grains and have suffered substantial physical compaction with burial as determined by stratigraphic studies these rocks were buried to maximum depths of 500 to 4 400 m initially the grain fraction grain volume as a percent of total rock volume must have been near 59 but increased to over 90 at depth statistical analysis reveals that the grain fraction gf is primarily a function of burial depth in meters d percent ductile grains such as phyllite rock fragments p and matrix content m multiple regression suggests the following linear equation gf 90 0 23p 0 72m 0 0018d simple grain rearrangement operates to a depth of 550 800 m and accounts for a 25 increase in grain fraction at greater depths compaction is by plastic deformation however the increase in grain fraction appears to fall off considerably below 1 000 m and compaction due to plastic deformation increases grain fraction on average by only 7 early quartz cementation never progressed beyond a beginning stage and thus was ineffective at preventing compaction on the other hand clay matrix drastically impedes compaction grain fraction for lithic wackes with an abundant matrix never exceeds 83 the compaction law derived in this study may prove useful in further exploration for petroleum reservoirs for example lithic arenites with a low ductile content should retain an intergranular porosity of 6 to a burial depth of 2 000 m

triple water isotopologue record from wais divide antarctica controls on glacial interglacial changes in 17oexcess of precipitation measurements of the 17oexcess of h2o were obtained from ice cores in west and east antarctica combined with previously published results from east antarctica the new data provide the most complete spatial and temporal view of antarctic 17oexcess to date there is a steep spatial gradient of 17oexcess in present day precipitation across antarctica with higher values in marine influenced regions and lower values in the east antarctic interior there is also a spatial pattern to the change in 17oexcess between the last glacial maximum lgm and holocene periods at coastal locations there is no significant change in 17oexcess at both the west antarctic ice sheet divide site and at vostok east antarctica the lgm to early holocene change in 17oexcess is about 20 per meg atmospheric general circulation model gcm experiments show that both the observed spatial gradient of 17oexcess in modern precipitation and the spatial pattern of lgm to early holocene change can be explained by kinetic isotope effects during snow formation under supersaturated conditions requiring a high sensitivity of supersaturation to temperature the results suggest that fractionation during snow formation is the primary control on 17oexcess in antarctic precipitation variations in moisture source relative humidity play a negligible role in determining the glacial interglacial 17oexcess changes observed in antarctic ice cores additional gcm experiments show that sea ice expansion increases the area over which supersaturating conditions occur amplifying the effect of colder temperatures temperature and sea ice changes alone are sufficient to explain the observed 17oexcess glacial interglacial changes across antarctica

chlorite meniscus cement implications for diagenetic mineral growth after oil emplacement the possible influence on diagenesis of oil migration into reservoir sandstones is important for understanding their development although many studies have addressed the question of continued growth of diagenetic minerals after oil emplacement there is little clear evidence of the timing of mineral growth relative to oil emplacement nor of the possible mechanisms of continued mineral growth this study presents an example of continued cementation by pore lining chlorite during and after the migration of oil into a palaeocene turbidite sandstone reservoir near the faroe islands chlorite developed in two different textures one texture occurs on framework grain surfaces adjacent to open pore space in contact with oil the other texture occurs close to framework grain contacts where contact with oil appears to have been prevented by residual pore water menisci the meniscus crystals are larger than those from open pore implying that diagenetic clay mineral growth has continued after oil emplacement

spatial distribution of 17o excess in surface snow along a traverse from zhongshan station to dome a east antarctica the influence of temperature on the triple isotopic composition of oxygen in water is still an open question and limits the interpretation of water isotopic profiles in antarctic ice cores the main limitation arises from the lack of 17o excess measurements in surface snow and especially for remote regions characterized by low temperature and accumulation rate in this study we present new 17o excess measurements of surface snow along an east antarctic traverse from the coastal zhongshan station to the highest point of the antarctic ice sheet at dome a the 17o excess data significantly decrease inland with a latitudinal gradient of 1 33 0 41 per meg degree an altitudinal gradient of view the mathml source and a temperature gradient of view the mathml source theoretical calculations performed using a rayleigh model attribute this inland decrease to kinetic isotopic fractionation occurring during condensation from vapor to ice under supersaturation conditions at low temperatures however large heterogeneity of 17o excess in antarctic precipitation cannot only be explained by temperature at condensation and or influences of relative humidity in the moisture source region

testing periodicity of depositional cyclicity cisco group virgilian and wolfcampian texas abstract periodicities of 15 cyclic sections of the cisco group are tested through gamma and spectral analyses the sections are composed of marine and nonmarine carbonate and siliciclastic rocks deposited on the eastern shelf north central texas in late pennsylvanian and early permian time gamma analysis generates facies dependent thickness time conversion factors s to construct corrected time series which are more realistic than the uncorrected series obtained by proportional conversion positive and stable s and improved resolution of the corrected spectra indicate successful tuning on the cisco spectra a short eccentricity peak 95 131 ky is the most prominent and persistent long eccentricity 413 ky and obliquity 34 and 43 ky peaks are persistent and precessional index peaks 17 and 21 ky are less persistent a persistent peak of period 50 80 ky may be a combination tone between obliquity and precessional index cycles non milankovitch peaks are sporadic the validity of gamma and spectral results is indicated by consistent estimated ages among the cisco sections and reasonable effective sedimentation rates calculated on the basis of milankovitch spectral calibrations the results combined with geologic evidence suggest that the cisco cyclicity was controlled dominantly by milankovitch climatic forcing

influence of the changing lunar orbit on the astronomical frequencies of pre quaternary insolation patterns the sensitivity of the frequencies of the earth s orbital elements involved in the astronomical theory of paleoclimates obliquity and climatic precession to the earth moon distance and consequently to the length of the day and to the dynamical ellipticity of the earth is investigated for the last billions of years but the new frequencies have been computed for the last 440 million years only a time interval over which data and a model of the earth were available moreover we consider only the impact of the changing precessional parameter assuming that the fundamental frequencies of the planetary motion are steady at the geological time scale under such hypotheses the shortening of the earth moon distance and of the length of the day back in time induces a shortening of the fundamental astronomical periods for obliquity and precession a shortening that is larger for longer periods

experimental mechanical compaction of reconstituted shale and mudstone aggregates investigation of petrophysical and acoustic properties of sw barents sea cap rock sequences abstract this study investigates petrophysical and acoustic properties of experimentally compacted reconstituted samples of seal sequences from the southwestern barents sea the aggregates were collected from drill cuttings of mudstone and shale formations of two exploration wells 7220 10 1 salina discovery and 7122 7 3 goliat field the washed and freeze dried samples were characterized for grain size distributions geochemical analyses and mineralogical compositions a total of 25 compaction tests 12 dry and 13 brine saturated were performed with a maximum effective vertical stress of 50 mpa the laboratory measurements demonstrated that petrophysical and acoustic properties of argillaceous sediments can change within a sedimentary basin and even within a given formation the results show that the collected aggregates from goliat field are compacted more compared to salina discovery the maximum and minimum compaction are measured in samples collected from snadd and fuglen formations respectively the final porosity of brine saturated specimens varies between 5 and 22 the ultrasonic velocity measurements depict that samples with the same porosity values can have a broad range of velocity values the resulting compaction trends in this study were compared to published compaction curves for synthetic mixtures of quartz and clay all compaction trends show higher porosity reduction than the silt fraction with 100 quartz comparison of experimental compaction result of each mudstone and shale aggregate with its corresponding acquired well log data helps to delineate the burial history and exhumation in the study area a net exhumation of 950 m and 800 m is estimated at salina and goliat wells respectively the outcomes of this study can provide insights for hydrocarbon prospect discovery in a pre mature sedimentary basin in terms of exploration and production and also for geological co2 storage sites the experimental results may provide information for well log and seismic interpretation basin modeling and seal integrity of investigated horizons

comment on record of δ18o and 17o excess in ice from vostok antarctica during the last 150 000 years by amaelle landais et al

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

evidence of astronomical forcing of the earth s climate in cretaceous and cambrian times abstract we have completed analyses of three sections of shallow marine sedimentary cycles that were deposited at about 80 ma and between about 510 ma and 530 ma in different parts of north america in spite of the antiquity of these strata and the differences in the age location and cycle types power spectra in all of the records have distinct spectral peaks with ratios that are consistent with forcing by eccentricity obliquity and climatic precession for two examples utah 530 ma and the southern canadian rockies 510 ma the spectra reveal prominent peaks corresponding to obliquity with a period closer to that estimated for early paleozoic time than for the modern these are the first results supporting recent calculations of changes in astronomical periods over geological time we were able to extract these intriguing results from rocks as old and as poorly dated as these only after applying a new method the gamma method which we have developed for scaling time in cyclic strata the results are evidence that astronomical forcing has left a robust signal in the geological record and that the astronomical periods observed in the geological data are consistent with recent calculations of the shifts in those periods over phanerozoic times

the effects of oil emplacement on diagenetic processes examples from the fulmar reservoir sandstones central north sea geologic note 1 sedimentary petrography and petroleum geochemistry studies have been done on fulmar formation upper jurassic cores from fulmar field central north sea the extent of quartz overgrowths and albitization of k feldspars is lower in oil saturated zones compared with water saturated zones however microthermometric studies of aqueous fluid inclusions in quartz overgrowths show similar temperature ranges for both oil and water saturated zones 85 125 degrees c with maximum recorded temperatures close to the current reservoir temperature about 130 degrees c this suggests that quartz overgrowth development in the oil saturated zones continued after oil emplacement but was retarded compared with development in the water saturated zones the quartz cement therefore mus have been essentially locally derived by pressure solution albitization of k feldspars is significantly less about 50 in the oil saturated zones compared with albitization in the water saturated zones because albitization of k feldspars depends on a supply of sodium and removal of potassium this process is more likely to be terminated by oil emplacement the observed difference in the albitization has been used to suggest that the fulmar reservoir probably was filled during the neogene this age estimate agrees broadly with timing determined independently by modeling of oil generation in adjacent source basins the fulmar sandstones contain little or no kaolinite and show no evidence of substantial leaching of feldspars or carbonates this is probably because the fulmar sandstones being partly turbiditic offshore sand deposits were not exposed to meteoric water their close proximity to the marginally mature to mature kimmeridge clay formation seems to have little effect in terms of developing secondary porosity suggesting that the role of organic acids or co 2 generated in these source rocks is of minor importance

seasonal variations of 17 o excess and d excess in snow precipitation at vostok station east antarctica the use of water isotopes in polar regions is essential for reconstructing past climate over glacial interglacial cycles in addition to d do rd 18 o linearly related to condensation temperature the second order parameters d excess and 17 o excess provide important information on the climatic conditions of the source of precipitations in order to best interpret the glacial interglacial records of d excess and 17 o excess in polar ice cores it is important to document their present variability especially in remote and cold regions of east antarctica indeed the current climatic conditions encountered in these regions provide a good analogy with glacial climatic conditions in a large part of antarctica here we present the first seasonal variations of 17 o excess and d excess at vostok station on an event basis i e samples were collected immediately after each precipitation event over 1 year these records show strong correlation between 17 o excess and d 18 o over the course of the year with an amplitude 40 per meg 10 3 in the 17 o excess seasonal cycle and strong anticorrelation between d excess and d 18 o with d excess variations up to 20 the d excess and 17 o excess variations can be explained by the influence of kinetic fractionation at very low temperatures the comparison with simple isotopic models confirms this explanation but cannot explain the link between 17 o excess d excess and temperature without 1 a particular relationship between condensation and surface temperature and or 2 seasonal changes in the climatic conditions of the source regions

micromorphological and chemical complexities of a lateritic profile from basalt jos plateau central nigeria abstract a ferricrete capped lateritic profile developed on basaltic saprolite is described within both ferricrete and the immediately underlying soft laterite two types of sesquioxidic cement are distinguished the isopachous goethitic intergranular cement which is predominant and is interpreted as an indication of past phreatic conditions and the fe stained meniscus cement which is interpreted as a sign of vadose conditions the observed features are suggested to reflect changing drainage conditions related to morphological rejuvenation of the profile in full accordance with previous hypotheses developed on the basis of geomorphological consideration

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 reduction by pressure solution a theoretical model for quartz arenites pressure solution and the subsequent precipitation of the dissolved material may play an important role in the lithification of sandstones and limestones this paper presents a one dimensional model for a well sorted quartz arenite sequence undergoing diagenesis the sedimentary sequence is treated as a saturated porous medium in which pressure solution occurs at individual grain contacts quartz dissolved at grain contacts precipitates on free surfaces of adjacent grains this solution and precipitation process reduces porosity and forces the migration of pore fluid permeability decreases with porosity hindering fluid migration and excess fluid pressures are generated distributions of porosity pore pressure and fluid flow rates are given for several sedimentation rates and thermal gradients complete reduction of the primary intergranular porosity was accomplished in all cases examined by a depth of 3 5 km

sources of ca2 in the major carbonate cements in eocene sandstones and conglomerates evidence from sr isotopes sr ca ratios and rare earth elements abstract different sources of calcium have important effects on the formation and distribution of carbonate cements on the basis of identifying all possible sources of calcium comprehensive quantitative analyses of sr c and o isotopic values and trace elements including rare earth elements were conducted to determine the ca2 sources and their changes in the same type of carbonate cements at different vertical positions in the same reservoirs ferroan calcite and ankerite were the major types of carbonate cements ca2 in ferroan carbonate cements in the sandstones near the mudstone sandstone m s interfaces was derived primarily from diagenetic fluids mixed with ca2 originating from the transformation of smectite to illite and from original sedimentary water in the adjacent mudstones however ca2 in the ferroan carbonate cements in the sandstones away from the m s interfaces originated from the transformation of smectite to illite the dissolution of carbonate minerals and original sedimentary water in source rocks the gradual change of ca source of the same type of ferroan carbonate cements indicates the diversity of migration direction of diagenetic paleo fluid in the same period during the transformation of smectite to illite concentration gradients were created from mudstones to sandstones ca2 mg2 fe2 and other cations in the mudstones diffused to the adjacent sandstones and provided sources of ferroan carbonate cements at the edge of the sandstones pore water with ca2 mg2 fe2 co32 and other cations in the source rocks was collectively charged into sandstones during oil emplacement and promoted the precipitation of ferroan carbonate cements in residual pore water due to oil emplacement and material supplied by the adjacent mudstones the content of ferroan carbonate cements in the sandstones was lower and the reservoir porosity was higher at a greater distance from the m s interfaces however in thick bedded sandstones due to lack of material supply from the adjacent mudstones ca2 mg2 fe2 and co32 in the ferroan carbonate cements were mainly derived from source rocks as a result no significant changes were observed in the content of ferroan carbonate cements and reservoir porosity in the vertical direction

stacking pattern of cyclic carbonate platform strata lower cretaceous of southern apennines italy detailed sedimentological analysis of aptian albian shallow water carbonate platform strata from monte faito southern italy reveal composite sea level change the organization of the lithofacies and their related diagenetic overprint have allowed the identification of 50 elementary cycles they often show exposure related diagenetic features superimposed on subtidal deposits suggesting drops of relative sea level elementary cycles are grouped into 16 bundles forming six superbundles the hierarchical organization of the cycles are interpreted to be related to composite eustatic sea level fluctuations driven by the earth s orbital perturbation in the milankovitch frequency band moreover the systematic variation of superbundle thickness and the general evolution of the lithofacies reflects variations of accommodation potential defining longer term environmental changes different lithofacies forming elementary cycles are frequently separated by bedding planes that when viewed in a sequence stratigraphic context are interpreted as transgressive surfaces maximum flooding surfaces or sequence boundaries this allows the recognition of lowstand transgressive and highstand deposits in the cyclic units considering both the bundle superbundle ratio and the sequence stratigraphic interpretation of the superbundles a chronostratigraphic diagram illustrating both gaps at bundle level and their expected location along the section was created it establishes that the section developed over about 2 4 ma

comparison of deeply buried paleoregolith profiles norwegian north sea with outcrops from southern sweden and georgia usa implications for petroleum exploration abstract for the first time on the norwegian continental shelf deeply buried paleoregolith profiles have been identified as part of the petroleum reservoirs in recently discovered oil fields on the utsira high norwegian north sea reservoir properties porosity and permeability in the granitic basement on the utsira high are mainly the result of physical and chemical alteration of the rock occurring in the near surface environment during sub aerial exposure of the high in the mesozoic evaluating the reservoir potential of altered basement rocks requires a different approach than in conventional petroleum exploration in this paper macroscopic mineralogical and micromorphological alteration features observed in two deeply buried paleoregolith profiles are compared with surface paleoregoliths from ivo klack sweden and georgia usa the paleoregolith profiles are subdivided into specific weathering facies altered coherent rock facies saprock facies and saprolite facies based on the rock fabric and mechanical strength the reservoir potential of each weathering facies is controlled by the type and degree of alteration in the altered coherent rock facies porosity and permeability is mainly controlled by joints and microfractures that developed prior to subaerial exposure of the granitic pluton in the saprock facies intensified chemical dissolution of plagioclase enhanced porosity and the development of mesofractures improved the connectivity between pores in the saprolite facies progressive dissolution of plagioclase creates porosity but the precipitation of clays within voids and mesofractures has a destructive effect on the overall reservoir properties the deeply buried paleoregolith profiles from the utsira high display comparable macroscopic mineralogical and micromorphological alteration features to what was observed in surface paleoregoliths from ivo klack and georgia outcrop studies may therefore be an important tool when evaluating the reservoir potential in subsurface paleoregoliths

a new method of testing periodicity in cyclic sediments application to the newark supergroup abstract we present a new quantitative method for analyzing cyclic sediments the gamma method gamma time thickness tests the assumption that the cycles are periodic and provides quantitative estimates for relative accumulation rates of the different sedimentary facies that compose the cycles as such it provides a means of estimating relative time at very high resolution application of the gamma method to hettangian sediments of the newark supergroup indicates that the primary cycles were deposited in response to precessional climatic forcing and that these cycles were modulated by an eccentricity cycle with period ratios very similar to those of today results of gamma analysis also suggest that relative accumulation rates of the laminated deep water lacustrine facies was about six times slower than that of the shallow water facies

the effect of grain coating microquartz on preservation of reservoir porosity clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones but few workers have realized that similar effects can be produced by microcrystalline quartz coatings this phenomenon can be expected only under special circumstances but in such cases it can have profound consequences for exploration in the central graben area of the southern north sea unusually high porosity 20 27 and permeability 100 1000 md are found in certain zones in upper jurassic sandstones at depths of 3 4 4 4 km the porosity in these zones is 5 15 higher than expected based on average porosity depth trends from brent and haltenbanken sandstones we propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0 1 2 m thick the distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules rhaxella which implies a sedimentological control on the reservoir quality we present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites high porosities in parts of various upper jurassic oil fields ula and gyda have previously been explained by inhibition of quartz cementation by early hydrocarbon charge we suggest that the microcrystalline quartz coatings provide a more plausible explanation

mechanical compaction of deeply buried sandstones of the north sea abstract sandstones experience mechanical compaction when the overburden load exceeds the compressive strength petrographic evidence is rarely sufficient to determine the timing of mechanical compaction it is often assumed from indirect evidence such as regional porosity depth trends that mechanical compaction is a process that occurs exclusively during shallow or intermediate burial mechanical compaction without grain fracturing may occur at depth following pervasive framework grain dissolution and or if anomalously high porosity has been preserved due to the presence of small amounts of cement we describe examples from the fulmar sandstone formation of the central graben north sea that experienced late stage mechanical compaction following sponge spicule dissolution and microcrystalline quartz cementation deep burial mechanical compaction involving grain crushing may occur if the rate of grain contact quartz dissolution and or quartz overgrowth development cannot compete with the rate of stress increase at grain contacts some rotliegendes sandstones of the southern north sea that have been buried to 4 5 km offer a good example where the suppression of chemical compaction due to the presence of grain coating clays resulted in pervasive grain fracturing mineral veins are frequently associated with sandstones that have experienced pervasive mechanical compaction during deep burial these may reflect the sudden development of overpressure resulting from the transfer of load to the fluid during collapse of the sandstone framework

interannual variation of water isotopologues at vostok indicates a contribution from stratospheric water vapor combined measurements of water isotopologues of a snow pit at vostok over the past 60 y reveal a unique signature that cannot be explained only by climatic features as usually done comparisons of the data using a general circulation model and a simpler isotopic distillation model reveal a stratospheric signature in the 17o excess record at vostok our data and theoretical considerations indicate that mass independent fractionation imprints the isotopic signature of stratospheric water vapor which may allow for a distinction between stratospheric and tropospheric influences at remote east antarctic sites

kaolinitic meniscus bridges as an indicator of early diagenesis in nubian sandstones sinai egypt abstract characteristic meniscus geometries formed by kaolinitic in ltration areobserved in nubian sandstones collected on the western coast of the sinaipeninsula egypt based on petrographic and scanning electron microscope sem observations the kaolinite forming the menisci consists of mixed sizediscrete platelets that gather into the corners around the framework grain to grain contacts speci cally the internal fabric of menisci indicates a generalorganizing trend from 1 the centre where the platelets coat the frameworkgrains 2 to the peripheral zone where they are oriented tangentially to grainsurfaces 3 to the pore linings with curvatures that are consistent withtheoretical considerations of air water interfaces this typical arrangementsuggests a detrital origin of kaolinite platelets by mechanical in ltration intosediments lying above the tablewater in vadose conditions this type of claycementation occurring during early diagenesis can prevent delay deep burialdiagenetic processes and therefore preserve excellent reservoir properties keywords clay minerals kaolinite nubian sandstone sandstone diagenesis introductionearly diagenesis of sediment can occur shortlyafter deposition depending on depositional envi ronment the early lithi cation i e the rstdeveloped bonding agent occurs as carbonateprecipitation or particle in ltration that linksframework grains by forming meniscus bridges such grain to grain bridges grow preferentially inthe corners of pore space close to grain contacts dunham 1971 speci cally by developing inconformity to the water air interface the menis cus as a result of differential capillarity pres sures these cements or in ltrating material areassumed to be related to vadose environments dunham 1971 the meniscus geometry forming pore roundinghabit has been described frequently in modernsedimentary environments where carbonate pre cipitation produces beachrocks from carbonatesands meyers 1987 hillga rtner et al 2001 these types of precipitated meniscus cementsconsist mainly of crypto to microcrystallinecalcite or aragonite that display laments whichgenerally grow perpendicular to grain rim miller 1988 rao et al 1994 hillga rtner et al 2001 incontrast precipitated menisci of goethitic ferru ginous material have been reported for a lateriticpro le in basalt horva th et al 1999 several studies have also dealt with the men iscus shape displayed by mechanically in ltratedclay particles in clastic sediments walker 1976 kessler 1978 meyers 1987 matlack et al 1989 moraes de ros 1990 owing to the micro metre scale size of particles the detrital clays aretransported along connected pores by uidscirculating between skeleton grains walker 1976 during a drying period the detrital claysgather where the capillarity pressure preserves uids therefore the accumulated clays exhibitgrain coatings and meniscus shaped bridgeslinking neighbouring grains thus the meniscuscements typically indicate vadose conditionswhere pore spaces are not permanently saturated i e water table level uctuates walker 1976 matlack et al 1989 moraes de ros 1990 sedimentology 2003 50 1221 1229

a fluid inclusion study of quartz cemented sandstones from offshore mid norway possible evidence for continued quartz cementation during oil emplacement abstract jurassic sandstones from the oil zone in a well from the haltenbanken area offshore mid norway contain both aqueous and hydrocarbon fluid inclusions within quartz overgrowths homogenization temperatures for the inclusions range from 92 c up to the present formation temperature of approximately 118 c since the inclusions mostly are located at the boundaries between quartz clasts and quartz overgrowths the homogenization temperatures indicate that quartz cementation started at temperatures slightly below 92 c poikilotopic ferroan calcite cement usually predates formation of quartz overgrowths and most calcite cement must be precipitated at temperatures below 90 c although in one calcite cemented zone ferroan calcite encloses quartz overgrowths with inclusions trapped at temperatures of up to 105 c none of the examined hydrocarbon inclusions homogenized below 102 c and this suggests that oil started entering the reservoir at temperatures of around 100 c the burial history of the examined sandstones indicates that temperatures of 100 c were not reached before the late pliocene and that filling of the reservoir has taken place during the last few million years homogenization temperatures approximately equal to present formation temperature suggest that quartz cementation may still be in progress and that emplacement of oil did not necessarily stop precipitation of quartz cement possible continued quartz cementation after oil filled the reservoir could be explained by dissolution of quartz at stylolites and grain contacts as transport of dissolved quartz from these sources might possibly take place by diffusion through irreducible water

stability of the astronomical frequencies over the earth s history for paleoclimate studies the expected changes over the past 500 million years in the principal astronomical frequencies influencing the earth s climate may be strong enough to be detectable in the geological records and such effects have been inferred in several cases calculations suggest that the shortening of the earth moon distance and of the length of the day back in time induced a shortening of the fundamental periods for the obliquity and climatic precession from 54 to 35 41 to 29 23 to 19 and 19 to 16 thousand years over the last half billion years at the same time the precessional constant increased from 50 to 61 arc seconds per year the changes in the frequencies of the planetary system due to its chaotic motion are much smaller their influence on the changes of the periods of climatic precession obliquity and eccentricity of the earth s orbit around the sun can be neglected eccentricity periods used for quaternary climate studies may therefore be considered to have been more or less constant for pre quaternary times

rayleigh convection mass transport and change in porosity in layers of sandstone the critical rayleigh number for the onset of thermal convection in a quartz rich sandstone layer is discussed for different thermal boundary conditions the mass transfer of silica due to rayleigh convection is computed assuming that the solution is saturated with respect to quartz and that the silica concentration is a function of temperature only since quartz solubility increases with temperature silica is dissolved from the sandstones in the lower part of the layer where the fluid is being heated and precipitated in the upper part where it is cooled the change in porosity is found as function of the vertical coordinate valid for small values of time it is shown that for small supercritical rayleigh numbers the time for the porosity to change up to 10 of its initial value is of the order 10 m y the spatial variation in porosity should be a characteristic signature of rayleigh convection and should be easy to recognize if convection has taken place for significant geological time

milankovitch climatic origin of mid cretaceous black shale rhythms in central italy the earth s orbital variations reflected in pleistocene ice volume were also recorded in non glacial times carbonate production in pelagic mid cretaceous sediments quantified by calcium carbonate and optical densitometry time series reflects the orbital eccentricity and precessional cycles minimal eccentricity brought deep sea anoxia most of the sedimentary variability of this 100 myr old sequence lies within the milankovitch orbital frequency band

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

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

relationship between reservoir diagenetic evolution and petroleum emplacement in the ula field north sea abstract the diagenetic and petroleum filling histories of the ula field have been studied by analysing aqueous and petroleum inclusions occurring in authigenic cements this study shows that diagenesis continues actively after the arrival of petroleum in the sandstones although the reaction rates and petroleum saturation remain obscure microthermometric measurements of fluid inclusions in authigenic quartz suggest an onset of extensive quartz cementation at temperatures around 110 c i e 2 5 km and that this cementation has continued to the present day synchronously with quartz cementation authigenic albite was formed as inter and intragranular cements large amounts of petroleum inclusions locally occur in the albite and quartz cements these inclusions were trapped in the cements as the main charge of petroleum arrived at the structure diffusion through thin water layers between the petroleum and the mineral surface would probably have dominated mass transfer of solutes for the precipitation of authigenic quartz and albite it is evident from analysis of the petroleum inclusions in early formed k feldspar overgrowths that some secondary migrated petroleum probably arrived in the reservoir at a shallow burial depth before the rapid subsidence in the last 25 my compared with petroleum in the free porosity of the ula formation the petroleum in the k feldspar inclusions is evidently sourced from a different source facies and most maturity parameters testify to the latter being less mature later trapped petroleum inclusions in quartz and albite have characteristics found both in k feldspar and in the ula formation dst oil and thus is likely to reflect the progressive change in the ula field petroleum charge which occurred during the time period of quartz diagenesis

documenting the reliability and utility of the γ method as applied to cyclic sections using forward modeling abstract the problem of determining if cyclic sections which are older than 5 m y in age were deposited in response to orbital control is discussed gamma analysis a method which uses the assumption that effective accumulation rates are facies dependent to minimize differences in cycle duration is presented as a possible solution to this problem while the assumptions of analysis are generally not met by geologic data the effective accumulation rates estimated may be instrumental in recovering a spectrum which reveals orbital control we address the problem of assessing the reliability of results by performing analyses on three cyclic time series generated by a forward model our results show that exact accumulation rates are not likely to be obtained by the method however the accumulation rates obtained markedly improve the time scale resulting in a spectrum from which one could distinguish orbital forcing if present only when spectra of cyclic data show predicted orbital peaks can analysis be considered to yield effective accumulation rates which are more nearly like the true effective accumulation rates than the assumption of a single effective accumulation rate for all facies

predictive permeability variations in a hermod sand reservoir stine segments siri field danish north sea abstract the hermod sand of the stine segments siri field danish north sea displays large permeability variations 1 600 md these permeability variations represent horizontal layering in the reservoir largely formed by diagenetic processes they have a significant impact on reservoir performance because horizontal producers in fairly thin reservoirs 30 m are protected against bottom water influx and can produce water free for long periods of time this paper presents the results of a detailed multidisciplinary study where the observed variation in permeability can be explained by two main diagenetic phases silica as opal cement and microquartz and berthierine a serpentine mineral that is closely related to chlorite opal microquartz and berthierine cements dominate in separate parts of the reservoir bodies opal microquartz cemented sands have high permeability typically 500 600 md berthierine cement has a different growth pattern in the oil and water zones resulting in intermediate permeability in the oil zone typically 50 100 md and very low permeability in the water zone typically 1 10 md

influence of grain boundary structural evolution on pressure solution creep rates intergranular pressure solution is a well known rock deformation mechanism in wet regions of the upper crust and has been widely studied especially in the framework of compaction of granular materials such as reservoir sandstones and fault rocks several analytical models exist that describe compaction creep by stress induced mass transport and the parameters involved are relatively well constrained by laboratory experiments while these models are capable of predicting compaction behavior observed at relatively high porosities they often overestimate compaction rates at porosities below 20 by up to several orders of magnitude this suggests that the microphysical processes operating at low porosities are different and are not captured well by existing models the implication is that available models cannot be extrapolated to describe compaction of sediments and fault rocks to the low porosities often reached under natural conditions to address this problem we propose a new thermodynamic model that describes the decline of pressure solution rates within individual grain contacts as a result of time averaged growth of asperities or islands and associated constriction of the grain boundary diffusion path here termed grain boundary evolution the resulting constitutive equations for single grain grain contacts are then combined and solved semianalytically the compaction rates predicted by the model are compared with those measured in high strain compaction experiments on wet granular halite a significant reduction in compaction rate is predicted when grain boundary evolution is considered which compares favorably with the experimental compaction data

on changes in porosity and volume during burial of argillaceous sediments abstract the porosity and hence volume of argillaceous sediments is determined by 1 the magnitude of the effective stress acting within the sediment 2 the previous stress history of the sediment and 3 at shallow depths of burial by features such as the mineralogy and the nature of the depositional environment stress paths and the critical state diagrams for a number of clays are used to investigate the range of porosities possible in argillaceous sediments as the effective stresses increase it is found that all porosity effective stress curves converge at large stresses the change in porosity is strongly dependent on the mean effective stress but largely independent of the deviatoric stress and thus is largely independent of the nature of the stress field acting on the basin compressional extensional etc because of the dependence of porosity on the mean effective stress no simple relationship exists between porosity and depth of burial but in the absence of overpressured pore fluids and assuming the sediment is not overconsolidated it is possible to contour the porosity effective stress diagram in terms of burial depths these data should assist in recalculating stratigraphic thicknesses for basin reconstruction and stratigraphic correlation studies

lack of inhibiting effect of oil emplacement on quartz cementation evidence from cambrian reservoir sandstones paleozoic baltic basin currently the question of whether or not the presence of oil in sandstone inhibits quartz cementation and preserves porosity is still debated data from a number of cambrian sandstone oil fields and dry fields have been studied to determine the effects of oil emplacement on quartz cementation the data show that the porosity distribution is not affected by the presence of oil in sandstones from oil fields and dry fields with similar porosity distribution from this it can be concluded that oil emplacement does not lead to preservation of primary porosity and silica supply for quartz cementation is derived from internal sources rather in spite of large variation in porosity and quartz cement content a regular pattern of porosity decrease is related to increasing temperature or depth the observed heterogeneity is due to local factors that influence the precipitation of quartz cement including sandstone architecture i e distribution of shales within the sandstone bodies and sandstone thickness heterogeneity is inherent to sandstone architecture and to the fact that silica for quartz cementation is derived from heterogeneously distributed local pressure solution models predicting reservoir properties should encompass facies and architecture as important independent factors

cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences over the past 25 yr the science of stratigraphy has evolved to include time correlative data from vastly disparate components of the earth system not least of these is the global signal afforded by cyclostratigraphy which has recorded the evolution of earth s astronomical milankovitch forcing of insolation and the paleoclimate system fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies geochemistry mineralogy rock magnetism color etc in step with the documentation of astronomically forced paleoclimate from ever older older geologic times innovations in computational science have provided ever longer high accuracy astronomical model targets that can be used for time scale calibration the earth s orbit is affected by motions of other planets notably the orbital perihelia of venus and jupiter which impose a dominant 405 k y eccentricity cycle on earth s orbital evolution the large mass of jupiter stabilizes this cycle over hundreds of millions of years the cyclostratigraphic record of 405 k y cycles is therefore often used to correct chronologies affected by variable sedimentation earth s shape and rotation rate are influenced by tidal dissipation and climate friction these effects affect earth s precession rate through time thus a record of earth moon evolution is also embedded in cyclostratigraphy the geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high precision radioisotope dating which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca 50 ma limit of accurate astronomical solutions and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time finally space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of mars with patterns suggestive of astronomical forcing analogous to earth

genesis and distribution pattern of carbonate cements in lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation in the dongying sag jiyang depression eastern china abstract the lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation are the main exploration target for hydrocarbons in the dongying sag eastern china carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep water gravity flow sandstone reservoirs the sandstones are mainly lithic arkose with an average framework composition of q43f33l24 the carbonate cements are dominated by calcite ferroan calcite ankerite and a small amount of dolomite the calcite and ferroan calcite are mainly poikilotopic blocky crystals while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores the relatively positive 13c values 2 to 3 9 of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon from 32 ma to 25 ma the pore water was rich in bicarbonate and ca2 due to carbonate dissolution in mudstone and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium to thick sandstone beds from 12 ma to present abundant ca2 fe3 fe2 mg2 and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium to thick bedded sandstones ankerite is mainly distributed in the reservoirs associated with oil migration or charge because change of fe3 to fe2 from oil charge may supply sufficient fe2 for ankerite precipitation the center of sandstone beds 0 6 m is with potential of high quality reservoirs in the research area carbonate cementation appears to be an important factor that controls the accumulation of oil in deep water gravity flow sandstone reservoirs in the study area

reply to comment by martin f miller on record of δ18o and 17o excess in ice from vostok antarctica during the last 150 000 years that this excess was lower in glacial than in interglacial times we further suggested that the change in 17 o excess indicates smaller effect of kinetics signifying higher normalized humidity in the oceanic source region during glacials in his comment miller 2008 questions the validity of these conclusions on three grounds first he emphasizes that the measurements are reported with respect to the vsmow standard and not with respect to ocean water and therefore 17 o excess with respect to the ocean may not exist we note however that this point was raised by us landais et al 2008 p 3 and we emphasized the need for precise calibration with respect to seawater recently we luz and barkan 2008 have made precise calibration and reported that the triple oxygen isotope composition of vsmow is similar to seawater therefore we conclude that antarctic precipitation unambiguously contains 17 o excess with respect to the ocean

generation of evenly spaced pressure solution seams during late diagenesis a kinetic theory stylolites and pressure solution seams can form during diagenesis of rocks subjected to stress through a feedback involving texture and pore fluid solute concentration this feedback destabilizes the state of uniform texture represented here only by porosity for simplicity a mathematical kinetic model that includes pressure solution diffusion and reprecipitation predicts the spontaneous generation even in initially uniform rocks of regions alternatingly in space more and less soluble a region that becomes more soluble also becomes simultaneously more porous the kinetic theory predicts that stylolites can form at roughly constant spacings this agrees with many field and petrographic observations bedding planes between beds that have initially different texture are shown by the theory to be the focus of a porosity instability that can lead to the formation of a stylolite and to the generation of more stylolites within each bed this prediction agrees with the observation that many stylolites have developed at bedding planes

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

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

kaolin coating of stylolites effect on quartz cementation and general implications for dissolution at mineral interfaces abstract the deeply buried 4 km 160 c quartz rich sandstones of the middle jurassic garn formation in well 6507 7 1 offshore mid norway typically contain 20 30 quartz cement and only 3 9 porosity the exception is a less than 1 5 m thick interval at the top of the formation where quartz cement volumes typically drop to 8 16 and porosities are in the range 7 19 the uppermost part of the garn formation is not isolated from the rest of the formation by any type of impermeable barrier the sandstones are water filled indicating that the change in cementation does not correspond to a hydrocarbon contact and there are no significant changes in grain size grain coating abundance grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1 5 m of the sandstones however in the samples from the less cemented interval illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete we propose that the cause of reduced quartz cementation in the top of the garn formation may be the coating of stylolites developed from primary illitic clay rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin rich rock adjacent to the stylolites is dissolved although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality our observations may help to unravel the mechanisms of quartz dissolution at stylolites specifically we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin quartz contacts similarly lack of dissolution at clean quartz quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica which may explain the extensive dissolution often seen at stylolites in limestones

illite distribution and morphology explaining basinal variations in reservoir properties of upper jurassic sandstones danish north sea abstract growth of fibrous illite may destroy an otherwise promising sandstone reservoir even sandstones characterized by microquartz coatings therefore detailed petrographical and diagenetic investigations were performed in order to understand the factors that control illite precipitation and other factors affecting the reservoir properties in the upper jurassic heno formation the heno formation is encountered in large parts of the danish central graben where its lower part the gert member was mainly deposited in back barrier environments and its upper part the ravn member was deposited in a shoreface environment in total 215 core samples from 15 wells were investigated petrographically by optical and scanning electron microscopy this was supplemented by plug porosity and permeability measurements and x ray diffraction of bulk and clay fraction the investigations show among other things that microquartz coatings may help to preserve porosity and permeability whereas the permeability is reduced when microquartz occurs combined with fibrous illite illite precipitation is largely governed by the amounts of k feldspar present during burial consequently the initial abundance of k feldspar in the sediment source area and its alteration during and immediately after deposition in particular during subaerial exposure at sequence boundaries has a major influence on the final reservoir properties of the sandstones the risk of illite precipitation can largely be predicted from the available sediment source material depositional environment and burial depth

record of δ18o and 17o excess in ice from vostok antarctica during the last 150 000 years 1 we measured 17 o and 18 o in recent antarctic snow and down the vostok ice core and calculated the excess of 17 o with respect to vsmow the magnitude of the 17 o excess in the holocene and the last interglacial is 45 per meg and it remains constant in a transect from the coast to the continental interior analysis of the transect data shows that the 17 o excess is not sensitive to temperature variations over the continent there are significant shifts in 70 excess from low values in glacial to high values in interglacial times the observed shifts suggest higher normalized relative humidity and or wind speeds over the source oceanic regions in glacial times

depositional sequences in shallow carbonate dominated sedimentary systems concepts for a high resolution analysis abstract oxfordian and berriasian sections representing shallow water carbonate dominated sedimentary systems have been studied in the swiss and french jura in spain and in normandy they all display a hierarchical stacking of depositional sequences facies evolution and stacking pattern allow to define elementary small scale medium scale and large scale sequences some depositional sequences display well marked sequence boundaries others are limited by transgressive or maximum flooding surfaces the hierarchical organisation of such sequence stratigraphic elements implies that sea level fluctuations were an important factor in their formation and that these fluctuations had different frequencies the superposition of high frequency sea level changes on a long term sea level trend leads to repetition of diagnostic surfaces defining sequence boundary and maximum flooding zones wherein the corresponding high frequency surfaces are well developed chronostratigraphic tie points permit us to estimate the duration of large scale sequences this time control and the observed hierarchical stacking suggest that the high frequency sea level changes were controlled by climatic cycles in the milankovitch frequency band the variability of stacking pattern and facies evolution between sections illustrates the complexity of the studied environments furthermore because of the minimal accommodation space available in these shallow water settings much of the geologic time is not recorded nevertheless detailed analysis of the depositional sequences allows the interpretation of the evolution of the sedimentary system with a high time resolution thus there is a potential to monitor sedimentological ecological and diagenetic processes on a time scale of 20 to 100 ka

cretaceous rhythmic bedding sequences a plausible link between orbital variations and climate abstract simple oscillation cycles have been recognized in cretaceous pelagic sequences as a marked periodicity in carbonate and or organic carbon content the estimated periodicities of these cycles resemble milankovitch type variations in orbital elements however during warm geologic periods the nature of the link between orbital variations and the sedimentary record has been problematic climate model studies show that the intensity of continental margin precipitation in specific low latitude regions is a function of land sea thermal contrast which is sensitive to orbital variations cretaceous climate model simulations are characterized by regions of intense precipitation the locations of which are controlled by geographic configuration in particular in the model simulation the northern margin of the tethys ocean has all the climatic characteristics necessary to exhibit a sensitivity to orbital variations detailed studies of the biota sedimentology and geochemistry of the bedding rhythms in greenhorn cyclothem in the western interior seaway of north america are best interpreted in terms of variations in precipitation runoff and stable stratification of the seaway combined the geologic and model studies support a link between orbital periodicities variations of intense precipitation associated with specific paleogeographic situations and cretaceous bedding patterns this hypothesis has important implications for the sensitivity of the atmosphere hydrosphere lithosphere system to external forcing

variations of 17o 16o and 18o 16o in meteoric waters the variations of d 17 o and d 18 o in recent meteoric waters and in ice cores have proven to be an important tool for investigating the present and past hydrologic cycle in order to close significant information gaps in the present distribution ofd 17 o and d 18 o of meteoric water we have run precise measurements with respect to vsmow on samples distributed globally from low to high latitudes based on the new and existing data we present the global meteoric water line gmwl ford 17 o andd 18 o as ln d 17 o 1 0 528 ln d 18 o 1 0 000033 r 2 0 99999 in addition to meteoric water we carried out the first measurements of seawater from the pacific and atlantic oceans with respect to vsmow the obtained results show that the slope of the trend line ln d 17 o 1 vs ln d 18 o 1 of seawater samples is 0 528 the same as for meteoric water but the regression intercept is 5 per meg thus the positive intercept in the gmwl indicates an excess of 17 o in meteoric waters with respect to the ocean an excess or depletion of 17 o in water is defined as 17 o excess ln d 17 o 1 0 528 d 18 o 1 most meteoric water samples have positive 17 o excess of varying magnitudes with an average of 37 per meg with respect to vsmow we explain how these positive values originate from evaporation of sea water into marine air which is undersaturated in water vapor and how subsequent increase of 17 o excess occurs when atmospheric vapor condenses to form liquid and solid precipitation we also clarify the effect of excessive evaporation on 17 o excess finally based on the new results on 17 o excess of seawater we recalculated the relationship of d 17 o vs d 18 o in vapor diffusion in air as 18 adiff 1 0096

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

a dual porosity model for evaluating petroleum resource potential in unconventional tight shale plays with application to utica shale quebec canada unconventional tight to shale reservoirs vary from tight sandstone siltstone to organic rich mudstone shale commonly with mixed lithologies in such reservoir systems matrix pores and organic pores with different origins and distinct physical and chemical properties co exist for hydrocarbon storage traditional resource assessment methods designed for conventional reservoirs cannot handle the two pore systems properly this study proposes a dual porosity model to respond to the need for a new method in assessing hydrocarbon resource potential in such reservoir systems the dual porosity model treats the two types of pores separately and derives the resource estimates from different sources of data thus better characterizing unconventional reservoirs with complicated pore systems the new method also has the flexibility of assessing resource potential for the entire spectrum of mixed lithologies ranging from a complete tight to a pure source rock organic rich shale mudstone reservoir the proposed method is illustrated through the assessment of the in place petroleum resource potential in the upper ordovician utica shale of southern quebec canada the results of the application suggest that the proposed approach effectively handles the two pore systems in tight shale reservoirs effectively and provides a useful tool for estimating resource potential in unconventional plays

provenance and burial history of cement in sandstones of the northbrook formation carboniferous western newfoundland canada a geochemical investigation abstract the carboniferous north brook formation western newfoundland canada consisting of fluvial lacustrine arkosic litharenites has undergone two cementation cycles the first cycle comprises chlorite followed by minor quartz and calcite mono phase aqueous fluid inclusions hosted in first cycle calcite reflect precipitation in a near surface meteoric diagenetic environment chlorite linings and cements generally inhibit precipitation of quartz overgrowths thus maintaining open pore space until a subsequent cycle when temperatures are within the oil window the second cycle comprises chlorite followed by calcite which hosts two phase fluid inclusions with homogenization temperatures ranging from 91 7 to 120 7 c and salinity from 6 3 to 8 4 eq wt nacl these results suggest conditions in line with a deep burial environment and calculated fluid inclusion isochors stipulate a trapping temperature of around 132 c and pressure of 500 bar 5 km hydrostatic pressure for these cements quantitative fluid inclusion gas analysis further confirms that fluids were sourced from evolved meteoric fluids with fluctuating co2 ch4 n2 ratios manganese analysis confirms an oxidizing environment for the first cycle calcite cement but a reducing one for the infilling second cycle cement both cements have similar ree signatures are slightly lree depleted and during migration the fluids were influenced by the sedimentary host rock although there is a prominent negative ce anomaly there is no eu anomaly in the cements the 13c and 18o values are mostly in the negative in the calcite cement hosted in the coarse grained clastics which were influenced by migrating brines and or hydrocarbons in the diagenetic history of this sedimentary basin the first cementation event is characterized by chlorite quartz and calcite emplaced under shallow oxidizing meteoric water conditions and appropriate temperatures a diagenetic hiatus ensued until the units were more deeply buried at this stage a second generation of cement characterized by minor chlorite and major calcite infilled vugs and voids under higher temperatures equivalent to the upper limit of the oil window and 5 km burial depth the lack of porosity precludes the north brook formation from being a petroleum reservoir although limited petroleum may have been sourced during the geologic history

a model for diagenetic mass transfer between adjacent sandstone and shale abstract there are numerous suggestions that mass transfer between sandstone and shale is an important factor in the diagenesis of sandstone reservoirs however advective transport models for mass transport have not produced results that support these proposals a model is developed that overcomes the problems of advective transport models by assuming that diffusive mass transfer is the principle transport mechanism the process occurs when the rate of dissolution of reactive detrital components exceeds the local rate of diffusive transport at that point in the burial history units with greater amounts of a reactive component will generate more solute per unit time than units with less reactive component the process forms chemical gradients that can rapidly transfer mass equivalent to several vol of rock over distances of at least 5 m the observed transport of potassium and silica between adjacent sandstone and shale in the north sea during late stage illitization was correctly predicted using this conceptual approach the conceptual framework from the illite model can be extended to other diagenetic reactions and suggests that the burial of adjacent sandstone shale units could produce episodic diffusion gradients that transfer mass and form multiple generations of cement in sandstone reservoirs

postaccumulation sandstone porosity evolution by mechanical compaction and the effect on gas saturation case study of the lower shihezi formation in the bayan aobao area ordos basin china abstract porosity evolution and reservoir quality are strongly affected by mechanical compaction however the compaction induced porosity reduction process in sandstone reservoirs during the postaccumulation period and its effect on gas saturation remain poorly understood by studying the sandstones in the bayan aobao area we were able to quantify the porosity evolution and gas saturation variations caused by mechanical compaction during the postaccumulation period through fluid inclusion analysis and mathematical modeling moreover we verified our findings by compaction experiments and geological statistics fluid inclusion microthermometry was employed to determine the hydrocarbon accumulation periods and the highest homogenization temperature of the two phase aqueous fluid inclusions coexisting with natural gas fluid inclusions i e 115 c with a corresponding geological age of 140 ma was regarded as the beginning of the postaccumulation stages with these data we constructed an inversion evolution model based on the geologic statistical model of sandstone mechanical compaction for the postaccumulation period and determined the gas saturation changes based on compaction porosity evolution the modeling results indicate that the sandstone porosity decreased greatly during postaccumulation subsidence and increased slightly during late stage uplift while the gas saturation increased with both the porosity decrease during the postaccumulation subsidence and the porosity increases during the later uplift the modeling results of subsidence are concordant with the compaction experiments which also show that the larger the initial porosity the greater the porosity reduction the uplift modeling results demonstrate that an erosion induced decrease in burial depth allowed the sandstone porosity to increase which is supported by some scholars findings e g neuzil and pollock 1983 jiang et al 2004 zhang 2013 and by the positive correlation between sandstone porosities and restored geologic unit thicknesses in the study area however the inversion modeling indicates that the reason for the porosity rebound in litharenites is that the increase in elastic porosity slightly exceeds the decrease in viscoplastic porosity which would have been different if the uplift induced erosion rate had been distinctly lower gas production test data and restored geologic unit thicknesses indicate that uplift and erosion may be able to enrich sandstone gas reservoirs to a certain extent consistent with the calculated gas saturation variation results but a full understanding of gas saturation evolution awaits further studies the inversion model can provide a powerful tool for quantitative analysis of the sandstone compaction process during the postaccumulation period which could be used for reference in other similar regions

whither cyclostratigraphy testing the gamma method on upper pleistocene deep sea sediments north atlantic deep sea drilling project site 609 1 abstract gamma analysis a procedure for tuning cyclic sediments in which gamma is defined as 1 sedimentation rate was applied to the upper pleistocene gray scale record of deep sea drilling project site 609 from the north atlantic the spectrum of the gamma tuned time series produces eccentricity obliquity and precession index signals when the primary periodicity is tuned to the precessional beat frequency however the timescales are significantly too long suggesting that some of the primary cycles used in the gamma tuning procedure have higher frequency than the precessional beat by filtering the specmap calibrated time series at i 0 12 kyr the spurious cy cles are removed from the tuning procedure in this case the resulting gamma corrected spectra look much more like that of the specmap tuned time series however the time series remains slightly too long forward modeling is consistent with the results from modeling the core that is the gamma method is quite robust at revealing orbital periodicity in an orbitally controlled re cord for which sedimentation rates are facies dependent however the method does not produce an accurate time series criteria for complete rejection of gamma results are clarified as either 1 least squares results include negative gamma values or 2 the resulting spectrum does not show an or bital signature variation on sedimentation rates greater than 5 result in incorrect time series however variations on sedimentation rates as high as 15 generally reproduce strong orbital spectra and occasionally the method can also generate an orbital spectrum with up to 25 noise the method is not as robust in predicting gamma values or sedimentation rates although it generally distinguishes facies with relatively high sedimentation rates from facies with relatively low sedimentation rates the values predicted may be incorrect by a factor of2 thus while gamma tuning of the time series is an improvement over observed thicknesses the resulting timescale is not correct the implications of these results for cyclostratigraphy are significant the generation of an orbital spectrum from a time series that is demonstrably incorrect casts doubt on cyclicity as a dating tool

precipitation regime influence on oxygen triple isotope distributions in antarctic precipitation and ice cores abstract the relative abundance of 17o in meteoric precipitation is usually reported in terms of the 17o excess parameter variations of 17o excess in antarctic precipitation and ice cores have hitherto been attributed to normalised relative humidity changes at the moisture source region or to the influence of a temperature dependent supersaturation controlled kinetic isotope effect during in cloud ice formation below 20 c neither mechanism however satisfactorily explains the large range of 17o excess values reported from measurements a different approach based on the regression characteristics of 10 3 ln 1 17 o versus 10 3 ln 1 18 o is applied here to previously published isotopic data sets the analysis indicates that clear sky precipitation diamond dust which occurs widely in inland antarctica is characterised by an unusual relative abundance of 17o distinct from that associated with cloud derived synoptic snowfall furthermore this distinction appears to be largely preserved in the ice core record the respective mass contributions to snowfall accumulation on both temporal and spatial scales provides the basis of a simple first order explanation for the observed oxygen triple isotope ratio variations in antarctic precipitation surface snow and ice cores using this approach it is shown that precipitation during the last major deglaciation both in western antarctica at the west antarctic ice sheet wais divide and at vostok on the eastern antarctic plateau consisted essentially of diamond dust only despite a large temperature differential and thus different water vapour supersaturation conditions at the two locations in contrast synoptic snowfall events dominate the accumulation record throughout the holocene at both sites

development of early diagenetic silica and quartz morphologies examples from the siri canyon danish north sea abstract the siri canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones the development in silica morphologies can be observed from very shallow 1700 m to increased burial depth 3000 m and increased proximity to the central graben distance from 0 to 65 km hydrocarbons and pore fluids now found in the siri fairway have at least partly originated from the central graben the siri canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass flows and sandy turbidites which originated on the stavanger platform several hydrocarbon exploration and production wells have been drilled in the siri canyon seven of which are included in this study nini 3 nini 1 na 2p sofie 1 siri 4 celilie 1a and augusta 1 the reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy the silica morphologies in some places are classic and well documented in the literature whereas others at least to our knowledge have never previously been described some of the silica morphologies presented here show gradual transition from one to another and others are stand alone forms without clear relationships to other forms the silica morphologies can be expressed in the following way 1 opal rims characteristic of the initial phase of the silica diagenesis in most sandstone units in the siri canyon thick opal rims characterise the sandstone parts adjacent to the mudstone units in the stine segment of the siri field 2 microquartz quartz crystals with a size of 1 5 m seen as coatings on the opal rims both ordered and random 3 cavity overgrowth found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases angular cavities in the microquartz coatings origin from dissolution of clinoptilolite possibly with a source in volcanic ash whereas dissolution of lepispheres probably resulted in circular cavities 4 quartz ridges syntaxial quartz overgrowths a few micro millimetre in width in parallel lines defined in this paper 5 quartz mountains syntaxial irregular 5 10 m large quartz crystals defined in this paper developed in the water filled sandstone intervals where fluids from the central graben were introduced possibly together with the hydrocarbon and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids 6 macroquartz syntaxial quartz overgrowths 20 m seen as continued growths from quartz mountains this takes place in the deeper reservoir sandstones promoted by fluids originating from the central graben and increased burial depth 7 quartz with sutured grain contacts occurring only in the deepest well 3000 m

overpressure preventing quartz cementation comment to stricker et al 2016 abstract there is increasing evidence that quartz cementation can be viewed as a process controlled by temperature and insensitive to effective stress this view of quartz cementation in sandstones is often referred to as the illite mica induced dissolution model imid which assumes quartz dissolution to occur along stylolites and clay laminae rather than at quartz quartz grain contacts in the present comment it is argued that the exceptional reservoir quality in the skagerrak formation of the heron cluster north sea is due to grain coatings and rapid cenozoic burial limiting the exposure to quartz cementation this line of reasoning implies overpressure has had neglectable porosity preserving effect in the heron cluster

diagenetic sequences of continuously deposited tight sandstones in various environments a case study from upper paleozoic sandstones in the linxing area eastern ordos basin china an integrated analysis of the petrographic characteristics and types and distribution of diagenetic alteration in the upper paleozoic benxi taiyuan shanxi and xiashihezi formations provides insights into the controlling factors on variations in porosity and permeability in tight sandstones 85 of the sandstone samples display porosity values 10 and 90 of the samples exhibit permeability 1 md diagenetic alteration includes mesogenetic compaction cementation by dolomite ankerite and quartz dissolution of feldspar and illitization of smectite eodiagenesis includes compaction development of smectite cementation by pore filling quartz and disordered kaolinite and precipitation of calcite and fe calcite chlorite and quartz preserve primary pores against damage whereas kaolinite illite smectite i s mixed layer and illite significantly diminish reservoir quality via permeability reduction chlorite and i s content decrease abruptly as depth increases whereas the kaolinite content remains elevated at depth because of the complete destruction of k feldspar the transformation from smectite to illite provides silica ions for the widely distributed quartz overgrowths as the depositional environment transformed from fluvial xiashihezi to deltaic shanxi and to epicontinental taiyuan and benxi the dissolution effect increased monotonically feldspar dissolution is dominant in the shanxi formation whereas the benxi and taiyuan formations commonly contain quartz dissolution pores the taiyuan formation has markedly higher porosities than in the overlying and underlying formations caused by strong dissolution and high silica content the decrease in porosity in the benxi formation results from the extensive formation of clay minerals caused by high frequency transgressions in a transitional environment

the effect of oil saturation on the mechanism of compaction in granular materials higher oil saturations lead to more grain fracturing and less pressure solution abstract whether addition of oil to sandstones slows stops or has no influence upon pressure solution and quartz cementation has long been disputed despite having major implications for petroleum exploration and appraisal strategies to elucidate the effect of addition of oil to compaction pressure solution and cementation processes this study utilizes an experimental approach simulating isochemical volumetric compaction using granular halite in the presence of variable brine oil mixtures each experiment at 500 kpa effective stress lasted 48 hours and involved repeated measurements of volumetric strain the lithified products of experiments were examined using sem techniques after 30 hours approximately 4 volumetric strain occurs when the brine oil ratio is 100 0 pure brine fractures are rare in samples from experiments undertaken with pure brine so that pressure solution and concomitant cementation must be the compaction mechanism when the brine oil ratio is decreased from 100 0 to 40 60 the volumetric strain remains about 4 but there are more fractures as quantified from sem image analysis these observations suggest that although pressure solution has occurred some of the volumetric strain is the result of fracturing implying that pressure solution has been less effective in the presence of some oil than it is in the presence of pure brine when the brine oil ratio is decreased further from 40 60 to 5 95 the volumetric strain increases up to 7 3 sem image analysis reveals that the strain is predominantly due to grain fracturing this change from pressure solution to grain fracturing is likely due to heterogeneous pressure solution resulting from heterogeneous distribution of stresses brine coated grain contacts undergo pressure solution but oil coated grain contacts experience increasing stress up to the point of failure as the surrounding grain pack compacts when the brine oil ratio is decreased to less than 5 95 volumetric strain decreased to about 1 with few induced fractures and little pressure solution compaction is negligible when the brine oil ratio is 0 100 only oil in the pores and fractures are negligible halite is more water soluble and more hygroscopic than quartz so that water will cling to halite surfaces more tenaciously than to quartz since even halite has its mechanism of volumetric strain affected when the brine oil ratio is less than 40 60 quartz pressure solution and subsequent cementation is even more likely to be affected by oil filling this experimental study suggests that the mechanism and extent of compaction and subsequent cementation in sandstones is strongly affected by the addition of oil

milankovitch climatic signals in lower triassic olenekian peritidal carbonate successions nanpanjiang basin south china abstract meter scale peritidal carbonate strata from an isolated platform in the nanpanjiang basin of south china were tested for the presence of periodic or quasi periodic climatic signals these signals provide information on factors controlling cyclic sedimentation in the equatorial eastern tethys during the early triassic greenhouse period parasequences are composed of shallowing upward successions of subtidal facies including skeletal packstone and grainstone calcimicrobial boundstone and oolitic packstone and grainstone intertidal laminated lutite and ribbon rock and rare supratidal microbial laminite parasequence stacking patterns display three orders of cyclicity suggesting hierarchical stratigraphic relationships gamma analysis of kominz and bond earth planet sci lett 48 1990 233 244 was used to estimate facies dependent thickness time conversion factors i e s and to construct tuned and untuned time series for two stratigraphic sections spectra of these time series indicate the presence of quasi periodic signals with prominent short eccentricity 94 9 131 2 kyr short obliquity 35 8 kyr and long precessional index 21 2 kyr peaks and minor long eccentricity 412 9 kyr long obliquity 45 3 kyr short precessional index 17 7 kyr and constructional tone 9 7 kyr peaks when calibrated on the tuned spectra thus milankovitch climatic forcing probably greatly influenced sedimentation the calibration indicates that the subtidal facies has a sedimentation rate of 24 6 30 7 cm kyr and the intertidal supratidal facies has a rate of 2 7 6 0 cm kyr the estimated duration of deposition of the two sections is 1139 1423 kyr and corresponds to a stratigraphic completeness of 32 92 the completeness is much greater than that of icehouse stratigraphic records we speculate that variations in carbonate productivity and environmental conditions driven by milankovitch climatic forcing combined with low amplitude sea level fluctuations were likely major controls on cyclic sedimentation furthermore evolutional spectra of the two sections show that dominant milankovitch climatic forcings varied from short eccentricity obliquity to long precessional index during the course of sedimentation some spectral differences between the two sections suggest variations in depositional recording mechanisms of milankovitch climatic signals between platform interior and windward platform margin

parameters influencing porosity in sandstones a model for sandstone porosity prediction thirteen parameters were reviewed for their influence on porosity in sandstone core material during compaction in basins of average geothermal gradients the first order parameters are age time of burial detrital quartz content maximum depth of burial and sorting overpressure may slow the compaction process depending on onset of overpressure during burial approximately 2 primary porosity may be preserved for every 1 000 psi 6 9 mpa above hydrostatic pressure depth related temperature and median grain size were found to be only second order parameters on the basis of 428 cases function coefficients were calculated for the relationship between porosity and the first order parameters this function permits porosity prediction with a high degree of accuracy th equation is valid for sandstones older than 3 ma with little or no cement no leaching depth of burial in excess of 500 m and little or no shear stresses secondary porosity is excluded and estimations will therefore be conservative where leaching occurred due to interrelationship of compaction and cementation the porosity reduction by cement is usually only a small fraction of the total reduction the function can therefore also be applied to little cemented sandstones

north sea chalk diagenesis cementation of chalks and healing of fractures abstract petrological and geochemical studies have been carried out on the chalk matrix and the fracture filling calcite from several oil fields in the great ekofisk area of the central graben norwegian north sea in order to understand the mechanism of chalk cementation and fracture healing a hypothesis involving an episode of ascending hot water from the underlying permian evaporites has been postulated by some workers to explain the healing of similar fractures in the chalk reservoirs of the norwegian and danish north sea and the danish sub basin however the results presented in this paper including cathodoluminescence microscopy major and trace element geochemistry c o and sr isotope composition and modelling of heating and compactional water flow rates suggest that the ascending hot water hypothesis does not satisfactorily explain the required water budget rate of flow and the source of the calcite cement heat flow simulations demonstrate that a fluid flow rate of 7 5 cm yr is required by the ascending hot water hypothesis this is much higher than the flow rates calculated for this and other sedimentary basins an alternative model is proposed which can account for the observed 18o values of the fracture filling calcite 12 7 to 8 5 pdb the chalk matrix 7 4 to 4 1 pdb and the formation water 4 smow in terms of closed to semi closed system diagenesis in this model calcite from the chalk matrix dissolves congruently and cement precipitates with an isotope ratio governed by the geothermal gradient similarity in the cathodoluminescence trace element composition and carbon isotope values between the fracture filling calcite and the chalk matrix also suggests that the cements in the fractures and the chalk matrix are similar and were probably derived locally by pressure solution processes

packing states of ideal reservoir sands insights from simulation of porosity reduction by grain rearrangement abstract the question being tackled in this study is to which extent grain rearrangement contributes to porosity reduction in very well sorted quartzose sands ideal reservoir sands a numerical model rampage an acronym of random packing generator has been developed to address this long standing problem rampage represents a synthesis of various algorithms designed to simulate packing of equal sized spheres which have been used to represent ideal solids liquids and gases as well as natural porous media the results of rampage simulations compare favourably to theoretical and experimental data from various disciplines and allow delineation of the field of gravitationally stable random packing of equal sized spheres in the 2 d state space of porosity p versus mean coordination number n three end member packing states have been identified random loose packing rlp p 45 4 n 5 2 random close packing rcp p 36 3 n 7 0 and bridged random close packing bridged rcp p 39 5 n 5 2 unlike previously proposed models rampage can simulate the transition from rlp to any other point in the stability field the rlp state is fully consistent with wet packed porosities of synthetic sands with lognormal mass size distributions reported in the literature the much higher in situ porosity values reported for modern air packed sands are unlikely to be preserved at depth on geological time scales data on the relation between intergranular volume and burial depth indicate that the observed intergranular volume reduction in the upper 800 m of the sediment column corresponds to the evolution of rlp to rcp and is thus fully explained by non destructive grain rearrangement

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

modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions abstract mathematical calculations of thermal convection have been carried out using a porous three player model to simulate pore water flow in a sedimentary basin with layers of different permeabilities the calculated flow lines demonstrate that even very thin layers eg around salt domes and igneous or hydrothermal intrusions analyses of formation waters from sedimentary basins like the north sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition this is evidence suggesting that large scale convection or other types of mixing does not take place and positive salinity depth gradient may help to physically stabilize the formation water the fact that many of the shallow reservoir rocks from the north sea have formation water with low salinity and negative 18 o values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water in the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross sections diagenetic reactions will therefore normally be relatively isochemical during deeper burial