porosity trends in the skagerrak formation central graben united kingdom continental shelf the role of compaction and pore pressure history this paper describes reservoir properties in the triassic skagerrak formation in the central north sea this prolific sandstone reservoir often possesses anomalously high porosity for its depth of burial simple statistical analysis of wire line log derived porosity data is used to derive empirical trends as a function of both depth and vertical effective stress that show variations between neighboring hydrocarbon fields and between different parts of the basin porosity data from the josephine j ridge quadrant 30 of the united kingdom continental shelf ukcs show a marked degradation with depth but the porosities are significantly higher than in similarly deeply buried areas such as the puffin high to the west quadrant 29 or the forties montrose high to the north quadrant 22 to understand the porosity patterns better the data have been analyzed by plotting against vertical effective stress this allows a better comparison to be made between fields and wells within the high pressure high temperature hpht realm high pressure here refers to fluid pressures above 10 000 psi whereas high temperatures are above 300 f 149 c results show that porosity and fractional effective reservoir the proportion of net sandstone with a porosity greater than a predetermined cutoff decrease systematically with increasing vertical effective stress data from the different j ridge fields fall on a common compaction trend even though they are derived from structures with marked variations in present day depth of burial and static formation overpressure trends from the other areas of the central graben the puffin and forties montrose highs indicate more indurate reservoir states the observed porosity trends are independent of fluid type within the reservoir and the absolute magnitude of overpressure the main observed hydrocarbon effect is the result of buoyancy forces the analysis supports the contention that after accounting for facies related grain size variations compaction controls average reservoir properties differences in compaction state between areas are postulated to relate primarily to structurally controlled timing of overpressure development relative to burial and how these affect the resultant vertical effective stress history both the puffin and forties montrose highs are directly attached to the basin margins across stepped faults these marginal terraces were open to lateral fluid flow for longer probably because across fault seals were only established late in the burial history when higher temperatures promoted cementation and the destruction of permeability within fault cores as a result they developed overpressures in the last 5 10 m y or so and are largely normally compacted the j ridge horst block is hydrologically more isolated within the basin center by across fault juxtaposition seals here overpressure development appears to have started earlier possibly between 50 and 60 ma retarding compaction and allowing preservation of higher porosities compaction continues to present day driven by the large static vertical effective stress gradients in these deeply buried reservoirs the observed empirical trends offer a means of predicting average reservoir properties in deep untested exploration targets

regional circulation characteristics associated with a cold surge event over east asia during winter monex abstract a case study is presented of the cold surge over east asia during 9 13 december 1978 using nine vertical levels of winter monex data the surge event is manifested by a rapid meridional mass flow in the lower troposphere from the midlatitudes 30 n to the equator within three days during the period studied the lower tropospheric circulation dramatically changed in extent and intensity with the onset of the surge event the main divergent maximum began shifting from the south china sea to southeast china at the same time the upper tropospheric circulation correspondingly changed in a reversed order from that of the lower troposphere and a direct vertical coupling between flows in the low and high troposphere was observed the time averaged meridional mass circulation between 100 and 126 e reveals a two cell structure the southern cell is located between the northern south china sea and the equator and the northern cell between midlatitudes and the northern south china sea analysis of s

exceptional reservoir quality in hpht reservoir settings examples from the skagerrak formation of the heron cluster north sea uk abstract as the exploration of hydrocarbons moves into more complex and deeper basinal settings the need to understand the effect of high temperatures and high pressures on reservoir quality and rock properties becomes even more important the fluvial channel sandstone reservoirs of the skagerrak formation in the central north sea exhibit anomalously high porosities and permeabilities considering their depth of burial 3500 m below sea floor despite the complex depositional setting diagenetic history high overpressure and temperatures encountered in the skagerrak formation hydrocarbons are currently being produced the skagerrak formation reservoirs used in this study have encountered overpressures of 40 mpa and temperatures up to 185 c at present day maximum burial to identify the role played by the high pressure and high temperature encountered in the reservoir sandstones a multidisciplinary approach involving petrographic fluid inclusion and burial history modelling studies has been adopted our interpretation of the results is that the generation of shallow overpressure in these fields limited mechanical compaction and also played an important role in minimizing pressure solution in the chemical compaction regime as evidenced by reduced quartz cementation fluid inclusions found in the quartz overgrowths indicate late stage development with temperatures of formation in the range 130 to 170 c coincident with late stage deeper burial hydrocarbon emplacement occurred after quartz cementation and has had little to no effect on porosity preservation the formation of well developed authigenic chlorite 70 surface coating and to a lesser extent illite clay coats with burial had a positive effect on porosity preservation even though permeability was marginally reduced in the illite rich sandstones a schematic porosity and quartz cement evolution model has been developed which allows for pre drill prediction of reservoir quality in the heron cluster and provide valuable insights for other complex high pressure high temperature reservoirs

thin section and s e m textural criteria for the recognition of cement dissolution porosity in sandstones cement porosity relationships are described from the lower triassic sherwood sandstone group and the middle jurassic ravenscar group in the united kingdom calcite cemented sandstones display a variety of replacement textures with preferential replacement of grains and of overgrowth faces with high free surface energy dolomite and siderite cemented sandstones display similar textures but replacement is less specific and euhedral overgrowth surfaces are commonly embayed by carbonates examination of the more porous sandstones with the scanning electron microscope reveals a range of pitting and embayment textures in authigenic overgrowths and in detrital grains these range from small v shaped notches and pits through regular and irregular shaped embayments into large depressions these textures appear to be morphologically similar to the quartz surfaces seen in thin sections of carbonate cemented sandstones and are interpreted to have been formed by the dissolution of pore filling and grain replacive authigenic carbonates this is confirmed by examination of experimentally exhumed overgrowth surfaces from carbonate cemented sandstones these textures indicate that part of the intergranular porosity in these sediments is secondary in origin and has been generated by the dissolution of carbonate cements the identification of such textures may lead to a more confident interpretation of the nature of intergranular porosity in the subsurface

the stable isotopic composition of modern soil carbonate and its relationship to climate the oxygen isotopic composition of modern soil carbonate is well correlated with the isotopic composition of local meteoric water the carbon isotopic cycle for co 2 in soils can be described in terms of the proportion of biomass using the c 4 photosynthetic pathway and the co 2 respiration rate of the soil at low soil respiration rates significant atmospheric co 2 mixing can occur in general the carbon isotopic composition of soil carbonate is related to the proportion of c 4 biomass present in soil but soils that freeze to the depth of carbonate formation often have a significant atmospheric component this suggests that freezing of the soil solution should be considered as another important mechanism for soil carbonate formation because of these relationships the isotopic composition of soil carbonate may be a paleoclimatic and paleoecologic indicator in cases in which diagenetic alteration has not occurred

climate instability during the last interglacial period recorded in the grip ice core isotope and chemical analyses of the grip ice core from summit central greenland reveal that climate in greenland during the last interglacial period was characterized by a series of severe cold periods which began extremely rapidly and lasted from decades to centuries as the last interglacial seems to have been slightly warmer than the present one its unstable climate raises questions about the effects of future global warming

comparison of imbrie kipp transfer function and modern analog temperature estimates using sediment trap and core top foraminiferal faunas we evaluate the reliability of statistical estimates of sea surface temperature sst derived from planktonic foraminiferal faunas using the modem analog method and the imbrie kipp method global core top faunas provide a calibration data set while modem sediment trap faunas are used for validation linear regression of core top predicted sst against atlas sst generated slopes close to one for both methods however the imbrie kipp transfer function temperature estimates had an intercept 1 3oc warmer than modem analog estimates and 1 7oc warmer than recorded atlas sst the rms error for the core top data set using the modem analog method 1 5oc was smaller than that of the imbrie kipp method 1 9oc sst errors for the sediment trap faunas were not statistically different from those of the core top data set regardless of method developing imbrie kipp transfer functions for limited regions reduced the rms variability but introduced residual structure not present in the global imbrie kipp transfer function dissolution simulations with the sediment trap sample which generated the warmest sst residual for both methods suggests that the loss of delicate warm water foraminifera from midlatitude sediments may be the cause of this thermal error we conclude that 1 the fauna structure of sediment trap and core top assemblages are similar 2 both methods estimate sst reliably for modem foraminifera flux assemblages but the modem analog method exhibits less bias and 3 both methods are relatively robust to samples with low communality but sensitive to selective faunal dissolution

slag remains from the na slupi site prague czech republic evidence for early medieval non ferrous metal smelting archaeological investigation of the na slupi site in the central part of prague the capital of the czech republic and located in the vicinity of vysehrad castle has demonstrated the presence of early medieval 12th century large scale smelting of non ferrous metals this study is focused on the investigation of smelting remains pb rich red and black slags using a combination of geochemical and mineralogical methods in order to understand the technologies and processes used at this site glassy red slags are composed of high temperature ca k and pb rich silicates wollastonite larnite kalsilite melilite and pb bearing glass and are associated with less abundant litharge fragments red slag impregnated furnace walls composed of litharge massicot pbo and cu2pbo2 they indicate that the cupellation process in small furnaces rather than in crucibles open vessels was used for ag refining at the studied site the black slags found in much lower quantity at this site are composed of maghemite fayalite clinopyroxene melilite and glass and are s poor we believe that they correspond to waste products from the first stage of ag purification prior to final cupellation where slagging additives such as fe rich material silica sand or ashes could be added to remove impurities the pb isotopic composition of both types of slags is similar to the isotopic signature of pb zn ores from the vicinity of cracow poland and supports the hypothesis that pb was probably traded between prague and poland in the 12th century and used for refining precious metals in metallurgical workshops

mass transfer and problems of secondary porosity creation in deeply buried hydrocarbon reservoirs abstract secondary porosity created by the post depositional leaching of detrital grains and cements can be of great economic importance the term secondary porosity is only descriptive and does not give any information on whether the porosity of the rock has been enhanced in reality there are two end member volumetric outcomes to the leaching process firstly the mineral can be completely dissolved and its components transported out of the system in such an instance the porosity of the rock will be enhanced secondly it is possible that a mineral will decompose and by product minerals will be precipitated which will occupy approximately the same volume as the original mineral the latter case is more likely where the pore waters are close to equilibrium with the majority of the minerals of the rock this will occur at elevated temperatures and or where the pore waters have been in contact with the rock for a long time these conditions are commonly met during intermediate to deep burial many metastable minerals such as high temperature feldspars probably decompose in this way a consequence of the decomposition of such metastable minerals is that primary porosity will be swapped for secondary and that older and more deeply buried rocks will contain a higher proportion of secondary porosity the mass transport equation may be used to examine the physico chemical environment where enhanced porosities most likely occur during leaching by using a simple non equilibrium mass transport model based on the silica system it is possible to demonstrate the relative importance of flow rate reaction rates specific surfaces and temperature in controlling the areal extent of leaching wide leaching zones are related to high flow rates which in turn promote a high throughput of undersaturated fluid and dispersion leaching over a wide area is also favoured by low reaction rates which will occur at low temperatures although it is true to say that less of a given silicate mineral can be dissolved by an undersaturated pore water at low temperature than at high temperature it is probable that pore fluids from deeply buried sediments are more likely to be close to equilibrium with a given mineral than for instance meteoric water thus the most likely environment for the production of widespread enhanced porosity is in the shallow subsurface the lower flow rates and higher temperatures that are encountered during deep burial diagenesis will result in far narrower more intense leaching zones as a result providing sufficient reactive mineral is present any undersaturated pore water will approach equilibrium in a short distance in order to transport undersaturated pore waters appreciable distances in the deep subsurface it is necessary to minimize their interaction with the host rock this could occur if the pore fluid were to move through a fracture system where it would interact with a far smaller surface area of reactive mineral than if it moved through the pore network

provenance of early bronze age metal artefacts in western switzerland using elemental and lead isotopic compositions and their possible relation with copper minerals of the nearby valais abstract ten early bronze age bza1 2200 2000 bc copper artefacts from the central valais region from switzerland were studied for their elemental composition and lead isotope ratios in order to answer the archaeological question of a local copper supply a database for copper minerals across the valais switzerland has been established this database contains 69 data on lead isotope ratios as well as additional information on the minerals and geochemical associations for copper minerals from 38 locations in the valais comparisons of the artefacts were also made with data pertaining to minerals from various deposits from europe and anatolia taken from the literature the provenance of the materials is very diverse some of the data are compatible with the data from the copper mineral deposits of the valais region moreover three copper lunulae were identified as possibly tuscan which suggests contacts between italy and the valais region this pattern also establishes a multiplicity of provenances for the metal and cultural influences in the alpine environment of the rhone valley of switzerland at the beginning of the early bronze age

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

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

origin distribution and significance of carbonate cements in the albert formation reservoir sandstones new brunswick canada abstract carbonate cements in the fluvio lacustrine reservoir sandstones of the albert formation mainly occur in three phases pre compaction calcite post compaction calcite and ankerite the precompaction calcite cement 25 to 40 is characterised by bright luminescence high concentrations of fe mn and sr means of 1 56 wt 1 42 wt and 0 25 wt respectively slightly heavy 13c and 18o values mean 1 79 pdb mean 14 99 pdb respectively and its selective occurrence in lacustrine sandstones suggestive of cementation at shallow depths from lacustrine connate water dull luminescence variable concentrations of fe mn and sr variable but slightly heavy 13c and 18o values mean of 2 97 pdb and 11 05 pdb respectively of the post compaction calcite as well as its occurrence in alluvial fan sandstones exhibiting high pressure solution of detrital grains suggest cementation from pore waters modified by water rock interactions we suggest that the ankerite cement was formed from upwardly migrating formation water based on the following evidence i ankerite cement filling the secondary pores created in the deep subsurface ii an absence of unconformity during ankerite cementation precluding involvement of meteoric water iii ankerite cementation in brecciated veins synchronous with ankerite precipitation in host sandstones iv occurrences of significant volumes of ankerite 20 to 30 in the shales indicating that the fe and mg ions expelled during smectite to illite transformation reactions were consumed within the shales and v the heavy 13c and 18o isotope values of the ankerite cements means of 2 2090 pdb and 11 05 pdb respectively

glacial holocene biogenic sedimentation patterns in the south china sea productivity variations and surface water pco2 a bathymetric transect of cores in the south china sea extending from 4200 m to less than 1000 m water depth has been examined for glacial interglacial changes in carbonate and organic carbon sedimentation typical pacific carbonate cycles high carbonate content during glacials and low carbonate content during interglacials characterize cores from water depths deeper than 3500 m in contrast atlantic carbonate cycles low carbonate during glacials and high carbonate during interglacials are observed in cores from depths shallower than 3000 m as a result of increased dilution of carbonate by terrigenous material during glacial low stands of sea level glacial interglacial changes in the carbonate chemistry of south china sea intermediate and deep waters resulted in significant changes in the positions of the carbonate compensation depth ccd and the aragonite compensation depth acd during the last glacial the ccd and acd were at least 400 and 1200 m deeper respectively than at present organic carbon accumulation rates in the south china sea were approximately 2 times higher during the last glacial than the holocene carbon isotopic analyses and c n ratios of the organic matter indicate that only a small fraction of the increase in glacial organic carbon accumulation can be attributed to input of terrestrial carbon on the basis of this we conclude that surface water productivity in the south china sea was approximately 2 times higher during the last glacial maximum this is consistent with previous studies which have demonstrated that glacial productivity was higher in low to mid latitude regions of the atlantic and eastern pacific the deglacial decrease in organic carbon accumulation is accompanied by a decrease in 13corg using the relationship between 13corg and co2 aq developed by popp et al 1989 we estimate that surface water pco2 values in the south china sea during the last 25 000 years were very similar to atmospheric co2 concentrations

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

prehistoric earthquakes along the shanchiao fault taipei basin northern taiwan abstract stratigraphic relations recorded in boreholes drilled along the shanchiao fault a normal fault that bounds the western margin of metropolitan taipei reveal evidence of early mid holocene subsidence and active extension of the taipei basin paleoseismic data suggest that the maximum displacement for each subsidence event ranges from 2 3 to 4 5 m despite large uncertainties the large offsets imply earthquake magnitudes that range in size from 6 9 to 7 1 our results suggest that large magnitude earthquakes along the shanchiao fault are one of the major seismic hazards in the metropolitan taipei area

teleconnections between the subtropical monsoons and high latitude climates during the last deglaciation the major deglacial intensification of the southwest monsoon occurred at 11 450 150 calendar years before present synchronous with a major climate transition as recorded in greenland ice an earlier event of monsoon intensification at 16 000 150 calendar years before present occurred at the end of heinrich layer 1 in the atlantic and parallels the initial rise in global atmospheric methane concentrations and the first abrupt climate changes in the antarctic thus the evolution of the monsoonal and high latitude climates show teleconnections but hemispheric asymmetries superimposed on abrupt events the monsoonal climate shows high frequency variability of 1785 1450 and 1150 year oscillations and abrupt climate change seems to occur when at least two of these oscillations are in phase

iceberg discharges into the north atlantic on millennial time scales during the last glaciation high resolution studies of north atlantic deep sea cores demonstrate that prominent increases in iceberg calving recurred at intervals of 2000 to 3000 years much more frequently than the 7000 to 10 000 year pacing of massive ice discharges associated with heinrich events the calving cycles correlate with warm cold oscillations called dansgaard oeschger events in greenland ice cores each cycle records synchronous discharges of ice from different sources and the cycles are decoupled from sea surface temperatures these findings point to a mechanism operating within the atmosphere that caused rapid oscillations in air temperatures above greenland and in calving from more than one ice sheet

sandstone reservoir quality prediction the state of the art joanna ajdukiewicz joined exxon production research company in 1980 she was reservoir quality assessment and prediction team lead there from 1991 to 1995 and at imperial oil research centre in calgary from 1995 to 1997 subsequently she has worked a variety of exploration company assignments in the north sea gulf of mexico and middle east her current interests are in predicting the distribution of early diagenetic controls on deep reservoir quality rob lander develops diagenetic models for geocosm llc he obtained his ph d in geology from the university of illinois in 1991 was a research geologist at exxon production research from 1991 to 1993 and worked for rogaland research and geologica as from 1993 to 2000 he is also a research fellow at the bureau of economic geology to guess is cheap to guess wrongly is expensive chinese proverb reservoir quality predictive models will be a useful element of risk analysis until remote sensing tools are invented that accurately measure effective porosity and permeability ahead of the bit this issue of the aapg bulletin highlights recent advances in a new generation of reservoir quality models that have successfully predicted porosity and permeability in diverse siliclastic reservoirs under many different burial conditions most previous attempts at predrill reservoir quality prediction have relied on empirical correlations or on first principle geochemical simulations that incorporate laboratory derived input parameters wood and byrnes 1994 the new reservoir quality models differ from previous approaches in that although incorporating theory inspired algorithms they include terms with values that are explicitly designed to be calibrated by and tested against data sets of high quality petrographic analyses that are linked to thermal and effective stress histories petrographic observations therefore provide essential constraints in these models on the types timing and rates of key geologic processes affecting sandstone pore systems this approach avoids the pitfalls inherent

reservoir quality and rock properties modeling triassic and jurassic sandstones greater shearwater area uk central north sea abstract the complex burial and diagenetic histories of the jurassic fulmar and triassic skagerrak sandstones in the uk central north sea present significant challenges with regard to reservoir quality and rock property prediction commercial reservoir quality is retained despite deep burial and associated high temperatures and pressures shallow marine fulmar sands are normally compacted mean igv 26 3 yet have porosities of 21 33 porosity was preserved through inhibition of quartz cementation by clay and microquartz coatings and enhanced by dissolution of framework grains 5 skagerrak fluvial sands are more compacted mean igv 23 2 exhibit minor feldspar dissolution we modeled reservoir quality evolution using the forward diagenetic model touchstone which simulates porosity loss due to compaction and quartz cementation quantitative petrographic analyses and burial history data were used to calibrate touchstone model parameters the results were applied to deeper prospects for pre drill prediction of porosity and permeability in parallel petrophysical data were used to characterize the elastic properties of the sandstones to provide a basis for quantitative seismic forward modeling experimental data and core calibrated petrophysical results reflecting variable in situ fluids and saturations were used to build an elastic properties model the model is robust and was used to generate fluid filled sandstone properties incorporating touchstone results for prospect specific seismic attribute modeling well results from exploration wells are in good agreement with pre drill touchstone and elastic properties model predictions

origin of quartz cements in some sandstones from the jurassic of the inner moray firth uk the extent of quartz cementation in shallow marine sandstones of the brora arenaceous formation oxfordian is closely related to the occurrence and abundance of rhaxella perforata sponge spicules three cement morphologies are identified chalcedonic quartz microquartz and mesoquartz chalcedonic quartz forms matrix supported cements which preserve moulds of rhaxella spicules chalcedonic quartz crystals have inequant development of crystal faces on average 0 1 m in diameter and are the first formed cement and reveal homogeneous dark grey tones on the sem cl bei microquartz forms 5 10 m diameter crystals which commonly grow on chalcedonic quartz substrates and show various grey tones under sem cl bei mesoquartz crystals grow in optical continuity with their host grains have 20 m a axial diameter crystals and exhibit distinctly zoned luminescence although no opaline silica is preserved the quartz cement is interpreted to have formed from an opaline precursor detrital quartz has an average 18o composition of 12 2 and mesoquartz syntaxial overgrowth has an average 18o composition of 20 0 estimates of the 18o compositions of microquartz and chalcedonic quartz are complicated by the problem of isolating the two textural types mixtures of the two give consistently higher 18o compositions than mesoquartz the higher estimate being 39 2 from oxygen isotope data the formation of quartz microquartz and chalcedonic quartz is interpreted to have taken place between 35 and 71 c in marine derived pore waters organic and inorganic maturation data constrain the upper temperature limit to less than 60 c

shell fluxes of solution resistant planktonic foraminifers as a proxy for mixed layer depth 1 shell fluxes of solution resistant planktonic foraminifera g scitula and n dutertrei collected by moored sediment traps are examined as a possible tool to reconstruct the surface water convection in the past subtropical western north pacific time series observation reveals that shell fluxes of these two species fluctuate in an opposite sense when surface water is warmed and well stratified n dutertrei fluxes reach a maximum in contrast g scitula increases in response to the deepening of the mixed layer to amplify the response of shell fluxes to climatological variables the shell flux ratio of g scitula to n dutertrei fg sci n dut is introduced the time series fg sci n dut pattern is similar to that of the monsoon index moi representing the strength of the east asian winter monsoon the fg sci n dut is also well correlated with the mixed layer depth these correlations allow reconstruction of the ancient moi and mld from the faunal ratio

correlation between climate events in the north atlantic and china during the last glaciation episodes of massive iceberg release heinrich events 1 3 into the north atlantic ocean during the last glaciation were associated with recurring episodes of unusually cold north atlantic surface water bond cycles 4 and cold air temperatures over greenland dansgaard oeschger events 5 6 four of the youngest of these cold events have also been reported in climate records from sites outside the north atlantic region7 but until now the entire suite has been identified only in north atlantic marine sediments greenland ice core records and tentatively in french lake sediments8 here we examine grain size data from chinese loess and intercalated accretionary palaeosols of last glacial age for evidence of similar climate signals remote from the north atlantic region we see grain size maxima with ages that match those of the last six heinrich events which we interpret as an indication of the changing strength of the east asian winter monsoon which largely controls the transport and deposition of central asian aeolian dust thus it seems that these heinrich events have left their signature in the chinese loess record this is consistent with simulations of the glacial climate9 which imply that the climates of the north atlantic and china were linked by the effect of westerly winds

origin of calcite cements and their impact on reservoir heterogeneity in the triassic yanchang formation ordos basin china a combined petrological and geochemical study abstract calcite cement is a typical product of fluid rock interaction in petroleum reservoirs and its migration has a major impact on reservoir heterogeneity a case study of a closed diagenetic system was carried out on the upper triassic yanchang formation of ordos basin china the most successful exploration region for tight sandstone oil in china the investigated sandstones are mainly lithic arkoses q31f42l27 three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality early calcite completely filling intergranular volumes late calcite filling pores after chlorite coating distributed in oil layers or locally replacing feldspar and crystal stock calcite typically distributed in dry layers 13c values of the sandstones are in the range 9 69 to 2 05 indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid providing sufficient ca2 mn2 fe2 and mg2 for calcite precipitation through interaction with feldspar and other silicates such as dissolution and replacement together with transformation of smectite to illite manganese was continuously enriched in calcite during burial diagenesis driven by compaction of interbedded mudstone and fluid overpressure calcite migrated widely into calcite cemented zones cczs where fluid rock interaction was strong such as in the margins of sandstone bodies intrastratal sandstones with original high permeability zones and thin sandstones thereby forming an important organic co2 sink potential high quality reservoirs are distributed mainly in the central part of the closed system restricted by calcite cemented zones and are an exploration target our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic co2 source sink system controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid rock interaction

on the structural evolution of the central trough in the norwegian and danish sectors of the north sea abstract the central trough of the north sea is not a simple rift graben it is an elongated area of regional subsidence which was initiated in mid cretaceous times and continued to subside through to the late tertiary its form is not representative of pre mid cretaceous tectonics in late permian times the north sea was divided into a northern and southern zechstein basin by the e w trending mid north sea ringkobing fyn high the latter was dissected by a narrow graben trending nnw through the tail end graben and the sogne basin the feda graben was a minor basin on the northern flank of the mid north sea high at this time this structural configuration persisted until end middle jurassic times when a new wnw trend separated the tail end graben from the sogne basin right lateral wrench movement on this new trend caused excessive subsudence in the tail end and feda grabens while the sogne basin became inactive upper jurassic subsidence trends continued during the early cretaceous causing the deposition of large thicknesses of sediments in local areas along the trend from mid cretaceous times the regional subsidence of the central trough was dominant but significant structural inversions occurred in those areas of maximum early cretaceous and late jurassic subsidence

the role of fluid pressure and diagenetic cements for porosity preservation in triassic fluvial reservoirs of the central graben north sea anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the triassic skagerrak formation in the central north sea at burial depths greater than 3200 m 10 499 ft from which hydrocarbons are currently being produced the aim of our study was to improve understanding of sandstone diagenesis in the skagerrak formation to help predict whether the facies with high porosity may be found at even greater depths the skagerrak sandstones comprise fine to medium grained arkosic to lithic arkosic arenites we have used scanning electron microscopy petrographic analysis pressure history modeling and core analysis to assess the timing of growth and origin of mineral cements with generation and the impact of high fluid pressure on reservoir quality our interpretation is that the anomalously high porosities in the skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the late triassic onward in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats increasing salinity of pore fluids during burial diagenesis led to pore filling halite cements in sustained phreatic conditions the halite pore filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases fluid flow associated with the migration of hydrocarbons during the neogene is inferred to have dissolved the halite locally dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35 at current production depths

on the structure and origin of major glaciation cycles 1 linear responses to milankovitch forcing time series of ocean properties provide a measure of global ice volume and monitor key features of the wind driven and density driven circulations over the past 400 000 years cycles with periods near 23 000 41 000 and 100 000 years dominate this climatic narrative when the narrative is examined in a geographic array of time series the phase of each climatic oscillation is seen to progress through the system in essentially the same geographic sequence in all three cycles we argue that the 23 000 and 41 000 year cycles of glaciation are continuous linear responses to orbitally driven changes in the arctic radiation budget and we use the phase progression in each climatic cycle to identify the main pathways along which the initial local responses to radiation are propagated by the atmosphere and ocean early in this progression deep waters of the southern ocean appear to act as a carbon trap to stimulate new observations and modeling efforts we offer a process model that gives a synoptic view of climate at the four end member states needed to describe the system s evolution and we propose a dynamic system model that explains the phase progression along causal pathways by specifying inertial constants in a chain of four subsystems solutions to problems involving systems of such complexity are not born full grown like athena from the head of zeus rather they evolve slowly in stages each of which requires a pause to examine data at great lengths in order to guarantee a sure footing and to properly choose the next step victor p starr

archaeological reconstruction of medieval lead production implications for ancient metal provenance studies and paleopollution tracing by pb isotopes abstract the identification of metal provenance is often based on chemical and pb isotope analyses of materials from the operating chain mainly ores and metallic artefacts such analyses however have their limits some studies are unable to trace metallic artefacts or ingots to their ore sources even in well constrained archaeological contexts possible reasons for this difficulty are to be found among a variety of limiting factors i problems of ore signatures ii mixing of different ores alloys iii the use of additives during the metallurgical process iv metal recycling and v possible pb isotopic fractionation during metal production this paper focuses on the issue of pb isotope fractionation during smelting to address the issue of metal provenance through an experimental reconstruction of argentiferous pb production in the medieval period an attempt was made to better understand and interpret the pb isotopic composition of ore smelting products it is shown that the measured differences outside the total external uncertainties of 0 005 2 sd for 206 pb 204 pb ratios in pb signatures measured between ores slag and smoke are not due to pb mass fractionation processes but to 1 ore heterogeneity 206 pb 204 pb slag ores 0 066 and 2 the use of additives during the metallurgical process 206 pb 204 pb slag ores 0 083 even if these differences are due to causes 1 and or 2 smoke from the ore reduction appears to reflect the ore mining area without a significant disturbance of its pb signature for all the isotopic ratios 206 pb 204 pb smokes ores 0 026 thus because the isotopic heterogeneity of the mining district and additives is averaged in slags slag appears as the most relevant product to identify ancient metal provenance whereas aiming at identifying a given mine seems beyond the possibilities provided by the method searching for the mining district through analysis of the smelting workshop materials should provide a more appropriate approach in cases where no archaeological evidence of ancient mining is available furthermore smoke pb isotopic composition does not seem to be significantly affected by the metallurgical process paleopollution recorded in peat deposits could help to detect ancient mining production and workshops integrated collaboration between mining archaeologists and geochemists appears crucial to achieve this goal

climate variations in europe over the past 140 kyr deduced from rock magnetism rapid shifts in climate during the last glacial are now well documented particularly from the oxygen isotope records of the two greenland ice cores grip1 2 and gisp23 in the grip record1 2 these climate events are also seen during the preceding eemian interglacial which may be an analogue for the future climate warmed by the greenhouse effect but these shifts are not found in the eemian section of the gisp2 core3 casting doubt on whether the rapid shifts in the grip oxygen isotope record really do represent a climate signal here we present magnetic susceptibility pollen and organic carbon records from maar lake deposits in the massif central france these data provide an independent record of past climate and we find that they correlate well with the ice core records during the last glacial during the eemian two rapid cooling events seen in our record also correlate with those seen in the grip ice core supporting the idea that rapid climate change did occur in the eemian interglacial and demonstrating that it extended to continental europe

high resolution climate records from the north atlantic during the last interglacial the two deep ice cores recovered by the grip1 and gisp22 projects at summit greenland agree in detail over the past 100 000 years3 and demonstrate dramatic climate variability in the north atlantic region during the last glacial before the current period of holocene stability this glacial climate instability has subsequently been documented in the marine sedimentary record of surface ocean conditions in the north atlantic4 before 100 kyr ago the two ice core records are discrepant however casting doubt on whether the oxygen isotope fluctuations during the last interglacial eemian seen in the grip core1 5 represent a true climate signal here we present high resolution records of foraminiferal assemblages and ice rafted detritus from two north atlantic cores for the interval 65 kyr to 135 kyr ago extending the surface ocean record back to the eemian the correlation between our records and the greenland ice core records is good throughout the period in which the two ice cores agree suggesting a regionally coherent climate response during the eemian our marine records show a more stable climate than that implied by the grip ice core suggesting that localized phenomena may be responsible for the variability in the latter record during the eemian

comparison of oxygen isotope records from the gisp2 and grip greenland ice cores recent results1 2 from the greenland ice core project grip summit ice core suggest that the climate in greenland has been remarkably stable during the holocene but was extremely unstable for the time period represented by the rest of the core spanning the last two glaciations and the intervening eemian inter glacial here we present the complete oxygen isotope record for the greenland ice sheet project 2 gisp2 core drilled 28 km west of the grip core we observe large rapid climate fluctuations throughout the last glacial period which closely match those reported for the grip core however in the bottom 10 of the cores spanning the eemian interglacial and the previous glacia tion there are significant differences between the two records it is possible that ice flow may have altered the chronological sequences of the stratigraphy for the bottom part of one or both of the cores considerable further work will be necessary to evaluate the likelihood of this and the extent to which it will still be possible to extract meaningful climate information from the lowest sections of the cores

geochemical constraints on the origins of calcite cements and their impacts on reservoir heterogeneities a case study on tight oil sandstones of the upper triassic yanchang formation southwestern ordos basin china calcite cementation has been identified as an active process in the upper triassic yanchang formation throughout its burial history and as a major diagenetic factor causing strong reservoir heterogeneities the origins of calcite cements and their relevance to reservoir heterogeneities were investigated using a suite of petrographic and geochemical methods including optical microscopy with fluorescence and cathodoluminescence scanning and backscattered electron microscopy with energy dispersive spectrometry x ray diffraction x ray fluorescence electron probe microanalysis quantitative evaluation of minerals by scanning electron microscopy fluid inclusion analysis and carbon and oxygen stable isotope analyses the sandstones are compositionally immature with relatively high amounts of volcanic rock fragments the two generations of calcite cements are ca i and ca ii the ca i calcites are distributed along the interface of sandstone and mudstone units and were formed during the late triassic to early jurassic at formation temperatures of approximately 90 c the ca ii calcite mainly developed in the lower part of the fining upward sandstone units and was formed in the late jurassic at higher temperatures of approximately 110 c the origins of calcite cements were constrained by geochemical and isotope measurements fluid inclusion homogenization temperature and in situ element analysis the ca i calcite cement originated from dissolution of the lacustrine depositional carbonates in the interbedded mudstones and reprecipitation in the adjacent sandstones the ca ii calcite was mainly related to organic matter decarboxylation with ca2 having been provided internally by volcanic fragment alteration and plagioclase dissolution calcite cementation had caused strong reservoir heterogeneities in the yanchang formation tight sandstones the ca i calcite cementation destroyed reservoir properties along the interface of sandstones and mudstones the lower parts of the fining upward sandstone units were tightly cemented by ca ii calcite although they originally had high porosity and permeability the middle upper parts of the fining upward sandstone units contain less calcite cements and thus have better preserved reservoir pores because of oil emplacement inhibiting the calcite cementation processes

mesozoic dissolution tectonics on the west central shelf uk central north sea abstract 3 d seismic mapping of the upper jurassic kimmeridge clay formation on the west central shelf in the central north sea reveals a complex fault array which is constrained by seismic interpretation and well control to be of late jurassic early cretaceous age fault shapes in plan view range from linear to circular linear fault lengths are 200 300 m to 5 km the strongly curved and circular faults range in diameter from 100 1000 m fault trends are apparently random and display no correlation in location or trend with basement sub zechstein structures there is however a strong link between this fault pattern and the structure of the top zechstein top salt surface linear faults occur at the edges of elongate salt walls and the circular faults lie directly above structures which have been interpreted here as tall steep sided salt chimneys the salt chimneys are present only in the thick elongate minibasins of triassic sediment which lie between the salt walls it is argued that salt dissolution controls the timing location orientation and shape of the late jurassic early cretaceous faults a model is provided to account for the development of both salt walls and chimneys we suggest that early triassic karstification of the zechstein evaporites led to development of an array of circular collapse features during the ensuing episode of triassic halokinesis which led to minibasin subsidence and salt wall growth salt passively intruded the circular collapse features within the subsiding minibasins to form narrow salt chimneys the resulting array of salt walls and chimneys was subject to dissolution during subsequent subaerial exposure and the late jurassic marine transgression of the basin creating the observed fault array prior to sealing of the salt from circulating groundwater by compaction of the upper jurassic and lower cretaceous shales which blanket the area

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

millennial scale dynamics of the winter cold tongue in the southern south china sea over the past 26 ka and the east asian winter monsoon millennial scale variations of the east asian winter monsoon eawm remain elusive due to sparse and controversial reconstructions by compiling a variety of alkenone based sea surface temperature sst estimates we find that the west east sst gradient in the southern south china sea scs well documents the temporal dynamics of the winter cold tongue off the southern vietnam and by inference variations in the eawm intensity over the past 26 ka our results reveal that the winter cold tongue ssts were significantly colder during heinrich event 1 and the younger dryas event resulting in an increased west east sst gradient in the southern scs due to a strengthened eawm within dating uncertainties an intensified eawm during cold stadials was coeval with the shutdown or a reduction in strength of the atlantic meridional overturning circulation amoc exhibiting a strong linkage between the amoc and the eawm system the west east sst gradient also indicates an enhanced eawm during the early holocene which may be induced by postglacial ice sheet dynamics and a strong seasonal contrast in solar insolation our findings suggest that the eawm was probably modulated by a complex interplay between the amoc solar insolation and ice sheet dynamics on sub orbital time scales

brief interstadial events in the santa barbara basin ne pacific during the past 60 kyr the instability of the northern hemisphere glacial climate over the past 100 kyr has been revealed by at least 20 brief warm interstadial episodes called dansgaard oeschger events recorded in greenland ice cores1 3 and in north atlantic sedimentary records4 5 a few of these events have been recognized elsewhere6 8 here we describe a record of ocean oxygenation and circulation from the santa barbara basin in the northeast pacific ocean which correlates well with the greenland ice core records we see 19 of the 20 dansgaard oeschger events in the form of laminated sediments deposited under anoxic conditions and we can correlate at least 16 of these with the 17 ice core interstadials of the past 60 kyr thus these short term events were not restricted to the north atlantic region the events had substantial ecological and oceanographic effects in the santa barbara basin including changes in benthic faunal populations and in the age and composition of bottom waters similar ventilation changes have been seen in the gulf of california9 10 suggesting that these changes may have been widespread and synchronous along the northeast pacific margin these results suggest sensitivity of broad areas of the ocean atmosphere cryosphere system to short term climate change

diagenesis and origin of calcite cement in the flemish pass basin sandstone reservoir upper jurassic implications for porosity development abstract the flemish pass basin is a deep water basin located offshore on the continental passive margin of the grand banks eastern newfoundland which is currently a hydrocarbon exploration target the current study investigates the petrographic characteristics and origin of carbonate cements in the ti 3 member a primary clastic reservoir interval of the bodhran formation upper jurassic in the flemish pass basin the ti 3 sandstones with average q 86 0 f 3 1 r 10 9 contain various diagenetic minerals including calcite pyrite quartz overgrowth dolomite and siderite based on the volume of calcite cement the investigated sandstones can be classified into 1 calcite cemented intervals 20 calcite and 2 poorly calcite cemented intervals porous petrographic analysis shows that the dominant cement is intergranular poikilotopic 300 500 m calcite which stared to form extensively at early diagenesis the precipitation of calcite occured after feldspar leaching and was followed by corrosion of quartz grains intergranular calcite cement hosts all liquid inclusions mainly in the crystal core but rare primary two phase liquid and vapor fluid inclusions in the rims with mean homogenization temperature t h of 70 2 4 9 c and salinity estimates of 8 8 1 2 eq wt nacl the mean 18 o and 13 c isotopic compositions of the intergranular calcite are 8 3 1 2 vpdb and 3 0 1 3 vpdb respectively whereas fracture filling calcite has more depleted 18 o but similar 13 c values the shale normalized rare earth element ree sn patterns of calcite are generally parallel and exhibit slightly negative ce anomalies and positive eu anomalies fluid inclusion gas ratios co 2 ch 4 and n 2 ar of calcite cement further confirms that diagenetic fluids originated from modified seawater combined evidence from petrographic microthermometric and geochemical analyses suggest that 1 the intergranular calcite cement precipitated from diagenetic fluids of mixed marine and meteoric riverine waters in suboxic conditions 2 the cement was sourced from the oxidation of organic matters and the dissolution of biogenic marine carbonates within sandstone beds or adjacent silty mudstones and 3 the late phases of the intergranular and fracture filling calcite cements were deposited from hot circulated basinal fluids calcite cementation acts as a main controlling factor on the reservoir quality in the flemish pass reservoir sandstones over 75 of initial porosity was lost due to the early calcite cementation the development of secondary porosity mostly enlarged moldic pores and throats by later calcite dissolution due to maturation of organic matters e g hydrocarbon and coals was the key process in improving the reservoir quality

the influence of calcite dissolution on reservoir porosity in miocene sandstones picaroon field offshore texas gulf coast abstract middle miocene sandstones occur in picaroon field corsair trend offshore texas gulf coast at depths of approximately 4 9 to 5 2 km 16 17 000 ft these deltaic sandstones contain evidence of the following sequence of diagenetic events 1 formation of chlorite coatings on detrital grains 2 partial dissolution of detrital feldspar 3 quartz cementation 4 calcite cementation 5 dissolution of calcite cement and grains 6 ankerite cementation the reservoir quality of the sandstones is largely a function of porosity enhancement by calcite dissolution calcite cement was emplaced at depths of approximately 1 8 to 2 6 kin the calcite has 87 86sr values of 0 7083 0 7086 eliminating coeval 15 myr seawater and marine carbonate 0 70873 0 70885 as the primary source of strontium the 87 86sr composition of calcite cement implies that mass transfer of calcite from older marine sources to younger sediments has occurred fluid inclusion measurements indicate that ankerite cement formed at minimum temperatures of 120 188 c 3 0 5 1 km oxygen isotope modeling predicts that at these depths shales would expel waters with 18osmow of 5 to 9 during smectite illite conversion ankerite 18opdb 7 8 would be in isotopic equilibrium with these waters at temperatures similar to those derived from fluid inclusions ankerite cements have relatively radiogenic 87 86sr ratios 0 7097 which are consistent with their formation from shale derived fluids calcite dissolution occurs between the precipitation of calcite and ankerite it is therefore concluded that calcite cement dissolution occurred at burial depths of 2 6 to 3 0 km 107 120 c

a late quaternary planktonic foraminifer faunal record of rapid climatic changes from the south china sea abstract a high resolution planktonic foraminifer record from a core recovered from the south china sea scs sonne 17938 2 19 47 2 n 117 32 3e 2840 m t 250 1000 years shows rapid millennial scale changes in the western pacific marginal sea climate during the last 30 000 years the scs is the largest western pacific marginal sea off the southeast asian continent the area today dominated by seasonal monsoon changes quantitative analyses of planktonic foraminifer faunal abundance data from the core indicate large downcore variations in the relative abundances of the dominant taxa since about 30 000 years ago in the isotope stage 3 further analyses indicate that the abundance of g inflata a good indicator species for cold sst 13 19 c and deep mld 100 125 m waters shows abrupt shifts during stages 2 and 3 the abundance record of g inflata tends to be punctuated by quasi periodic short intervals 2000 3000 yrs where its abundance reaches 15 or greater superimposed on generally low 5 10 background values this pattern suggests an instability of surface ocean conditions of the scs during the past 30 000 years the abrupt abundance changes of g inflata correlate well with similar climatic changes observed from a gisp2 ice core 18o and north atlantic core dsdp 609 n pachyderma s and lithic grain abundances during heinrich events these results suggest that the millennial scale variability of climate is not peculiar to the atlantic region apparently the rapid scs climatic changes during heinrich events are driven by effective mechanisms of particularly the effects of shifts in the latitudinal position of the siberia high pressure system

copper production at baratti populonia southern tuscany in the early etruscan period 9th 8th centuries bc abstract populonia is commonly regarded as one of the most important iron working centres in europe in the 1st millennium bc however multiple layers of copper slags have been recently discovered at the bottom of the slag deposit occurring along the shoreline of the underlying baratti gulf copper slag layers have been radiocarbon dated to the 9th 8th centuries bc i e to an early metallurgical stage predating the later large scale iron production the overall features of copper slags indicate a single step smelting process of copper rich ores containing variable amounts of zn and pb chemical and textural features of copper slags indicate an optimised high efficiency metallurgical process

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

the role played by carbonate cementation in controlling reservoir quality of the triassic skagerrak formation norway anomalously high porosities up to 30 at burial depth of 3000 m along with varying amounts and types of carbonate cements occur in the fluvial channel sandstone facies of the triassic skagerrak formation central graben norway however porosities of the skagerrak formation are lower in the norwegian sector than in the uk sector in this study petrographic analysis core examination scanning electron microscopy elemental mapping carbon and oxygen isotope fluid inclusion and microgeometry analysis are performed to determine the diagenesis and direct influence on reservoir quality with particular focus on the role played by carbonate cementation the sandstones are mainly fine grained lithic arkosic to sub arkosic arenites and display a wide range of intergranular volumes 2 3 43 7 with an average of 23 6 porosity loss is mainly due to compaction av 26 6 with minor contribution from cementation av 12 1 the carbonate cements are patchy in distribution from trace to 20 7 and appear as various types e g calcretes i e calcareous concreted gravels poikilitic sparite and sparry ferroan dolomite and euhedral or and aggregated ankerite ferroan dolomite crystals this study highlights the association of carbonate precipitation with the remobilisation of carbonate from intra skagerrak calcretes during early burial stage i e 500 m during deeper burial compaction is inhibited by carbonate cements resulting high intergranular volume of up to 32 and 29 for fine and medium grained sandstones respectively carbonate cement dissolution probably results from both meteoric water flow with co2 during shallow burial and organic co2 and carboxylic acid during deep burial the maximum intergranular volume enhanced by dissolution of early carbonate cements is calculated to 8 and 5 for fine and medium grained sandstones respectively compaction continues to exert influence after dissolution of carbonate cements which results in a loss of 6 intergranular volume for fine and medium grained sandstones reservoir quality of the norwegian sector is poorer than that of the uk sector due to a lower coverage of clay mineral coats e g chlorite later and deeper onset of pore fluid overpressure lower solubility of carbonate compared to halite and a higher matrix content

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

hemipelagic sediment accumulation rates in the south china sea related to late quaternary sea level changes sediments of 13 piston cores from opposite continental slopes of the south china sea off southern china and sabah northern borneo were analyzed by sedimentological methods and dated by oxygen isotope stratigraphy sediments mostly consist of hemipelagic clay with 20 carbonate off sabah and 40 off china we calculated terrigenous and carbonate accumulation rates for up to 11 time slices from the holocene to oxygen isotope stage 6 terrigenous accumulation rates generally increase with water depth and reach a maximum at the middle slope off sabah and at the lower continental slope off china during glacial and interglacial times this distribution pattern did not markedly change despite an increase of accumulation rates for glacial periods by a factor of 2 to 5 compared to interglacial periods rates are negatively correlated with positions of sea level which controls the partition of fluviatile terrigenous material for deposition on shelf slope and abyssal plain carbonate accumulation rates are higher off china by a factor of 2 compared to sabah probably indicating higher calcareous plankton productivity

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

molecular stratigraphy of cores off northwest africa sea surface temperature history over the last 80 ka the molecular stratigraphy of biogeochemical oceanic flux study core 31k 19 n 20 10 w and ocean drilling program hole 658c 20 45 n 18 35 w has been studied for c37 alkenone abundances over the past 80 ka at high resolution circa 200 500 years the derived uk37 sea surface temperature record for both cores shows a range of temperatures from about 18 c during the last glacial to 21 5 c during the early holocene both records also reveal changes in sea surface temperature as much as 2 4 c over a few hundred years which correlate well with similar abrupt climatic changes observed in cores from elsewhere in the ne atlantic associated with heinrich events our results indicate that meltwater produced by these ice rafting events was transmitted southward by the canary current where it had considerable impact on sea surface temperatures in the subtropical eastern atlantic

regional distribution and geometry of salt diapirs and supra zechstein group faults in the western and central north sea abstract regional mapping of major faults above the zechstein group salt in the western and central parts of the north sea basin has shown that there exist at least three discrete salt tectonic zones on the platform west of the central graben there is zone 1 an area of extensional faulting above the salt roughly parallel to the margin of the central graben c 60 km wide and continuous along strike for more than 180 km within zone 1 extension c 6 has created arcuate and en echelon listric growth faults which sole within the salt layer beneath and bound blocks typically 3 7 km wide 2 3 km thick and 7 10 km long zone 2 occurs in the central graben area where there is considerably less faulting above the zechstein group salt and a much greater thickness of salt tall and narrow salt diapirs occur exlusively in the three deepest parts of the central graben and some penetrate high into the cenozoic section between zone 1 and zone 2 there is a third area zone 3 characterised by having relatively few supra zechstein faults and no major salt diapirism zone 1 formed mainly due to gravity driven break up of the supra zechstein sedimentary wedge diapirism in zone 2 resulted from increased relative bouyancy of salt at greater depths and a relatively greater supply of salt zone 3 represents a relatively stable region with possibly some movement of salt and supra salt sediments down the regional slope

late quaternary east asian monsoon variability in the south china sea evidence from planktonic foraminifera faunal and hydrographic gradient records the surface hydrographic conditions of the south china sea scs are influenced strongly by fluctuations of the east asian monsoon eam long term fluctuations of the eam have been previously investigated through analysis of several proxy records obtained from scs sediment cores however in this marginal sea interpretation of these records is complicated by ice volume sea level effects here we have extracted east west hydrographic gradients from two images core records in the scs which are proposed to indicate monsoon variability this strategy minimizes the effect of ice volume sea level on the monsoon signal highresolution records of planktonic foraminifer faunal assemblages and sea surface temperature sst of the last 135 kyr have been generated for two cores taken off the vietnam margin md012394 in the western scs and near palawan island md972142 in the southeastern scs these core sites represent contrasting hydrographic conditions in modern monsoon climates where the winter eam drives cold and strong mixing conditions in the western scs but maintains warm and deep thermocline conditions in the southeastern scs a planktonic foraminifera hydrographic index pfhi is established based on the abundance ratios of warm thermocline and cold mixing species assemblages and sst estimates from a modern analogue technique mat the records of the east west gradients for pfhi pfhi and for sst sst between the two cores md012394 and md972142 show a dominance of 23 kyr precession cycles in the precession cycles the pfhi and sst increased at the december 21 perihelion which is associated with seasons of minimum northern hemisphere winter and maximum southern hemisphere summer insolation cross spectral analysis between pfhi sst and the precession cycles indicates coherent and in phase relationships considering that the december 21 perihelion of the cycle of precession accorded with the time of maximum summer insolation in the southern hemisphere we suggest that the scs variability of hydrographic gradients for the last 135 kyr has been dominated by winter monsoon dynamics in response to geographic forcing controlled by the strength of the australian summer monsoon

spatial distribution and controlling factors of planktonic foraminifera in the modern western pacific abstract studies of the spatial distribution of living planktonic foraminifera in modern water columns are still lacking in the western pacific in this study 40 plankton tows at 8 stations were conducted focusing on the terminus of the north equatorial current nec and its two bifurcated branches in the western pacific the results show that planktonic foraminifera were in high abundance around the kuroshio current kc source region and the mindanao current mc system but in low abundance in the nec the planktonic foraminiferal assemblage was dominated by g sacculifer and g ruber with relative abundances of 44 0 and 27 3 respectively according to a cluster analysis of the results of seven stations the stations can be divided into three groups namely group nec group kc and group mc which displayed different features of planktonic foraminiferal assemblages the low abundance of planktonic foraminifera that characterized the nec stations may be induced by the thick mixed layer and deep thermocline nutricline in this region however stations in the kc region which also had thick mixed layers and deep thermoclines showed higher abundances of planktonic foraminifera than the nec stations this result was probably caused by the higher terrigenous input the planktonic foraminifera show the highest abundance at the mc stations which may be the result of existing upwelling conditions

slags from ruda śląska poland as a large scale laboratory for the crystallization of rare natural rocks melilitolites and paralavas abstract zinc and lead smelting slags from ruda l ska are unique in their chemistry and phase composition which resemble rare natural rocks such as paralavas and melilitolites moreover considering its size we can treat a pyrometallurgical slag dump as a geological body in slags from ruda l ska the assemblage melilite pseudowollastonite wollastonite plagioclase was discovered in glassy slag high temperature experiments were performed to determine the temperature conditions and to reconstruct the crystallization of such an assemblage two slag samples were subjected to complete melting and crystallization with controlled thermal gradients of 53 25 c h 15 20 c h and 7 60 c h the results showed that crystal nucleation started at temperatures of 1250 1300 c depending on the fluctuations of chemical composition in both samples the thermal gradient only partly influenced the phase differentiation being rather responsible for the disappearance of the primary glass moreover even slight differences in chemical composition resulted in changes in phase assemblages under the same temperature conditions mll gls vs pwol pl mll gls it was proven that it is due to the combination of undercooling conditions and solidus dependences in the akermanite gehlenite solid solution the occurrence of such phenomena should be considered in natural rocks with similar composition in the case of the ruda l ska slags it explains the dominance of glassy slag in that location the experiments gave us an opportunity to observe and precisely analyze crystallization in real time providing new insights into the creation of slags and their natural analogues however the study has also shown that possible variations of the original crystallization should always be assessed

thermogenetic degradation of early zeolite cement an important process for generating anomalously high porosity and permeability in deeply buried sandstone reservoirs abstract diagenetic explanations to anomalously high porosities and permeabilities in deeply buried sandstones have been presented for many reservoirs for example grain coatings of microquartz or chlorite can preserve porosity by preventing succeeding quartz overgrowths and dissolution of grains or cement can generate secondary porosity anomalously high porosity and permeability up to 25 and 500 md respectively characterize the deeply buried 5 km upper jurassic shoreface sandstones of the gert member heno formation in the hejre field in the danish central graben optical cathodoluminescence and scanning electron microscopy were used to characterize the petrography and diagenetic changes that resulted in this high pressure and high temperature hpht reservoir this was supplemented by chemical analyses icp oes and icp ms mineralogical composition xrd and oxygen isotope composition of quartz and calcite cement sims the petrographic investigation shows that none of the known explanations for anomalously high porosity and permeability can be applied hence a new hypothesis for deeply buried sandstones in contact with lavas and volcaniclastic lithologies is proposed the new hypothesis includes reducing mechanical compaction due to early patchy zeolite cement which was sourced from adjacent lavas and volcaniclastic rocks locally stylolites developed where zeolite cement was less effective hence the present overpressure formed at a later stage increased temperatures during burial resulted in thermal instability of the zeolite cement which was dissolved and authigenic k feldspar and quartz precipitated instead this resulted in a reduction of the cement volume of up to 70 if the zeolite was k heulandite and a corresponding relative porosity increase thus a reduction in cement volume was generated by replacing zeolite of an open crystal structure with mineral phases k feldspar and quartz of denser crystal structures the thermogenetic breakdown of zeolite was forced by increased temperatures during burial this process may explain previously enigmatic highly porous and permeable deeply buried sandstone reservoirs

enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the triassic skagerrak central graben north sea uk current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation however the importance of overpressure developed during initial shallow burial in maintaining high primary porosity during subsequent burial has generally not been appreciated where pore fluid pressures are high and the vertical effective stress is low the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic the deeply buried fluvial sandstone reservoirs of the triassic skagerrak formation in the central graben north sea show anomalously high porosities at depths greater than 3500 metres below sea floor mbsf pore pressures can exceed 80 mpa in the upper part of the skagerrak formation at depths of 4000 5000 mbsf where temperatures are above 140 c the skagerrak reservoirs commonly have high primary porosities of up to 35 little macroquartz cement and variable amounts of diagenetic chlorite grain coats this research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the skagerrak formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial

importance of vertical effective stress for reservoir quality in the skagerrak formation central graben north sea abstract the complex fluvial sandstones of the triassic skagerrak formation are the host reservoir for a number of high pressure high temperature hpht fields in the central graben north sea all the reservoir sandstones in this study comprise of fine grained to medium grained sub arkosic to arkosic sandstones that have experienced broadly similar burial and diagenetic histories to their present day maximum burial depths despite similar diagenetic histories the fluvial reservoirs show major variations in reservoir quality and preserved porosity reservoir quality varies from excellent with anomalously high porosities of up to 35 at burial depth of 3500 m below seafloor to non economic with porosities this study has combined detailed petrographic analyses core analysis and pressure history modelling to assess the impact of differing vertical effective stresses ves and high pore fluid pressures up to 80 mpa on reservoir quality it has been recognised that fluvial channel sandstones of the skagerrak formation in the uk sector have experienced significantly less mechanical compaction than their equivalents in the norwegian sector this difference in mechanical compaction has had a significant impact upon reservoir quality even though the presence of chlorite grain coatings inhibited macroquartz cement overgrowths across all skagerrak formation reservoirs the onset of overpressure started once the overlying chalk seal was buried deeply enough to form a permeability barrier to fluid escape it is the cumulative effect of varying amounts of overpressure and its effect on the ves history that is key to determining the reservoir quality of these channelised sandstone units the results are consistent with a model where vertical effective stress affects both the compaction state and subsequent quartz cementation of the reservoirs

diagenesis of carbonate cements in permo triassic sandstones in the wessex and east yorkshire lincolnshire basins uk a stable isotope study

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

calcretes related to phreatophytic vegetation from the middle triassic otter sandstone of south west england carbonate cementation in the triassic otter sandstone of budleigh salterton sw england occurs in two distinct forms large vertical concentrically zoned cyclinders and thin subhorizontal sheets the former represent rhizocretions of the tap roots of phreatophytic plants which colonized bars and abandoned channels on a large braidplain the sheets represent cementation around ancient water tables the precipitation of the rhizocretions took place at least in part during the life of the plants and 13c and 18o values support the view that evapo transpiration induced carbonate precipitation palaeosol profiles are rare in the otter sandstone reflecting the geomorphological instability of the braidplain surfaces in contrast rhizocretionary and sheet calcretes which formed several metres below the active depositional surface are abundant because they had a high preservation potential

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

sediment mixing and accumulation rates in the sulu and south china seas implications for organic carbon preservation in deep sea environments abstract recent rates of carbon burial in two deep sea basins the sulu and south china seas are contrasted to determine factors influencing organic carbon preservation in deep sea sediments the sulu sea is an oxygen deficient warm water basin whereas the adjacent south china sea is more typical of open ocean conditions high resolution sea bed profiles of 14c reveal that accumulation rates for the sulu sea are 2 3 times higher than the south china sea at any given water depth e g 1 3 g cm 2 kyr 1 versus 4 3 g cm 1 kyr 1 respectively at 3000 m higher accumulation rates for the sulu sea are attributed to higher terrigenous input and better carbonate preservation in the warmer bottom waters benthic mixing rates determined using 210pb geochronology generally are lower in the sulu sea 0 004 0 254 cm2 yr 1 as compared with the south china sea 0 17 1 57 cm2 yr 1 especially at the 3000 m and 4000 m sites in the sulu sea where benthic mixing is negligible despite the higher bottom water oxygen levels in the south china sea organic carbon concentration profiles are strikingly similar for the two basins at any given water depth with similar concentration profiles higher accumulation rates in the sulu sea result in equivalently higher carbon preservation rates as primary production is thought to be similar for both basins if anything slightly higher for the south china sea the production rate can not explain the observed differences in carbon preservation in addition benthic mixing rates apparently do not play a major role in carbon preservation as extremely low rates at the deeper sulu sea would predict relatively low carbon preservation contrary to our observations we conclude that sediment accumulation rate is the dominant control on carbon preservation in this area of the western equatorial pacific

evidence for general instability of past climate from a 250 kyr ice core record recent results1 2 from two ice cores drilled in central greenland have revealed large abrupt climate changes of at least regional extent during the late stages of the last glaciation suggesting that climate in the north atlantic region is able to reorganize itself rapidly perhaps even within a few decades here we present a detailed stable isotope record for the full length of the greenland ice core project summit ice core extending over the past 250 kyr according to a calculated timescale we find that climate instability was not confined to the last glaciation but appears also to have been marked during the last interglacial as explored more fully in a companion paper3 and during the previous saale holstein glacial cycle this is in contrast with the extreme stability of the holocene suggesting that recent climate stability may be the exception rather than the rule the last interglacial seems to have lasted longer than is implied by the deep sea specmap record4 in agreement with other land based observations5 6 we suggest that climate instability in the early part of the last interglacial may have delayed the melting of the saalean ice sheets in america and eurasia perhaps accounting for this discrepancy

calcite cement in permian deep water sandstones delaware basin west texas origin distribution and effect on reservoir properties calcite cement is the dominant control on reservoir quality in turbidite sandstones of the upper permian bell canyon formation delaware basin these well sorted very fine grained arkoses were deposited in a basin floor setting by channel levee systems terminating in broad lobes calcite cement distribution in the east ford and geraldine ford fields was mapped using core log and thin section data calcite is concentrated in tightly cemented zones that are mostly less than 1 ft 0 3 m thick areas that have high percentages of calcite cemented sandstone 20 occur along the margins of the sandstone bodies in overbank and lobe deposits where sandstone pinches out into siltstone areas that have the lowest percentage of calcite cemented sandstone 10 occur where the sandstone is thickest in the channel facies isotopic composition of the calcite 13c 1 8 to 3 0 relative to the peedee belemnite pdb 18o 4 6 to 6 3 pdb is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris because internal sources of calcite were apparently insufficient to account for the cement volume cement components are interpreted as having been transported into the sandstones from organic rich basinal siltstones and limestones feldspars buffered acidic formation waters near where they entered the sandstone resulting in calcite concentrated near the sandstone margins the calcite formed near maximum burial depths of 4800 ft 1 5 km and temperatures of 104f 40c from marine pore waters with 18o of approximately 0 relative to standard mean ocean water most of the calcite cemented zones are interpreted as being concretions that extend no more than a few meters laterally production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft 300 m and cause vertical reservoir compartmentalization laterally extensive calcite layers may be associated with the base of turbidite deposits

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

eemian climate fluctuations observed in a european pollen record recent ice core data from greenland1 2 suggest that the climate during the last interglacial period the eemian was more unstable than that of the holocene about 10 000 years ago to the present being characterized in particular by a series of cold episodes each lasting about 70 to 750 years subsequent analysis of a second greenland ice core3 4 however failed to corroborate the details of these eemian climate fluctuations a result that may be attributable to the effects of ice flow4 to resolve this discrepancy it is imperative to seek alternative sources of information about the eemian climate here we present climate reconstructions from pollen data from the annually laminated eemian lake sediment record at bispingen5 and from the eemian and holocene peat records at la grande pile6 the former record indicates that an initially warm period of 2 900 yr was followed by cooling and a series of colder episodes one of which had winter temperatures comparable to those at the end of the preceding cold stage the latter records show greater climate instability during the eemian than the holocene these results are in broad agreement with those from the grip ice core but contrast both with the gisp2 core3 4 and with recent high resolution marine records from the north atlantic7 8

prediction of diagenetic facies using well logs a case study from the upper triassic yanchang formation ordos basin china abstract understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration as a typical tight sandstone reservoir the seventh unit of the upper triassic yanchang formation in the ordos basin chang 7 unit central china is an important oil producing interval results of helium porosity and permeability and petrographic assessment from thin sections x ray diffraction scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction cementation by carbonate quartz clay minerals and dissolution of feldspar and lithic fragments the sandstones comprise silt to medium grained lithic arkoses to feldspathic litharenites and litharenites which have low porosity 0 5 13 6 with an average of 6 8 and low permeability 0 009 10 3 m 2 to 1 818 10 3 m 2 with an average of 0 106 10 3 m 2 this study suggests that diagenetic facies identified from petrographic observations can be up scaled by correlation with wire line log responses which can facilitate prediction of reservoir quality at a field scale four diagenetic facies are determined based on petrographic features including intensity of compaction cement types and amounts and degree of dissolution unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values and those sandstones show the highest reservoir quality tightly compacted sandstones siltstones which tend to have high gamma ray readings and relatively high bulk density values show the poorest reservoir quality a model based on principal component analysis pca is built and show better prediction of diagenetic facies than biplots of well logs the model is validated by blind testing log predicted diagenetic facies against petrographic features from core samples of the upper triassic yanchang formation in the ordos basin which indicates it is a helpful predictive model

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

a 20 000 year record of ocean circulation and climate change from the santa barbara basin much of the evidence for climate driven fluctuations in ocean circulation during the past 20 000 years has come from studies of the north atlantic region1 6 the extent to which such interactions have occurred in other ocean basins and any associated teleconnec tions between basins is poorly understood here we present high resolution palaeoclimate and palaeoceanographic records from a 20 000 year sedimentary sequence from the santa barbara basin on the eastern margin of the north pacific ocean the sequence shows oscillations of the benthic environment between low oxygen conditions laminated sediments during periods of warm climate and higher oxygen conditions non laminated bioturbated sediments during cool intervals age differences between coexisting benthic and planktonic foraminifers indicate climate related changes in the age and source and hence oxygen content of basin bottom waters relatively young bottom waters are associated with the cooler intervals and are considered to reflect high proportions of intermediate waters derived from proximal sources conversely older bottom waters are associated with the warmer intervals and were derived from more distal sources these climate driven variations in ocean circulation appear to be synchronous with the main ocean climate fluctuations in the north atlantic region1 6 suggesting that a tight coupling mechanism operates between the two basins

late quaternary paleoceanography of the south china sea surface circulation and carbonate cycles paleoceanographic information from 34 sediment cores is summarized to investigate the glacial interglacial variations in sea surface circulation and late quaternary carbonate cycles in the south china sea judging from the distribution pattern of deposition rates the enormous terrigenous supply by rivers is responsible for the high rate of hemipelagic sedimentation which was even higher during glacial periods paleotemperature maps based on transfer function temperature estimates have revealed a e w gradient of sst for glacial summer and a s n gradient for glacial winter and this agrees well with the clockwise surface circulation in summer and counterclockwise in winter during glacials as shown by a preliminary computer modelling the high resolution paleotemperature curves of core v36 3 from the northern slope show an abrupt cooling event which may belong to the younger dryas carbonate percentage in sediments and preservation of foraminifers and pteropods are used to study the carbonate cycles in the marginal sea it turns out that two mechanisms are functioning there being a basin with large drainage areas vast amount of river borne terrigenous input dilutes carbonate and its intensification during glacial decreases the glacial cac03 being a basin connected with the pacific the fluctuations of lysocline ccd and acd in the scs are related to those in the pacific ocean and dissolution is strengthened at interglacial thus above the present ccd the dilution factor predominates and the atlantic cycles are produced below the present ccd the dissolution factor prevails resulting in the pacific pattern of carbonate cycles

silver lining evidence for inka silver refining in northern chile prehistoric silver purification using lead cupellation has been documented in multiple places throughout the andes but direct evidence of the inka use of this technology has remained elusive in this study we use x ray fluorescence scanning electron microscopy and electron microprobe analysis to document direct evidence of inka period ad 1400 1532 silver purification using lead cupellation in the tarapaca valley of northern chile local metalworkers used wind driven huayra furnaces to produce pure lead metal sustaining temperatures of ca 900 1100 c to smelt lead bearing ores that may have included galena the lead metal was then used in open vessel cupellation of silver bearing ores some of which may have been cupriferous and derived from the nearby inka mines at huantajaya phase analyses of the slagged interiors of bowl shaped ceramic vessels used for cupellation indicate that the metalworkers maintained the oxidizing environment and temperatures between 800 and 1100 c requisite for cupellation we argue that the inka introduced this technique to tarapaca metalworkers the absence of finished silver artifacts in local valley contexts suggests that the refined silver was removed from the valley for use elsewhere in the empire

glacial climate instability in the northeast pacific ocean recent climate records from greenland ice cores1 2 and north atlantic sediments3a 5 have challenged the long held notion that pleistocene climate fluctuates between two relatively stable states glacials and interglacials it has been appreciated for some time that the transitions from one state to another are not smooth6 but the new records indicate that the glacial and interglacial periods themselves appear to be punctuated by significant climate variability several short interstadial events punctuated the last glacial period for example but it has not been clear whether this climate instability is a global phenomenon or is peculiar to the north atlantic region here we present climate proxy records from sediment cores from the eastern margin of the north pacific ocean which indicate that climate in this region was also highly unstable during the last glaciation our observations suggest that glacial climate instability throughout the northern hemisphere might be linked to rapid changes in the size of the laurentide ice sheet and associated changes in atmospheric circulation

fluid rock interaction and its effects on the upper triassic tight sandstones in the sichuan basin china insights from petrographic and geochemical study of carbonate cements abstract tight sandstone gas is an important unconventional hydrocarbon resource but its exploration is made difficult by the compactness and heterogeneity of the sandstone fluid rock interaction and subsequent cementation play key roles in developing this heterogeneity we conducted a petrographic trace element and isotopic study of the upper triassic xujiahe sandstones from the sichuan basin to investigate fluid rock interactions and associated diagenetic effects carbonate fragments and cements are widespread in these sandstones and are the primary control on the nature of the xujiahe sandstone reservoirs the carbonate cements primarily comprise of calcite and are typically distributed around the margins of sandstone bodies and along fine grained beds carbonate minerals in the centre or at the margin of sandstone units have heavy 13ccarb values of ca 0 whereas 13ccarb is markedly depleted in the transition zone with values as low as 10 reflecting a contribution from organic carbon manganese strontium and barium generally become enriched in the carbonate cements with diagenesis and their concentrations correlate with porosity and 13ccarb redox sensitive mn and fluid mobile sr and ba were analysed to reveal the diagenetic processes and environment of carbonate precipitation we propose that fluids flowed preferentially along the central parts of sandstone units associated with mineral dissolution whereas cementations took place near the margins of the units fluid rock interaction especially that involving late hydrocarbon related fluids greatly enhances the heterogeneity of sandstones and can potentially create high quality reservoirs in the central parts of tight sandstone units

on the structure and origin of major glaciation cycles 2 the 100 000 year cycle climate over the past million years has been dominated by glaciation cycles with periods near 23 000 41 000 and 100 000 years in a linear version of the milankovitch theory the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity but the 100 000 year radiation cycle arising from eccentricity variation is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing we present phase observations showing that the geographic progression of local responses over the 100 000 year cycle is similar to the progression in the other two cycles implying that a similar set of internal climatic mechanisms operates in all three but the phase sequence in the 100 000 year cycle requires a source of climatic inertia having a time constant similar to 15 000 years much larger than the other cycles similar to 5 000 years our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source when these ice sheets forced by precession and obliquity exceed a critical size they cease responding as linear milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses in our model the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity driven modulations in the 23 000 year sea level cycle during an interval when sea level is forced upward from a major low stand by a milankovitch response acting either alone or in combination with an internally driven higher frequency process ice sheets grounded on continental shelves become unstable mass wasting accelerates and the resulting deglaciation sets the phase of one wave in the train of 100 000 year oscillations whether a glacier or ice sheet influences the climate depends very much on the scale the interesting aspect is that an effect on the local climate can still make an ice mass grow larger and larger thereby gradually increasing its radius of influence

a high resolution late quaternary upwelling record from the anoxic cariaco basin venezuela results are presented of a high resolution study of the planktonic foraminiferal faunas from two piston cores recovered from the cariaco basin in the southern caribbean sea the cariaco basin is a small anoxic marine basin on the northern continental margin of venezuela in an area today characterized by both seasonal trade wind induced upwelling and pronounced dry and wet seasons our data indicate that large changes in the intensity of upwelling and hence trade wind strength occurred in this region during the last glacial interglacial transition and throughout the holocene during the last glacial lowstand of sea level the cariaco basin was effectively isolated from the open caribbean along its northern margin by the then largely emergent tortuga bank oxic conditions existed in the deep cariaco basin at this time and surface productivity was low about 12 600 years ago the abrupt initiation of strong upwelling over the basin and the onset of permanent anoxia in the deep waters are coincident with the rapid rise of sea level that accompanied the peak interval of meltwater discharge from the laurentide ice sheet into the gulf of mexico strong upwelling between 12 600 and about 10 000 years ago may be related to intensified trade winds resulting in part from cooler sea surface temperatures in the caribbean and gulf of mexico after about 10 000 years ago upwelling intensity was reduced though highly variable a preliminary frequency domain analysis of the holocene portion of the cariaco basin time series suggests that solar forcing may explain a significant component of the century scale variability observed in the record of upwelling and trade wind strength

a 50 000 year record of climate oscillations from florida and its temporal correlation with the heinrich events oscillations of pinus pine pollen in a 50 000 year sequence from lake tulane florida indicate that there were major vegetation shifts during the last glacial cycle episodes of abundant pinus populations indicate a climate that was more wet than intervening phases dominated by quercus oak and ambrosia type ragweed and marsh elder the pinus episodes seem to be temporally correlated with the north atlantic heinrich events which were massive periodic advances of ice streams from the eastern margin of the laurentide ice sheet possible links between the tulane pinus and heinrich events include hemispheric cooling the influences of mississippi meltwater on sea surface temperatures in the gulf of mexico and the effects of north atlantic thermohaline circulation on currents in the gulf