reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the jiyang super depression bohai bay basin china abstract a great difference exists between the hydrocarbon charging characteristics of different tertiary lacustrine turbidites in the jiyang super depression of the bohai bay basin east china based on wireline log data core observation and thin section analyses this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs the lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0 5 10 km2 with a thickness of 5 20 m the sandstones of the tertiary turbidites in the jiyang super depression have been strongly altered diagenetically by mechanical compaction cementation and mineral dissolution the effect of compaction caused the porosity to decrease drastically with the burial depths especially during the early diagenesis when the porosity was reduced by over 15 the effect of cementation and mineral dissolution during the late stage diagenesis is dominated by carbonate cementation in sandstones high carbonate cement content is usually associated with low porosity and permeability carbonate dissolution secondary porosity zone and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone two stages of carbonate cementation were identified the precipitation from pore water during sedimentation and secondary precipitation in sandstones from the organic acid dissolved carbonate minerals from source rocks petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs it is also noticed that interstitial matter content affects the oil bearing property to some degree there are three essential elements for high oil bearing turbidite reservoirs excellent pore types low carbonate cement 15 and average permeability 10 md

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

evaluating the mechanical compaction of quartzarenites the importance of sorting llanos foreland basin colombia abstract mechanical compaction is the main porosity reducing process in sandstones including high reservoir quality rigid grain sandstones for such sandstones the extrapolation of theoretical or experimental compaction algorithms needs calibration with rocks having well constrained burial histories evaluating the compaction of these rocks is achieved by comparing current intergranular volume igv with depositional igv which is strongly dependent on sorting however because sandstone sorting is difficult to measure accurately its impact on depositional porosity and compaction state is largely underestimated we use the quartzarenites of the oligocene carbonera formation in the subsurface of the hydrostatically pressured llanos basin to illustrate the importance of sorting when evaluating the compaction of rigid grain sandstones igv and sorting were measured in core samples using a combination of transmitted light and cathodoluminescence images resulting in improved accuracy over standard procedures the compaction state of clean quartzarenites at given depths is best described using igv versus sorting plots which are used to derive compaction curves for specified sorting values the igv versus sorting trends are displaced to lower igv values with increasing burial depth the differences in igv caused by differences in sorting exceed the differences in igv resulting from 1000 m of burial illustrating the high impact of sorting when evaluating compaction contrasting with published experimental results the compaction of the llanos basin ductile grain poor quartzarenites is independent of grain size and grain rearrangement is the main compaction mechanism during the first 1 6 km of burial based on the llanos data we have generated igv versus depth curves for clean pure quartzarenites of specific sorting which can be used to predict their maximum primary porosity up to moderate burial depths differences with other published burial curves are probably related to unaccounted variations in sorting ductile grain content and framework strengthening cements however the llanos basin quartzarenites contain virtually no cements explaining their high degree of compaction relative to other rigid grain sandstones and making them ideal to isolate the effects of compaction on the igv of quartzarenites the llanos basin data suggest that below 2 5 km of depth clean well to moderately well sorted quartzarenites continue reducing their igv by mechanical compaction below the 26 limit which should apply only to extremely well sorted rigid grain uncemented sandstones

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

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

petroleum generation and charge history of the northern dongying depression bohai bay basin china insight from integrated fluid inclusion analysis and basin modelling abstract the petroleum generation and charge history of the northern dongying depression bohai bay basin was investigated using an integrated fluid inclusion analysis workflow and geohistory modelling one and two dimensional basin modelling was performed to unravel the oil generation history of the eocene shahejie formation es3 and es4 source rocks based on the reconstruction of the burial thermal and maturity history calibration of the model with thermal maturity and borehole temperature data using a rift basin heat flow model indicates that the upper interval of the es4 source rocks began to generate oil at around 35 ma reached a maturity level of 0 7 ro at 31 30 ma and a peak hydrocarbon generation at 24 23 ma the lower interval of the es3 source rocks began to generate oil at around 33 32 ma and reached a maturity of 0 7 ro at about 27 26 ma oil generation from the lower es3 and upper es4 source rocks occurred in three phases with the first phase from approximately 30 20 ma the second phase from approximately 20 5 ma and the third phase from 5 ma to the present day the first and third phases were the two predominant phases of intense oil generation samples from the es3 and es4 reservoir intervals in 12 wells at depth intervals between 2677 7 m and 4323 0 m were investigated using an integrated fluid inclusion workflow including petrography fluorescence spectroscopy and microthermometry to determine the petroleum charge history in the northern dongying depression abundant oil inclusions with a range of fluorescence colours from near yellow to near blue were observed and were interpreted to represent two episodes of hydrocarbon charge based on the fluid inclusion petrography fluorescence spectroscopy and microthermometry data two episodes of oil charge were determined at 24 20 ma and 4 3 ma respectively with the second episode being the predominant period for the oil accumulation in the northern dongying depression the oil charge occurred during or immediately after the modelled intense oil generation and coincided with a regional uplift and a rapid subsidence suggesting that the hydrocarbon migration from the already overpressured source rocks may have been triggered by the regional uplift and rapid subsidence the expelled oil was then charged to the already established traps in the northern dongying depression the proximal locations of the reservoirs to the generative kitchens and the short oil migration distance facilitate the intimate relationship between oil generation migration and accumulation

genesis and depositional model of subaqueous sediment gravity flow deposits in a lacustrine rift basin as exemplified by the eocene shahejie formation in the jiyang depression eastern china abstract subaqueous sediment gravity flow ssgf deposits have recently been recognized as a major reservoir plays in lacustrine basins research on their genesis and depositional models are crucial to understand gravity flow evolution and the distribution of their deposits this study analysed ssgf deposits in the third member of the shahejie formation in the jiyang depression eastern china based on 3 d seismic data well log data and core observations the types and origins of ssgf the distribution patterns and depositional models of ssgf deposits have been investigated the dominant types of ssgf include muddy debris flows sandy debris flows high density turbidity currents surge like low density turbidity currents and quasi steady low density turbidity currents which are mainly caused by river floods and sediment failures the overall distribution of ssgf deposits is controlled by the sediment supplies basin structure and sequence stratigraphic framework paleotopography flood energy triggering mechanism and the slope angle of the delta front are also important ssgf deposits associated with river floods can be further subdivided into elongated and fan shaped deposits elongated sandstones are composed predominantly of bedload dominated and suspension load dominated hyperpycnal flow deposits the fan shaped sandstones can be subdivided into an inner fan a middle fan and an outer fan inner fan deposits are dominated by gully filling debrites middle fan deposits are composed predominantly of channel fill deposits while outer fan deposits consist of thin bedded and laterally extensive lobe intervals ssgf deposits triggered by sediment failures can be further subdivided according to their proximity to slumping proximal intermediate and distal parts the proximal deposits are composed predominantly of slide and slump deposits the intermediate deposits are dominated by sandy debrite tongues while the distal deposits are composed predominantly of thin bedded surge like low density turbidites this study offers some insight into the types origin of gravity flow as well as the distribution of their deposits in a lacustrine rift basin

the origin and timing of multiphase cementation in carbonates impact of regional scale geodynamic events on the middle jurassic limestones diagenesis paris basin france the middle jurassic carbonates of the eastern part of the paris basin display surprisingly low values of porosity and permeability 15 and k 0 5 md the main objective of this study is to determine the causes and timing of the cementation that altered the petrophysical properties of these carbonates thereby destroying their potential as oil reservoirs a fate that did not befall their equivalents in deeper central parts of the paris basin using petrographic and geochemical analyses stable o and c isotopes sr isotopes major elements we identify six calcitic spar stages two dolomite stages and several episodes of fracturing and stylolitization ordered in paragenetic sequence cement quantification shows the predominance of two blocky calcite cement stages 75 of total cementation o and sr isotopes from these calcite cements suggest that the parent fluids resulted either from a mixing of trapped jurassic seawater and meteoric water or from buffered meteoric waters in the geological history of the paris basin major meteoric water inputs were possible during the early cretaceous when the middle jurassic carbonates cropped out at its northern and eastern borders lateral meteoric recharge may have occurred as a result of two separate uplift events late cimmerian unconformity and late aptian unconformity and the related exposures of carbonates to the north of the study area this palaeohydrological circulation brought about a significant reduction of porosity from 40 to 10 through calcite cement precipitation the early cretaceous events are of great importance in the diagenetic evolution of the sedimentary basins bordering the london brabant massif across all of northwestern europe a subsequent dolomite and calcite cementation stage accounts for about 5 of the total cement volumes this late cementation may have been caused by hydrothermal fluids ascending along permeable fractures during the late oligocene extension episode

cretaceous deep water formation in the indian sector of the southern ocean 1 the role that meridional overturning circulation moc patterns played in poleward heat transport during the extreme warmth of the early to late cretaceous is a fundamental and unresolved question in climate dynamics in order to address this question we must determine where deep waters formed and how they may have circulated during periods of extreme warmth here we present late albian through maastrichtian 105 to 65 ma nd isotope records from deep sea drilling project dsdp and ocean drilling program odp sites in the proto indian ocean and the tropical pacific comparison of these data with previously published records indicates deep water formation in the indian sector of the southern ocean began at least 105 ma extending the record of high latitude convection back into the early cretaceous prior to the peak warmth of the mid cretaceous the growing body of data supports a mode of moc in part characterized by high latitude downwelling during the peak of greenhouse warmth of the mesozoic and cenozoic however this mode of moc likely was characterized by numerous locations of deep convection that were regionally important but not significant in terms of a globally overturning circulation due to paleogeographic and bathymetric barriers

anomalous porosity and permeability preservation in deeply buried tertiary and mesozoic sandstones in the cusiana field llanos foothills colombia reply abstract porosity in deeply buried 14000 ft 4200 m sandstones of the lower tertiary mirador formation in the cusiana field llanos foothills colombia is less than 10 porosity units p u however permeability is as much as 1000 md which is believed to be due to the simple quartzose detrital grain composition and large grain size the low porosity has resulted from pervasive quartz cement 10 15 fluid inclusion microthermometry and intergranular grain volumes indicate that the quartz cement precipitated at a temperature of around 100 c during deep burial surprisingly thick 400 ft 120 m sandstones of the underlying upper cretaceous guadalupe group in the cusiana field have porosity as high as 20 p u these sandstones are lithic sandstones composed of quartz and a variety of phosphate grain types and are poorly cemented quartz cement is not abundant and occurs as rims of microcrystalline quartz on quartz and phosphate grains in contrast interbedded quartz arenites within the guadalupe group are heavily cemented by quartz cement and have porosity similar to the overlying mirador formation the major control on porosity in these sandstones appears to be the presence of quartz cement the quartz cement is controlled by grain composition the reasons for the lack of quartz cement in the lithic sandstones of the guadalupe group are uncertain but phosphate poisoning inhibiting quartz cement growth is a possibility the presence of high porosity sweet spots in deeply buried sandstones may be linked to the occurrence of phosphate beds a primary depositional control

hydrocarbon filling and leakage history of a deep geopressured sandstone fulmar formation united kingdom north sea diagenetic minerals in a water filled borehole from the jurassic fulmar formation united kingdom central north sea record two phases of hydrocarbon filling and emptying initial oil charge was during the late cretaceous at shallow burial depths of about 1 5 km 0 9 mi as we consider that hydrocarbon has preserved porosity during burial this has implications for the understanding of the porosity evolution of the fulmar formation which in other locations is an important hydrocarbon reservoir the early oil charge as recorded by illite k ar ages progressively filled the structure from 84 to 59 ma and possibly precipitated bitumen because of biodegradation the first oil predated many of the burial diagenetic reactions within the sandstone after leak off at ca 60 ma diagenetic reactions continued in an open geochemical system with possible import of co2 products of these reactions include ankerite and quartz overgrowth cements hydrocarbon staining postdates these phases and provides evidence of a hydrocarbon charge probably gas condensate the second hydrocarbon charge also leaked off and the sandstone is now water bearing previous work on the fulmar formation has incorrectly placed all the diagenetic reactions as predating the first arrival of hydrocarbon the present day pore fluids are high salinity high 18o fluids derived from the underlying permian zechstein evaporates these fluids entered the reservoir during a phase of overpressure release that caused fracturing of the framework quartz grains possibly coincident with the second phase of hydrocarbon leak off at 2 5 ma mark wilkinson studied sandstone hosted calcite concretions for his ph d and has subsequently tried to take a more holistic view of the diagenesis of sedimentary basins his career has been funded by a variety of oil companies although he is currently involved in geological disposal of some of the co2 that he was partly responsible for producing stuart haszeldine is a professor at edinburgh university he received his ph d on clastic and coal sedimentology in 1981 from the university of strathclyde after an attempted reeducation in the oil industry he has collaborated with many companies to research on hydrocarbon reservoir quality radioactive waste disposal and coal resources he currently heads the united kingdom s largest group investigating carbon dioxide sequestration tony fallick was educated at the university of glasgow scotland in physics b sc and chemistry ph d he did research at mcmaster university ontario canada and cambridge england before moving in 1980 to the scottish universities research and reactor center where he is currently the director

summary of the aapg research symposium on lacustrine basin exploration in china and southeast asia in this paper we review the work presented at the aapg shengli petroleum administration research symposium dealing with lacustrine basin exploration in china and southeast asia this meeting held in the fall of 1995 revealed that there has been an expansion of the available knowledge base associated with these basins this increase in information has resulted in a better understanding of lacustrine basin fill properties and characteristics even though many of the causal mechanisms for these properties have not yet been established the many papers presented suggest that there are two primary causes for economic failure within many of these basins 1 communication between source and reservoir and 2 the limited reservoir potential within the lacustrine sequence of these basins more effective petroleum systems are found in basins where marine sandstones are in communication with lacustrine sources the discussions also clearly demonstrated that lacustrine basins are sensitive to environmental changes causing complex sedimentary facies successions each facies however displays unique characteristics that can be mapped several presenters also suggested that within extensional lacustrine settings the stratigraphic succession follows a distinct pattern although most of these basins are predominantly oil prone several lacustrine basins within the region contain significant volumes of gas included within these gas reserves are biogenic accumulations that appear to have formed nearly penecontemporaneously with deposition recent work on several of the chinese basins suggests the possibility for immature hydrocarbon generation and expulsion in a manner different than that associated with marine type ii s kerogens even with the progress that this symposium revealed there are several key questions that remain including questions associated with how technology might be used to improve the economics of many of these systems that display limited reservoir potential

geometry styles and quantification of inversion structures in the jiyang depression bohai bay basin eastern china abstract inversion structures of jiyang depression evolved during mesozoic and cenozoic are classified into positive and negative inversion geometry styles the geometry styles of the reactivated faults in the study area are attributed to their multi phase history which includes at least four phases of compressive deformation and subsequent subsidence caused by extensional reactivation of faults since mesozoic in the case of negative inversion style the negative inversion ratios cannot be calculated by the same method of calculating the degree of positive inversion thus a new method of quantifying the negative inversion ratio is presented in this paper negative inversion ratio is defined as the ratio of extensional displacement to contractional displacement measured along the fault this method provides a useful complement to conventional estimation of the degree of inversion and adapts to any negative inversion faults based on this method we obtained the negative inversion ratios of some faults in the jiyang depression which indicates a decrease in deformational degree from east to west in the whole region

water isotopes in precipitation data model comparison for present day and past climates abstract variations of hdo and h2 18 o concentrations are observed in precipitation both on a geographical and on a temporal basis these variations resulting from successive isotopic fractionation processes at each phase change of water during its atmospheric cycle are well documented through the iaea wmo network isotope concentrations are in middle and high latitudes linearly related to the annual mean temperature at the precipitation site paleoclimatologists have used this relationship to infer paleotemperatures from isotope paleodata extractable from ice cores deep groundwater and other such sources for this application to be valid however the spatial relationship must also hold in time at a given location as the location undergoes a series of climatic changes progress in water isotope modeling aimed at examining and evaluating this assumption has been recently reviewed jouzel et al 1997 with a focus on polar regions and more specifically on greenland we extend this review in comparing the results of two different isotopic agcms nasa giss and echam and in examining with a more global perspective the validity of the above assumption i e the equivalence of the spatial and temporal isotope temperature relationships these results confirm the dominating role of local temperature changes on the paleo isotope signal in most regions however the exact calibration of this valuable paleothermometer is biased by for example the seasonality of precipitation and other factors we forced the two models by the climatic boundary conditions of the mild holocene at 6 kyr bp which only slightly differs from today s climate the isotope response on this weak forcing is quite heterogeneous the only robust common response is the intensification of the hydrological cycle in low latitudes and therefore isotopically more depleted precipitation in the tropics and subtropics we also examine recent progress made in modeling the relationship between the conditions prevailing in moisture source regions for precipitation and the deuterium excess of that precipitation

meteoric water diagenesis in late cretaceous canyon fill turbidite reservoirs from the espirito santo basin eastern brazil abstract parameters controlling the diagenetic evolution of passive margin marine turbidites which are important targets of hydrocarbon exploration are poorly constrained in the literature this study aims to unravel the conditions of diagenesis and its impact on the reservoir quality evolution from late cretaceous canyon filling turbiditic sandstones of the onshore portion of espirito santo basin eastern brazil kaolinization 18 o 13 3 to 15 2 d 96 6 to 79 6 and dissolution of framework silicate grains is attributed to meteoric water incursion during eodiagenesis in response to a considerable fall in relative sea level eogenetic alterations also include cementation by siderite average 18 o 7 2 13 c 9 3 and pyrite progressive sediment burial present depths 1530 2027 m resulted in the formation of poikilotopic calcite ferroan dolomite ankerite average 18 o 7 9 13 c 2 9 minor amounts of quartz overgrowths and in partial dickitization of kaolinite isotopic values of calcite and dolomite ankerite follow two trends of co variance of 13 c with decreasing 18 o and increasing temperature from a composition closer to marine 0 one trend goes towards positive 13 c values up to 22 4 for calcite 18 6 for dolomite ankerite indicating increasing input of carbonate from methanogenic fermentation the other trend develops towards negative 13 c values down to 17 2 for calcite 15 for dolomite ankerite suggesting increasing contribution from thermal decarboxylation with increasing temperature and depth despite the presence of various cement types mechanical compaction was more important than cementation in reducing depositional porosity in the onshore urucutuca sandstones

carbonate cementation patterns and diagenetic reservoir facies in the campos basin cretaceous turbidites offshore eastern brazil massive turbidite arkoses contain more than 80 of the oil reserves of the campos basin the main petroleum province of brazil the porosity and permeability distribution in the namorado albian cenomanian and carapebus turonian santonian sandstones is

seismic attenuation and pore fluid viscosity in clay rich reservoir sandstones the frequency dependence of seismic attenuation in a suite of clay rich reservoir sandstones was investigated in the laboratory compressional and shear wave velocities vp and vs and quality factors qp and qs were measured as functions of pore fluid viscosity at an effective pressure of 50 mpa and at an experimental frequency of about 0 8 mhz using the pulse echo technique the experimental viscosity ranged from 0 3 to 1000 centipoise which gives equivalent frequencies for a water saturated sandstone of 2 6 mhz to 780 hz assuming a global flow loss mechanism two types of behavior were observed high permeability greater than 100 millidarcies sandstones tend to show variable qp and qs which are similar in magnitude to those predicted by the biot theory over the viscosity range 0 3 to about 20 centipoise equivalent frequency range 2 6 mhz to about 39 khz low permeability less than 50 millidarcies sandstones tend to show almost constant qp and qs over the experimental viscosity range that are

an approach to estimate lower jurassic seawater oxygen isotope composition using δ18o and mg ca ratios of belemnite calcites early pliensbachian northern spain palaeotemperature estimates from the oxygen isotope compositions of belemnites have been hampered by not knowing ancient seawater isotope compositions well enough we have tackled this problem using mg ca as a proxy for temperature and here we present a 2 ma record of paired mg ca and d 18 o measurements of jurassic early pliensbachian belemnites from the asturian basin as a palaeo proxy of seawater oxygen isotope composition from the combined use of the two approaches we suggest a d 18 ow composition of about 0 1 for the jamesoni ibex zones this value may have been increased by about 0 6 during the davoei zone due to the effect of waters with a different d 18 ow composition these findings illustrate the inaccuracy of using a globally homogeneous ice free value of d 18 ow 1 for d 18 ocarbbased palaeotemperature reconstructions our data suggest that previous palaeotemperatures calculated in the region from d 18 o values of belemnites may have been underestimated as the seawater oxygen isotopic composition could have been higher

late paleocene early eocene tethyan carbonate platform evolution a response to long and short term paleoclimatic change abstract the early paleogene experienced the most pronounced long term warming trend of the cenozoic superimposed by transient warming events such as the paleocene eocene thermal maximum petm the consequences of climatic perturbations and associated changes on the evolution of carbonate platforms are relatively unexplored today modern carbonate platforms especially coral reefs are highly sensitive to environmental and climatic change which raises the question how sub tropical reef systems of the early paleogene reacted to gradual and sudden global warming eutrophication of shelf areas enhanced co 2 levels in an ocean with low mg ca ratios the answer to this question may help to investigate the fate of modern coral reef systems in times of global warming and rising co 2 levels here we present a synthesis of tethyan carbonate platform evolution in the early paleogene 59 55 ma concentrating on coral reefs and larger foraminifera two important organism groups during this time interval we discuss and evaluate the importance of the intrinsic and extrinsic factors leading to the dissimilar evolution of both groups during the early paleogene detailed analyses of two carbonate platform areas at low egypt and middle spain paleolatitudes and comparison with faunal patterns of coeval platforms retrieved from the literature led to the distinction of three evolutionary stages in the late paleocene to early eocene tethys stage i late paleocene coralgal dominated platforms at low to middle paleolatitudes stage ii a transitional latest paleocene platform stage with coralgal reefs dominating at middle paleolatitudes and larger foraminifera dominated miscellanea ranikothalia assilina platforms at low paleolatitudes and stage iii early eocene larger foraminifera dominated alveolina orbitolites nummulites platforms at low to middle paleolatitudes the onset of the latter prominent larger foraminifera dominated platform correlates with the paleocene eocene thermal maximum the causes for the change from coral dominated platforms to larger foraminifera dominated platforms are multilayered the decline of coralgal reefs in low latitudes during platform stage ii is related to overall warming leading to sea surface temperatures in the tropics beyond the maximum temperature range of corals the overall low occurrence of coral reefs in the paleogene might be related to the presence of a calcite sea at the same time larger foraminifera started to flourish after their near extinction at the cretaceous paleogene boundary the demise of coralgal reefs at all studied paleolatitudes in platform stage iii can be founded on the effects of the petm resulting in short term warming eutrophic conditions on the shelves and acidification of the oceans hampering the growth of aragonitic corals while calcitic larger foraminifera flourished in the absence of other successful carbonate producing organisms larger foraminifera were able to take over the role as the dominant carbonate platform inhabitant leading to a stepwise tethyan platform stage evolution around the paleocene eocene boundary this szenario might be also effective for threatened coral reef sites

early diagenetic pyrite morphology in a mudstone dominated succession the lower jurassic cleveland ironstone formation eastern england abstract diagenetic pyrite in the mudstones and ironstones of the lower jurassic cleveland ironstone formation of eastern england exhibits two distinct morphologies framboidal pyrite commonly associated with organic matter and euhedral pyrite associated with detrital clay pellets these two morphologies are mutually exclusive in occurrence framboidal pyrite is present in clay rich mudstones ooidal ironstones apatite rich units and some silt rich mudstones euhedral pyrite is present in silt rich and sand rich mudstones 34s isotopic analysis of six samples of pyrite suggests that both types of pyrite morphology precipitated during early diagenesis from porewaters with open access to overlying sea water although both probably acted as sites for continued pyrite precipitation during burial it is proposed that framboidal pyrite precipitated from iron dominated porewaters at sites of sulfide supply i e in the region of organic matter as a result of bacterial sulfate reduction where locally sulfide production rates were high enough for porewaters to reach supersaturation with respect to fes euhedral pyrite also precipitated from iron dominated porewaters but sulfide production rates from organic matter was such that fes saturation was not reached at the sites of sulfide production instead euhedral pyrite was precipitated directly from porewater when fes2 saturation was reached the control over pyrite morphology was probably the amount and reactivity of the organic matter within the deposited sediments the sand rich mudstones contained less reactive organic matter due to clastic dilution and deposition in shallower environments with o2 rich bottom waters the ironstones and apatite rich units were deposited under very low sedimentation rates and as a result organic matter contents were very low and iron reduction dominated early diagenesis which inhibited sulfate reduction the presence of minor framboidal pyrite within these units however suggests that sulfide reduction took place in micro environments during early diagenesis

testing the cretaceous greenhouse hypothesis using glassy foraminiferal calcite from the core of the turonian tropics on demerara rise glassy turonian foraminifera preserved in clay rich sediments from the western tropical atlantic yield the warmest equivalent 18o sea surface temperatures ssts yet reported for the entire cretaceous cenozoic we estimate turonian ssts that were at least as warm as conservative mean 30 c to significantly warmer warm mean 33 c than those in the region today however if independent evidence for high middle cretaceous pco2 is reliable and resulted in greater isotopic fractionation between seawater and calcite because of lower sea surface ph our conservative and warm sst estimates would be even higher 32 and 36 c respectively our new tropical ssts help reconcile geologic data with the predictions of general circulation models that incorporate high cretaceous pco2 and lend support to the hypothesis of a cretaceous greenhouse our data also strengthen the case for a turonian age for the cretaceous thermal maximum and highlight a 20 40 m y mismatch between peak cretaceous cenozoic global warmth and peak inferred tectonic co2 production we infer that this mismatch is either an artifact of a hidden turonian pulse in global ocean crust cycling or real evidence of the influence of some other factor on atmospheric co2 and or ssts a hidden pulse in crust cycling would explain the timing of peak cretaceous cenozoic sea level also turonian but other factors are needed to explain high frequency 10 100 k y instability in middle cretaceous ssts reported elsewhere

facies and climate environmental changes recorded on a carbonate ramp a sedimentological and geochemical approach on middle jurassic carbonates paris basin france a detailed sedimentological geochemical and mineralogical study is carried out on the early bajocian to early callovian middle jurassic limestones of the paris basin objectives are to document and explain the facies changes in the context of the climate environmental evolution at the nw european scale deposits include 18 lithofacies which are stacked into 10 third order depositional sequences at a greater time scale 4 biosedimentary packages are distinguished by their allochems associations 1 an intracratonic carbonate environment with coral reefs and crinoid rich facies is typical of the early bajocian 2 a major facies change occurred at the early late bajocian transition with a shift from crinoid and coral rich facies to ooid rich facies during the late bajocian a southward dipping ooid ramp with successive progradational trends was emplaced 3 a large lagoon protected by ooid shoals developed during the bathonian in a muddy ramp setting 4 at the bathonian callovian boundary a second major change occurred with the lagoonal facies being superseded by an ooid bioclastic crinoids corals ramp associated with a waning of carbonate productivity retrogradational trend our geochemical study including a compilation of bibliographic data allows us to refine the existing sea surface palaeotemperature pattern a cooling from the latest early bajocian to the late bajocian and a subsequent warming from the earliest callovian to the early middle callovian transition are described using a proxy of sea surface temperatures together with 13c and mineralogical data our new 18o values suggest a palaeo climatic palaeo environmental control of facies in this shallow carbonate ramp environment

implications of trace element composition of syntaxial quartz cements for the geochemical conditions during quartz precipitation in sandstones a petrographic investigation revealed polyphase quartz cementation in the finefrau sandstone upper carboniferous western germany and the solling sandstone lower triassic central germany three different cements could be distinguished in each sandstone based on their cathodoluminescence and trace element composition the first quartz generation is suggested to have been formed during eogenesis due to dissolution and replacement of feldspar the mesogenetic paragenesis comprises two generations of quartz and illite which are accompanied by albite in the solling sandstone sharp luminescence zoning in quartz overgrowths points to distinct episodes of cementation in both sandstones significant amounts of al li and h and traces of ge and b have been detected in the quartz overgrowths the al content of the quartz cements in the finefrau sandstones exceeds that in the quartz cements in the solling sandstone by a factor of five it is suggested that this compositional variation reflects the conditions in the pore water such as temperature and ph the al concentration is generally correlated to the li content with the exception of the latest quartz generation in the finefrau sandstones which is also most enriched in trace elements the ratio of li al varies between 0 11 and 0 25 in the two sandstones the li h ratio which ranges from 0 12 to 0 3 is controlled by the activity ratio of li and h in the pore fluid clay minerals are the most important source for li and high salinities favour the mobilization of li during diagenesis thus a relatively low salinity and low ph are responsible for the low li h ratio in the finefrau sandstone while high salinity and neutral to alkaline ph results in a high li h ratio for the solling sandstone the ge contents are generally near the average of detrital quartz and indicate that pressure dissolution is a major source for quartz cementation different chemical compositions of distinct quartz generations indicate changes in the physico chemical conditions and point to mobilization of silica from different sources for example pressure solution and clay mineral transformations

quantification of a greenhouse hydrologic cycle from equatorial to polar latitudes the mid cretaceous water bearer revisited article this study aims to investigate the global hydrologic cycle during the mid cretaceous greenhouse by utilizing the oxygen isotopic composition of pedogenic carbonates calcite and siderite as proxies for the oxygen isotopic composition of precipitation the data set builds on the aptian albian sphaerosiderite 18 o data set presented by ufnar et al 2002 by incorporating additional low latitude data including pedogenic and early meteoric diagenetic calcite 18 o ufnar et al 2002 used the proxy data derived from the north american cretaceous western interior basin kwib in a mass balance model to estimate precipitation evaporation fluxes we have revised this mass balance model to handle sphaerosiderite and calcite proxies and to account for longitudinal travel by tropical air masses we use empirical and general circulation model gcm temperature gradients for the mid cretaceous and the empirically derived 18 o composition of groundwater as constraints in our mass balance model precipitation flux evaporation flux relative humidity seawater composition and continental feedback are adjusted to generate model calculated groundwater 18 o

oil generation as the dominant overpressure mechanism in the cenozoic dongying depression bohai bay basin china the dongying depression in the bohai bay basin is a young prolific oil producing basin in china the gray to black mudstones calcareous mudstones and oil shales of the eocene shahejie formation es3 and es4 are the major source rocks that are primarily dominated by type i kerogens with a high total organic carbon of up to 18 6 the es3 interval is characterized by a sedimentation rate of up to 500 m m y widespread overpressures are present in the eocene es3 and es4 intervals in the depression with pressure coefficients up to 1 99 from drillstem tests among the sonic resistivity and density logs only the sonic log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress pore pressures are accurately predicted in the dongying depression using an eaton 1972 exponent of 2 0 by comparing the predicted pressure coefficients with measured pressure coefficients disequilibrium compaction has been previously proposed as the sole cause of the high magnitude overpressures in the eocene strata of the dongying depression because of rapid deposition of fine grained sediments however our data indicate that the overpressures are caused by oil generation from the source rocks within the es3 and es4 intervals the overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density the apparent lack of correlation between mudstone densities and effective vertical stress and the overpressured reservoir sandstones showing no anomalous high matrix porosities or anomalous geothermal gradient the depths to the top of the overpressure intervals range from 2000 to 3000 m 6562 9843 ft following closely source rock depths all the overpressured reservoirs and source rocks have a minimum temperature of approximately 87c and overpressured source rocks generally have vitrinite reflectance ro values of 0 6 or higher overpressures are not found in the strata within which the ro values are less than 0 5 the overpressured reservoirs in the es3 and es4 intervals are predominantly oil saturated or oil bearing organic rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure the calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal the presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing

hydrothermally influenced cementation of sandstone an example from deeply buried cambrian sandstones from bornholm denmark abstract cambrian sandstones on bornholm denmark are tightly cemented by quartz cementation occurred after deep burial as evidenced by a very strong compaction partly mediated by intensive grain fracturing the quartz cement has a complex cathodoluminescence sem cl zonation which is related to variable substitution of al in the quartz lattice based on the substitution pattern and sem cl zonation three phases of quartz growth can be identified phase 1 cement has a low sem cl level and its volume corresponds approximately to the estimated volume of press solved detrital quartz it is related to the late compaction phase 2 cement is the volumetrically most important it has an intricate sem cl zonation and associated al substitution it is referred to hydrothermal activity related to the permo carboniferous magmatic and tectonic event the cementation gradually reduced the influence of hydrothermal flow and late phase 3 cement has a low intensity and more simple sem cl pattern the almost fully cemented sandstone was fractured and the fractures were filled by hydrothermal chlorite sulphides and quartz these minerals are also found sporadically as cement in the sandstones

cretaceous sea surface temperature evolution constraints from tex86 and planktonic foraminiferal oxygen isotopes it is well established that greenhouse conditions prevailed during the cretaceous period 145 66 ma determining the exact nature of the greenhouse gas forcing climatic warming and climate sensitivity remains however an active topic of research quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on cretaceous climate in particular reconstructions of cretaceous sea surface temperatures ssts have been revolutionised firstly by the recognition that clay rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen isotope analyses and secondly by the development of the organic palaeothermometer tex86 based on the distribution of marine archaeal membrane lipids here we provide a new compilation and synthesis of available planktonic foraminiferal 18o 18opl and tex86 sst proxy data for almost the entire cretaceous period the compilation uses ssts recalculated from published raw data allowing examination of the sensitivity of each proxy to the calculation method e g choice of calibration and places all data on a common timescale overall the compilation shows many similarities with trends present in individual records of cretaceous climate change for example both sst proxies and benthic foraminiferal 18o records indicate maximum warmth in the cenomanian turonian interval our reconstruction of the evolution of latitudinal temperature gradients low 48 palaeolatitudes reveals temporal changes in the valanginian aptian the low to higher mid latitudinal temperature gradient was weak decreasing from 10 17 c in the valanginian to 3 5 c in the aptian based on tex86 ssts in the cenomanian santonian reconstructed latitudinal temperature contrasts are also small relative to modern 14 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts compared with 20 c for the modern in the mid campanian to end maastrichtian latitudinal temperature gradients strengthened 19 21 c based on low latitude tex86 and 18opl ssts minus higher latitude 18opl ssts with cooling occurring at low middle and higher palaeolatitude sites implying global surface ocean cooling and or changes in ocean heat transport in the late cretaceous these reconstructed long term trends are resilient regardless of the choice of proxy tex86 or 18opl or calibration this new cretaceous sst synthesis provides an up to date target for modelling studies investigating the mechanics of extreme climates

middle late cretaceous climate of the southern high latitudes stable isotopic evidence for minimal equator to pole thermal gradients a detailed 18 o and 13 c stratigraphy has been generated from analysis of well preserved albian early maastrichtian foraminifera from deep sea drilling project dsdp sites 511 and 327 falkland plateau 58 s 62 s paleolatitude in the southern south atlantic and cenomanian and coniacian santonian foraminifera from dsdp site 258 naturaliste plateau 58 s paleolatitude in the southern indian ocean these results when combined with previously published maastrichtian stable isotope data from ocean drilling program odp site 690 weddell sea 65 s paleolatitude provide new insight into the climatic and oceanographic history of the southern high latitudes during middle late cretaceous time the planktonic foraminifer 18 o curves reveal a gradual warming of surface waters from the albian through the cenomanian followed by extremely warm surface waters from the turonian through the early campanian long term cooling of surface waters began in the late early campanian and continued through the end of the maastrichtian the benthic foraminifer 18 o record generally parallels changes in the oxygen isotopic curves defined by shallow dwelling planktonic foraminifera the vertical oxygen and carbon isotopic gradients were relatively low during the albian cenomanian high from the turonian early campanian and then low during the late campanian and maastrichtian foraminiferal oxygen isotopic data from published sources and this study are averaged for each site corrected for latitudinal changes in salinity based on modern day surface water values and plotted versus paleolatitude for the late albian coniacian santonian and late maastrichtian differences between lowand high latitude surface water paleotemperatures are estimated at 14 c during the late albian and late maastrichtian but the coniacian santonian reconstruction reveals only a 0 4 c latitudinal temperature gradient uncertainty regarding cretaceous salinity gradients and possible diagenetic alteration of 18 o values introduce error into our estimates of paleolatitudinal thermal gradients however apparent low equator to pole temperature differences could indicate much higher poleward heat transport than at present

diagenetic evolution and porosity destruction of turbiditic hybrid arenites and siliciclastic sandstones of foreland basins evidence from the eocene hecho group pyrenees spain abstract this study aims to unravel the impact of diagenetic alterations on porosity loss of foreland basin turbiditic hybrid arenites and associated siliciclastic sandstones of the eocene hecho group south central pyrenees spain in this succession hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement 18ovpdb 10 0 to 5 8 th mode 80 c salinity mode 18 8 wt eq nacl fe dolomite 18ovpdb 8 5 to 6 3 and trace amounts of siderite the extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains which provided nuclei and sources for the precipitation and growth of carbonate cements other diagenetic alterations such as pyrite and albitization had no impact on reservoir quality scarce early diagenetic cements coupled with abundant ductile carbonate and siliciclastic framework grains have led to rapid porosity loss owing to compaction conversely abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction reservoir quality was affected by mesogenetic cementation by quartz overgrowths calcite and dolomite intergranular pressure dissolution of quartz grains and formation of fracture filling calcite cement 18ov pdb values from 10 4 to 7 8 th temperatures of 150 c which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism the results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition particularly the proportion and types of extrabasinal noncarbonates extrabasinal carbonates and intrabasinal carbonate grains these insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones degradation of reservoir quality makes these hybrid arenites calclithites and sandstones suitable as tight gas reservoirs but only if fracture porosity and permeability develop during tectonic deformation

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

diagenesis and reservoir quality of the lower cretaceous quantou formation tight sandstones in the southern songliao basin china abstract the lower cretaceous quantou formation in the southern songliao basin is the typical tight oil sandstone in china for effective exploration appraisal and production from such a tight oil sandstone the diagenesis and reservoir quality must be thoroughly studied first the tight oil sandstone has been examined by a variety of methods including core and thin section observation xrd sem cl fluorescence electron probing analysis fluid inclusion and isotope testing and quantitative determination of reservoir properties the sandstones are mostly lithic arkoses and feldspathic litharenites with fine to medium grain size and moderate to good sorting the sandstones are dominated by feldspar quartz and volcanic rock fragments showing various stages of disintegration the reservoir properties are quite poor with low porosity average 8 54 and permeability average 0 493 md small pore throat radius average 0 206 m and high displacement pressure mostly higher than 1 mpa the tight sandstone reservoirs have undergone significant diagenetic alterations such as compaction feldspar dissolution quartz cementation carbonate cementation mainly ferrocalcite and ankerite and clay mineral alteration as to the onset time the oil emplacement was prior to the carbonate cementation but posterior to the quartz cementation and feldspar dissolution the smectite to illite reaction and pressure solution at stylolites provide a most important silica sources for quartz cementation carbonate cements increase towards interbedded mudstones mechanical compaction has played a more important role than cementation in destroying the reservoir quality of the k 1 q 4 sandstone reservoirs mixed layer illite smectite and illite reduced the porosity and permeability significantly while chlorite preserved the porosity and permeability since it tends to be oil wet so that later carbonate cementation can be inhibited to some extent it is likely that the oil emplacement occurred later than the tight rock formation with the porosity close to 10 however thicker sandstone bodies more than 2 m constitute potential hydrocarbon reservoirs

warm tropical ocean surface and global anoxia during the mid cretaceous period the middle of the cretaceous period about 120 to 80 myr ago was a time of unusually warm polar temperatures1 repeated reef drowning in the tropics2 and a series of oceanic anoxic events oaes that promoted both the widespread deposition of organic carbon rich marine sediments and high biological turnover3 4 5 6 7 8 the cause of the warm temperatures is unproven but widely attributed to high levels of atmospheric greenhouse gases such as carbon dioxide7 8 9 10 11 12 in contrast there is no consensus on the climatic causes and effects of the oaes with both high biological productivity and ocean stagnation being invoked as the cause of ocean anoxia3 4 5 6 7 8 here we show using stable isotope records from multiple species of well preserved foraminifera that the thermal structure of surface waters in the western tropical atlantic ocean underwent pronounced variability about 100 myr ago with maximum sea surface temperatures 3 5 c warmer than today this variability culminated in a collapse of upper ocean stratification during oae 1d the breistroffer event a globally significant period of organic carbon burial that we show to have fundamental stratigraphically valuable geochemical similarities to the main oaes of the mesozoic era our records are consistent with greenhouse forcing being responsible for the warm temperatures but are inconsistent both with explanations for oaes based on ocean stagnation and with the traditional view reviewed in ref 12 that past warm periods were more stable than today s climate

too low δ18o of paleo meteoric low latitude water do paleo tropical cyclones explain it abstract isotopic analysis of rock samples showed anomalously 18 o depleted reconstructed isotopic composition of paleo meteoric water over southern israel between the late cretaceous and the early miocene with 18 o w as low as 14 smow this range is significantly lighter than the expected values for rain at the relevant paleo latitudes 8 n to 25 n it is similar or lighter than areas with meteoric water at the present climate in which rainfall contribution from tropical cyclone tc is significant rainfall from tcs is 18 o depleted because of the recirculation of the vapor into repeated precipitation or distillation cycles a paleo geographic reconstruction of the region shows that throughout the period there was a warm ocean to the east and southeast of the relevant area thus tcs formed over the ocean and made landfall on the coastline at a distance of 300 to 400 km to the east the reconstructed composition of the rainwater implies dominance of tcs at these times furthermore the combination of warmer seas and lighter rainwater than at present suggests that the tcs were more dominant than in the most tc prone coastal areas at the present climate the results of the present study suggest that climate during the relevant time periods was very different from the present with tc activity similar or exceeding the most tc prone locations in the world at the present climate this might also hint that warmer sea waters in the geological past were conducive to greater tc activity than in the present climate answering this question requires additional research

growth and demise of the jurassic carbonate platform in the intracratonic paris basin france interplay of climate change eustasy and tectonics it is usually very difficult to identify and quantify the relative influence of tectonics eustasy and climateon carbonate system evolution from sedimentary records in order to improve our understanding of these mechanisms we have traced for the first time the evolution of the eastern paris basin platform throughout the entire jurassic period this carbonate platform underwent eight successive growth and demise phases with different depositional profiles ranging from ramps to flat topped geometries the eight carbonate growth periods are compared with the standard sea level curves local tectonic regimes and recently published oxygen isotope and or clay mineralogy databases prograding heterozoan facies along ramp profiles mark periods dominated by second order eustatic sea level rise relatively cool sea surface temperatures and mesotrophic and humid conditions hettangian pliensbachian late oxfordian tithonian during these periods variable detrital contents in the sedimentary succession hampered the efficiency of shallow marine carbonate factories higher sea surface temperatures oligotrophic and humid conditions associated with either eustatic sea level rise or very high local subsidence occurred during the early bajocian and the mid oxfordian these seawater properties seem to have favoured the aggradation of scleractinian corals forming dome shaped bioherm buildups an oolitic and lime mud carbonate system deposited during the bathonian second order eustatic sea level fall is characterised by miliolid rich micritic facies on a rimmed ramp under stable cooler and drier conditions the secondorder maximum flooding associated with a sea surface temperature decline and or a seawater eutrophication caused at least five carbonate demise periods i e toarcian earliest late bajocian callovian oxfordian transition earliest late oxfordian and kimmeridgian

significance of trace elements in syntaxial quartz cement haushi group sandstones sultanate of oman abstract trace element contents and distributions in authigenic quartz cement in deeply buried 2500 4000 m haushi group sandstones from wells in oman have been investigated in order to determine the factors that control trace element uptake during precipitation scanning electron microscope cathodoluminescence images show well developed growth zones within the quartz cement which correlate with chemical zonations observed in electron microprobe al distribution maps the most abundant trace elements are al 50 3000 g g 1 li 1 100 g g 1 na 1 40 g g 1 and ge 0 3 5 g g 1 with a strong linear correlation between li and al and a weaker one between ge and al the molar concentration of li na accounts only for 15 of the charge compensation for al 3 substitution of si 4 though h was not measured in this study these data indicate a major role of h in charge balancing al 3 the samples belong to the same stratigraphic unit and have similar petrography but show considerable variability in absolute trace element concentrations between different wells this variability does not correlate with either sample depth or temperature and shows no regional pattern but seems to reflect petrophysical and tectonic differences within the sedimentary basin petrographic observations of the cogenetic mineral assemblages and hydrochemical modelling indicate that a change from the equilibrium assemblage quartz kaolinite dolomite to quartz illite dolomite reflects a decrease in the co 2 concentration and concurrent variations of the al concentration it is concluded that changes in the co 2 concentrations are responsible for fluctuations in fluid al concentrations and thus likely also in the investigated quartz cements

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

diagenesis and reservoir quality evolution of palaeocene deep water marine sandstones the shetland faroes basin british continental shelf abstract the palaeocene deep water marine sandstones recovered from six wells in the shetland faroes basin represent lowstand transgressive and highstand systems tract turbiditic sediments mineralogic petrographic and geochemical analyses of these siliciclastics are used to decipher and discuss the diagenetic alterations and subsequent reservoir quality evolution the middle upper palaeocene sandstones subarkoses to arkoses from the shetland faroes basin british continental shelf are submarine turbiditic deposits that are cemented predominantly by carbonates quartz and clay minerals carbonate cements intergranular and grain replacive calcite siderite ferroan dolomite and ankerite are of eogenetic and mesogenetic origins the eogenetic alterations have been mediated by marine meteoric and mixed marine meteoric porewaters and resulted mainly in the precipitation of calcite 18 o v pdb 10 9 and 3 8 trace amounts of non ferroan dolomite siderite 18 o v pdb 14 4 to 0 6 as well as smectite and kaolinite in the lowstand systems tract lst and highstand systems tract hst turbiditic sandstone below the sequence boundary minor eogenetic siderite has precipitated between expanded and kaolinitized micas primarily biotite the mesogenetic alterations are interpreted to have been mediated by evolved marine porewaters and resulted in the precipitation of calcite 18 o v pdb 12 9 to 7 8 and fe dolomite ankerite 18 o v pdb 12 1 to 6 3 at temperatures of 50 140 and 60 140 c respectively quartz overgrowths and outgrowth which post and pre date the mesogenetic carbonate cements is more common in the lst and tst of distal turbiditic sandstone discrete quartz cement which is closely associated with illite and chlorite is the final diagenetic phase the clay minerals include intergranular and grain replacive eogenetic kaolinite smectite and mesogenetic illite and chlorite kaolinite has been subjected to mesogenetic replacement by dickite the k feldspar and plagioclase grains have been albitized dissolution of calcite cement and of framework grain feldspar volcanic fragments and mud intraclasts has resulted in a considerable enhancement of reservoir quality

isotopic tracers in polar seas and glacier ice the relationship between isotope concentration and salinity provides important evidence on the origin of water masses in polar regions it can be used to deduce the isotopic composition of melting ice we discuss a source of error implicit in the use of isotope salinity diagrams for this purpose we argue that conventional 18o and d salinity diagrams should be abandoned in favor of 1 s 18o and 1 s d salinity diagrams which are physically more meaningful and which introduce no significant error

heat transport deep waters and thermal gradients coupled simulation of an eocene greenhouse climate for the first time a coupled general circulation model with interactive and dynamical atmospheric oceanic and sea ice components is used to simulate an eocene 50 ma greenhouse climate we introduce efficient ocean spin up methods for coupled paleoclimate modeling sea surface temperatures ssts and salinities evolve unconstrained producing the first proxy data independent estimates for these eocene climate parameters tropical and extratropical model predicted ssts are warmer than modern values by 3 and 5 c respectively salinity driven deep water formation occurs in the north atlantic and tethys the zonal average overturning circulation is weaker than modern eocene ocean heat transport is 0 6 pw less than modern in the northern hemisphere and 0 4 pw greater in the southern hemisphere the model predicted near modern vertical and meridional eocene temperature gradients imply that the dominant theory for maintaining low gradients increased ocean heat transport is incorrect or incomplete and other mechanisms should be explored

climatic fluctuations and seasonality during the late jurassic oxfordian early kimmeridgian inferred from δ18o of paris basin oyster shells abstract oxygen isotope data from biostratigraphically well dated oyster shells from the late jurassic of the eastern paris basin are used to reconstruct the thermal evolution of western tethyan surface waters during the early oxfordian early kimmeridgian interval seventy eight oyster shells were carefully screened for potential diagenetic alteration using cathodoluminescence microscopy isotope analyses were performed on non luminescent parts of shells n 264 intra shell 18 o variability was estimated by microsampling along a transect perpendicular to the growth lines of the largest oyster shell the sinusoidal distribution of the 18 o values along this transect and the dependence of the amplitude of variations with bathymetry suggest that intra shell variability reflects seasonal variations of temperature and or salinity average amplitudes of about 5 c in shallow water environments and of about 2 3 c in deeper offshore environments are calculated these amplitudes reflect minimum seasonal temperature variation our new data allow to constrain existing paleotemperature trends established from fish tooth and belemnite 18 o data and are in better agreement with paleontological data more specifically a warming trend of about 3 c is reconstructed for oceanic surface waters during the early to middle oxfordian transition with maximum temperatures reaching 24 c in the transversarium zone late middle oxfordian from the transversarium zone to the bimmamatum zone a cooling of about 7 c is indicated whereas from the bimmamatum zone temperatures increased again by about 7 c to reach 24 c in average during the cymodoce zone early kimmeridgian

possible atmospheric co2 extremes of the middle cretaceous late albian turonian 1 atmospheric carbon dioxide co2 estimates for the middle cretaceous mk have a range of 4000 ppm which presents considerable uncertainty in understanding the possible causes of warmth for this interval this paper examines the problem of mk greenhouse forcing from an inverse perspective we estimate upper ocean water temperatures from oxygen isotope measurements of well preserved late albian turonian planktonic foraminifera and compare these against temperatures predicted by general circulation model gcm experiments with co2 concentrations of 500 7500 ppm at least 4500 ppm co2 is required to match maximum temperatures inferred from well preserved planktonic foraminifera approximately 900 ppm co2 produces a good match between the model and the minimum temperature estimates for the mk an ocean model forced by these two extremes in surface conditions brackets nearly all available bottom water temperature estimates for this interval the climate model results support nearly the entire range of mk co2 estimates from proxy data the ocean model suggests possible mk oceanographic changes from deep water formation in the high latitude region of one hemisphere to the other hemisphere in response to changes in atmospheric temperatures and hydrologic cycle strength we suggest that rather than contradicting one another the various proxy co2 techniques especially those with high temporal resolution may capture true variability in co2 concentrations and that mk co2 could have varied by several thousand ppm through this interval

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

open system chemical behavior in deep wilcox group mudstones texas gulf coast usa abstract wilcox group mudstones have been mechanically and geochemically transformed over a temperature range of 20 200 c our research controlled for provenance and age by sampling from five wells parallel to the paleodepositional axis all within the houston delta system across the sampled depths mudstone porosity has been reduced from 25 to

evidence for deep water production in the north pacific ocean during the early cenozoic warm interval the deep ocean circulation is responsible for a significant component of global heat transport in the present mode of circulation deep waters form in the north atlantic and southern oceans where surface water becomes sufficiently cold and dense to sink polar temperatures during the warmest climatic interval of the cenozoic era approximately 65 to 40 million years myr ago were significantly warmer than today and this may have been a consequence of enhanced oceanic heat transport however understanding the relationship between deep ocean circulation and ancient climate is complicated by differences in oceanic gateways which affect where deep waters form and how they circulate here i report records of neodymium isotopes from two cores in the pacific ocean that indicate a shift in deep water production from the southern ocean to the north pacific approximately 65 myr ago the source of deep waters reverted back to the southern ocean 40 myr ago the relative timing of changes in the neodymium and oxygen isotope records indicates that changes in cenozoic deep water circulation patterns were the consequence not the cause of extreme cenozoic warmth

cathodoluminescent textures and the origin of quartz silt in oligocene mudrocks south texas abstract in subsurface mudrocks of the oligocene frio formation in south texas a substantial proportion of silt size quartz 10 62 m manifests cathodoluminescence cl that varies greatly in intensity within each particle giving rise to textures that differ greatly from grain to grain in contrast most quartz grains of sand size 62 m have cl that is relatively homogeneous within individual grains a substantial percentage of the quartz within silt grains is very weakly luminescent no depth trend in the occurrence of luminescent textures in detrital quartz is observed suggesting that the component of dark possibly low temperature silt size quartz is not related to chemical processes in the present burial setting paleogene rhyolites of west texas are a partial model for the source rocks that supplied sediment to the frio formation phenocrystic quartz in these volcanics has uniform cl similar to that observed in the frio sand fraction whereas complex cl structure seen in the associated groundmass bears many similarities to the cl textures observed in frio silts this observation further supports the idea that cl structure of detrital quartz in the frio is most likely an inherited feature if the quartz with dark cl is of low temperature origin it would help to elucidate the relatively 18o enriched isotopic values reported for mudrock quartz in the frio

deep sea paleotemperature record of extreme warmth during the cretaceous oxygen isotope analyses of well preserved foraminifera from blake nose 30 n paleolatitude north atlantic and globally distributed deep sea sites provide a long term paleotemperature record for the late albian maastrichtian interval that is difficult to reconcile with the existence of significant cretaceous ice sheets given reasonable assumptions about the isotopic composition of cretaceous seawater our results suggest that middle bathyal water temperatures at blake nose increased from 12 c in the late albian through middle cenomanian to a maximum of 20 c during the latest cenomanian and earliest turonian bottom waters were again 12 c during the middle campanian and cooled to a minimum of 9 c during the maastrichtian correlative middle bathyal foraminifera from other ocean basins yield paleotemperature estimates that are very similar to those from blake nose comparison of global bottom water temperatures and latitudinal thermal gradients suggests that global climate changed from a warm greenhouse state during the late albian through late cenomanian to a hot greenhouse phase during the latest cenomanian through early campanian then to cool greenhouse conditions during the mid campanian through maastrichtian

evaluation of shark tooth diagenesis screening methods and the application of their stable oxygen isotope data for palaeoenvironmental reconstructions the 18o po4 of shark teeth can be used as a geochemical proxy for the palaeoenvironment during tooth mineralization a well preserved state of the teeth is necessary for this approach we used polarization cathodoluminescence cl and scanning electron microscopy to investigate two fossil shark teeth sphenodus nitidus from the late jurassic nusplingen plattenkalk the data are compared with those for two recent shark teeth hemipristis elongata lamiopsis temminckii 18o po4 of six further jurassic shark teeth was measured and is compared with 18o data from co occurring belemnite rostra and carbonate sediments the enameloid of both the fossil and the recent shark teeth shows orange red cl which is due to incorporation of mn during biomineralization the 18o po4 values of the fossil enameloid are constant around 21 50 v smow whereas those of the fossil dentine are more negative with values around 20 v smow which indicates that the enameloid is well preserved palaeotemperatures calculated from the enameloid are like those derived from belemnites we demonstrate that cl microscopy should only be used cautiously as a diagenesis screening method for fossil biophosphates geochemical and microstructural analyses of tissue types with different susceptibility to diagenetic alteration on the other hand provide an effective way to identify well preserved tissues supplementary material polarization microscopy images energy dispersive x ray data and cathodoluminescence spectra are available at https doi org 10 6084 m9 figshare c 4373720

the evolution of late cretaceous deep ocean circulation in the atlantic basins neodymium isotope evidence from south atlantic drill sites for tectonic controls 1 the south atlantic basins during the late cretaceous were characterized by overall expansion and the opening of deep water connections eventually permitting deep ocean circulation with the indian and north atlantic basins however the evolving pattern of deep ocean circulation through the late cretaceous particularly the timing of the connection between various basins is not well constrained here we present new neodymium isotope data from five sites in the south atlantic to reconstruct water mass chemistry and circulation patterns the new data combined with previously published data indicate deep water formation occurred in the high latitude south atlantic throughout the late cretaceous tectonic boundaries restricted the circulation of southern component water until the opening of the equatorial atlantic gateway and the subsidence of rio grande rise between 79 and 75 ma subsidence of these features permitted the northward flow of southern component water which then ventilated the deep north atlantic nd isotope data indicate that the south atlantic water column during the cenomanian to santonian was vertically stratified between intermediate and deep waters with increased mixing evident during the campanian and maastrichtian

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

the middle to upper jurassic stable isotope record of madagascar linking temperature changes with plate tectonics during the break up of gondwana abstract stable isotope 18o 13c analyses were performed on well preserved belemnites oysters and rhynchonellid brachiopods from the middle to upper jurassic of the morondava basin in southern madagascar both brachiopods and oysters indicate similar average temperatures of 18 7 to 19 3 c in the early callovian followed by a temperature decrease towards the middle oxfordian 13 9 c and a minimum in the early kimmeridgian 12 3 c in contrast belemnites from the oxfordian show lower average temperatures of 10 0 c which is likely caused by specific conditions for these organisms e g different fractionation or life habits additionally three oysters from the upper oxfordian and lower kimmeridgian were used for high resolution stable isotope analyses the data show seasonal fluctuations of 6 c around averages between 14 4 and 14 7 c latitudinal temperature gradients for the callovian and kimmeridgian are similar to today at the examined low latitudes of the southern hemisphere the observed cooling of around 5 c from the callovian to the oxfordian kimmeridgian can be attributed to a concurrent southward drift of madagascar during the break up of gondwana thus the study underlines the importance of considering palaeogeography in interpreting stable isotope data as well as the potential of detecting and timing palaeogeographic events by using stable isotope analyses

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

extreme polar warmth during the cretaceous greenhouse paradox of the late turonian δ18o record at deep sea drilling project site 511 1 oxygen isotope data for upper turonian planktonic foraminifera at deep sea drilling project site 511 falkland plateau 60 s paleolatitude exhibit an 2 excursion to values as low as 4 66 vienna peedee belemnite standard pdb coincident with the warmest tropical temperature estimates yet obtained for the open ocean the lowest planktonic foraminifer 1 8 o values suggest that the upper ocean was as warm as 30 32 c this is an extraordinary temperature for 60 s latitude but is consistent with temperatures estimated from apparently coeval mollusc 1 8 o from nearby james ross island 65 s paleolatitude glassy textural preservation a well defined depth distribution in site 511 planktonics low sediment burial temperature 32 c and lack of evidence of highly depleted pore waters argue against diagenesis even solid state diffusion as the cause of the very depleted planktonic values the lack of change in benthic foraminifer 1 8 o suggests brackish water capping as the mechanism for the low planktonic 1 8 o values however mixing ratio calculations show that the amount of freshwater required to produce a 2 shift in ambient water would drive a 7 psu decrease in salinity the abundance and diversity of planktonic foraminifera and nannofossils high planktonic benthic ratios and the appearance of keeled foraminifera argue against lower than normal marine salinities isotope calculations and climate models indicate that we cannot call upon more depleted freshwater 1 8 o to explain this record without more late turonian data especially from outside the south atlantic basin we can currently only speculate on possible causes of this paradoxical record from the core of the cretaceous greenhouse

effect of changes in d18o content of the surface ocean on estimated sea surface in past warm climate 1 using a coupled climate model of intermediate complexity including oxygen 18 climber 2 we investigate the evolution of the distribution of surface water 18o composition under warm climate conditions we then determine the impact of changes of the surface water 18o distribution on ocean surface temperatures inferred from calcite oxygen 18 our results show that published temperature reconstructions based on oxygen 18 from calcite are systematically biased by 2 to 4 c in the absence of major oceanic circulation changes and up to 7 c in the presence of major oceanic circulation changes as the bias introduced is shown to vary with latitude our work has major implications on past latitudinal temperature gradient reconstructions based on oxygen 18 measurements

warm saline bottom water in the ancient ocean considerable isotopic evidence indicates that oceanic bottom water was much warmer in the geological past a simple convection model driven by multiple turbulent buoyant plumes and observations of present deep water formation have led to the development of a theory for the formation of warm saline bottom water it is suggested that changes in the size and configuration of marginal seas in net evaporation zones due to lithospheric plate motions and eustatic sea level change caused these seas to become sources of warm saline bottom water evidence from the palaeotemperature record and palaeogeography during the late cretaceous is used to reinforce the hypothesis climatic and chemical consequences of the formation of warm saline bottom water are discussed

high paleolatitude late cretaceous paleotemperatures new data from james ross island antarctica oxygen isotope analysis of well preserved macrofossils from the santonian campanian of james ross island and the maastrichtian of vega island antarctica indicates that cool high paleolatitude temperatures prevailed during the late cretaceous and suggests that cooling occurred between the santonian campanian and the maastrichtian although more than 50 of the material showed diagenetic alteration 52 unaltered aragonite and calcite samples were analyzed mean delta sup 18 o and calculated paleotemperature values were minus 0 23 per thousand and 13 6 c respectively for the santonian campanian and 0 66 per thousand and 11 7 c respectively for the masstrichtian in conjunction with recent late cretaceous paleoclimatic data from high northern paleolatitudes these data indicate the presence of cool polar regions with broad climatic zonation during the late cretaceous this may have partly controlled faunal distributions

evolution of early cenozoic marine temperatures the equator to high southern latitude sea surface and vertical temperature gradients are reconstructed from oxygen isotope values of planktonic and benthic foraminifers for the following five time intervals late paleocene early eocene early middle eocene late eocene and early oligocene paleotemperatures are calculated using standard oxygen isotope temperature equations with adjustments to account for 1 variations in sea water 18o related to changes in global ice volume over time and 2 latitudinal gradients in surface water 18o these reconstructions indicate that sea surface temperatures sst of the southern oceans in the early eocene were as high as 15 c whereas temperatures during the late paleocene and early middle eocene reached maximum levels of 10 12 c by the late eocene and early oligocene high latitude sst had declined to 6 and 4 c respectively for most of the early paleogene low latitude sub tropical temperatures remained constant and well within the range of holocene temperatures 24 ndash 25 c but by the late eocene and early oligocene declined to values in the range of 18 to 22 c the late paleogene apparent decline in tropical temperatures however might be artificial because of dissolution of near surface foraminifera tests which biased sediment assemblages toward deeper dwelling foraminifera moreover according to recent plate reconstructions it appears that the majority of sites upon which the late eocene and early oligocene tropical temperatures were previously established were located either in or near regions likely to have been influenced by upwelling global deepwater temperature on average paralleled southern ocean sst for most of the paleogene we speculate based on the overall timing and character of marine sea surface temperature variation during the paleogene that some combination of both higher levels of greenhouse gases and increased heat transport was responsible for the exceptional high latitude warmth of the early eocene

atmospheric carbon dioxide linked with mesozoic and early cenozoic climate change carbon isotopes of fossil plants and model simulations suggest that atmospheric carbon dioxide levels were variable during the period 200 to 60 million years ago the large decreases in the partial pressure of co2 coincide with glaciations providing evidence against climate co2 decoupling during the mesozoic

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

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

jiggling the tropical thermostat in the cretaceous hothouse modern open ocean sea surface temperatures rarely exceed 28 29 8c and the same has been thought to represent a rough maximum for past tropical climates however new isotopic estimates from the uppermost cenomanian in the tropical western north atlantic suggest that mixed layer temperatures reached 33 34 8 c 62 8c during the middle cretaceous hothouse uppermost cenomanian tropical sea surface temperatures may have been as much as 4 7 8c warmer than the highest modern mean annual temperatures such extreme conditions suggest that warm tropical oceans could have driven substantially intensified atmospheric heat transport near the cenomanian turonian boundary the tropical thermostat was set higher than today challenging the hypothesis of tropical climate stability

possible role of oceanic heat transport in early eocene climate increased oceanic heat transport has often been cited as a means of maintaining warm high latitude surface temperatures in many intervals of the geologic past including the early eocene although the excess amount of oceanic heat transport required by warm high latitude sea surface temperatures can be calculated empirically determining how additional oceanic heat transport would take place has yet to be accomplished that the mechanisms of enhanced poleward oceanic heat transport remain undefined in paleoclimate reconstructions is an important point that is often overlooked using early eocene climate as an example we consider various ways to produce enhanced poleward heat transport and latitudinal energy redistribution of the sign and magnitude required by interpreted early eocene conditions our interpolation of early eocene paleotemperature data indicate that an approximately 30 increase in poleward heat transport would be required to maintain eocene high latitude temperatures this increased heat transport appears difficult to accomplish by any means of ocean circulation if we use present ocean circulation characteristics to evaluate early eocene rates either oceanic processes were very different from those of the present to produce the early eocene climate conditions or oceanic heat transport was not the primary cause of that climate we believe that atmospheric processes with contributions from other factors such as clouds were the most likely primary cause of early eocene climate

isotopic evaluation of ocean circulation in the late cretaceous north american seaway the role of the north american seaway in creating and maintaining late cretaceous global greenhouse conditions has been unclear an isotopic analysis of marine turtle and fish fossils from western kansas and the mississippi embayment reveals that the inflow of tethyan surface waters to the seaway was limited during the mid and late cretaceous period north america was split by the north south oriented western interior seaway its role in creating and maintaining late cretaceous global greenhouse conditions remains unclear different palaeoceanographic reconstructions portray diverse circulation patterns1 2 3 the southward extent of relatively cool low salinity low 18o surface waters critically distinguishes among these models but past studies of invertebrates could not independently assess water temperature and isotopic compositions here we present oxygen isotopes in biophosphate from coeval marine turtle and fish fossils from western kansas representing the east central seaway and from the mississippi embayment representing the marginal tethys ocean our analyses yield precise seawater isotopic values and geographic temperature differences during the main transition from the coniacian to the early campanian age 87 82 myr and indicate that the seaway oxygen isotope value and salinity were 2 and 3 lower respectively than in the marginal tethys ocean we infer that the influence of northern freshwater probably reached as far south as kansas our revised values imply relatively large temperature differences between the mississippi embayment and central seaway explain the documented regional latitudinal palaeobiogeographic zonation4 5 and support models with relatively little inflow of surface waters from the tethys ocean to the western interior seaway2 3

oxygen 18 salinity relationship simulated by an oceanic general circulation model the distribution of h218o in the ocean is simulated using an oceanic general circulation model gcm surface isotopic fluxes prescribed to this model are derived from an atmospheric gcm which ensures their consistency the oceanic gcm is initialized with a homogeneous composition and redistributes isotopic gradients so as to equilibrate with the atmosphere a good agreement is found between the observations and the simulation concerning i the interbasin h218o gradients especially the atlantic being more enriched than the pacific as well as the meridional gradients and ii the h218o salinity relationship

thermal evolution of cretaceous tethyan marine waters inferred from oxygen isotope composition of fish tooth enamels 1 the evolution of subtropical 30 35 n upper ocean temperatures through the cretaceous is inferred from the oxygen isotope compositions of 64 fish teeth enamel coming from the western tethyan platform mean 18o values of 22 at the berriasian valanginian boundary decrease with oscillations to 18 5 around the cenomanian turonian boundary and progressively increase to 21 5 by the end of the cretaceous the similarity of this oxygen isotope curve for bioapatites from platform environments with those for foraminifera and bulk carbonates that were deposited in deeper waters and at other paleolatitudes indicates that they record global climatic signals major cooling events at the million year scale can be distinguished 1 at the berriasian valanginian boundary and 2 during the earliest late valanginian a third cooling event is detected during the earliest aptian these events already proposed as icehouse interludes during the lower cretaceous are also recorded at subtropical latitudes a progressive warming is identified from the aptian to the cenomanian turonian interval that corresponds to a thermal optimum and then upper ocean temperatures decreased to the maastrichtian minimum isotopic temperatures range from 15 c to 28 c assuming a 18oseawater of 1 for an ice free world taking more realistic 18oseawater values of 0 for tropical waters during glacial periods within the berriasian valanginian interval and earliest aptian or with above average salinities possibly the maastrichtian temperatures are increased by 4 5 c temperature differences between climatic extremes of the valanginian and cenomanian turonian are estimated to have been 10 c latitudinal thermal gradients for the albian cenomanian turonian and maastrichtian were 0 2 0 3 c latitude and thus weaker than modern oceanic values at about 0 4 c latitude

evidence for thermohaline circulation reversals controlled by sea level change in the latest cretaceous fluctuations in oxygen 18 o and carbon 13 c isotope values of benthic foraminiferal calcite from the tropical pacific and southern oceans indicate rapid reversals in the dominant mode and direction of the thermohaline circulation during a 1 m y interval 71 70 ma in the maastrichtian at the onset of this change benthic foraminiferal 18 o values increased and were highest in low latitude pacific ocean waters whereas benthic and planktic foraminiferal 13 c values decreased and benthic values were lowest in the southern ocean subsequently benthic foraminiferal 18 o values in the indo pacific decreased and benthic and planktic 13 c values increased globally these isotopic patterns suggest that cool intermediate depth waters derived from high latitude regions penetrated temporarily to the tropics the low benthic 13 c values at the southern ocean sites however suggest that these cool waters may have been derived from high northern rather than high southern latitudes correlation with eustatic sea level curves suggests that sea level change was the most likely mechanism to change the circulation and or source s of intermediate depth waters we thus propose that oceanic circulation during the latest cretaceous was vigorous and that competing sources of intermediate and deep water formation linked to changes in climate and sea level may have alternated in importance

sensitivity of seawater oxygen isotopes to climatic and tectonic boundary conditions in an early paleogene simulation with giss modele r 1 an isotope enabled ocean atmosphere general circulation model giss modele r is used to estimate the spatial gradients of the oxygen isotopic composition of seawater 18osw where is the deviation from a known reference material in per mil during the early paleogene 45 65 ma understanding the response of 18osw to changes in climatic and tectonic boundary conditions is important because records of carbonate 18o document changes in hydrology as well as changes in temperature and global ice volume we present results from an early paleogene configuration of modele r which indicate that spatial gradients of surface ocean 18osw during this period could have been significantly different to those in the modern ocean the differences inferred from modele r are sufficient to change early paleogene sea surface temperature estimates derived from primary carbonate 18o signatures by more than 2 c in large areas of the ocean in the north atlantic indian and southern oceans the differences in 18osw inferred from our simulation with modele r are in direct contrast with those from another 18o tracing model study which used different but equally plausible early paleogene boundary conditions the large differences in 18osw between preindustrial and early paleogene simulations and between models emphasizes the sensitivity of 18osw to climatic and tectonic boundary conditions for this reason absolute estimates of eocene paleocene temperature derived from carbonate 18o alone are likely to have larger uncertainties than are usually assumed

diagenesis and reservoir quality of lacustrine deep water gravity flow sandstones in the eocene shahejie formation in the dongying sag jiyang depression eastern china the lacustrine deep water gravity flow sandstone reservoirs in the middle of the third member of the shahejie formation 43 7 38 2 ma are the main exploration targets for hydrocarbons in the dongying sag jiyang depression eastern china the impact of diagenesis on reservoir quality of these sandstones has been established by a variety of methods petrographically the sandstones consist mainly of lithic arkose with 43 of quartz 33 of feldspar and 24 of lithic fragment with reservoirs having average porosity and permeability values of 17 1 and 38 1 md respectively in addition to their primary intergranular pores secondary pores are well developed from feldspar dissolution with pore throat radii ranging from 0 01 to 40 m the dissolution of feldspar and transformation of smectite to illite may be the sources of silica for the first phase of quartz overgrowth whereas illitization of kaolinite may be responsible for the second phase of quartz overgrowth in addition pressure dissolution of quartz is also an important source of silica for quartz overgrowth dissolution of feldspar has little impact on reservoir quality whereas compaction and carbonate cementation are the main factors significantly reducing reservoir quality oil charge may have retarded carbonate cementation and generated overpressure that retarded compaction thus preserving porosity precipitation of vermicular kaolinite resulting from feldspar dissolution promoted a change in the wettability of the reservoir from water wet to oil wet which is beneficial for oil entrapment the central parts of medium to thick bedded sandstones that have experienced oil charge twice appear to have the highest quality reservoirs based on petrophysical properties this work provides a new interpretation of the formation mechanism of high quality lacustrine deep water gravity flow sandstones reservoirs that have experienced multiple periods of oil charge

oceanic oxygen 18 at the present day and lgm equilibrium simulations with a coupled climate model of intermediate complexity abstract an earth system model of intermediate complexity climber 2 is used to simulate the oxygen 18 content of the water masses h182o in the oceans firstly we forced climber 2 with the fluxes from the atmospheric general circulation model echam simulated oceanic 18o fields for the present day are in good agreement with data secondly a water isotope module was developed to transport 18o in the atmosphere on a large scale and compute the 18o fluxes to the ocean at the atmosphere ocean interface using only variables already computed by climber 2 for the present day we successfully represent oxygen 18 distribution in the atlantic indian and pacific oceans and close agreement is also found when we compare modelled and observed 18ow salinity relationships during the last glacial maximum lgm we find that the major differences in the 18o oceanic fields apart from the global oceanic enrichment due to ice sheet build up are due to surface condition changes surface temperature shift in bottom water formation zones and that no drastic changes occurred in the 18ow salinity spatial relationship in addition we compute a calcite 18oc field for the atlantic and compare it to the available data to assess the variation between the lgm and the present day

late jurassic kimmeridgian tithonian stable isotopes δ18o δ13c and mg ca ratios new palaeoclimate data from helmsdale northeast scotland abstract the data presented here provide the first detailed stable isotope 18 o 13 c and geochemical mg ca investigation of kimmeridgian tithonian belemnites from the helmsdale coast scotland uk oxygen and carbon stable isotope values from well preserved specimens range from 2 8 to 0 3 and from 2 3 to 2 8 respectively the oxygen isotope data are consistent with palaeotemperatures of up to 24 c in the early kimmeridgian cymodoce zone and down to 11 c in the mid tithonian rotunda fittoni zones these estimates are strongly supported by the mg ca data which also indicate a cooling episode and very similar palaeotemperatures 11 22 c at this time the cooling event is associated with an approximately 5 decline in 13 c ratios this shift towards more negative 13 c values has also been recorded in the tethys and on the russian platform confirming that this event was of global origin such globally synchronous records can provide a valuable tool for stratigraphic correlation the helmsdale 13 c data are therefore combined here with other published belemnite isotope data from the jurassic of scotland from the callovian kimmeridgian of the isle of skye and the toarcian aalenian of raasay the resultant belemnite 13 c curve provides a detailed record of the toarcian tithonian british boreal realm that can be compared with coeval records to investigate regional differences in jurassic carbon isotope stratigraphy

the maastrichtian record from shatsky rise northwest pacific a tropical perspective on global ecological and oceanographic changes we present new isotopic and micropaleontological data from a depth transect on shatsky rise that record the response of the tropical pacific to global biotic and oceanographic shifts during the mid maastrichtian results reveal a coupling between the upper ocean characterized by a weak thermocline and low to intermediate productivity and intermediate waters during the earliest maastrichtian oxygen and neodymium isotope data suggest a significant contribution of relatively warm intermediate water from the north pacific isotopic shifts through the early maastrichtian suggest that this warmer water mass was gradually replaced by cooler waters originating in the southern ocean although the cooler water mass remained dominant through the remainder of the maastrichtian it was displaced intermittently at shallow intermediate depths by north pacific intermediate water the globally recognized mid maastrichtian event similar to 69 ma manifested by the brief appearance of abundant inoceramid bivalves over shallow portions of shatsky rise is characterized by an abrupt increase similar to 2degrees 3degrees c in sea surface temperatures a greater flux of organic matter out of the surface ocean and warmer similar to 4degrees c intermediate waters results implicate simultaneous changes in surface waters and the sources distribution patterns of intermediate water masses as an underlying cause for widespread biotic and oceanographic changes during mid maastrichtian time