how are diagenesis and reservoir quality linked to depositional facies a deltaic succession edgeoya svalbard abstract middle to late triassic strata exposed at edgeoya eastern svalbard represents an uplifted part of the northwestern corner of the barents sea this interval is characterized by a predominantly mud dominated deltaic depositional system where rocks with petroleum reservoir potential are expected both in delta front and channelized sandstone deposits recent drilling campaigns into time equivalent rocks in the barents sea have however been disappointing with porosity and permeability below expectation this study improves the current understanding of reservoir potential within depositional systems of this kind by examining the link between different depositional facies diagenesis and their impact on reservoir quality five depositional facies were mapped and correlated channel floodplain shallow marine prodelta and offshore our study suggests that diagenetic signatures that control the quality of reservoir rocks vary systematically with these depositional facies mechanical compaction was the main cause of porosity destruction in the channel shallow marine and floodplain samples while early carbonate cementation occludes the intergranular volume in the prodelta samples moreover quartz cement reduced the porosity in the shallow marine depositional facies the estimated maximum burial temperature of 124 c indicates burial to several km depths depositional settings within a deltaic to shallow marine system do not exert a strong control on the abundance of chlorite but it does impact whether the chlorite is pore filling or grain coating and porosity preserving efficient chlorite coating have aided in preserving porosities up to 32 in the very well well sorted medium to fine grained sandstones deposited by fluvial dominated channels

the salinity temperature and δ18o of the glacial deep ocean we use pore fluid measurements of the chloride concentration and the oxygen isotopic composition from ocean drilling program cores to reconstruct salinity and temperature of the deep ocean during the last glacial maximum lgm our data show that the temperatures of the deep pacific southern and atlantic oceans during the lgm were relatively homogeneous and within error of the freezing point of seawater at the ocean s surface our chloride data show that the glacial stratification was dominated by salinity variations in contrast with the modern ocean for which temperature plays a primary role during the lgm the southern ocean contained the saltiest water in the deep ocean this reversal of the modern salinity contrast between the north and south atlantic implies that the freshwater budget at the poles must have been quite different a strict conversion of mean salinity at the lgm to equivalent sea level change yields a value in excess of 140 meters however the storage of fresh water in ice shelves and or groundwater reserves implies that glacial salinity is a poor predictor of mean sea level

predictive distribution of shallow marine low porosity pseudomatrix rich sandstones in a sequence stratigraphic framework example from the ferron sandstone upper cretaceous usa reservoir quality prediction can be considerably improved with an integrated approach between sequence stratigraphy and diagenesis here we demonstrate that the occurrence of low porosity sandstones pseudomatrix rich i e matrix formed by mechanical compaction and squeezing of soft clasts in intergranular pores deposited in shallow marine settings can be predicted in a sequence stratigraphic framework these sandstones occur as lags bounding parasequences and thus may form baffles and barriers for vertical fluid flow and reservoir compartments between two sandstone bodies of adjacent parasequences the prediction of their occurrence and precise spatial distribution is therefore very useful for production of shallow marine reservoirs

basin scale diagenetic alteration of shoreface sandstones in the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah abstract leached zones whitecaps and dolomite cement bodies are volumetrically and areally significant features in shoreface sandstones of the upper cretaceous spring canyon and aberdeen members blackhawk formation book cliffs utah quantitative spatial and stratigraphic data of their distribution together with petrography and geochemistry allow development of a model for their formation within a sequence stratigraphic framework leached zones up to 5 m thick and 15 km in lateral extent are present beneath coals capping parasequences concretionary dolomite cement is present in shoreface sandstones downdip of parasequence capping coals and associated whitecaps these concretions are up to 2 m thick and 6 m in length and are present for up to 7 km downdip of these coals petrographic and field data indicate an early diagenetic timing for whitecap and dolomite cement formation 13c data suggest carbonate derivation from detrital dolomite whereas 18o data suggest that cement precipitated from porewater with a significant meteoric component although later diagenetic re equilibration and recyrstallization cannot be rejected we propose that organic acid generation during early diagenesis of coals led to the leaching of detrital dolomite from underlying sandstones and the formation of whitecaps meteoric water flowed basinward in response to base level lowering and sequence boundary development in the upper part of the aberdeen member remobilizing dolomite into shoreface sandstones dolomite precipitation resulted from the mixing of these meteoric fluids with downdip marine pore fluids and the presence of detrital dolomite in the shoreface sandstones that acted as nucleation sites for dolomite precipitation the geometry and extent of dolomite cement in the spring canyon and aberdeen members is in marked contrast to the much greater spatial extent of such cement in low accommodation space settings such as the stratigraphically younger desert member blackhawk formation the relatively simple aggradational to progradational evolution of the spring canyon and aberdeen member parasequences with only minor sequence boundary development in its upper part resulted in spatially limited meteoric fluid input into shoreface sandstones this resulted in dolomite cementation of shoreface sandstones up to a maximum of 7 km downdip of leached zones in the desert member extensive meteoric fluid migration as a result of major sequence boundary development resulted in dolomite cementation of shoreface sandstones for over 25 km downdip of leached zones this highlights that the degree and extent of early fluid diagenesis can be significantly controlled by the nature of sequence development having clear implications for developing predictive models for porosity permeability evolution in sandstone reservoirs

wave ravinement surfaces classification and key characteristics abstract the wave ravinement surface wrs is a surface of sequence stratigraphic significance that forms in shallow water settings during transgression it develops progressively landward during shoreface retreat due to wave action which erodes the substrate and leads to sediment bypass in the offshore direction the wrs may rework previously formed surfaces such as the maximum regressive surface also known as the transgressive surface and the subaerial unconformity becoming a systems tract and or sequence boundary due to the associated lithological contrast between underlying and overlying deposits and the presence of lags and or shell pavements the wrs has a very good appearance in both outcrop and cores and is often used as the boundary of shallow marine stratigraphic sequences the wrs therefore represents an important lithological and paleo environmental break and its recognition is significant for reconstructing stratigraphic architectures and the history of relative sea level change as well as for reservoir characterization several controlling factors govern the development of the wrs and of the associated deposits they include the accommodation to sedimentation a s ratio environmental energy topographic gradient roughness and lithology of the substrate relatively low rates of relative sea level rise and sediment supply high wave energy steep topographies and easily erodible substrates favor the development of the wrs these factors interact leading to a high variability in the amount of erosion shape length along depositional dip and deposits associated with these surfaces wrss may be classified based on either underlying or overlying deposits or a combination of both a classification based on the deposits that underlie the wrs helps to determine the location of the studied outcrop along depositional dip whereas a classification based on the deposits that overlie the wrs afford insights into the physical conditions that accompanied its formation however the best description of the wrs in terms of location of a considered outcrop physical conditions under which the wrs developed and their lateral variability type of substrate as well as its practical significance for reservoir modeling is achieved with a combination of the two criteria

old radiocarbon ages in the southwest pacific ocean during the last glacial period and deglaciation marine radiocarbon 14c dates are widely used for dating oceanic events and as tracers of ocean circulation essential components for understanding ocean climate interactions past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep water and surface water reservoirs but the apparent age of surface waters currently approximately 400 years in the tropics and approximately 1 200 years in antarctic waters might not be constant through time as has been assumed in radiocarbon chronologies and palaeoclimate studies here we present independent estimates of surface water and deep water reservoir ages in the new zealand region since the last glacial period using volcanic ejecta tephras deposited in both marine and terrestrial sediments as stratigraphic markers compared to present day values surface reservoir ages from 11 900 14c years ago were twice as large 800 years and during glacial times were five times as large 2 000 years contradicting the assumption of constant surface age furthermore the ages of glacial deep water reservoirs were much older 3 000 5 000 years the increase in surface to deep water age differences in the glacial southern ocean suggests that there was decreased ocean ventilation during this period

excess sediment 230th transport along the sea floor or enhanced water column scavenging 1 the sedimentary inventory of 230th is often employed to distinguish between areas which receive excess sediment as the result of along bottom transport from those which lose sediment as a result of winnowing this process is referred to as focusing a case is made here that at least in the western equatorial pacific the excess 230th in on equator sediment is more likely scavenged from the overlying water column by the rain of organic matter than retransported along the bottom if so lateral variations in biological productivity may well contribute to the excess 230th inventories elsewhere as well

high resolution sequence stratigraphy of clastic shelves iii applications to reservoir geology abstract high frequency clastic shelf sequences contain porous deposits consisting of coastal to shallow water sandstones and conglomerates which may represent very good reservoirs and therefore critical targets for exploration and production as well as for fluid hydrocarbon water or co2 storage the study of these reservoirs following a sequence stratigraphic approach is essential to predict their features and distribution within systems tracts forming high frequency sequences which in turn compose higher rank systems tracts and sequences a critical aspect to improve the exploitability of the reservoirs is their heterogeneity which directly affects the volume of the reservoirs and determine internal compartmentalization the heterogeneity can be primary related to depositional processes influencing grain size and facies distribution and secondary related to diagenesis in particular the primary heterogeneity is referred to the facies variability from distal to proximal settings to the accumulation of condensed shell beds and to facies characteristics of bedsets whereas the secondary heterogeneity derives from diagenetic processes at sequence stratigraphic surfaces and facies contacts and within sandbodies both the distribution of reservoirs and their heterogeneity can be predicted following a high resolution sequence stratigraphic approach ultimately the integration of subsurface data with the analysis of outcrop analogs is essential to better understand the features of clastic shelf reservoirs and to develop 3d models of fluid migration

muddy and dolomitic rip up clasts in triassic fluvial sandstones origin and impact on potential reservoir properties argana basin morocco abstract the significance of rip up clasts as sandstone framework grains is frequently neglected in the literature being considered as accessory components in bulk sandstone composition however this study highlights the great value of muddy and dolomitic rip up clast occurrence as a information source about low preservation potential from floodplain deposits and b key element controlling host sandstone diagenetic evolution and thus ultimate reservoir quality high resolution petrographic analysis on triassic fluvial sandstones from argana basin t6 and t7 t8 units highlights the significance of different types of rip up clasts as intrabasinal framework components of continental sediments from arid climates on the basis of their composition and ductility three main types are distinguished a muddy rip up clasts b dolomitic muddy rip up clasts and c dolomite crystalline rip up clasts spatial distribution of different types is strongly facies related according to grain size origin of rip up clasts is related to erosion of coeval phreatic dolocretes in different development stages and associated muddy floodplain sediments cloudy cores with abundant inclusions and clear outer rims of dolomite crystals suggest a first replacive and a subsequent displacive growth respectively dolomite crystals are almost stoichiometric this composition is very similar to that of early sandstone dolomite cement supporting phreatic dolocretes as dolomite origin in both situations sandstone diagenesis is dominated by mechanical compaction and dolomite cementation a direct correlation exists between 1 muddy rip up clast abundance and early reduction of primary porosity by compaction with irreversible loss of intergranular volume igv and 2 occurrence of dolomitic rip up clasts and dolomite cement nucleation in host sandstone occluding adjacent pores but preserving igv both processes affect reservoir quality by generation of vertical and 3d fluid flow baffles and barriers that compartmentalize the reservoir these findings may provide quantitative useful data for the better understanding of reservoir quality in analogous hydrocarbon bearing basins such as the bay of fundy nova scotia canada

deltaic coals of ferron sandstone member of mancos shale predictive model for cretaceous coal bearing strata of western interior the upper cretaceous ferron sandstone member of the mancos shale the coal bearing unit of the emery coalfield in central utah shows accumulation of clastic sediments in a lobate river dominated deltaic system that existed along the western shoreline of the interior cretaceous seaway during late turonian time five cycles of deltaic sedimentation each containing one major coal bed or coal zone are represented a clear genetic relationship exists between the geometries of the major coal beds of the emery field and the geometries of the delta front sandstone units with which they are associated the thicker part of each coal bed extends from the vicinity of the landward pinch out of the associated delta front sandstone landward to a distance of about 10 km this genetic relation forms the basis of a predictive model that can be used in designing more cost effective exploration programs in coal bearing strata of cretaceous age in the western interior

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

distribution of carbonate cements within depositional facies and sequence stratigraphic framework of shoreface and deltaic arenites lower miocene the gulf of suez rift egypt abstract this study aims to unravel the spatial and temporal distribution of diagenetic alterations of the mheiherrat member the rudeis formation lower miocene of the gulf of suez rift egypt within depositional facies and sequence stratigraphy the mheiherrat member is represented by shoreface calcarenites and hybrid arenites transgressive and highstand systems tracts tst and hst respectively and deltaic rudites and coarse grained calcarenites and hybrid arenites lowstand systems tracts lst petrographic stable o and c isotopes mineral chemical and geochemical analyses have revealed that the arenites are pervasively cemented by eogenetic carbonates and to small extent by zeolite and pyrite as well as by telogenetic palygorskite the shoreface tst and hst calcarenites and hybrid arenites were dominantly cemented by microcrystalline grain rimming and inter and intragranular pore filling calcite 18 o v pdb 3 6 to 0 3 and 13 c v pdb 2 3 to 0 7 and rhombic dolomite 18 o v pdb 3 9 to 0 9 and 13 c v pdb 2 5 to 0 7 these cements are interpreted to be formed by marine to brackish waters with 18 o v smow 1 2 to 3 2 at temperature of 20 55 c the deltaic lst coarse grained calcarenites and hybrid arenites were dominantly cemented by coarse crystalline inter and intragranular pore filling calcite 18 o v pdb 4 4 to 2 3 and 13 c v pdb 2 8 to 1 3 which are interpreted to have precipitated from pore waters with 18 o v smow 3 5 to 5 5 at temperatures of greater than 55 c such pervasive cementation by carbonates is attributed to the occurrence of abundant intrabasinal carbonate grains the carbonate cement texture is suggested to be controlled by changes in pore water chemistry owing to transgression and regression events this case study revealed that better understanding of factors controlling the extent and textural habits of carbonate cements can be achieved when it is linked with depositional facies and sequence stratigraphy

an experimental evaluation of shaw s paleointensity method and its modifications using late quaternary basalts abstract absolute paleointensity experiments were carried out using shaw s method geophys j res astr soc 39 1974 133 and its modifications geophys j res astr soc 54 1978 241 geophys j int 80 1985 773 geophys j int 118 1994 781 on 49 samples belonging to six late quaternary basaltic flows from central mexico samples were selected from a large collection because of their low viscosity index stable remanent magnetization and close to reversible continuous thermomagnetic curves moreover they previously yielded high quality thellier paleointensity results which makes them good candidates to assess the reliability of shaw s paleointensity method only 13 samples yielded acceptable results using shaw s original method arm 2 arm 1 ratio varies from 0 95 to 1 05 for accepted determinations although 6 samples do not pass the validity test proposed by tsunakawa and shaw geophys j int 118 1994 781 and thus should be rejected for paleointensity analyses rolph and shaw s geophys j int 80 1985 773 method gives reliable determination only in one case and no single determination was obtained by kono s geophys j res astr soc 54 1978 241 modification our results indicate an extremely low success rate of shaw s paleointensity method which may be due to magneto chemical changes that occurred during heating of samples above their curie temperatures

meteoric water incursion into marine turbditic sandstones evidence from the andrew formation paleocene uk central graben north sea abstract petrographic and stable carbon and oxygen isotope study revealed extensive meteoric water incursion during diagenesis of deep water marine paleocene turbidite sandstones in the north sea andrew formation uk central graben meteoric water diagenesis resulted in the dissolution and kaolinitization of framework silicates and cementation by calcite and methanogenic siderite and fe dolomite ankerite incursion is envisaged to have occurred over large distances along well connected permeable shelf slope and turbidite sand bodies during major fall in sea level which resulted in exposure of the continental shelf this work urges for consideration of the impact of undersaturated meteoric waters on reservoir quality modifications during hydrocarbon exploration in marine turbidite sandstones

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

radiocarbon and stable isotope constraints on last glacial maximum and younger dryas ventilation in the western north atlantic 1 foraminiferal abundance 14c ventilation ages and stable isotope ratios in cores from high deposition rate locations in the western subtropical north atlantic are used to infer changes in ocean and climate during the younger dryas yd and last glacial maximum lgm the 18o of the surface dwelling planktonic foram globigerinoides ruber records the present day decrease in surface temperature sst of 4 c from gulf stream waters to the northeastern bermuda rise if during the lgm the modern 18o salinity relationship was maintained this sst contrast was reduced to 2 c with lgm to interglacial 18o changes of at least 2 2 ssts in the western subtropical gyre may have been as much as 5 c colder above 2 3 km glacial 13c was higher than today consistent with nutrient depleted younger bottom waters as identified previously below that 13c decreased continually to 0 5 about equal to the lowest lgm 13c in the north pacific ocean seven pairs of benthic and planktonic foraminiferal 14c dates from cores 2 5 km deep differ by 1100 340 years with a maximum apparent ventilation age of 1500 years at 4250 m and at 4700 m apparent ventilation ages are presently unavailable for the lgm 2 5 km because of problems with reworking on the continental slope when sea level was low because lgm 13c is about the same in the deep north atlantic and the deep north pacific and because the oldest apparent ventilation ages in the lgm north atlantic are the same as the north pacific today it is possible that the same water mass probably of southern origin flowed deep within each basin during the lgm very early in the yd dated here at 11 25 0 25 n 10 conventional 14c kyr bp equal to 12 9 calendar kyr bp apparent ventilation ages 2 3 km water depth were about the same as north atlantic deep water today below 2 3 km four yd pairs average 1030 400 years the oldest apparent ventilation age for the yd is 1600 years at 4250 m this strong contrast in ventilation which indicates a front between water masses of very different origin is similar to glacial profiles of nutrient like proxies this suggests that the lgm and yd modes of ocean circulation were the same

search for a glacial age 14c depleted ocean reservoir 1 measurements of the radiocarbon age difference between mixed benthic foraminifera and several coexisting planktic entities were made in hopes of finding evidence for the existence of an isolated glacial age abyssal ocean reservoir whose demise would explain the large drops in the 14 c to c ratio in atmosphere and surface ocean carbon which occurred during the period of deglaciation forty six new radiocarbon results were obtained at seven depths covering the full range of glacial time i e 27 to 15 14 c kyrs from an equatorial pacific sediment core at 4 4 km water depth even though age reversals for foraminifera but not for coccoliths at two of the eight depths and large age differences between g sacculifer and three submixed layer planktic species mar this data set there remains little likelihood that such a reservoir existed

qualitative and quantitative prediction of diagenetic facies via well logs abstract drilled cores thin section petrography x ray diffraction xrd scanning electron microscopy sem routine core analysis rca and mercury injection capillary pressure micp tests were used to establish a predictable model for diagenetic facies and reservoir quality using well logs in sandstones of the paleogene dongying formation in nanpu sag bohai bay basin eastern china the results show that the sandstones are texturally and compositionally immature and have primary intergranular pores secondary intergranular and intragranular pores moldic pores and micro pores associated with diagenetic clay minerals the sandstones have undergone low to moderate mechanical compaction but a high degree of framework grain dissolution diagenetic minerals are dominantly of carbonates calcite dolomite and fe dolomite and clay minerals illite mixed layer illite smectite and kaolinite four types of diagentic facies are recognized tightly compacted carbonate cemented clay mineral filling and dissolution facies porosity permeability and oil saturation ranges among these diagenetic facies were determined from routine core analyses the diagenetic facies were then translated to conventional well logs by core log calibration and the well log expressions in terms of natural gamma ray neutron porosity bulk density sonic transit time and resistivity logs of various diagenetic facies were summarized the mineralogical composition obtained from the elemental capture spectroscopy ecs logs was then used for the diagenetic facies prediction the diagenetic facies predicted with the comprehensive conventional log analyses were combined with the ecs log profile and were calibrated with the xrd sem analysis and logging interpretation results for wells without ecs log the diagenetic facies can be quantitatively predicted by the mathematical calculations of reservoir quality index rqi via the gr ac and den log curves reservoir quality or oil bearing properties can be evaluated and predicted by translating the diagenetic facies to well logs

transgressive regressive cycles and the occurrence of coal in some upper cretaceous strata of utah cenomanian santonian strata contain about 60 of the known cretaceous coal resources of utah the areal and temporal distribution of these resources can be related to the transgressive regressive history of the western shoreline of the interior cretaceous seaway two orders of transgressive regressive cycles the larger of which corresponds to third order cycles of vail et al are deemed important in analyzing the distribution of coal in utah the largest coal fields are associated with extensive vertical stacking of the deposits of what are here termed fourth order cycles at the transgressive and regressive maxima of third order cycles deposits of transgressive and regressive phases of third order cycles contain only minor amounts of coal this genetic relationship may prove useful in predicting areas of important cretaceous coal resources in deeper parts of structural basins in the rocky mountain region

carbonate cementation in a sequence stratigraphic framework upper cretaceous sandstones book cliffs utah colorado abstract three macroscopic diagenetic features can be recognized in the sandstones of the upper cretaceous desert member of the blackhawk formation and castlegate sandstone of the mesaverde group exposed in the book cliffs utah each of which have distinctive form geometry and stratigraphic distribution diagenetic alterations are 1 leached zones whitecaps up to 10 m thick beneath coal beds 2 large up to 8 m concretionary carbonate cemented bodies in amalgamated shoreface and thin fluvial sandstones and 3 thin up to 2 m laterally extensive carbonate cemented horizons beneath major marine flooding surfaces each feature has distinct petrographic and geochemical signatures and formed through discrete diagenetic processes large isolated carbonate cemented bodies are composed of ferroan dolomite most of which precipitated during early diagenesis field and petrographic data coupled with stable isotope data early cements 13c 2 5 to 3 4o oo vpdb 18o 7 8 to 12 0o oo vpdb 87sr 86sr 0 7078 later cements 13c 3 1 to 5 7o oo vpdb 18o 12 0 to 15 1o oo vpdb 87sr 86sr 0 7093 suggest precipitation from meteoric fluids input into sediments during times of relative sea level fall the source of carbonate for the dolomite cement was dissolution of detrital dolomite from beneath coals by organic acids and subsequent mobilization by meteoric fluids carbonate precipitation in laterally extensive cement horizons appears to have started as a result of hiatus in sediment accumulation during marine flooding events relative sea level rise cement precipitation in these horizons continued through sediment burial as a result of organic matter oxidation reactions in overlying organic rich mudstones the results of this study show a link between sedimentation related to changes in relative sea level and diagenesis leading to the potential for the development of process based predictive models of early diagenesis in depositional successions

distribution of diagenetic alterations in fluvial deltaic and shallow marine sandstones within a sequence stratigraphic framework evidence from the mullaghmore formation carboniferous nw ireland abstract the distribution of diagenetic alterations in the fluvial deltaic and shallow marine arkosic to subarkosic sandstones average q72f26l02 of the mullaghmore formation carboniferous nw ireland can be predicted within a sequence stratigraphic framework eogenetic calcite 18opdb 13 3 to 6 5 13cpdb 3 0 to 3 4 and 87sr 86sr 0 706721 to 0 709227 and ferron dolomite feco3 8 12 mol 18opdb 14 2 to 7 8 13cpdb 1 4 to 1 0 and 87sr 86sr 0 709051 to 0 709167 occur in bioclast rich transgressive lag deposits at parasequence boundaries and transgressive surfaces and in wave influenced deltaic highstand systems tract hst deposits mesogenetic illite chlorite baroque dolomite feco3 16 mol 18opdb 14 2 to 12 7 13cpdb 3 8 to 1 0 quartz and calcite 18opdb 15 7 to 12 5 13cpdb 5 8 to 3 7 and 87sr 86sr 0 709016 to 0 709122 were formed mainly in the bioclast poor deposits which were not pervasively cemented by carbonates during near surface eodiagenesis these deposits include fluvial incised valley sandstones of lowstand systems tract lst and fluvial dominated deltaic sandstones of transgressive systems tract tst and hst illite is the dominant diagenetic clay mineral in the fluvial incised valley sandstones of lst possibly because of simultaneous albitization of k feldspars conversely chlorite dominates in the fluvial dominated deltaic sandstones of tst and hst because of the presence of suitable precursor clays the integration of diagenesis into sequence stratigraphic framework of clastic sequences should improve the ability to predict the spatial and temporal distribution of diagenetic alterations and related reservoir quality modifications of sandstone deposits

radiocarbon age anomaly at intermediate water depth in the pacific ocean during the last deglaciation 1 benthic and planktonic 14c ages are presented for the last glacial termination from marine sediment core vm21 30 from 617 m in the eastern equatorial pacific the benthic planktonic 14c age differences in the core increased to more than 6000 years between heinrich 1 time and the end of the younger dryas period several replicated 14c ages on different benthic and planktonic species from the same samples within the deglacial section of the core indicate a minimal amount of bioturbation scanning electron microscopy reveals no evidence of calcite alteration or contamination the oxygen isotope stratigraphy of planktonic and benthic foraminifera does not indicate anomalously old glacial age values and there is no evidence of a large negative stable carbon isotope excursion in benthic foraminifera that would indicate input of old carbon from dissociated methane it appears therefore that the benthic 14c excursion in this core is not an artifact of diagenesis bioturbation or a pulse of methane a benthic 14c stratigraphy reconstructed from the 14c ages from the deglacial section of vm21 30 appears to match that of baja margin core mv99 mc19 gc31 pc08 705 m but the magnitude of the low 14c excursion is much larger in the vm21 30 record this would seem to imply that the vm21 30 core was closer to the source of 14c depleted waters during the deglaciation but the source of this co2 remains elusive

packstones and floatstones ambiguous textures and origins in need of critical appraisal abstract the depositional textural classification of allochthonous limestones by dunham 1962 and its modifications by embry iii and klovan 1971 are widely used by carbonate petrologists during petrographic field studies and core examinations the origin and textures of packstones and floatstones used in this classification scheme need re appraisal towards their unified and unequivocal usage by carbonate petrologists in academia and the petroleum industry such usage will help avoiding inadequate interpretations of the depositional sequence stratigraphic or reservoir quality distribution models based on examination of jurassic cretaceous and holocene carbonate deposits from united arab emirates and oman it is suggested here that packstones should not be used to interpret energy of depositional environments because the majority are originally deposited as grainstones i e free of mud matrix thus sand and mud sized particles are expected to be hydraulically separated i e not deposited simultaneously therefore packstones might be originally grainstones that have incorporated variable amounts of mud matrix by various post depositional processes primarily bioturbation and or compaction deformation of peloids known as pseudomatrix dickinson 1970 we further observed that the 2 mm allochems in floatstones float in grainstones and packstones and not in mud wackestone matrix

geomagnetic field intensity during the last 60 000 years based on 10be and 36cl from the summit ice cores and 14c abstract based on 10 be 36 cl and 14 c records we reconstruct past changes in the geomagnetic field intensity under the assumption that radionuclide records are mainly influenced by changes in their production rates subsequent comparison of these records with independent geomagnetic field reconstructions allows us to assess the reliability of our approach and to identify periods where our method or the geomagnetic field reconstructions might be problematic the geomagnetic field derived from the 10 be flux to summit in central greenland agrees well with independent geomagnetic field intensity reconstructions however compared to independent palaeomagnetic data the radionuclide record suggests increased geomagnetic field intensities especially for the period from 20 000 to 30 000 years bp 36 cl from the grip ice core agrees well with the 10 be record for the last ice age and hence also the reconstructed geomagnetic field based on the tree ring radiocarbon record it is possible to reconstruct geomagnetic field intensity changes during the holocene a comparison of radiocarbon based and independent geomagnetic field reconstructions indicates that changes in the carbon cycle significantly influenced the atmospheric radiocarbon concentration before 10 000 years bp

impact of sequence stratigraphy depositional facies and diagenesis on reservoir quality a case study on the pennsylvanian taiyuan sandstones northeastern ordos basin china abstract the pennsylvanian taiyuan formation of the shenmu field in the northeastern ordos basin of china contains tight sandstone reservoirs that have recently been commercially produced an evaluation of the reservoir quality of the taiyuan sandstones will play a critical role in the development of the field an investigation was conducted to explore possible effects of the sequence stratigraphy depositional facies and diagenetic alterations on the reservoir quality of the taiyuan sandstones based on core descriptions the examination of thin sections and core plug and scanning electron microscope analyses the taiyuan formation consists of lowstand system tracts lst transgressive system tracts tst and highstand system tracts hst the lst sandstones are principally medium to coarse grained and are interpreted to have been deposited in the dominant distributary channels of a delta plain the early tst fine to coarse grained sandstones were largely deposited in the underwater distributary channel and mouth bar of a delta front and the middle tst fine to medium grained sandstones were deposited in the shoreface to foreshore environment the hst silt to medium grained sandstones were mostly deposited in the mouth bar of a delta front and shoreface to foreshore the taiyuan sandstones which are composed of sublitharenites quartzarenites and litharenites suffered from extensive compaction that resulted in a large loss of intergranular porosity before the sandstones were further cemented severely the sandstone cements are mostly composed of illite ankerite calcite and quartz the taiyuan sandstones have a particularly low average core plug porosity of 7 41 and horizontal permeability of 0 916 md which results from that most of the pores of the sandstones are micropores and secondary pores with poor connectivity the lst dominant distributary channel early tst underwater distributary channel and proximal mouth bar and hst shoreface to foreshore sandstones with relatively coarser grain sizes mostly medium to coarse grained have relatively higher porosities and permeabilities than the sandstones with relatively finer grain size which is ascribed to a widespread early dissolution by meteor water leaching and lower content of cements within the coarser grained sandstones however the lst dominant distributary channel sandstones had undergone a more effectively early dissolution and less carbonate cementation than the tst underwater distributary channel and proximal mouth bar and hst shoreface and foreshore sandstones had which lead to the relatively best reservoir quality of the lst dominant distributary channel sandstones among the taiyuan sandstones the study revealed that the reservoir quality of the tight taiyuan sandstones was controlled by sequence stratigraphy framework depositional facies grain size and diagenetic alterations together the sequence stratigraphy framework played a key role in controlling the reservoir quality of the taiyuan sandstones by governing the depositional facies and corresponding diagenetic products while the grain size affected the reservoir quality of the sandstones by controlling the content of cement within the sandstones

sequence stratigraphic distribution of diagenetic alterations in coal bearing paralic sandstones evidence from the rio bonito formation early permian southern brazil linking siliciclastic diagenesis to sequence stratigraphy allows a better understanding of the parameters controlling the spatial and temporal distribution of diagenetic alterations and hence of reservoir quality a study of the coal bearing alluvial deltaic estuarine and shallow marine sandstones of the rio bonito formation early permian parana basin southern brazil reveals that the distribution of diagenetic alterations and of related reservoir quality evolution can be constrained within a sequence stratigraphic framework calcite dolomite siderite kaolinite and pyrite cementation is consistently linked to sequence and parasequence boundaries transgressive and maximum flooding surfaces and is systematically distributed within lowstand transgressive and highstand systems tracts diagenesis of coal layers at parasequence boundaries has promoted the formation of stratabound calcite detectable in resistivity wire line logs concretionary pyrite and kaolinite and of silicate grain dissolution in sandstones located above and below these boundaries particularly in the transgressive systems tract meteoric water diagenesis caused grain dissolution and the formation of kaolinite in sandstones below sequence boundaries and in lowstand systems tract sandstones carbonate bioclasts and low sedimentation rates in lag deposits at parasequence boundaries transgressive and maximum flooding surfaces favoured the formation of grain rimming siderite the results of this study are relevant to the exploration of coal bed methane and other coal bearing reservoirs where it is crucial to unravel and predict the distribution and quality of reservoirs and compartments

radiocarbon age offsets of foraminifera resulting from differential dissolution and fragmentation within the sedimentary bioturbated zone 1 shells of coexisting species of planktonic foraminifera from the ontong java plateau reveal radiocarbon age offsets of up to 2200 years similar offsets are found between fragments and whole shells of single species steady state modelling of dissolution and bioturbation within the sedimentary mixed layer predicts age differences of up to several kiloyears due to the interplay between differential dissolution and fragmentation of foraminifer shells and bioturbation the observation that fragile foraminiferal shells are systematically older than those of more robust species is more difficult to explain mechanisms of chemical erosion interface dissolution and sediment redistribution are all apparently unable to explain this phenomenon a possible solution is presented in which a particular species may be represented by two distinct classes of shells which are more or less robust in this case differential dissolution and fragmentation causes an increase in the mean age as the fragile class contributes less to the remaining intact shells this study highlights the vulnerability of low sedimentation rate cores to the effects of dissolution and bioturbation

firmgrounds key surfaces in the recognition of parasequences in the aptian lower greensand group isle of wight southern england aptian lower greensand group exposures in the cliffs of the isle of wight southern england display a consistent coarsening up cyclicity on the scale of centimetres to tens of metres that reflects the bed bed set parasequence parasequence set and sequence hierarchy these coarsening up cycles are most commonly recognized at the scale of parasequences 20 cm to 10 m thick genetically related groups of which form parasequence sets both parasequences and parasequence sets contain the succession of biofacies that culminate in firmground development numerous episodes of erosion deposition and colonization are recorded reflecting multiple erosion bypass events the increase in mean grain size through each cycle is reflected by changes in physical sedimentary structures ichnofauna or bioturbational fabric fossil fauna and diagenesis interbedded mudstones siltstones and sandstones in the lower beds of each cycle display a variety of structures ranging from low angle hummocky or tabular cross strata sandstone filled erosional gutters and planar lamination the cleaner sandstones found in the upper parts to each cycle are often completely bioturbated with only rare stratification and pebble plant debris filled scours preserved bioturbational fabrics change upward through each cycle from small subhorizontal mud or sandstone filled burrows to large branching clay filled or cemented burrow systems the top surface of each cycle is marked by a fossil epifauna indicative of firm to hard substrate conditions concentrations of bivalves brachiopods bryozoa crinoids and corals are preferentially cemented by iron oxide carbonate or phosphate such cements were early and thus utilized by firm or hard substrate dwellers this fossiliferous cemented sandstone is overlain by a flooding surface marked by the mudstone and silt rich sandstones at the base of the next cycle together the fauna and ichnofauna in each cycle represent the gradual development of firm substrate conditions culminating in the diverse firmground fauna preserved at the top of each cycle the fauna and changing substrate conditions reflect the hiatuses developed during successive episodes of marine flooding high species diversity is matched by complex patterns of taphonomic feedback in the mature firmground faunas that mark major flooding surfaces increasing faunal maturity allows recognition of a hierarchy of hiatuses this hierarchy is analogous to the parasequence parasequence set division the stratigraphic condensation of firmgrounds can be used to empirically define the condensed section the thickness of sediment between firmgrounds being a function of sediment supply and water depth accommodation space

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

tree rings and ice cores reveal c 14 calibration uncertainties during the younger dryas the younger dryas interval during the last glacial termination was an abrupt return to glacial like conditions punctuating the transition to a warmer interglacial climate despite recent advances in the layer counting of ice core records of the termination the timing and length of the younger dryas remain controversial also a steep rise in the concentration of atmospheric radiocarbon at the onset of the interval recorded primarily in the cariaco basin has been difficult to reconcile with simulations of the younger dryas carbon cycle here we discuss a radiocarbon chronology from a tree ring record covering the late glacial period that has not been absolutely dated we correlate the chronology to ice core timescales using the common cosmic production signal in tree ring 14c and ice core 10be concentrations the results of this correlation suggest that the cariaco record may be biased by changes in the concentration of radiocarbon in the upper ocean during the early phase of the younger dryas climate reversal in the cariaco basin this bias in the marine record may also affect the accuracy of a widely used radiocarbon calibration curve over this interval our tree ring based radiocarbon record is easily reconciled with simulated production rates and carbon cycle changes associated with reduced ocean ventilation during the younger dryas attaching a floating tree ring chronology to ice core records that cover the abrupt younger dryas cold interval during the last glacial termination provides a better estimate of the onset and duration of the radiocarbon anomaly the chronology suggests that marine records may be biased by changes in the concentration of radiocarbon in the ocean which may affect the accuracy of a popular radiocarbon calibration program during this interval

upper cretaceous ferron sandstone major coalbed methane play in central utah recent drilling for coalbed gas in the upper cretaceous ferron sandstone member of central utah has resulted in one of the most successful plays of this kind exploration to date has resulted in three fields and a potential fairway 6 10 mi 10 16 km wide and 20 60 mi 32 96 km long corresponding to shallow coal occurrence at depths of about 1800 3500 ft 545 1060 m in the ferron a sequence of interbedded fluvial deltaic sandstone shale and coal in the lower part of the cretaceous mancos shale coalbed methane cbm reservoirs in this interval consist of thin to moderately thick 3 10 ft 1 3 m coal beds of relatively low rank high volatile b bituminous and variable gas content ranging from 100 scf ton or less in the south to as high as 500 600 scf ton in the north productive wells have averaged more than 500 mcf day and after several years continue to typically show negative production declines in the major productive area drunkards wash unit the first 33 producers averaged 974 mcf and 85 bbl of water per day after five years of continuous production estimated ultimate recoverable reserves for individual wells in this unit range from 1 5 to 4 bcf based on several criteria including gas content thermal maturity and chronostratigraphy the play is divided into northern and southern parts the northern part is characterized by coals that have the following characteristics 1 high gas contents 2 moderate thermal maturity e g vitrinite reflectance ro values of 0 6 0 8 3 good permeabilities 5 20 md 4 lack of exposure and 5 overpressuring due to artesian conditions southern coals have much lower average gas contents 100 scf ton and lower thermal maturity ro 0 4 0 6 and they are exposed along an extensive 35 mi 56 km outcrop belt that may have allowed a degree of flushing these coals however are also thicker and more extensive than those to the north and thus may retain significant begin page 200 potential northern coals appear to contain a mixture of gas from three sources in situ thermogenic methane migrated thermogenic methane from more mature sources and late stage biogenic gas current development is focused on the northern part of the stated fairway where well control and an existing infrastructure are present indications are that cbm exploration in the ferron will expand considerably in the near future

linking diagenesis to sequence stratigraphy in fluvial and shallow marine sandstones evidence from the cambrian ordovician lower sandstone unit in southwestern sinai egypt abstract by integrating diagenesis and sequence stratigraphy the distribution of diagenetic alterations and their impact on reservoir quality was investigated within a sequence stratigraphic framework using the fluvial and shallow marine sandstones in the cambrian ordovician succession of southwest sinai the perographic and geochemical analysis of the studied sandstone revealed that the eogenetic alterations display fairly systematic spatial and temporal distribution patterns within the lowstand system tract and transgressive system tract as well as along the sequence stratigraphic surfaces i e sequence boundaries transgressive surfaces and parasequence boundaries during relative sea level fall percolation of meteoric waters through sandstones of the lst and below sequence and parasequence boundaries resulted in extensive dissolution of detrital grains and formation of kaolinite authigenic k feldspar and feldspar overgrowths as well as formation of mechanical infiltrated clays around the detrital grains during relative sea level rise invasion of marine water into the sandstones as a consequence of landward migration of the shoreline as well as low sedimentation rates encountered in the tst resulted in the formation of glauconite apatite and pyrite development of pseudomatrix which was formed by mechanical compaction of mud intraclasts is mostly abundant along transgressive surfaces and parasequence boundaries of the tst and is related to the abundance of mud intraclasts in the transgressive lag deposits the types and extent of eogenetic alterations have an important impact on the distribution of the mesogenetic alterations including the formation of quartz overgrowths and dickite distribution of mesogenetic quartz overgrowths in the sandstones was controlled by the distribution of mechanically infiltrated clays and the presence of eogenetic cement sandstones that remained poorly cemented during eodiagenesis and that have thin or discontinuous infiltrated clay rims around the detrital grains were cemented during mesodiagenesis by quartz the absence of extensive eogenetic cements in the sandstones suggested that the partial deterioration of porosity was mainly due to mechanical compaction partial transformation of kaolinite to dickite which indicates neomorphic change to a better ordered and more stable crystal structure at the elevated temperatures during mesodiagenesis is partially a function of distribution of kaolinite during eodiagenesis the conceptual model developed in this study shows the diagenetic evolutionary pathways in the reservoir sandstones within a sequence stratigraphic context which in turn provides some insights into the controls on reservoir heterogeneity

toward better paleocarbonate ion reconstructions new insights regarding the caco3 size index 1 the reconstruction of paleocarbonate ion concentrations provides an important constraint on the contribution of the caco3 cycle to the decrease in atmospheric co2 content during glacial time such reconstructions have been challenging because each of the existing paleo co32 indices has serious limitations in this study we reexamine the broecker clark caco3 size index by analyzing the 20 m 20 to 38 m and 38 to 63 m fractions in sediments from the ontong java plateau and the ceara rise scanning electron microscope analyses demonstrate that the less than 20 m caco3 is dominated by coccoliths and the greater than 20 m caco3 is dominated by foraminifera our results clearly indicate that the coccoliths are far more resistant to dissolution than the foraminifera referenced to a core top sample from 2 31 km depth in a core top sample from 4 04 km depth on the ontong java plateau 70 of the foraminifera caco3 was dissolved as opposed to only 7 of the coccolith caco3 we found that the dissolution of foraminifera shells did not produce a significant amount of fragments smaller than 63 m in size and thus the broecker clark size index is not a measure of the extent of fragmentation rather it is a measure of the extent of differential dissolution of foraminifera relative to coccoliths on the basis of these results we propose a new dissolution index which involves the ratio of dissolution susceptible foraminifera caco3 to total caco3

ventilation of the glacial deep pacific ocean measurements of the age difference between coexisting benthic and planktic foraminifera from western equatorial pacific deep sea cores suggest that during peak glacial time the radiocarbon age of water at 2 kilometers depth was no greater than that of today these results make unlikely suggestions that a slowdown in deep ocean ventilation was responsible for a sizable fraction of the increase of the ratio of carbon 14 14c to carbon in the atmosphere and surface ocean during glacial time comparison of 14c ages for coexisting wood and planktic foraminifera from the same site suggests that the atmosphere to surface ocean 14c to c ratio difference was not substantially different from today s

the vertical compartmentalization of reservoirs an example from an outcrop analog crotone basin southern italy the lower pliocene shallow marine successions deposited within half graben subbasins exposed in the northern crotone basin southern italy are good examples illustrating the variable development of small scale cycles 2 15 m 6 6 49 ft thick within the hanging wall blocks of normal growth faults shallow marine small scale cycles forming the successions deposited near the immediate hanging wall of half graben subbasins show a clear alternation between well cemented shell rich intervals and weakly cemented siliciclastic sandstones in contrast the shallow marine successions deposited in hanging wall dip slope locations and in the footwalls show a poorly preserved cyclicity related to lower subsidence rates and a higher degree of cycle amalgamation the vertical alternation between well cemented carbonate rich intervals and less cemented sandstones creates a marked heterogeneity in the permeability of these shallow marine half graben fills which are sealed by impermeable offshore and lagoonal mudstones and may be considered as an outcrop analog of potential reservoirs as a consequence of the well developed cyclicity therefore the potential reservoir is vertically compartmentalized this feature tends to vanish toward the hanging wall dip slopes where the cyclicity is remarkably less appreciable the correct prediction of geometry and effective volume of reservoirs and in particular the recognition of their vertical compartmentalization have an important impact on hydrocarbon exploration and production the developed conceptual model of compartmentalized half graben fill may be useful in several contexts to study fault bounded basins from a reservoir point of view

synchronous radiocarbon and climate shifts during the last deglaciation radiocarbon data from the cariaco basin provide calibration of the carbon 14 time scale across the period of deglaciation 15 000 to 10 000 years ago with resolution available previously only from holocene tree rings reconstructed changes in atmospheric carbon 14 are larger than previously thought with the largest change occurring simultaneously with the sudden climatic cooling of the younger dryas event carbon 14 and published beryllium 10 data together suggest that concurrent climate and carbon 14 changes were predominantly the result of abrupt shifts in deep ocean ventilation