permeability prediction from geologic models permeability is a critical parameter for the petroleum geologist by simulating the processes of compaction and cementation in a model porous medium we have gained a new understanding of how permeability is controlled in reservoir sandstones this understanding can be used predictively for simple sandstones the geometry of the model pore system is completely defined so permeability can be calculated directly using a network model for flow the calculation is based on first principles and is physically rigorous in contrast to many previous efforts to predict permeability there are no adjustable parameters in the calculations and no additional measurements or correlations e g capillary pressure data or pore system data from thin sections are required the model derived porosity permeability trend for a compacted or quartz cemented sandstone or a sandstone having a combination of these processes closely matches measurements on fontainebleau sandstone samples whose permeabilities span nearly five orders of magnitude the model also correctly predicts mercury injection measurements of pore throat size distribution for the fontainebleau sandstone pore scale geometric features of the model are found to be spatially correlated and this departure from randomness significantly affects macroscopic properties such as permeability the agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and consequently uncorrelated or arbitrarily correlated models of transport in such media are unlikely to be physically representative we also discuss extending the model to predict properties of more complicated rocks

paleoclimatic analyses of middle eocene through oligocene planktic foraminiferal faunas quantitative faunal analyses and oxygen isotope ranking of individual planktic foraminiferal species from deep sea sequences of three oceans are used to make paleoceanographic and paleoclimatic inferences species grouped into surface intermediate and deep water categories based on 18o values provide evidence of major changes in water mass stratification and individual species abundances indicate low frequency cool warm oscillations these data suggest that relatively stable climatic phases with minor cool warm oscillations of 0 5 m y frequency are separated by rapid cooling events during middle eocene to early oligocene time five major climatic phases are evident in the water mass stratification between middle eocene through oligocene time phase changes occur at p14 p15 p15 p16 p20 p21 and p21 p22 zone boundaries and are marked by major faunal turnovers rapid cooling in the isotope record hiatuses and changes in the eustatic sea level a general cooling trend between middle eocene to early late oligocene is indicated by the successive replacement of warm middle eocene surface water species by cooler late eocene intermediate water species and still cooler oligocene intermediate and deep water species increased water mass stratification in the latest eocene p17 indicated by the coexistence of surface intermediate and deep dwelling species groups suggest that increased thermal gradients developed between the equator and poles nearly coincident with the development of the psychrosphere this pattern may be related to significant ice accumulation between late eocene and early late oligocene time

quantitative reconstruction of middle and late eocene paleoclimate based on palynological records from the huadian basin northeastern china evidence for monsoonal influence on oil shale formation abstract the eocene was an interval of climate evolution when the extreme high temperatures of the earliest cenozoic gave way to a global cooling trend fine grained lacustrine sediments of eocene age from northeastern china represent an excellent archive to study long term environmental and climatic changes at high resolution the huadian basin is a cenozoic fault bounded basin in jilin province northeastern china though small in area it contains numerous oil shale and coal bearing layers which provide an abundant middle to late eocene palynoflora eighty five palynomorph genera and one hundred palynomorph species were identified in the huadian formation allowing to distinguish 4 palynozones the nearest living relatives of the recovered palynomorph taxa suggest the presence of subtropical evergreen and deciduous broad leaved forests during deposition of the huadian formation using the coexistence approach ca we calculated seven paleoclimatic parameters which indicate a subtropical warm and humid climate with a mean annual temperature above 13 6 c and a mean annual precipitation of at least 887 mm however the climate became cooler and drier during the transition from the lower oil shale member to the upper carbonaceous shale member palaeoclimatic data from the various middle and late eocene basins of northeastern china i e the huadian fushun yilan hunchun and jishu basins were compared with data from north american and european basins middle eocene climate in the northern hemisphere was generally warm and humid and the mean annual temperature and precipitation in northeastern china were similar to those in north america though slightly lower than those of europe similar trends in temperature and precipitation on different continents support the existence of a global mid eocene cooling event this paper is the first systematic study of middle to late eocene seasonality in the huadian basin our results show an apparent differentiation in both temperature and precipitation between summer and winter supporting the view that the east asian monsoon was established in northeastern china already during the middle to late eocene this middle and late eocene monsoon was likely driven by the development of pacific trade winds and an alternating seasonal high and low pressure system over siberia the climatic changes observed in our dataset especially the precipitation changes appear to have played an important role in controlling oil shale formation in the huadian basin a warmer and more humid climate with abundant precipitation and high subsidence rates formed a deeper huadian depositional lake high productivity and stable stratification provided the abundant organic matter and favorable preservation conditions necessary for oil shale accumulation

global cooling accelerated by early late eocene impacts at ocean drilling program site 689 maud rise southern ocean 18o records of fine fraction bulk carbonate and benthic foraminifers indicate that accelerated climate cooling took place following at least two closely spaced early late eocene extraterrestrial impact events a simultaneous surface water productivity increase as interpreted from 13c data is explained by enhanced water column mixing due to increased latitudinal temperature gradients these isotope data appear to be in concert with organic walled dinoflagellate cyst records across the same microkrystite bearing impact ejecta layer in the mid latitude massignano section central italy in particular the strong abundance increase of thalassiphora pelagica is interpreted to indicate cooling or increased productivity at massignano because impact induced cooling processes are active on time scales of a few years at most the estimated 100 k y duration of the cooling event appears to be too long to be explained by impact scenarios alone this implies that a feedback mechanism such as a global albedo increase due to extended snow and ice cover may have sustained impact induced cooling for a longer time after the impacts

similarities between planktonic and larger foraminiferal evolutionary trends through paleogene paleoceanographic changes abstract the paleogene evolutionary records of planktonic foraminifera and larger benthic foraminifera show parallel patterns in development that may in part reflect changes in nutrient flux to surface waters following the terminal cretaceous extinctions faunas were dominated by a few cosmopolitan opportunistic species more specialized k strategists in both groups diversified during the paleocene dominating into the early eocene consistent with he globally mild climatic regime and probable limited rates of nutrient flux to surface waters high latitude cooling intensified in the middle eocene promoting diversification of less specialized taxa characteristic of cooler and deeper waters higher latitudes meridional upwelling zones and boundary currents geographic ranges of warm water taxa diminished and fragmented yet many species survived thus global diversities during the middle eocene were the highest in the cenozoic as cooling and increased rates of nutrient flux further intensified in the late eocene extinction rates of warm water planktonic and larger benthic species exceeded origination rates of cooler water and less specialized forms so diversity dropped to intermediate levels where it remained through the late eocene and oligocene previous observations that planktonic foraminiferal faunas dominated by warm water taxa are often replaced by cool water faunas even though oxygen isotopes only recorded relatively minor cooling can be reconciled by the explanation that the biotic communities responded to changes in trophic resources and euphotic habitats that accompany the temperature reduction as well as to temperature change itself

natural convection in porous permeable media sheets wedges and lenses abstract any fluid saturated porous permeable media subject to a temperature field such that there is a net upward heat flow and the isotherms across it are non horizontal will be subject to convective circulation analytical solutions for the fluid velocity exist only for the restricted case of an inclined layer in which the upper and lower surfaces are maintained at constant temperature this result has been much cited in the geological literature although the conditions under which the analytical solution holds are unrealistic in a geological situation numerical modelling is used to investigate how closely the analytical solution holds under conditions which are more geologically realistic it is found that for an homogeneous aquifer the analytical solution is a reasonable approximation provided the inclunation of the layer is replaced by the inclination of the isotherms in the expression for the fluid velocity in general the magnitude of will be much smaller than that of and depending on the effective thermal conductivity of the inclined layer relative to its surroundings may be of the opposite sign in this instance circulation is in the reverse direction to that obtained from the analytical solution for a vertically heterogeneous system in which the aquifer is divided by a series of thin continuous low permeability layers conditions exist in which the flow may be approximated by that in a homogeneous but anisotropic medium the model is also extended to cover other simple geometrics for which analytical solutions are not available

nearshore carbonate deposits in lake tanganyika an exceptionally wide variety of carbonate fades dominated by high magnesian calcite occurs along the littoral and shallow sublittoral zones 50 m of lake tanganyika in central africa these facies include exposed and submerged calcite cemented ridges of nearshore terrigenous sand ooid sand shoals and lithified oolite ridges chara meadows of bioturbated calcareous silts gastropod shell blankets and related coquinas and extensive thrombolitic microbial reefs though texturally and compositionally similar to many modern and ancient shallow water marine facies these deposits record carbonate deposition and cementation in a large tropical deep water lake of tectonic origin lithofacies along lake tanganyika represent the broadest spectrum of carbonate deposits yet reported from any modern lake and serve as important analogues for lacustrine carbonate sequences in the stratigraphic record

eocene oligocene paleoceanographic changes in the stratotype section massignano italy clues from rock magnetism and stable isotopes 1 we have conducted high resolution paleomagnetic and rock magnetic studies in addition to stable isotope analyses of the massignano sedimentary section which is the global stratotype section and point gssp for the eocene oligocene boundary our research builds upon the many past studies of the massignano section in seeking to understand the timing and nature of the paleoenvironmental variations that occurred during the transition for the earth s climate system from greenhouse to icehouse the new paleomagnetic results provide a refined magnetostratigraphy of the section and new age for the eocene oligocene boundary at 33 7 ma abrupt and large alternations in magnetic concentration composition and grain sizes that occur in the high resolution rock magnetic record are interpreted to be the result of rapid bimodal shifts in deep sea circulation that affect sediment sources or transport we speculate that currents flowing through the gateway between the atlantic and indo pacific oceans may have turned on and off as the gateway was progressively closing resulting in the deposition of two different assemblages of magnetic minerals at massignano finally stable isotope 18o and 13c data collected on ostracod valves also suggest significant changes in sea bottom circulation in the neo tethys ocean at and about 2 m y before the eocene oligocene boundary

the eocene oligocene planktic foraminiferal transition extinctions impacts and hiatuses biostratigraphic study and re examination of 22 late eocene to early oligocene sections provides data in support of three and possibly a fourth late eocene impact events in the g index zone during a period of about 1 ma of the middle priabonian between 34 7 and 35 7 ma no major species extinctions or significant species abundance changes directly coincide with these impact events species extinctions are gradual and selective affecting primarily cool temperature intolerant surface dwellers these extinctions began with the onset of global cooling during the early middle eocene and culminated near the eocene oligocene boundary the global cooling was associated with the isolation of antarctica as australia moved northward the development of a circum antarctic circulation growth of antarctic ice sheet beginning by late middle eocene time and a change from a thermospheric to thermohaline circulation we find no evidence that the multiple late eocene impact events directly contributed to the climatic deterioration already in progress

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

facies relationships and sedimentation in large rift lakes and implications for hydrocarbon exploration examples from lakes turkana and tanganyika abstract two african lakes turkana and tanganyika illustrate a spectrum of sediments and facies architectures which can occur in large rift lacustrine basins they demonstrate contrasts between sediments deposited in a semi arid moderately deep mainly volcanic basin turkana and a semi humid extremely deep basin without significant volcanism tanganyika turkana deposits are dominated by rapidly accumulating organic poor terrigenous muds coarse clastic input is in part limited by ponding in marginal subbasins due to volcanic barriers profundal coarse clastic deposition is rare sediments contain little biogenic components except in the sediment starved south basin sediments are also poor in cartonates tanganyika deposits are dominated by organic rich biogenic muds steep fault escarpments and uniform slopes along shelf platform and rift axis lake margins promote the efficient transport of sands into profundal settings total clastic sediment supply into the basin is severely restricted by basin morphology with steep relief backslopping a limited drainage basin area and presettling in upstream lake kivu sediment ponding along margins is rare littoral carbonates are common a consequence of low net clastic sedimentation rates many of the differences between facies in the two lakes can be explained by contrasts in basin margin slope and relief volcanic activity and climate between the two lakes the results of this study suggest that lake deposits formed in tanganyika type rift basins are more appropriate exploration target for hydrocarbon exploration than those from turkana type basins

bathyal ostracodes from the florida hatteras slope the straits of florida and the blake plateau abstract epibathyal ostracodes from the florida hatteras slope the blake plateau and the straits of florida were studied to determine the relationship of numerous genera and species to bottom water environmental conditions such as dissolved oxygen and bottom water temperatures from a total of 100 samples 44 samples evenly distributed between 200 and 1100 m water depth and having an average of 325 specimens were examined in detail using occurrence data from the adjacent continental shelf carapace preservation rose bengal staining and population data indigenous death assemblages were distinguished from transported or reworked fossil specimens the percent of transported specimens varied as follows blake plateau 90 of the samples and usually constitute 10 to 30 of each trachyleberidea bairdoppilata saida paranesidea ambocythere bythocypris cytherella bradleya henryhowella and polycopidae occur in 45 to 80 of the samples in varying percentages the upper depth limits of 39 taxa occur at or just below the thermocline suggesting a relationship to temperature australoecia quasibuntonia cytheropteron ruggieriella saida ambocythere trachyleberidea macrocypris krithe thalassocythere andcytherella are most common or restricted to the o2 minimum zone conversely anchistrocheles bradleya henryhowella androckallia are most common below 750 m in well oxygenated water with temperatures below 8 c the results show that 1 ostracodes display a narrow depth zonation controlled by dissolved oxygen and water temperature 2 species diversity is very high for a bathyal zone 3 ostracodes can be used to identify the source of sediment that has been transported downslope and 4 some taxa are useful in recognizing low oxygenated water in cenozoic deposits

ostracod faunas and water masses across the continental margin off southwestern africa changes in composition of modern benthic ostracod faunas across the continental margin of southwestern africa occur at boundaries between and within major water masses a mixed layer antarctic intermediate water aaiw boundary at 200 m an aaiw salinity minimum zone at 650 m an aaiw bathyal thermocline at 1000 m and the aaiw north atlantic deep water nadw boundary at 1500 m in addition two population changes occur within the nadw at 1 8 2 0 km and 2 0 3 0 km the antarctic bottom water assemblage is sparse and poorly preserved

numerical analysis of seawater circulation in carbonate platforms ii the dynamic interaction between geothermal and brine reflux circulation density driven seawater circulation may occur in carbonate platforms due to geothermal heating and or reflux of water of elevated salinity in geothermal circulation lateral contrasts in temperature between seawater and platform groundwa ters warmed by the geothermal heat flux result in upward convective flow with colder seawater drawn into the platform at depth with reflux circulation platform top waters concentrated by evaporation flow downward displacing less dense underlying ground waters we have used a variable density groundwater flow model to examine the pattern magnitude and interaction of these two different circulation mechanisms for mesosaline platform top waters 50 and brines concentrated up to saturation with respect to gypsum 150 and halite 246 geothermal circulation most active around the platform margin becomes restricted and eventually shut off by reflux of brines from the platform interior towards the margin the persistence of geothermal circulation is dependent on the rate of brine reflux which is proportional to the concentration of platform top brines and also critically dependent on the magnitude and distribution of permeability low permeability evaporites can severely restrict reflux whereas high permeability units in hydraulic continuity enhance brine transport reduction in permeability with depth and anisotropy of permeability kv kh focuses flow laterally in the shallow subsurface 1 km resulting in a horizontally elongated brine plume aquifer porosity and dispersivity are relatively minor controls on reflux platform brines can entrain surficial seawater when brine generating conditions cease but the platform top remains submerged a variant of reflux we term latent reflux brines concentrated up to gypsum saturation have relatively long residence times of at least 100 times the duration of the reflux event they thus represent a long term control on post reflux groundwater circulation and conse quently on the rates and spatial patterns of shallow burial diagenesis such as dolomiti zation

reactive transport modeling of brine reflux dolomitization anhydrite precipitation and porosity evolution abstract both geothermal convection and brine reflux drive circulation of sea water derived fluids through carbonate platforms during early burial but dynamic interactions between heat and solute transport and resulting diagenesis are at present poorly understood this paper describes high resolution reactive transport model rtm simulations that suggest that reflux of 85 ppt brines rapidly restricts geothermal convection to the platform margin with flow focused in the more permeable shallow carbonates in a baseline simulation involving an elongate 25 km wide grain dominated packstone platform brine reflux resulted in complete dolomitization beneath the 5 km wide brine pool in 335 ky the dolomite body then extends downward at c 22 m 100 ky into an underlying broad area of partial dolomitization this process enhances porosity at shallow depth but beneath the dolomite body precipitation of anhydrite occludes porosity and limits the depth of reflux in contrast geothermal convection at the platform margin forms a smaller partially dolomitized body over a longer time 60 dolomite after 1 my with very minor associated anhydrite cementation reflux diagenesis is sensitive to platform geometry with higher rates of fluid flow increasing the depth of alteration beneath the brine pool for a circular platform compared to the linear baseline fluid flow across thermal gradients enhances reaction rates and ignoring heat transport by 85 ppt brine reflux underestimates the extent of reflux dolomite by 25 and associated anhydrite by 90 the depth and rate of anhydritization is sensitive to the geothermal heat flux whereas platform top temperatures affect dolomitization rate reflux diagenesis is also sensitive to brine density which affects both fluid flow and reaction rates sediment permeability and reactive surface area rsa are key intrinsic controls on diagenesis where the permeability structure permits sufficient fluid flow diagenesis preferentially affects more reactive fine grained sediments however as flow rates decline reactions become focused in the more permeable but less reactive sediments simulations thus shed light on why in some settings reflux preferentially dolomitizes muddy sediments but elsewhere favors grainstones once active reflux ceases brines continue to flow in the subsurface but this latent reflux causes only minor dolomitization due to prior mg2 consumption at shallow depth

the terminal eocene event and the polish connection abstract the eocene oligocene boundary in europe is marked by major discontinuities in all environments the grande coupure in continental mammals the elimination of semitropical elements from high latitude floras the virtually complete replacement of the shallow marine malacofauna and an extraordinary downslope excursion of carbonate deposition in deep ocean basins drop in the ccd these phenomena collectively represent the terminal eocene event tee in the carpathian mountains the tee is manifested in the thin but regionally persistent globigerina marl a calcareous unit containing abundant cool water microplankton that occurs within very thick siliceous bathyal flysch sequences in southern poland the marl is of very latest eocene age within planktonic foraminifera zone p17 calcareous nannoplankton zone np19 20 and the zone of the dinoflagellate rhomdodinium perforatum zircons from bentonites bracketing the marl are dated by fission track analysis at polany two underlying bentonites are 41 7 and 39 8 ma and at znamirowice two overlying bentonites are 34 6 and 28 9 ma in sequence this accords with glauconite k ar ages in western europe by which the eo oligocene boundary age is estimated at 37 38 ma global correlations indicate that the tee corresponds to a major glacio eustatic regression with a duration of about 0 5 ma in which a large antarctic ice cap was formed the ocean circulation was permanently changed to the psychrospheric condition and world climate shifted irreversibly towards the modern state

the eocene oligocene boundary and the psychrospheric event in the tethys as recorded by deep sea ostracods from the massignano global boundary stratotype section and point central italy abstract latest eocene earliest oligocene deep water ostracods from the massignano global boundary stratotype section and point gssp central italy were studied from a palaeoecological perspective to investigate the psychrospheric event in the tethys although quantitative analyses were hampered by the low incidence of fauna the high resolution sampling used in this study 148 samples distributed over a 23 m thick succession spanning 3 m y allowed the following inferences 1 overall features e g abundance diversity composition of the assemblages suggest a middle lower bathyal environment with no significant palaeobathymetrical changes 2 today a similar fauna notably characterised by the common occurrence of the genus agrenocythere would likely be encountered in upper psychrospheric conditions water masses beneath the permanent thermocline with a temperature range of 4 10 c 3 although no noticeable faunal turnover has been recognised throughout the succession minor faunal changes seem to occur in the first part of the section probable appearance of legitimocythere presequenta and to a lesser extent also across the eocene oligocene boundary the presumed appearance of l presequenta latest eocene correlates quite well in timing and possible significance with the faunal changes observed in other microfossil groups foraminifers and dinocysts this event is possibly linked to a cooling pulse which might have been caused by the arrival or increasing advection of cold bottom waters in the study area faunal evolution across the eocene oligocene boundary is thought to be consistent with an intensification of the cooling trend and an increase of primary productivity which fits fairly well with the 13c record relationships between the subtle ostracod faunal changes observed in the massignano gssp and the main late eocene early oligocene psychrospheric event can be only speculative nevertheless we found no evidence that precludes the possibility of interpreting the massignano ostracod pattern as a subdued local expression of this major event although speculative this is a reasonable hypothesis because the investigated area is situated in western tethys far from the main circulation of deep ocean waters

prediction of elastic wave velocities in sandstones using structural models elastic wave propagation in fluid saturated sandstones depends upon two sets of rock features 1 the volume fractions and elastic constants of the rock constituents quartz clay water etc and 2 microstructural geometry grain contacts pore aspect ratios while the former data are usually obtainable the latter are relatively inaccessible we present a new method for determining microstructural data using idealized but physically representative models of sandstone the key to the method is the simulation of certain depositional and diagenetic processes in a manner that completely specifies the geometry of the resulting models hence the geometric features of the grain space and void space required for various theories of elastic propagation can be calculated directly from the models we find good agreement between predictions and measurements of compressional and shear wave velocities in both clean and clay bearing saturated sandstones in contrast with previous efforts at predicting velocitie

carbonate porosity creation by mesogenetic dissolution reality or illusion many authors have proposed that significant volumes of porosity are created by deep burial dissolution in carbonate reservoirs we argue however that this model is unsupported by empirical data and violates important chemical constraints on mass transport because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow even if the initial generation of such undersaturated pore water could occur a review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment in consequence the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality

petroleum reservoir porosity versus depth influence of geological age average porosity values for the producing zones of oil and gas fields worldwide are examined as a function of the present depth for sandstone and carbonate lithologies divided into 10 groupings by reservoir depositional age precambrian silurian to pliocene pleistocene the wide variations in average reservoir porosity within each depth range reflect the extreme ranges in porosity controlling factors such as depositional facies early

pore networks in holocene carbonate sediments abstract permeabilities of holocene carbonate sediments from florida and the bahamas determined for 74 samples by water flow rates in a variable head permeameter range from 0 6 to 57 000 millidarcies porosities range from 40 to 78 percent these parameters are related to depositional texture as follows table the muddiest sediments i e the finest grained have the highest porosities but lowest permeabilities this negative correlation between porosity and permeability is the reverse of the situation found in carbonate rocks even as young as pleistocene high porosity and low permeability in sediments show a strong correlation with percentage of fines 62 m from capillary pressure curves it is inferred that many of the pore throats of muddy carbonate sediments are less than 1 m in diameter at least after drying measured permeabilities are used to calculate cementation rates for simple models of upper phreatic zone cementation the calculated rates would require excessive time to produce the degree of cementation seen in late pleistocene rocks of florida and the bahamas the amounts of rainfall and evaporation are the most important factors in the degree of cementation

biotic response of tethyan bathyal ostracodes through the eocene oligocene transition the composite faunal record from the massicore and massignano global stratotype section and point east central italy abstract the eocene oligocene transition eot is marked by a double stepped positive 18 o excursion of 1 3 and a deepening of the calcium compensation depth ccd interpreted to represent marked antarctic glaciation and global ocean atmosphere cooling i e oi 1 event a 0 5 m 55 70 kyr resolution ostracode faunal study of the massicore drilled up section from the italian massignano quarry containing the eocene oligocene global stratotype section and point gssp revealed a relative increase in krithe and decreases in faunal evenness and shannon index correlative to the oi 1 event these transient faunal changes may reflect local water column changes e g export production or intensified thermohaline flow of cooler deep waters through the paleotethyan seaway comparison of these massicore ostracode faunal data to previously published massignano quarry ostracode faunal data reveals similar relative abundances among common taxa where the study intervals overlap and thereby provide a more complete tethyan ostracode record spanning all of c13n and the broader eot at the gssp locality

glacial eustasy during the cenozoic sequence stratigraphic implications a newly constructed composite oxygen isotope record is a proxy for eustasy that extends back to the cretaceous tertiary boundary and provides an independent test of sequence stratigraphic based eustatic curves the isotope record shows several eustatic episodes that are consistent with the geological record of ice sheet evolution the first evidence for the existence of an ice sheet in east antarctica occurs near the lower middle eocene boundary base of the lutetian stage there is no evidence for a large ice sheet on antarctica prior to this time however strata of this age are lacking over most of the continent the isotope curve also indicates that the ice sheet experienced phases of growth during the late eocene and middle oligocene followed by a decrease in volume in the early miocene the ross sea stratigraphic record indicates initial evolution of the west antarctica ice sheet during the early miocene by the middle miocene the ice sheet spread across the ross sea weddell sea and antarctic peninsula continental shelves the pliocene pleistocene record of glaciation in antarctica includes numerous glacial erosion surfaces on the continental shelf indicating repeated advance and retreat of both east and west antarctica ice sheets these volume changes in the antarctica ice sheet were in response to the rise and fall of sea level caused by expanding and contracting northern hemisphere ice sheets there is a reasonable correlation between eustatic curves derived from sequence stratigraphic studies and the composite oxygen isotope record since the middle eocene this correlation indicates that glacial eustasy has been the principal factor regulating stratal stacking patterns on a global scale since the middle eocene

subsurface seepage of seawater across a barrier a source of water and salt to peripheral salt basins many of the great phanerozoic salt basins share a common paleogeography in which a deep lake or sea is separated from the ocean by a narrow barrier examples include the aptian evaporites of the south atlantic rift basins and the messinian salt deposits of the mediterranean sea and the north caspian depression marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins we test an alternate hypothesis that subsurface seepage of seawater through the barrier was a significant source of water influencing both water level and water composition our model permits flux of water and salt into and out of the basin via 1 seepage through the barrier 2 evaporation from the water surface 3 rainfall and or rivers and 4 incorporation into basin sediments our investigation focuses on three settings a modern analog in the gregory rift of northern kenya the messinian mediterranean basin and the early cretaceous aptian salt basins of the south atlantic we test our model through a numerical simulation applying reasonable values for the dimensions of the barrier differences in water level and hydraulic conductivities to calculate flow rates through the barrier modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate size of the peripheral basin and hydraulic conductivity of the barrier the flux of salt is significant over the full range of modeled hydraulic conductivities producing brines in the peripheral lake and or sea within 0 5 m y in most cases this demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions and that marine transgressions across the barrier are not necessary to explain the apparent high salinities

distribution of paleogene planktonic foraminifera analogies with the recent abstract distribution patterns and morphologies of modern and paleogene planktonic foraminifera in the atlantic ocean were compared although many paleogene and modern planktonic foraminifera are homeomorphs they are not ecomorphs morozovellid homeomorphs of the modern higher spired globorotaliids inhabited warmest layers of the mixed layer the planorotalitids homeomorphic with modern low trochospiral globorotaliids have different geographic ranges they predominated at temperature and higher latitudes where they grew to largest sizes the cool water low spired subbotinids little resemble the non spinose modern neogloboquadrinids which dominate polar faunas today spinosity of paleogene polar forms indicates that they could have been carnivorous or omnivorous rather than herbivorous like the modern neogloboquadrinids early paleogene polar seas may have been relatively oligotrophic for at least part of the year some paleogene genera may have been ecomorphic with their modern analogues the dentoglobigerinids and globoquadrinids occupied equatorial and offshore upwellings much as neogloboquadrina dutertrei does today because the globorotaloids and catapsydracids deposited calcitic crusts and register most enriched oxygen isotope values they may have lived within and below the thermocline like some modern globorotaliids globorotaloids and catapsydracids may have been strictly herbivorous as are most modern non spinose species their predominance in later paleogene lower latitude upwellings but their near exclusion from polar regions indicates that subeuphotic equatorial waters may have been more eutrophic paleogene type warming in zones p1d p3a p5 p6 7 late p12 late p16 late p19 and early p22 involves proliferation evolutionary diversification and poleward migration of thermophilic forms but decreased species abundance at low latitudes and diminished numbers of fertility indices the guembelitrids biserial heterohelicids globigerinathekids globoquadrinids or dentoglobigerinids neogene type warmings in zones p4 and probably p alpha and p21a involved diversification of cooler water taxa retreat of thermophilic forms from higher latitudes and their concentration in lower latitudes and proliferation of fertility indices under these conditions oxygen isotope records include lower latitude warming but cooling of temperature and higher latitudes and tropospheric waters planktonic foraminiferal diversity and population structure reflect changes in oceanic fertility and the depth of the thermocline cooling and eutrophication resulting in a contraction of the trc trophic resource continuum in atlantic surface waters from late middle eocene through mid oligocene time explain the low diversities extinctions of morphologically complex thermophilic species increased dominance of opportunistic subeuphotic and temperature species proliferation of euphotic and subeuphotic fertility indices and geographic expansion of siliceous primary producers

cenozoic deep sea benthic foraminifers tracers for changes in oceanic productivity from late middle eocene through earliest oligocene high latitude regions cooled and by the end of the period continental ice sheets existed in antarctica diversity of planktonic microorganisms declined and modern groups of terrestrial vertebrates originated coeval faunal changes in deep sea benthic foraminifers have been related to cooling of deep waters and increased oxygenation cooling however occurred globally whereas species richness declined at high latitudes and not in the tropics the late eocene and younger lower diversity high latitude faunas typically contain common epistominella exigua and alabaminella weddellensis opportunistic phytodetritus exploiting species that indicate a seasonally fluctuating input of organic matter to the sea floor we speculate that the species richness gradient and increase in abundance of phytodetritus exploiting species resulted largely from the onset of a more unpredictable and seasonally fluctuating food supply especially at high latitudes

shallow marine ostracode turnover during the eocene oligocene transition in mississippi the gulf coast plain usa abstract the eocene oligocene transition eot is associated with a major eustatic sea level fall sea surface cooling change in ocean stratification and enhanced seasonality while these changes are well known to have significantly restructured open ocean ecosystems comparatively little is known about how these multiple environmental changes affected coastal biotas here we describe ostracode faunas in shelf deposits in the gulf coast of mexico through the late eocene to early oligocene analyzing the biostratigraphy and paleoecology of ostracodes from the mossy grove core mississippi mossy grove ostracode faunas suffered gradual extinction and diversity loss in the 150 kyr between 33 96 and 33 81 ma during the turnover 10 of 21 species disappeared seven of the 12 disappearing species became extinct while others are lazarus taxa that later re appeared in the vicksburgian 33 7 ma during the sea level fall associated with the eocene oligocene e o boundary 33 90 ma six of seven outer shelf taxa disappeared while three of four inner shelf taxa increased in abundance the major changes in ostracode assemblages occur during the eot 1 and eot 2 events prior to the oi1 isotope excursion and appear to reflect a combination of impacts from eustatic sea level fall and local enhanced surface ocean seasonality shown in stable isotope records from the st stephen s quarry core alabama

fate of reflux brines in carbonate platforms active reflux may occur during periods of platform top brine generation but the role and fate of these brines after reflux events are uncertain we have used a numerical flow model to investigate and quantify the response of reflux brines to changes in platform top salinity simulations suggest that reflux brines originally concentrated to gypsum saturation 150 have a relatively long platform residence time on the order of 100 times the duration of the reflux event when brine generating conditions cease brines will continue to sink through the platform entraining seawater a variant of reflux circulation we term latent reflux mesosaline brines intercepted by drilling of carbonate margins by the ocean drilling program may have originated from pleistocene reflux event s on the adjacent platform top and be currently moving by latent reflux latent reflux circulation could deliver a significant quantity of dissolved reactants to platform carbonates including mg for dolomitization

astronomic calibration of the late eocene early oligocene massignano section central italy we present an astronomically tuned polarity timescale for the late eocene early oligocene based on a cyclic limestone marls sedimentary succession exposed at massignano italy gssp for the eocene oligocene boundary here we applied spectral analysis techniques to the high resolution magnetic susceptibility record the results provide clear evidence of a regular and cyclic signal that allows us to discard a stochastic mechanism as controlling these fluctuations conversely these outcomes reveal the presence of stable and notable periodicities that are in tune with those associated with orbital forcing through a cyclostratigraphic analysis of the rhythmic sedimentary alternations and combination with the results of time series analysis of the proxy record we refine the late eocene portion of the gpts through astronomical tuning of the rhythmic sedimentary alternations to recent astronomically calculated variations of earth s orbit we recalibrated the polarity boundaries ages and the main stratigraphic events recognized throughout the sedimentary record among these an age of 33 714 ma is proposed for the e o boundary

environmental magnetic record of paleoclimate change from the eocene oligocene stratotype section massignano italy 1 a high resolution environmental magnetic study of the massignano section italy global stratotype section and point for the eocene oligocene boundary has been performed to test whether a clear magnetic signature associated with climatic change is recognizable in this record our results reveal the existence of alternating intervals with high and low magnetic mineral concentrations similar to the pattern of rock magnetic property variations observed from an environmental magnetic study of the ciros 1 sediment core from antarctica these results suggest that an external forcing mechanism drove the sedimentary response to global climate change prior to the major oi 1 cooling event at the eocene oligocene boundary

marine biotic signals across a late eocene impact layer at massignano italy evidence for long term environmental perturbations the eocene oligocene transition marks the passage from greenhouse conditions to an icehouse state with progressive global cooling starting in the early middle eocene the late eocene presents substantial evidence for extraterrestrial impacts whose effects on living organisms and climatic changes are still not completely clear a high resolution microfloral and faunal investigation has been carried out in a 4 m thick segment of the massignano global stratotype section and point for the eocene oligocene boundary the studied interval includes a late eocene 35 7 0 4 myr old impactoclastic layer containing several cosmic signatures the impact event recorded at massignano had no abrupt dramatic effects on marine biota in terms of extinction however significant quantitative changes in the calcareous plankton and dinoflagellate cyst assemblages occurred 60 kyr after the impact event the observed pattern is intepreted as reflecting a long term re organization of water structure

critical grain size parameters for predicting framework and floating grains in sediments abstract in unconsolidated sediments some grains form a load bearing framework while others may float that is occupy volume without contributing to mechanical strength the latter grains affect the relationship between acoustic velocity and porosity as well as the permeability of the sediment we propose that characteristic features of the grain size distribution are indicators of the fraction of floating grains the criterion for floating involves grain scale geometry not accessible from macroscopic models of grain packing so we support this proposition with an examination of computer generated model sediments dense disordered packings of spheres having two sizes of spheres the analysis reveals two thresholds first the fraction of floating grains is larger when the volume fraction of small grains is below 40 in packings satisfying the first threshold a second threshold exists if the large grain radius is sufficiently greater than the small grain radius by a factor of 3 or half a decade on a logarithmic scale the fraction of floating grains is significantly larger these thresholds are qualitatively consistent with field observations of anomalously slow p wave velocities in several poorly consolidated to semiconsolidated reservoirs the grain size distributions in these reservoirs are sufficiently broad to satisfy the two thresholds and the emergence of a second mode in the distribution correlates with greater anomaly in the velocity porosity trend the maximum fraction of floating grains in the model sediments is comparable to the value needed to explain the anomaly quantitatively significantly the thresholds for floating grains are closely related to those known to affect trends of macroscopic properties of sediments porosity permeability the grain scale model can thus serve more generally as a microstructural complement to established macroscopic models

deep sea benthonic foraminiferal faunal turnover near the eocene oligocene boundary abstract eocene oligocene deep sea benthonic foraminifera in d s d p site 277 in the southwest pacific have been analyzed to determine the benthonic foraminiferal response to the development of the psychrosphere near the eocene oligocene boundary biostratigraphic ranges of 41 taxa show that 23 taxa are found throughout the late eocene to early oligocene sequence while 18 taxa exhibit first or last occurrences comparison of the faunal changes in site 277 with a benthonic foraminiferal oxygen isotope record shows that the development of the psychrosphere did not have a profound effect upon the benthonic foraminifera and the overall faunal change preceding and subsequent to the bottom water circulation event occurred gradually the inferred water mass event affected the relative abundance of one species epistominella umbonifera the lack of major faunal changes at the eocene oligocene boundary in site 277 probably reflects either wide environmental tolerances of the benthonic foraminifera or a bottom water temperature change less than 3 c examination of previously published benthonic foraminiferal biostratigraphic data from d s d p sites 167 171 357 360 363 and 400a and deep sea ostracode data from d s d p leg 3 show faunal changes occurred during discrete intervals in the middle eocene early oligocene the faunal patterns from these data and from site 277 show that the eocene oligocene cooling event did not cause rapid catastrophic changes of the benthonic faunas of the open ocean although significant faunal changes are associated with the water mass event in sites 167 171 and 400a the benthonic faunal changes in middle eocene early oligocene time are consistent with the gradual decrease of inferred bottom water temperatures based on previously published oxygen isotopic data the 18 o eocene oligocene enrichment of 0 76 is a major event in the southern ocean oxygen isotopic record but is considerably less in magnitude than the 1 75 2 00 change that occurred gradually from mid early eocene to the eocene oligocene boundary the benthonic foraminiferal and isotopic data indicate that bottom water circulation may have developed during the middle eocene to early oligocene interval with the 3 c bottom water cooling near the eocene oligocene boundary representing part of this development

late cretaceous and cainozoic bathyal ostracoda from the central pacific dsdp site 463 cainozoic deep sea ostracod assemblages from the summits of mid pacific guyots point to high levels of endemism possibly as a result of their bathymetric separation from the surrounding sea floor however the interpretation of these fossil assemblages is hampered by the paucity of comparative material from surrounding non guyot sites fifteen ostracod assemblages from dsdp site 463 late cretaceous pleistocene were studied to compare with those from nearby guyots three distinct faunal assemblages are recognised at site 463 assemblage a maastrichtian eocene assemblage b oligocene upper miocene and assemblage c upper miocene pleistocene although the palaeoenvironmental significance of these units is unclear sixty two ostracod species are identified the thirteen most abundant are discussed in the taxonomic section five of which are described as new between 30 and 100 of the species encountered in each sample are considered as endemic to site 463 while some of the remaining species were previously thought to be endemic to individual guyots similarly high levels of endemism on nearby guyots probably reflect an incomplete knowledge of deep sea ostracod faunas rather than the establishment of geographically or bathymetrically restricted populations the presence of globally pandemic and geographically widespread taxa on sites such as the mid pacific mountains surrounded by abyssal depths which lie below the ccd indicates that some faunal exchange or migration of ostracods does take place this must be achieved within the intermediate waters and probably occurs passively

evolution of an early eocene pull apart basin in the central pontides northern turkey new insights into the origin of the north anatolian shear zone although the north anatolian shear zone is one of the main lithospheric scale strike slip deformation zones in the world playing a prominent role in the complex geodynamic interaction between the eurasian anatolian and arabian plates the onset time of its activity remains highly controversial here we tackle this issue by utilizing nannofossil biostratigraphy on deposits from the tascilar basin a pull apart basin that we have identified inside the north anatolian shear zone overprinting the intra pontide suture zone the syn tectonic sedimentary succession of the tascilar basin developed completely during the early eocene ypresian cne4 cne5 zones the strike slip faulting related to the initial onset of the north anatolian shear zone can likely be constrained within the ypresian suggesting that the westward escape of the anatolian plate along the north anatolian shear zone started in the early eocene this article is protected by copyright all rights reserved

free thermohaline convection beneath allochthonous salt sheets an agent for salt dissolution and fluid flow in gulf coast sediments basinward migration of jurassic salt in the u s gulf of mexico has resulted in the emplacement of large allochthonous salt sheets into shallow miocene to holocene sediments although comparatively little direct information is available on the environment below these salt bodies it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below we derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures from these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport even if the underlying sediments have permeabilities as low as 10 17 m2 0 01 md the calculated maximum darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability when the vertical permeability of the underlying sediment is 10 17 m2 0 01 md salt is dissolved from the base of the sheet at an average rate of 3 5 m m y 1 the corresponding fluid velocities are such that over a 10 m y period the integrated fluid flux in the underlying sediments would be 104 m3 m 2 by comparison integrated fluid flux for compactive expulsion is 103 m3 m 2 thus for the offshore sediments of the gulf of mexico thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport diagenesis and salt tectonics

sedimentary talc in neoproterozoic carbonate successions abstract mineralogical petrographic and sedimentological observations document early diagenetic talc in carbonate dominated successions deposited on two early neoproterozoic 800 700 million years old platform margins in the akademikerbreen group svalbard talc occurs as nodules that pre date microspar cements that fill molar tooth structures and primary porosity in stromatolitic carbonates in the upper fifteenmile group of the ogilvie mountains nw canada the talc is present as nodules coated grains rip up clasts and massive beds that are several meters thick to gain insight into the chemistry required to form early diagenetic talc we conducted precipitation experiments at 25 c with low so4 synthetic seawater solutions at varying ph mg2 and sio2 aq our experiments reveal a sharp and reproducible ph boundary at 8 7 only above which does poorly crystalline mg silicate precipitate increasing mg2 and or sio2 aq alone is insufficient to produce the material the strong ph control can be explained by mg silica complexing activated by the deprotonation of silicic acid above 8 6 8 7 ft ir tem and xrd of the synthetic precipitates reveal a talc like 2 1 trioctahedral structure with short range stacking order hydrothermal experiments simulating burial diagenesis show that dehydration of the precipitate drives a transition to kerolite hydrated talc and eventually to talc this formation pathway imparts extensive layer stacking disorder to the synthetic talc end product that is identical to neoproterozoic occurrences early diagenetic talc in neoproterozoic carbonate platform successions appears to reflect a unique combination of low al concentrations and by inference low siliciclastic input near modern marine salinity and mg2 elevated sio2 aq and ph 8 7 because the talc occurs in close association with microbially influenced sediments we suggest that soluble species requirements were most easily met through microbial influences on pore water chemistry specifically ph and alkalinity increases driven by anaerobic fe respiration

mesogenetic dissolution its role in porosity development in carbonate reservoirs models of porosity formation in carbonate rocks have stressed subaerial exposure and attendant shallow meteoric diagenesis porosity formation also occurs in deep burial or mesogenetic settings as a result of dissolution enlargement of preexisting pores porosity enhancement and creation of new pore systems brines charged with organic acids carbon dioxide and or hydrogen sulfide derived from organic matter diagenesis and thermochemical sulfate reduction are the likely fluids causing significant mesogenetic dissolution enhanced and newly created mesogenetic pore types can mimic pore types formed in shallow meteoric environments and therefore the mesogenetic origin of some porosity may go unrecognized

benthic ostracods and deep water masses in the atlantic ocean ostracod faunas at six locations are compared and related to distributions in an atlantic ocean wide data base five widely developed vertical faunal sequences are recognised at particular levels within deep water masses henryhowella fauna lower part of antarctic intermediate water krithe fauna upper north atlantic deep water poseidonamicus bosquetina fauna upper part of lower nadw dutoitella fauna lower part of lower nadw legitimocythere fauna antarctic bottom water these faunas are correlated with previously established deep water benthic foraminiferal assemblages and their possible palaeo oceanographic use is discussed

paleoceanographic events and deep sea ostracodes eight recognized or theorized paleoceanographic events during the past 70 million years were tested against changes in the global deep sea benthic ostracode fauna two events the sudden cretaceous tertiary boundary event at 66 million years ago and the more gradual 40 million year event formation of the psychrosphere show up most dramatically before the 40 million year event ostracodes freely radiated into the deeper water regions but were provincial the development of thermal stratification isolated these deep water taxa mostly the survivors of the cretaceous tertiary boundary event which adapted to a new free flowing but more frigid ecosystem and spread rapidly throughout the world

reactive transport modeling of early burial dolomitization of carbonate platforms by geothermal convection reactive transport models rtms permit quantitative investigation of diagenesis and its effects on reservoir quality the rtm toughreact is used to investigate diagenesis in an isolated platform driven by geothermal kohout convection of seawater which has been invoked to explain dolomitization during early burial previous short 0 1 m y rtm simulations suggested that convection can drive dolomitization mostly at greater than 50c and anhydritization but complete dolomitization requires greater than 30 60 m y our more extended rtm simulations 30 m y indicate significant nonlinearities in the system consistent with high temperature experiments with parts of the platform completely dolomitized within 10 15 m y as dolomitization proceeds the process becomes predominantly flux controlled with development of a wedge shaped dolomite body which thins from the margin to the interior at considerably shallower depth and cooler temperatures 20 30c than suggested by short simulations dolomitization is relatively insensitive to boundary conditions such as relative sea level and platform geometry but is significantly slower in circular than elongate platforms sediment permeability and reactive surface area commonly inversely related are key controls dolomitization is limited to the margin of low permeability muddy platforms despite a high reactive surface area dolomitization of more permeable grainy platforms is limited by a lower reactive surface area occurring only in the platform core due to widespread cooling sedimentary layering produces a complex diagenetic stratigraphy dolomitization favoring more reactive beds at shallow depth where permeability is not limiting but switching to more permeable beds at depth bank marginal fracturing limits dolomitization of the platform interior whether the fractures are baffles or conduits for flow

compaction and decompaction algorithms for sedimentary carbonates abstract the mechanical compaction behavior of shallow marine carbonate sediments has been investigated to develop carbonate compaction algorithms and decompaction curves that can be used in stratigraphic restoration backstripping analyses and quantitative models for simulation of carbonate stratigraphy deep water chalks and calcareous oozes are not treated here empirical porosity depth data for carbonate sediments have been compiled and compared with experimentally derived compaction curves to derive appropriate porosity depth curves for shallow water carbonate muds and sands the curves are exponential in form this review indicates that mud rich shallow marine carbonates behave differently than sand rich grain supported sediments with regard to reduction in porosity and thickness quantitative analysis of compaction behavior in carbonate sediments reveals that barring early cementation carbonate muds suffer significant thickness reduction early in their burial history and at shallow depths e g 50 thickness reduction with about 150 200 m overburden however primary porosities of carbonate muds approximately 70 80 are reduced only to about 42 under that overburden thus the potential exists for significant retention of primary porosity in carbonate muds in the form of microporosity this indicates that interparticle cementation with or without chemical compaction is required to transform muddy carbonate sediments into lithified rock in the absence of early lithification carbonate sands compact more slowly than muds primary interparticle porosities of about 40 45 can be maintained in the first 200 300 m of burial with little thickness reduction

geothermal convection in south atlantic subsalt lacustrine carbonates developing diagenesis and reservoir quality predictive concepts with reactive transport models prolific hydrocarbon discoveries in the subsalt commonly known as the presalt section of brazil and the conjugate african margin have created a business imperative to predict reservoir quality in lacustrine carbonates geothermal convection is a style of groundwater flow known to occur in rift settings which is capable of diagenetic modification of reservoir quality we simulated variable density groundwater flow coupled with chemical reactions to evaluate the potential for diagenesis driven by convection in subsalt carbonates rates of calcite diagenesis are critically controlled by temperature gradient and fluid flux following the principles of retrograde solubility simulations predict that convection could operate in rift carbonates prior to salt deposition but with rates of dissolution in the reservoir interval only on the order of 0 01 vol m y which is too low to significantly modify reservoir quality the exception is around permeable fault zones and or unconformities where flow is focused and dissolution rates are amplified to 1 to 10 vol m y and could locally modify reservoir quality after salt deposition simulations also predict convection with a critical function for salt rugosity the greatest potential for dissolution at rates of 0 1 to 1 vol m y occurs where salt welds overlying permeable carbonates thin to 500 m 1640 ft or less with tens of million years residence times feasible convection under these conditions could locally result in reservoir sweet spots with porosity modification of 1 to 10 and potentially an order of magnitude or more in reservoir permeability integrating quantitative model derived predictive diagenetic concepts with traditional subsurface data sets refines exploration to production scale risking of carbonate reservoir presence and quality

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