reactive transport modelling of reflux dolomitization in the arab d reservoir ghawar field saudi arabia it has long been recognized that the arab d reservoir in ghawar field has been significantly dolomitized and that the distribution of dolomites is highly heterogeneous across this reservoir previous studies indicated that dolomite occurs with either a stratigraphic or non stratigraphic distribution when mapped dolomite tends to form several parallel linear trends across the field although stratigraphic dolomite was suggested to be formed early from highly evaporated pore fluids sourced from overlying evaporite deposits non stratigraphic dolomite was thought to be generated primarily from hydrothermal fluids sourced from below this study focuses primarily on these non stratigraphic dolomites and proposes that i these dolomites initially formed via seepage reflux but were reinforced by late stage hydrothermal dolomitization and ii reflux is also responsible for the formation of parallel linear trends of dolomite the reflux model hypothesizes that an evaporative lagoon which is the source of dolomitizing fluids formed during the falling stage systems tract of a depositional sequence and that with continuing sea level fall this lagoon migrated progressively towards deeper parts of an intrashelf basin adjacent to the ghawar field leaving behind lines of dolomite bodies along a series of temporary coastlines two dimensional reactive transport models have been built to test this hypothesis and have resulted in a predicted pattern of dolomite bodies that agrees with both the observed vertical distribution of non stratigraphic dolomite as well as the mapped lateral distribution of the dolomite trends in addition the major ion compositions of late jurassic seawater are calculated based on fluid inclusion data in the literature using jurassic seawater in current models leads to the absence of anhydrite cements and less potential of over dolomitization than using modern seawater

an oscillating trend in phanerozoic non skeletal carbonate mineralogy textural and compositional data on ancient ooids and cements suggest the existence of an oscillating phanerozoic temporal trend in non skeletal carbonate mineralogy the trend shows good correlation with eustatic climatic oscillations proposed by others the most likely mechanism appears to be plate tectonically influenced oscillations in pco2 the vapour pressure of co2 rather than change in oceanic mg ca ratio

the major ion composition of silurian seawater one hundred fluid inclusions in silurian marine halite were analyzed in order to determine the major ion composition of silurian seawater the samples analyzed were from three formations in the late silurian michigan basin the a 1 a 2 and b evaporites of the salina group and one formation in the early silurian canning basin australia the mallowa salt of the carribuddy group the results indicate that the major ion composition of silurian seawater was not the same as present day seawater the silurian ocean had lower concentrations of mg2 na and so42 and much higher concentrations of ca2 relative to the ocean s present day composition furthermore silurian seawater had ca2 in excess of so42 evaporation of silurian seawater of the composition determined in this study produces kcl type potash minerals that lack the mgso4 type late stage salts formed during the evaporation of present day seawater the relatively low na concentrations in silurian seawater support the hypothesis that oscillations in the major ion composition of the oceans are primarily controlled by changes in the flux of mid ocean ridge brine and riverine inputs and not global or basin scale seawater driven dolomitization the mg2 ca2 ratio of silurian seawater was 1 4 and the k ca2 ratio was 0 3 both of which differ from the present day counterparts of 5 and 1 respectively seawaters with mg2 ca2 2 e g modern seawater facilitate the precipitation of aragonite and high magnesian calcite therefore the early paleozoic calcite seas were likely due to the low mg2 ca2 ratio of seawater not the pco2 of the silurian atmosphere

active circulation of saline ground waters in carbonate platforms evidence from the great bahama bank measurements of salinity temperature and ground water discharges within the great bahama bank provide evidence of active circulation of near normal ocean water beneath north andros island elevated salinity waters 38 42 derived by density refulx from the great bahama bank flow eastward beneath the island and mix with normal salinity cold ground waters 19 20 c from deeper than 250 m in the adjacent oceans east to west flow of water from the straits of florida may be driven by head differences generated by the florida current which impinges on the western margins of the platform alternatively density differences between the reflux and tongue of the ocean sea waters may cause more local circulation on the eastern flanks these flows have important diagenetic implications particularly in the explanation of pervasive secondary dolomitization widely reported at shallow depths in the bahamas

something old something new something borrowed something blue a hindsight and foresight of sedimentary geology abstract sedimentary geology is rooted in the origin of stratigraphy around 1800 in spite of scattered early flashes of genius sedimentology did not gain identity until about 1930 heavy mineral petrography and grain size analysis came into their own in the 1920s and the promise of these plus micropaleontology for application in the petroleum industry stimulated the creation of the society of economic paleontologists and mineralogists sepm in 1926 the journal of sedimentary petrology and several treatises on sedimentary petrography appeared before world war ii after the war lithostratigraphy emerged apter 150 years of index fossil dominance studies of modern environments were slow to develop until after world war ii when deltas barriers reefs and carbonate banks were examined these and similar later investigations have added enormous catalogues of information about both marine and non marine systems but it is often difficult to integrate the results with ancient sediments through stimulus by engineering experiments in fluvial transport and experimental turbidity currents the rigorous study of sedimentary processes also began in the 1950s soon vertical sequence analysis and formulation of facies models commenced today we are experiencing a model mania but there is little agreement on what a sedimentary model really should be whither next mudrocks will arrive at last and sophisticated instruments promise continuing advances in studies of grains chemical sedimentation diagenesis etc even more exciting however is that the wedding of sequence stratigraphy which evolved from lithostratigraphy and event stratigraphy which evolved from process and facies sequence studies promises an overdue synthesis of sedimentary geology at last it should be possible to construct true quantitative and predictive models that can evaluate stochastic and deterministic as well as episodic and periodic phenomena at all scales from local events to global eustasy

a comparison of gcm simulated cretaceous greenhouse and icehouse climates implications for the sedimentary record abstract two general circulation model simulations of mid cretaceous climate are presented one representing a greenhouse and the other representing an icehouse world through a comparison of greenhouse and icehouse palaeoclimates differences in processes which are intrinsically linked with climate such as continental weathering and snow and ice accumulation may be observed a number of conspicuous and discrete climatic differences can be seen between the two simulations which may be translated into a number of identifiable signatures in the sedimentary record the climate at mid high latitudes in the icehouse simulation was more arid than the greenhouse simulation and could result in deposition of smectite dominated clay suites in contrast the climate of low latitudes of the icehouse simulation resulted in a moister climate than the greenhouse simulation which could affect the weathering regimes and result in deposition of kaolin dominated clay suites thus cool climate episodes do not necessarily result in a global increase in aridity the icehouse simulation predicts a small permanent icecap at high latitudes which could account for the origin of dropstones in marine sediments furthermore a sea level fall as a consequence of an accumulation of ice at high latitudes may also increase continentality and surface albedo which may also contribute to the causes of the drier climate at high latitudes hurricanes are likely to be more frequent in greenhouse than icehouse climates mid latitude depressions also important for generating storms are likely to be both intense and frequent during icehouse and greenhouse periods of the middle cretaceous

a sequence of events across the cretaceous tertiary boundary the lithological and biological sequence of events across the cretaceous tertiary k t as developed in thick and complete landbased sections and termed the standard k t event sequence is also found in many i sdp cores from all over the globe microtektite like spherules have been found in almost every core or outcrop with an iridium anomaly forming a worldwide strewnfield these microtektites are an excellent indicator of the k t boundary level the basal paleocene globigerina eugubma zone is nov established in all major ocean basins the difference in faunas between the pacific and tethys atlantic oceans may point to different recovery of the oldest paleocene planktonic foraminiferal assemblages in different ocean basins planktonic foraminifera and nannoplankton apparently show a different extinction recovery pattern at the k t boundary which suggests that the nannoplankton underwent final extinction later and recover later than planktonic foraminifera these patterns may be important for the evaluation of the type and range of environmental stress resulting from the hypothesized impact s at the k t boundary blocking of sunlight by the dust cloud still can account for the clear mass mortality at the k t boundary but seems less likely as explanation for the final nannoplankton extinction as photosynthetic nannoplankton appear to survive 1000 10 000 years into the tertiary

stable warm tropical climate through the eocene epoch earth s climate cooled from a period of extreme warmth in the early eocene epoch ca 50 ma to the early oligocene ca 33 ma when a large ice cap first appeared on antarctica evidence from the planktonic foraminifer oxygen isotope record in deep sea cores has suggested that tropical sea surface temperatures declined by 5 10 degrees over this interval eventually becoming much cooler than modern temperatures here we present paleotemperature estimates from foraminifer isotopes and the membrane lipids of marine crenarcheota from new drill cores in tanzania that indicate a warm and generally stable tropical climate over this period we reinterpret the previously published isotope records in the light of comparative textural analysis of the deep sea foraminifer shells which shows that in contrast to the tanzanian material they have been diagenetically recrystallized we suggest that increasingly severe alteration of the deep sea plankton shells through the eocene produced a diagenetic overprint on their oxygen isotope ratios that imparts the false appearance of a tropical sea surface cooling trend this implies that the long term eocene climatic cooling trend occurred mainly at the poles and had little effect at lower latitudes

the stability of ca na pyroxene in low grade metabasites of high pressure intermediate facies series minor metamorphic ca na pyroxene occurs as minute crystals and as thin rims around primary augites in metabasites intermediate between the prehnite pumpellyite pp and pumpellyite actinolite pa facies in the sanbagawa belt shikoku japan their compositions vary systematically with mineral paragenesis from augite to acmite with a constant jadeite component of about 20 molvo the ca na pyroxenes in the pumpellyite bearing assemblages contain a very low acmite component acaaugssjd2s which increases through ac25aug55jd2s in the alkali amphibole bearing and ac66augzsjdrs in the hematite bearing assemblages finally attaining an aegirine composition ac65aug1sjd25 in the magnetite bearing assemblages they are distinguished from relict igneous augites chemically by very low opx and tio2 contents and morphologically by fibrous crystal habit the pyroxene bearing metabasites contain assemblages of chlorite pyroxene pumpellyite epidote actinolite alkali amphibole stilpnomelane hematite and magnetite which are incompatible with the higher grade and widely developed pumpellyiteactinolite facies assemblage epidote chlorite xr 0 50 actinolite pumpellyite the reaction augite chlorite pumpellyite actinolite h2o has been suggested for such a transition the stability relations of augite chlorite pumpellyite were analyzed from comparative mineral parageneses in low grade metabasites from new caledonia new zealand and the appalachians the assemblage may have a wedge shaped p t space bounded against the high tpumpellyite actinolite facies by the above reaction and against the low t prehnite pumpellyite facies by the reaction prehnite actinolite augite chlorite with probable negative slope from the mineral parageneses in the metabasites of the sanbagawa belt and elsewhere it is concluded that the ca na pyroxenes occur within a restricted p t space between the prehnite pumpellyite and pumpellyite actinolite facies

addendum to inclusion of the weathering of volcanic rocks in the geocarbsulf model r a berner 2006 v 306 p 295 302 in the paper by berner 2006a 1 the compiled data presented by dessert and others 2001 2 for different values of the relative weathering rates of basalts versus granites was missed the term granite is used here to refer to all granitoid rocks including gneisses it is the primary

dolomitization of the capitan formation forereef facies permian west texas and new mexico seepage reflux revisited interpretation of seepage reflux dolomitization is commonly restricted to intervals containing evaporites even though several workers have modelled reflux of mesosaline brines this study looked at the partially dolomitized forereef facies of the capitan formation to test the extent of reflux dolomitization and evaluate the possible role of the near backreef mesosaline carbonate lagoon as an alternative source of dolomitizing fluids the capitan formation forereef facies ranges from 10 to 90 dolomite most of the dolomite is fabric preserving and formed during early burial after marine cementation before and or during evaporite cementation and before stylolitization within the forereef facies dolomite follows depositional units with debris flow and grain flow deposits the most dolomitized and turbidity current deposits the least the amount of dolomite increases with stratigraphic age and decreases downslope within the reef facies dolomite is restricted to haloes around fractures and primary cavities except where the reef facies lacks marine cements and in contrast is completely dolomitized this dolomite distribution supports dolomitization by sinking fluids oxygen isotopic values for fabric preserving dolomite 1 8 o 0 9 1 0 n 101 support dolomitization by sea water to isotopically enriched sea water these values are closer to the near backreef dolomite 1 8 o 2 1 0 7 n 48 than the hypersaline backreef dolomite 1 8 o 3 6 0 9 n 11 therefore the fabric preserving dolomite is consistent with dolomitization during seepage reflux of mainly mesosaline brines derived from the near backreef carbonate lagoon the occurrence of mesosaline brine reflux in the capitan formation has important implications for dolomitization in forereef facies and elsewhere first any area with a restricted carbonate lagoon may be dolomitized by refluxing brines even if there are no evaporite facies present secondly such brines may travel significant distances vertically provided permeable pathways such as fractures are present therefore the absence of immediately overlying evaporite or restricted facies is not sufficient cause to eliminate reflux dolomitization from consideration

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

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

dolomitization anhydrite cementation and porosity evolution in a reflux system insights from reactive transport models significant volumes of world hydrocarbon reserves occur in dolostones and the majority of these reservoirs are interpreted to be of reflux origin we used a two dimensional numerical reactive transport model to investigate systematically the temporal and spatial distribution of replacement dolomitization dolomite cementation anhydrite cementation and porosity evolution in a reflux system we tested the sensitivity of reflux dolomitization to brine concentration mesohaline to near halite saturated brines near surface temperature flow rate porosity permeability feedback relationships reactive surface area and the effect of initial porosity permeability heterogeneity simulations support the contention that hypersaline reflux is capable of extensive pervasive dolomitization and that mesohaline reflux dolomitization is viable reflux generated a tabular dolomite body that is thickest close to the brine source and thins basinward incorporation of initial porosity and permeability heterogeneity resulted in a dolomite body with pronounced lateral fingers replacement dolomitization increased porosity up to 8 but postreplacement reflux resulted in the precipitation of minor dolomite cements overdolomitization anhydrite cements that were spatially and temporally associated with replacement dolomitization caused a significant porosity reduction of up to 25 simulated rates of replacement dolomitization by reflux are fast up to three orders of magnitude faster than seawater dolomitization the rate of dolomitization and anhydrite mineralization proved to be critically sensitive to the flow rate and the brine chemistry temperature and reactive surface area were important controls on the rate of dolomitization whereas the feedback relationship between porosity and permeability was a relatively moderate control our reactive transport models predict the general spatial and temporal trends in dolomite porosity and anhydrite observed in some major dolomite reservoirs

dolomitization of the cambrian carbonate platform southern canadian rocky mountains dolomite front geometry fluid inclusion geochemistry isotopic signature and hydrogeologic modelling studies

dolomite occurrence evolution and economically important associations abstract dolomite is not a simple mineral it can form as a primary precipitate a diagenetic replacement or as a hydrothermal metamorphic phase all that it requires is permeability a mechanism that facilitates fluid flow and a sufficient supply of magnesium dolomite can form in lakes on or beneath the shallow seafloor in zones of brine reflux and in early to late burial settings it may form from seawater from continental waters from the mixing of basinal brines the mixing of hypersaline brine with seawater or the mixing of seawater with meteoric water or via the cooling of basinal brines bacterial metabolism may aid the process of precipitation in settings where sulfate reducing species flourish and microbial action may control primary precipitation in some hypersaline anoxic lake settings dolomite is a metastable mineral early formed crystals can be replaced by later more stable phases with such replacements repeated a number of times during burial and metamorphism each new phase is formed by the partial or complete dissolution of an earlier dolomite this continual re equilibration during burial detracts from the ability of trace elements to indicate depositional conditions and resets the oxygen isotope signature of the dolomite at progressively higher temperatures because subsurface dolomite evolves via dissolution and reprecipitation a bed of dolomite can retain or create porosity and permeability to much greater burial depths and into higher temperature realms than a limestone counterpart dolomitization also creates new crystals with new rhomb growth following the dissolution of less stable precursors repetition of this process without complete pore cementation can generate intercrystalline porosity a number of times in the rock s burial history intercrystalline porosity is a highly interconnected style of porosity that gives dolomite reservoirs their good fluid storage capacity and efficient drainage the fact that many dolomite reservoirs formed via brine reflux means that they sit beneath an evaporite seal in both platform and basinwide evaporite settings the same association of evaporites sulfate source and entrained hydrocarbons means that burial conditions are also suitable for thermochemical sulfate reduction and the precipitation of base metals this tends to occur at higher temperatures 60 c 80 c and so the resulting dolomites tend to be ferroan and consist of saddle shaped crystals

outcrop constrained hydrogeological simulations of brine reflux and early dolomitization of the permian san andres formation our hydrogeologic model tests the effectiveness of brine reflux as the mechanism for early dolomitization of the permian san andres formation brine circulation is constrained by sequence stratigraphic parameters and a heterogeneous distribution of petrophysical properties based on outcrop data the model simulates accumulation of the san andres platform and calculates fluid flow and solute transport in response to relative sea level fluctuations it tracks porosity loss caused by compaction and the concomitant permeability feedback the amount of dolomite potentially formed is calculated by means of a magnesium mass balance between brine and rock results show that 1 brine reflux is an effective mechanism to deliver magnesium to dolomitize large carbonate successions 2 relative sea level controlled transient boundary conditions result in intricate flow and salinity patterns that can generate irregular dolomite bodies with complex spatial distributions 3 pervasive dolomitization can result from several short lived reflux events by the amalgamation of brine plumes sourced in different locations and times and 4 the model successfully recreates the dolostone and limestone patterns observed in san andres outcrops

calcium isotope record of phanerozoic oceans implications for chemical evolution of seawater and its causative mechanisms a total of 280 brachiopods of ordovician to cretaceous age complemented by published data from belemnites and planktonic foraminifera are used to reconstruct the evolution of calcium isotope composition of seawater 44 40casw over the phanerozoic the compiled 44 40casw record shows a general increase from 1 3 nist srm 915a at the beginning of the ordovician to 2 at present superimposed on this trend is a major long term positive excursion from the early carboniferous to early permian as well as several short term mostly negative oscillations a numerical model of the global cycles of calcium carbon magnesium and strontium was used to estimate whether the recorded 44 40casw variations can be explained by varying magnitudes of input and output fluxes of calcium to the oceans the model uses the record of marine 87sr 86sr ratios as proxy for seafloor spreading rates a record of oceanic mg ca ratios to estimate rates of dolomite formation and reconstructed atmospheric co2 discharge and erosion rates to estimate continental weathering fluxes the model results indicate that varying magnitudes of the calcium input and output fluxes cannot explain the observed 44 40casw trends suggesting that the isotope signatures of these fluxes must also have changed as a possible mechanism we suggest variable isotope fractionation in the sedimentary output flux controlled by the dominant mineralogy in marine carbonate deposits i e the oscillating calcite aragonite seas the ultimate control of the calcium isotope budget of the phanerozoic oceans appears to have been tectonic processes specifically variable rates of oceanic crust production that modulated the hydrothermal calcium flux and the oceanic mg ca ratio which in turn controlled the dominant mineralogy of marine carbonates hence the 44 40casw as to the causes of the short term oscillations recorded in the secular 44 40casw trend we tentatively propose that these are related to variable rates of dolomite formation and or to changing chemical composition of the riverine flux in particular and ratios induced by variable proportions of silicate vs carbonate weathering rates on the continents

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

evaluating seawater chemistry from fluid inclusions in halite examples from modern marine and nonmarine environments fluid inclusions from marine halites have long been studied to determine the chemical composition of ancient seawater chemical analyses of the major ions in fluid inclusions in halites from the solar saltwork of great inagua island bahamas and from the supratidal sabkha baja california mexico show that modern marine halites faithfully record the chemical signature of seawater the major ions in great inagua and baja california fluid inclusions display distinctive linear trends when plotted against one another ie na k and so42 vs mg2 and cl which track the evaporation path of seawater as it evolved during the crystallization of halite these evaporation paths defined for the major ions by fluid inclusions in halite overlap findings of computer simulations of the evaporation of modern seawater by the harvie moller and weare hmw computer program the close match between the hmw seawater evaporation paths and the great inagua fluid inclusion data is not surprising considering the carefully controlled inflow evaporation and discharge of seawater at the great inagua saltwork the major ion chemistry of fluid inclusions from the baja california halites matches the hmw seawater evaporation paths in most respects but one baja fluid inclusion has lower concentrations of mg2 than evaporated seawater nonmarine inflows and syndepositional recycling of preexisting salts in the baja california supratidal setting were not large enough to override the chemical signature of evaporating seawater as the primary control on the baja fluid inclusion compositions fluid inclusions in halites from the nonmarine qaidam basin qinghai province western china have a distinctly different major ion chemical signature than does global seawater the fluid inclusion chemistries from the qaidam basin halites do not lie on the evaporation pathways defined by modern seawater and can clearly be differentiated from fluid inclusions containing evaporated seawater if fluid inclusions in halites from modern natural settings contain unmistakable samples of evaporated seawater then evaluation of the chemistry of ancient seawater by chemical analysis of fluid inclusions in ancient marine halites by means of the same approach should be valid

the geochemistry of carbonate diagenesis the past present and future stable carbon and oxygen isotopes 18o and 13c values and trace elements have been applied to the study of diagenesis of carbonate rocks for over 50 years as valuable as these insights have been many problems regarding the interpretation of geochemical signals within mature rocks remain for example while the 18o values of carbonate rocks are dependent both upon the temperature and the 18o value of the fluid and additional information including trace element composition aids in interpreting such signals direct evidence of either the temperature or the composition of the fluids is required such information can be obtained by analysing the 18o value of any fluid inclusions or by measuring the temperature using a method such as the clumped isotope technique such data speak directly to a large number of problems in interpreting the oxygen isotope record including the well known tendency for 18o values of carbonate rocks to decrease with increasing age unlike the 18o 13c values of carbonates are considered to be less influenced by diagenesis and more a reflection of primary changes in the global carbon cycle through time however many studies have not sufficiently emphasized the effects of diagenesis and other post depositional influences on the eventual carbon isotopic composition of the rock with the classic paradigm that the present is the key to the past being frequently ignored finally many additional proxies are poised to contribute to the interpretation of carbonate diagenesis although the study of carbonate diagenesis is at an exciting point with an explosion of new proxies and methods care should be taken to understand both old and new proxies before applying them to the ancient record

carbonates and relative changes in sea level abstract in the geologic record some of the most accurate gauges of changes in sea level are the sediment type geometry and diagenesis of carbonate shelves and platforms this is because carbonates frequently occur at or very near sea level and are usually less compacted than siliciclastics world wide changes in relative sea level the sum of eustatic sea level changes sedimentation and crustal movements have occurred repeatedly and cyclicly through geologic time producing characteristic responses in carbonates 1 1 relative rises in sea level usually caused by the cumulative effect of tectonic subsidence and eustatic rise may result in the following 1 1 a drowned carbonate reefs or platforms here carbonate growth potential is exceeded by relative sea level rise and is characterized by shallow water sediments overlain by hardgrounds and or deep water sediments some of which may be condensed sequences 1 2 b platforms where only the fast growing rim and patches of the interior are able to match sea level rise while the remainder of the platform is drowned temporarily 1 3 c platforms which keep up and maintain a flat top at sea level and contain shallow water sediments whose thickness at the least matches the height of the sea level rise if terrigenous supply is limited prograding sheets of shelf carbonate occur frequently capped by supratidal evaporites with prograding shelf margin carbonate clinoforms and turbidites if terrigenous supply is high the shelf carbonates encroach on deltaics 2 2 relative drops in sea level caused by crustal uplift or by subsidence being outpaced by a eustatic drop in sea level cause karst and soil development over shelves and platforms deposition of deep water evaporites in adjacent semi enclosed basins and in open marine basins the deposition of deltaic and aeolian clastics that bypassed the shelf falls are accompanied by platform wide fresh water diagenesis during relative sea level rises marine diagenesis is common in the subtidal portions of the shoaling upward carbonates and fresh water diagenesis and dolomitization and sulphate deposition is common in their intertidal and supratidal portions the stratigraphic significance of these reponses to relative sea level change is that many are tied to eustatic events and so are predictable within a basin of deposition

origin of dolomite in the arab d reservoir from the ghawar field saudi arabia evidence from petrographic and geochemical constraints a significant proportion of oil production from the kim meridgian arab d strata in the ghawar field saudi arabia originates from dolomitized rocks stratigraphic petrographic and geochemical data suggest that at least four episodes of dolomitization affected these sediments the lower portion of the arab d zone 3 is only partially dolomitized with the dolomite frequently being associated with firm grounds we propose that these dolomites were formed on an outer ramp setting with a maximum water depth of 50 m during a period of nondeposition with the dolomitization process being promoted by the oxidation of organic material and the diffusion of mg 21 from the overlying seawater the dolomites in zone 2b are geochemically dis tinct compared to those in zone 3 in that they have relatively positive oxygen isotope compositions 2 1t o22 compared to 26 5 the relatively positive oxygen isotope composition and the geochemical sim ilarity of zone 2b to the dolomites in zone 1 which are intimately associated with the overlying evaporites has led us to conclude that the zone 2 dolomites probably formed by the reflux of hypersaline fluids through the sediments these hypersaline fluids bypassed zone 2a by moving through the grain dominated sediments early cemen tation and dolomite formation made these units more susceptible to later fracturing that affected the entire arab d formation this frac turing allowed higher temperature fluids to leach the dolostones there by removing any remaining calcite and partially resetting the oxygen isotope composition of some of the dolomites as a result of this later dolomitization event rocks that were only partially dolomitized were leached creating units with extremely high permeability and porosity super k intervals dolomites in the lower zone 3 were recrystallized during burial by the normal geothermal gradient leading to the pre sent negative oxygen isotope values zone 1 dolomites are petrograph ically distinct from zone 2 dolomite in that they are mimetic and fabric preserving although they are geochemically similar this mimetic style of dolomitization occurs immediately adjacent to the overlying anhy drite and is interpreted to have formed very shortly after deposition from hypersaline brines recovery and their presence is inferred from a combination of production data and the diameter of the bore hole as measured using the caliper log as a result of the importance of dolomite in the formation of the super k intervals within the arab d an understanding of the paragenesis of these zones is important not only in predicting super k formation within the arab d but also as an analogue for understanding other dolomitized res ervoirs with high permeability and porosity previous characterization work of dolostones from the arab d in the ghawar field cantrell et al 2004 has identified three types of dolomite with distinctive petrographic stratigraphic and geochemical characteristics fabric preserving non fabric preserving and baroque dolomite fabric pre serving dolomite is very finely crystalline in which petrographic details of the original limestone fabric are usually well preserved fabric preserving dolomite contains low concentrations of fe average 247 ppm fe and relatively positive oxygen isotope values average

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