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

early diagenesis of shallow water periplatform carbonate sediments leeward margin great bahama bank ocean drilling program leg 166 mineralogic petrographic and geochemical analyses of sediments recovered from two leg 166 ocean drilling program cores on the western slope of great bahama bank 308 m and 437 m water depth are used to characterize early marine diagenesis of these shallow water periplatform carbonates the most pronounced diagenetic products are well lithified intervals found almost exclusively in glacial lowstand deposits and interpreted to have formed at or near the seafloor i e hardgrounds hardground cements are composed of high mg calcite 14 mol mgco 3 and exhibit textures typically associated with seafloor cementation geochemically hardgrounds are characterized by increased 18 o and mg contents and decreased 13 c sr and na contents relative to their less lithified counterparts despite being deposited in shallow waters that are supersaturated with the common carbonate minerals it is clear that these sediments are also undergoing shallow subsurface diagenesis calculation of saturation states shows that pore waters become undersaturated with aragonite within the upper 10 m at both sites dissolution and likely recrystallization of metastable carbonates is manifested by increases in interstitial water sr and sr ca profiles with depth we infer that the reduction in mineral saturation states and subsequent dissolution are being driven by the oxidation of organic matter in this fe poor carbonate system precipitation of burial diagenetic phases is indicated by the down core appearance of dolomite and corresponding decrease in interstitial water mg and the presence of low mg calcite cements observed in scanning electron microscope photomicrographs

self organization and nonlinear dynamics in sedimentary basins and discussion patterns of mineralization may develop during diagenesis that are not a direct reflection of sedimentary features or stresses and fluids imposed on the boundaries of a basin these patterns occur through the amplification of omnipresent non uniformities of textural or compositional variables amplification occurs through feedback involving the coupling of diagenetic processes the spatial scale of these patterns varies from microns to kilometres examples to be discussed include intracrystalline oscillatory zoning millimetre to metre scale differentiated layering and stylolites liesegang bands reaction front fingering pulsatile fluid release from overpressurized zones in deep basins and kilometre scale basin compartmentation

record of late holocene debris avalanches and lahars at iliamna volcano alaska abstract iliamna volcano is a 3053 meter high glaciated stratovolcano in the southern cook inlet region of alaska and is one of seven volcanoes in this region that have erupted multiple times during the past 10 000 yr prior to our studies of iliamna volcano little was known about the frequency magnitude and character of holocene volcanic activity here we present geologic evidence of the most recent eruptive activity of the volcano and provide the first outline of late holocene debris avalanche and lahar formation iliamna has had no documented historical eruptions but our recent field investigations indicate that the volcano has erupted at least twice in the last 300 yr clay rich lahar deposits dated by radiocarbon to 1300 and 90 yr bp are present in two major valleys that head on the volcano these deposits indicate that at least two large possibly deep seated flank failures of the volcanic edifice have occurred in the last 1300 yr noncohesive lahar deposits likely associated with explosive pyroclastic eruptions date to 2400 1300 1500 300 and debris avalanche deposits from recent and historical small volume slope failures of the hydrothermally altered volcanic edifice cover most of the major glaciers on the volcano although these deposits consist almost entirely of hydrothermally altered rock debris and snow and ice none of the recently generated debris avalanches evolved to lahars a clay rich lahar deposit that formed

water rock interaction and reactive transport modeling using elemental mass balance approach i the methodology water rock interaction and reactive transport modeling is an important tool for deciphering chemical and physical reactions occurring in sediments and rocks the modeling methodology calls for solving conservation equations to account for the interaction between mass transfer and reaction processes using a sequential iteration numerical method the formalism presented here improves upon the traditional approach by reformulating conservation equations to express chemical elemental mass evolution the elemental conservation equations describe mass change through mass transfer and kinetic reactions of solids and solute solute interaction equilibrium reaction expressions the system of equations is solved using a sequential iteration method whereby globally discretized mass transfer coefficients of conservation equations are injected explicitly into the conservation equations and solved locally at each node the result is a mathematically simple approach that eliminates the traditionally required use of primary and secondary species classification the formalism also allows the use of solute specific diffusivities in addition the tight coupling between mass transfer and reactions achieved by the methodology also allows nonlinear self organization or pattern forming phenomena to be captured the methodology is demonstrated with simulations of 1 a diffusive reaction system involving patterned hematite nucleation and 2 a flow through system where co2 charged water interacts with formation water in a sandstone reservoir both are isothermal one dimensional simulations in the first example convergent diffusive infiltration of fe and o2 from two opposite ends produces patterned precipitates a natural analog of such phenomena is the occurrence of iron oxide concretions in navajo sandstones of utah and arizona the second example demonstrates the formation of a diffusive solute migration front ahead of advective effluent displacement and reaction fronts such phenomena are observed when large quantities of co2 are injected into geologic formations the early arrival of diffusive solute fronts at nearby monitoring wells are noted by decreasing water ph and raised alkalinity some time before the arrival of the effluent

stratigraphic framework of holocene volcaniclastic deposits akutan volcano east central aleutian islands alaska akutan volcano is one of the most active volcanoes in the aleutian arc but until recently little was known about its history and eruptive character following a brief but sustained period of intense seismic activity in march 1996 the alaska volcano observatory began investigating the geology of the volcano and evaluating potential volcanic hazards that could affect residents of akutan island during these studies new information was obtained about the holocene eruptive history of the volcano on the basis of stratigraphic studies of volcaniclastic deposits and radiocarbon dating of associated buried soils and peat a black scoria bearing lapilli tephra informally named the akutan tephra is up to 2 m thick and is found over most of the island primarily east of the volcano summit six radiocarbon ages on the humic fraction of soil a horizons beneath the tephra indicate that the akutan tephra was erupted approximately 1611 years b p at several locations the akutan tephra is within a conformable stratigraphic sequence of pyroclastic flow and lahar deposits that are all part of the same eruptive sequence the thickness widespread distribution and conformable stratigraphic association with overlying pyroclastic flow and lahar deposits indicate that the akutan tephra likely records a major eruption of akutan volcano that may have formed the present summit caldera noncohesive lahar and pyroclastic flow deposits that predate the akutan tephra occur in the major valleys that head on the volcano and are evidence for six to eight earlier holocene eruptions these eruptions were strombolian to subplinian events that generated limited amounts of tephra and small pyroclastic flows that extended only a few kilometers from the vent the pyroclastic flows melted snow and ice on the volcano flanks and formed lahars that traveled several kilometers down broad formerly glaciated valleys reaching the coast as thin watery hyperconcentrated flows or water floods slightly cohesive lahars in hot springs valley and long valley could have formed from minor flank collapses of hydrothermally altered volcanic bedrock these lahars may be unrelated to eruptive activity

cenozoic dolomites of carbonate islands their attributes and origin abstract dolomites found on and below carbonate islands atolls and oceanic platforms provide useful insights into the origin of dolomite insights that may not be attainable from the study of more ancient cratonic dolomites for this reason they have been the subject of study for decades a critical mass of case studies now exist and some significant conclusions can be drawn from the cumulative data in most cases the association with an island is causal and not genetic nevertheless these dolomite occurrences are referred to herein as island dolomites one type of dolomite on carbonate islands is penecontemporaneous dolomite a phase that forms while the host sediment is in its original depositional setting island examples are holocene in age occur in holocene sediments and originate as a direct precipitate from either normal or evaporated seawater these are microcrystalline poorly ordered ca rich 18o enriched and geochemically unstable phases that are susceptible to recrystallization in the setting in which they formed post depositional dolomite is the other type of dolomite associated with carbonate islands this type of dolomite replaces older precursors and forms as cement distinctive characteristics include a dominance of fabric preserving texture pore lining cement rims that may exhibit micron scale banding with low mg calcite and formation in association with precursor dissolution sr isotopic dating indicates that all examples are neogene or quaternary in age those formed during the middle miocene through pliocene are massive laterally continuous and often multigenerational in contrast younger examples tend to be localized partial replacements of a single generation sr isotope ages also suggest global synchroneity in many dolomitization events which suggests a connection between dolomitization global eustacy and or global climatic factors geochemical attributes of post depositional island dolomites are ca enrichment positive 18o and 13c low sr contents 150 300 ppm and low fe global similarity in petrography and geochemistry of replacive island dolomites argues for a similar origin inferred origins however depend primarily on how 18o and sr data are interpreted values for 18o and dsr must be assumed due to uncertainties in oxygen isotope fractionation and sr partitioning there is no consensus or uniformity in those assumptions thus interpretations can vary and be biased to a desired result covariant trends in 13c and 18o some negative 13c values and high sr all favor a mixing zone origin but examples with these attributes are few lack of covariance in the isotopes mean 18o of 2 0 to 3 5 and low sr future studies should standardize the type of data collected and the analytical techniques employed multiple geochemical attributes should be measured on microsampled components and quantitative modeling should be employed in order to constraint interpretations as much as possible also needed are a better understanding of the kinetic processes that form these dolomites more careful assessment of their recrystallization status and an improved understanding of dsr and 18o at low temperature

formation of lateral patterns in rock properties by dolomitization evidence from a miocene reaction front bonaire netherlands antilles abstract petrophysical and geochemical properties of dolomites may exhibit a nested set of lateral patterns typically at the scales of a few meters to 20 m this study assesses whether those lateral patterns are inherited from the limestone precursor or formed during dolomitization sampling was done across a preserved limestone to dolomite reaction front in grainstones of the miocene seroe domi formation bonaire netherlands antilles post dolomitization diagenetic overprinting in the two rock types is absent or minimal two 65 m lateral transects were drilled at 30 cm spacing in two separate beds porosity permeability and geochemical 18o 13c mn sr and na analyses were performed on all 287 recovered samples and 287 thin sections were point counted for petrographic attributes variography shows that dolomite abundance and porosity contain three scales of lateral variability much of the signal is random sample to sample noise as 65 of the total spatial variance occurs at 30 cm spacing however porosity and dolomite abundance also exhibit a short scale correlation length of 3 7 m and a long range oscillatory pattern hole effect at 16 m both of which are also periodic trace elements mn sr and permeability exhibit short range correlation to 5 8 m but no long range pattern mole mg and 13c show increasing variance with distance and all other dolomite attributes exhibit no spatial patterns no limestone attributes show evidence for any short range or long range oscillatory patterns this indicates that the lateral patterns in dolomite abundance porosity permeability sr and mn contents were not inherited from the limestone precursor dolomitization is interpreted to be the cause of the observed spatial patterns as that is the only geologic process that has affected the dolomite but not the limestone on the other side of the front geochemical self organization during the dolomitization process is suggested to be the mechanism by which the spatial patterns in the dolomites emerge a model for that self organizing process is proposed that involves positive feedbacks between precipitation dissolution textural properties reaction rates carbon transport and the evolving flow field relative to mississippian permian and eocene dolomites that have been studied previously the seroe domi dolomites have similar porosity variance at 30 cm spacing and hole effects with slightly greater wavelengths and much larger magnitudes any differences are interpreted to be related to diagenetic history the older dolomites have been affected by burial diagenesis and or weathering which would likely increase sample to sample variability therefore increasing short scale randomness decreasing the magnitude of hole effects and shortening wavelengths of hole effects the lateral patterning formed by dolomitization may thus not be as strongly expressed in ancient dolomites that have complex burial histories

bed scale spatial patterns in dolomite abundance part ii effect of varied fluid chemistry flow rate precursor mineralogy temperature textural heterogeneity nucleation density and bed geometry

controls on reflux dolomitisation of epeiric scale ramps insights from reactive transport simulations of the mississippian madison formation montana and wyoming abstract prediction of the geometry and petrophysical properties of dolomite geobodies depends on understanding both the hydrological system supplying reactive fluids and the chemistry of these fluids however patterns are complicated by the non linear response of the diagenetic system to depositional texture which controls both fluid flux via permeability architecture and reaction rate via effective surface area this study explores interactions between extrinsic controls spatial distribution of brine composition and temperature and intrinsic controls permeability and reactivity using local and regional scale reactive transport models of sequential episodes of brine reflux that resulted in partial dolomitisation of the mississippian madison ramp inter well scale models show preferential early dolomitisation of fine grained more reactive beds pervasive dolomitisation can occur most readily beneath the brine pool where flow is perpendicular to bedding and is most rapid at high brine fluxes down dip of the brine pool bedding parallel flow is focused in relatively permeable coarse grained beds providing reactants for strongly preferential alteration of intervening more reactive fine grained beds in contrast thicker sequences of fine grained beds dolomitise more slowly limited by the rate of supply of magnesium regional scale models with injection of brines of increasing salinity towards the ramp interior reproduce the observed pattern of dolomitisation however more realistic simulations in which reflux is driven by lateral density contrasts generate flow rates orders of magnitude too low for significant dolomitisation simulations suggest pervasive dolomitisation of epeiric scale ramps by a platform wide reflux circulation as often invoked is not feasible rather dolomitisation of such extensive systems critically requires local scale flow systems such as may result from topographically controlled variations in restriction of platform top seawater circulation

lateral periodic variations in the petrophysical and geochemical properties of dolomite mississippian dolowackestones contain periodic oscillations in the lateral distribution of trace element concentrations porosity and permeability random variations at 30 cm spacing account for 50 70 of the total variability the remainder of the variability occurs in short and long range oscillatory patterns with periods of 1 2 7 6 m which can only be resolved by high resolution sampling of an 150 m lateral transect possible origins for these patterns are 1 inheritance from the depositional precursor 2 for mation by self organizing processes during dolomitization or 3 overprinting by late dia genesis these oscillatory patterns have up to now been unrecognized and addressing their origin and meaning s represents a new approach to the study of dolomites under standing the lateral distribution of petrophysical properties can also improve models of fluid flow in dolomite petroleum reservoirs and contaminant transport between matrix and conduits in dolomite aquifers further if 30 50 of the variability in a geochemical attribute in any bed is due to lateral periodicity one must ask if that variability is too great to assume a spot sample will be a suitable proxy for ancient geologic processes and conditions

a model for calcium magnesium and sulfate in seawater over phanerozoic time a theoretical model has been constructed for calculating the concentrations of dissolved calcium magnesium and sulfate in seawater over the past 550 million years phanerozoic time it is demonstrated that the weathering of ca carbonates and silicates by carbonic acid can be generally ignored in the modeling a previous model of the carbon cycle is used to guide input to the oceans of mg from carbonate and silicate weathering by h 2 co 3 similarly a previous model of the sulfur cycle is used to guide input fluxes to the ocean by calcium sulfate weathering and the weathering of ca and mg carbonates and silicates by sulfuric acid the sulfuric acid is derived from the oxidation of pyrite during weathering plus the oxidation of sulfur gases derived from the metamorphic volcanic decomposition of pyrite and caso 4 calcium is removed from seawater as sedimentary carbonates and sulfates so 4 is removed as sedimentary pyrite and caso 4 and mg is removed by exchange for ca via basalt seawater reaction and or dolomitization plus minor burial in sediments with pyrite as anoxic dolomite by adjusting starting conditions modeling results can be found that agree well with results of studies of salt fluid inclusions for variations of ca and so 4 this is especially true of the timing when ca was more abundant in seawater than so 4 and vice versa general agreement was also found with fluid inclusion results for changes in the mg ca ratio thereby distinguishing calcite seas from aragonite seas because of this overall agreement the modeling offers a new perspective on why oceanic ca mg and so 4 have varied over phanerozoic time

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

outcrop analog for cyclic shelf reservoirs san andres formation of permian basin stratigraphic framework permeability distribution geostatistics and fluid flow modeling vertical and horizontal transects were sampled from core and outcrop of the san andres formation at lawyer canyon guadalupe mountains new mexico to assess permeability variation in a geologic framework of upward shallowing carbonate cycles and to show the potential effect these variations have on viscous dominated flow behavior in analogous reservoirs these cycles occur in a ramp crest facies tract are 3 13 m 10 45 ft thick and contain both vertical and lateral variation of lithofacies thicker cycles consist of a basal dolomudstone which is overlain by burrowed dolopackstone and capped by bar flank ooid peloid dolograinstone and bar crest ooid dolograinstones in vertical transects permeability is extremely variable about the mean yet upward increasing trends coinciding with the succession of lithofacies typify a given cycle semi variance analysis shows permeability to be uncorrelated vertically at distances greater than 5 5 m 18 ft which is the average cycle thickness suggesting that the cycles may equate to a fluid flow unit in a reservoir semi variance analysis of measurements collected along a horizontal transect within bar crest dolograinstones of a single cycle shows permeability is uncorrelated at distances greater than 3 6 m 12 ft this correlation distance appears to be controlled by alternating porous and tightly cemented zones that formed during dolomitization vertical and lateral variogram models were fit to the spatial parameters to generate a variety of conditionally simulated permeability fields fluid flow simulations show viscous dominated flow behavior is compartmentalized by both the individual cycles and groups of cycles the basal dolomudstones are potential baffles to flow crossover between cycles but poorly developed cycles i e those that are mud rich and lack well developed bar flank and bar crest facies result in the greatest compartmentalization of fluid flow within a succession of cycles

permeability loss with depth in the cenozoic carbonate platform of west central florida relationships between matrix permeability and depth in shallow buried 500 m cenozoic limestones of florida were investigated using more than 12 000 minipermeameter measurements on 1210 m of slim hole core these data are considered representative of the matrix permeability reduction that accompanies early burial of shallow water subtidal to peritidal nonreefal carbonates of sta ble cratonic margins platforms shelves and ramps measured matrix permeabilities range from less than 4 0 to 6450 md for both limestones and dolostones although the median permeability of limestones 88 md exceeds that for dolostones 15 md sucrosic dolostones which have high intercrystalline po rosities have median matrix permeabilities of 1000 md non sucrosic dolostones which are much less porous have median matrix permeabilities of only 13 md neither population of dolostones exhibits a permeability with depth trend low permeability limestones consist of mudstones wackestones tidal laminites pedogenetically altered limestone lime mud rich breccia and well cemented grain supported rocks these low permeability limestones all exhibit median matrix permeabilities less than 35 md and none exhibit permeability with depth trends in contrast high mud packstones low mud packstones very low mud packstones and grainstones exhibit larger median matrix permeabilities 69 129 181 and 418 md respectively and a systematic reduction with depth in their mean and maximum permeabilities least squares exponential regressions indicate that burial depth explains 22 31 of the permeability variance in these grain supported rocks the balance of the variability within each textural class is attributed to variation in facies grain types grain sizes sorting etc and the amount of early preburial cementation in the absence of permeability enhancement by deep subsurface diagenesis or fracturing extrapolation of the permeability with depth trends suggests that the limestones that have the best reservoir potential in the deeper subsurface are grain supported limestones that have higher than begin page 1254 average matrix permeabilities in the near surface realm petrographic analyses and consideration of diagenetic histories suggest that matrix permeability of the dolostones and low permeability limestones was determined by early near surface dolomitization and cementation respectively and has yet to be affected by burial processes petrographic observations also suggest that the depth signal in grain supported limestones results from mechanical compaction and the onset of grain to grain pressure solution extrapolation of the matrix permeability data indicates that permeability loss with burial occurs faster than the documented burial related porosity loss for the same group of rocks this indicates that limestone permeability is more sensitive to burial compaction than limestone porosity thus preservation of reservoir quality rock during burial is more dependent on permeability preservation than on porosity preservation

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

volcanic mixed avalanches a distinct eruption triggered mass flow process at snow clad volcanoes a generally unrecognized type of pyroclastic deposit was produced by rapid avalanches of intimately mixed snow and hot pyroclastic debris during eruptions at mount st helens nevado del ruiz and redoubt volcano between 1982 and 1989 these mixed avalanches traveled as far as 14 km at velocities up to 27 m s involved as much as 10 7 m 3 of rock and ice and left unmelted deposits of single flow units as thick as 5 m during flow downslope heat transfer from hot rocks to snow produced meltwater that partially saturated the mixtures apparently giving these mixed avalanches mobilities equal to or greater than those of dry debris avalanches of similar volume the avalanches studied for this report began as snow avalanches triggered during explosive phases of the eruptions by ballistic bombardment of snow slopes by hot pyroclastic debris erosion by the avalanches incorporated additional snow fragments of glacier ice and other rock debris mixed avalanche deposits were massive very poorly sorted and ungraded to inversely graded prior to melting although rock debris composed as much as 70 wt of the frozen samples the bulk volumetric content of rock debris was only 9 to 36 snow and ice composed between 36 and 72 of the frozen samples with void space making up the remainder of the volume deformation and consolidation after melting of the ice content reduced the deposits to loose porous layers that were only a fraction of their original thickness and that later gave little hint of their origin after melting and desiccation the deposits are highly susceptible to erosion and unlikely to be well preserved in the stratigraphic record

dolomite reservoirs porosity evolution and reservoir characteristics systematic analyses of the published record of dolomite reservoirs worldwide reveal that the majority of hydrocarbon producing dolomite reservoirs occurs in 1 peritidal dominated carbonate 2 subtidal carbonate associated with evaporitic tidal flat lagoon 3 subtidal carbonate associated with basinal evaporite and 4 nonevaporitic carbonate sequence associated with topographic high unconformity platform margin buildup or fault fracture reservoir characteristics vary greatly from one dolomite type to another depending upon the original sediment fabric the mechanism by which dolomite was formed and the extent to which early formed dolomite was modified by postdolomitization diagenetic processes e g karstification fracturing and burial corrosion this paper discusses the origin of dolomite porosity and demonstrates the porosity evolution and reservoir characteristics of different dolomite types

quantitative diamondoid analysis indicates oil cosourcing from a deep petroleum system onshore niger delta basin the niger delta basin is one of the most prolific hydrocarbon provinces in the world yet the origin of the vast amounts of oil and gas found in the numerous sub basins across the delta remains contested a total of 180 oil samples from more than 40 oil fields in the niger delta were analyzed by gas chromatography gc gc mass spectrometry gc ms and selected samples by gc tandem mass spectrometry gc msms the interpreted thermal maturity and source depositional environments of these hydrocarbons show significant variation depending on the components analyzed and allow no clear correlation to a single source rock but rather imply extensive mixed contributions diamondoid hydrocarbon parameters were used for the first time on these paleogene neogene reservoired oils to investigate source thermal maturity and mixing effects and to perform cross correlations of these oils the diamondoid abundances and distributions support the hypothesis of co sourcing of oil from a thermally cracked sub delta type ii marine source which was then mixed with oils of relatively lower maturities in the paleogene neogene reservoirs future geochemical interpretations should treat the niger delta oils as potential mixtures of oils of variable maturities from different sources often with the most important source biomarkers depleted because of the extent of thermal cracking

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

formation of bed scale spatial patterns in dolomite abundance during early dolomitization part i mechanisms and feedbacks revealed by reaction transport modelling dolomites of varied ages exhibit metre scale nested patterns of lateral periodic variation in permeability and porosity and by inference dolomite abundance as most examples are 100 dolomite two dimensional reaction transport modelling simulations of bed scale dolomitization were used to assess whether those patterns in dolomite abundance could form during near surface replacement dolomitization simulations used a 2 m high and 18 m long model domain a low mg calcite grainstone precursor and an evaporated mississippian seawater brine 430 parts per thousand salinity as the dolomitizing fluid the domain was initially populated with random variations in porosity and or grain size results reveal that spatial patterns in dolomite abundance emerge when there is as little as 1 dolomite formed with similarities between the modelled patterns and outcrop documented patterns the nested patterns include a near random component that constitutes 40 of the total variance short range correlation ranging from 1 5 to 3 3 m and a longer range periodic trend with a wavelength up to 6 5 m the emergence of pattern in dolomite abundance is the result of an autogenic self organizing phenomenon it is triggered by variation in initial calcite reactive surface area that occurs due to the random heterogeneities in initial porosity and or grain sizes the pattern develops due to a combination of kinetic disequilibrium reactions dolomite precipitation and calcite dissolution and positive feedbacks between dolomite growth calcite dissolution and fluid flow flow is around loci of higher dolomite lower porosity and higher reactive surface areas but through loci of lower dolomite higher porosity and lower reactive surface areas the resulting less porous more dolomite and more porous less dolomite structures at the metre scale arise from those localized interactions this self organizing mechanism for pattern formation constitutes a new model for geochemical self organization during dolomitization and is the only self organization model that is proven applicable to the formation of metre scale patterns during early near surface dolomitization this article is protected by copyright all rights reserved

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

scales of lateral petrophysical heterogeneity in dolomite lithofacies as determined from outcrop analogs implications for 3 d reservoir modeling petrophysical data from dolomite outcrops of the mississippian madison formation at sheep canyon wyoming exhibit three scales of lateral variability in single rock fabric units these include a near random component nugget effect a short range structure and a long range cyclic trend hole effect the nugget effect is high and accounts for 31 39 and 48 50 of the variance in porosity and permeability respectively short range lateral variability is reflected by correlation lengths of 6 5 16 ft 2 5 5 m laterally long range periodicities are equivalent to approximately 10 of the petrophysical variance and have wavelengths of 31 and 140 ft 9 5 and 42 6 m for porosity and permeability 55 ft 16 8 m for log10 permeability respectively cross sectional and plan view petrophysical models and streamline simulations explore the effects of these scales of heterogeneity on fluid flow although short range variability accounts for most of the petrophysical heterogeneity the longer range trends can significantly affect fluid flow behavior results indicate that breakthrough time and sweep efficiency vary depending on the magnitude of the lateral long range petrophysical variability that exists in a dolomite reservoir as the component of the long range periodicity hole effect increases from approximately 0 to 25 of the total petrophysical variability a corresponding increase in breakthrough time and sweep efficiency occurs however as the magnitude of the lateral long range petrophysical variability increases beyond 25 of the total petrophysical variability e g from 25 to 50 a corresponding reduction in breakthrough time occurs because the spatial continuity of permeability is greater results indicate that heterogeneity caused by lateral petrophysical cyclicity should be incorporated into dolomite reservoir models for hole effect magnitudes that are greater than 10 of the petrophysical variance to properly characterize and model these scales of variability in a petroleum reservoir outcrop analogs are essential to provide accurate quantitative descriptions of lateral variability in dolomite rock fabrics

spatial patterns of diagenesis during geothermal circulation in carbonate platforms geothermal convection of seawater deep in carbonate platforms could provide the necessary supply of magnesium for dolomitization at temperatures high enough to overcome kinetic limitations we used reactive transport simulations to predict the rates and spatial patterns of dolomitization during geothermal convection in a platform that was 40 km across and 2 km thick in the simulations porosity and permeability decrease with depth to account for sediment compaction dolomitization of a platform consisting of medium grained 0 0 5 mm sediments occurred in a broad band ranging from 2 5 km depth near the margin to 1 5 km depth near the platform center the area of dolomitization is deep enough that temperatures exceed 50 c but not so deep that low permeabilities restrict mass transport complete dolomitization in the center of this zone is estimated to require at least 60 my incorporation of permeability contrasts permeable beds and reactive beds focused dolomitization strongly and reduced the estimated time required for dolomitization by as much as 50 percent dolomitization created magnesium depleted calcium rich fluids in less than 10 ky and results support a link between dolomitization and anhydrite precipitation where adequate sulfate is available

model of seawater composition for the phanerozoic we present an inverse model of phanerozoic seawater com position calibrated against updated paleoseawater compositions from fluid inclusions in marine halites the model considers step wise alteration of seawater composition via 1 variable input of river water 2 variable rates of alteration of seawater through reactions at mid ocean ridges and 3 variable rates of alteration of seawater through reactions on ridge flanks and across the ocean floor in general the model achieves agreement with paleoseawater fluid inclusion data for na 1 c a 2 1 s o 4 2 2 and k 1 particularly when variable runoff is considered variable rates of basalt seawater interactions at both ridges and ridge flanks are required to understand the evolution of seawater particularly the observed near constant concentration of k 1 through time

integrated characterization of carbonate ramp reservoirs using permian san andres formation outcrop analogs the san andres formation permian guadalupian of the permian basin is representative of carbonate ramp reservoirs in that it has highly stratified character complex facies and permeability structure and generally low recovery efficiencies of 30 of original oil in place the approach used here to describe carbonate ramp reservoirs such as the san andres formation produces detailed reservoir models based on integration of sequence stratigraphic analysis petrophysical quantification through definition of rock fabric flow units and fluid flow simulation synthesis of these subdisciplines clarifies which aspects of the geologic petrophysical model are most significant in predicting reservoir performance and ultimately in understanding the location of remaining oil satur tion the san andres formation crops out along the algerita escarpment a long oblique dip continuous shelf to basin exposure in the central guadalupe mountains these outcrops provide a unique opportunity to study lateral relationships in geologic and petrophysical structure analogous to those occurring between wells in subsurface reservoirs on the basis of sequence stratigraphic analysis three scales of cyclicity are recognized depositional sequences high frequency sequences and cycles examination of the cycles in two detailed window areas provides a practical scale for petrophysical quantification and fluid flow simulation an understanding of cycle position within the high frequency sequence framework also provides predictive information petrophysical analysis revealed six rock fabric groups dominated by intergranular separate vug or dense intercrystalline pore types comparison of these rock fabric groups with facies descriptions produced a rock fabric flow unit model that honors the geologic structure of the cycle and sequence framework permeability data were averaged within rock fabric flow units using a geometric mean approach based on fine scale fluid flow modeling of deterministic and stochastically generated permeability fields two dimensional black oil fluid flow models illustrate that 1 major differences in sweep efficiency and fluid flow performance are predicted when linear interwell interpolations are compared with actual interwell scale geologic structure as determined by outcrop geologic and petrophysical mapping 2 an understanding of static geologic petrophysical conditions provides only a partial understanding of reservoir performance defined by the interaction of these static properties and dynamic properties of fluid flow interaction within the flow unit architecture and 3 because of the orderly distribution of high and low permeability facies within cycle stacks of high frequency sequences this larger scale of geologic description can give a reasonable first order approximation of fluid flow patterns and early breakthrough end page 181

high porosity cenozoic carbonate rocks of south florida progressive loss of porosity with depth porosity measurements by borehole gravity meter in subsurface cenozoic carbonates of south florida reveal an extremely porous mass of limestone and dolomite which is transitional in total pore volume between typical porosity values for modern carbonate sediments and ancient carbonate rocks a persistent decrease of porosity with depth similar to that of chalks of the gulf coast occurs in these rocks we make no attempt to differentiate depositional or diagenetic facies which produce scatter in the porosity depth relationship the dominant data trends thus are functions of carbonate rocks in general rather than of particular carbonate facies carbonate strata with less than 20 porosity are absent from the rocks studied here aquifers and aquicludes cannot be distinguished on the basis of porosity although aquifers are characterized by great permeability and well developed vuggy and even cavernous porosity in some intervals they are not exceptionally porous when compared to other tertiary carbonate rocks in south florida permeability in these strata is governed more by the spacial distribution of pore space and matrix than by the total volume of porosity present dolomite is as porous as or slightly less porous than limestones in these rocks this observation places limits on any model proposed for dolomitization and suggests that dolomitization does not take place by a simple ion for ion replacement of magnesium for calcium dolomitization may be selective for less porous limestone or it may involve the incorporation of significant amounts of carbonate as well as magnesium into the rock the great volume of pore space in these rocks serves to highlight the inefficiency of early diagenesis in reducing carbonate porosity and to emphasize the importance of later porosity reduction which occurs during the burial or late near surface history of limestones and dolomites

secular variations in seawater chemistry controlling dolomitisation in shallow reflux systems insights from reactive transport modelling dolomitization often plays a critical role in the pore network development of platform carbonates with implications for reservoir quality distribution understanding both the hydrological system driving dolomitization and the chemistry of the fluids involved is fundamental to constrain predictions of the geometry and the petrophysical properties of dolomite bodies here the role of secular variations in seawater mg ca as a control on dolomitization and early porosity modification was evaluated using one dimensional reactive transport models and fluids based on modern aragonite sea mississippian and aptian calcite sea seawaters the sensitivity of dolomitization to a range of extrinsic controls brine salinity temperature fluid flow rate and pco2 and to intrinsic reactivity of the sediments effective reactive surface area was also explored simulations suggest faster calcite replacement by dolomite for seawaters with higher mg ca indicating that dolomitization potential is determined more by mg ca rather than saturation index increasing evaporative concentration enhances reaction rate independent of the effect of enhanced density driven fluid flux in addition to brine composition effective surface area of precursor sediments and temperature exert a critical control on replacement rate while secular variations of ph and carbonate alkalinity associated with changes in pco2 are only secondary controls above flow rates of 0 01 m yr 1 replacive dolomitization is reaction limited rather than flux limited favouring alteration of fine grained carbonates and suggesting that preferential alteration of grainstone units is rare unless head gradients are low post replacement dolomite cementation is flux dependent and thus favoured in areas of high head gradient and high permeability sediments and contrary to replacement supersaturation is a more important driver than mg ca while uncertainties remain regarding low temperature dolomitization kinetics the capability of numerical simulations to decouple individual controls provides new insights which can be used in conjunction with traditional comparative sedimentology to generate more rigorous conceptual models for individual reservoir settings

application of numerical modelling to a case of compaction driven dolomitization a jurassic palaeohigh in the po plain italy dolomitization of a carbonate platform can occur at different times and in different diagenetic environments from synsedimentary to deep burial settings numerical simulations are valuable tools to test and select the model that among different hypotheses compatible with field and geochemical data best honour mass balance kinetic and thermodynamic constraints moreover the simulation can predict the distribution of the dolomitized bodies in the subsurface and evaluate porosity changes valuable information for the oil industry this study is the first attempt to reproduce and investigate the compaction dolomitization model the diagenetic study of the jurassic carbonate basin and palaeohigh system of the po plain indicates that the carbonates of the palaeohighs were dolomitized by basin compaction fluids the main goal of the simulations is to evaluate the origin and evolution of the dolomitizing fluids and to provide insights regarding the distribution of the potential reservoir dolomitized bodies in the po plain the modelling process is subdivided into two steps basin modelling and reactive transport modelling the sebe3 basin simulator eni proprietary was used to create a three dimensional model of the compacting system the results include compaction fluid flow rate from the basin to the palaeohigh compaction duration and a determination of the total amount of fluid introduced into the palaeohigh these data are then used to perform reactive transport modelling with the toughreact code sensitivities on dolomite kinetic parameters suggest that dolomitization was an efficient process even at low temperatures with differences mainly related to the dynamics of the process fluid composition is one of the main constraints the sea water derived compaction fluid is proven to be efficient for dolomitization due to its relatively high mg content simulations also confirmed that permeability is the most important factor influencing fluid flow and consequently the dolomite distribution in the formation permeable fractured zones have a strong influence diverting the dolomitizing fluids from their normal path towards overlying or lateral zones moreover the simulations showed that after dolomite replacement is complete the dolomitizing fluids can precipitate dolomite cement causing over dolomitization with related localized plugging effects in the zone of influx mass balance calculations indicate that in the dolomitization compaction model the amount of compaction water fluxed from the basin to the carbonate is the main constraint on dolomitization efficiency this observation implies that the ratio between the volume of the basin undergoing compaction and the volume of the palaeohigh is a limiting factor on the final size of the dolomitized bodies an isolated palaeohigh could be an ideal site for pervasive replacement dolomitization due to the large volume of compaction fluids available compared with the carbonate rock volume in the case of large platforms the more permeable margin lithofacies are the most likely sites for compaction model dolomitization the combined use of a basin simulator and reactive transport modelling has proved to be a successful method to verify model reliability and it provides insights into the volumetric distribution of diagenetic products

integrated depositional model for the cenomanian turonian organic rich strata in north africa abstract during the late cenomanian early turonian c t oceanic anoxic event oae2 organic rich strata was deposited in rift shelf basins and slopes across north africa and in deep sea basins of the adjacent oceans based on a review of published and unpublished information this paper documents the distribution and organic richness of c t strata across the whole region within a palaeogeographic framework and systematically analyses the conditions and processes which controlled their deposition previously the c t in north africa has been most intensively studied in southern morocco tarfaya and tunisia only little data is availabe for other parts of north africa namely algeria libya and egypt because distribution of c t corg strata there becomes more patchy a general decrease in peak organic richness and black shale thickness occurs from west to east partly as a result of upwelling along the moroccan atlantic coast and the absence of upwelling in the eastern mediterranean area furthermore in the confined central atlantic the oxygen minimum zone omz in many places reached down to the deep sea floor 3 4 km while the lower limit of the omz along the north african tethys was much shallower and underlain by oxic water masses as documented by high resolution biostratigraphic and chemostratigraphic data c t black shale deposition in most areas outside the upwelling zone are restricted to a strong eustatic latest cenomanian transgressive phase triggered by this sea level rise the omz impinged onto the north african continental shelf and the margins of intrashelf basins which mostly formed during the early cretaceous as halfgrabens important units containing c t organic rich strata in the region are the atlantic tarfaya black shales morocco western sahara black shales and phtanites in the moroccan and algerian atlas the bahloul fm in the se constantine basin and in northern and central tunisia the etel fm in the sirte basin the al hilal fm in cyrenaica libya the abu roash fm in the abu gharadig and fayium basins western desert egypt the daliyya fm along the ne sinai palestine isreal coast and the shueib fm in west central jordan

the hydrocarbon systems of north africa abstract north africa s hydrocarbon reserves are spread over a wide range of reservoirs and play types but derive from only a few key source rock intervals particularly those in the silurian late cretacous and eocene source rock maturity together with the timing of the generation relative to trap forming and destructive events seem to be the key controlling factors on success or failure at a basin level in most cases generation clearly post dates trap formation although a more complex relationship is evident in various parts of algeria where some traps did not form until gas was being generated source rock deposition and maturation can be tied to two main extensional tectonic cycles which ended in the compressional and destructive events of the hercynian and alpine orogenies remaining exploration potential in mature provinces may be tied largely to subtle and stratigraphic traps in addition significant frontier potential can be identified in two main fairways corresponding to regions where silurian devonian and cretaceous eocene sources respectively are likely to have generated petroleum critical factors in these areas are timing of generation and preservation the gas discoveries in the ahnet basin provide analogues for palaeozoic generation preservation models in the interior silurian devonian source fairway similarly the key to success for the mesozoic basins of the atlas region lies in demonstrating a favourable relationship between trap formation destruction and generation from late cretaceous and eocene source rocks

reactive transport modeling an essential tool and a new research approach for the earth sciences abstract reactive transport modeling is an essential tool for the analysis of coupled physical chemical and biological processes in earth systems and has additional potential to better integrate the results from focused fundamental research on earth materials appropriately designed models can describe the interactions of competing processes at a range of spatial and time scales and hence are critical for connecting the advancing capabilities for materials characterization at the atomic scale with the macroscopic behavior of complex earth systems reactive transport modeling has had a significant impact on the treatment of contaminant retardation in the subsurface the description of elemental and nutrient fluxes between major earth reservoirs and in the treatment of deep earth processes such as metamorphism and magma transport active topics of research include the development of pore scale and hybrid or multiple continua models to capture the scale dependence of coupled reactive transport processes frontier research questions that are only now being addressed include the effects of chemical microenvironments coupled thermal mechanical chemical processes controls on mineral fluid reaction rates in natural media and scaling of reactive transport processes from the microscopic to pore to field scale

sedimentation cycles and their diagenesis on the slope of a miocene carbonate ramp bahamas odp leg 166 numerous small scale depositional cycles are present in the miocene sediments of seismic sequence m along the margin of great bahama bank odp leg 166 these cycles consist of decimetre to metre scale alternations between light grey and dark grey black wackestones packstones the light grey layers are well cemented and nearly uncompacted they contain planktonic and benthic foraminifera and bioclasts bioturbation in these layers is moderate the dark grey wackestones packstones are uncemented strongly compacted and normally strongly bioturbated the main components are planktonic foraminifera and fine grained bioclasts the dark layers are rich in aragonite and organic carbon and contain around 80 carbonate the light layers show low aragonite and organic carbon contents combined with carbonate values of up to 97 5 light 13c and slightly negative 18o values were observed in the dark uncemented layers while the cemented intervals show heavy 13c values and slightly more positive 18o values the carbon isotope signal between the dark and the light layers shows variations of up to 1 45 both 13c and 18o co vary with carbonate content the formation microscanner images reflect the changes in carbonate mineralogy the natural gamma ray tool shows that variations in siliciclastic content of the examined succession displays a different frequency than the cyclic alternations in carbonate mineralogy the internal stacking pattern of the cycles is closely tied to sea level the dark layers are deposited during rising sea level while the light layers reflect sediment production and export during highstand and falling sea level these cycles thus represent a rather continuous sediment shedding pattern that is clearly related to the ramp morphology and differs from the highstand shedding pattern typical for rimmed flat topped platforms most measured parameters suggest that primary sediment composition played an important role in the cementation process the primary differences in composition were enhanced during shallow burial diagenesis the initially high content of metastable carbonate phases in the periplatform sediments triggered rapid cementation of the primary aragonite rich layers leading to the formation of the light beds dissolution of metastable high mg calcite and aragonite was followed by in situ precipitation of more stable dolomite and calcite diagenesis was enhanced by the coarse grain size of the primary aragonite rich layers in contrast the more pelagic dark calcite rich layers were only slightly affected by diagenesis and could preserve their aragonite content the uncemented layers subsequently were subject to strong mechanical compaction whereas the cemented beds stayed nearly uncompacted

geocarbsulf a combined model for phanerozoic atmospheric o2 and co2 abstract a model for the combined long term cycles of carbon and sulfur has been constructed which combines all the factors modifying weathering and degassing of the geocarb iii model berner r a kothavala z 2001 geocarb iii a revised model of atmospheric co2 over phanerozoic time am j sci 301 182 204 for co2 with rapid recycling and oxygen dependent carbon and sulfur isotope fractionation of an isotope mass balance model for o2 berner r a 2001 modeling atmospheric o2 over phanerozoic time geochim cosmochim acta 65 685 694 new isotopic data for both carbon and sulfur are used and new feedbacks are created by combining the models sensitivity analysis is done by determining 1 the effect on weathering rates of using rapid recycling rapid recycling treats carbon and sulfur weathering in terms of young rapidly weathering rocks and older more slowly weathering rocks 2 the effect on o2 of using different initial starting conditions 3 the effect on o2 of using different data for carbon isotope fractionation during photosynthesis and alternative values of oceanic 13c for the past 200 million years 4 the effect on sulfur isotope fractionation and on o2 of varying the size of o2 feedback during sedimentary pyrite formation 5 the effect on o2 of varying the dependence of organic matter and pyrite weathering on tectonic uplift plus erosion and the degree of exposure of coastal lands by sea level change 6 the effect on co2 of adding the variability of volcanic rock weathering over time berner r a 2006 inclusion of the weathering of volcanic rocks in the geocarbsulf model am j sci 306 in press results show a similar trend of atmospheric co2 over the phanerozoic to the results of geocarb iii but with some differences during the early paleozoic and for variable volcanic rock weathering lower co2 values during the mesozoic atmospheric oxygen shows a major broad late paleozoic peak with a maximum value of about 30 o2 in the permian a secondary less broad peak centered near the silurian devonian boundary variation between 15 and 20 o2 during the cambrian and ordovician a very sharp drop from 30 to 15 o2 at the permo triassic boundary and a more or less continuous rise in o2 from the late triassic to the present

the significance of 24 norcholestanes 4 methylsteranes and dinosteranes in oils and source rocks from east sirte basin libya abstract the present paper involves a detailed evaluation of specific steroid biomarkers by gas chromatography mass spectrometry gc ms and gc metastable reaction monitoring mrm analyses of several crude oils and source rocks from the east sirte basin 24 norcholestanes dinosteranes 4 methyl 24 ethylcholestanes and triaromatic steroids have been identified in both source rocks and crude oils of the east sirte basin diatoms dinoflagellates including those potentially associated with corals and or their direct ancestors are amongst the proposed sources of these biomarkers these biomarker parameters have been used to establish a mesozoic oil source correlation of the east sirte basin hydropyrolysis of an extant coral extract revealed a similar distribution although immature of dinosteranes and 4 methyl 24 ethylcholestanes also observed in the sirte oils and source rocks this is consistent with the presence of dinoflagellates present during the deposition of the mesozoic aged east sirte basin formations a good data correlation for the rock extracts revealed a similar distribution of 3 24 dimethyl triaromatic steroids 3 methyl 24 ethylcholestanes 4 methyl 24 ethylcholestanes and 2 methyl 24 ethylcholestanes observed in one of the oil families and associated source rocks for the east sirte basin

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

modeling reservoir heterogeneity within outer ramp carbonate facies using an outcrop analog san andres formation of the permian basin variably cyclic fusulinid rich outer ramp facies of the permian san andres formation are exposed along the algerita escarpment guadalupe mountains new mexico we have used the outcrop exposures and cored wells drilled adjacent to the outcrop to assess reservoir and interwell scale variability of permeability as a potential analog for carbonate reservoirs in the permian basin and elsewhere permeability distribution was evaluated using a field permeameter and conventional measurements on small core plugs taken along vertical and horizontal outcrop traverses and from the slim hole cores permeability is related to the variable development of depositional cycles within a small scale sequence and also to diagenetic overprinting geostatistical models of permeability variation honoring the geologic and petrophysical data were constructed and input into a waterflood simulator to understand the interactions between heterogeneity and flow variograms constructed from conventional plug analyses of permeability for a vertical outcrop transect contain a significant small scale signal noise as seen in the high nugget effect approximately 50 of the overall sample variance and have correlation ranges of less than 4 5 m 15 ft different vertical variogram characteristics are displayed by cyclic and less distinctly cyclic parts of the san andres variograms constructed for horizontal transect data from three distinct stratigraphic units have nearly identical properties overall the ranges of correlation are short 3 3 5 m 10 12 ft when compared to typical interwell distances supporting a nearly uncorrelated and highly variable permeability model using observed short ranges of vertical and horizontal correlation and honoring the vertical transect data cross sectional conditionally simulated permeability fields were generated and used in simulated waterfloods to investigate the sensitivities to an oil recovery model and overall fluid injection rate for this style of stratigraphy and cyclicity cyclic parts of the section are characterized by a potential for early water breakthrough and relatively high vertical sweep efficiencies within the less distinctly cyclic section waterflood fronts have a fingerlike profile and vertical sweep efficiency is generally poorer

geochemical and petrographic analyses of new petroleum source rocks from the onshore upper jurassic and lower cretaceous of lebanon abstract the recent interest in the lebanese petroleum potential led to new studies mainly concerning marine petroleum source rocks without taking in consideration the dysodiles and other associated facies from the upper jurassic and the lower cretaceous these organic rich outcrops of metric thickness max 3 m are of limited lateral extension and often found next to volcanic deposits sixty three samples were collected from 25 different localities and were subdivided into three facies that are presented in this paper i dysodiles formed by the superposition of millimetric layers that are easy to separate ii dysodiles like with distinct but more compacted layers iii non dysodiles where the layering is not visible the aim of this work is to assess the origin of these organic rich rocks and to evaluate their source rock potential we used herein rock eval pyrolysis elemental analysis palynofacies observation organic petrography and molecular geochemical analyses rock eval pyrolysis reveals a very good to excellent source rock potential for all above cited rock types with average total organic carbon toc of 14 7 for the dysodiles 19 12 for the dysodiles like and 13 9 for the non dysodiles hydrogen index hi and oxygen index oi values indicate a type i to ii kerogen for the dysodiles average hi 649 162 mg hc g toc type i to ii kerogen for the dysodiles like average hi 340 182 mg hc g toc and type iii kerogen for the non dysodiles average hi 145 42 mg hc g toc the dominance of fluorescent amorphous organic matter aom and alginite macerals confirm an algal bacterial source of organic matter for the dysodiles the presence of opaque aom phytoclasts sporinites and cutinites macerals suggests a variable terrestrial organic matter input for the dysodiles like and non dysodiles maturity parameters such as tmax values on average 428 7 c for dysodiles 429 6 c for dysodiles like and 424 10 c for non dysodiles vitrinite reflectance 1 indicate that the organic matter is still thermally immature the present study sheds light on the long forgotten dysodile shales and similar material and their importance as potential source rocks or oil shales a comparison with local and regional source rocks is provided in order to emphasize the specific character of the dysodile deposits