two dimensional variations of exchangeable cation composition in the terrigenous sediment eastern flank of the juan de fuca ridge abstract the exchangeable cation compositions of organic poor terrigenous sediments containing smectite as primary ion exchanger from a series of holes along odp leg 168 transect on the eastern flank of the juan de fuca ridge have been examined as a function of distance from the ridge axis and burial depth the total cation exchange capacity cec values of the sediments ranged from 2 to 59 meq 100 g increasing with increases in the wt smectite at the seafloor the exchangeable cation compositions involving na k mg and ca expressed in terms of equivalent fraction are nearly constant regardless of the different transect sites x na 0 21 0 04 x k 0 08 0 01 x mg 0 33 0 09 and x ca 0 38 0 09 the calculated selectivity coefficients of the corresponding quaternary exchange reactions calculated using porewater data are in log units 5 45 0 39 for na 1 97 0 49 for k 0 42 0 41 for mg and 3 06 0 69 for ca the exchangeable cation compositions below the seafloor change systematically with distance from the ridge crest and burial depth conforming to the trends of the same cations in the porewaters the selectivities for na and mg are roughly constant at temperatures from 2 to 66 c indicating that the equivalent fractions of these two cations are independent of sediment alteration taking place on the ridge flank unlike na and mg the temperature influence is significant for k and ca with ca selectivity decreases being coupled with increases in k selectivity although potentially related to diagenetic and or hydrothermal mineral precipitation or recrystallization no evidence of such alteration was detected by xrd and tem in sites where upwelling of hydrothermal fluids from basement is occurring the k selectivity of the sediment is appreciably higher than at the other sites and corresponds to the formation of fe mg rich smectite and zeolites our study indicates that local increases in k selectivity at hydrothermal sites are caused by the formation of these authigenic minerals

episodic petroleum fluid migration in fault zones of the northwestern junggar basin northwest china evidence from hydrocarbon bearing zoned calcite cement hydrocarbon bearing zoned calcite cements occur widely in jurassic cretaceous fault zone cores and sandstone outcrops of the northwestern junggar basin northwest china hydrocarbon bearing bands alternate with nearly hydrocarbon free bands at a micron scale analytical results from biomarker organic geochemistry fourier transform infrared microspectroscopy and trace element geochemistry on these zoned cements suggest that at least three different types of fluids have participated in their formation the first fluid type is probably primary unmodified lacustrine formation water from which the hydrocarbon poor bands are formed and are characterized by mg rich calcite the other two types of fluids include basinal fluids e g hot hydrocarbon bearing fluids and meteoric water the hydrocarbon rich bands in which the hydrocarbons have been biodegraded and the mn content is relatively high suggest a mixture of hydrocarbon bearing basinal fluid and meteoric water the alternating growth of hydrocarbon bearing and hydrocarbon free bands of calcite cements implies that the cement formation is episodic it is related to alternating episodes of mixed petroleum bearing fluid and unmodified primary formation waters respectively the fault appears to have been a mixing zone where seismic pumping during the movement of associated regional faults occurred thus in the northwestern junggar basin the micron scale hydrocarbon bearing zoned structure of the calcite cements is likely a reflection of episodic petroleum fluid migration in fault zones

intermediate term predictions of earthquakes in italy algorithm m8 large earthquakes in italy are preceded by a specific seismic activation which could be diagnosed by a reproducible intermediate term earthquake prediction method a modification for lower seismic rate areas of the algorithm known as m8 keilis borok and kossobokov 1990 use has been made of the pfg ing catalog of earthquakes compiled on a regular basis to determine areas and times of increased probability for occurrences of m 6 earthquakes in retroactive simulation of forward prediction for the period 1972 1995 both the 1976 friuli m 6 1 and the 1980 irpinia m 6 5 earthquakes are predicted in the experiment where priority magnitude scale is used the times of increased probability for a strong earthquake to occur tips occupy less than a quarter of the total magnitude space time domain and are rather stable with respect to positioning of circles of investiga tion successful stability tests have been made considering a recently compiled catalog cci97 peresan et al 1997 in combination with the cn algorithm results costa et al 1996 the spatio temporal uncertainty of the prediction could be reduced to 5 the use of m8 for the forward prediction requires the computations to be repeated each half year using the updated catalog

diagenetic cycling of arsenic in amazon shelf sediments in order to help elucidate major natural controls on the diagenetic behavior of as vertical profiles of total dissolved and solid phase as were measured in amazon shelf deposits a deltaic region little influenced by human activity arsenic distributions are typically characterized by subsurface porewater maxima and surficial solid phase enrichment maximum porewater as concentrations 2 4 m are an order of magnitude greater than levels reported in most other coastal marine environments and elevated concentrations extend several meters below the sediment surface dissolved as varies inversely with easily leachable as 40 140 nmol g 1 in the solid phase and directly with increases in solid phase fe ii these patterns reflect a strong redox coupling with fe whereby oxidized as is associated with fe oxyhydroxides in surface sediments then subsequently reduced and released upon burial in the extensive suboxic zone upward diffusion and intense physical reworking of sediments presumably releases the dissolved arsenic fraction to the water column or allows for re adsorption in surface sediments arsenic porewater concentrations generally increase away from the river in some cases regular seasonal variations are evident apparently corresponding to nonsteady state ingrowth of diagenetic profiles following extensive sediment reworking during rising river flow the massive cycling of as in amazon shelf sediments is likely due to the combined effects of intense chemical weathering in regions of the drainage basin production of oxides and a turbulent physical regime present on the shelf both of which promote sedimentary cycling of fe and coupled elements during early diagenesis

carbonate cementation patterns potential mass transfer and implications for reservoir heterogeneity in eocene tight oil sandstones dongying depression bohai bay basin china evidence from petrology geochemistry and numerical modeling this paper investigates transport mechanisms involving carbonate cementation in eocene tight oil sandstones in bohai bay basin china to determine potential mass transfer between adjacent mudstones and sandstones evidence from petrology geochemistry and numerical modeling suggests two generations of carbonate cementation 1 early nonferroan calcite formed at 28 c 41 c and dolomite formed at 45 c 63 c and 2 later ferroan calcite formed at 105 c 124 c and ankerite formed at 101 c 137 c based on a one dimensional model for a coupled sandstone mudstone system under low and high temperatures different distribution patterns of carbonate cements reflect episodic concentration gradients that led to diffusive transport of bicarbonate species during progressive burial firstly extensive precipitation of early nonferroan calcite followed by dolomite at or near mudstone sandstone contacts resulted from initial concentration gradients related to different compositions in primary mineral assemblages secondly introduction of aqueous co2 from adjacent mudstones into sandstones resulted in dissolution of early nonferroan carbonates and led to diffusive transport of bicarbonate species these bicarbonate species were incorporated with fe2 and subsequently reprecipitated as ferroan carbonate minerals at distances greater than 2 m 6 6 ft from sandstone mudstone contacts therefore short distance diffusive transport is inferred to have been the predominant transport mechanism associated with carbonate cementation large scale mass transfer between sandstones and adjacent mudstones occurred in a relatively open geochemical system on a very local scale numerical model results show that low porosity zones 2 6 5 1 exhibit coherence with high abundances of carbonate cements 13 9 21 2 tightly cemented intervals were created by different generations of carbonate cementation and resulted in destruction of sandstone reservoir porosity

evidence for deep anaerobic biodegradation associated with rapid sedimentation and burial in the beaufort mackenzie basin canada abstract formation waters of the 14 km thick late cretaceous cenozoic beaufort mackenzie basin were examined as part of a larger project to better understand the petroleum potential of the region where early exploration defined petroleum reserves of 744 109 bbls recoverable crude oil and 11 74 tcf gas historical water analyses 2583 samples from 250 wells drilled up to 5 km depth were compiled and culled to remove incomplete and poor quality samples the resultant database shows a broad range of salinity and water chemistry that has no systematic relationship with depth three main water types are defined paleo seawater and freshwaters related to a miocene age gravity driven flow system and low tds high alkalinity waters high alkalinity waters are isolated in overpressured fault blocks that were rapidly buried by post miocene iperk shale deposition the high alkalinities up to 9000 mg l are interpreted to be related to in situ co2 generation through anaerobic methanogenesis in response to freshwater invasion the dominant control on biogenic gas generation appears to be maximum burial temperature rather than the modern temperature distribution this is consistent with the paleopasteurization model that suggests once critical burial temperatures are reached sterilized rocks are inhibited from further biodegradation even when temperatures subsequently drop back into the habitable zone

diagenesis and formation water chemistry of triassic reservoir sandstones from southern tunisia the fluvial triassic reservoir subarkoses and arkoses 2409 5 2519 45 m of the el borma oilfield southern tunisia were subjected to cementation by haematite anatase infiltrated clays kaolinite and k feldspar at shallow burial depths from meteoric waters subsequently basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths magnesian siderite minor ferroan magnesite and anhydrite the enrichment of siderite in 12c isotope 13cpdb 14 5 to 9 results from derivation of carbon from the thermal decarboxylation of organic matter during further burial the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred followed by the formation of microcrystalline k feldspar and quartz chlorite and illite prior to the emplacement of oil present day formation waters are na ca cl brines evolved by the evaporation of seawater and water mineral interaction and are in equilibrium with the deep burial 3 1 km minerals these waters are suggested to be derived from the underlying silurian and devonian dolomitic mudstones

isotope geochemistry of fluid inclusions in permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters abstract d and 18o values have been determined for fluid inclusions in 45 samples of permian halite the inclusions are enriched in 18o relative to the meteoric water line but are depleted in d relative to ocean water inclusions with the more positive values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a d 18o diagram inclusions with more negative values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines the isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that permian sea water was isotopically similar to modern sea water connate evaporite brines can have negative values because of the probable hooked isotope trajectory of evaporating sea water and or synsedimentary mixing of evaporite brines with meteoric waters subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested

dissolution and time dependent compaction of albite sand experiments at 100 c and 160 c in ph buffered organic acids and distilled water abstract aqueous fluids are important in the diagenesis and deformation of crustal rocks both chemical and physical interactions are involved and often they are strongly coupled for example pore waters not only dissolve transport and precipitate chemical species but they also substantially affect the mechanical behavior of the rocks that contain them stresses magnified at grain contacts by differences in pore fluid pressure p p and confining pressure p c can in turn influence the rate and extent of chemical exchange to begin investigation of these coupled systems compaction experiments were conducted using albite sand 250 500 m and distilled water ph 5 8 0 07 m acetate ph 4 7 and 0 07 m acetate 0 005 m citrate ph 4 4 solutions in a hydrothermal flow through system at conditions that simulate diagenesis pore fluid chemistry and pore volume loss were monitored to quantify the effects of organic acids on time dependent compaction rates the effects of stress and fluid chemistry on the dissolution kinetics were also examined albite dissolution rates monitored by steady state fluid chemistry increased when an effective pressure p e p c p p was applied probably due to increases in total surface area caused by grain breakage at contacts these effects were transient in distilled water however si and al concentrations remained elevated in the acetate pore fluid the average si based release rates indicate 35 increase in reactive surface area by application of p e 34 5 mpa at 100 c with p e 34 5 mpa steady state si concentrations were 2 3 times higher in 0 07 m acetate and 5 8 times higher in 0 07 m acetate 0 005 m citrate than in distilled water al increased by even larger factors 3 in the acetate buffer and 10 in the citrate solution these changes in fluid chemistry are attributed to both ph and ligand enhanced reactions albite dissolution appears to be controlled by surface complexation reactions at al sites rapid dissolution of albite in the organic acid solutions is probably due to the ability of organic acid ligands to selectively complex with aluminum time dependent compaction was observed at 100 and 160 c with p e 34 5 mpa strain rates increased with temperature from 10 9 s l at 100 c to 10 8 s l at 160 c and decreased with strain in all pore fluids especially at 100 c compaction rates in distilled water and in the acetate solution had similar magnitudes and strain dependencies however small amounts of citrate species apparently enhance compaction compared to the other fluids at similar strains textural data indicate that time dependent compaction of the albite sand occurred primarily by brittle mechanisms at these temperatures however the deformation is clearly thermally activated and may be chemically assisted by the aqueous pore fluid

deposition and diagenesis of upper miocene arkoses yowlumne and rio viejo fields kern county california upper miocene arkoses of the yowlumne and rio viejo fields southern san joaquin valley california are channel fill turbidites characterized by the dominance of incomplete massive a or ab turbidite sequences they occur in a depth range of 11 250 to 14 300 ft 3 430 to 4 360 m quartz and feldspar constitute more than 90 of the detrital mineralogy however in the yowlumne and stevens sandstones k feldspars dominate over plagioclase by more than a 2 1 ratio and the plagioclase has an average composition of an14 or less in the 10 4 sandstone this ratio is 1 1 and the average plagioclase composition is an32 diagenesis during deep burial results in extensive feldspar alteration and clay authigenesis the clay minerals vary in mineralogy abundance and sequence of formation in the different units these variations were considered geochemically and we concluded that the effect of detrital feldspar composition on the pore fluid chemistry is the dominant control porosity and permeability of these arkoses are dependent on the abundance and type of authigenic clays the overall averages are 14 7 and 3 8 md for the 10 4 sandstone compared to 16 9 and 38 md for the yowlumne and stevens sandstones thus development of reservoir quality is strongly influenced by detrital feldspar composition and diagenesis

the structural and diagenetic evolution of injected sandstones examples from the kimmeridgian of ne scotland injected sandstones occurring in the kimmeridgian of ne scotland along the bounding great glen and helmsdale faults formed when basinal fluids moved upward along the fault zones fluidizing oxfordian sands encountered at shallow depth and injecting them into overlying kimmeridgian strata the orientation of dykes in addition to coeval faults and fractures was controlled by a stress state related to dextral strike slip along the bounding fault zones diagenetic studies of cements allow the reconstruction of the fluid flow history the origin of deformation bands in sandstone dykes and sills was related to the contraction of the host rocks against dyke and sill walls following the initial stage of fluidized flow and these deformation bands are the earliest diagenetic imprint early non ferroan calcite precipitated in injection structures at temperatures between 70 and 100 c indicating that it precipitated from relatively hot basinal fluids that drove injection coeval calcite filled fractures show similar temperatures suggesting that relatively hot fluids were responsible for calcite precipitation in any permeable pathway created by dextral simple shear along the faults during progressive burial percolating sea water was responsible for completely cementing the still relatively porous injected sandstones with a second generation of ferroan calcite which contains fluid inclusions with homogenization temperatures below 50 c during this phase depositional host sandstones were also cemented

in situ stable isotopic evidence for protracted and complex carbonate cementation in a petroleum reservoir north coles levee san joaquin basin california u s a abstract knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation the stevens sands is a sequence of marine shales and deep sea fan sands that was deposited within the miocene monterey formation in the south central part of the san joaquin basin california during the upper miocene 10 6 ma rapid high precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi collection mode in conjunction with electron microprobe analyses indicate that carbonate cement zones within the stevens sands at north coles levee ncl have had a complex and protracted fluid history three main generations of carbonate cement were identified the relative timing of carbonate cement precipitation within the stevens sands at ncl was estimated using the thermal and burial history of the san joaquin basin in situ oxygen isotope data and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite water and dolomite water precipitation of these cement zones began soon after sediment deposition 7 ma and is ongoing early dolomite was precipitated at a temperature of 10 c near the sediment water interface and soon after sediment deposition calcite cements which are the most abundant variety precipitated semicontinuously between 4 ma and 5 ma at temperatures between 50 c and 65 c and depths of 800 m to 1300 m fe dolomite which is paragenetically late appears to have precipitated at temperatures near 100 c in response to pore pressure reduction which accompanied exploitation of the gas cap within the last 35 years carbon in these cements was likely derived from several sources including marine maturing hydrocarbons and a zone of methanogenesis

carbonate equilibria and ph buffering by organic acids response to changes in pco2 abstract organic acid anions such as acetate a natural product of the thermal breakdown of kerogen may buffer the ph of natural formation waters the extent of ph buffering by acetate should determine whether an increase in the partial pressure of carbon dioxide p co 2 will lead to calcite dissolution porosity enhancement or calcite precipitation porosity destruction successful prediction of porosity therefore requires a quantitative understanding of this process this paper presents computer simulations of the response of the acetic acid carbonate system to changes in p co 2 for typical reservoir fluid compositions and temperatures the effects of variations in temperature ionic strength initial ca 2 concentration and the magnitude of the change in p co 2 were investigated natural formation waters from cenozoic reservoirs in the gulf coast basin and california show maximum organic acid concentrations at 100 20 c at 100 c the simulations show that for concentrations of acetic acid up to 0 06 m 3600 mg l 1 increases in the p co 2 of initially calcite saturated solutions will promote calcite undersaturation at higher temperatures equivalent buffering responses occur at lower acetic acid concentrations but acetic acid concentrations in natural formation waters also decrease rapidly with increasing temperature since present day concentrations of acetic acid in formation waters from cenozoic reservoirs rarely exceed 0 06 m increases in p co 2 will generally promote calcite dissolution by these waters in the gulf coast basin variation in organic acid concentrations as a function of reservoir age suggests that reductions in dissolved organic acid concentrations over geologic time have been relatively small it can be concluded therefore that at any point in the histories of these reservoirs an increase in p co 2 would have promoted calcite dissolution despite the effects of ph buffering by acetate since increases in p co 2 will in most cenozoic basins promote calcite dissolution knowledge of the location and timing of co 2 production in the subsurface can more easily be applied to the prediction of calcite cementation and dissolution

diagenesis of dissolved aluminum in organic rich estuarine sediments abstract the sedimentary geochemistry of dissolved al is complicated by a number of different reactions in this study we show that complexation by organic matter adsorption to fe oxyhydroxides and reaction with si in solution have important effects on the distribution of dissolved al in sediments in the absence of physical resuspension of sediment into overlying waters dissolved al is rapidly consumed at the sediment water interface and is subsequently released upon reduction of fe oxyhydroxides this release does not cause noticeable perturbations in dissolved al concentrations in sediments because of rapid consumption reactions which mask the true mobility of al results suggest that one of the consumption reactions may be due to formation of an fe al silicate the amount of authigenic aluminosilicate formed in estuarine sediments must be very small relative to the detrital component in the deep sea however the long residence time of fe oxyhydroxides at the sediment water interface with resulting greater accumulation of adsorbed al may explain the abundance of al in fe smectites reported from many different areas

inhibition of dissolution within shallow water carbonate sediments impacts of terrigenous sediment input on syn depositional carbonate diagenesis the early diagenetic chemical dissolution of skeletal carbonates has previously been documented as taking place within bioturbated shallow water tropical carbonate sediments the diagenetic reactions operating within carbonate sediments that fall under the influence of iron rich terrigenous sediment input are less clearly understood such inputs should modify carbonate diagenetic reactions both by minimizing bacterial sulphate reduction in favour of bacterial iron reduction and by the reaction of any pore water sulphide with iron oxides thereby minimizing sulphide oxidation and associated acidity to test this hypothesis sediment cores were taken from sites within discovery bay north jamaica which exhibit varying levels of fe rich bauxite sediment contamination at non impacted sites sediments are dominated by caco3 up to 99 by weight pore waters from the upper few centimetres of cores show evidence for active sulphate reduction reduced so4 cl ratios and minor caco3 dissolution increased ca2 cl ratios petrographic observations of carbonate grains specifically halimeda and amphiroa show clear morphological evidence for dissolution throughout the sediment column in contrast at bauxite impacted sites the sediment is composed of up to 15 non carbonate and contains up to 6000 g g 1 fe pore waters show no evidence for sulphate reduction but marked levels of fe ii suggesting that bacterial fe iii reduction is active carbonate grains show little evidence for dissolution often exhibiting pristine surface morphologies samples from the deeper sections of these cores which pre date bauxite influence commonly exhibit morphological evidence for dissolution implying that this was a significant process prior to bauxite input previous studies have suggested that dissolution driven by sulphate reduction and sulphide oxidation can account for the loss of as much as 50 of primary carbonate production in localized platform environments the finding that chemical dissolution is minor in a terrigenous impacted carbonate environment therefore has significant implications for carbonate budgets and cycling and the preservation of carbonate grains in such sediment systems

petrogenesis of the proterozoic athabasca basin northern saskatchewan canada and its relation to diagenesis hydrothermal uranium mineralization and paleohydrogeology abstract in the athabasca basin proterozoic sandstones unconformably overlie archean and aphebian rocks multiple fluid events involving isotopically and chemically distinct fluids which have migrated laterally for considerable distances and along fault zones have produced a paragenetically identifiable assemblage of clay silicate and oxide minerals in the basin and basement rocks altered sandstones and the underlying metasedimentary rocks proximal to fault zones host high grade u ni co unconformity type uranium ore deposits and contain an alteration assemblage of highly crystalline 2m 3t illite 1m kaolinite clinochlore and sudoite hematite dravite and euhedral quartz in the sandstones 18o values of coeval quartz hematite quartz illite quartz dravite illite hematite and kaolinite hematite fluid inclusion temperatures in diagenetic quartz and d values of 1m kaolinite 2m 3t illite dravite and in fluid inclusions in the quartz indicate formation from a basinal fluid that had d values of 60 10 and which underwent salinity temperature and 18o increases from 5 to 34 wt nacl 120 to 240 c and 5 to 3 respectively as a result of increasing burial depth and sustained water rock interactions with the basinal sediments over substantial time periods 18 o 16 o and d h ratios of sudoite and clinochlore in meta sedimentary basement rocks within the alteration haloes indicate formation from a basement derived fluid with 18o and d values of 4 4 and 30 10 respectively that was isotopically distinct from the basinal fluids 18o and d values of late kaolinites in reactivated fault zones indicate formation mainly from low temperature 50 25 c fluids having 18o and d values of 16 and 130 respectively similar to relatively modern meteoric waters in the area some kaolinites have 18o and d values which suggest they formed from meteoric waters at mid latitudes and have experienced varying degrees of oxygen and hydrogen isotope exchange with relatively modern meteoric waters in the re activated faults illite sudoite and dravite have d values that vary by 100 without concurrent changes to their 18o values indicating preferential hydrogen isotope exchange with relatively modern meteoric waters at low temperatures and under conditions of high integrated water rock ratios rb sr isochron and model ages of 1477 57 and 952 ma respectively for the crystalline 2m 3t illites indicate the timing of the high temperature basinal fluid events associated with fluid mixing rb sr and k ar ages on illites with the lowest d values are as young as 300 ma and indicate late stage resetting of their radiogenic isotope systematics as a result of substantial interactions with relatively modern meteoric waters the evolution of sr isotopic compositions of fluids in the basin and basement rocks as reflected by the 87 sr 86 sr ratios of chlorite illite and dravite indicates that mixing of two isotopically distinct fluids in the vicinity of faults was the process by which uranium was precipitated similar to that suggested by oxygen and hydrogen isotope systematics in the clays and silicate minerals nd ratios in the high temperature diagenetic and hydrothermal clay and silicate minerals and the cogenetic uranium ores suggest they all have formed from similar fluids which have interacted substantially with the basin sediments in general the isotopic chemical microthermometric and petrologic data indicate that the athabasca basin has had a protracted fluid history comparable to that documented for more modern sedimentary basins the data clearly show that the major sandstone aquifers have been affected by widespread lateral flow of diagenetic fluids over distances of hundreds of kilometers and also that these fluid migration paths have been modified by cross formational fluid flow near active fault zones

distribution and occurrence of aliphatic acid anions in deep subsurface waters abstract this study reports 144 new analyses of short chain aliphatic acid anions acetate propionate butyrate and valerate in formation waters from eight localities eastern venezuelan basin denver basin eastern green river basin red desert and washakie basins wamsutter arch san juan basin piceance basin raton basin gulf coast basin and the western overthrust reservoir temperature does not predict total or relative abundance of aliphatic acid anions but does predict maximum total concentrations of these species maximum concentrations increase to 90 c above 90 c maximum concentrations decrease above 250 c maximum concentrations should not exceed 1 mg 1 the general order of dominance is acetate propionate butyrate valerate but for coal associated waters is propionate acetate butyrate valerate lack of longer chain aliphatic acid anion dominance over acetate at low reservoir temperatures may suggest hydrologie communication with deeper reservoirs

fluid flow and related diagenetic processes in a rift basin evidence from the fourth member of the eocene shahejie formation interval dongying depression bohai bay basin china the purpose of this paper is to relate diagenetic processes in deeply buried sandstones in the fourth member of the eocene shahejie formation interval bohai bay basin china to pore fluid flow changes with progressive burial based on petrographic mineralogical and geochemical analysis distribution patterns of authigenic minerals are recognized that reflect 1 the sources and patterns of fluid flow and 2 fluid flow in an evolving open to closed system partial to extensive precipitation of calcite and dolomite at or near mudstone sandstone contacts during eogenesis was a result of large scale mass transfer between sandstones and adjacent mudstones this process was driven by steep diffusion gradients from adjacent mudstones in a relatively open geochemical system on the local scale support for this model is provided by large sulfur isotope fractionation between framboidal pyrite and precursor gypsum dissolution of feldspar grains and dissolution of nonferroan carbonate cements during early mesogenesis are spatially associated with quartz and ferroan carbonate cementation respectively this process was related to organic carbon dioxide expelled from adjacent source rocks and indicates a relatively open system during late mesogenesis dissolution of evaporitic cements related to thermochemical sulfate reduction tsr generated ankerite and nodular pyrite cements in adjacent pores a lack of sulfur isotope fractionation between parent anhydrite and late stage nodular pyrite during tsr supports a relatively closed fluid flow system because the velocities of pore fluid flow were low during mesogenesis large scale thermal convection and advection probably did not occur instead diffusion over short distances is inferred as the predominant transport mechanism for dissolved solids that were precipitated as other phases either in situ or in adjacent pores

diffusive cation fluxes in deep sea sediments and insight into the global geochemical cycles of calcium magnesium sodium and potassium the biogeochemical cycles of the ocean s major cations calcium magnesium sodium and potassium are linked to the ocean s alkalinity budget through terrestrial weathering and the subsequent formation and burial of calcium carbonate in the ocean chemical reactions within ocean sediments play a critical role in the biogeochemical cycles of the major cations as indicated by geochemical gradients both positive and negative in the concentrations of these cations within the pore fluid system i e fluid trapped between sediment particles here we review the biogeochemical cycles of calcium magnesium and sodium and provide new estimates of the diffusive fluxes of these cations within marine sediments to explore the importance of these sedimentary processes we quantify these fluxes by compiling a global database of pore fluids from the various ocean drilling programs deep sea drilling program dsdp ocean drilling program odp international ocean drilling program iodp comprising nearly 700 locations which allows a wider geographic coverage and therefore better integrated flux estimates than previous work has allowed the myriad of subseafloor chemical reactions that may influence the concentrations of the major cations in pore fluids include authigenic carbonate precipitation carbonate dissolution clay mineral formation and ion exchange as previous work has shown we confirm that these integrated fluxes are globally significant because the dsdp odp iodp cores begin sampling one meter below the sediment water interface additional studies of the processes within the top meter are needed to accurately calculate total cation fluxes across the sediment water interface delineating the various processes that control the major cation chemistry of seawater over geologic time scales remains critical for understanding the operation of the co2 silicate weathering thermostat on geologic timescales

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to discussion by p d lundegard and l s land abstract macgowan and s urdam 1990a suggested some modifications to the model of lundegard and land 1989 to make it more geologically and geochemically reasonable the predictive power of such a geochemical model is wholly dependent on the species modeled and the constants used any model that excludes important species or important thermodynamic data or one that couples certain reactions in an unrealistic way may produce results which are not geologically or geochemically reasonable w k harrison pers commun 1988 y k kharaka pers commun 1991 we have long recognized that under early burial diagenetic conditions aluminosilicate hydrolysis generally controls formation water ph surdam and eugster 1976 mariner and surdam 1970 taylor and surdam 1981 and that during intermediate burial either aqueous co2 or caa species in the absence of aqueous s species or other weak conjugate acid base pairs will dominate formation water alkalinity and control ph surdam et al 1989c we reassert that the model of lundegard and land 1989 does not take into account the relative importance of pco2 and of concentrations of both ca2 and caa and their relative organic metal complexes to carbonate mineral stability in sandstones in the zone of intermediate burial clastic diagenesis cf the models of surdam et al 1984 and surdam and crossey 1985 the usefulness of such models is predicted on the completeness of the model and the use of the best most accurate thermodynamic data also geologically realistic concentrations of critical species are required for reasonable modeling to be done although their model is vigorously defended in the discussion of lundegard and land 1989 lundegard and land 1993 we continue to disagree that their analysis of their model conditions are either geologically or geochemically satisfying we agree with the fundamental approach and philosophy of lundegard and land 1989 lundegard and land 1993 it is of the utmost importance to determine from experimental geochemical petrographic and geological data what the controls on ph and alkalinity in formation waters are as well as the exact thermodynamic speciation of aqueous moieties and the stability of detrital and authigenic minerals lundegard and land 1993 raise an additional point about caa reaction with carbonate minerals in shales although fisher and lewan 1989 lewan 1989 and macgowan and surdam 1990b have demonstrated that caa generated in shale likely migrate in the oil phase along incipient shale microfractures to the sandstone reservoir and thus are likely to not react much with the shale finally we agree with lundegard and land that these areas require much additional experimental and field analysis and petrographic study

origin and migration of brines from paleozoic strata in central tarim china constraints from 87sr 86sr δd δ18o and water chemistry chemistry of major and minor elements 87sr 86sr d and 18o of oilfield waters and 87sr 86sr of whole rock were measured from paleozoic strata in the central tarim basin nw china the aim is to elucidate the origin and migration of formation water and its relation to petroleum migration high salinity oilfield waters in carboniferous silurian and ordovician reservoirs have maintained the same na cl ratio as seawater indicative of subaerially evaporated seawater two possible sources of evaporitic water are carboniferous cii and cambrian both of which contain evaporitic sediments geographic and stratigraphic trends in water chemistry suggest that most of the high salinity water is from the cambrian strontium h and o isotopes as well as ion chemistry indicate at least 3 end member waters in the basin high salinity cambrian evaporitic water was expelled upward into ordovician silurian and carboniferous reservoirs along faults and fractures during compaction and burial meteoric water has likely invaded the section throughout its history as uplift created subaerial unconformities meteoric water certainly infiltrated silurian and older strata during development of the ciii unconformity and again in recent times modern meteoric water enters carboniferous strata from the west and flows eastward mixing with the high salinity cambrian water and to a lesser degree with paleometeoric water the third end member is highly radiogenic shale derived water which has migrated eastward from the awati depression to the west enrichment of ca and sr and depletion of k mg and so4 relative to the seawater evaporation trajectory suggest waters were affected by albitization of feldspars dolomitization illitization of smectite and so4 reduction the mixing of meteoric water occurred subsequently to seawater evaporation main water rock interactions and brine migration the direction of brine migration is consistent with that of petroleum migration suggesting water and petroleum have followed the same migration pathways

spatial and temporal variations in early diagenetic organic matter oxidation pathways in lower jurassic mudstones of eastern england abstract the lower liassic succession of the east midlands platform eastern england comprises shallow marine mudstones and oolitic ironstones in these sediments temporal and spatial variations in the relative importance of early diagenetic anaerobic organic matter oxidation pathways are recorded the ironstones are stratigraphically condensed and were deposited upon the market weighton swell mws a topographic high in the basin in shallow water and high energy conditions at times of low relative sea level at times of higher sea level stratigraphically condensed mudstones were deposited upon the mws away from the mws stratigraphically expanded mudstones were deposited throughout the lower liassic the ironstones contain berthierine ooids and pore filling cement and siderite pore filling cement with rare pyrite replacing berthierine and siderite mudstones deposited away from the mws contain pyrite as the sole early diagenetic iron mineral contrastingly mudstones deposited upon the mws contain both early diagenetic pyrite and mn rich siderite the presence of berthierine and siderite in the ironstones indicates that sulphide was absent from early diagenetic pore waters and that fe iii reduction dominated anaerobic organic matter oxidation in contrast in mudstones deposited away from the mws the presence of pyrite indicates that sulphate reduction was a predominant pathway for anaerobic organic matter oxidation in mudstones deposited upon the mws the presence of both pyrite and mn rich siderite suggests that both sulphate reduction and fe iii reduction were important during early diagenesis it is proposed that this spatial and temporal variation in relative importance of early diagenetic anaerobic organic matter oxidation reactions was a result of variation in physical sedimentary parameters sediment accumulation rate and energy of environment at times of low net sediment accumulation rates in shallow water high energy environments suboxic diagenesis fe iii and mn iv reduction dominated giving rise to the characteristic berthierine siderite ironstone assemblage at times of higher sedimentation accumulation rates in deeper lower energy environments sulphate reduction dominated leading to pyrite formation at intermediate conditions fe iii mn iv and sulphate reduction were all important leading to precipitation of both pyrite and mn rich siderite

evidence for meteoric recharge in the san joaquin basin california provided by isotope and trace element chemistry of calcite abstract calcite cements were analysed petrographically and geochemically to document patterns of meteoric recharge into marine sandstones in the san joaquin basin california calcite cements in shallow marine sandstones in the eastern basin are pure 96 mol ca 32 and 18 o pdb 18 o and point counted volumes of 26 27 appear 5 m apart the purity of low temperature calcite 8 10 9 m 3 and rates of locally variable dissolution reactions and calcite crystallization exceeded the flushing rates of reservoirs if meteoric recharge began shortly after the deposition of marine shelf sandstones approximately 23 ma then calcite cements record downdip infiltration at an average rate of 1 mm year less than some recorded rates of gravity driven flow

depositional and diagenetic controls on the reservoir quality of lower cretaceous pendencia sandstones potiguar rift basin brazil the quality of the reservoirs of the lower cretaceous pendencia formation of the potiguar basin northeastern brazil is directly controlled by depositional facies related carbonate cementation and compaction the study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and in particular the role of carbonate cementation in poorly understood continental systems the pendencia formation is a thick sequence of fan deltaic fluvial deltaic turbiditic and lacustrine sandstones conglomerates and shales deposited during the rift stage of the basin the sandstones are predominantly arkoses average q49f40l11 with subordinate plutonic and volcaniclastic feldspathic litharenites compaction and cementation had similar importance in the destruction of porosity with a dominance of cementation in the turbidites and of compaction in the fluvial deposits carbonate cementation in pendencia reservoirs increases progressively with depth and is facies controlled eodiagenetic nonferroan calcite i d18opdb 10 7 to 4 0 d13cpdb 17 5 to 8 5 mesodiagenetic rhombohedral ferroan dolomite ankerite d18opdb 9 3 to 3 9 d13cpdb 1 7 to 1 1 and ferroan calcite ii d18opdb 17 2 to 6 8 d13cpdb 13 6 to 2 3 were precipitated at three distinct temperature intervals calculated from the d18o values 21 to 58 c 70 to 79 c and 85 to 150 c respectively according to the d13c values dissolved carbonate for calcite i was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter the shales were a major source of dissolved begin page 1720 carbonate as indicated by the isotopic similarity between their calcite d13cpdb 0 2 to 1 8 87sr 86sr 0 719 and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones as a result of this cementation pattern thin turbiditic and deltaic sandstone beds are pervasively cemented the best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales alluvial fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity however porosity enhancement was very limited due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis abstract carboxylic acid anions caa in formation waters are of interest to studies of clastic diagenesis because of their ability to buffer formation water eh and ph and thus substantially contribute to controls on carbonate mineral stability and their ability to complex and transport al and si from the site of aluminosilicate mineral dissolution during diagenesis carboxylic acid anions are also extremely important to the aqueous geochemistry of ca fe mn pb and zn some formation waters from sedimentary basins contain high concentrations of caas the analyses of 20 formation waters from the san joaquin basin california and 20 formation waters from the louisiana gulf coast basin presented in this study show concentrations as high as 8100 ppm monofunctional and 370 ppm difunctional cca by comparison previously reported analyses indicate monofunctional caa occir in concentrations up to 10 000 ppm and difunctional caa may occur in concentrations up to 2610 ppm analyses of drilling muds and scale soaps presented in this study show that few if any difunctional caa in the study area can be attributed to contamination from these sources additionally aqueous extracts of crude oils contain both mono and difunctional acid anions as do the aqueous and petroleum phases of hydrous pyrolysates previously unreported dissolution experiments equilibrium computer simulations and hydrous pyrolysis experiments support those already published and suggest that caa are generated during thermal maturation of kerogen and expelled from the shale along oil wet microfractures upon entering the water wet sandstone pore the hydrophilic caa partition into the aqueous phase organic inorganic reactions may occur which from caa metal complexes because the complexes are hydrophobic they partition in the petroleum phase where present carboxylic acid anions are of great importance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity certainly no other viable mechanism has been advanced which adequately explains the observed aluminosilicate mineral dissolution with subsequent mass transfer of a1 as well as the carbonate mineral diagenetic successions observed in sand shale systems world wide the utility of modeling these observations of organic inorganic diagenesis is limited only by the ability to model the concentration and distribution of caa through space and time

geochemistry of metal rich brines from central mississippi salt dome basin u s a abstract oil field brines are the most favored ore forming solutions for the sediment hosted mississippi valley type ore deposits detailed inorganic and organic chemical and isotope analyses of water and gas samples from six oil fields in central mississippi one of the very few areas with high metal brines were conducted to study the inorganic and organic complexes responsible for the high concentrations of these metals the samples were obtained from production zones consisting of sandstone and limestone that range in depth from 1900 to 4000 m 70 120 c and in age from late cretaceous to late jurassic results show that the waters are dominantly bittern brines related to the louann salt the brines have extremely high salinities that range from 160 000 to 320 000 mg l total dissolved solids and are na ca cl type waters with very high concentrations of ca up to 48 000 mg l and other alkaline earth metals but with low concentrations of aliphatic acid anions the concentrations of metals in many water samples are very high reaching values of 70 mg l for pb 245 mg l for zn 465 mg l for fe and 210 mg l for mn the samples with high metal contents have extremely low concentrations 2 s samples obtained from the smackover formation limestone have low metal contents that are more typical of oil field waters but have very high concentrations up to 85 mg l of h 2 s computations with the geochemical code solmineq 87 give the following results 1 both pb and zn are present predominantly as aqueous chloride complexes mainly as pbcl 4 2 and zncl 4 2 respectively 2 the concentrations of metals complexed with short chained aliphatic acid anions and reduced s species are minor 3 organic acid anions are important in controlling the concentrations of metals because they affect the ph and buffer capacity of the waters at subsurface conditions and 4 galena and sphalerite solubilities control the concentrations of pb and zn in these waters

hydrogeochemical analysis of salinization processes in the coastal aquifer of oropesa castellon spain the coastal aquifer of oropesa is affected by salinization processes undoubtedly associated with intense groundwater exploitation for agriculture supply the aquifer corresponds geologically to a tectonic depression with plioquaternary fill hydrogeologically this aquifer is detrital with intergranular porosity which receives substantial recharge from adjacent mesozoic aquifers contact with the sea in addition to the presence of cultivated soil requiring extreme exploitation of groundwater frequently give rise to processes of seawater intrusion the present research is an attempt to understand the saltwater intrusion in this aquifer using hydrochemical analyses of the behavior of certain minor ions that could help in the characterization process in the case of the oropesa sector groundwater salinization does not appear to be attributable solely to the intrusion of seawater but there are also anomalies related largely to the geology of the sector and its surroundings the type of recharge the hydrodynamic conditions in the specific area etc

depositional and diagenetic controls on deeply buried eocene sublacustrine fan reservoirs in the dongying depression bohai bay basin china abstract this paper investigates the reservoir potential of deeply buried eocene sublacustrine fan sandstones in the bohai bay basin china by evaluating the link between depositional lithofacies that controlled primary sediment compositions and diagenetic processes that involved dissolution precipitation and transformation of minerals this petrographic mineralogical and geochemical study recognizes a complex diagenetic history which reflects both the depositional and burial history of the sandstones eogenetic alterations of the sandstones include 1 mechanical compaction and 2 partial to extensive non ferroan carbonate and gypsum cementation typical mesogenetic alterations include 1 dissolution of feldspar non ferroan carbonate cements gypsum and anhydrite 2 precipitation of quartz kaolinite and ferroan carbonate cements 3 transformation of smectite and kaolinite to illite and conversion of gypsum to anhydrite this study demonstrates that 1 depositional lithofacies critically influenced diagenesis which resulted in good reservoir quality of the better sorted middle fan but poor reservoir quality in the inner and outer fan lithofacies 2 formation of secondary porosity was spatially associated with other mineral reactions that caused precipitation of cements within sandstone reservoirs and did not greatly enhance reservoir quality and 3 oil emplacement during early mesodiagenesis temperatures 70 c protected reservoirs from cementation and compaction

magnetotactic bacteria and single domain magnetite in hemipelagic sediments remanent magnetism in marine sediments has been used extensively over the past twenty years to calibrate the geological time scale study geomagnetic reversals and secular variation and measure the rates of seafloor spreading although these sediments may contain different magnetic minerals magnetite is the most commonly observed and magnetically stable phase and its size shape and post depositional fate affect the magnetic remanence of the sediments biogenic magnetites are single domain with a high natural magnetic remanence nrm and have been suggested as a significant source of magnetic remanence in marine sediments we have studied surface sediments from the santa barbara basin and report the occurrence of living magnetotactic bacteria and the deposition of biogenic ultra fine grained single domain magnetite using a novel extraction technique transmission electron microscopy and squid magnetometry we show that these bacteria and the magnetite they produce are the major source of stable remanent magnetism in these sediments

times of increased probability of strong earthquakes m 7 5 diagnosed by algorithm m8 in japan and adjacent territories this paper continues to explore the possibility of identifying the times of increased probability tips of occurrence of a strong earthquake by premonitory intermediate term seismic activation in lower magnitude range as defined in the algorithm m8 a tip refers to a 5 year period and an area whose linear size is several times larger than that of an incipient earthquake in the course of the global test of the algorithm we applied it retrospectively to the data from the national earthquake information center worldwide earthquake catalog since the regional japan meteorological agency catalog was unavailable to authors related to the segment of circum pacific belt between latitudes 20 n and 48 n from the south of kuril arc to the taiwan and izu bonin arc the worldwide catalog allows us to diagnose tips for earthquakes above magnitude 7 5 from 1975 on five out of six earthquakes with m 7 5 occurred within the tips diagnosed by the algorithm m8 with a priori fixed parameters the missed earthquake occurred at the final stage of formation of the corresponding tip altogether the tips occupy less than 20 and times of expectation occupy about 8 of the total space time domain considered there are also current tips covering about half of the territory application of the additional algorithm of localization named mendocino scenario indicates that the southern part of the japan trench is an area deserving particular attention the results obtained here by no means constitute the definitive earthquake prediction but merely outline the territories for more detailed and comprehensive analysis if confirmed by regional data the results may be used for implementation of some safety measures and evaluation of other precursors including short term ones

early diagenesis of biogenic silica in the amazon delta alteration authigenic clay formation and storage deltaic environments are commonly assumed to be relatively minor sites of biogenic silica burial because of the small quantities of opaline silica detected by most operational analytical techniques rapid conversion of biogenic silica into authigenic silicates is also often discounted as a significant control on oceanic silica budgets a variety of evidence for extensive early diagenetic alteration of biogenic silica in rapidly accumulating amazon delta sediments indicates that both of these general assumptions are unjustified apparent lack of significant biogenic silica storage in deltaic environments particularly in the tropics may be largely an artifact of operational definitions that do not include early diagenetic products of biogenic silica biogenic silica particles buried in suboxic amazon delta deposits can be unaltered partially dissolved covered with aluminosilicate or metal rich coatings or completely reconstituted into authigenic k fe rich aluminosilicate minerals pore water k mg f si and solid phase distributions direct observations of particles laboratory experiments and depositional context indicate that authigenic clays form rapidly 1 yr in the seasonally reworked surface layer 0 5 2 m of the delta topset and are disseminated during sediment remobilization fe al oxide rich debris derived from the tropical drainage basin is an abundant reactant and thus the supply of biogenic silica is a major control on the amount of clay formed the mild 1 na2co3 alkaline leach procedure commonly used to estimate biogenic silica was modified to include an initial mild leach step with 0 1n hcl to remove metal oxide coatings and to activate poorly crystalline authigenic phases for alkaline dissolution well crystallized clays are not significantly affected by this modification nor is bulk amazon river bed sediment the two step procedure indicates that 90 of the biogenic silica originally present in deposits is converted to clay or otherwise altered raising the effective quantity of biogenic silica stored from 33 to 296 mol si g 1 1 8 sio2 biogenic si stored in the delta increases away from the river mouth across shelf and along the dispersal system where primary production is highest the k si ratio of labile authigenic material is 0 19 mol mol 1 far higher than amazon river suspended matter 0 07 mol mol 1 diagenetic models indicate formation rates in the mobile sediment layer of 2 8 mol k g 1 yr 1 16 mol si g 1 yr 1 inclusion of authigenic alteration products of biogenic silica in estimates of reactive si burial increases the deltaic storage of riverine si to 22 of the amazon river input the rapid formation of aluminosilicates from biogenic sio2 seawater solutes and remobilized fe al oxides represents a form of reverse weathering rapid reverse weathering reactions in tropical muds and deltaic deposits the largest sediment depocenters on earth confirms the general importance of these processes in oceanic elemental cycles

a critical look at iron paleoredox proxies new insights from modern euxinic marine basins enrichments in reactive iron occur under euxinic marine conditions that is where dissolved sulfide is present in the water column these enrichments result primarily from the export of remobilized iron from the oxic shelf which is scavenged from the euxinic water column during syngenetic pyrite formation and deposited in the underlying sediments strongly elevated ratios of highly reactive iron to total iron fehr fet and total iron to aluminum fet al and high degrees of pyritization dop are each products of this enrichment process these paleoredox proxies are among the most faithful recorders of ancient euxinia contrary to previous arguments iron enrichment is decoupled from biogenic sediment inputs but it does appear to be a uniquely euxinic phenomenon in other words we can rule out a major contribution from preferential physical transport of fehr rich detrital sediment to the deep basin which could also operate under oxic conditions furthermore enrichment via the shuttling of iron remobilized from oxic shelves appears to be limited by inefficient transport and trapping processes in deep oxic basins elevated fet al ratios in the euxinic sediments also cannot be a product of internal enhancement of the reactivity of the detrital iron pool without net fehr addition these conclusions are supported by observations in the modern black sea orca basin and effingham inlet fet al ratios are unambiguous recorders of paleoredox even in sediments that have experienced high degrees of metamorphic alteration however this study suggests that high siliciclastic accumulation rates can swamp the enrichment mechanism resulting in only intermediate dop values for euxinic sediments and fet al ratios that mimic the oxic shelf such dilution effects are well expressed in black sea basinal turbidites and rapidly accumulating muds on euxinic basin margins under conditions of persistent euxinia varying extents of fehr enrichment can illuminate spatial and temporal gradients in siliciclastic sedimentation the magnitude of enrichment is a function of the source shelf to sink ocean basin areal ratio suggesting that iron proxies can also record ocean scale paleoenvironmental properties through muted enrichments at times of very widespread euxinia for the first time manganese data are interpreted in light of the redox shuttle model as for the iron data the black sea orca basin and effingham inlet show enrichments in total manganese in the deep euxinic basin suggesting export from the suboxic porewaters of the oxic shelf and scavenging and burial in the basin the black sea data reveal iron and manganese enrichment across the broad deep euxinic basin suggesting efficient lateral transport and deep water mixing tied to the physical properties of the water column

origin of light hydrocarbons in gases from mud volcanoes and ch4 rich emissions abstract this paper presents new chemical data of hydrocarbon rich gases discharged from mud volcanoes and ch4 rich emissions located in different areas of italy northern apennines and sicily the determination of c5 c10 alkanes cyclics and aromatics was carried out by gc ms while the main gas species and c1 c4 hydrocarbons were analyzed by gc tdc and gc fid respectively methane is by far the most abundant component of all the investigated gas emissions with the exception of part of those discharging close to mt etna volcano which are co2 rich the gas samples collected from the emilia apennine and sicily show d ch4 and 13c ch4 values typical of thermogenic gases most gases from the romagna apennine discharge methane with a biogenic isotopic signature non methane hydrocarbons of biogenic gases are almost exclusively c2 c4 alkanes with minor amounts fraction of mol mol of c4 compounds including few aromatics and cyclics likely deriving from minor thermogenic contribution not evidenced by the 13c ch4 and d ch4 values the etnean gases whose r ra and 13c co2 values indicate a strong contribution from a hydrothermal fluid source related to the nearby volcanic system show a significant enrichment of aromatics compounds up to 36 of the non methane organic gas fraction which is likely produced by catalytic reforming processes such as dehydrocyclization of alkanes the thermogenic gases from the apennines and southwestern sicily are characterized by the presence of more than 20 different cyclic compounds with concentrations up to several mol mol cyclic compounds are likely formed by i thermal cracking and ii uncompleted aromatization of alkanes occurring at depth 3 km and temperatures not exceeding 120 150 c

evolution of interstitial waters along the passive margin of the southeast basin of france welcom well chemical on line monitoring applied to balazuc 1 well ardeche abstract as part of the geologie profonde de la france gpf deep geology of france programme investigating the passive margin of the southeast basin of france studies are being made on the possibility of modelling mass transfer in the summer of 1990 when the first hole in the area balazuc 1 was being drilled along a basement rooted fault chemical monitoring of the drilling fluids welcom enabled the composition of the interstitial fluids contained within the intersected rocks to be reconstructed the computations were compared to on core leaching carried out during the drilling following which the welcom results were calibrated the interstitial fluids from the drilled formations show very contrasted compositions from the top to the bottom of the hole it has been possible to distinguish diagenetically evolved seawater within the hettangian limestone modified continental fluids within the upper part of the triassic sandstone reservoirs and evaporated seawater within the middle triassic evaporites several zones of intensive fluid circulation are also indicated by dolomitization of the basal hettangian limestone circulation of fluids expelled from the evaporites within the upper part of triassic sandstone reservoirs and continental derived hydrothermal fluid flow within the lower triassic sandstone reservoir and carboniferous silty shale the results show that chemical logging of drilling fluids can provide valuable data concerning interstitial fluids where other techniques are not available

formation water geochemistry of the sverdrup basin implications for hydrocarbon development in the high arctic abstract the sverdrup basin holds significant reserves of discovered and undiscovered petroleum that have not been produced to date given its remote arctic setting loss of perennial sea ice increases the prospects for producing reserves from this region potential renewed drilling and production requires knowledge of formation water geochemistry to aid both exploration as well as regulatory development historic data shows formation waters typically have a seawater origin although there is high variability in formation water geochemistry salt diapirs lead to localized brine generation with salinities over an order of magnitude higher than average for the basin evidence also shows fresh water influx occurred along the basin margins that initiated biogenic gas generation

early diagenetic alteration of lateritic particle coatings in amazon continental shelf sediment abstract intense chemical weathering in the tropical amazon river basin strips crustal material of alkali and alkaline earth metals leaving a residue composed predominantly of oxyhydroxides of fe hydroxides of al and cation poor clays this lateritic residue often coats soil particles within the marine environment lateritic particle coatings on sand undergo or have the capacity to undergo rapid reaction and mineralogical alteration 1 at least 20 of fe in coatings can be readily liberated during short term incubation in anoxic sediment 2 coatings change color from red white to green yellow during burial implying fe3 fe2 3 average mg cation content of coatings increases net addition from 1 85 near the amazon s mouth to 4 76 120 km from the mouth or with burial 4 a narrowing of the measured range and a slight increase of si al occurs near mouth to offshore 5 mg fe al layer silicates are probably forming by transformation of kaolinite or other cation stripped material of similar composition in coatings mg uptake by lateritic particle coating reactions assuming comparable involvement of all particle sizes could account for up to 67 of the mg discharged in solution by the amazon river evidence for mg fe al layer silicate formation supports the hypothesis that ancient ironstone minerals formed in some cases by direct alteration of lateritic material

calcium mass transport and sandstone diagenesis during compaction driven flow stevens sandstone san joaquin basin california sediment compaction provides a limited source of fluids for diagenesis and drives fluid flow at average rates of only 1 mm yr nevertheless many geochemical and petrographic studies of diagenesis provide evidence of significant mass transport in systems where fluid flow appears to be compaction driven this apparent discrepancy could arise because diagenetic studies generally are concerned with relatively permeable petroleum reservoirs focused fluid flow through permeable zones could increase local flow rates and allow mixing of fluids from different sources this study uses the stevens sandstone of the san joaquin basin to explore the potential diagenetic effects of fluid focusing during compaction driven flow over a 5 m y period reactive transport simulations incorporate a new kinetic expression for plagioclase dissolution and suggest that rate limited plagioclase dissolution drove calcium enrichment of pore fluids and caused precipitation of calcite kaolinite and albite in spite of the importance of this rate limited reaction simulations show that influx of fluids from compacting shale could limit the distribution of calcite and kaolinite in adjacent sandstones although calcium mass transport is predicted on a scale of kilometers upward flow of calcium rich fluids from deep beds does not significantly increase calcite volume relative to closed system predictions increased transverse dispersivity increases mixing which further limits precipitation of calcite and kaolinite results are consistent with field observations of fluid chemistry although simulations account for

model for the evolution of oxygen isotope ratios in the pore fluids of mudrocks during burial abstract the determination of oxygen isotopic ratios in authigenic silicates and pore fluids provides vital information on the nature of water rock interactions in the subsurface and the origin of porosity occluding phases in reservoir rocks the 18 o values of pore fluids generally increase during burial through water rock interaction the transformation of detrital smectite to illite is thought to have a major influence on the isotopic evolution of pore fluids but previous attempts to model this effect have failed to accurately reproduce the observed water rock isotopic trends with depth a revised model has been developed which incorporates an empirical rate equation for the illitization reaction the input variables include the initial mineral and pore water isotopic compositions time porosity depth functions and geothermal histories and the assumption is made that the isotopic equilibration of the solid phases with pore waters only takes place during reaction the model is tested against the most complete data set available from the texas gulf coast usa and produces a good fit with the available pore fluid and mineral data the sensitivity of the model is tested for variations in the following parameters the illite water fractionation factor the fractionation of oxygen isotopes between the smectitic and illitic components of the detrital illite smectite the oxygen isotope fractionation between pore waters and the interlayer water within the smectite and the porosity depth function the results of the model are shown to be strongly dependent on the porosity depth function the best fit between the model and real data is provided by the dickinson equation for undercompacted shale modified to have a constant porosity below 2600 m to simulate the effects of overpressuring the model is also sensitive to the degree of oxygen isotopic fractionation between the illite and smectite within the sediment at the time of burial but not to the extent of fractionation between interlayer water within smectite and the adjacent pore waters the uncertainty within the illite water fractionation factor has only a minor effect on the results of the model although only incorporating a single mechanism for water rock interaction the model provides a tool for testing the interpretations of subsurface pore fluid evolution trends during sediment diagenesis

interactions between organic acids anions in formation waters and reservoir mineral phases abstract formation waters from the norwegian continental shelf have been analysed for short chain organic acid anions acetic acid is the major component and the observed levels are in the range 0 3 20 mm in uncontaminated formation water samples studies of individual reservoirs and formations over time and area show a quite uniform distribution of the acids within each unit for large volumes of fluid the organic acid anion levels are generally low in reservoirs containing biodegraded oil statistical analysis has been used to find correlations between the organic and inorganic ion compositions and the reservoir conditions and shows a dominating negative correlation between the organic acid anion levels and the levels of inorganic ions indicating that the main effect of the organic ions is to increase the stability of mineral phases laboratory experiments have also been performed and show similar effects for sandstone samples under oxidizing conditions the buffering capacity of the organic acid species is also important as it prevents the development of extreme ph conditions caused by pyrite oxidation in carbonate systems few systematic effects were observed the results both for the natural systems and the laboratory experiments show that the presence of organic acids in the formation waters does not contribute to the development of secondary porosity in the reservoir rock the observed relationships rather indicate decreasing concentration levels of inorganic ions in the formation waters with increasing levels of organic anions and thus an increase in mineral stability

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis reply to critical comment by j s hanor l s land and g l macpherson

miocene stable isotope record a detailed deep pacific ocean study and its paleoclimatic implications deep sea drilling project site 289 in the western equatorial pacific has yielded an extremely detailed record of the carbon and oxygen isotopic changes in the miocene deep ocean the isotopic record reflects major changes in paleoclimate and paleoceanography probably dominated by a major phase of antarctic ice cap growth the transition from a relatively unglaciated world to one similar to today occurred between 16 5 x 10 6 and 13 x 10 6 years before the present with the greatest change occurring between approximately 14 8 x 10 6 and 14 0 x 10 6 years before the present

carboxylic acid anions in formation waters san joaquin basin and louisiana gulf coast u s a implications for clastic diagenesis critical comment the analyses of carboxylic acid anions caas in san joaquin basin and louisiana gulf coast formation waters published by macgowan and scrdam 1990 do little to resolve the important question of whether caas are of great significance to clastic diagenesis over the temperature range in which they dominate fluid alkalinity as advocated by these two authors comparison of their analyses with analyses of formation waters from the same wells previously published by three independent research groups hanor and workman 1986 land et al 1988 and land and macpherson 1989 and fisher and boles 1990 a body of data not discussed in their article reveals that most of the macgowan and surdam values are grossly inflated relative to the previous analyses table 1 fig 1 cedures of llco et al 1982 while field procedures for caas now normally include the addition of a bactericide to the sample an independent check on the upper limit of total caas which could have been present in this water is provided by the results of an alkalinity titration to ph 2 5 also published by hanor and workman 1986 which show that the total concentration of weak acid anions bicarbonate plus caas could not have exceeded approximately 428 mg 1 the alkalinity determinations were done shortly after collection on separate untreated aliquots stored in gas tight glass containers further details are given in workman 1985

testing earthquake prediction algorithms statistically significant advance prediction of the largest earthquakes in the circum pacific 1992 1997 abstract algorithms m8 and msc i e the mendocino scenario were used in a real time intermediate term research prediction of the strongest earthquakes in the circum pacific seismic belt predictions are made by m8 first then the areas of alarm are reduced by msc at the cost that some earthquakes are missed in the second approximation of prediction in 1992 1997 five earthquakes of magnitude 8 and above occurred in the test area all of them were predicted by m8 and msc identified correctly the locations of four of them the space time volume of the alarms is 36 and 18 correspondingly when estimated with a normalized product measure of empirical distribution of epicenters and uniform time the statistical significance of the achieved results is beyond 99 both for m8 and msc for magnitude 7 5 10 out of 19 earthquakes were predicted by m8 in 40 and five were predicted by m8 msc in 13 of the total volume considered this implies a significance level of 81 for m8 and 92 for m8 msc the lower significance levels might result from a global change in seismic regime in 1993 1996 when the rate of the largest events has doubled and all of them become exclusively normal or reversed faults the predictions are fully reproducible the algorithms m8 and msc in complete formal definitions were published before we started our experiment keilis borok v i kossobokov v g 1990 premonitory activation of seismic flow algorithm m8 phys earth and planet inter 61 73 83 kossobokov v g keilis borok v i smith s w 1990 localization of intermediate term earthquake prediction j geophys res 95 19763 19772 healy j h kossobokov v g dewey j w 1992 a test to evaluate the earthquake prediction algorithm m8 u s geol surv ofr 92 401 m8 is available from the iaspei software library healy j h keilis borok v i lee w h k eds 1997 algorithms for earthquake statistics and prediction vol 6 iaspei software library

zeolites in petroleum and natural gas reservoirs zeolites have been reported in petroleum and natural gas reservoirs of a wide range of geological ages from many parts of the world such zeolite bearing oil and gas reservoirs are not comparatively large in number and scale however where present in petroleum and natural gas reservoirs zeolites can have profound effects on the quality of the reservoirs affecting rock porosity and permeability zeolite containing reservoir rocks vary widely lithologically including not only porous pelitic sedimentary rocks but also volcanogenic rocks and even fractured intrusive and plutonic rocks several different genetic types of zeolite occurrences have been distinguished and classified e g hay 1966 1986 1995 iijima 1980 1986 as hay and sheppard discussed and summarized in an earlier chapter this volume in addition the organic origin theory for the formation of economic oil and gas resources seems to have been universally accepted including the prominent kerogen decomposition theory summarized by tissot and welte 1978 sedimentary basins filled by very thick sequences of sedimentary strata containing organic rich source rocks and porous reservoir rocks are considered necessary for producing hydrocarbon resources such sedimentary basins develop primarily through rifting or in relation to arc formation at active plate boundaries and they are often accompanied by heating and extensive volcanic activity most types of zeolite occurrence are consequently constrained to burial diagenesis uplift related late stage diagenesis surdam and boles 1979 boles 1993 and hydrothermal alteration as will be discussed in this chapter zeolites in hydrocarbon reservoir rocks occur commonly as cements and grain replacements in the form of disseminated microcrystalline aggregates and occasionally as macroscopic crystals in vugs and veins it was in 1950s that coombs and co workers pioneered the petrologic study of rock forming zeolites through their classic research on burial metamorphism of the triassic geosynclinal deposits in south syncline new

lithofacies of the silurian keefer sandstone east central appalachian basin usa abstract fourteen lithofacies of the keefer sandstone in the area of northeastern west virginia and western maryland were identified depositional environments of the lithofacies include storm dominated marine shelf basin barrier island and backbarrier tidal flat and lagoon coastal lithofacies document a dominant basinward progradation with some sea level rise which placed backbarrier lithofacies above barrier island lithofacies shelf lithofacies as stacked coarsening upward sequences document as many as four fluctuations within the major progradation coarsening upward sequences are typically capped by transgressive sands which represent material eroded from relatively landward environments and transported basinward during coastal retreat deposition of cycles within the keefer occurred under conditions of tectonic quiescence dominant steady or falling sea level followed by rise in sea level and in a climate which was warm and humid

diagenetic carbonate in miocene sandstone reservoir san joaquin basin california stevens arkosic sandstones deposited about 7 ma in a deep marine environment form the north coles levee reservoir dolomite and calcite cements occur as zones averaging 1 ft thick and having less than 300 ft of lateral continuity based on dgr18o measurements and estimated pore fluids between 0 and 2 8 the dolomite formed at 30 40 c an early calcite cement formed at 50 60 c and a late pore filling and plagioclase replacing calcite formed at 70 80 c corresponding to these temperature estimates dolomite formed in the zone of methanogenesis dgr13c 4 to 8 and calcites in the zone of decarboxylation dgr13c 3 to 12 dolomite has between 4 and 12 mole fe and calcites contain up to 8 mole o fe mg mn with a characteristic fe mn mg ratio of about 3 1 the latest calcite is the most ca rich due in part to ca sourced from plagioclase dissolution calcite cements in nonmarine sections have markedly high mn contents to 5 mole compared with the marine section at north coles levee the volume of cement systematically decreases from about 40 20 over the cementation temperature interval suggesting progressive compaction of the sandstones present day mean porosity of the reservoir 15 appears to be largely a result of mechanical compaction a very late dolomite cement with high fe content 13 mole and light carbon dgr13c 14 is found in the cleavage flakes of crushed biotite the dolomite is calculated to have formed from a fluid with a dgr18o of 0 9 at present reservoir temperatures of 105 c suggesting precipitation in the past 20 years during release of the gas cap and injection of isotopically light water calcite also forms thick scales in production tubing primarily as a result of pressure drops from 4 900 to 400 psi in the tubing string the calcites have high mg contents 7 mole due to the composition of the injection water calculated distribution coefficients for fe and mn are similar to published values but mg is two times higher than published values at 90 c similar results are obtained for late calcite cements a time temperature plot for north coles levee constrains the timing of dolomite cementation between 5 and 6 ma and the calcite cementation between 4 and 2 5 ma plagioclase dissolution and hydrocarbon emplacement occurred within the past few million years suggesting that the presence of the hydrocarbons and associated byproducts of kerogen maturation enhance porosity despite the narrow window for diagenesis minimum fluid fluxes of only 0 02 cm3 yr per 100 cm3 rock are required to account for the volume of dissolved plagioclase

rock solute reaction mass balance of water flowing within an aquifer system with geochemical stratification abstract aquifers possess distinct mineralogy diagenetic history and geochemical constraints which control the pore water s hydrochemistry whenever aquifers exchange groundwater the incoming groundwater establishes a new chemical driving force imposing new reactions related to rock water interactions this study developed a simple approach to assess the mass balance of the rock water interaction of the groundwater exchanged between interconnected aquifers to illustrate the suitability of this approach it was applied to the bauru aquifer system located in sao paulo state in southeastern brazil the hydrochemical characterization identified three major stratified hydrochemical facies with typical chemical compositions as well as the authigenic mineralogy assemblages the first and shallowest facies was composed of a variable groundwater composition along with highly diluted water from a recent recharge the second facies was composed of unconfined bicarbonate calcium rich water with a mineralogy mainly composed of k feldspar mg montmorillonite and calcite the third facies which was confined and the deepest was composed of bicarbonate sodium rich water and mineralogically dominated by na mg and ca montmorillonite calcite and analcime although the replacement of na by ca suggests an extensive cation exchange in the deep confined portion sodium rich silicate dissolution and calcium rich clay precipitation were identified as the operating mechanisms promoting na enrichment and ca consumption in the water of the closed system this finding is supported by the authigenic mineral characterization 13c of the dissolved inorganic compounds and numerical analysis the approach developed for this study which used numerical simulations to calculate the mass balance was capable of consistently reproducing the water composition and mineralogy changes

experimental simulation of plagioclase diagenesis at p t conditions of 3 5 km burial depth dissolution of plagioclase under the physical conditions at shallow to intermediate burial depths is a prime candidate for secondary porosity generation in feldspathic siliciclastic sediments the diagenetic behavior of granular aggregates of plagioclase feldspar and quartz has been investigated by experimentation performed in a bridgeman type pressure vessel the experiments each of two weeks duration simulated pressure temperature conditions approximating 3 5 km burial depth by using a double acting pore fluid reservoir solutions of various chemistries were cycled through samples composed of oligoclase or labradorite feldspar and quartz 90 10 wt respectively

intermediate term earthquake prediction an earthquake of magnitude m and linear source dimension l m is preceded within a few years by certain patterns of seismicity in the magnitude range down to about m 3 in an area of linear dimension about 5l 10l prediction algorithms based on such patterns may allow one to predict approximately 80 of strong earthquakes with alarms occupying altogether 20 30 of the time space considered an area of alarm can be narrowed down to 2l 3l when observations include lower magnitudes down to about m 4 in spite of their limited accuracy such predictions open a possibility to prevent considerable damage the following findings may provide for further development of prediction methods i long range correlations in fault system dynamics and accordingly large size of the areas over which different observed fields could be averaged and analyzed jointly ii specific symptoms of an approaching strong earthquake iii the partial similarity of these symptoms worldwide iv the fact that some of them are not earth specific we probably encountered in seismicity the symptoms of instability common for a wide class of nonlinear systems

the influence of water column hypoxia on the behaviour of manganese and iron in sandy coastal marine sediment the influence of bottom water hypoxia on manganese iron and sulfur biogeochemistry was examined in sandy sediment from the shallow coastal lagoon faellesstrand denmark the organic poor sediment at faellesstrand experiences occasional coverage of floating macroalgae and variable degrees of hypoxia at the sediment water interface resulting in dramatic changes in metal behaviour the narrow peaks and steep gradients in mn and fe oxides as well as porewater mn 2 and fe 2 observed in the upper 2 3 cm of the sediment under fully oxic conditions indicate intense metal reduction oxidation cycles the fe zones were generally displaced about 1 cm downwards compared with the mn zones due to differences in reactivity at lowered o2 conditions in the overlying water mn oxides gradually disappeared followed by fe oxides the subsequent diffusive loss of mn 2 and fe 2 to the overlying water was inversely related to the o2 concentration in the overlying water the ability of the sediment to retain upward diffusion of h2s sulfide retaining capacity gradually disappeared at lowered o2 concentrations in a temporal pattern closely related to the changes in reactive mn and fe present the sulfide retaining capacity is sustained for about 14 days under anoxia in faellesstrand sediment after 28 days of anoxia 30 35 of the total mn and fe pools initially present in the sediment was lost despite the relatively low metal content this organic poor sediment may withstand hypoxic conditions in the bottom water e g caused by coverage with floating macroalgae and is thus capable of maintaining an intact benthic community for extended periods of time

influence of detrital composition on the diagenetic history of tight sandstones with implications for reservoir quality examples from the permian xiashihezi formation and carboniferous taiyuan formation daniudi gas field ordos basin china abstract tight gas reservoirs characterized by low porosity and low permeability are widely considered to be the product of post depositional diagenetic processes associated with progressive burial this study utilizes a combination of thin section petrography scanning electron microscopy microprobe and back scatter electron analysis stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history including porosity formation within sandstones of the second member of carboniferous taiyuan formation c3t2 and the first member of permian xiashihezi formation p1x1 in the ordos basin in central china in the p1x1 member relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition produced more smectite for transformation to illite 50 120 c this reaction was driven by dissolution of unstable minerals k feldspar and rock fragments during the early to middle stages of mesodiagenesis and consumed all k feldspar abundant intragranular porosity average values of 2 8 and microporosity in kaolinite average values of 1 5 formed at these burial depths with chlorite and calcite developed as by products in the c3t2 member relatively low abundances of metamorphic rock fragments coupled with an incised valley coastal plain environment of deposition resulted in less smectite for transformation to illite high k h ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction under these conditions k feldspar was partially preserved during the middle to late stages of mesodiagenesis k feldspar breakdown produced secondary intragranular average values of 1 4 and intergranular pores average values of 1 2 release of k ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by products this study has demonstrated that whereas both members are typical tight gas sandstones they are characterized by quite different diagenetic histories controlled by the primary detrital composition especially during mesodiagenesis types of secondary porosity vary between the two members and developed at different stages of progressive burial the content of unstable detrital components notably feldspar was the key factor that determined the abundance of secondary porosity

significance of early diagenetic water rock interactions in a modern marine siliciclastic evaporite environment salina ometepec baja california although marine brines are a significant component of pore waters in sedimentary basins there are few geochemical studies of modern analogues of such systems especially in siliciclastic settings for these reasons we chose the evaporite associated siliciclastic sediments deposited in the salt flats of the salina ometepec baja california for an integrated investigation of sediment pore water and overlying brine geochemistry here the detrital components include quartz k feldspar plagioclase chlorite biotite and smectite and authigenic minerals are dominated by halite gypsum and k rich magnesium smectite thermal and saline stresses on the sediments of the salina ometepec keep both organic and inorganic carbon concentrations in the sediments unusually low relative to other coastal marine environments sediment pore waters exhibit little microbial sulfate reduction and dissolved inorganic c contents are also very low as a result we did not observe carbonate and sulfide mineral authigenesis in the salina ometepec sediments instead pore water geochemical evolution is largely controlled by evaporative concentration of seawater evaporite mineral dissolution and recrystallization and diagenetic alteration of detrital aluminosilicates evaporite mineral recycling affects the compositional evolution of surficial brines even before they infiltrate the sediment specifically na and cl concentrations are increased owing to halite dissolution we see significant br enrichment relative to expected seawater evaporation trends in near surface pore water secondary to dissolution of k and mg salts because bacterial sulfate reduction is inhibited in the salina ometepec sediments sulfate concentrations are more accurate indicators of the degree of evaporation than br a usually conservative element during geochemical reactions pore waters exhibit down core increases in dissolved mg2 k and batchmode documentclass fleqn 10pt legalpaper article usepackage amssymb usepackage amsfonts usepackage amsmath pagestyle empty begin document h 4 sio 4 0 end document over the upper 1 m authigenic k rich mg smectite formation is promoted by the concurrent processes of brine concentration selective dissolution of k and mg bearing salts and dissolution of detrital aluminosilicates pore waters at a depth of 1 m have 87sr 86sr ratios that require input of sr that is less radiogenic than that of gulf of california seawater this sr is likely derived from weathering of detrital aluminosilicates from nearby volcaniclastic sources these results show that significant chemical interactions among marine brines evaporite minerals and detrital aluminosilicates can occur relatively soon after sedimentation

geochemical constraints from formation water analyses from the north sea and the gulf coast basins on quartz feldspar and illite precipitation in reservoir rocks

nacacl saline formation waters frio formation oligocene south texas usa products of diagenesis abstract saline formation waters from reservoirs in oligocene deltaic clastic sedimentary rocks frio formation south texas are derived from three sources mixtures characterize most samples accounting for the wide range in chemistry low salinity water is derived from cenozoic shales in contrast saline water having variable br cl ratios is allochthonous and is derived from at least two sources in mesozoic strata which underly the cenozoic units saline water in this area regardless of br cl ratio contains variable high concentrations of ca calcium is derived primarily from dissolution of plagioclase as demonstrated by petrography 87 sr 86 sr analyses and chemical modelling the mccl 2 or carpenter function of saline formation waters in this area thus can be derived from water rock interaction and cannot be used to identify the ultimate source of solutes reverse osmosis is an unimportant processes in this area and 18 o and d values converge on rock buffered values of approximately 8 o and 30 o respectively

burial evolution of evaporites with implications for sublacustrine fan reservoir quality a case study from the eocene es4x interval dongying depression bohai bay basin china abstract eocene sublacustrine fan sandstones that developed in a rift basin are important tight gas reservoirs in the dongying depression bohai bay basin northeastern china two units of evaporites developed at the top and bottom of the lower unit of the es4 interval es4x consist predominantly of anhydrite with subordinate gypsum evaporite and related diagenetic processes greatly influenced reservoir quality 13 c values for micritic dolomite cements in es4x are depleted 7 45 to 2 57 due to microbial sulfate reduction msr under shallow burial conditions and this interpretation is supported by large 34 s fractionation between anhydrite and framboidal pyrite precipitation temperatures for micritic dolomite are calculated as 57 5 72 8 c anhydritization of gypsum probably occurred at 100 5 145 2 c during progressive burial as evidenced by homogenization temperatures of aqueous inclusions within anhydrite cements this process resulted in dehydration fractures within anhydrite cements that increased reservoir permeability by connecting isolated pores thermochemical sulfate reduction tsr probably resulted in dissolution of gypsum and anhydrite cements under relatively deep burial conditions ankerite cements are replaced by anhydrite cements and are enclosed by solid bitumen in es4x ankerite cements likely were derived from tsr as reflected in negative 13 c values 7 12 to 3 70 and high calculated temperatures 121 3 185 1 c dissolution by products e g saddle dolomite ankerite nodular pyrite related to tsr precipitated in adjacent pores a lack of significant 34 s fractionation between parent sulfate and nodular pyrite indicates that tsr occurred in a relatively closed system therefore dissolution of gypsum and anhydrite related to tsr contributed little to reservoir quality middle fan lithofacies with better sorting porosity and permeability than inner and outer fan lithofacies constitute high quality reservoirs in es4x

water rock interaction in tertiary sandstones san joaquin basin california u s a diagenetic controls on water composition three water types are recognized among petroleum associated waters of the southern san joaquin basin 1 meteoric water 2 diagenetically modified connate water and 3 mixed meteoric modified connate water the diagenetically modified connate marine water can be further subdivided into three types 1 fully mg depleted mg 1 mole of the sum of na k ca and mg 2 partially mg depleted and 3 ca enriched enrichment in ca which is the sharpest chemical divide found for the diagenetically modified connate water is the result of feldspar albitization the ca enriched waters are also rich in total dissolved al up to 6 mg l 1 and exhibit lighter 18o values than other diagenetically modified connate waters the total dissolved al abundance in these waters may be due to the presence of organic complexing agents simple and aromatic organic acids and phenolics the lighter 18o values are probably due to both albitization and the lack of substantial clay diagenesis illitization of smectite

intermediate term earthquake prediction in the area of greece application of the algorithm m8 the 3 strongest earthquakes m 7 0 which have occurred since 1973 in the area of greece were preceded by a specific increase of the earthquake activity in the lower magnitude range this activation is depicted by algorithm m8 this algorithm of intermediate term earthquake prediction was originally designed for diagnosis by times of increased probability tips of the strongest earthquake m 8 0 worldwide keilis borok andkossobokov 1984 at present the algorithm is retrospectively tested for smaller magnitudes in different seismic regions keilis borok andkossobokov 1986 1988 a tip refers to a time period of 5 years and an area whose linear size is proportional and several times larger than that of the incipient earthquake source altogether the tips diagnosed by the algorithm m8 in the area of greece occupy less than 20 and the times of expectation tes about 10 of the total space time domain considered also there is a current tip for the southeastern aegean sea and 1988 1992 it may specify the long term prediction given inwyss andbaer 1981a b

syndepositional formation of fe rich clays in tropical shelf sediments san blas archipelago panama solid phase chemistry of modern carbonate siliciclastic marine sediments from the san blas archipelago panama reveals the occurrence of an fe mg authigenic clay facies that is dominated by a 7 1 a ferric clay mineral these fe rich clays are highly susceptible to hcl attack and can comprise a large fraction up to 5 4 wt fe of the sediment some of the clays are clearly forming in the marine environment as they coat grains infill carbonate micropores and replace faecal pellets and carbonate shells the mode of occurrence structure and ferric nature of the clay assemblage identifies the facies as verdine which has odinite a dioctohedral trioctohedral fe3 rich 1 1 clay mineral as its key member the verdine facies in san blas occurs in suboxic sediments where iron reduction can be a prevalent organic carbon oxidation pathway sediment radiocarbon ages show that the san blas verdine clays formed within the last few thousand years and in some cases the clays have been extensively converted into pyrite the verdine clays most likely grew by incorporating terrestrially derived fe and al seawater mg and pore water si derived from biogenic opal dissolution similar clay formation processes may be common in other tropical nearshore environments where sediments are derived from highly weathered lateritic soils and may prove to be an important oceanic elemental sink

rates and mechanisms of shoreface progradation and retreat downdrift of the amazon river mouth field surveys of the 350 km shoreline adjacent to the amazon river mouth reveal three distinct types erosional mud accretionary sand and accretionary mud formation of these zones is controlled by the delivery of amazon suspended sediment mediated by the hydrodynamic regime erosional mud shorelines extend from rio araguari near the amazon river mouth northwestward 280 km to 3 5 n near cabo cassipore shoreface 5 m water depth retreat annually yields 1 4 106 tons of fine grained sediment deposited during an earlier phase of shoreface progradation sandbodies up to 5 m thick overlie erosional mud shorefaces for 10 30 km downdrift of small rivers the sand is supplied from these rivers and released by shoreline retreat of the river mouth areas amazon river sand is restricted in the coastal zone from its mouth north to the cabo norte shoal area where it mixes with sand carried by rio araguari from the guiana highlands north of 3 5 n mud aggradation and progradation is taking place on underconsolidated low gradient tidal flats backed by mangrove swamps 210pb and 14c geochronology of vibracores from the mudflats indicate that sediment accumulation is rapid 0 24 2 0 cm yr landward of the 2 m isobath supplied from a thick 50 150 cm seasonal surface layer shoreface progradation is episodic separated by decadal hiatuses fine grained suspended sediment flux from the amazon and minor amounts of sand and mud from the local rivers supply sediment to the mudflats shore normal tidal currents and solitary waves rework the surface mud layer preferentially transporting available sand landward into the mangrove fringe and producing very fine grained accumulation on the tidal flat 10 12 mean grain size lateral accretion of features 10 100 km long termed mudcapes produces tens of kilometers of seaward coastal plain addition along the northern coast similar features are identified downdrift along the guianas coast as far as the orinoco river 1600 km the northern amapa shoreface deposits are a locus of modern sediment accumulation which progrades over subaqueous deltaic strata

short chain carboxylates in fluid inclusions in minerals abstract the carboxylate formate acetate propionate and oxalate and common inorganic anions f cl and so 2 4 compositions for aqueous fluid inclusion leachates from 17 mineral samples collected from various deposits have been determined using ion chromatography in conjunction with microthermometric measurements on the fluid inclusions of their host minerals the minerals quartz fluorite barite beryl and a few ore minerals wolframite pyrite and galena came from hydrothermal vein type deposits in felsic igneous rocks or archean metamorphic rocks the results indicate that short chain carboxylates are common components in hydrothermal fluids and can be present in considerable amounts formic acid as formate is the dominant species over other carboxylic acids the present study raises new questions about the origin and geochemical significance of carboxylates in hydrothermal ore forming processes

times of increased probability of earthquakes of ml 5 5 in greece diagnosed by algorithm m8 an attempt is made to explore the possibility of identifying the times of increased probability tips of occurrence of earthquakes with ml 5 5 by premonitory intermediate term seismic activation in lower magnitude ranges as defined in the algorithm m8 a tip refers to a 5 year period and an area whose length is several times greater than that of an incipient earthquake this algorithm has been already tested retrospectively for the strongest earthquakes of magnitude greater than or equal to 7 0 which have occurred in greece since 1973 in the present study the same algorithm is also applied retrospectively by considering smaller earthquakes of local magnitude ml greater than or equal to 5 5 the obtained results show that ten out of eleven events with m 5 5 which took place in greece and surrounding region from january 1 1977 till december 31 1990 occurred within the tips diagnosed by the algorithm m8 retrospectively only the thessaloniki earthquake of june 20 1978 did not occur within tip the m8 algorithm failed in four cases namely those that occurred in regions for which our catalogue was not complete altogether the tips occupy less than 25 and times expectation about 8 of the total space time considered there is also a current tip started at the end of 1986 or at the beginning of 1987 for the south southwestern part of thehellenic arc indicating that this area deserves particular attention if the algorithm m8 works reliably then this tip will be filled by the end of 1992 this result by no means constitute the definitive earthquake prediction but merely outline a territory for more detailed and comprehensive analysis

speciation and spatial distribution of solid phase iron in surface sediments of the east china sea continental shelf abstract speciation and reactivity characterization of solid phase fe in marine sediments are of significance to understanding its heterogeneous mineralogy and crystallinity the diagenetic cycling of fe and its regulating roles on many other elements in sediments in this study a combination of sequential and single step extractions was used for the determination of seven fe pools in surface sediments of the east china sea ecs continental shelf 1 carbonate associated fe fe ii carb plus acid volatile sulfide fe fe ii avs 2 easily reducible amorphous poorly crystalline fe oxides fe ox1 3 reducible crystalline fe oxides fe ox2 4 magnetite fe mag 5 poorly reactive sheet silicate fe fe prs 6 pyrite fe fe py and 7 unreactive silicate fe fe u total fe fe t in the sediments is largely determined by terrestrial aluminosilicate particles as indicated by a great similarity of the fe t with that of the yangtze river and global riverine particulates the size of fe prs is found to be the largest pool followed by fe u fe ox2 fe mag fe ii avs carb fe ox1 and fe py the large fe prs may result from neoformation of fe rich clay minerals via reverse weathering and subsequent ageing the small sizes of fe ii avs carb and fe py pools is believed to be the result of low so 4 reduction due to generally low labile organic matter together with the oxic suboxic dynamic environments of the surface sediments the occurrence of fe ox1 fe ox2 and fe prs in the sediments is closely associated with the clay fraction as indicated by a high spatial correlation between the former and the latter highly reactive fe fe hr in the sediments is comparable to that in global marine sediments but apparently lower than in the yangtze river and global riverine particulates due probably to sequestration in the yangtze estuary the ratios of fe hr fe t fe pr fe t and fe u fe t in the ecs surface sediments consistently show more similarity to those in the yangtze river particulates than in the global continental margin or deep sea sediments the surface sediments maintain a high level of buffering capacity toward sulfidation suggested by a large fraction of highly reactive fe iii oxides fe iii hr in fe hr

diagenetic mass transport in the southern san joaquin basin california u s a implications from the strontium isotopic composition of modern pore fluids abstract three types of chemically and isotopically distinct pore fluids from the southern san joaquin basin previously recognized by j b fisher and j r boles also have distinctive 87 sr 86 sr ratios and sr concentrations meteoric fluids have stable isotopic compositions which lie on or near the meteoric water line and low chlorinities sr concentrations are between 0 01 and 2 6 mg l 1 and 87 sr 86 sr ratios range from 0 7061 to 0 7078 diagenetically modified connate marine fluids have d and 18o values more positive than 35 and 0 respectively and have chlorinities generally comparable to seawater sr concentration are much higher than the meteoric group 16 198 mg l 1 although the 87 sr 86 sr ratios 0 7070 0 7081 are not distinctive mixed meteoric modified connate fluids have d 18o and chlorinity intermediate between the meteoric and modified connate groups sr concentrations are also intermediate between 16 and 22 mg l 1 but 87 sr 86 sr ratios 0 7080 0 7087 are generally more radiogenic than either the meteoric or modified connate groups all of the fluids have 87 sr 86 sr ratios comparable to or lower than tertiary seawater alteration of detrital plagioclase is the probable origin of the low isotopic ratios mass balance calculations based on the sr data suggest that essentially no transport of sr occurred during diagenesis of sandstones containing modified connate pore fluids while large amounts of sr have been transported out of meteoric reservoirs by fluid flow the chemically anomalous mixed meteoric modified connate fluids contain the most radiogenic strontium in the basin these fluids are spatially associated with major faults and may represent clay mineral dehydration waters which have been transported upward from greater depth these results suggest that the three types of fluids identified by fisher and boles represent three distinct mass transport regimes a largely stagnant deep basin system containing modified connate pore fluids an actively recharging meteoric system along the basin flanks and a third system restricted to the southern basin which may be characterized by largescale cross formational fluid flow rather than dilution by meteoric waters

stratigraphic significance of ooidal ironstones from the cretaceous western interior seaway the peace river formation alberta canada and the castlegate sandstone utah u s a ooidal ironstones form prominent units and an appreciation of their sedimentological and stratigraphic significance is important for the interpretation of sedimentary successions ooidal ironstones from the albian paddy member of the peace river formation alberta canada and the campanian castlegate sandstone of the mesaverde group in colorado have been analyzed the ooidal ironstones are associated with a major sequence boundary and subsequent transgressive surface both are interpreted to have been deposited under conditions of low net sediment accumulation in well oxygenated bottom water conditions with episodic storm events reworking the sediments the paddy member ironstone is interpreted to have been deposited in an estuarine environment and the castlegate ironstone is interpreted to have been deposited in a shallow marine environment both ironstones are dominated by berthierine ooids and grain rimming and pore filling siderite with later ferroan dolomite and calcite in the castlegate ooidal ironstone petrographic geochemical and isotopic evidence coupled with thermodynamic considerations indicates that the ooidal ironstone mineralogy formed during suboxic diagenesis the conditions required for suboxic diagenesis were extensive sediment reworking and slow net sediment accumulation rates we propose that these conditions arose as a result of marine transgression during the initial relative sea level rise following the development of a sequence boundary ooidal ironstone formation ceased once sedimentation rates increased and transgression deepened water depths considerably these results illustrate the association between ooidal ironstones and major stratal surfaces in sedimentary successions and highlights the complexity that early diagenesis at such surfaces can take

geochemical modelling of sandstone diagenesis and its consequences on the evolution of porosity abstract geochemical modelling of fluid rock interaction is one theoretical approach that can be used for understanding diagenetic processes and their effects on reservoir quality results of successive modelling of dissolution and heating using two specific path calculation codes dissol and thermal from 25 to 160 c are presented if one considers one volume of reacting solution the diagenetic effects are very important for fluid chemistry while mass transfers have little effect on porosity only cumulative effects due to the circulation of fluids in reservoirs may explain the extent of diagenetic processes on minerals comparison between the computed data this study and available data from the north sea from gulf coast sandstones from pattani basin gulf of thailand from san joaquin basin california u s a and from a triassic sandstone reservoir in the rhine graben cronenbourg france show interesting similarities in the chemical evolution of the fluid the modelling has interesting implications for the relations between ph p co 2 conditions and the diagenetic stability of calcic plagioclase

distribution and significance of dicarboxylic acid anions in oil field waters the origin distribution and interactions of low molecular weight organic acid anions have become an intensively studied field in geochemistry since their widespread occurrence in formation waters of sedimentary basins was first documented by carothers and kharaka 1978 high concentrations up to 10 000 mg 1 of monocarboxylic mainly acetate propionate and butyrate and dicarboxylic mainly oxalate malonate and succinate acid anions have been reported from many sedimentary basins with the highest values present in relatively young cenozoic age petroleum reservoir rocks at subsurface temperatures of 80 120 c see lundegard and kharaka 1990 for a recent review with references geochemical interest in these organic anions stems mainly from their important role in mineral diagenesis in sedimentary basins kharaka et al 1986 macgowan and surdam 1990 in particular these species act as sources or sinks of protons as a source of co2 and as ph and eh buffering agents lundegard and kharaka 1990 they also form complexes with cations and metals such as ca a1 fe pb and zn kharaka et al 1985 harrison and thyne 1992 data on the concentrations of monocarboxylic acid anions in subsurface waters from many sedimentary basins worldwide are available and are generally accepted as representing the aqueous concentrations at depth data on the concentrations of dicarboxylic acid anions are much more limited some reported values are controversial and the total values reported range widely from 0 to 2640 mg 1 surdam et al 1984 kharaka et al 1986 barth 1987 macgowan and surdam 1988 1990 because of the wide range in the reported concentrations and because dicarboxylic acid anions generally form stronger complexes with a1 fe and other cations than do monocarboxylic acid anions we resampled four oil wells in the san joaquin basin california in order to better assess the role of these acid anions in water rock interactions in sedimentary basins

an optimization of using the m8 algorithm for prediction of major m7 0 earthquakes in the iranian plateau the casualties and financial losses caused by large earthquakes have led to an awareness of prediction importance of such earthquakes the earthquake prediction is divided into four categories long term intermediate term short term and immediate the m8 algorithm is one of the intermediate term middle range prediction algorithms primarily used to predict earthquake of magnitude 8 or more and is applied later for smaller magnitudes the iranian plateau is less exposed to earthquake with magnitude 8 or more and it is observed that the seismicity rate in this region is generally low thus the original m8 is not suitable for applying in this region the objective of this study is to modify the m8 algorithm for prediction of major m7 0 earthquakes in the iranian plateau the major earthquake of magnitude 7 or more in the iranian plateau from 1975 to 2018 is considered as the target earthquakes the hit rate times 1 minus alarm rate is defined as objective function and the particles swarm optimization meta heuristic algorithm is used to maximize it the optimum m8 could predict 14 out of 17 large earthquakes in the iranian plateau while occupying 31 7 of the spatio temporal space as the alarm the results show that by employing an optimization algorithm we can modify the m8 algorithm for efficient prediction of the target magnitudes less than 8 in the regions with low seismicity rate

testing an earthquake prediction algorithm the 2016 new zealand and chile earthquakes the 13 november 2016 m7 8 54 km nne of amberley new zealand and the 25 december 2016 m7 6 42 km sw of puerto quellon chile earthquakes happened outside the area of the on going real time global testing of the intermediate term middle range earthquake prediction algorithm m8 accepted in 1992 for the m7 5 range naturally over the past two decades the level of registration of earthquakes worldwide has grown significantly and by now is sufficient for diagnosis of times of increased probability tips by the m8 algorithm on the entire territory of new zealand and southern chile as far as below 40 s the mid 2016 update of the m8 predictions determines tips in the additional circles of investigation cis where the two earthquakes have happened thus after 50 semiannual updates in the real time prediction mode we 1 confirm statistically approved high confidence of the m8 msc predictions and 2 conclude a possibility of expanding the territory of the global test of the algorithms m8 and msc in an apparently necessary revision of the 1992 settings