multiple fluid flow events and the formation of saddle dolomite case studies from the middle devonian of the western canada sedimentary basin abstract in western canada sedimentary basin wcsb large scale and focused fluid flow that caused hydrothermal dolomitization have been suggested with different timing and intensity in this study we conducted a petrographic and geochemical comparison between the middle devonian sulphur point and slave point carbonates from northwestern alberta the results demonstrate the presence of both an early fluid flow event associated with the late devonian to mississippian antler orogeny as well as a later event coincident with the late cretaceous to early tertiary laramide orogeny early fluid flow event is characterized by high salinity fluids and high temperatures and oxygen isotopic values of marine or slightly enriched values as demonstrated in saddle dolomite from the slave point formation in contrast later fluids that caused the precipitation of saddle dolomite in the sulphur point formation are characterized by having slightly saline values comparable homogenization temperatures but more enriched 18 o values and slightly depleted 13 c signatures geochemical data indicate that this later hydrothermal fluid was mixture of middle devonian brines and radiogenic basement fluids

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

dolomitization of the latemar platform fluid flow and dolomite evolution abstract the anisian ladinian latemar platform northern italy presents a spectacularly exposed outcrop analogue for dolomitized carbonate reservoirs in relation to fracture controlled igneous intrusions although the latemar is one of the best studied carbonate platforms worldwide timing and evolution of dolomitization and the link to fractures and dikes have not been explored in detail previous dolomite observations are based on a stratigraphically limited portion of the platform this study extends observations to the complete exposed interval in which dolomite bodies occur including those within the less accessible valsorda valley numerous parallel mafic dikes crosscut the latemar platform and border several of its large dolomite bodies 50 m wide 100 m high within dikes and along dike carbonate contacts there are abundant dolomite veins that are geochemically related to surrounding dolomite bodies dolomitization is the result of limestone interaction with hydrothermal fluids delivered along these dikes at dike boundaries impermeable marble aureoles exist derived from contact metamorphism the marble aureoles have locally shielded surrounding limestone from dolomitizing fluid dolomite occurs only where the protective marble is missing or crosscut by fractures based on geometric relationships we conclude that dikes and their damage zones formed the pathways for the dolomitizing fluids and functioned as boundaries for dolomite bodies from field observations and petrography we established a detailed paragenesis dolomitization started shortly after dike emplacement there is an evolution in the fe content of matrix dolomite and dolomite veins from highly ferroan dolomite to non ferroan saddle dolomite alternating with episodes of silica cementation non ferroan calcite precipitation followed dolomitization possibly indicating concurrent depletion in mg this stage likely resulted in further limestone recrystallization rather than dolomitization stable and radiogenic isotopes suggest that the dolomitizing fluid comprised carnian seawater with elevated fe and mg from interaction with other lithologies possibly the nearby predazzo intrusion

synthesizing shallow seasonal snow covers a method of synthesizing a snowfield from point field measurements which is based on the fractal structure of the water equivalent is presented two steps are involved in the synthesis first the method of fractal sum of pulses is used to generate the spatial distribution of the snow water equivalent in an array second the synthetic snowfield is adjusted to take on the statistical properties of the natural snow cover the two parameter lognormal probability density function fitted to field data is used for this purpose examples of the application of the lognormal distribution for describing the water equivalent of snow covers on various landscapes encountered in a prairie environment are presented the geometrical properties fractal dimension s of snow patches that result from melting a synthetic snow cover agree closely with those of patches of an ablated natural snow cover

numerical models of fluid flow in carbonate platforms implications for dolomitization abstract the role of fluid flow in dolomitization is evaluated through hydrologic simulations designed to capture and test numerically the essential elements of some common dolomitization models a two dimensional finite difference code bethke 1985 was used to simulate single phase flow of variable salinity fluids driven by compaction convection and topography in carbonate platforms and adjacent basinal sediments a simple hydrostratigraphic model consisting of a platform reef complex surrounded by basin filling shale was constructed to examine the magnitude and direction of compaction flow during the initial phase of subsidence most pore fluids in the shale migrate to the surface at velocities less than 0 1 mm yr and basin derived fluids are focused into the reef from only a limited distance of 10 km relatively porous and permeable basinal limestones and sandstones may act as conduits for updip fluid flow into the platform reef complex magnesium mass balance constraints coupled with hydrologic simulations suggest that burial compaction is not an efficient mechanism for regional dolomitization two layer hydrostratigraphic models involving variable salinity fluids show that evaporation of seawater results in downward and seaward fluid migration the magnitude of flow is a function of fluid density even fluids at salinities marginally elevated above normal seawater values are capable of penetration several hundreds of meters into the underlying sediments albeit at rates of less than 1 cm yr paleohydrologic simulations coupled with thermodynamic arguments support that reflux of surface derived waters evaporated past gypsum saturation is a viable dolomitization mechanism calculations utilizing rates of mg mass flux show that dolomitization of large carbonate platforms 10 km3 by fluids below halite saturation requires millions of years of reflux driven flow p thermal convection of seawater through carbonate platforms has been simulated by varying the basal heat flow the thermal pump produces a convection cell in which lateral and vertical flow achieve rates of 75 0 and 13 0 mm yr respectively the potential for dolomitization is high because a large reservoir of mg seawater is tapped elevated temperatures drive fluids further into the stability field for dolomite and kinetic inhibition of dolomite nucleation and growth is decreased simulations of high frequency glacio eustatic sea level oscillations suggest that a dynamic hydrologic regime may be established a sea level fall of 2 m lasting for 10 000 yr can drive lateral flow at rates up to 0 15 m yr dolomitization is possible because marine waters migrate through the carbonate platform in response to gradients of hydraulic head created by eustatic sea level fluctuations

dolomitization by penesaline sea water in early jurassic peritidal platform carbonates gibraltar western mediterranean peritidal carbonates of the lower jurassic liassic gibraltar limestone formation which form the main mass of the rock of gibraltar are replaced by fine and medium crystalline dolomites replacement occurs as massive bedded or laminated dolomites in the lower 100 m of an 460 m thick platform succession the fine crystalline dolomite has d 18 o values either similar to or slightly higher than those expected from early jurassic marine dolomite and d 13 c values together with 87 sr 86 sr ratios that overlap with sea water values for that time indicating that the dolomitizing fluid was early jurassic sea water absence of massive evaporitic minerals and or evaporite solution collapse breccias in these carbonate rocks indicates that the salinity of sea water during dolomitization was below that of gypsum precipitation the occurrence of peritidal facies a restricted microbiota and rare gypsum pseudomorphs are also consistent with penesaline conditions salinity 72 199 the medium crystalline dolomite has some d 18 o and d 13 c values and 87 sr 86 sr ratios similar to those of early jurassic marine dolomites which indicates that ambient sea water was again a likely dolomitizing fluid however the spread of d 18 o d 13 ca nd 87 sr 86 sr values indicates that dolomitization occurred at slightly increased temperatures as a result of shallow 500 m burial or that dolomitization was multistage these data support the hypothesis that penesaline sea water can produce massive dolomitization in thick peritidal carbonates in the absence of evaporite precipitation taking earlier models into consideration it appears that replacement dolomites can be produced by sea water or modified sea water with a wide range of salinities normal penesaline to hypersaline provided that there is a driving mechanism for fluid migration the gibraltar dolomites confirm other reports of significant early jurassic dolomitization in the western tethys carbonate platforms

boundary layer growth over snow and soil patches field observations much of the snowmelt season is characterized by a patchy surface differential heating of the snow and snow free surfaces results in a significant horizontal transport of energy that affects and contributes to the snowmelt the calculation of the rate of energy advection requires some knowledge of the behaviour of the thermal boundary layer over the patches of snow and snow free surfaces we present the results from a series of field observations of the rate of growth of the thermal boundary layer over snow and snow free patches the results confirm that the boundary layer growth can be described by a power function of the distance from the leading edge of the patch for the case of the thermal boundary layer over a snow patch within a bare field the boundary layer growth is affected by the upwind surface roughness the thermal boundary layer over a snow patch within a rough field grows much more quickly than that in a smooth field relationships are derived and presented for the parameterization of the boundary layer growth as a function of distance and upwind surface roughness copyright 2006 crown in the right of canada published by john wiley sons ltd

petrologic and geochemical constraints on the origin of subsurface dolomite enewetak atoll an example of dolomitization by normal seawater dolomite is present 1 250 1 400 m below sea level in eocene strata of enewetak atoll petrographically the deep enewetak dolomite postdates brittle compaction of rigid grains in the host eocene strata the 87 sr 86 sr ratio of these dolomites 0 70865 0 70901 indicates that they formed at one or more times between the middle miocene and the present since the top of the lower miocene is more than 900 m above the deep dolomitic interval the dolomite must have formed at a minimum burial depth of 900 m stable oxygen isotope determinations suggest dolomite precipitation from cold marine water lower miocene and eocene carbonate strata on the atoll are apparently in open communication with cold modern ocean water suggesting that those same strata were in open communication with ocean water during dolomitization at a depth of about 1 000 m modern pacific ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite therefore it is proposed that the deep enewetak dolomite precipitated from cold deep ocean water undersaturated with respect to calcite at a burial depth of more than 900 m

origin of dolomite in the third member of feixianguan formation lower triassic in the jiannan area sichuan basin china abstract the third member of feixianguan formation jiannan area consists of marine carbonates parts of which were dolomitized to form high quality reservoirs based on petrographical features three types of dolomite phases are distinguished including very finely finely crystalline dolomite type d1 medium crystalline dolomite type d2 and coarse crystalline saddle dolomite type d3 type d1 dolomite has characteristics of micritic to fine crystals 18 o values imply partly diagenetic stabilitization during burial type d2 dolomite has characteristics of mainly medium 100 250 m subhedral crystals with zoned to uniform dull red cl the estimates from 18 o values suggest precipitation at depths of 760 1100 m values of 13 c and 87 sr 86 sr of type d2 are similar to values of host limestone suggesting that dolomitization was mediated by t 1 f 3 seawater like fluids type d3 is strictly associated with type d2 high homogenization temperatures and limited distributions indicate that type d3 formed in a deep closed environment dolostone made of type d1 dolomite is volumetrically minor and is characterized by micropores and low permeability while type d3 as a void filling cement slightly reduces the reservoir porosity high quality reservoirs are only intimately associated with dolostone made of type d2 a burial compaction model is proposed for type d2 mg 2 was derived from seawater derived compaction fluid that was expelled from the trough strata the potential high quality reservoirs are likely to occur in opposing margin shoals

reequilibration of fluid inclusions in low temperature calcium carbonate cement calcium carbonate cements precipitated in low temperature near surface vadose environments contain fluid inclusions of variable vapor to liquid ratios that yield variable homogenization temperatures cements precipitated in low temperature phreatic environments contain one phase all liquid fluid inclusions neomorphism of unstable calcium carbonate phases may cause reequilibration of fluid inclusions stable calcium carbonate cements of low temperature origin which have been deeply buried contain fluid inclusions of variable homogenization temperature and variable salt composition most inclusion fluids are not representative of the fluids present during cement growth and are more indicative of burial pore fluids therefore low temperature fluid inclusions probably reequilibrate with burial fluids during progressive burial reequilibration is likely caused by high internal pressures in inclusions which result in hydrofracturing the resulting fluid inclusions population could contain a nearly complete record of burial fluids in which a particular rock has been bathed

multiple episodes of dolomitization and dolomite recrystallization during shallow burial in upper jurassic shelf carbonates eastern swabian alb southern germany abstract the upper jurassic of the eastern swabian alb is composed of oolitic platform sands with associated microbe siliceous sponge mounds at the platform margins they are surrounded by argillaceous or calcareous mudstones and marl limestone alternations deposited in adjacent marl basins partial to complete dolomitization is predominantly confined to the mound facies six types of dolomite as well as one type of ankerite document a complex diagenetic history during shallow burial with multiple episodes of dolomite formation and recrystallization the earliest massive matrix dolomitization is ca rich has slightly depleted oxygen isotope values relative to late jurassic seawater and carbon isotopic values in equilibrium with late jurassic seawater this initial massive matrix dolomitization occurred during latest jurassic to earliest cretaceous and is related to pressure dissolution during very shallow burial at temperatures of at least 50 c hydrologic conditions and mass balance calculations indicate that burial compaction provided sufficient fluids for dolomitization mg is derived from negligibly modified seawater that was expelled from the adjacent off reef strata into the mound facies position of the mounds along the platform margins controlled the distribution of the shallow burial dolomite covariant trends between textural modification increasing stoichiometry partial changes in trace element content mn fe sr and depletion in stable isotopes as well as distinctive cl pattern illustrate two recrystallization phases of the precursor matrix dolomite during further burial at elevated temperatures strong sr enrichment of the second phase of recrystallized dolomite is ascribed to sr rich meteoric waters descending from overlying aragonite bearing reef limestones or evaporite bearing peritidal carbonates late stage coarsely crystalline dolomite cements occur as vug and fracture fillings and formed during burial ankerite associated with sulphide and sulphate minerals and saddle dolomite are assumed to have formed from hydrothermal waters that moved to higher stratigraphic levels along fracture conduit systems that developed during late cretaceous to tertiary alpine orogenesis

boundary layer growth and advection of heat over snow and soil patches modelling and parameterization essery rlh rj granger and jw pomeroy 2006 boundary layer growth and advection of heat over snow and soil patches modelling and parametrization hydrological processes 20 953 967

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

geochemical and sr nd pb isotopic compositions of cretaceous granitoids constraints on tectonic framework and crustal structure of the dabieshan ultrahigh pressure metamorphic belt china abstract the dabie orogenic belt can be divided into four tectonic and lithological units they are from north to south the north huaiyang nhy unit the northern dabie complex ndc unit the ultrahigh pressure uhp metamorphic unit and high pressure hp metamorphic unit cretaceous granitoids from the four tectonic units irrespective of tectonic unit and lithology show surprisingly similar major and trace element and particularly sr nd pb isotopic compositions indicating a similar crustal source for their magma genesis they have initial 87 sr 86 sr t values of 0 7068 0 7091 and nd depleted mantle model ages of 1 6 2 4 ga and e nd t values at the time of their emplacement 120 ma ranging from 13 to 26 their present day pb isotopic compositions are 206 pb 204 pb 15 62 17 06 207 pb 204 pb 15 21 15 44 and 208 pb 204 pb 36 59 37 81 with comparison to the sr nd pb isotopic compositions of the basement rocks in the dabie orogenic belt the isotopic compositions of the cretaceous granitoids indicate a source that is similar to the northern dabie complex but distinct from the ultrahigh pressure metamorphic unit this strongly suggests that the exposed northern dabie complex extends in the deep crust southward beneath the uhp hp metamorphic units and northward beneath the north huaiyang unit the northern dabie complex is best interpreted as the core of a dome within the dabie orogenic belt the uhp hp metamorphic unit is only a thin skinned slab confined in shallower crustal levels over the northern dabie complex present tectonic framework for the uhp hp terrain was formed by extensional tectonics and post the uhp hp metamorphism the northern dabie complex has an affinity to the yangtze craton the similar source for the cretaceous granitoids also suggests that the suture between the north china and the yangtze cratons is marked by the xiaotian mozitang fault separating the north huaiyang unit and the northern dabie complex rather than the previously thought wuhe shuiko fault which separates the northern dabie complex and the uhp unit

saddle dolomite as a by product of chemical compaction and thermochemical sulfate reduction petrographic stable and radiogenic isotope trace element and fluid inclusion data from the devonian nisku reef trend in the subsurface of alberta canada suggest that saddle dolomite was a by product of chemical compaction and thermochemical sulfate reduction in subsiding dolomitized rocks furthermore thermochemical sulfate reduction may be representative of other processes that increase the amount of saddle dolomite via an increase in carbonate alkalinity the data also confirm that saddle dolomite is most likely to form at elevated temperatures from hypersaline brines

rotation period of venus estimated from venus express virtis images and magellan altimetry abstract the 1 02 m wavelength thermal emission of the nightside of venus is strongly anti correlated to the elevation of the surface the virtis instrument on venus express has mapped this emission and therefore gives evidence for the orientation of venus between 2006 and 2008 the magellan mission provided a global altimetry data set recorded between 1990 and 1992 comparison of these two data sets reveals a deviation in longitude indicating that the rotation of the planet is not fully described by the orientation model recommended by the iau this deviation is sufficiently large to affect estimates of surface emissivity from infrared imaging a revised period of rotation of venus of 243 023 0 002 d aligns the two data sets this period of rotation agrees with pre magellan estimates but is significantly different from the commonly accepted value of 243 0185 0 0001 d estimated from magellan radar images it is possible that this discrepancy stems from a length of day variation with the value of 243 023 0 002 d representing the average of the rotation period over 16 years

the strontium isotopic composition of ordovician and silurian brachiopods and conodonts relationships to geological events and implications for coeval seawater utilized for strontium isotope measurements the results document a gradual decrease in 87 sr 86 sr from 0 7091 to 0 7087 from tremadoc to llandeilo a sharp decline to 0 7078 during the late llandeilo early caradoc little change during caradoc and the ashgil and a steady rise to 0 7087 through the silurian these long term 10 7 yr variations with magnitudes in the range of 10 23 are interpreted to be controlled primarily by continental collisional tectonics and its associated erosion and weathering the gradual decrease in 87 sr 86 sr ratio during the early ordovician may record the reduction in uplift and weathering rates due to waning of the pan african orogenies the rapid decline near the llandeilo caradoc boundary suggests a strong hydrothermal flux likely due to increased sea floor spreading and a possible superplume event the latter may have caused the prominent transgressive phase the largest in the phanerozoic which would have muted continental flux input although the caradoc was the main interval for the taconic orogeny its impact on the sr continental flux may have been delayed until the early llandovery this effect complemented by reworking of glacial deposits near the ordovician silurian boundary and enhanced by phases of the silurian salinic orogeny may have combined to give the progressive increase in the strontium isotope ratio through the silurian the scale and directionality of these changes makes the strontium isotope curve valuable for dating and correlation purposes copyright 1998 elsevier science ltd

recrystallization versus neomorphism and the concept of significant recrystallization in dolomite research abstract the term neomorphism originally introduced in 1965 as a substitute for the term recrystallization in limestones is now in use also for dolostones however only very few dolomite researches use the term neomorphism even though it is well and strictly defined many do not know what this term means and using the term commonly leads to confusion among carbonate researchers as well as between them and other geologists partially or largely for these reasons most dolomite researchers use the term recrystallization rather than neomorphism in addition the term neomorphism does not encompass all the important characteristics of recrystallization known today it is recommended therefore to abandon the term neomorphism in dolomite research and apply the term recrystallization with an extended definition that includes those characteristics that commonly are involved in recrystallization and that can be measured in dolomites and dolostones textural changes size shape structural changes ordering strain compositional changes stoichiometry isotopes trace elements including zoning fluid inclusions and changes in the paleomagnetic properties for practical applications the absence presence and or degree of recrystallization is often important for a genetic interpretation of dolostones if changes via recrystallization in texture structure composition and or paleomagnetic properties results in data ranges that are larger than the original ones which must be known for reference a dolomite dolostone is said to be significantly recrystallized and the reset properties are no longer representative of the fluid and environment of dolomitization but characterize the last event of recrystallization however not all measurable properties must be reset during recrystallization in this case inherited properties represent the event of dolomitization whereas reset properties represent the fluids and process of recrystallization in recrystallization resets only one of 10 properties e g 87 sr 86 sr ratios beyond the range of the pristine samples within the analytical errors this dolomite is significantly recrystallized with respect to sr isotopes yet insignificantly recrystallized with respect to the other nine properties in either case values of those properties that are identical to the pristine reference values can be used for genetic interpretations of dolomitization the concept of significant recrystallization is of great use in genetic interpretations of dolomites dolostones in particular a reliable interpretation of the chemistry of the dolomitizing fluids no longer depends on an absolute recognition of recrystallization rather it is sufficient to recognize that a dolomite is insignificantly recrystallized application of this principle also shows that many ancient dolomites and dolostones are insignificantly recrystallized finally the concept of significant recrystallization is independent of mineralogy hence it can and should be applied also to limestones

sensible heat flux and local advection over a heterogeneous landscape at an arctic tundra site during snowmelt during snowmelt over a continuous snow cover the vertical turbulen t exchanges of sensible and latent energy arc influenced by reo ional ai r mass chara cteris tics which exert a strong conlrol on air temperature in hig b h iatitude sites the melting surface rapidly becomes heterogeneous with patches of snow and snow free areas local advection occurs when near surface air layers are warmed due to sensible heat flux from the snow free areas with the resulting heat transferred horizontally to adjacent snow patches this advection greatly increases the rate of snow melt along the leading edges of the snowpat ches in order to estimate correctly the average melt rates of the snowpatches and the bulk energy balancc of the entire landscape it is necessary to estimate the local advection component to date few studies have dealt with this problem this paper reports results from an arctic tundra site located approximately 55 km northeast of inuvik northwest 1erritories canada the importance of local ad cction is estimated by comparing the sensible heat flux of the snowpatches to estimates of sensible heat with out local advection this latter term is derived from a relationship between upper air temp erature and sensible heat flux over a continuous snow cover this work has imponalll lmpllcalions for de eloplng models that correctly represent the cryosphere of tundra regions and in developing appropriate scaling techniques for heterogeneous landscapes

processes involved in the origin and accumulation of hydrocarbon gases in the yuanba gas field sichuan basin southwest china abstract natural gases in the superimposed sichuan basin commonly experienced a history of remigration in marine carbonate reservoirs since the late cretaceous the reservoir in the changxing formation p 2 c in the yuanba gas field in the sichuan basin is characterized by a great burial depth of 6200 7000 m and a high temperature about 165 c the gas dryness is 99 73 99 99 and 13 c values of methane and ethane are 31 0 to 28 9 and 29 9 to 25 6 respectively the chemical and isotopic compositions of natural gases abundant reservoir solid bitumen and high reservoir temperature maximum to 240 c indicate that the p 2 c gases are of sapropelic origin and are derived from oil cracking the paleo oil layers recognized by solid bitumen distribution were mainly developed in high position traps when the paleo oil accumulated during the early jurassic reconstructed structural evolution shows the northwest was uplifted sharply and southern part dipped gently to the north in the gas field after oil cracking fluid potential analyses based on changes in the structural configuration imply that gas should re migrate mainly to the northwest the observations that paleo oil water contacts are mainly above the present day gas water contacts in the northwest traps and are below present day gas water contacts in the middle and eastern traps also confirm the gas remigration trend currently high gas production wells are mainly located in northwest traps and in high positions in the middle and eastern traps systematic analyses on early paleo oil accumulation and late gas remigration processes can reduce the economic risks associated with natural gas exploration in the northeastern sichuan basin

fluid inclusions in sedimentary and diagenetic systems abstract some of the major problems in sedimentary geology can be solved by using fluid inclusions in sedimentary and diagenetic minerals important fluids in the sedimentary realm include atmospheric gases fresh water of meteoric origin lake water seawater mixed water evaporated water formation waters deep in basins oil and natural gas preserving a record of the distribution and composition of these fluids from the past should contribute significantly to studies of paleoclimate and global change research is essential for improving understanding of diagenetic systems and provides useful information in petroleum geology applications of fluid inclusions to sedimentary systems are not without their complexities some fluid inclusions exposed to natural conditions of increasing temperature may be altered by thermal reequilibration which results in stretching or leakage and refilling of some fluid inclusions similarly overheating in the laboratory can also cause reequilibration of fluid inclusions so fluid inclusions from the sedimentary realm must be handled carefully and protected from overheating natural overheating of fluid inclusions must be evaluated through analysis of the most finely discriminated events of fluid inclusion entrapment fluid inclusion assemblages fia consistency in homogenization temperatures within a fluid inclusion assemblage consisting of variably sized and shaped inclusions is the hallmark of a data set that has not been altered through thermal reequilibration in contrast fluid inclusion assemblages yielding variable data may have been altered through thermal reequilibration if a fluid inclusion assemblage has not been altered by thermal reequilibration its fluid inclusions may be useful as geothermometers for low and high temperature systems or useful as geobarometers applicable throughout the sedimentary realm if a fluid inclusion assemblage has been altered partially by thermal reequilibration techniques for distinguishing between altered and unaltered fluid inclusions may be applied in studies of global change fluid inclusions can be used as sensitive indicators of paleotemperature of surface environments fluid inclusions also preserve microsamples of ancient seawater and atmosphere the analysis of which could figure prominently into discussions of past changes in chemistry of the atmosphere and oceans in petroleum geology fluid inclusions have proven to be useful indicators of migration pathways of hydrocarbons they can delineate the evolution of the chemistry of hydrocarbons and they remain important in understanding the thermal history of basins and relating fluid migration events to evolution of reservoir systems in studies of diagenesis fluid inclusions can be the most definitive record most diagenetic systems are closely linked to temperature and salinity of the fluid thus fluid inclusions are sensitive indicators of diagenetic environments

impact of sedimentology diagenesis and solid bitumen on the development of a tight gas grainstone reservoir in the feixianguan formation jiannan area china implications for gas exploration in tight carbonate reservoirs abstract tight gas grainstone reservoirs in the third member of the feixianguan formation jiannan area evolved from a paleo oil accumulation as evidenced from abundant solid reservoir bitumen porosity evolution of the grainstones was studied by evaluating relative influences of sedimentology diagenesis and solid bitumen formed during cracking of accumulated oils grainstones exhibited regional distinct effectiveness for paleo oil and present gas accumulations during oil window and subsequent gas window diagenesis in the southern zone where grainstones were not subjected to subaerial exposure and meteoric diagenesis in the early diagenetic stage paleoporosity at the time of oil charge was mainly controlled by sedimentologic factors e g grain size sorting and grain type and paleo oil reservoirs only occurred in the ooid dominated grainstones with good sorting and coarse grain size in contrast in the northern zone meteoric diagenesis was responsible for paleoporosity preservation due to the early mineral stabilization of grains and meteoric calcite cementation which caused grainstones greater resistance to compaction hence most of the grainstones in the northern zone regardless of textural variables formed effective reservoirs for paleo oil accumulation as the oil cracked to gas with increasing depth and temperature during the late oil window and initial gas window solid bitumen occluded reservoir pores to varying degrees and caused paleo oil reservoirs to be significantly heterogeneous or completely ineffective for gas accumulation in contrast most grainstones that were once ineffective oil reservoirs transformed into effective gas reservoirs due to no or minor influence of solid bitumen precipitation the model of reservoir transformation development of tight grainstones provides a plausible explanation for key observations concerning the diagenetic and distribution differences between paleo oil and present gas reservoirs it is useful in predicting the distribution of potential reservoirs in carbonate strata in future exploration

the fate of co2 derived from thermochemical sulfate reduction tsr and effect of tsr on carbonate porosity and permeability sichuan basin china abstract this article discusses the role of methane in thermochemical sulfate reduction tsr the fate of tsr derived co2 and the effect of tsr on reservoir porosity and permeability and the causes of the anomalously high porosity and permeability in the lower triassic soured carbonate gas reservoirs in the northeast sichuan basin southwest china the lower triassic carbonate reservoirs were buried to a depth of about 7000 m and experienced maximum temperatures up to 220 c before having been uplifted to the present day depths of 4800 to 5500 m but they still possess porosities up to 28 9 and permeabilities up to 3360 md the present day dry gas reservoirs evolved from a paleo oil accumulation and experienced varying degrees of tsr alteration as evidenced from the abundant sulfur rich solid bitumens and varying h2s and co2 concentrations tsr occurred mainly within the oil and condensate wet gas windows with liquid hydrocarbons and wet hydrocarbon gases acting as the dominant reducing agents responsible for sulfate reduction sulfur rich solid bitumen and h2s generation and calcite precipitation methane dominated tsr was a rather late event and had played a less significant role in altering the reservoirs intensive h2s and co2 generation during tsr resulted in calcite cementation rather than carbonate dissolution which implies that the amount of water generated during tsr was volumetrically insignificant 13c depleted co2 derived from hydrocarbon oxidation preferentially reacted with ca2 to form isotopically light calcite cements and the remaining co2 re equilibrated with the 13c enriched water rock systems with its 13c rapidly approaching the values for the host rocks which accounted for the observed heavy and relatively constant co2 13c values the carbonate reservoirs suffered from differential porosity loss by tsr involved solid bitumen generation and tsr induced calcite and pyrite precipitation intensive tsr significantly reduced the porosity and permeability of the intervals expected to have relatively high sulfate contents the evaporative platform dolostones and the platform margin shoal dolostones immediately underlying the evaporative facies early oil charge and limited intensity of tsr alteration together with very low phyllosilicate content and early dolomitization accounted for the preservation of anomalously high porosities in the reservoirs above the paleo oil water contact a closed system seems to have played a special role in preserving the high porosity in the gas zone reservoirs below the paleo oil water contact the closed system which is unfavorable for deep burial carbonate dissolution and secondary porosity generation was favorable for the preservation of early formed porosity in deeply buried carbonates especially sucrosic and vuggy dolostones have a high potential to preserve such porosity

boundary layer integration approach to advection of sensible heat to a patchy snow cover in the calculation of the melting of a patchy snow cover the energy advected from the adjacent bare soil to the snow surface is an important consideration the quantity or rate of energy advected depends on the fetches and sizes of snow and bare ground patches any successful method to estimate advection will necessarily require the incorporation of relationships describing the same complex boundary layer methods require detailed spatial knowledge of the patch sizes wind direction and fetch distances and are computationally intensive a physically based approach that can be spatially applied using distributions of snow patch geometry is required this paper presents a new approach in which boundary layer integration is used to provide a means of calculating the amount of energy removed by the snow patch surface as warmer air moves over it the method is reduced to a simple parametric form and the relationships describing the coefficients required for its application are developed the applicability of this new approach is discussed in light of the fractal nature of snow patches the relationship between the individual patch length and area and the distribution of patch sizes as they develop and disappear on the landscape

dolomitization of the lower ordovician catoche formation implications for hydrocarbon exploration in western newfoundland the lower ordovician st george group in western newfoundland consists of a sequence of subtidal and peritidal carbonates which are extensively dolomitized the current study investigates the diagenetic evolution of the catoche formation from the port aux choix and port au port peninsulas in order to study the controls on reservoir quality in western newfoundland the catoche formation dolomites are classified into three main generations early and pervasive replacement dolomite d1 indicates that dolomitization began during early stages of diagenesis stable isotope and trace element data indicate significant variations between d1 dolomite on the port aux choix and port au port peninsulas the depleted 18o signature of d1 dolomite fluids 8 7 1 3 vpbd on the port aux choix peninsula is consistent with partial dolomitization associated with mixing of seawater and meteoric waters on the flanks of structural highs in contrast 18o values 6 1 0 7 vpbd and trace element data from the port au port peninsula indicate that pervasive d1 was associated with mixing of possibly post evaporitic brines with meteoric waters later stage replacement dolomites d2 are associated with enhancement in porosity through the development of intercrystalline pores while latest stage saddle dolomite d3 significantly occluded the pores in some horizons d2 dolomite formed due to the influx of warm 100 c saline 15 eq wt nacl fluids intercrystalline porosity in d2 formed due to the dolomitization of precursor calcite due to the lower molar volume of dolomite compared to calcite therefore porosity development is lower on the port au port peninsula with no significant volume change during the recrystallization of the pervasive early d1 dolomicrite similarly extensive porous horizons on the port aux choix peninsula are related to the limited extent of d1 dolomitization this suggests that the quality of a potential dolomite reservoir is strongly controlled by tectonic and diagenetic history of host carbonates

contrasting geochemical and sm nd isotopic compositions of archean metasediments from the kongling high grade terrain of the yangtze craton evidence for cratonic evolution and redistribution of ree during crustal anatexis abstract twenty three clastic metasediments from the kongling high grade terrain of the yangtze craton south china were analyzed for major trace and rare earth elements and sm nd isotopic ratios associated dioritic tonalitic trondhjemitic dtt and granitic gneisses as well as amphibolites were also analyzed in order to constrain provenance the results show that the clastic metasediments can be classified into 3 distinct groups in terms of mineralogical geochemical and sm nd isotopic compositions group a is characterized by having no to slight negative eu anomalies eu eu 0 82 1 07 being high in cr 191 396 ppm and ni 68 137 ppm and low in th 3 3 7 8 ppm and ree ree 99 156 ppm these characteristics are similar to those of metasediments from archean greenstone belts in addition the group a metasediments have the value of the chemical index of alteration ciw close to felsic gneisses their sm nd isotopic ree and trace element compositions can be interpreted by mixtures of the dtt gneisses and amphibolites dating of detrital zircons from 2 group a samples by shrimp reveals a major concordant age group of 2 87 3 0 ga which is identical to the age of the trondhjemitic gneiss these results strongly suggest that group a was principally the first cycle erosion product of the local kongling dtt gneiss and amphibolite moreover the higher than amphibolite cr content and slight eu depletion exhibited by some samples from this group infer that ultramafic rocks like komatiite and granite of probably 3 0 3 3 ga in age also played a role group b is characterized by the presence of graphite and shows a more evolved composition similar to post archean shales with a prominent negative eu anomaly eu eu 0 48 0 77 and high ciw on paired cr th vs la co and co th plots group b samples conform to a two end member mixing line of the kongling granitic gneiss and amphibolite however data on nd model age and ciw suggest that the granite component should be younger than the sampled granitic gneiss and derived from a distal source both groups a and b exhibit a clear positive correlation between ciw and tdm and a negative one between ciw and eu eu these correlations point to the crustal evolution of the yangtze craton towards coupled increasing ciw and eu depletion with decreasing age this in turn reflects the change of granitoid magmatism from local na rich dioritic tonalitic trondhjemitic rocks to widespread k feldspar granite the change led to the intracrustal differentiation stabilization and growth of the craton group c is restite and contains abundant sillimanite and garnet and unusually high ilmenite 7 11vol which can be seen to be dehydration melting products of biotite under the microscope this group shows extremely varied ree distributions from lree enriched to depleted and from negative to strong positive eu eu 1 63 eu anomalies compared to groups a and b group c is severely depleted in na2o k2o lree rb and ba whereas tio2 co v sc and hree and y are considerably enriched this is accompanied by anomalous high sm nd 0 21 0 28 147sm 144nd 0 1361 0 1738 and 143nd 144nd 0 511589 0 511958 ratios tdm correlates clearly with sm nd ratio and 2 out of 3 samples give significantly older to unrealistic tdm 3 9 4 9 ga the results document redistribution of ree and an open behavior of the sm nd isotope system during the biotite dehydration melting of metasediments

new distribution coefficient for the incorporation of strontium into dolomite and its implications for the formation of ancient dolomites late tertiary dolomites form the bahamas exhibit a strong inverse correlation between sr and mol mgco sub 3 it is proposed that this relation is a result of a distribution coefficient dependent on the concentration of ca in dolomite because these dolomites precipitated from water with a sr sup 2 ca sup 2 ratio similar to that of seawater a distribution coefficient d sub sr sup dolomite of 0 0118 is calculated for sr in stoichiometric dolomite this coefficient increases by about 0 0039 for every additional mol caco sub 3 in nonstoichiometric dolomite these data have three important implication first the new d sub sr sup dolomite is substantially lower than previously suggested second a relation has been established among the sr sup 2 ca sup 2 ratio of dolomite forming waters the major element composition of dolomites and the sr content of dolomites allowing the determination of the sr sup 2 ca sup 2 ratio of fluids involved in the formation of ancient dolomites third some published data sets on dolomites show a similar relation between sr content and major element composition suggesting that these dolomites may also have formed from seawater other data sets deviate from the correlation defined by little bahama bank dolomites indicating either dolomitization in fluids other than seawater or changes ofmore the sr sup 2 ca sup 2 ratio of seawater through time less