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

volcanic investigations of the puna ridge hawai i relations of lava flow morphologies and underlying slopes abstract previous investigators have demonstrated that submarine lava flow morphologies are the result of a complex interplay between lava rheology effusion rate and underlying slope here we specifically investigate the relation between underlying slope and submarine lava flow morphology along three study sites of the puna ridge kilauea volcano hawai i using electronic still camera esc images and detailed bathymetry collected by argo ii we can compare the lava morphology and the associated underlying slope our results indicate that rubble is most common on the steepest slopes 25 and that pillowed and lobate flows dominate on slopes 20 sheet flows were only seen on relatively shallow slopes 15 these observations disagree with the results obtained from previous laboratory analog experiments suggesting that the laboratory results are directly applicable for lavas emplaced on relatively low

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

the puguang gas field new giant discovery in the mature sichuan basin southwest china the giant puguang gas field with a proven original in place gas volume of 350 109 m3 12 36 tcfg was discovered in 2003 in the eastern sichuan fold thrust belt of the mature sichuan basin southwest china the field is a combination structural stratigraphic trap closed by lateral depositional change and fault closure the trap evolved from a paleo oil reservoir originating in the triassic jurassic the entrapment of thermal gas which was derived from lower middle silurian and permian source rocks occurred during deep burial in the jurassic cretaceous tertiary quaternary compression transformed the paleotrap into the present gas reservoir gas is contained in the lower triassic feixianguan and the upper permian changxing reservoirs which consist predominantly of dolomitized oolites deposited in shelf and platform margin shoal and backreef environments reservoir quality is characterized by porosity of 1 29 and permeability of 0 01 9664 md with buried depth greater than 5000 m 16 400 ft the discovery of the puguang field exemplifies the successful application of a new play concept and new technology in a mature basin the discovery resulted from a shift in exploration strategy from structures to stratigraphic traps in reef and shoal dolomites and benefited from advanced high resolution seismic techniques the discovery will not only broaden the exploration scope in the sichuan basin but also provide an excellent analog for exploration in other fold thrust belts worldwide

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

holocene depositional history of the burdekin river delta of northeastern australia a model for a low accommodation highstand delta abstract the burdekin river of northeastern australia has constructed a substantial delta during the holocene delta plain area 1260 km2 the vertical succession through this delta comprises 1 a basal coarse grained transgressive lag overlying a continental omission surface overlain by 2 a mud interval deposited as the coastal region was inundated by the postglacially rising sea in turn overlain by 3 a generally sharp based sand unit deposited principally in channel and mouth bar environments with lesser volumes of floodplain and coastal facies the holocene burdekin delta was constructed as a series of at least thirteen discrete delta lobes formed as the river avulsed each lobe consists of a composite sand body typically 5 8 m thick the oldest lobes formed during the latter stages of the postglacial sea level rise 10 5 5 kyr bp are larger than those formed during the highstand 5 5 3 kyr bp which are in turn larger than those formed during the most recent slight sea level lowering and stillstand 3 0 kyr bp radiocarbon ages and other stratigraphic data indicate that inter avulsion period has decreased through time coincident with the decrease in delta lobe area the primary control on holocene delta architecture appears to have been a change from a pluvial climate known to characterize the region 12 4 kyr bp to the present drier enso dominated climate in addition to decreasing the sediment supply via lower rates of chemical weathering this change may have contributed to the shorter avulsion period by facilitating extreme variability of discharge more frequent avulsion may also have been facilitated by the lengthening of the delta plain channels as the system prograded seaward

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

use of strontium isotopes to constrain the timing and mode of dolomitization of upper cenozoic sediments in a core from san salvador bahamas the sup 87 sup 86 sr ratios and the activity ratios of sup 234 u sup 238 u and sup 230 th sup 238 u have been measured in dolomites from a 168 m deep core taken on the island of san salvador bahamas these data suggest two periods of dolomitization the first episode dolomitized miocene age sediments during the latest miocene and the second dolomitized the pliocene portion of the core and was still active as recently as 150 ka the late timing of the second episode argues against penecontemporaneous models of dolomitization for the pliocene sediments instead dolomitization is favored either as a result of mixing zone development during the large pleistocene sea level changes or by movement of seawater through the platform

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

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

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

large perturbations of the carbon cycle during recovery from the end permian extinction high resolution carbon isotope measurements of multiple stratigraphic sections in south china demonstrate that the pronounced carbon isotopic excursion at the permian triassic boundary was not an isolated event but the first in a series of large fluctuations that continued throughout the early triassic before ending abruptly early in the middle triassic the unusual behavior of the carbon cycle coincides with the delayed recovery from end permian extinction recorded by fossils suggesting a direct relationship between earth system function and biological rediversification in the aftermath of earth s most devastating mass extinction

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

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

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

origin and petrophysics of dolostone pore space abstract the common claim that dolomitization creates 12 porosity is based on the mole for mole replacement equation however in the past 50 years data have been collected demonstrating that dolomitization does not create porosity instead dolostones inherit the porosity and fabric of the precursor limestone and porosity is reduced by overdolomitization the porosity of the precursor limestone depends on the diagenetic history up to the time of dolomitization data show that 1 carbonates are born with high porosity and lose porosity gradually over a long period and 2 mud dominated fabrics compact more readily than grain dominated fabrics the problem of estimating the time of dolomitization is minimized by confining observations to young limestones and associated dolostones limited data from holocene dolomitic sediments suggest no change in porosity with dolomitization study of plio pleistocene carbonates in bonaire netherlands antilles demonstrates that precursor limestones are more porous than dolostones limestones average 25 porosity whereas dolostones average 11 porosity data from the neogene of the great bahama bank show that dolostones and adjacent limestones both have 40 porosity porosity studies of the jurassic arab d formation show that dolostones and associated grain dominated limestones have similar porosity ranges and that the decrease in porosity with increasing dolomitization results from compaction of the mud dominated fabrics these data suggest that porosity in dolostone is not created by a mole for mole replacement mechanism instead dolostone porosity is 1 inherited from the precursor limestone and 2 occluded by the process of overdolomitization palaeozoic dolostones however are commonly more porous than juxtaposed limestones the explanation for this observation is that limestones lose porosity through compaction and cementation whereas dolostones resist compaction and retain much of their porosity permeability studies have demonstrated that dolomitization of grain dominated limestones usually does not change porosity permeability relationships instead precursor limestone fabric controls pore size distribution the dolomite crystal size of a muddominated dolostone may however be larger than the carbonate mud size improving the porosity permeability relationship substantially hence there is a predictable relationship between interparticle grains or crystals porosity permeability precursor grain size and dolomite crystal size

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

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

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

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

pervasive subsurface dolomitization of the nisku formation in central alberta abstract the nisku formation in the subsurface of central alberta canada contains pervasive dolomites that occur mainly as a replacement of reefal limestones the degree of pervasive dolomitization increases downdip along a structural homocline from about 25 to 100 vol and associated replacive anhydrites increase from about 5 to 20 vol pervasive dolomites occur as two types one type consists of almost non porous and impermeable mosaics of cloudy crystals that have diameters of 5 300 microns the other type has significant intercrystalline porosity and permeability and consists of cloudy and clear crystals ranging in size from 100 500 microns cathodoluminescence zonation is rare but modulated structures observed with tem are common both types of dolomite have strong xrd ordering refl ctions and are nearly stoichiometric sr concentrations 25 125 ppm increase and mn concentrations 20 400 ppm decrease upwards in several cores the boron contents of both dolomite types 20 50 ppm are enriched relative to other diagenetic phases 18 ppm the sr isotope ratios vary from 0 7089 0 7107 and are enriched relative to devonian and younger seawater oxygen and carbon isotope ratios cluster around 5 8 permil pdb and 2 0 permil pdb respectively and are similar to those of remnant calcites from the combination of stratigraphic petrographic petrophysical and geochemical data it is inferred that both types of matrix dolomite formed contemporaneously from the same fluids in a flow regime with predominantly upward fluid movement at temperatures between about 40 50 c and at burial depths of about 300 1 000 m most anhydrites formed originally as gypsum during the advanced stages and after matrix dolomitization and later recrystallized to the anhydrites now present based on all accumulated data and on mass balance calculations at least two types of subsurface flow could have caused pervasive dolomitization and gypsification anhydritization 1 expulsion of burial compaction water i e chemically modified seawater with or without additional contributions of m bearing formation waters from deeper parts of the basin e g middle devonian brines and 2 thermal convection of formation fluids originally underlying and or overlying the nisku

evidence against the dorag mixing zone model for dolomitization along the wisconsin arch a case for hydrothermal diagenesis ordovician carbonates near the wisconsin arch represent the type locality in ancient rocks for the dorag or mixing zone model for dolomitization field petrographic and geochemical evidence suggests a genetic link between the pervasive dolomite trace mississippi valley type mvt minerals and potassium k silicate minerals in these rocks which preserve a regional hydrothermal signature constraints were placed on the conditions of water rock interaction using fluid inclusion methods cathodoluminescence and plane light petrography stable isotopic analyses and organic maturity data homogenization temperatures of two phase aqueous fluid inclusions in dolomite sphalerite and quartz range between 65 and 120c freezing data suggest a na ca mg cl h2o fluid with salinities between 13 and 28 wt nacl equivalent the pervasive dolomitization of paleozoic rocks on and adjacent to the wisconsin arch was the result of water rock interaction with dense brines at elevated temperatures and it was coeval with regional trace mvt mineralization and k silicate diagenesis a reevaluation of the dorag mixing zone model for dolomitization in conjunction with convincing new petrographic and geochemical evidence has ruled out the dorag model as the process responsible for pervasive dolomitization along the wisconsin arch and adds to the abundant body of literature that casts serious doubt about the viability of the dorag model in general

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

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

diagenesis of the devonian southesk cairn carbonate complex alberta canada marine cementation burial dolomitization thermochemical sulfate reduction anhydritization and squeegee fluid flow abstract one of the largest geological features in the deep part of the alberta basin in western canada is the middle to upper devonian southesk cairn carbonate complex sccc the sccc is located at depths of about 5000 m adjacent to the disturbed belt of the rocky mountains rising to depths of 3000 m in a northeastward direction over a distance of 150 km the sccc produces oil and gas with up to about 30 h2s from several stratigraphic levels the diagenesis of the sccc was investigated by means of petrography and geochemistry stable isotope compositions were determined for fibrous calcite cements matrix dolomite saddle dolomite and sparry calcite fibrous calcites have stable isotope values of 18o 7 5 to 4 5 pdb and 13c 2 0 1 0 pdb matrix dolomite samples have stable isotope values of 18o 8 1 to 4 9 pdb and 13c 0 to 1 0 pdb saddle dolomite samples range from 18o 9 8 to 7 1 pdb and 13c 1 0 to 0 pdb sparry calcite samples have 18o 13 7 to 7 2 pdb and 13c 24 0 to 0 pdb the sr isotope values of all analyzed samples span a broad range from 0 7080 to 0 7320 showing considerable scatter but also one distinctive pattern i e values higher than 0 7220 occur only at depths in excess of about 4000 m whereas lower values scatter throughout the entire depth range furthermore the sr isotope values of sparry calcites decrease northeastward similarly the fluid inclusion homogenization temperatures of these calcites display a general decrease updip whereas the salinities show no general spatial trends the uncorrected homogenization temperatures range from about 85 to 180 c and the nacl equivalent salinities range from about 17 to 24 weight the d values of brines obtained from seven wells range from 79 to 20 smow and their 18o values range from 1 5 to 8 7 smow the sulfur isotope values of the dissolved sulfate ranges from 34s 10 1 to 24 5 cdt the sr isotope values of the brine samples range from 0 7109 to 0 7255 elemental sulfur isotope values cluster tightly around 34s 18 0 to 18 4 cdt the values of secondary anhydrites range from 34s 18 4 to 26 7 cdt values for solid bitumen vary from 34s 19 5 to 20 2 cdt integration of all data reveals a complex diagenetic evolution from the middle devonian to the recent the paragenetic sequence consists of 24 distinct diagenetic phases which have been grouped into 5 diagenetic stages stage i density driven convection in near surface settings likely superimposed on and or aided by tidal pumping facilitating marine cementation stage ii cellular convection facilitating pervasive matrix dolomitization aided by compaction stage iii hydrocarbon migration stage iv thermochemical sulfate reduction forming sour gas and regional squeegee flow forming sparry calcite cements and localized flow through faults stage v fluid flow driven by erosional rebound in the overlying mesozoic strata and remnant squeegee flow this study yielded several important results 1 the prolific repertoire of diagenetic textures found in the southesk cairn carbonate complex serves as a means of comparison for petroliferous carbonate complexes elsewhere especially in sour gas provinces close to orogenic fronts 2 the diagenetic evolution contains long periods of relative tranquility that were punctuated by several episodes of intense diagenetic activity 3 both thermochemical sulfate reduction and squeegee type fluid flow are recorded in late diagenetic sparry calcite cements 4 the effect of the various burial diagenetic episodes on the hydrocarbon reservoir properties throughout the entire reef complex was highly variable specifically matrix dolomitization thermochemical sulfate reduction and the emplacement of secondary and tertiary anhydrites affected the hydrocarbon reservoir properties throughout the entire reef complex on the other hand squeegee fluid flow was insignificant in terms of affecting the reservoir properties

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

dolomitization of gas reservoirs the upper permian changxing and lower triassic feixianguan formations northeast sichuan basin china abstract the lower triassic feixianguan formation and to a lesser extent the upper permian changxing formation contain more than ten giant recently discovered dolomite gas fields on either side of a paleo embayment kaijiang liangping bay sichuan basin china this study documents the origin of the dolomite using a combination of petrology fluid inclusion trace element geochemistry and stable isotope and radiogenic isotope analysis of samples from outcrop and core samples from producing gas fields three stages of dolomitization are revealed dolomicrite from the lagoon facies in the feixianguan formation and minor fabric retentive dolomite from reef facies in the changxing formation on the northeast side of kaijiang liangping bay represent the initial dolomite phase caused by reflux of feixianguan brine with salinity close to gypsum saturation 18 wt salinity during the first stage of diagenesis at 30 c the second and dominant dolomite phase fabric retentive dolomite in both the feixianguan and changxing formations on the both sides of kaijiang liangping bay is the result of reflux of diagenetic fluids with salinity ranging from normal feixianguan seawater 3 5 salinity up to mesohaline water 6 5 salinity during early burial diagenesis at about 35 40 c as shown by the dominance of single phase aqueous inclusions shallow formed stylolites that cut the second phase of dolomite and the early triassic marine carbon oxygen and strontium isotope and rare earth element and yttrium ree y signals the subordinate third dolomite phase fabric obliterative dolomite and dolomite cement was due to burial diagenetic recrystallization between 80 and 140 c in the feixianguan and changxing formations on both sides of kaijiang liangping bay the third phase of dolomitization involved water expelled from the slightly younger lower triassic jialingjiang formation marine evaporites and carbonates on the northeast side of kaijiang liangping bay revealed by the high salinity of the aqueous inclusions that homogenize at 80 to 140 c and elevated strontium isotope data the carbon isotope and ree y signals of the third phase of dolomite are similar to earlier dolomite cements confirming the lack of exotic input future exploration should be focused on dolomitized high energy sediments e g reef and oolitic shoal and bank in similar triassic tethyan paleogeographic settings that are associated with restricted facies but underlying dolomitized high energy permian strata may also be good exploration targets

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