the effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs abstract thermochemical sulfate reduction tsr is a well known process that can lead to sour h2s rich petroleum accumulations most studies of tsr have concentrated upon gas chemistry in this study we have investigated palaeoformation water characteristics in a deep anhydrite bearing dolomite sour gas reservoir of permian age in abu dhabi using fluid inclusion stable isotope petrographic and gas chemical data the data show that low salinity isotopically distinct water was generated within the reservoir by reaction between anhydrite and methane the amount of water added to the reservoir from tsr indicated by reduced fluid inclusion salinity and water 18o values varied systematically with the extent of anhydrite reaction with methane water salinity and isotope data show that the original formation water was diluted by between four and five times by water from tsr thus we have shown that large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement the salinity of formation water in evaporite lithologies is therefore not necessarily high modelling based upon a typical khuff gas reservoir rock volume suggests that initial formation water volumes can only be increased by about three times as a result of tsr the extreme local dilution shown by the water salinity and 18o data must therefore reflect transiently imperfect mixing between tsr water and original formation water the creation of large volumes of water has important implications for the mechanism and rate of thermochemical sulphate reduction and the interpretation of gas volumes using petrophysical logging tools

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 geochemistry of the trotternish sills isle of skye crustal contamination in the british tertiary volcanic province geochemical and isotopic data are presented for a suite of palaeocene picrites picrodolerites and crinanites from the trotternish sills northern skye the whole rock geochemistry of the trotternish sills is comparable with the overlying skye main lava series although the sills are relatively mg rich up to 30 wt mgo due to abundant cumulus olivine rare earth element patterns exhibit overall lree enrichment relative to chondrite and suggest an alkali basalt parental magma incompatible element patterns show ba and k enrichment relative to nb th and u sr nd and pb isotope ratios indicate that crinanites which are the most fractionated magmas are the most contaminated isotope ratios suggest up to 22 amphibolite facies lewisian gneiss may have been assimilated during fractionation in the upper crust prior to multi phase intrusion as sills this is in contrast to the skye main lava series which show evidence of crustal contamination by granulite facies material in the lower crust but is comparable with the skye preshal mhor basalts and the rhum intrusion the trotternish sills and the overlying plateau lavas therefore have a more complicated genetic relationship than was previously supposed

isotopic evidence for the age and origin of pitchstones and felsites isle of eigg nw scotland an 8 point whole rock and mineral rb sr isochron for the sgurr of eigg yields an age of 52 1 1 0 ma 2 this is the youngest accurate date yet reported for igneous activity in the british tertiary province and indicates that sub acid rocks of the small isles represent one of the latest episodes of hebridean magmatism pb isotope compositions of these rocks yield an apparent age of 2660 75 ma 2 interpreted as a mixing line between lewisian torridonian and mantle derived pb and indicating the presence of lewisian basement under eigg pb sr and nd isotopic evidence points to contamination of basaltic differentiates and granulite facies lewisian melts by torridonian sandstone but precludes significant involvement of amphibolite facies lewisian which may have been largely removed by erosion before deposition of the torridonian sediments in the small isles

multiphase dolomitization of deeply buried cambrian petroleum reservoirs tarim basin north west china cambrian dolostone reservoirs in the tarim basin china have significant potential for future discoveries of petroleum although exploration and production planning is hampered by limited understanding of the occurrence and distribution of dolomite in such ancient rocks buried to nearly 8 km the study herein accessed new drill core samples which provide an opportunity to understand the dolomitization process in deep basins and its impact on cambrian carbonate reservoirs this study documents the origin of the dolostone reservoirs using a combination of petrology fluid inclusion microthermometry and stable and radiogenic isotopes of outcrop and core samples an initial microbial dolomitization event occurred in restricted lagoon environments and is characterized by depleted 13c values dolomicrite from lagoonal and sabkha facies some fabric retentive dolomite and fabric obliterative dolomite in the peloidal shoal and reef facies show the highest 18o values these dolomites represent relatively early reflux dolomitization the local occurrence of k feldspar in dolomicrite indicates that some radiogenic strontium was contributed via terrigenous input most fabric retentive dolomite may have precipitated from seawater at slightly elevated temperatures suggested by petrological and isotopic data most fabric obliterative dolomite and medium to coarse dolomite cement formed between 90 c and 130 c from marine evaporitic brine saddle dolomite formed by hydrothermal dolomitization at temperatures up to 170 c and involved the mixing of connate brines with sr enriched hydrothermal fluids intercrystalline moldic and breccia porosities are due to the early stages of dolomitization macroscopic intergranular vuggy fracture and dissolution porosity are due to burial related dissolution and regional hydrothermal events this work has shown that old for example cambrian or even precambrian sucrosic dolomite with associated anhydrite buried to as much as 8000 m can still have a high potential for hosting substantial hydrocarbon resources and should be globally targeted for future exploration

trace element and isotope constraints on crustal anatexis by upwelling mantle melts in the north atlantic igneous province an example from the isle of rum nw scotland sr and nd isotope ratios together with lithophile trace elements have been measured in a representative set of igneous rocks and lewisian gneisses from the isle of rum in order to unravel the petrogenesis of the felsic rocks that erupted in the early stages of palaeogene magmatism in the north atlantic igneous province naip the rum rhyodacites appear to be the products of large amounts of melting of lewisian amphibolite gneiss the sr and nd isotopic composition of the magmas can be explained without invoking an additional granulitic crustal component concentrations of the trace element cs in the rhyodacites strongly suggests that the gneiss parent rock had experienced cs and rb loss prior to palaeogene times possibly during a caledonian event this depletion caused heterogeneity with respect to 87sr 86sr in the crustal source of silicic melts other igneous rock types on rum dacites early gabbros are mixtures of crustal melts and and primary mantle melts forward rare earth element modelling shows that late stage picritic melts on rum are close analogues for the parent melts of the rum layered suite and for the mantle melts that caused crustal anatexis of the lewisian gneiss these primary mantle melts have close affinities to mid oceanic ridge basalts morb whose trace element content varies from slightly depleted to slightly enriched crustal anatexis is a common process in the rift to drift evolution during continental break up and the formation of volcanic rifted margins systems the early felsic later mafic volcanic rock associations from rum are compared to similar associations recovered from the now drowned seaward dipping wedges on the shelf of se greenland and on the voring plateau norwegian sea these three regions show geochemical differences that result from variations in the regional crustal composition and the depth at which crustal anatexis took place

isotopic and geochemical evidence for magma mixing in the petrogenesis of the coire uaigneich granophyre isle of skye n w scotland field relations petrographic features major and selected trace element compositions and sr and pb isotope characteristics indicate that the coire uaigneich granophyre cug was formed by the mixing of two magmas one of these a liquid of high 87sr 86sr ratio ca 0 731 formed by the anatexis of late precambrian torridonian sediments contained relict quartz and zircon the other liquid was an acid differentiate of basaltic magma and was enriched in incompatible elements such as zr and y the two magmas mixed in the proportions of approximately two parts anatectic melt to one part acid differentiate hydrothermal metamorphism had no significant effect on the bulk chemistry of the cug

tsr origin of sulfur in permian and triassic reservoir bitumen east sichuan basin china solid bitumen h2s pyrite and anhydrite samples from the lower triassic feixianguan formation and upper permian changxing formation and source rock kerogen samples from the cambrian ordovician silurian and permian in the east sichuan basin were analyzed for their sulfur content and d 34 s values to determine if the measurements were consistent with a thermochemical sulfate reduction tsr origin of the produced h2s and solid bitumen the results show that the solid bitumen samples have sulfur contents from 2 6 to 12 5 wt and d 34 s values from 13 4 to 22 1 the d 34 s values are significantly heavier than those of potential upper permian source rock kerogens that average 1 3 n 3 suggesting that most of the sulfur in the solid bitumen is derived from isotopically heavy h2s of tsr origin from 12 0 to 13 7 interestingly two gases produced from gas to water transition zones during drill stem testing dst show significantly lighter d

dolomitization history and porosity evolution of a giant deeply buried ediacaran gas field sichuan basin china abstract the deeply buried 7 km upper ediacaran sinian dengying formation ca 551 1 541 ma in the sichuan basin china is the largest precambrian dolostone gas reservoir worldwide gas exploration from the dengying formation however is hampered by a limited understanding of its complex dolomitization history and porosity evolution new petrological geochemical and petrophysical analyses were performed and are discussed here to develop a better understanding of the formation s complexities microbialite dolo grainstone and crystalline dolostone lithologies from the platform margin have high primary porosities relative to dolo mudstone and less common microbialite lithologies from the low energy platform interior spatially variable primary porosity was subsequently overprinted by meteoric dissolution and dolomitization often inducing secondary porosity the various forms of dolomitization sabkha reflux and burial dolomitization and cementation increased the resistivity of these rocks to chemical and physical compaction and porosity destruction in the deep burial domain medium to coarse crystalline dolomite cements and saddle dolomite precipitated at fluid temperatures of 120 160 c and 160 220 c respectively the significance of deep burial dissolution is critically discussed and constrained by i corrosion of late diagenetic minerals ii pores cross cutting or forming around stylolites and iii the occurrence of solid bitumen in the center of secondary pores vuggy inter and intracrystalline pores developed during a late and deep burial stage and comprise ca 20 of the overall dengying reservoir porosity mechanisms which induced corrosion at this burial stage include hydrothermal pulses and thermochemical sulfate reduction similar to many carbonate reservoirs in the phanerozoic this study documents that the platform margin has better porosities and higher gas production compared to the platform interior due to its specific carbonate rock properties and diagenesis data shown here document the complex multiphase dolomitization history and high potential for gas production from deeply buried precambrian dolostone reservoirs

evidence of three dimensional acoustic propagation in the catoche tongue this paper presents observations of two classes of acoustic arrivals recorded on a sparsely populated vertical line array vla moored in the center of the catoche tongue a major reentrant in the campeche bank in the southeastern gulf of mexico the acoustic signals were generated by signals underwater sound sus located 50 80 km from the vla the first class of arrivals was identified as resulting from a direct non horizontally refracted path then following a quiescent period a second more diffuse class of arrivals is observed and is believed to be the result of horizontal refraction from the margin of the tongue a spectral analysis of the measured data revealed that both classes of arrivals were characterized by the source spectrum associated with sus additionally the difference in time between the onset of the first and second class of arrivals observed as a function of range from the vla is consistent with the relative difference in the length of the direct and refracted paths the observations are further supported by a three dimensional 3d acoustic propagation computation that reproduces many of the features of the measured data and provides additional insight into the details of the 3d propagation

perth bermuda sound propagation 1960 adiabatic mode interpretation a 1960 experiment is examined in which sound from three underwater explosions near perth australia was detected near bermuda a recent attempt munk et al j phys ocean 18 1876 1988 to calculate propagation paths for this event included rotational flattening of the earth and horizontal refraction determined from the vertical sound speed minimum that calculation left bermuda in a shadow zone the current work invokes adiabatic mode theory to include refraction due to horizontal variations in the vertical mode structure these results include separate horizontal rays for each of the first few vertical modes using an archival data set of 230 ocean sound profiles to generate the modes numerically where appropriate interaction with bathymetry is included this solution possesses two eigenray groups group a passes just south of the cape of good hope at which point group b is almost 1000 km to the south intermediate rays are blocked by islands group a proceeds unimpeded to bermuda for a total t

modeling three dimensional propagation in a continental shelf environment an acoustic propagation model is applied to predict measurements of three dimensional 3 d effects recorded off the southeast coast of florida the measured signal is produced by a low frequency source that is towed north parallel to the shelf from a fixed receiving array the acoustic data show the direct path arrival at the bearing of the tow ship and a second refracted path arrival as much as 30 inshore of the direct arrival notably the refracted arrival has a received level more than 25 db greater than that of the direct arrival a geoacoustic model of the environment is created to explain the data it is shown that the topography of the seafloor plays the largest role in controlling horizontal refraction effects whereas the range dependent sediment properties have the most influence on the received level the modeling approach is based on a 3 d adiabatic mode technique in which the horizontal refraction equation is solved using a parabolic equation in cartesian coordinates a modal decomposition of the field provides insight into the variability in the arrival angle and received level of the measured signal

isotopic and geochemical investigation of unit 10 from the eastern layered series of the rhum intrusion northwest scotland sr and nd isotopic compositions together with whole rock sr and nd abundances and olivine compositions are presented for cumulates from unit 10 of the eastern layered series of the rhum intrusion northwest scotland the results show an nd sr isotopic correlation consistent with contamination by high 87 sr 86 sr low 143 nd 144 nd crust there is a strong correlation between the degree of contamination sr and nd whole rock abundance and fe enrichment in cumulus olivine the allivalites are the most contaminated while the basal peridotites are uncontaminated this suggests that the cumulates crystallized from a primitive uncontaminated magma and the allivalites from an evolved contaminated magma subsequently movement of contaminated feldspathic intercumulus liquid has perturbed the isotopic trace element and olivine composition of the upper peridotites

srndpb isotopic evidence for crustal contamination in the rhum intrusion abstract sr nd and pb isotope ratios measured on cumulates from the eastern layered series els of the rhum intrusion indicate that the rhum magma chamber was contaminated by upper crustal amphibolite facies lewisian gneiss this is in marked contrast to the skye main lava series of skye which shows extensive contamination by lower crustal granulite facies lewisian gneiss 3 the amount of contamination involved was about 7 cryptic sr isotope profiles through the els show that contamination was extremely variable and may be controlled by mixing between fractionated contaminated magma and periodic injections of primitive magma into the chamber

rhythmic layering formed by deposition of plagioclase phenocrysts from influxes of porphyritic magma in the cuillin centre isle of skye the outer bytownite gabbros of the cuillin centre on the isle of skye include a 175 m rhythmically layered sequence consisting of alternating layers of coarse grained massive gabbro and finer grained laminated gabbro we infer that the massive layers formed from repeated influxes of plagioclase phyric magmas which flowed into an existing magma chamber and deposited their entrained phenocrysts on the chamber floor the phenocrysts accumulated rapidly trapping large amounts of pore liquid in the cumulus pile during the intervals between influxes laminated gabbro layers formed by crystallization of the resident magma these layers accumulated more slowly by crystal settling or in situ growth allowing sufficient time for their pore liquids to be expelled by compaction or adcumulus crystallization the resulting fractions of trapped pore liquid were510 in laminated gabbros and 20 40 in massive gabbros high porosity in the massive layers is attributable to rapid accumulation of a lowdensity cumulus assemblage with trapping of pore liquids facilitated by the confining effects of impermeable laminated cumulates above and below among layers whole rock sr and nd isotope ratios reveal variable degrees of contamination by the archean crust beneath skye in the lower part of the section influxes of plagioclase phyric magma are associated with a shift to more primitive mineral compositions lower sr sr and higher nd nd indicating that the replenishing magmas were initially more primitive and less contaminated than those already residing in the chamber in accordance with this interpretation massive layers have lower sr sr than laminated layers consistent with derivation of the plagioclase phenocrysts from a less contaminated source in the upper part of the section the shift toward more primitive and less contaminated compositions is reversed suggesting that over time the replenishing magmas underwent increasing degrees of fractionation and contamination within the magmatic plumbing system that fed the intrusion these observations and inferences coupled with published evidence for other isotopic reversals support a model for growth of the cuillin centre by frequent replenishment of a small volume magma chamber with pulses of magma that underwent complex histories of fractionation and contamination during their transit through the archean crust periods dominated by assimilation and fractional crystallization within the feeder system alternated with periods of vigorous recharge by relatively primitive and less contaminated magmas these cycles were superimposed on an overall trend toward less contamination through time consistent with increasing dominance of the system by uncontaminated tholeiitic magmas as the hebridean magmatic province matured

petrological and geochemical constraints on diagenesis and deep burial dissolution of the ordovician carbonate reservoirs in the tazhong area tarim basin nw china abstract deeply buried 4500 7000 m ordovician carbonate reservoirs in the tazhong area tarim basin nw china show obvious heterogeneity with porosity from null in limestones and sweet dolostones to 27 8 in sour dolostones from which economically important oils sour gas and condensates are currently being produced petrographic features c o sr isotopes were determined and fluid inclusions were analyzed on diagenetic calcite dolomite and barite from ordovician reservoirs to understand controls on the porosity distribution ordovician carbonate reservoirs in the tazhong area are controlled mainly by initial sedimentary environments and eo genetic and near surface diagenetic processes however vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction filling and cementation in some locations or wells especially zg5 zg7 oilfield nearby zg5 fault burial diagenesis e g thermochemical sulfate reduction tsr probably played an important role in quality improvement towards high quality reservoirs c2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures ths from 86 to 113 c from 96 to 128 c and from 128 to 151 c respectively we observed petrographically corroded edges of these high temperature minerals with oil inclusions indicating the dissolution must have occurred under deep burial conditions the occurrence of tsr within ordovician carbonate reservoirs is supported by c3 calcite replacement of barite and the association of sulfur species including pyrite anhydrite or barite and elemental sulfur with hydrocarbon and 12c rich as low as 7 2 v pdb c3 calcite with elevated ths 135 153 c the tsr may have induced burial dissolution of dolomite and thus probably improved porosity of the sour dolostones reservoirs at least in some locations in contrast no significant burial dissolution occurred in limestone reservoirs and non tsr dolostone reservoirs the deeply buried sour dolostone reservoirs may therefore be potential exploration targets in tarim basin or elsewhere in the world

origin and distribution of hydrogen sulfide in the yuanba gas field sichuan basin southwest china abstract the yuanba gas field in the permian changxing formation p2c which exhibits wide variations in its hydrogen sulfide h2s concentration 1 20 12 16 is a typical sour gas field in the northern sichuan basin the sulfur rich reservoir s solid bitumen atomic s c ratios are 0 032 0 142 and late calcite cement 13c values which are smaller than the 13c values of the host dolostone indicate that the h2s originated from thermal sulfate reduction tsr and oil was involved in tsr the gas souring index gsi of p2c s gases is generally lower than 0 1 the ethane 13c values increase as the gsi increases although no obvious increase was observed in the methane 13c values the calcite cements 13c values 15 36 to 4 56 in dolostone are heavier than the typical reported values which implies that only limited heavy hydrocarbon gases were involved in tsr no anhydrites developed in p2c s reservoirs and dissolved sulfate anions so42 were mainly enriched during dolomitization insufficient dissolved so42 most likely caused the lower h2s concentrations in the permian to triassic reservoirs in the northeastern sichuan basin compared to the permian khuff formation in saudi arabia and the jurassic smackover formation in mississippi except for the so42 in residual water in paleo oil zones so42 from bottom water may also be involved in tsr therefore oil reservoirs with bottom water have more so42 and can produce more h2s than pure oil reservoirs this phenomenon may be the main cause of the great difference in the h2s concentrations between reservoirs while gravitational differentiation during late uplift most likely creates differences in h2s concentrations in a single reservoir carbon dioxide co2 which has a relatively heavy 13c value 3 9 to 0 3 may be the combined result of tsr the balance between co2 and inorganic fluid systems and carbonate decomposition

isotope and trace element geochemistry of proterozoic natkusiak flood basalts from the northwestern canadian shield abstract the proterozoic continental tholeiites of natkusiak province 723 ma old of the northwestern canadian shield are part of the franklin magmatic event they resemble phanerozoic flood basalts in their incompatible trace element and sr nd pb isotopic variations as well as in their large volumes and a relatively short period of eruption the wide variations in sr nd pb isotopic and incompatible trace element ratios e g initial nd 5 6 to 5 7 in the natkusiak basalts together with the correlations between the isotopic and trace element ratios imply the involvement of a crustal component in their genesis the natkusiak basalts are inferred to be derived from the subcontinental upper mantle and contaminated by precambrian basement during their ascent

limited thermochemical sulfate reduction in hot anhydritic sour gas carbonate reservoirs the upper jurassic arab formation united arab emirates abstract limited thermochemical sulfate reduction tsr in hot 130 160 c and anhydrite rich sour gas reservoir carbonates of the arab formation upper jurassic is manifested by rare calcitization of anhydrite with slightly lower 13cvpdb values 3 2 to 0 1 than calcite precipitated in equilibrium with late jurassic seawater fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that tsr occurred between 130 c and 160 c the lack of evidence for extensive tsr despite the suitable current temperatures and abundant sulfates in the gas reservoir coupled with the relatively more common tsr related calcite in the flanks water zone than crest gas zone indicate that 1 gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited tsr in the crest even when it subsequently reached depths where extensive tsr would occur and 2 h2s up to 38 vol has migrated from the underlying permo triassic and or jurassic sulfate carbonate deposits this study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial thermal history is crucial for a better understanding of the origin and distribution of h2s in hot anhydrite rich sour gas reservoirs

braided peridotite sills and metasomatism in the rum layered suite scotland the rum eastern layered intrusion eli scotland is an open system layered intrusion constructed of 16 macro rhythmic units each of the macro rhythmic units consists of a peridotite base and a troctolite gabbro top previously attributed to the fractional crystallisation of a single magma batch this classic paradigm has been challenged however with evidence presented for the emplacement of peridotite sills in units 9 10 and 14 such as cross cutting relationships upward oriented apophyses and lateral discontinuities to test whether the other major peridotites within the eli represent sills we have carried out new field petrographic and mineral chemical analyses of the peridotites in units 7 8 and 9 the peridotites display large and small scale cross cutting relationships with the overlying troctolite indicative of an intrusive relationship the peridotites also show large scale coalescence and lateral spatial discontinuities such that the eli unit divisions become arbitrary harrisite layers and cr spinel seams found throughout units 7 8 and 9 suggest the peridotites were constructed incrementally via repeated injections of picritic magma our observations allow for distinct subtypes of peridotite to be defined separated by intrusive contacts allowing for their relative chronology to be determined older poikilitic peridotite rich in clinopyroxene is truncated by younger well layered peridotite containing abundant harrisite layers in addition to the new peridotite subtypes defined in this study we find strong evidence for laterally oriented metasomatism within clinopyroxene rich wehrlites at the top of the unit 8 peridotite the wehrlites and surrounding peridotites record a complex series of metasomatic reactions that transformed thin picrite sills into clinopyroxene rich wehrlites without any evidence for the sort of vertical melt movement typically posited in layered intrusions the observations presented in this study from the eli cannot be reconciled with the classic magma chamber paradigm and are better explained by the emplacement of composite sills into pre existing feldspathic cumulate gabbro or troctolite the evidence for sill emplacement presented here suggests that the layered complex was constructed by a combination of sill emplacement and metasomatism forming many of the unusual often clinopyroxene rich lithologies that surround the sills the broad scale formation of the layered peridotites via incremental sill emplacement suggested by the occurrence of upward oriented apophyses coalescence and lateral discontinuity could be applied to much larger ultramafic intrusions which might have formed by similar mechanisms

pyrometamorphism and the contamination of basaltic magma at tieveragh co antrim a tertiary dolerite plug at tieveragh co antrim cuts old red sandstone tuffaceous sedimentary rocks glassy buchites have formed by disequilibrium partial melting induced by convective heat transfer from the basaltic magma variation in bulk rock chemistry has produced glasses coexisting with variable proportions of pyroxene cordierite ilmenite hematite magnetite mullite plagioclase and alkali feldspar mixing of the basaltic magma with acid melt illustrates how up to 10 contamination may occur without radically altering the course of crystallization mineral geothermometers indicate maximum temperatures at the contact around 1100 c falling to only 940 at 10 m

oxygen isotope evidence for major fluid flow along the contact zone of the rum ultrabasic intrusion recent studies indicate that the rum tertiary ultrabasic intrusion formed in situ and was not emplaced as a fault bounded plug the suggestion that the main ring fault was the primary pathway for the flow of meteoric hydrothermal fluids on rum is therefore seriously flawed oxygen isotope evidence is presented indicating that the contact zone of the intrusion was the major pathway for meteoric fluids during cooling of the pluton 18 o depletions of over 12 correlate with hydrothermal alteration assemblages indicating that the bulk of the interaction with meteoric fluids took place at low temperatures 200 450 c

long range acoustic measurements of an undersea volcano a seamount 8 km southeast of sarigan island erupted on 29 may 2010 and was visually observed the recordings on two sets of hydrophones operated by international monitoring system ims of the comprehensive test ban treaty organization ctbto are analyzed each array is a triplet of axial single hydrophones deployed as a 2 km triangle measurements of acoustic intensity for the path to the southern triplet are on the order of 6 db lower than those received on the northern triplet temporal cross correlation beamforming estimation is performed and the estimated arrival angles for the two arrays 265 and 267 were consistent with the predicted geodesic arrival of 264 6 and 267 8 respectively cross correlation between single phones on the northern and southern arrays reveals a peak at 266 with a cross correlation of 0 1 nx2d parabolic equation modeling predicts complete blockage due to seamount interaction along the geodesic path overprediction of the seamount blockage indicates that the 2d approximation is incorrect and three dimensional propagation must be used to explain the observations this is demonstrated by the computation of the adiabatic mode parabolic equation transmission loss which predicts a 5 10 db lower reception at the southern site

comparison of differential diagenesis of two oolites on the lower triassic platform margin ne sichuan basin implications for the co evolution of rock structure and porosity abstract two oolites of the lower triassic feixianguan formation on the margins of the kaijiang liangping k l trough in the ne sichuan basin were deposited in the upper parts of similar shallowing upward sequences and are capped by a subaerial exposure surface however they have contrasting diagenetic histories and reservoir qualities the sour oolitic dolostone reservoirs on the eastern side of the k l trough have significantly higher porosities than the sweet oolitic limestone reservoirs on the western side a petrologic and geochemical comparison of the two oolites was carried out so as to investigate the role of diagenetic processes on the co evolution of oolite structure and pore network on both sides of the k l trough eogenetic meteoric digenesis led to an inversion of texture with ooid dissolution and interparticle cementation in the oolite layers close to the exposure surface the layers relatively far from the exposure surface mostly underwent recrystallization in association with connate seawater resulting in neomorphic fabrics in the ooid cortices and less interparticle calcite cement during the subsequent dolomitization the early formed oolite structure were mostly inherited by the dolostones on the eastern side with fabric retentive textures in the layers close to the exposure surface and fabric destructive textures in the layers far from the exposure surface during deeper burial the oolitic limestones were subjected to extensive calcite cementation and porosity loss whereas the dolostones generally retained more porosity and have better connectivity of the micropore network the higher porosity of the dolostones is ascribed to lesser chemical compaction and associated cement precipitation rather than the creation of new pores porosity enhancement is closely dependent on the absence of the calcite cement related to pressure solution the heterogeneity of the dolostone fabrics and the porosity was further enhanced by the subsequent recrystallization and the dissolution reprecipitation process induced by thermochemical sulfate reduction during which small amounts of net porosity are calculated to have been created the oolitic limestones are the best reservoirs close to the exposure surface whereas the most porous dolostones with the highest degree of fabric destruction are commonly some distance from the exposure surface these results provide some insights into the diagenetic controls on oolite structure and porosity and have implications for predicting the porosity of deeply buried oolitic reservoirs so have global theoretical and practical significance

geochemistry and origin of sour gas accumulations in the northeastern sichuan basin sw china significant natural gas reserves have recently been discovered in the lower triassic oolitic reservoirs from northeastern sichuan basin sw china in the wake of the december 2003 sour gas well blow out this study presents an overview on the petroleum geology and geochemistry of the sour gas accumulations in the study area two types of natural gas accumulations were identified in the lower triassic oolitic reservoirs both containing highly mature thermogenic gases with their hydrocarbon source rocks in upper permian strata natural gases from the area south of the ancient kaijiang liangping seaway are generally sweet gases formed as the result of thermal maturation whereas those discovered from north of the seaway are products of both thermal maturation and thermochemical sulfate reduction of early accumulated oils in the feixianguan formation reservoirs the proposed origins of the gases are supported by their chemical and stable carbon isotope compositions as well as the presence or absence of pyrobitumens in the reservoir the distribution of gas accumulations is controlled predominantly by the combination of lithologic and structural factors the regional variation in the concentrations of h2s in the gases appears related to the presence and thickness of anhydrite bearing evaporitic rocks interbedded or intercalated with the oolitic reservoirs

thermochemical sulphate reduction can improve carbonate petroleum reservoir quality abstract interest in the creation of secondary pore spaces in petroleum reservoirs has increased because of a need to understand deeper and more complex reservoirs the creation of new secondary porosity that enhances overall reservoir quality in deeply buried carbonate reservoirs is controversial and some recent studies have concluded it is not an important phenomenon here we present petrography geochemistry fluid inclusion data and fluid rock interaction reaction modeling results from triassic feixianguan formation sichuan basin china core samples and explore the relative importance of secondary porosity due to thermochemical sulphate reduction tsr during deep burial diagenesis we find that new secondary pores result from the dissolution of anhydrite and possibly from dissolution of the matrix dolomite assuming porosity before tsr was 16 and the percentage of anhydrite was 6 modelling shows that due to tsr 1 6 additional porosity was created that led to permeability increasing from 110 md range 72 168 md within a 95 confidence interval to 264 md range 162 432 md within a 95 confidence interval secondary porosity results from the density differences between reactant anhydrite and product calcite the addition of new water during tsr and the generation of acidity during the reaction of new h2s with the siderite component in pre existing dolomite in the reservoir fluid pressure was high during tsr and approached lithostatic pressure in some samples this transient overpressure may have led to the maintenance of porosity due to the inhibition of compactional processes an additional 1 6 porosity is significant for reserve calculations especially considering that it occurs in conjunction with elevated permeability that results in faster flow rates to the production wells

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

a strontium and neodymium isotopic investigation of the laramie anorthosites wyoming usa implications for magma chamber processes and the evolution of magma conduits in proterozoic anorthosites abstract a strontium and neodymium isotopic investigation of plagioclase rich cumulate rocks in the 1 43 ga laramie anorthosite complex lac provides new insights into the evolution of magma chambers and underlying magma plumbing systems during the crystallization of proterozoic anorthosites the 725 km2 lac intruded the boundary between archean rocks of the wyoming province in the north and proterozoic island arc terranes in the south the cheyenne belt at pressures of 3 4 kilobars initial strontium and end isotopic ratios from the three large composite anorthositic intrusions poe mountain chugwater and snow creek and late troctolitic intrusions range from 0 7034 0 7055 and 0 9 to 4 9 respectively the isotopic ratios in the cumulates vary as a result of 1 the isotopic composition of the parental mantle derived high al gabbroic magmas 2 the amount of contamination by archean rocks during ascent through the crust and 3 mixing of replenishing magmas and resident magmas with different isotopic compositions in the magma chamber at the final level of emplacement strontium isotopic and normative an variations across the 5 7 km thick stratigraphic section of layered plagioclase rich cumulates in the 200 km2 poe mountain anorthosite indicate that multiple inputs of magma are needed to construct even relatively small intrusions in proterozoic anorthosites the earliest and stratigraphically lowest intrusion was emplaced as a plagioclase rich magma that crystallized large volumes of compositionally homogeneous anorthosite at least three subsequent chamber wide episodes of magma replenishment followed by mixing are indicated by abrupt shifts in isr and or normative an in the stratigraphically higher levels of the poe mountain anorthosite strontium and neodymium isotopic disequilibrium between a high al clinopyroxene megacryst and the layered cumulates is consistent with a high pressure origin for al rich pyroxene megacrysts in proterozoic anorthosites the megacryst crystallized in a magma chamber at pressures of 10 12 kilobars from relatively uncontaminated basaltic parent magma high al gabbro and was transported through the crust in progressively contaminated anorthositic magma the range of observed isotopic compositions in the anorthositic rocks is nearly identical to that of high al gabbroic dikes in the lac supporting the proposition that high al gabbros are parental to anorthosite mitchell et al 1995 the isotopic data require that the anorthositic parental magmas were contaminated during ascent through the crust by archean orthogneisses and or metapelitic rocks decreasing isr and increasing end with a relative decrease in age of the intrusions snow creek poe mountain chugwater troctolites indicates that the magma conduit became increasingly insulated from crustal contamination over time this study indicates that the strontium and neodymium isotopic systems can be used to distinguish between processes that occurred at lower crustal upper mantle pressures during the ascent of magma diapirs and within individual magma chambers in proterozoic anorthosites and underscores the need for stratigraphic control when addressing the origin of plagioclase rich cumulates

mineral scale sr isotope constraints on magma evolution and chamber dynamics in the rum layered intrusion scotland sr isotopic zoning within single plagioclase crystals from rocks from unit 9 of the rum layered intrusion is used to infer events during crystal growth in a magma undergoing contamination the 87sr 86sr diversity among minerals and between cores and rims of plagioclase crystals increases as the boundary between unit 9 and the overlying unit 10 peridotite is approached models of near solidus interaction of the cumulate with a fluid or melt or large scale textural re equilibration cannot easily account for the systematic differences in 87sr 86sr between small crystals and the rims of larger crystals

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

basaltic replenishment of the rhum magma chamber evidence from unit 14 a detailed geochemical and lithological profile through unit 14 of the rhum eastern layered series is presented and interpreted to trace the evolution of this cyclic peridotite troctolite unit the lower 15 5 m of the unit are peridotite which has an abrupt contact with the upper 35 5 m of troctolite numerous thin subsidiary peridotites occur within the troctolite variation in the modes whole rock 87 sr 86 sr ratios and ni and fo in olivine provide most information about the evolution of the unit discontinuities in these parameters divide the unit into six subunits i the lower peridotite ii the middle main peridotite iii the upper main peridotite and lowermost troctolite iv the middle troctolite v the upper troctolite and vi the disturbed troctolite these subunits are interpreted as the result of magma replenishment by picritic and by basaltic magmas field evidence suggests that the subsidiary peridotites are later sills intruded into solid or semi consolidated troctolite the olivine fo and ni values are similar to those of surrounding troctolite suggesting either re equilibration with or assimilation of the troctolite comparison with unit 10 shows that the unit 14 resident basaltic liquid was more mafic than that of unit 10 this factor along with the basaltic replenishment can explain many of the differences in geochemical and lithological variation between the two units

isotopic composition of lead and strontium in lavas and coarse grained blocks from ascension island south atlantic an addendum abstract new sr and pb isotope data are presented for a selection of lavas and associated coarse grained blocks from ascension island k ar dates for the lavas range up to1 5 0 2ma initial87sr 86sr ratios are consistent with earlier measurements and for most rocks are ca 0 7029 but range up to 0 7135 in the case of the most evolved lavas and blocks pb isotope data are also consistent with earlier measurements but the pb in two gabbroic blocks is less radiogenic than pb in the other rocks it is suggested that these gabbroic blocks crystallized from a magma of tholeiitic composition whose source was similar to that of mid oceanic ridge basalt whereas the lavas and other blocks crystallized from mildly alkaline magmas derived from a source further from the crest of the mid atlantic ridge the high87sr 86sr ratios result from contamination of the most silicic magma by radiogenic sr from pelagic sediments these data and their interpretation are consistent with the petrological and geochemical observations that the granite blocks are the coarse grained equivalents of the volcanic suite 11 and not fragments of relict continental material 2 3

diagenesis of an evaporite related carbonate reservoir in deeply buried cambrian strata tarim basin northwest china many hydrocarbon shows have been reported from potential evaporite related carbonate reservoirs in the lower to middle cambrian units in the tarim basin petrography facies and geochemical analyses from outcrop and core samples were integrated to document the effect of diagenetic evolution on these evaporite related carbonate reservoirs a simplified depositional model has been developed that reveals a restricted carbonate platform dominated by an evaporatic inner platform lagoon with shoal and reef facies developed around the platform margins grainstones and packstones were deposited at the platform margin and on inner platform shoals whereas carbonate mudstones and wackestones were deposited in the lagoon associated with evaporites early dolomitization and dissolution occurred related to the reflux of evaporated seawater breccia associated fracture porosity caused by a short period of meteoric water induced dissolution is the predominant type of effective porosity in the anhydrite bearing dolomudstone reservoir thermochemical sulfate reduction tsr occurred in the deep subsurface diagenetic environment characterized by high temperature 100 c 160 c and high salinity 16 26 wt aqueous fluid inclusions in tsr calcite that has relatively negative 13c values as low as 12 vienna peedee belemnite and elevated 87sr 86sr values the porous dolomites contain solution enlarged pores caused by dissolution of anhydrite and carbonate as a secondary consequence of tsr this study shows that deeply buried evaporite related ancient carbonate rocks may be effective reservoirs because of a combination of fracture porosity and tsr induced secondary porosity evaporitic cambrian carbonates buried to over 5000 m 16 500 ft can thus be considered as targets for future exploration

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

carbon isotope fractionation during methane dominated tsr in east sichuan basin gasfields china a review thermochemical sulfate reduction is considered to result in h2s 10 and high dryness coefficient values in the lower triassic and upper permian carbonate reservoirs in the ne sichuan basin the gases pro duced from gasewater transition and water intervals were measured to have h2s higher than 30 and are calculated to have significantly higher h2s and co2 contents than a gas produced from a gas interval and thus are not used to reflect tsr extents methane and ethane were shown to shift positively in carbon isotopes as a result of tsr however the fractionation has not quantitatively described a loga rithmic relationship is found to give the best fit for methane d 13 c 1 and 1 h2s

halogen cycling and precious metal enrichment in sub volcanic magmatic systems insights from the rum layered intrusion scotland abstract the cycling of the halogens in sub volcanic systems may play an important role in magmatic ore deposit formation and the emission of climate altering gases via volcanic outgassing however the mobile and reactive nature of the halogens as well as their typically low abundances in most intrusive igneous rocks means that their behaviour in shallow crustal level magmatic systems is difficult to constrain we present data from the 60 ma rum layered suite nw scotland utilising the neutron irradiation noble gas mass spectrometry ni ngms technique to provide insight into the provenance and behaviour of cl br and i in magma chamber and precious metal enrichment processes as a well characterised and superbly exposed example of an open system sub volcanic magmatic body the rum intrusion is an ideal locality to investigate halogen behaviour in this context our data reveal concentrations of

the role of burial diagenesis in hydrocarbon destruction and h2s accumulation upper jurassic smackover formation black creek field mississippi organic inorganic interactions during burial of the smackover formation at black creek field mississippi have resulted in nearly complete destruction of hydrocarbons the formation has been buried to a depth of 6 km has experienced temperatures of over 200 degrees c and presently contains 78 h 2 s 20 co 2 and 2 ch 4 three distinct stages of burial diagenesis correspond to three phases of organic matter maturation pre oil window diagenesis was dominated by precipitation of prebitumen calcite cement diagenesis in the oil window was characterized by precipitation of saddle dolomite and anhydrite in water filled layers and by formation of solid bitumen in the oil column diagenesis in the gas window was dominated by thermochemical sulfate reduction tsr resulting in hydrocarbon destruction anhydrite dissolution large amounts of h 2 s co 2 and s generation and postbitumen calcite cementation during tsr anhydrite reacted with h 2 s to produce s which in turn reacted with ch 4 to generate more h 2 s in a self reinforcing cycle the lack of metal cations to stabilize h 2 s as metal sulfides availability of sufficient sulfate to generate h 2 s and a closed system to prevent h 2 s from escaping resulted in the continuation of the tsr cycle until nearly all hydrocarbons were consumed in mississippi concentration of h 2 s is nearly zero in smackover hydrocarbon reservoirs that have experienced temperatures of 120 degrees c for more than 50 m y suggesting that tsr is not a kinetic time dependent process high h 2 s concentrations initiate at temperatures above 140 degrees c and increase with temperature indicating that tsr is a thermodynamic phenomenon reported high h 2 s concentrations at low temperatures 80 120 degrees c from other locations may be explained by the following processes 1 migration of h 2 s into these reservoirs 2 high geothermal gradients or local thermal perturbations in the past 3 a biochemical origin for the h 2 s or 4 exposure of these reservoirs to temperatures greater than 150 degrees c and a rapid uplift in black creek field burial cementation and pressure solution resulted in total destruction of porosity and permeability in limestone reservoirs but not in dolomite reservoirs which still possess up to 20 porosity and 100 md permeability secondary porosity was not created as a result of hydrocarbon migration abundant co 2 derived during hydrocarbon destruction resulted in calcite cementation rather than carbonate dissolution late secondary porosity development in carbonates may be related to acids generated by metal sulfide precipitation

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

rare earth element and yttrium rey geochemistry in carbonate reservoirs during deep burial diagenesis implications for rey mobility during thermochemical sulfate reduction abstract the impact of burial diagenesis especially deep burial related processes such as thermochemical sulfate reduction tsr on the distribution of rare earth elements and yttrium rey in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material trace element and rey concentrations 87 sr 86 sr 18 o and 13 c have been determined for limestone host dolomite pore filling calcite calcite 2 and late stage fracture filling calcite calcite 3 barite barite 2 and anhydrite anhydrite 3 from lower triassic feixianguan formation reservoirs in northeast sichuan basin china calcite 2 calcite 3 barite 2 and anhydrite 3 precipitated during deep burial from 110 c to 220 c demonstrated by petrology and fluid inclusion thermometry sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical 87 sr 86 sr ratios to triassic seawater indicating negligible input of sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals carbon isotope analysis showed that calcite 2 has relatively low 13 c values down to 18 9 v pdb suggesting that they are tsr calcites with the carbonate derived from oxidized isotopically light hydrocarbons unlike the seawater like rey patterns of the limestone and dolomites calcite 3 and anhydrite 3 are enriched in rare earth elements ree and show light rare earth element enrichment and heavy rare earth element depletion exhibiting a chevron like pattern of shale normalized rey trends in contrast tsr calcite calcite 2 has similar ree to the host dolomite but a relatively flat rey sn pattern suggesting strong variations in elemental and ree compositions of the burial fluids significantly tsr calcite shows a prominent positive eu anomaly and an unusually high chondritic y ho ratio both yttrium versus holmium fractionation and eu 2 oxidation to eu 3 must have occurred during thermochemical sulfate reduction hence a positive eu anomaly and an elevated y ho ratio may be used as effective proxies to differentiate calcite resulting from tsr from ordinary calcite cement this is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite

wettability and connectivity of overmature shales in the fuling gas field sichuan basin china the fuling gas field in sichuan basin china has produced greater than 1 5 1010 m3 0 53 tcf of natural gas from overmature upper ordovician wufeng and lower silurian longmaxi shales to systemically investigate the characteristics of wettability and connectivity and to understand the underlying causes of production behavior we study five samples of wufeng and longmaxi shales with different total organic carbon contents and mineral compositions complementary approaches include mercury intrusion capillary pressure micp contact angle measurement spontaneous imbibition and saturated diffusion and tracer both nonsorbing and sorbing migration mapped via laser ablation inductively coupled plasma mass spectrometry according to measured contact angles and imbibition tests conducted on aqueous deionized water and brine and oleic n decane phases wufeng and longmaxi shales are strongly oil wet and moderately strong water wet the lower boundary of estimated permeability obtained from n decane imbibition can reach 137 nd which is higher than the geometric mean permeability derived from the micp method 5 5 68 8 nd effective diffusion coefficients of the wufeng and longmaxi shales are in the range of 10 13 m2 s 1 1 10 12 ft2 s tests of imbibition and saturated diffusion using tracer containing brine show that concentrations of nanometer sized tracers decrease rapidly a factor of 10 over a migration distance of a few millimeters from the sample edge suggesting the presence of poorly edge connected water wet pores sparsely connected hydrophilic pores mixed wettability and highly restricted pathways collectively contribute to the limited migration of nano sized tracers which probably results in the production behavior of initial steep decline and low overall recovery in the fuling gas field

pb isotope geochemistry of a massif type anorthositic charnockitic body the hidra massif rogaland s w norway abstract the hidra massif rogaland complex s w norway is a massif type anorthositic charnockitic body it consists of undeformed anorthosites and leuconorites grading into fine grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope a stockwork of charnockitic dykes cross cuts the massif the pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites 206 pb 204 pb from 18 079 to 19 307 207 pb 204 pb from 15 568 to 15 657 and 208 pb 204 pb from 37 617 to 38 493 these values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates but show the incorporation of lower crustal material u depleted and thus less radiogenic the charnockitic dykes have significantly less radiogenic pb isotopic compositions 206 pb 204 pb from 17 472 to 19 171 207 pb 204 pb from 15 489 to 15 620and 208 pb 204 pb from 36 991 to 40 922 which can be explained by a larger proportion of lower crustal contamination material the contaminant could be the granulite facies gneisses of the metamorphic envelope this interpretation is compatible with the k rb relationships of these rocks and with the o and sr isotopic geochemistry the proportion of contaminating lead in the charnockitic dykes can be estimated at 55 15 considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

the unit 12 allivalite eastern layered intrusion isle of rum a textural and geochemical study of an open system magma chamber the eastern layered intrusion of the rum central complex nw scotland comprises 16 macro units broadly defined by a basal peridotite body overlain by gabbros and troctolites the local term for the upper plagioclase bearing cumulates is allivalite the unit 12 allivalite contains five subsidiary peridotite horizons each continuous on the kilometre scale field textural and geochemical evidence points to the unit 12 allivalite forming by progressive accumulation of crystals from up to seven separate magma batches varying from picrite to basalt the thickness of the magma lens in the sill like chamber was as low as 30 m during a significant part of the development of the unit 12 allivalite and evidence supports one eruption from the chamber during its accumulation

dynamics and thermodynamics of magma mixing insights from a simple exploratory model the mixing of magmas of distinct temperature bulk composition mineralogy and physical properties plays a central role in explaining the diversity of magma types on earth and in explaining the growth of continental and oceanic crust magma mixing is also of practical importance for example the mixing of distinct magmas has been cited as an important process in creation of economically important horizons in layered intrusions as well as a triggering mechanism for initiation of volcanic eruptions the motivation for better quantifying the dynamics and thermodynamics of magma mixing and its attendant plutonic and volcanic products is clear the degree of magma mixing which spans a continuum from mingling to complete hybridization depends upon initial and boundary conditions magma properties driving forces and time available for mixing magma mingling produces a heterogeneous mixture of discrete clumps of the end member magmas whereas complete hybridization involves the thermodynamic equilibration of two distinct magmas to form a third qualitatively mixing occurs via reduction in the size of compositional heterogeneities i e clumps through stretching and folding by viscous flow followed by homogenization once shear has reduced the size of compositional anomalies to diffusive length scales quantification of this process relies on two statistical measures the linear scale of segregation defined as the spatial integral of the compositional correlation function related to the size distribution of the segregated clumps within the mixture and the intensity of segregation i a measure that quantifies how much the composition at each location differs from the average the mixing dynamics of a layered system are analyzed in terms of the parameters governing mixing rayleigh lewis and buoyancy numbers and viscosity ratio to estimate how the timescale for magma hybridization h compares to solidification recharge diffusive and assimilation timescales this analysis illustrates that hybridization times can be shorter than or comparable to thermal solidification and replenishment timescales thus formation of hybridized or nearly hybridized magmas is one anticipated outcome of mixing the machinery of thermodynamics can be used to compute the hybrid magma state an exploratory model for the thermochemistry of hybridization is developed based on binary eutectic phase relations and thermodynamics eight thermodynamic parameters define the phase diagram and associated energetics and six parameters initial temperatures compositions mass ratio of mixing magmas and an enthalpy parameter are necessary and sufficient to determine the state of hybrid magma uniquely while relevant combinations of 14 thermodynamic and mixing parameters might suggest that the number of mixing outcomes i e products is too high to systematize monte carlo simulations using the exploratory model document how millions of arbitrary initial states evolve into five possible final mixed states such an analysis implies that a magma mixing taxonomy that defines possible mixed product states can be developed and tied to petrologic indicators of mixing additional insights gained from this exploratory model that are supported by independent results from a multicomponent multiphase thermodynamic model of magma mixing magma chamber simulator include 1 the proclivity of invariant point hybrid states which may explain some instances of compositionally monotonous melts associated with mixed magma eruptions 2 a surprising thermal effect such that the temperature of hybridized magma can be significantly less than the initial temperature of either of the mixing magmas this type of magma mixing may result in crystal resorption thus invalidating an assumption that resorption textures in crystals are typically the result of a magma heating event 3 illustration of the differing effects of stoped block temperature and composition on hybrid magma temperature and phase state and 4 illustration of a cessation of crystallization effect that may pertain to the morb pyroxene paradox differences between adiabatic or r hybridization and diabatic or rfc hybridization are also explored the model can be used to elucidate the thermodynamic principles underlying magma mixing in the hybridization limit these principles are of general applicability and carry over to more compositionally complicated systems

galena mineralization in the orcadian basin scotland geological and isotopic evidence for sources of lead galena and other sulphides are widespread in the old red sandstone devonian orcadian basin syndiagenetic occurrences are associated with organic rich sedimentary rocks especially algal limestones epigenetic galenas are often spatially related to these syndiagenetic occurrences particularly in the vicinity of permo carboniferous igneous intrusions mineralization is concentrated about the granite gneiss basement inlier in southwest orkney

meteoric calcite cementation diagenetic response to relative fall in sea level and effect on porosity and permeability las negras area southeastern spain abstract a dolomitized upper miocene carbonate system in southeast spain contains extensive upper and lower zones of calcite cementation that cut across the stratigraphy cement textures including isopachous and circumgranular which are consistent with phreatic zone cementation cements in the upper cemented zone are non luminescent whereas those in the lower cemented zone exhibit multiple bands of luminescent and non luminescent cements in the upper cemented zone isotopic data show two meteoric calcite lines mcl with mean 18 o at 5 1 and 5 8 vpdb whereas no clear mcl is defined in the lower cemented zone where mean 18 o for calcite cement is at 6 7 vpdb 13 c values in both cement zones are predominantly negative ranging from 10 to 2 vpdb suggestive of carbon from soil gas or decayed organics measurements of tm ice in primary fluid inclusions yield a mode of 0 0 c in both zones indicating calcite cementation from fresh water these two zones define the positions of two different paleo water tables that formed during a relative sea level fall and erosional downcutting during the plio pleistocene the upper cemented zone pre dated the lower cemented zone on the basis of known relative sea level history meteoric calcite cementation reduced porosity and permeability but measured values are inconsistent with simple filling of open pore space each texture boundstone grainstone packstone wackestone produces a different relationship between percent calcite cement and porosity permeability distribution of cements may be predictable on the basis of known sea level history and the effect of the cementation can be incorporated into subsurface geomodels by defining surfaces of rock boundaries that separate cemented zones from uncemented zones and applying texture specific relationships among cementation porosity and permeability

gas souring by thermochemical sulfate reduction at 140 c reply natural gas in the permian triassic khuff formation of abu dhabi contains variable amounts of h2s gas souring occurred through thermochemical sulfate reduction of anhydrite by hydrocarbon gases sour gas is observed only in reservoirs hotter than a critical reaction temperature 140 c petrographic examination of core from a wide depth range showed that the anhydrite reactant has been replaced by calcite reaction product only in samples deeper than 4300 m gas composition data show that only reservoirs deeper than 4300 m contain large quantities of h2s i e 10 at present day geothermal gradients 4300 m is equivalent to 140 c fluid inclusion analysis of calcite reaction product has shown that calcite growth only became significan at temperatures greater than 140 c thus three independent indicators all show that 140 c is the critical temperature above which gas souring by thermochemical sulfate reduction begins the previously suggested lower temperature thresholds for other sour gas provinces 80 130 c derive from gas composition data that may not allow adequately either for the reservoir temperature history or for the migration of gas generated at higher temperatures into present traps conversely published proposals for higher threshold temperature 180 200 c derive from short duration experimental data that are not easily extrapolated to geologically realistic temperatures and time scales therefore the temperature of 140 c derived from our study of the khuff formation may be th best estimate of temperature required for in situ thermochemical sulfate reduction to produce the high h2s concentrations encountered in deep carbonate gas reservoirs

numerical analysis of three dimensional acoustic propagation in the catoche tongue analysis of modeled time series data is presented to provide insight into propagation physics of horizontally refracted sound in the catoche tongue region of the gulf of mexico the analysis is motivated by the observation of out of plane arrivals in measured time series data in particular the extended duration of the refracted arrivals is shown to be caused by interaction with multiple locations along the steep sides of the tongue comparison of the modeled time series is made to previous work by sturm j acoust soc am 117 3 1058 1079 2005 who examined the frequency dependence of out of plane modal arrivals for the wedge shaped ocean

constraining the fluid history of a co2 h2s reservoir insights from stable isotopes ree and fluid inclusion microthermometry reservoirs that host co2 h2s bearing gases provide a key insight into crustal redox reactions such as thermochemical sulphate reduction tsr despite this there remains a poor understanding of the extent duration and the factors limiting this process on a reservoir scale here we show how a combination of petrography fluid inclusion rare earth element ree and carbon 13c oxygen 18o and sulphur 34s stable isotope data can disentangle the fluid history of the world s largest co2 accumulation the labarge field in wyoming usa the carbonate hosted labarge field was charged with oil around 80 ma ago which together with nodular anhydrite represent the reactants for tsr the nodules exhibit two distinct trends of evolution in 13c with both 34s and 18o that may be coupled to two different processes the first trend was interpreted to reflect the coupled dissolution of anhydrite and reduction to elemental sulphur and the oxidation of organic compounds and associated precipitation of calcite during tsr in contrast the second trend was interpreted to be the result of the hydrothermal co2 influx after the cessation of tsr in addition mass balance calculations were performed to estimate an approximate tsr reaction duration of 80 ka and to identify the availability of organic compounds as the limiting factor of the tsr process such an approach provides a tool for the prediction of tsr occurrence elsewhere and advancing our understanding of crustal fluid interactions

measurements of three dimensional propagation in a continental shelf environment although a significant amount of theoretical and numerical modeling effort has been put into the study of three dimensional 3d acoustic propagation on a coastal wedge including the development of the asa 3d benchmark problem set there have been few observations of the predicted 3d propagation effects significant horizontal multipath arrivals were observed in a pair of acoustic transmission tests on the continental shelf off the east coast of florida in september 2007 and february 2008 for many transmissions arrivals were received coming from nearly the global positioning system gps bearing of the ship as well as up to 30 deg inshore of the true bearing the inshore path was up to 25 db stronger than the direct path in some cases the experimental waveforms transmitted included continuous wave transmissions ranging in frequency from 24 to 415 hz as well as wideband linear frequency modulation pulses 20 420 hz horizontal multipath arrivals were observed for source ranges from 10 to 80 km sourc

the influence of rock fabric and mineralogy on thermochemical sulfate reduction khuff formation abu dhabi abstract thermochemical sulfate reduction tsr is the reaction between anhydrite and petroleum fluids at elevated temperatures to produce h2s and calcite in this study of the dolomite hosted hydrocarbon gas reservoirs in the permo triassic khuff formation abu dhabi a geochemically well constrained rock gas system we demonstrate for the first time a clear influence of rock texture and mineralogy on the rate and extent of tsr reactions and thus on h2s concentration in the gas phase the controls on the rate on h2s accumulation were tsr reaction kinetics tsr became significant as temperature exceeded 140 c a critical threshold temperature for chemical reaction between aqueous sulfate and aqueous methane anhydrite dissolution rate once initiated the subsequent reaction rate was controlled by the rate of supply of aqueous sulfate to the reaction site sulfate limitation is indicated by the lack of fractionation of sulfur isotopes between sulfate and sulfide i e suggesting total reaction for each unit of sulfate that dissolves also finely crystalline anhydrite began reacting at a lower temperature than coarse crystalline anhydrite likely a result of relative surface area suggesting that anhydrite dissolution was the rate limiting step transport rates are unlikely to have been rate limiting at the earliest stage of reaction because anhydrite was replaced in situ by calcite on the very edges of anhydrite nodules and crystals transport rate later as tsr proceeded it became transport controlled as calcite growing on the surface of anhydrite crystals began to isolate them from dissolved methane tsr ceased once calcite had effectively armor plated totally isolated the remaining anhydrite finely crystalline anhydrite underwent more extensive and more rapid tsr than coarser anhydrite crystals because these had a greater ratio of surface area to volume allowing more and faster dissolution and requiring more calcite to isolate them from methane localized loss of h2s occurred in the reservoir by reaction with indigenous fe bearing clays consequently reservoirs with a relatively high siliciclastic content have less h2s than would be expected from the advanced state of anhydrite replacement by calcite in order to predict tsr related h2s concentration in hydrocarbon gases it is thus important to understand the diagenetic and textural characteristics of the reservoir as well as the thermal and petroleum emplacement history

diagenesis and its impact on a microbially derived carbonate reservoir from the middle triassic leikoupo formation sichuan basin china the uppermost middle triassic leikoupo formation in the western sichuan basin of china has recently been shown to host as much as 5 3 tcf 1 5 1012 m3 of natural gas resources the reservoir rocks composed mainly of microbially derived dolomudstone e g thrombolites and stromatolites are characterized by low porosity 8 and permeability 0 001 to 10 md the limestone is commonly tight and not of reservoir quality because of abundant meteoric calcite cementation whereas the dolostone has various types of pores dominated by solution enlarged pores and vugs microbial framework pores and micropores breccias are well developed in places probably because of dissolution of underlying evaporites e g anhydrite by an influx of low salinity fluids e g freshwater and seawater during an early burial stage early dolomitization created micropores in the dolomudstone and subsequent diagenetic events were dominated by calcite dolomite quartz cementation pyrite replacement compaction fracturing and development of stylolites localized hydrothermal activity has been evidenced by high homogenization temperatures 160 c 200 c obtained from fluid inclusions in fracture filling cements bacterial sulfate reduction probably resulted in h2s generation pyrite precipitation and solution enlarged pore and vug formation whereas part of the current h2s in these reservoirs may have been sourced from thermochemical sulfate reduction or an underlying formation e g the feixiangguan formation development of microfractures and associated micropores was probably the final diagenetic event which improved pore interconnectivity this study confirms the effect of diagenesis on the development of a microbial dolomudstone reservoir which may be applicable to other similar microbial carbonate reservoirs elsewhere for example middle triassic sections of the tethys region and offshore brazil

generation of isotopically and compositionally distinct water during thermochemical sulfate reduction tsr in carbonate reservoirs triassic feixianguan formation sichuan basin china abstract thermochemical sulfate reduction tsr the reaction of petroleum with anhydrite in reservoirs resulting in the growth of calcite and the accumulation of h2s has been documented in the feixianguan formation dolomite reservoir in the sichuan basin china fluid inclusion salinity and homogenization temperature data have shown that tsr results in a decrease in salinity from a pre tsr value of 25 wt down to 5 wt as a result of water created as a byproduct of progressive tsr we have studied the isotopic character of the water that resulted from tsr in the feixianguan formation by analyzing the oxygen isotopes of tsr calcite and determining the oxygen isotopes of the water in equilibrium with the tsr calcite at the temperatures determined by aqueous fluid inclusion analysis we have compared these tsr waters to water that would have been in equilibrium with the bulk rock also at the temperatures determined by aqueous fluid inclusion analysis we have found that the tsr waters are relatively depleted in oxygen isotopes by up to 8 compared to what would be expected at equilibrium between the bulk rock and water since this type of water was specifically derived from anhydrite the generation of relatively large volumes of low salinity low 18o water associated with advanced tsr in the feixianguan formation has also been reported in the permian khuff formation in abu dhabi and from sour devonian fields in the western canada basin this suggests that tsr derived water may be a common phenomenon the effects of which on mesogenetic secondary porosity and reservoir quality have previously been underappreciated

methane dominated thermochemical sulphate reduction in the triassic feixianguan formation east sichuan basin china towards prediction of fatal h2s concentrations abstract new sour pools have recently found in the lower triassic feixianguan fm carbonate reservoirs in the east sichuan basin in china with h 2 s up to 17 4 by volume a recent blowout from a well drilled into this formation killed hundreds of people as a result of the percentage concentrations of h 2 s in order to assess the origin of fatal h 2 s as well as the cause of petroleum alteration h 2 s concentrations and the isotopes 34 s and 13 c have been collected and measured in gas samples from reservoirs anhydrite pyrite and elemental sulphur 34 s values have been measured for comparison the high concentrations of h 2 s gas are found to occur at depths 3000 m temperature now at 100 c in evaporated platform facies oolitic dolomite or limestone that contains anhydrite nodule occurrence within the reservoirs where h 2 s concentrations are greater than 10 its 34 s values lie between 12 0 and 13 2 cdt this is within the range of anhydrite 34 s values found within the feixianguan fm 11 0 to 21 7 average 15 5 3 5 cdt thus h 2 s must have been generated by thermochemical sulphate reduction tsr locally within the reservoirs burial history analysis and fluid inclusion data reveal that the temperature at which tsr occurred was greater than about 130 140 c suggesting that the present depth temperature minimum is an artifact of post tsr uplift both methane and ethane were actively involved in tsr since the petroleum became almost totally dry no alkanes except methane and methane 13 c values become significantly heavier as tsr proceeded methane 13 c difference thus reflects the extent of tsr while it is tempting to use a present day depth control 3000 m to predict the distribution of h 2 s in the feixianguan fm this is an invalid approach since tsr occurred when the formation was buried some 1000 2000 m deeper than it is at present the likelihood of differential uplift across the basin means that it is important to develop a basinal understanding of the thermal history of the feixianguan fm so that it is possible to determine which parts of the basin have been hotter than 130 140 c

magma ascent along a major terrane boundary crustal contamination and magma mixing at the drumadoon intrusive complex isle of arran scotland the composite intrusions of drumadoon and an cumhann crop out on the se coast of the isle of arran scotland and form part of the larger british and irish palaeogene igneous province a subset of the north atlantic igneous province the intrusions shallow level dykes and sills comprise a central quartz feldspar phyric rhyolite flanked by xenocryst bearing basaltic andesite with an intermediate zone of dark quartz feldspar phyric dacite new geochemical data provide information on the evolution of the component magmas and their relationships with each other as well as their interaction with the crust through which they travelled during shallow crustal emplacement the end member magmas mixed isotopic evidence shows that both magmas were contaminated by the crust prior to mixing the basaltic andesite magma preserves some evidence of contamination within the lower crust whereas the rhyolite mainly records upper crustal contamination the highland boundary fault divides arran into two distinct terranes the neoproterozoic to early palaeozoic grampian terrane to the north and the palaeozoic midland valley terrane to the south the drumadoon complex lies within the midland valley terrane but its isotopic signatures indicate almost exclusive involvement of grampian terrane crust therefore although the magmas originated at depth on the northern side of the highland boundary fault they have crossed this boundary during their evolution probably just prior to emplacement

calcite vein system and its importance in tracing paleowater flow and hydrocarbon migration in the ordovician carbonates of the tazhong area tarim basin china it is difficult to determine flow pathways of paleofluids in sedimentary basins petrography fluid inclusion and isotopic compositions from different phases of calcite veins in the ordovician in the tazhong area were studied our results reveal a complex diagenesis process with two importantly crossformational fluid movements coupled with three phases of calcite veins c1 c2 and c3 precipitation and thermochemical sulfate reduction by hydrocarbons fluids that precipitated c2 and c3 calcites were saline and enriched in 18o and thus may have originated from underlying evaporates whereas fluid that precipitated c3 had an additional contribution from 87sr rich ba rich and zn rich precambrian basinal clastic units fluid inclusion homogenization temperatures salinities and 18owater values of c2 veins show decreasing trends with increasing distances from strike slip faults this may indicate that c2 fluid might migrate into ordovician carbonate reservoirs through strike slip faults within 6 km during late hercynian orogeny late permian the similarly decreasing trends are observed in c3 veins indicating that the eastern part of no 1 fault seemed to be the main fluid flow pathway for c3 calcite precipitation during himalayan orogeny hydrocarbon may have migrated along the similar pathways as supported by 1 abundant primary hydrocarbon bearing fluid inclusions in the c2 and c3 calcites and 2 well documented hydrocarbon migration and mixing this is an ideal case showing how spatial variation in geochemical data from fracture fillings can be used to trace paleowater and likely hydrocarbon flows in sedimentary basins it thus may be useful in predicting fluid flow and distribution of hydrocarbons in the tarim basin and other basins

lewisian gneiss geochemistry and archaean crustal development models abstract the geochemistry of lewisian amphibolite facies gneisses from northwest scotland is described with particular reference to the rare earth elements ree and compared with the geochemistry of lewisian granulite facies gneisses the results show that there are no significant differences between laxfordian amphibolite facies and scourian granulite facies gneisses in terms of ree and other immobile trace elements at equivalent silica levels although the mobile radioactive heat producing elements k rb th u are significantly lower in the granulites in both types the basic gneisses have moderately fractionated ree patterns while the intermediate and acid gneisses have strongly fractionated ree patterns with low heavy ree abundances and decreasing levels of total ree with increasing sio 2 the most silicic gneisses develop large positive europium anomalies these gross chemical similarities between gneisses from intermediate amphibolite facies and lower granulite facies crustal levels constrain models for the evolution of the archaean crust the depletion of k rb cs th and u in granulites but not other incompatible trace elements cannot be explained by magmatic processes the positive eu anomaly in the more siliceous gneisses of both facies is a function of the primary processes of crustal generation and not secondary processes such as intracrustal melting or fractional crystallisation fractionation of radioactive heat producing elements from other trace elements is a result of granulite facies metamorphism with these elements being removed by an active fluid phase the apparent lack of partial melting in lower crustal granulites suggests a model for archaean crustal growth largely through underplating by primary tonalitic magmas

successive episodes of reactive liquid flow through a layered intrusion unit 9 rum eastern layered intrusion scotland we present a detailed microstructural and geochemical study of reactive liquid flow in unit 9 of the rum eastern layered intrusion scotland unit 9 comprises an underlying lens like body of peridotite overlain by a sequence of troctolite and gabbro termed allivalite with some local and minor anorthosite the troctolite is separated from the overlying gabbro by a distinct sub horizontal undulose horizon the major wavy horizon higher in the stratigraphy is another similar horizon the minor wavy horizon that separates relatively clinopyroxene poor gabbro from an overlying gabbro to the north of the peridotite lens both troctolite and gabbro grade into poikilitic gabbro clinopyroxene habit in the allivalite varies from thin rims around olivine in troctolite to equigranular crystals in gabbro and to oikocrysts in poikilitic gabbro the poikilitic gabbros contain multiple generations of clinopyroxene with cr rich 1 1 wt cr2o3 anhedral cores with moderate ree concentrations core1 overgrown by an anhedral ree depleted second generation with moderate cr 0 7 wt cr2o3 core2 these composite cores are rimmed by cr poor 0 2 wt cr2o3 and ree poor to moderate clinopyroxene we interpret these microstructures as a consequence of two separate episodes of partial melting triggered by the intrusion of hot olivine phyric picrite to form the discontinuous lenses that comprise the unit 9 peridotite loss of clinopyroxene saturated partial melt from the lower part of the allivalite immediately following the early stages of sill intrusion resulted in the formation of clinopyroxene poor gabbro the spatial extent of clinopyroxene loss is marked by the minor wavy horizon a second partial melting event stripped out almost all clinopyroxene from the lowest allivalite to form a troctolite with the major wavy horizon marking the extent of melting during this episode the poikilitic gabbro formed from clinopyroxene saturated melt moving upwards and laterally through the remobilized cumulate pile and precipitating clinopyroxene en route this process called reactive liquid flow is potentially important in open magma chambers

seismic stratigraphy of the mid cretaceous unconformity mcu in the central gulf of mexico basin a widespread high amplitude reflection seen on seismic data throughout the gulf of mexico has been called the mid cretaceous unconformity mcu this reflector seems to be a major stratigraphic boundary in the gulf of mexico basin it is believed to correspond to vail s type 1 unconformity of mid cenomanian age 94 ma which records an eustatic drop in sea level of approximately 60 100 m the study area includes all of the gulf of mexico with the exception of areas of thick and highly deformed salt where following the mcu with any degree of confidence becomes impossible the mcu is easy to follow in multichannel seismic reflection profiles collected in the gulf of mexico by the institute for geophysics of the university of texas at austin in the central deeper part of the gulf reflectors above and below the mcu are parallel in the southern and eastern rims of the gulf along the campeche and florida escarpments reflectors are truncated below the mcu and show onlap relationships above it the mcu

raman spectroscopic study of polysulfanes h2s in natural fluid inclusions abstract sulfur bearing methane inclusions with up to 68 mol h2s in metamorphic quartzite from bastar craton india exhibit common 1 vibration modes of h2s and ch4 at 2609 cm 1 and 2919 cm 1 respectively associated with unknown raman bands at 2488 2503 2574 and 2899 cm 1 the fluid inclusions generate molecular hydrogen and surplus sulfur during irradiation with a 532 nm 25 mw laser thus indicating photolytic breakdown of polysulfanes h2sn quantum chemistry modeling based on the density functional theory was conducted to better understand the origin of the unusual raman bands in the sh stretching region vibrations of simple h2s h2s2 h2s3 h2s4 h2s5 s6 8 molecules were simulated together with selected complexes of n 1 4 polysulfanes with the crown shaped orthorhombic cycloocta sulfur identified in the fluid inclusions according to intense raman bands at 60 62 79 80 149 151 218 220 441 442 and 472 474 cm 1 theoretical calculations confirmed that the vibrations at 2488 2503 and 2574 cm 1 reflect symmetric and asymmetric h bonded and non h bonded h2s2 s8 h2s2 h2s2 s8 and h2s s8 h2s complexes in the crystallized sulfur melt a small band at 2899 cm 1 is tentatively attributed to ch3 s8 and or ch2 s8 bonds which also diminished during irradiation with green laser phantom bands occasionally occurring at 2580 cm 1 were identified as artificial bands resulting from an imprecise angle of incidence of laser beam to rotatable holographic grating another small 2579 1 cm 1 band in raman spectra recorded using longer acquisition times represents a combination band of host quartz rather than the a12 4 resonant vibration of ch4 located at the same frequency

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

thermochemical sulfate reduction and fluid evolution of the lower triassic feixianguan formation sour gas reservoirs northeast sichuan basin china the dolomite hosted lower triassic feixianguan formation from the northeast sichuan basin china is an economically important reservoir that contains sour natural gas these reservoirs were initially filled with oil later replaced by gas during burial to 7000 m 22 965 ft followed by uplift to about 4000 m 13 123 ft we have studied the souring process thermochemical sulfate reduction tsr and diagenetic evolution of the feixianguan formation using detailed petrology fluid inclusion studies and stable isotope data from carbonate minerals pre tsr diagenesis included in time order the eodiagenetic main stage of dolomitization by a reflux mechanism fracture related calcite cementation barite quartz celestite and fluorite mineralization and a dolomite recrystallization stage thermochemical sulfate reduction resulted in anhydrite replacement by calcite petroleum destruction formation of sulfur rich pyrobitumen and elemental sulfur and generation of large volumes of and diagenesis during tsr can be subdivided into oil stage tsr and gas stage tsr with oil stage tsr defined by the presence of primary oil and bitumen inclusions in the tsr calcite based on aqueous inclusion homogenization temperatures oil stage tsr commenced at a temperature of 116 c with a mode between 130 c and 140 c gas stage tsr started at a temperature of 135 c and continued to maximum burial temperatures of about 220 c trace amounts of pyrite barite quartz and celestite grew during tsr post tsr diagenesis was dominated by fracture related calcite precipitation as well as celestite and anhydrite crystallization formation water salinity increased from depositional values 3 5 wt up to 24 wt during pre tsr dolomite recrystallization probably because of an influx of evaporite associated water from the overlying jialingjiang formation although pre tsr barite quartz celestite and fluorite mineralization was associated with a transient decrease in water salinity during tsr formation water salinity decreased from 26 wt to as low as 4 wt as a result of water being produced during tsr reactions

eigenray finding and time series simulation in a layered bottom ocean a ray theory approach for simulating the propagation of broadband signals interacting with a layered ocean bottom is presented the range invariant environment consists of the ocean and an arbitrary number of bottom layers each bottom layer has profiles of compressional and shear wave velocity attenuation and density eigenrays including those with multiple interactions and refractions in the ocean and bottom layering are found using a thorough and efficient algorithm based on an analysis of the sound velocity profile and the dependence of ray path geometry on the snell invariant the time series at a receiver due to an arbitrary source waveform is obtained by constructing a frequency domain transfer function from the eigenray characteristics several example applications demonstrate the potential use of this approach and show that reflection from relatively simple layering can severely distort a received time series

burial diagenesis and thermochemical sulfate reduction smackover formation southeastern mississippi salt basin the smackover formation in the southeastern mississippi salt basin has been buried from 3 03 to 6 06 km and subjected to temperatures of 100 to 200c pre hydrocarbon migration burial diagenesis is characterized by precipitation of calcite cement followed by saddle dolomite and late anhydrite these phases formed at temperatures of less than 100c the oxygen isotopic composition of calcite ranges from minus 5 to minus 8 4 pdb and carbon isotopic composition ranges from 3 5 to 6 0 pdb the sulfur isotopic composition of late anhydrites ranges from 18 1 to 19 1 cdt post hydrocarbon migration burial diagenesis is dominated by replacement of previously formed late anhydrite by calcite or calcite plus elemental sulfur and precipitation of post bitumen calcite cement these diagenetic phases formed at temperatures above 150c and are present only in the deep part of the basin the oxygen isotopic compositions of calcites that replace anhydrite range from minus 3 0 to minus 4 0 pdb and carbon isotopic compositions range from minus 2 0 to 4 2 pdb the oxygen isotopic compositions of post bitumen calcites range from minus 3 4 to minus 5 6 pdb and carbon isotopic compositions range from minus 1 6 to minus 16 3 pdb two samples of elemental sulfur have sulfur isotopic compositions of 15 8 and 16 1 cdt the high temperature replacement of anhydrite by calcite more the presence of post bitumen calcite cement the sulfur isotopic composition of sulfates and elemental sulfur and high concentration of hydrogen sulfide indicate that rapid thermochemical reduction of solid sulfates is taking place in the deep part of the mississippi salt basin at temperatures as low as 150c less

basin structure and tertiary magmatism on skye nw scotland the emplacement of igneous material into upper crustal rocks of sedimentary basins is likely to be strongly controlled by the geometry of the pre existing basin structures these controls are investigated using examples from the tertiary igneous complexes of skye part of the sea of hebrides basin of nw scotland the basin consists of an array of half graben related to se dipping normal faults these pre volcanic mesozoic structures are traced near the igneous complexes using geological relationships preserved unconformably beneath the widespread basaltic lava fields the unconformity represents a period of cretaceous uplift and denudation of the basin and its flanks entirely pre dating the tertiary volcanism of nw scotland this unconformity seals stratigraphically the major basin faults preserving field relationships that permit the tracing of these faults in the country rocks to the tertiary intrusions the major camasunary fault is separated from the raasay fault via a series of minor graben linked by a series of steep nw se trending faults that transfered mesozoic displacements between the principal fault strands a broad range of igneous material of various compositions was intruded into part of the mesozoic sea of hebrides basins and their flanks during palaeocene times different emplacement styles and different structural controls are found the major gabbroic centres do not appear to be controlled by upper crustal structures having been emplaced into the footwalls of major faults however minor synmagmatic displacements on the basin faults may have been sufficient to generate dilatational sites in these footwall positions thereby facilitating emplacement in contrast the granitic melts have been emplaced as sheets and domed into the sediments and overlying lava pile reactivating segments of the basin fault network doming occurred from an array of sills the stratigraphic levels of which can be reconstructed using structural relationships preserved in the roofs and walls of the intrusions the sill levels and their transgressive forms are strongly related to inferred mesozoic basin structures the major fold structures of tertiary age in southern skye are interpreted as accommodating granitic emplacement rather than crustal shortening the nw se mesozoic transfer fault trend appears to have strongly influenced the segmentation of the granite domes these interpretations are illustrated using field relationships mapped in the vicinity of the coire uaigneich granophyre it is concluded that although the higher parts of the basin faults were reactivated to facilitate the doming of granitic intrusions the deeper levels of the mesozoic faults show no evidence of substantial reactivation

a ce nd isotope study of crustal contamination processes affecting palaeocene magmas in skye northwest scotland twelve138ce 136ce isotope determinations 31 nd isotope analyses and 31 ree profiles are presented for tertiary basic to intermediate igneous rocks from the isle of skye nw scotland the aim of this work is to precisely identify the contamination mechanisms of basic magmas emplaced through old crust and to test the effectiveness of ce isotope analysis as a petrogenetic tool

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

experimental confirmation of horizontal refraction of cw acoustic radiation from a point source in a wedge shaped ocean environment analysis of experimental data obtained in the region of the east australian continental slope has been found to be consistent with theoretical predictions of energetic horizontal refraction due to multiple reflections from a sloping bottom the observed azimuthal characteristics are in accordance with both normal mode and ray theoretical solutions the experiment conducted with two ships one towing a source and the other an array started in water 400 to 500 m deep with an initial separation of 34 km and proceeded to deeper water on a diverging but constant line of bearing course bearing shifts deduced from beamformed data show changes of tens of degrees in as little as half an hour these bearing shifts demonstrate the dominance of horizontally refracted arrivals for the geometry a combination of source array positions and critical angle effects served to limit both the azimuthal extent of energy received and the total number of modes arriving at any one position

a sr nd and pb isotopic investigation of the transition between two megacyclic units of the bjerkreim sokndal layered intrusion south norway abstract the bjerkreim sokndal layered intrusion forms part of the proterozoic south rogaland igneous complex of southwest norway the intrusion has been divided into two parts an anorthositic to gabbro noritic layered series and a generally more massive mangeritic quartz mangeritic to charnockitic upper part on the basis of repeated lithological sequences the layered series has been divided into a number of megacyclic units which are interpreted as reflecting magma chamber replenishment initial 940 ma sr nd and pb isotope ratios for plagioclases separated from samples crossing the transition between megacyclic unit iii and megacyclic unit iv range from 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 to 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 the isotopic compositions vary systematically with stratigraphic height and the interplay between the three isotopic systems is used to divide the stratigraphic profile into two parts replenishment and post replenishment across the replenishment interval isotopic variations are consistent with mixing between a resident and a replenishing magma the replenishing magma was characterized by 87 sr 86 sr i 0 7048 143 nd 144 nd i 0 51156 end 2 6 and 206 pb 204 pb i 17 50 the positive end of this magma indicates that its source region was either in the mantle or was itself recently derived from the mantle in contrast the resident magma had 87 sr 86 sr i 0 7061 143 nd 144 nd i 0 51143 end 0 1 and 206 pb 204 pb i 17 73 indicating that it had assimilated crustal material in addition the isotopic correlations across the replenishment interval indicate that prior to mixing the concentrations of nd and pb in the resident magma had been strongly reduced probably by fractionation of apatite above the replenishment interval nd and pb isotopic variations are apparently decoupled from sr isotopic variations two mechanisms are proposed for achieving this the first is heterogeneous mixing of up to 20 of a third magmatic component into an already isotopically stratified magma chamber the second is the presence of 10 20 of post cumulus plagioclase that crystallized from a migrating intercumulus melt

weathering fluxes and sediment provenance on the sw scottish shelf during the last deglaciation the reconstruction of past ice sheet dynamics can shed a light on long term ice stream activity and in turn provide constraints on the response of modern ice sheets to climate change the hebrides ice stream his flowed across part of the western scottish shelf to the shelf break during the last glacial cycle and drained a large portion of the northern sector of the british irish ice sheet to investigate the deglacial dynamics of the his following the last glacial maximum lead pb isotope records were extracted from the femn oxyhydroxide and detrital fractions of recovered laminated glacimarine mud sequences to monitor the changing activity of his during its retreat these provide timing and some source information of glacially weathered inputs to the marine environment the femn oxyhydroxide fraction in the samples is dominated by allochthonous particles pre formed and shows a marked decrease from radiogenic 20 05 206pb 204pb at 21 cal ka bp to less radiogenic pb isotope compositions 19 48 towards the windermere interstadial 15 4 13 ka this decrease represents a reduction in the flux of subglacially derived radiogenic pb to the continental shelf is interpreted as being associated with the break up of the ice stream in western scotland around that time the pb sr and nd isotopic signatures of the detrital fraction indicate a preponderance of moine sourced fine sediments originated from the nw highlands in the core locations from 21 to 15 cal ka bp dimlington stadial windermere interstadial most likely dictated by the orientation of ice flow tidal and oceanic current directions and sediment delivery in contrast ice rafted debris in a 21 cal ka bp old basal diamicton contains volcanic derived material suggesting different provenance for different grain sizes the femn oxyhydroxide 208pb 204pb ratio shows an unusual inversion relative to the other pb isotope ratios and is attributed to the introduction of secondary weathering phases from a source with contrasting 208pb 204pb but similar 206pb 204pb and 207pb 204pb in the detrital fraction the inversions are constrained to periodic spikes which may indicate an increased contribution from a high th u source potentially the neighbouring archaean amphibolitic lewisian basement in the outer hebrides this study demonstrates how geochemical investigation on continental shelves can be used to constrain the timing activity and flow sources of palaeo ice streams