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

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

an integrated analytical approach for determining the origin of solid bitumens in the mcarthur basin northern australia abstract an integrated analytical approach using petrological geochemical and isotopic techniques has been used to determine the origin of solid bitumens in the middle proterozoic roper group mcarthur basin northern australia sandstone and dolerite samples which in hand specimen contain dark material believed to be solid bitumens were examined from six wells under ultraviolet illumination non fluorescent solid bitumens can be distinguished from two generations of crude oil one of lower maturity orange fluorescence the other of higher maturity white fluorescence other petrographic observations indicate that the precursor hydrocarbons to the solid bitumens and the subsequent charges of crude oil postdate all diagenetic clay and quartz cementation geochemical analyses indicate that many of the sandstones contain both an aliphatic rich oil and solid bitumen mixed in varying proportions thus corroborating the petrological results on the basis of n alkane distributions the oil appears to have two 3ubtly different source inputs one of which is typified by a greater proportion of high molecular weight n alkanes which have a slight even carbon number predominance and an isotopically lighter aliphatic hydrocarbon fraction one sandstone sample contains solid bitumen in a fracture which is characterized by a high abundance of asphaltenes polar compounds and sulphur and may have formed by deasphalting of a precursor oil the solid bitumens recovered from fractures in dolerite sills were formed by thermal processes which in one sample resulted in an unusual series of ketone isomers however most solid bitumens particularly where present in the coarser sandstones are related to the generation of secondary porosity the large proportion of unresolved complex mixture the lack of n alkanes and the isotopically heavier aliphatic hydrocarbon fractions of samples containing predominantly solid bitumens indicate that bitumen formation occurred mainly by biodegradation of a precursor oil

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

comment on step pool streams adjustment to maximum flow resistance by athol d abrahams gang li and joseph f atkinson the writer wishes to congratulate the authors for their very interesting development on step pool streams recent advances on stepped channel flows with fixed bed provide additional information on the topic the discusser will show that the maximum flow resistance condition implies a characteristic flow pattern basically the bed profile of maximum flow resistance on step pool streams is achieved for flow conditions near the transition between nappe and skimming flow

origin of high h2s concentrations in the upper permian changxing reservoirs of the northeast sichuan basin china abstract intense thermochemical sulfate reduction tsr and up to 18 h 2 s are found in the upper permian changxing formation p 3 ch in the northeast ne sichuan basin china despite that rare gypsum or anhydrite was found in this formation here we present new concentration data of carbonate associated sulfate cas from carbonate host rocks c o and sr isotope data for tsr related calcites and s isotope data for sulfur compounds obtained during this study these data along with spatial temporal changes in palaeogeopressure conditions hydraulic conductivity and the physical capacity indicate that the h 2 s was generated locally from tsr within the p 3 ch reservoirs we propose that the reactive sulfates were derived from cas released during dolomitization and recrystallization of earlier dolomite within the p 3 ch fm and from the cross formational migration of evaporative brines from the lower triassic feixianguan formation t 1 f to p 3 ch fm our calculation shows that the two sources could provide enough so 4 2 for the generation of h 2 s within the p 3 ch reservoirs early downward migration of sulfate rich evaporative brines from the t 1 f formation occurred in near surface and shallow burial diagenetic settings mainly 3 ch fm subsequently tsr calcites precipitated from the pore water have high sr concentrations up to 7767 ppm close to the t 1 f tsr calcites and 87 sr 86 sr ratios mainly from 0 7074 to 0 7078 which are significantly higher than those of late permian seawater but within the range of early triassic seawater

gas genetic type and origin of hydrogen sulfide in the zhongba gas field of the western sichuan basin china abstract natural gases and associated condensate oils from the zhongba gas field in the western sichuan basin china were investigated for gas genetic types and origin of h 2 s by integrating gaseous and light hydrocarbon geochemistry formation water compositions s isotopes 34 s and geological data there are two types of natural gas accumulations in the studied area gases from the third member of the middle triassic leikoupo formation t 2 l 3 are reservoired in a marine carbonate sequence and are characterized by high gas dryness high h 2 s and co 2 contents slightly heavy c isotopic values of ch 4 and widely variable c isotopic values of wet gases they are highly mature thermogenic gases mainly derived from the permian type ii kerogens mixed with a small proportion of the triassic coal type gases gases from the second member of the upper triassic xujiahe formation t 3 x 2 are reservoired in continental sandstones and characterized by low gas dryness free of h 2 s slightly light c isotopic values of ch 4 and heavy and less variable c isotopic values of wet gases they are coal type gases derived from coal in the triassic xujiahe formation the h 2 s from the leikoupo formation is most likely formed by thermochemical so 4 reduction tsr even though other possibilities cannot be fully ruled out the proposed tsr origin of h 2 s is supported by geochemical compositions and geological interpretations the reservoir in the leikoupo formation is dolomite dominated carbonate that contains gypsum and anhydrite petroleum compounds dissolved in water react with aqueous so 4 species which are derived from the dissolution of anhydrite burial history analysis reveals that from the temperature at which tsr occurred it was in the late jurassic to early cretaceous and tsr ceased due to uplift and cooling thereafter tsr alteration is incomplete and mainly occurs in wet gas components as indicated by near constant ch 4 13 c values wide range variations of ethane propane and butane 13 c values and moderately high gas dryness the 34 s values in so 4 elemental s and h 2 s fall within the fractionation scope of tsr derived h 2 s high organo s compound concentrations together with the occurrence of 2 thiaadamantanes in the t 2 l reservoir provide supplementary evidence for tsr related alteration

geochemical and carbon isotopic studies of crude oil destruction bitumen precipitation and sulfate reduction in the deep smackover formation abstract crude oil generated by the lower smackover source facies migrated to upper smackover reservoirs where slow thermal cracking of crude oil resulted in the formation of gas condensate and late solid bitumen ultimately only pyrobitumen methane and nonhydrocarbon gases including hydrogen sulfide persist in the deepest smackover reservoirs the carbon isotopic compositions of crude oils became heavier during crude oil destruction the carbon isotopic compositions of asphaltenes nso compounds and saturated hydrocarbons in late solid bitumen and the lower smackover source facies became isotopically lighter during crude oil destruction it is suggested that some isotopically light components from crude oils were incorporated in late solid bitumen by reactions involving thermochemical sulfate reduction thermochemical sulfate reduction and crude oil destruction occurred over a long span of geologic time at temperatures in the 120 150 c range

origins and fates of h2s in the cambrian and ordovician in tazhong area evidence from sulfur isotopes fluid inclusions and production data abstract gases oils solid bitumen samples and fluid inclusions were analyzed for chemical composition and 13 c and 34 s values and production data were collated to elucidate the origin and fates of the h 2 s in the tazhong area the h 2 s in the ordovician was measured to have 34 s values from 12 1 to 23 4 which are significantly lighter than the counterpart in the cambrian 33 0 indicating that the h 2 s did not migrate from the cambrian but may have been derived from thermochemical sulfate reduction in situ as partially supported by 12 c rich co 2 in fluid inclusions and tsr calcites with 13 c values from 3 6 to 17 7 in the ordovician the h 2 s rich gas subsequently mixed with h 2 s poor 13 c rich methane dominated gas as indicated by the roughly negative correlations of h 2 s content to dryness and gas oil ratio and ch 4 from fluid inclusions having 13 c values 1 2 3 6 lighter than the associated free gas the h 2 s has been dissolved in formation water precipitated as pyrite and incorporated into oils and solid bitumens resulting in the generation of alkylthiolanes alkylthiols and alkyl 2 thiaadamantanes and increase in 34 s values and s c ratios of the solid bitumen the content of h 2 s produced from the water intervals or petroleum water transition zones is significantly contributed from exsolved gas and thus higher than that of the gas cap gas in the reservoirs thus h 2 s contents cannot be used to indicate tsr extents in the ordovician however in the cambrian whole oil and saturates show positive shifts in 13 c values with increasing h 2 s contents indicating that the h 2 s in the cambrian was derived from tsr by liquid hydrocarbons

sulfur and carbon isotopic compositions of the permian to triassic tsr and non tsr altered solid bitumen and its parent source rock in ne sichuan basin abstract reservoir solid bitumens from the permian to triassic and potential source rocks in the northeastern sichuan basin were measured for elemental and stable carbon 13c and sulfur 34s isotopic compositions to determine the chemical and isotopic composition of non tsr and tsr altered solid bitumen and to develop proxies to reflect the extent of bitumen alteration by tsr non tsr altered reservoir bitumen samples showed s c atomic ratios 0 03 and had 13c values closer to longtan formation p3l kerogen than to the lower silurian and cambrian kerogens and 34s values slightly more enriched than p3l kerogens the tsr altered solid bitumens have incorporated significant amounts of tsr derived h2s resulting in s c atomic ratios 0 047 0 074 and 34s values 13 4 24 0 being significantly higher than those of the solid bitumen not altered by tsr s c 0 013 0 030 34s 5 8 9 6 and the potential source rock kerogen the incorporation has produced a decrease in n c ratio and negative shift of 2 7 in 13c values that resulted from increasing tsr h2s back reactions with 12c rich n alkanes from the gas phase soluble solid bitumen or paleo oil to generate organic sulfur compounds such as ethanethiol that were incorporated into the solid bitumens which is followed by further aromatization and polymerization the solid bitumens show positive relationships of s c ratios and 34s values to the associated gas h2s h2s c1 6 or tsr extents and thus the s c ratios and 34s values can be used as proxies to reflect tsr extents

stratigraphic control of the geochemistry of holocene inner shelf facies great barrier reef received 15 august 1994 revision accepted 28 june 1995 abstract cleveland bay and halifax bay are adjacent embayments situated on the inner shelf region o 20 m of the central great barrier reef shelf off townsville northeast australia these bays contain holocene sediments up to 5 m in thickness deposited during the last stages of the post glacial sea level rise x ray diffraction and x ray fluorescence techniques were used to examine the mineralogy and geochemistry of the main holocene facies sampled in 40 vibrocores the sediments of southern halifax bay and cleveland bay consist mainly of quartz ca 50 alkali feldspars ca 20 clay minerals ca 20 consisting of mixed layer clays smectite kaolinite and illite and carbonate ca 10 including aragonite and calcite the holocene facies sequence comprises in order of increasing age modern bay shoreline and mangrove sediments the oxides of the major components sioz a o fe203 mgo cao k o na o p o in halifax bay show the predicted linear correlations with geochemically related minor elements pb rb sr y zr ga zn ni co mn cr ti sc v ba and can be used to discriminate holocene facies however in cleveland bay these elements are poorly correlated within each facies and provide very poor facies discrimination these findings probably result from the presence of sandy shoreline sediments up to 2 m in thickness in the holocene sequence of cleveland bay with the consequent development of a weak oxic zone and migration of elements between adjacent facies a similar oxic zone is poorly developed in halifax bay because the shoreline facies there is either thin or absent that the chemical signatures of holocene inner shelf facies are spatially variable and are strongly influenced by the stratigraphy has wide implications for geochemical studies of marine sediments

timing of the permian triassic biotic crisis implications from new zircon u pb age data and their limitations abstract the most profound biotic crisis in the earth s history causing the near extinction of both terrestrial and marine life occurred at the end of the permian period about 253 myr ago and marks the paleozoic mesozoic era boundary the cause of this event is still a matter of vigorous debate with both brief and catastrophic as well as gradual mechanisms having been proposed similar to a recent landmark study this study uses the u pb method on zircons from the uppermost permian lowermost triassic ash fall deposits at meishan zhejiang province se china in order to examine time and rate constraints for these events the results of both this study and previous work show that for these ash layers the effects of pb loss are combined with varying amounts and sources of inheritance resulting in an age scatter which prohibits the extraction of a statistically robust age in many cases though the effects of pb loss on the zircons analyzed in this study were reduced by leaching the grains in hydrofluoric acid as opposed to commonly applied air abrasion prior to analysis the presence within a single ash layer of multiple generations of older xenocrysts in many cases only slightly older than the depositional age has made quantitative interpretation even more difficult when these combined phenomena bias individual zircon ages by less than a percent they are extremely difficult to deconvolute and if multi grain analyses are used can become impossible to recognize because of the resulting age averaging monte carlo simulations using actual measurements of individual zircon crystals show that age excursions due to pb loss and xenocrystic contamination for the meishan bentonites are easily homogenized to the point of undetectability when replicate analyses of multi grain zircon samples are compared thus this study uses only high precision analyses of single crystals whether from our work or that of previous studies three main conclusions have emerged first our data require a significant increase in the age of the permian triassic boundary by more than 2 myr compared to the previous study which shifts the age to a value older than 253 ma second neither our data nor those from previous work can confirm or negate the possibility of a very abrupt biotic crisis third even large suites of very high quality single zircon u pb analyses for these tuffs cannot in most cases yield objective reliable and robust dates with accuracies at the sub myr level though the temptation to perform arbitrary selection of subsets of the analyses for that purpose is almost irresistible the last conclusion is not an indictment of zircon u pb dating in general other rocks and other zircon populations can and do behave very differently and further technical advances will likely improve our ability to prepare grains or sub grains of adequately enhanced quality for analysis consequently the results of the present study strongly suggest that for problems requiring time scale accuracy inferences from zircon u pb dating must be based on sufficiently large suites of single crystal or crystal domain high precision analyses

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

sulfur cycle in buried evaporites sulfur isotopes are potent indicators of the way in which sulfur behaves chemically during diagenesis we have studied sulfur isotope ratios 34s 32s from a number of minerals and compounds across the permian triassic boundary in the khuff formation abu dhabi the 34s in dissolved marine sulfate increased by 10 from the late permian to the early triassic despite precipitation of gypsum from permian and triassic seawater and its subsequent dehydration to anhydrite at depths greater than about 1000 m the primary marine stratigraphic sulfur isotope variation has been preserved in anhydrite in the khuff formation a combination of biostratigraphic and absolute age data show that this 10 shift occurred over 2 m y gypsum dehydration to anhydrite has not involved significant isotopic fractionation or diagenetic redistribution of material in the subsurface the sulfur isotope variation across the permian triassic boundary is also present in elemental sulfur and h2s at depths greater than 4300 m formed by reaction of anhydrite with hydrocarbons via thermochemical sulfate reduction this demonstrates that sulfate reduction has not led to isotope fractionation it also demonstrates that significant mass transfer has not occurred at least in the vicinity of the permian triassic boundary even though elemental sulfur and h2s are both fluid phases at depths greater than 4300 m thus despite two major diagenetic processes that converted the sulfur in gypsum into elemental sulfur and h2s by 4300 m burial and the potentially mobile nature of some of the reaction products the primary differences in sulfur isotopes have been preserved in the rocks and fluids all reactions occurred in situ there was no significant sulfur isotope fractionation and only negligible sulfur was added subtracted or moved internally within the system

the role of scour in shipwreck site formation processes and the preservation of wreck associated scour signatures in the sedimentary record evidence from seabed and sub surface data fully submerged shipwreck sites act as open systems with the exchange of material sediment water organic and inorganic objects and energy wave tidal storm across system boundaries formation processes at wreck sites are driven by some combination of chemical biological and physical processes with physical processes dominant in initial phases of site formation scouring and associated depositional patterns that form in response to hydrodynamic forcing are commonly the dominant physical processes acting at shipwreck sites scour is initiated by the introduction of a shipwreck to the seafloor leading to increases in flow velocity and turbulent intensity around the structure near field and far field scour pits form at wreck sites due to the interaction of horseshoe and lee wake vortices with the mobile substrate the morphology of resultant scour signatures are controlled by the orientation of the wreck structure in relation to the prevailing hydrodynamic regime the morphology and size of the wreck and individual site components the hydrodynamic regime currents waves or combined waves and currents bathymetry and the geology of the site seafloor and sub surface conditions individual objects or artefacts may act as nuclei to promote scour at a local scale under high energy conditions groups of artefacts and or disarticulated structural components emanating from a wreck may compound natural scour processes by rolling or sliding under suitable environmental conditions wreck associated scour features can be preserved in the sedimentary record

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

origins of palaeozoic oils in the tarim basin evidence from sulfur isotopes and biomarkers abstract crude oil h 2 s and pyrite samples from palaeozoic formations in the tarim basin have been analyzed for 34 s values together with oil samples for biomarkers and fluid inclusions for chemical composition to elucidate the origin of sulfur in the oil and to address potential applications in oil source rock correlation the results show that crude oil samples have sulfur contents from 0 02 to 2 47 and 34 s values from 11 9 to 28 2 non degraded oils without associated h 2 s gas have 34 s values from 11 9 to 20 5 and are relatively rich in c 28 20r sterane aryl isoprenoids and or gammacerane the features are well correlated with the cambrian and lower ordovician source rocks with 34 s values from 10 4 to 19 4 non degraded oils with associated h 2 s gas have 34 s values from 15 1 to 19 1 close to the h 2 s from 15 0 to 18 5 suggesting that the h 2 s was incorporated into the oils leading to the generation of 2 thiaadamantanes and likely alkyl thiolanes in the oil with a 34 s value of 18 5 heavily biodegraded oils have the highest sulfur contents and show 34 s values from 20 0 to 28 2 significantly heavier than those of any of the potential source rocks 15 3 to 19 4 thus it is likely that isotopically heavy sulfides have been incorporated into the biodegraded oils resulting in 34 s values of the oils becoming closer to cambrian and ordovician age seawater sulfates the sulfides may have originated from thermochemical sulfate reduction tsr as evidenced by the occurrence of h 2 s rich fluid inclusions and late stage and fracture filling pyrite with 34 s values mainly from 25 to 34 in the palaeozoic reservoirs

a theory of the ka band radar imaging mechanism of a submerged wreck and associated bed forms in the southern north sea the ka band radar imaging mechanism of the submerged wreck sand ribbon of the motor vessel m v birkenfels in the southern north sea is investigated by applying the quasi specular scattering theory and considering the capillary as well as the gravity wave ranges of the wave energy density spectrum for the imaging of wrecks and other oceanographic and meteorological phenomena at the sea surface it is assumed that quasi specular scattering becomes dominant at higher radar frequencies like ka and x band and wind speeds 7 8 m s 1 multibeam echo sounder images of the birkenfels wreck and associated sand ribbons as well as other available environmental in situ data have been analyzed the formation of sand ribbons at the sea bed and the manifestation of its radar signatures at the water surface are caused by an elliptical vortex or helical flow cell triggered by unidirectional tidal current flow interacting with the wreck the difference between simulated and measured normalized radar cross section nrcs modulation as a function of the space variable is less than 31 6 results are presented for nrcs simulations dependent on position for different effective incidence angles unidirectional current speeds wind speeds and relaxation rates the calculated current gradient or strain rate of the imaging theory has the same order of magnitude as those obtained for marine sand waves this implies that the responsible hydrodynamic interaction mechanism is able to produce radar signatures of submerged wrecks sand ribbons and make them visible at the sea surface

distinguishing cambrian from upper ordovician source rocks evidence from sulfur isotopes and biomarkers in the tarim basin abstract the reported source rocks for the abundant petroleum in the tarim basin china range from cambrian to lower ordovician and or upper ordovician in age however the difference between the two groups of source rocks is not well characterized in this study pyrite was removed from eleven mature to over mature kerogen samples from source rocks using the method of crcl 2 reduction and grinding the kerogen and coexisting pyrite samples were then analyzed for 34 s values results show that the kerogen samples from the cambrian have 34 s values between 10 4 and 19 4 the values are significantly higher than those from the lower ordovician kerogen 34 s of between 6 7 and 8 7 which in turn are generally higher than from the upper ordovician kerogen samples 34 s of between 15 3 and 6 8 the associated pyrite shows a similar trend but with much lower 34 s values this stratigraphically controlled sulfur isotope variation parallels the evolving contemporary marine sulfate and dated oil 34 s values from other basins suggesting that seawater sulfate and source rock age have an important influence on kerogen and pyrite 34 s values the relatively high 34 s values in the cambrian to lower ordovician source rocks are associated with abundant aryl isoprenoids gammacerane and c 35 homohopanes in the extractable organic matter indicating that these source rocks were deposited in a bottom water euxinic environment with water stratification compared with the upper ordovician the cambrian to lower ordovician source rocks show abundance in c 28 20r sterane c 23 tricyclic terpanes 4 23 24 trimethyl triaromatic dinosteroids and depletion in c 24 tetracyclic terpane c 29 hopane thus 34 s values and biomarkers of source rock organic matter can be used for distinguishing the cambrian and upper ordovician source rocks in the tarim basin

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

in situ geochemical characterization of pyrite crystals in burial dolomites of st george group carbonates sims was used to measure the 34s of pyrite disseminated in burial dolomite matrix of st george group at three localities in western newfoundland at main brook 34s values for the pyrite grains show wide variation 15 to 20 n 20 but are mostly negative combined with a paucity of two phase fluid inclusions in the host burial dolomite the depleted 34s values suggest that the pyrite is a by product of bsr predominantly negative 34s values was also obtained for sampled pyrite at daniel s harbour and port au choix however relatively high homogenization temperatures 100oc of two phase fluid inclusions in the host dolomites is incompatible with a bsr process for pyrite formation more so daniel s harbour is a site for main stage sulfide mineralization mvt system hosted in similarly burial dolomite and that has been previously constrained to be associated with tsr a relative proximity of the currently studied pyrite samples to this mvt system deposit is thus inconsistent with an in situ

contrasting diagenetic evolution patterns of platform margin limestones and dolostones in the lower triassic feixianguan formation sichuan basin china abstract deeply buried carbonate reservoirs from the lower triassic feixianguan formation in the sichuan basin of china host extensive natural gas resources these reservoirs are predominantly found in oolitic shoals with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts here we present a combination of petrographic isotopic fluid inclusion and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of dolostones and limestones the porosity of limestones has been reduced to 7 5 due to calcite cementation whereas the porosity in oolitic dolostones is not cemented with calcite and typically has 23 5 porosity dolomitization and concurrent early diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic dolostone first by stabilizing the rock fabric to inhibit loss of porosity during burial and secondly through the generation of new porosity by dissolution of carbonate and anhydrite a negative shift of 18 o and salinity values 13 c values 20 vpdb and 18 o 10 vpdb together with elevated calcite precipitation temperatures 110 c it is likely that tsr was responsible for the formation of enlarged dissolution vugs that increased porosity by 2 in dolostones due to i anhydrite dissolution ii production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite iii generation of h 2 s and co 2 and the consequent reaction of h 2 s with the siderite feco 3 component in calcite and dolomite this study demonstrates the importance of diagenesis in the formation of deeply buried high quality reservoirs in ooid dominated grainstones influenced by the presence of evaporites our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings

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

integrated geophysical surveys of the french frigate la surveillante 1797 bantry bay co cork ireland between 1998 and 2000 a series of geophysical surveys were conducted over the site of la surveillante a french frigate wrecked in 1797 in bantry bay co cork ireland the objectives of the surveys were to accurately relocate the wreck site delineate the extent of site map the seafloor and subsurface lithologies in the wreck environs and provide basemaps for more detailed diver investigations the integrated surveys employed a suite of equipment consisting of a digital echo sounder side scan sonar chirp sub bottom profiler and an overhause effect magnetometer integration and interpretation of the geophysical data indicate la surveillante was wrecked on a fine grained substrate in a low energy environment with site distribution limited to an area measuring 50 x 25 m centred on the wreck integration of geophysical and diver survey data with tidal and sediment records indicates site formation is dominated by biological and chemical processes

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

formation and preservation of a giant petroleum accumulation in superdeep carbonate reservoirs in the southern halahatang oil field area tarim basin china giant petroleum accumulations worldwide with burial depths more than 7000 m 23 000 ft occur mostly in mesozoic and cenozoic reservoirs and yield predominantly natural gas recently however a giant oil accumulation with reservoir depths between 7000 m 23 000 ft and 8000 m 26 000 ft was discovered in the lower paleozoic section in the southern part of the halahatang region in the tarim basin china petroleum sourced from lower paleozoic rocks is contained in ordovician karst fracture cave reservoirs and sealed by middle upper ordovician limestones and mudstones the newly discovered superdeep accumulation is among the deepest black single phase oil accumulations worldwide and opens up new avenues for petroleum exploration in deep marine carbonate reservoirs reservoir pressures are between 75 mpa 10 878 psi and 85 mpa 12 328 psi with pressure coefficients between 1 2 and 1 7 and temperatures ranging between 140 c 284 f and 172 c 342 f charging and accumulation of petroleum occurred during the late hercynian orogeny followed by subsequent gradual deep burial which took place before rapid subsidence beginning circa 5 ma following subsidence the thickness of overlying strata increased by more than 2000 m 6600 ft before finally attaining current depth therefore this oil accumulation represents a well preserved ancient petroleum system based on the geochemical features of oils and gases the crude oils can be classified as mature sourced from mixed marine organofacies of shale marl and carbonate whereas the gases were cogenerated with oils despite very high present day reservoir temperatures no oil cracking has occurred because of the relatively short exposure of oils to high temperatures in a low geothermal gradient regime thus there is significant exploration potential under similar conditions for liquid petroleum in superdeep strata faults and reservoirs are major factors controlling petroleum accumulation interlayer karsts with excellent fracture cavity connectivity developed adjacent to faults generally resulting in the enrichment of oil and gas along fault zones high quality reservoirs in this area are easy to identify because they exhibit strong bead like amplitude features in seismic sections wells located near faults produce relatively large amounts of oil and gas effective karst fracture cave reservoirs with noncracked oil may exist below 8000 m 26 000 ft in the tarim basin and represent a significant exploration target in china

dating and characterizing primary gas accumulation in precambrian dolomite reservoirs central sichuan basin china insights from pyrobitumen re os and dolomite u pb geochronology abstract primary gas accumulation with trillions of cubic meters of gas volumes was discovered recently in the precambrian dengying formation of the gaoshiti moxi block central sichuan basin china in this study we investigated the pervasive saddle dolomite cements and solid bitumens within karst vugs in the dengying dolomite reservoirs using laser ablation inductively coupled plasma mass spectrometry la icp ms in situ u pb dating we obtained a high precision absolute age of 259 4 3 0 ma for the saddle dolomite cementation consistent in timing with the eruption of emeishan flood basalts 262 251 ma combined with high homogenization temperatures th and salinities of primary fluid inclusion as well as rare earth element ree compositions we consider that the saddle dolomite precipitation was likely related to hydrothermal activity driven by the emeishan mantle plume the identification of rare surviving oil inclusions and extensive solid bitumens within the dolomite vugs is indicative of the presence of paleo oil reservoirs these bitumens are characterized by high equivalent vitrinite reflectance requ no fluorescence and deformed fibrous textures typical of highly mature pyrobitumens gas geochemistry data indicates that only minor thermochemical sulfate reduction tsr has occurred thus the pyrobitumen was interpreted to result mainly from the thermal cracking of paleo oils and its associated re os isochron age of 154 21 ma representing the timing of dry gas formation petrographical evidence suggests that the paleo oil charge must have taken place after sparry dolomite cementation but prior to pyrobitumen generation this combined with coeval aqueous inclusion th data and published thermal burial models constrains the time of paleo oil emplacement to ca 222 205 ma suggesting an oil filling event related to the indosinian orogeny this study documented the first chronological data constraining the complex evolution history of a petroleum system in one of the oldest primary gas reservoirs in the world

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

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 origin of pyrobitumens in upper devonian leduc formation gas reservoirs alberta canada an optical and eds study of oil to gas transformation pyrobitumens are common within deep basin devonian carbonate gas reservoirs of the western canada sedimentary basin incident light microscopy and sem eds have been used to study the origin of pyrobitumens from upper devonian isolated reefs of the leduc formation west central alberta reflectance properties romax and romin coke microtextures and relative s c ratios are used to classify the pyrobitumens in portions of reservoir units where nso enriched low aromaticity crudes may have been dominant oil to gas transformations produced mainly low reflecting isotropic to fine grained mosaic coke microtextures within the reservoir pyrobitumens type a in contrast where normal crudes were most likely dominant in the reservoir units prior to thermal cracking pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate type b oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance relative concentrations of type a and b pyrobitumens vary vertically and temporally in some of the reefal buildups the variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation e g tar mats in some zones whereas other zones may have been altered contemporaneously with crude oil cracking the later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction this reaction is prevalent within devonian evaporite carbonate strata of the western canada sedimentary basin generating copious volumes of h2s the interaction between crude oil and sulphur enriched highly alkaline fluids is suggested by the presence of high s c very fine grained late stage calcined needle cokes within brecciated pyrobitumens

petrological and geochemical constraints on porosity difference between lower triassic sour and sweet gas carbonate reservoirs in the sichuan basin abstract petrographic features carbon and oxygen isotopes and trace elements were determined and fluid inclusions were analyzed on late diagenetic cements and vug fillings from the lower triassic carbonates to account for much higher porosities in the sour dolomite reservoirs than in the sweet limestones in northeastern sichuan basin in the ne side to the kaijiang liangping trough dolostone reservoirs have porosities mainly from 6 to 12 up to 28 and contain 9 20 h 2 s the dolomite includes pre bitumen re crystallized silt sized crystals without occurrence of fluid inclusions and late burial dolomite the late burial dolomite has been measured to have fluid inclusions homogenization temperatures hts from 72 to 128 c part of the dolomites with hts up to 121 c show corroded edges or replacement by tsr calcites tsr calcites show 13 c values from 2 3 to 18 9 feo from null to 810 g g and so 3 from 50 to 18 700 g g vug filling tsr calcites have much lower feo and so 3 than the replacement calcites of anhydrite suggesting that dissolved fe 2 released from the dissolution of dolomite and anhydrite as a result of tsr may have reacted with tsr h 2 s and precipitated as pyrite the dissolution may have released 13 c rich co 2 resulting in the present day co 2 having 13 c values significantly heavier than those of the tsr calcites as light as 18 9 the dolomite and anhydrite dissolution during late stage of tsr may have enlarged or redistributed porosity in the ne side dolostones in contrast oolitic limestone reservoirs in the sw side contain significantly lower sulfates and thus no significant late diagenetic dissolution and tsr the porosity difference in the two sides resulted from original physical property due to early dolomitization and subsequent improvement from anhydrite and dolomite dissolution as a result of tsr

the role of computational fluid dynamics in understanding shipwreck site formation processes studies indicate that physical processes commonly dominate the initial stages of wreck site formation detailed knowledge and understanding of hydro and sediment dynamics are therefore imperative for studies dealing with site formation and in situ preservation in this investigation the results of computational fluid dynamic modelling over a shipwreck site are presented using a high resolution surface derived from multi beam echo sounder data and boundary conditions constrained by field measurements sediment samples and flow measurements simulation of the 3 dimensional flow velocity field around the wreck site and secondary products derived from the computational model confirm that flow velocity and turbulence are both amplified by the presence of the wreck causing changes in the morphology of the flow regime flow contraction the formation of lee wake vortices behind the structure accompanied by vortex shedding and increased turbulence are all observed shear stress and tke amplification three to four times greater than ambient values are recorded downstream of the wreck structure benefits of this approach for studies of site formation and in situ conservation include the inexpensive open source and desk based nature of the investigation

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

a new process based model for wreck site formation abstract in order for a model of wreck disintegration to have some predictive ability it is advantageous that such a model should be process orientated scale independent i e be applicable to the wreck as a whole and to individual wreck components and or spatially and temporally independent existing models and flowcharts for wreck site formation focus on the products rather than the processes of wreck disintegration the major classes of environmental processes affecting wreck site formation are physical biological and chemical which are each closely linked to the depositional environment here a modified flow chart for wreck evolution is presented in which the major variables influencing wreck site formation are the wreck itself sediment supply and the hydrodynamic environment the rate of wreck disintegration dd dt may be given by the sum of the rates of disintegration due to physical biological and chemical processes gsdp dt db dt dc dt a model is developed where wreck disintegration may be plotted against the relative sedimentation rate ds dt which is a first order control on the forces of degradation the model possesses the advantageous features noted above and acts to illustrate the characteristics of a wreck site and provide a framework in which to begin prediction of the nature of the material preserved although its application to marine wreck is outlined the model is applicable to both terrestrial and marine wreck site formation

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

stable carbon isotopes of alkane gases from the xujiahe coal measures and implication for gas source correlation in the sichuan basin sw china abstract the upper triassic xujiahe formation in the sichuan basin sw china consists of a series of coal measures the first third and fifth members of this formation are dominated by gas prone dark mudstones and coals the mudstones contain type ii and iii kerogens with average organic carbon contents around 1 96 these source rocks are mature in the central sichuan and highly mature in the western sichuan basin characterized by gas generation with subordinate amounts of light oil or condensate oils the source rocks are intercalated with the sandstone dominated second fourth and sixth members of the xujiahe formation thus leading to three separate self contained petroleum systems in the region the proven gas reserves in the xujiahe formation are only less than that of the triassic feixianguan formation and the xujiahe formation has the second largest gas field guang an gas field in the basin gases derived from the xujiahe formation coals generally show a normal stable carbon isotopic trend for c1 c4 n alkanes with the highest 13c2 values among the nine gas pay zones in the basin 20 7 to 28 3 and 13c1 values as low as 43 0 in the central sichuan gas accumulations with an oil leg have also been found in the eastern and southern sichuan where the thickness of the xujiahe formation is significantly reduced gases in these accumulations tend to show low 13c2 values 30 0 to 36 3 characteristic of oil prone source rocks

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

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

sulfur isotopic evidence for chemocline upward excursions during the end permian mass extinction the latest permian was a time of major change in ocean chemistry accompanying the greatest mass extinction of the phanerozoic to examine the nature of these changes samples from two well studied marine sections that span the permian triassic boundary have been analyzed the meishan and shangsi sections located in southern china isotopic analysis of the carbonate associated sulfate in these samples provides a detailed record of several isotopic shifts in 34scas approaching and across the ptb ranging from 30 to 15 vcdt with repeated asynchronous fluctuations at the two locations we interpret the patterns of isotopic shifts in conjunction with other data to indicate a shallow unstable chemocline overlying euxinic deep water which periodically upwelled into the photic zone these chemocline upward excursion events introduced sulfide to the photic zone stimulating a bloom of phototrophic sulfur oxidizing bacteria we hypothesize that elemental sulfur globules produced by these organisms and 34s depleted pyrite produced in the euxinic water column were deposited in the sediment later oxidation led to incorporation as cas this created the large changes to the 34scas observed in the latest permian at these locations

transition flow in step pool systems pressure distributions and drag forces abstractstep pool systems occur naturally in steep mountain streams and as man made structures in steep channel sections where they serve as energy dissipating structures three different flow regimes may occur the dominant flow regime is the cascading nappe flow regime at higher discharges the skimming flow regime develops with an almost plane water surface in between these flow regimes a transition flow regime occurs a physical model was used to measure the pressure distribution around a single step for a particular step pool geometry and for different flow regimes nappe transition skimming to achieve this 14 simultaneously recording piezoresistive pressure transducers were distributed around the step the results showed that the regime change from nappe to transition flow is associated with a discontinuity of the average froude number of a step pool unit as well as drops in pressures and water depth relative instantaneous drag forces reached a maximum for this regime change this implies

evidence for cross formational hot brine flow from integrated 87sr 86sr ree and fluid inclusions of the ordovician veins in central tarim china abstract rare earth elements 34s 87sr 86sr and 18o were determined and fluid inclusions were analyzed of fracture filling barite anhydrite fluorite calcite and quartz from upper ordovician carbonates in central tarim china the aim was to assess the origin evolution and flow of fluid in the fractures there was mixing of relatively hot ba rich and 87sr rich fluid with in situ basinal water and fresh water which was relatively cool and more radiogenic the hotter fluid is hydrothermal as indicated by homogenization temperatures hts about 20 c higher than the formation and was most likely derived from precambrian to lower cambrian clastic rocks or and from the basement through faults this is because almost all of 87sr 86sr ratios of late stage fracture fillings mainly 0 70889 0 71036 and present brines are higher than that of upper ordovician limestones the hydrothermal fluid was enriched in f eu2 ba2 sr2 and zn2 during migration as indicated by the positive relationships of eu to sr ba and zn when the hydrothermal fluid mixed with local basinal water it precipitated isotopically normal s anhydrite fluorite and calcite with positive eu anomalies the precipitation of these ca minerals was followed by thermochemical so4 reduction tsr of dissolved sulphates by hydrocarbons it was likely that only part of the so 4 2 was reduced and the generated h2s leaked out thus barite deposited from residual so 4 2 has abnormally high 34s values from 42 to 47 and the generated h2s is isotopically light from 15 to 18 subsequently hydrothermal fluid was likely to have mixed with relatively cool and more 87sr rich freshwater resulting in precipitation of quartz and fluorite with salinities as low as 0 5 wt nacl equiv and 87sr 86sr ratios up to 0 71036

study of thermochemical sulfate reduction of different organic matter insight from systematic tsr simulation experiments abstract a series of thermochemical sulfate reduction tsr simulation experiments were carried out involving different organic matter crude oil solid bitumen type ii kerogen type iii kerogen and different sulfate species anhydrite and mgso4 to address the chemical and carbon isotopic variations of the hydrocarbon h2s and co2 the increase of main peak carbons of residual saturated hydrocarbon and the decrease of gaseous hydrocarbons from control treatment to corresponding sulfate treatments suggest that tsr promote the consumption of both liquid and gaseous hydrocarbons the 13c1 3 values generally shift positively from control treatment to sulfate treatments and the 13cethane 13cmethane values in sulfate treatments are higher than that in control treatments the variation of 13c1 3 in tsr is controlled by the isotope fractionation during the generation and consumption of c1 3 in tsr mgso4 is more reactive than anhydrite in tsr a certain amount of h2s is incorporated into solid bitumen as the result of secondary alteration we believe that the reactivity order of different organic matter in tsr is crude oil solid bitumen type ii kerogen type iii kerogen the reactivity of organic matter in tsr depends on the hydrocarbon generation kinetics of each organic matter the dissolution decomposition and precipitation of carbonate control the yield of co2 in sour reservoirs the negative shift of 13co2 with increasing tsr extent is mainly due to the inheritance effect of carbon isotope from hydrocarbons inorganic co2 sourced from the thermal decomposition or acid dissolution of carbonate mineral impose significant influence on 13co2 the h2s yields decrease with ch4 yields and increase with 13ch4 value the 13ch4 values increase with residual amount of gaseous alkane 1 h2s residual alkane h2s suggesting that methane acted as reactant in tsr the 13co2 13ch4 values decrease significantly with increasing temperature and the 13co2 is even more negative than 13ch4 in 450 c mgso4 treatments involving type ii and type kerogen accordingly methane acted as a predominant reactant in 450 c sulfate treatments in our experiment it is possible for natural gas with high gas dryness to experience methane dominated tsr in geological reservoirs

flow regimes bed morphology and flow resistance in self formed step pool channels received 3 july 2008 revised 28 december 2008 accepted 13 january 2009 published 25 april 2009 1 we used a mobile bed flume with scaled grain size distribution channel geometry and flow to examine morphology and hydraulics of stepped channels we hypothesized that 1 step geometry and flow resistance differs significantly as a function of the range of grain sizes present 2 a transition from nappe to skimming flow occurs in stepped channels with mobile beds for conditions similar to stepped spillways and 3 the partitioning of flow resistance changes significantly when flow passes from nappe to skimming conditions results support each of these hypotheses and help to illuminate the complexity of v q relationships in stepped channels in which a dramatic decrease in flow resistance and increase in velocity accompany the transition from nappe to skimming flow near step forming events therefore a single flow resistance equation applicable to both ordinary and large floods may not be ideal in stepped channels nonetheless models based on dimensionless velocity and unit discharge appear more robust compared to those based on the darcy weisbach friction factor

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

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

the role of time lapse bathymetric surveys in assessing morphological change at shipwreck sites abstract data from time lapse single beam and multi beam bathymetric surveys are used to assess the rate and scale of morphological change at shipwreck sites caused by anthropogenic forcing the technique is illustrated by characterizing changes in wreck site morphology over two sites located on the east coast of ireland the drogheda boat site on the river boyne and the arklow bank site in the irish sea results indicate that repeated bathymetric surveys accurately and rapidly capture morphological change with time lapse accretion erosion models indicating possible erosion rates of up to 0 04 m per day and accretion rates of 0 36 m per day at dynamic riverine and marine sites in response to external triggers these data are valuable in understanding site formation processes at fully submerged archaeological sites and in the derivation of high resolution site formation models

alteration of solid bitumen by hydrothermal heating and thermochemical sulfate reduction in the ediacaran and cambrian dolomite reservoirs in the central sichuan basin sw china abstract solid bitumen in reservoir rocks is an allochthonous organic matter formed from bitumen or liquid hydrocarbons by various alteration processes in this study two types of altered solid bitumen have been identified in both the ediacaran dengying and cambrian longwangmiao formations in the central sichuan basin using petrographic microthermometric chemical and isotopic data the hydrothermally altered bitumen type 1 is associated with the pore filling saddle dolomite and quartz in which homogenization temperatures of some fluid inclusions are more than 10 c higher than the maximum burial temperature of the dengying formation it has various anisotropic textures with significantly high bireflectance values rbmax rbmin between 3 89 and 9 21 moreover the hydrothermal alteration led to 13c enrichment in the type 1 bitumen compared to the normally matured bitumen derived from the same source on the other hand the thermochemical sulfate reduction tsr altered bitumen type 2 has s c atomic ratios of up to approximately 0 06 the 34s values of type 2 bitumen are close to those of the carbonate associated sulfate and similar to the tsr derived h2s in the same reservoirs a strong negative correlation exists between s c atomic ratios and 13c values in the type 2 bitumen in the dengying formation which is interpreted as a result of an increasing incorporation of the 13c depleted ethanethiol into the bitumen during tsr this interpretation is consistent with the fact that the remaining ethane became isotopically heavier as tsr proceeded in addition the reactive sulfate for tsr was primarily derived from the hydrothermal sulfate such as barite this is evidenced by the replacement of hydrothermal barite by the 34s enriched pyrite and the small differences in 34s value between the barite and the type 2 bitumen or the tsr derived h2s in the same strata we suggest that tsr in the ediacaran and cambrian hydrocarbon reservoirs was probably initiated by hydrothermal activity

morphological effects of local scouring in step pool streams step pool morphology characterizes many high gradient streams in a variety of natural settings but formative processes and evolutionary dynamics are still poorly understood in this paper natural step pool geometry is compared with steep alluvial channels where grade control structures such as check dams and bed sills make the stream profile resemble a natural stepped stream along these channels local scouring due to falling jets forms plunge pools under each structure analogous to natural steps determining the formation of pools in order to test the hypothesis that natural pools are analogous to pools formed below grade control works with respect to their dimensions shape and formative dynamics 37 natural pools and 73 artificial pools were surveyed in 10 mountain streams of the eastern italian alps pools below grade control works featured a transitional zone between the scour hole and the downstream sloping bed marked by a depositional berm when geometric parameters such as maximum pool depth length and step berm distances are normalized to the jet virtual energy no statistically significant differences were detected between natural and artificial systems these results lend support to an upstream forced cascade model for step pool formation where the energy of falling jets controls the geometry of the pools and is therefore regarded as the most important scaling independent variable copyright 2005 john wiley sons ltd