precipitation of dolomite using sulphate reducing bacteria from the coorong region south australia significance and implications dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral hypersaline dolomitic lakes of the coorong region south australia analyses of lake and pore water samples document rapid geochemical changes with time and depth in both dolomitic and non dolomitic lakes extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate reduction zone whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water ultimately sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments most probable number calculations of lake sediment samples record the presence of large populations of sulphate reducing bacteria whereas sulphur isotope analyses of lake water samples indicate microbial fractionation in all the lakes studied viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory samples were then injected into vials containing sterilized clastic or carbonate grains or glass beads immersed in a solution that simulated the lake water falls in the levels of sulphate and rising ph in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate within 2 months sub spherical sub micron size crystals of dolomite identical to those of lake sediments were precipitated it is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs and by generating elevated carbonate concentrations in a hypersaline strongly electrolytic solution the results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments it is proposed that dolomite formation through bacterial sulphate reduction provides a process analogue applicable to thick platformal dolostones of the past where benthic microbial communities were the sole or dominant colonizers of shallow marine environments

origin and evolution of oilfield waters in the tazhong oilfield tarim basin china and their relationship to multiple hydrocarbon charging events abstract we conducted a chemical and isotopic study of oilfield waters from ordovician limestone reservoirs in the tazhong oilfield tarim basin china to trace the origin and evolution of the waters and to demonstrate their possible relationship with hydrocarbon charging the elemental chemistry cl br so4 i k na ca mg and sr and multiple isotopic compositions d 18o 87sr 86sr and 129i i of the oilfield waters were determined the results show that these oilfield waters contain basinal brines that infiltrated in three stages along with a lesser contribution from meteoric water the meteoric water appears to have infiltrated downwards during the late silurian and carboniferous the earliest brine had very low i contents 50 mg l was derived from cambrian argillaceous source rocks and migrated upwards into ordovician reservoirs this stage was related to the main period of migration of crude oils out of the source rocks into reservoirs during the silurian and devonian both of these brines incorporated constant levels of fissiogenic 129i from the limestone reservoir rocks 11 8 47 0 atom l finally brine with very low i contents but higher 129i 106 491 atom l began to migrate upwards and infiltrate reservoirs given the relatively high fissiogenic 129i this stage may have occurred during the late cenozoic and was associated with migration of dry gas from the same source rocks our results combined with those of a previous study of other oilfields in the basin suggest that the evolutionary of oilfield waters can be used to constrain hydrocarbon accumulation events and both of which were ultimately controlled by regional tectonism and local geological factors

microbial nucleation of mg rich dolomite in exopolymeric substances under anoxic modern seawater salinity new insight into an old enigma sulfate reducing bacteria are known to mediate dolomite formation under hypersaline conditions but details of the crystal nucleation process are still poorly constrained our laboratory study demonstrates for the first time that desulfobulbus mediterraneus a marine sulfate reducing bacterium mediates primary precipitation of mg rich dolomite under anoxic conditions in media replicating modern seawater chemistry at low temperature 21 c precipitation of crystals was associated with extracellular polymeric substances in a monospecific biofilm providing templates for nucleation by altering the molar mg ca ratio after initial nucleation of single nanospherulites 50 nm growth was mediated by aggregation resulting in spherulites of 2 3 m in diameter nucleation led to differences in mg ca ratios and 44 40ca values among the organic material i e biofilm including cells and extracellular polymeric substances 0 87 0 01 2 sd and 0 48 0 11 2 se respectively the crystals 1 02 0 11 2 sd and 0 08 0 24 2 se respectively and the liquid bulk medium after mineral precipitation 4 53 0 04 2 sd and 1 10 0 24 2 se respectively these data indicate a two step fractionation process involved in the sequestration of ca from the solution into the crystal lattice of the mineral precipitated our results demonstrate the capability of extracellular polymeric substances to overcome kinetic inhibition fostering the formation of kinetically less favorable mg rich dolomite and they also question the applicability of the ca isotopic system as a proxy for paleogeochemistry of seawater

chemistry and environments of dolomitization a reappraisal abstract dolomitization of calcium carbonate can best be expressed by mass transfer reactions that allow for volume gain preservation or loss during the replacement process experimental data as well as textures and porosities of natural dolomites indicate that these reactions must include co 3 2 and or hco 3 supplied by the solution to the reaction site since dolomite formation is thermodynamically favoured in solutions of a low ca 2 mg 2 ratios b low ca 2 co 3 2 or ca 2 hco 3 ratios and c high temperatures the thermodynamic stability for the system calcite dolomite water is best represented in a diagram with these three parameters as axes kinetic considerations favour dolomitization under the same conditions and additionally at low as well as at high salinities if thermodynamic and kinetic considerations are combined the following conditions and environments are considered chemically conducive to dolomitization 1 environments of any salinity above thermodynamic and kinetic saturation with respect to dolomite i e freshwater seawater mixing zones normal saline to hypersaline subtidal environments hypersaline supratidal environments schizohaline environments 2 alkaline environments i e those under the influence of bacterial reduction and or fermentation processes or with high input of alkaline continental groundwaters and 3 many environments with temperatures greater than about 50 c subsurface and hydrothermal environments whether or not massive replacive dolostones are formed in these environments depends on a sufficient supply of magnesium and thus on hydrologic parameters most massive dolostones particularly those consisting of shallowing upward cycles and capped by regional unconformities have been interpreted to be formed according to either the freshwater seawater mixing model or the sabkha with reflux model however close examination of natural mixing zones and exposed evaporitic environments reveals that the amounts of dolomite formed are small and texturally different from the massive replacive dolostones commonly inferred to have been formed in these environments many shallowing upward sequences are devoid of dolomite it is therefore suggested that massive replacive dolomitization during exposure is rare if not impossible rather only small quantities of dolomite cement or replacement are formed which may act as nuclei for later subsurface dolomitization alternatively large scale dolomitization may take place in shallow subtidal environments of moderate to strong hypersalinity the integration of stratigraphic petrographic geochemical and hydrological parameters suggests that the only environments capable of forming massive replacive dolostones on a large scale are shallow hypersaline subtidal environments and certain subsurface environments

the role of sulphate reducing bacteria and cyanobacteria in dolomite formation in distal ephemeral lakes of the coorong region south australia abstract the dolomitic distal ephemeral lakes of the coorong region of south australia are dynamic systems in which the concentrations of solutes generally increase during late spring and summer when the rate of evaporation greatly exceeds aqueous input during late evaporation active bacterial sulphate reduction occurs in uppermost anoxic lake sediments as rising salinity and then h2s toxicity kills off the macrobiota these hypersaline lakewaters are characterised by high ph and elevated concentrations of carbonate and sulphate ions magnesium concentrated in cyanobacterial cells and sheaths is released into the strongly electrolytic alkaline lake brines from lake margin stromatolites and mats during desiccation and may explain the remarkable increase in mg levels recorded at this time continued evaporation of the lakewaters produces a halite crust overlying the dolomite sulphate free residual puddles and the absence of gypsum indicates that all sulphate is removed by sulphate reducing bacteria dolomite forms in situ as a primary precipitate from these microbially mediated lake porewaters and both the morphology and size distribution of dolomite grains is identical to bacterial cells it is not proven exactly when dolomite precipitation occurs but the submicron sized grains and multiple defects in a disordered lattice indicate that precipitation is rapid and follows bacterial removal of sulphate which is known to inhibit dolomite formation bacterial sulphate reduction and cyanobacterial degradation may thus lead to the removal of all kinetic inhibitors to dolomite formation and provide a mechanism for dolomite precipitation around bacterial cells in shallow subsurface sediments during the late stages of evaporation in adjacent non dolomitic lakes sulphate is present throughout evaporation so that other minerals principally aragonite and hydromagnesite are precipitated

the contribution of loess magnetism in china to the retrieval of past global changes some problems abstract in the loess plateau of china the stratigraphical position of the brunhes matuyama boundary was not found in loess unit l8 at luochuan and jixian as is commonly accepted the interbedded palaeosols may be differentiated from the parent loess not only by their high magnetic susceptibility but also by their enhanced remanence properties these contrasts mainly result from concentration changes of the inherited magnetic minerals grain size changes and authigenic formation of magnetic minerals which are all linked to climate controlled soil formation processes

a route for the direct crystallization of dolomite the direct crystallization of dolomite from an aqueous solution at temperatures between 60 220 c was followed in situ through time resolved synchrotron based energy dispersive x ray diffraction combined with offline high resolution imaging x ray diffraction and infrared spectroscopy crystalline camg co3 2 phases form through a three stage process in the first stage a nanoparticulate magnesium deficient amorphous calcium carbonate mg acc with a nominal formula of ca0 606mg0 394co3 1 37h2o forms after a temperature dependent induction time during stage 2 the mg acc partially dehydrates and orders prior to its rapid 5 min crystallization to non stoichiometric proto dolomite this occurs via the dissolution of mg acc followed by the secondary nucleation of proto dolomite from solution the proto dolomite crystallization proceeds via spherulitic growth that follows a growth front nucleation mechanism with a de nuovo and continuous formation of nanocrystalline proto dolomite subunits that form spherical aggregates in stage three of the reaction the proto dolomite transforms to highly crystalline and stoichiometric dolomite on a much longer timescale hours to days via an ostwald ripening mechanism such a three stage crystallization can explain microbially induced proto dolomites observed in modern hypersaline settings and may also be the route by which the cryogenian cap dolomite deposits of the neoproterozoic formed

catalytic effect of microbially derived carboxylic acids on the precipitation of mg calcite and disordered dolomite implications for sedimentary dolomite formation abstract the genesis of dolomite is a contentious issue partly due to the difficulty in its synthesis at ambient temperature certain types of microorganisms have been demonstrated to be effective in promoting the precipitation of disordered dolomite an important precursor of sedimentary ordered dolomite in contrast to a growing body of research on the catalytic role of microbial exopolymers in the crystallization of disordered dolomite the role of other microbial exudates e g carboxylic acids remains unknown to fill in this knowledge gap precipitation experiments mimicking the carbonation process within microbial mats were conducted in saline solutions containing 0 30 mm succinic acid or citric acid which are commonly produced by microbes the starting salinities of experiment solutions were set to 35 and 70 in order to evaluate the effect of solution salinity on dolomite formation our results showed that both succinic acid and citric acid enhanced the incorporation of mg2 into growing ca mg carbonates solution salinity also played a positive role in enhancing mg signature in ca mg carbonates disordered dolomite with 40 92 mol mgco3 was detected under the conditions of 30 mm succinic acid and 70 salinity whereas in other reactors with succinic acid mg calcites formed citric acid was more effective in loading mg2 into ca mg carbonates compared with succinic acid as evidenced by the predominant occurrence of disordered dolomite with mgco3 content ranging from 40 91 mol to 46 75 mol in most conditions tested the results of this study have implications for the formation mechanism of sedimentary dolomite

analysis of secondary migration of hydrocarbons in the ordovician carbonate reservoirs in the tazhong uplift tarim basin china the process and mechanisms of secondary hydrocarbon migration in the tazhong uplift tarim basin were investigated based on the analysis of the regional structure and by integrating geologic hydrodynamic and geochemical parameters parameters successfully analyzed included the fluid potential fluid properties production outputs and diamantane index the results indicated that hydrocarbons migrated into the tazhong uplift from the northern part of the manjiaer depression through a series of injection points ips during four orogenies that is the early caledonian 510 ma the late caledonian 439 ma the late hercynian indosinian 290 ma and the yanshanian himalayan 208 ma a total of six ips were identified at the intersections of the northeast trending faults and the northwest trending flower strike faults the hydrocarbons migrated from the ips into traps along regional trends from northwest to southeast and from northeast to southwest the hydrocarbon migration process and patterns determined the distribution of hydrocarbon properties and production rates in the tazhong uplift with increasing distance from the ips daily hydrocarbon production decreases and the hydrocarbons become progressively heavier and display lower gas oil ratios

secondary migration of hydrocarbons in the zhujiang formation in the huixi half graben pearl river mouth basin south china sea the process and mechanisms of secondary hydrocarbon migration in the huixi half graben pearl river mouth basin were investigated on the basis of geological analysis of the strata and study of the porosity and permeability of the reservoir rocks fluid potential oil properties and geochemistry of oil source correlation the results suggest that the hydrocarbons of the zhujiang formation in the huixi half graben were derived from source rocks of the eocene wenchang formation and the eocene oligocene enping formation in the huizhou sag the hydrocarbons migrated laterally from northeast to southwest the sandstone in the upper member of the zhujiang formation exhibited superior physical properties porosity and permeability and connectivity than the lower member thin sandstone beds with good physical properties and stable distribution in the upper member of the zhujiang formation were the main carrier beds for lateral hydrocarbon migration

application of diamondoids to distinguish source rock facies abstract diamondoid distributions in solvent extracts from marine siliciclastic shales marine carbonates and coals of a wide variety of maturities were determined by gc msd for this sample set published diamondoid maturity parameters did not change systematically throughout the oil window it was found that several novel diamondoid ratios were indicative of source it was possible to distinguish between terrigenous marine carbonate and marine siliciclastic facies the proposed dimethyl diamantane facies ratios are virtually unaffected by maturity while the ethyl adamantane ratio appears to be affected by maturity in the late gas window range

petroleum migration pathways and charge concentration a three dimensional model petroleum migration pathways through a basin are determined by the three dimensional distribution of discontinuous sealing surfaces which are usually parallel to bedding the petroleum migrates below the sealing surface taking the structurally most advantageous route the three dimensional distribution of migration pathways within the petroleum system can be modeled on a personal computer using a program based on the parameters discussed in this paper application of the model to the paris and williston basins demonstrates that a good correlation between predicted pathways and discovered accumulations can be made using simple models pathways form a dense network overlying generating areas in the central parts of basins toward the basin margins these routes commonly become increasingly focused into discrete pathways by the sealing surface morphologies eventually these pathways may reach the surface as seepages it is important to integrate surface outcrops of migration routes surface seepages into migration modeling deflection of the pathways from the structurally most advantageous route below the sealing surface may be caused by lateral sealing barriers due to facies variation in the carrier rock below the seal fault juxtaposition or cross formational seals such as salt intrusions deflection of pathways also occurs where there are hydrodynamic conditions in response to topography driven groundwater flow zones of vertical migration are associated with facies changes along the horizon of the sealing surface into a nonsealing facies or juxtaposition to nonsealing strata by faults vertical migration from either normally or abnormally pressured strata is most likely to occur into normally or lesser pressured strata at intrabasinal highs where hydrocarbons can be stored and transferred at times of temporary seal rupture reverse modeling of individual pathways from fields and seepages to potential sources is a powerful exploration tool it assists in predicting the boundaries of the leaking accumulations or areas of petroleum expulsion

presence of sulfate does not inhibit low temperature dolomite precipitation abstract the hypothesis that sulfate inhibits dolomite formation evolved from geochemical studies of porewaters from deep sea sedimentary sequences and has been tested with hydrothermal experiments we examined the sulfate inhibition factor using aerobic culture experiments with virgibacillus marismortui and halomonas meridiana two moderately halophilic aerobic bacteria which metabolize independent of sulfate concentration the culture experiments were conducted at 25 and 35 c using variable so42 concentrations 0 14 28 and 56 mm and demonstrate that halophilic aerobic bacteria mediate direct precipitation of dolomite with or without so42 in the culture media which simulate dolomite occurrences commonly found under the earth s surface conditions hence we report that the presence of sulfate does not inhibit dolomite precipitation further we hypothesize that if sedimentary dolomite is a direct precipitate as in our low temperature culture experiments the kinetic factors involved are likely to be quite different from those governing a dolomite replacement reaction such as in hydrothermal experiments consequently the occurrence and presumably growth of dolomite in so42 rich aerobic cultures may shed new light on the long standing dolomite problem

dolomite abundance and stratigraphic age constraints on rates and mechanisms of phanerozoic dolostone formation increasing dolomite abundance with age relative to limestone has long been accepted as one of the general truisms of sedimentary geology a relationship which suggests that dolomitization is a cumulative process occurring slowly over the full burial history of a carbonate sequence however reevaluation of data in support of this assumption indicates that although relative dolomite abundances do change significantly throughout phanerozoic carbonate sequences no data set demonstrates any direct correlation between dolomite content and age whereas tabulation of available data does suggest greater amounts of dolomite formation during periods of continental flooding global mass balance calculations of potential magnesium sources also indicate that most dolomite has formed from marine or marine derived fluids at the earth9s surface this conclusion is indirectly supported by significant short term period scale changes in phanerozoic dolomite abundance a variation which would be more attenuated if dolomitization were dominantly a burial process dolomitization may be enhanced during times of global transgression higher atmospheric pco 2 and lower calcite saturation state in shallow marine settings

aerobic microbial dolomite at the nanometer scale implications for the geologic record microbial experiments are the only proven approach to produce experimental dolomite under earth s surface conditions although microbial metabolisms are known to induce dolomite precipitation by favoring dolomite growth kinetics the involvement of microbes in the dolomite nucleation process is poorly understood in particular the nucleation of microbially mediated dolomite remains a matter for investigation because the metabolic diversity involved in this process has not been fully explored herein we demonstrate that halomonas meridiana and virgibacillus marismortui two moderately halophilic aerobic bacteria mediate primary precipitation of dolomite at low temperatures 25 35 c this report emphasizes the biomineralogical implications for dolomite formation at the nanometer scale we describe nucleation of dolomite on nanoglobules in intimate association with the bacterial cell surface a combination of both laboratory culture experiments and natural samples reveals that these nanoglobule structures may be 1 the initial step for dolomite nucleation 2 preserved in the geologic record and 3 used as microbial tracers through time and or as a proxy for ancient microbial dolomite as well as other carbonate minerals

the abundance and distribution of diamondoids in biodegraded oils from the san joaquin valley implications for biodegradation of diamondoids in petroleum reservoirs abstract the biodegradability of diamondoids was investigated using a collection of crude oil samples from the san joaquin valley california that had been biodegraded to varying extent in the reservoir our results show that diamondoids are subjected to biodegradation which is selective as well as stepwise adamantanes are generally more susceptible to biodegradation than other diamondoids such as diamantanes and triamantanes we report a possible pathway for the microbial degradation of adamantane this cage hydrocarbon possibly breaks down to a metabolic intermediate through the action of microbes at higher levels of biodegradation in petroleum reservoirs microbial alteration has only a minor effect on diamondoid abundance in oil at low levels of biodegradation our results suggest that most diamondoids with the exception of adamantane are resistant to biodegradation like the polycyclic terpanes e g c19 c24 tricyclic terpanes hopanes gammacerane oleananes ts tm c29 ts steranes and diasteranes microbial alteration of diamondoids has a negligible impact on the quantification of oil cracking achieved using the diamondoid biomarker method

geochemical characteristics and origin of natural gas in tarim basin china the tarim basin located in northwest china is one of the largest basins in the world that has not been well explored in recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1 1 ratio of oil to gas resource this means that the natural gas resource is very important for the hydrocarbon resource in the tarim basin in this basin most of the discovered gases are condensate associated gas and oil associated gas research on chemical components and isotope composition of carbon and hydrogen of natural gases from the tarim basin shows that all discovered gases in this basin are thermogenic with respect to source organic matter there are two types of gases 1 oil type gas generated from sinian to lower paleozoic marine source rocks with the sapropelic organic matter with 44 5 to 33 8o oo of d13c1 42 0 to 28 1o oo of d13c2 35 4 to 28 4o oo of d13c3 and heavier than 200o oo of dd1 and 2 coal type gas generated from mesozoic terrestrial source rocks with the humic organic matter with 40 5 to 33 1o oo of d13c1 29 7 to 21 3o oo of d13c2 26 3 to 20 3o oo of d13c3 and lighter than 200o oo of dd1

microbial precipitation of dolomite in methanogenic groundwater we report low temperature microbial precipitation of dolomite in dilute natural waters from both field and laboratory experiments in a freshwater aquifer microorganisms colonize basalt and nucleate nonstoichiometric dolomite on cell walls in the laboratory ordered dolomite formed at near equilibrium conditions from groundwater with molar mg ca ratios of 1 dolomite was absent in sterile experiments geochemical and microbiological data suggest that methanogens are the dominant metabolic guild in this system and are integral to dolomite precipitation we hypothesize that the attached microbial consortium reacts with the basalt surface releasing mg and ca into solution which drives dolomite precipitation via nucleation on the cell wall these findings provide insight into the long standing dolomite problem and suggest a fundamental role for microbial processes in the formation of dolomite across a wide range of environmental conditions

microbially catalyzed dolomite formation from near surface to burial abstract over the past century a number of models have been developed to explain the vast stratigraphic distribution of authigenic dolomite these models include hydrogeological geochemical and geomicrobiological interpretations that shed light on kinetic constraints on low temperature dolomite formation and in particular how certain microbes can overcome such limitations in this review we focus on the direct and indirect mechanisms by which microbes facilitate the nucleation and shallow burial diagenetic stabilization of dolomite in marine sediments we also consider the likely effects of local and global scale biogeochemical perturbations over the distribution and activities of subsurface microbes and thus potentially over the stratigraphic distribution of dolomite considering the many advances that have been made in the fields of geomicrobiology and biogeochemistry during the past 20 years investigating the important role microbes play in sedimentary systems will undoubtedly contribute to a more complete interpretation of dolomite formation in the rock record

oil composition variation and reservoir continuity unity field sudan abstract a suite of oils from stacked reservoirs in the unity field in sudan has been analyzed by various geochemical techniques for molecular information to elucidate the geological processes which cause variations in oil composition and their resulting oil fingerprints in different reservoir units analyses of these highly paraffinic oils indicate that the chromatographic fingerprint variations are due to differences in the abundances of saturated compounds including branched and cyclic alkanes neither aromatics nor nso compounds have any significant effect on the observed fingerprint variations this association of saturates instead of aromatics and nso compounds with the fingerprint variations precludes rock fluid interactions as a cause of the variations biomarker analyses show that variations in thermal maturity and organic facies of the source rock are responsible for the fingerprint variations thermal maturity increases with the depth of the reservoir suggesting a multiple charge process for the oils to fill these reservoirs over an extended period of time apparently the source rock generated and expelled progressively more mature oils and little mixing occurred during migration thus knowledge of oil compositional variations from one reservoir to another organic facies variation and source rock maturity combined with tectonic history may help explain charging and timing of oil emplacement

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

testing the basin centered gas accumulation model using fluid inclusion observations southern piceance basin colorado the upper cretaceous mesaverde group in the piceance basin colorado is considered a continuous basin centered gas accumulation in which gas charge of the low permeability sandstone occurs under high pore fluid pressure in response to gas generation high gas pressure favors formation of pervasive systems of opening mode fractures this view contrasts with thatofothermodelsoflow permeabilitygasreservoirsinwhich gas migrates by buoyant drive and accumulates in conventional traps with fractures an incidental attribute of these reservoirs we tested the aspects of the basin centered gas accumulation model as it applies to the piceance basin by determining the timing of fracturegrowth and associated temperature pressure and fluid composition conditions using microthermometry and raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth trapping temperatures of methane saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185 c and then decrease to approximately 158 c over time which indicates fracture growth during maximum burial conditions calculated pore fluid pressures for methanerich aqueous inclusions of 55 to 110 mpa 7977 15 954 psi indicate fracture growth under near lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge lack of systematic pore fluid pressure trends

tectono thermal evolution of cambrian ordovician source rocks and implications for hydrocarbon generation in the eastern tarim basin nw china abstract the tectono thermal evolution is an important control on petroliferous basin development however this evolution is difficult to reconstruct in sedimentary basins subjected to multiple tectono thermal events the tarim basin is the largest petroliferous basin in northwestern china and hosts a complex oil gas system comprising several sets of source reservoir and cap rocks that have experienced multiple tectonic thermal and sedimentary events large oil and gas reserves have been discovered in the central and northern tarim basin during the last 30 years but none in the eastern part of the basin this is generally attributed to the high thermal maturity reached by source rocks during the late ordovician and subsequent hydrocarbon loss therefore reconstructing the cambrian ordovician tectono thermal evolution is critical in facilitating successful oil and gas exploration in this region we compiled equivalent vitrinite reflectance data n 178 of source rocks from seven wells in the tarim basin which for cambrian ordovician source rocks are 1 3 beyond the hydrocarbon generation threshold and in the main gas and dry gas generation stages the cambrian ordovician source rocks experienced rapid burial and heating during the caledonian period entered the hydrocarbon generation stage during the middle late ordovician and reached peak hydrocarbon generation at the end caledonian period they might have experienced secondary hydrocarbon generation after the jurassic homogenization temperatures of fluid inclusions n 616 from wells gc4 lx1 md1 and yd2 as well as the modeled thermal evolution reveal a main hydrocarbon generation event during the caledonian period and a secondary event in the yanshanian and himalayan period in other wells lx1 md1 and yd2 hercynian and indosinian uplift and exhumation in the eastern tarim basin controlled the main structures and faulting created favorable conditions for secondary hydrocarbon generation and migration at this time since the yanshanian and himalayan the tarim basin has experienced further sedimentation and heating and the tectono thermal evolution might have been related to the formation of the tibetan plateau in summary our reconstruction of the tectono thermal evolution of cambrian ordovician strata and the hydrocarbon generation history of the tarim basin suggests that the eastern tarim basin may also have hydrocarbon generation potential and can guide future oil and gas exploration

diamondoid hydrocarbon ratios novel maturity indices for highly mature crude oils abstract diamondoid hydrocarbons have been identified in oil and source rock samples from the tarim yinggehai qiongdongnan and other chinese basins two diamondoid hydrocarbon ratios have been used as novel high maturity indices to evaluate the maturation and evolution of crude oils and condensates in these chinese basins the two ratios 1 ma 1 ma 2 ma mai methyl adamantane index and 4 md 1 md 3 md 4 md mdi methyl diamantane index were found to increase with increasing vitrinite reflectance the initial values for the ratios are ca 50 and 30 respectively diamondoid hydrocarbons are products of rearrangement of polycyclic hydrocarbons and their ratios are found to be affected by organic input to a minor extent based on this study crude oils from the tarim basin are generally believed to be very mature ro 1 1 the cambrian ordovician source beds are the major source rocks of crude oils and natural gases in this basin condensates from the tarim yinggehai and qiongdongnan basins have higher maturities equivalent to ro values of 1 6 2 0 gases and condensates from the yinggehai and qiongdongnan basins are generated mainly from tertiary source rocks below 4000 5000 m

characterization of reservoirs fluids and productions from the ordovician carbonate condensate field in the tarim basin northwestern china the tz no1 gas field in the tarim basin is the largest condensate field in china however it has not developed efficiently because of low and unpredictable production during the past 20 yr cores logging interpretation seismic descriptions fluid properties and production data indicate that this ordovician carbonate field is different from conventional stratigraphic oil and gas fields as follows 1 the hydrocarbon bearing area covers a large region of 2000 km2 2 matrix reservoirs have low porosity 20 per year an examination of the carbonate reservoir and hydrocarbon accumulation history suggests that this large scale stratigraphic accumulation formed during hydrocarbon emplacement in the early paleozoic with the reservoirs gradually evolving into tight reservoirs as a result of intense diagenesis variable amounts of neogene gas charged the tight carbonate reservoirs and formed unconventional accumulations the tz no1 gas field is characterized by a strongly heterogeneous tight matrix reservoir with a superimposed fracture cave reservoir and a complicated unconventional fluid distribution which provides insights into the exploitation challenges of unconventional carbonate resources

evolution of the late cenozoic tectonic stress regime in the shanxi rift central north china plate inferred from new fault kinematic analysis abstract the shanxi rift an intracontinental belt along the eastern margin of the ordos block north china plate has undergone multi stage extensional deformation during the late cenozoic in this study new fault kinematic analysis of boundary faults in the shanxi rift constrained by geochronological data defines a three stage tectonic stress regime since the late miocene providing new structural evidence for deciphering the northeastward growth of the tibetan plateau the earliest stage involved a transtensional stress regime of nw se extension and ne sw compression in the late miocene early pleistocene ca 10 1 8 ma and characterized by mean principal stress axes of 1 at 344 7 87 2 2 at 217 6 01 1 and 3 at 128 1 01 0 the early transtensional stress regime was responsible for the opening of the shanxi rift and was very likely associated with intensive right lateral pull apart activity in the eastern ordos block due to the northeastward growth of the tibetan plateau during the second stage the stress regime changed to less intense ne sw extension with mean stress orientations of 323 3 85 2 for 1 127 5 02 9 for 2 and 214 9 02 6 for 3 in the early late pleistocene ca 1 8 0 11 ma this stage is marked by a distinct shift from fluvio lacustrine to fluvio piedmont sedimentary facies at the end of the early pleistocene and the widespread eruption of basalt in the datong basin north of the shanxi rift this short lived extension appears to be the result of relaxation in the midst of crustal shortening during growth of the tibetan plateau the third stage was transtensional characterized by nnw sse extension and wnw ese compression 1 at 060 88 9 2 at 074 9 03 9 and 3 at 164 00 since the late pleistocene ca 0 11 ma causing dextral shear deformation in the shanxi rift and the disappearance of its paleo lake this stage resulted from regional ene wsw shortening due to northeastward growth of the tibetan plateau

microbial dolomite precipitation using sulfate reducing and halophilic bacteria results from qinghai lake tibetan plateau nw china past studies have revealed the important roles of certain types of microbes in precipitating dolomite in coastal hypersaline lakes and lagoons and marine sediments however it is still poorly known if other types of microbes can precipitate dolomite in low salinity lakes without any marine influence the objective of this study was therefore to expand the microbial dolomite precipitation model by investigating the origin of dolomite in the sediments of qinghai lake an inland slightly saline lake salinity 12 5 g l on the tibetan plateau nw china this objective was accomplished via an integrated approach combining geochemical investigations of natural dolomite present in the lake sediments with laboratory precipitation experiments a 5 m sediment core was collected and pore water chemistry was analyzed calculation of saturation index indicated that dolomite was oversaturated in sediment pore water sub spherical grains of dolomite with abundant pores and microstructures similar to microbially precipitated dolomite were found in the sediments suggesting a biogenic origin dolomite precipitation experiments were set up in the laboratory to simulate the natural conditions of the qinghai lake dolomite was precipitated using sulfate reducing bacterium srb desulfotomaculum ruminis and aerobic halophilic bacterium halomonas marina present in the qinghai lake sediments aqueous geochemistry and mineral characterization data collectively suggest that dolomite precipitation was accomplished via increase of ph and alkalinity to overcome the kinetic inhibitors and exopolymeric substances eps and cell walls as possible nucleation sites these results expand the current microbial dolomite precipitation model and have implications for dolomite formation in slightly saline inland lake environments

temperature and pressure characteristics of ordovician gas condensate reservoirs in the tazhong area tarim basin northwestern china this paper investigates the temperature and pressure distribution characteristics and their evolution in the upper ordovician strata in the tazhong area tarim basin northwestern china based on hundreds of temperature data from boreholes the present day temperature ranges from 100 c to 130 c increasing from the southeast to the northwest and having a cool belt in the central fault horst pore pressures in the upper ordovician reservoirs range from 35 to 62 mpa 5076 to 8992 psi overpressures developed in the southwestern region and along the eastern segment of the central fault horst to the northern slope the lower ordovician reservoirs in the east reached 140 c during the late ordovician but those in the west heated continuously and reached the maximum temperature 130 c during present day paleopressures reconstructed from fluid inclusion data indicate that overpressures caused by hydrocarbon generation were formed in the areas away from fault belts but were absent in the area around the no 1 fault belt the temperature evolution indicates that the middle lower cambrian source rocks reached gas generation stage vitrinite reflectance ro 2 0 at the end of the ordovician whereas the middle upper ordovician source rocks are still in the high maturity stage ro 1 3 at the present day deep oil and gas migration is driven by overpressure and commercial reservoirs are predominantly distributed in the normally pressured zones along the no 1 fault belt the formation of gas condensate is controlled by specific temperature and pressure characteristics

paleozoic oil source rock correlations in the tarim basin nw china we studied a suite of 40 oils and extracts of purported source rocks from the tarim basin in nw china the main group of oils comes from tazhong and tabei wells which sample the largest known petroleum accumulations in the basin these oils can be statistically correlated with extracts of ordovician rocks based upon high relative concentrations of 24 isopropylcholestanes and low relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes in contrast extracts from cambrian rocks have low relative concentrations of 24 isopropylcholestanes with high relative concentrations of dinosteranes triaromatic dinosteroids and 24 norcholestanes although some tarim basin cambrian rocks yield high total organic carbon contents we see little evidence in the analyzed oil samples to suggest that they came from cambrian source rocks

magnetostratigraphy of the dali basin in yunnan and implications for late neogene rotation of the southeast margin of the tibetan plateau 1 the rotation pattern and fault activity in the southeast margin of the tibetan plateau semtp provide meaningful constraints on the geodynamic evolution of the plateau however the lack of cenozoic paleomagnetic studies and accurate age constraints on neogene sediments prevents a better understanding of the late cenozoic tectonic activity in this area to clarify the tectonic rotation pattern and deformation history of the semtp we report new magnetostratigraphic data from a late neogene sedimentary sequence in the dali basin northwestern yunnan province china rock magnetic analyses indicate that both magnetite and hematite are the main carriers of the characteristic remanent magnetizations chrms magnetostratigraphic results show that the sedimentary profile spans from chron c4n 1r to chron c2n the age of the sedimentary sequence in the dali basin can thus be paleomagnetically constrained to an interval from late miocene to early pleistocene the basal age of the sediments is 7 6 ma which indicates that the unroofing of diancang shan and activation of the dali fault system were initiated at this time the appearance of conglomerates and syntectonic sediments suggests the reactivation of the dali fault system at 2 5 ma moreover the overall mean chrm direction suggests that the dali basin experienced significant 4 4 2 5 but minor post late miocene rotation this indicates that most of the clockwise rotation demonstrated by previous paleomagnetic studies in the semtp occurred prior to late miocene and may be concentrated between eocene and miocene which is contemporaneous with the sinistral slip of the ailao shan red river fault

bacterially induced dolomite precipitation in anoxic culture experiments to study the process of microbial mediated dolomite formation growth experiments were carried out with selected bacterial cultures under anoxic environmental conditions simulating those found in lagoa vermelha a hypersaline lagoon in brazil where dolomite precipitation occurs specifically we report the isolation of a particular strain of sulfate reducing bacteria lvform6 from lagoa vermelha sediment which apparently promotes the formation of nonstoichiometric dolomite sulfate reducing bacteria grown in a synthetic liquid medium produced dolomite during 30 days incubation at 30 c the precipitates have morphologies similar to those observed in lagoa vermelha sediment our results demonstrate that sulfate reducing bacteria can influence dolomite precipitation under controlled lowtemperature anoxic conditions and imply that anaerobic microorganisms can play an important role in carbonate sedimentation they may have been particularly significant in earth s earliest history when a more reducing atmosphere existed

microbes produce nanobacteria like structures avoiding cell entombment microsedimentary structures referred to as nanobacteria like particles were described from modern carbonate environments where they form in close spatial association with sulfate reducing bacteria srb however the exact mechanism of their formation as well as their paleontological significance remains controversial here we report on an investigation of microbe mineral interactions in experimentally produced carbonate globules the experiments were carried out under anoxic conditions at 30 c with desulfovibrio brasiliensis a srb known to mediate dolomite formation we observed that extracellular polymeric substances eps secreted by the microbial community play a key role in the mineralization process nanobacteria like particles represent the early stage of carbonate nucleation within the eps which progressively evolve to larger globules displaying a grainy texture we excluded the possibilities that these structures are fossils of nanobacteria dissolution surfaces or artifacts created during sample preparation d brasiliensis cells are predominantly located outside of the eps aggregates where mineral growth takes place as a result they remain mobile and are rarely entombed within the mineral this self preservation behavior may not be limited to d brasiliensis other microbes may produce or may have produced during the geological past biogenic minerals through a similar process mineralization within eps explains why microbial relics are not necessarily present in biogenic carbonates

dolomite mediating bacterium isolated from the sabkha of abu dhabi uae terra nova 24 248 254 2012 abstract the study of microbe mineral interactions and the identification of biominerals in sedimentary rocks provide crucial information on the coevolution of life and earth surface environments desulfovibrio brasiliensis a sulfate reducing bacterium isolated from lagoa vermelha brazil is known for its ability to precipitate dolomite a mineral that is common in the geological record but difficult to obtain in laboratory experiments simulating earth s surface conditions here we report on a new bacterial strain capable of precipitating mg calcite and ca dolomite isolated from a microbial mat in the sabkha of abu dhabi uae the 16s rrna gene sequence of the isolate revealed a 99 6 identity i e same species with desulfovibrio brasiliensis the presence of the same microbial species at two geographically distant dolomite forming locations is difficult to explain in terms of pure coincidence rather it suggests that the ability of precipitating dolomite may be a unique characteristic associated with specific strains

dolomite abundance and stratigraphic age constraints on rates and mechanisms of phanerozoic dolostone formation perspectives abstract increasing dolomite abundance with age relative to limestone has long been accepted as one of the general truisms of sedimentary geology a relationship which suggests that dolomitization is a cumulative process occurring slowly over the full burial history of a carbonate sequence however reevaluation of data in support of this assumption indicates that although relative dolomite abundances do change significantly throughout phanerozoic carbonate sequences no data set demonstrates any direct correlation between dolomite content and age whereas tabulation of available data does suggest greater amounts of dolomite formation during periods of continental flooding global mass balance calculations of potential magnesium sources also indicate that most dolomite has formed from marine or marine derived fluids at the earth s surface this conclusion is indirectly supported by significant short term period scale changes in phanerozoic dolomite abundance a variation which would be more attenuated if dolomitization were dominantly a burial process dolomitization may be enhanced during times of global transgression higher atmospheric pco2 and lower calcite saturation state in shallow marine settings

mineralogy nucleation and growth of dolomite in the laboratory and sedimentary environment a review dolomite camg co3 2 forms in numerous geological settings usually as a diagenetic replacement of limestone and is an important component of petroleum reservoir rocks rocks hosting base metal deposits and fresh water aquifers dolomite is a rhombohedral carbonate with a structure consisting of an ordered arrangement of alternating layers of ca2 and mg2 cations interspersed with co32 anion layers normal to the c axis dolomite has r3 symmetry lower than the caco3 r3 c symmetry of calcite primarily due to ca mg ordering high magnesium calcite also has r3 c symmetry and differs from dolomite in that ca2 and mg2 ions are not ordered high magnesium calcite with near dolomite stoichiometry 50 mol mgco3 has been observed both in nature and in laboratory products and is referred to in the literature as protodolomite or very high magnesium calcite many dolomites display some degree of cation disorder ca2 on mg2 sites and vice versa which is detectable using transmission electron microscopy and x ray diffractometry laboratory syntheses at high temperature and pressure as well as studies of natural dolomites show that factors affecting dolomite ordering stoichiometry nucleation and growth include temperature alkalinity ph concentration of mg and ca mg to ca ratio fluid to rock ratio mineralogy of the carbonate being replaced and surface area available for nucleation in spite of numerous attempts dolomite has not been synthesized in the laboratory under near surface conditions examination of published x ray diffraction data demonstrates that assertions of dolomite synthesis in the laboratory under near ambient conditions by microbial mediation are unsubstantiated these laboratory products show no evidence of cation ordering and appear to be very high magnesium calcite elevated temperature and elevated pressure experiments demonstrate that dolomite nucleation and growth always are preceded by very high magnesium calcite formation it remains to be demonstrated whether microbial mediated growth of very high magnesium calcite in nature provides a precursor to dolomite nucleation and growth analogous to reaction paths in high temperature experiments

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 1 oil family classification abstract a series of palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the tarim basin nw china have been examined to delineate oil families crude oils from the mesozoic foreland kuqa depression were also analyzed for comparison the genetic family was assessed using bulk hydrocarbon isotopic composition alkylation pattern of polycyclic aromatic hydrocarbons pahs and biomarker distributions detailed oil oil correlation suggests two distinct families occur in the cratonic region despite the diversity of the reservoir rocks and some variability in oil physical properties the tabei and tazhong oils are classified in general into a single family with a possible indication for secondary subdivision these oils are characterized by low pristane phytane pr ph ratio high dibenzothiophene phenanthrene dbt p ratio light bulk isotopic composition high degree of pah alkylation high proportion of c23 and c24 tricyclic terpanes and low abundance of c24 tetracyclic terpane and triaromatic dinosteroids this family is inferred to have a middle upper ordovician source rock the tadong family distinctly different from the tabei and tazhong families is characterized by relatively high pr ph ratio low dbt p ratio slightly heavier bulk isotopic compositions and low degree of pah alkylation it also has a unique biomarker signature with a high proportion of triaromatic dinosteroids possibly indicating cambrian lower ordovician source rocks the oils from the kuqa depression are easily distinguished from the other two families due to their terrigenous origin the major characteristics of the kuqa oils are high pr ph ratio very low dbt p ratio heavy bulk isotopic composition low pah alkylation degree high proportion of c20 and c21 tricyclic terpanes and c24 tetracylic terpane and abundance of triaromatic dinosteranes and 4 methyl 24 ethyltriaromatic cholestanes these characteristics are consistent with terrestrial organic source rocks deposited under sub oxic conditions

a relationship between d104 value and composition in the calcite disordered dolomite solid solution series x ray diffraction has been widely used in analyzing ca mg carbonates compositions of biogenic and inorganic ca mg co3 crystals are often calculated by comparing their d 104 values with published empirical curves however previous studies suggested that these curves do not apply to very high mg calcite and disordered dolomite based on synthesized high mg calcite and disordered dolomite a new empirical curve between values of magnesian calcite d 104 and mgco3 content in the calcite disordered dolomite solid solution series is constructed this new curve is consistent with the significant cell parameter changes accompanying the mg ca cation disorder in dolomite and it can help the characterization of the mgco3 content of both natural and synthetic magnesian calcite and disordered dolomite especially for the mineral mixtures that are not suitable for other analysis methods

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

high salinity facilitates dolomite precipitation mediated by haloferax volcanii ds52 abstract although most modern dolomites occur in hypersaline environments the effects of elevated salinity on the microbial mediation of dolomite precipitation have not been fully evaluated here we report results of dolomite precipitation in association with a batch culture of haloferax volcanii ds52 a halophilic archaeon under various salinities from 120 to 360 and the impact of salinity on microbe mediated dolomite formation the mineral phases morphology and atomic arrangement of the precipitates were analyzed by xrd sem and tem respectively the amount of amino acids on the archaeal cell surface was quantified by hplc ms the xrd analysis indicated that disordered dolomite formed successfully with the facilitation of cells harvested from cultures with relatively high salinities 200 and 280 but was not observed in association with cells harvested from cultures with lower salinity 120 or the lysates of cells harvested from extremely high salinity 360 the tem analysis demonstrated that the crystals from cultures with a salinity of 200 closely matched that of dolomite importantly we found that more carboxyl groups were presented on the cell surface under high salinity conditions to resist the high osmotic pressure which may result in the subsequent promotion of dolomite formation our finding suggests a link between variations in the hydro chemical conditions and the formation of dolomite via microbial metabolic activity and enhances our understanding about the mechanism of microbially mediated dolomite formation under high salinity conditions

geochemistry of alkylbenzenes in the paleozoic oils from the tarim basin nw china a b s t r a c t monoaromatic hydrocarbons alkylbenzenes in paleozoic oils condensates from the tarim basin were quantitatively analyzed using gas chromatography mass spectrometry their relative abundances and distributions show large variation which are most likely controlled by the temperature and duration of the catagenesis stage of petroleum maturation relatively high abundance of long chain linear substituted alkylbenzenes and alkyltoluenes in the studied samples implies that source rocks of these oils condensates contain high contents of algal material the dominance of dodecylbenzene in these n alkylbenzenes suggests the formation of alkylbenzenes from fatty acids via lactone formation dehydration the 1 2 3 4 tetramethylbenzene temb 1 methyl 3 phytanylbenzene 1 m 3 pyb and aryl isoprenoids are mainly derived from carotenoids produced by chlorobiaceae and their occurrence may indicate a highly reducing depositional environment however the concentration of these compounds diminishes significantly at high maturity with increasing thermal maturity the prominent trend of demethylation of alkylbenzenes is accompanied by shortening of alkyl side chains large variation in the c4 alkylbenzene isomer distribution is also controlled by thermal maturity the 1 2 3 5 temb and 1 2 4 5 temb isomers are more stable than other temb isomers and the overall thermodynamic stability of tembs is lower than that of dimethylethylbenzenes therefore several maturity parameters can be derived based on alkylbenzene stability absolute concentrations of alkylbenzenes generally increase with increasing thermal maturity but concentrations of 1 m 3 pyb and aryl isoprenoids decrease significantly at high maturity maturity parameters obtained from alkylbenzenes are especially useful for highly mature oils and condensates when most biomarkers are depleted caution is advised when interpreting the paleoenvironment and thermal maturity of an oil from the alkylbenzene signatures because they are affected by various other processes such as evaporative fractionation thermal cracking and biodegradation as well

dolomite mountains and the origin of the dolomite rock of which they mainly consist historical developments and new perspectives beginning in the late 18th century the dolomite mountains in northern italy have been the location for major sedimentological developments from the discovery of the mineral dolomite to the formulation of the coral reef hypothesis to explain the origin of the massive dolomite structures that define the splendid scenery of the region further the dolomite mountains have inspired voluminous research into the origin of dolomite questioning whether dolomite is a primary precipitate or a secondary replacement product recently with the recognition that microbes can mediate dolomite precipitation a new geomicrobiological approach combining the study of modern natural environments with bacterial culture experiments is now being used to calibrate or interpret microbial evidence derived from the dolomite rock record this three pronged methodology applied to the study of dolomite formation holds great promise for future research into the dolomite problem and provides a new impetus to revisit the dolomite mountains in the 21st century

microbially influenced formation of mg calcite and ca dolomite in the presence of exopolymeric substances produced by sulphate reducing bacteria some species of sulphate reducing bacteria srb are known to mediate the formation of dolomite and mg calcite however their exact role in the mineralization process remains elusive here we present the result of a laboratory experiment that was designed to test whether formation of carbonate minerals by srb can occur in the absence of living cells through passive mineralization of their exopolymeric substances eps srb capable of mediating dolomite were cultivated in the laboratory allowing them to secrete eps microbial activity within the cultures was subsequently inhibited with antibiotics only after this step ca2 and mg2 were added to the solution and carbonate minerals could form mg calcite and disordered ca dolomite precipitated in association with eps the mol of mg2 in the crystals increased with longer incubation times this result demonstrates that organic compounds produced by srb can mediate the formation of ca mg carbonates in the absence of an active metabolism

paleomagnetism of the yuanmou basin near the southeastern margin of the tibetan plateau and its constraints on late neogene sedimentation and tectonic rotation abstract new paleomagnetic investigation was carried out on the late neogene fluviolacustrine sequence of the yuanmou basin located near the southeastern margin of the tibetan plateau magnetostratigraphic results indicate nine reverse magnetozones r1 to r9 and eight normal magnetozones n1 to n8 in the sedimentary profile which can be correlated to the geomagnetic polarity timescale from c3n 3r to c1r 1r the age of the sedimentary sequence of the yuanmou basin can thus be paleomagnetically constrained to an interval from early pliocene to pleistocene with sedimentation rates varying from 12 5 to 55 cm kyr in addition to its highly resolved magnetostratigraphic sequence the yuanmou basin provides a record of plio pleistocene tectono and climato sedimentary processes the mean declinations of the seventeen polarity units excluding samples with transitional directions can be grouped into three distinct directional intervals group i 2 58 1 37 ma group ii 4 29 2 58 ma and group iii 4 91 4 29 ma these directions indicate that the yuanmou basin has probably experienced vertical axis clockwise rotation of about 12 from 1 4 ma to 4 9 ma which may be related to slip activity of the red river fault to the southwest and the xianshuihe xiaojiang fault to the east

molecular organic geochemistry of the tarim basin northwest china we conducted organic geochemical analyses on the largest suite of oils and source rock extracts from the tarim basin northwest china currently available statistical cluster analysis of the entire suite of tarim oils distinguishes at least seven genetic groups of oils the largest group of oils was collected from the tazhong and tabei uplifts and originated from marine middle upper ordovician anoxic marls that mark slope facies at the margins of structural uplifts two other genetic groups most likely originated from marine middle upper ordovician source rocks but of distinct facies with one an oxic shale rich source west of the bachu uplift and the other an anoxic shale source at tazhong

ordered low temperature dolomite mediated by carboxyl group density of microbial cell walls abundant in the ancient rock record early dolomite remains scarce in modern systems at low temperatures 50c even those systems supersaturated with respect to dolomite this scarcity is attributed to kinetic inhibition including complexation of mg2 by water and sulfate carbonate activity and mg ca ratio recent investigations point to a function for microbial metabolisms and surfaces in which disordered phases are formed here we report the precipitation of primary ordered dolomite at 30c facilitated solely by the cell walls of two nonmetabolizing archaea from saline solutions with an mg ca ratio of 1 1 5 1 and 10 1 and slightly saturated with respect to dolomite control experiments using bacteria and functionalized microspheres did not precipitate dolomite archaeal cell wall functional groups were approximately one order of magnitude higher than the bacteria and spheres used in this study from these results we propose a mechanistic model in which carboxyl groups associated with cell wall biomass and exopolymeric substances dehydrate mg ions further promoting carbonation and leading to dolomite nucleation these data explain reports of low temperature dolomite formation associated with numerous microbial metabolic guilds including bacteria and archaea and those reported in association with exopolymeric substances or cell wall surfaces and identify a key and widespread mechanism in the formation of disordered dolomite and ordered primary phases of dolomite at low temperature importantly the functionalized dead and nonmetabolizing biomass is the key in low temperature dolomite precipitation not active microbial metabolism these observations may lead to new predictive models for the distribution of dolomite

study of an ordovician carbonate with alternating dolomite calcite laminations and its implication for catalytic effects of microbes on the formation of sedimentary dolomite

evaluation method and application of the relative contribution of marine hydrocarbon source rocks in the tarim basin a case study from the tazhong area abstract in total 2 37 million tons of marine crude oil originating from mixed source rocks has been discovered in the tarim basin geological and geochemical analyses have confirmed that these mixed hydrocarbons are mainly from two sets of source rocks including the cambrian lower ordovician and middle upper ordovician hydrocarbon source rocks in this study we determined the set of source rocks primarily responsible for the mixed hydrocarbons and the next location to be explored differences in n alkane carbon isotopes in end member oils from cambrian lower ordovician and middle upper ordovician source rocks were examined a material balance model and simulation methods were used to evaluate the relative amounts contributed by each source the results from known reserves in the tazhong area show that the mixing ratio or contribution is up to 65 from cambrian lower ordovician source rocks and is generally higher than that from middle upper ordovician source rocks the discovery of deep hydrocarbons has caused the total oil contribution from the cambrian lower ordovician to increase the mixing ratio of cambrian lower ordovician oil varies depending on the well formation and block it increases from west to east horizontally and from top to bottom vertically hydrocarbons from cambrian lower ordovician source rocks migrate upward along faults and the mixing ratio decreases as the distance from the oil source fault increases favorable areas for cambrian lower ordovician hydrocarbon exploration are deep layers and areas near the fault zone that are connected to deep layers the material balance model for carbon isotopes and evaluation methods for relative contributions considered differences in relative concentration and carbon isotope structure of n alkanes herein new methods for the identification and evaluation of hydrocarbons in the petroleum system of this superimposed basin are presented

vertical and lateral fluid flow related to a large growth fault south eugene island block 330 field offshore louisiana data from sediments in and near a large growth fault adjacent to the giant south eugene island block 330 field offshore louisiana indicate that the fault has acted as a conduit for fluids whose flux has varied in space and time core and cuttings samples from two wells that penetrated the same fault about 300 m apart show markedly different thermal histories and evidence for mass flux sediments within and adjacent to the fault zone in the u s department of energy pennzoil pathfinder well at about 2200 m sstvd subsea true vertical depth showed little paleothermal or geochemical evidence for throughgoing fluid flow the sediments were characterized by low vitrinite reflectances ro averaging 0 3 ro moderate to high d18o and d13c values and little difference in major or trace element composition between deformed and undeformed sediments in contrast faulted sediments from the a6st well which intersects the a fault at 1993 m sstvd show evidence for a paleothermal anomaly 0 55 ro and depleted d18o and d13c values sodium is depleted and calcium is enriched in a mudstone gouge zone at the top of the fault cut in the well this effect diminishes with distance from this gouge zone cuttings from other wells in south eugene island block 330 show slightly elevated vitrinite reflectance in fault intercepts relative to sediments outside the fault zone overall indicators of mass and heat flux indicate the main growth fault zone in south eugene island block 330 has acted as a conduit for ascending fluids although the cumulative fluxes vary along strike this conclusion is corroborated by oil and gas distribution in downthrown sands in blocks 330 and 331 which identify the fault system in northwestern block 330 as a major feeder simple modeling of coupled heat and mass flux indicates the paleothermal anomaly in the fault zone intersected by a6st well was short lived having a duration less than 150 yr the anomaly could have been produced by a 2 106 m3 pulse of fluid ascending the fault at an actual velocity of over 1 km yr darcy flux of 330 m yr from 3 km deeper in the basin simple darcy law computation indicates a transient fault permeability on the order of 110 md during this flow pulsing of fluid up the fault was probably the norm although most flow did not produce such strong thermal anomalies as the one detected in the a6st well analysis of fluid pressures shows that the main fault is a profound lateral permeability barrier having up to 1800 psi of water pressure differential across it the hydrocarbon sealing capacity of the fault depends on the pressure difference across the fault fault permeability is best understood in terms of effective stress under ambient conditions the fault is at high pressure relative to downthrown reservoirs a pulse of high pressure fluid ascending end page 244 the fault lowers effective stress in the fault zone sufficiently to produce a significant transient increase in permeability if the fluid is in an area of the fault adjacent to downthrown relatively low pressure reservoir sands the fluid will discharge into them permeability in and adjacent to the fault then decreases such that fluid cannot reenter the fault zone and escape from the reservoir

secondary migration of petroleum along syncline axes a case study of the southern kuqa foreland basin tracing petroleum migration pathways for petroleum exploration in a sedimentary basin is challenging paleo uplifts are generally considered to represent the primary migration directions of petroleum here we propose a model for the kuqa foreland basin in which petroleum migrates along the axes of synclines between paleo uplifts based on an integrated analysis of seismic geochemical and production data interpretation of detailed seismic data indicates that petroleum is unlikely to migrate to the paleo uplifts because of their relatively low positions fault disruptions or a lack of mesozoic reservoirs geochemical parameters including the ratios of alkyl dibenzothiophenes and the gas wetness parameter show that petroleum maturity gradually decreases along the axes of synclines between paleo uplifts from northeast to southwest petroleum production data and gas oil ratios also show decreasing trends from northeast to southwest this evidence suggests that petroleum is likely to migrate along the ax

calcium and magnesium limited dolomite precipitation at deep springs lake california dolomite ca mg co3 2 precipitation from supersaturated ionic solutions at earth surface temperatures is considered kinetically inhibited because of the difficulties experienced in experimentally reproducing such a process nevertheless recent dolomite is observed to form in hypersaline and alkaline environments such recent dolomite precipitation is commonly attributed to microbial mediation because dolomite has been demonstrated to form in vitro in microbial cultures the mechanism of microbially mediated dolomite precipitation is however poorly understood and it remains unclear what role microbial mediation plays in natural environments in the study presented here simple geochemical methods were used to assess the limitations and controls of dolomite formation in deep springs lake a highly alkaline playa lake in eastern california showing ongoing dolomite authigenesis the sediments of deep springs lake consist of unlithified clay fraction dolomite ooze based on d 18 o equilibria and textural observations dolomite precipitates from oxygenated and agitated surface brine the na so4 dominated brine contains up to 500 mm dissolved inorganic carbon whereas mg 2 and ca 2 concentrations are ca 1 and 0ae 3m m respectively precipitation in the subsurface probably is not significant because of the lack of ca 2 below 0ae01 mm under such highly alkaline conditions the effect of microbial metabolism on supersaturation by ph and alkalinity increase is negligible a putative microbial effect could however support dolomite nucleation or support crystal growth by overcoming a kinetic barrier an essential limitation on crystal growth rates imposed by the low ca 2 and mg 2 concentrations could favour the thermodynamically more stable carbonate phase which is dolomite to precipitate this mode of unlithified dolomite ooze formation showing d 13 c values near to equilibrium with atmospheric co2 ca 3 contrasts the formation of isotopically light organically derived hard lithified dolomite layers in the subsurface of some less alkaline environments inferred physicochemical controls on dolomite formation under highly alkaline conditions observed in deep springs lake may shed light on conditions that favoured extensive dolomite formation in alkaline precambrian oceans as opposed to modern oceans where dolomites only form diagenetically in organic c rich sediments

the application of diamondoid indices in the tarim oils this study presents new data for the identification of the source and assessment of the thermal maturity of oils based on the diamondoid indices of oils from the tazhong and luntai uplifts in the tarim basin in northwest china the oil samples were divided into three groups according to biomarker characteristics and the abundance and distribution of diamondoids group i oils are located along the tazhong no 1 fault zone which contained abundant diamondoids and are of high thermal maturity suggesting they are late charged hydrocarbons that were derived from a middle upper ordovician source group ii oils are mainly located in blocks close to the tazhong no 1 fault zone and are dominated by early formed hydrocarbons from cambrian lower ordovician source rocks group iii lacustrine oils comprise relatively low concentrations of diamondoids compared with group i and group ii oils group iii oils were sourced from jurassic or possibly triassic units and are hosted by the low relief yingmaili section of the luntai uplift the thermal maturity of the oils in each group was evaluated using diamondoid parameters a few group i oils exhibit intense thermal cracking to estimate the extent of oil cracking using the concentrations of diamondoids in oils this study proposes a practical approach that facilitates the determination of baseline 4 and 3 methyldiamantane concentrations application of this method indicates that the tarim basin marine oils contain a baseline concentration of approximately 69 ppm

microbial fossilization in carbonate sediments a result of the bacterial surface involvement in dolomite precipitation recent dolomitic sediment samples from lagoa vermelha brazil were examined microscopically to study the process of bacterial fossilization in carbonate sediments bacteria like bodies were intimately associated with carbonate mineral surfaces and coatings on the former demonstrate the calcification of single bacterial cells the bacterial fossilization process in lagoa vermelha sediments was simulated in the laboratory by cultivation of mixed and pure cultures of sulphate reducing bacteria which were isolated from the lagoa vermelha sediments these cultures produced carbonate minerals that were studied to provide insight into the initiation of the fossilization process in mixed culture experiments bacterial colonies became calcified whereas in pure culture experiments single bacterial cells were associated with dolomite surfaces dolomite nucleated exclusively in bacterial colonies intimately associated with extracellular organic matter and bacterial cells electrophoretic mobility measurements of the bacterial cells in electrolyte solutions demonstrated the specific adsorption of ca 2 and mg 2 onto the cell surfaces indicating the role of the bacterial surface in carbonate nucleation and bacterial fossilization the affinity of the cells for mg 2 was related to the capability of the strains to mediate dolomite formation combined with sulphate uptake which dissociates the mgso 4 o ion pair and increases the mg 2 availability the concentration of mg 2 ions in the microenvironment around the cells where the conditions are favourable for dolomite precipitation may be the key to overcome the kinetic barrier to dolomite formation these results demonstrate that bacterial fossilization is a consequence of the cell surface involvement in carbonate precipitation implying that fossilized bacterial bodies can be used as a tool to recognize microbially mediated carbonates

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

dolomite formation within microbial mats in the coastal sabkha of abu dhabi united arab emirates microbial mediation is the only demonstrated mechanism to precipitate dolomite under earth surface conditions a link between microbial activity and dolomite formation in the sabkha of abu dhabi has until now not been evaluated even though this environment is cited frequently as the type analogue for many ancient evaporitic sequences such an evaluation is the purpose of this study which is based on a geochemical and petrographic investigation of three sites located on the coastal sabkha of abu dhabi along a transect from the intertidal to the supratidal zone this investigation revealed a close association between microbial mats and dolomite suggesting that microbes are involved in the mineralization process observations using scanning electron microscopy equipped with a cryotransfer system indicate that authigenic dolomite precipitates within the exopolymeric substances constituting the microbial mats in current models microbial dolomite precipitation is linked to an active microbial activity that sustains high ph and alkalinity and decreased sulphate concentrations in pore waters such models can be applied to the sabkha environment to explain dolomite formation within microbial mats present at the surface of the intertidal zone by contrast these models cannot be applied to the supratidal zone where abundant dolomite is present within buried mats that no longer show signs of intensive microbial activity as no abiotic mechanism is known to form dolomite at earth surface conditions two different hypotheses can reconcile this result in a first scenario all of the dolomite present in the supratidal zone formed in the past when the mats were active at the surface in a second scenario dolomite formation continues within the buried and inactive mats in order to explain dolomite formation in the absence of active microbial metabolisms a revised microbial model is proposed in which the mineral template properties of exopolymeric substances play a crucial role

molecular correlation of free oil adsorbed oil and inclusion oil of reservoir rocks in the tazhong uplift of the tarim basin china the free adsorbed and inclusion oils were recovered by sequential extraction from eleven oil and tar containing reservoir rocks in the tazhong uplift of tarim basin the results of gas chromatography gc and gc mass spectrometry analyses of these oil components and seven crude oils collected from this region reveal multiple oil charges derived from different source rocks for these oil reservoirs the initially charged oils show strong predominance of even over odd n alkanes in the range n c12 to n c20 and have ordinary maturities while the later charged oils do not exhibit any predominance of n alkanes and have high maturities the adsorbed and inclusion oils of the reservoir rocks generally have high relative concentrations of gammacerane and c28 steranes similar to the cambrian lower ordovician source rocks in contrast the free oils of these reservoir rocks generally have low relative concentrations of gammacerane and c28 steranes similar to the middle upper ordovician source rocks there are two interpretations of this result 1 the initially charged oils are derived from the cambrian lower ordovician source rocks while the later charged oils are derived from the middle upper ordovician source rocks and 2 both the initially and later charged oils are mainly derived from the cambrian lower ordovician source rocks but the later charged oils are contaminated by the oil components from the silurian tar sandstones and the middle upper ordovician source rocks

precipitation of protodolomite facilitated by sulfate reducing bacteria the role of capsule extracellular polymeric substances abstract the origin of dolomite has long puzzled geologists it has recently been documented that sulfate reducing bacteria srb are capable of catalyzing the formation of protodolomite a previously proposed precursor of ordered sedimentary dolomite however the catalytic mechanism of srb remains incompletely understood this experimental study is aimed at probing the effect of capsule extracellular polymeric substances capsule eps from srb on the crystallization of protodolomite in vivo the capsule eps tested herein was isolated from a protodolomite mediating srb desulfotomaculum ruminis and added into a solution wherein the degree of oversaturation was close to the growth medium of d ruminis at stationary phase the solid products were characterized with x ray diffraction xrd raman spectroscopy and scanning and transmission electron microscopy sem and tem our results indicated that aragonite emerged in the reactors without capsule eps while ca mg carbonates mg calcite and protodolomite were produced in the systems amended with capsule eps the incorporation amount of mg2 in ca mg carbonates was enhanced with the increasing concentration of capsule eps the predominant occurrence of protodolomite was found in the reactor with 140 mg l capsule eps these resulting protodolomites were spherical in shape and composed of numerous nano particles the catalytic influence of capsule eps on the precipitation of protodolomite might be attributed to their strong mg2 binding capacity potentially diminishing mg hydration which is a potent inhibitor of protodolomite crystallization the results of fourier transformation infrared ft ir spectra showed that mg2 was bonded with carboxyl and hydroxyl groups on capsule eps this inferred adsorption capacity of capsule eps was also supported by new calculations of complexation chemistry between mg h2o complex and organic compounds present in capsule eps

paleomagnetic age of the tangxian planation surface northwestern zhongtiao shan of the shanxi graben system north china abstract a numerical formation age for the tangxian planation surface txps in north china is still controversial the txps beveled across paleoarchean basement rocks is overlain by late cenozoic conglomerate aeolian red clay and loess the magnetostratigraphy of two late cenozoic deposition sections is established and two other sections are recorded and measured in this work the magnetostratigraphy indicates six normal magnetozones n1 to n6 and six reverse magnetozones r1 to r6 these magnetozones are correlated to chron c1n through c2an 1r indicating that the txps was formed at 3 12 3 03 ma the characteristics of the zhongtiao shan tectonic uplift and its relationship with the tibetan plateau and ordos block are discussed in terms of tectonic uplift the txps formation and accumulation of the late cenozoic sediments are mainly controlled by tectonic activities the formation and uplift of the txps with a lag in the shanxi graben system is consistent with the activities of the tibetan plateau and ordos block implying that the ne extension of the tibetan plateau epeirogenic uplift advances successively

dissolved sulfide catalyzed precipitation of disordered dolomite implications for the formation mechanism of sedimentary dolomite abstract dolomite is a common mineral in the rock record however the rarity of modern dolomite and the notorious difficulty in synthesizing dolomite abiotically under normal earth surface conditions result in the long standing dolomite problem in sedimentary geology some modern dolomites are associated with sediments where microbial sulfate reduction is active however the role of sulfate reducing bacteria in dolomite formation is still under debate in this study we tested the effect of dissolved sulfide on the precipitation of ca mg carbonates which has been never explored before although dissolved sulfide is one of the major products of microbial sulfate reduction our results demonstrated that dissolved sulfide with a concentration of as low as several millimoles can enhance the mg2 incorporation into the calcitic structure and promote the crystallization of high magnesian calcite and disordered dolomite we also conducted seeded precipitation in experimental solutions containing dissolved sulfide which showed that calcite seeds can inhibit the precipitation of aragonite and monohydrocalcite caco3 h2o and induce more mg2 incorporation we propose that accumulated dissolved sulfide in pore waters in organic rich sediments may trigger the precipitation of disordered dolomite which can be considered as a precursor of some sedimentary dolomite our adsorption experiments revealed a strong adsorption of dissolved sulfide onto calcite faces we suggest that adsorbed dissolved sulfide can lower the energy barrier to the dehydration of mg2 water complexes on the growing carbonate surfaces this study sheds new light on understanding the role of sulfate reducing bacteria in dolomite formation and the formation mechanism of sedimentary dolomite

the tazhong hybrid petroleum system tarim basin china abstract the tazhong petroleum system is geographically located in the central tarim basin including the tazhong uplift the north slope and the southern area of the manjiaer depression the total thickness of early palaeozoic to cenozoic sediments reaches 6500 9500 m the manjiaer depression and the north slope are the two major petroleum source areas with three sets of source rocks cambrian black shales and ordovician dark carbonates of the manjiaer depression and ordovician argillaceous limestones of the north slope there are three sets of reservoir cap rock combinations in this petroleum system the main periods for oil generation from these source rocks occurred during the middle silurian to early devonian late carboniferous to early triassic and tertiary there were three corresponding phases of petroleum migration well developed thrust faults and unconformity surfaces in this area provided the main pathways for petroleum migration as some of the faults cut through all the lower palaeozoic strata the oil in any given reservoir rock may be actually a mixture of oils from different sources complicating the origin and properties of the hydrocarbons in the reservoir and forming a hybrid petroleum system the formation of a typical oil and gas pool in this petroleum system may be attributed to one or all of three petroleum events which were closely associated with the regional tectonics the first event occurred in the early palaeozoic tectonic cycle and the generated oil accumulated in lower silurian sandstone the second event occurred in the late palaeozoic tectonic cycle and formed light oil pools mainly in lower silurian and ordovician reservoirs the third event took place during the cenozoic tectonic cycle and formed normal crude oils the oil and gas generated from different sources in different phases may co exist with one another in a reservoir rock however the commercial oil and gas pools are related to the cenozoic and late palaeozoic events