evidence for deep anaerobic biodegradation associated with rapid sedimentation and burial in the beaufort mackenzie basin canada abstract formation waters of the 14 km thick late cretaceous cenozoic beaufort mackenzie basin were examined as part of a larger project to better understand the petroleum potential of the region where early exploration defined petroleum reserves of 744 109 bbls recoverable crude oil and 11 74 tcf gas historical water analyses 2583 samples from 250 wells drilled up to 5 km depth were compiled and culled to remove incomplete and poor quality samples the resultant database shows a broad range of salinity and water chemistry that has no systematic relationship with depth three main water types are defined paleo seawater and freshwaters related to a miocene age gravity driven flow system and low tds high alkalinity waters high alkalinity waters are isolated in overpressured fault blocks that were rapidly buried by post miocene iperk shale deposition the high alkalinities up to 9000 mg l are interpreted to be related to in situ co2 generation through anaerobic methanogenesis in response to freshwater invasion the dominant control on biogenic gas generation appears to be maximum burial temperature rather than the modern temperature distribution this is consistent with the paleopasteurization model that suggests once critical burial temperatures are reached sterilized rocks are inhibited from further biodegradation even when temperatures subsequently drop back into the habitable zone

water rock interaction and methanogenesis in formation water in the southeast huaibei coalfield china abstract the hydrogeochemical characteristics of aquifers in coal bearing strata can provide an important foundation for understanding the evolution of the aquifers and recognizing the formation process and occurrence of biogenic gas by analyzing the hydrogeochemical characteristics of water produced from the cenozoic the limestone and the coal strata aquifers and the coalbed gas wells this study focused on water rock interactions and methanogenesis in the formation water of the suzhou mining area huaibei coalfield china tectonically this is one of the most complex coalfields in china the dissolution of evaporites silicates and carbonates controlled the chemical composition of the formation water the cenozoic aquifers have a relatively stable water quality type and balanced ionic composition but more complex water rock interactions occur in the coal strata and the limestone aquifers extensive methanogenic activity and methanogenesis could be recognized by the meteoric water recharge positive 13 c signatures for dissolved inorganic carbon dic 15 4 26 2 lower sulfate concentrations

br cl na systematics in illinois basin fluids constraints on fluid origin and evolution the authors present here bromide chloride and sodium data for fluids from reservoirs of ordovician through pennsylvania age in the illinois basic which suggest that remnant marine fluids contribute significantly to fluid cl budgets cl br and nabr ratios for ordovician through devonian formation fluids are relatively uniform and near those for seawater despite greater than a factor of ten range in cl concentration in contrast fluids from mississippian and pennsylvanian reservoirs separated from older reservoirs by the new albany shale group have more variable fluid cl br and na br ratios most of which are significantly greater then those of seawater the 1 1 stoichiometry of cl and na increases for mississippian and pennsylvanian formation fluids is consistent with halite dissolution nevertheless br systematics and mass balance considerations indicate that he overall cl budget of illinois basin formation fluids appears to be more significantly influenced by the contribution from subaerially evaporated seawater than by halite dissolution

structural geology of impact craters abstract the formation of impact craters is a highly dynamic and complex process that subjects the impacted target rocks to numerous types of deformation mechanisms understanding and interpreting these styles of micro meso and macroscale deformation has proved itself challenging for the field of structural geology in this paper we give an overview of the structural inventory found in craters of all size ranges on earth and look into the structures of craters on other planetary bodies structural features are discussed here that are caused by i extremely high pressures and temperatures that occur during the initial passage of the shock wave through the target rock and projectile ii the resulting flow field in the target that excavates and ejects rock materials and iii the gravitationally induced modification of the crater cavity into the final crater form a special focus is put on the effects that low angle impacting bodies have on crater formation we hope that this review will help both planetary scientists and structural geologists understand the deformation processes and resulting structures generated by meteorite impact

coupled hydrology and biogeochemistry of paleocene eocene coal beds northern gulf of mexico thirty six formation waters gas and microbial samples were collected and analyzed from natural gas and oil wells producing from the paleocene to eocene wilcox group coal beds and adjacent sandstones in north central louisiana usa to investigate the role hydrology plays on the generation and distribution of microbial methane major ion chemistry and cl br relations of wilcox group formation waters suggest mixing of freshwater with halite derived brines high alkalinities up to 47 8 meq l no detectable so4 and elevated 13c values of dissolved inorganic carbon up to 20 5 vienna peedee belemnite vpdb and co2 up to 17 67 vpdb in the wilcox group coals and adjacent sandstones indicate the dominance of microbial methanogenesis the 13c and d values of ch4 and carbon isotope fractionation of co2 and ch4 suggest co2 reduction is the major methanogenic pathway geochemical indicators for methanogenesis drop off significantly at chloride concentrations above 1 7 mol l suggesting that high salinities inhibit microbial activity at depths greater than 1 6 km formation waters in the wilcox group contain up to 1 6 modern carbon a14c to at least 1690 m depth the covariance of d values of co produced h2o and ch4 indicate that the microbial methane was generated in situ with these late pleistocene or younger waters the most enriched carbon isotope values for dissolved inorganic carbon dic and co2 and highest alkalinities were detected in wilcox group sandstone reservoirs that were co2 flooded in the 1980s for enhanced oil recovery leading to the intriguing hypothesis that co2 sequestration may actually enhance methanogenesis in organic rich formations

sedimentological analysis of resurge deposits at the lockne and tvaren craters clues to flow dynamics the lockne and tvren craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement the target water depth for tvaren apparent basement crater diameter d 2 km was probably not over 150 m and for lockne d 7 5 km recent best fit numerical simulations suggest the target water depth of 500 700 m lockne has crystalline ejecta that partly cover an outer crater 14 km diameter apparent in the target sediments tvren is eroded with only the crater infill preserved we have line logged cores through the resurge deposits within the craters in order to analyze the resurge flow the focus was clast lithology frequencies and size sorting we divide the resurge into resurge proper with water and debris shooting into the crater and ultimately rising into a central water plume anti resurge with flow outward from the collapsing plume and oscillating resurge not covered by the line logging due to methodological reasons with decreasing flow in diverse directions at lockne the deposit of the resurge proper is coarse and moderately sorted whereas the anti resurge deposit is fining upwards and better sorted the tvren crater has a smoothly fining up section deposited by the resurge proper and may lack anti resurge deposits at lockne the content of crystalline relative to limestone clasts generally decreases upwards which is the opposite of tvren this may be a consequence of factors such as crater size i e complex versus simple and the relative target water depth the mean grain size i e the mean phi value per meter and standard deviation i e size sorting for both craters can be expressed by the equation 0 60 1 25

comparison of fluid geochemistry and microbiology of multiple organic rich reservoirs in the illinois basin usa evidence for controls on methanogenesis and microbial transport abstract microbial methane in sedimentary basins comprises approximately 20 of global natural gas resources yet little is known about the environmental requirements and metabolic rates of these subsurface microbial communities the illinois basin located in the midcontinent of the united states is an ideal location to investigate hydrogeochemical factors controlling methanogenesis as microbial methane accumulations occur 1 in three organic rich reservoirs of different geologic ages and organic matter types upper devonian new albany shale up to 900 m depth pennsylvanian coals up to 600 m depth and quaternary glacial sediments shallow aquifers 2 across steep salinity gradients and 3 with variable concentrations of so 4 2 for all three organic rich reservoirs aqueous geochemical conditions are favorable for microbial methanogenesis with near neutral ph so 4 2 concentrations 80 c in nutrient poor environments necessitating the re introduction of microbes into the subsurface via groundwater transport

evidence for bacterially generated hydrocarbon gas in canadian shield and fennoscandian shield rocks hydrocarbon rich gases found in crystalline rocks on the canadian and fennoscandian shields are isotopically and compositionally similar suggesting that such gases are a characteristic feature of precambrian shield rocks gases occure in association with saline groundwaters and brines in pressurized pockets formed by sealed fracture systems within the host rocks when released by drilling activities gas pressures as high as 5000 kpa have been recorded typical gas flow rates for individual boreholes range from 0 25 l min to 4 l min the highest concentrations of ch4 are found in the deepest levels of the boreholes associated with ca na cl and na ca cl brines n2 is the second major component of the gases and with ch4 accounts for up to 80 to 90 vol higher hydrocarbon c2 concentrations range from 1 to 10 vol with c1 c2 c3 ratios from 10 1000 isotopically the gases show a wide range of values overall 13c 57 5 to 41 1 d 245 to 470 but a relatively tight cluster of values within each sampling locality the enonkoski mine methanes are unique with 13c values between 65 4 and 67 3 and d values between 297 and 347 the shield gases are not readily reconcilable with conventional theories of methanogenesis the range of c1 c2 c3 ratios for the shield gases is too low to be consistent with an entirely bacterial origin in addition dch4 values are in general too depleted in the heavy isotope to be produced by thermogenic methanogenesis or by secondary alteration processes such as bacterial oxidation or migration however isotopic and compositional evidence indicates that bacterially derived gas can account for a significant component of the gas at all shield sites conventional bacterial gas accounts for 75 94 vol of the occurrences at enonkoski mine in finland at each of the other shield sites bacterial gas can account for up to 30 50 vol of the total gas accumulation this study and other recent evidence of active bacterial communities in deep hydrogeological environments emphasize the need for more comprehensive investigation of the role of microorganisms in the deep subsurface

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

organic geochemistry in pennsylvanian tidally influenced sediments from sw indiana abstract tidal rhythmites are vertically stacked small scale sedimentary structures that record daily variations in tidal current energy and are known to overlie some low sulfur coals in the illinois basin tidal rhythmites from the pennsylvanian brazil formation in indiana have been analyzed sedimentologically petrographically and geochemically in order to understand the character and distribution of organic matter om preserved in an environment of daily interactions between marine and fresh waters the concentration of organic matter toc ranges from traces to 6 9 and sulfur rarely exceeds 0 1 in individual laminae angular vitrinite is the major organic matter type accounting for 50 90 of total om the c s ratio decreases as the vertical distance from the underlying coal increases a decreasing c s ratio coupled with decreases in pr ph pr n c17 ph n c18 ratios and a shift of carbon isotopic composition towards less negative values suggest an increase in salinity from freshwater in the mudflat tidal rhythmite facies close to the coal to brackish marine in the sandflat tidal rhythmite facies further above from the coal within an interval spanning one year of deposition toc and s values show monthly variability on a daily scale toc and s oscillations are still detectable but they are of lower magnitude than on a monthly scale these small scale variations are believed to reflect oscillations in water salinity related to tidal cycles

mercury content of the springfield coal indiana and kentucky abstract with pending regulation of mercury emissions in united states power plants its control at every step of the combustion process is important an understanding of the amount of mercury in coal at the mine is the first step in this process the springfield coal middle pennsylvanian is one of the most important coal resources in the illinois basin in indiana and western kentucky hg contents range from 0 02 to 0 55 ppm the variation within small areas is comparable to the variation on a basin basis considerable variation also exists within the coal column ranging from 0 04 to 0 224 ppm at one kentucky site larger variations likely exist since that site does not represent the highest whole seam hg nor was the collection of samples done with optimization of trace element variations in mind estimates of hg capture by currently installed pollution control equipment range from 9 53 capture by cold side electrostatic precipitators esp and 47 81 hg capture for esp flue gas desulfurization fgd the high cl content of many illinois basin coals and the installation of selective catalytic reduction of no x enhances the oxidation of hg species improving the ability of esps and fgds to capture hg

microbial methane in the shallow paleozoic sediments and glacial deposits of illinois u s a abstract methane formed by the microbial decomposition of buried organic matter is virtually ubiquitous in the groundwaters of illinois chemical and carbon isotopic compositions are reported for gas samples collected from over 200 private and municipal water wells and from 39 small gas wells completed in glacial deposits drift gas wells carbon and hydrogen isotopic data for methane carbon dioxide and water show that these gases were formed by the carbon dioxide reduction pathway the same mechanism which has been previously shown to be responsible for microbial methane formation in the marine environment the isotopic composition of methane in these samples can be closely correlated with the chemical composition of the gas and with water chemistry the data are interpreted as indicating that isotopically very light methane is found in waters where the residence time of groundwater in the methanogenesis zone was very short relative to the methane production rate

stable isotopic and molecular composition of desorbed coal seam gases from the walloon subgroup eastern surat basin australia this study used compositional and stable isotopic analysis to test hypotheses on the distribution and origins of walloon subgroup coal seam gas csg in the eastern surat basin queensland australia the middle jurassic walloon subgroup play differs from many other low rank csg plays particularly in methane carbon isotopic signature i e the csg is not as microbial as could be expected the carbon isotope compositions of desorbed methane from three cored appraisal wells fall within the generally accepted range for thermogenic or mixed gas 13c 58 5 to 45 3 the 13c ch4 values from stratigraphically placed coal core samples increased became more thermogenic from the top of the upper juandah coal measures to the base of the tangalooma sandstone below the tangalooma sandstone in the lower taroom coal measures the 13c ch4 values decreased with increasing depth these positively parabolic 13c profiles tracked total measured gas content in two out of the three wells studied the third well displayed lower variance of 13c ch4 and gas content increased uniformly with depth a genetic classification based on methane stable carbon isotopes alone might interpret this pattern as a transition from microbially to thermogenically sourced methane in the central coal seams however a 13c co2 profile for one well tracks total gas content and 13c ch4 and exhibits an inverse relationship with d ch4 these results together with the mostly dry nature of the gas samples c1 c2 c3 ratios up to 10 000 and relatively uniform d ch4 values d 238 to 202 suggest that microbial co2 reduction is the primary source of walloon subgroup methane as such stratigraphic variations in gas content mainly reflect the extent of microbial methanogenesis we suggest that peak microbial utilisation of h2 co2 occurred at the tangalooma sandstone level enriching the residual co2 pool and derived methane in 13c carbon 13c co2 ch4 and deuterium isotopic differences d h2o ch4 and cross plots of d h2o and 18o h2o are also consistent with kinetic isotope fractionation during microbial mediated carbonate reduction the results are relevant for applying microbially enhanced coal bed methane mecom in the surat basin

regional fluid flow as a factor in the thermal history of the illinois basin constraints from fluid inclusions and the maturity of pennsylvanian coals vitrinite reflectance measurements on pennsylvanian coals in the illinois basin indicate significantly higher thermal maturity than can be explained by present day burial depths an interval of additional sedimentary section now removed by erosion has been suggested to account for the discrepancy although burial could indeed account for the observed maturity levels of organic matter fluid inclusion temperatures provide a stringent additional constraint in this article we combine measurements of coal maturity with fluid inclusion temperatures from three sites to constrain the basin s thermal and burial history the fluorspar district at the illinois basin s southern margin the upper mississippi valley zinc district at the basin s northern margin and a north central location two dimensional numerical modeling of a north south cross section through the basin tests scenarios both with and without regional fluid flow vitrinite reflectance values can be matched assuming burial by 1 8 2 8 km of southward thickening additional post pennsylvanian sedimentary section in the central and northern illinois basin however these burial depths and temperatures are not sufficient to account for the fluid inclusion data to account for both parameters with burial alone does not appear feasible in contrast our best hypothesis assumes a wedge of post pennsylvanian sediment thickening southward to about 1 2 km and a brief period of magmatism in the fluorspar district significant advective heat redistribution by northward regional fluid flow accounts for fluid inclusion temperatures and coal maturities throughout the basin the modeling results demonstrate the potential contribution of advective heat transport to the thermal history of the illinois basin begin page 258

fate of injected co2 in the wilcox group louisiana gulf coast basin chemical and isotopic tracers of microbial brine rock co2 interactions the 2800 sandstone of the olla oil field is an oil and gas producing reservoir in a coal bearing interval of the paleocene eocene wilcox group in north central louisiana usa in the 1980s this producing unit was flooded with co2 in an enhanced oil recovery eor project leaving 30 of the injected co2 in the 2800 sandstone post injection this study utilizes isotopic and geochemical tracers from co produced natural gas oil and brine to determine the fate of the injected co2 including the possibility of enhanced microbial conversion of co2 to ch4 via methanogenesis stable carbon isotopes of co2 ch4 and dic together with mol co2 show that 4 out of 17 wells sampled in the 2800 sandstone are still producing injected co2 the dominant fate of the injected co2 appears to be dissolution in formation fluids and gas phase trapping there is some isotopic and geochemical evidence for enhanced microbial methanogenesis in 2 samples however the co2 spread unevenly throughout the reservoir and thus cannot explain the elevated indicators for methanogenesis observed across the entire field vertical migration out of the target 2800 sandstone reservoir is also apparent in 3 samples located stratigraphically above the target sand reservoirs comparable to the 2800 sandstone located along a 90 km transect were also sampled to investigate regional trends in gas composition brine chemistry and microbial activity microbial methane likely sourced from biodegradation of organic substrates within the formation was found in all oil fields sampled while indicators of methanogenesis e g high alkalinity 13c co2 and 13c dic values and oxidation of propane were greatest in the olla field likely due to its more ideal environmental conditions i e suitable range of ph temperature salinity sulfate and iron concentrations

influence of groundwater flowpaths residence times and nutrients on the extent of microbial methanogenesis in coal beds powder river basin usa water and natural gas samples were collected from coalbed methane wells and a surface coal mine in the powder river basin prb and analyzed for solute chemistry isotopes and gas composition to test hypotheses about the timing and source of recharge importance of nutrient influx and extent of microbial methanogenesis in coalbeds recharge to the wyodak anderson coal zone in the central portions of the prb appears to be from the southern basin margin based on 18o h2o values and hydraulic gradients coal zones along the eastern margin near the coal outcrop represent mixing between surface water and deeper circulating groundwater coalbeds along the northwestern basin margin may contain high elevation recharge from the bighorn mountains with flow patterns likely affected by local faults detectable 14c 0 39 to 4 13 pmc in coal waters throughout the prb indicates they were recharged 50 000 bp correlation of d ch4 and d h2o values suggests that methane has accumulated since the late pleistocene nutrient concentrations c n and p species are low throughout the prb and do not correlate to groundwater flowpaths or methanogenic pathways nutrients may be sourced from in situ leaching of coals carbon isotope values of ch4 and co2 likely indicate the extent of methanogenesis i e early versus late stage rather than the dominant metabolic pathway i e acetate fermentation versus co2 reduction coal gasses from the northwestern margin of the basin are isotopically depleted and suggest early stage methanogenesis where the reactant carbon reservoir has not been significantly fractionated due to relatively rapid groundwater flow coal gasses from the central basin and southeastern margin are isotopically enriched and suggest late stage methanogenesis where the reactant carbon reservoir has been significantly depleted likely due to longer groundwater residence times

formation waters from mississippian pennsylvanian reservoirs illinois basin usa chemical and isotopic constraints on evolution and migration we have analyzed a suite of seventy four formation water samples from mississippian and pennsylvanian carbonate and siliciclastic strata in the illinois basin for major minor and trace element concentrations and for strontium isotopic composition a subset of these samples was also analyzed for boron isotopic composition data are used to interpret origin of salinity and chemical and sr isotopic evolution of the brines and in comparison with a similar data set from an earlier study of basin formation waters from silurian devonian reservoirs systematics of cl br na show that present mississippian pennsylvanian brine salinity can be explained by a combination of subaerial seawater evaporation short of halite saturation and subsurface dissolution of halite from an evaporite zone in the middle mississippian st louis limestone along with extensive dilution by mixing with meteoric waters additional diagenetic modifications in the subsurface interpreted from cation br ratios include k depletion through interaction with clay minerals ca enrichment and mg depletion by dolomitization and sr enrichment through caco3 recrystallization and dolomitization ste genevieve limestone middle mississippian formation waters show 87sr86sr ratios in the range 0 70782 0 70900 whereas waters from the siliciclastic reservoirs are in the range 0 70900 0 71052 inverse correlations between 87sr86sr and b li and mg concentrations suggest that the brines acquired radiogenic 87sr through interaction with siliciclastic minerals completely unsystematic relations between 87sr86sr and 1 sr are observed sr concentrations in ste genevieve and aux vases middle mississippian waters appear to be buffered by equilibrium with respect to srso4 although there are many similarities in their origin and evolution these formation waters are distinguished from silurian devonian brines in the basin by elevated clbr and nabr ratios and by unsystematic sr isotope relationships thus waters from these two major segments of the illinois basin stratigraphic column form distinct geochemical regimes which are separated by the new albany shale group devonian mississippian regional aquitard geochemical evolution appears to have been influenced significantly by paleozoic and mesozoic hydrologic flow systems in the basin

the obolon impact structure ukraine and its ejecta deposits the obolon impact structure 18 km in diameter is situated at the northeastern slope of the ukrainian shield near its margin with the dnieper donets depression the crater was formed in crystalline rocks of the precambrian basement that are overlain by marine carboniferous and continental lower triassic deposits the post impact sediments comprise marine middle jurassic bajocian and bathonian and younger mesozoic and cenozoic deposits today the impact structure is buried beneath an about 300 meter thick sedimentary rock sequence most information on the obolon structure is derived from two boreholes in the western part of the crater the lowest part of the section in the deepest borehole is composed by allogenic breccia of crystalline basement rocks overlain by clast rich impact melt rocks and suevites abundant shock metamorphic effects are planar deformation features pdfs in quartz and feldspars kink bands in biotite etc coesite and impact diamonds were found in clast rich impact melt rocks crater fill deposits are a series of sandstones and breccias with blocks of sedimentary rocks that are covered by a layer of crystalline rock breccia crystalline rock breccias conglomeratic breccias and sandstones with crystalline rock debris have been found in some boreholes around the obolon impact structure to a distance of about 50 km from its center those deposits are always underlain by lower triassic continental red clay and overlain by middle jurassic marine clay the k ar age of impact melt glasses is 169 ma which corresponds to the middle jurassic bajocian age the composition of crater fill rocks within the crater and sediments outside the obolon structure testify to its formation under submarine conditions

pleistocene recharge to midcontinent basins effects on salinity structure and microbial gas generation the hydrogeochemistry of saline meteoric water interface zones in sedimentary basins is important in constraining the fluid migration history chemical evolution of basinal brines and physical stability of saline formation waters during episodes of freshwater recharge this is especially germane for interior cratonic basins such as the michigan and illinois basins although there are large differences in formation water salinity and hydrostratigraphy in these basins both are relatively quiescent tectonically and have experienced repeated cycles of glaciation during the pleistocene exploration for unconventional microbial gas deposits which began in the upper devonian age antrim shale at the northern margin of the michigan basin has recently extended into the age equivalent new albany shale of the neighboring illinois basin providing access to heretofore unavailable fluid samples these reveal an extensive regional recharge system that has profoundly changed the salinity structure and induced significant biogeochemical modification of formation water elemental and isotope geochemistry new formation water and gas samples were obtained from devonian mississippian strata in the illinois basin these included exploration wells in the new albany shale an organic rich black shale of upper devonian age and formation waters from over and underlying regional aquifer systems siluro devonian and mississippian age the hydrostratigraphic relations of major aquifers and aquitards along the eastern margin of the illinois basin critically influenced fluid migration into the new albany shale the new albany shale formation water chemistry indicates significant invasion of meteoric water with d values as low as 46 05 into the shale the carbon stable isotope system 13c values as high as 29 4 coupled with 18o d and alkalinity of formation waters alkalinity 24 08 meq kg identifies the presence of microbial gas associated with meteoric recharge regional geochemical patterns identify the underlying siluro devonian carbonate aquifer system as the major conduit for freshwater recharge into the fractured new albany shale reservoirs recharge from overlying mississippian carbonates is only significant in the southernmost portion of the basin margin where carbonates directly overlie the new albany shale recharge of dilute waters cl 1000 mm into the siluro devonian section has suppressed formation water salinity to depths as great as 1 km across the entire eastern illinois basin margin taken together with salinity and stable isotope patterns in age equivalent michigan basin formation waters they suggest a regional impact of recharge of 18o and d depleted fluids related to pleistocene glaciation devonian black shales at both basin margins have been affected by recharge and produced significant volumes of microbial methane this recharge is also manifested in different salinity gradients in the two basins because of their large differences in original formation water salinity given the relatively quiet tectonic history and subdued current topography in the midcontinent region it is likely that repeated cycles of glacial meltwater invasion across this region have induced a strong disequilibrium pattern in fluid salinity and produced a unique class of unconventional shale hosted gas deposits

hydrogeologic constraint on the origin of deep subsurface microorganisms within a triassic basin the thermal history of the taylorsville basin indicates that the thermophilic anaerobic bacteria extracted from triassic strata at 2800 m below the land surface within the basin probably migrated to their current location in order to ascertain the most probable scenario for microbial transport a two dimensional transient fluid flow and heat transport model of the taylorsville basin was developed using the finite element method the numerical model was constrained by tectonic and thermal histories of the basin coupled with the permeability and porosity data extracted from core samples the model demonstrates that a topographically driven groundwater flow system caused by the tectonic uplift and erosion during the jurassic can explain the observed differential cooling of the basin the computed groundwater flow rates during this period were on the order of 1 to 100 mm yr combined with the cooling history of the microbially sampled strata such rates provided a possible mechanism for the introduction of ancient surface or subsurface microorganisms to the deep subsurface this tectogenetic mechanism may explain the origins of deep subsurface microorganisms for other regions of the earth

primary to re equilibrated fluids and geochemical signatures for the evolution of the nagthat siliciclastics in the tons valley lesser himalaya india abstract medium to coarse grained neo proterozoic nagthat siliciclastic rocks form a part of the krol formation in the lesser himalayan geotectonic zone fluid inclusion and geochemical studies have been carried out on the nagthat siliciclastics from the tons valley to determine their provenance during the proterozoic and their recrystallisation during maximum burial to uplift fluid inclusion studies have been carried out on detrital recrystallised quartz grains and quartz overgrowths major and trace element analyses of the siliciclastics the relationships of sio 2 with various trace elements and the association of various trace elements with mineral species suggest a granitic source for these siliciclastics primary q1 aqueous brine inclusions and q3 h 2 o co 2 fluid with 0 9 gm cm 3 co 2 density in detrital quartz grains characterised the protolith of the sandstone as granite or metamorphic rocks h 2 o nacl fluids participated in the cementation history temperatures of quartz overgrowth from 198 to 232 c show the effect of maximum burial the re equilibration of the primary fluid due to elevated internal pressure confining pressure is evident from features like c shaped cavities stretching of the inclusions their migration decrepitation clusters etc during recrystallisation these inclusions were equilibrated at 187 and 235 c in a restricted fluid of aqueous moderately saline composition the observed inclusion morphology is attributed to a decrease in external pressure related to isothermal decompression uplift

characterization of the origin of coalbed gases in southeastern illinois basin by compound specific carbon and hydrogen stable isotope ratios abstract coalbed gases and waters from exploratory and production gas wells in the southeastern illinois basin were sampled to assess geochemically the origin of coalbed gases with an emphasis on the springfield and seelyville coal members that are commercially targeted for coalbed methane production in indiana on line analyses of hydrocarbon gases methane to butanes c 1 c 2 c 3 n c 4 i c 4 and co 2 yielded gas concentrations plus d and 13 c values the low thermal maturity of indiana coals with vitrinite reflectance r 0 0 6 is in agreement with an overwhelmingly biogenic isotopic signature of coalbed gases containing 96 methane generated via bacterial co 2 reduction in contrast thermogenic gas was generated in the stratigraphically equivalent coal beds in western kentucky s rough creek graben zone where higher maturities of up to r 0 0 8 were reached owing to tectonic and hydrothermal activity no secondary biogenic methane was observed in more mature western kentucky coal beds where greater burial depth limits the recharge of meteoric water biogenic and thermogenic coalbed gases represent two end members that are compositionally and isotopically distinct microbial biodegradation of thermogenic c 2 hydrocarbon gases in indiana coal beds preferentially targets c 3 and introduces isotope fractionation whereby remaining c 3 is enriched in deuterium and 13 c

problems in applying preservation facies and sequence models to sinian neoproterozoic glacial sequences in australia and asia abstract the sinian 800 530 ma is an extraordinary period of earth history characterized by the transition from simple to complex multicellular organisms and by extreme variations in climate eustasy and possibly tectonics understanding sinian stratigraphy may help interpret the co evolution of life and environment during the period though sinian successions are poorly dated and difficult to correlate preservation facies and sequence models based on mesozoic recent successions may be used to interpret them preservation models based on basin subsidence permit estimates of how thick and complete successions can be in different tectonic settings and how long it took for them to be deposited facies models for rift craton shelf slope and basin environments help in identification of these different tectonic settings sequence models facilitate interpretation of the interplay of tectonic subsidence eustasy rates of sedimentation and climate such models applied to the sinian successions of australia and asia indicate the following great shallowing upwards megacycles occur in sinian successions and each ends with regional glaciations deep rift basins formed around 850 800 ma preserve the oldest megacycles but these basins soon were filled with rapidly deposited glaciogenic sediments relatively complete sequences are found only in deep basins subject to repeated rifting cratons and shallow shelves preserve only the youngest megacycles older deposits have been removed by younger glacial advances deep shelves and slopes preserve one to three megacycles depending on the timing of the start of passive margin subsidence megasequences can be correlated based on limited radiometric dating and inferred regional distribution of unique rock types but these sequences are not developed to the same extent in all blocks problems in correlating the four great megacycles recognized elsewhere with those of asia will persist until the requisite facies analysis and u pb dating of interbedded volcanics are done

constraining the timing of microbial methane generation in an organic rich shale using noble gases illinois basin usa abstract at least 20 of the world s natural gas originates from methanogens subsisting on organic rich coals and shales however in situ microbial methane production rates are unknown methanogens in the upper devonian new albany shale in the illinois basin extract hydrogen from low salinity formation water to form economic quantities of natural gas because of this association constraining the source and timing of groundwater recharge will enable estimation of minimum in situ metabolic rates thirty four formation water and gas samples were analyzed for stable isotopes oxygen and hydrogen chloride tritium 14 c and noble gases chloride and 18 o spatial patterns reveal a plume of water with low salinity 0 7 to 2154 mm and 18 o values 0 14 to 7 25 penetrating 1 km depth into evapo concentrated brines parallel to terminal moraines of the laurentide ice sheet suggesting glacial mediated recharge however isotopic mixing trends indicate that the recharge endmember 7 18 o is higher than the assumed bulk ice sheet value 18 o and similar to modern local precipitation 7 5 to 4 5 18 o continental paleoprecipitation records reveal that throughout the pleistocene 18 o of precipitation in the region ranged from 10 to 5 suggesting that the dilute groundwater was primarily sourced from paleoprecipitation with minor contributions from glacial meltwater for the majority of samples the atmosphere derived 4 he contribution is negligible and the 4 he is dominated by a crustal radiogenic source with near complete transfer of dissolved noble gases to the gas phase in addition mantle derived helium is negligible for all samples 18 o data however all groundwater in the study area was influenced by pleistocene recharge estimated in situ microbial methane production rates range from 10 to 1000 tcf ma 10 4 to 10 6 times slower than average laboratory rates from coals findings from this study have implications for targeting undeveloped microbial gas accumulations improving natural gas reservoir estimates the potential of in situ methanogen stimulation and understanding biologic cycling of carbon in subsurface reservoirs

methanogenesis and sulfate reduction in marine sediments a new model a number of studies have shown that methanogens are active in the presence of sulfate under some conditions this phenomenon is especially exemplified in carbonate sediments of the southern australian continental margin three sites cored during ocean drilling program odp leg 182 in the great australian bight have high concentrations of microbially generated methane and hydrogen sulfide throughout almost 500 m of sediments in these cores the sulfate reducing and methanogenic zones overlap completely that is the usual sulfate methane transition zone is absent amino acid racemization data show that the gassy sediments consist of younger carbonates than the low gas sites high concentrations of the reduced gases also occur in two odp sites on the margin of the bahamas platform both of which have similar sedimentary conditions to those of the high gas sites of leg 182 co generation of these reduced gases results from an unusual combination of conditions including 1 a thick quaternary sequence of iron poor carbonate sediments 2 a sub seafloor brine and 3 moderate amounts of organic carbon the probable explanation for the co generation of hydrogen sulfide and methane in all these sites as well as in other reported environments is that methanogens are utilizing non competitive substrates to produce methane within the sulfate reducing zone taken together these results form the basis of a new model for sulfate reduction and methanogenesis in marine sediments the biogeochemical end members of the model are 1 minimal sulfate reduction 2 complete sulfate reduction followed by methanogenesis and 3 overlapping sulfate reduction and methanogenesis with no transition zone

cleats and their relation to geologic lineaments and coalbed methane potential in pennsylvanian coals in indiana abstract cleats and fractures in pennsylvanian coals in southwestern indiana were described statistically analyzed and subsequently interpreted in terms of their origin relation to geologic lineaments and significance for coal permeability and coalbed gas generation and storage these cleats can be interpreted as the result of superimposed endogenic and exogenic processes endogenic processes are associated with coalification i e matrix dehydration and shrinkage while exogenic processes are mainly associated with larger scale phenomena such as tectonic stress at least two distinct generations of cleats were identified on the basis of field reconnaissance and microscopic study a first generation of cleats that developed early on during coalification and a second generation that cuts through the previous one at an angle that mimics the orientation of the present day stress field the observed parallelism between early formed cleats and mapped lineaments suggests a well established tectonic control during early cleat formation authigenic minerals filling early cleats represent the vestiges of once open hydrologic regimes the second generation of cleats is characterized by less prominent features i e smaller apertures with a much less pronounced occurrence of authigenic mineralization our findings suggest a multistage development of cleats that resulted from tectonic stress regimes that changed orientation during coalification and basin evolution the coals studied are characterized by a macrocleat distribution similar to that of well developed coalbed methane basins e g black warrior basin alabama scatter plots and regression analyses of meso and microcleats reveal a power law distribution between spacing and cleat aperture the same distribution was observed for fractures at microscopic scale our observations suggest that microcleats enhance permeability by providing additional paths for migration of gas out of the coal matrix in addition to providing access for methanogenic bacteria the abundance distribution and orientation of cleats control coal fabric and are crucial features in all stages of coalbed gas operations i e exploration and production understanding coal fabric is important for coal gas exploration as it may be related to groundwater migration and the occurrence of methanogenic bacteria prerequisite to biogenic gas accumulations likewise the distribution of cleats in coal also determines pathways for migration and accumulation of thermogenic gas generated during coalification

facies mosaic in the ghuneri member jurassic of the bhuj formation western kutch india abstract in marginal marine environments facies mosaics become complicated through time as and when the regular autocyclic changes are frequently intervened by allocyclic changes the facies mosaic of the marginal marine tithonian ghuneri member 476 m thick exposed in western kutch india is likewise fairly complex with frequent lateral as well as vertical alternations between six facies the basic motif built up of autocyclic facies transitions of the bar lagoon spectrum in response to shift of the shoreline is time and again interfered with by allocyclic changes caused by storm surges these storm surges overtopped the bar and laid their sediments over the indigenous facies as washover the autocyclic facies association is composed of three facies namely 1 bar 2 lagoon margin and 3 lagoon interior the allocyclic facies association comprises 4 proximal storm washover 5 distal storm washover and 6 proximal grain flow of insignificant occurrence facies statistical analysis indicates that the vertical facies transitions between these six facies are fairly unpredictable elimination of the products of instantaneous deposition storm washover facies from the same data base however renders the sequence of congruous autocyclic facies predictable

variability of geochemical properties in a microbially dominated coalbed gas system from the eastern margin of the illinois basin usa this study outlines gas characteristics along the southeastern margins of the illinois basin and evaluates regional versus local gas variations in seelyville and springfield coal beds our findings suggest that high permeability and shallow 100 250 m depths of these indiana coals allowed inoculation with methanogenic microbial consortia thus leading to widespread microbial methane generation along the eastern marginal part of the illinois basin low maturity coals in the illinois basin with a vitrinite reflectance ro 0 6 contain significant amounts of coal gas 3 m 3

the lawn hill annulus an ordovician meteorite impact into water saturated dolomite the lawn hill impact structure lhis is located 250 km n of mt isa in nw queensland australia and is marked by a highly deformed dolomite annulus with an outer diameter of similar to 18 km overlying low metamorphic grade siltstone sandstone and shale along the ne margin of the georgina basin this study provides detailed field observations from sections of the lawn hill annulus and adjacent areas that demonstrate a clear link between the deformation of the dolomite and the lawn hill impact ar 40 ar 39 dating of impact related melt particles provides a time of impact in the ordovician 472 8 ma when the georgina basin was an active depocenter the timing and stratigraphic thickness of the dolomite sequence in the annulus suggest that there was possibly up to 300 m of additional sedimentary rocks on top of the currently exposed thorntonia limestone at the time of impact the exposed annulus is remarkably well preserved with preservation attributed to postimpact sedimentation the lhis has an atypical crater morphology with no central uplift the heterogeneous target materials at lawn hill were probably low strength porous and water saturated with all three properties affecting the crater morphology the water saturated nature of the carbonate unit at the time of impact is thought to have influenced the highly brecciated nature of the annulus and restricted melt production the impact timing raises the possibility that the lawn hill structure may be a member of a group of impacts resulting from an asteroid breakup that occurred in the mid ordovician 470 6 ma

evaluation of coalbed gas potential of the seelyville coal member indiana usa abstract the seelyville coal member of the linton formation in indiana potentially contains 0 03 trillion m3 1 1 tcf of coalbed gas the gas content determined by canister desorption technique ranges from 0 5 to 5 7 cm3 g on dry ash free basis 15 4 to 182 2 scf ton the controls on gas content distribution are complex and cannot be explained by the coal rank alone ash content and the lithology of the overlying strata among other factors may influence this distribution

stable isotope geochemistry of coal bed and shale gas and related production waters a review coal bed and shale gas can be of thermogenic microbial or of mixed origin with the distinction made primarily on the basis of the molecular and stable isotope compositions of the gases and production waters methane ethane carbon dioxide and nitrogen are the main constituents of coal bed and shale gases with a general lack of c2 hydrocarbon species in gases produced from shallow levels and more mature coals and shales evidence for the presence of microbial gas include 13c ch4 values less than 50 covariation of the isotope compositions of gases and production water carbon and hydrogen isotope fractionations consistent with microbial processes and positive 13c values of dissolved inorganic carbon in production waters the co2 reduction pathway is distinguished from acetate methyl type fermentation by somewhat lower 13c ch4 and higher d ch4 but can also have overlapping values depending on the openness of the microbial system and the extent of substrate depletion crossplots of 13c ch4 versus 13c co2 and d ch4 versus 13c h2o may provide a better indication of the origin of the gases and the dominant metabolic pathway than the absolute carbon and hydrogen isotope compositions of methane in the majority of cases microbial coal bed and shale gases have carbon and hydrogen isotope fractionations close to those expected for co2 reduction primary thermogenic gases have 13c ch4 values greater than 50 and 13c values that systematically increase from c1 to c4 and define a relatively straight line when plotted against reciprocal carbon number although coals and disseminated organic matter in shales represent a continuum as hydrocarbon source rocks current data suggest a divergence between these two rock types at the high maturity end in deep basin shale gas reversals or rollovers in molecular and isotopic compositions are increasingly reported in what is effectively a closed shale system as opposed to the relative openness in coal measure environments detailed geochemical studies of coal bed and shale gas and related production waters are essential to determine not only gas origins but also the dominant methanogenic pathway in the case of microbial gases

stratigraphy and depositional setting of the upper mcnamara group lawn hills region northwest queensland the mesoproterozoic upper mcnamara group forms the uppermost division of the lawn hill platform in the western mount isa block several zinc and lead occurrences including the century deposit are hosted by clastic and carbonate sediments within this interval the upper mcnamara group is subdivided into four formations the shady bore quartzite riversleigh siltstone termite range formation and lawn hill formation the shady bore quartzite comprises up to 460 m of sandstone and dolomitic siltstone deposited in a marginal to shallow subaqueous environment the riversleigh siltstone is up to 2 900 m thick in the southern lawn hill region where four members r 1 to r 4 are recognized member r 1 to r 3 sandstone carbonate and shale were mostly deposited in mid and outer shelf environments indicating a regional transgression above the shady bore quartzite sediment transport during the r 1 to r 3 interval was mostly from the south member r 4 shale and sandstone represent deeper subaqueous deposits riversleigh siltstone sedimentation focused on a depocenter in the southern lawn hill region all members display an abrupt decrease in thickness and facies changes to the north of the termite range and barramundi faults which are interpreted as active structures during deposition the termite range formation is up to 1 300 m thick and comprises two fining upward sequences t 1 and t 2 both dominated by sandstone in the lower part and by shale in the upper part the termite range formation is sandiest in the north and thins to the south and east sandstone beds are interpreted as high concentration turbidites deposited in a deep subaqueous environment t 1 and t 2 are interpreted as two major episodes of turbidite fan sedimentation sourced from the northwest fault activity to the west of the lawn hill region at about 1640 ma is inferred to account for deposition of sandstone and the major shift in provenance from the south to the northwest the lawn hill formation is 2 200 m thick in the southern lawn hill region where six members h 1 to h 6 are recognized member h 1 shale and sandstone were deposited in outer shelf and deeper environments member h 2 and h 3 tuffaceous sediments were deposited in a mid shelf environment indicating a regional regression member h 1 to h 3 sediments were sourced from the west member h 4 shale and siltstone were deposited at outer shelf and greater depths indicating a regional transgression above member h 3 member h 5 sandstone beds are interpreted as high concentration turbidites deposited in prograding turbidite fans member h 1 to h 3 sedimentation occurred throughout the lawn hill region but a major depocenter was located in the northeast at this time member h 1 h 2 and h 3 sediments rapidly thicken to the north of the termite range fault which is interpreted as an active structure during deposition of this interval the northward shift of the depocenter with respect to the riversleigh siltstone can be attributed to a reversal in movement along the termite range fault member h 5 sandstone beds are widespread throughout the northern lawn hill platform and their deposition indicates another significant change in the distribution and style of sedimentation deposition in response to uplift in the south can be interpreted by the age of the sandstone 1595 ma which may coincide with the onset of the d 1 phase of the isan orogeny this study provides a framework for the identification of prospective areas for base metal mineralization within the upper mcnamara group based on a knowledge of sediment architectures and their controls

microbial generation of economic accumulations of methane within a shallow organic rich shale although methane of bacterial origin is ubiquitous in marine and freshwater sediments economic accumulations of bacterial gases occur mainly at depths of several kilometres in tertiary basins that had high sedimentation rates1 2 here we present an integration of geochemical and isotopic data from gas and water extracted from the upper devonian antrim shale along the northern margin of the michigan basin which demonstrates that significant volumes of bacterial gas have been generated in organic rich shales at depths of less than 600 metres the antrim shale is mainly a self sourced reservoir in contrast to conventional gas deposits that have migrated from a source to a reservoir and has become one of the most actively exploited gas reservoirs3 in the united states the gas forming processes operating at shallow depths in the antrim shale are not unique4 and an understanding of these processes should lead to the identification and development of other economic non conventional gas deposits around the world

timing of biogenic gas formation in the eastern qaidam basin nw china abstract quaternary biogenic gas from the qaidam basin nw china is regarded as one of the representative examples of early generation biogenic gas gases formed by microbial activity during sedimentation of source rocks however new age determinations of natural gas and formation waters make this generally accepted assumption questionable gas in the qaidam basin is dominantly methane 99 0 and has 13 c ch 4 values in the range of 68 to 65 reflecting the microbial origin carbon and hydrogen isotope values of methane coupled with carbon isotope values of carbon dioxide and dissolved inorganic carbon dic reflect water derived hydrogen in the methane and a dominant co 2 reduction metabolic pathway for the formation of biogenic methane coproduced formation waters have hydrogen and oxygen isotope values similar to surface water from the golmud and ortmuren rivers t test 0 75 but different from the original formation water t test 1 sequence around 1 35 ma bp this corresponds to a regional uplift erosion and water influx event economic accumulations of biogenic gas were thus most likely generated late in the qaidam basin rather than at the same time as sediment deposition fresh water recharge may be critical to enhancing biogenic methane generation by introducing nutrients and or microbes into the reservoir

paleowaters in silurian devonian carbonate aquifers geochemical evolution of groundwater in the great lakes region since the late pleistocene abstract changes in the climatic conditions during the late quaternary and holocene greatly impacted the hydrology and geochemical evolution of groundwaters in the great lakes region increased hydraulic gradients from melting of kilometer thick pleistocene ice sheets reorganized regional scale groundwater flow in paleozoic aquifers in underlying intracratonic basins here we present new elemental and isotopic analyses of 134 groundwaters from silurian devonian carbonate and overlying glacial drift aquifers along the margins of the illinois and michigan basins to evaluate the paleohydrology age distribution and geochemical evolution of confined aquifer systems this study significantly extends the spatial coverage of previously published groundwaters in carbonate and drift aquifers across the midcontinent region and extends into deeper portions of the illinois and michigan basins focused on the freshwater saline water mixing zones in addition the hydrogeochemical data from silurian devonian aquifers were integrated with deeper basinal fluids and brines in upper devonian black shales and underlying cambrian ordovician aquifers to reveal a regionally extensive recharge system of pleistocene age waters in glaciated sedimentary basins elemental and isotope geochemistry of confined groundwaters in silurian devonian carbonate and glacial drift aquifers show that they have been extensively altered by incongruent dissolution of carbonate minerals dissolution of halite and anhydrite cation exchange microbial processes and mixing with basinal brines carbon isotope values of dissolved inorganic carbon dic range from 10 to 2 87sr 86sr ratios range from 0 7080 to 0 7090 and 34 s so 4 values range from 10 to 30 a few waters have elevated 13cdic values 15 from microbial methanogenesis in adjacent organic rich upper devonian shales radiocarbon ages and 18o and d values of confined groundwaters indicate they originated as subglacial recharge beneath the laurentide ice sheet 14 50 ka bp 15 to 13 18o these paleowaters are isolated from shallow flow systems in overlying glacial drift aquifers by lake bed clays and or shales the presence of isotopically depleted waters in paleozoic aquifers at relatively shallow depths illustrates the importance of continental glaciation on regional scale groundwater flow modern groundwater flow in the great lakes region is primarily restricted to shallow unconfined glacial drift aquifers recharge waters in silurian devonian and unconfined drift aquifers have 18o values within the range of holocene precipitation 11 to 8 and 7 to 4 5 for northern michigan and northern indiana ohio respectively carbon and sr isotope systematics indicate shallow groundwaters evolved through congruent dissolution of carbonate minerals under open and closed system conditions 13cdic 14 7 to 11 1 and 87sr 86sr 0 7080 0 7103 the distinct elemental and isotope geochemistry of pleistocene versus holocene age waters further confirms that surficial flow systems are out of contact with the deeper basinal scale flow systems these results provide improved understanding of the effects of past climate change on groundwater flow and geochemical processes which are important for determining the sustainability of present day water resources and stability of saline fluids in sedimentary basins

generation accumulation and resource potential of biogenic gas biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms more than 20 of the world s discovered gas reserves are of biogenic origin a higher percentage of gases of predominantly biogenic origin will be discovered in the future biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread large quantities at shallow depths in rapidly accumulating marine sediments a succession of microbial ecosystems leads to the generation of biogenic gas after oxygen is consumed by aerobic respiration sulfate reduction becomes the dominant form of respiration methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted the most important mechanism of methane generation in marine sediments is the reduction of co2 by hydrogen electrons produced by the anaerobic oxidation of organic matter co2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures the factors that control the level of methane production after sediment burial are anoxic environment sulfate deficient environment low temperatu e availability of organic matter and sufficient space the timing of these factors is such that most biogenic gas is generated prior to burial depths of 1 000 m in marine sediments most of the biogenic gas formed can be retained in solution in the interstitial pore waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column under certain conditions of high pressures and or low temperatures biogenic methane combines with water to form gas hydrates biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses the hydrocarbon fraction of biogenic gas consists predominantly of methane the presence of as much as 2 of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low temperature degradation of organic matter the amounts of hydrocarbon components other than methane generally are proportional to temperature age and organic matter content of the sediments biogenic methane is enriched in the light isotope 12c dgr13c1 lighter than 55 ppt owing to kinetic isotope fractionation by methanogens the variations in isotopic composition of biogenic methane are controlled primarily by dgr13c of the original co2 substrate which reflects the net isotopic effect of both addition and removal of co2 the methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas the possible complicating factors require that geologic chemical and isotopic evidence be considered in attempts to interpret the origin of gas accumulations accumulations of biogenic gas have been discovered in canada germany italy japan trinidad the united states and ussr in cretaceous and younger rocks at less than 3 350 m of burial and in marine and nonmarine rocks other gas accumulations of biogenic origin have undoubtedly been discovered however data that permit their recognition are not available

fluvial tidal and storm sedimentation in the chilhowee group lower cambrian northeastern tennessee u s a the lower cambrian chilhowee group of northeastern tennessee consists of the unicoi hampton and erwin formations and is divided into four facies the conglomerate facies occurs only within the lower 200 m of measured section the unicoi formation and consists of fine grained to pebbly quartz wacke with rare thin beds of laminated siltstone low angle to horizontally laminated fine grained sandstone with laminae and lenses of granules and pebbles represents upper flow regime overbank deposition within a braided stream system that was close to a coastline medium scale planar tabular cross stratified conglomerate in which megaripple bedforms are preserved is interpreted as representing deposition in interbar pools of braided channels as flood stage waned and larger bedforms ceased to migrate large scale planar tabular cross stratified conglomerate beds represent migration of large transverse bars within a broad braided stream channel during high flood stage the sandstone facies occurs throughout the chilhowee group and is therefore interbedded with all other facies it consists of mainly medium to very coarse grained subarkosic to arkosic arenite thinly interbedded laminated siltstone and sandstone which may exhibit wavy or lenticular bedding represents deposition during slack water periods between ebb and flood tides large scale planar tabular and trough cross stratification reflects deposition within the deepest areas of subtidal channels whereas medium scale cross stratification represents deposition in shallower water on shoals separating channels fining and thinning upward sequences most likely resulted from the longshore migration of channels and shoals the hummocky facies occurs only in the erwin formation and consists of horizontally laminated to hummocky stratified fine grained arkosic to subarkosic arenite interbedded with equal amounts of bioturbated mudstone it represents deposition between storm and fairweather wave base by combined flow storm currents the quartz arenite facies is characterized by an absence of fine grained units and lithologically consists of a super mature medium to coarse grained quartz arenite large scale planar tabular cross stratification and abundant low angle cross stratification with rare symmetrical ripples lower quartz arenite facies occurs interbedded with the braided fluvial conglomerate facies and was deposited within either a ridge and runnel system or a system of nearshore bars large scale planar tabular cross stratification upper quartz arenite facies which forms the top of two 40 m thick coarsening upward sequences of the type hummocky faciessandstone faciesquartz arenite facies probably represents deposition on sand ridges that formed on a sand starved shelf as transgression caused the detachment and reworking of shoreface channel shoal couplets palaeocurrent data for the chilhowee group are unimodal but widely dispersed from 0 to 180 and exhibit a minor mode to the west the data are interpreted to reflect the influence of longshore tidal and storm currents the ichnofossil assemblage changes upsection from one characterized only by paleophycus to a skolithos ichnofacies and finally to a cruziana ichnofacies the facies sequence biogenic and palaeocurrent data reflect the interaction through time of i non marine and marine processes and 2 transgression coupled with shoreline progradation the chilhowee group represents an overall deepening from terrestrial deposition to a marine shoreface that experienced both longshore and tidal currents and finally to a storm shelf environment that periodically shoaled upward

lopatin analysis of maturation and petroleum generation in the illinois basin a modified lopatin approach was used to evaluate the present day maturity of paleozoic source rock units across the illinois basin timing of generation regional porosity trends and basin paleostructure during major generative events ten cases were modeled at 100 locations to test assumed paleogeothermal gradients post pennsylvanian overburden thicknesses and rates of erosional stripping lopatin predicted maturities for the herrin number sign 6 coal and the new albany shale are in good agreement plus minus 0 02 r sub o with measured maturities if 500 3 000 ft of post middle pennsylvanian strata and were deposited and subsequently eroded between the permian and mid cretaceous and if paleogeothermal gradients were within a few degree c km of present day gradients predicted mean reflectance levels range from 1 0 to 4 0 r sub o at the base of the potsdam megagroup 0 7 to 3 5 at the base of the know megagroup and 0 6 to 1 3 at the base of the maquoketa shale excluding only a small high maturity area in southeastern illinois the knox and potsdam section attained oil generation 475 300 ma while the maquoketa and the younger new albany shale reached the oil window much later 300 250 ma because most significant structures in the basin formed after 300 ma any pre maquoketa source rocks were alreadymore within the gas zone and may have been largely spent by the time known structures formed any know or deeper traps in the basin will probably contain gas be restricted to old structures earlier than 300 ma or stratigraphic traps and will hold pre 300 ma generated hydrocarbons which subsequently cracked to gas less

carbonate platform growth and cyclicity at a terminal proterozoic passive margin infra krol formation and krol group lesser himalaya india the infra krol formation and overlying krol group constitute a thick 2 km carbonate rich succession of terminal proterozoic age that crops out in a series of doubly plunging synclines in the lesser himalaya of northern india the rocks include 18 carbonate and siliciclastic facies which are grouped into eight facies associations 1 deep subtidal 2 shallow subtidal 3 sand shoal 4 peritidal carbonate complex 5 lagoonal 6 peritidal siliciclastic carbonate 7 incised valley fill and 8 karstic fill the stromatolite rich peritidal complex appears to have occupied a location seaward of a broad lagoon an arrangement reminiscent of many phanerozoic and proterozoic platforms growth of this complex was accretionary to progradational in response to changes in siliciclastic influx from the south eastern side of the lagoon metre scale cycles tend to be laterally discontinuous and are interpreted as mainly autogenic variations in the number of both sets of cycles and component metre scale cycles across the platform may result from differential subsidence of the interpreted passive margin apparently non cyclic intervals with shallow water features may indicate facies migration that was limited compared with the dimensions of facies belts correlation of these facies associations in a sequence stratigraphic framework suggests that the infra krol formation and krol group represent a north to north westfacing platform with a morphology that evolved from a siliciclastic ramp to carbonate ramp to peritidal rimmed shelf and finally to open shelf this interpretation differs significantly from the published scheme of a basin centred on the lesser himalaya with virtually the entire infra krol krol succession representing sedimentation in a persistent tidal flat environment this study provides a detailed neoproterozoic depositional history of northern india from rift basin to passive margin and predicts that genetically related neoproterozoic deposits if they are present in the high himalaya are composed mainly of slope basinal facies characterized by fine grained siliciclastic and detrital carbonate rocks lithologically different from those of the lesser himalaya

palaeoenvironmental analysis of the late proterozoic nagthat formation nw kumaun lesser himalaya india abstract the late proterozoic nagthat formation 750 m of the jaunsar group exhibits extensive development of multistoried medium to thick bedded sandstone impersistent conglomerate pebbly beds and siltstone shale it represents a shoreface to a proximal inner shelf deposit developed under the dominant influence of tidal and occasional storm conditions the major lithofacies recognised are the following a the coarse grained siliciclastic facies cgs represented by discontinuous beds of coarse grained sandstone with lenses of conglomerates the important sedimentary features observed in this facies are medium to large scale trough tabular cross stratifications and crudely graded conglomerates etc the coarse grained sandstone is frequently punctuated by numerous broadly undulatory or locally more or less flat erosional surfaces a proximal shoreface environmental set up has been assigned to this facies b the interbedded medium to fine grained siliciclastic facies imfs consisting of decimetre to metre thick alternations of medium to fine grained subarkosic to sublithic arenite with subordinate quartz arenite and quartz wacke the striking sedimentary features are the herringbone cross stratification and rare hummocky cross stratification hcs the rareness of the hcs suggests a frequent reworking of storm laden sediments by fair weather processes in the intermediate shoreface zone c the fine grained siliciclastic facies fgs represented by parallel to cross laminated silty sandstone and shale is interpreted as the distal shoreface to proximal inner shelf deposit the study of these facies reveals the three prominent associations viz fining upward cgs imfs fgs and cgs fgs and coarsening upward fgs imfs the first two facies associations have resulted mainly from tidal activity whereas the last one provides evidence of storm activity the overall coarsening upward stratigraphic sequence suggests gradual shallowing of the basin and thus indicates a prograding behaviour of the nagthat formation