correlation of reservoired gases using the carbon isotopic compositions of wet gas components the carbon isotopic compositions of the wet gas components particularly propane isobutane and normal butane have been found to be particularly valuable for correlating one reservoired gas with another the usefulness of these components for correlation results from their carbon isotopic compositions reflecting both the nature of their source and their maturity this source control is strongest for gases derived from the more highly structured types of kerogen i e woody coaly type iii although the wet gas components carbon isotopic compositions of most gases are at least partly controlled by their source for levels of maturity below the point at which thermal destruction of the wet components occurs as a result the wet gas components are found to provide more ositive correlations than is methane alone three exploration examples illustrate the use of the carbon isotopic compositions of the wet gas components for correlation the leduc reef trend of alberta canada the sleipner area in the north sea and the lena field offshore louisiana united states the three examples also illustrate the need to integrate geochemical interpretations with regional geology to obtain a good understanding of the hydrocarbon source

geochemical characteristics of the extremely high thermal maturity transitional shale gas in the southern north china basin sncb and its differences with marine shale gas abstract a great number of studies have been carried out on the geochemical characteristics of marine shale gas from the lower silurian longmaxi formation in the sichuan basin however a systematic study on geochemical and isotopic characteristics of transitional shale gas gas from the alternating marine terrigenous shale facies has not been well addressed in the present study transitional shale gas from the lower permian shanxi p1s and taiyuan p1t formation in the southern north china basin sncb were collected and analyzed for their geochemical and isotopic compositions the hydrocarbon gases are dominated by methane with small amounts of ethane without propane and butane which is consistent with the extremely high thermal maturity of the gas shales ro values between 3 2 and 4 2 compared with marine shale gas the relatively high non hydrocarbon components may be associated with the humic source rocks the 13c1 values range from 31 6 to 21 0 and the 13c2 values range from 35 9 to 26 1 the 2hch4 values range from 221 to 138 these results indicate that the gases are of thermogenic origin methane with anomalous heavy carbon isotopic and light hydrogen isotopic values was interpreted as redox reactions of gases with transition metals and water at maximum burial resulting in rayleigh type fractionation the difficulties of identifying marine and transitional shale gas may be due to the complete or partial carbon isotopic reversal caused by secondary alteration and the extremely high thermal maturity of the gas shales co2 concentration and 13c co2 values suggest that co2 in the transitional shale gas was mainly thermogenic origin and formed together with hydrocarbon generation furthermore significant differences can be observed from the co2 in different types of shale gas co2 in the transitional shale gas is mainly distributed in areas with co2 5 and 13c co2 8 the 15n n2 values and high illite and illite smectite mixed clay i s content suggest that nitrogen in transitional shale gas was possibly generated during thermo ammoniation of organic matter and or the breakdown of nh4 rich clay minerals in addition based on the geochemical characteristics of marine and transitional shale gas a geochemical pattern of shale gas is presented in this paper

hydrothermal hydrocarbon gases in the sediments of the king george basin bransfield strait antarctica thermogenic hydrocarbons formed by the thermal alteration of organic matter are encountered in several piston core stations in the king george basin anatarctica these hemipelagic sediments are being deposited in an area of active hydrothermalism associated with the back arc spreading in the bransfield strait the lateral extent of sediments infiltrated by the hydrothermally influenced interstitial fluids is characterized by basalt diapirix intrusions and is delineated by an acoustically turbid zone in the sediments of the eastern part of the basin iron sulphide bearing veins and fractures cut across the sediment in several cores they appear to be conduits for flow of hydrothermally altered fluids these zones have the highest c2 and ethene contents the thermogenic hydrocarbons have molecular c1 c2 c3 ratios typically 50 and 13ch4 values between 38 and 48 indicating an organic source which has undergone strong thermal stress several sediment cores also have mixed gas signatures which indicate the presence of substantial amounts of bacterial gas predominantly methane hydrocarbon generation in the king george basin is thought to be a local phenomenon resulting from submarine volcanism with temperatures in the range 70 150 c there are no apparent seepages of hydrocarbons into the water column and it is not believed that significant accumulation of thermogenic hydrocarbons reside in the basin

the caldwell group lavas of southern quebec morb like tholeiites associated with the opening of iapetus ocean the caldwell group belongs to the internal nappe domain of the humber zone and consists of basaltic lavas quartzo feldspathic sandstones and mudslates the lavas are clinopyroxene plagioclase olivine phyric tholeiites and are typically altered to epidote chlorite carbonate and or hematite rich secondary assemblages in most cases the high field strength elements do not appear to have been perturbed by the alteration and preserve magmatic signatures most caldwell basalts exhibit coupled major and trace element variations compatible with low to medium pressure 10 kbar where 1 kbar 100 mpa fractional crystallization paleotectonic discriminants imply an ocean floor or normal mid ocean ridge basalt n morb affinity most basalts have flat n morb normalized profiles except for the highly incompatible elements ba th nb which show slight relative enrichment melting models suggest that most of these lavas formed by about 20 melting from a mantle slightly less depleted than fertil

forward potential field modeling of the appalachian orogen in the gaspe peninsula quebec canada implications for the extent of rift magmatism and the geometry of the taconian orogenic wedge the nature and extent of potential source bodies associated with prominent regional scale gravity and magnetic anomalies in the northern quebec appalachians are investigated through forward modeling potential field anomalies are interpreted to be the geophysical expression of basalt flows extruded during the neoproterozoic to cambrian rifting of laurentia prior to opening of iapetus geophysi cal modeling provides substantial new constraints on seismic and structural cross sections and the map distribution of volcanism this study indicates that the volcanism was significantly more widespread than previously recognized basaltic rocks form a sheet up to 4 kilometers thick transported on the main taconic decollement most of this sheet is deeply buried and overlain by a thick pile of paleozoic sediments later tectonic modifications by backthrusting and hinterland directed normal faulting as well as by acadian strike slip faulting substantially changed the geometry of the transported volcanic rocks and the structural style both across and along the strike of the orogen the extension of the gravity anomaly under the st lawrence river also suggests that the ordovician deformation front is 20 to 30 kilometers further to the north than previously interpreted

natural seepage of shale gas and the origin of eternal flames in the northern appalachian basin usa abstract natural hydrocarbon gas seeps are surface expressions of petroleum seepage systems whereby gas is ascending through faults from pressurized reservoirs that are typically associated with sandstones or limestones a spectacular eternal flame in western new york state marks a gas macroseep of dominantly thermogenic origin emanating directly from deep shale source rocks which makes this a rare case in contrast to most petroleum seepage systems where gas derives from conventional reservoirs the main flaming seep releases about 1 kg of methane per day and may feature the highest ethane and propane c 2 c 3 concentration ever reported for a natural gas seep 35 vol the same gas is also released to the atmosphere through nearby invisible and diffuse seepages from the ground the synopsis of our data with available gas geochemical data of reservoir gases in the region and the stratigraphy of underlying shales suggests that the thermogenic gas originates from upper devonian shales without intermediation of a conventional reservoir a similar investigation on a second eternal flame in pennsylvania suggests that gas is migrating from a conventional sandstone pool and that the seep is probably not natural but results from an undocumented and abandoned gas or oil well the large flux of the emitted shale gas in new york state implies the existence of a pressurized gas pool at depth tectonically fractured shales seem to express naturally fracked characteristics and may provide convenient targets for hydrocarbon exploration gas production from tectonically fracked systems might not require extensive artificial fracking

composition and stable isotope geochemistry of natural gases from kansas midcontinent u s a abstract more than 28 1012 ft 3 79 1010 m3 of natural gas and 5 3 109 bbl 8 4 108 m3 of oil have been produced in kansas u s a from paleozoic carbonate and sandstone reservoirs on structural uplifts and shallow embayments along the northern margin of the anadarko basin a heavily explored geologically well characterized state kansas is an excellent place to study hydrocarbon migration and to test geochemical models for the origin of natural gases immature to marginally mature rocks of eastern kansas cherokee and forest city basins produce mixed microbial and thermogenic gases gases in this region have wetness 0 03 51 methane 13c 65 to 43 and methane d 260 to 150 gases from central and western kansas nemaha uplift to hugoton embayment are entirely thermogenic and have wetness 4 51 methane 13c 48 to 39 and methane d 195 to 140 ethane and propane 13c values throughout kansas vary from 38 to 28 and from 35 to 24 respectively mature thermogenic gas generated from source rocks in southwestern kansas and the anadarko basin with 1 0 ro 1 4 is recognized throughout the state lateral migration into shallow reservoirs on the central kansas and northern nemaha uplifts and in the cherokee basin probably occurred along basal pennsylvanian conglomerates and weathered lower paleozoic carbonates at the regional sub pennsylvanian unconformity early thermogenic gas generated by local source rocks with ro 0 7 is recognized in isolated fields in the salina and forest city basins in ordovician reservoirs beneath the sub pennsylvanian unconformity in the cherokee basin and in reservoirs generally above the unconformity in the cherokee and sedgwick basins the eastern central kansas uplift and the hugoton embayment

carbon isotope geochemistry of hydrocarbons in the cerro prieto geothermal field baja california norte mexico hydrocarbon abundances and stable isotopic compositions were measured in wells m5 m26 m35 and m102 which represent a range of depths 1270 2000 m and temperatures 275 330 degrees c in the field in order to simulate the production of the geothermal hydrocarbons gases were collected from the pyrolysis of lignite in the laboratory this lignite was obtained from a well which sampled rock strata which are identical to those occurring in the field but which have experienced much lower subsurface temperatures in both the well and the laboratory observations high temperature environments favored higher relative concentrations of methane ethane and benzene and generally higher delta 13c values in the individual hydrocarbons the best correlation between the laboratory and well data is obtained when laboratory produced gases from experiments conducted at lower 400 degrees c and higher 600 degrees c temperatures are mixed this improved correlation suggests that the wells are sampling hydrocarbons produced from a spectrum of depths and temperatures in the sediments

geochemical characteristics and origin of natural gas from wufeng longmaxi shales of the fuling gas field sichuan basin china abstract as the first giant shale gas field in china the fuling gas field has recently been regarded as one of the most important regions for natural gas exploration and production in the sichuan basin however the origin of natural gas from upper ordovician wufeng and the bottom of lower silurian longmaxi wl shales in the fuling gas field is poorly understood to limit a comprehensive understanding of gas generation accumulation and exploration in this work based on molecular and stable carbon isotopic composition of a total of 24 gas samples from five shale gas wells in the fuling field we analyzed the geochemical characteristics and gas origin and discussed the cause for the geochemical anomalies carbon isotopic reversals molecular composition results show that gases from the fuling gas field are dry and mainly composed of methane 97 9 98 9 with a very low level of ethane c2h6 propane c3h8 and non hydrocarbon gases mainly co2 and n2 these dry gases are classified as oil associated gas and mainly derived from secondary cracking due to the lack of gas samples across a maturation gradient from immature to late mature the wl gases in the fuling field show an unclear evolution trend between the 13c2 and wetness values however all these samples are located in the isotopic reversal zone carbon isotopes of gaseous alkanes clearly display full isotopic reversals 13c1 13c2 13c3 which is indicative of a relatively high thermal maturity and consistent with the measured vitrinite reflectance and modeled values 3 0 ro the observed complete carbon isotopic reversals in the wl gases are caused by a combination of several mechanisms in which isotope exchange at high temperature is the primary controlling factor other secondary factors include rayleigh type fractionation of c2h6 and c3h8 secondary cracking and gas diffusion mixing of gases at different thermal maturity levels

origin of conventional and shale gas in sinian lower paleozoic strata in the sichuan basin relayed gas generation from liquid hydrocarbon cracking large amounts of natural gas have been discovered recently both in carbonate reservoirs and in shales in the sinian silurian strata of the sichuan basin southern china although numerous studies have been conducted to learn about the accumulation of conventional and shale gas the gas generation history the origins of the two types of gas and gas resources in the basin remain unclear in this study the hydrocarbon generation histories of cambrian and silurian source rocks were reconstructed on the basis of thermal history and hydrocarbon generation kinetic parameters intensive cracking of crude oil to generate considerable amounts of secondary cracking gas in silurian and cambrian formations occurred during deep burial the high abundance of overmature pyrobitumen in carbonate reservoirs and shale demonstrates the occurrence of large scale in situ cracking of liquid hydrocarbons a mixing model was proposed to quantitatively determine the contribution of kerogen and oil cracking gas in shale gas accumulations the mixing of two thermogenic gases with different maturities resulted in a reversal and a rollover of the carbon isotope compositions of shale gas moreover gas resources from the cracking of three occurrence states of liquid hydrocarbons including dispersed residual bitumen in source rocks dispersed oil outside the source and accumulated oil in reservoirs were evaluated in sinian cambrian and silurian petroleum systems in the sichuan basin providing a guide for future deep gas exploration of similar formations in china and elsewhere

biochemical evolutionary significance of ordovician oils and their sources abstract geochemical data from several ordovician oils and kerogens from distinct basins in canada united states estonia and australia show some unique compositional characteristics the oils analyzed by gas chromatography and gas chromatography mass spectrometry are characterized by an odd dominance in the c9 c19 molecular weight range of normal paraffins and an odd dominated homologous series of cyclohexyl substituted normal paraffins the oils contain an unusually small amount of c20 components and pristane and phytane are conspicuous by their virtual absence the compositions of the oils and kerogens correlate closely and display characteristics that have not been observed in younger sediments the predominant organism found in the source kerogens for these unique oils is gloeocapsamorpha prisca a primitive prokaryotic organism data from the analysis of gloeocapsamorpha rich kerogens and the association of this organism with ordovician source rocks indicate a genetic relationship exists between the organism and the unusual oil chemistry the chemistry of the oils and kerogens suggests that gloeocapsamorpha was either non photosynthetic or that it fixed light using a chlorophyll that did not contain a phytyl side chain e g a bacteriochlorophyll the widespread occurrence of oils and kerogens possessing such a uniform composition appears unique to the ordovician certainly to the early paleozoic this suggests that paleocological conditions of ordovician anoxic environments were very similar and g prisca was particularly well adapted to these conditions

origin of natural gas in sacramento basin california the remnant of a late mesozoic early cenozoic fore arch basin the sacramento valley is principally a dry gas province complex stratigraphic relations widespread deformation and faulting and the presence of igneous intrusives testify to an active tectonic history sediments are organically immature down to an average of 10 000 ft 3 000 m and are geopressured throughout much of the basin small fields with nitrogen concentrations as high as 88 are found along the shallow eastern margin gases were sampled from 94 producing wells methane dgr13c and dgrd values range from 61 to 15 pmil and 200 to 96 pmil respectively indicating a wide range of source rock maturities wet gases in the western delta area sgrc2 c5 sgrc1 c5 12 5 have ethane dgr13c values between 31 and 20 pmil and propane dgr13c values between 27 and 12 pmil dry gases elsewhere in the basin sgrc2 c5 sgrc1 c5 1 6 have ethane dgr13c values as light as 58 pmil the chemical and stable isotope data are consistent with four principal sources gases in the western delta area were generated from mature source rocks within the oil and condensate window gases in the northern and southeastern basin are mixtures of indigenous gas mostly microbial methane with small quantities of diagenetic ethane dry thermogenic gas migrated from post mature source rocks in the basin deep and nitrogen rich gases thought to originate in metasedimentary rocks deep beneath the sediment these results place logical constraints on the location of productive traps with high btu gas and suggest caution in the interpretation of stable isotope data from other tectonically active basins due to dynamic migration and mixing processes

analysis of light hydrocarbons in soil gases lost river region west virginia relation to stratigraphy and geological structures analyses of 471 near surface soil gas samples for light hydrocarbons c1 c4 c2l c3l and h2 from the lost river gas field in hardy county west virginia reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases which occur directly over the faulted eastern limb of the whip cove anticline compositional changes in the soil gases data clearly define major changes in the maturity and locations of potential source beds grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil gas constituents large concentrations coupled with high saturate to olefin ratios further confirms that this active seepage is near macroseep levels variations in soil gas compositional trends separate the soil gas data into two domains with oilier compositions to the west and gassier compositions to the east although the composition of the shallow soil gases above the lost river gas field are oilier than the reservoir gases they occur directly over the eastern faulted limb of the producing anticlinal structure suggesting that the dry gases from the oriskany reservoir are probably mixed with oilier gases from organic rich strata among devonian shales the eastern anomalies are much gassier and are very similar to the oriskany gases produced by the lost river gas field the eastern anomalies directly overlie near vertical beds of devonian and older age formations that are likely conduits for deeper mature thermal gases

middle ordovician organic matter assemblages and their effect on ordovician derived oils two distinct middle ordovician oil prone organic matter assemblages occur in thermally immature to marginally mature petroleum source rocks from iowa the substantially more oil prone assemblage a generates the unique ordovician oil fingerprint which has been associated with the organic walled microfossil gloeocapsamorpha prisca the second assemblage b generates a more ordinary signature the two assemblages which are mixed or interbedded in many ordovician sediments of north america explain the variations in oils derived from ordovician source rocks these mixtures also aid in interpretation of the chemistry of g prisca core samples examined are from three wells in iowa penetrating the primarily carbonate sequence from the guttenberg member of the decorah formation down to the st peter sandstone microscopy rock eval pyrolysis and capillary gas chromatography on source rock bitumen extracts and nearby oils were used to evaluate the two assemblages solvent extracts from rocks containing predominantly assemblage a have the diagnostic ordovician gas chromatographic signatures with an odd preference in c11 c19 normal alkanes low amounts of heavier normal alkanes and virtual absence of isoprenoids including pristane and phytane although assemblage a occurs only in very thin beds it is potentially so oil generative that thicknesses in inches may be significant extracts from r cks containing predominantly assemblage b show reduced odd preference a full profile of normal alkanes and conspicuous occurrence of isoprenoids including pristane and phytane these samples from iowa are representative of middle ordovician source rocks found in most paleozoic basins of north america as well as baltic europe and australia therefore the conclusions drawn from these middle ordovician rock samples from iowa provide a basis to explain variations in ordovician derived oils found in many other areas

origin of gaseous hydrocarbons noble gases carbon dioxide and nitrogen in carboniferous and permian strata of the distal part of the polish basin geological and isotopic approach abstract sixteen natural gas samples collected from pennsylvanian and permian rotliegend and zechstein limestone strata of the distal part of the polish basin in wielkopolska and lower silesia were analysed for molecular composition and stable carbon isotope composition of hydrocarbons ch 4 c 2 h 6 c 3 h 8 n c 4 h 10 and i c 4 h 10 and co 2 stable hydrogen isotope composition of methane stable nitrogen isotope composition of n 2 and stable isotope composition of noble gases he ne ar kr and xe thirteen of analysed hydrocarbon gases reveal complete 13 c ch 4 13 c c 2 h 6 13 c c 3 h 8 and partial inversed isotopic trends from methane to propane thus they have a very complicated generation migration and accumulation history and range of their source rock horizons two source rock horizons occur ne from wolsztyn ridge they contain mixed types iii ii and ii iii kerogens of pennsylvanian age and mississippian and or devonian age respectively one source rock horizon occurs sw from wolsztyn ridge it contains type iii and mixed type iii ii kerogens of pennsylvanian age the kerogens in all source rock horizons generated gaseous hydrocarbons at two separate maturity stages about 0 5 to 0 8 in vitrinite reflectance scale at the first maturity stage and over 1 3 in vitrinite reflectance scale at the second maturity stage high concentrations of he in analysed gases are mostly a product of decay of u and th enriched in crustal materials a small contribution of he and ne of mantle origin to the gas reservoirs was inferred radiogenic 4 he 40 ar ratios are higher than the average production rate ratio of about 5 for crustal materials which might have been caused by a selective supply of lighter he through crustal rocks surrounding the gas reservoirs or higher u th k ratio than the crustal average co 2 from analysed gases was mainly generated during thermogenic processes of transformation of organic matter although some gases can contain components from endogenic processes and from thermal destruction of zechstein limestone and probably precambrian carbonates n 2 was mainly generated during thermal transformation of organic matter and had a large component released from nh 4 rich illites very high n 2 40 ar for the gases might be caused by accelerated thermogenic production of n 2 under the condition of high heat flux from volcanic activity at late pennsylvanian and early permian age deep seated faults mainly framing the wolsztyn ridge perform important role in migration and mixed in different proportions of noble gases originated from upper mantle and from mineralisation by radioactive minerals their fault surfaces and occurrence of pennsylvanian lower permian volcanites in crust as well as hydrocarbon gases co 2 and n 2 also from thermal destruction of nh 4 rich illites from gas generation kitchens of source rock horizons to the traps

geochemistry origin and provenance of upper proterozoic to upper cambrian alkaline to transitional basaltic rocks in and contiguous to a sector of the appalachian humber zone canada

chemical and isotopic tracers of the contribution of microbial gas in devonian organic rich shales and reservoir sandstones northern appalachian basin in this study the geochemistry and origin of natural gas and formation waters in devonian age organic rich shales and reservoir sandstones across the northern appalachian basin margin western new york eastern ohio northwestern pennsylvania and eastern kentucky were investigated additional samples were collected from mississippian berea sandstone silurian medina sandstone and ordovician trenton black river group oil and gas wells for comparison dissolved gases in shallow groundwaters in devonian organic rich shales along lake erie contain detectable ch4 0 01 50 55 mol with low 13c ch4 values 74 68 to 57 86 and no higher chain hydrocarbons characteristics typical of microbial gas nevertheless these groundwaters have only moderate alkalinity 1 14 8 72 meq kg and relatively low 13c values of dissolved inorganic c dic 24 8 to 0 6 suggesting that microbial methanogenesis is limited the majority of natural gases in devonian organic rich shales and sandstones at depth 168 m in the northern appalachian basin have a low ch4 to ethane and propane ratios 3 35 mol c1 c2 c3 and high 13c and d values of ch4 53 35 to 40 24 and 315 0 to 174 6 respectively which increase in depth reservoir age and thermal maturity the molecular and isotopic signature of these gases show that ch4 was generated via thermogenic processes despite this the geochemistry of co produced brines shows evidence for microbial activity high 13c values of dic 10 slightly elevated alkalinity up to 12 01 meq kg and low so4 values 1 mmole l in select devonian organic rich shale and sandstone formation water samples suggest the presence of methanogenesis while low 13c dic values 22 and relatively high so4 concentrations up to 12 31 mmole l in many brine samples point to so4 reduction which likely limits microbial ch4 generation in the appalachian basin together the formation water and gas results suggest that the vast majority of ch4 in the devonian organic rich shales and sandstones across the northern appalachian basin margin is thermogenic in origin small accumulations of microbial ch4 are present at shallow depths along lake erie and in western ny

origin and enrichment factors of natural gas from the lower silurian songkan formation in northern guizhou province south china abstract the northern guizhou area located near the southwestern margin of the yangtze block and south of the sichuan basin is a promising area for shale gas exploration and development the discovery of a natural gas accumulation in the lower silurian songkan formation in the study area is the first discovery of natural gas in this stratum in china marking an exciting breakthrough in natural gas exploration in northern guizhou area this paper presents an investigation of this accumulation using gas geochemistry including gas composition carbon and hydrogen isotopes in context with geological information available for the study area our results indicate that the natural gas accumulation in songkan formation is probably sourced from the lower silurian wufeng longmaxi shale the 13 c ch 4 values range from 33 9 to 33 2 the 13 c c 2 h 6 values range from 37 0 to 36 2 and the 2 h ch 4 values range from 157 to 144 these results indicate that the gases are of thermogenic origin and are oil derived furthermore isotopic rollovers seen in the natural gas is similar to that seen in the natural gas from the local wufeng longmaxi shale which is identified as carbon exchange at high temperature the 13 c co 2 values range from 20 8 to 17 1 suggesting that carbon dioxide was mainly generated by thermogenic processes the fracture investigation indicates that the large development of horizontal fractures in calcareous mudstone is one of the reasons for the high yield of natural gas in songkan formation the petrophysical parameters suggest that limestone with low porosity and permeability effectively prevents natural gas escaping and seals natural gas into calcareous mudstone with horizontal fractures

carbon isotope fractionation during natural gas generation from kerogen abstract in petroleum exploration it is important to be able to determine the origin of any gas which is found this paper describes a new method of estimating the source type and maturity of a gas based on a rayleigh fractionation model kerogen is divided conceptually into a labile dominantly oil generating fraction and a refractory gas prone component 13 c of methane from either kerogen type and ethane propane and butane for gases from labile kerogen can be defined as a function of 13 c of the gas precursor groups in kerogen a kinetic isotope fractionation factor k and the extent of gas generation the isotopic ratio of the methane precursors relative to bulk kerogen determined from laboratory pyrolysis are 17 5 for labile kerogen and 1 4 for refractory kerogen values for ethane propane and butane from labile kerogen based on field correlations are 4 9 2 2 and 1 6 respectively the corresponding fractionation factors are 0 9892 0 9919 0 9947 and 0 9975 for methane ethane propane and butane respectively from labile kerogen and 0 9984 for methane from refractory kerogen using these parameters summary diagrams are constructed which allow differentiation of these sources from each other and from biogenic gases and cracked oil and recognition of gases of mixed origin if an independent estimate of 13 c for the source kerogen is possible then 13 c of the gas components can be used to estimate maturity in terms of the gas generation index the fraction of gas potential which has been realized

geochemistry and origin of some natural gases in the plateau province central appalachian basin pennsylvania and ohio the appalachian plateau is a large elongate structural basin in the western part of the central appalachians in western pennsylvania and eastern ohio the plateau is a mostly wet gas mean c sub 2 c sub 5 c sub 1 c sub 5 6 08 district which has produced approximately one third of all the natural gas produced in the entire appalachian basin we sampled gases from 22 wells and 5 cores to determine the possible utility of stable isotope geochemistry for interpreting the origin of the gases and correlating them with their probable sources methane delta sup 13 c and delta c and delta d values range from 55 1 to 27 24 and 303 to 150 respectively corresponding ethane and propane values of selected samples range from 41 79 to 29 61 and 38 21 to 26 71 several gas samples exhibit isotopic reversals amoung the c sub 1 to c sub 3 hydrocarbons the isotope data provide some potential insight into the origin of these gases and the timing and direction of their migration into different reservoirs with few exceptions the gases we analyzed are secondary i e mixtures of gases varying origins or primary gases original gas from a single active source rock which were altered by geological processes most samples originated as associated thermogenic gases generated frommore types i ii kerogens at source rock maturities ranging from early mature to overmature coalbed gases and some upper devonian gases appear to be mixtures of thermogenic and microbial hydrocarbons we interpret several other gases from various cambrian through devonian reservoirs as mixtures of thermogenic gases less

mixing of thermogenic natural gases in northern appalachian basin stable isotopic studies were undertaken to investigate the origin of 20 oil associated and non associated gases from western and central new york state in the northern appalachian basin gas wetness c2 c5 c1 c5 methane dgr13c and methane dgrd values range from 0 8 to 26 9 47 9 to 30 7 pmil and 305 to 143 pmil respectively corresponding ethane and propane dgr13c values range from 40 9 to 32 0 pmil and 34 9 to 27 9 pmil these data are interpreted to indicate a thermogenic origin with source rock maturities ranging from early mature equivalent vitrinite reflectance ro approximately 0 5 to post mature equivalent ro 2 0 inferred gas maturities generally in rease with increasing reservoir age from late to middle devonian marcellus and overlying rocks middle devonian to latest silurian oriskany onondaga akron silurian lockport herkimer medina to ordovician queenston and black river oil and gas in many late to middle devonian reservoirs may have migrated from interbedded organic rich shales in other reservoirs particularly those of ordovician and silurian age unusual isotopic reversals methane dgr13c ethane dgr13c suggest that gas generated locally from equivalent age rocks within the oil window mixed with post mature gas migrated from deeply buried sources to the south and southeast the post mature gas was probably sourced from middle to late ordovician shales regional migration of middle ordovician gas may have occurred along the middle ordovician knox unconformity gas was presumably channeled into overlying silurian and devonian reservoirs along faults and fractures

hydrocarbon gas generation from pyrolysis of extracts and residues of low maturity solid bitumens from the sichuan basin china abstract two sets of pyrolysis experiments were performed on bitumen a ba and extracted organic matter er from low maturity solid bitumen at heating rates of 2 c h and 20 c h in confined systems gold capsules the main observations can be listed as follows 1 ba and er have high generation potential of hydrocarbons and were a significant secondary hydrocarbon source 2 both have similar evolutionary characteristics of hydrocarbon gas generation and have high c1 c1 5 values than those from an ordovician normal oil ea distributions of the methane generated from pyrolysis of ba and er is far lower than that of ordovician normal oil 3 the evolution of stable carbon isotope values of individual hydrocarbon gases can be divided into three stages plots of ln c1 c2 versus ln c2 c3 13c1 13c2 versus ln c1 c2 and 13c2 13c3 versus ln c2 c3 clearly illustrate the characteristics of hydrocarbon gases generated from ba and er at different thermal maturity stages 4 basin modelling reconstructions of the hydrocarbon generation history of ba and er in sinian and cambrian strata respectively indicate hydrocarbon gases generation from er started in the middle jurassic and ended in the early cretaceous with a maturity equivalent to easy ro 2 2 the final conversion rate of c1 5 and c1 are 0 71 and 0 48 respectively hydrocarbon gas generation from ba started in the early jurassic and ended in the early cretaceous with an easy ro 3 0 the final conversion rate of c1 5 and c1 are 0 90 and 0 81 respectively

geochemistry and origin of natural gas in the petroliferous mahu sag northwestern junggar basin nw china carboniferous marine and permian lacustrine gas systems abstract the mahu sag located in the northwestern junggar basin of nw china is a renowned carboniferous marine permian lacustrine hydrocarbon depocenter that in theory could host both oil and gas however as ongoing hydrocarbon exploration and production are focused solely on oil few gas prospects have been discovered in this paper we discuss the possible occurrence of natural gas deposits based on a comprehensive study of gas geochemistry including gas composition carbon isotopes and compositions of light hydrocarbons c5 c7 and biomarkers in retrograde condensates combined with geological information available for the study area our results indicate that three types of natural gas occur in the region a sapropelic type gas derived from the lower permian fengcheng formation p1f a humic type gas sourced from carboniferous strata c and the overlying lower permian jiamuhe formation p1j and a humic type gas derived from the middle permian lower wuerhe formation p2w the p1f sourced sapropelic type gas accumulated during at least two distinct periods of time yielding low maturity oil associated gases and mature to high maturity kerogen oil cracking gases the carboniferous and p1j sourced humic type gases are mainly high maturity to over mature kerogen oil cracking gases without obvious geochemical evidence for a multi stage accumulation the p2w sourced gas is a mature humic type gas of these three types of gas the former two have the potential to form significant gas deposits and constitute valid exploration targets making the mahu sag a prospective area for gas exploration favorable areas for gas occurrence include deep anticlines hosting both kerogen and oil cracking gases the inferred exploration targets occur at significant depths which is why little drilling has been undertaken in these areas and major gas discoveries have yet to be made

geochemical characteristics of gases from the largest tight sand gas field sulige and shale gas field fuling in china abstract this study performed a detailed geochemical analyses of the components stable carbon isotopes of alkane gas and co2 stable hydrogen isotopes of alkane gas and helium isotopes of reproducing gas from the largest tight gas field sulige and shale gas fuling field in china the comparative study shows that tight gas from the sulige gas field in the ordos basin is of coal derived origin which is characterized by a positive carbon and hydrogen isotopic distribution pattern 13c1 13c2 13c3 13c4 2h1 2h2 2h3 i e the carbon and hydrogen isotopes increase with increasing carbon numbers carbon dioxide from this field are of biogenic origin and the helium is crust derived shale gas from the fuling shale gas field belongs to oil derived gas which has complete carbon and hydrogen isotopic reversal of secondary alteration origin 13c1

multiple origins of methane in the earth methane occurrences in the earth s crust are predominantly of biogenic origin i e their ultimate source is biologically formed organic matter methane can also form through inorganic reactions and is consequently termed abiogenic biogenic methanes can either form through bacterial or thermogenic processes bacterial processes follow a co2 reduction and or fermentation pathway the fermentation processes are quantitatively more important in recent fresh sediments and swamps methane formed by co2 reduction however is most common in older sediments and commercial gasfields temperature organic substrate and age may be the major factors controlling the relative importance of the two pathways stable isotope concentrations in thermogenic methanes seem to be controlled by the extent of conversion of organic matter the timing of gas expulsion and trapping the different character of methane in individual sedimentary basins may be a result of the geologic history geothermal methanes are most likely derived from pyrolysis of organic matter abiogenic methane occurs in hydrothermal vents and ophiolite complexes inorganic reactions either surficial or deep seated are the likely source of such methanes a uniform mantle origin of methane is not supported by the observed isotope variations in naturally occurring methanes

geochemistry of the sinian cambrian gas system in the sichuan basin china abstract based on the outcrops core samples drilling cuttings seismic and geochemical data natural gases of the sinian cambrian gas system were derived from four sets of over mature high quality source rocks the cambrian qiongzhusi source rock is sapropelic with toc of 1 94 ro eq of 1 83 3 90 and a thickness of 140 m on average the sinian 620 541 ma doushantuo source rock is sapropelic with toc of 1 69 ro eq of 2 08 3 83 and a thickness of 11 m on average the 3rd member of the dengying fm is also sapropelic with toc of 1 43 ro eq of 3 16 3 21 and a thickness of 13 m on average the 2nd and 4th members of the dengying fm are dominated by algal dolomite with toc of exhibiting 0 39 ro eq of 1 97 3 46 and a thickness of 246 m on average most of the 13c2 values are lower than 28 and gas dryness is 99 indicating that the natural gas is oil derived with high maturity the high contents of isoparaffins and cycloparaffin suggest that natural gas in the sinian cambrian reservoir was originated from secondary oil cracking the cambrian longwangmiao gas was mainly derived from the cambrian qiongzhusi source rock and the gas within the sinian dengying formation mainly originated from the three sinian source rock layers the area with gas generation intensity superimposed the four sets of source rocks exceeding 6 109 m3 km2 covered 80 of the sichuan basin the proved reserves exceed 1 0 1012 m3 in the sinian cambrian gas system and the accumulated gas resource in the sinian cambrian gas system might exceed 5 1012 m3

origin and migration of hydrocarbon gases and carbon dioxide bekes basin southeastern hungary the bekes basin is a sub basin within the pannonian basin containing about 7000 m of post cretaceous sedimentary rocks natural gases are produced from reservoirs precambrian to tertiary in age located on structural highs around the margins of the basin gas composition and stable carbon isotopic data indicate that most of the flammable gases were derived from humic kerogen contained in source rocks located in the deep basin the depth of gas generation and vertical migration distances were estimated using quantitative source rock maturity carbon isotope relationships for methane compared to known neogene source rock maturity depth relationships in the basin these calculations indicate that as much as 3500 m of vertical migration has occured in some cases isotopically heavy 7 0 co2 is the predominant species present in some shallow reservoirs located on basin margin structural highs and has probably been derived via long distance vertical and lateral migration from thermal decompositon of carbonate minerals in mesozoic and older rocks in the deepest parts of the basin a few shallow reservoirs 2000m contain isotopically light 50 to 60 0 methane with only minor amounts of c2 homologs 3 v v this methane is probably mostly microbial in origin above normal pressures occuring at depths greater than 1800 m are believed to be the principal driving force for lateral and vertical gas migration these pressures are caused in part by active hydrocarbon generation undercompaction and thermal decomposition of carbonates

isotope reversals and universal stages and trends of gas maturation in sealed self contained petroleum systems abstract isotope geochemistry is now a tool for shale gas exploration largely due to the association of isotope reversals with mature highly productive shale gas its utility however depends on an understanding of the isotope systematics for the particular region of interest as well as for shale gas maturation in general this paper reviews and re examines isotope data from four published papers that include shale gas from the barnett and fayetteville shales rodrigez and philp 2010 zumberge et al 2012 and gas from fractured reservoirs in the appalachians burruss and laughrey 2010 and the foothills of the western canada sedimentary basin wcsb tilley et al 2011 new shale and tight sandstone gas data are also presented for the wcsb comparisons of these data show that the progression through three stages of gas maturation pre rollover zone rollover zone and post rollover zone is universal in sealed self contained petroleum systems and that each zone has characteristic isotopic relationships and trends that are seen in all areas examined gases in the pre rollover zone are isotopically normal 13 c methane 13 c ethane 13 c propane unless mixing of gases from different sources has occurred in the rollover zone 13 c ethane and 13 c propane become progressively more negative as 13 c methane becomes less negative and ethane and methane are reversed 13 c ethane 13 c methane only towards the most mature portion of the rollover zone at the beginning of the post rollover zone 13 c ethane 13 c methane but as 13 c ethane and 13 c propane become increasingly less negative at varying rates ethane and propane may or may not be reversed with respect to each other at the highest maturities 2 h methane in gases of the post rollover zone approaches isotopic equilibrium with local formation water generally becoming more negative with increasing maturity correct assignment of maturity stage could be of importance because the rollover zone may represent the peak of high productivity shale gas whereas the post rollover zone may represent a decline in productivity burruss and laughrey 2010

origin source mixing and thermal maturity of natural gases in the panyu lower uplift and the baiyun depression pearl river mouth basin northern south china sea multiple natural gas fields have been discovered in the baiyun depression and the adjoining panyu lower uplift in the pearl river mouth basin northern south china sea the natural gases are associated with condensate and are characterized by relatively heavy carbon isotopes with methane and ethane 13c values ranging from 44 2 to 33 6 and 30 0 to 25 4 respectively nearly all methane and ethane are derived from oil prone type ii kerogen in the wenchang formation source rock whereas the heavy hydrocarbon gases propane butanes and pentanes are derived from both the wenchang and enping type iii kerogen formations based on an integrated comparison of carbon isotopic compositions of the natural gases typical type i ii and type iii kerogen derived gases and the enping and wenchang kerogens the gases from the eastern parts of the baiyun depression and the panyu lower uplift mainly originate from secondary oil cracking and primary kerogen cracking respectively the gases from the northern slope of the baiyun depression are a mixture of oil cracking and kerogen cracking gases both oil cracking and kerogen cracking gases were mainly generated from the wenchang formation source rock in the maturity range of 1 5 2 5 vitrinite reflectance with a corresponding present day depth range of 5400 6500 m 17 700 21 300 ft the apparent contribution of the wenchang formation to the discovered gas accumulations demonstrates that it is the most important source rock in the area instead of the enping formation the search for more gas derived from oil cracking will be the next natural gas exploration direction in the baiyun depression

genesis and controls of hydrothermal dolomitization in sandstones of the appalachian thrust belt quebec canada implications for associated galena barite mineralization abstract the appalachian thrust belt of eastern quebec hosts six epigenetic barite galena sphalerite occurrences in interdigitated carbonate cemented sandstones carbonate clast bearing conglomerates and minor siltstones of the upper cambrian st damase formation two of these the st fabien and cap enrage deposits consist of veins and disseminations enveloped by dolomitized zones and contain ankerite as an essential gangue mineral in contrast the other occurrences consist exclusively of calcite cemented veins and are conspicuously devoid of dolomite the domains of the st fabien and cap enrage deposits are intensely fractured and contain abundant dolomite barite sulfide cemented interconnected fractures and pores whereas those of the other occurrences display relatively minor fractures and pores which are only rarely interconnected this difference in porosity leads to the conclusion that differences in paleo permeability were the most likely reasons for the presence of dolomite and disseminated mineralization in the two deposits and their absence from the vein only occurrences mass balance calculations relating altered and unaltered rock suggest that dolomitization was accompanied by large gains of cao mgo and co 2 and corresponding losses of sio 2 and k 2 o under conditions of constant volume the mass of silica removed from the altered sandstone at st fabien 14 5 g per 100 g of rock when considered in the context of quartz solubility implies a minimum water rock ratio of 2635 a similar value 2500 is indicated by the content of lead of a representative sample of the st fabien deposit and typical concentrations of lead in mississippi valley type fluids the ph of the mineralizing dolomitizing fluid was calculated from the solubility of dolomite and fluid inclusion estimates of m ca 2 m mg 2 and m co 2 to be between 3 9 and 4 6 fluid inclusion data suggest that dolomitization and mineralization at st fabien and cap enrage were the products of interactions between local sedimentary rocks and minor metabasaltic dikes with hydrothermal brines detailed x ray diffraction analysis of the phyllosilicates in the host rocks in and around the st fabien deposit mainly sandstones and minor metabasalt indicates that these brines could have acquired virtually all magnesium and all iron manganese needed for dolomite precipitation as a result of the replacement of chlorite by illite since p t conditions during deposition of the two types of mineralization were similar to each other it is concluded that these conditions did not govern the formation of dolomite

a review of alkane gas geochemistry in the xujiaweizi fault depression songliao basin abstract a large suite 172 of gases from the yingcheng fm in the xujiaweizi fault depression has been studied chemically and isotopically the results show that they are mainly type iii kerogen coal type gas and have been generated and accumulated under the influence of hydrothermal fluids both measured and calculated vitrinite reflectance data show that they have been generated mainly at source maturity higher than 2 0 ro many gases show carbon isotopic reversals typically 13 c 1 13 c 2 and 13 c 2 13 c 3 the origin of carbon isotopic reversals is discussed in this paper and the results show that previous interpretations including a mixing and b the occurrence of abiogenic gases are incorrect based on iso butane n butane data wet gas cracking may have occurred but it is not the main cause of carbon isotopic reversals here we propose that a process of abiogenic polymerization of c 2 gases with methane is the main cause of the carbon isotopic reversals in this basin

revised chronostratigraphy and biostratigraphy of the john day formation turtle cove and kimberly members oregon with implications for updated calibration of the arikareean north american land mammal age abstract although the arikareean north american land mammal age was first typified in the great plains the succession there contains significant unconformities a generally poor magnetic record relatively sparse radioisotopic calibration and a major faunal hiatus in the john day valley of central oregon however is a thick remarkably complete sequence of oligocene through early miocene strata the john day formation potentially amenable to addressing these shortcomings and long known to harbor one of the richest records of mid tertiary mammals in north america since prothero and rensberger s first magnetostratigraphic study of the john day formation in 1985 new advances in geochronology together with a more comprehensive suite of paleomagnetic sections keyed to new radioisotopic and biostratigraphic data have greatly enhanced chronostratigraphic precision in our attempt to refine john day chronostratigraphy we sampled nearly 300 sites for magnetostratigraphy over a 500 m thick interval and us

geology of the devonian black shales of the appalachian basin abstract black shales of devonian age in the appalachian basin are a unique rock sequence the high content of organic matter which imparts the characteristic lithology has for years attracted considerable interest in the shales as a possible source of energy the recent energy shortage prompted the u s department of energy through the eastern gas shales project of the morgantown energy technology center to underwrite a research program to determine the geologic geochemical and structural characteristics of the devonian black shales in order to enhance the recovery of gas from the shales geologic studies by federal and state agencies and academic institutions produced a regional stratigraphic network that correlates the 15 ft black shale sequence in tennessee with 3000 ft of interbedded black and gray shales in central new york these studies correlate the classic devonian black shale sequence in new york with the ohio shale of ohio and kentucky and the chattanooga shale of tennessee and southwestern virginia biostratigraphic and lithostratigraphic markers in conjunction with gamma ray logs facilitated long range correlations within the appalachian basin basinwide correlations including the subsurface rocks provided a basis for determining the areal distribution and thickness of the important black shale units the organic carbon content of the dark shales generally increases from east to west across the basin and is sufficient to qualify as a hydrocarbon source rock significant structural features that involve the black shale and their hydrocarbon potential are the rome trough kentucky river and irvine paint creek fault zone and regional decollements and ramp zones

geochemistry and origin of the giant quaternary shallow gas accumulations in the eastern qaidam basin nw china abstract this study provided an overview of the geological setting and geochemical characteristics of the pleistocene shallow gas accumulations in the eastern qaidam basin nw china the five largest gas accumulations discovered in this region have a combined enclosure area of about 87 km 2 and 7 9 trillion cubic feet tcf of proven gas reserves the dominance of methane mostly more than 99 9 and the 13 c and d values of methane 68 51 to 65 00 and 227 55 to 221 94 respectively suggest that these gases are biogenic derived from the degradation of sedimentary organic matter by methanogens under relatively low temperatures 15 tds and strong stratification the deposition and extensive lateral occurrence of lacustrine shoreline sands silts in beach sand sheets and sand bars provided excellent reservoirs for the biogenic gas generated from adjacent rocks effective but dynamic gas seals are provided by a combination of factors such as the intermittent vertical variation in the sediment lithologies hydraulic trapping due to the mudstone water saturation the hydrocarbon gradient created as the result of gas generation from potential caprocks and the presence of a regional caprock consisting of 400 800 m of muds and evaporites it appears that the most favorable traps for large gas accumulations occur on structural slopes near the major gas kitchen and the prolific gas pools are often those large gentle anticlines with little faulting complication

geochemistry of natural gases in deep strata of the songliao basin ne china chemical composition and stable carbon isotopic studies were undertaken for 27 gas samples from deep strata of the xujiaweizi depression in the songliao basin to investigate their origin gas molecular and carbon isotopic compositions show great variety methane is the main component for all studied samples and its content ranges from 57 4 to 98 2 with an average of 90 1 gas wetness ranges from 0 8 to 16 7 with an average of 2 7 the main non hydrocarbon gases are carbon dioxide and nitrogen with an average of 4 0 and 3 2 respectively carbon isotope data suggest that these deep strata gases are mainly coal type gases mixed with minor amounts of associated oil type gases coal type gases are characterized by heavier carbon isotopic values and drier chemical compositions these gases were generated from the lower cretaceous shahezi formation coals interbedded shales with type iii kerogen during the postmature stage of hydrocarbon generation oil type gases are characterized by lighter carbon isotope and higher wetness which were generated from the lower cretaceous shales with type ii kerogen in the shallow strata during the early mature stage of hydrocarbon generation mixing of two different gases causes unusual carbon isotopic distribution patterns with lighter isotopic values in higher numbered carbons in most gases the discovery of coal type gases in the songliao basin provides new prospects for the exploration in this region

black shale source rocks and oil generation in the cambrian and ordovician of the central appalachian basin usa nearly 600 million bbl of oil mmbo and 1 to 1 5 trillion ft3 tcf of gas have been produced from cambrian and ordovician reservoirs carbonate and sandstone in the ohio part of the appalachian basin and on adjoining arches in ohio indiana and ontario canada most of the oil and gas is concentrated in the giant lima indiana field on the findlay and kankakee arches and in small fields distributed along the knox unconformity based on new geochemical analyses of oils potential source rocks bitumen extracts and previously published geochemical data we conclude that the oils in both groups of fields originated from middle and upper ordovician black shale utica and antes shales in the appalachian basin moreover we suggest that approximately 300 mmbo and many trillions of cubic feet of gas in the lower silurian clinton sands of eastern ohio originated in these same source rocks oils from the cambrian and ordovician reservoirs have similar saturated hydrocarbon compositions biomarker distributions and carbon isotope signatures regional variations in the oils are attributed to differences in thermal maturation rather than to differences in source total organic carbon content genetic potential regional extent and bitumen extract geochemistry identify the black shale of the utica and antes shales as the most plausible source of the oils other cambrian and ordovician shale and carbonate units such as the wells creek formation which rests on the knox unconformity and the rome formation and conasauga group in the rome trough are considered to be only local petroleum sources tmax cai and pyrolysis yields from drill hole cuttings and core indicate that the utica shale in eastern and central ohio is mature with respect to oil generation burial thermal and hydrocarbon generation history models suggest that much of the oil was generated from the utica antes source in the late paleozoic during the alleghanian orogeny a pervasive fracture network controlled by basement tectonics aided in the distribution of oil from the source to the trap this fracture network permitted oil to move laterally and stratigraphically downsection through eastward dipping impermeable carbonate sequences to carrier zones such as the middle ordovician knox unconformity and to reservoirs such as porous dolomite in the middle ordovician trenton limestone in the lima indiana field some of the oil and gas from the utica antes source escaped vertically through a partially fractured leaky upper ordovician shale seal into widespread lower silurian sandstone reservoirs

gas maturity and alteration systematics across the western canada sedimentary basin from four mud gas isotope depth profiles abstract carbon isotope and molecular compositions of mississippian to upper cretaceous mud gases have been examined from four depth profiles across the western canada sedimentary basin wcsb the profiles range from the shallow oil sands in the east r 0 0 25 to the very mature sediments in the overthrust zone to the west r 0 2 5 in the undisturbed wcsb 13 c 1 13 c 2 and 13 c 2 13 c 3 cross plots show three maturity and alteration trends 1 pre cretaceous gas sourced from type ii kerogen 2 cretaceous colorado group gas and 3 lower cretaceous mannville group biodegraded gas a fourth set of distinctly different maturity trends is recognized for lower cretaceous gas sourced from type iii kerogen in the disturbed belt of the wcsb displacement of these latter maturity trends to high 13 c 2 values suggests that the sampled gas was trapped after earlier formed gas escaped probably as a result of overthrusting unusually 13 c enriched gas 13 c 1 34 13 c 2 13 and 13 c 3 0 from the gething formation in the disturbed belt is the result of late stage gas cracking in a closed system in general gas maturity is consistent with the maturity of the host sediments in the wcsb suggesting that migration and mixing of gases was not pervasive on a broad regional and stratigraphic scale the deep basin portion of the wcsb is an exception here extensive cross formational homogenization of gases has occurred in addition to updip migration along the most permeable stratigraphic units

deciphering fracturing and fluid migration histories in northern appalachian basin an integrated structural and geochemical analysis of mineralized faults and fractures revealed that the most widespread fracture systems in the northern appalachian basin resulted from two periods of post alleghenian deformation fluid inclusion homogenization measurements on calcite samples from pre syn and post kinematic structural settings bracketed the temperature ranges predominantly a function of depth of burial for each period of fracturing the first period of deformation produced northwest west northwest and east northeast fractures and related faults that experienced recurrent strike slip movements during a cooling event from 176 to 120 c 349 to 248 f this cooling episode was associated with the waning of an elevated thermal grad ent attained during dewatering caused by late paleozoic burial metamorphism at depths of 3 to 3 5 km 9 800 to 11 500 ft later normal faulting reopened the earlier strike slip fault zones during subsequent cooling of the system from 112 c 234 f to below 76 c 169 f this second cooling stage resulted from the 1 to 2 km 3 300 to 6 600 ft of uplift and erosion that occurred throughout the eastern united states during the cretaceous the ubiquitous occurrence of carbonate silicate and sulfide epigenetic mineralization in many environments allows for widespread application of the dating techniques demonstrated in this study basic to this approach is the analysis of mineral assemblages deposited during increases in hydrothermal circulation resulting from enhanc d permeability associated with periods of faulting and fracturing the presence of different mineral assemblages representing a range of ages and crystallization environments on various generations of brittle structures provides a means for developing detailed faulting and circulation histories and the determination of approximate as well as precise relative ages for each event

cause of shale gas geochemical anomalies and mechanisms for gas enrichment and depletion in high maturity shales abstract this article reviews the abnormal characteristics of shale gases natural gases produced from organic rich shales and discusses the cause of the anomalies and mechanisms for gas enrichment and depletion in high maturity organic rich shales the reported shale gas geochemical anomalies include rollover of iso alkane normal alkane ratios rollover of ethane and propane isotopic compositions abnormally light ethane and propane 13 c values as well as isotope reversals among methane ethane and propane these anomalies reflect the complex histories of gas generation and associated isotopic fractionation as well as in situ mixing and accumulation of gases generated from different precursors at different thermal maturities a model was proposed to explain the observed geochemical anomalies gas generation from kerogen cracking at relatively low thermal maturity accounted for the increase of iso alkane normal alkane ratios and ethane and propane 13 c values normal trend simultaneous cracking of kerogen retained oil and wet gas and associated isotopic fractionation at higher maturity caused decreasing iso alkane normal alkane ratios lighter ethane and propane 13 c and corresponding conversion of carbon isotopic distribution patterns from normal through partial reversal to complete reversal relatively low oil expulsion efficiency at peak oil generation low expulsion efficiency at peak gas generation and little gas loss during post generation evolution are necessary for organic rich shales to display the observed geochemical anomalies high organic matter richness high thermal maturity high degrees of kerogen gas and oil gas conversions and late stage the stage of peak gas generation and post generation evolution closed system accounted for gas enrichment in shales loss of free gases during post generation evolution may result in gas depletion or even undersaturation total gas content lower than the gas sorption capacity in high maturity organic rich shales

serpentinization and the origin of hydrogen gas in kansas hydrogen gas occurs in ten kansas wells near the mid continent rift system since 1982 two wells have yielded small amounts of gases containing an average of 29 37 mole h2 the remainder being chiefly n2 with only traces of hydrocarbons isotopic compositions for hydrogen dgrd 740 to 836 pmil imply near ambient about 10 c equilibration temperatures for the gases which are among the most deuterium depleted in nature and resemble the h2 rich gases described from ophiolites in oman isotopic values for the kansas n2 differ slightly from those of the atmosphere but not enough to rule out an atmospheric origin because they are low in ch4 and co2 expected byproducts of biogenic activity the gases are probably abiogenic in origin the existence of such gases near a major rift system containing mafic rocks and not far from known kimberlites is consistent with an origin from reactions involving fe 2 oxidation for example during serpentinization because the gases may be associated with kimberlites and deep seated rifting mantle outgassing is possible but such an origin would be difficult to reconcile with the low isotopic temperatures the h2 gases from kansas and elsewhere seem to be too low in pressure to have commercial value however neither the kansas gases nor those from other h2 occurrences have been adequately examined to assess their importance as potential resources

organic geochemistry and oil source correlations paleozoic of ohio routine analytical methods and new kerogen kerogen pyrolyzate techniques used for detailed petroleum geochemical studies permit the correlation of oils to their respective source rocks in ohio in the paleozoic section four good to excellent source units were identified the mississippian sunbury shale the devonian ohio shale and olentangy shale and the shales of the ordovician point pleasant formation these marine shales are dominantly oil prone with maturation levels that span the immature to peak oil generation range no source rocks examined exceeded an ro of 1 0 1 1 or equivalent the reservoired oils in ohio are mostly supermature based on normal alkanes pristane n c17 to phytane n c18 ratios low asphaltene and sulfur contents and high paraffin indices three basic oil groups were identified using oil characterization procedures group 1 consists of cambrian ordovician and some silurian oils group 2 consists of some silurian devonian one mississippian and some pennsylvanian oils and group 3 consists of oil from the mississippian berea sandstone oil source rock correlation techniques permitted the assignment of probable source rocks to each oil group group 1 oil was probably derived from the point pleasant shales group 2 oil was derived from the ohio shale and olentangy shale and group 3 oil came from the sunbury shale because the supermature oils in ohio occur in reservoirs with maturities generally under 0 8 ro the oils would have had to migrate from deep within the appalachian basin migration could have followed fracture zones unconformities or extensive and permeable units such as the silurian clinton sands

vertical movements of the crust case histories from the northern appalachian basin evidence of former deep burial of ordovician to devonian strata of the northern appalachian basin has been obtained from various techniques of study including fluid inclusion homogenization temperatures 18 o and vitrinite reflectance diagenetic minerals indicate paleotemperatures of 100 200 c maximum depths of burial were calculated from the estimated paleotemperatures a gradient of 26 c km was assumed silurian strata of the basin are interpreted to have reached maximum burial depths of 5 0 km devonian strata in the catskill mountains had former burial depths of 6 5 km lower ordovician carbonate sequences were buried to 7 km middle ordovician strata had paleodepths of 5 km and devonian carbonate strata had paleodepths from 4 5 to 5 km if these strata were buried deeper than previously thought unexpectedly large amounts of uplift and erosion ranging from 4 3 to 7 km must also have taken place to bring these strata to the present land surface the occurrence of such large scale vertical movements of the crust and lithosphere must be recognized in paleogeographic reconstructions such drastic changes represent isostatic unroofing with widespread implications for paleogeography of a kind unrecognized at present

character origin and occurrence of natural gases in the anadarko basin southwestern kansas western oklahoma and texas panhandle u s a abstract natural gas production in the anadarko basin comes from three geographically separated areas that can be differentiated by age of reservoir and by inferred nature of organic thermal origin of the gases in the central basin non associated gases are produced mainly from upper mississippian and pennsylvanian sandstones gas samples are from reservoirs as much as 6588 m deep gases become isotopically heavier 13 c 1 values range from 49 8 to 33 2 and chemically drier c 2 values range from 1 33 with increasing level of thermal maturity gases were generated mainly from interbedded shales with type iii kerogen during the mature and post mature stages of hydrocarbon generation deviations from the trend are due to vertical migration and mixing of gases generated at different levels of thermal maturity over the past 250 myr in the giant panhandle hugoton field non associated gases are generally produced from permian carbonates at depths of 13 c 1 value is 43 2 mean c 2 value is 14 because organic rich mature source rocks are not present in the area gases probably were generated in the central basin from pennsylvanian or older source rocks during the mature stage of hydrocarbon generation this interpretation implies migration over distances as much as several hundred kilometers in the sooner trend associated gases are produced from silurian devonian and mississippian carbonates at depths as great as 2950 m and were generated from type ii kerogen during the mature stage of hydrocarbon generation associated oil usually correlates with extracts of the upper devonian and lower mississippian woodford shale gases are isotopically lighter mean 1 3 c 1 value is 43 9 and chemically wetter mean c 2 value is 14 than those derived from type iii kerogen at an equivalent level of thermal maturity

possible late middle ordovician organic carbon isotope excursion evidence from ordovician oils and hydrocarbon source rocks mid continent and east central united states oils generated by middle ordovician rocks are found throughout the mid continent and east central regions of the united states gas chromatographic characteristics of these oils include a relatively high abundance of n alkanes with carbon numbers less than 20 a strong predominance of odd numbered n alkanes between c10 and c20 and relatively small amounts of branched and cyclic alkanes saturated and aromatic hydrocarbon fractions of 43 ordovician oils from the anadarko ardmore forest city illinois michigan salina sedgwick and williston basins and the iowa shelf demonstrate a wide range in carbon isotope composition dgr13csat 24 9 pmil to 33 9 pmil dgr13carom 24 3 pmil to 33 7 pmil saturated and aromatic hydrocarbons extracted from late middle ordovician shales 17 core samples show ranges in dgr13c similar to that of the oils the wide ranges in dgr13c for oils and rock extracts reflect a major positive excursion s 6 9 per pmil in organic matter dgr13c in late middle ordovician rocks this excursion has at least a regional significance in that it can be documented in sections 480 mi 770 km apart in south central kansas and eastern iowa the distance may be as much as 930 mi 1 500 km if the carbon isotope variations observed in michigan basin ordovician oils and in organic matter from late middle ordovician rocks in southwestern ontario are related to the same carbon isotope excursion organic matter dgr13c in core samples from south central kansas and eastern iowa is not directly related to variations in quantity or quality of organic matter or maceral compositi n the positive excursion in organic matter dgr13c is a possible result of increased organic matter productivity and or preservation the parallel shifts in organic and carbonate dgr13c in core samples from 1 e m greene well washington county iowa imply changes in the isotope composition of the ocean atmosphere carbon reservoir differences in the magnitude of the carbon isotope shifts between organic matter 8 8 pmil and carbonate 4 2 pmil in this core suggest a decrease either locally or regionally in available dissolved co2 possibly a result of high organic matter productivity and or limited circulation in the late middle ordovician seas

correlation of natural gas by use of carbon isotopic distribution between hydrocarbon components the natural distribution of carbon isotopes between hydrocarbon gas components is used for 1 determining a gas s maturity 2 correlating a reservoired gas to its source 3 correlating one reservoired gas with another and 4 recognizing gas mixtures calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single continuous diagram independent of source type actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock levels of organic metamorphism ranging from 8 to 13 covering the entire range of oil and wet gas generation at greater maturities the wet gas components are found to undergo thermal degradation losing their usefulness for correlation three examples showing indigenous gas west texas non indigenous gas gippsland basin australia and gas mixtures from multiple sources southeastern alberta illustrate exploration applications

oil generation in the michigan basin a biological marker and carbon isotope approach abstract palaeozoic strata in the michigan basin produced crude oils which despite their rather small volume and economic value are interesting because of the considerable age of the basin and because of the opportunity to study generation and migration of oils within a relatively simple but ancient geological setting based on n alkane profiles biological marker distributions and carbon isotope ratios the oils belong to three main families of different genetic origin and a few less important mixed types silurian oils from salina and niagara limestones have broad n alkane distributions and abundant isoprenoid hydrocarbons a strong phytane over pristane predominance and the lack of diasteranes indicate a carbonate source for these oils chemical maturity parameters show that they are more mature than the oils from the other main families from which they are also clearly distinguished by carbon isotope ratios of hydrocarbon fractions and single n alkanes oils found in the calcareous ordovician trenton formation contain n alkane cyclohexylalkane and alkyl phenanthrene distributions typical of immature oils many oils from the devonian dundee reservoirs are very similar in overall composition to the trenton oils devonian traverse oils are considered to be mainly from a devonian source with some contribution of ordovician type oil and are of intermediate maturity based on calculations using kinetic parameters of biological marker reactions considerably deeper subsidence of the devonian source rock in the past is implied

carbon isotopes of hydrocarbons and carbon dioxide in natural gases in china abstract 130 natural gases in the continent of china were determined for the abundance and carbon isotopes of hydrocarbons and carbon dioxide the 13c pdb values of methane range from 68 to 24 4 and the largest isotopic variability for hydrocarbons is of 13c1 there is no direct evidence for the presence of abiogenic hydrocarbon in natural gases in china one sample from songliao basin shows a heavier 13c1 24 and reverse isotopic distribution pattern 13c1 13c2 13c3 concerning the occurrence of mantle derived he ne ar and xe in the south songliao basin we propose that the methane mantle derived and heavy hydrocarbons ethane propane butane etc formed from polymerization of the mantle derived methane the 13c values for carbon dioxide range from 13 9 to 13 5 suggesting the multiple origins gases with co2 concentrations greater than 10 have a narrow range of 13c values from 8 to 2 and show higher 1he 4he ratios 0 1 times the atmospheric value suggesting that a mantle derived component is diluted by co2 derived from carbonate and or organic matter positive 13c values 4 14 indicate co2 reduction to ch2 co2 with trace concentration in ch4 rich gases can be attributed to microbial decomposition of organic matter in sedimentary rocks

geochemistry and habitat of natural gases in po basin northern italy three types of natural gases can be distinguished in the po basin 1 bacterial gases dgr13cch4 60 ppt dgrdch4 180 to 200 ppt c2 0 2 which are found predominantly in pliocene pleistocene reservoirs 2 mixed gases dgr13cch4 60 to 50 ppt dgrdch4 180 to 200 ppt c2 0 2 to 5 which are predominantly in reservoirs of messinian upper miocene age but also in a few places in older and younger reservoirs 3 thermogenic gases dgr13c 50 ppt dgrd 200 to 150 ppt c2 5 which commonly are associated with oil and are produced predominantly from miocene and mesozoic reservoirs a quantitative assessment of gas in place for these three gas types is 1 80 2 10 and 3 10 respectively the areal distribution and amount of the three gas types are controlled strictly by geologic factors synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area where 70 of the bacterial gas in place is produced here bacterially formed methane is produced from depths of up to 4 500 m 15 000 ft and is the deepest bacterial gas so far reported in the literature in the pedealpine homocline lower sedimentation rates and a weak deformation of pliocene pleistocene sediments prevented appreciable accumulations only 15 of the bacterial gas in the basin is produced in this area thermogenic gases are found in the vicinity of the apennines where strong tectonic movements have favored its migration from deeper mesozoic sources into tertiary reservoirs or into deep mesozoic carbonate reservoirs of the pedealpine homocline

genetic characterization of natural gases natural gases can be characterized genetically using four properties c2 concentration carbon and hydrogen isotope variations in methane and carbon isotope variation in ethane three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters in these diagrams compositional fields have been defined for primary gases biogenic thermogenic associated and thermogenic nonassociated and for gases which result from mixing of these gases these fields are strictly empirical and comprise compositional variations found in about 500 natural gases the isotopic and compositional variations in natural gases can be described in terms of 1 processes during formation of the gases such as bacterial fermentation or maturation of organic matter and 2 processes during secondary migration mixing of primary gases is an important and common process migration of gases predominantly affects the c2 concentration whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged allowing an assessment of the origin of migrated gases and properties of their source rocks the formation of gas from humic organic matter and coals is not yet clear from published data the diagrams use data from various basins and areas interstitial gases from the gulf of california are entirely of bacterial origin traces of thermogenic gases are formed only in the vicinity of dolerite sills gases in the south german molasse basin and in the vienna basin are of bacterial mixed and thermogenic origins data from the north italian po basin provide examples for genetic characterization of migrated gases

early cretaceous uplift and erosion of the northern appalachian basin new york based on apatite fission track analysis abstract the thermal history of outcropping devonian sediments of the northern appalachian basin new york has been investigated using fission track analysis of detrital apatites from 57 sandstone samples based on lengths and apparent age measurements using fission tracks in apatite it is concluded that lower devonian sediments presently at the surface in the catskill region were cooled rapidly from temperatures higher than about 110 c during early cretaceous times 120 140 ma ago in the western part of new york wellsville buffalo data from late devonian sediments are consistent with cooling at the same time as that identified for the catskill region but from lower temperatures in the range of approximately 80 110 c the maximum temperature these sediments experienced since deposition for a pre uplift paleogeothermal gradient of 25 35 c km the confined track length data indicates uplift and erosion of 2 3 km for western new york and greater than 3 4 km for the catskill region a differential uplift pattern which is consistent with the historical stratigraphic data from the region this conclusion is at variance with earlier interpretations put forth by others rapid broad scale uplift and erosion of the scale identified imply that large volumes of sediment could have been supplied from the northern appalachian basin during the early cretaceous this timing for the dominant post devonian cooling phase in the basin is not accounted for by recent models of the tectonic evolution of the appalachian orogen but is compatible with the change from carbonate to siliciclastic deposition in the atlantic coastal plain it is suggested that this style of broad regional uplift without significant deformation is characteristic of a tectonic regime associated with and subsequent to continental rifting apatite fission track analysis is shown to be a basic tool in providing fundamental limits for thermal history assessment in regional tectonic problems

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

anomalies of natural gas compositions and carbon isotope ratios caused by gas diffusion a case from the donghe sandstone reservoir in the hadexun oilfield tarim basin northwest china abstract natural gases in the carboniferous donghe sandstone reservoir within the block hd4 of the hadexun oilfield tarim basin are characterized by abnormally low total hydrocarbon gas contents 13c ethane c2

the diagenetic origin and depositional history of the cherry valley member middle devonian marcellus formation abstract we report 13c 18o 34s records coupled with elemental pyrite morphology and fluid inclusion data spanning the cherry valley member in the middle devonian marcellus shale from two cores located across a depth gradient in the appalachian basin the goal of the study was to evaluate the diagenetic and redox history of this carbonate interbedded layer within a prominent shale gas system three lithological units were identified in the cherry valley member a nodular clay rich marlstone lower unit a fossiliferous limestone middle unit and a calcareous mudstone upper unit low 13c and 18o values of the carbonate matrix display a mixing trend among marine carbonate authigenic carbonate and late stage diagenetic cementation the petrology of barite nodules and associated 34sbarite values suggest that a shoaling sulfate methane transition zone created by methane advection contributed to the nodule growth and pyritization along with other early and late diagenetic processes this mechanism provided the main carbonate source for the lower unit due to elevated rates of aerobic methane oxidation the 34spyrite values u and mo contents and pyrite framboid size distributions together constrain the redox history of the cherry valley member an anoxic euxinic marine environment rather than a fully oxygenated environment continued through the lower unit in areas with deeper paleo bathymetry until it was interrupted by an episode of dysoxia during the deposition of the middle unit basin scale anoxia was then renewed during a subsequent 3rd order sea level rise in the upper unit the entire cherry valley member was ultimately altered by warm burial fluid interaction through vertically distributed veins the combined body of evidence indicates that the duration and spatial distribution of oxygenated marine conditions during the deposition of cherry valley member within the marcellus formation may have been overestimated

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

isotopic reversals with respect to maturity trends due to mixing of primary and secondary products in source rocks abstract carbon isotopic composition 13 c reversals with respect to maturity trends have been reported for ethane and propane in barnett shale gas in the fort worth basin u s a this trend also exists in some conventional gas reservoirs such as in the ordos basin china and appalachian basin u s a fractionation during mass transport is unlikely to be a significant mechanism because its effect is minor under geological conditions it would have caused more obvious reversal for methane than for ethane and propane and mass transport can not account for the occurrence of the trend both inside and outside source rocks this paper demonstrates that the reversal may result from mixing of indigenous primary gas generated directly from kerogen and secondary gas generated from oil and condensate within source rocks and presents deconvolution results using end members derived from a kinetic isotope fractionation model the results including the wetness and mixing ratio of primary and secondary gases are consistent with geological conditions condensates may be the major precursor of the secondary gas in shale when the reversal occurs a complete trend of 13 c variations in methane and ethane with maturity is proposed that is based on the indigenous mixing mechanism

organic geochemistry of mid continent middle and late ordovician oils ordovician oils in mohawkian and cincinnatian reservoirs of the united states mid continent retain the biochemical imprint of middle and upper ordovician oceanic life before the evolution of land plants and most vertebrates thus these oils have some geochemical features that distinguish them from younger oils these features include 1 a predominance of n c15 n c17 and n c19 alkanes in the saturated hydrocarbon fraction 2 relatively low amounts of longer chain n alkanes 3 low amounts of chlorophyll derived isoprenoids such as pristane and phytane and 4 abundant c29 sterane relative to c27 with rearranged forms diasteranes predominant over normal steranes ordovician oils also generally contain little sulfur and have a somewhat variable light stable carbon isotopic composition with dgr13csat and dgr13caro values of 28 to 31 pmil pdb but these features are typical of many marine oils the unusual chemistry of these ordovician oils supports the interpretation of reed illich and horsfield 1986 that prokaryotic organisms provided the organic matter for most ordovician oils although their claim for gloeocapsamorpha a problematic unicellular prokaryote possibly a blue green alga or an unusually large bacterium cannot be proven from oil chemistry alone knowing that indigenous mid continent ordovician oils were derived from prokaryotic organisms may aid in future exploration for these reservoirs

unique chemical and isotopic characteristics and origins of natural gases in the paleozoic marine formations in the sichuan basin sw china isotope fractionation of deep and high mature carbonate reservoir gases abstract large quantities of natural gases have recently been discovered in the marine carbonate formations in the sichuan basin southern china the origins and isotopic compositions of the gases are poorly understood and highly controversial we examined the chemical and isotopic compositions of the carbonate reservoir gases in marine formations from the sinian to lower triassic as well as shale gas from the silurian and cambrian formation in the sichuan basin it is found that the carbon isotopes of the carbonate reservoir gases had been evolved through three stages corresponding to pre rollover rollover and post rollover in the plot of 13c2 versus wetness which is consistent with that found in shale gas by tilley and muehlenbachs 2013 the unique carbon isotopic compositions indicate that both kerogen and oil cracking gas have contributed to the accumulation of the carbonate reservoir gases in the sichuan basin a mixing of two thermogenic gases with different maturities can cause the 13c reversal between methane and ethane which is commonly observed in the carbonate reservoir gases in the carboniferous permian and triassic formations and the shale gas in the silurian and cambrian formations in the sichuan basin we established two mixing models for quantitatively determining the contribution of each type gas for the natural gases in different formations kinetic extrapolation based on thermal history of the anyue gas field demonstrates that ethane cracking indeed occurred in the deep reservoir formations which may explain the disappearance of the 13c reversal for methane and ethane in the carbonate reservoir gases in the post rollover zone it is proposed that organic inorganic interactions may be responsible for the presence of d isotope rollover for methane in carbonate reservoir gases with ultra high maturity the organic inorganic interactions include thermal reactions between water and residual organic matters or carbons and hydrogen isotope exchange between water and methane and their mechanism needs further investigations in future

the origin and accumulation of multi phase reservoirs in the east tabei uplift tarim basin china abstract a giant deep strata oil gas field with complex fluid characteristics was recently discovered in the hade yuke area of the east tabei uplift tarim basin nw china the deep fluids show a lateral co existence of several gas condensate and oil phase reservoirs characterized through the integration of complimentary geochemical e g bulk composition biomarker and stable carbon isotope and geological data typical features of the gas and condensate reservoirs which were mainly distributed in the eastern parts of the study site included high gor gas to oil ratio hydrocarbon gases with relatively heavy 13c values and crude oils with high wax content in contrast oil reservoirs which mostly occurred in more westerly locations had lower gor values lighter 13c values and lower wax content indicating gradual change of fluid characters the petroleum accumulations derive from two hydrocarbon charge events an early oil charge in the late hercynian and a later gas charge in the late himalayan which migrated from east to west leading to a gradual variation in fluid phase and characteristics alteration of paleo oil reservoirs by this secondary gas charge was therefore primarily responsible for the complex fluid character and multi phase reservoirs presently in place secondary condensates were formed from the retrograde condensation of paleo reservoirs our re construction of the hydrocarbon accumulation process encourages future exploration endeavours to target commercially viable gas accumulations in the deep east strata and oil reservoirs in the west