late cenozoic evolution of the east china continental margin insights from seismic gravity and magnetic analyses abstract seismic reflection profiles reveal the structural characteristics beneath the east china sea shelf margin and the okinawa trough and provide new constraints on the late cenozoic evolution of the east china continental margin the frontal shelf basin between the taiwan sinzi uplift and the trough axis occupies the western half of the northern middle okinawa trough in this basin the middle late miocene sediments are confined to grabens or half grabens dominated by listric faults whereas the overlying pliocene quaternary sequence is characterized by a uniform thickness and dense planar type faults suggesting that rifting of the northern middle okinawa trough initiated during the middle miocene but slowed down during the earliest pliocene since that time the opening of the okinawa trough has been dominated by diffuse rifting the southern okinawa trough is predominately filled by quaternary sediments indicating that its back arc rifting began during the earliest pleistocene contractional structures identified in the pre quaternary sequence beneath the continental slope along with an erosional pleistocene pre pleistocene unconformity in the southern okinawa trough demonstrate the existence of pre rifting compression and uplifting in this region we use this evidence and previously published results to propose an evolutionary model of the east china continental margin during the late cenozoic the northern middle okinawa trough began rifting during the middle miocene on a paleo uplift the luzon arc initially impinged upon the eurasian continental margin during the late miocene near the southeastern end of the miyako fault belt and activated the proto taiwan orogeny in today s southern okinawa trough and adjacent regions during the late miocene pliocene the orogeny quickly propagated southwestward along with the west northwest moving philippine sea plate subsequently the rifting of the southern okinawa trough was initiated during the earliest pleistocene and propagated southwestward following the post collisional collapse of the proto taiwan orogen this geodynamic scenario resembles that of present day northern taiwan

comparison of hydrocarbon gases c1 c5 production from carboniferous donets ukraine and cretaceous sabinas mexico coals abstract the main purpose of this contribution is to compare the ability of carboniferous coals from the donets basin of the ukraine and cretaceous coal from the sabinas basin of the mexico to generate hydrocarbon gases c1 c5 two bituminous coals from the donets basin 2c10yd and 1l1dim 0 55 and 0 65 rr respectively and one bituminous coal from the sabinas basin olmos 0 92 rr were studied using heating experiments in a confined pyrolysis system the highest rank reached during the heating experiments corresponds to the anthracite stage 2 78 and 2 57 rr for the 2c10yd and 1l1dim coals and 2 65 rr for the olmos coal the composition of the generated c1 c5 gases was evaluated using a thermodesorption multidimensional gas chromatography the results show that the carboniferous donets coals produced more wet gas and methane during pyrolysis than the cretaceous olmos coal this is probably due to their higher liptinite 6 20 and collodetrinite content and to the loss of a major part of the petroleum potential of the olmos coal during natural coalification c2 c5 compounds are mainly derived from the cracking of liquid hydrocarbons ethane is the most stable compound and formed from the cracking of higher hydrocarbon component large amounts of methane up to 81 mg g coal for the donets coals and 50 mg g coal for the sabinas coal were formed at high temperatures by cracking of previously formed heavier hydrocarbons and by dealkylation of the coal matrix a linear relationship was observed between methane generation and the maturity level of both coal types

biodegradation of sedimentary organic matter associated with coalbed methane in the powder river and san juan basins u s a the powder river basin and san juan basin u s a are two of the most productive coalbed methane reserves in the world of particular interest is the microbial biodegradation of coal beds associated with this natural gas production biogenic methane production is indicated as a significant component to the total gas resources in the san juan basin and as the nearly sole source for the shallow coals of the powder river basin molecular and isotopic signatures indicate a microbial origin for the gas geochemical characteristics of formation waters such as elevated alkalinity and 13c enriched dissolved inorganic carbon dic further support extensive microbial degradation of coal organic matter associated with methanogenesis extractable organic matter isolated from coals in both basins point to patterns of hydrocarbon biodegradation in coals restricted to specific depths to some extent biodegradation patterns are similar to those observed in methanogenic biodegraded black shales of the mid continent of the united states specifically both coals and shales exhibit near quantitative removal of straight chain and acyclic isoprenoid hydrocarbons however loss of aromatic hydrocarbons in the coals proceeds prior to the extensive removal of the saturated hydrocarbons in contrast to what is conventionally observed in biodegraded petroleum systems or in black shales in addition previous thermal maturation histories in both the fruitland and fort union coalbed methane systems have little impact on more recent hydrocarbon biodegradation instead localized hydrologic conditions and subsurface geology likely play important roles in controlling the extents of biodegradation and methanogenesis these results suggest that biodegradation of hydrocarbons coupled with methanogenesis may develop regardless of organic matter source across a range of inherited thermal maturities

a seismic event on august 20 1983 double explosion or a single earthquake abstract an interesting weak seismic event occurred on august 20 1983 in the caspian sea region in southwestern u s s r which produced at gauribidanur array two distinct seismic signals viz the leading p signal and a similar signal after an interval of nearly 12 s preliminary data corresponding to these two signals led to an initial assumption that the event was a double underground nuclear explosion however the focal depth of this event from p arrival time data of 21 stations was found to be 45 km further processing of the gauribidanur array seismograms confirmed that the second signal detected exactly 11 85 s after the first arrival was none other than the surface reflected p pulse which lent excellent support to the source depth estimate by using three discriminants viz signal complexity third moment of frequency and relative efficiency of generation of core reflected p the event was objectively compared with 10 small presumed underground nuclear explosions in southwestern u s s r to conclude that the august 20 1983 event was a shallow focus small earthquake in the caspian sea area

motion of the pacific plate relative to eurasia and its potential relation to cenozoic extension along the eastern margin of eurasia extension along the eastern margin of eurasia has been regarded commonly as a far field effect of the india eurasia collision however some aspects of the timing and location of this extension make a link between the collision and extension difficult we suggest that some extensional features commonly interpreted as effects of the collision may be related more simply to changes in plate convergence rates along the eastern plate boundary of eurasia an analysis of the motion of the pacific plate relative to eurasia suggests that the rate of pacific eurasia convergence varied significantly during the tertiary period from a late cretaceous convergence rate of 120 140 mm yr the rate declined substantially during early tertiary time and reached a minimum in eocene time of 30 40 mm yr in oligocene to earliest miocene time the average convergence rate increased moderately to 70 95 mm yr then decreased again to 65 70 mm yr during early to middle miocene time from late miocene to the present the rate of convergence increased to an average of 100 110 mm yr the paleocene through middle miocene episode of relatively slow convergence correlates with a period of widespread extension along the eastern margin of eurasia decreased convergence may have been related to a net reduction in horizontal compressional stress transmitted between the pacific and eurasian plates which resulted in widespread extension adjacent to the margin of eurasia

review of seismic source models for underground nuclear explosions abstract a number of seismic source models for underground nuclear explosions have been developed over the past 2 decades these models include the spherically symmetric compressional source model the wave conversion source model the tectonic strain release source model the spall slapdown source model and the near regional source model these model are reviewed in this study and are shown to be inconsistent with various geophysical data associated with underground nuclear explosions in particular the rayleigh and love wave signals generated by underground nuclear explosions have not been explained satisfactorily by any of these source models to explain the observed explosion data it may be necessary to model the explosion seismic source as a sequence of mechanisms producing seismic signals these mechanisms all act within the first few seconds following the explosion detonation one of the most important of these mechanisms is probably explosion induced thrust faulting

the genesis of coal from the viewpoint of coal petrology abstract the genesis of the microscopic constituents of coal macerals and of maceral associations microlithotypes representing various coal facies is discussed in the light of recent advances that have been made in the study of coals and modern peats peat microscopy especially promoted the understanding of humification and biological gelification both of which are decisive for the genesis of huminites vitrinites in brown coals and hard coals respectively microbial activity during peatification has been documented by chemists microbiologists and petrologists and recently by electron microscopical observations in coals this activity leads to the incorporation of lipid substances into the humic matter and later influences the chemical physical and technological properties of vitrinites geochemical gelification transforms the huminites of brown coals into the vitrinites of bituminous coals and is probably caused by the onset of bitumen generation in coal in this process the hydrogel brown coal is transformed to the bitumogel bituminous coal coal as a source rock for oil has become an important research subject for coal petrologists and oil exploration the formation of more or less liquid bitumen and its later cracking to gaseous hydrocarbons leads to the coalification jumps of the macerals especially liptinites and vitrinites and to the generation of secondary macerals like exsudatinite bitumen and micrinite dead carbon bituminization in coal has been revealed by the use of fluorescence microscopy which also found the presence of new liptinite macerals bituminite fluorinite and exsudatinite and suggests a higher proportion of algal material in coals than was thought earlier the genesis of various types of inertinite macerals will probably become an increasingly important subject of research especially with regard to rank inertinites that seem to attain their high inertinitic reflectance initially during coalification besides the origin of macerals the origin of various coal facies in terms of maceral associations is considered in relation to the palaeogeographic depositional milieu the peat forming vegetation palaeoclimate water and nutrient supply acidity marine and calcareous influence and fire with examples from euramerican carboniferous coals gondwana coals and tertiary brown coals vegetation water and oxygen supply eh conditions are most important for the generation of coal facies vegetation has been reconstructed with the help of palaeobotany including palynology cuticular analysis and wood anatomy in brown coals and with the study of petrified permineralized peats preserved in coal balls the microscopical investigations of botanically recognizable plant remains isolated from coals or accompanying rocks has been especially informative however the results of palynological and cuticular analyses should be evaluated with caution considering the varying resistance of palynomorphs and cuticles and in the case of palynomorphs their possible extrapaludal origin microscopical studies of modern peats that were derived from different types of vegetation growing in different hydrological environments have been very helpful in interpreting coal facies especially in brown coals the much disputed origin of brown coal lithotypes is considered in detail for hard coals there seems to be agreement that coal layers rich in vitrites and liptinite poor clarite were deposited in wet forest swamps that microlayered liptinite rich clarites are sub aquatic humic muds and that inertinite rich durites represent oxidized peats the origin of liptinite rich durites is controversial either subaquatic muds or oxidized peats the latter perhaps from raised bogs a recent trend in the literature is the assumption that many if not all mineable coal seams have been deposited in raised bogs under ombrogenous oligotrophic conditions although raised bogs with a dense arborescent vegetation are known from borneo it seems not probable that most seams which were deposited in foredeeps or steadily sinking graben zones are of ombrogenous origin although a succession from relatively wet and eutrophic to relatively dry and oligotrophic conditions may be true for occasional seams but as in modern peats striking changes of coal facies are commonly caused by hydrological events e g inundations as a consequences of crevasses or of fires that burned the peat the origin of the inertinite rich gondwana coals is discussed in relation to their specific climatic conditions and a comparison with sub arctic peats is recommended coals influenced by marine and calcareous conditions are characterized by high contents of sulfur and pyrite as well as by hydrogen rich vitrinites with a relatively low reflectance and high fluorescence all due to a strong bacterial activity in peats of low acidity proposals for further research are made in the conclusions

influence of paleoenvironmental factors on preservation of organic matter in middle cretaceous greenhorn formation pueblo colorado sedimentological and geochemical studies of the upper cretaceous hartland shale and overlying bridge creek limestone members of the greenhorn formation reveal close associations between abundance of current induced sedimentary structures extent of bioturbation and type of preserved organic matter abundant hydrogen rich organic matter 4 5 wt and low sulfur to organic carbon ratios are characteristic of the laminated facies which lack current induced sedimentary structures sparse hydrogen poor organic matter 0 1 0 5 wt and relatively high sulfur to organic carbon ratios are characteristic of the bioturbated facies which contain numerous sedimentary structures indicative of currents the concentration of oxygen in the benthic environment and the degree of bio urbation of the sediment apparently were determined principally by the rate that oxygen was supplied advectively by currents rather than by the rate of oxygen consumption by decomposition thus paleoclimatic and paleo oceanographic factors that influenced mixing and current strength in the water column profoundly affected the amount and type of organic matter preserved in these units it is proposed that episodes of high river discharge lead to density stratification of the water column and in turn to quiescent oxygen depleted bottom water and high preservation of organic matter thus accounting for the intermittent widespread deposition of organic rich strata during maximum transgression and depth of the greenhorn sea the lateral association of voluminous deltaic deposits with dee water black shales in other elongate or restricted marine basins may have causes similar to those envisioned for the hartland shale and bridge creek limestone members

geochemical evaluation of upper cretaceous fruitland formation coals san juan basin new mexico and colorado geochemical analyses of coal samples from the upper cretaceous fruitland formation in the san juan basin of new mexico and colorado were used to determine thermal maturity type of kerogen and hydrocarbon generation potential mean random vitrinite reflectance rm of the fruitland coal ranges from 0 42 to 1 54 rock eval pyrolysis data and saturated to aromatic hydrocarbon ratio indicate that the onset of thermal hydrocarbon generation begins at about 0 60 rm and peak generation occurs at about 0 85 rm several samples have hydrogen index values between 200 and 400 indicating some potential for liquid hydrocarbon generation and a mixed type iii and ii kerogen pentacyclic and tricyclic terpanes steranes aromatic steroids and methylphenanthrene maturity parameters were observed through the complete range of thermal maturity in the fruitland coals aromatic pentacyclic terpanes similar to those found in brown coals of australia were observed in low maturity samples but not found above 0 80 rm n alkane depleted coal samples which occur at a thermal maturity of approx 0 90 rm paralleling peak hydrocarbon generation are fairly widespread throughout the basin depletion of n alkanes in these samples may be due to gas solution stripping and migration fromthe coal seams coincident with the development of pressure induced fracturing due to hydrocarbon generation however biodegradation may also effect these samples

thermogenic and secondary biogenic gases san juan basin colorado and new mexico implications for coalbed gas producibility the san juan basin is the most prolific coalbed gas basin in the world with 1992 production exceeding 440 gcf footnote 12 4 billion m3 resources of approximately 50 tcf 1 4 trillion m3 and proved reserves of over 6 tcf 170 billion m3 coalbed gas wells with the highest production initial potential greater than 10 mcf day or 0 28 million m3 day occur in the overpressured north central part of the basin hydrologic analysis indicates that overpressure in the fruitland formation is artesian in origin and represents repressuring that developed during the middle pliocene highly permeable laterally continuous coal beds override abandoned shoreline pictured cliffs sandstones and extend to the elevated recharge area in he northern basin to form a dynamic regionally interconnected aquifer system coal rank and basin hydrodynamics control the composition of fruitland coalbed gases which varies significantly across the basin chemically dry gases in the north central part of the basin coincide with meteoric recharge and regional overpressure the consistency of methane dgr13c values across the basin the presence of isotopically heavy carbon dioxide in coalbed gases and bicarbonate in formation waters and biodegraded n alkane distributions of some coal extracts indicate that coalbed gases in the north central basin are a mixture of thermogenic 25 50 secondary biogenic 15 30 and migrated thermogenic 12 60 gases migrated conventionally and hydrodynamically trapped gases in situ generated secondary biogenic gases and solution gases result in gas content that plot on or above the coal sorption isotherm bacteria transported basinward in groundwater flowing from the elevated northern basin margins metabolized wet gas components n alkanes and organic compounds in the coal and generated secondary biogenic methane and carbon dioxide subsequent to coalification uplift erosion and cooling these gases may be limited to basin margins where shallow depths and structural deformation result in higher permeability or may extend more than 35 mi 56 km basinward from the recharge zone the presence of appreciable secondary biogenic gas indicates an active dynamic flow system with overall permeability sufficient for high productivity basin hydrogeology reservoir heterogeneity location of permeability barriers no flow boundaries and the timing of biogenic gas generation and trap devel pment are critical for exploration and development of unconventional gas resources in organic rich rocks

coal as a source rock for oil a review the geological debate about whether and to what extent humic coals have sourced oil is likely to continue for some time despite some important advances in our knowledge of the processes involved it is clear that not only liptinites but also perhydrous vitrinites have the potential to generate hydrocarbon liquids in the course of natural coalification some liptinites especially alginite cutinite and suberinite contain a higher proportion of aliphatic moieties in their structure than other liptinites such as sporinite and resinite and are therefore more oil prone it is of potential value to be able to predict the several environments of deposition in which coals with high liptinite contents or containing perhydrous vitrinites may have been formed review of the distribution of oil prone coals in time and space reveals that most are jurassic tertiary with key examples from australia new zealand and indonesia methods based both on experimental simulations and the examination of naturally matured samples have been used to determine the order of generation of hydrocarbons from different macerals results are not entirely consistent among the different approaches and there is much overlap in the ranges of degradation but it seems probable that in the natural environment vitrinites begin to generate early followed by labile liptinites such as suberinite then cutinite sporinite and finally alginite petroleum potential may be determined by experimental simulation of natural coalification or inferred through various micro techniques especially fluorescence and infrared ir spectroscopy or bulk techniques such as elemental analysis and 13c nmr spectroscopy the latter three techniques enable a measure of the polymethylene component of the coal which now appears to be one of the best available approaches for determining petroleum potential no method of experimental simulation of petroleum generation from coals is without criticism and comparative results are highly variable however hydrous pyrolysis confined pyrolysis and forms of open system hydrous pyrolysis approach acceptable simulations whether and to what degree generated liquid hydrocarbons are expelled has long been the central problem in oil from coal studies the structure of vitrinite was believed until recently to contain an interconnected microporous network in which generated oil would be contained until an expulsion threshold was attained recent studies show the pores are not interconnected combined with a dynamic model of pore generation it now seems that expulsion of hydrocarbons is best explained by activated diffusion of molecules to maceral boundaries and ultimately by cleats and fractures to coal seam boundaries the main reason for poor expulsion is the adsorption of oil on the organic macromolecule which may be overcome 1 if coals are thin and interbedded with clastic sediments or 2 if the coals are very hydrogen rich and generate large quantities of oil the existence of oil in vitrinite is attested to by solvent extractions fluorescence properties and by microscopic observations of oil and bitumen experimental simulation of expulsion of oil from coals has only recently been attempted the relative timing of release of generated co2 and ch4 could have considerable importance in promoting the expulsion of liquid hydrocarbons but the mechanism is unclear as it is universally agreed that dispersed organic matter dom in some shales readily generates and expels petroleum it is curious that few consistent geochemical differences have been found between coal macerals and dom in interbedded shales unambiguous evidence of expulsion from coals is limited and in particular only a few commercial oil discoveries can be confidently correlated to coals these include upper cretaceous fruitland formation coals in the usa from which oil is produced new zealand tertiary coals and middle jurassic coals from the danish north sea it is likely that coals have at least contributed to significant oil discoveries in the gippsland basin australia in the turpan basin china and in the kutei and ardjuna basins in indonesia but this remains unproven early reports that early jurassic coals in mid norway were a major source of the reservoired oils have been shown to be inaccurate none of the proposed rules of thumb for generation or expulsion of petroleum from coals seem particularly robust decisions on whether a particular coal is likely to have been an active source for oil should consider all available geological and geochemical information the assumptions made in computational models should be well understood as it is likely with new understandings of processes involved that some of these assumptions will be difficult to sustain

early cenozoic two phase extension and late cenozoic thermal subsidence and inversion of the bohai basin northern china abstract the bohai basin is one of a family of early cenozoic extensional basins that lie along the eastern margin of asia from russia to vietnam initial extension was probably triggered by subduction roll back of the oceanic pacific plate from the asian continent there were two phases in the bohai basin s rift history the earlier paleocene early eocene phase resulted in the deposition of the kongdian formation and the fourth lowest member of the shahejie formation in a series of elongate half grabens these half grabens have master faults with a nne ssw orientation secondary normal faults are typically clockwise oblique to the master faults indicating a component of dextral transtension deposition was focused in the west and south of the present basin these rocks are mainly alluvial fan and fluvial red beds the architecture of the basin underwent an important change at ca 43 45 ma middle eocene beginning with the deposition of the third member of the shahejie formation in part these sediments were deposited in the same half grabens as the kongdian formation but the bozhong depression in the central part of the basin originated at this time and became the major depocentre the bozhong depression superficially resembles a pull apart basin it formed when continued transtension of the earlier tertiary fault systems to the east and west created an extensional overlap between them during this phase the basin as a whole had a geometry with elements typical of both pull apart and transtensional basins regional extension in many east asian basins ended at the end of the oligocene probably because of the onset of transpression within eastern asia caused by the collision of australia with the philippine sea plate dextral transpression caused minor inversion of some of the earlier normal faults in bohai but as a whole the basin began to subside in a post rift phase of thermal subsidence that has lasted until the present day

palaoecologic trends and petroleum potential of upper carboniferous coal seams of western germany as revealed by their petrographic and organic geochemical characteristics abstract petrographic compositions of microscopically homogeneous vertical sections in coal profiles from the ruhr basin northwestern germany are reported the studied coal seams of late westphalian age are between 0 4 and 2 4 m thick mean vitrinite reflectivities in the range of 0 1 0 thus indicating high volatile a bituminous coal changes in the organic facies within the coal profiles are common and can be linked to changes of plant communities and preservation conditions in the depositional environments also associated variations in mineral content are usually found although patterns of changes in maceral and mineral contents are complex some general trends have been observed continuous development from seat earth to coal with vitrinitic roots and detritic liptinites and inertinites representing the initial stage of peat and coal evolution just above the base thick vitrinite rich sections with abundant mudstone layers are overlain by inertinite rich sections with low mineral and high sporinite contents the latter may represent highly degraded oxidized peats that accumulated at morphologically elevated areas deposition of clastic roof strata had only a minor influence on organic facies coals and dispersed organic matter in these clastic rocks are in the main similar in maceral group composition organic geochemical characterization of selected lithotypes revealed significant differences between coals and dispersed organic matter this can be explained by trapping of generated hydrocarbons in the coals differences in elemental compositions of coals of the same organic facies at different ranks vitrinite reflectance of 0 70 and 0 92 respectively reflect a loss of about 20 wt co 2 bitumen water during this maturation stage gas chromatographic mass spectrometric analyses of the saturated hydrocarbon fraction in solvent extracts separated from selected samples reveal that all samples contain the same classes of compounds namely straight chain alkanes hopanoids and isoprenoids but that there is a significant difference in the relative abundance of these classes of compounds in the different coals

characteristics and origins of coal cleat a review abstract cleats are natural opening mode fractures in coal beds they account for most of the permeability and much of the porosity of coalbed gas reservoirs and can have a significant effect on the success of engineering procedures such as cavity stimulations because permeability and stimulation success are commonly limiting factors in gas well performance knowledge of cleat characteristics and origins is essential for successful exploration and production although the coal cleat literature spans at least 160 years mining issues have been the principal focus and quantitative data are almost exclusively limited to orientation and spacing information few data are available on apertures heights lengths connectivity and the relation of cleat formation to diagenesis characteristics that are critical to permeability moreover recent studies of cleat orientation patterns and fracture style suggest that new investigations of even these well studied parameters can yield insight into coal permeability more effective predictions of cleat patterns will come from advances in understanding cleat origins although cleat formation has been speculatively attributed to diagenetic and or tectonic processes a viable mechanical process for creating cleats has yet to be demonstrated progress in this area may come from recent developments in fracture mechanics and in coal geochemistry

characterization of coal derived hydrocarbons and source rock potential of coal beds san juan basin new mexico and colorado u s a coal beds are considered to be a major source of nonassociated gas in the rocky mountain basins of the united states in the san juan basin of northwestern new mexico and southwestern colorado significant quantities of natural gas are being produced from coal beds of the upper cretaceous fruitland formation and from adjacent sandstone reservoirs analysis of gas samples from the various gas producing intervals provided a means of determining their origin and of evaluating coal beds as source rocks the rank of coal beds in the fruitland formation in the central part of the san juan basin where major gas production occurs increases to the northeast and ranges from high volatile b bituminous coal to medium volatile bituminous coal rm values range from 0 70 to 1 45 on the basis of chemical isotopic and coal rank data the gases are interpreted to be thermogenic gases from the coal beds show little isotopic variation 13c1 values range 43 6 to 40 5 ppt are chemically dry c1 c1 5 values are 0 99 and contain significant amounts of co2 as much as 6 these gases are interpreted to have resulted from devolatilization of the humic type bituminous coal that is composed mainly of vitrinite the primary products of this process are ch4 co2 and h2o the coal generated methane rich gas is usually contained in the coal beds of the fruitland formation and has not been expelled and has not migrated into the adjacent sandstone reservoirs in addition the coal bed reservoirs produce a distinctive bicarbonate type connate water and have higher reservoir pressures than adjacent sandstones the combination of these factors indicates that coal beds are a closed reservoir system created by the gases waters and associated pressures in the micropore coal structure in contrast gases produced from overlying sandstones in the fruitland formation and underlying pictured cliffs sandstone have a wider range of isotopic values 13c1 values range from 43 5 to 38 5 ppt are chemically wetter c1 c1 5 values range from 0 85 to 0 95 and contain less co2 2 these gases are interpreted to have been derived from type iii kerogen dispersed in marine shales of the underlying lewis shale and nonmarine shales of the fruitland formation in the underlying upper cretaceous dakota sandstone and tocito sandstone lentil of the mancos shale another gas type is produced this gas is associated with oil at intermediate stages of thermal maturity and is isotopically lighter and chemically wetter at the intermediate stage of thermal maturity as compared with gases derived from dispersed type iii kerogen and coal this gas type is interpreted to have been generated from type ii kerogen organic matter contained in coal beds and carbonaceous shales of the fruitland formation has hydrogen indexes from rock eval pyrolysis between 100 and 350 and atomic h c ratios between 0 8 and 1 2 oxygen indexes and atomic o c values are less than 24 and 0 3 respectively extractable hydrocarbon yields are as high as 7 000 ppm these values indicate that the coal beds and carbonaceous shales have good potential for the generation of liquid hydrocarbons voids in the coal filled with a fluorescent material that is probably bitumen is evidence that liquid hydrocarbon generation has taken place preliminary oil source rock correlations based on gas chromatography and stable carbon isotope ratios of c15 hydrocarbons indicate that the coals and or carbonaceous shales in the fruitland formation may be the source of minor amounts of condensate produced from the coal beds at relatively low levelsof thermal maturity rm 0 7

petrographic characteristics and methane sorption dynamics of coal and shaly coal samples from ib valley basin odisha india abstract hydrocarbon generation potential methane sorption capacity maturity and petrographic composition of ten borehole 2 shaly coals and 8 coals samples from ib valley india were determined rock eval pyrolysis and total organic carbon toc analysis indicate that all the samples have excellent toc content and hydrocarbon generation potential though tmax values indicate the samples are mostly immature in nature vitrinite reflectance analysis shows that the samples are within the vitrinite reflectance oil window moreover vt60 vitrinite grains having reflectance greater than 0 60 indicates that thermogenic gas generation is occurring to some degree in almost all sample intervals toc content of the samples 25 35 58 15 wt shows good positive relation with langmuir volume vl 0 481 0 551 mmol g rock and methane sorption capacity msc 0 317 0 379 mmol g rock indicating that organic matter abundance is the significant factor in gas sorption capacity of these samples while fourier transform infrared spectrometry ftir spectrum of coal samples shows the presence of aromatic h the same was not observed in the case of shaly coal samples which may be attributed to their lesser organic matter and greater content of clay

oil generating coals of the san juan basin new mexico and colorado u s a abstract coal beds of the upper cretaceous fruitland formation in the san juan basin of northwestern new mexico and southwestern colorado have significant liquid hydrocarbon generation potential as indicated by typical rock eval hydrogen indexes in the range of 200 400 mg hydrocarbon g organic carbon type ii and iii organic matter small non commercial quantities of oil have been produced from the coal beds at several locations the oils are characterized by high pristane phytane ca 4 and pristane n c17 ratios ca 1 2 abundant c21 alkanes in the c10 fraction with a slight predominance of odd carbon numbered n alkanes abundant branched chain alkanes in the c15 region and a predominance of methylcyclohexane in the c4 c10 fraction the oils are indigenous to the fruitland formation coals and probably migrated at thermal maturities corresponding to vitrinite reflectance values in the range 0 7 0 8 although the oils found to date are not present in commercial amounts these findings illustrate the potential of some coals to generate and expel oil under conditions of moderate thermal heating

composition and origin of coalbed gases in the lower silesian basin southwest poland abstract coalbed gases in the lower silesian coal basin lscb of poland are highly variable in both their molecular and stable isotope compositions geochemical indices and stable isotope ratios vary within the following ranges hydrocarbon chc index chc ch4 c2h6 c3h8 from 1 1 to 5825 wet gas c2 index c2 c2h6 c3h8 c4h10 c5h12 ch4 c2h6 c3h8 c4h10 c5h12 100 from 0 0 to 48 3 co2 ch4 cdmi index cdmi co2 co2 ch4 100 from 0 1 to 99 9 13c ch4 from 66 1 to 24 6 d ch4 from 266 to 117 13c c2h6 from 27 8 to 22 8 and 13c co2 from 26 6 to 16 8 isotopic studies reveal the presence of 3 genetic types of natural gases thermogenic ch4 higher gaseous hydrocarbons and co2 endogenic co2 and microbial ch4 and co2 thermogenic gases resulted from coalification processes which were probably completed by late carboniferous and early permian time endogenic co2 migrated along the deep seated faults from upper mantle and or magma chambers minor volumes of microbial ch4 and co2 occur at shallow depths close to the abandoned mine workings late stage microbial processes have commenced in the upper cretaceous and are probably active at present however depth related isotopic fractionation which has resulted from physical and physicochemical e g diffusion and adsorption desorption processes during gas migration cannot be neglected the strongest rock and gas outbursts occur only in those parts of coal deposits of the lscb which are dominated by large amounts of endogenic co2

microbial origin of australian coalbed methane bituminous coal seam gases from the permian sydney and bowen basins australia are characterized by 1 methane ethane ratios greater than or equal to 1000 2 dgr13c and dgrd values for methane of 60 10 pmil pdb and 217 17 pmil smow respectively 3 carbon dioxide contents of less than 5 and 4 dgr13c co 2 ch4 values of 55 10 pmil pdb these data suggest that microbial reduction of co2 rather than traditional thermogenic reactions is mainly responsible for gas composition invasions by co2 of deep seated external origins are readily recognized by their isotopic compositions 7 2 pmil pdb

influence of geochemical organo petrographical and palynofacies assemblages on hydrocarbon generation a study from upper oligocene coal and shale of the makum coal basin assam india abstract the makum coal basin of northeast india comprises important coal and shale reserves of upper oligocene age a total of twenty samples were collected to evaluate the kerogen type hydrocarbon generation potential and thermal maturity through geochemically organo petrographically and palynofacies analyses the samples are characterized by high toc total organic carbon and large s2 remaining heavier hydrocarbons meet the required standards as good to excellent generation potential the coal and shale are marked by mixed type iii ii kerogen with minor occurrence of type i kerogen moderate to high hydrogen index hi values and are considered to generate thermogenic gas with some condensate oil the reflectance values indicate samples are mostly immature and close to the oil birth line the amount of vt60 grains also suggests that the samples can produce some condensate oil along with thermogenic gas the rock eval pyrolysis rep data stands with the interpretation through petrographic palynofacies and fourier transform infrared spectroscopy ftir in determining kerogen type and generation potential the generation potential is also controlled by the degree of organic matters reactivity the estimated reactivity indicates that besides vitrinite and liptinite up to 70 vol of the inertinite macerals have the potential to generate hydrocarbon the occurrence of a substantial amount of reactive macerals i e collotelinite vitrodetrinite collodetrinte corpogelinite alginite resinite and bituminite within the samples makes them more potential to generate hydrocarbons the diagram proposed based on our data of macerals and palynofacies assemblages may also be a convenient tool in detecting kerogen type s and nature of hydrocarbons

isotopic geochemistry and habitat of the natural gases from the upper carboniferous žacleř coal bearing formation in the nowa ruda coal district lower silesia poland the nowa ruda coal district is located in the se part of the lower silesian coal basin the analysed coals have vitrinite reflectance from 1 12 to 1 56 and 13c values from 24 6 to 23 2 natural gases accumulated in coal seams reveal considerable variability of molecular and isotopic compositions geochemical indices and stable isotope ratios vary in the following ranges c2 wet gas index from 0 6 to 40 7 carbon dioxide methane index cdmi from 1 9 to 99 9 13c methane from 66 1 to 28 6 d methane from 256 to 211 13c carbon dioxide from 24 4 to 7 0 and 13c ethane from 27 8 to 25 5 stable isotope studies allow distinction between thermogenic endogenic magmatic and or upper mantle derived and probably even bacterial gases the intensity of migration mixing and accumulation of the natural gases of various origins appear to be variable in time and space the variability of both the molecular and the isotopic compositions of the analysed gases probably results from the specific primary reactions active during the generation of the thermogenic gases in coal seams decomposition of coal seam matrix and or cracking of n alkanes and or in humic dispersed organic matter of coaly mudstones an isotopic fractionation may also develop during the migration of the natural gases through the ultra fine pores of coals the results of this paper suggest that in the case of gases connected with coal seams the existing gas classifications cannot be directly applied to their genetic interpretation

organic geochemistry and petrology of coals and carbonaceous shales from western venezuela abstract the aim of this study is to provide geochemical and petrological data to characterize coals and carbonaceous shales from western venezuela in terms of both compositional and thermal maturity parameters the samples collected in outcrop belong to the paleocene marcelina formation the eocene oligocene carbonera formation the paleocene orocue group and the lower miocene cerro pelado formation geochemical results are based on total organic carbon toc content rock eval pyrolysis and proximate analyses whereas petrographic data include vitrinite reflectance rr and maceral composition thermal evolution of all samples is low varying from immature to early mature coals and carbonaceous shales from the marcelina and cerro pelado formations are humic the former having larger amounts of inertinite nevertheless both groups of samples are compositionally homogeneous samples from the carbonera formation and orocue group show a great compositional scatter varying from humic to sapropelic our geochemical profile for marcelina formation coals allowed the detection of the first coalification jump whereas the cerro pelado formation profile shows an abrupt decrease of organic matter content toward the top of the studied interval results shown here confirm data from previous studies relative to the high oil source rock potential of carbonera formation and orocue group coals and carbonaceous shales

relation of natural gas composition to thermal maturity and source rock type in san juan basin northwestern new mexico and southwestern colorado san juan basin is a roughly circular asymmetric structural depression located in northwestern new mexico and southwestern colorado ultimate recoverable reserves of predominantly nonassociated gas 23 tcf 0 65 1012 m3 are present in the structurally low part of the central basin the major producing intervals are low permeability sandstone reservoirs in the upper cretaceous dakota sandstone mesaverde group and pictured cliffs sandstone lesser amounts of oil and or gas are produced from pennsylvanian jurassic and cretaceous rocks along the southern and western flanks of the basin the gases display a trend of becoming isotopically heavier dgr13c1 values range from 48 7 to 31 4 pmil and chemically drier c1 c1 5 values range from 0 75 to 0 99 with increasing depth these changes are assumed to be the result of thermal cracking processes and the gases are interpreted to have been generated during the mature and post mature stages of hydrocarbon generation however there is considerable scatter in the data which is interpreted to result from a difference in source rock type gases generated from nonmarine humic source rocks are isotopically heavier and chemically drier than those generated from marine sapropelic source rocks at equivalent levels of maturity the gases also become isotopically heavier and chemically drier to the northeast following the trend of increasing maturity of all units in that direction the increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the san juan mountains area maximum burial and heat flow occurred during the oligocene which probably coincided with peak hydrocarbon generation lack of oil in the central basin is believed to be the result of two factors first gas in reservoirs such as the dakota sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature wet gas condensate and post mature dry gas stages of hydrocarbon generation second gas in reservoirs such as mesaverde group and pictured cliffs sandstone is nonassociated and probably was generated from nonmarine coaly organic matter during the mature and post mature stages minor amounts of condensate instead of oil may have been generated from nonmarine source rocks during the mature stage

carbon cycling in the pliocene velenje coal basin slovenia inferred from stable carbon isotopes abstract stable isotopes of carbon were used to trace organic and inorganic carbon cycles and biogeochemical processes especially methanogenesis within different geologic substrates of the pliocene lignite bearing velenje basin in northern slovenia lithotypes of lignite coalbed gases calcified woods xylites carbonate rich sediments and groundwaters were investigated carbon isotope 13 c values of the different lignite lithotypes ranged from 28 1 to 23 0 with the variability likely a function of the original isotopic heterogeneity of the source plant materials and subsequent biogeochemical processes i e gelification fusinitization mineralization of organic matter during the early stage of biomass accumulation and diagenesis in the lignite seam co 2 and ch 4 were the major gas components with small amounts of n 2 the carbon isotope values of co 2 13 c co2 and ch 4 13 c ch4 were highly variable ranging from 9 7 to 0 6 and 70 5 to 34 2 respectively carbon dioxide is likely sourced from a mixture of in situ microbial activity and external co 2 while ch 4 is dominantly sourced from microbial methanogenesis with possible addition of thermogenic gas from deeper formations and the influence of microbial oxidation of methane calcified xylites enriched with 13 c 13 c values up to 16 7 indicate that microbial methanogenesis was active during formation of the basin the 13 c dic values from 17 4 to 3 2 of groundwaters recharging the basin from the triassic aquifer are consistent with degradation of organic matter and dissolution of dolomite groundwaters from the pliocene sandy and lithotamnium carbonate aquifers have 13 c dic values from 9 1 to 0 2 suggestive of degradation of organic matter and enrichment via microbial reduction of co 2

numerical modeling of late miocene tectonic inversion in the xihu sag east china sea shelf basin china abstract the east china sea shelf basin is an important oil and gas bearing basin in the west pacific continental margin this region was affected by subduction of the pacific plate and the philippine plate in cenozoic and experienced multi stage tectonic inversions this paper presents results from a numerical simulation by finite element method to the xihu sag in the east china sea shelf basin and neighboring areas in an attempt to evaluate the wnw directed compression on the sag during late miocene based on comprehensive structural analysis of a large number of seismic profiles we determine the structural geometry of the sag including the basement of the basin the sedimentary cover and 29 major faults in the xihu sag simulation results show that under continuous wnw directed compression tectonic inversion occurred firstly in the longjing and yuquan tectonic zones in the sag based on quantitative analysis of vertical displacement field of the xihu sag and peripheral areas and its stress intensity evolution we identify a compressional regime in the longjing anticline zone with a gradually propagated uplifting from south to north whereas the propagation of uplifting in the yuquan anticline zone is from north to south the inversion intensity decreases from north to south the formation of the tectonic inversion zone in the xihu sag is not only correlated to the direction of compression and fault patterns in the basin but also closely related to the spatial configuration of fault surfaces of the xihu jilong fault in the xihu sag

developmental geology of coalbed methane from shallow to deep in rocky mountain basins and in cook inlet matanuska basin alaska u s a and canada abstract the rocky mountain basins of western north america contain vast deposits of coal of cretaceous through early tertiary age coalbed methane is produced in rocky mountain basins at depths ranging from 45 m 150 ft to 1 981 m 6 500 ft from coal of lignite to low volatile bituminous rank although some production has been established in almost all rocky mountain basins commercial production occurs in only a few despite more than two decades of exploration for coalbed methane in the rocky mountain region it is still difficult to predict production characteristics of coalbed methane wells prior to drilling commonly cited problems include low permeabilities high water production and coals that are significantly undersaturated with respect to methane sources of coalbed gases can be early biogenic formed during the early stages of coalification thermogenic formed during the main stages of coalification or late stage biogenic formed as a result of the reintroduction of methane generating bacteria by groundwater after uplift and erosion examples of all three types of coalbed gases and combinations of more than one type can be found in the rocky mountain region coals in the rocky mountain region achieved their present ranks largely as a result of burial beneath sediments that accumulated during the laramide orogeny late cretaceous through the end of the eocene or shortly after thermal events since the end of the orogeny have also locally elevated coal ranks coal beds in the upper part of high volatile a bituminous rank or greater commonly occur within much more extensive basin centered gas deposits which cover large areas of the deeper parts of most rocky mountain basins within these basin centered deposits all lithologies including coals sandstones and shales are gas saturated and very little water is produced the interbedded coals and carbonaceous shales are probably the source of much of this gas basin centered gas deposits become overpressured from hydrocarbon generation as they form and this overpressuring is probably responsible for driving out most of the water sandstone permeabilities are low in part because of diagenesis caused by highly reactive water given off during the early stages of coalification coals within these basin centered deposits commonly have high gas contents and produce little water but they generally occur at depths greater than 5 000 ft and have low permeabilities significant uplift and removal of overburden has occurred throughout the rocky mountain region since the end of the eocene and much of this erosion occurred after regional uplift began about 10 ma the removal of overburden generally causes methane saturation levels in coals to decrease and thus a significant drop in pressure is required to initiate methane production the most successful coalbed methane production in the rocky mountain region occurs in areas where gas contents were increased by post eocene thermal events and or the generation of late stage biogenic gas methane generating bacteria were apparently reintroduced into the coals in some areas after uplift and erosion and subsequent changes in pressure and temperature allowed surface waters to rewater the coals groundwater may also help open up cleat systems making coals more permeable to methane if water production is excessive however the economics of producing methane are impacted by the cost of water disposal

occurrence and geochemistry of natural gases piceance basin northwest colorado the piceance basin is a hydrocarbon rich province that has natural gas production from reservoirs ranging in age from late jurassic to eocene and large undeveloped resources of natural gas in coal beds and tight sandstone reservoirs of cretaceous age gases from all producing intervals are of predominantly thermal origin and become isotopically heavier delta isotope 13 c 1 51 3 to 29 1 o oo and chemically drier c 1 c 1 5 0 26 to 1 00 with increasing thermal maturity of reservoirs r o 0 45 to 2 40 over a depth range of 1100 to 11 702 ft 335 3567 m scatter in trend is attributed to source rock differences and considerable vertical and lateral migration based on chemical and isotopic composition three major types of gases can be distinguished those generated from mixed type ii and iii kerogens those from dispersed type iii kerogen and those from coal gases generated from mixed type ii and iii kerogens are produced from the upper jurassic morrison formation the lower cretaceous cedar mountain formation the upper cretaceous dakota sandstone the upper cretaceous mancos b producing interval and marginal marine sandstones of the upper cretaceous iles producing interval these gases are associated with minor amounts of oil and probably were generated from kerogen in the marine mancos shale gases generated from dispersed type iii kerogen are produced from nonmarine sandstones of the upper cretaceous williams fork producing inter al and from thermally immature reservoirs in the overlying paleocene and eocene fort union and wasatch formations these nonassociated gases contain large amounts of co 2 and probably were generated from carbonaceous shales in the williams fork producing interval their presence in immature fort union and wasatch reservoirs implies considerable vertical migration the third type of gas is methane rich is produced by devolatilization of humic coal and is generally in coal beds of the cameo fairfield zone of the williams fork producing interval these gases are not the major source for adjacent sandstone reservoirs a fourth distinct type of isotopically light thermogenic gas occurs in immature reservoirs of the eocene green river formation this gas is inferred to have migrated from u identified deeper more mature source rocks

stable carbon isotopic ratios of ch4 co2 bearing fluid inclusions in fracture fill mineralization from the lower saxony basin germany a tool for tracing gas sources and maturity abstract the stable carbon isotopic ratios 13 c of methane ch 4 and carbon dioxide co 2 of gas rich fluid inclusions hosted in fracture fill mineralization from the southern part of the lower saxony basin germany have been measured online using a crushing device interfaced to an isotopic ratio mass spectrometer irms the data reveal that ch 4 trapped in inclusions seems to be derived from different source rocks with different organic matter types the 13 c values of ch 4 in inclusions in quartz hosted by carboniferous rocks range between 25 and 19 suggesting high maturity coals as the source of methane methane in fluid inclusions in minerals hosted by mesozoic strata has more negative carbon isotope ratios 45 to 31 and appears to represent primary cracking products from type ii kerogens i e marine shales there is a positive correlation between increasing homogenization temperatures of aqueous fluid inclusions and less negative 13 c ch 4 values of in co genetic gas inclusions probably indicating different mtaturity of the potential source rocks at the time the fluids were released the co 2 isotopic composition of ch 4 co 2 bearing inclusions shows slight negative or even positive 13 c values indicating an inorganic source e g water rock interaction and dissolution of detrital marine calcite for co 2 in inclusions we conclude that the 13 c isotopic ratios of ch 4 co 2 bearing fluid inclusions can be used to trace migration pathways sources of gases and alteration processes furthermore the 13 c values of methane can be used to estimate the maturity of the rocks from which it was sourced results presented here are further supported by organic geochemical analysis of surface bitumens which coexist with the gas inclusion rich fracture fill mineralization and confirm the isotopic interpretations with respect to fluid source type and maturity

comparison of microbial gases from the middle america trench and scripps submarine canyon implications for the origin of natural gas chemical and isotopic compositions have been measured on 62 microbial gases from tertiary hemipelagic sediments in the middle america trench off guatemala and from decaying kelp and surf grass currently accumulating in scripps submarine canyon off southern california gases from the middle america trench have been generated primarily by the reduction of carbon dioxide methane 13c varies from 84 to 39 methane d varies from 208 to 145 and carbon dioxide 13 c varies from 27 to 28 gases from scripps submarine canyon have been generated primarily by acetate dissimilation methane 13 c varies from 63 to 43 methane d varies from 331 to 280 and carbon dioxide 13c varies from 17 to 3 methane 13c values as heavy as 40 appear to be uncommon for gases produced by carbon dioxide reduction and in the middle america trench are associated with unusually positive carbon dioxide 13c values however based on the 25 intramolecular fractionation between acetate car yl carbon and methyl carbon estimated from the scripps submarine canyon data methane produced by acetate dissimilation may commonly have heavy 13c values the d of methane derived from acetate is more negative than natural methanes from other origins microbial methane d values appear to be controlled primarily by interstitial water d and by the relative proportions of methane derived from carbon dioxide and acetate the chemical and isotopic compositions of microbial gas and thermogenic gas overlap making it difficult to determine the origins of many commercial natural gases from methane 13c and c2 hydrocarbon concentrations alone measurements of methane d and carbon dioxide 13c can provide useful additional information and together with ethane 13c data help identify gases with mixed microbial and thermogenic origins

a novel approach for recognition and quantification of hydrocarbon migration effects in shale sandstone sequences a detailed organic geochemical study of over 150 samples from two cores with a total combined length of 320 m 1 050 ft through sequences of interbedded source rock type shales 0 84 rm maturity and reservoir sandstones allowed recognition and quantitation of a number of migration effects detailed gas chromatography mass spectrometry of steranes and triterpanes was used to insure that samples being compared to investigate migration effects contain organic matter of a similar type thin shales interbedded in sands and the edges of thick shale units are depleted in petroleum range hydrocarbons to a much higher degree than the centers of thick shale units for the alkanes expulsion occurs with pronounced compositional fractionation effects shorter chain length n alkanes are expelled preferentially and isoprenoid alkanes are expelled to a lesser degree than their straight chain isomers based on material balance calculations expulsion efficiencies were determined and found to be very high in certain instances for thin interbedded shales they decrease from about 80 around c15 to near zero in the c25 region there is no evidence for significant redistribution of steranes and triterpanes in the two sequences compared to c15 to c25 n alkanes they appear relatively immobile the composition of the hydrocarbons impregnating parts of the reservoir sandstones is in agreement with expulsion occurring with pronounced fractionation based on molecular chain length hence consideration of bulk expulsion efficiencies gives an unrealistic picture furthermore the impregnation of a siltstone cap rock from an underlying hydrocarbon accumulation seems to have occurred by bulk oil migration and without significant fractionation the degree of hydrocarbon depletion of some of the shales of both sequences appears to be controlled by compaction and the primary migration process appears to have occurred with chromatographic separation the migration phenomena observed in both sequences lead us to propose that the main phase of expulsion can be preceded by an earlier sta e during which the edges of thick shales and thin interbedded shales appear to be slowly and continuously depleted by the chromatographic processes the composition of the hydrocarbon product accumulating in the reservoirs at this stage appears to be controlled primarily by physical processes rather than by the type and maturity of the organic matter in the generating source rock by this mechanism the origin of accumulations of light oils and gas condensates in low mature sequences bearing predominantly terrestrial derived organic matter can be explained finally the migration effects documented in this study have some consequences for interpretation of geochemical data e g the pristane n c17 ratio a commonly accepted maturity parameter has been shown to be also controlled by the degree of hydrocarbon expulsion

a source discrimination study of a chinese seismic event of may 4 1983 abstract a weak seismic event that occurred on may 4 1983 close to the lop nor underground test site in southern sinkiang province china has been studied for source discrimination digital waveforms obtained at gauribidanur array india of this event of six known shallow focus earthquakes from southern sinkiang and of two known underground nuclear explosions in lop nor were processed for further analysis the estimates of signal complexity and tmf third moment of frequency parameters of the may 4 event were found to be quite different from those of the known explosions whereas they were consistent with those of the known earthquakes on this basis the event under investigation appeared to be a crustal focus earthquake

analogue modelling and mechanism of tectonic inversion of the xihu sag east china sea shelf basin abstract the east china sea shelf basin lies between the pacific subduction and indian eurasian collision tectonic domains and records cenozoic tectonic inversion especially in the xihu sag to improve the understanding of the evolution and mechanism of tectonic inversion this paper employs analogue modelling to reproduce the evolutionary process combined with the structural analysis of seismic profile this paper determines the pattern of basement involved faults simulation results show that under the transtension two subsidence centers developed and a number of normal faults assembled in two flower structures when the stress field turned into transpression the geometry and deformation of inversion basin inherited the previous transtensional basin and pre existing faults respectively the geometry and fault patterns in models are well consistent with those observed in the xihu sag which indicates the plausibility of similar deformation controls the formation of the tectonic inversion is related to the variation in stress field caused by the changes in the rates and directions of the subduction of the pacific plate and the collision of the indian plate with eurasian plate

hydrocarbon generation as a mechanism for overpressuring in rocky mountain region overpressured reservoirs are present in the deeper parts of most basins in the rocky mountain region a variety of conditions can cause overpressuring however most high pressures in the region are caused by present day or recently active oil and gas generation or generation in the last few million years in low permeability rock successions that still contain organic matter capable of yielding thermally generated hydrocarbons rocks with above normal pressure range from devonian to tertiary and commonly are associated with low permeability tight reservoirs most of the overpressuring occurs in cretaceous and tertiary rocks overpressuring in rocks older than cretaceous is rare and where present only occurs in strata with very rich hydrocarbon source beds such as the d vonian mississippian bakken formation most organically lean pre cretaceous source beds are no longer capable of yielding enough hydrocarbons to maintain abnormal pressure nearly all overpressured reservoirs and source rocks have temperatures of approximately 200 f 93 c or higher in addition available data indicate that hydrocarbon related overpressuring does not generally occur if vitrinite reflectance ro values are less than 0 6 in oil prone strata or less than 0 7 in gas prone strata overpressured gas bearing strata generally have ro values of 0 8 or higher maximum reservoir pressures are about equal to the fracture gradient pressure in the reservoir rocks some vertical fractures in reservoir rocks are probably caused by rapid expulsion of hydrocarbons from the source beds high pressures in the rocky mountain region means that the rock succession is gas or oil saturated and discrete hydrocarbon water contacts are generally not present classical concepts of hydrodynamics do not appear to be applicable to these overpressured successions the optimum depths for exploration are generally the upper 1 000 2 000 ft 305 610 m of the overpressured succession

geochemistry of coalbed gas a review coals are both sources and reservoirs of large amounts of gas that has received increasing attention in recent years as a largely untapped potential energy resource coal mining operations such as ventilation of coalbed gas from underground mines release coalbed ch4 into the atmosphere an important greenhouse gas whose concentration in the atmosphere is increasing because of these energy and environmental issues increased research attention has been focused on the geochemistry of coalbed gas in recent years this paper presents a summary review of the main aspects of coalbed gas geochemistry and current research advances

assessment of organic richness and hydrocarbon generation potential of raniganj basin shales west bengal india abstract high energy prices and severe energy shortage has led shale gas to become the focus of study and exploration in many countries india like many other countries around the world with shale gas potential recognizes the strategic importance of developing its shale gas resources for the purpose of shale gas assessment the authors have selected sixty six borehole shale samples of different formations from different parts of raniganj basin west bengal india rock eval pyrolysis and total organic carbon toc analysis and petrographic characterization of the shales were carried out shales from barakar lower permian barren measures upper permian and raniganj formation upper permian are marked by good to excellent toc content input of type iii organic matter and are capable of generating oil and thermogenic gas upon thermal cracking igneous intrusives dykes and sills in the formations occurring in and around the areas of sitarampur si and kulti ku in western part of the basin might have resulted in cooking of the shales increasing their maturities estimated vitrinite reflectance vro as indicated by the barren measures shales of shallow depths from the above mentioned areas majority of the samples are marked by low oxygen index oi values

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

the seismic disturbance of 1976 march 20 elast kazakhstan earthquake or explosions summary on 1976 march 20 a seismic disturbance took place near an explosion test site in east kazakhstan ussr on the mb ms criterion this disturbance is identified as an earthquake however it has been suggested that the source may have been one or more explosions it is shown here that the polarities and relative amplitudes of the main arrivals on the short period p wave seismograms of the disturbance can be accounted for by assuming the source was an earthquake in a crust with some lateral variation in structure there is no evidence that the disturbance was anything other than a single earthquake

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

focal mechanism of the 20 august 1983 caspian sea earthquake abstract the short period array seismogram at gba india of a central caspian sea seismic event on 20 august 1983 was described by some workers as explosion like though they finally favoured an earthquake source on focal depth and p wave complexity spectral criteria we subject gba and other array records to a quantitative test for compatibility with a double couple source the p pp sp relative amplitude method applied in conjunction with a moving depth window process to test separately a wide range of p pp sp times the only sections of coda consistent with being pp and or sp are large amplitude arrivals 12 15 s behind p they indicate a normal fault striking wnw ese and downthrown to the north at a depth of some 30 km in good agreement with the trend of mapped faults and local seismicity and focal depth derived from p onset times

assessment of hydrocarbon source rock potential of polish bituminous coals and carbonaceous shales abstract we analyzed 40 coal samples and 45 carbonaceous shale samples of varying thermal maturity vitrinite reflectance 0 59 to 4 28 from the upper carboniferous coal bearing strata of the upper silesian lower silesian and lublin basins poland to evaluate their potential for generation and expulsion of gaseous and liquid hydrocarbons we evaluated source rock potential based on rock eval pyrolysis yield elemental composition atomic h c and o c and solvent extraction yields of bitumen an attempt was made to relate maceral composition to these source rock parameters and to composition of the organic matter and likely biological precursors a few carbonaceous shale samples contain sufficient generation potential pyrolysis assay and elemental composition to be considered potential source rocks although the extractable hydrocarbon and bitumen yields are lower than those reported in previous studies for effective type iii source rocks most samples analysed contain insufficient capacity for generation of hydrocarbons to reach thresholds required for expulsion primary migration to occur in view of these findings it is improbable that any of the coals or carbonaceous shales at the sites sampled in our study would be capable of expelling commercial amounts of oil inasmuch as a few samples contained sufficient generation capacity to be considered potential source rocks it is possible that some locations or stratigraphic zones within the coals and shales could have favourable potential but could not be clearly delimited with the number of samples analysed in our study because of their high heteroatomic content and high amount of asphaltenes the bitumens contained in the coals are less capable of generating hydrocarbons even under optimal thermal conditions than their counterpart bitumens in the shales which have a lower heteroatomic content

source depth as a useful parameter in the discrimination of earthquakes and underground explosions abstract in order to identify whether observed seismic signals are generated by an underground nuclear explosion or an earthquake it is adequate to rely on one efficient identifier that provides a reasonably good clue in an unambiguous way although it is generally accepted that multi station multi parameter discrimination can provide separation between explosions and earthquakes it has been observed that cases do arise where signal characteristics cannot be established distinctly and satisfactorily in the so called difficult cases which are associated with some ambiguity in deducing the nature of the source using single station seismograms it is shown in this paper that a reliable estimate of source depth proves extremely useful out of the eleven typical examples of not easy to discriminate events recorded at the gauribidanur short period seismic array in southern india seven could be successfully identified as earthquakes and the remaining four as probable underground explosions on the basis of focal depth estimates from multi station data

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

a preliminary study of regional seismic discrimination in central asia with emphasis on western china abstract in support of a comprehensive test ban treaty we have started to evaluate regional seismic event discrimination capabilities for central asia emphasizing western china we have measured noise and signal levels of over 260 earthquakes and 26 underground nuclear explosions recorded at the broadband digital station wmq in western china and over 120 earthquakes and five nuclear explosions at station aak in kyrgyzstan the explosions are from the kazakh test site kts and lop nor china the earthquakes are mostly from northwest china event magnitudes measured as either m b or m l range between 2 5 and 6 5 and maximum event station distance is about 1700 km we formed phase spectral cross spectral short period long period and long period ratios to test many possible event discriminants all ratios were corrected for distance effects before forming ratio versus magnitude discrimination plots the most consistent discriminants for separating earthquakes from explosions are p s phase ratios for frequencies above 3 to 4 hz p wave f 1 hz to rayleigh wave 0 03 hz f p s ratios separation between earthquakes and explosions increases with frequency and for the p r ratios separation increases with increasing p frequency the p 3 to 6 hz s 0 75 to 1 5 hz cross spectral ratios also separate earthquakes and explosions these cross spectral ratios may prove quite useful because due to weak high frequency l g signals from explosions the high frequency p l g ratio sometimes cannot be evaluated but the p 3 to 6 hz l g 0 75 to 1 5 hz can be evaluated successfully p spectral ratios 3 to 6 hz 0 75 to 1 5 hz also separate earthquakes and explosions but not quite as consistently as the p s ratios however p spectral ratios may prove useful in cases where path effects block or highly attenuate s n and l g waves rendering the p s ratios unavailable in contrast to nevada test site nts events the l g 3 to 6 hz 0 75 to 1 5 hz spectral ratio does not separate earthquakes and nuclear explosions in central asia this observation may be a source effect as asian explosions are thought to be detonated in highly lithified rocks below the water table while most of the smaller m b for station aak the short period spectral and cross spectral ratios identify a few earthquakes not recorded at wmq from the north pamir region that plot with the lop nor explosions the waveforms from these earthquakes lack surface waves and a distinct l g phase these events which we interpret as having occurred in a subducted slab do separate from the explosions on high frequency f 4 hz p s discrimination plots for station wmq long period r l rayleigh wave love wave ratios separate kts explosions and asian earthquakes however r l ratios fail to identify the large lop nor explosions recorded at station aak because the love waves are as large as the rayleigh waves suggesting that strong tectonic release occurs with these explosions because we have examined only one path at a single station for these explosions source radiation pattern may be influencing this observation

geology of the chinese nuclear test site near lop nor xinjiang uygur autonomous region china abstract the chinese underground nuclear test site in the kuruktag and kyzyltag mountains of the xinjiang uygur autonomous region of northwest china is the location of sixteen underground tests that occurred between 1969 and 1992 the largest test to date conducted on 21 may 1992 had a reported yield of about one megaton geophysical properties of the rocks and a large scale geologic map of part of the test area were published by the chinese in 1986 and 1987 and are the first site specific data available for this test site in areas of low relief underground nuclear testing has occurred below the water table in shafts drilled vertically into dense low porosity paleozoic granitic and metasedimentary rocks additional testing in areas of more rugged terrain has occurred in horizontal tunnels probably abovethe water table at least one of these tunnels was driven into granite the upper 50 m of the rock in the area of the vertical tests is weathered and fractured these conditions have been shown to influence the magnitude of the disturbance of the land surface after a nuclear explosion these descriptions suggest hard rock coupling at depth and a closer resemblance to the former soviet test site in eastern kazakhstan than to the u s test site in nevada

mesozoic cenozoic basin inversion and geodynamics in east china a review abstract east china experienced alternating tectonic extension and compression with ambiguous tectonic settings in the mesozoic cenozoic interval long term extensional episodes associated with the subduction retreat of the paleo pacific and pacific slabs have been widely studied the compressional events of which basin inversion is a typical case are less concerned but important in understanding the tectonic evolution and geodynamics in east china we made a systematic study involving tectonics sedimentation apatite fission track and dynamic topography on the mesozoic cenozoic basin inversion in east china four phases of basin inversion were concluded 1 early late cretaceous inversion occurred at 115 100 ma in southeast china and at 100 89 ma in northeast china showing a northward migration due to the diachronous collision of the northward moving okhotomorsk block with east asia 2 the latest cretaceous early paleogene inversion was widely developed throughout east china it is associated with the increasingly younger subducted oceanic crust of the izanagi plate which caused the progressively increasing buoyancy beneath east china and an uplift in dynamic topography cenozoic inversion showed an eastward migration from 3 the western part of each basin at the late paleogene to 4 the eastern part at the neogene due to the subduction retreat of the pacific plate