late miocene environmental change in nepal and the northern indian subcontinent stable isotopic evidence from paleosols neogene sediments belonging to the siwalik group crop out in the himalayan foothills along the length of southern nepal carbon and oxygen isotopic analyses of siwalik paleosols from four long siwalik sections record major ecological changes over the past 11 m y the carbon isotopic composition of both soil carbonate and organic matter shifts dramatically starting ca 7 0 ma marking the displacement of largely c3 vegetation probably semi deciduous forest by c4 grasslands by the beginning of the pliocene all the flood plains of major rivers in this region were dominated by monsoonal grasslands the floral shift away from woody plants is also reflected by the decline and final disappearance of fossil leaves and the decrease in coal logs in the latest miocene a similar carbon isotopic shift has been documented in the paleosol and fossil tooth record of pakistan and in terrigenous organic matter from the bengal fan showing that the floral shift was probably continentwide the latest miocene also witnessed an average change of 4 in the oxygen isotopic composition of soil carbonate as observed previously in pakistan the reasons for this are unclear if not diagenetic a major environmental change is indicated perhaps related to that driving the carbon isotopic shift recently described pollen and leaf fossils from the surai khola section show that evergreen forest was gradually displaced by semi deciduous and dry deciduous forest between 11 and 6 ma the gradual nature of this floral shift which culminated in the rapid expansion of c4 grasses starting 7 0 m y ago is difficult to explain by a decrease in atmospheric pco2 alone cerling et al 1993 but fits well with a gradual onset of monsoonal conditions in the late miocene in the northern indian subcontinent himalayan uplift driving both monsoonal intensification and consumption of co2 through weathering may be the common cause behind major late miocene environmental change globally however the decline of effective moisture associated with monsoon development has probably slowed not increased the rate of consumption of co2 by chemical weathering of himalayan sediments

oil and gas developments in western canada in 1989 there were 5513 wells drilled in western canada during 1989 30 fewer than in 1988 total meterage drilled was 6 459 797 m for an average well depth of 1172 m exploratory drilling in 1989 dropped by 13 and development drilling by 40 from 1988 levels british columbia was the only area in western canada with increased activity the industry continued to acquire more acreage for less total bonus at provincial crown sales throughout western canada during 1989 3 564 827 ha were acquired for a total bonus of 484 872 000 footnote 5 although the total disposition of crown lands decreased only 4 the average bonus dropped to 136 ha down 26 from 1988 in alberta the most active exploration areas were the southern and eastern mesozoic gas play the western deep devonian gas play the oil plays on and adjacent to the peace river arch and the middle devonian carbonate oil plays in the northwest in british columbia the most active exploration areas were the mesozoic and paleozoic gas plays in the northeastern portion of the province for the first time in 5 years crown drilling rights were issued within the fraser valley in the southwest there was a focus on gas exploration in the province as pipeline reversal was implemented permitting access to canadian and california markets through the alberta transportation network in manitoba exploration and development activity continued to decline in northern canada exploratory drilling success offering of crown lands for exploration approval of gas exports and indications that many of the native lands claims may be approaching settlement all provided exploration encouragement for the next decade exploration activities occurred throughout the northwest territories from the offshore beaufort to the southern border in saskatchewan active exploration areas were shallow mesozoic gas plays in the western portions of the province and deep paleozoic oil plays in the south

application of organic facies studies to sedimentary basin analysis a case study from the yitong graben china abstract an organic facies is defined in this paper as a mappable stratigraphic unit containing organic matter om of a particular genetic type in certain abundance several concepts concerning organic facies are proposed and their significance in basin analysis is discussed in detail a combined sedimentological and organic geochemical investigation has been carried out in the yitong graben and four organic facies and one organic parafacies have been recognized distinct organic basin fill sequences obfs and organic facies association ofa were developed in different sub units of this graben which indicates that the yitong graben had undergone two tectonic subsidence cycles such tectonic subsidence patterns could not have been found out without detailed organic facies analysis confirming that organic facies especially obfs and ofa study is a very useful tool not only for prediction and three dimensional mapping of petroleum source rocks but also for sedimentary basin analysis

pore water geochemistry in shallow sediments from the northeastern continental slope of the south china sea abstract the northeastern continental slope of the south china sea is one of the promising areas for gas hydrates which is characterized by its variable topography and complex tectonics in this paper two gravity piston cores were sampled from the shallow sediments in the vicinity of the region where various morphologies of gas hydrate was recovered during the second gas hydrate expedition of guangzhou marine geological survey conventionally the identification of gas hydrate was primarily dependent on the bottom simulating reflector bsr apart from bsr the anomalies in pore water geochemistry which are highly related to various microbially mediated processes and features of fluid from deep hydrocarbon reservoir are also helpful for the gas hydrate exploration in this study upward methane flux may be responsible for distinct characteristics of pore waters observed at the two sample sites a reactive transport model was accordingly established to compare contrastive fluid features and to estimate the upward methane flux since fluid migration is typically associated with complex tectonic settings of the study area it is proposed that heterogeneous distribution of gas hydrates along the continental margin should be well considered

eocene oligocene proto cascades topography revealed by clumped δ47 and oxygen isotope δ18o geochemistry chumstick basin wa usa the topography of the present day washington cascades impacts atmospheric circulation and precipitation patterns in the pacific northwest introducing a pronounced orographic rain shadow in the lee of the mountain range the temporal development of cascade topography however remains largely unconstrained for the early cenozoic based on coupled carbonate clumped isotope 47 and oxygen isotope 18o measurements we reconstruct 18o values of eocene groundwater 18owater in the chumstick basin central washington today located in the cascade rain shadow 47 paleo thermometry indicates a systematic change in basin burial temperatures from 110 c to 70 c depending on burial depth in the basin these data are in good agreement with low t thermochronological and vitrinite reflectance data and further constrain the basin burial and exhumation history in concert with field observations microstructural analysis and 18o values of the analyzed carbonates we suggest that the 47 temperatures and 18o values reflect open system carbonate cement recrystallization in meteoric derived groundwaters during early burial diagenesis assuming open system behavior reconstructed mean 18owater values of 7 middle eocene to 9 late eocene early oligocene are consistent with a low elevation origin of the corresponding meteoric waters that permeated the sandstone conglomerate members of the eocene sedimentary units in light of the paleogeographic setting of the chumstick basin the reconstructed 18owater values agree well with pacific derived moisture that did not experience strong rainout the absence of a rain shadow effect therefore permits only moderate eocene oligocene elevations at least for the southern part of the washington proto cascades

do stable isotopes in carbonate cement of mio pleistocene himalayan sediments record paleoecological and paleoclimatic changes non pedogenic carbonates such as carbonate cement and nodules in the sandstones are quite common in the terrestrial geological record unlike pedogenic carbonates their stable isotope ratios lack investigations for paleo climatic reconstructions the present investigation therefore explores the possibility of use of stable isotope studies of non pedogenic carbonates from the mb pleistocene siwalik group of sediments exposed in the ramnagar sub basin of the nw himalaya petrographic studies reveal the dominance of micrite fabric in carbonate nodules both of pedogenic and non pedogenic samples irrespective of specific stratigraphic unit however calcite as cement in the sandstones shows the dominance of micrite fabric in the younger in age sediments seventy two non pedogenic carbonate samples from the carbonate nodules and cement in the siwalik sandstones ranging in age between similar to 1 ma and 12 2 ma were analyzed for delta c 13 and delta o 18 values the delta c 13 values vary from 24 77 parts per thousand to 1 1 parts per thousand and delta o 18 values vary from 15 34 parts per thousand to 7 81 parts per thousand pedogenic and non pedogenic carbonates ranging in age between similar to 1 ma and 6 ma have largely similar delta c 13 values and the range of delta c 13 values indicate the dominance of c 4 type of vegetation however unlike pedogenic carbonates which showed the dominance of c 3 type of vegetation pre 7 ma on the basis of delta c 13 depleted isotopic values singh et al 2011 delta c 13 values are largely enriched in the corresponding aged non pedogenic carbonates revealing no information on specific type of vegetation likewise paleoprecipitational reconstructions from delta o 18 values in pedogenic carbonates showed a progressive increase in aridity from similar to 12 ma to recent excluding short term increases in rainfall monsoon intensity at around 10 ma 5 ma and 1 8 ma singh et al 2012 on the contrary such reconstructions are not possible from the delta o 18 values of non pedogenic carbonates and indeed the delta o 18 values of non pedogenic carbonates are largely depleted to as much as 6 parts per thousand from the corresponding pedogenic carbonates such differences in delta c 13 and delta o 18 values of non pedogenic carbonates from pedogenic carbonates are primarily due to the dependence of the former on groundwater conditions responsible for precipitating carbonate further a comparison of isotopic values between non pedogenic and pedogenic carbonates can be interpreted that post 6 ma and pre 6 ma non pedogenic carbonates were largely formed by shallow and deep groundwater conditions respectively the result of these investigative studies therefore suggests that the stable delta c 13 and delta o 18 values of non pedogenic carbonates unlike the pedogenic carbonates and irrespective of nature of calcite fabric showed their little importance in paleoclimatic and paleoecological reconstructions c 2014 elsevier b v all rights reserved

thermal regime interreservoir compositional heterogeneities and reservoir filling history of the dongfang gas field yinggehai basin south china sea evidence for episodic fluid injections in overpressured basins the dongfang gas field located in a diapir structure zone is the largest gas field found in the yinggehai basin a strong thermal anomaly caused by hydrothermal fluid flows occurs in the gas field as evidenced from drill stem test and fluid inclusion homogenization temperatures rock eval tmax vitrinite reflectance and clay mineral transformation profiles such a thermal anomaly suggests focused rapid flow of deeply sourced hydrothermal fluids which has shifted the threshold depth to the onset of hydrocarbon generation upward by about 500 m the dongfang gas field shows considerable variation in nitrogen and carbon dioxide content with nitrogen content ranging from less than 5 to 31 2 and carbon dioxide content ranging from less than 1 to 88 9 hydrocarbon gases and associated condensates show high maturities and have been generated most probably from the meishan and sanya formations of miocene age carbon dioxide in gases with co2 content less than 10 is organic in origin whereas carbon dioxide in gases with co2 content higher than 10 is inorganic in origin and has been generated from high temperature decomposition of carbonates most gases display negative d15n values gases with nitrogen content higher than 15 always contain organic co2 co2d13c values lighter than 10o oo and the nitrogen contents decrease as the d13c values for methane and ethane become heavier suggesting an organic origin of the nitrogen gas generated in the catagenetic stage source rock ro 2 0 before the significant thermal decomposition of carbonate took place systematic interreservoir compositional heterogeneities occur in the gas field which along with the thermal regime and fluid inclusion homogenization temperature measurements give a clear suggestion of the reservoir filling history methane dominated gases with relatively high nitrogen content and a small amount of organic co2 accumulated first and carbon dioxide rich gases with hydrocarbon components of higher maturity injected into the reservoir later interreservoir compositional heterogeneities which have the advantage of being unaffected by in reservoir mixing processes can be effective indicators of the field filling history especially when they are studied in combination with fluid inclusion and reservoir diagenesis analyses the short lived transient nature of the thermal effect of fluid flow and the wide variation of the toluene n heptane values seem to suggest episodic fluid injections from the overpressured systems into the reservoirs

need for re apprehension of basin tectono depositional set up during initial stage of himalayan orogeny from pedogenic evidences abstract the collision and suturing of indian and eurasian plates about 50 ma ago closed the tethys ocean initiated himalayan orogen and a foreland basin developed towards its south the last phase of marine sedimentation in indian sub continent during paleocene eocene is represented by subathu formation overlain by continental dagshai sediments and nature of this transition named as passage beds sensu bhatia and mathur 1965 is controversial further it is poorly understood both in terms of depositional environment and age surprisingly these beds show evidences of pedogenesis but no such palaeopedological studies have been reported so far the degree and nature of pedogenesis is widely used in solving diverse geological problems in complex geological settings therefore pedogenesis of these beds exposed in the shimla hills of subathu sub basin has been worked out morphological and micromorphological studies divide them into older green greenish grey pedofacies gp and younger red reddish brown pedofacies rp units pedons are up to tens of centimeters thick structure largely comprises sub angular to sub rounded blocky peds a suite of pedogenic features include burrows root traces infillings mottling cutans segregations and nodules which vary in dominance from gp to rp unit greenish grey pedons are characterized by mottles fe mn oxide segregations whereas reddish brown pedons by root traces commonly filled with carbonates presence of carbonate segregations and cutans detailed field and laboratory studies at various local and regional exposures can reveal the nature and extent of pedogenesis which will be significant to enact depositional environment as well as stratigraphic position of passage beds co relatable at different localities throughout length of the himalaya from pakistan to nepal because of their distinctive lithology and stratigraphic position such studies bear global significance in demonstrating the use of palaeosols as a geologic archive in complex geological settings

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

overpressure retardation of organic matter maturation and petroleum generation a case study from the yinggehai and qiongdongnan basins south china sea three superimposed pressure systems developed in the yinggehai basin south china sea as indicated by seismic data well logs and direct pressure measurements the organic maturation profile is nonlinear with three nonparallel segments that correspond to the shallow normal pressured system the intermediate overpressured system and the deep strongly overpressured system respectively the intermediate and deep overpressured systems have abnormally low r0 gradients the organic maturity of these overpressured rocks is significantly lower than the maturity of normal pressured source rocks in nearby wells with similar thermal histories and does not match the thermal histories of the rocks such an organic maturity anomaly is distinctly different from those aused by variation in activation energies conductivity contrasts and hydrologic effects and is confirmed to be the result of overpressure retardation the degree to which the organic matter maturation is retarded expressed as the difference between predicted and measured vitrinite reflectance increases exponentially with increasing fluid pressure confirming that pressure increases the activation energies of organic matter maturation reactions overpressure retardation has been proven to be conditional and quite important for clearly understanding petroleum generation migration and accumulation in overpressured sedimentary basins

overcoming pitfalls of vitrinite reflectance measurements in the assessment of thermal maturity the case history of the lower congo basin abstract vitrinite reflectance has been always considered the most reliable thermal maturity indicator in sedimentary basins nevertheless the reliability of vitrinite reflectance measurements can be affected by many external factors that change from basin to basin e g organic facies temperature and pressure conditions leading to wide uncertainties in thermal modelling calibration and thus in the evaluation of timing and amount of hydrocarbons generation we demonstrate that in case of vitrinite suppression and or retardation in siliciclastic successions the classical sweeney and burnham kinetics do not match the actual thermal evolution of the basin in these cases thermal evolution can be successfully modelled using a complementary approach based on smectite illitization kinetics or more appropriate kinetics e g presro tm and t p ro to evaluate the effect of pressure on organic matter maturation where pressure data are available this approach was applied in the lower congo basin where 32 cuttings from a 5 km deep well drilled in the upper oligocene miocene malembo formation have been analysed to derive organic and inorganic constraints to model the basin thermal evolution vitrinite and bitumen reflectance measurements show values between 0 3 and 1 0 along the well whilst the illite content of mixed layer i s ranges between 35 and 88 passing from r0 to r1 to r3 stacking order with increasing depth present day temperature distribution through the well indicates a geothermal gradient of about 40 c km and measured pressures show an overpressured interval between 3 and 4 km depth retardation and suppression phenomena affected vitrinite reflectance in this interval but did not influence smectite illitization the comparison between thermal maturity scenarios constrained by organic and inorganic thermal indicators shows differences in the calculation of the depth of the onset of hydrocarbon generation that is overestimated of about 500 m in the lower miocene interval when only suppressed retarded vitrinite reflectance values are used to calibrate models

the effect of oil expulsion or retention on further thermal degradation of kerogen at the high maturity stage a pyrolysis study of type ii kerogen from pingliang shale china high maturity oil and gas are usually generated after primary oil expulsion from source rocks especially from oil prone type i ii kerogen however the detailed impacts of oil expulsion or retention in source rock on further thermal degradation of kerogen at the high maturity stage remain unknown in the present study we collected an ordovician pingliang shale sample containing type ii kerogen the kerogens which had previously generated and expelled oil and those which had not were prepared and pyrolyzed in a closed system to observe oil expulsion or oil retention effects on later oil and gas generation from kerogen the results show that oil expulsion and retention strongly impacts on further oil and gas generation in terms of both the amount and composition in the high maturity stage gas production will be reduced by 50 when the expulsion coefficient reaches 58 and gas from oil expelled kerogen less oil retained is much drier than that from fresh kerogen the oil expulsion also causes n alkanes and gas compounds to have heavier carbon isotopic compositions at high maturity stages the enrichment of 13c in n alkanes and gas hydrocarbons are 1 and 4 6 respectively compared to fresh kerogen oil expulsion may act as open system opposite to the oil retention that influences the data pattern in crossplots of 13c2 13c3 versus c2 c3 13c2 13c3 versus 13c1 and 13c1 13c2 versus ln c1 c2 which are widely used for identification of gas from kerogen cracking or oil cracking these results suggest that the reserve estimation and gas source correlation in deep burial basins should consider the proportion of oil retention to oil expulsion the source rocks have experienced

tectonics or climate what drove the miocene global expansion of c4 grasslands investigations into the most plausible causes which triggered the late miocene global expansion of c4 grasslands have reached no consensus and are still being debated the global decline in co2 levels in the late miocene has been thought to be the most common driver although c4 grassland expansion was largely confined to the late miocene their first appearance varied significantly in different parts of the old and new world and therefore emphasizes role of local and or regional factors may or may not be in combination with one or more global factors the dynamic himalayan orogen constitutes a significant global tectonic event that is believed to have affected global climate hence a study of the himalayan foreland basin sediments could help in gleaning out possible causes behind this major paleoecologic event which could perhaps be directly or indirectly related stable carbon isotope analyses of a total of 141 pedogenic carbonates in the mio pleistocene siwalik paleosols of the ramnagar sub basin are coherent providing a better insight into paleovegetational changes across the sub basin on temporal and spatial scales furthermore paleovegetational history during oligocene is reconstructed from dagshai paleosols exposed in the subathu sub basin the isotopic results show dominance of c3 vegetation pre 7 ma and dominance of c4 vegetation post 5 ma percentage abundance of c4 vegetation was less than 20 pre 7 ma but increased to more than 40 post 5 ma reaching up to 100 in the youngest analyzed sediments there is exclusive dominance of c3 vegetation during oligocene these results conform to the pattern of change in vegetation documented in other parts of the himalayan belt the global expansion of c4 grasslands largely during late miocene have long been linked with then climate changes particularly brought by declining atmospheric co2 level large scale fires intensification of monsoon seasonality and aridity as demonstrated by various researchers these major hypotheses explaining expansion of c4 grasslands during late miocene are not very convincing due to significant shortcomings associated with them for example expansion of c4 grasslands in non monsoonal regions put serious constraints on the monsoonal intensification hypothesis likewise aridity as being the primary driver of c4 grassland expansion seems hard to support since there is no substantial evidence that could suggest global onset of arid conditions at about same geologic time and some most arid periods like the triassic show no evidence of c4 plants we suggest that the initial lowering of co2 below 450 ppmv created an environment for the beginning of c4 vegetation but the persistence of this threshold value for a considerable time during late miocene appears to be the probable cause of the late miocene global expansion of c4 grasslands irrespective of their time of first appearance it has been deduced through the correlation of himalayan tectonic events with atmospheric co2 levels and paleovegetational changes since upper miocene times that it was indeed the late miocene continuous intense tectonic instability of the himalayas that significantly decreased atmospheric co2 levels and which perhaps played a key role in changing the nature of photosynthetic pathways

occurrence and origin of carbon dioxide in the fushan depression beibuwan basin south china sea abstract natural gases produced from the fushan depression beibuwan basin south china sea are generally rich in carbon dioxide the 13 c values of methane in these gases are within the range of 41 18 to 47 00 this together with the linearity of the 13 c values of methane to butane in the natural gas plots indicates that the hydrocarbon gases are of organic origin the 13 c co 2 values of these gases are in the range of 5 1 to 7 5 suggesting an inorganic origin a dominant mantle source for the co 2 in these gases was determined on the basis of a simple binary mixing model using the 13 c co 2 values and 3 he 4 he ratios of associated helium in these gases the primary geological controls on the occurrence of co 2 rich gas accumulations in the fushan depression are discussed following a comparison with other sedimentary basins in the region

mio pliocene monsoonal record from himalayan foreland basin indian siwalik and its relation to vegetational change carbon and oxygen isotope ratios of soil carbonate nodules and carbon isotope ratio of associated organic matter were measured from three indian siwalik stratigraphic successions in order to reconstruct vegetational history and change in contemporaneous precipitation 13c values of soil carbonates show that from 10 5 to 6 ma the vegetation was c3 type and around 6 ma c4 grasses appeared the 18o variations of soil carbonates suggest that the monsoon system intensified with one probable peak at around 10 5 ma and a clear onset at 6 ma with peak at 5 5 ma after 5 5 ma monsoon strength decreased and attained the modern day condition with minor fluctuations which is supported by marine proxy of upwelling in the arabian sea and sedimentary morphology in siwalik the covariation between 18o and 13c data suggests that a change in precipitation pattern was partly responsible for expansion of c4 grasses in a mixed c3 c4 environment estimation of abundance of c3 and c4 plants using soil carbonate in one case and residual soil organic matter in another may differ this can be explained by assuming that the plant respired co2 is the main contributor of carbon in soil carbonate and may have different isotopic composition from that of the residual organic matter in the soil in addition the abundance estimate of c3 c4 plants also shows variation with time probably caused by change in growing season conditions through time

the stable isotope composition of early ground water cements from sandstone in paleoecological reconstruction abstract the carbon isotope composition of soil carbonates in paleosols records the global expansion of cm4 biomass in the late miocene however soil carbonate is not present in many geological sections or if present is often difficult to distinguish from other carbonate cements here we report on the 13c values of sparry early ground water cements in sandstones from the siwalik group in pakistan that record like the soil carbonates from the same sections the dramatic late miocene expansion of c4 vegetation the early cements are confined to densely cemented nodules in the sandstones but can also be found reworked as nodules into adjacent paleo channels demonstrating their formation shortly after burial the early cements are isotopically distinct from later higher temperature calcite cements our results demonstrate that some early ground water cements can be used in a manner similar to soil carbonates to reconstruct vegetation and perhaps pco2 changes in the past

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

india eurasia collision chronology has implications for crustal shortening and driving mechanism of plates the motion of the indian plate is determined in an absolute frame of reference and compared with the position of the southern margin of eurasia deduced from palaeomagnetic data in tibet the 2 600 900 km of continental crust shortening observed is shown to have occurred in three different episodes subduction of continental crust intracontinental thrusting and internal deformation and lateral extrusion the detailed chronology of the collision and plate reorganizations in the indian and pacific oceans supports the hypothesis that slab pull is a dominant driving mechanism of plate tectonics

the structure and formation of diapirs in the yinggehai song hong basin south china sea abstract the occurrence of shale diapirs in the yinggehai song hong ygh sh basin is well documented as is their association with big petroleum fields in order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high resolution two and three dimensional seismic reflection data as well as drilling data that cover the diapirs in ygh sh basin as many as 18 diapirs were identified and are arranged in six n s striking vertical en echelon zones on seismic reflection sections gas chimney structures diapiric faults and palaeo craters are genetically linked with the process of diapirism here we use geophysical and geological observations to propose a three stage model for diapirism initiation emplacement and collapse during these three stages different diapiric structure styles are formed which we describe in detail these include buried diapirs piercing diapirs and collapsed diapirs we link the diapirism to activity on the offshore continuation of the red river fault as shown on our high resolution seismic reflection data which is also related to a high paleogeothermal gradient caused by crustal thinning we also recognize the role of loading by the very large volume of sediment eroded from the edges of the tibetan plateau and delivered by the red river to the basin

tectono geomorphic and environmental set up deduced during deposition of mio pleistocene sediments in nw himalaya india detailed pedofacies characterization along with lithofacies investigations of the mio pleistocene siwalik sediments exposed in the ramnagar sub basin have been studied so as to elucidate variability in time and space of fluvial processes and the role of intra and extra basinal controls on fluvial sedimentation during the evolution of the himalayan foreland basin hfb dominance of multiple moderately to strongly developed palaeosol assemblages during deposition of lower siwalik similar to 12 10 8 ma sediments suggest that the hfb was marked by upland set up of thomas et al 2002 activity of intra basinal faults on the uplands and deposition of terminal fans at different times caused the development of multiple soils further detailed pedofacies along with lithofacies studies indicate prevalence of stable tectonic conditions and development of meandering streams with broad floodplains however the middle siwalik similar to 10 8 4 92 ma sub group is marked by multistoried sandstones and minor mudstone and mainly weakly developed palaeosols indicating deposition by large braided rivers in the form of megafans in a lowland set up of thomas et al 2002 significant change in nature and size of rivers from the lower to middle siwalik at similar to 10 ma is found almost throughout of the basin from kohat plateau pakistan to nepal because the himalayan orogeny witnessed its greatest tectonic upheaval at this time leading to attainment of great heights by the himalaya intensification of the monsoon development of large rivers systems and a high rate of sedimentation hereby a major change from the upland set up to the lowland set up over major parts of the hfb an interesting geomorphic environmental set up prevailed in the ramnagar sub basin during deposition of the studied upper siwalik similar to 4 92 to 1 68 ma sediments as observed from the degree of pedogenesis and the type of palaeosols in general the upper siwalik sub group in the ramnagar sub basin is subdivided from bottom to top into the purmandal sandstone 4 92 4 49 ma nagrota 4 49 1 68 ma and boulder conglomerate 1 68 ma formations on the basis of sedimentological characters and change in dominant lithology presence of mudstone a few thin gravel beds and dominant sandstone lithology with weakly to moderately developed palaeosols in the purmandal sandstone fm indicates deposition by shallow braided fluvial streams the deposition of mudstone dominant nagrota fm with moderately to some well developed palaeosols and a zone of gleyed palaeosols with laminated mudstones and thin sandstones took place in an environment marked by numerous small lakes water logged regions and small streams in an environment just south of the piedmont zone perhaps similar to what is happening presently in the upland region the upper gangetic plain this area is locally called the trai region pascoe 1964 deposition of boulder conglomerate fm took place by gravelly braided river system close to the himalayan ranges activity along the main boundary fault led to progradation of these environments distal ward and led to development of on the whole a coarsening upward sequence c 2014 elsevier b v all rights reserved

characteristics and origin of the gas and condensate in the yinggehai basin offshore south china sea evidence for effects of overpressure on petroleum generation and migration three superimposed pressure systems have developed in the yinggehai basin the strongly overpressured rocks have abnormally low vitrinite reflectance ro and rock eval tmax values and display abnormal c15 hydrocarbon distributions such organic maturity anomalies were confirmed to be the result of overpressure retardation commercial gas fields with carbon dioxide content up to 93 and gas shows with nitrogen content more than 50 have been found in the basin methane and carbon dioxide dominated gases have low c2 hydrocarbon contents less than 4 and heavy methane 13c and d values and the associated condensates have high diamondoid indices and very low c25 saturated hydrocarbon concentration at the same time the methane and carbon dioxide dominated gases have high branched and cyclic light hydrocarbon contents and associated condensates have abnormally low heptane and isoheptane values low sterane isomerization values and abnormally high concentrations of isoprenoid hydrocarbons relative to adjacent n paraffins these distinct characteristics seem to result from the effects of overpressure on both hydrocarbon generation and migration

palaeovegetational reconstruction in late miocene a case study based on early diagenetic carbonate cement from the indian siwalik abstract carbon isotope ratio of early diagenetic carbonate cement edcc from sandstones was measured from mohand rao age 9 to 4 5 ma and haripur khol 6 to 1 8 ma sections of siwalik group of india to reconstruct palaeovegetation the 13 c of cement from mohand rao section varies from 10 5 to 0 2 with progressive increase in values from 9 to 7 3 ma indicating gradual change of existing c 3 type vegetation to c 4 type vegetation post 7 3 ma the 13 c value is anchored around zero per mil indicating mixed c 3 c 4 environment with c 4 dominating the ecosystem in haripur khol section the 13 c value of edcc from sandstone indicates presence of both c 3 and c 4 type of plants with dominance of c 4 in the ecosystem which corroborates the results of our previous study based on carbon isotope ratio of soil carbonates the oxygen isotope ratio of edcc from sandstones does not show any systematic variation with time the 18 o values of edcc from mohand rao section ranges from 8 9 to 13 6 and in haripur khol section from 9 9 to 13 6 at a given stratigraphic level the average 18 o value of edcc of sandstones is lower up to 4 compared to the average 18 o of soil carbonate from the same or adjacent level using our earlier published data from haripur khol section the depletion in 18 o of edcc may be due to contribution of contemporary river water infiltrating the groundwater system in the post monsoon period carbon and oxygen isotope ratio of soil carbonate nodules and carbon isotope ratio of associated organic matter from the same nodules were also measured from the mohand rao section in a few cases n 9 from 9 to 8 ma the carbon isotope ratio of soil carbonates varies from 10 8 to 7 8 indicating dominance of c 3 type vegetation in the flood plain in contrast from 5 4 to 4 8 ma the 13 c ranges from 0 1 to 4 3 indicating that the vegetation consisted of mixed c 3 c 4 plants with c 4 dominating the ecosystem the carbon isotope ratio of the organic matter from the same soil carbonate nodules ranges from 25 2 to 24 4 from 9 to 8 ma and 17 4 to 24 6 from 5 4 to 4 8 ma corroborating the above results the average 18 o value of soil carbonate nodules for the time period 9 to 8 ma is 8 8 for 5 4 to 4 8 ma the value is 7 9 these average 18 o values are comparable with previously published oxygen isotope results of soil carbonates from haripur khol and kangra valley

hierarchies of overpressure retardation of organic matter maturation case studies from petroleum basins in china the effects of overpressure on different aspects of organic matter maturation have been examined in three basins using multiple parameters overpressure in the yinggehai basin has retarded kerogen maturation hydrocarbon generation and thermal cracking of long chain normal hydrocarbons as well as evolution of isoprenoid hydrocarbons overpressure in the dongpu depression bohai bay basin seems to have retarded the maturation of bulk kerogens reflected by rock eval tmax the thermal cracking of long chain normal hydrocarbons and the evolution of isoprenoid hydrocarbons but seems to have had no detectable effects on vitrinite reflectance overpressure in the qiongdongnan basin has had no detectable retardation effects on all aspects of organic matter maturation the observed phenomena suggest differential retardation of organic matter maturation that is different organic matter maturation reactions and different maturity parameters have been retarded by overpressure to different degrees in the same overpressured system and four hierarchies of overpressure retardation hierarchy i retardation is defined as the state where the overpressure has retarded all aspects of organic matter maturation and is proposed to have been caused by the combination of high formation pressure and excess pressure a high water organic matter ratio and strong retention of reaction products in an early developed strong persistent overpressured system hierarchy ii retardation refers to the case in which all aspects of organic matter maturation except the maturation of hydrogen poor kerogens especially vitrinite have been retarded and seem to have been caused by early developed moderate to strong persistent overpressure hierarchy iii retardation refers to the situation where overpressure has retarded only the thermal cracking of liquid hydrocarbons but has had no detectable effect on the maturation of both hydrogen rich and hydrogen poor kerogens and is proposed to have been caused by moderate to strong persistent overpressures that began to develop around peak oil generation hierarchy iv retardation is defined as the situation where overpressure has had no detectable retardation on all aspects of organic matter maturation and may have been caused by cases wherein the overpressure developed too late the intensity of the overpressure was too low the duration of the overpressure was too short or overpressured fluids had been expelled frequently resulting from both the diversities of organic matter maturation reactions and the complexities of overpressure generation and development the hierarchies of overpressure retardation in different basins may be quite different and it is necessary to use multiple parameters to understand the evolution of the petroleum systems in overpressured basins

depositional origin of tuffaceous units in the pliocene upper siwalik subgroup jammu india nw himalaya during late pliocene times extrabasinally derived acidic volcanic ashes were deposited as distal pyroclastic fallout in upland interfluvial lacustrine settings in the northwestern part of the siwalik basin these ash beds occur as a bentonitized tuff band and tuffaceous mudstones in the jammu region of india we located and described 12 outcrops of this conspicuous geographically widespread bentonitized tuff band and tuffaceous mudstone association at the same stratigraphic level coincident with the gauss matuyama boundary at 2 48 ma this bentonitized tuff band and tuffaceous mudstone association represents a stratigraphic marker horizon in the diachronous and sporadic vertebrate fauna yielding siwalik strata the claystone siltstone and sandstone units and embedded bentonitized tuff band and tuffaceous mudstone beds represent a coarsening upward sequence reflecting deposition in lacustrine proximal and distal bottomsets pro delta foresets and mouth bar facies capped by fluvial topsets this study reports a chain of four contemporaneous palaeolake basins at this stratigraphic level which ranged in length from 2 to 7 km

classification and thermal history of petroleum based on light hydrocarbons abstract classifications of oils and kerogens are described two indices are employed termed the heptane and isoheptane values based on analyses of gasoline range hydrocarbons the indices assess degree of paraffinicity and allow the definition of four types of oil normal mature supermature and biodegraded the values of these indices measured in sediment extracts are a function of maximum attained temperature and of kerogen type aliphatic and aromatic kerogens are definable only the extracts of sediments bearing aliphatic kerogens having a specific thermal history are identical to the normal oils which form the largest group 41 in the sample set this group was evidently generated at subsurface temperatures of the order of 138 149 c 280 300 f defined under specific conditions of burial history it is suggested that all other petroleums are transformation products of normal oils

the distribution and discrimination of shallow authigenic carbonate in the pliocene pleistocene palomas basin southern rio grande rift nine types of authigenic carbonate are present in the pliocene pleistocene alluvial fan and axial fluvial sediment of the palomas half graben in the southern rio grande rift pedogenic and other vadose carbonate includes 1 pedogenic carbonate of stage ii and stage iii morphology underlying bw and bt horizons 2 mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone 3 pedogenic calcic nodules and tubules in eolian sand and 4 gully bed cement of proximal fan conglomerates formed by infiltration and evaporation of surface runoff shallow groundwater carbonate exists as 5 thin 30 50 cm massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe 6 thick 1 5 3 m massive beds deposited by lateral flow of groundwater or at springs and 7 thin 30 cm calcified root mats associated with near surface water saturated sediment or springs phreatic spar cements 8 occupy the interstices of conglomerates and sandstones and locally exist as 9 oriented concretions groundwater carbonates are best developed near the toes of the large hanging wall derived alluvial fans whereas phreatic cement preferentially exists in footwall derived alluvial fan conglomerates pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons a predominantly vertical arrangement of root traces peds desiccation cracks and calcic tubules and by gradational contacts the 13 c and 18 o values are similar among pedogenic and shallow groundwater carbonate although locally nodules in eolian sand and gully bed cement have higher 18 o values perhaps due to the effects of evaporation some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of 13 c and 18 o authigenic carbonate in footwall derived alluvial fan sediment has consistently higher values of 13 c than that in hanging wall derived sediment which may reflect differences in vegetative type and or density on either side of the basin

carbon isotopic composition of individual light hydrocarbons evolved from pyrolysis of source rocks from ying qiong basins china abstract light hydrocarbons are an important part of natural gas and are commonly used in gas source rock correlation this study determines carbon isotopic compositions of individual light hydrocarbons generated from three potential source rocks pyrolyzed at different temperatures by using gc irms technique with combined glass tube pyrolysis the results indicate that for the natural gases from the ying qiong basins it is difficult to further identify the origin of gas in terms of carbon isotope data alone light hydrocarbons from pyrolysates can be divided into two main generation stages at the lower temperature stage light hydrocarbons are formed directly from primary cracking of kerogen and their carbon isotopic distributions accord with isotope kinetic fractionation at the higher temperature stage isotopic distributions of light hydrocarbons become more complex due to the mixing of light hydrocarbons with different generation mechanisms e g secondary cracking of heavy hydrocarbons in addition experimental results also prove that an accumulative effect may be one of major factors that cause the part reversal in carbon isotope ratios among light hydrocarbon components from the ying qiong basins

expansion of c4 grasses in the late miocene of northern pakistan evidence from stable isotopes in paleosols stable isotopic clay mineralogic and bulk chemical analyses were conducted on paleosols of the neogene siwalik sections in northern pakistan in order to reconstruct floodplain environments over the past 17 ma the stable carbon isotopic composition of soil carbonate mean 13c pdb 10 2 and associated organic matter mean 13c pdb 24 1 in paleosols representing 17 7 3 ma reveal that floodplain vegetation was dominated by c3 plants at 7 3 ma a shift toward more positive carbon isotopic values began signaling the gradual expansion of c4 grasses onto the floodplain from 6 ma to present carbon isotopic values for paleosol carbonate mean 13c pdb 0 6 and organic matter mean 13c pdb 14 4 are uniformly enriched in 13c indicating the presence of nearly pure c4 grassland the scarcity of kaolinite and abundance of smectite and pedogenic carbonate in most paleosols suggest that rainfall in the region remained 1 0 1 25 m yr or less for the entire 17 ma of record paleosols in the lower portion of the section lack organic a horizons but have reddish b horizons often containing secondary iron oxide nodules leaching depths of soil carbonate in these older paleosols are typically greater than those in the plio pleistocene part of the section where organic a horizons are common and b horizons are markedly more yellow the combined evidence suggests that the mature paleosols in the pre 7 3 ma part of the record are dominantly calcareous alfisols or mollisols that once underlay nearly pure c3 vegetation perhaps trees and shrubs while calcareous mollisols underlying c4 grassland dominate the upper part of the record the carbon and oxygen isotopic trends in the paleosol record in pakistan are also evident in the diet of fossil mammals and in paleosols from nepal thus demonstrating that these paleoenvironmental changes in floodplain vegetation may be continent wide local effects such as the development or intensification of the asian monsoon driven by uplift of the tibetan plateau may have led to the expansion of c4 grasses if however the expansion of c4 grasses proves globally synchronous then a larger scale cause such as a marked decrease in co2 may be the driving mechanism

early diagenesis atherton formation quaternary a guide for understanding early cement distribution and grain modifications in nonmarine deposits abstract the atherton formation represents an extensive sheet of outwash and related materials of quaternary age these sand q77f8r15 and gravel sediments are locally lithified and present an excellent opportunity for evaluating the effects of early diagenesis on a terrigenous deposit the principal cement present is calcium carbonate 98 occurring as euhedral to anhedral grain contact pore lining and porefilling varieties limonite and gypsum are present as rare cements cement distribution was controlled at various levels by 1 stratification character 2 grain size and sorting 3 framework grain composition and 4 proximity to near surface processes initial cement development and subsequent early growth were primarily parallel to laminae within beds this precipitation site preference is related to initially higher relative permeabilities estimated at 2 to 2 5 x parallel versus normal to stratification when associated with trough cross strata cements preferentially are located in the lower portion of troughs or directly below trough set boun aries there is a slight preference for early formed cements to be present along the axis instead of the flanks of a trough about 50 of pebble gravel and sandy pebble gravel units contain locally cemented zones while 17 of finer grain size strata are locally lithified over 95 of locally cemented units are within 7 m of the present ground surface suggestive of a causal relation to near surface perhaps soil related processes the sources for the various constituents composing cements are related to the dissolution of previously cemented layers and framework components such as carbonate rock fragments calcareous pelitic rock fragments and iron bearing silicate grains the examination of oversized voids elongate pores and intraparticle porosity suggests that at least locally multiple episodes of cementation followed by dissolution have taken place reactions between carbonate cement and limestone rock fragments have destroyed a high percentage of detrital carbonate grain boundaries such reactions combined with early dissolution can result in an information loss in excess of 50 of the limestone rock fragment component in a sandstone a combination of organic acids and co2 mixed with meteori waters is likely responsible for the dissolution and transportation of cement constituents

geology compositional heterogeneities and geochemical origin of the yacheng gas field qiongdongnan basin south china sea the yacheng gas field is located in the footwall of the no 1 fault the boundary fault between the yinggehai and qiongdongnan basins the main reservoir is the fan delta sandstones in the lingshui formation the seals are meishan for m ation shales near the no 1 fault and lingshui formation shales away from the no 1 fault all strata are normally pressured in the gas field except for the meishan formation the meishan formation is overpressured near the no 1 fault in the gas field and in the adjacent yinggehai basin away from this fault into the qiongdongnan basin the overpressure diminishes an obvious thermal anomaly occurs below 3600 m in the gas field this anomaly characterized by an abrupt increase in drill stem test and fluid inclusion homogenization temperatures vitrinite reflectance ro and rock eval tmax and by an abnormally low temperature ro tmax gradient diminishes away from the yinggehai basin the gases and condensates have abnormally high aromatic hydrocarbon contents and show obvious heterogeneities away from the no 1 fault the c2 hydrocarbon content and c2 scn increase carbon dioxide content decreases d13c values for methane ethane and carbon dioxide become lighter the heptane and isoheptane values decrease and the relative contents of aromatic hydrocarbons both in c6 c7 light hydrocarbons and in the condensates decrease such heterogeneities reflect the reservoir filling process and origin of the gas field the gas field was charged from both the qiongdongnan and the yinggehai basins hydrocarbons sourced from the qiongdongnan basin have relatively low maturities whereas hydrocarbons from the yinggehai basin have relatively higher maturities and seem to have been in association with hydrothermal fluids the hydro thermal fluids from the yinggehai basin in which methane ethane carbon dioxide and especially aromatic hydrocarbons dissolved under the high temperature and high pressure subsurface conditions migrated along the no 1 fault and caused the abnormally high concentration of aromatic hydrocarbons as well as the thermal anomalies in the gas field especially near the no 1 fault

evidence for multiple stages of oil cracking and thermochemical sulfate reduction in the puguang gas field sichuan basin china the puguang gas field is the largest gas field found in marine carbonates in china marine carbonate reservoirs in this field were buried to a depth of about 7000 m 22 966 ft and experienced maximum temperature up to 220c before uplift to the present day depth of 5000 5500 m 16 404 18 045 ft with present day thermal maturity between 2 0 and 3 0 equivalent vitrinite reflectance ro sulfur rich pyrobitumens with reflectance up to 3 5 are widespread in the reservoirs and resulted from thermal cracking of crude oils most likely generated from upper permian source rocks and thermochemical sulfate reduction tsr natural gases in the puguang gas field have wide variations in nonhydrocarbon gas contents with h2s contents between 5 1 and 58 3 and co2 contents between 7 9 and 18 0 the hydrocarbon gases originated mainly from thermal cracking of accumulated oil but were altered by tsr thermochemical sulfate reduction in the study area exerted different effects on the isotope compositions of different hydrocarbon gas components at different tsr stages the differential increase of 13c values for different gas components reflects transformation from a heavy hydrocarbon gas dominated tsr stage to a methane dominated tsr stage this caused a decrease of 13cmethane13cethane values and a corresponding conversion from reversed to normal isotope distributions thermochemical sulfate reduction in the study area appears to have been limited by sulfate concentrations in the reservoirs a successive three stage tsr series namely liquid hydrocarbon involved tsr heavy hydrocarbon gas dominated tsr and methane dominated tsr occurred in reservoirs with sufficient sulfate concentration methane can be the dominant organic reactant for tsr but only at elevated temperature and after most c2 hydrocarbons are exhausted

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

a tracer study of the floridan aquifer in southeastern georgia implications for groundwater flow and paleoclimate distributions of stable isotopes of water radiocarbon noble gases and chloride c1 in groundwater from the upper floridan aquifer in southeastern georgia suggest that down gradient of the gulf trough this aquifer contains waters representative of both regional and local groundwater flow systems in this area locally recharged waters tend to remain near the top of the aquifer and do not mix substantially with the regional groundwater flow system noble gas temperatures suggest that this region of georgia was 4 0 0 6 c cooler during the last glacial period lgp similar temperature changes have been reported for southern texas and northern new mexico suggesting that the southern united states cooled uniformly during the lgp stable isotopes of water became enriched down gradient from the recharge area these enrichments which have been observed before appear to result from local influx of shallow groundwater into the regional aquifer system rather than representing a climate change signal an inland gradient of the stable isotope composition 0 60 0 14 100 km in 18o was found in young holocene water after correcting for the change in the stable isotope composition of the ocean during the lgp water that was recharged during the lgp was found to be slightly depleted in stable isotopes relative to modern recharge 18o 0 6 0 4 assuming the modern inland gradient the change in 18o is consistent with a shift in the coastline which was caused by the lower sea level during the lgp

stable isotopic studies of palaeosol sediment from upper siwalik of himachal himalaya evidence for high monsoonal intensity during late miocene abstract palaeosols from sedimentary section exposed 2800 m at haripur kolar on somb river in himachal himalaya were studied for stable isotopes of carbon oxygen and hydrogen along with sr isotopic ratios stable isotopic studies of middle and upper siwalik sections from pakistan and nepal region have been used for inferring the palaeoclimate of the region during early and late miocene time the aim of our work was to extend this type of study to himachal himalaya india carbon isotopes in palaeosol carbonate nodule and associated organic matter indicate an abrupt transient of c3 vegetation dominance around 4 ma this transient disrupted the general trend of c4 ecological dominance observed from about 8 to 4 ma in this region of asian subcontinent we attribute this observation with the higher monsoonal intensity which also is supported by different archives from the region g bulloides abundance from dsdp core from the arabian sea and grass pollens abundances on land we propose that greater moisture availability around 4 ma due to higher monsoonal activity caused an abrupt transient of c3 dominance in this region the proposition is further supported by our data on 18o of carbonates and d of clay showing depletion around 4 ma observed high 87sr 86sr ratios in carbonates around the same time period suggest increased weathering and in turn higher rainfall activity

oxygen isotopic composition of carbonate concretions from the lower cretaceous of victoria australia implications for the evolution of meteoric waters on the australian continent in a paleopolar environment oxygen isotopic data from carbonate cements in concretions have been used to infer the isotopic composition of meteoric fluids present at the time of concretion growth in terrestrial sediments that were deposited within the early cretaceous south polar circle at 75 80 s carbon and oxygen isotope compositions have been determined on over 135 samples of carbonate from 45 concretions taken from 24 localities aptian albian in age in the terrestrial sedimentary basins associated with the otway and strzelecki groups southeastern australia the carbonate cements include calcite having 26 4 13c 19 6and3 6 18o 29 6 or siderite having17 6 18o 30 8 calcite cemented concretions are more abundant and are interpreted to represent early near surface cementation events on the basis of textural evidence such as high 30 porosities at the time of cementation and mineralogical evidence such as the preferential preservation within concretions of labile detrital grains including plagioclase pyroxene and amphibole the oxygen isotopic data indicate that meteoric fluids with very low 18o certainly less than 15 and probably on the order of 20 were involved in the precipitation of the early calcites the extremely low 18o values of the fluids involved in the early diagenesis of both the otway and strzelecki groups suggest that the catchment area of the river system that carried sediments to these basins had a cold high latitude climate with mean annual temperatures less than 5 c and quite possibly below freezing by analogy with the relationship between modern18o distribution of meteoric fluids and climate these new data suggest that the early cretaceous polar regions may not have been ice free

maturation and source rock evaluation of kimmeridge clay norwegian north sea using a slightly modified version of the lopatin method of determining the level of thermal maturation of source rocks it is found that major hydrocarbon generation from the kimmeridge clay began in the late cretaceous early paleocene along the deeper parts of the viking and central grabens throughout the tertiary maturation progressed as subsidence and deposition continued as the maturation in the deep graben areas passed from the oil generation stage into wet gas and or condensate generation and finally into the dry gas stage the areas of strong active oil generation migrated from the graben areas into shallower areas toward the continental craton applying the results of the maturity calculations in conjunction with chemical analyses of the kimmeridge clay allows computations of the original generative capacity hydrogen index total organic carbon kerogen type and the volume of hydrocarbons generated the kimmeridge clay is mainly characterized by type ii kerogen with type i kerogen occurring in the deeper graben areas and type iii in the platform areas it is heterogeneous as a source with original generative capacities ranging from essentially zero to over 35 000 ppm and averaging about 13 300 ppm

geology and geothermal origin of grant canyon and bacon flat oil fields railroad valley nevada eastern nevada s grant canyon and bacon flat oil fields show strong evidence of formation in a still active moderate temperature geothermal system modern manifestations of this system include unusually elevated oil reservoir temperature at shallow depth 116 122 c at 1 1 1 6 km and dilute na hco3 cl thermal waters directly associated with hot oil hydrogen and oxygen isotopic compositions indicate that these thermal waters are meteoric in origin but were probably recharged prior to the holocene before 10 ka the waters apparently ascended to oil reservoir elevations after deep heating in response to the normal regional thermal gradient there is no evidence for a modern magmatic heat source the beginning of oil reservoir evolution at both fields is recorded by late stage fracture filling quartz in the vuggy brecciated paleozoic dolostone reservoir rocks oil and aqueous solutions were trapped as fluid inclusions in the quartz at temperatures comparable to those now prevailing in the reservoirs apparent salinities of the aqueous inclusions closely match the actual concentrations of current oil field waters and the quartz has the oxygen isotopic composition predicted for its crystallization from these waters at contemporary temperatures present day and fluid inclusion temperatures define essentially coincident isothermal profiles through and beneath the oil reservoir interval a phenomenon consistent with near constant convective heat transfer since inception of the eothermal system textural and mineralogic clues indicate that hot waters circulating in this system also increased porosity by dissolving carbonate minerals and helped seal reservoir margins by precipitating silica and kaolin more importantly the rising thermal waters may have aided oil transport and accelerated source rock maturation through an increase in the shallow 3 km local thermal budget along with the aforementioned fluid inclusion and isotopic evidence radiometrically dated life spans for numerous extinct geothermal systems epithermal ore deposits in the basin and range make it likely that the grant canyon bacon flat system and associated oil reservoirs are no older than 2 5 ma

development of asian monsoon revealed by marked ecological shift during the latest miocene in northern pakistan carbon isotopes from soil carbonate1 4 and soil organic matter5 6 yield palaeoecological information because the carbon in the soil carbonate forms in isotopic equilibrium with local soil co2 refs 1 7 the isotopic composition of which is in turn determined by local plant cover siwalik group sediments in northern pakistan contain a well exposed palaeosol record spanning the past 18 myr here we report on stable carbon isotope results from associated pedogenic carbonate which indicate a dramatic ecological shift from to dominated floodplain biomass beginning 7 4 7 0 myr ago the earlier c3 floodplain biomasses were probably mainly composed of trees and shrubs whereas c4 grasslands dominated in the plio pleistocene oxygen isotopes also exhibit a shift in the latest miocene probably corresponding to a major climate change which may have induced the forest to grassland transition this dramatic ecological shift in the latest miocene may mark the inception or a marked strengthening of the asian monsoon system

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

concretions in glacial sediments at seglvatnet norway abstract concretions in pleistocene glacial sediments at seglvatnet norway include a wide variety of forms the simplest of which apparently reflects relatively uniform growth rates in different directions the preferred orientation of others may be associated with water movement through the sediments concretion shape generally is influenced by the grain size and bedding of the enveloping sediments some concretions may have formed during or soon after deposition of the sediments others are forming at present

overpressure generation and evolution in lower paleozoic gas shales of the jiaoshiba region china implications for shale gas accumulation abstract overpressured shale gas plays have been found in the lower paleozoic marine shales of jiaoshiba region in the sichuan basin china the black shales from the upper ordovician wufeng group o3w and the lower part of lower silurian longmaxi group s1l are the producing reservoirs overpressure is widespread in the o3w s1l shales with the pressure coefficient up to 1 74 fluid inclusion microthermometric and raman spectroscopic techniques pvt and basin modeling are used to investigate overpressure generation and evolution in the o3w s1l shales the results show that thermocatalytic dry gas generation is the main genetic mechanism for overpressure development in the o3w s1l shales with specific characteristics as follows 1 the maximum burial depth of overpressured shales is more than 6200 m without anomalously low density and high primary porosity according to wireline log and core data 2 the over mature o3w s1l shales with the mean toc values ranging from 3 36 wt to 4 48 wt and the vitrinite reflectance values of 2 7 2 9 ro determined by kerogen ftir fourier transform infrared spectroscopy characteristics has excellent potential to produce dry gas 3 overpressured reservoirs are characterized by high gas saturations and the pressure coefficients positively correlate with gas saturation 4 the abnormally high pressures recorded in the fluid inclusions are coincident with high homogenization temperatures which is supportive of overpressure occurring during dry gas generation and provides convincing in situ evidence for gas generation overpressuring the calculated pore fluid pressures for methane inclusions within the maximum buried o3w s1l shales are approximately 90 3 139 4 mpa suggesting that gas generation produced a maximum pressure gradient of 23 1 mpa km which is equivalent to the lithostatic magnitudes the overpressure evolution in the o3w s1l shales is noticed to have evolved through four stages a rapidly increasing overpressure caused by thermocatalytic gas generation 145 85 ma a rapid pressure release 85 61ma overpressure preservation 61 20 ma and a relatively slow pressure release 20 ma present during post generation evolution 85 ma present of the o3w s1l shales both the overpressure and pressure coefficients decreased with time due to the intensive tectonic uplifts and denudation and approximately 19 0 28 5 of the initial total gas content was continuously released until the present day overpressure state the study of overpressure evolution in the o3w s1l shales provides basic information for the further study of the conversion between adsorbed gas and free gas and the method and technique used in this study to stress the overpressure generation and evolution may be generally applied to other overpressured or normally pressured shale gas systems

mechanisms of shale gas storage implications for shale gas exploration in china this article reviews the mechanisms of shale gas storage and discusses the major risks or uncertainties for shale gas exploration in china at a given temperature and pressure the gas sorption capacities of organic rich shales are primarily controlled by the organic matter richness but may be significantly influenced by the type and maturity of the organic matter mineral composition especially clay content moisture content pore volume and structure resulting in different ratios of gas sorption capacity gsc to total organic carbon content for different shales in laboratory experiments the gsc of organic rich shales increases with increasing pressure and decreases with increasing temperature under geologic conditions assuming hydrostatic pressure gradient and constant thermal gradient the gsc increases initially with depth due to the predominating effect of pressure passes through a maximum and then decreases because of the influence of increasing temperature at greater depth this pattern of variation is quite similar to that observed for coals and is of great significance for understanding the changes in gsc of organic rich shales over geologic time as a function of burial history at an elevated temperature and pressure and with the presence of moisture the gas sorption capacities of organic rich shales are quite low as a result adsorption alone cannot protect sufficient gas for high maturity organic rich shales to be commercial gas reservoirs two models are proposed to predict the variation of gsc and total gas content over geologic time as a function of burial history high contents of free gas in organic rich shales can be preserved in relatively closed systems loss of free gas during postgeneration uplift and erosion may result in undersaturation the total gas contents lower than the sorption capacity and is the major risk for gas exploration in marine organic rich shales in china

geochemistry and origins of natural gases in the yinggehai and qiongdongnan basins offshore south china sea abstract four gas fields and a number of gas bearing structures have been discovered in the yinggehai and qiongdongnan basins offshore south china sea chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins biogenic gas thermogenic gas and gases with mixed origin thermogenic gases produced from the yacheng field are characterized by relatively high contents of benzene and toluene relatively low 13c values of toluene and high abundance of bicadinanes and oleanane in the associated condensates showing good correlation with the coal bearing sequence of the yacheng formation in the qiongdongnan basin in contrast the gases from the df1 1 and ld gas fields contain high amounts of n2 and co2 low to moderate amounts of benzene and toluene with relatively high 13c values of toluene these characteristics correlate well with the miocene neritic shales in the yinggehai basin analyses on potential source rock samples indicate excellent gas source potential for both the coal bearing sequence in the yacheng formation and the miocene neritic shales containing type iii ii2 kerogens as the result of recent rapid subsidence and sedimentation high temperature and overpressure systems are well developed in the yinggehai and qiongdongnan basins the rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata whereas the strongly overpressured systems led to retarded organic matter maturation postponing the time when the source rocks reached peak gas generation results of 1 d modeling indicate that the coal bearing yacheng formation in the yacheng sag reached peak gas generation during mid miocene pliocene time after the deposition of upper oligocene to lower miocene reservoir rocks the lower mid miocene marine shales reached their peak gas generation stage during the pliocene quaternary in the central yinggehai basin supplying abundant gases to charge the pliocene reservoirs more importantly focused episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time the gas source relationship for the yacheng field suggests dominantly short distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the qiongdongnan basin

foreland sedimentation in himalayan tectonic regime a relook at the orogenic process by raiverman v 2002 bishen singh mahendra pal singh publishers 23 a connaught place p o box 137 dehradun 248 001 india 378 pp price us 125

a review of heterogeneous materials and their implications for relationships between kinematics and dynamics in continents 1 the variation of mechanical properties as a function of depth in the lithosphere controls the relationship between surface deformation and whole lithosphere deformation where mechanical competence elastic strength or viscous stiffness is a continuous function with depth surface deformation can be used to constrain either force balance or rheological parameters for the lithosphere however where competence is vertically discontinuous the surface deformation and deformation at depth may have a range of relationships including complete independence therefore both inversions and forward models designed to test either rheology or stress state are highly sensitive to the a priori choice of a mechanical model

origin of nitrogen rich natural gases in the california great valley evidence from helium carbon and nitrogen isotope ratios abstract dry natural gases produced from the sacramento and northern san joaquin basins contain up to 87 nitrogen 15n measurements for gases with n2 8 vary from 0 9 to 3 5 methane 13c values vary from 61 to 15 3 he 4 he ratios indicate the presence of mantle derived helium in addition to radiogenic crustal helium and suggest that the gases may originate from mixing between three principal components a hydrocarbon rich gas with 3 he 4 he 0 1r a derived from the microbial and thermogenic alteration of sedimentary organic matter a hydrocarbonrich gas with 3 he 4 he 2 75r a derived from the localized pyrolysis of sediments by magmatic volatiles and a nitrogenrich gas with 3 he 4 he 1r a the nitrogen rich component is characterized by methane 13c less negative than 25 and n 2 ar ratios as high as 22 000 nitrogen concentrations tend to increase towards the basement along the shallow eastern margin of the valley suggesting that the nitrogen rich component is derived from metasedimentary rocks deep within the crust the coast ranges fraciscan complex which was thrust well beneath the great valley during late mesozoic and early cenozoic subduction appears to be the most likely source

palaeoprecipitation record using o isotope studies of the himalayan foreland basin sediments nw india abstract the himalayan foreland basin sediments were deposited during a crucial time period in the himalayan orogeny this period in the tertiary has become major focus of research because of the large scale uplift of the himalayan range which has greatly modified the climate of the asian continent the present south asian monsoonal climate is believed to be a consequence of himalayan uplift during this period also the himalayan orogeny comprises a major regional tectonic event of the earth hence the study of these himalayan foreland basin sediments are not only significant in revealing the palaeoclimatic history of the asian continent but also paves a way for understanding of the effects of himalayan orogeny on global climate here we report a palaeoprecipitation history of the ramnagar sub basin since miocene times and its regional correlation to other sub basins of the himalayan mountain belt an attempt has been made to use oxygen isotope ratios of pedogenic carbonates in siwalik palaeosols as proxy palaeo recorders to deduce the palaeoclimatic history of the jammu region in the ramnagar sub basin for the first time a total of 141 pedogenic carbonates have been analyzed from three siwalik sections covering a lateral stretch of approximately 40 50km along strike the 18 o values vary from 12 31 to 5 89 and when plotted against age indicate that 18 o values on average become 2 enriched from the oldest to the youngest analyzed sediments except for largely depleted 18 o values around 10 ma 5 ma and 1 8 ma this 2 increase in calculated 18 o values could have been possible by an 3 c increase in mean annual air temperature conversely the largely negative 18 o values indicate periods of intense rainfall or stronger monsoonal conditions an increase of 2 6 in 18 o values has been found from the 10 ma rainfall event to that at 1 8 ma which indicates a reduction of 173 mm in rainfall intensity and amount hence precipitation reconstructions indicate a progressive increase in aridity from 12 ma to recent excluding a short term increases in rainfall or monsoon intensity at around 10 ma 5 ma and 1 8 ma this pattern of palaeoprecipitation change is similar to the changes documented in other sub basins of the himalayan mountain range further monsoon intensification at around 10 ma 5 ma and 1 8 ma is also observable in various other proxy records thereby suggesting the role of significant regional tectonism in the evolution and intensification of the present south asian monsoon climate our results indicate that from the mid miocene to recent except for the periods of largely depleted 18 o values the average 18 o value increased by 2 the general increase in average 18 o values from lower to upper siwalik sediments can be explained by changes in both continentality and latitude of the moisture source it seems likely that this long term increase in 18 o values was due to the uplift of the himalayas and the tibetan plateau in the north which acted as a barrier to the passage of airmasses from central asia in the north thereupon with time restricting the source of airmasses to the south from the indian ocean

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

the influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis because fluid pressures are transient in sedimentary basins over geologic time the effect of increasing fluid pressure on organic matter metamorphism is difficult to determine and conflicting opinions exist concerning its influence properly performed aqueous pyrolysis experiments can closely simulate hydrocarbon generation and maturation in nature and thus offer an excellent way to study the influence of pressure such experiments carried out on the retort phosphatic shale member of the lower permian phosphoria formation type ii s organic matter at different constant temperatures demonstrated that increasing pressure significantly retards all aspects of organic matter metamorphism including hydrocarbon generation maturation and thermal destruction this conclusion results from detailed quantitative and qualitative analyses of all products from hydrocarbon generation from the c1 to c4 hydrocarbon gases to the asphaltenes and also from analyses of the reacted rocks we have documented that our aqueous pyrolysis experiments closely simulated natural hydrocarbon generation and maturation thus the data taken as a function of pressure have relevance to the influence of normal and abnormal fluid pressures as related to 1 depths and temperatures of mainstage hydrocarbon generation 2 the thermal destruction of deposits of gas or light oil or their preservation to unexpectedly high maturation ranks and 3 the persistence of measurable to moderate concentrations of c15 hydrocarbons in fine grained rocks even to ultra high maturation ranks

principles of geochemical prospect appraisal our current understanding of the geochemical processes responsible for the creation of commercial accumulations of oil and gas can be reduced to simple rules and equations to allow the petroleum geologist to use geochemistry to estimate before drilling the likely volumes and compositions of oil and gas delivered to the trap in most sedimentary basins oil is expelled from source rocks between 120 and 150 c whereas most gas and gas condensate are released between 150 and 230 c when the initial potential of source rocks exceeds 10kg mt oil expulsion efficiencies are between 60 and 90 expelled oil and gas migrate as petroleum rich phases driven by fluid potential gradients most flow is laterally updip in beds with effective horizontal permeabilities greater than 1 md in lower permeability rocks the petroleum fluids move vertically up or down along the path of least resistance i e the most permeable or thinnest beds separating them from a high permeability lateral carrier bed the residual saturations of petroleum left behind along the migration pathway are of the order of a few percent of the porosity geochemical analysis of source rocks helps determine the yields and compositions of petroleum fluids expelled in the catchment area of a prospect these fluid masses are converted to volumes from knowledge of the phase relations and densities of petroleum fluids evaluation of the total volume of rock through which the petroleum potentially can migrate and the mean porosity of the rocks that constitute the migration pathway allow migration losses to be assessed these losses constrain both the range of migration of petroleum fluids away from the mature source rock and the volumes of petroleum remaining for entrapment in the prospect

paleomagnetic chronology fluvial processes and tectonic implications of the siwalik deposits near chinji village pakistan a 2800 m section of siwalik strata containing the stratotypes for both the chinji and nagri formations has been dated by magnetic polarity stratigraphy and the observed polarity zonation has securely been correlated with the chron 17 7 segment of the time scale the base of the section is the base of the kamlial formation which occurs near the top of chron 17 18 3 m y the kamlial chinji formation boundary occurs in the middle of chron 15 14 3 m y the chinji nagri boundary near the bottom of chron 10 10 8 m y and the nagri dhok pathan boundary at the chron 8 9 boundary 8 5 m y the siwalik deposits near chinji village consist of four distinct classes of fluvial cycles each with a characteristic periodicity a first order of 10 7 years a second order of 10 6 years a third order of 10 4 10 5 years and a fourth order of 10 0 10 4 years the river system responsible for siwalik sedimentation flowed from west to east and had properties much like the modern ganges river syste

contrasting diagenetic histories of concretions and host rock lion mountain sandstone cambrian texas white elliptical calcite cemented concretion nuclei up to 2 m long contrast markedly in color composition and diagenetic history from more glauconite rich concretion rinds and from dark green glaucarenite host rocks concretion nuclei are placers of loosely packed trilobite carapaces and minor quartz and glauconite tightly cemented with an average of 58 calcite cement the nuclei are shell lag deposits that were cemented by calcite at the sea floor or after burial of a few meters concretion rinds composed of sub equal amounts of quartz and compactionally deformed glauconite have an average of only 32 cement which consists of minor quartz overgrowths and later pore occluding calcite cement the rinds underwent burial for several million years to tens of million years to depths of several hundred meters before they were cemented the host rock is predominantly glauconite with very minor quartz and calcite cement 87 sr 86 sr ratios of host rock calcite cement are variable 0 7084 to 0 7093 but the lowest value suggests precipitation during the middle ordovician in the absence of significant amounts of carbonate cement the host rock underwent complete dissolution of trilobite carapaces and maximum compaction with total loss of porosity through squashing of glauconite grains maximum burial during this stage was completed by the end of ordovician time carbon and oxygen isotopic values of calcite from 16 samples show no differences between concretion nuclei rinds and host rocks 13 c ranges from 2 2 to 2 9 pdb and probably reflects derivation of carbon from trilobite carapaces and other skeletal grains 18 o ranges from 4 6 to 6 2 pdb and reflects either the original composition of the cement and trilobites or the modification of original grains by meteoric dominated ground water

oriented concretions ionian coast italy evidence of groundwater flow direction abstract highly elongate aligned calcite cemented concretions are present in exposed shallow marine pleistocene sands and gravels along the basilicata coast of the ionian sea southern italy concretions are shaped like rods pencils and cigars 0 5 3 cm in diameter and thin blades 031 cm thick and 1 5 cm wide most have aspect ratios greater than 5 and some reach 30 most concretions are in parallel laminated beds but a few are in cross stratified and ripple cross laminated beds concretions cut across foresets and inclined laminae the standard deviations of long axes at each of six localities with more than eight measurements are less than 7 the standard deviation of the orientation of concretions from 30 localities over a 400 km2 area is less than 17 previous interpretations for elongate carbonate concretions in sandstones include growth in the direction of groundwater flow growth parallel with current oriented grains and or paleocurrents and selective cementation in the troughs of wave ripples the basilicata concretions are oriented perpendicular to the local coastline and parallel with the direction of present day groundwater flow the groundwater flow direction was probably the same during the pleistocene the evidence indicates that the concretions grew parallel with groundwater flow and were not influenced by sedimentary structures or grain fabric the isotopic values 13c and 18o range from 10 to 1 pdb and from 5 to 3 pdb respectively and textures of calcite cement in the concretions indicate that the calcite was precipitated from meteoric groundwater nannobacteria may have mediated precipitation of calcite

petroleum potential and history of yitong basin china abstract the yitong basin in northeast china is an asymmetric elongate pull apart graben it underwent intense subsidence during the tertiary forming four sub depressions filled with thick alluvial lacustrine clastic sequences the sedimentation of the basin is characteristic of rapid near source deposition during intermontane flooding the source rock consists of dark claystones and shales deposited in the eocene kerogens are mainly of types iib to iii with abundant non fluorescent amorphous organic matter the average total organic carbon is 0 8 1 4 the hydrogen index is about 80 150 mg g toc biomarker distributions in the extracts and oil show high proportions of c29 hopane and moretane tm sesquiterpanes and 4 methyl steranes but little gammacerane these data indicate important contributions from higher plants the yitong basin has an average thermal gradient of 38 c km reconstruction of its burial history shows that oil generation begins at 1900 2200 m and condensate is formed deeper than 4000 m the yitong basin produces condensate and paraffinic oils with volumetric oil gas ratios of 200 600 these oils are mature to overmature with c29 sterane ratios of 0 45 sterone ratios of 0 45 and 20s r s sterane ratios of 0 55 compared with the nearby coal bearing shulan depression the yitong basin has a higher subsidence rate but similar source organic matter

mantle helium in sacramento basin natural gas wells abstract helium isotope ratios in sacramento basin natural gas wells show a strong mantle signal the3he 4he ratios range from 0 11 times the atmospheric ratio 0 11 ra in the rio vista field to 2 75 ra in the moon bend field indicating that 1 to 34 of the helium is mantle derived 3he 4he versus ch4 4he ratios provide evidence of two component mixing between crustal and magmatic end members extrapolation of the linear regression line to ch4 4he 0 gives a hypothetical magmatic end member 3he 4he ratio of 3 84 ra half the typical mantle ratio this indicates that the magmatic end member may actually represent a mixture of mantle and crustal helium gases which deviate from the simple two component mixture can be explained by addition of pure methane radiogenic 4he or a high n2 he component with 3he 4he 0 6 ra to 1 0 ra the ch4 3he ratio of the magmatic end member remains poorly constrained 0 to 3 109 and one cannot rule out the possibility that a significant proportion of the methane in some fields may be of deep earth origin however fields with the highest 3he 4he ratios are associated with buried plio pleistocene intrusives which have up arched sediments to form hydrocarbon traps the methane in these fields may have been produced by rapid thermal alteration of the intruded sediment elsewhere the methane appears either to have migrated from deeply buried sediments in the western basin or to have been produced by local microbial activity

variations in vitrinite reflectance with organic facies examples from pennsylvanian cyclothems of the midcontinent u s a abstract significant variations in vitrinite reflectance r0 with organic facies have been demonstrated from pennsylvanian aged cyclothems of the midcontinent u s a thermal alteration index tai and rock eval tmax values remain relatively constant through these thin lithologic successions irregardless of facies and are in reasonable agreement with r0 measurements on interbedded coal organic rich black and phosphatic black shales which contain a significant component of liptinitic organic matter of presumed marine algal derivation display r0 values which are suppressed relative to the coal benchmark a distinct correlation between increased hydrogen index hi and liptinite content with suppressed reflectivities is apparent more oxic facies e g calcareous and fosiliferous gray shales and limestones often display r0 s that are considerably higher than for coal soxhlet extraction prior to maceration yielded no variation in reflectance compared to non extracted kerogens suggesting bitumen adsorption was not the cause of the apparent suppression in the organic rich black shales lower total organic carbon toc normalized bitumen contents and rock eval transformations ratios in liptinite rich facies suggest different reaction kinetics for kerogen types with terrestrial organic matter maturing at an earlier stage than hydrogen rich marine derived organic matter

depositional environment sediment provenance and oxygen isotope paleoaltimetry of the early paleogene greater green river basin southwestern wyoming u s a depositional process sediment provenance and paleotopography are critical to paleogeographic reconstruction during the coeval laramide and sevier orogenies in the cordillera orogenic system here we conduct lithofacies analysis sediment provenance and paleoelevation studies in the fluvial lower paleogene fort union and the main body of the wasatch formations in the center of the greater green river basin in order to reconstruct paleogeography the depositional environment changed from low energy floodplains and associated meandering river during the paleocene to braided rivers during the earliest eocene the lowermost eocene strata have more precambrian basement derived feldspar grains compared to the paleocene strata this change in sediment provenance was associated with the change to dominant northwestward paleoflow direction suggesting major uplift of the laramide uinta mountains during the earliest eocene the 18o values of authigenic carbonates were stable during the early paleocene but decreased gradually from 10 permil during the late paleocene to 18 permil during the earliest eocene suggesting the lack of highland precipitation in the basin catchment during the early paleocene and significant surface uplift and arrival of highland precipitation from the uinta mountains during the earliest eocene the reconstructed paleoelevation of the uinta mountains was at least 3 km possibly greater than 3 5 km and the paleoelevation of the greater green river basin floor was at most 1 km and the paleorelief between the greater green river basin and uinta mountains was 2 km during the earliest eocene our record of the earliest eocene establishment of high topography of the uinta mountains through renewed uplift is 4 to 6 myr older than previously thought if the carbonate cements in the fort union formation record the isotope compositions of surface water from the sevier orogen their high 18o values likely suggest that the sevier orogen had rugged topography during the paleocene

the equivalent elastic thickness te seismicity and the long term rheology of continental lithosphere time to burn out creme brulee insights from large scale geodynamic modeling abstract depending on the conditions and time scale the lithosphere exhibits elastic brittle plastic or viscous ductile properties as suggested by rock mechanics experiments a large part of the long term lithospheric strength is supported in the ductile regime unfortunately these data validated for strain rates 10 6 s 1 small scales few cm and simplified conditions cannot be univocally interpolated to geological time and spatial scales strain rates 10 17 10 13 s 1 100 1000 km spatial scales complex conditions without additional parameterization an adequate parameterization has to be based on real time observations of large scale deformation indeed for the oceanic lithosphere the goetze and evan s brittle elastic ductile yield strength envelopes derived from data of experimental rock mechanics were successfully validated by a number of geodynamic scale observations such as the observations of plate flexure and the associated t e equivalent elastic thickness estimates yet for continents the uncertainties of flexural models and of the other data sources are much stronger due to the complex structure and history of continental plates for example in one continental rheology model dubbed jelly sandwich the strength mainly resides in the crust and mantle while in another dubbed creme brulee the mantle is weak and the strength is limited to the upper crust these models have arisen because of conflicting results from distributed earthquake elastic thickness t e and rheology data we address these problems by examining the plausibility of each rheological model from general physical considerations we review the elastic thickness t e estimates and their relationship to the seismogenic layer thickness t s to show that these two quantities have no direct physical relation we also show that some of small t e must be artifacts of inconsistent formulation of the mechanical problem in some free air anomaly admittance models we point out that this does not necessarily detract from the admittance method itself but refers to its incorrect application in the continental domain we then explore by analytical and numerical thermo mechanical modeling the implications of a weak and strong mantle for tectonic structural styles we conclude that rheological models such as creme brulee which invoke a weak lithosphere mantle are generally incompatible with observations the jelly sandwich is in better agreement and we believe provides a useful first order explanation for the long term support of the earth s main surface features

geochemical composition and origin of tertiary oils in the yinggehai and qiongdongnan basins offshore south china sea abstract this study comprehensively characterizes the geochemical compositions of tertiary crude oils in the yinggehai and qiongdongnan basins offshore south china sea and differentiates their source properties using chemical and isotopic data the oils in the two basins have distinct source characteristics the majority of the oils from the yacheng yc field in the qiongdongnan basin are commonly marked by high pr ph ratios mostly from 4 5 to 7 0 unusually abundant angiosperm derived compounds and a domination of c29 in diasteranes suggesting a predominantly terrigenous organic source in contrast the oils from the dongfang df and ledong ld fields in the yinggehai basin have relatively lower pr ph values 2 5 5 5 and a low amount of bicadinanes moreover some oils in this basin contain abundant 4 methylsteranes and detectable c30 desmethyl steranes the difference in the molecular composition is also apparent with regard to the aromatic hydrocarbons the former oils contain more abundant biphenyls fluorenes and 2 phenylnaphthalene than the latter ones the oils from the qiongdongnan basin are relatively depleted in 13c and have 13c values of the whole oil in the range of 24 9 to 26 9 consistent with the great terrigenous input in contrast the yinggehai oils are isotopically heavier with 13c values from 21 6 to 23 6 typical of a marine isotope signature correspondingly their n alkane isotopes have different 13c ranges and exhibit distinct distribution profiles based on the oil source rock correlation the oils in the yc field were generated from the coastal plain swamp and shallow marine strata of the oligocene in the qiongdongnan basin while the yinggehai oils were sourced from the miocene shallow marine mudstones in the basin