methane released from groundwater the source of natural gas accumulations in northern west siberia the west siberian basin is the largest continental sedimentary basin on earth and contains about one third of the world s conventional reserves of natural gas from gas compositional and isotope data this gas can be described as a mixture of thermogenic gas from lower cretaceous rocks 13c1 36 c2 10 vol and almost pure methane 98 vol with the relatively light stable carbon isotope signature of 51 13c1 on average the proportion of the isotopically light methane increases from the lower cretaceous to the top of the cretaceous hydrocarbon system top of the pokur formation where the main reservoirs are situated the composition and the huge volume of gas is difficult to explain with the established models of gas generation in sediments the hydrogeology of the basin is determined by thick aquifer systems with an almost basin wide lateral continuity the largest hydrodynamic system is the cretaceous aquifer in which 98 of the natural gas is trapped the groundwater within the cretaceous aquifer is saturated with the same isotopically light methane as that found in the reservoirs of the pokur formation based on petrophysieal data a detailed hydrogeological model of the cretaceous aquifer beneath the largest gas accumulation the urengoy gas field has been developed and the contribution of gas release from groundwater was compared with the volume of gas in the reservoirs the potential methane release from groundwater has been evaluated as a result of 1 cooling of sediments in quaternary times 2 the lateral gradient of the hydraulic pressure under the gas field and 3 the pressure and temperature decrease due to cenozoic uplift the role of quaternary cooling is insignificant and according to our calculations the contribution of the effect by groundwater flow is small the volume of isotopically light methane released from groundwater due to a neogene uplift is very large and can explain the entire gas in place the source of this gas must be located south of the gas field province

burial history and kinetic modeling for hydrocarbon generation part i the galo model this paper discusses the principles and applications of the galo computer program which generates numerical simulations of the evolution of a sedimentary basin the program calculates variations in the sedimentary bed thickness predicts the thermal regime in the sedimentary cover or in the basement and estimates the potential hydrocarbon yield of the source rocks alternative methods for determining the magnitude of tectonic subsidence of the basement surface within the framework of a local isostasy model are applied to provide a detailed model of the sequential tectonic thermal events involved in the development of the basin the program determines reaction parameters controlling the maturation of the organic matter in source rocks these parameters are based largely on the hydrocarbon pyrolysis yield data and the stage of maturation of the organic matter entered into the program neglecting this stage drastically displaces the reaction spectrum toward higher energies leading to considerable underestimation of hydrocarbon output

geochemical characteristics of oils and source rocks in the yamal peninsula west siberia russia abstract eight oil and condensate samples and four samples of possible source rocks from the yamal peninsula northernmost west siberia have been characterized geochemically in order to determine source rocks and their maturity level the high content of c29 steranes m z 217 in extracts combined with geological data indicate nearshore marine and continental depositional environments for cretaceous argillites jurassic source rocks with a predominance of c27 steranes and pr ph ratios of 2 4 2 6 have been deposited in the deeper marine basin and contain oil prone type ii kerogen a number of parameters 20s 20s 20r steranes m z 217 22s 22s 22r hopane hopane moretane m z 191 ratios of tri and monoaromatic steroids m z 231 m z 253 and methylphenanthrene ratios show an increase in maturity with depth from immature cretaceous source rocks to moderately mature jurassic sources bazhenov and tyumen formations oil oil and oil source rock correlations based on genetic parameters and maturity indicators have demonstrated that fluids found in jurassic and cretaceous reservoirs including heavy biodegraded oil from the cenomanian belong to a single family and could be derived jointly from the bazhenov j3 and tyumen j1 2 formations the origin of naphthenic condensate found at shallow depth is still controversial although earlier work has suggested that biodegradation has been a dominant factor in the formation of naphthenic condensates their formation from cretaceous source rocks with type iii kerogen could also be possible

upper jurassic black bituminous shales in western siberia abstract the bazhenovskaya formation of volzkski tithonian berriasian age is one of the most interesting and unique sedimentary formations of the mesozoic section of the western siberian platform it is a shaly sequence characterized by a uniformity of lithology high content of organic matter broad regional distribution in excess of 1 106 km2 and relatively uniform thickness the formation shows high electrical resistivity up to 5000 and its natural radioactivity exceeds background radiations by 10 mes or more because of these features the bazhenovskaya formation is the leading marker horizon in the sedimentary cover on the west siberian platform

a new evaluation of palaeo heat flows and eroded thicknesses for the carboniferous ruhr basin western germany palaeo heat flows and thicknesses of eroded carboniferous overburden in the coal bearing ruhr basin western germany were estimated using one dimensional basin modelling techniques thermal and burial histories for 11 localities were calibrated by comparing measured and calculated vitrinite reflectance data based on the assumption of pre tectonic maturation in contrast to former studies the kinetic easy r0 approach applied for the calculation of vitrinite reflectance lopatin s tti algorithm was used in addition for one well in order to demonstrate the impact of the selection of the algorithm for vitrinite reflectance calculation on the simulation results simulated palaeo heat flows for the time of maximum burial most probably in the stephanian range from 64 to 83 mw m2 and decrease towards the south these heat flows are lower than values computed during earlier studies but are still relatively high in comparison to recent foreland basins the regional distribution of palaeo heat flows indicates that the crust beneath the ruhr basin was relatively thin and thickened towards the south at the end of the carboniferous after the stephanian 2200 3500 m of carboniferous overburden were eroded an observed southward increase in eroded thicknesses is caused by an uplift of the area studied which decreased towards the north at the end of the variscan folding large thicknesses of deposited and later eroded carboniferous sediments have to be postulated to explain the maturity data even along the present southern margin of the ruhr basin this indicates that the late carboniferous sediments originally extended far to the south geological arguments suggest that the deposition of carboniferous sediments in the entire ruhr basin continued until the stephanian

sources and mechanisms of formation of gaseous hydrocarbons in sedimentary rocks abstract accepted concepts of principal stages of hydrocarbon formation fail to satisfactorily explain the accumulation of supergiant gasfields in west siberia u s s r such as for example urengoy which comprise 30 of the proven gas reserves of the world a new mathematical model has been developed which applies the concept of activation energy distribution to the formation of gases characteristic differences in the molecular structure of humic and sapropelic source organic matter are expressed in different activation energy distributions the model allows the calculation of isotope values for different types of organic matter and their dependence on maturity application of the model to west siberia suggests that the supergiant gas accumulations in urengoy are more likely formed from terrestrial organic matter through thermochemical reactions at moderate depths and are not the result of either deep overmature or bacteriogenic processes the new mathematical model suggests that humic organic matter in general has a high methane generating capacity at comparably low maturities of organic matter equivalent to 0 5 0 7 on the vitrinite reflectance scale

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

geochemistry of selected oils and rocks from the central portion of the west siberian basin russia six analyzed oils produced from middle jurassic to upper cretaceous strata in the middle ob region of the west siberian basin show biomarker and stable carbon isotope compositions indicating an origin from the upper jurassic bazhenov formation the chemical compositions of these oils are representative of more than 85 of the reserves in west siberia kontorovich et al 1975 bazhenov sourced oil in cenomanian strata in the van egan field underwent biodegradation in the reservoir resulting in a low api gravity an altered homohopane distribution and the appearance of 25 norhopanes without alteration of the steranes high api gravity oil from the salym field has surpassed the peak of the oil window consistent with abnormally high temperatures and pressures in the bazhenov source rock from which it is produced the remaining oils are very similar including samples from valanginian and bathonian callovian intervals in a sequence of stacked reservoirs in the fedorov field bazhenov rock samples from the study area contain abundant oil prone marine organic matter preserved under anoxic conditions organic matter in a bazhenov core from the pokachev 58 well was compared with the oils because it is thermally mature and shows total organic carbon toc 13 8 wt and hydrogen index values hi 489 mg hc g toc representative of the average for the formation 10 7 wt for 840 samples 420 mg hc g toc for 75 samples other rocks in the middle ob region are far less likely to have generated these oils than the bazhenov formation no prospective source rocks older than middle jurassic were available geochemical analyses indicate a lack of thermally mature hauterivian or younger source rocks in the study area core samples from the lower cretaceous frolov formation either are thermally immature hauterivian or show little oil generative potential berriasian valanginian the upper jurassic vasyugan formation shows lower oil generative potential than the bazhenov formation the average toc for vasyugan rocks is 3 20 wt 240 samples average atomic h c ratios for vasyugan and bazhenov kerogens are 0 90 10 samples and 1 13 25 samples respectively the vasyugan formation cannot be excluded as a source rock because insufficient sample was available for biomarker analysis core from the lower to middle jurassic tyumen formation in the yemyegov 15 well was compared with the oils because it is thermally mature and shows toc and hi values 2 78 wt and 137 mg hc g toc respectively indicating slightly more favorable oil generative characteristics than the average for the formation 2 75 wt for 720 samples 95 mg hc g toc for 25 samples the core contains terrigenous gas prone organic matter that shows no relationship with the analyzed oils

extrapolation of the kinetics of oil and gas formation from laboratory experiments to sedimentary basins it is generally accepted that the formation of oil and gas from sedimentary organic matter or kerogen is adequately described by kinetic models based on first order kinetics with rate constants obeying an arrhenius equation these models which are extensively used in oil exploration to assess the petroleum potential of sedimentary basins are based on geological observations1 2 3 4 5 6 but are formally quite similar to models designed to simulate oil shale pyrolysis7 8 9 or coal processing10 11 12 the idea of blending these two approaches is not new see for example ref 2 but until now the most sophisticated attempts reproduced only the gross features of kerogen cracking on a geological timescale5 or were uncontrolled by geochemical measurements13 14 here we show that the kinetic parameters determined from the laboratory pyrolysis of an immature kerogen sample apply to the conditions of sedimentary basins altough the time and temperature scale are completely different

comparative organic petrology of interlayered sandstones siltstones mudstones and coals in the upper carboniferous ruhr basin northwest germany and their thermal history and methane generation in the coal mining ruhr area upper carboniferous rocks ca 4000 m consist of interlayered sandstones siltstones mudstones and coals they were deposited in a tropical paralic environment where alternating fluvial sedimentation occasional marine ingressions and swamp growth resulted in an irregular cyclic succession the total sedimentary package contains on an average 6 vol of organic matter about 70 vol of the organic matter occurs in coal seams the rest as dispersed organic matter in clastic rocks the organic matter is autochthonous in the coals and allochthonous in associated sandstones and siltstones it consists of about 70 vitrinite 20 inertinite and 10 liptinite the overall maceral group composition is the same for coals and dispersed organic matter this surprising similarity is caused by a nearly exclusive input of land plant derived organic matter to swamps and fluvial systems and a similar degree of preservation highest average liptinite contents of total macerals were found in unrooted mudstones highest average inertinite contents in coarse grained siltstones and highest average vitrinite percentages in sandstones

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

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

diffusion of methane and ethane through the reservoir cap rock implications for the timing and duration of catagenesis discussion in the july 1995 issue of this journal nelson and simmons 1995 presented a computation of diffusive losses from natural gas accumulations in their contribution they pointed out correctly that oil and gas fields must be considered as dynamic systems regrettably they omitted almost systematically all references to earlier work in which the dynamic character of natural gas accumulations on the geologic time scale was emphasized we acknowledge that the approach is generally well documented and the corresponding computations are consistent with the definitions we disagree however with certain aspects of the approach used by nelson and simmons 1995 as a conclusion of their case study on the mcclave gas field in southeastern colorado nelson and simmons 1995 suggested that high turnover rates less than 1 m y up to several million years due to diffusive losses are a characteristic feature of natural gas accumulations and they consequently postulated continuous and rapid gas generation as a prerequisite for the persistence of natural gas reservoirs however by using a rather arbitrary set of parameters as the basis of their calculations they depicted a scenario that must be considered as quite exceptional in terms of reservoir geometry and pay zone thickness their conclusionsmore on the dynamics of natural gas accumulations are therefore questionable and must be considered with caution before discussing this issue in detail we address two aspects of the computational approach used by nelson and simmons 1995 the definition of the effective diffusion coefficient and the method of calculating the fractional reduction of the diffusion coefficient less

reaction kinetics of gas generation in selected source rocks of the west siberian basin implications for the mass balance of early thermogenic methane abstract about one third of the actual global recoverable natural gas reserves is accumulated in the west siberian basin in several super giant and mega giant as well as numerous smaller gas fields the elucidation of the nature of this gas its mechanisms of formation and accumulation are still a matter of discussion for instance it is still not yet clear whether the enormous dry gas accumulations in cenomanian reservoirs are of microbial early thermogenic or catagenetic metagenetic origin or if they represent a mixture of these possible formation mechanisms the present contribution provides new results on the thermal gas generation properties of selected source rocks of the cretaceous pokur formation in the west siberian basin eight selected source rock core samples of the pokur formation and a recent peat sample of the same area were submitted to open system programmed temperature pyrolysis in order to quantify the formation of individual gas compounds i e methane as well as c2 and c3 hydrocarbons the rock samples were selected according to geological age and maturity i e from the aptian albian and cenomanian with the most immature samples from the latter organic carbon contents comprise the whole range from 3 to more than 60 the elevated values representing typical lignites a reaction kinetic model system was applied to the generation curves of the gases resulting in activation energy distributions and the corresponding pre exponential arrhenius factors the kinetic data were combined with the temperature history of two selected wells of the urengoy area obtained from basin modelling in order to calculate the gas generation on the geological time scale it was found that even for the low temperatures occurring in the pokur formation early gas generation could have taken place to a certain yet small degree under the assumption of a respective catchment area and the accumulation of this gas in the reservoirs however methane quantities are in the order of magnitude of the urengoy gas accumulation thus early thermogenic methane could at least partially contribute to the gas accumulated in the reservoirs

determination of gross kinetic parameters for petroleum formation from jurassic source rocks of different maturity levels by means of laboratory experiments abstract solvent extracted core samples of lower toarcian carbonate rich shales posidonia shale lias of the hils syncline area lower saxony basin northern germany covering a wide maturity range vitrinite reflectance rr 0 48 0 88 and 1 45 and a kimmeridge mudstone rr 0 45 are pyrolyzed at three different heating rates 0 1 0 7 and 5 0 k min by application of non isothermal first order reaction kinetics to the resulting hydrocarbon evolution curves activation energy distributions between 46 and 70 kcal mol and pre exponential arrhenius factors are obtained by an iteration method hypothetical hydrocarbon generation curves are calculated for the lias assuming three geological heating rates 0 5 5 and 50 k 106a peak hydrocarbon generation temperatures range from 135 to 165 c

gas exsolution during fluid migration and its relation to overpressure and petroleum accumulation abstract petroleum systems in the tertiary sections of the gulf of mexico and niger delta among other basins are characterized by the close association between petroleum accumulation and over pressure although overpressure is generated mainly by compaction disequilibrium of the low permeability sediments exsolution of gases from water and petroleum during migration over great vertical distance may create overpressure or modify the existing overpressure in this paper the author combined relative permeability data and petroleum phase behavior modelling to explain and predict the presence of an overpressure and petroleum baffle due to gas exsolution generalized isoperm diagrams for oil gas and water phases in sandy and silty shaly sediments were created based on published data aqueous solubility of methane and carbon dioxide as a function of temperature pressure and salinity is summarized based on literature data phase diagrams of petroleum with a wide range of composition api gravity and gor were produced using phase behavior software with calibration using field data these tools were integrated to model overpressure and petroleum phase variations in two oil gas fields in the gulf of mexico the pressure gradients in these fields are likely modified by gas exsolution from the petroleum which migrated and accumulated within and below the pressure transition zone ptz petroleum trapped within ptz tends to be fractionated whereas that accumulated below ptz tends to be single phase with high api gravity these results provide a model which explains both the effect of gas exsolution on ptz and the occurrence of different types of petroleum in these fields these tools may be integrated with geological and seismic data to assess more accurately the economic value of petroleum accumulations in petroleum systems governed by overpressure and petroleum phase separation and fractionation

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

generation of nitrogen and methane from sedimentary organic matter implications on the dynamics of natural gas accumulations nitrogen n2 contents of natural gases in rotliegend and buntsandstein reservoirs of the north german basin regionally approach 100 a review is given of the various hypotheses primordial origin volcanic or magmatic origin radiogenic origin atmospheric origin organic origin inorganic nitrogen in sedimentary rocks presented to account for nitrogen anomalies in this area and other parts of the world the objective of the present study was to investigate sedimentary organic matter in particular coals as a potential source of molecular nitrogen in the subsurface comparison of reservoir sizes and gas generation potentials indicates that carboniferous coal measures which are considered as the source of the natural gas in the north german basin can readily account for the nitrogen quantities found in present day reservoirs laboratory pyrolysis experiments were carried out to investigate the kinetics of generation of methane and molecular nitrogen from coals of different type and rank under experimental conditions nitrogen is formed at higher temperatures than methane supporting the concept of a fractional generation of methane and nitrogen in natural systems based on the kinetic parameters derived from laboratory experiments methane and nitrogen generation rates from coals were calculated for geologic heating rates gas containing more than 50 nitrogen is generated under these conditions at temperatures in excess of 300 c nitrogen rich gases are thus formed only in the final stage of gas generation after methane generation has practically ceased it is concluded that the amounts of gas encountered in nitrogen rich gas accumulations represent only a small fraction possibly 1 of the total gas generation potential of this area while the bulk of the generated gas has escaped to the atmosphere the present day composition of the reservoir gases reflects the composition of only the most recently generated gas on a geologic time scale

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

the morphology of small fresh craters on mars and the moon the depth diameter ratio for new meter to decameter scale martian craters formed in the last 20 years averages 0 23 only slightly deeper than that expected for simple primary craters on rocky surfaces large variations in depth diameter d d between impact sites indicate that differences between the sites such as target material properties impact velocity angle and physical state of the bolide s are important in determining the depth of small craters in the strength regime on the moon the d d of random fresh small craters with similar diameters averages only 0 10 indicating that either the majority of them are unrecognized secondaries or some proportion are degraded primaries older craters such as these may be shallower due to erosional infilling which is probably not linear over time but more effective over recently disturbed and steeper surfaces processes that are not yet acting on the new martian craters brand new meter to decameter scale craters such as the martian ones studied here are statistically easily distinguishable as primaries but the origins of older craters of the same size such as the lunar ones in this study are ambiguous

tracing of deeply buried source rock a case study of the wc9 2 petroleum pool in the pearl river mouth basin south china sea abstract the identification of a deeply buried petroleum source rock owing to the difficulty in sample collection has become a difficult task for establishing its relationship with discovered petroleum pools and evaluating its exploration potential in a petroleum bearing basin this paper proposes an approach to trace a deeply buried source rock the essential points include determination of the petroleum charging time of a reservoir reconstruction of the petroleum generation history of its possible source rocks establishment of the spatial connection between the source rocks and the reservoir over its geological history identification of its effective source rock and the petroleum system from source to trap and evaluation of petroleum potential from the deeply buried source rock a case study of the w9 2 petroleum pool in the wenchang a sag of the pearl river mouth basin south china sea was conducted using this approach the w9 2 reservoir produces condensate oil and gas sourced from deeply buried source rocks the reservoir consists of a few sets of sandstone in the zhuhai formation and the possible source rocks include an early oligocene enping formation mudstone and a late eocene wenchang formation mudstone with a current burial depth from 5000 to 9000 m the fluid inclusion data from the reservoir rock indicate the oil and the gas charged the reservoir about 18 3 5 ma and after 4 5 ma respectively the kinetic modeling results show that the main stages of oil generation of the wenchang mudstone and the enping mudstone occurred during 28 20 ma and 20 12 ma respectively and that the 13 c 1 value of the gas generated from the enping mudstone has a better match with that of the reservoir gas than the gas from the wenchang mudstone results from a 2d basin modeling further indicate that the petroleum from the enping mudstone migrated upward along the well developed syn sedimentary faults in the central area of the sag into the reservoir but that the petroleum from the wenchang mudstone migrated laterally first toward the marginal faults of the sag and then migrated upward along the faults into shallow strata the present results suggest that the trap structure in the central area of the sag is a favorable place for the accumulation of the enping mudstone derived petroleum and that the wenchang mudstone derived petroleum would have a contribution to the structures along the deep faults as well as in the uplifted area around the sag

gas generation and accumulation in the west siberian basin we appreciate the discussion by murris 2001 and take the opportunity to clarify some of his objections and discuss some of the still open questions in more detail murris s main objections to the conclusions of littke et al 1999 concern first the influence of overpressure in the lower cretaceous and upper jurassic on hydrocarbon distribution and second the model of gas release by exsolution degassing from deep ground water as the main mechanism for accumulating the huge gas fields of northern west siberia the first objective of murris s discussion is how the overpressure in jurassic strata might have influenced the distribution of hydrocarbons in the overlying cretaceous sediments two crucial points regarding this question are the reason and timing of overpressuring and its influence on hydrocarbon movement we agree with murris that the effect of compaction disequilibrium is not sufficient to explain the extent of overpressure in the urengoy area and that hydrocarbon generation in the bazhenov formation has contributed to the buildup of overpressure two dimensional basin modeling along a seismic line through the urengoy field however showed that hydrocarbon generation in jurassic source rocks bazhenov and the tyumen formations did only generate sufficient overpressure to explain the recent situation if the models were laterally completely sealed this sealing appears to be highly unlikely furthermore hydrocarbon generation ceased during the miocene when the tectonic movement started to lift the source rocks into shallower depths where lower temperatures were reached thus any overpressure caused by petroleum generation must have been preserved over a period of about 5 20 m y in addition the overpressure is not restricted to the jurassic bazhenov formation but extends into the cretaceous including the achimov sands uplift might have contributed to the overpressure however maximum pore pressure from the stage of deepest burial might have

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried