petrophysical inversion of borehole array induction logs part i numerical examples we have developed a new methodology for the quantitative petrophysical evaluation of borehole array induction measurements the methodology is based on the time evolution of the spatial distributions of fluid saturation and salt concentration attributed to mud filtrate invasion we use a rigorous formulation to account for the physics of fluid displacement in porous media resulting from water base mud filtrate invading hydrocarbon bearing rock formations borehole array induction measurements are simulated in a coupled mode with the physics of fluid flow we use inversion to estimate parametric 1d distributions of permeability and porosity that honor the measured array induction logs as a byproduct the inversion yields 2d axial symmetric spatial distributions of aqueous phase saturation salt concentration and electrical resistivity we conduct numerical inversion experiments using noisy synthetic wireline logs the inversion requires a priori knowledge of several mud petrophys ical and fluid param

an approximate 3d inversion method for inversion of single well induction logging responses electromagnetic logging is a technique used to probe differences in electric conductivity around a measurement device electromagnetic logging while drilling can contribute to proactive geosteering to improve well placement in reservoirs because such measurements can serve as an indication for the structure of the surrounding geology we have developed a novel way of predicting the 3d conductivity distribution around a drilling tool based on the contrast source inversion method in which we have replaced the full integral equation approach by the single spherical scatterer sss approximation the approximation took into account the dominant features of the diffusive electromagnetic field this allowed for a substantial gain in computational speed and storage of the inversion method for reconstruction of the conductivity distribution in view of the limited range of the electromagnetic probing the overall reconstruction can be segmented in several local windows this reduced the computational speed requirements and the storage requirements dramatically while safeguarding the overall 3d character of reconstruction we have synthesized 3d electromagnetic logging data using synthetic models and conductivity maps from a hydrocarbon north sea reservoir model reconstructions were made for multiple source frequencies and the results were compared with the results obtained from the born approximation we have observed that reconstruction based on the sss approximation was superior to the one based on the born approximation our algorithm helped us determine the feasibility of producing reconstructions of reservoir sections in a short time frame which allows for real time decision making during drilling operations

preliminary evaluation of trace hydrocarbon speciation and abundance as an exploration tool for footwall style sulfide ores associated with the sudbury igneous complex ontario canada deposits of cu ni platinum group elements pge rich sulfides occur along the north range footwall of the sudbury igneous complex ontario canada the deposits have been modified chemically mineralogically and texturally by multiple syn to postmagmatic hydrothermal events and are hosted mainly within sudbury breccia a contact metamorphosed and hydrothermally altered impact breccia the composition of c1 to c6 unsaturated and saturated hydrocarbons in bulk fluid inclusion volatiles in the matrix of the sudbury breccia was investigated using in line rock crushing gas chromatography gc in order to compare the hydrocarbon composition of this rock type in breccia zones containing economic footwall sulfide deposits at the morrison deposit levack mine north range sudbury ontario with barren breccia zones trace hydrocarbons possibly abiogenic in origin occur throughout the sudbury igneous complex footwall and country rocks investigated however there are differences in the composition and abundance of bulk hydrocarbons released from mineralized and barren sudbury breccia when fluid inclusions are opened volatiles released from samples of sudbury breccia from sulfide mineralized and sulfide barren environments showed the following 1 higher average abundances of light saturated hydrocarbons c1 c4 and an approximately 2 higher total abundance of hydrocarbons mol g of rock basis associated with samples from the mineralized study area and 2 comparable average unsaturated hydrocarbon abundances in both settings but a larger range in unsaturated hydrocarbon abundances in the mineralized study area differences in bulk hydrocarbon abundance and composition reflect differences in the abundance and hydrocarbon content of fluid inclusions trapped in the barren and mineralized sudbury breccia zones in addition the compositional modification of circulating fluids by processes and conditions unique to the mineralized sudbury breccia environment may have occurred prior to inclusion entrapment e g modification by mineral catalysts differences in temperature and oxygen fugacity preliminary spatial analysis of the hydrocarbon distribution within and surrounding the morrison deposit shows that the total hydrocarbon abundance in the sudbury breccia does not change with orebody proximity within a single host breccia zone however volatiles trapped in the sudbury breccia matrix near the top of the footwall deposit where it is physically connected to contact style ni cu pge ores have elevated unsaturated hydrocarbon abundances e g propene compared to other parts of the same breccia zone distal to this zone of physical connection samples of the pge enriched sudbury breccia and mineralized veins containing alteration assemblages characteristic of the footwall ore styles have higher unsaturated hydrocarbon abundances and unsaturated saturated ratios compared to sudbury breccia and vein samples that are pge poor localized enrichments in unsaturated hydrocarbons in sudbury breccia zones may be the result of specific ore minerals and metal complexes involving the pge and group viii metals that acted as catalytic agents modifying the composition of initially light saturated hydrocarbon species prior to entrapment in fluid inclusions to apply the analysis of hydrocarbon compositions in trapped fluid inclusions to discriminating mineralized from barren zones of sudbury breccia samples from targeted areas were compared to samples of barren sudbury breccia background although a large data set is presented here to establish these compositional fields analysis of smaller sample subsets from each environment yields a statistically significant separation between the two environments total hydrocarbon abundance c3 c1 ratios and unsaturated saturated hydrocarbon ratios are useful indicators of sulfide ore proximity and may help delineate regions enriched in pge

late paleozoic depositional history of the tarim basin northwest china an integration of biostratigraphic and lithostratigraphic constraints this study provides the first detailed lithostratigraphic and biostratigraphic constraints for improving stratigraphic resolution for hydrocarbon prospecting and exploration in the tarim basin a total of 49 stratigraphic units 38 formations and 11 members ranging in age from the latest devonian to permian are reviewed or redefined in terms of nomenclatures lithology age constraints and lateral distributions based on the detailed field works or newly published data of these the piqiang formation new formation is proposed to include the reefal carbonates of asselian sakmarian age from the northern tarim the subsurface upper paleozoic stratigraphic framework of the desert areas of the basin is also established for the first time the high resolution basinwide stratigraphic correlations reveal that the sedimentation of the basin in the late paleozoic was extremely uneven of these the famennian to changhsingian successions are completely recorded in the southwestern margin areas of the basin here five eustatic sedimentary cycles are well recognizable suggesting the sedimentation was more eustatically controlled and little affected by local tectonism the late paleozoic successions of both kalpin and taklimakan regions are commonly interrupted by major hiatuses at various horizons suggesting that the sedimentation was apparently modified by local tectonism of these the northward movement of the tarim block and its subsequent collision with the yili microcontinent part of the kazakhstan plate may be principally accountable for the discrepancy in the sedimentation of the various regions in the basin in the late paleozoic

the complexity secondary geochemical process genetic mechanism and distribution prediction of deep marine oil and gas in the tarim basin china abstract most paleozoic marine craton basins in china are rich in natural gas except the tarim basin where both oil and gas are prolific various hydrocarbon types have been found in this basin including deep ultra heavy oil heavy oil normal oil volatile oil condensate and natural gas petroleums with diverse types even coexist within a single reservoir generally the ordovician and cambrian reservoirs are deeply buried 5500 8000 m and intensively altered after initial oil and gas accumulations bringing challenges to physical property prediction before drilling and evaluation of exploration targets based on recent exploration discoveries and experimental data we systematically analyzed the origin of oil and gas demonstrated the mechanisms and processes of secondary geochemical alteration e g biodegradation gas washing fractionation tsr and high temperature cracking moreover the preservation mechanism of ultra deep normal oil was also clarified and the distribution patterns of oil and gas were predicted tarim basin has been uplifted and shallowly buried in the early accumulation stage then rapidly and deeply buried in the late stage resulting in various geochemical alteration processes at different burial stages through the reconstruction of complex deep strata oil and gas accumulation histories the spatial distribution of deep petroleum types were predicted this review has provided detailed assessment with case studies in the tarim basin and shed light on the preservation mechanisms and the alteration processes of deep liquid petroleums in addition to massive gas accumulations liquid oils still have great exploration potential in strata deeper than 8000 m in the tarim basin

geochemical characteristics and origin of natural gas in tarim basin china the tarim basin located in northwest china is one of the largest basins in the world that has not been well explored in recent years the large scale of petroleum exploration in the basin has led to the discovery of a series of oil and gas fields with an approximate 1 1 ratio of oil to gas resource this means that the natural gas resource is very important for the hydrocarbon resource in the tarim basin in this basin most of the discovered gases are condensate associated gas and oil associated gas research on chemical components and isotope composition of carbon and hydrogen of natural gases from the tarim basin shows that all discovered gases in this basin are thermogenic with respect to source organic matter there are two types of gases 1 oil type gas generated from sinian to lower paleozoic marine source rocks with the sapropelic organic matter with 44 5 to 33 8o oo of d13c1 42 0 to 28 1o oo of d13c2 35 4 to 28 4o oo of d13c3 and heavier than 200o oo of dd1 and 2 coal type gas generated from mesozoic terrestrial source rocks with the humic organic matter with 40 5 to 33 1o oo of d13c1 29 7 to 21 3o oo of d13c2 26 3 to 20 3o oo of d13c3 and lighter than 200o oo of dd1

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

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

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

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

petrophysical inversion of borehole array induction logs part ii field data examples we describe the application of alpak et al s 2006 petrophysical inversion algorithm to the interpretation of borehole array induction logs acquired in an active north american gas field layer by layer values of porosity and permeability were estimated in two closely spaced vertical wells that penetrated the same horizontal rock formation the wells were drilled with different muds and overbalance pressures and the corresponding electromagnetic induction logs were acquired with different tools rock core laboratory measurements available in one of the two wells were used to constrain the efficiency of gas displacement by water based mud during the process of invasion estimated values of porosity and permeability agree well with measurements performed on rock core samples in addition to estimating porosity and permeability the petrophysical inversion algorithm provided accurate spatial distributions of gas saturation in the invaded rock formations that were not possible to obtain with conventional procedures based solely on the use of density and resistivity logs

geochemistry and origins of natural gases in the central junggar basin northwest china abstract the petroliferous central junggar basin in northwest china is predominantly an oil exploration region however its gas exploration also might have good prospects thus to assist in gas exploration the geochemistry and origins of gases are discussed in this paper based on relatively comprehensive analyses of compositions carbon isotopes and light hydrocarbons of gases based on the results the gas genetic types are grouped into families and combined with the geological setting e g biomarkers of retrograde condensates and source rock characteristics we show that there are four representative genetic types of gases the first consists of gases derived from permian lacustrine mudstones with type i ii kerogen and type iii kerogen sources in the penyijingxi sag their representative geochemical feature is 13c2 ranging from 31 4 to 24 7 the second is gas sourced from carboniferous tufaceous mudstones of type iii kerogen in the dishuiquan sag whose representative geochemical feature is the heaviest values of 13c1 in the studied samples ranging from 32 0 to 30 4 the third consists of gases sourced from jurassic coals and mudstones in the shawan fukang sag the light hydrocarbon fingerprints of these gases are similar to those of gases and oils typically derived from jurassic source rocks in the southern junggar basin the fourth is gas most likely generated from the degradation of crude oil it is mainly found in the luliang area and has dryness values as much as 0 999 and 13c1 ranging from 54 8 to 43 2 among these four types of gases the first mainly sourced from the permian lacustrine mudstones in the penyijingxi sag is the predominant type

variations in denitrification and ventilation within the arabian sea oxygen minimum zone during the holocene the continental slope of india is exposed to an intense perennial oxygen minimum zone omz supporting pelagic denitrification sediments that are presently in contact with the lower boundary of the denitrification zone indicate marked changes in the intermediate and bottom waters ventilation of omz during the past 9 500 years the 15n of sediment suggests that the omz waters were less ventilated during the early holocene between 9 5 and 8 5 ka bp resulting in intensified denitrifying conditions with an average 15n value of 7 8 while at the same time stable mo isotope composition average 98mo of 0 02 indicates that the bottom waters that were in contact with the sediments were better oxygenated by the mid holocene omz became more oxygenated suppressing denitrification average 15n of 6 2 while bottom waters gradually became less oxygenated average 98mo of 1 7 the mid holocene reduction in denitrification coincided with a global decrease in atmospheric n2o as inferred from ice core records which is consistent with a decreased contribution from the arabian sea since 5 5 ka bp omz waters have again been undergoing progressive deoxygenation accompanied by increasing denitrification

estimating petrophysical parameters and average mud filtrate invasion rates using joint inversion of induction logging and pressure transient data we introduce an inversion approach for determining the water based mud filtrate invasion profile as well as the formation porosity and horizontal permeability from the induction logging data the inversion is constrained by a multiphase fluid flow simulator that simulates the mud filtrate invasion process to obtain the spatial distributions of the water saturation and the salt concentration which are in turn transformed into the formation resistivity using a resistivity saturation formula by ignoring the diffusion effect we assume that the mud filtrate invasion process is mainly convective so that it can be equivalently simulated by providing an average invasion rate and the duration of invasion the average invasion rate can be directly inverted from the fluid flow constrained inversion of induction logging data we also obtain the mud filtrate invasion profile which is consistent with the fluid flow physics the reconstructed mud filtrate invasion profile benefits the interpretation of the formation test when the pressure transient data are available this approach can be also used to jointly invert both induction logging data and pressure transient data to obtain the mud filtrate invasion profile as well as a parametric distribution of the ti anisotropic formation permeability and porosity assuming a vertical well penetrating horizontal formations the fluid flow problem is solved using an implicit black oil finite difference simulator with brine tracking option based on a cylindrical axially symmetric grid whereas the response of the induction logging tool is simulated using a frequency domain finite difference solver based on a cartesian grid a gauss newton inversion scheme using the multiplicative regularization technique is used for either the fluid flow constrained inversion or the joint inversion the reliability of the inversion results depends on the accuracy of a priori knowledge of the reservoir which needs to be confirmed via sensitivity analysis

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

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

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

substratum transverse faults in kuqa foreland basin northwest china and their significance in petroleum geology abstract the importance of transverse faults in basin evolution and petroleum geology has been often emphasized however the transverse faults in the kuqa foreland basin the most important gas producing area in china have rarely been studied interpreted seismic sections and earthquake focal mechanism solutions as well as other geological and geophysical data allow us to identify a few widely spaced approximately ns trending transverse strike slip faults separating major structural units and to clarify the geometry and kinematics of these transverse faults hidden below thrust faults in the kuqa foreland basin two major structural domains can be distinguished transverse faults in the northern kuqa foreland basin are mainly nne trending sinistral indicating clockwise rotation of fault bounded blocks in contrast the predominant nw trending dextral faults in the southern kuqa foreland basin imply counterclockwise rotation of fault bounded blocks we propose a tectonic model in which crustal blocks are bounded by strike slip faults in a zone of simple shear rotation about vertical axis the strike slip faulting and thrust faulting in the kuqa foreland basin suggest that some of the convergence between south tianshan and tarim blocks may have been accommodated not only by obvious crustal shortening and thickening along thrust faults but also by rotation and possible lateral movement of the crust along transverse faults controlled by the remote collision of indian block with eurasian block since the miocene these reactivated substratum faults which may inherit from paleozoic deformation control various elements relevant to gas accumulation in the kuqa foreland basin which should expect to be paid more attention in the future these elements include maturity of tertiary and jurassic source rocks a difference in the regional cap of kumugelimu salt beds from east to west reservoir bed properties gas migration channels and traps formation in addition the strike slip faulting of the transverse faults may affect deformation of the sedimentary cover including making the western qiulitag structural belt to form an arcuated structure curved to the ssw

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

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

estimating reservoir permeability with borehole radar in oil drilling mud filtrate penetrates into porous formations and alters the compositions and properties of the pore fluids this disturbs the logging signals and brings errors to reservoir evaluation drilling and logging engineers therefore deem mud invasion as undesired and attempt to eliminate its adverse effects however the mud contaminated formation carries valuable information notably with regard to its hydraulic properties typically the invasion depth critically depends on the formation porosity and permeability therefore if adequately characterized mud invasion effects could be used for reservoir evaluation to pursue this objective we have applied borehole radar to measure mud invasion depth considering its high radial spatial resolution compared with conventional logging tools which then allows us to estimate the reservoir permeability based on the acquired invasion depth we investigate the feasibility of this strategy numerically through coupled electromagnetic and fluid modeling in an oil bearing layer drilled using freshwater based mud time lapse logging is simulated to extract the signals reflected from the invasion front and a dual offset downhole antenna mode enables time to depth conversion to determine the invasion depth based on drilling coring and logging data a quantitative interpretation chart is established mapping the porosity permeability and initial water saturation into the invasion depth the estimated permeability is in a good agreement with the actual formation permeability our results therefore suggest that borehole radar has significant potential to estimate permeability through mud invasion effects