the western desert versus nile delta a comparative molecular biomarker study abstract the western desert and the nile delta are prolific petroleum producing basins the correlation and matching between their petroleum systems have been briefly studied overall geochemical and biological marker analyses of twelve crude oil and condensate samples taken from seven oil gasfields located in the nile delta and the western desert basins of egypt were effective in identifying and geographically defining two major oil families related by age and source rock depositional environment these samples were classified respectively as late cretaceous tertiary and jurassic early cretaceous the source rocks giving rise to the late cretaceous tertiary oils are associated with abundant terrestrial organic matter the source rocks giving rise to jurassic early cretaceous oil are presumably generated from a mixed terrestrial marine source meanwhile the western desert oil samples show prominent c 30 steranes elevated c 27 monoaromatic steroids and a distinct homohopane distribution suggesting this oil type is derived from a more marine influenced source rock than terrigenous sourced oil in the nile delta the most crucial geochemical fingerprinting biomarkers that have been used to differentiate between nile delta and the western desert oil are oleananes highly branched isoprenoids hbi sterane isomerization 24 norcholestanes aromatic steroid hydrocarbons and c 30 24 n propylcholestanes

hydrocarbon sources of cold seeps off the vesteralen coast northern norway abstract we investigated active methane seeps in a water depth of 200 m in the hola area off the coast of vesteralen northern norway to assess 1 hydrocarbon sources 2 migration pathways and 3 the influence of hydrocarbon seepage on sediment pore water and water column chemistry the seepage area is characterised by the presence of gas flares in the water column as revealed by hydro acoustic surveys and elevated methane concentrations of up to 42 nm ca 5 m above the seafloor pore water analyses of three gravity cores from the seepage area show varying depths of the sulphate methane transition zone smtz between 80 cm and 250 cm indicating spatially heterogeneous methane ascent the isotopic composition of methane 13 c from 40 to 63 and 2 h from 191 to 225 and 13 c depth profiles of methane and dissolved inorganic carbon show that the hydrocarbons are predominantly of thermogenic origin consistent with 13 c values of c 2 to c 4 hydrocarbons isotope data also indicate considerable biodegradation of propane seismic profiles from the study area reveal major faults and steeply dipping unconformities between the basement and overlying mesozoic sedimentary rocks we propose that these act as migration pathways for the hydrocarbons from late jurassic to early cretaceous source rocks

seismic stratigraphy and sedimentary facies analysis of the pre and syn messinain salinity crisis sequences onshore nile delta egypt implications for reservoir quality prediction abstract this study provides the first high resolution outlines of the messinian sequence in the nile delta based on an integrated sedimentological petrophysical and seismic stratigraphic approach the distribution pattern and architecture of the messinian facies deposited before during and after the messinian salinity crisis msc have been investigated in seismic the pre msc qawasim formation constitutes parallel oblique seismic facies interpreted as deltaic system prograding northward from prodelta distal sand bars into subaqueous distributary channels and mouthbars and subaerial delta plain distributary channels the msc abu madi formation consists of three seismic stratigraphic units infilling the eonile canyon a seismically chaotic lower unit paving the canyon floor is overlain by the continuous high amplitude reflectors of the middle unit which is capped by semi chaotic low amplitude facies of the upper unit the lower unit corresponds to the subaerial gravity flow deposits synchronously accumulated during the eonile incision whereas the middle unit comprises the aggradational progradational fluvial facies and the upper unit constitutes the retrogradational estuarine facies the superimposed diagenetic alterations on these various depositional facies produced complex reservoir heterogeneity fluid flow conduits are restricted to the coarse grained delta distributary channels and fluvial facies with limited marine influence and minimal depositional clay content and traces of diagenetic kaolinite and chlorite on the other hand flow baffles and barriers are common in the fine grained marine influenced facies with pervasive depositional clays diagenetic calcite and glauconite

gas migration below gas hydrates controlled by mass transport complexes offshore mauritania abstract interpretation of 3d seismic data from offshore mauritania reveals many localized high amplitude anomalies haas above mass transport complexes mtcs these haas are located at 400 600 m burial depth and have circular or elongate shapes in map view the reflections at the tops of these haas are of negative polarity we divide these haas into two populations aligned and non aligned the aligned haas comprise three linear groups above the structural lineaments of underlying mtcs suggesting a genetic link the aligned haas in the western group are often located in depressions and are interpreted to be caused by both gas migration and coarser grained sediments the other two groups of aligned haas are interpreted to be caused by residual gas after gas migration fed along the margins of underlying mtcs in contrast the non aligned haas are generally smaller located above the mtc bodies they are interpreted to be caused by gas which migrated from underlying mtc bodies rather than along the margins without any evidence for lithology change we propose that the locations of the haas both aligned and non aligned indicate gas migration pathways up mtc margins or local faults associated with the underlying mtcs one possible gas source could be from hydrate dissociation due to the upward resetting of the base of gas hydrates since buried mtcs and gas hydrates can coexist in marine environments the model of buried mtcs channelling fluids upwards may occur widely which can supply gas to the hydrate systems

paleo pressure evolution and its origin in the pinghu slope belt of the xihu depression east china sea basin abstract the xihu depression in the east china sea basin is one of the prime gas and light oil producing regions of eastern china overpressure evolution and origin in the sandstone reservoirs of the pinghu e2p formation in the pinghu slope belt of the xihu depression were investigated through a combination of fluid inclusion analyses and basin numerical modeling combined with measured pore pressures drill stem and wireline formation tests well loggings gas geochemistry and casting thin section data four oil inclusion types exhibiting different api gravities and hydrocarbon gas inclusions were identified using inclusion petrography microscopic fluorescence and raman spectra homogenization and ice melting temperatures of coeval aqueous inclusions determined using microthermometry combined with burial thermal histories determined by basin numerical modeling suggested multiple episodes of oil and natural gas charge in the e2p reservoirs from the miocene to the present day pore pressure evolution history reconstructed with fluid inclusion pvtx analysis indicated that the pinghu formation experienced three phases overpressure generation followed by overpressure release and then further overpressure generation the first phase of weak overpressure was generated during burial in the middle late miocene currently it is hard to definitely determine the origin of the first phase of overpressuring nevertheless it can be speculated that the origin may be related to disequilibrium compaction oil generation and tectonic compression based on burial tectonic and hydrocarbon generation histories following this a sudden uplift of formations led to overpressure release in the late miocene and then the second phase of moderate overpressure generation occurred during burial in the late pliocene and quaternary based on well log signatures vitrinite reflectance data of mudstones and gas geochemistry combining with seismic analysis the second phase of overpressuring was interpreted to have been mainly caused by fluid expansion which resulted from local gas generation by kerogen maturation and long distance vertical and lateral pressure transfer

microbial alteration of subsurface natural gas accumulations bacterial alteration of subsurface reservoired hydrocarbon accumulations extends into the range of the c1 c4 gaseous hydrocarbons preferentially attacking the wet gas components in most instances during the initial stages of bacterial gas alteration propane seems to be selectively attacked resulting in anomalous propane carbon isotopic compositions as well as unusual carbon isotopic distributions among the wet gas components extensive bacterial alteration appears to be capable of removing all but traces of the wet gas components resulting in a dry gas that could be confused with overmature dry gas whose wet components have undergone extensive thermal cracking recognition of microbial alteration in subsurface reservoired hydrocarbon gas accumul tions is important for both correlation purposes and maturity estimates

some observations on the origins of large volumes of carbon dioxide accumulations in sedimentary basins large volumes of carbon dioxide i e up to 90 v v have been encountered in sedimentary basins from various geological settings but their origins remain enigmatic evidence from carbon and noble gas isotope analyses indicate a variety of interpretations for the origins of the co2 including mantle degassing metamorphic reactions or magmatic processes the formation of large accumulations of large volumes of co2 in sedimentary basins is discussed with specific reference to published data from the pannonian basin hungary the cooper eromanga basin australia and the south viking graben north sea these basins have occurrences of large volumes i e over 15 v v of isotopically heavy carbon dioxide thought to be of inorganic origin and although they differ somewhat in terms of sedimentary fill all have major structural features close to the gas accumulations the available geochemical data are currently neither sufficient to confirm the specific origin s of the co2 e g mantle derived carbonate metamorphism nor unscramble co2 mixed from two or more sources ongoing studies especially those involving noble gas analyses should provide fuller answers

origins of hydrocarbon gas seeping out from offshore mud volcanoes in the nile delta abstract this paper discusses the origin of gas seepages free gas or dissolved gas in ground water or brine sampled with the nautile submarine during the nautinil cruise at the seafloor of the deep water area of the nile turbiditic system on different mud volcanoes and brine pools generally the gas is wet and includes c1 c2 c3 ic4 nc4 co 2 these gas samples show no evidence of biodegradation which is not the case of the gas present in the deep hydrocarbon accumulations at depth it indicates that the gas expelled by the mud volcanoes is not issued from direct leakages from deep gas fields the collected gas samples mainly have a thermogenic origin and show different maturities some samples show very high maturities indicating that these seepages are sourced from great depths below the messinian salt moreover the different chemical compositions of the gas samples reflect not only differences in maturity but also the fact that the gas finds its origin in different deep source rocks carbon dioxide has an organic signature and cannot result from carbonate decomposition or mantle fluids the crustal derived radiogenic isotopes show that the analyzed gas samples have suffered a fractionation processes after the production of the radiogenic isotopes due either to oil occurrence at depth interacting with the flux of gas and or fractionation during the fluid migration

alteration of natural gas during leakage from a north sea salt diapir field oil and gas are leaking from a salt diapir field in the norwegian sector of the central north sea the gas is being biodegraded as it migrates to the surface the seeping gas is pervasive in the sediments above the diapir and clearly visible on shallow and deep seismic records and on wireline logs an exploration well was drilled to test a cretaceous chalk reservoir above the diapir and provided an opportunity to sample the gas along the seepage pathway carbon isotope ratios of gases released from sealed cuttings taken from the gas cloud interval above the diapir were determined to test the hypothesis that biodegradation of leaking hydrocarbons produces secondary methane with a biogenic isotope signature generally high but variable concentrations of co2 were detected all the way from the surface to the salt diapir this results from biodegradation of hydrocarbons and particularly oil the source of oxygen for this process is problematic since most of the migration takes place through shales and there is no obvious aquifer source of dissolved oxygen the effect of biodegradation of gases escaping from this structure results in a comparable increase in 13c for both methane and ethane up to 10 with a lesser effect on propane and less still for butane and pentane this conflicts with published data on such effects which suggest that methane and propane should be most affected with little effect on ethane isotopically light methane was found only in the very deepest samples within the salt of the diapir itself and this is thought to be biogenic gas trapped in the salt at the time of deposition no evidence was found that any secondary biogenic methane is being produced during gas seepage and biodegradation at this site

recognising biodegradation in gas oil accumulations through the δ13c compositions of gas components a large suite of natural gases 93 from the north west shelf and gippsland and otway basins in australia have been characterised chemically and isotopically resulting in the elucidation of two types of gases about 26 of these gases have anomalous stable carbon isotope compositions in the c1 c4 hydrocarbons and co2 components and are interpreted to have a secondary biogenic history the characteristics include unusually large isotopic separations between successive n alkane homologues up to 29 pdb and isotopically heavy co2 up to 19 5 pdb irrespective of geographic location these anomalous gases are from the shallower accumulations 600 1700 m where temperatures are lower than 75 c the secondary biogenic gases are readily distinguishable from thermogenic gases 74 of this sample suite which should assist in the appraisal of hydrocarbons during exploration where hydrocarbon accumulations are under 2000 m while dissolution effects may have contributed to the high 13c enrichment of the co2 component in the secondary biogenic gases the primary signature of this co2 is attributed to biochemical fractionation associated with anaerobic degradation and methanogenesis correlation between biodegraded oils and biodegraded dry gas supports the concept that gas is formed from the bacterial destruction of oil resulting in secondary biogenic gas furthermore the prominence of methanogenic co2 in these types of accumulations along with some isotopically depleted methane provides evidence that the processes of methanogenesis and oil biodegradation are linked it is further proposed that biodegradation of oil proceeds via a complex anaerobic coupling that is integral to and supports methanogenesis

sources and turnover of organic carbon and methane in fjord and shelf sediments off northern norway to better understand the present and past carbon cycling and transformation processes in methane influenced fjord and shelf areas of northern norway we compared two sediment cores from the hola trough and from ullsfjorden we investigated 1 the organic matter composition and sedimentological characteristics to study the sources of organic carbon corg and the factors influencing corg burial 2 pore water geochemistry to determine the contribution of organoclastic sulfate reduction and methanogenesis to total organic carbon turnover and 3 the carbon isotopic signature of hydrocarbons to identify the carbon transformation processes and gas sources high sedimentation and corg accumulation rates in ullsfjorden support the notion that fjords are important corg sinks the depth of the sulfate methane transition smt in the fjord is controlled by the supply of predominantly marine organic matter to the sediment organoclastic sulfate reduction accounts for 60 of the total depth integrated sulfate reduction in the fjord in spite of the presence of ethane propane and butane we suggest a purely microbial origin of light hydrocarbons in the sediments based on their low 13c values in the hola trough sedimentation and corg accumulation rates changed during the deglacial to post glacial transition from approximately 80 cm ka 1 to erosion at present thus corg burial in this part of the shelf is presently absent low organic matter content in the sediment and low rates of organoclastic sulfate reduction only 3 of total depth integrated sulfate reduction entail that the shallow depth of the smt is controlled mostly by ascending thermogenic methane from deeper sources this article is protected by copyright all rights reserved

hydrocarbon exploration in a complex incised valley fill an example from the late messinian abu madi formation nile delta basin egypt since the early 1960s when the nile delta basin started to show its potentiality generations of geologists and geophysicists have applied different concepts and methodologies keeping pace with the latest available technologies regional gravity surveys were followed by more and more extended and sophisticated 2 d seismic surveys up to almost routine 3 d acquisition in the 1990s among the oil companies more active in the area agip through its affiliate international egyptian oil company ieoc drilled in 30 years more than 50 new field wildcat wells 29 of which had messinian targets following this technological and conceptual evolution the rate of technical success approached the 100 in the last exploratory phase of the abu madi formation when 3 d seismic data and seismic attributes were extensively used to predict the sand distribution within the valley

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

natural gas formation in the western nile delta eastern mediterranean thermogenic versus microbial abstract the offshore part of the nile delta is an emerging gas and condensate province and one of the most promising areas for future petroleum exploration and supply in northern africa while the nile delta s hydrocarbon potential and its geological and stratigraphic pattern has been the focus of numerous studies the origin of its gas has been poorly studied in order to characterize the natural gas and to determine whether microbial or thermogenic processes are the dominant generation pathways of methane a regional geochemical study was performed on 38 test gases collected from 9 neogene western nile delta discoveries main constituents of the gases are methane 89 4 99 4 up to pentane with traces of carbon dioxide and nitrogen methane carbon isotopic composition ranges from 66 0 to 39 6 displaying a mixed microbial and thermogenic character for most of the gases microbial gas contributions range from 15 to 95 the mixed and the more thermogenic gases are frequently accompanied by light oils and condensates 41 59 api calculated maturities of the thermogenic gas proportions suggest source rock maturities between 0 8 and 1 4 ro which are actually present in the pre miocene section evidence for microbial alteration of petroleum accumulations in the nile delta is largely manifested in anomalously heavy isotopic compositions of both wet gas components propane and n butane and preferential removal of higher n alkanes from condensates the hydrocarbon accumulations affected by biodegradation are trapped within shallow pliocene reservoirs

impact of hot fluid advection on hydrocarbon gas production and seepage in mud volcano sediments of thick cenozoic deltas hydrocarbon seeps are ubiquitous at gas prone cenozoic deltas such as the nile deep sea fan ndsf2 where seepage into the bottom water has been observed at several mud volcanoes mvs3 including north alex mv namv4 here we investigated the sources of hydrocarbon gases and sedimentary organic matter together with biomarkers of microbial activity at four locations of namv to constrain how venting at the seafloor relates to the generation of hydrocarbon gases in deeper sediments at the centre high upward flux of hot 70 c hydrocarbon rich fluids is indicated by an absence of biomarkers of anaerobic oxidation of methane aom and nearly constant methane ch4 concentration depth profile the presence of lipids of incompatible thermal maturities points to mixing between early mature petroleum and immature organic matter indicating that shallow mud has been mobilized by the influx of deep sourced hydrocarbon rich fluids methane is enriched in the heavier isotopes with values of 13c 46 6 vpdb and d 228 vsmow and is associated with high amounts of heavier homologues c2 suggesting a co genetic origin with the petroleum on the contrary at the periphery a lower but sustained ch4 flux is indicated by deeper sulphate methane transition zones and the presence of 13c depleted biomarkers of aom consistent with predominantly immature organic matter values of 13c ch4 60 vpdb and decreased concentrations of 13c enriched c2 are typical of mixed microbial ch4 and biodegraded thermogenic gas from plio pleistocene reservoirs of the region the maturity of gas condensate migrated from pre miocene sources into miocene reservoirs of the western ndsf is higher than that of the gas vented at the centre of namv supporting the hypothesis that it is rather released from the degradation of oil in neogene reservoirs combined with the finding of hot pore water and petroleum at the centre our results suggest that clay mineral dehydration of neogene sediments which takes place posterior to reservoir filling may contribute to intense gas generation at high sedimentation rate deltas

new u pb age constraints for the laxford shear zone nw scotland evidence for tectono magmatic processes associated with the formation of a paleoproterozoic supercontinent the lewisian gneiss complex in north west scotland is a part of the extensive network of archaean cratonic areas around the margins of the north atlantic it is considered to be made up of a number of terranes with differing protolith ages which have been affected by a range of different metamorphic events a major shear zone the laxford shear zone forms the boundary between two of these terranes new dates presented here allow us to constrain the timing of terrane assembly related to the formation of palaeoproterozoic supercontinents early deformation along the laxford shear zone and primary accretion of the two terranes occurred during the inverian event at c 2480 ma this was followed by extension and the intrusion of the mafic scourie dykes subsequently renewed silicic magmatic activity occurred at c 1880 ma producing major granite sheets considered to have formed as part of a continental arc a further collisional event began at c 1790 ma and was followed by slow exhumation and cooling this laxfordian event caused widespread crustal melting metamorphism and deformation and is considered to represent the final assembly of the lewisian gneiss complex within the major supercontinent of columbia or nuna

biodegradation of oil in uplifted basins prevented by deep burial sterilization biodegradation of crude oil by bacterial activity which has occurred in the majority of the earth s oil reserves is known to reduce greatly the quality of petroleum in reservoirs for economically successful prospecting for oil it is therefore important to understand the processes and conditions in geological formations that lead to oil biodegradation although recent studies speculate that bacterial activity can potentially occur up to temperatures as high as 150 degrees c refs 3 4 it is generally accepted that effective petroleum biodegradation over geological timescales generally occurs in reservoirs with temperatures below 80 degrees c ref 2 this appears however to be at odds with the observation that non degraded oils can still be found in reservoirs below this temperature here we compile data regarding the extent of oil biodegradation in several oil reservoirs and find that the extensive occurrence of non biodegraded oil in shallow cool basins is restricted to those that have been uplifted from deeper hotter regions of the earth we suggest that these petroleum reservoirs were sterilized by heating to a temperature around 80 90 degrees c during deep burial inactivating hydrocarbon degrading organisms that occur in the deep biosphere even when such reservoirs are subsequently uplifted to much cooler regions and filled with oil degradation does not occur implying that the sterilized sediments are not recolonized by hydrocarbon degrading bacteria

ha py field a pliocene bright spot example from the nile delta egypt amoco has been exploring for gas in the nile delta basin of egypt since 1993 the pliocene section of the offshore nile delta has long been recognized as a potential exploration target however only recently has an exploration effort focused on this promising trend since mid 1995 several important discoveries have been made in the offshore ras el barr and adjacent temsah concessions by a partnership of amoco and ieoc agip the pliocene trend has yielded an exploration success rate of nearly 100 ha py field is the largest of these pliocene discoveries and is a classic example of the fields within this bright spot trend the field name comes from the ancient egyptian god of floods and the nile pictured above

microbial cycling migration and leakage of light alkanes in the nile delta tertiary fan abstract we present a geochemical dataset which helps unravel the complex set of processes by which light alkanes are sourced charged mixed leaked and altered in the plio pleistocene nile delta thermogenic gas derives from a sub messinian source and mixes with microbial hydrogenotrophic methane rich gas in the plio pleistocene section there is strong isotopic evidence for both microbial generation and microbial degradation of ethane and propane but these signals are only recorded where the rates of biological processes outstrip the rate of thermogenic charge high resolution gas geochemistry profiles through four 500 1100 m sequences of mixed channel channel levee mass flow deposits and hemipelagites shed light on migration and leakage pathways we see limited geochemical evidence for migration along or across faults which cross cut mud rich sequences lateral migration along sands silts delivers gas to structural highs where leakage occurs vertically along focussed pathways through heterogeneous mud rich sediments we propose that in situ generation of microbial gas within mud rich sequences facilitates percolation of migrating gases that replenishes c2 components and sustains the biodegradation pathway in syntrophic microbial communities high resolution changes in gas composition suggest an unequilibrated system which is being actively charged in some structures rates of charge and of microbial alteration of gas are broadly similar in others rates of charge exceed rates of microbial alteration our results have important implications for the understanding of both basin wide fluid flow and the ecology of the deep biosphere

plate motions in the north andean region repeated geodetic measurements with the global positioning system gps provide direct measurements of displacements due to plate motions and active crustal deformation in central america and northern south america an area of complex interaction of the nazca cocos caribbean and south american plates the displacement rates for the period 1988 1991 obtained from the results of the first three central and south america casa gps campaigns are in general agreement with the predictions of the nuvel 1 plate motion model but there are differences in detail between the observations and the model the nazca north andes convergence rate vector measured by gps is different from the nuvel 1 vector at 95 confidence the difference implies that the north andes are moving northward relative to south america the measured convergence between the caribbean plate and the north andes suggests that the southern margin of the caribbean plate is located in the south caribbean deformed belt the april 1991 costa rica earthquake and the cocos caribbean convergence rate determined by gps suggest the possibility of significant ongoing deformation between central america and the stable interior of the caribbean plate our gps results are consistent with deformation of the overriding plates at the convergent margins of central and south america and confirm that active convergence is occurring around much of the southern margin of the caribbean plate from colombia west to costa rica costa rica and panama are not part of the stable caribbean plate instead the south caribbean deformed belt and the north panama fold belt probably represent the southern margin of the caribbean plate

origin and secondary processes in petroleum in the eastern part of the polish outer carpathians a geochemical study of crude oil and natural gas from producing wells and seeps within the lower cretaceous oligocene sandstone reservoirs of the most eastern part of the polish outer carpathians was conducted for the evaluation of their origin migration pathways and the effects of secondary processes biodegradation water washing and evaporative fractionation the petroleum was generated from mixed marine terrigenous organic matter type ii iii kerogen occurring mostly in oligocene menilite beds in the silesian and skole nappes shales within upper cretaceous palaeocene istebna and lower cretaceous ve ovice lgota and spas beds are considered as an additional source of hydrocarbons mainly gaseous biodegradation processes were recorded in numerous oils especially occurring in seeps but the extent of these processes is not severe because steranes and terpanes were not affected all oils are more or less water washed while the evaporative fractionation processes are developed only on a minimal scale the natural gases are primarily of thermogenic origin usually co genetic with oil and often contain components of secondary microbial degradation of both oil and natural gas 13c enriched co2 and 13c depleted ch4 gases with the highest microbial methane contribution have been recorded in petroleum accumulated in dwernik and odyna fields

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

regional geologic and tectonic setting of the maracaibo supergiant basin western venezuela this special issue contains eight topical studies on the structure stratigraphy and petroleum system of the maracaibo basin a supergiant basin in western venezuela most of the work reported in this special issue is the product of thesis related research by master s and doctoral level students at the jackson school of geosciences of the university of texas at austin during a collaborative relationship with the venezuelan national oil company petrleos de venezuela s a that was initiated in the late 1980s this introductory article presents a regional overview of the tectonic setting and geology of the maracaibo basin with a cumulative oil production of more than 30 billion bbl since the first production well was drilled in 1914 and estimated ultimate oil reserves of more than 44 billion bbl the maracaibo basin is the most prolific hydrocarbon basin in the western hemisphere unlike the more extensive gulf of mexico giant hydrocarbon provinces the relatively small size 50 000 km2 19 305 mi2 relative simplicity in its structure and stratigraphy and wealth of surface and subsurface data make the maracaibo basin an attractive target for basinwide synthesis the objective of this article is to present a regional compilation of two dimensional 2 d and three dimensional 3 d seismic data wells and outcrop data at a basinwide scale to reveal the basin s 3 d structure and stratigraphy moreover we show regional tectonic reconstructions regional geologic maps and basin subsidence history to better constrain four major tectonic events that affected the basin and that are critical for understanding the timing and distribution of major unconformities and clastic wedges the distribution of the reservoir rocks the reactivation of older fault trends and the timing of maturation for underlying source rocks many of these topics are discussed in greater detail in the other eight articles in this special issue

microbial production and modification of gases in sedimentary basins a geochemical case study from a devonian shale gas play michigan basin an expanded data set for gases produced from the antrim shale a devonian black shale in the michigan basin united states has allowed for a detailed examination of the related chemical and isotopic compositional changes in the solid gas liquid systems that discriminate between microbial and thermogenic gas origin in the antrim shale economic microbial gas deposits are located near the basin margins where the shale has a relatively low thermal maturity and fresh water infiltrates the permeable fracture network the most compelling evidence for microbial generation is the correlation between deuterium in methane and coproduced water along the basin margins there is also a systematic enrichment in 13c of ethane and propane with decreasing concentrations that suggests microbial oxidation of these thermogenic gas components microbial oxidation accounts not only for the shift in 13c values for ethane but also in part for the geographic trend in gas composition as ethane and higher chain hydrocarbons are preferentially removed this oxidation is likely an anaerobic process involving a syntrophic relationship between methanogens and sulfate reducing bacteria the results of this study are integrated into a predictive model for microbial gas exploration based on key geochemical indicators that are present in both gas and coproduced water one unequivocal signature of microbial methanogenesis is the extremely positive carbon isotope values for both the dissolved inorganic carbon in the water and the coproduced co2 gas in contrast the 13c value of methane is of limited use in these reservoirs as the values typically fall between the commonly accepted fields for thermogenic and microbial gas in addition the confounding isotopic and compositional overprint of microbial oxidation increasing the values to typically thermogenic values may obscure the distinction between methanogenic and thermogenic gas

genetic characterization of gas shows in the east mediterranean offshore of southwestern israel drilling in the eastern continental margin of the levant basin southeastern mediterranean offshore southwestern israel encountered light oil shows numerous vertically stacked gas pockets and more recently commercial gas discoveries chemical and stable isotopic compositions obtained for 24 gas samples from middle jurassic to pliocene reservoir rocks suggest that they represent a few genetic systems which are distributed vertically in the section in accordance with the stratigraphy these groups include bacterial gas in the pliocene a mixture of bacterial and thermogenic gas in the lower cretaceous and thermogenic gas in the jurassic section the bacterial gases in the pliocene and the lower cretaceous appear not to be cogenetic and likewise the thermogenic gas in the lower cretaceous and the jurassic sections excluding one case where the thermogenic gas appears to represent phase separation from a probably cogenetic light oil the relationship of either the bacterial or other thermogenic gas samples to particular potential source rocks in the section or to light oil shows encountered in the middle jurassic section is at this stage inconclusive

evidence for large scale methane venting due to rapid drawdown of sea level during the messinian salinity crisis we report the discovery from three dimensional seismic mapping of a field of pockmarks buried at present depths of 4 km in the levant basin southeastern mediterranean the pockmarks cover an area of 1000 km 2 have diameters as great as 2 km and erode as much as 200 m into their substrate of deep water clastic sediments which immediately predate the messinian salinity crisis msc ca 6 2 5 5 ma these craters are filled by the basal units of the messinian evaporites thus implying they formed at or close to the time of the earliest major drawdown of sea level all the pockmarks are developed at a single regionally correlatable surface horizon n which is observed throughout the mediterranean to coincide with the onset of the msc in the study area this surface formed by erosion and drawdown with a magnitude of 900 1000 m we propose that this rapid basinal drawdown led to a dramatic increase of the shallow subsurface pore fluid overpressure regime in the mostly fine grained pre messinian sediments subsequent pressure release then resulted in high flux fluid venting sediment remobilization and pockmark formation the spatial association of the craters with an underlying canyon suggests that their formation is linked to biogenic methane generation the model of drawdown induced overpressuring and remobilization may be applicable to many other evaporitic basins and represents a novel mechanism for inducing large scale sediment remobilization of shallow buried depositional systems at the earliest stages of salt basin development

morphostructure growth patterns and tectonic control of the rhone and nile deep sea fans a comparison the rhone and the nile rivers shape the sediment distribution of the ligurian and the levantine basins in the mediterranean sea respectively both rivers cut huge canyons in their bed rock during the messinian desiccation of the mediterranean and the subsequent early pliocene marine transgression reached far inland both in france and in egypt the sediment supply of both rivers was affected by climatic variations during the pleistocene but whereas glacial periods were associated with reduced water flow in the rhone they can be correlated with enhanced flow of the nile the sediment distribution and accumulation patterns of both rivers built large deltas on their continental shelves during high sea level stands during lowstands both rivers flowed as far as the shelf e ge and transported their sedimentary load through the continental slopes directly to their deep sea fans the patterns of sediment distribution and accumulation in the marine basins of the ligurian and the levantine seas were affected also by pliocene quaternary tectonic activity and halokinetic offsets that led to sediment instability the present hydrographic and sedimentological regimes of both rivers do not represent their natural potential due to artificial interference the principal economic significance of the deep sea fan accumulation process is the transportation of medium and coarse grained sediments into the deep marine basin considering the effect of these sediments on stratal permeabilities and hydrocarbon potential the presented comparative overview emphasizes recent and subrecent sedimentological aspects that are critical to petroleum exploration in active and extinct deep sea depositional environments

tectonic evolution of the eastern mediterranean basin and its significance for hydrocarbon prospectivity in the ultradeepwater of the nile delta the northeast mediterranean deepwater area 41 500 km2 was awarded to shell egypt in july 1998 operations started on 1 february 1999 to acquire a 7000 km 2 d 6 km cable 8 s 120 fold seismic survey key pre 1999 data included a 1500 km 2 d seismic 1973 vintage in december 1999 exxon exploration and production egypt acquired a 25 interest in the concession

the spatial pattern and drainage cell characteristics of a pockmark field nile deep sea fan over 25 300 seabed pockmarks were mapped from the rosetta channel region of the western niledeepseafan ndsf using concurrent high resolution 2d chirp profiler and multibeam bathymetry data which spans the holocene pleistocene period within the region apockmarkfield containing 13 800 pockmarks was analysed using spatial statistics to determine the distribution of pockmarks within the field pockmarks within the field are small 16 m diameter shallow 0 5 m deep circular depressions which formed within the last 6500 years the fluid source for the field is identified as an accumulation generation of gas beneath a hemipelagic seal c 20 40 ms beneath the seabed the position of the pockmarks is shown to be unrelated to the depth to the fluid source and an irregular high amplitude acoustic anomaly which is tentatively interpreted as a possible carbonate precipitate of biogenic microbial activity statistical spatial analysis of the field confirms the distribution of pockmarks is not random an exclusion zone surrounding each individual pockmark is identified the exclusion zone is a unique minimum radius around each pockmark which is not penetrated by any other pockmark the exclusion zone works in unison with self organised criticality soc in determining the spatial distribution of pockmarks within the field the exclusion zone is interpreted as apockmark drainagecell a conceptual model for apockmarkdrainagecell is proposed whereby pockmark formation dissipates a radius area of fluid and overpressure thereby preventing the formation of another pockmark within that cell consequently pockmarks are observed to separate or produce anti clustering tendencies within the field

geochemistry and accumulation of carbon dioxide gases in china we propose the following identification indicators for co2 of different origins based on data obtained from analyzing the molecular composition of 1198 gas samples the co2 carbon isotope values of 390 samples and the helium isotope composition of 256 samples collected from wells of 16 oil and gas bearing basins and gas seepages in china and reviewing literature on organic and inorganic co2 throughout the world co2 is of organic origin if co2 content 13cco22 is of inorganic origin if dgr13cco2 8 pmil or co2 60 and co2 is of either organic or inorganic origin or a mixtur of organic and inorganic origins if 10 pmil 13cco213cco2 values of organic co2 mainly range from 10 to 22 pmil in tectonically stable oil and gas bearing basins where deep faults and magmatism are undeveloped organic co2 occurs almost exclusively in tectonically active oil and gas bearing basins where active faults and magmatism are present or in areas of modern volcanic activity the dgr13cco2 values of inorganic co2 range from 2 to 8 pmil and are distributed along faults the inorganic co2 with a dgr13cco2 value of 0 3 pmil is of metamorphic origin and that with dgr13cco2 values of 6 2 pmil is mantle derived and of magmatic origin in china the dgr13cco2 values of inorganic co2 gas pools range from 3 41 to 8 44 pmil and the helium ratio r ra is 1 which indicates that these gases are typically derived from the mantle

origin and migration of oil and natural gas in the western part of the ukrainian outer carpathians geochemical and geological approach abstract we established the generation and migration processes as well as the influence of secondary processes of oil and natural gas accumulated in lower cretaceous oligocene strata of the western part of the ukrainian outer carpathians based on the results of organic geochemical analyses of oil and natural gas related to the geological setting and source organic matter the oligocene menilite beds from the boryslav pokuttya nappe are the main source rock for oil and natural gas accumulated in the fields of the boryslav pokuttya and skiba nappes co sourcing of oils and natural gas accumulated in the skiba nappe in the study area by both menilite and spas beds of this nappe cannot be excluded in some cases e g novoshidnytsya field oil and natural gas originated within the oligocene menilite beds of the boryslav pokuttya nappe migrated through thrusts and fissure zones and accumulated in the lower cretaceous reservoirs of the skiba nappe oil from small vovche field and pohar seep of the krosno nappe was generated by menilite source rocks of this tectonic unit and migrated to the traps sporadically oils underwent secondary processes mainly biodegradation and occasionally water washing hydrocarbon gases and carbon dioxide were generated mainly during low temperature thermogenic processes from type ii kerogen and partly during the microbial co2 reduction process only natural gas from the mala volosyanka oil and gas seep of the krosno nappe was documented to have undergone biodegradation

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

petroleum and source rock characterization based on c7 star plot results examples from egypt the main types of egyptian oils condensates have been reviewed and characterized by light end hydrocarbon c7 based geochemical star plots the technique relies on high resolution gas chromatography of the c7 compounds and compositional differences are evaluated by using crossplots and multivariate polar plots c7 results of oils condensates are presented for the three main hydrocarbon provinces in egypt the gulf of suez the western desert and the nile delta and nearby mediterranean for the gulf of suez there are two main oil types which were derived from 1 cretaceous eocene carbonate rich and 2 miocene siliciclastic rich source rocks for the western desert there are three distinct oil types two of these oil types probably originated from middle jurassic carbonaceous marine shale rich source rocks whereas the third oil was produced from upper cretaceous argillaceous limestones for the nile delta there are two types of oil the majority of the oils condensates from this region are derived from tertiary shale rich source rocks but some may have a cretaceous jurassic source the given examples show that the c7 based star plot is very effective as a discriminating tool and that the c7 results are supportive and supplementary to other more standard geochemical oil condensate correlation methods the c7 data give reliable results over a wide range of thermal maturities but some compounds may be sensitive to evaporative fractionation water washing and biodegradation special attention is given to what extent the c7 data provide reliable information on source rock typing for the egyptian petroleums it appears that marine carbonate rich type ii derived oils have very different c7 compound concentrations than terrestrial clastic type iii ii derived oils for any oil condensate source correlation study be it on an exploratory regional or production field scale the light end c7 data may provide extremely useful geochemical information

maturation history of neogene quaternary sediments nile delta basin egypt geologic note the present nile delta area covers approximately 60 000 sq km fields in this area provide two thirds of the gas production in egypt geological knowledge of the nile delta is still limited because of insufficient subsurface data gas is generated and accumulates at stratigraphic levels ranging from the lower miocene to the lower pliocene the highest levels of organic maturation in the neogene quaternary section are in the northern part of the onshore area such as in the abu madi well and in the eastern part of the offshore area such as in the el temsah well where gas reservoirs occur in the lower pliocene sandstones abu madi formation and in the underlying sidi salem formation and lower miocene rocks here the sidi salem formation is probably generative having n r o of approximately 0 65 lom levels of organic metamorphism of 9 5 to 9 7 and tti time temperature index of 9 7 to 15 2 by contrast a different thermal history clearly existed in the western and eastern parts of the onshore area such as in the monaga and damanhur wells where the organic maturities are significantly lower than maturities elsewhere in the basin r o 0 38 and 0 29 respectively the predicted maturities obtained by using the lom method seem to fit much closer to the observed maturities than the predicted maturities obtained by using the lopatin tti method

origin and migration of oil and natural gas in the central part of the ukrainian outer carpathians geochemical and geological approach the generation and migration processes as well as the influence of secondary processes on oil and natural gas accumulated in lower cretaceous lower miocene strata of the central part of the ukrainian outer carpathians are discussed type ii and ii iii kerogens from the oligocene lower menilite beds of the boryslav pokuttya nappe generated oil and natural gas mainly during the middle stage of the oil window these hydrocarbons accumulated in the fields of the boryslav pokuttya and skiba nappes water washing is the dominant secondary process occurring in many accumulations regardless of reservoir rock whereas biodegradation is insignificant and is clearly evident only in natural gas from one seep in the abandoned starunia ozocerite mine all hydrocarbon gases are thermogenic and were generated from mixed organic matter containing various proportions of type ii and iii kerogens only one natural gas accumulation in the pivnichna dolyna field contains some microbial hydrocarbon contribution carbon dioxide mainly originated from thermogenic kerogen decarboxylation and sporadically from microbial processes molecular nitrogen was mainly generated during low temperature thermal transformation of organic matter and could also partly originate from nh4 rich illites of the clayey facies of the oligocene lower miocene menilite beds the integration of the geochemical data with geological setting and maturation history of source rocks enabled us to reconstruct hydrocarbon migration paths revealing the complicated trap filling process

variability of geochemical properties in a microbially dominated coalbed gas system from the eastern margin of the illinois basin usa this study outlines gas characteristics along the southeastern margins of the illinois basin and evaluates regional versus local gas variations in seelyville and springfield coal beds our findings suggest that high permeability and shallow 100 250 m depths of these indiana coals allowed inoculation with methanogenic microbial consortia thus leading to widespread microbial methane generation along the eastern marginal part of the illinois basin low maturity coals in the illinois basin with a vitrinite reflectance ro 0 6 contain significant amounts of coal gas 3 m 3

recent depositional patterns of the nile deep sea fan from echo character mapping the nile deep sea fan was surveyed in 1998 using swath multibeam bathymetry backscatter imagery and 3 5 khz and seismic profiling on the basis of this new data set the fan has been divided into three main provinces western central and eastern recent sedimentary patterns as deduced from echo character mapping show that gravity induced sedimentary deposits are predominant and are expressed through either slides debris flows or turbidites turbidity processes are particularly active in the westernmost province through a well developed network of deep sea channels this network controls turbiditic flows that subsequently overflow levees mass wasting processes are extremely efficient all over the nile deep sea fan in the upper central province a broad allochthonous tongue of mass flow deposits generated by slope destabilizations may have led to channel avulsions and migrations in the eastern province a northwest southeast deformed belt resulting from combined deep seated and salt related tectonics contains large proportions of destabilized sediments i e originating from destabilization processes such as mass movements probably settled by recent tectonic activity this article highlights 1 the influence of both thin skinned and thick skinned tectonics on sedimentary distribution and 2 the predominance of gravity induced sediments turbidites slides and mass flows in such submarine fans the distribution of debris flows and sedimentary deposits settled by turbidity currents appears fundamental in predicting reservoir geometry and economic potential

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

origin secondary processes and migration of oil and natural gas in the central part of the polish outer carpathians abstract the origin migration pathways and the influence of secondary processes of oil and natural gas accumulated in lower cretaceous to lower miocene strata of the central part of the polish outer carpathians are investigated based on organic geochemical analyses oil and thermogenic hydrocarbon gases were mainly generated from both types ii and iii kerogens in the oligocene menilite beds of the silesian skole and dukla nappes moreover some gases originated from type iii kerogen of lower cretaceous ve ovice shales in the silesian nappe and lower cretaceous spas shales in the skole nappe these gases migrated and subsequently mixed with oil and gas produced from type ii kerogen in the menilite beds the most extensive biodegradation and water washing were observed in oils from harklowa and iwonicz fields and in oil from the bf franek dug well in bobrka rogi field one of the world s oldest with exploitation started in 1854 of the silesian nappe oils most affected by evaporative fractionation come from kro cienko and iwonicz zdroj fields natural gas originated mainly from low mature thermogenic processes oil window with some primary microbial gas contribution primary carbon dioxide reduction and many gas samples show secondary alteration caused by biodegradation carbon dioxide originated from both thermogenic kerogen decarboxylation and microbial processes

deep biogenic methane and drilling associated gas artifacts influence on gas based characterization of petroleum fluids classical interpretation of drilling mud gas molecular and isotopic data in petroleum systems can be impacted by both natural and engineering processes which can introduce allochthonous and or isotopically unrepresentative gas species this study investigates 1 widespread primary biogenic methane c1 and its impact on mud gas based petroleum system interpretation and on the properties of the petroleum fluids in the subsurface 2 differentiation of migration mixing versus diffusion leakage in petroleum systems and 3 bias introduced by gases generated at the drill bit including alkenes and isotopically changed gaseous alkanes our goal was to provide solutions to flag impacted intervals by the above processes minimize resulting interpretative bias and extract additional information about the fluid using advanced mud gas logging amgl tools and workflows an amgl interpretative methodology was developed using a combination of logs of c1 to pentane c5 concentrations and 13c c1 this approach allows identification of subsurface intervals containing nearly pure background primary biogenic c1 and can quantify its concentration per rock volume consequently thermogenic end member fluid types can be differentiated from mixtures with the biogenic c1 and assess its impact on the gas oil ratio of the mixture additionally two different processes can be responsible for mixing trends apparent on classical interpretation charts e g modified schoell diagram the presented analysis of amgl allows the deciphering of migration mixing versus diffusion leakage mixing trends using studied wells as examples another bias of mud gas based interpretation originates from artificial gases generated by drill bit metamorphism dbm evidence that dbm produced artificial alkanes in addition to alkenes via cracking of oil based drilling mud included strong correlation of concentrations among alkenes furthermore the concentration of alkenes strongly correlated with the magnitude of shift in 13c of c1 and ethane c2 between mud gas and solution gas from bottomhole fluids sampled at equivalent depths the artificial alkanes contributed by the drilling process can strongly bias and interfere with petroleum systems early interpretation based on mud gas molecular and isotopic ratios the bias depends on relative contributions of petroleum reservoir liberated mud gases and dbm gases which in this study reached half of the total c1 and c2 in the analyzed mud gas conversely alkene concentrations can be used as mud gas quality check parameters and early indicators of excess heat generated at the drill bit

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

natural gas washing induces condensate formation from coal measures in the pinghu slope belt of the xihu depression east china sea basin insights from fluid inclusion geochemistry and rock gold tube pyrolysis abstract a systematic study into the origin and potential formation processes of condensates in the pinghu slope belt of the xihu depression east china sea basin was conducted using fluid inclusion analysis petroleum geochemistry and rock gold tube pyrolysis grains containing oil inclusions goi technique well logging and formation testing together indicate that the present day gas condensate layer has evolved from a paleo oil layer biomarkers from oil inclusions indicate that these paleo oils were generated from coal measures at a moderate maturity level based on organic macerals elemental ratios of kerogens rock eval pyrolysis and results from gold tube pyrolysis experiments the coal measures are evaluated as source rocks prone to gas generation and capable of oil formation at moderate levels of maturity this maturity level coincides with present day condensate maturities determined from a series of molecular maturity ratios vitrinite reflectance 0 7 1 0 ro biomarkers related to specific sources suggest that the condensates are also coal derived these observations suggest that the condensates are derived from secondary alteration following paleo oil accumulation rather than being related to the thermal degradation of resin rich organic matter at a low maturity level or the thermal cracking of kerogen at a high maturity level some condensates are characterized by a significant loss of n alkanes with low carbon numbers and relatively high toluene n heptane tol nc7 ratios this indicates that the paleo oil reservoirs have been subjected to varying degrees of gas washing which resulted in condensate formation as opposed to the process of oil cracking under high temperatures subsequent interactions between gas and paleo oils resulted in oil inclusions showing fluorescence varying from near yellow to blue variable vapor volume fractions measured in the same oil assemblage are the result of two phase trapping of oil and gas the widely observed presence of bitumen asphaltene in the reservoir pores and bitumen bearing oil inclusions is considered to be due to gas deasphalting homogenization temperatures th of associated aqueous inclusions and burial thermal curves suggest that the initial paleo oil reservoirs formed in the late miocene excess gas composed of local mature gas and heterochthonous high to over mature gas migrated into the paleo oil reservoirs later in the quaternary this process not only resulted in the formation of condensates but also led to vertical differentiation of density and wax content in the reservoirs

petroleum migration alteration and remigration within troll field norwegian north sea troll field represents the largest petroleum discovery within the entire north sea area in oil equivalents with 74 of the accumulated petroleum present as dry gas and 26 as a heavy biodegraded oil leg the field is divided into several provinces based on the distribution of gas and oil and the gas and oil have been suggested to be cogenetic the migration and filling model presented in this paper suggests that the oil and gas represent two different migration phases and that gas migration and filling predate oil emplacement two different oil populations have been characterized and mapped in troll field applying conventional geochemical techniques we suggest that the two oil populations migrated into the structure through two different migration systems oil and gas were subsequently biodegraded within the reservoir the two oil populations have been found in neighboring oil and gas discoveries and an oil oil correlation with these discoveries has been used to determine the location of field filling points and regional migration routes when oil biodegradation terminated fresh oil continued to migrate into the reservoir and mixed with the residue of the biodegraded oil the field was tilted downward to the west in the neogene and oil and gas remigrated within the field with a possible spillage of gas tilting resulted in a dominantly upward movement of the oil phase whereas gas migrated laterally residual oils in the water zone copyright 1997 the american association of petroleum geologists all rights reserved 1manuscript received july 18 1995 revised manuscript received march 14 1996 final acceptance august 16 1996 2saga petroleum a s kjorboveien 16 p o box 490 n 1301 sandvika norway 3newcastle research group drummond building university of newcastle upon tyne newcastle upon tyne ne1 7ru united kingdom we would like to thank the partners in the troll license who allowed us to publish these results per erling johansen and tone molvik are acknowledged for the analytical work and snorre olaussen nigel mills and terje hellem are thanked for helpful discussions and for assistance with the back tilting of the irap reservoir field model jill sonrier is acknowledged for typing we acknowledge k m wolgemuth john f brint and two anonymous reviewers for helpful comments that improved the paper significantly

methanogenic biodegradation of petroleum in the west siberian basin russia significance for formation of giant cenomanian gas pools approximately 1700 tcf 48 trillion m3 of dry gas 99 methane reserves and resources occur in western siberia mostly in shallow 1500 m 4921 ft cenomanian pools in the northern part of the basin this dry gas constitutes about 11 of the world s conventional gas endowment and about 17 of the annual gas production the origin of the dry gas has been debated extensively over the last 45 yr but remains controversial widely discussed hypotheses on the origin include early mature thermogenic gas from coal primary microbial gas from dispersed organic matter or coal and thermogenic gas from deep source rocks however all these hypotheses are in some ways inconsistent with the molecular or isotopic composition of the gases or the results of basin and petroleum systems modeling here i present geochemical and geological evidence that a significant although yet not quantified part of the shallow dry gas in the northern west siberian basin originated from methanogenic biodegradation of petroleum circumstantial evidence includes the occurrence of heavily biodegraded oil legs and residual oil in many cenomanian gas pools as well as geochemical evidence of heavy to slight biodegradation in jurassic albian reservoirs commonly underlying the cenomanian pools direct evidence includes most importantly 13c enriched co2 in pools with biodegraded oil although data are limited which indicates 40 70 wt conversion of oil derived co2 to secondary microbial methane distinctive hydrocarbon molecular and isotopic compositions of most gases in cenomanian pools average dryness c1 sum c1 c5 is 0 9976 average 13c of methane is 51 8 suggest that they represent mixtures of biodegraded thermogenic gases from deep mainly jurassic source rocks and secondary microbial methane with an occasional small addition of primary microbial methane contribution of early mature coal derived gas is possible in areas with the most significant thermal stress of hauterivian aptian sediments but remains speculative review of petroleum habitats of five representative oil gas condensate fields in western siberia including the world s second largest gas field urengoyskoe suggests that methanogenic biodegradation may best explain the observed distribution and properties of fluids in the shallow reservoirs of those fields recognition of secondary microbial gas in western siberia helps explain the observed dominance of gas in the shallow cool northern part of the basin where conditions were more favorable for prolonged petroleum biodegradation than in the central and southern parts of the basin secondary microbial gas has been recognized worldwide and may 1 represent a volumetrically significant exploration target in shallow reservoirs perhaps more significant than primary microbial gas and 2 indicate effective thermogenic petroleum systems in the deeper sections large volumes up to 66 500 tcf 1884 trillion m3 of secondary microbial methane could have been generated from biodegraded petroleum accumulations worldwide although a part of that gas accumulated as oil dissolved free and hydrate bound gas most gas apparently escaped into the overburden atmosphere and ocean and could have affected global climate in the geologic past