the effects of thermochemical sulfate reduction upon formation water salinity and oxygen isotopes in carbonate gas reservoirs abstract thermochemical sulfate reduction tsr is a well known process that can lead to sour h2s rich petroleum accumulations most studies of tsr have concentrated upon gas chemistry in this study we have investigated palaeoformation water characteristics in a deep anhydrite bearing dolomite sour gas reservoir of permian age in abu dhabi using fluid inclusion stable isotope petrographic and gas chemical data the data show that low salinity isotopically distinct water was generated within the reservoir by reaction between anhydrite and methane the amount of water added to the reservoir from tsr indicated by reduced fluid inclusion salinity and water 18o values varied systematically with the extent of anhydrite reaction with methane water salinity and isotope data show that the original formation water was diluted by between four and five times by water from tsr thus we have shown that large volumes of very low salinity water were generated within the gas reservoirs during diagenesis following gas emplacement the salinity of formation water in evaporite lithologies is therefore not necessarily high modelling based upon a typical khuff gas reservoir rock volume suggests that initial formation water volumes can only be increased by about three times as a result of tsr the extreme local dilution shown by the water salinity and 18o data must therefore reflect transiently imperfect mixing between tsr water and original formation water the creation of large volumes of water has important implications for the mechanism and rate of thermochemical sulphate reduction and the interpretation of gas volumes using petrophysical logging tools

estimation of the hydrocarbon loss through major tectonic events in the tazhong area tarim basin west china abstract the tazhong area located in the central part of the tarim basin is the major commercial oil and gas province in the tarim basin hydrocarbons have been discovered in the ordovician silurian and carboniferous reservoirs most of the hydrocarbon reservoirs in the tazhong area have experienced post charge and accumulation readjustment or loss associated with multiple post charge tectonic movements the relationship between the destroyed hydrocarbon proportion dhp caused by tectonic events and tectonic movement intensity tmi in 15 well blocks has been established for the entire tazhong area using tmi analysis tmi can be quantitatively characterized by the denudation thickness and effective caprock thickness ratio whereas the dhp can be quantitatively determined by the variations of the palaeo and current oil water contacts there appears to be a good relationship between dhp and tmi with a correlation coefficient of 0 9 most of the original ordovician hydrocarbon accumulations formed in the late ordovician had been lost with an average dhp of up to 70 the tectonic movement is much weaker in the late silurian and late permian period with the dhps for the silurian reservoirs and carboniferous reservoirs in the eastern tazhong area averaging 50 all the dhps show a gradual increase from west to the east figure 17 figure 18 figure 19 the preservation condition of the reservoirs is generally better in west than in the east

shallow gravity driven flow in a poro elastic layer by combining biot s theory of poro elasticity with standard shallow layer scalings a theoretical model is developed to describe axisymmetric gravity driven flow through a shallow deformable porous medium motivated in part by observations of surface uplift around co2 sequestration sites the model is used to explore the injection of a dense fluid into a horizontal deformable porous layer that is initially saturated with another less dense fluid the layer lies between a rigid base and a flexible overburden both of which are impermeable as the injected fluid spreads under gravity the matrix deforms and the overburden lifts up the coupled model predicts the location of the injected fluid as it spreads and the resulting uplift of the overburden due to deformation of the solid matrix in general the uplift spreads diffusively far ahead of the injected fluid if fluid is injected with a constant flux and the medium is unbounded both the uplift and the injected fluid spread in a self similar fashion with the same similarity variable r t1 2 the asymptotic form of this spreading is established results from a series of laboratory experiments using polyacrylamide hydrogel particles to create a soft poro elastic material are compared qualitatively with the predictions of the model

geochemical variations among eastern saudi arabian paleozoic condensates related to different source kitchen areas abstract large reserves of gas and condensate are found in paleozoic reservoirs khuff unayzah and jauf in the ghawar area of eastern saudi arabia the source rock for these hydrocarbons is the basal organic rich hot shale of the qusaiba member of the silurian qalibah formation this is the same prolific source rock that generated the sweet light crude oil and condensate found in the unayzah reservoirs in central saudi arabia thermal maturity modeling of the source rock identifies two kitchen areas to the east and west of the ghawar structure the west kitchen was well into the oil window by 210 ma while the east kitchen was just entering the oil window large areas of both kitchens were in the wet gas window by 125 ma and by 90 ma most of the area north of ghawar was generating dry gas in eastern saudi arabia paleozoic oil and condensate are found in early formed structures carboniferous and early triassic growth whereas late cretaceous and tertiary structures contain only dry gas the geochemistry of the oils and condensates indicates two groups each generated from a different hydrocarbon kitchen suggesting that the two kitchens differ slightly in organic facies within each group variations among hydrocarbons are related to the thermal maturity of the source rock during generation within the khuff reservoir gas souring is caused by thermochemical sulfate reduction

origin of the unusually high dibenzothiophene oils in tazhong 4 oilfield of tarim basin and its implication in deep petroleum exploration abstract unusually high dibenzothiophene dbt concentrations are present in the oils from the tazhong 4 oilfield in the tazhong uplift tarim basin positive ion electrospray ionization fourier transform ion cyclotron resonance mass spectrometry ft icr ms was used in combination with conventional geochemical approaches to unravel the enrichment mechanisms significant amounts of s1 species with relatively low dbe values 0 8 i e sulfur ethers mercaptans thiophenes and benzothiophenes were detected in three lower ordovician oils with high thermal maturity which were suggested to be the products of thermochemical sulfate reduction tsr in the reservoir the occurrence of tsr was also supported by the coexistence of thiadiamondoids and abundant h2s in the gases associated with the oils obviously low concentrations of the dbe 9 s1 species mainly equivalent to c0 c35 dbts compared to its homologues were observed for the three oils which were probably altered by tsr indicating that no positive relationship existed between tsr and dbts in this study the sulfur compounds in the tazhong 4 oils are quite similar to those in the majority of lower ordovician oils characterized by high concentrations of dbts and dominant dbe 9 s1 species with only small amounts of sulfur compounds with low thermal stability dbe 0 8 suggesting only a small proportion of sulfur compounds were derived from tsr it is thermal maturity rather than tsr that has caused the unusually high dbt concentrations in most of the lower ordovician oils we suggest that the unusually high dbt oils in the tazhong 4 oilfield are caused by oil mixing from the later charged lower ordovician or perhaps even deeper with high dbt abundances from the earlier less mature oils which was supported by our oil mixing experiments and previous relevant investigations as well as abundant authigenic pyrite of hydrothermal origin we believe that tsr should have occurred in the tazhong uplift based on our ft icr ms results however normal sulfur compounds were detected in most oils and no increase of 13c2h6 13c4h10 was observed for the gas hydrocarbons suggesting only a slight alteration of the oils by tsr currently and or recently we suspect that the abnormal sulfur compounds in the lower ordovician oils might also be a result of deep oil mixing which might imply a deeper petroliferous horizon i e cambrian with a high petroleum potential this study is important to further deep petroleum exploration in the area

an investigation of the in reservoir conversion of oil to gas compositional and kinetic findings from closed system programmed temperature pyrolysis abstract the thermal alteration of reservoired petroleum upon burial was simulated by closed system non isothermal pyrolysis samples of a medium gravity oil from the norwegian north sea central graben were pyrolysed in microscale sealed glass or quartz vessels at heating rates of 0 1 0 7 and 5 0 k min 1 up to temperatures varying from 300 to 650 c the composition of the oil and gas in each vessel was determined by a single step on line gas chromatographic analysis with increasing rate of heating the onset of all oil degradation reactions was shifted to higher temperatures the same successive compositional changes occurred in each case increase in the total yield of gc detectable compounds significant gas c1 c4 generation accompanied by a decrease in yield of heavy components aromatisation and attainment of maximum gas yield and finally a cracking of the c2 gas components the stable final mixture consisted of methane aromatic hydrocarbons pyrobitumen and possibly hydrogen kinetic modelling of the oil to gas conversion resulted in a narrow gas potential vs activation energy distribution between 66 and 70 kcal mol assigning 35 of the total gas potential 460 mg per g of oil to an energy of 66 kcal mol and 29 to 67 kcal mol pre exponential factor 1 1 1016 s 1 by extrapolation to natural maturation conditions the onset of gas generation is predicted to occur between 160 and 190 c for geological heating rates between 0 53 and 5 3 k ma 1 predictions from the model are in accordance with the observed preservation of liquid hydrocarbons in a deep hot 165 c petroleum reservoir from the saga 2 4 14 well norwegian continental shelf

serpentine iron rich phyllosilicates and fayalite produced by hydration and mg depletion of peridotite duluth complex minnesota usa

the tectonic significance of dikes of irregular fold like shape the effects of postemplacement deformation on originally nonplanar dikes were tested experimentally and compared to field data from the rainy lake zone ontario canada where irregular fold like structures are observed the analyses reveal that two processes are responsible for the irregular dike geometries first dikes intrude with varying orientations as they propagate across layers with rheology contrasts thus developing nonplanar profiles second with postemplacement ductile deformation dikes that are more competent than the host rocks are folded and refracted in variable amounts with the amount depending on the initial dike geometry the heterogeneities in the host and the regional strain deformed dikes and veins are often used as kinematic markers to evaluate regional tectonics using methods that generally assume that veins had planar walls prior to deformation the experimental results indicate that caution should be taken when using this approach and they provide criteria to distinguish between intrusive and deformation structures in intensely deformed areas where distinction between the two is not readily apparent

late precambrian series and structures in the navalpino variscan anticline central iberian peninsula the navalpino anticline is a major variscan structure in the central iberian zone of spain three lithological groups are defined in the pre ordovician rocks of this anticline the rifean or lower vendian extremeno dome group is unconformably overlain by the upper vendian ibor navalpino group this latter group presents two different facies separated by a nw se trending synsedimentary fault the lower cambrian valdelacasa group unconformably overlies both the extremeno dome and the ibor navalpino groups

early generation characteristics of a sulfur rich monterey kerogen this paper documents the critical role of organic sulfur in the early formation of bitumen and oil from a sulfur rich kerogen from the miocene monterey formation in california monterey kerogen commonly contains abundant chemically bound sulfur 8 14 wt which appears responsible for early low temperature generation of heavy sulfur rich petroleum due to the lower energy required to break carbon sulfur bonds compared to carbon carbon bonds hydrous pyrolysis was used to simulate the natural generation of petroleum from a sulfur rich type ii kerogen from the phosphatic member of the monterey shale the amount and distribution of sulfur in the kerogen bitumen and expelled oil generated at various temperatures indicate that 1 initial bitumen and oil generation is directly related to cleavage of weak sulfur linkages and 2 most of the expelled oil is formed by decomposition of bitumen and not directly from the kerogen finally comparison of hydrous pyrolysis results with other type ii kerogens suggests that the temperature of peak bitumen generation is inversely related to the heteroatom content primarily sulfur of the kerogen low temperature bitumen generation is an important exploration concept because kerogen rich in sulfur can generate producible oil at thermal maturities significantly below conventionally accepted values furthermore established relationships between thermal maturity sulfur content and other oil properties such as api gravity can be used to rank exploration prospects

physical and chemical controls on the composition of waters in sedimentary basins abstract the composition of subsurface waters is determined not only by diagenetic reactions but also by physical processes of fluid convection and hydrodynamic dispersion the south west louisiana gulf coast provides an instructive field example of the net effects of diagenetic reaction and solute transport on pore water compositions in a regional siliciclastic sequence most formation waters here have compositions totally unlike the compositions of the connate meteoric and marine fluids that were buried with their host sediments at the time of deposition dissolved chloride has been generated by the subsurface dissolution of salt domes and has been pervasively transported by fluid flow throughout most of the upper 3 km of the sedimentary section the simultaneous systematic variation in dissolved na k mg ca and alkalinity with chloride in these waters supports the hypothesis that metastable thermodynamic buffering by silicate carbonate mineral assemblages is a first order control on fluid compositions the chemical potential of chloride or alternatively total anionic charge appears to be a master variable which ranks in importance with such other variables as pressure and temperature in driving diagenesis in this region this variable is controlled largely by physical processes of advection and dispersion in the upper 3 km of the section and by dehydration reactions in deeper mudstone dominated sediments where the composition of the fluid is largely rock buffered the ultimate origin of the fluid and its pathway of chemical evolution may be obscured at least in terms of major solute composition non buffered components such as cl and br or isotopic compositions are more likely to retain information on original end member fluid compositions and reaction pathways

application of sulfur and carbon isotopes to oil source rock correlation a case study from the tazhong area tarim basin china abstract up until now it has been assumed that oil in the palaeozoic reservoirs of the tazhong uplift was derived from upper ordovician source rocks oils recently produced from the middle and lower cambrian in wells zs1 and zs5 provide clues concerning the source rocks of the oils in the tazhong uplift tarim basin china for this study molecular composition bulk and individual n alkane 13 c and individual alkyl dibenzothiophene 34 s values were determined for the potential source rocks and for oils from cambrian and ordovician reservoirs to determine the sources of the oils and to address whether 13 c and 34 s values can be used effectively for oil source rock correlation purposes the zs1 and zs5 cambrian oils and six other oils from ordovician reservoirs were not significantly altered by tsr the zs1 oils and most of the other oils have a v shape in the distribution of c 27 c 29 steranes bulk and individual n alkane 13 c values predominantly between 31 to 35 vpdb and bulk and individual alkyldibenzothiophene 34 s values between 15 to 23 vcdt these characteristics are similar to those for some cambrian source rocks with kerogen 13 c values between 34 1 and 35 3 and 34 s values between 10 4 and 21 6 the oil produced from the lower ordovician in well ym2 has similar features to the zs1 cambrian oils these new lines of evidence indicate that most of the oils in the tazhong uplift contrary to previous interpretations were probably derived from the cambrian source rocks and not from the upper ordovician conversely the 13 c and 34 s values of zs1c cambrian oils have been shown to shift to more positive values due to thermochemical sulfate reduction tsr thus 13 c and 34 s values can be used as effective tools to demonstrate oil source rock correlation but only because there has been little or no tsr in this part of the section

gas geochemistry of the mobile bay jurassic norphlet formation thermal controls and implications for reservoir connectivity the mobile bay gas field is located offshore alabama in the northern gulf of mexico production is from eolian dunes of the jurassic norphlet sandstone at depths exceeding 6100 m 20 000 ft and temperatures greater than 200c reservoir connectivity and compositional variation including the distribution of nonhydrocarbon gases h2s and co2 are critical factors in production strategy to evaluate the controls on compositional variation and connectivity detailed molecular and isotopic analyses were conducted for 29 wells analysis of volatiles in fluid inclusions suggests that the field was originally filled with oil that subsequently cracked to gas in addition to the thermal destruction cracking of oil the process of thermochemical sulfate reduction tsr continues to destroy the remaining hydrocarbons through oxidation of gas and reduction of sulfate to form h2s and co2 the variable extent of the tsr process at mobile bay results in a wide range of hydrocarbon and h2s compositions condensates are almost exclusively composed of diamondoids whose composition appears controlled by h2s concentrations in contrast to hydrocarbon and h2s contents co2 concentrations are relatively constant throughout the field carbon isotopic ratios for co2 correlate positively with those for wet gas hydrocarbons but are heavier than expected for co2 originating from hydrocarbon oxidation via tsr the narrow range of co2 contents and heavy isotope ratios suggests that co2 is regulated by water rock equilibration and carbonate precipitation the destruction of the hydrocarbon gas and mineralization of the carbon dioxide product create a volume reduction and an associated drop in reservoir pressure this process creates several internal sinks or exits that may control the spill direction for gas in the field

thermochemical sulphate reduction in cambro ordovician carbonates in central tarim h2s and co2 are found in elevated concentrations in palaeozoic reservoirs in the tarim basin in china we have carried out analyses on gas petroleum mineral cement and bulk rock compositions and isotope ratios together with petrography and fluid inclusion to assess the origin of the h2s a deep crustal e g volcanic origin of the h2s and co2 is unlikely since the inert gases n2 and he have isotope ratios totally uncharacteristic of this source organic sources are also unlikely since the source rock has low a sulphur content and the sulphur isotope ratio of the petroleum correlates positively with the sulphur content the opposite of what would be anticipated from petroleum derived h2s bacterial sulphate reduction is unlikely because temperatures are too high for bacteria to have survived thermochemical sulphate reduction of petroleum fluids by anhydrite in lower ordovician and cambrian carbonate reservoirs is the most likely source of both the h2s and the co2 causing isotopically characteristic pyrite co2 gas and calcite cement h2s and possibly co2 migrated into silurian sandstone reservoirs by cross formational flow the h2s with the same sulphur isotope ratio as ordovician anhydrite was partially lost from the fluid phase by extensive growth of late diagenetic pyrite similarly the co2 was partially lost from the fluid phase by precipitation of late diagenetic calcite the h2s that resulted from tsr underwent reaction with the remaining petroleum resulting in locally elevated organic sulphur concentrations in the petroleum and the progressive adoption of the ordovician anhydrite sulphur isotope ratio

comment on paleozoic reactivation structures in the appalachian ouachita marathon foreland far field deformation across pangea by j craddock d malone r porter j compton j luczaj a konstantinou j day and s johnston earth science reviews 169 2017 1 34

tsr versus non tsr processes and their impact on gas geochemistry and carbon stable isotopes in carboniferous permian and lower triassic marine carbonate gas reservoirs in the eastern sichuan basin china abstract the palaeozoic and lowermost mesozoic marine carbonate reservoirs of the sichuan basin in china contain variably sour and very dry gas the source of the gas in the carboniferous permian and lower triassic reservoirs is not known for certain and it has proved difficult to discriminate and differentiate the effects of thermal cracking and tsr related processes for these gases sixty three gas samples were collected and analysed for their composition and carbon stable isotope values the gases are all typically very dry alkane gases being 97 5 methane with low 2 s and co 2 carboniferous gas is negligibly sour while the lower triassic gas tends to be most sour the elevated h 2 s up to 62 is due to thermochemical sulphate reduction with the most sour triassic and permian reservoirs being deeper than 4800 m the non tsr affected carboniferous gas is a secondary gas that was derived from the cracking of sapropelic kerogen derived oil and primary gas and is highly mature carboniferous and non sour triassic and permian gas has unusual carbon isotopes with methane and propane being isotopically heavier than ethane a reversal of typical low to moderate maturity patterns the gas in the non sour triassic and permian reservoirs has the same geochemical and isotopic characteristics and therefore the same source as the carboniferous gas tsr in the deepest triassic reservoirs altered the gas composition reaching 100 dryness in the deepest most sour reservoirs showing that ethane and propane react faster than methane during tsr ethane evolves to heavier carbon isotope values than methane during tsr leading to removal of the reversed alkane gas isotope trend found in the carboniferous and non sour triassic and permian reservoirs however methane was directly involved in tsr as shown by the progressive increase in its carbon isotope ratio as gas souring proceeded co 2 increased in concentration as gas souring proceeded but typical co 2 carbon isotope ratios in sour gases remained about 4 v pdb showing that it was not solely derived from the oxidation of alkanes instead co 2 may partly result from reaction of sour gas with carbonate reservoir minerals such as fe rich dolomite or calcite resulting in pyrite growth as well as co 2 generation

genetic origin of sour gas condensates in the paleozoic dolomite reservoirs of the tazhong uplift tarim basin abstract twenty gas condensates six oils and their associated formation water from the tazhong uplift in the tarim basin nw china were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates thermochemical sulfate reduction tsr is the main secondary alteration process relevant to the generation of sour gas condensates those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations the hydrogen sulfide h 2 s concentrations in gas phase show a strong positive correlation to gas oil ratio organosulfur compound osc concentrations in the liquid phase while methane isotopic values are mainly controlled by thermal maturity wet gas components show unusual 13 c enrichment at a low degree of thermal cracking geochemical anomalies occur mainly in the lower ordovician yingshan fm where more h 2 s has been discovered while the upper ordovician lianglitage fm shows less irregularity and h 2 s concentration is much lower generation of sour gas condensate is most likely caused by tsr alteration which could oxidize liquid hydrocarbons and form a large quantity of oscs isotopic enrichment of 34 s in formation water sulfate and elevated osc concentrations provide supplemental evidence of tsr alteration both burial history reconstructions based on fluid inclusion analysis and pvt simulation indicate that in situ tsr reactions mainly occurred around last 10 ma during the late himalayan orogeny and overall tsr extent is relatively low in the tazhong uplift

the influence of thermochemical sulphate reduction on hydrocarbon composition in nisku reservoirs brazeau river area alberta canada abstract the upper devonian nisku formation reservoirs of the brazeau river area of west central alberta canada produce oil and sweet and sour gas condensate generally oil pools are located updip in the study area and sour 6 31 h2s gas condensate downdip h2s in the study area is formed by thermochemical sulphate reduction tsr all liquid hydrocarbons probably have one source with the duvernay formation being the most likely candidate assessing the relative maturity of the oils and condensates is difficult because of the wide variation in thermal maturity and the effects of tsr but the ratio of pristane n heptadecane does appear to decrease with increasing maturity for this sample set with increasing tsr the following changes were noted decrease in the saturate aromatic hydrocarbon ratio increase in the relative abundance of organo sulphur compounds e g benzothiophenes 34s values of liquid hydrocarbon samples approached the values for anhydrite of the nisku formation in the study area and an increase in 13c of the saturate fraction h2s concentrations in hydrodynamically isolated pools provide a good estimate of the extent of tsr in these reservoirs however other pools have anomalously high h2s concentrations for their depth suggesting that the h2s was generated at greater depths and migrated updip into these pools

origin and quantitative source assessment of deep oils in the tazhong uplift tarim basin abstract a large amount of deep oil has been discovered in the tazhong uplift tarim basin whereas the oil source is still controversial an integrated geochemical approach was utilized to unravel the characteristics origin and alteration of the deep oils this study showed that the lower cambrian oil from well zs1c 1x was featured by small or trace amounts of biomarkers unusually high concentration of dibenzothiophenes dbts high 34 s of dbts and high 13 c value of n alkanes these suggest a close genetic relationship with the cambrian source rocks and tsr alteration on the contrary the middle cambrian oils from well zs1 2a were characterized by low 13 c of n alkanes and relatively high 34 s of individual sulfur compounds and a general v shape of steranes indicating a good genetic affinity with the middle upper ordovician source rocks the middle cambrian salt rock separating the oils was suggested to be one of the factors responsible for the differentiation it was suggested that most of the deep oils in the tazhong uplift were mixed source based on biomarkers and carbon isotope which contain tsr altered oil in varied degree the percentage of the oils contributed by the cambrian lower ordovician was in the range of 19 100 average 57 controlled by several geological and geochemical events significant variations in the 34 s values for individual compounds in the oils were observed suggesting a combination of different extent of tsr and thermal maturation alterations the unusually high dbts concentrations in the tazhong 4 oilfield suggested as a result of mixing with the zs1c oil 1x and lower ordovician oils based on 34 s values of dbt this study will enhance our understanding of both deep and shallow oil sources in the tazhong uplift and clarify the formation mechanisms of the unusually high dbts oils in the region

fracture cleavage in the duluth complex northeastern minnesota an unusual cleavage is developed in troctolitic rocks of the late precambrian duluth complex in northeastern minnesota it is localized in a zone 250 m wide and it is parallel to the axes of gentle folds in rhythmic banding the rocks show no textural signs of ductile deformation and the cleavage is defined by fractures that are fairly straight and evenly spaced but on a fine scale somewhat irregular multiple anastomosing and locally crosscutting textures indicate a purely extensional mode of development cleavage refraction across interfaces of bands of different composition is common serpentine coats the fracture surfaces and suggests that fluids were present to produce this alteration during formation of cleavage the geological relationships and serpentinization suggest conditions of cleavage formation of 1 5 1 8 kb and 350 to 500 c the presence of extensional fractures indicates that cleavage formation involved crack propagation at high fluid pressures the absence of shear fractures or ductile deformation features and the nature of the cleavage refraction indicate that differential stresses were low localization of the cleavage seems to be due to the presence of a structure probably a dike below the layered intrusive rocks this could have served to localize stresses and reactive fluids at high pressure during a late stage of continental rifting

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 3 thermal cracking of liquid hydrocarbons and gas washing as the major mechanisms for deep gas condensate accumulations abstract the lunnan lower uplift in the tarim basin nw china contains a composite petroleum system with mainly biodegraded heavy oil in the west normal oil in the center and gas condensate in the east twenty three gas samples and 37 oil samples taken from three major hydrocarbon bearing intervals in the lunnan lower bulge were analyzed for their stable carbon isotopes and molecular biomarkers on the basis of their isotopic compositions biomarkers especially diamondoids and integrating the physical properties of the hydrocarbons including densities gor and pvt relationships it has been concluded that the recently discovered deep 6500 m eastern lungu giant ordovician gas condensate pool with an estimated reserve of 723 million bbl oil equivalent is a secondary hydrocarbon accumulation derived from the mixing of an early formed oil and a late formed gas the extremely dry gases with a gas dryness of 0 98 were derived from thermal cracking of crude oils and charged to an existing oil reservoir causing extensive gas washing and secondary alteration compared with most of the hydrocarbons in the lunnan area the gas from the eastern lungu gas condensate pool has a heavier carbon isotopic composition higher dryness and a higher maturity level the unique physical chemical and isotopic compositions of the gas condensate are believed to be a direct result of gas flushing of an early formed eastern lungu oil pool by a late formed dry gas from oil cracking the eastern lungu gas condensate pool is presently characterized by containing dry gas heavy oil abundant aromatics and high wax

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

simple kinetic models of petroleum formation part i oil and gas generation from kerogen modern basin analysis uses kinetic models to predict the extent of petroleum generation within potential source rocks the global kinetic model presented here assigns kinetic parameters based on gross depositional environment and stratigraphic age this is useful in areas of low geochemical knowledge including exploration frontiers five kerogen kinetic organofacies each characterized by a specific organic matter input and early diagenetic overprint can broadly be related to sedimentary facies age associations even using seismic sequence stratigraphy alone a aquatic marine siliceous or carbonate evaporite any age b aquatic marine siliciclastic any age c aquatic non marine lacustrine phanerozoic d e terrigenous non marine ever wet coastal mesozoic and younger and f terrigenous non marine coastal late palaeozoic and younger routine pyrolysis data allow partitioning of organic carbon in the immature source rock into four initial components oil oil generative gas generative and inert separate kinetic parameters for the oil and gas generative fractions allow computation of the evolving concentration and composition of the products it was assumed that the activation energy distributions for each fraction were normally distributed non linear regression of large combined laboratory and field data sets varying by up to 12 orders of magnitude in heating rate established optimum values for the required ten 5 2 kinetic parameter sets a emean e differences in kinetic parameters can be reconciled with known chemical properties although our knowledge base is still inadequate to construct kinetic models from first principles mean activation energies governing oil generation increase systematically in the order a f causing a corresponding increase in generation temperature at a reference heating rate 2 c ma 1 the oil generation window 10 90 oil generative kerogen degraded increases from ca 95 135 c to 145 175 c organofacies c has the tightest energy distribution and narrowest oil generation window the gas generation window ranges from ca 105 155 c to 175 220 c thermal stress results from a combination of temperature and time an order of magnitude increase decrease in heating rate elevates depresses reaction temperatures by ca 15 c heating rates in subsiding sedimentary basins can vary by two orders of magnitude dramatic differences in generation temperature can result if the heating rate and organofacies effects compound thus global oil and gas generation temperature thresholds constitute blunt instruments with which to screen the petroleum potential of sedimentary basins because unexpelled oil provides a potential feedstock for oil to gas cracking in the source rock quantitative models of petroleum generation become really useful only when coupled with models of oil gas cracking and expulsion for example the frequently observed association between gas provinces and coals cannot be explained solely by their generation behaviour

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

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

geochemistry of paleozoic marine oils from the tarim basin nw china part 4 paleobiodegradation and oil charge mixing abstract fifty three oil samples from paleozoic marine reservoirs of the tarim basin were analyzed by gc fid and gc ms to elucidate the influence of biodegradation and mixing on molecular compositions biodegradation has a major impact on many classes of compounds but the ultimate compositions and physical properties of the oil are influenced strongly by mixing with late oil charge subsequent to the process of biodegradation almost all samples display biodegradation signatures characterized by variable size of the unresolved complex mixture ucm and variable amounts of 25 norhopanes 25 nhs and 17 nortricyclic terpanes 17 nts the ubiquitous occurrence of n alkanes suggests that the petroleum accumulated in these fields is mainly a mixture of previously biodegraded oils with later migrated fresh oils the unaltered late charge of oil which has commingled with the initial biodegraded charge gives the wide range of oil gravities and asphaltene contents observed most commonly used maturity and depositional environment diagnostic parameters in the saturated hydrocarbon fraction are affected by biodegradation based on the intensity of 25 nhs and 17 nts oils from the tabei uplift appear to have experienced a much greater degree of biodegradation than those from the tazhong uplift however we caution against the simplistic use of molecular indicators to define a specific level of biodegradation in this case study as it may be somewhat misleading for physical property prediction a wide range of estimated maturity values from different components provides an important clue for multiple charge and mixing interpretations the relationship between the absolute concentrations of compound class and peak hump ratios of the samples can potentially qualitatively differentiate the pulse of inputs to the mixed oils the positive correlation between the concentrations of a given compound class and peak hump ratios may suggest that that class of compounds is most likely derived from the late charge after biodegradation while the negative correlation may indicate compounds largely inherited from the early charge before biodegradation some compounds show almost no correlation between their concentrations and peak hump ratios implying more similar intensity of charge inherited from early pulse and amalgamated from later pulses in some extremely biodegraded cases where ucm hump can also be diminished not in present case study the correlation between compound class concentrations and peak hump ratios will be more complicated

paleozoic fault systems of the tazhong uplift tarim basin china abstract this report presents interpretations developed from a detailed study of new three dimensional 3 d high resolution reflection seismic data in a portion of tarim basin the tectonic history began with oceanic spreading during the cambrian early ordovician and continues beyond the silurian devonian time of oceanic closure paleozoic faults of the tazhong uplift in the hinterland of tarim basin are capped by thick and undisturbed meso cenozoic sedimentary sequences four paleozoic fault systems have been recognized 1 the cambrian early ordovician extensional faulting 2 the late ordovician nww trending thrust faulting 3 the silurian devonian nne strike slip faulting and 4 the permian plutonism influenced by pre existing fault planes zones of weakness created during cambrian early ordovician extensional faulting influenced subsequent tectonic movements the late ordovician fault system divides the tazhong uplift into several deformation zones their mechanical characteristics vary across the study area with stronger thrusting in the east the silurian devonian strike slip fault system consists of three components main faults subordinate en echelon faults and fault troughs the main faults appear as steeply dipping almost vertical offsets on seismic map view with associated flower structures on seismic profiles together with other levels of faults the permian magmatic plugs have a spotty or bandy distribution and are interpreted to have utilized former faults

segmented curved faults the example of the balduini thrust zone northern apennines italy abstract meso and micro structural studies of the well exposed balduini thrust northern apennines italy indicate that the structure formed during a single folding event contemporaneous with diagenesis and is a zone comprising curved en echelon fault segments the geometry of each segment is arcuate with pure compression at one end and right lateral displacement along the other the thrust developed during the tortonian within a single mud rich formation the upper eocene upper oligocene scaglia cinerea but rheological variations within the unit led to differences in deformation style zones of scaly fabric are discontinuous and calcite veins vary in abundance the mesoscopic morphology of the veins and the distribution of calcium carbonate along the formation indicate variations in the distribution of fluids at the time of deformation which affected both diagenesis and the structural response of the material systematic variations of mechanical properties within the thickness of the scaglia cinerea formation account for the curvature of the propagating thrust together with the heterogeneity of the stress field the confinement of the arcing thrust to this single weak unit lead to repeated initiation of new fractures and hence segmentation of the propagating thrust geometric analysis of the calcite veins and their microscopic characters suggests that hydrofracturing was involved with the scaglia cinerea formation experiencing high fluid pressure followed by rapid water expulsion the balduini thrust is therefore an example of a fluid driven refracted compound shear zone the analysis presented here provides insights into the three dimensional arrangement of fault zones and fluid migration patterns during regional faulting