sedimentary pyrite formation an update sedimentary pyrite formation during early diagenesis is a major process for controlling the oxygen level of the atmosphere and the sulfate concentration in seawater over geologic time the amount of pyrite that may form in a sediment is limited by the rates of supply of decomposable organic matter dissolved sulfate and reactive detrital iron minerals organic matter appears to be the major control on pyrite formation in normal non euxinic terrigenous marine sediments where dissolved sulfate and iron minerals are abundant by contrast pyrite formation in non marine freshwater sediments is severely limited by low concentrations of sulfate and this characteristic can be used to distinguish ancient organic rich fresh water shales from marine shales under marine euxinic conditions sufficient h2s is produced that the dominant control on pyrite formation is the availability of reactive iron minerals calculations based on a sulfur isotope model indicate that over phanerozoic time the worldwide average organic carbon to pyrite sulfur ratio of sedimentary rocks has varied considerably high cs ratios during permo carboniferous time can be explained by a shift of major organic deposition from the oceans to the land which resulted in the formation of vast coal swamps at that time low cs ratios compared to today during the early paleozoic can be explained in terms of a greater abundance of euxinic basins combined with deposition of a more reactive type of organic matter in the remaining oxygenated portions of the ocean the latter could have been due to lower oceanic oxygen levels and or a lack of transportation of refractory terrestrial organic matter to the marine environment due to the absence of vascular land plants at that time

variations in pore characteristics in high volatile bituminous coals implications for coal bed gas content abstract the seelyville coal member of the linton formation pennsylvanian in indiana was studied to 1 understand variations in pore characteristics within a coal seam at a single location and compare these variations with changes occurring between the same coal at different locations 2 elaborate on the influence of mineral matter and maceral composition on mesopore and micropore characteristics and 3 discuss implications of these variations for coal bed gas content the coal is high volatile bituminous rank with r 0 ranging from 0 57 to 0 60 bet specific surface areas determined by nitrogen adsorption of the coals samples studied range from 1 8 to 22 9 m 2 g bjh adsorption mesopore volumes from 0 0041 to 0 0339 cm 3 g and micropore volumes determined by carbon dioxide adsorption from 0 0315 to 0 0540 cm 3 g the coals that had the largest specific surface areas and largest mesopore volumes occur at the shallowest depths whereas the smallest values for these two parameters occur in the deepest coals micropore volumes in contrast are not depth dependent in the coal samples examined for this study mineral matter content influenced both specific surface area as well as mesopore and micropore volumes it is especially clear in the case of micropores where an increase in mineral matter content parallels the decrease of micropore volume of the coal no obvious relationships were observed between the total vitrinite content and pore characteristics but after splitting vitrinite into individual macerals we see that collotelinite influences both meso and micropore volume positively whereas collodetrinite contributes to the reduction of mesopore and micropore volumes there are large variations in gas content within a single coal at a single location because of this variability the entire thickness of the coal must be desorbed in order to determine gas content reliably and to accurately calculate the level of gas saturation

high thermal maturity in the lower saxony basin intrusion or deep burial abstract the lower saxony basin lsb is situated at the southern rim of the northern permian basin the most important hydrocarbon bearing province in europe in its southern part areas of high thermal maturation up to 6 rr occur which were previously explained as being caused by magmatic intrusions of turonian to santonian age that affected carboniferous to lower cretaceous rocks in this area 2d numerical basin modelling was carried out in order to reconstruct the temperature histories along two parallel n s trending seismic sections along these sections new data about maturity distribution and temperature history were obtained by vitrinite reflectance measurements and apatite fission track analysis for the areas with highest maturity values low coalification gradients were established which agree well with earlier published data on wells further towards the west this new information does not support previous hypotheses on high heat flows from magmatic intrusions even if lateral heat transport is taken into account instead comparisons with a recently published map of the subsurface structure of the lsb indicate that maturity distribution may be closely related to major tectonic structures deep burial of individual structures of the lsb which are bordered by deep e w to ese wnw trending faults is further suggested by a rapidly increasing thicknesses of preserved cretaceous strata towards the south and b structural restoration of one of the seismic sections to achieve a fit between measured and calculated palaeo temperature and vitrinite reflectance data deep burial of carboniferous source rocks during cretaceous time has to be assumed due to an inversion phase in the early and middle upper cretaceous major amounts of sediment were removed and further uplift occurred in tertiary times in contrast the northern part of the lsb experienced maximum burial and temperatures during tertiary times

geochemical comparison of reservoir solid bitumens with diverse origins abstract geochemical and petrologic comparison of solid bitumens with various origins from reservoirs in zaire canada and kazakhstan were studied to provide insight into their formation processes and their physical and chemical properties results of this study can help develop methods for treatment and removal of solid bitumen and evaluating producibility of associated hydrocarbon fluids characterization of these solid bitumens reveals a wide range in quantity and composition of extractables soluble material and in solid bitumen reflectance which indicate significant differences in formation processes hydrocarbon source and thermal maturity the zaire and kazakhstan samples represent end members of the spectrum of solid bitumens studied solid bitumens from the motoba field zaire are highly extractable and low in thermal maturity 0 2 0 3 bitumen reflectance reservoir mixing of a cretaceous lacustrine sourced oil and a tertiary marine sourced oil of different thermal maturities appears to be responsible for their formation in contrast solid bitumens from the tengiz field kazakhstan are characterized by their extremely low extractables high bitumen reflectance 1 and low atomic h c ratios

widespread oxidized and hydrated amorphous silicates in cr chondrites matrices implications for alteration conditions and h2 degassing of asteroids abstract the cr chondrites carry one of the most pristine records of the solar nebula materials that accreted to form planetesimals they have experienced very variable degrees of aqueous alteration ranging from incipient alteration in their matrices to the complete hydration of all of their components in order to constrain their chemical alteration pathways and the conditions of alteration we have investigated the mineralogy and fe oxidation state of silicates in the matrices of 8 cr chondrites from type 3 to type 1 fe l edge x ray absorption near edge structure xanes was performed on matrix fib sections using synchrotron based scanning transmission x ray microscopy stxm the fe 3 fe ratio of submicron silicate particles was obtained and coordinated with tem observations in all the least altered cr chondrites que 99177 eet 87770 eet 92042 lap 02342 gra 95229 and renazzo we find that the matrices consist of abundant submicron fe rich hydrated amorphous silicate grains mixed with nanometer sized phyllosilicates the fe 3 fe ratios of both amorphous and nanocrystalline regions are very high with values ranging from 68 to 78 in the most altered samples al rais and gro 95577 fine grained phyllosilicates also have a high fe 3 fe ratio around 70 whereas the coarse micrometer sized phyllosilicates are less oxidized down to 55 and have a lower iron content these observations suggest the following sequence submicron fe2 amorphous silicate particles were the building blocks of cr matrices after accretion they were quickly hydrated and oxidized leading to a metastable amorphous gel like phase nucleation and growth of crystalline phyllosilicates was kinetically limited in most type 3 and 2 crs but increased as alteration became more extensive in al rais and gro 95577 the decreasing fe 3 fe ratio is interpreted as a result of the transfer of fe3 from silicates to oxides during growth while aqueous alteration progressed higher temperature longer duration change of fluid composition in a fully closed system equilibrium thermodynamics suggest that the water to rock ratios typically assumed to be low fe 3 fe ratio such a high fe 3 fe value could be accounted for however if the system was partially open at least with respect to h2 and other gases as well rapid degassing of the fluid would have favored more oxidizing fluid conditions recently proposed scenarios involving some degree of water d h increase through rayleigh isotopic fractionation are supported by these results

in situ analysis of solid bitumen in coal examples from the bowen basin and the illinois basin solid bitumen and associated vitrinite from selected coals from the bowen basin and the illinois basin were studied using electron microprobe and micro ftir techniques the coal studied covers a range of vitrinite reflectance from 0 59 to 1 33 carbon content in the bitumen is generally lower than in vitrinite in coals with vitrinite reflectance below 0 67 in coals with reflectance above 0 67 carbon content of bitumen is higher than in vitrinite reflecting higher aromaticity due to hydrocarbon generation sulfur and iron content are comparable between vitrinite and bitumen functional group distribution suggests the presence of two types of bitumen in the illinois basin coals the more aliphatic variety occurring in veins and cleats is interpreted as pre gas generation bitumen and the more aromatic variety filling cells and voids in inertinite as post gas generation bitumen

focused ion beam fib combined with sem and tem advanced analytical tools for studies of chemical composition microstructure and crystal structure in geomaterials on a nanometre scale abstract in order to make meaningful interpretations of trace element and isotopic information from geomaterials prior characterization of the material of interest with respect to structural and chemical homogeneity radiation damage and microstructure is often required transmission electron microscopy tem is a powerful analytical tool that may be used to fully characterise a wide range of synthetic and natural materials the focused ion beam technique fib is an ideal tool for tem sample preparation that allows for the fabrication of electron transparent foils with typical dimensions of 15 10 0 150 m from any region of interest i e site specific and in virtually any material fib sample preparation consumes only a small volume of material approximately 2300 m 3 for a standard foil with the dimensions 15 10 0 150 m leaving most of the material essentially unaffected by the sampling process modern tem facilitates the measurement of chemical composition with unsurpassed spatial resolution e g nanometer scale high resolution imaging and electron diffraction provide important complimentary structural information it is the combination of crystal structure information and chemical composition that allows for the complete characterization of the volume of interest high resolution elemental mapping or line scans may be used to acquire complete energy dispersive x ray analysis edx spectra of a material pixel by pixel subsequent evaluation of the data allows for the correlation of the edx spectrum with the respective elemental map or line scan with an image thus illustrating spatially resolved chemical composition variations at the 0 5 1 0 wt level frequently minerals enclose sub micrometer or even nanometre sized inclusions of minerals fluids or silicate melt which are of great interest to geoscientists nanoinclusions e g in diamond or olivine are of particular interest because they may have preserved their original crystal structure thus providing a geobarometer recording the pressure conditions at the time of formation of the inclusion fib tem are appropriate techniques to sample and subsequently define the chemical composition and the structural state of mineral inclusion on the nanometre scale the combination of fib with an sem allows for 3d information to be obtained from samples including 3d imaging e g phase distribution and volume of the individual phases in symplectites 3d distribution of elements in a specific volume 3d elemental maps and three dimensional texture analysis using electron backscatter diffraction ebsd

the importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs abstract the effect of shale composition and fabric upon pore structure and ch 4 sorption is investigated for potential shale gas reservoirs in the western canadian sedimentary basin wcsb devonian mississippian d m and jurassic shales have complex heterogeneous pore volume distributions as identified by low pressure co 2 and n 2 sorption and high pressure hg porosimetry thermally mature d m shales 1 6 2 5 vro have dubinin radushkevich d r co 2 micropore volumes ranging between 0 3 and 1 2 cc 100 g and n 2 bet surface areas of 5 31 m 2 g jurassic shales which are invariably of lower thermal maturity ranging from 0 9 to 1 3 vro than d m shales have smaller d r co 2 micropore volumes and n 2 bet surface areas typically in the range of 0 23 0 63 cc 100 g co 2 and 1 9 m 2 g n 2 high pressure ch 4 isotherms on dried and moisture equilibrated shales show a general increase of gas sorption with total organic carbon toc content methane sorption in d m shales increases with increasing toc and micropore volume indicating that microporosity associated with the organic fraction is a primary control upon ch 4 sorption sorption capacities for jurassic shales however can be in part unrelated to micropore volume the large sorbed gas capacities of organic rich jurassic shales independent of surface area imply a portion of ch 4 is stored by solution in matrix bituminite solute ch 4 is not an important contributor to gas storage in d m shales structural transformation of d m organic matter has occurred during thermal diagenesis creating and or opening up microporosity onto which gas can sorb as such d m shales sorb more ch 4 per weight percent wt toc than jurassic shales inorganic material influences modal pore size total porosity and sorption characteristics of shales clay minerals are capable of sorbing gas to their internal structure the amount of which is dependent on clay type illite and montmorillonite have co 2 micropore volumes of 0 78 and 0 79 cc 100 g n 2 bet surface areas of 25 and 30 m 2 g and sorb 2 9 and 2 1 cc g of ch 4 respectively dry basis a reflection of microporosity between irregular surfaces of clay platelets and possibly related to the size of the clay crystals themselves mercury porosimetry analyses show that total porosities are larger in clay rich shales compared to silica rich shales due to open porosity associated with the aluminosilicate fraction clay rich sediments low si al ratios have unimodal pore size distributions

authigenic albite in carbonate rocks a tracer for deep burial brine migration euhedral post depositional albite from the eastern and western alps the western carpathians and some greek islands was examined petrographically and geochemically to gain insights into the nature of feldspar reactions in carbonate rocks this study focuses on coarsely crystalline homogeneously nucleated albite in order to avoid problems related to the presence of inseparable detrital material in fine grained albite varieties all albite samples show a very restricted compositional variability and are typically 99 mol ab component unit cell parameters determined by rietveld analysis are slightly more variable than previously accepted but confirm high al si ordering characteristic of low albite the oxygen isotopic composition of albite ranges from 19 4 to 28 3 vsmow there is no direct relationship between the 18o value and the inferred temperature of albite formation nor is there one with stoichiometry the coarse crystal size up to several millimetres in diameter petrographic evidence showing albite cross cutting stylolites greater abundance of albite in carbonate rocks subject to high grade diagenetic or weak metamorphic overprinting and available fluid inclusion data suggest that albite precipitation is favoured at higher temperatures in carbonates than in sandstones pore fluids were invariably brines as suggested by the inferred high positive 18ofluid values the common association of albite bearing carbonates and evaporites and reports of saline fluid inclusions in albite the presence of authigenic albite may thus be a useful tracer of palaeobrine carbonate reactions particularly in deep burial and incipient metamorphic settings

organic geochemistry and petrography of lower cretaceous wealden black shales of the lower saxony basin the transition from lacustrine oil shales to gas shales abstract ninety seven wealden black shale samples from three wells in the lower saxony basin have been studied by organic geochemical and organic petrographical methods to determine their maturity organic facies and depositional environment the maturities of the three wells range from early mature ex a late to postmature ex c to overmature ex b as determined by vitrinite reflectance measurements diamondoid ratios and other geochemical maturity parameters ex c and ex b show distinct petrographic features related to oil generation and migration in particular the occurrence of dispersed solid bitumen replacing initial type i kerogen suggests a formerly active petroleum system structural and textural differences between early mature alginites and solid bitumen in postmature to overmature samples show an alteration of the pore system with increasing maturity a freshwater depositional environment is indicated by widespread occurrence of botryococcus algae and other small alginite particles predominating in the immature well these alginites are absent in the more mature gas shales of wells ex c and ex b geochemical evidence of algae and phytoplankton in general is provided by numerous biomarker parameters while the occurrence of carotane in some samples indicates events of increased salinity although no hypersaline conditions are inferred due to very low gammacerane indices increased amounts of vitrinite and inertinite in samples of ex b suggest locally significant terrigenous input of organic matter for some periods during wealden shale deposition high sulfur organic carbon ratios provide evidence for sulfate rich waters and partly anoxic bottom water conditions while the lower mature lacustrine source rocks generate paraffinic waxy oils gas and condensates are produced at post mature stages furthermore maturity distribution maps from 3d numerical petroleum systems modeling reveal substantial differences in respect to petroleum generation

geochemical constraints on the origin and volume of gas in the new albany shale devonian mississippian eastern illinois basin this study involved analyses of kerogen petrography gas desorption geochemistry microporosity and mesoporosity of the new albany shale devonian mississippian in the eastern part of the illinois basin specifically detailed core analysis from two locations one in owen county indiana and one in pike county indiana has been conducted the gas content in the locations studied was primarily dependent on total organic carbon content and the micropore volume of the shales gas origin was assessed using stable isotope geochemistry measured and modeled vitrinite reflectance values were compared depth of burial and formation water salinity dictated different dominant origins of the gas in place in the two locations studied in detail the shallower owen county location 415 433 m 1362 1421 ft deep contained significant additions of microbial methane whereas the pike county location 832 860 m 2730 2822 ft deep was characterized exclusively by thermogenic gas despite differences in the gas origin the total gas in both locations was similar reaching up to 2 1 cm3 g 66 scf ton lower thermogenic gas content in the shallower location lower maturity and higher loss of gas related to uplift and leakage via relaxed fractures was compensated for by the additional generation of microbial methane which was stimulated by an influx of glacial melt water inducing brine dilution and microbial inoculation the characteristics of the shale of the maquoketa group ordovician in the pike county location are briefly discussed to provide a comparison to the new albany shale

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

mineral composition and seal condition implicated in pore structure development of organic rich longmaxi shales sichuan basin china abstract shale pore structure morphology type size distribution and porosity is important in controlling the storage and movement of oil and gas characterization of pore structure is one of the main research foci for unconventional resources however factors influencing its heterogeneity are poorly understood with total organic carbon ranging at 0 52 4 23 23 core samples from three wells jy2 wy1 and yz1 in different tectonic units of the longmaxi shale in sichuan basin were studied using a combination of petrographic geochemical and pore structure characterization techniques the shale porosity shows positive relationships with quartz and total organic carbon contents in jy2 and wy1 but not in yz1 because of its different pore structure pore structure differences are primarily attributed to the different contributions of organic matter om hosted nanopores which are controlled by preservation conditions due to the differences in the duration and intensity of tectonic activities the jy2 and wy1 areas are characterized by over pressured conditions with respective pressure coefficients of about 1 5 and 1 4 conversely the yz1 area has a normal pressure condition with a pressure coefficient of about 1 0 indicating a poor seal condition under good seal conditions generated hydrocarbons cannot escape easily thus the accumulation and expansion of shale gas trapped within organic pores increase the pore pressure to abnormally high levels which in turn protect the shale reservoir from compaction and support om hosted pore development the om pore evolution of the longmaxi shale in the sichuan basin during burial was generally uniform thus the differences in pore structure reflect subsequent tectonic uplift the siliceous shales of yz1 did not develop many om hosted pores because of the lack of seal conditions during the high maturity stage before uplift most of the pores shrink or close due to an escape of hydrocarbons mineral composition thermal maturity and seal conditions are identified to influence pore development and preservation in fine grained sedimentary rocks

development of organic porosity in the woodford shale with increasing thermal maturity abstract using a combination of focused ion beam milling and scanning electron microscopy we describe the evolution of secondary organic porosity in eight woodford shale late devonian early mississippian samples with mean random vitrinite reflectance values ranging from 0 51 ro to 6 36 ro organic porosity was observed to be absent in samples with vitrinite reflectance values of up to 0 90 ro with the first appearance of secondary pores starting with the 1 23 ro sample porosity in the organic matter was unexpectedly absent in a sample with a vitrinite reflectance of 2 00 ro however organic pores were again found in samples with higher thermal maturities porosity when present did not appear to be uniformly distributed among the organic matter that was within less than a micron of each other suggesting important differences in composition of the organic matter thin regions of organic matter were observed between grains raising the possibility that small amounts of the deposited organic matter were compacted between grains to form thin layers and or the structures are part of the secondary organic matter interpreted to be post oil bitumen which was left behind as a residue during oil migration through the shale some regions of porous organic matter appeared to be grain protected whereas others did not which indicates that these non protected porous organic regions may be stress supporting with porosity intact under in situ reservoir conditions these observations suggest that thermal maturity alone is insufficient to predict porosity development in organic shales and other factors such as organic matter composition complicate porosity development

low temperature hydrothermal oil and associated biological precursors in serpentinites from mid ocean ridge abstract the origin of light hydrocarbons discovered at serpentinite hosted mid ocean hydrothermal fields is generally attributed to the abiogenic reduction of carbon di oxide by molecular hydrogen released during the progressive hydration of mantle derived peridotites these serpentinization by products represent a valuable source of carbon and energy and are known to support deep microbial ecosystems unrelated to photosynthesis in addition the pool of subsurface organic compounds could also include materials derived from the thermal degradation of biological material we re investigate the recently described relics of deep microbial ecosystems hosted in serpentinites of the mid atlantic ridge 4 6 n in order to study the ageing and hydro thermal degradation of the preserved biomass an integrated set of high resolution micro imaging techniques scanning electron microscopy high resolution transmission electron microscopy raman and fourier transform infra red microspectroscopy confocal laser scanning microscopy and scanning transmission x ray microscopy at the carbon k edge has been applied to map the distribution of the different organic components at the micrometer scale and to characterize their speciation and structure we show that biologically derived material containing aliphatic groups along with carbonyl and amide functional groups has experienced hydrothermal degradation and slight aromatization in addition aliphatic compounds up to c 6 c 10 with associated carboxylic functional groups wet the host bastite and the late serpentine veins crosscutting the rock these compounds represent a light soluble organic fraction expelled after biomass degradation through oxidation and thermal cracking the detected complex organic matter distribution recalls a typical petroleum system where fossil organic matter of biological origin maturates expelling the soluble fraction which then migrates from the source to the reservoir ecosystem hosting serpentinites can thus be seen as source rocks generating a net transfer of hydrocarbons and or fatty acids issued from oxidative processes and primary cracking reactions then migrating upward through the serpentine vein network this finally suggests that deep thermogenic organic compounds of biological origin can be a significant contributor to the organic carbon balance at and far below peridotite hosted hydrothermal fields

oil retention and porosity evolution in organic rich shales petroleum is retained in shales either in a sorbed state or in a free form within pores and fractures in shales with oil resource potential organic matter properties i e richness quality and thermal maturity control oil retention in general in gas shales organic pores govern gas occurrence although some pores may originate via secondary cracking reactions it is still largely unclear as to how these pores originate here we present case histories mainly for two classic shales the mississippian barnett shale texas and the toarcian posidonia shale lower saxony germany in both cases shale intervals enriched in free oil or bitumen are not necessarily associated with the layers richest in organic matter but are instead associated with porous biogenic matrices however for the vast bulk of the shale hydrocarbon retention and porosity evolution are strongly related to changes in kerogen density brought about by swelling and shrinkage as a function of thermal maturation secondary organic pores can form only after the maximum kerogen retention swelling ability is exceeded at tmax the temperature at maximum rate of petroleum generation by rock eval pyrolysis around 445 c 833 f approximately 0 8 vitrinite reflectance shrinkage of kerogen itself leads to the formation of organic nanopores and associated porosity increase in the gas window

sulphur cycling in organic rich marine sediments from a scottish fjord in this study the biogeochemical transformations of sulphur in organic rich marine sediments in a scottish fjord are investigated by a combination of pore water and sediment geochemistry with sulphide diffusive gradient thin film probes and sulphate isotopic data 34s and 18o particular attention is paid to sulphur cycling in the upper sediment profile where sulphate reduction occurs but free sulphide is below the detection limits of conventional pore water geochemical analysis but quantifiable by sulphide diffusive gradient thin film in the uppermost part of the sediment core 18o sulphate decreased from near sea water values to 7 indicating that anoxic sulphide oxidation dominated during this interval sulphate 34s remained unchanged as there was no net sulphate reduction i e reduction was balanced by re oxidation below 4 cm depth there was a slight increase in sulphate 34s from 20 to 23 associated with minor accumulation of iron sulphide the 18o of the sulphate also increased to around 10 at 10 cm depth as a result of the isotopic exchange of sulphate oxygen with pore water and or sulphur disproportionation reactions mediated during sulphur cycling these processes continued to increase the 18o of the sulphate to 14 at 20 cm depth with no further change in the 34s of the sulphate below 20 cm depth free sulphide is detectable in pore waters and both the 34s of the sulphate and sulphide increase with depth with an offset controlled by kinetic fractionation during bacterial sulphate reduction the 34s of the sedimentary organic fraction shifted towards lower more bacteriogenic values with depth in the profile without any increase in the size of this sulphur pool thus the organic sulphur fraction was open to interaction with bacteriogenic sulphide without the occurrence of net addition therefore caution should be exercised when using sulphur isotopic compositions to infer simple net addition of bacteriogenic sulphide to the organic sulphur fraction

thermal cracking of kerogen in open and closed systems determination of kinetic parameters and stoichiometric coefficients for oil and gas generation abstract the purpose of the study is to compare artificial maturation of kerogens representative of the main types of organic matter kerogen types i ii ii s and iii in open and anhydrous closed pyrolysis systems the generated compounds are fractionated according to their thermal stability into six chemical classes c 1 c 2 c 5 c 6 c 14 c 15 saturates c 15 aromatics and nsos which include resins and asphaltenes in both systems primary cracking of kerogen can be described by two main reactions oil generation together with early gas generation followed by late gas production mass balances obtained in the two pyrolysis systems are reasonably similar although some secondary cracking of the nsos and underestimation of methane potential due to incomplete pyrolysis may occur in the open system kinetic parameters of thermal cracking are derived from experiments in the open system using an optimization procedure which determines a unique frequency factor and a weight distribution of chemical classes for a discrete series of activation energies the resultant frequency factors are checked with those obtained for specific molecular tracers such as n c 12 and n c 24 alkanes generated from kerogens in open and closed systems under isothermal conditions the results show that using a unique frequency factor as determined by optimization is correct for type i ii and ii s kerogens this factor is more questionable however when the distribution of activation energies is very broad such as for type iii coals based on this study a strategy for determination of kinetic parameters on any new source rock sample is proposed

microfracturing and microporosity in shales shales are ubiquitous rocks in sedimentary basins where their low permeability makes them efficient seals for conventional oil and gas reservoirs and underground waste storage repositories waste waters co2 nuclear fuels moreover when they contain organic matter they form source rocks for hydrocarbons that may escape towards a more porous reservoir during burial a process referred to as primary migration and when the hydrocarbons cannot escape these rocks can be exploited as oil or shale gas reservoirs while the presence of fractures at the outcrop scale has been described the existence of fractures at smaller scales their link with microporosity the mechanisms that created them their persistence over geological times and their effect on the petrophysical properties of shales represent scientific challenges for which drillings in various sedimentary basins over the past decades may hold timely key data here we review and synthetize the current knowledge on how microfractures and micropores in shales can be imaged and characterized and how they control their anisotropic mechanical properties and permeability one question is whether such microfractures when observed in outcrops or in drilled core samples extracted from boreholes are related to decompaction and do not exist at depth another question is whether veins observed in shales represent microfractures that were open long enough to have acted as flow paths across the formation the mechanisms of microfracture development are described some have an internal origin fracturing by maturation of organic matter dehydration of clays while others are caused by external factors tectonic loading importantly the amount of microfracturing in shales is shown to depend strongly on the content in 1 organic matter and 2 strong minerals the nucleation of microfractures depends on the existence of mechanical heterogeneities down to the nanometer scale their propagation and linkage to create a percolating network will depend on the presence of heterogeneities at the meso to macro scales such percolating microfracture networks could control both the long term sealing capabilities of cap rocks and the further propagation of hydraulic fracturing cracks finally possible areas of research for describing the mechanism of microfracture formation in greater detail and how this impacts the transport and mechanical properties of shales are also discussed

hydrocarbon bearing fluid inclusions in calcite filled horizontal fractures from mature posidonia shale hils syncline nw germany abstract hydrocarbon bearing fluid inclusions in calcite filled horizontal fractures from mature posidonia shale 0 88 r o were investigated by microscopic microthermometric and chemical methods the fluid inclusions predominantly contain both a liquid hydrocarbon phase which shows bright uv fluorescence colours yellow brownish light blue and a non fluorescent hydrocarbon gas phase the different uv fluorescence colours probably reflect the different gross chemical compositions of the trapped liquid hydrocarbons the saturated hydrocarbon fraction of the fluid inclusions mostly secondary inclusions predominantly consists of low molecular weight n alkanes c 9 c 12 while c 12 c 22 n alkanes are more abundant in the adjacent rock matrix the molecular composition of the fluid inclusions appears to reflect a mobile hydrocarbon phase which was entrapped within the calcitic fill of the horizontal fractures pressure corrected trapping temperatures were estimated for primary fluid inclusions 123 162 c this temperature range is in agreement with the results from numerical simulation studies which indicate a maximum temperature of 148 c coinciding with the period of fracture formation calcite filling and entrapment of primary fluid inclusions

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

evolution of porosity and pore geometry in the permian whitehill formation of south africa a fe sem image analysis study abstract since the advent of technologically efficient exploitation of economic hydrocarbon reservoirs in shales increasingly more research has been devoted to identifying and characterizing pore systems within shales however only a handful of these studies focused on the development of porosity in thermally mature unconventional reservoirs in this study the evolution of porosity and pore geometry in the permian whitehill formation is addressed with the aid of ultrathin sections 2 3 cm 10 20 m thick and field emission scanning electron microscopy fe sem on samples with mean random vitrinite reflectance values ranging from 1 03 to 4 07 ro we document a strong positive covariation of porosity and total organic carbon content toc in all localities however for samples with vitrinite reflectance values greater than 2 88 ro porosity per unit toc decreased by over 25 relative to samples with lower thermal maturities the positive covariation of thermal maturity and total porosity recorded here is unsurprising and have been documented previously in many gas shales however the dramatic decrease in porosity restricted to samples from localities that experienced advanced maturation ro 2 88 is viewed as an evidence that porosity decrease is directly related to late thermal decarboxylation of organic matter this is supported by the presence of pores and micro fractures infilled by fibrous grains including carbonates clays silicates and phosphates and residual fluid inclusions these grains were likely generated from re precipitation of framework grains previously dissolved by organic acids carboxylic phenolic that were generated during thermochemical decarboxylation of the om our findings do not only fill important gaps in the understanding of organic pore development including processes that create preserve and destroy porosity the porosities described here are also key to gas transfer from shale matrix to induced fractures during fracture stimulation programs

source rock evaluation by well log analysis lower toarcian hils syncline abstract the feasibility of conducting petroleum source rock evaluations from well log data was examined using three continuously cored sections immature mature and postmature stages of the lower toarcian posidonia shale northern germany logs depicting natural gamma emission k u th bulk density sonic travel time and resistivity were utilised as was the borehole televiewer for fracture control selected data from these logs were correlated with parameters of source richness hydrocarbon content and maturity the resulting three dimensional cross plot shows at a glance well by well comparison of facies maturity and hydrocarbon content a simple correlation between organic geochemical and petrophysical parameters is of course not perfect since log response is complex and affected by mineralogical and pore fluid properties of the rock as well as by organics however source rock characterization by wireline logging is still a fast and therefore economical method relative quick look comparisons from well to well are possible without calibration to core data and can easily be performed at once at the drill site the present application is limited to clastic source rocks in basins with simple geologic histories

diagenetic albitization of detrital k feldspars in jurassic lower cretaceous and tertiary clastic reservoir rocks from offshore norway ii formation water chemistry and kinetic considerations abstract formation waters from clastic reservoirs offshore norway are saturated or supersaturated with respect to albite deeper than 2 5 km and support the view of saigal et al 1988 that albitization of k feldspar is presently active in these sandstones thermodynamic and mass constraints show that the albitization process is more sensitive to potassium removal than sodium supply the temperature of albitization commencement coincides more or less with the estimated onset temperature of illitization of smectite 70 80 c interbedded shales seem to be the major potassium sink and source of sodium kinetic considerations indicate that at temperatures below 145 c the dissolution of k feldspar is always faster than albite growth combined with replacement constraints the kinetic data also explain the observed coarsening of growing albite crystals with increasing burial temperature

phenols in hydrothermal petroleums and sediment bitumen from guaymas basin gulf of california the aliphatic aromatic and polar nso fractions of seabed petroleums and sediment bitumen extracts from the guaymas basin hydrothermal system have been analyzed by gas chromatography and gas chromatography mass spectrometry free and silylated the oils were collected from the interiors and exteriors of high temperature hydrothermal vents and represent hydrothermal pyrolyzates that have migrated to the seafloor by hydrothermal fluid circulation the downcore sediments are representative of both thermally unaltered and thermally altered sediments the survey has revealed the presence of oxygenated compounds in samples with a high degree of thermal maturity phenols are one class of oxygenated compounds found in these samples a group of methyl dimethyl and trimethyl isoprenoidyl phenols c27 c29 is present in all of the seabed nso fractions with the methyl and dimethyl isoprenoidyl phenols occurring as major components and a trimethyl isoprenoidyl phenol as a minor component a homologous series of n alkylphenols c13 c33 has also been found in the seabed petroleums these phenols are most likely derived from the hydrothermal alteration of sedimentary organic matter the n alkylphenols are probably synthesized under hydrothermal conditions but the isoprenoidyl phenols are probably hydrothermal alteration products of natural product precursors the suites of phenols do not appear to be useful tracers of high temperature hydrothermal processes

increasing maturity of kerogen type ii reflected by alkylbenzene distribution from pyrolysis gas chromatography mass spectrometry abstract a series of late devonian to early mississippian type ii kerogens with vitrinite reflectance values ro 0 29 2 41 were analyzed using py gc ms in addition a low maturity kerogen with ro 0 44 was separated into fractions via density gradient centrifugation followed by py gc ms of the alginite and amorphinite maceral concentrates alkylbenzenes and n alk 1 ene n alkane doublets represented the main compound classes identified in all pyrolysates the pyrolysate from alginite featured 1 2 4 trimethylbenzene and toluene as the two most prominent alkylbenzenes in contrast alkylbenzenes in pyrolysates from amorphinite and low maturity bulk kerogens with ro 0 29 0 63 were dominated by 1 2 3 4 tetramethylbenzene with increasing thermal maturity pyrolysates were increasingly dominated by i alkylbenzenes with fewer methyl groups namely by tri and dimethylbenzenes at medium maturity ro 0 69 1 19 and ii by toluene at higher maturity ro 1 30 2 41 with increasing maturity of kerogen type ii the decreasing abundance of highly methyl substituted alkylbenzenes and the parallel increase in less methyl substituted alkylbenzenes in flash pyrolysates suggest that demethylation is an important chemical process in the thermal maturation of kerogen type ii

multiscale characterization of pyritized plant tissues in blueschist facies metamorphic rocks pyritized plant tissues with well preserved morphology were studied in rocks from vanoise western alps france that experienced high pressure low temperature metamorphic conditions in the blueschist facies during the alpine orogeny organic and inorganic phases composing these fossils were characterized down to the nanometer scale by raman microspectroscopy scanning transmission x ray microscopy and transmission electron microscopy the graphitic but disordered organic matter composing these fossils is chemically and structurally homogeneous and mostly contains aromatic functional groups its original chemistry remains undefined likely because it was significantly transformed by diagenetic processes and or thermal degradation during metamorphism various mineral phases are closely associated with this organic matter including sulphides such as pyrite and pyrrhotite carbonates such as ankerite and calcite and iron oxides a tentative time sequence of formation of these diverse mineral phases relative to organic matter decay is proposed the absence of traces of organic matter sulphurization the pervasive pyritization of the vascular tissues and the presence of ankerite suggest that the depositional diagenetic environment of these metasediments was likely rich in reactive iron fe sulphides and ankerite likely precipitated early and might have promoted the preservation of the fossilized biological soft tissues by providing mechanical resistance to compaction during diagenesis and subsequent metamorphism in contrast iron oxides which form rims of 100 nm in thickness at the interface between organic matter and fe sulphides may result from metamorphic processes this study illustrates that it may be possible in some instances to deconvolve metamorphic from diagenetic imprints and opens new avenues to better constrain processes that may allow the preservation of organic fossils during diagenesis and metamorphism

impact of biomineralization on the preservation of microorganisms during fossilization an experimental perspective abstract the biogenicity of fossil microbial biomorphs is often debated because their morphologies are poorly informative and the chemical structural and isotopic signatures of putative biogenic organic molecules have been altered during their incorporation into the sediments and the geological history of the host rock here we investigated the effect of encrustation by biominerals on the morphological and chemical degradation of escherichia coli cells during experimental thermal treatments non calcified e coli cells and e coli cells encrusted by calcium phosphates were exposed to heating under an argon atmosphere at two different temperatures 300 c 600 c for 20 h two additional experiments were performed on non calcified e coli cells at 300 c for 2 h and 100 h to discuss the influence of experiment duration organic residues of all experiments were characterized at a multiple length scale using a combination of scanning electron microscopy transmission electron microscopy raman microspectroscopy electron paramagnetic resonance epr spectroscopy fourier transform infrared ft ir spectroscopy and x ray absorption near edge structure spectroscopy xanes at the carbon k edge in the absence of encrusting biominerals the morphological structure of the organic residues of e coli cells was completely lost after heating at 300 or 600 c even after short 2 h long heating experiments the content of aromatic functional groups of the organic residues of non calcified e coli cells increased during heat treatment at 600 c for 20 h while the amide functional groups were lost as indicated by ft ir spectroscopy consistently the epr spectrum of these organic residues indicated important transformation as a comparison this spectrum appeared similar to epr spectra of ancient organic carbons such as carbons from the apex chert ca 3460 myr indicating a similar concentration of aromatic moieties in contrast calcified e coli exposed to the same conditions showed only limited morphological alteration as observed by electron microscopy as well as lower chemical transformation as detected by ft ir and epr spectroscopies despite the difficulties to relate experimental conditions directly to geological conditions these experiments evidence the influence of cell encrustation by minerals on their chemical and morphological preservation potential during fossilization processes

new insights into the volume and pressure changes during the thermal cracking of oil to gas in reservoirs implications for the in situ accumulation of gas cracked from oils previous pressure volume calculations during oil cracking to gas based on the conventional model that presupposes oil cracking to be completed by approximately 150c underestimate the potential for gas accumulation in petroleum reservoirs in this article a compositional kinetic model of gas generation from oil cracking is suggested based on pyrolysis data using sealed gold tubes and the pressure volume changes are recalculated based on the new kinetic model under various geological conditions the kinetic modeling of oil cracking confirms that crude oil begins cracking at about 160c for a heating rate of 2c m y and that the oil cracking process has two distinct stages with significant differences in gas composition the first stage is characterized by dominant c2 5 wet gases whereas the second is characterized by the recracking of c2 5 wet gases to methane and pyrobitumen leading to a progressive increasing dryness of the gas the pressure volume temperature simulations of oil cracking to gas show that initial oil saturation temperature pressure gradients and openness of reservoirs are important geological factors that control gas accumulation in original petroleum reservoirs for a reservoir that is geologically open and saturated with 100 oil gas spills out of the trap at 196c the gas loss at 240c is almost 50 of the total gas far lower than the 75 based on the conventional model of oil destruction with lower oil saturation the gas loss decreases because the gas water contact can shift downward and the gas loss occurs mainly by solution for effectively isolated reservoirs oil cracking readily exceeds lithostatic pressure leading to reservoir fracturing which becomes more obvious when oil saturation decreases the calculated fracturing temperatures for 100 and 50 vol oil saturations correspond to oil destructions of 95 and 86 4 greatly exceeding the value of 1 as suggested by previous studies a conceptual model of gas accumulation and loss in isolated and open geological conditions for a reservoir with 50 oil saturation is suggested on the basis of this model the triassic carbonate gas pool in northeastern sichuan basin was discussed as a typical example for in situ accumulation of gas cracked from reservoired oils the present model infers that the reservoired oils were completely cracked into gas at 87 6 ma and that 75 85 of the gas has been preserved in the original reservoir rocks to form the in situ gas pools with a huge amount of gas resources we believe that gas accumulation from oil cracking in original petroleum reservoirs is much more prospective than previously thought and that gas cracked from oil has great potential in other areas of the sichuan basin and the eastern tarim basin

distinguishing gases derived from oil cracking and kerogen maturation insights from laboratory pyrolysis experiments kerogen separated from an immature type i mudstone from the third section of the tertiary shahejie formation es 3 3 in the dongying depression of bohai bay basin was subjected to stepwise isothermal pyrolysis products were extracted fractionated into chemical groups and re mixed to obtain a synthetic oil with a group composition similar to reservoir oil and a pseudo kerogen composed of 83 kerogen and 17 residual soluble organic matter referred as s oil and p kerogen respectively to distinguish them from a produced reservoir oil and a real kerogen the two samples were pyrolyzed in sealed gold tubes under constant pressure 50 mpa and non isothermal heating 300 600 c conditions and their generated gases were analyzed the two gases are quite different in their chemical and isotopic composition compared with the gas derived from the p kerogen the s oil derived gas is enriched in c2 c5 hydrocarbons in its early cracking stages and c1 c3 hydrocarbons are depleted in d 13 c throughout the cracking stages the maximum wetness c2 5 c1 5 of the s oil gas and the p kerogen gas is 0 62 and 0 35 respectively and the carbon isotopic ratio differences between the two type gases can reach 10 d 13 c1 14

organic matter maturation under the influence of a deep intrusive heat source a natural experiment for quantitation of hydrocarbon generation and expulsion from a petroleum source rock toarcian shale northern germany four shallow boreholes were drilled in the hils syncline northern germany in order to determine quantitatively the amount of hydrocarbons generated and expelled during maturation of a typical kerogen type ii bearing source rock the holes penetrated the carbonceous lias shales posidonia shale lower toarcian and part of the adjacent dogger and lias mudstones the maturity of the organic matter in the cores recovered ranges from immature 0 48 r0 to overmature 1 45 r0 due to location of the hils syncline in the vicinity of the vlotho massif which is deep seated intrusive body facies variations of the lias within the short geographical distances in the study area are negligible organic matter mass balance calculations were based on detailed organic geochemical analyses of residual material in the lias shales kerogen bitumen etc and on the evidence of a uniform initial composition of these sediments in the study area dead carbon determinations supported this latter criterion but were not used as a parameter in the calculations about 50 of the initial kerogen was transformed into oil gas and inorganic compounds during the vitrinite reflectance increase from 0 48 to 0 88 ro and only marginally more during the maturity increase from 0 88 to 1 45 ro only a small portion of the generated material remained in the source rock even at a relatively early stage of generation 0 68 ro expulsion efficiency of oil plus gas reached a value of 86 at the end of the main generation stage 0 88 ro

simple kinetic models of petroleum formation part ii oil gas cracking kinetic schemes describing oil gas cracking are a required component of models which predict the masses compositions and phases of petroleum expelled from source rocks an additional application is in predicting the temperature zone over which reservoired oil deposits will be degraded to gas this simple first order kinetic model calibrated on closed system laboratory pyrolysis of 16 different source rock samples describes the bulk conversion of oil c6 molecular range to gas c1 5 molecular range cracking rates vary markedly between samples depending on the saturate to aromatic ratio of the generated oil the initial hydrogen index hl0 of the sample taken as an indirect measure of the saturate to aromatic ratio of the oil it generates correlates strongly with optimized rate constants for oil cracking in individual samples thus cracking rates are predictable using a simple routinely performed geochemical screening measurement source rocks with high hl0 tend to generate saturate rich oils with a high mean activation energy and a tight distribution of bond energies which crack relatively slowly over a high but relatively narrow temperature range source rocks with low hl0 generate aromatic rich oils with a low mean activation energy and a broad distribution of bond energies which crack relatively rapidly over a lower but relatively wide temperature range this is an important contributing factor in the observed gas proneness of type iii source rocks with low hl0 the projected cracking window defined limits at 10 90 of initial oil degraded reference heating rate 2 c ma 1 varies from 155 to 205 c in very high quality source rocks hl0 600 mg g c 1 to 115 205 c in very poor quality source rocks hl0 50 mg g c 1 an order of magnitude increase decrease in heating rate elevates depresses these temperature windows by ca 15 c the extrapolations to the geological subsurface are subject to a confidence limit no better than 7 c extrapolation to the reservoir environment requires caution because kerogen an important potential catalyst and hydrogen donor allowing 100 cracking efficiency in the source rock is absent furthermore fractionation during expulsion causes saturate enrichment of expelled ultimately reservoired oils thus in reservoir cracking should be at least as slow as cracking in good quality source rocks and coke or pyrobitumen formation is needed to conserve the hydrogen balance a case history based on deep petroleum pools of the uk central north sea confirms these projections demonstrating no positive evidence of significant in reservoir cracking at temperatures as least as high as 174 c and perhaps even 195 c previous studies have overestimated the importance of in reservoir cracking future studies aiming to predict petroleum composition in traps should concentrate on understanding the charge composition expelled from the kitchen

pore throat sizes in sandstones tight sandstones and shales pore throat sizes in siliciclastic rocks form a continuum from the submillimeter to the nanometer scale that continuum is documented in this article using previously published data on the pore and pore throat sizes of conventional reservoir rocks tight gas sandstones and shales for measures of central tendency mean mode median pore throatsizes diameters are generally greater than2 mm in conventionalreservoir rocks range from about 2 to 0 03 mm in tight gas sandstones and rangefrom0 1to0 005 mminshales hydrocarbonmolecules asphaltenes ring structures paraffins and methane form another continuum ranging from 100 a 0 01 mm for asphaltenes to 3 8 a 0 00038 mm for methane the pore throat size continuum provides a useful perspective for considering 1 the emplacement of petroleum in consolidated siliciclastics and 2 fluid flow through fine grained source rocks now being exploited as reservoirs

quantification of loss of calcite pyrite and organic matter due to weathering of toarcian black shales and effects on kerogen and bitumen characteristics abstract comparison of geochemical data on posidonia shale early toarcian from a shallow unweathered core and from adjacent weathered exposures of the same fades was performed results revealed that quantitatively the most affected petrographic constituent is pyrite and that organic matter and carbonate were also altered by weathering due to this process of weathering the bulk composition of organic matter especially the molecular composition of soluble organic matter and the fluorescence colour of organic particles are changed estimates of weathering rates reveal that the annual release of sulphur and organic carbon from black shales may significantly add to anthropogenic pollution

influence of in situ bitumen on the generation of gas and oil in new zealand coals new zealand coals of peat to high volatile bituminous rank have been studied in order to assess the effect of in situ bitumen soluble in a methanol acetone chloroform azeotrope on petroleum potential during progressive maturation both non extracted and extracted coals as well as the extracted bitumen itself were analyzed using rock eval and pyrolysis gas chromatography the difference in yields between non extracted and extracted coals attributable to the influence of bitumen were compared with direct measurements made on the bitumen itself major compositional changes in pyrolysis products were seen when the bitumen was finely dispersed throughout the kerogen matrix notably second order reactions between kerogen and bitumen during pyrolysis reduce primary gas yield and enhance both oil yield and potential secondary gas yield the stabilization of free radicals and prevention of recombination repolymerization processes have been attributed to the presence of the dispersed in situ bitumen these results demonstrate that petroleum formation in coals likely proceeds via intermolecular interactions and not simply first order cracking the common procedure of employing solvent extraction prior to pyrolysis is clearly not appropriate when assessing the petroleum potential of coal

sem petrography of eastern mediterranean sapropels analogue data for assessing organic matter in oil and gas shales abstract examination of organic matter hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter om as seen in high resolution sem images field emission sem imaging of eastern mediterranean sapropels pliocene pleistocene age from ocean drilling program drill sites was performed on samples prepared by ar ion cross section polishing the immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation hydrocarbon generation kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores the most abundant organic matter is in the form of microns thick flaky or stringy material largely nonporous and internally amorphous and lacking the well defined shapes of discrete particles this dominant om has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt and clay size grains a portion of the larger mineral hosted pores remain open unfilled by ductile om minor nanometer scale pores of uncertain origin occur in the dominant om silt and clay size mineral crystals are distributed in organic matter of most types and it is difficult to fully ascertain if the om mineral mixing has occurred during compaction versus during aggregation during gravity settling marine snow sedimentation or through sediment accumulation in om rich microbial mats results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial these observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature om rich mudrocks it is clear that petrographic discrimination of diagenetically generated organic matter bitumen and related om types from ductile dispersed amorphous detrital om kerogen using sem data will be challenging in the absence of compositional information

intraformational redistribution of selected trace elements in the posidonia shale hils syncline nw germany caused by the thermal influence of the vlotho massif selected samples of nw german posidonia shale hils syncline area were separated into five fractions bitumen kerogen pyrite carbonates silicates and chemically analyzed for trace element concentration using icp oes analyses in immature posidonia shale 0 53 ro co cu mo ni and zn occur mainly in the kerogen and pyrite fractions in mature posidonia shale 1 45 ro these trace elements are mainly fixed in the silicate fractions we postulate a thermally induced intraformational redistribution of co cu mo ni and zn caused by heat from a deep seated igneous intrusion the vlotho massif

kerogen maturation in a reference kerogen type ii series the toarcian shales of the hils syncline nw germany abstract studies on the generation and migration of petroleum in the reference type ii source rocks of the hils syncline were published some years ago by rullkotter et al 1988 and leythaeuser et al 1988 in the present study kerogens of these reference samples ranging from 0 48 to 1 45 vitrinite reflectance r o were isolated and analysed after having confirmed their similarity with the toarcian shales of the paris basin series an immature kerogen of this latter series was added to the german natural series this same kerogen was heated in sealed gold tubes at various temperatures for one day these two series of kerogens allowed us to follow the thermal maturation both under natural and artificial conditions and to compare them to find common maturation parameters comparison of samples relies both on bulk analyses of kerogens and on molecular analyses of their thermal degradation products bulk analyses comprise elemental analysis 13 c nmr spectroscopy and rock eval pyrolysis molecular analyses are based on mass balance of saturated and aromatic hydrocarbons in extracts and structural analyses of saturated hydrocarbons by gas chromatography and mass spectrometry these analyses show that the evolution of the natural series is adequately reproduced by the heated samples the latter having the advantage of recovering all the compounds formed during kerogen maturation from methane to coke without losses during sampling or by migration phenomena as in natural samples

distinct evolution trends of nanometer scale pores displayed by the pyrolysis of organic matter rich lacustrine shales implications for the pore development mechanisms abstract organic matter om compositions greatly affect hydrocarbon generation and expulsion processes which are critical for the organic porosity development in shale lacustrine shale samples of low thermal maturity were pyrolyzed using two pyrolysis systems closed and semi closed systems pore development was measured by low pressure gas adsorption and field emission scanning electron microscopy fe sem and bulk porosity was modeled utilizing organic geochemical data the gas adsorption analysis indicated that shale samples of different om compositions differed in pore volume evolution with thermal maturity mainly related to the different amounts of hydrocarbon generated and expelled residual bitumen significantly reduced the pore volume of om rich oil generative shale samples and restricted the micro meso and macro pore volumes to different extents calculations and experiments both showed that om rich and oil generative shale experienced a greater increase in pore volume after the oil peak in addition it was observed that variations in the main pore types were associated both with shale compositions and with exerted overburden pressure increase in overburden pressure were found to greatly facilitate the development of nanometer size spongy and complex om pores in shales containing type kerogens possibly as a result of the expulsion of gaseous hydrocarbons by contrast the shale with abundant type kerogen tended mainly to develop relatively large pores following oil expulsion regardless of overburden pressure the modeled organic porosity of pyrolyzed samples of type iii om was similar to the porosity of geological shale but the porosity of om rich oil generative shale samples with high expulsion efficiency at the oil generation stage was two to three times the measured porosity of the geological shale in some cases the higher modeled shale porosity might be related to the higher expulsion efficiency for geological shales of expulsion efficiency comparable to the pyrolyzed samples geological processes e g compaction and cementation may have greatly reduced the om associated pore volume

geochemical effects of petroleum migration and expulsion from toarcian source rocks in the hils syncline area nw germany for mature oil prone source rocks of toarcian age in the hils syncline area nw germany the geochemical effects of primary migration of petroleum are shown expulsion has occurred at high efficiencies primarily via a network of macro and micro fractures the effects of this fracture controlled expulsion with respect to gross and molecular composition of c15 soluble organic matter are documented samples from fractured intervals have expelled total c15 soluble organic matter more efficiently than adjacent unfractured intervals a systematic comparison of samples taken at cm intervals away from macro fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures extracts of shale samples directly at fracture walls are enriched in nso compounds and highly polar asphaltene like fractions the composition of residual petroleum fluids extracted from the fracture fill materials is variable in some cases there is a close match with that of the shale at some distance away from the fractures in other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes rapid heating by a nearby intrusive body at shallow depths when the toarcian sediments still contained about 15 20 pore water has played an important role overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature toarcian sections finally the closely interdependent relationships between petroleum generation and petroleum migration processes are documented especially the evolution of these relationships with maturity progress from immature 0 48 rm to mature 0 68 and 0 88 rm and overmature 1 45 rm stages is shown for example in the maturity interval from 0 68 to 0 88 rm most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum for some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities

non connected pores of the longmaxi shale in southern sichuan basin of china abstract the characteristics of non connected pores of four longmaxi shale samples from the southern sichuan basin of china were investigated from the studies of pore size distribution psd pore volumes pore types using helium pycnometry gas injection porosimetry gip and gri gas research institute method nuclear magnetic resonance nmr nuclear magnetic resonance cryoporometry nmrc and spontaneous water and oil imbibition experiments gri porosity on particle sizes less than 5 mm sample and a 2 mpa helium injection pressure provides information on the total porosity including both non connected and connected pores gip porosity using core plugs only measures connected pores which extend to the surface of the sample accessible by helium so the difference between gri and gip porosities is used to estimate the volumes of non connected pores and the volumes of non connected pores accounts for 17 5 40 3 of the total pore volume for nmrc test results total pore volume increases with sample size decrease and the great difference of pore volume with a pore diameter range from 5 nm to 31 nm between small cylinder sample and 0 15 0 075 mm crushed sample is caused by non connected pores opened with a decrease of length for cubic samples the increase in volume and rate of oil imbibition is greater than that of water imbibition indicating that the non connected pores are considered to be mainly oil wet in addition the percentage of non connected pores has a good correlation with toc total organic carbon content therefore this evidence implies that non connected pores are mainly within the organic matter nmr tests at different drying temperatures and a nmrc experiment provide a quantitative classification of the shale pore system which indicates that the exploitable pore diameters are larger than 4 25 nm so non connected pores with a diameter of 5 31 nm could be effectively exploited if connected by fracturing the more the non connected pores are opened the better the pore connectivity of shale reservoir will be with a less possibility of water blocking in the process of shale gas development attention should be paid to the interaction between fracturing fluid and rock components in order to release more non connected pores therefore to increase shale gas extraction efficiency more effort should be directed towards identifying natural and creating induced porosity

ketones in hydrothermal petroleums and sediment extracts from guaymas basin gulf of california the polar compound nso fractions of seabed petroleums and sediment extracts from the guaymas basin hydrothermal system have been analyzed by gas chromatography and gas chromatography mass spectrometry the oils were collected from the interiors and exteriors of high temperature hydrothermal vents and represent hydrothermal pyrolysates that have migrated to the seafloor by hydrothermal fluid circulation the downcore samples are representative of both thermally unaltered and thermally altered sediments the survey has revealed the presence of oxygenated compounds correlated with samples exhibiting a high degree of thermal maturity several homologous series of related ketone isomers are enriched in the interiors of the hydrothermal vent samples or in hydrothermally altered sequences of the downcore sediments dsdp holes 477 and 481a the n alkanones range in carbon number from c11 to c33 with a cmax from 14 to 23 distributions that are similar to those of the n alkanes the alkan 2 ones are usually in highest concentrations with lower amounts of 3 4 5 6 7 and higher alkanones and they exhibit no carbon number preference there is an odd carbon number preference of alkanones observed for downcore samples the alkanones are enriched in the interiors of the hydrothermal vent spires or in downcore hydrothermally altered sediments indicating an origin at depth or in the hydrothermal fluids and not from an external biogenic deposition minor amounts of c13 and c18 isoprenoid ketones are also present simulation of the natural hydrothermal alternation process by laboratory hydrous pyrolysis techniques provided information regarding the mode of alkanone formation hydrous pyrolysis of n c32h66 at 350 c for 72 h with water only or water with inorganic additives has been studied using a stainless steel reaction vessel in each experiment oxygenated hydrocarbons including alkanones were formed from the n alkane the product distributions indicate a reaction pathway consisting of n alkanes and olefins as primary cracking products with internal olefins and alkanones as secondary reaction products hydrous pyrolyses of messel shale spiked with molecular probes have been performed under similar time and temperature constraints to produce alkanone distributions like those found in the hydrothermal vent petroleums

the origin of pyrobitumens in upper devonian leduc formation gas reservoirs alberta canada an optical and eds study of oil to gas transformation pyrobitumens are common within deep basin devonian carbonate gas reservoirs of the western canada sedimentary basin incident light microscopy and sem eds have been used to study the origin of pyrobitumens from upper devonian isolated reefs of the leduc formation west central alberta reflectance properties romax and romin coke microtextures and relative s c ratios are used to classify the pyrobitumens in portions of reservoir units where nso enriched low aromaticity crudes may have been dominant oil to gas transformations produced mainly low reflecting isotropic to fine grained mosaic coke microtextures within the reservoir pyrobitumens type a in contrast where normal crudes were most likely dominant in the reservoir units prior to thermal cracking pyrobitumens with high bireflectance and very anisotropic coarse flow to domain cokes dominate type b oil to gas cracking within some of the reservoir units resulted in overpressuring and is represented by highly deformed pyrobitumens with fibrous coke textures and high bireflectance relative concentrations of type a and b pyrobitumens vary vertically and temporally in some of the reefal buildups the variations in the dominant pyrobitumen types are attributed to crude oil alteration which preceded crude oil to gas transformation e g tar mats in some zones whereas other zones may have been altered contemporaneously with crude oil cracking the later may have resulted from the oxidation and sulphur enrichment of the crude oil during thermochemical sulphate reduction this reaction is prevalent within devonian evaporite carbonate strata of the western canada sedimentary basin generating copious volumes of h2s the interaction between crude oil and sulphur enriched highly alkaline fluids is suggested by the presence of high s c very fine grained late stage calcined needle cokes within brecciated pyrobitumens

the electrical conductivity of posidonia black shales from magnetotelluric exploration to rock samples we carried out a magnetotelluric field campaign in the south east lower saxony basin germany with the main goal of testing this method for imaging regional posidonia black shale sediments two dimensional inversion results of the magnetotelluric data show a series of conductive structures correlating with brine saturated sediments but also with deeper anthracitic westphalian namurian coals none of these structures can be directly related with the posidonia black shale which appears to be generally resistive and therefore difficult to resolve with the magnetotelluric method this assumption is supported by measurements of electrical resistivity on a set of posidonia shale samples from the hils syncline in the lower saxony basin these rock samples were collected in shallow boreholes and show immature 0 53 ro oil 0 88 ro and gas 1 45 ro window thermal maturities none of the black shale samples showed low electrical resistivity particularly those with oil window maturity show resistivity exceeding 104 m moreover we could not observe a direct correlation between maturity and electrical resistivity the harderode samples showed the highest resistivity whereas the haddessen samples showed the lowest a similar trend has been seen for coals in different states of thermal maturation saturation of the samples with distilled and saline water solutions led to decreasing electrical resistivity moreover a positive correlation of electrical resistivity with porosity is observed for the wickensen and harderode samples which suggests that the electrical resistivity of the posidonia black shale is mainly controlled by porosity

geochemical evolution of organic rich shales with increasing maturity a stxm and tem study of the posidonia shale lower toarcian northern germany hydrocarbon generation and retention processes occurring within gas shales as a response to increases in thermal maturation are still poorly constrained while efforts have been directed at unravelling the resource potential composition and textures of these economically important unconventional systems their spatial variability in chemistry and structure is still poorly documented at the sub micrometer scale here we have characterized samples of the lower toarcian posidonia shale samples from northern germany at varying stages of thermal maturation using a combination of compositional organic geochemistry and spectromicroscopy techniques including synchrotron based scanning transmission x ray microscopy stxm we document geochemical and mineralogical heterogeneities down to the nanometer scale within the investigated samples as a function of their level of thermal maturity in particular authigenic albite crystals containing nanometric halite inclusions have been documented within the investigated mature and overmature samples the presence of such tracers of palaeobrine carbonate interactions supports a maturation scenario for the lower toarcian posidonia shale intimately related to ascending brine fluids rather than a maturation scenario solely resulting from high heat flows in addition various types of asphaltene and nso rich bitumen have been detected within the same samples very likely genetically derived from thermally degraded organic precursors furthermore the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity likely resulting from the formation of gaseous hydrocarbons by providing in situ insights into the fate of bitumen and pyrobitumen as a response to the thermal evolution of the macromolecular structure of kerogen the results reported here constitute an important step towards better constraining hydrocarbon generation processes during natural shale gas maturation

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

ftir absorption indices for thermal maturity in comparison with vitrinite reflectance r0 in type ii kerogens from devonian black shales abstract ftir absorbance signals in kerogens and macerals were evaluated as indices for thermal maturity two sets of naturally matured type ii kerogens from the new albany shale illinois basin and the exshaw formation western canada sedimentary basin and kerogens from hydrous pyrolysis artificial maturation of the new albany shale were characterized by ftir good correlation was observed between the aromatic aliphatic absorption ratio and vitrinite reflectance r 0 ftir parameters are especially valuable for determining the degree of maturity of marine source rocks lacking vitrinite with increasing maturity ftir spectra express four trends i an increase in the absorption of aromatic bands ii a decrease in the absorption of aliphatic bands iii a loss of oxygenated groups carbonyl and carboxyl and iv an initial decrease in the ch 2 ch 3 ratio that is not apparent at higher maturity in naturally matured samples but is observed throughout increasing r 0 in artificially matured samples the difference in the ch 2 ch 3 ratio in samples from natural and artificial maturation at higher maturity indicates that short term artificial maturation at high temperatures is not fully equivalent to slow geologic maturation at lower temperatures with increasing r 0 the carboxyl carbonyl aromatic carbon ratio generally decreases except that kerogens from the exshaw formation and from hydrous pyrolysis experiments express an intermittent slight increase at medium maturity ftir derived aromaticities correlate well with r 0 although some uncertainty is due to the dependence of ftir parameters on the maceral composition of kerogen whereas r 0 is solely dependent on vitrinite

the influence of solid reservoir bitumen on reservoir quality 1 reservoir bitumen precipitates in pore systems from the alteration of trapped or migrating oil in carbonate and siliciclastic reservoirs reservoir bitumen is not readily identified on typical log suites where it is read as open porosity which has hampered its broad recognition as a reservoir altering cement when present as a solid it can be as influential on reservoir quality as carbonate silica or authigenic clay cements and should therefore be evaluated as part of the diagenetic evolution of any pore system whenever encountered five morphotypes of reservoir bitumen have been identified and named droplets carpets peanut brittle vesicular and digitate data from east texas gulf coast and west african examples indicate that reservoir bitumen can significantly reduce total effective porosity permeability can be significantly reduced by restricted or closed pore throats and by fines migration even when bitumen occurs in only moderate amounts the occurrence and distribution of bitumen on well field and regional scales can range from uniform and predictable to irregular and unpredictable but mappable through examination of core and cuttings within a trap bitumen can cause heterogeneity and form permeability barriers not related to depositional facies or pre bitumen diagenesis and reservoir quality it economic importance must be recognized when reservoir parameters are adversely affected impacting reserves calculations recovery factors and secondary recovery programs on a regional scale predictive models can be developed to determine the areas of a trend or portions of a stratigraphic section that are most likely to be affected and these models can be incorporated with other data for evaluating expected reservoir quality in regional play assessments

three common source rock evaluation errors made by geologists during prospect or play appraisals geologists are frequently called on to evaluate the source rocks associated with their exploration prospects or plays the three most common questions asked and answered about the source rock during project reviews are what s the total organic carbon toc what kerogen type does rock eval indicate and what maturity level does the vitrinite reflectance data point to the answers to these seemingly innocuous questions may in fact be providing a false sense of security about the source rock in question understanding how this line of questioning can lead you astray and make you the victim of the toc myth if i have high toc i have a good source rock the rock eval fallacy the rock eval data tell me what kind of kerogen is in my source rock and the vitrinite reflectance deficiency vitrinite reflectance will tell me if my source rock is generating is important some of the solutions to these problems include fully integrating toc and rock eval data supplementing rock eval data with pyrolysis gas chromatography and using burial history diagrams to help interpret vitrinite reflectance harry dembicki jr is a senior geological advisor for geochemistry in the geological technology group at anadarko petroleum corporation where he provides technical support to both exploration and development teams he holds a b s degree 1973 in geology from the state university college of new york at new paltz and a ph d 1977 in geology with emphasis in organic geochemistry from indiana university he previously worked as an organic geochemist for conoco and marathon

influence of experimental temperature and duration of laboratory confined thermal maturation experiments on the evolution of the porosity of organic rich source rocks abstract to investigate the influence of the duration temperature pairs chosen for laboratory thermal maturations on the evolution of the organic matter om and porosity of the clay rich source rocks with increasing maturity long duration 104 days thermal maturations bulk molecular geochemical om characterization rock eval gc tcd gc ms and nitrogen adsorption porosimetry were applied on six type ii mudstones from the kimmeridge clay formation these results were compared with shorter duration experiments 72 h previously carried out the results show that the increase in the duration of thermal maturation experiments enhanced the degradation of heavy polar om components into saturated and aromatic hydrocarbons leading to the production of similar amounts of extractible om eom enriched in saturated and aromatic hydrocarbons then in preventing the early breaking of low energy bonds the lower temperature used for the longer duration experiments led to a less pronounced early cracking of om resulting in a different timing of gas generation this process did not influence the general evolution of the pore volume which increased during gas generation whatever the duration temperature pair chosen for maturation experiments highly oil prone samples formed lower pore volumes during gas generation increasing the duration of thermal maturation seemed nevertheless to influence the amplitude and the timing of pore alteration events that occur during gas generation leading to slightly different pore size distributions and pore volumes between short and long duration experiments however these differences remain limited suggesting that results from different time duration experiments can be easily compared

mississippian barnett shale fort worth basin north central texas gas shale play with multi trillion cubic foot potential the mississippian barnett shale serves as source seal and reservoir to a world class unconventional natural gas accumulation in the fort worth basin of north central texas the formation is a lithologically complex interval of low permeability that requires artificial stimulation to produce at present production is mainly confined to a limited portion of the northern basin where the barnett shale is relatively thick 300 ft 92 m organic rich presentday total organic carbon 3 0 thermally mature vitrinite reflectance 1 1 and enclosed by dense limestone units able to contain induced fractures the most actively drilled area is newark east field currently the largest gas field in texas newark east is 400 mi 2 1036 km 2 in extent with more than 2340 producing wells and about 2 7 tcf of booked gas reserves cumulative gas production from barnett shale wells through 2003 was about 0 8 tcf wells in newark east field typically produce from depths of 7500 ft 2285 m at rates ranging from 0 5 to more than 4 mmcf day estimated ultimate recoveries per well range from 0 75 to as high as 7 0 bcf efforts to extend the current barnett play beyond the field limits have encountered several challenges including westward and northward increases in oil saturation and the absence of lithologic barriers to induced fracture growth patterns of oil and gas occurrence in the barnett in conjunction with maturation and burial history data indicate a complex multiphased thermal evolution with episodic expulsion of hydrocarbons and secondary cracking of primary oils to gas

kerogen asphaltene sulfur relationships in sulfur rich monterey oils abstract monterey oils in the santa maria basin onshore and offshore are generally low maturity oils because of low thermal exposure relatively shallow burial for short times they are generated as heavy high sulfur oils because of the composition of the unusual type ii source kerogens which are exceptionally rich in organic sulfur 8 14 these high sulfur kerogens which we designate type ii s appear to generate oil at significantly lower thermal exposures than ordinary type ii kerogens with less than 6 s in contrast to common type ii kerogens which generate oil largely by cleavage at oxygen bridge groups and carbon carbon bonds eliminating co2 h2o and hydrocarbon fragments type ii s kerogens have an additional complement of weak bonds associated with bound sulfur the preferential cleavage at weak sulfur linkages tends to produce larger fragments leading to high initial amounts of asphaltenes resins and sulfur rich aromatics together with smaller amounts of saturated hydrocarbons elemental analyses of monterey kerogens and asphaltenes clearly support the concept that the asphaltenes are essentially fragments of kerogen cleaved sufficiently to be soluble or peptized in this heavy oil system solubilization of asphaltenes is attributed to their interactions with the abundant resins and aromatics this sysstem is sufficiently stable under source and reservoir conditions to allow expulsion and migration for reasonable distance without significant fractionation or changes in gross properties initial asphaltenes have essentially the same atomic h c ratios as the source kerogen but have significantlyy lower o c ratios and slightly lower s c ratios the n c ratios are nearly the same for kerogens and their derived asphaltenes with increasing maturity the h c ratios in asphaltenes decrease much like maturation changes in kerogen the s c ratio also decreases significantly however the o c ratios initially low in asphaltenes and n c ratios increase slightly due to material balance caused by other losses these element composition and atomic ratio relationships allow some important inferences concerning source kerogen composition from analysis of asphaltenes alone this can be useful when potential source rock samples are not available for analysis and or when a choice between alternate source rock possibilities must be made

kerogen origin evolution and structure kerogen commonly defined as the insoluble macromolecular organic matter om dispersed in sedimentary rocks is by far the most abundant form of om on earth this fossil material is of prime importance as the source of oil and natural gas moreover kerogen can provide essential information on major topics such as past environments climates and biota this review reports the main advances in kerogen studies since the comprehensive synthesis edited by durand durand b ed kerogen insoluble organic matter from sedimentary rocks editions technip paris 1980 it is organized into eight sections the first two are concerned with the successive definitions of kerogen and the definition used here the different techniques used for kerogen isolation without loss or degradation and basic kerogen analysis the third and fourth focus on sedimentary om sources and preservation processes in relation to depositional environment including sedimentation conditions favourable for kerogen accumulation and extrapolation to past geological time great strides have been made in the latter topics over the last 25 years owing to a combination of classical studies in organic geochemistry and studies in other domains such as biogeochemistry oceanography hydrology and soil science along with the development of powerful analytical tools the next two sections deal with the different kerogen classifications by type and kerogen evolution and maturation upon burial in sediments structural modelling of coal and kerogen based on physical and or chemical structural analysis is described in the following section although only statistical the models thus derived provide a synthetic view of the main structural resemblances and differences among various samples in relation to source maturity or physicochemical properties finally the last section explores some of the advances in kerogen understanding expected for the near future the review includes a list containing about 500 references

microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in toarcian source rocks of different maturities abstract this study is part of a comprehensive research program on the generation and migration of petroleum from the organic rich posidonia shale lower toarcian jurassic of the hils syncline northern germany four cores of the complete posidonia shale with varying amounts of underlying pliensbachian and overlying aalenian mudstones provided the sample base depending upon their distance from a presumed subjacent heat source the vlotho massif the cores display progressive levels of increasing thermal maturation with r 0 values 0 48 0 68 0 88 and 1 45 which span the limits of the oil generation window primary stratigraphic lithologic and petrographic attributes of the four sections are all very similar each includes a lower marlstone member about 5 m thick overlain by 11 31 m of calcareous clay shale clay minerals calcite pyrite kerogen and quartz are the principal constituents thus there is no evidence that depositional conditions of the posidonia shale varied significantly over the sampled area the omnipresence of framboidal pyrite clearly formed from the activity of sulphate reducing bacteria at or near the depositional interface is strong evidence that uniform reducing anoxic conditions were persistent at all localities this conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry organic macerals and other properties are directly related to the variable levels of thermal maturation kerogen from the least mature section r o 0 48 is type ii and composed of alginite a alginite b liptodetrinite bituminite and some terrestrial sporinite vitrinite and inertinite macerals with increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties alginite of the least mature section displays a strong green fluorescence at 0 68 r o alginite macerals exhibit yellow orange fluorescence and are structurally well preserved although geochemical data indicate generation and loss of large amounts of oil at 0 88 r o hydrocarbon generation is almost complete and few primary alginites remain they have been transformed into secondary liptinites designated as metaalginite which have similar fluorescence colours at the highest maturation level 1 45 r o metaalginites are only weakly fluorescent with a shift toward red colours calcite and or bitumen filled bedding plane micro and macrofractures occur exclusively in the organic rich posidonia shale at maturity levels of 0 68 r o and above such features are totally absent in the immature posidonia section and in the adjacent organic poor mudstones this observation indicates that the fractures developed in direct response to overpressuring i e pore pressure exceeding lithologic load related to oil and gas generation in the source rock the fractures undoubtedly provided important expulsion conduits for generated products

natural and artificial maturation of biological markers in a toarcian shale from northern germany steroid and terpenoid biological marker evolution was studied in a maturity sequence of lias toarcian shales and adjacent strata from three shallow boreholes in the hils syncline northern germany excellent facies control within the study area ensures the comparability of the organic matter type among the stratigraphically equivalent samples of the natural maturation series in the early hydrocarbon generation stage 0 68 r0 a strong facies dependence of some sterane and hopane isomerisation values was found within the same borehole at only a few metres depth difference the extent of sterane isomerisation at c 14 and c 17 was close to 80 in the toarcian shale and between about 55 and 65 in the adjacent dogger and lias sediments similar differences were observed for the 17 h hopane moretane and 22 29 30 trisnor 18 h neohopane 22 29 30 trisnor 17 h hopane ratios whereas the 20s 20s 20r sterane isomerisation ratio did not show this difference the observed differences mostly disappear by the end of the main oil generation phase 0 88 ro where several reactions appear to have reached completion hydrous pyrolysis experiments by variations of time and temperature were used to simulate the natural evolution of the biological markers results for sterane isomerisation in the high temperature range were compared to those published by mackenzie and mckenzie geol mag 120 417 470 1983 for natural sediments kinetic data derived for this reaction from the hydrous pyrolysis results alone or from the combination of hydrous pyrolysis and natural data sets differ markedly from those determined by mackenzie and mckenzie 1983 the hydrous pyrolysis data alone yield an activation energy for sterane isomerisation at c 20 of 131 kj mol the combined data of 170 kj mol the frequency factors are in the 105 108 s 1 range the significance of the data is discussed in detail

experimental investigation of oil generation retention and expulsion within type ii kerogen dominated marine shales insights from gold tube nonhydrous pyrolysis of barnett and woodford shales using miniature core plugs abstract although oil retention has recently emerged as a key topic of unconventional shale resource assessment oil retention and expulsion controls in organic rich shales during thermal maturation remain poorly constrained this study presents an experimental comparison of oil generation retention and expulsion in two immature type ii kerogen dominated marine shales the mississippian barnett shale and the upper devonian lower mississippian woodford shale mainly with respect to the combined effects of the organic macerals and rock fabric involved in both cases miniature core plugs drilled from the given samples were isothermally pyrolyzed at 130 to 425 c for 72 h under a confining pressure of 68 mpa during gold tube nonhydrous pyrolysis corresponding to the thermally immature early stage of the oil window the main stage of the oil window the late stage of the oil window the main stage of oil cracking to wet gas and the late stage of oil cracking yields of generated oil retained oil and expelled oil for the two studied samples were systematically quantified on the basis of mass balance calculation of measured oil and gas yields as well as rock eval analyses on pyrolyzed subsamples through the six stages of petroleum formation investigated the principal difference in oil generation was observed in the two studied samples with approximately 38 to 68 greater yields of generated oil equivalent to 130 mg oil g toco for the woodford shale when it evolved into the main and late stages of the oil window these elevated yields of generated oil for the woodford shale were compensated for by additional oil generation resulting from conversion of abundant type i kerogen like algae such as tasmanites and leiosphaeridia which lag in onset and have a shorter period of petroleum generation upon maturation as a response to the difference in oil generation oil retention was found to be significantly enhanced for the woodford shale at equivalent stages with 0 2 to 1 7 times more free oil equivalent to 24 105 mg oil g toco and 0 7 to 3 9 times more sorbed oil equivalent to 58 76 mg oil g toco being retained than that of the barnett shale although this effect was not pronounced for oil expulsion in contrast to the barnett shale relatively low expelled oil yields and expulsion efficiencies both indicate highly limited oil expulsion in the woodford shale implying that the woodford shale whose mineral composition and lithofacies are similar to those of the barnett shale may have a relatively low permeability rock fabric to prevent oil from being expelled furthermore not only significantly higher oil saturation index osi values but also a wider range of maturity at which the oil crossover effect osi 100 mg g toc occurs is expected for the woodford shale when extrapolation to a geological setting occurs these data suggest that the presence of abundant type i kerogen like algae and relatively low permeability rock fabric in the woodford shale are critical to significant oil retention during oil generation and expulsion which jointly raise the possibility of potential commercial shale oil within type ii kerogen dominated marine shales

pore structure characterization of shales using synchrotron saxs and nmr cryoporometry abstract the characterization of shale pore structure is extremely important to shale gas exploitation various techniques have been employed to characterize the pore size distribution psd in shales small angle x ray scattering saxs is a relatively new technique that can be applied to obtain the psd of porous materials however little work has been done on shales by synchrotron saxs in this paper scanning electronic microscopy sem nuclear magnetic resonance cryoporometry nmrc and synchrotron small angle x ray scattering saxs were used to comprehensively characterize pore structure in three shales with a total organic carbon content of 0 68 3 68 and 4 3 the processing and postprocessing for synchrotron saxs data were executed based on background deduction a positive deviation correction of saxs data was conducted sem was combined with synchrotron saxs to explore whether the sample is a single dispersion two models gauss distribution model and the maximum entropy model for calculating the psd by synchrotron saxs were compared with the results by nmrc the psd from the gauss distribution presents good consistency with the psd from the nmrc with a diameter of less than 15 nm notably a psd obtained by maximum entropy model and nmrc is more consistent for pores with a diameter of 15 48 nm nanopore structure was also explored and discussed based on the porod theory methods for quantitative characterization of pore size proposed in this paper are applicable to other two phase rock media

a reproducible and linear method for separating asphaltenes from crude oil abstract petroleum asphaltenes exhibit strong structural similarities to their parent kerogen thus they can be used to help determine the source rock characteristics of active kitchen areas and to reconstruct the filling histories of reservoirs the analysis of asphaltenes from compartmentalised reservoirs with complex charging and alteration histories presents a particularly difficult challenge because the in situ compositional variations are sometimes subtle it is therefore of paramount importance that asphaltene separation methods be utilised which are highly reproducible and linear thereby minimising experimental uncertainty and providing high resolution interpretative capability the asphaltene separation method described here is built on many that have been proposed before different parameters for asphaltene separation were investigated which influence the asphaltene amount and properties with the main focus on reproducibility and linearity gravimetric and thermal analysis techniques were used to evaluate yield and quality the developed asphaltene separation method described in detail has four sequential steps 1 dissolution of the oil in dichloromethane 1 vol methanol 2 addition of an excess of n hexane 3 settling time and 4 filtration purging and weighing this sequence of steps is repeated three times results concerning the reproducibility depending on solvent type solvent amount and aging time as well as the linearity depending on sample amount are presented

ancient graphite in the eoarchean quartz pyroxene rocks from akilia in southern west greenland i petrographic and spectroscopic characterization because all known eoarchean 3 65 ga volcano sedimentary terranes are locked in granitoid gneiss complexes that have experienced high degrees of metamorphism and deformation the origin and mode of preservation of carbonaceous material in the oldest metasedimentary rocks remain a subject of vigorous debate to determine the biogenicity of carbon in graphite in such rocks carbonaceous material must be demonstrably indigenous and its composition should be consistent with thermally altered biogenic carbon as well as inconsistent with abiogenic carbon here we report the petrological and spectroscopic characteristics of carbonaceous material typically associated with individual apatite grains but also with various other minerals including calcite in a 3 83 ga granulite facies ferruginous quartz pyroxene unit qp rock from the island of akilia in southern west greenland in thin sections of the fine grained parts of akilia qp rock sample g91 26 mapped apatites were found to be associated with graphite in about 20 of the occurrences raman spectra of this carbonaceous material had strong g band and small d band absorptions indicative of crystalline graphite three apatite associated graphites were found to contain curled graphite structures identified by an anomalously intense second order d band or 2d band raman mode these structures are similar to graphite whiskers or cones documented to form at high temperatures raman spectra of apatite associated graphite were consistent with formation at temperatures calculated to be between 635 and 830 c which are consistent with granulite facies metamorphic conditions three graphite targets extracted by focused ion beam fib methods contained thin graphite coatings on apatite grains rather than inclusions sensu stricto as inferred from transmitted light microscopy and raman spectroscopy tem analyses of graphite in these fib sections showed a 0 0 0 2 interplanar spacing between 3 41 and 3 64 a for apatite associated graphite which is larger than the spacing of pure graphite 3 35 a and may be caused by the presence of non carbon heteroatoms in interlayer sites samples analyzed by synchrotron based scanning transmission x ray microscopy stxm also confirmed the presence of crystalline graphite but abundances of n and o heteroatoms were below detection limit for this method graphite in the akilia qp rock was also found to occur in complex polyphase mineral assemblages of hornblende calcite sulfides magnetite that point to high temperature precipitation from carbon bearing fluids these complex mineral assemblages may represent another generation of graphitization that could have occurred during the amphibolite facies metamorphic event at 2 7 ga several observations point to graphitization from high temperature fluid deposition for some of the akilia graphite and our results do not exclude a biogenic source of carbon in graphite associated with apatite but ambiguities remain for the origin of this carbon

chemistry of oil field brines in relation to diagenesis of reservoirs 2 reconstruction of palaeo water composition for modelling illite diagenesis in the greater alwyn area north sea abstract input data for water composition are required in order to numerically simulate diagenetic episodes in rock dominated closed system modelling the composition of water initially present in the pores has at most only a minor influence on the stability of the system and is controlled by the mineral composition in open system modelling the composition of infiltrating water can play an important role brent water salinities of the greater alwyn area exhibit a regional trend from the north to the south the k cl ratio of these waters varies along two trends it is higher in the north than in the south alkalinity and published pco2 values are used to calculate ph under subsurface conditions very few silica measurements are available aluminium analyses of pore water are not available some reservoirs of the greater alwyn area experienced a major reduction in permeability due to illite and quartz precipitation coeval with kaolinite and k feldspar dissolution saturation of present day brent waters with respect to these minerals is examined the method proposed in part 1 is applied to reconstruct a range of possible compositions for ancient waters in brent reservoirs between several extreme conditions one of these extremes is provided by buffering water with a set of minerals chosen from petrographic considerations the other extremes correspond to reasonable states of over saturation with respect to particular minerals e g quartz and concern the values of dissolved si and al which are the least well characterised concentrations from the routine analyses of formation waters

pore characterization of lower silurian shale gas reservoirs in the middle yangtze region central china abstract organic rich shales from lower silurian are widely distributed in the middle yangtze region central china however the lack of fundamental data for shale gas reservoirs increases the difficulty of gas exploration in this study 34 core samples were collected to characterize the shale pore structure and conduct a preliminary evaluation of the shale gas reservoir the toc total organic carbon content of the successively deposited black shales range from 1 6 to 5 9 while the total porosity range from 0 5 to 4 2 the positive correlation between toc and porosity indicates that toc is the key factor determining porosity the major component of the mineral matrix is quartz content of 21 4 69 2 followed by clay minerals content of 16 7 44 5 field emission scanning electron microscopy fe sem and energy dispersive x ray spectroscopy eds results illustrate that organic matter mixed with clay minerals can form an organo clay complex containing many nanopores furthermore larger organic pores are developed in organo clay complexes with higher clay content than in those with lower clay content correlational analyses between pore volume or pore surface area and toc or clay content demonstrate that micropores are associated with organic matter while mesopores and macropores are probably associated with clay minerals many of the clay related nanopores are organic in nature and are developed in organo clay complexes containing both organic matter and clay minerals overall the toc content controls development of nanopores in the shale pore structure followed by clay content the dft derived psd indicates that the pore volume is comprised primarily of pores having widths larger than 10 nm while the surface area is comprised primarily of micropores when considering the gas in place model and mechanisms of shale gas storage further shale gas exploration in central china should aim to the deep 1000 m and well preserved longmaxi shales

role of water in hydrocarbon generation from type i kerogen in mahogany oil shale of the green river formation abstract hydrous and anhydrous closed system pyrolysis experiments were conducted on a sample of mahogany oil shale eocene green river formation containing type i kerogen to determine whether the role of water had the same effect on petroleum generation as reported for type ii kerogen in the woodford shale the experiments were conducted at 330 and 350 c for 72 h to determine the effects of water during kerogen decomposition to polar rich bitumen and subsequent bitumen decomposition to hydrocarbon rich oil the results showed that the role of water was more significant in bitumen decomposition to oil at 350 c than in kerogen decomposition to bitumen at 330 c at 350 c the hydrous experiment generated 29 more total hydrocarbon product and 33 more c 15 hydrocarbons than the anhydrous experiment this is attributed to water dissolved in the bitumen serving as a source of hydrogen to enhance thermal cracking and facilitate the expulsion of immiscible oil in the absence of water cross linking is enhanced in the confines of the rock resulting in formation of pyrobitumen and molecular hydrogen these differences are also reflected in the color and texture of the recovered rock despite confining liquid water pressure being 7 9 times greater in the hydrous experiments than the confining vapor pressure in the anhydrous experiments recovered rock from the former had a lighter color and expansion fractures parallel to the bedding fabric of the rock the absence of these open tensile fractures in the recovered rock from the anhydrous experiments indicates that water promotes net volume increase reactions like thermal cracking over net volume decrease reactions like cross linking which results in pyrobitumen the results indicate the role of water in hydrocarbon and petroleum formation from type i kerogen is significant as reported for type ii kerogen

fractured shale gas systems the first commercial united states natural gas production 1821 came from an organic rich devonian shale in the appalachian basin understanding the geological and geochemical nature of organic shale formations and improving their gas producibility have subsequently been the challenge of millions of dollars worth of research since the 1970s shale gas systems essentially are continuous type biogenic predominant thermogenic or combined biogenic thermogenic gas accumulations characterized by widespread gas saturation subtle trapping mechanisms seals of variable lithology and relatively short hydrocarbon migration distances shale gas may be stored as free gas in natural fractures and intergranular porosity as gas sorbed onto kerogen and clay particle surfaces or as gas dissolved in kerogen and bitumen five united states shale formations that presently produce gas commercially exhibit an unexpectedly wide variation in the values of five key parameters thermal maturity expressed as vitrinite reflectance sorbed gas fraction reservoir thickness total organic carbon content and volume of gas in place the degree of natural fracture development in an otherwise low matrix permeability shale reservoir is a controlling factor in gas producibility to date unstimulated commercial production has been achievable in only a small proportion of shale wells those that intercept natural fracture networks in most other cases a successful shale gas well requires hydraulic stimulation together the devonian antrim shale of the michigan basin and devonian ohio shale of the appalachian basin accounted for about 84 of the total 380 bcf of shale gas produced in 1999 however annual gas production is steadily increasing from three other major organic shale formations that subsequently have been explored and developed the devonian new albany shale in the illinois basin the mississippian barnett shale in the fort worth basin and the cretaceous lewis shale in the san juan basin in the basins for which estimates have been made shale gas resources are substantial with in place volumes of 497 783 tcf the estimated technically recoverable resource exclusive of the lewis shale ranges from 31 to 76 tcf in both cases the ohio shale accounts for the largest share

organic chemical differentiation within fossil plant cell walls detected with x ray spectromicroscopy organic matter preserved in cell walls of permineralized plant fossils was analyzed by using scanning transmission x ray microscopy and spectroscopy at energies near the 1s absorption edge of carbon microchemical analyses were performed directly on cellulose acetate peels of the fossils preserving information on the anatomical distribution of organic materials individual tracheid walls in both eocene and early devonian fossils exhibit spatially distinct chemical zoning inherited from original wall biopolymers and cell wall microstructure molecular analysis of submicrometer domains using carbon x ray absorption near edge spectroscopy documents the differential distribution of hydroxylated aromatic and alcohol and or ether carbon in the inner and outer regions of tracheid walls this zonation reflects the deposition of lignin and structural polysaccharides in devonian plants indicating biochemical and developmental pathways similar to those of living tracheophytes

morphology genesis and distribution of nanometer scale pores in siliceous mudstones of the mississippian barnett shale research on mudrock attributes has increased dramatically since shale gas systems have become commercial hydrocarbon production targets one of the most significant research questions now being asked focuses on the nature of the pore system in these mudrocks our work on siliceous mudstones from the mississippian barnett shale of the fort worth basin texas shows that the pores in these rocks are dominantly nanometer in scale nanopores we used scanning electron microscopy to characterize barnett pores from a number of cores and have imaged pores as small as 5 nm key to our success in imaging these nanopores is the use of ar ion beam milling this methodology provides flat surfaces that lack topography related to differential hardness and are fundamental for high magnification imaging nanopores are observed in three main modes of occurrence most pores are found in grains of organic matter as intraparticle pores many of these grains contain hundreds of pores intraparticle organic nanopores most commonly have irregular bubblelike elliptical cross sections and range between 5 and 750 nm with the median nanopore size for all grains being approximately 100 nm internal porosities of up to 20 2 have been measured for whole grains of organic matter based on point count data from scanning electron microscopy analysis these nanopores in the organic matter are the predominant pore type in the barnett mudstones and they are related to thermal maturation nanopores are also found in bedding parallel wispy organic rich laminae as intraparticle pores in organic grains and as interparticle pores between organic matter but this mode is not common although less abundant nanopores are also locally present in fine grained matrix areas unassociated with organic matter and as nano to microintercrystalline pores in pyrite framboids intraparticle organic nanopores and pyrite framboid intercrystalline pores contribute to gas storage in barnett mudstones we postulate that permeability pathways within the barnett mudstones are along bedding parallel layers of organic matter or a mesh network of organic matter flakes because this material contains the most pores

exceptional preservation of fossil plant spores in high pressure metamorphic rocks it is commonly believed that high grade metamorphism erases all traces of fossils and more generally of biogenicity in rocks to address this issue we report in this study x ray spectromicroscopy raman microspectroscopy and electron microscopy observations at the nm scale of fossil lycophytes megaspores found in triassic metasedimentary rocks from the french alps these rocks have undergone high pressure metamorphic conditions not vert similar 14 kbar not vert similar 360 c during the alpine orogeny corresponding to a burial of not vert similar 35 km during subduction despite such metamorphism the fossil megaspores are remarkably well preserved texturally an unusual mineralogical zonation consisting of magnetite ankerite and calcite is superposed on the structures of the spore walls in parallel chemical heterogeneities of the organic matter om composing the spore walls were detected we discuss different interpretations for our observations in particular a scenario in which these features are remnants of the original structural heterogeneities in the spores and provide insights on the fossilization processes under high grade metamorphic conditions whatever the processes leading to such heterogeneities our observations oppose the usual assumption that high grade metamorphism erases all indications of biogenicity in rocks and open new avenues for investigation at the nm scale of the evolution of organic matter during diagenesis and metamorphism

alteration of submarine basaltic glass from the ontong java plateau a stxm and tem study frequent observations of tubular to vermicular microchannels in altered basalt glass have led to increasing appreciation of a possible significant role of microbes in the low temperature alteration of seafloor basalt we have examined such microchannel alteration features at the nanoscale in basalt glass shards from the ontong java plateau using a combination of focused ion beam milling transmission electron microscopy and scanning transmission x ray microscopy three types of materials were found in ultrathin cross sections cut through the microchannels by fib milling fresh basalt glass amorphous si rich rims surrounding the microchannels and palagonite within the microchannels x ray absorption spectroscopy at the c k edge and fe l2 3 edges showed the presence of organic carbon in association with carbonates within the microchannels and partial oxidation of iron in palagonite compared with basalt glass although these observations alone cannot discriminate between a biotic or abiotic origin for the microchannels they provide new information on their mineralogical and chemical composition and thus better constrain the physical and chemical conditions prevailing during the alteration process 2007 published by elsevier b v