empirical and theoretical comparisons of the chicxulub and sudbury impact structures chicxulub and sudbury are 2 of the largest impact structures on earth research at the buried but well preserved chicxulub crater in mexico has identified 6 concentric structural rings in an analysis of the preserved structural elements in the eroded and tectonically deformed sudbury structure in canada we identified ring like structures corresponding in both radius and nature to 5 out of the 6 rings at chicxulub at sudbury the inner topographic peak ring is missing which if it existed has been eroded reconstructions of the transient cavities for each crater produce the same range of possible diameters 80110 km the close correspondence of structural elements between chicxulub and sudbury suggests that these 2 impact structures are approximately the same size both having a main structural basin diameter of 150 km and outer ring diameters of 200 km and 260 km this similarity in size and structure allows us to combine information from the 2 structures to assess the production of shock melt melt produced directly upon decompression from high pressure impact and impact melt shock melt and melt derived from the digestion of entrained clasts and erosion of the crater wall in large impacts our empirical comparisons suggest that sudbury has 70 more impact melt than does chicxulub 31 000 versus 18 000 km3 and 85 more shock melt 27 000 km3 versus 14 500 km3 to examine possible causes for this difference we develop an empirical method for estimating the amount of shock melt at each crater and then model the formation of shock melt in both comet and asteroid impacts we use an analytical model that gives energy scaling of shock melt production in close agreement with more computationally intense numerical models the results demonstrate that the differences in melt volumes can be readily explained if chicxulub was an asteroid impact and sudbury was a comet impact the estimated 70 difference in melt volumes can be explained by crater size differences only if the extremes in the possible range of melt volumes and crater sizes are invoked preheating of the target rocks at sudbury by the penokean orogeny cannot explain the excess melt at sudbury the majority of which resides in the suevite the greater amount of suevite at sudbury compared to chicxulub may be due to the dispersal of shock melt by cometary volatiles at sudbury

evolution of methane sorption capacity of coal seams as a function of burial history a case study from the campine basin ne belgium based on extensive data sets of high pressure sorption isotherms and canister desorption data from two central european coal basins campine and ruhr basins a computational scheme has been developed to calculate the maximum coal bed methane cbm sorption capacity of coal seams as a function of pressure temperature and coal rank in addition the effects of in situ moisture content and maceral composition have been tentatively implemented using this algorithm it is possible to explicitly take into account variations in sorption capacity of the coal seams in sedimentary basins over geologic time as a function of burial history and thermal evolution the procedure has been applied to model the evolution and the present day coal bed methane content of the campine basin ne belgium it is demonstrated how the present day gas content of the campine basin is controlled by the burial history of the coal layers throughout geologic time the maximum gas sorption capacity typically occurs at a depth range between 500 and 1000 m during periods of uplift and erosion 250 and 90 ma before present the uppermost coal layers have lost methane due to a reduction of gas storage capacity while their storage capacity has increased during periods of burial 300 and 180 ma ago and present additionally the present day gas content is controlled by the gas generation history coal layers which have lost storage capacity during geologic time will stay undersaturated if not replenished by late stage e g microbial gas the gas content profile of test well kb206 in the ne campine basin established from canister desorption tests can be reproduced by assuming that undersaturation of the coals is due to erosion and re burial and that no significant gas generation e g microbial gas has taken place after the time of maximal burial isotopic data d 178 13c 59 indicate the presence of a small portion of microbial gas the absolute gas contents in this well are lower than the calculated maximum present day sorption capacity this may be due to an underestimation of the effect of water content on sorption capacity or result from degassing via nearby faults enforced by fluid circulation

high pressure methane and carbon dioxide sorption on coal and shale samples from the parana basin brazil abstract an experimental study has been conducted to assess the potential for coalbed methane production and carbon dioxide storage in coals carbonaceous shales and source rocks in the parana basin in brazil high pressure sorption tests with methane and carbon dioxide were performed on coal and carbonaceous shales from the santa terezinha coalfield and samples from two principal petroleum source rocks measured excess sorption capacities ranged from 0 03 to 0 47 mmol g for methane and 0 14 to 0 81 mmol g for carbon dioxide showing a decrease with decreasing organic matter content linear regression lines for methane sorption capacity vs toc extrapolated to approximately zero whereas for carbon dioxide the intercept of the regression line indicated a residual sorption capacity of 0 2 mmol g on the mineral matter present day gas contents of coals collected from the first cbm well in the santa terezinha coalfield correspond to 13 38 of the measured maximum sorption capacities carbon dioxide sorption capacities exceed methane sorption capacities by a factor of 1 9 to 6 9 for these coals free sorption capacities of the under saturated coals in combination with preferential sorption of carbon dioxide could favour co 2 enhanced methane recovery and co 2 storage in coals and shales of the study area based upon the calculated coal reserves gas contents and measured sorption capacities a total storage potential of 15 4 gt co 2 was estimated for an area of 20 40 km 2 in the santa terezinha coal field assuming a combined co 2 enhanced coalbed methane ecbm production and co 2 storage operation to fully evaluate the potential for carbon dioxide storage and coalbed methane production further studies are required to assess producibility of methane and efficiency of long term co 2 storage in the study area

formation of nanoporous pyrobitumen residues during maturation of the barnett shale fort worth basin abstract hydrocarbon generation reactions occur within organic rich shales as a response to thermal maturation here we report observations on samples from the organic rich mississippian barnett shale gas system fort worth basin texas usa at varying stages of thermal maturation a multiscale characterization was achieved using a combination of compositional organic geochemistry and spectromicroscopy techniques including synchrotron based scanning transmission x ray microscopy stxm and transmission electron microscopy tem we document the chemical evolution of the macromolecular structure of barnett shale kerogen with increasing maturity from an immature kerogen containing a significant aliphatic component and an important concentration of oxygen and sulfur containing functional groups to an overmature kerogen dominated by poorly condensed aromatic structures in addition to the presence of bitumen in samples of oil window maturity very likely genetically derived from thermally degraded kerogen the formation of nanoporous pyrobitumen has been inferred for samples of gas window maturity likely resulting from the formation of gaseous hydrocarbons by secondary cracking of bitumen compounds

a tem investigation of shock metamorphism in quartz from the vredefort dome south africa the origin of the vredefort structure in south africa is still debated several causes have been discussed namely asteroid impact internal gas explosion or tectonic processes evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains shatter cones veins of pseudotachylite and occurrence of coesite and stishovite high pressure quartz polymorphs a number of these characteristics is widely believed to support an impact origin however the planar features in quartz which are generally considered as one of the strongest indicators of impact are in the vredefort case considered as anomalous when compared with those from accepted impact structures we have investigated by optical and transmission electron microscopy tem the defect microstructures in quartz grains from different lithologies sampled at various places at the vredefort structure whatever the locality only thin mechanical brazil twin lamellae in the basal plane are observed by tem so far such defects have only been found in quartz from impact sites but always associated with sets of thin glass lamellae in rhombohedral planes 10 1n with n 1 2 3 and 4 at the scale of the optical microscope brazil twins in 0001 are easily detected in vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them similar alignments of tiny fluid inclusions parallel to other planes are also detected optically but at the tem scale no specific shock defects are detected along their traces if these inclusion alignments initially were shock features they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae fine grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen but stishovite was not found even in areas where its occurrence was previously reported finally definite evidence of high temperature annealing was observed in all the samples these observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the vredefort structure most of the original shock defects are now overprinted by an intense post shock annealing episode only the thin mechanical twin lamellae in the basal plane have survived

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

observations and interpretations at vredefort sudbury and chicxulub towards an empirical model of terrestrial impact basin formation the structural topographic and other characteristics of the vredefort sudbury and chicxulub impact structures are described assuming that the structures originally had the same morphology the observations interpretations for each structure are compared and extended to the other structures this does not result in any major inconsistencies but requires that the observations be scaled spatially in the case of vredefort and sudbury this is accomplished by scaling the outer limit of particular shock metamorphic features in the case of chicxulub scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring the observations interpretations are then used to construct an integrated empirical kinematic model for a terrestrial peak ring basin the major attributes of the model include a set of outward directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor some of which are pre existing structures that have been reactivated during transient cavity formation inward directed motions along the same outermost structures and along a set of structures at intermediate radial distances during transient cavity collapse structural uplift in the center followed by a final set of radially outward directed thrusts at the outer edges of the structural uplift during uplift collapse the rock displacements on the intermediate inward and innermost outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift

relationships between the vredefort structure and the witwatersrand basin within the tectonic framework of the kaapvaal craton as interpreted from regional gravity and aeromagnetic data abstract processed gravity and aeromagnetic data covering the witwatersrand basin and adjacent areas have been used to delineate the boundary of the basin and to investigate other features such as the vredefort structure the bethlehem gravity high and the colesberg magnetic anomaly trend a model is proposed in which the regional aeromagnetic and gravity signatures of these features can be related to the disposition of a semi continuous mid crustal magnetite rich layer which is exposed or has a shallow suboutcrop in three regions in the centre of the vredefort structure along the kaapvaal craton boundary and centrally through the craton northwards from colesberg the bethlehem high is thought to be due to elevation of lower levels of the crust along a nw trending geanticlinal arch the apex of this broad arch is termed the vredefort axis and plunges northwestwards beneath the witwatersrand basin the vredefort structure lies at the intersection of this axis and a major nne trending axis of crustal downwarp which also constitutes the long axis of the witwatersrand basin two contrasting scenarios for the development of these major axes are discussed with either scenario genesis of the vredefort structure should be integrated with the genesis and evolution of the vredefort axis and with the tectonic evolution of the kaapvaal craton

organic matter clay mineral intergranular pores in the lower cambrian lujiaping shale in the north eastern part of the upper yangtze area china a possible microscopic mechanism for gas preservation abstract gas shales with a high gas content were encountered in the lower cambrian lujiaping formation in the northeast part of the upper yangtze area china in this area the tectonic condition is complex and the organic matter is over matured so conventional oil and gas exploration have not previously been considered during a recent shale gas exploration canister desorbed gas contents of more than 1 m 3 t were detected from 490 to 825 m depth with a maximum value of 3 18 m 3 t at 762 m hence to understand the mechanism of preservation of the gas is important the gas shales have a total organic carbon toc content of between 0 44 and 6 91 and vitrinite like macerals reflectance r o between 3 3 and 4 3 with type ii kerogen the porosity of 22 gas shale samples ranges between 0 4 and 2 29 and their permeability is between 6 9 10 5 and 3 8 10 1 md to investigate the shale s pore structure and its relation to gas storage behavior we studied the microscopic mechanism of gas preservation using a combination of pore characterization techniques including focused ion beam scanning electron microscope fib sem imaging and comparison of gas sorption and free gas storage capacity folding thrusting and subsequent rapid uplifting movements occurred after most of the dry gas had been originally generated and the present gas is a mixture originated from the cracking of liquid hydrocarbons at high maturity and kerogen pyrolysis in the early hydrocarbon generation phase sem observations combined with low temperature gas adsorption and mercury injection capillary pressure analyses show that pores within the organic matter or the mineral particles in the lujiaping shale are rare while the nanometer sized pores 1 4 nm between organic matter and clay mineral particles especially in cleavage domains are greatly developed thus the gas in these shales is mainly adsorbed gas due to the limited free gas storage space this is unlike most other shale gas reservoirs pores within the organic matter and pores between or within mineral particles are dominant moreover the desorbed gas content correlates well with toc content and the microporous and mesoporous specific surface area determined by low temperature co 2 and n 2 respectively suggesting that the nanometer sized pores associated with the organic matter make up the fundamental storage space for this shale the calculations based on fib sem observation showed that the nanometer sized intergranular pores have large specific surface areas with their small pore throat diameters resulting in high capillary pressure these characteristics have contributed to the preservation of shale gas in this complex tectonic area

shatter cones branched rapid fractures formed by shock impact 1 shatter cones are rock discontinuities known only from sites of extraterrestrial impacts and they are assumed to be formed by impact induced shock waves here we characterize the structure of shatter cones by field and microanalyses and explain their formation by dynamic fracture mechanics our analyses reveal that shatter cones always occur as multilevel three dimensional networks 0 01 100 m in size with hierarchal branched fractures a typical individual shatter cone is a curved oblate branch that bifurcates from its parent fracture e g a larger shatter cone and expands to form a spoon like surface the unique shatter cone striations are arranged in v shaped pairs whose enclosed angle is constant for a given sample we propose that shatter cones are the natural consequence of tensile rock fracturing at extreme velocities first the structure of shatter cone networks is strikingly similar to the structure of branched networks of experimental dynamic fractures that propagate at high velocities velocities that approach the rayleigh wave speed vr second fracture front waves generated experimentally by the interaction of a rapidly moving tensile fractures and material inclusions create tracks on the fracture surface that correspond to the v shaped striations of shatter cones third applying the front wave concept to our field measurements vredefort impact south africa shows that the shatter cones propagated at velocities of 0 98 0 90vr with a systematic velocity decrease from the impact center these extreme asymptotic velocities require the intense energy flux of impacts our model explains all of the structural features of shatter cones curved surfaces cone directivity unique striations hierarchic multilevel structure and their exclusive occurrence at impact sites

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

organic matter hosted pore system marcellus formation devonian pennsylvania the marcellus formation of pennsylvania represents an outstanding example of an organic matter om hosted pore system most pores detectable by field emission scanning electron microscopy fe sem are associated with om instead of mineral matrix in the two wells studied here total organic carbon toc content is a stronger control on om hosted porosity than is thermal maturity the two study wells span a maturity from late wet gas vitrinite reflectance ro 1 0 to dry gas ro 2 1 samples with a toc less than 5 5 wt display a positive correlation between toc and porosity but samples with a toc greater than 5 5 wt display little or no increase in porosity with a further increasing toc in a subset of samples 14 across a range of toc 2 3 13 6 wt the pore volume detectable by fe sem is a small fraction of total porosity ranging from 2 to 32 of the helium porosity importantly the fe sem visible porosity in om decreases significantly with increasing toc diminishing from 30 of om volume to less than 1 of om volume across the range of toc the morphology and size of om hosted pores also vary systematically with toc the interpretation of this anticorrelation between om content and sem visible pores remains uncertain samples with the lowest om porosity higher toc may represent gas expulsion pore collapse that was more complete as a consequence of greater om connectivity and framework compaction whereas samples with higher om porosity lower toc correspond to rigid mineral frameworks that inhibited compactional expulsion of methane filled bubbles alternatively higher toc samples may contain om low initial hydrogen index relatively unreactive that is less prone to development of fe sem detectable pores in this interpretation om type controlled by sequence stratigraphic position is a factor in determining pore size distribution

experimental investigation of changes in methane adsorption of bitumen free woodford shale with thermal maturation induced by hydrous pyrolysis this study quantifies the effects of organic matter om thermal maturity on methane ch4 sorption on the basis of five samples that were artificially matured through hydrous pyrolysis achieved by heating samples of immature woodford shale under five different time temperature conditions ch4 sorption isotherms at 35 degrees c 50 degrees c and 65 degrees c and pressures up to 14 mpa on dry solvent extracted samples of the artificially matured woodford shale were measured the results showed that ch4 sorption capacity normalized to toc varied with thermal maturity following1 the trend maximum oil 367 degrees c oil cracking 400 degrees c maximum bitumen early oil 333 degrees c early bitumen 300 degrees c immature stage 130 degrees c the langmuir constants for the samples at maximum oil and oil cracking stages are larger than the values for the bitumen forming stages the total pore volume determined by n 2 physisorption at 77 k increases with increased maturation mesopores 2 50 nm in width were created during the thermal conversion of organic matter and a dramatic increase in porosity appeared when maximum bitumen and maximum oil generation stages were reached a linear relationship between thermal maturity and brunauer emmett teller bet surface area suggests that the observed increase in ch4 sorption capacity may be the result of mesopores produced during om conversion no obvious difference is observed in pore size distribution and pore volume for samples with pores 2 0 nm with the increase of thermal maturity based on co2 physisorption at 273 k the isosteric heat of adsorption and the standard entropy for artificially matured samples ranged from 17 9 kj mol 1 to 21 9 kj mol 1 and from 85 4 j mol 1 k 1 to 101 8 j mol 1 k 1 respectively these values are similar to the values of immature woodford kerogen concentrate previously observed but are larger than naturally matured organic rich shales high temperature hydrous pyrolysis might have induced lewis acid sites on both organic and mineral surfaces resulting to some extent in chemical interactions between the adsorption site and the methane c h bonds the formation of abundant mesopores 2 50 nm within organic matter during organic matter thermal maturation makes a great contribution to the increase in both bet surface area and pore volume and a significant increase in 2 6 nm pores occurs at maximum oil generation and oil cracking to gas ultimately controlling the methane adsorption capacity therefore consideration of pore size effects and thermal maturity is very important for gas in place gip prediction in organic rich shales c 2014 elsevier ltd all rights reserved

polyphase thermal evolution in the infra cambrian ara group south oman salt basin as deduced by maturity of solid reservoir bitumen petrographic and organic geochemical analyses on solid reservoir bitumen from the late precambrian to early cambrian ara group indicate polyphase thermal evolution in the south oman salt basin sosb the ara group constitutes a complex petroleum system in the deep subsurface 3 5 km of interior oman salt diapirs encase isolated partly highly overpressured carbonate bodies intra salt stringers which represent a predominantly self charging hydrocarbon system a large number of stringer cores show that reservoir quality is decreased by the presence of solid reservoir bitumen by using different microscopy techniques this solid reservoir bitumen has been identified within pores pore throats and microfractures of the carbonates solid bitumen reflectance measurements brr were converted to vitrinite reflectance vrr using a new improved calibration vitrinite reflectance geothermometry shows that maximum paleo temperatures have a highly heterogeneous distribution paleo temperatures of 380 c were obtained from coke like solid reservoir bitumen which most likely formed from thermal cracking of oil in our model this process is related to hydrothermal fluids deriving from deeper pre salt strata of the sosb which entered the intra salt carbonate stringers during times of basement tectonic activity via hydrofractures this caused the formation of high maturities pyrobitumens in close vicinity to the fractures and precipitation of migrabitumens with increasing distance to the ht source which is supported by the geochemical composition of the solid reservoir bitumens the influx of these fluids into the carbonate stringers is considered to represent a major contribution to their strong overpressures 2007 elsevier ltd all rights reserved

dynamic fracture by large extraterrestrial impacts as the origin of shatter cones a large impact by a comet or meteorite releases an enormous amount of energy which evaporates melts and fractures the surrounding rocks distinctive features of such impacts are shatter cones deformed rocks characterized by hierarchical striated features although such features have been used for decades as unequivocal fingerprints of large body impacts the process by which shatter cones form has remained enigmatic here we show that the distinctive shatter cone striations naturally result from nonlinear waves front waves that propagate along a fracture front this explains the observed systematic increase of striation angles with the distance from the impact shatter cone networks typically spanning many scales can be understood as hierarchical bifurcations of the fracture front which is generated by the immense energy flux carried by the initial impact generated shock waves our quantitative predictions based on this theory are supported by field measurements at the kentland and vredefort impact sites these measurements indicate that shatter cones near to the impact site were formed by fractures propagating at nearly the rayleigh wave speed of the host rocks whereas the furthest shatter cones observed about 40 km from the impact site were formed by fronts moving more slowly these results provide insight into impact dynamics as well as dissipative mechanisms in solids subjected to sudden extremely intense fluxes of energy

paleomagnetic and rock magnetic study of the vredefort impact structure and the johannesburg dome kaapvaal craton south africa implications for the apparent polar wander path of the kaapvaal craton during the mesoproterozoic abstract new paleomagnetic and rock magnetic results are presented for various pre syn and post impact lithologies from the region of the 2023 4 ma originally ca 250 km diameter vredefort impact structure on the kaapvaal craton of south africa after removal of a viscous remanent magnetization vrm component the characteristic remanent magnetization chrm component from 16 vredefort granophyre and pseudotachylitic pt breccia samples d 18 3 i 54 8 95 8 1 was isolated and a paleomagnetic pole at 25 1 n and 43 5 e a 95 10 6 obtained since such a chrm component was isolated also from the archean basement rocks of the vredefort dome we consider its nature as a primary component of the impactites proven the pole falls onto the paleoproterozoic part of the apparent polar wander path of the kaapvaal craton rock magnetic analysis and scanning electron microscopy sem revealed that the pseudotachylitic breccia and granophyre samples contain two distinct magnetite phases one ultra small micrometer size and one altered and larger 50 m phase the ultra small magnetite is interpreted as the carrier of the chrm and the larger grains as the carriers of the vrm component additionally rock magnetic and petrophysical data reveal unusually high koenigsberger ratios q values in all pre impact lithologies in some vredefort impactite samples and in the much younger 1 1 ga anna s rust sheet ars gabbro samples as the high q values which had also been reported by previous studies of vredefort lithologies are now also seen in samples from the johannesburg dome a direct link to the vredefort impact can be ruled out this is also supported by the hysteresis data of this study as the observed magnetization is rather hard and shows multiple components of remanent magnetization we exclude lightning as a cause for all observed high q values except in case of ars gabbros it is instead suggested that the cause of the high q values could be related to the high temperatures of the rocks that were uplifted by the impact event from a mid crustal original setting and to fluid circulation within the two domes that made the rocks vulnerable to acquire high thermochemical remanence moreover paleomagnetic analysis of the rocks around the ars gabbro intrusion in the northern part of the vredefort structure revealed the presence of either a shallow north or a shallow south direction which is tentatively related to emplacement of the umkondo large igneous province analysis of all rocks including the vredefort impactites yields occasionally distinct great circle paths towards these shallow directions a likely explanation for this overprint direction is the heating caused by now eroded ars type gabbro in the area or by regional ca 1 1 1 0 ga orogenic effects no evidence of karoo type 0 18 ga overprint is seen

aspects of the dynamic and thermal metamorphic history of the vredefort cryptoexplosion structure implications for its origin abstract the vredefort structure is the oldest and the largest known cryptoexplosion structure on earth an approximately 36 km deep section through the archean sialic crust and the overlying precambrian strata of the kaapvaal craton is exposed in the core of the structure the geology presented in the exposed section includes all the principal metamorphic facies in the crust and records a long and complex thermo tectonic history which dates back to at least 3 5 ga the petrographie and geological observations in the basement rocks indicate that there is a complex interrelationship between the archean geology and the 2 0 ga dynamic and thermal metamorphic overprint some of which are postulated to be indicative of impact processes the dynamic and thermal metamorphic effects do not increase progressively towards the centre of the structure as found at known impact structures in particular dynamic deformation phenomena such as pseudotachylite and planar features in quartz reach maximum intensity in the rocks close to the vredefort discontinuity a brittle ductile shear zone which separates upper crustal amphibolite facies rocks from lower crustal granulites in certain other lithological zones deformation phenomena are noticeably absent or diminished we suggest that changes in the physical and chemical properties of the rocks from margin to centre of the basement may account for the variation in the intensity of the 2 0 ga thermal and dynamic metamorphic effects observed at vredefort in conclusion our overall impression of the vredefort structure is that it is a relic of an ancient meteorite impact crater but that there were thermo tectonic events which occurred both prior to and after the postulated impact event which complicates the interpretation of its origin

microscopic planar deformation features in quartz of the vredefort structure anomalous but still suggestive of an impact origin abstract the orientation and distribution of diagnostic shock produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the vredefort structure in south africa these features are widespread occurring in all but four samples the majority of the planar features have a basal 0001 orientation with a few higher pressure so called features and three features occurring near the center of the structure their relative distribution and preservation are however anomalous compared to those at other large terrestrial impact structures and recorded shock pressures calculated on the basis of planar feature orientation do not show a regular decrease radially outward as observed at other known impact structures the apparently anomalous development of planar features is interpreted as a result of post shock recrystallization if the effect of recrystallization of quartz which generally increases inward toward the core of the structure is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure then higher pressures are recorded in the core compared to the collar rocks no evidence of a previously reported additional post recrystallization shock event is found in this study although previous researchers report that pseudotachylite which may or may not be associated with the shock event cuts recrystallized quartzites in is concluded that although anomalous the evidence from planar features in quartz is still consistent with the vredefort structure being an erosional remnant of a large complex impact structure

impact tectonics in the core of the vredefort dome south africa implications for central uplift formation in very large impact structures the 80 km wide vredefort dome presents a unique opportunity to investigate the deep levels of the central uplift of a very large impact structure exposure of progressively older strata in the collar of the dome and of progressively higher grade metamorphic rocks toward its center is consistent with differential uplift however the deepest levels exposed correspond to pre impact mid crust rather than lower crust as has been suggested previously pre impact archean gneissic fabrics in the core of the dome are differentially rotated with the angle of rotation increasing sharply at a distance of 1619 km from the center the present asymmetric dips of the collar strata with layering dipping outward at moderate angles in the southeastern sector but being overturned and dipping inward in the northwestern sector and the eccentric distribution of the pre impact metamorphic isograds around the core of the dome can be reconciled with symmetric rotation of an initially obliquely nw dipping target sequence during central uplift formation the rocks in the core of the dome lack distinctive megablocks or large slip magnitude faults such as have been described in other central uplifts we suggest that the large scale coherent response of these rocks to the central uplift formation could have been accommodated by small scale shear and or rotation along pervasive pseudotachylitic breccia vein fractures

constraints on the size of the vredefort impact crater from numerical modeling the vredefort structure in south africa was created by a meteorite impact about two billion years ago since that time the crater has been deeply eroded so to estimate its original size researchers have had to rely heavily upon comparison to other terrestrial impact structures recent estimates of the original crater diameter range from 160 km to as much as 400 km in this study we combined the capabilities of both hydrocode and finite element modeling using the former to predict where the pressure of an impact generated shock wave would have been high enough to form planar deformation features pdfs and shatter cones and the latter to follow the subsequent displacement of these shock isobars during the collapse of the crater we established constraints on the sizes of the projectile and the transient crater and thus on the size of the final crater by comparing the observed locations of pdfs around vredefort to the results of our simulations of impacts by projectiles of various sizes these simulations indicate that a rocky projectile with a diameter of 10 km impacting vertically at a velocity of 20 km s generates shock pressures that are consistent with the distribution of pdfs around vredefort these projectile parameters correspond to a transient crater 80 km in diameter or a final crater 120 160 km in diameter allowing for uncertainties in our modeling procedures we consider final craters 120 to 200 km in diameter to be consistent with the observed locations of pdfs at vredefort the shock pressure contour corresponding to the formation of shatter cones is almost horizontal near the surface making the locations of these features less useful constraints on the crater size however they may provide a constraint on the amount of erosion that has occurred since the impact

mid crustal granulite facies metamorphism in the central kaapvaal craton the bushveld complex connection abstract the numerous greenstone remnants which occur as inclusions in the archaean gneissic granitoid basement exposed in the core of the vredefort dome display evidence of very high grades of granulite facies metamorphism and an unusually complex three stage metamorphic history 1 in metapelites migmatites and refractory restites were produced in a mid crustal 0 5 gpa m1 anatectic event at temperatures in excess of 900 c in the restites 2 in the migmatites the peak metamorphic assemblages are variably overprinted by a high grade retrograde reaction which involved crystallizing anatectic melts m2 3 both the peak metamorphic assemblages and the retrogression textures are cross cut by pseudotachylitic breccia veins which mark a shock metamorphic event associated with the formation of the vredefort dome and 4 the peak metamorphic assemblages and the m2 retrogression textures are overprinted by new generations of remarkably fine grained post shock high grade phases which crystallised at a pressure some 0 25 gpa lower than that of both the peak metamorphic conditions and the retrogression m3 these data indicate an anticlockwise p t evolution where mid crustal anatexis most probably resulted from the intraplating of bushveld complex related ultramafic magmas at 2 06 ga a period of approximately isobaric cooling and retrogression was followed by the vredefort catastrophe which resulted from the impact of a large meteorite into the terrane at 2 02 ga this shock event halted the natural mid crustal metamorphic progression prior to the terrane cooling below 650 c and resulted in the rapid exhumation of the terrane by some 9 km in the process it provided a window into the deep levels of the central kaapvaal craton not seen elsewhere in the region

gold mineralization within the witwatersrand basin south africa evidence for a modified placer origin and the role of the vredefort impact event the chemical composition of gold within the archaean metasedimentary rocks of the witwatersrand supergroup displays significant heterogeneity at the micro meso and regional scales a detailed electron microbeam analytical and petrological study of the main auriferous horizons in the central rand group throughout the witwatersrand basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres contemporaneous chlorite formation was strongly rock buffered gold mobilization occurred under fluid poor conditions at temperatures that did not exceed 350 c widespread circulation of mineralizing fluids within the central rand group is not supported by the gold and chlorite chemical data brittle deformation that affects most of the paragenetic sequence of the central rand group late in its post depositional history is followed by sequences of mineral growth and dissolution that appear throughout the central rand group and have consistent textural relationships with gold the consistent location within the paragenetic sequence the wide regional and stratigraphic extent of the brittle deformation together with mineral chemical and petrological data suggest that the vredefort impact event 2 02 ga was the cause of this late deformation and that post impact fluid poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event

methane adsorption characteristics of overmature lower cambrian shales of deepwater shelf facies in southwest china abstract the lower cambrian shales of deepwater shelf facies comprise an important part of the marine black shales in southern china in this study twenty four lower cambrian shale samples from southwest china were subjected to analysis of geochemistry pore structure and methane adsorption capacity the samples have total organic carbon toc contents ranging from 0 25 to 11 3 and equivalent vitrinite reflectance eqvro values in the range of 2 51 4 23 specific surface areas determined from n2 adsorption at 77 4 k and specific micropore volumes from co2 adsorption at 273 15 k are in the range of 3 0 38 6 m2 g rock and 2 5 11 4 cm3 kg rock respectively and both of them have positive relationships with toc values the langmuir volumes of these samples at 60 c range from 0 47 cm3 g rock to 5 96 cm3 g rock and are positively correlated with toc values which on the one hand confirms the toc control on methane adsorption capacity and on the other hand also suggests a weak control of thermal maturity on methane adsorption capacity at overmature stages although both specific surface area and specific micropore volume are positively correlated with methane adsorption capacity the latter seems to have a stronger effect and the specific micropore volume determined from subcritical co2 adsorption may be considered as a proxy for the volume of adsorbed methane in dry shales for the extrapolation of measured methane excess adsorption isotherms to geological conditions the use of a constant or varying langmuir volume makes no significant difference with respect to the methane storage capacity of shales when the density of the adsorbed methane is allowed to change with temperature however the absolute methane adsorption quantity at subsurface is obviously dependent on the use of a constant or varying langmuir volume which subsequently affects the estimation of the relative percentages of adsorbed and free methane

investigation of pore structure and fractal characteristics of organic rich shale reservoirs a case study of lower cambrian qiongzhusi formation in malong block of eastern yunnan province south china abstract in order to study pore structure and fractal characteristics of the organic rich marine shale fourteen shale samples from lower cambrian qiongzhusi formation in malong block of eastern yunnan province were investigated by organic geochemical analysis total organic carbon content analysis and thermal maturity analysis x ray diffraction xrd analysis porosity and permeability tests field emission scanning electron microscopy fe sem low pressure nitrogen adsorption and methane adsorption experiments fractal dimensions d 1 and d 2 at relative pressure of 0 0 5 and 0 5 1 respectively were obtained from the nitrogen adsorption data using the fractal frenkel halsey hill fhh method not only have the relationships among pore structure parameters of shale the relationships between toc content mineral compositions pore structure parameters and fractal dimensions been discussed but also the significance of two fractal dimensions d 1 and d 2 and the impact of fractal dimensions on adsorption capacity have been investigated the results showed that fourteen shale samples have toc content ranging from 1 25 to 7 72 two fractal dimensions both increase with the increasing toc content and gradually come to a standstill the curves present the shape of parabola the major mineralogical compositions of shales are quartz and clay minerals the quartz contents are between 25 5 and 42 7 the clay contents are between 26 6 and 44 2 fractal dimension d 1 has a negative correlation with quartz contents and a positive correlation with clay minerals contents but fractal dimension d 2 has no apparent relationship with quartz and clay minerals contents the specific surface area is in the range of 4 98 m 2 g 19 66 m 2 g the total pore volume is between 0 00479 cm 3 g and 0 01765 cm 3 g and the average pore diameter is between 3 37 nm and 6 02 nm two fractal dimensions increase with the increasing surface area and pore volume and also increase with the decreasing average pore diameter because of the complicated pore surface and structure of small pores further investigation indicates that d 1 represents fractal characteristics from the irregular pore surface while d 2 represents fractal characteristics related to the complicated pore structure and shale samples with larger fractal dimensions have higher methane adsorption capacity therefore fractal analysis is helpful to have a better understanding of pore structure and adsorption capacity of marine shale

shock re equilibration of fluid inclusions in crystalline basement rocks from the ries crater germany this study examines the effects of shock metamorphism on fluid inclusions in crystalline basement target rocks from the ries crater germany the occurrence of two phase fluid inclusions decreases from shock stage 0 to shock stage 1 while single phase inclusions increase likely as a result of re equilibration in shock stages 2 and 3 both two phase and single phase inclusions decrease with increasing shock stage indicating that fluid inclusion vesicles are destroyed due to plastic deformation and phase changes in the host minerals however quartz clasts entrained in shock stage 4 melts contain both single phase and two phase inclusions demonstrating the rapid quenching of the melt and the heterogeneous nature of impact deformation inclusions in naturally shocked polycrystalline samples survive at higher shock pressures than those in single crystal shock experiments however fluid inclusions in both experimental and natural samples follow a similar trend in re equilibration at low to moderate shock pressures leading to destruction of inclusion vesicles in higher shock stages this suggests that shock processing may lead to the destruction of fluid inclusions in many planetary materials and likely contributed to shock devolatilization of early planetesimals

the nature of porosity in organic rich mudstones of the upper jurassic kimmeridge clay formation north sea offshore united kingdom abstract analyses of organic rich mudstones from wells that penetrated the upper jurassic kimmeridge clay formation offshore united kingdom were performed to evaluate the nature of both organic and inorganic rock constituents and their relation to porosity in this world class source rock the formation is at varying levels of thermal maturity ranging from immature in the shallowest core samples to mature in the deepest core samples the intent of this study was to evaluate porosity as a function of both organic macerals and thermal maturity at least four distinct types of organic macerals were observed in petrographic and sem analyses and they all were present across the study area the macerals include in decreasing abundance 1 bituminite admixed with clays 2 elongate lamellar masses alginite or bituminite with small quartz feldspar and clay entrained within it 3 terrestrial vitrinite fusinite semifusinite grains and 4 tasmanites microfossils although pores in all maceral types were observed on ion milled surfaces of all samples the pores largely nanopores with some micropores vary as a function of maceral type importantly pores in the macerals do not vary systematically as a function of thermal maturity insofar as organic pores are of similar size and shape in both the immature and mature kimmeridge rocks if any organic pores developed during the generation of hydrocarbons they were apparently not preserved possibly because of the highly ductile nature of much of the rock constituents of kimmeridge mudstones clays and organic material inorganic pores largely micropores with some nanopores have been observed in all kimmeridge mudstones these pores particularly interparticle i e between clay platelets and intraparticle i e in framboidal pyrite in partially dissolved detrital k feldspar and in both detrital and authigenic dolomite are noteworthy because they compose much of the observable porosity in the shales in both immature and mature samples the absence of a systematic increase in organic porosity as a function of either maceral type or thermal maturity indicates that such porosity was probably unrelated to hydrocarbon generation instead much of the porosity within mudstones of the kimmeridge appears to be largely intraparticle and interparticle adjacent to inorganic constituents so the petroleum storage potential in these organic rich mudstones largely resides in inorganic pores

investigations on the methane sorption capacity of marine shales from sichuan basin china abstract high pressure methane sorption isotherms measured at 30 50 and 80 c and pressure up to 20 mpa were measured on lower silurian marine shales from sichuan basin of china the effect of total organic carbon content temperature thermal maturity mineral composition and pore structure on methane sorption capacity has been investigated a linear combination approach has been developed to predict the langmuir sorption capacity of shales based on the mass fractions of their organic and inorganic components this information can be used to estimate the in situ sorption capacity of shale layers as a function of burial depth formation pressure and temperature and composition methane sorption capacity of the dry shales shows a positive relationship with toc and specific surface area the langmuir pressure decreases with increasing maturity but the toc normalized langmuir sorption capacity of over mature shales also decreases with increasing thermal maturity this may be related to the carbonization of highly over mature organic matter for the samples investigated approximately 16 3 46 7 average 28 6 of the methane sorption can be attributed to clay minerals and 46 5 81 5 to average 67 6 organic matter in these shales respectively using the linear combination approach the calculated langmuir sorption capacity function matches the measured values reasonably methane sorption capacity computed as a function of depth for a mean hydrostatic pressure gradient of 0 01 mpa m and a geothermal gradient of 0 03 c m shows a rapid increase and reaches a maximum between 900 and 1800 m followed by a slow decrease with increasing depth at shallow burial depths 900 1800 m sorption capacity is controlled by the geothermal gradient higher geothermal gradients result in a steeper decline of sorption capacity

shock metamorphism of quartz in nature and experiment ii significance in geoscience the occurrence of shock metamorphosed quartz is the most common petrographic criterion for the identification of terrestrial impact structures and lithologies its utility is due to its almost ubiquitous occurrence in terrestrial rocks its overall stability and the fact that a variety of shock metamorphic effects occurring over a range of shock pressures have been well documented these shock effects have been generally duplicated in shock recovery experiments and thus serve as shock pressure barometers after reviewing the general character of shock effects in quartz the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored the short pulse lengths in experiments may account for the difficulty in synthesizing the high pressure polymorphs coesite and stishovite compared to natural occurrences in addition post shock thermal effects are possible in natural events which can affect shock altered physical properties such as refractive index and cause annealing of shock damage and recrystallization the orientations of planar microstructures however are unaffected by post impact thermal events except if quartz is recrystallized and provide the best natural shock barometer in terms of utility and occurrence the nature of planar microstructures particularly planar deformation features pdfs is discussed in some detail and a scheme of variations in orientations with shock pressure is provided the effect of post impact events on pdfs is generally limited to annealing of the original glass lamellae to produce decorated pdfs resulting from the exsolution of dissolved water during recrystallization basal 0001 pdfs differ from other pdf orientations in that they are multiple mechanical brazil twins which are difficult to detect if not partially annealed and decorated the occurrence and significance of shock metamorphosed quartz and its other phases namely coesite stishovite diaplectic glass and lechatelierite are discussed for terrestrial impact structures in both crystalline non porous and sedimentary porous targets the bulk of past studies have dealt with crystalline targets where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures in sedimentary targets the effect of pore space results in an inhomogeneous distribution in recorded shock pressure and temperature which requires a different classification scheme for the variation of recorded shock compared to that in crystalline targets this is discussed along with examples of variations in the relative abundances of planar microstructures and their orientations which are attributed to textural variations in sedimentary target rocks examples of the shock metamorphism of quartz in distal ejecta such as at the k t boundary and from nuclear explosions are illustrated and are equivalent to that of known impact structures except with respect to characteristics that are due to long term post shock thermal effects finally the differences between the deformation and phase transformation of quartz by shock and by endogenic tectonic and volcanic processes are discussed we confirm previous conclusions that they are completely dissimilar in character due to the vastly different physical conditions and time scales typical for shock events compared to tectonic and volcanic events well characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment

the methane sorption capacity of paleozoic shales from the sichuan basin china the methane sorption capacities of paleozoic shales from the sichuan basin were investigated the primary results show they have sorption capacities ranging between 0 94 and 4 29 cm3 grock at stp comparable to the sorption capacity 1 27 2 41 cm3 grock at stp of barnett shale however the sorption isotherms indicate that most of the sichuan basin shales would reach their approximate sorption equilibrium at 3 4 mpa in contrast to the continuously upward trend of north american shales a general positive relationship between toc content and methane sorption capacity indicates that toc content is the major controlling factor for the methane sorption of the sichuan shales a negative relationship between the clay mineral content and the sorption capacity of these shales may suggest that clay minerals play an insignificant role in the methane sorption of these paleozoic shales sorption capacities and variation ranges are generally ordered as to geological age viz cambrian silurian permian

geochemical characteristics of marine and terrestrial shale gas in china abstract although the annual production of shale gas in china was 13 10 8 m 3 in 2014 a systematic study on geochemical and isotopic characteristics of these unconventional gases has not been well addressed in the present study almost all shale gas samples available in china including marine shale gas from the wufeng longmaxi formation in the sichuan basin o 3 w s 1 l and terrestrial shale gas from chang 7 member t 3 y 7 in the ordos basin were collected and analyzed for their geochemical and isotopic compositions the shale gas from the wufeng longmaxi shale is dry gas with an average methane content of 98 38 and records a highest content of ch 4 in the world 99 59 which is consistent with the very high thermal maturity levels of the gas shales that have equivalent vitrinite reflectance eqvro values between 2 4 and 3 6 the 13 c 1 values are correspondingly heavy and record a heaviest 13 c 1 values 26 7 for the shale gases found in the world as well the average values of 13 c 1 13 c 2 and 13 c 3 for the wufeng longmaxi shale gas are 31 3 35 6 and 47 2 respectively displaying a complete carbon isotopic reversal i e 13 c 1 13 c 2 13 c 3 2 h ch4 and 2 h c2h6 has an average value of 148 and 173 respectively also yielding a hydrogen isotopic reversal i e 2 h ch4 2 h c2h6 the chang 7 shale has an average toc content of 13 81 with vitrinite reflectance vro values between 0 7 and 1 2 the chang 7 shale gas is wet gas with an average methane content of 84 90 and is rich in heavy gaseous hydrocarbons c 2 c 5 the respective values of 13 c 1 13 c 2 and 13 c 3 are 48 7 36 4 and 31 3 displaying a positive carbon isotopic distribution pattern i e 13 c 1 13 c 2 13 c 3 the average 2 h ch4 2 h c2h6 and 2 h c3h8 values are 256 244 and 188 respectively and are characterized by a positive distribution pattern i e 2 h ch4 2 h c2h6 2 h c3h8 the differences in gas wetness and carbon and hydrogen isotopic distribution patterns between the shale gases from the wufeng longmaxi and chang 7 shale are attributed to variations in thermal maturity levels co 2 is present in low content in both the wufeng longmaxi and chang 7 shale gases mostly less than 1 13 c values for the co 2 in the wufeng longmaxi formation are between 8 9 and 9 2 indicating an inorganic origin from the cracking of carbonate mineral in the shales under high temperatures in contrast 13 c values of the co 2 in the chang 7 shale gas range from 8 2 to 22 7 indicating an organic origin from the thermal degradation of organic matter helium in both the wufeng longmaxi and chang 7 shale gases is dominantly of curst origin in terms of their r ra ratios that vary from 0 01 to 0 08 positive carbon isotopic distribution pattern is typical of primitive thermogenic gas however it can be converted into complete or partial carbon isotopic reversal patterns due to secondary alteration the causes that yield carbon isotopic reversal include 1 mixing of gases with the same source but different thermal maturity levels 2 secondary cracking of oil or wet gas 3 formation water involved reactions 4 gas diffusion and 5 carbon isotope exchange between alkane gases at high temperature among them carbon isotopic exchange between alkane gases at high temperature is a key factor nine plots have been drawn based on the shale gases from china usa and canada among them the plot of 13 c 2 versus wetness demonstrates a lying s shape with two inflection points on the gas wetness axis the wetness value of 1 4 represents a critical point from pyrolytic gas primary cracking gas to cracking gas secondary cracking gas and whereas the wetness value of 6 marks the end of oil generation on the diagram of wetness versus 13 c shale gases with wetness values greater than 1 6 are characterized by positive carbon isotopic distribution pattern whereas a complete or partial carbon isotopic reversals are observed for shale gases with wetness values less than 1 6

a schematic model of crater modification by gravity models are proposed to account for the formation of slump terraces central peaks peak rings and concentric rings in large impact structures crater slumping and central peak formation is attributed to the process of acoustic fluidization which endows a volume of the target surrounding the crater with a bingham plastic rheology for a short time after the impact the parameters of this bingham plastic model are remarkably constant throughout the solar system the bingham yield stress ranges from a maximum of 80 bars on mercury to a minimum of 7 bars on callisto the bingham viscosity is between 109 and 1010 p at the onset of central peak formation for all bodies studied although it appears to rise to 1011 p as crater diameter grows to a few hundred kilometers asymmetric or scarp bounded concentric rings are due to a fundamentally different process they form when the target planet s lithosphere fractures under stresses imposed by the flow of the asthenosphere toward the transient crater cavity lithosphere yield strengths of less than a few hundred bars are implied by observed ring development on the moon and callisto

the organic matter distribution and methane capacity of the lower cretaceous strata of northeastern british columbia canada abstract the methane sorption capacity of a succession of sandstone siltstone shale and coal from the lower cretaceous fort st john group of northeastern british columbia was investigated average organic matter om content for all formations is 2 3 wt with a minimum of 0 6 wt and maximum of 10 1 wt the methane sorption capacity ranges between 0 19 and 2 74 cm3 g at 6 mpa and at 30 c total gas capacity free and sorbed gas ranges between 2 2 and 16 6 cm3 g at 6 mpa micropore volumes range between 0 43 and 1 69 cm3 100 g a positive correlation exists between the om content micropore volume and the methane capacity of shales shales with high methane capacities have either high contents of inertodetrinite or vitrinite moisture content shows a positive relationship with om content micropore volume and methane capacity the negative relationship between moisture and surface area derived from n2 adsorption suggests the moisture is located within the microporosity of the om and also explains the positive relationship between methane capacity and moisture the high inertodetrinite and vitrinite contents correlate with sea level regressions that delivered terrestrial om from coastal plains located south of the study area into the basin

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

original size of the vredefort structure implications for the geological evolution of the witwatersrand basin abstract historically there have been a range of diameter estimates for the large deeply eroded vredefort impact structure within the witwatersrand basin south africa here we estimate the diameter of the transient cavity at the present level of erosion as 124 140 km based on the spatial distribution of shock metamorphic features in the floor of the structure and downfaulted transvaal outliers taking erosion into account 6 km and scaling to original final rim diameter an estimate of close to 300 km for the rim diameter is obtained independent estimates of the final rim diameter based on an empirical relation of central uplift diameter to rim diameter spatial distribution of pseudotachylites and concentric large scale structural patterns give a similar estimate of close to 300 km for the original final rim diameter an impact structure of this size is expected to have had an original multi ring form at this size the vredefort impact structure encompasses the bulk of the witwatersrand basin which appears to owe its preservation to the vredefort impact in addition the vredefort impact event may have been the thermal driver for some of the widespread hydrothermal activity in the area which in recent interpretations is believed to be a component in the creation of the world class gold deposits of the witwatersrand basin

porosity of devonian and mississippian new albany shale across a maturation gradient insights from organic petrology gas adsorption and mercury intrusion the evolution of porosity in shales with increasing maturity was examined in a suite of five new albany shale samples spanning a maturity range from immature vitrinite reflectance ro 0 35 to postmature ro 1 41 devonian to lower mississippian new albany shale samples from the illinois basin used in this study contain marine type ii kerogen having total organic carbon contents from 1 2 to 13 0 wt organic petrology co2 and n2 low pressure adsorption and mercury intrusion capillary pressure techniques were used to quantify pore volumes pore sizes and pore size distributions increasing maturity of the new albany shale is paralleled by many changes in the characteristics of porosity the total porosity of 9 1 vol in immature new albany shale decreases to 1 5 vol in the late mature sample whereas total pore volumes decrease from 0 0365 to 0 0059 cm3 g in the same sequence reversing the trend at even higher maturity the postmature new albany shale exhibits higher porosity and larger total pore volumes compared to the late mature sample with increasing maturity changes in total porosity and total pore volumes are accompanied by changes in pore size distributions and relative proportions of micropores mesopores and macropores porosity related variances are directly related to differences in the amount and character of the organic matter and mineralogical composition but maturity exerts the dominant control upon these characteristics we conclude that organic matter transformation due to hydrocarbon generation and migration is a pivotal cause of the observed porosity differences

numerical modeling of impact heating and cooling of the vredefort impact structure large meteorite impacts such as the one that created the vredefort structure in south africa 2 ga ago result in significant heating of the target the temperatures achieved in these events have important implications for post impact metamorphism as well as for the development of hydrothermal systems to investigate the post impact thermal evolution and the size of the vredefort structure we have analyzed impact induced shock heating in numerical simulations of terrestrial impacts by projectiles of a range of sizes thought to be appropriate for creating the vredefort structure when compared with the extent of estimated thermal shock metamorphism observed at different locations around vredefort our model results support our earlier estimates that the original crater was 120 160 km in diameter based on comparison of predicted to observed locations of shock features the simulations demonstrate that only limited shock heating of the target occurs outside the final crater and that the cooling time was at least 0 3 myr but no more than 30 myr

mechanisms of shale gas storage implications for shale gas exploration in china this article reviews the mechanisms of shale gas storage and discusses the major risks or uncertainties for shale gas exploration in china at a given temperature and pressure the gas sorption capacities of organic rich shales are primarily controlled by the organic matter richness but may be significantly influenced by the type and maturity of the organic matter mineral composition especially clay content moisture content pore volume and structure resulting in different ratios of gas sorption capacity gsc to total organic carbon content for different shales in laboratory experiments the gsc of organic rich shales increases with increasing pressure and decreases with increasing temperature under geologic conditions assuming hydrostatic pressure gradient and constant thermal gradient the gsc increases initially with depth due to the predominating effect of pressure passes through a maximum and then decreases because of the influence of increasing temperature at greater depth this pattern of variation is quite similar to that observed for coals and is of great significance for understanding the changes in gsc of organic rich shales over geologic time as a function of burial history at an elevated temperature and pressure and with the presence of moisture the gas sorption capacities of organic rich shales are quite low as a result adsorption alone cannot protect sufficient gas for high maturity organic rich shales to be commercial gas reservoirs two models are proposed to predict the variation of gsc and total gas content over geologic time as a function of burial history high contents of free gas in organic rich shales can be preserved in relatively closed systems loss of free gas during postgeneration uplift and erosion may result in undersaturation the total gas contents lower than the sorption capacity and is the major risk for gas exploration in marine organic rich shales in china

a multidisciplinary approach in determining the maceral kerogen type and mineralogical composition of upper cretaceous eagle ford formation impact on pore development and pore size distribution abstract maceral kerogen or organic matter type and mineralogical composition with the associated porosity of the upper cretaceous eagle ford formation karnes county texas was investigated in karnes county the calcareous mudstone of the eagle ford formation was deposited in an open marine outer shelf to upper slope environment to adequately characterise all components of this mudrock a multidisciplinary approach was implemented to analyse 26 core samples from a single producing well that completed within the eagle ford formation analyses included organic petrology organic geochemistry light and electron microscopy and spectroscopy pore size distribution analyses and x ray diffraction the bitumen reflectance ranges between 1 94 and 2 08 calculated vr o 1 60 1 69 indicating the mudrock is in the condensate rich gas window the toc content ranges between 1 7 and 7 2 with an average of 3 9 mineral composition is dominated in descending order by calcite clay quartz and plagioclase calcite varies between 32 and 87 with an average of 57 porosity measured by helium pycnometry ranges between 3 5 and 10 3 with an average of 7 5 organic petrology coupled with backscattered and field emission scanning electron microscopy indicates matrix bitumen as the most significant contributor to the toc content and porosity matrix bitumen contains significant meso and microporosity and is enhanced by the secondary cracking of the bitumen an increase in bioclastic mostly foraminifera and siliciclastic components correlates with lower toc content due to the lower volume of organic rich matrix peloids fecal pellets are calcareous organic rich and lack terrigenous sediment indicating a zooplankton source pelagic deposition of peloids is interpreted as a primary mechanism of amorphinite deposition which is converted to hydrocarbons and matrix bitumen the mixed siliciclastic calcareous matrix that surrounds the peloids is comprised of matrix bitumen and dispersed organics from land plants vitrinite fusinite and inertodetrinite the siliciclastic component increases in the middle of the eagle ford and marks the boundary maximum flooding surface between the lower tst and upper hst section of the formation the inorganic and organic components suggest a mixed terrigenous and marine source with both inorganic and organic sediments being derived from the proximal ouachita thrust belt and from pelagic organisms within the overlying water column the lower tst eagle ford formation contains more micro and mesopores than the upper hst eagle ford formation due to higher toc content in the lower eagle formation these results illustrate that a multidisciplinary approach is necessary to identify how pore network are distributed within mudrocks and how this distribution is controlled by organic inorganic deposition and hydrocarbon generation processes

re os isotope and geochemical study of the vredefort granophyre clues to the origin of the vredefort structure south africa the origin of granophyric rock dikes the vredefort granophyre at the 2 ga vredefort structure in south africa has been controversial they have been interpreted as either impact melt or igneous intrusions most vredefort granophyre samples have considerably higher os contents than the country rocks and 187re 188os and 187os 188os ratios that scatter about a 2 ga isochron their initial 187os 188os ratios at 2 ga overlap the meteoritic data range indicating that all the granophyre samples contain some meteoritic os the re os isotopic composition of the granophyre is significantly different from that of local precursor rocks and indicates that up to 0 2 of a chondritic component could be contained in the granophyre our results confirm the interpretation that the vredefort granophyre represents an impact melt rock

characterization of methane adsorption on overmature lower silurian upper ordovician shales in sichuan basin southwest china experimental results and geological implications abstract a series of methane adsorption isotherms were measured at 35 4 c 50 6 c and 65 4 c at pressures up to 15 0 mpa for eight dried overmature lower silurian upper ordovician shale samples collected from the sichuan basin with toc values in the range of 1 87 5 74 the measured maxima of excess adsorption capacity of methane range from 1 25 to 2 50 cm 3 g rock at 65 4 c the maxima are slightly enhanced at 35 4 c but all are positively correlated with total organic carbon toc both the supercritical dubinin radushkevich sdr and langmuir based excess adsorption models were found to represent the experimental excess adsorption isotherms equally well within the experimental range the temperature dependent densities of adsorbed methane resulting from the parameter fit of the sdr based excess adsorption model are in the range of 297 415 mg cm 3 for the langmuir based excess adsorption model the adjusted densities range from 386 mg cm 3 to 1027 mg cm 3 and most of them are much larger than the liquid density of methane at its boiling point 424 mg cm 3 nevertheless the maxima of absolute methane adsorption capacity fitted by both models are not significantly different and are linearly correlated one of the contributors to the uncertainty of the gas in place estimation in geological conditions is the inconsistent utilization of experimental excess sorption data as absolute sorption values particularly at high pressures however the choice of adsorption model itself langmuir or sdr based and the fitting procedure assuming either constant or temperature dependent adsorbed phase density and maximum sorption capacity do not significantly affect the estimated gips for the geological system studied here with depths of less than 4000 m

palaeomagnetism of the vredefort meteorite crater and implications for craters on mars magnetic surveys of the martian surface have revealed significantly lower magnetic field intensities over the gigantic impact craters hellas and argyre than over surrounding regions the reduced fields are commonly attributed to pressure demagnetization caused by shock waves generated during meteorite impact in the absence of a significant ambient magnetic field lower than average magnetic field intensities are also observed above the vredefort meteorite crater in south africa yet here we show that the rocks in this crater possess much higher magnetic intensities than equivalent lithologies found elsewhere on earth we find that palaeomagnetic directions of these strongly magnetized rocks are randomly oriented with vector directions changing over centimetre length scales moreover the magnetite grains contributing to the magnetic remanence crystallized during impact which directly relates the randomization and intensification to the impact event the strong and randomly oriented magnetization vectors effectively cancel out when summed over the whole crater seen from high altitudes as for martian craters the magnetic field appears much lower than that of neighbouring terranes implying that magnetic anomalies of meteorite craters cannot be used as evidence for the absence of the planet s internally generated magnetic field at the time of impact

soil gas co 2 ch 4 and h 2 distribution in and around las canadas caldera tenerife canary islands spain abstract diffuse degassing of co 2 ch 4 and h 2 was investigated at the surface environment of canadas caldera canary islands during the gas survey carried out in the summer of 1995 soil co 2 concentration varied significantly from atmospheric levels to 30 while soil ch 4 and h 2 contents ranged from 5 to 851 ppm and from 0 5 to 620 ppm respectively soil co 2 ch 4 and h 2 distribution suggests that high diffuse degassing at canadas caldera is volcanic structurally controlled anomalous soil h 2 concentrations were identified at the summit of teide and outside caldera boundaries where the most recent eruption of tenerife island occurred 13 c co 2 data showed a magmatic mixed magmatic biogenic and biogenic origin while a biogenic origin is suggested for soil ch 4 at canadas caldera and its surroundings by coupling the co 2 3 he ratio with the 3 he 4 he ratio of fumarolic gas samples from the summit of teide we propose three possible sources for carbon morb type organic carbon and carbonate

a preliminary study on the pore characterization of lower silurian black shales in the chuandong thrust fold belt southwestern china using low pressure n2 adsorption and fe sem methods although lower silurian black shales within and around the sichuan basin have received renewed attention during the past few years because of their potential prospectivity for shale gas studies associated with shale gas reservoir evaluation are still rare in this study eight black shale core samples were collected from a well drilled recently in the chuandong thrust fold belt southwestern china and their geochemistry and pore structures were investigated using low pressure n2 adsorption analysis and field emission scanning electron microscopy fe sem observations the results show that the black shales have total organic carbon toc values ranging from 1 01 to 3 98 and their equivalent vitrinite reflectance values are in the range of 2 84 3 05 both mineral matrix and organic matter pores are well developed with pore sizes ranging from several to several hundred nanometers the total porosity for the eight samples ranges from 2 60 to 4 74 and the percentages of organic matter pores are estimated to be in the range of 31 62 the total surface area ranges from 5 06 m2 g to 19 32 m2 g and the micropore 2 nm surface area estimated by the t plot method ranges from 3 13 m2 g to 9 27 m2 g the toc values have positive relationships with the total porosity total surface area and the micropore 2 nm volume and surface area indicating toc may be an effective parameter for shale gas reservoir evaluation in the studied area

occurrence and orientation of shatter cones in pretoria group quartzites in the collar of the vredefort dome impact origin precluded shatter cones have been found cutting completely indurated fault breccia associated with the down faulting of previously overturned pretoria group quartzites around the vredefort structure the evidence indicates that the shatter cones formed later than the overturning hence they cannot be considered to have been formed by an impact generated shock wave which penetrated the strata just before overturning

recognition of eroded astroblemes abstract the morphology of lunar craters degrades only slowly by the formation of new impacts and their ejecta it is not the same on earth because of weathering and tectonism generally the typical crater topography has disappeared and the term impact structure or astrobleme is applied on planetary surfaces as on the moon two classes of craters are recognized based on their depth to diameter ratio simple bowl shaped and complex readjusted flat bottomed craters with a sharp transition from simple to complex pike 1977 1980 on earth small simple craters retain some of their typical rimmed pit morphology because they are generally recent many are associated with meteorite fragments large impact structures or astroblemes in addition to their specific circular features are recognized as such only by the presence of shock metamorphism criteria such as dense minerals coesite stishovite diaplectic minerals and glass maskelynite planar deformation features in minerals principaly in quartz and feldspars and shattercones which are commonly used these criteria are abundant in allochthonous breccia particularly in fall back breccia among which there are glass fragments such as tectite or microtectite glass spherules shards droplets rods etc but on the floor of the astroblemes the criteria of shock metamorphism are restricted to the central part which is often covered or eroded using the measurements of nine astroblemes visited a scaling model is presented for two types of craters readjusted along listric slip surfaces a well readjusted model based on data from charlevoix and manicouagan canada and a partly readjusted model based on data from the ries germany both possess a central uplift and a ring graben there is also on the floor of the astrobleme a specific type of breccia mylolisthenite and pseudotachylite associated with the displacement along the autochthonous masses of rocks in very large impact structures the shock metamorphism is able to melt a part of the floor grieve and cintala 1992 the heat resulting from the superheated impact melt destroys a part of the shock metamorphism criteria

pore characterization of organic rich lower cambrian shales in qiannan depression of guizhou province southwestern china abstract nine organic rich shale samples of lower cambrian black shales were collected from a recently drilled well in the qiannan depression guizhou province where they are widely distributed with shallower burial depth than in sichuan basin and their geochemistry and pore characterization were investigated the results show that the lower cambrian shales in qiannan depression are organic rich with toc content ranging from 2 81 to 12 9 thermally overmature with equivalent vitrinite reflectance values in the range of 2 92 3 25 and clay contents are high and range from 32 4 to 53 2 the samples have a total helium porosity ranging from 2 46 to 4 13 and total surface area in the range of 9 08 37 19 m2 g the estimated porosity in organic matters defined as the ratio of organic pores to the volume of total organic matters based on the plot of toc vs helium porosity is about 10 for the lower cambrian shales in qiannan depression and is far lower than that of the lower silurian shales 36 in and around sichan basin this indicates that either the organic pores in the lower cambrian shale samples have been more severely compacted than or they did not develop organic pores as abundantly as the lower silurian shales our studies also reveal that the micropore volumes determined by dubinin radushkevich dr equation is usually overestimated and this overestimation is closely related to the non micropore surface area of shales i e the surface area of meso and macro pores however the modified bet equation can remove this overestimation and be conveniently used to evaluate the micropore volumes surface area and the non micropore surface areas of micropore rich shales

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

geochemical characterization and methane adsorption capacity of overmature organic rich lower cambrian shales in northeast guizhou region southwest china abstract the niutitang shale lower cambrian is another set of organic rich shale in addition to the lower silurian longmaxi shale in southwest china in this study methane adsorption capacities of both whole rock and organic matter were investigated based on adsorption isotherms measured at various temperatures 40 120 c and pressures up to 35 0 mpa for eleven niutitang shale samples collected from northeast guizhou province the investigated samples cover a wide range of toc contents varying from 1 8 to 11 3 with an average equivalent vitrinite reflectance eqvro value of approximately 4 0 their bet specific surface areas and micropore volumes range from 11 1 to 26 2 m2 g rock and between 4 3 and 11 2 cm3 kg rock respectively both of which are positively correlated to toc content model fitted methane maximum absolute adsorption capacities n range from 1 80 to 5 85 cm3 g rock at 60 c and are found to largely increase with increasing toc this suggests that the toc control on methane adsorption capacity obtained by other authors for other shales with relative low thermal maturity levels may be extended to a very high thermal maturity level e g eqvro 4 0 the methane adsorption capacity of organic matter om in our shale samples however is somehow reduced as compared to the lower silurian longmaxi shale and other shales that display eqvro values smaller than 3 0 indicating thermal maturity may adversely affect methane adsorption capacity of shales when it evolves to be eqvro 4 0 nevertheless this reduction in adsorption capacity may be compensated by the very enrichment of om in the niutitang shale in addition a higher thermal maturity level results in a lower langmuir pressure value which makes it more difficult for methane desorption at reservoirs conditions the temperature dependency of adsorption parameters was also investigated which could be used to predict the changes of methane adsorption with burial depth under geological conditions beyond experimental temperature and pressure ranges

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

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

relationships between porosity organic matter and mineral matter in mature organic rich marine mudstones of the belle fourche and second white specks formations in alberta canada abstract this study presents approaches for evaluating hybrid source rock reservoirs within tight rock petroleum systems the emerging hybrid source rock reservoir shale play in the upper cretaceous second white specks and belle fourche formations in central alberta canada is used as an example to evaluate organic and inorganic compositions and their relationships to pore characteristics nineteen samples from a 77 5 m long core were analyzed using organic petrography organic geochemistry several methods of pore characterization and x ray powder diffraction xrd the lower part of the studied section includes quartz and clay rich mudrocks of the belle fourche formation with low carbonate content whereas the upper portion contains calcareous mudrocks of the second white specks formation strata are mineralogically composed of quartz plus albite 18 56 wt carbonates calcite dolomite ankerite 1 65 wt clays illite kaolinite chlorite 15 46 wt and pyrite 2 12 wt petrographic examinations document that organic matter represents marine type ii kerogen partly biodegraded with limited terrestrial input vitrinite reflectance r o 0 74 0 87 t max values 438 446 c and biomarkers indicate mid maturity within the oil window the relatively poor remaining hydrocarbon potential expressed as an s2 value between 2 1 and 6 5 mg hc g rock may result from an estimated 60 83 of the original kerogen having been converted to hydrocarbons with the bulk having migrated to adjacent sandstone reservoirs however the present day remaining total organic carbon toc pd content remains relatively high 1 7 3 6 wt compared with the estimated original toc o of 2 4 5 0 wt the calculated transformation ratio of 60 83 suggests that the remaining 17 40 wt of kerogen is able to generate more hydrocarbons the studied section is a tight reservoir with an average swanson permeability of 3 37 10 5 md measured on two samples and total porosity between 1 7 and 5 0 vol 3 vol on average the upper part of the sandy belle fourche formation with slightly elevated porosity values 3 5 5 vol likely represents the interval with the best reservoir properties in the studied core interval total pore volume ranges between 0 0065 and 0 0200 cm 3 g measured by a combination of helium pycnometry and mercury immersion mesopores 2 50 nm are the most abundant pores and occupy 34 67 of total porosity or a volume of 0 0030 0 0081 cm 3 g in comparison micropores 3 g and macropores 50 nm reach up to 57 with the exception of some samples failing to indicate the presence of this pore fraction volume 0 0000 0 0107 cm 3 g macroporosity is mostly responsible for variations in total porosity as suggested by macroporosity s strongest correlation with total porosity within the section the relatively narrow ranges of toc and minerals contents among measured samples limit our ability to further deconvolute factors that influence changes in total porosity and pore size distribution

geological models of gas in place of the longmaxi shale in southeast chongqing south china abstract gas in place gip is one of the primary controlling factors in shale gas production but studies examining gip have been lacking for the lower silurian longmaxi shale in south china in the present study a suite of longmaxi shale samples was collected from an exploratory well in southeast chongqing south china and the adsorption parameters were fitted using a supercritical dubinin radushkevich sdr model based on the high pressure methane adsorption experiment data for the samples the results show that the adsorbed phase density and the adsorbed gas capacity of the samples have a positive correlation with the content of total organic carbon toc but a negative correlation with temperature combined with the geological characteristics of the longmaxi shale in southeast chongqing gip models were constructed under three different fluid pressure conditions the absolute adsorbed amount of the samples increases and later decreases with increasing depth with a maximum corresponding to depths between 800 and 1200 m the fluid pressure coefficient has no obvious effect on the absolute adsorbed amount when burial depth is over 2000 m but controls the free gas content overpressure primarily increases the free gas content and thus increases the total gas content the free gas content of the longmaxi shale in the pengshui block is reduced to 47 58 of that in the fuling block which is the main reason for its low gas production further exploration of the longmaxi formation should be expanded to deeper burial shales in the eastern area of southeast chongqing