characteristics of mud diapirs and mud volcanoes and their relationship to oil and gas migration and accumulation in a marginal basin of the northern south china sea mud diapir and mud volcano have the same formation mechanism and similar development and evolution characteristics but different geological features and forms in this study the development and characteristics of a mud diapir in yinggehai basin northwestern south china sea and a mud volcano in southwest taiwan basin northeastern south china sea were investigated the geochemical characteristics of their associated natural gases were also analyzed the results indicated that the yinggehai basin mud diapir is mainly distributed in the deep and large depression of the central basin and presents five lines in echelon arrangement along a northwest direction its development and evolution characteristics are low density and speed and abnormally high temperature and overpressure in contrast the southwest taiwan basin mud volcano is mainly distributed in a deep water slope area of the south sag and tainan kaohsiung onshore mud volcanoes are mainly distributed along the deep and large fault zone and their morphological characteristics are similar to venting volcanoes the seabed morphology of offshore mud volcano seems similar but current research is still insufficient there are many types of associated hydrocarbon gases in mud diapir and mud volcano but the high maturity coal type hydrocarbon gas is the main one the associated non hydrocarbon gas is rich in co2 and n2 which is mainly derived from a co2 source from inorganic crust the source of both diapir and volcano is thick marine miocene and pliocene age mud with hydrocarbon generating potential the mature hydrocarbon generation mud diapir mud volcano migration and deposition into an appropriate reservoir as well as diapir associated structures constitute a unique oil gas migration and accumulation system which thus adds to our understanding of the distribution of such mud diapir volcano type oil gas reservoir

ankerite cementation in deeply buried jurassic sandstone reservoirs of the central north sea abstract upper jurassic fulmar formation shelf sandstones of the high temperature high pressure franklin and elgin fields south central graben north sea contain abundant disseminated and concretionary ankerite in contrast most jurassic north sea reservoirs contain only minor amounts of dispersed ankerite disseminated ankerite cement in the franklin and elgin fields has a fairly uniform isotopic composition 18o 10 to 12 5o oo pdb 13c 3 to 5o oo pdb ankerite concretions have 18o values similar to disseminated cements but a wider range of 13c values 1 to 5 5o oo pdb they also have highly variable intergranular volumes which together with the 13c data are interpreted as a combination of pore filling cementation and in situ replacement of comminuted bioclastic debris by ankerite fluid inclusion 18o and paragenetic evidence suggests that ankerite formed during deep burial c 3 5 to 4 5 km 140 170 c after the onset of overpressuring but before hydrocarbon emplacement in the reservoirs the regionally consistent 18o data suggest that ankerite formed via a temperature influenced mechanism and the relatively uniform 13c cement value indicates that organic matter and marine bioclastic carbonate contributed to the dissolved carbon reservoir in constant proportion this can be explained by calcite dissolution in response to ph decrease during thermal breakdown of organic acids such acids were derived from adjacent mudrocks undergoing hydrocarbon maturation and clay mineral transformations and are likely to have been transported in pore fluids with mg2 and fe2 the presence of these cations in solution upon thermal decarboxylation is inferred to have stabilized ankerite at the expense of calcite a relative paucity of ankerite in other fulmar formation reservoirs may reflect different sedimentological compositions less bioclastic debris and or lower burial temperatures less advanced decarboxylation

discrimination of biogenic and detrital magnetite through a double verwey transition temperature magnetite occurs widely in natural environments in both inorganic and biogenic forms discrimination of the origin of magnetite has important implications from searching for past microbial activity to interpreting paleomagnetic and environmental magnetic records in a wide range of settings in this study we present rock magnetic and electron microscopic analyses of marine sediments from the continental margin of oman low temperature magnetic data reveal two distinct verwey transition tv temperatures that are associated with the presence of biogenic and inorganic magnetite this interpretation is consistent with room temperature magnetic properties and is confirmed by electron microscopic analyses our study justifies the use of two distinct tv temperatures as a diagnostic signature for discriminating inorganic and biogenic magnetite simple low temperature magnetic measurements therefore provide a tool to recognize rapidly the origin of magnetite within natural samples in addition our analyses reveal progressive down core dissolution of detrital and biogenic magnetite but with preservation of significant amounts of fine grained magnetite within sediments that have been subjected to severe diagenetic alteration we demonstrate that preservation of magnetite in such environments is due to protection of fine grained magnetite inclusions within silicate hosts our results therefore also provide new insights into diagenetic processes in marine sediments

dawsonite formation in the beier sag hailar basin ne china tuff a natural analog for mineral carbon storage abstract we determined the rock types the authigenic minerals the paragenetic sequence and the origin of dawsonite in pyroclastic rocks from the yimin formation of beier sag in the hailar basin china dawsonite a diagenetic mineral is thought to result from a large influx of co 2 and therefore this system represents a natural analogue for in situ mineral carbon storage the studied host rocks are mainly tuffs tuffaceous sandstones which now contain up to 70 vol authigenic carbonates including dawsonite ankerite and siderite the initial alteration of the tuffs yielded minor siderite kaolinite illite and mixed illite smectite then formed as product phases dawsonite and quartz subsequently precipitated in response to co 2 influx apparently coupled to feldspar and perhaps kaolinte dissolution dawsonite reaches a maximum 25 vol of the bulk rock mass balance suggests that this co 2 influx was coupled to the external import of sodium and export of sio 2 ankerite and additional siderite precipitated during the late stage alkaline diagenesis the carbon isotope values of the dawsonite are in the range 4 1 to 2 2 indicating the magmatic origin of the co 2 vitrinite reflectance and thermal gradient constraints suggest that the dawsonite at this location formed at a temperature of 75 c

quartz cementation in late cretaceous mudstones northern north sea changes in rock properties due to dissolution of smectite and precipitation of micro quartz crystals abstract late cretaceous mudstones from two wells located in the northern north sea and the norwegian sea have been examined with respect to quartz cement two different types of quartz cement type 1 and type 2 have been identified using sem eds cl analysis of drill bit cuttings at depths 2370 2670 m 80 85 c type 1 appears as relatively large aggregates 30 100 m of depth temperature related crypto or microcrystalline to macrocrystalline irregular quartz cement formed by local re crystallization of biogenic silica the cl responses of type 1 quartz cement give a clear indication of an authigenic origin type 2 quartz cement represents relatively high amounts of extremely fine grained micro sized 1 3 m crystals embedded as discrete short chains or small clusters nests within the illitized clay matrix the cl responses of micro quartz crystals indicate an authigenic origin the micro quartz is most probably sourced from silica released during the smectite to illite dissolution precipitation reaction the petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones the silica released produce a subtle inter connected micro quartz network interlocked with aggregates of micro quartz and authigenic clay crystals this micro quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime this is indicated by an abrupt increase in well log velocity vp and change in seismic facies close to 2500 m 80 85 c

the paleomagnetism of single silicate crystals recording geomagnetic field strength during mixed polarity intervals superchrons and inner core growth 1 the basic features of the geomagnetic reversal chronology of the last 160 million years are well established the relationship between this history and other features of the field however has been elusive the determination of past field strength paleointensity is especially challenging commonly accepted results have come from analyses of bulk samples of lava historic lavas have been shown to faithfully record the past field strength when analyzed using the thellier double heating method data from older lavas however tend to show effects of in situ and laboratory induced alteration here we review an alternative approach single plagioclase crystals can contain minute magnetic inclusions 50 350 nm in size that are potential high fidelity field recorders thellier experiments using plagioclase feldspars from an historic lava on hawaii provide a benchmark for the method rock magnetic data from older lavas indicate that the feldspars are less susceptible to experimental alteration than bulk samples this resistance is likely related to the lack of clays in addition magnetic minerals are sheltered by the encasing silicate matrix from natural alteration that can otherwise transform the well defined thermoremanent magnetization into an irresolute chemical remanent magnetization if there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons intervals with few or no reversals thellier data sets based on single plagioclase crystals from lavas erupted during the cretaceous normal polarity superchron 83 120 million years ago suggest a strong 12 1022 am2 stable field consistent with an inverse relationship between reversal frequency and paleointensity superchrons may represent times when the pattern of core mantle boundary heat flux allows the geodynamo to operate at peak efficiency as suggested in some numerical models thellier data from single plagioclase crystals formed during times of moderate 10 reversals million years reversal occurrence suggest a weaker and more variable field these paleointensity data together with a consideration of paleomagnetic directions suggest that geomagnetic reversals field morphology secular variation and intensity are related the linkages over tens of millions of years imply a lower mantle control on the geodynamo on even longer timescales the magnetization held by plagioclase and other silicate crystals can be used to investigate the proterozoic and archean geomagnetic field during the onset of growth of the solid inner core data from plagioclase crystals separated from mafic dikes together with directional data from whole rocks indicate a dipole dominated field similar to that of the modern 2 5 2 7 billion years ago older archean rocks are of great interest for paleomagnetic and paleointensity investigations because they may record a time when the compositionally driven convection of the modern dynamo may not have been operating and a solid inner core did not play its current role in controlling the geometry of outer core flow most rocks of this age have been affected by low grade metamorphism investigations using single silicate grains provide arguably our best hope of seeing through secondary geologic events and reading the early history of the geodynamo absolute paleointensity measurements of the oldest rocks on the planet will require the further development of methods to investigate silicate crystals with exsolved magnetic minerals that address the uncertainties posed by thermocrystallization remanent magnetization anisotropy and slow cooling fortunately prior work in rock magnetism together with advances in analytical equipment and techniques provides a solid foundation from which these frontier issues can be approached

potbellies wasp waists and superparamagnetism in magnetic hysteresis because the response of a magnetic substance to an applied field depends strongly on the physical properties of the material much can be learned by monitoring that response through what is known as a magnetic hysteresis loop the measurements are rapid and quickly becoming part of the standard set of tools supporting paleomagnetic research yet the interpretation of hysteresis loops is not simple it has become apparent that although classic single domain pseudo single domain and multidomain loops described in textbooks occur in natural samples loops are frequently distorted having constricted middles wasp waisted loops or spreading middles and slouching shoulders potbellies such complicated loops are often interpreted in oversimplified ways leading to erroneous conclusions the physics of the problem have been understood for nearly half a century yet numerical simulations appropriate to geological materials are almost unavailable in this paper we discuss results of numerical simulations using the simplest of systems the single domain superparamagnetic sd sp system examination of the synthetic hysteresis loops leads to the following observations 1 wasp waisting and potbellies can easily be generated from populations of sd and sp grains 2 wasp waisting requires an sp contribution that saturates quickly resulting in a steep initial slope and potbellies require low initial slopes the sp contribution approaching saturation at higher fields the approach to saturation is dependent on volume hence the cube of grain diameter therefore there is a very strong dependence of hysteresis loop shape on the assumed threshold size 3 we were unable to generate potbellies using an sp sd threshold size as large as 30 nm and wasp waists cannot be generated using a threshold size as small as 8 nm the occurrence of both potbellies and wasp waists in natural samples is consistent with a room temperature threshold size of some 15 nm 5 4 simulations using a threshold size of 15 20 nm with populations dominated by sp grain sizes that is with a small number of sd grains produce synthetic hysteresis loops consistent with measured hysteresis loops and transmission electron microscopic observations from submarine basaltic glass 5 simulations and measurements using two populations with distinct coercivity spectra can also generate wasp waisted loops a relatively straightforward analysis of the resulting loops can distinguish the latter case from wasp waisting resulting from sp sd behavior

experimental study on water rock interactions during co2 flooding in the tensleep formation wyoming usa abstract the conditions for mineral alteration and formation damage during co2 treatment of tensleep sandstone reservoirs in northern wyoming usa were examined through core flooding laboratory experiments carried out under simulated reservoir conditions 80 c and 166 bars subsurface cores from the tensleep sandstone which were cemented by dolomite and anhydrite and synthetic brines were used the brines used were ca mg na so4 nacl solution 9 69 g l total dissolved solids for run 1 and a 0 25 mol l nacl solution for run 2 the solution used in run 1 was saturated with respect to anhydrite at run conditions which is characteristic of tensleep formation waters three major reactions took place during flooding including 1 dissolution of dolomite 2 alteration of k feldspar to form kaolinite and 3 precipitation in run 1 or dissolution in run 2 of anhydrite all sample solutions remained undersaturated with respect to carbonates the permeability of all the cores except one used in run 2 decreased during the experiments despite the dissolution of authigenic cement kaolinite crystal growth occurring in pore throats likely reduced the permeability application of the experimental results to reservoirs in the tensleep formation indicates that an injection solution will obtain saturation with respect to dolomite and anhydrite in the immediate vicinity of the injection well the injection of nacl type water which can be obtained from other formations causes a greater increase in porosity than the injection of tensleep formation waters because of the dissolution of both dolomite and anhydrite cements

magnetic properties of sedimentary greigite fe3s4 an update greigite fe3s4 is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction it can also grow at any time when dissolved iron and sulfide are available during diagenesis greigite is important in paleomagnetic environmental biological biogeochemical tectonic and industrial processes much recent progress has been made in understanding its magnetic properties greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites the crystallographic c axis is the easy axis of magnetization with magnetic properties dominated by magnetocrystalline anisotropy robust empirical estimates of the saturation magnetization anisotropy constant and exchange constant for greigite have been obtained recently for the first time and the first robust estimate of the low field magnetic susceptibility is reported here the curie temperature of greigite remains unknown but must exceed 350 c greigite lacks a low temperature magnetic transition on the basis of preliminary micromagnetic modeling the size range for stable single domain behavior is 17 200 nm for cubic crystals and 17 500 nm for octahedral crystals gradual variation in magnetic properties is observed through the pseudo single domain size range we systematically document the known magnetic properties of greigite at high ambient and low temperatures and with alternating and direct fields and illustrate how grain size variations affect magnetic properties recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records including widespread remagnetizations

signature of magnetic enhancement in a loessic soil in nebraska united states of america our study of a loessic soil profile from east central nebraska shows that the a horizons of the modern soil are characterized by higher concentrations of fine grained b0 1 am magnetic minerals this pedogenic magnetic component leads to higher values of concentration dependent parameters such as magnetic susceptibility v isothermal remanent magnetization irm and anhysteretic remanent magnetization arm combined with increases in frequency dependent susceptibility vfd and arm irm ratios hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component the magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite analyses of bsoftq irm sirm and bhardq irm hirm however do show that approximately 80 90 of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri and antiferromagnetic minerals the pedogenic magnetic component is most likely caused by the conversion of paramagnetic iron bearing minerals to ferriand antiferromagnetic minerals soil compaction lessivage or decalcification cannot explain the observed magnetic soil properties magnetic analyses of loess paleosol sequences from the midwestern united states may yield valuable information about regional variability of paleoclimate based on the fine grained nature of the pedogenic magnetic component we expect grain size dependent proxies arm arm irm vfd to yield better paleoclimatic information than low field magnetic susceptibility used in previous analyses d 2004 elsevier b v all rights reserved

seismic imaging to help understand and manage water quality in coastal benin west africa abstractwe collected seismic data along 15 transects to characterize the geometry of a coastal aquifer in benin west africa that is being contaminated by saltwater we used standard high resolution seismic methods to image the upper 200 m using a sledgehammer source and a 120 channel recording system three transects were processed with an iterative updating flow that includes prestack depth migration residual moveout analysis and reflection tomography and the remaining 12 transects were processed with routine processing flows and poststack time migration we identified one unconfined aquifer and three confined aquifers separated by reflective confining clay layers some transects showed areas of missing reflectors which we interpreted as sand filled channels that could provide potential high permeability conduits for saltwater flow to the godomey well field

influence of localised igneous activity on cleat dawsonite formation in late permian coal measures upper hunter valley australia abstract stable 13 c and 18 o and radiogenic 87 sr 86 sr isotopic data have been used to investigate the origin of cleat dawsonite naalco 3 oh 2 in the late permian wittingham coal measures of the upper hunter region in the sydney basin new south wales the 13 c pdb values have a narrow range 1 7 to 2 4 with an average of 0 3 suggesting a magmatic source for the carbon in contrast 18 o smow values have a wide range 13 6 to 19 8 and decrease systematically with decreasing distance from a major intrusion this systematic variation reflects establishment of localised hydrothermal cells water rock interaction between fluids associated with these hydrothermal cells and rb poor volcaniclastic detritus in the coal measures produced mantle like 87 sr 86 sr 0 705032 to 0 706464 in the dawsonite

co2 percolation experiment through chlorite zeolite rich sandstone pretty hill formation otway basin australia underground co2 sequestration is highly recommended as an effective means of significantly decreasing co2 concentration in the atmosphere mineral storage is the more secure technology but requires the presence of high concentration of divalent cations in the pore fluid results from co2 percolation experiments through chlorite zeolite rich sandstone samples from the pretty hill formation otway basin australia are presented the dissolution of the laumontite 7 wt and chamosite 7 wt are the potential sources of calcium iron and magnesium required for carbonate precipitation the percolation experiment was setup to reproduce at laboratory scale the in situ temperature and pressure conditions t 95 degrees c and p 10 mpa the fluid injected at constant flow rate is a rock equilibrated brine subsequently enriched in co2 up to partial pressure of 6 mpa we observe feldspars laumontite and chamosite dissolution kaolinite and silica precipitation and a noticeable sink of co2 in the sample which is attributed to the precipitation of both amorphous carbon due to the reduction of co2 and fe rich carbonate permeability decreases of about one order of magnitude due to the localization of the kaolinite precipitation in the main flow paths while porosity increases the high reactivity of this sandstone makes this reservoir a valuable target for co2 mineralization but the associated permeability decrease may limit the injection rate and the spreading of the co2 in the reservoir

combined application of dc and tem to sea water intrusion mapping a geoelectric survey using a dc resistivity method and a transient electromagnetic induction tem method was carried out from 1986 to 1988 it was used to help map the lateral and vertical distributions of the freshwater salt water interface in the pei kang area on the west coast of taiwan the dc and tem soundings were performed at 79 localities over an area of 240 km2 of quaternary alluvium significant changes in pore water conductivity at some places were detected by these two methods a low resistivity value 1 5 ohm m implies salt water contamination of groundwater the results of spatial distribution of apparent resistivity indicate the salt water affected aquifers are confined to the southwest part of the study area and the affected aquifers are confined to the top of two aquifers a geoelectric model with three to four layers is inferred from the joint inversion of dc and tem data based on a modified archie s law suggested by the authors an empirical relation between pore water resistivity

rock magnetic properties of uncultured magnetotactic bacteria abstract understanding the magnetic properties of magnetite crystals fe3o4 produced by magnetotactic bacteria mtbs is of fundamental interest in fields of geosciences biomineralization fine particle magnetism and planetary sciences the database of bulk magnetic measurements on mtbs is however still too sparse to allow for generalizations due to difficulties in obtaining bacteria cells in sufficient quantities from natural environments and the fact that only a few cultivable strains are available here we report the first series of magnetic measurements on two air dried samples containing solely mtbs wild type cocci and magnetobacterium bavaricum which were directly isolated from carbonaceous lake sediments systematic rock magnetic studies show that 1 the magnetosomes in cells are dominated by single domain sd magnetite 2 the samples have delta ratios fc zfc higher than 2 3 the measured low temperature remanence cycling curves as well as the first order reversal curve forc diagrams are significantly different to those measured on synthetic sd magnetite samples and 4 the verwey transition temperature tv 100 k of mtb cells is distinctly lower than that from stoichiometric magnetite tv 120 125 k our results provide new insights on the magnetic properties of bacterial magnetite and advance the use of magnetic proxies for decoding the paleomagnetic signals of sediments containing bacterial magnetite

rock magnetic proxies of climate change in the loess palaeosol sequences of the western loess plateau of china summary results of the first detailed study of the climate proxy record in the loess palaeosol sequence at xining one of the few palaeoclimate sites in the currently arid western loess plateau of china illustrate the importance of making many types of rockmagnetic measurements other than susceptibility a multiparameter approach yielded confirmation that here as elsewhere in the loess plateau the susceptibility enhancement in palaeosols was caused primarily by ultrafine magnetite and maghaemite nevertheless magnetic enhancement was caused not exclusively by changes in relative grain size but also by variations in concentration and mineralogy of the magnetic fraction the effects of concentration variations were removed through normalization of susceptibility and anhysteretic remanence with saturation magnetization and saturation remanence respectively the resulting signal was ascribed more confidently to variation in magnetic grain size which in turn was interpreted as a better proxy of pedogenesis than simple susceptibility variations in magnetic mineralogy were also determined to constrain interpretations further the data were then used to discuss climate history at xining finally results from xining were compared with other western sites and contrasted with eastern sites in summary 1 data is presented from a new loess plateau site which also appears to yield a global climate signal 2 a demonstration is made of a more rock magnetically robust way to separate concentration composition and grain size controls on susceptibility and other magnetic parameters and 3 models are provided for inter regional comparisons of palaeoclimate proxy records

petroleum seepages and impregnations in great britain abstract this paper documents 173 occurrences of surface petroleum seepages and impregnations in great britain a brief description is given of the seven main areas where they are found the descriptions range from pore fillings of bitumen to steady flows of oil and gas the principal areas in which seepages are found are as follows the orcadian basin of north east scotland the midland valley of scotland the flanks of the pennines the welsh borderlands cornwall the dorset coast of the wessex basin and the south eastern weald stratigraphically the seepages range from caledonide basement to holocene peat the principal sources for the petroleum are devonian oil shales orcadian basin carboniferous shales and coals midland valley pennines welsh borderlands devon and cornwall and the liassic wessex and wealden basins several anomalous seepages are recorded from welsh and scottish basement there are three clearly defined styles in which the seepages occur many are found where permeable carrier beds unconformably overlie impermeable basement around the margins of basins orcadian basin midlands carboniferous basin others are fault related especially in the wessex and wealden basins the seepages in the cornish tin mines are best explained as sourced from the culm series and drawn by the convective flow of connate waters into the thermal chimneys of the granite intrusions

anomalous magnetic fabrics of dikes in the stable single domain superparamagnetic threshold this research has been partly funded by project cgl2008 04264 bte and by grant prx14 00303 to carles soriano

petrographic and stable isotopic evidences of co2 induced alterations in sandstones in the lishui sag east china sea basin china abstract dawsonite bearing sandstones in the lishui sag east china sea basin china have been investigated as a natural occurrence for co2 induced diagenesis using petrography x ray diffraction and stable isotopes the sandstones are predominantly litharenite with the order of cements growth comprising clay coatings and pyrite siderite pyrite quartz overgrowths ankerite kaolinite and microcrystalline quartz dawsonite and late calcite dawsonite which has recently been considered as trapping mineral after massive co2 charging constituted up to 7 of the whole rock volume occurring as pore filling clusters or replacement of detrital grains stable isotopic data suggest that the co2 in equilibrium with dawsonite and calcite display a mantle magmatic origin and is in close proximity to those of the co2 gas now present in the reservoir indicating the same origin for both the elevated co2 concentrations induced by a mass influx of magmatic co2 into host rock was favourable for dawsonite cementation plagioclase was then considered as the precursor of dawsonite k feldspar could also serve as the aluminum source of dawsonite with an external supply of na in a geochemically open system

direct observation of ferrimagnetic ferroelastic domain interactions in magnetite below the verwey transition abstract the magnetic behaviour of magnetite at low temperatures is dominated by its transformation to a monoclinic crystal structure that is simultaneously ferrimagnetic ferroelastic and ferroelectric below 125 k the verwey transition here we use electron microscopy to reveal the relationship between ferrimagnetic and ferroelastic domain structures in monoclinic magnetite we present dynamic observations of magnetite during heating and cooling across the verwey transition revealing a diversity of unexpected interaction phenomena between crystallographic twins and magnetic domain walls this study provides a new understanding of the low temperature magnetic properties of magnetite that will affect a broad range of rock magnetic studies from the interpretation of magnetic remanence in terrestrial rocks and sediments to the search for biogenic magnetite in extraterrestrial materials

illite smectite formation and potassium mass transfer during burial diagenesis of mudrocks a study from the texas gulf coast paleocene eocene abstract mudrock samples from a well penetrating eocene and paleocene strata 1235 4455 m of the texas gulf coast have been studied to assess geochemical redistribution in mudrocks during burial diagenesis and the mechanism by which detrital smectitic illite smectite i s is transformed into diagenetic illitic i s idealized smectite and illite end member compositions for interstratified i s are estimated to be k0 00x 10 56mg0 39fe0 57al1 13si3 90o10 oh 2 and k0 53x 10 18mg0 17fe0 16al2 28 si3 40o10 oh 2 respectively there is no obvious difference in the amount of k addition to i s in random and ordered interstratified i s aluminum substitution into both tetrahedral and octahedral sites suggests that the smectite to illite reaction is a complete dissolution precipitation reaction rather than a solid state k 1 al 3 for si 4 substitution reaction depth relat d mineral trends and mass balance calculations suggest that illitic i s may form from kaolinite and possibly illite or mica as well as from smectitic i s although most whole rock element abundances are invariant over the sampled depth interval k2o content increases from ca 2 0 weight percent at 1500 m of burial to ca 3 8 weight percent at 4000 m some of this increase is probably caused by import of k into the shale via fluids derived from interbedded sandstones implying that gulf coast mudrocks behave as open chemical systems during burial diagenesis if this is the case a minimum of 103 pore volumes of fluid must have passed through the most k enriched shales to have introduced the added potassium provided that the k content of present day formation water is representative of that of ancient formation water

changes in remanence coercivity and domain state at low temperature in magnetite abstract submicron magnetite crystals with mean sizes of 0 037 0 10 and 0 22 m undergo major changes in hysteresis properties and domain states in crossing the verwey transition t v 120 k the 0 037 m crystals are single domain sd both in the cubic phase at room temperature t 0 and in the monoclinic phase below t v the 0 10 and 0 22 m crystals have a mixture of sd and two domain 2d states at room temperature t 0 but mainly sd structures below t v in agreement with micromagnetic calculations coercive force h c increases on cooling through t v by a factor 3 5 in the submicron magnetites and 40 in a 1 3 mm single crystal because of the high crystalline anisotropy and magnetostriction of monoclinic magnetite as a result domain walls and sd moments are so effectively pinned below t v that all remanence variations in warming or cooling are reversible however between 100 k and t 0 remanence behavior is variable saturation remanence sirm produced in monoclinic magnetite at 5 k drops by 70 100 in warming across t v with minor recovery in cooling back through t v ultimate levels at 5 k of 23 37 for the submicron crystals and 3 for the 1 3 mm crystal in contrast sirm produced in the cubic phase at 300 k decreases 5 35 submicron or 95 1 3 mm during cooling from 300 to 120 k due to continuous re equilibration of domain walls but there is little further change in cooling through t v itself however the submicron magnetites lose a further 5 15 of their remanence when reheated through t v these irreversible changes in cycling across t v and the amounts of the changes have potential value in determining submicron magnetite grain sizes the irreversibility is mainly caused by 2d sd transformations on cooling through t v which preserve or enhance remanence while sd 2d transformations on warming through t v cause remanence to demagnetize

thin film dawsonite in jurassic coal measure strata of the yaojie coalfield minhe basin china a natural analogue for mineral carbon storage in wet supercritical co2 abstract a large amount of thin film dawsonite has developed in coal cleats and also in fractures within the roof sandstone in jurassic co 2 rich coal measure strata of the yaojie coalfield minhe basin china the co 2 in these strata has been in a supercritical state since natural emplacement in this study the yaojie coalfield is considered a natural analogue site for co 2 capture utilization and storage ccus the thin film dawsonite in coal measure strata was analyzed to investigate the evolution of carbon in supercritical co 2 after ccus as it provides a possible example of the formation of carbonate minerals from a wet supercritical co 2 fluid the cleat and fracture filling dawsonite takes the form of white or silver white radial thin film crystals generally 2 and co 2 derived from the decomposition of basement marble which resulted from dynamic thermal metamorphism of the f19 fault zone the carbon isotopic compositions of the cleat filling dawsonite from different carbon sources are transitional and the carbon source for all of the sandstone fracture filling dawsonite is the co 2 released from the decomposed basement marble the morphological and mineralogical characteristics of the thin film dawsonite show that it most likely nucleated and grew directly in the liquid like water film formed by wet supercritical co 2 on solid phase surfaces rather than by epitaxial growth on the rocks surrounding the cleats and fractures probably via layer by layer crystallization the sodium and aluminum in the dawsonite were derived from formation water the formation of dawsonite in the coal measure strata of the yaojie coalfield indicates that some of the free state co 2 which increases in content due to desorption of the coal matrix can ultimately be converted into carbonate minerals during ccus

diagenesis and reservoir quality of overpressured deep water sandstone following inorganic carbon dioxide accumulation upper miocene huangliu formation yinggehai basin south china sea abstract turbidite deposits overpressure hydrothermal fluid activity and co 2 accumulations are the basic geologic characteristics of the upper miocene huangliu formation sandstone reservoir in the dongfang gas field yinggehai basin this study aims to unravel the timing and origin of dissolution in sandstone and evaluate the effects of inorganic co 2 accumulation on reservoir quality a comparative study between area dfx 1 co 2 rich and area dfx 2 co 2 poor has illustrated the differences in diagenetic alteration and the effects of co 2 accumulation on the sandstone reservoir diagenetic alteration of sandstones in area dfx 1 was characterised by intense dissolution and medium cementation dissolution of silicates following inorganic co 2 charging in the dfx 1 area created a higher percentage of dissolution porosity 41 0 53 9 of total visible porosity compared with minor dissolution porosity in the dfx 2 area 16 7 29 6 of total visible porosity small amounts of kaolinite authigenic quartz ankerite and siderite were precipitated by co 2 water rock reactions in area dfx 1 these minerals are almost absent in area dfx 2 the carbonate cement of the huangliu formation sandstone in area dfx 1 is characterised by light 13 c 3 3 to 1 6 pdb which is consistent with formation from inorganic carbon the timing of dissolution by inorganic co 2 is suggested to be at about 0 4 ma based on the previous simulation experiments in the two areas studied overpressure was caused by pressure transmission along deep open faults and was not related to mudstone undercompaction overpressure did a little to contribute to porosity preservation co 2 accumulating at low partial pressures related to hydrothermal fluid flow cannot make a net increase in porosity in the huangliu formation sandstone of the dongfang gas field

the different climatic response of pedogenic hematite and ferrimagnetic minerals evidence from particle sized modern soils over the chinese loess plateau abstract in recent years increasing interest in loess studies has focused on qualitative and quantitative paleoclimatic reconstruction using the imperfect antiferromagnetic mineral hematite however the linkage between the hematite formation and climatic variables remains controversial here we present the results of a comprehensive investigation of the magnetic properties and statistical analysis of a suite of clay and silt fractions of modern soil samples from 179 sites across the chinese loess plateau clp and adjacent regions our objective was to clarify the relationships between modern climatic variables and pedogenic hematite as well as pedogenic ferrimagnetic minerals first order reversal curve measurements were also conducted on representative particle sized subsamples from a n s transect to understand the differences in magnetic mineralogy between the two fractions our results show that pipette extraction separates the fine grained superparamagnetic sp and most of the single domain sd magnetic grains into the clay fraction and that the remaining silt fraction displays the magnetic properties of coarse pseudo single domain psd or a mixture of multidomain md psd and a few sd particles only the pedogenic clay fraction shows a strong correlation with climatic variables the application of redundancy analysis helps to distinguish the climate variables controlling the formation of ferrimagnetic minerals and hematite during pedogenesis on the clp pedogenic ferrimagnetic minerals are sensitive to mean annual precipitation while pedogenic hematite formation is preferentially dependent on mean annual temperature the confirmation of the temperature dependent nature of hematite on the clp provides a new possibility for quantitatively reconstructing the paleotemperature history of chinese loess paleosol sequences

depth characterization of shallow aquifers with seismic reflection part ii prestack depth migration and field examples it is common in shallow seismic studies for the compressional wave velocity in unconsolidated sediments to increase by a factor of four or more at the transition from dry or partial water saturation to full saturation under these conditions conventional nmo velocity analysis fails and leads to large depth and layer thickness estimates if the dix equation is assumed valid prestack depth migration psdm is a means of improving image accuracy a comparison of psdm with conventional nmo processing for three field examples from differing hydrogeologic environments illustrates that psdm can significantly improve image quality and accuracy

hydrocarbon seeps in petroliferous basins in china a first inventory abstract natural hydrocarbon seepage is a widespread phenomenon in sedimentary basins with important implications in petroleum exploration and emission of greenhouse gases to the atmosphere china has vast petroleum oil and gas bearing sedimentary basins but hydrocarbon seepage has rarely been the object of systematic studies and measurements based on the available chinese literature we report a first inventory of 932 hydrocarbon seeps or seepage zones 710 onshore seeps and 222 offshore seeps including 81 mud volcanoes 449 oil seeps 215 gas seeps and 187 solid seeps bitumen outcrops the seeps are located within the main 20 mesozoic cenozoic petroliferous sedimentary basins especially along the marginal regional and local faults the type of manifestations oil gas or mud volcano reflects the type and maturity of the subsurface petroleum system and the sedimentary conditions of the basin oil seeps are particularly abundant in the junggar basin gas seeps mostly developed in the lunpola basin in smaller basins of the eastern guizhou and yunnan provinces onshore taiwan and in the offshore yinggehai basin mud volcanoes developed in basins junggar qaidam qiangtang onshore and offshore taiwan that experienced rapid sedimentation which induced gravitative instability of shales and diapirism in comparison to available global onshore seep data bases china results to be the country with the highest number of seeps in the world the massive gas seepage in china could represent a considerable natural source of methane to the atmosphere and a key process that may drive future hydrocarbon exploration

late miocene provenance change on the eastern margin of the yinggehai song hong basin south china sea evidence from u pb dating and hf isotope analyses of detrital zircons abstract understanding the provenance of the reservoir in a sedimentary basin is of great importance in hydrocarbon exploration and production the yinggehai song hong basin is one of the most important cenozoic petroliferous basins in the south china sea to better understand the provenance characteristics of the upper miocene huangliu formation tortonian messinian on the eastern margin of the basin and test the influence of uplift of the tibetan plateau in the south china sea 172 single zircons from two sandstone samples in the lingtou gas field were dated by u pb chronometer and 54 of these grains were spot analyzed for hf isotopes the results indicate that the upper member of the huangliu formation in this gas field shows two major peaks of u pb ages at ca 250 ma and ca 432 ma along with four subordinate peaks at ca 757 ma ca 1926 ma ca 2529 ma and ca 2775 ma the initial hf isotope ratios ehf t of these zircons are negative to positive for each age group suggesting that existing crustal materials were mixing with new mantle melts in some magmatic episodes in contrast the lower member of the huangliu formation show two major age peaks at ca 98 ma and 248 ma with a subordinate peak at ca 1453 ma the ehf t values are concentrated between 11 6 and 3 4 revealing that they were derived from older crust comparing with the source characteristics of tectonic units surrounding the basin the source of the upper member in the gas field was mainly from the southern yangtze block whereas the lower member was derived from the hainan uplift together with the change of climate and sedimentation rate after 10 ma on the northern margin of the south china sea we attribute the provenance change to the uplift of the tibetan plateau at the same time

the structure and formation of diapirs in the yinggehai song hong basin south china sea abstract the occurrence of shale diapirs in the yinggehai song hong ygh sh basin is well documented as is their association with big petroleum fields in order to better understand how and why the diapirs form we performed a detailed geophysical analysis using a new regional compilation of high resolution two and three dimensional seismic reflection data as well as drilling data that cover the diapirs in ygh sh basin as many as 18 diapirs were identified and are arranged in six n s striking vertical en echelon zones on seismic reflection sections gas chimney structures diapiric faults and palaeo craters are genetically linked with the process of diapirism here we use geophysical and geological observations to propose a three stage model for diapirism initiation emplacement and collapse during these three stages different diapiric structure styles are formed which we describe in detail these include buried diapirs piercing diapirs and collapsed diapirs we link the diapirism to activity on the offshore continuation of the red river fault as shown on our high resolution seismic reflection data which is also related to a high paleogeothermal gradient caused by crustal thinning we also recognize the role of loading by the very large volume of sediment eroded from the edges of the tibetan plateau and delivered by the red river to the basin

screening and ranking of sedimentary basins for sequestration of co2 in geological media in response to climate change sedimentary basins are suitable to different degrees for co2 geological sequestration as a result of various intrinsic and extrinsic characteristics of which the geothermal regime is one of the most important warm basins are less favorable for co2 sequestration than cold basins because of reduced capacity in terms of co2 mass and because of higher co2 buoyancy which drives the upward co2 migration a set of 15 criteria with several classes each has been developed for the assessment and ranking of sedimentary basins in terms of their suitability for co2 sequestration using a parametric normalization procedure a basin s individual scores are summed to a total score using weights that express the relative importance of different criteria the total score is ranked to determine the most suitable basin or region thereof for the geological sequestration of co2 the method is extremely flexible in that it allows changes in the functions that express the importance of various classes for any given criterion and in the weights that express the relative importance of various criteria examples of application are given for canada s case and for the alberta basin in canada

first order reversal curve forc diagrams for pseudo single domain magnetites at high temperature abstract the recently developed first order reversal curve forc technique for rapidly examining magnetic domain state has great potential for paleomagnetic and environmental magnetic investigations however there are still some gaps in the basic understanding of forc diagrams in particular the behavior of pseudo single domain psd grains and the contribution of magnetostatic interactions in this paper we address some of these problems we report the first forc diagrams measurements on narrowly sized and well characterized synthetic psd through multidomain md magnetite samples the forc diagrams evolve with grain size from single domain sd like to md like through the psd grain size range since each sample contains grains of essentially a single size individual psd grains evidently contain contributions from both sd like and md like magnetic moments in proportions that vary with grain size the evolving forc diagrams cannot be due to physical mixtures of sd and md grains of widely different sizes the forc diagrams were all asymmetric small psd samples have forc diagrams with a distinctive closed contour structure the distributions of the larger md grains display no peak and lie closer to the interaction field axis to assess the effect of magnetostatic interactions we measured forc diagrams between room temperature and the curie temperature on heating the forc distributions contract without changing shape until 500 c above this temperature the diagrams become more md like and in addition become more symmetric the temperature dependence of the interaction field parameter is proportional to that of the saturation magnetization in accordance with neel s interpretation of the preisach diagram the decrease in asymmetry with heating suggests that the origin of the asymmetry lies in magnetostatic interactions the magnetic hysteresis parameters as a function of temperature were determined from the forc curves as the grain size decreased the normalized coercive force was found to decrease more rapidly with temperature

the thermodynamic effect of nonhydrostatic stress on the verwey transition abstract experimental results on the change in the verwey transition temperature in magnetite under compression exhibit puzzling variability with slopes ranging from 6 to 16 k gpa our thermodynamic analysis of the verwey transition in magnetite explains much of this variability in terms of the transformation strain as magnetite changes in crystal structure from cubic to monoclinic because this strain involves a much larger change in shape than in volume the change in temperature of the phase boundary t v can be much more sensitive to nonhydrostatic stress than to hydrostatic pressure uniaxial compression and tension both are predicted to increase t v an effect that is opposite in sign and for favorable orientations up to six times greater per gigapascal than for pressure moreover because the monoclinic twin orientation with highest t v is thermodynamically favored our treatment also shows how any desired twin may be selected uniquely by applying stress of appropriate orientation thereby removing obstacles to understanding many kinds of low temperature phenomena in magnetite caused by the presence of multiple twins a similar analysis for the thermodynamic effects of strong magnetic fields is outlined predicting that t v will generally be lowered again the twin for which t v is highest i e is lowered the least will be the most stable twin selectivity per gigapascal of stress is up to two to three times stronger than that per tesla of magnetic field

thermochemical remanent magnetization in precambrian rocks are we sure the geomagnetic field was weak 1 thellier paleointensity determinations from two dikes of the early proterozoic 2 46 ga matachewan dike swarm canada yield field values of 2 14 0 18 and 9 81 1 65 t corresponding values of virtual axial dipole moment are very low 0 54 0 05 1022 and 2 49 0 42 1022 am2 respectively when compared with the modern field the characteristic remanent magnetization chrm was isolated over a narrow range of high unblocking temperatures 520 580 c detailed rock magnetic analyses indicate that the chrm is carried by nearly stoichiometric pseudo single domain magnetite scanning electron microscopy sem reveals that the magnetite is in the form of fine intergrowths with ilmenite formed by oxyexsolution during cooling the high temperature oxidation defined in the sem images could have continued at temperatures below the curie point of magnetite in this case the chrm would be a thermochemical remanent magnetization tcrm rather than a thermal remanent magnetization trm estimates of the tcrm trm ratio show that the thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria this uncertainty in trm fidelity translates into a potential range of field values that spans that defined by the modern field 8 1022 am2 and proposed low precambrian levels 2 1022 am2 therefore understanding further how tcrm is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in thellier studies aimed at defining the strength of the precambrian field

magnetic hysteresis of pseudo single domain and multidomain magnetite below the verwey transition abstract hysteresis loops have been measured as a function of temperature between 10 k and room temperature for two size fractions of crushed magnetite 1 5 m pseudo single domain psd and 100 150 m multidomain md the material under study appears to be fairly close to stoichiometric magnetite with a verwey transition temperature of 115 k for the md sample and 118 k for the psd sample for both samples hysteresis properties below the verwey transition depend critically on whether or not the samples were cooled through the transition in the magnetic field for the psd sample an even more complex behavior has been observed temperature dependences of coercive force obtained after zero field cooling differ significantly for the demagnetized versus magnetized starting magnetic state

origin and migration of hydrocarbon gases and carbon dioxide bekes basin southeastern hungary the bekes basin is a sub basin within the pannonian basin containing about 7000 m of post cretaceous sedimentary rocks natural gases are produced from reservoirs precambrian to tertiary in age located on structural highs around the margins of the basin gas composition and stable carbon isotopic data indicate that most of the flammable gases were derived from humic kerogen contained in source rocks located in the deep basin the depth of gas generation and vertical migration distances were estimated using quantitative source rock maturity carbon isotope relationships for methane compared to known neogene source rock maturity depth relationships in the basin these calculations indicate that as much as 3500 m of vertical migration has occured in some cases isotopically heavy 7 0 co2 is the predominant species present in some shallow reservoirs located on basin margin structural highs and has probably been derived via long distance vertical and lateral migration from thermal decompositon of carbonate minerals in mesozoic and older rocks in the deepest parts of the basin a few shallow reservoirs 2000m contain isotopically light 50 to 60 0 methane with only minor amounts of c2 homologs 3 v v this methane is probably mostly microbial in origin above normal pressures occuring at depths greater than 1800 m are believed to be the principal driving force for lateral and vertical gas migration these pressures are caused in part by active hydrocarbon generation undercompaction and thermal decomposition of carbonates

changes in reservoir heterogeneity and quality as a response to high partial pressures of co2 in a gas reservoir new zealand abstract the kapuni field is the largest onshore petroleum field in new zealand and produces co2 rich gas c 40 45 mol co2 diagenesis within the reservoir is dominated by co2 related reactions that have resulted in localised precipitation of kaolin abundant quartz calcite dolomite and siderite along with localised generation of secondary porosity most of the co2 in the reservoir is thought to have been sourced from intraformational coals with subsequent up dip migration to the crest of the kapuni structure during migration co2 will have dissolved into undersaturated pore fluids and the resulting acidic pore fluids catalysed feldspar and minor carbonate dissolution thereby providing ions for precipitation of authigenic minerals timing of the diagenetic reactions as determined by paragenetic observations and fluid inclusion analysis suggests that both quartz and carbonate formed at a very late stage 100 c corresponding to 0 4 ma which is consistent with modelled maturation and expulsion of co2 from intraformational source rocks 5 ma to present the carbon isotope composition of carbonate cements median 13cpdb 12 4 is similar to that of the reservoired co2 13cpdb 14 5 and this is supportive of late stage cement formation with carbon sourced primarily from thermal maturation reactions the degree of co2 related reactions observed within the kapuni field appears dependent upon grain size lithofacies and stratigraphic position in the reservoir coarse grained high energy facies have undergone the most feldspar dissolution coupled with formation and local preservation of secondary porosity this is interpreted to be due to preferential migration of large volumes of co2 rich fluids through permeable beds thereby further enhancing reservoir quality by comparison much less feldspar reaction but locally significant authigenic mineral precipitation occurs within finer grained lower energy sandstone facies this is considered to be due to relatively low permeability prior to co2 circulation restricting the degree of fluid circulation and resulting in net mineral precipitation authigenic kaolin has precipitated close to the mineral source whilst ions required to precipitate calcite dolomite cements are likely to have migrated further resulting in a complex distribution of carbonate cements with pervasive cements observed at gas water contacts results from this study demonstrate how co2 related reactions are dependent on grain size and mineralogy we show that these reactions can affect vertical heterogeneity of a reservoir by increasing the degree of compartmentalisation through development of sweet spots due to mineral dissolution and tight beds due to mineral cementation

dawsonite cement in the triassic lam formation shabwa basin yemen a natural analogue for a potential mineral product of subsurface co2 storage for greenhouse gas reduction abstract the origin and permeability effects of carbonate cement blamed for poor reservoir quality in the upper triassic turbidite sandstones of the lam formation in yemen have been investigated using petrography x ray diffraction stable isotopes fluid inclusions and thermodynamic modelling in order of growth in the sandstone the carbonate cements include ferroan calcite dawsonite naalco3 oh 2 ferroan dolomite and siderite ferroan dolomite is the most abundant cement there is an inverse correlation between permeability and total carbonate cement volume supporting the earlier assertion that carbonate cements are the main control on reservoir quality in these sandstones ferroan calcite replaced and was sourced by marine limestone rock fragments that are abundant in the turbidite sandstone the later cements were probably also at least partly sourced from the marine limestone rock fragments stable isotope data from ferroan calcite ferroan dolomite and siderite show that diagenesis occurred in formation water with oxygen isotopes typical of closed system diagenesis and that only minor organic derived carbon dioxide was involved in diagenesis dawsonite while not common as a diagenetic mineral in sandstones has recently been suspected of being a routine consequence of co2 storage in the subsurface for greenhouse gas reduction in the lam formation dawsonite found at concentrations of up to 8 vol of the rock grew at the expense of detrital feldspar minerals under conditions of elevated partial pressure of carbon dioxide isotopic data show that dawsonite grew in the presence of carbon dioxide with a 13c value of about 4 to 2 this is distinctly different from the carbon isotope ratio of the gas phase carbon dioxide now present in the sandstone 27 which had an organic source rock origin the dawsonite 13c data probably represent a mixture between carbonate from detrital marine limestone rock fragments and carbon dioxide from a moderately negative source possibly of magmatic or basement origins dawsonite may thus be the result of a mass influx of deep e g magmatic carbon dioxide that pushed the lam formation into the dawsonite stability field at the expense of albite

multiphase natural gas migration and accumulation and its relationship to diapir structures in the df1 1 gas field south china sea abstract the dongfang1 1 df1 1 gas field is the largest commercial gas field yet discovered in the yinggehai basin south china sea the gas reservoirs in the field are normally pressured pliocene shallow marine facies fine grained sandstones and the main source rocks are highly over pressured miocene mudstones and shales the natural gases in the reservoirs consist mainly of hydrocarbon gas co 2 and n 2 geochemical data for the natural gases show that the hydrocarbon rich gases except for a small amount of biogas were derived from these source rocks distributed within over pressured zones but gases rich in co 2 mainly originate from the thermal decomposition of miocene calcareous shales and pre tertiary carbonates at higher temperature fluid inclusion data for the reservoirs were used together with the regional geology distribution of different types of reservoirs and the evolution of diapir structures to determine the episodes of gas migration and accumulation in the df1 1 gas field we believe that the field was formed by four episodes of gas migration and accumulation fluid paleopressure simulation software designed and developed by cnooc china limited was used to simulate the paleopressure history of the strata at the top of the present day high pressure zone a depth of 2800 m in the df1 1 gas field it predicts that the first episode occurred before about 2 ma the second and the third episodes between 2 and 0 5 ma and that the co 2 rich gases migrated into the reservoirs after 0 5 ma the formation and evolution of diapir structures in the yinggehai basin had an important influence on the migration and accumulation of natural gases in the df1 1 gas field rapid deposition gas generation in the source rocks and thermal expansion of pore fluids caused over pressuring in the miocene shales in the center of the depression this over pressuring together with active faulting formed diapir structures during 5 0 5 ma the faults and fractures related to the diapir structures provided a pathway for gas vertical migration from the deep source rocks into the df1 1 reservoirs and controlled the geological distribution of different types of natural gases on this basis a model for gas migration and accumulation is suggested for the df1 1 gas field

miocene shallow marine cold seep carbonate in fold and thrust western foothills sw taiwan a mound shaped authigenic carbonate buildup 50 m wide and 5 m high occurred in the middle part of the late miocene shallow marine succession the hunghuatzu formation exposed along the nantzuhsien river sw taiwan the carbonate concretions are classified into four types based on morphology which appeared to develop in upward sequence with vague boundaries 1 flat pipe shaped nodules highly dolomitic small pipes or nodules subparallel to the host strata 2 bulb shaped nodules discrete or combined small mesoclots with subspherical to irregular shapes 3 mushroom shaped concretions large mushroom like or irregular shaped blocks with chimney or vent structures and 4 carbonated layers with sedimentary structures these concretions are mainly composed of dolomite micrites quartz feldspar muscovite and clay minerals ubiquitously interweaved with blackish brown colored low magnesium calcite lmc veins low delta c 13 vpdb values 51 8 parts per thousand to 29 8 parts per thousand of all types of the carbonates reveal that the carbonates were mainly derived from anaerobic oxidation of hydrocarbons many samples are also o 18 depleted delta o 18 vpdb 11 5 parts per thousand to 1 00 parts per thousand due to the mixture with isotopically light lmc veins likely precipitated from meteoric waters there are numerous chemosymbiotic bivalve fossils genus monitilora diameter 3 4 cm in growth position just above the carbonate buildup according to the geological and isotopic signatures the hunghuatzu carbonates are related to a cold seep pseudobioherm which formed in a very shallow marine environment like tidal flat in a passive margin setting exposures of the hunghuatzu pseudobioherm may provide clues for tracking and comparing the evolution history of gas reservoir systems and hints of active tectonics of the sw taiwan region c 2012 elsevier ltd all rights reserved

paleomagnetic evidence for fossil biogenic magnetite in western crete abstract previous paleomagnetic research has shown that the concentration of fine grained magnetite preserved in miocene marine clays of western crete drops during two successive geomagnetic transitions although the mechanism responsible for this effect was not known magnetite precipitation by magnetotactic bacteria offers a straightforward explanation for this decrease reduced field strengths during a reversal will strain the competitive advantage of having intracellular magnetite and will force bacteria to precipitate correspondingly more magnetite if they are to maintain the same alignment in the geomagnetic field their ability to do this is limited by several magnetophysical constraints which should act to reduce the numbers of magnetite precipitating cells particularly in fields of less than about 12 of the present level this would lead to a decrease in the supply of bacterial magnitude to the sediments recovery of intact magnetotactic bacterial fossils may lead to a new method for estimating paleointensities from sedimentary rocks

becoming effective velocity model builders and depth imagers part 1 the basics of prestack depth migration prestack depth migration psdm is the most accurate seismic solution for imaging subsurface structures because of its ability to focus and position reflections in the context of strong lateral velocity variations velocity model accuracy is the key to successful imaging using contemporary interval velocity model building ivmb methods because only an accurate velocity model can allow migration algorithms properly to account for the seismic wave propagation and raypath bending in the depth domain this is to say that the velocity model accuracy is in turn confirmed by the image quality

dongfang 1 1 gas field in the mud diapir belt of the yinggehai basin south china sea abstract the dongfang 1 1 df1 1 mud diapir field is the largest gas field discovered in the yinggehai basin up to now with a proven gas reserve of 99 68 10 9 m 3 the gas pay zones are within the pliocene fine sandstones and siltstones of the littoral and sand bar facies with normal pressure regimes the gases include variable amounts of co 2 and n 2 in addition to hydrocarbon gases the hydrocarbon gases with the exception of biogenic methane and nitrogen originated from the marine source rocks within the overpressured lower to middle miocene strata while co 2 is most likely the product of thermal decomposition of the tertiary calcareous mudstones and pre tertiary carbonates through relatively long distance vertical migration the field has received at least four phases of natural gas charging occurring relatively late most likely during the post pliocene himalayan orogeny diapiric faults act as the main conduit for upward movement of fluids released from the deep high pressure system with abnormally high pressures providing the significant driving force for gas migration because of the episodic nature of the deep high pressured fluid release and the diapir fault activity this shallow diapir gas field is characterized by multiple gas sources and episodic fluid accumulation

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

geochemistry origin and accumulation of co2 in natural gases of the yinggehai basin offshore south china sea geochemical and isotopic data indicate the presence of co2 of both organic and inorganic origin in the natural gas reservoirs of the yinggehai basin the natural gases with inorganic co2 commonly show a high content of co2 ranging from 15 to 85 a heavier carbon isotope value of co2 from 0 56 to 8 16 and a lower 3he 4he ratio ranging from 0 20 to 6 79 107 indicating a crustal origin these gases occur locally commonly related to diapir structures natural gases rich in hydrocarbons occur widely and are characterized by a low co2 content from 0 1 to 5 0 and a lighter c1 carbon isotope value from 10 59 to 20 7 indicating an organic origin geological background and geochemical data indicate that the sanya and meishan formations are the main source of hydrocarbon gases and the organic co2 pyrolysis experiments on tertiary calcareous shales and thermal history modeling both suggest that the calcareous shales occurring in the lower miocene strata are the main source of the inorganic co2 gas whereas thermal contact metamorphism of the paleozoic carbonates and or magmatic co2 may have made only a small contribution abnormally high paleogeothermal gradients 4 25 4 56c 100 m 12 09 12 26f 100 ft and a rapid heating rate caused the lower miocene calcareous shales to reach the threshold temperature about 300c 570f of their thermal decomposition at the burial depth of about 6500 m 21 300 ft and to generate great volumes of inorganic co2 gas diapir faults acted as the main pathways for the upward migration of deep inorganic co2 gases into reservoirs connected with shale diapirism along the central yinggehai basin the heavier carbon isotope values of associated methanes and a strong thermal anomaly in the co2 rich gas reservoirs provide evidence that the inorganic co2 gas migrated into the reservoirs later than their associated hydrocarbon rich gases this suggests that the earlier formed traps and sandstone reservoirs distant from shale diapir structures may have greater potential in the exploration for hydrocarbon rich gases

on the possibility of single domain pseudo single domain magnetic particles existing in the lower crust of mars source of the strong magnetic anomalies 1 this paper shows that single domain or pseudo single domain sd psd magnetite grains can exist in the upper 30 45 km of the martian crust the upper 50 km of the martian crust have likely formed by volcanic eruptions on a stagnant lithosphere a given lava flow cools very rapidly at the surface but is then gradually heated up through burial heating as it is overlain by other successive flows the thermal evolution models of a lava flow show that its temperature remains below the magnetic blocking temperatures of magnetite 480 580 c until it reaches 30 45 km depth the highly magnetic sd psd magnetite grains can easily explain the strong magnetic anomalies of mars less than 1 by volume of magnetite particles is sufficient

spatial distribution and geochemistry of the nearshore gas seepages and their implications to natural gas migration in the yinggehai basin offshore south china sea abstract about 120 gas seepage vents were documented along the west and southwest coast of the hainan island south china sea in water depths usually less than 50 m the principal seepage areas include the lingtou promontory the yinggehai rivulet mouth yazhou bay the nanshan promontory and the tianya promontory they occur along three major zones reflecting the control by faults and lateral conduits within the basement it is estimated that the total gas emission from these seepage vents is 294 956 m 3 year the seepage gases are characterized by a high ch 4 content 76 heavy 13 c 1 values 38 to 33 and high c 1 c 1 5 ratios 0 95 1 0 resembling the thermogenic gases from the diapiric gas fields of the yinggehai basin hydrocarbon source correlation shows that the hydrocarbons in the sediments from seepage areas can be correlated with the deeply buried miocene source rocks and sandstone reservoirs in the central depression the 2d basin modeling results based on a section from the source rock center to the gas seepage sites indicate that the gas bearing fluids migrated from the source rocks upward through faults or weak zones encompassed by shale diapirism or in up dip direction along the sandstone rich strata of huangliu formation to arrive to seabed and form the nearshore gas seepages it is suggested that the seepage gases are sourced from the miocene source rocks in the central depression of the yinggehai basin this migration model implies that the eastern slope zone between the gas source area of the central depression and the seepage zone is also favorable place for gas accumulation

overpressuring mechanisms in the yinggehai basin south china sea yinggehai basin is an elongate cenozoic rift basin on the northwestern margin of the south china sea continental shelf its thick 17 km basin fill is characterized by high geothermal gradient and high overpressure overpressure associated with nonequilibrium compaction mainly occurs at depths more than 2800 m at the basin center and more than 4000 m at the basin margin because the shallow buried neogene and quaternary strata lack effective seals this regional overpressure distribution however is disrupted at basin center where high overpressure occurs in permeable formations at a depth as shallow as 1400 m on top of a series of deep seated faults and fractures we studied the processes and mechanisms of overpressuring via numerical modeling that couples basin filling sediment compaction and thermal and pressure fields to approach the origin of the shallow high overpressure model results indicated that an increase of fluid volume due to natural gas generation by organic cracking is not large enough to generate the overpressure because of the limited amount of organic matter the shallow overpressure has probably been generated allogenically deep open faults have served as vertical hydraulic conduits and channeled the deep high pressure into shallow permeable formations

the red river fault zone in the yinggehai basin south china sea abstract the red river fault a major strike slip fault of onshore southeast asia is suggested to be closely related to the formation of the south china sea and uplift of the tibetan plateau yet published documentation of the offshore segment of the red river fault is sparse high resolution seismic reflection and borehole data obtained by the petroleum industry along the northwestern margin of the south china sea permit mapping of the offshore segment of the red river fault in the yinggehai basin as well as demonstration of the timing of its shear movement and slip reversal two boundary faults and two basin center faults are mapped three successive deformation phases for the offshore red river fault zone include sinistral movement from 30 to16 ma slip reversal between 16 and 5 5 ma and dextral movement with low rates after 5 5 ma although no robust piecing points were observed for the sinistral movement period the horizontal displacement of dextral movement is estimated to be tens of kilometers this study supports a two stage model for the evolution of the india asia collisional orogen before the middle miocene sinistral movement of the red river fault likely was linked to spreading of the south china sea consistent with the concept of continental extrusion after the middle miocene distributed shortening better explains a low rate of dextral accommodation of the red river fault in response to the continuing india asia collision

low temperature magnetic properties of magnetite using first order reversal curve analysis implications for the pseudo single domain state 1 low temperature magnetic hysteresis properties of nearly stoichiometric magnetite were studied using the first order reversal curve forc technique forc distributions were measured at 20 k after cooling in zero magnetic field or cooling in the presence of a strong 1 5 t magnetic field five polycrystalline samples ranged in mean grain size and shape representing mostly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the forc distributions measured from psd magnetite after zero field cooling zfc manifest an unusual behavior characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram in contrast the low temperature forc distributions for sd and md samples have a single maximum located at the hu 0 axis the appearance of two maxima on low temperature forc distributions is interpreted as a manifestation of a perminvar like behavior due to stabilization of the magnetic domain structure by irregularities of crystalline lattice induced by twinning in monoclinic magnetite the experimental data suggest that the effect of monoclinic twinning on remagnetization is only pronounced within a certain grain size range the double maxima zfc forc distribution appears to be a distinctive property of psd hysteresis behavior which cannot be reproduced by mixing sd and md magnetites

reactivity of micas and cap rock in wet supercritical co2 with so2 and o2 at co2 storage conditions abstract seal or cap rock integrity is a safety issue during geological carbon dioxide capture and storage ccs industrial impurities such as so2 o2 and nox may be present in co2 streams from coal combustion sources so2 and o2 have been shown recently to influence rock reactivity when dissolved in formation water buoyant water saturated supercritical co2 fluid may also come into contact with the base of cap rock after co2 injection supercritical fluid rock reactions have the potential to result in corrosion of reactive minerals in rock with impurity gases additionally present there is the potential for enhanced reactivity but also favourable mineral precipitation the first observation of mineral dissolution and precipitation on phyllosilicates and co2 storage cap rock siliciclastic reservoir core during water saturated supercritical co2 reactions with industrial impurities so2 and o2 at simulated reservoir conditions is presented phyllosilicates biotite phlogopite and muscovite were reacted in contact with a water saturated supercritical co2 containing so2 or so2 and o2 and were also immersed in the gas saturated bulk water secondary precipitated sulfate minerals were formed on mineral surfaces concentrated at sheet edges so2 dissolution and oxidation resulted in solution ph decreasing to 0 74 through sulfuric acid formation phyllosilicate dissolution released elements to solution with 50 fe mobilized geochemical modelling was in good agreement with experimental water chemistry new minerals nontronite smectite hematite jarosite and goethite were saturated in models a cap rock core siltstone sample from the surat basin australia was also reacted in water saturated supercritical co2 containing so2 or in pure supercritical co2 in the presence of so2 siderite and ankerite were corroded and fe chlorite altered by the leaching of mainly fe and al corrosion of micas in the cap rock was however not observed as the ph was buffered by carbonate dissolution ca sulfate and fe bearing precipitates were observed post so2 co2 reaction mainly centered on surface cracks and an illite rich illite smectite precipitate quantified water saturated impure supercritical co2 was observed to have reactivity to rock forming biotite muscovite and phlogopite mineral separates in the cap rock core however carbonates and chlorite were the main reacting minerals showing the importance of assessing actual whole core

relationships between seepage tectonics and subsurface petroleum reserves abstract the relationship between visible petroleum seepage and commercial subsurface reserves has been investigated by a study of the distribution of documented seeps in south east asia this is accompanied and integrated with a broader global study the intensity of flowing seepage is controlled largely by present day tectonics and by the timing of petroleum migration when such influences are reduced by isolating sets of tectonically similar basins correlations are found between numbers of seeps and basin reserves at a sub basinal scale seeps are observed to concentrate over tectonic features such as active diapirs active faults and uplifted basin margins it is relatively rare for seeps to directly overlie major fields although many seeps may be linked to downdip accumulations by long tortuous paths the best seep accumulation relationships are observed in active compressional and diapiric settings particularly those involving shallow reservoirs seeps are rare where traps are buried beneath thick undisturbed overburdens the prime value of visible seeps in most frontier basins lies at the regional level in the clues they give to the nature extent and quantitative hydrocarbon potential of a basin s source system

experimental evaluation of magnetic interaction in pyrrhotite bearing samples pyrrhotite bearing metamorphic limestones have recently experienced an increasing relevance in paleomagnetic research simple univectorial remanences document the metamorphic uplift whereas more complex multicomponent ptrms may constrain its age for a successful application of the latter it is important to estimate the degree of magnetic interactions to ensure the additivity of individual ptrm segments we therefore have subjected the sized dispersed suite 5 250m of tte pyrrhotite to forc analysis and

geochemistry and episodic accumulation of natural gases from the ledong gas field in the yinggehai basin offshore south china sea abstract the ledong gas field consisting of three gas pools in a shale diapir structure zone is the largest gas discovery in the yinggehai basin the gases produced from the pliocene and quaternary marine sandstone reservoirs show a considerable variation in chemical composition with 5 4 88 ch 4 0 93 co 2 and 1 23 7 n 2 the co 2 enriched gases often display heavier methane 13 c values than those with low co 2 contents the 15 n values of the gases range from 8 to 2 and the n 2 content correlates negatively with the co 2 content the high geothermal gradient associated with a relatively great burial depth in this area has led to the generation of hydrocarbon and nitrogen gases from the lower middle miocene source rocks and the formation of abundant co 2 from the tertiary calcareous shales and pre tertiary carbonates the compositional heterogeneities and stable carbon isotope data of the produced gases indicate that the formation of the ld221 gas field is attributed to three phases of gas migration initially biogenic gas followed by thermogenic hydrocarbon gas and then co 2 rich gas the filling processes occurred within a short period approximately from 1 2 to 0 1 ma based on the results of the kinetics modeling geophysical and geochemical data show that the diapiric faults that cut through miocene sediments act as the main pathways for upward gas migration from the deep overpressured system into the shallow normal pressure reservoirs and that the deep overpressure is the main driving force for vertical and lateral migration of the gases this gas migration pattern implies that the transitional pressure zone around the shale diapir structures was on the pathway of upward migrating gases and is also a favorable place for gas accumulation the proposed multiple sources and multiple phases of gas migration and accumulation model for the ledong gas field potentially provide useful information for the future exploration efforts in this area

petrology and isotopic geochemistry of dawsonite bearing sandstones in hailaer basin northeastern china abstract the co2 gas reservoir sandstones in the hailaer basin contain abundant dawsonite and provide an ideal laboratory to study whether any genetic relationship exists between dawsonite and the modern gas phase of co2 the origins of dawsonite and co2 in these sandstones were studied by petrographic and isotopic analysis according to the paragenetic sequence of the sandstones dawsonite grew later than co2 charging at 110 85 ma the dawsonite 18o value is 7 4 smow and the calculated 18o values of the water present during dawsonite growth are from 11 4 to 9 2 smow this combined with the nahco3 dominated water linked to dawsonite growth suggests meteoric water being responsible for dawsonite growth the 13c values of gas phase co2 and the ratios of 3he 4he of the associated he suggest a mantle magmatic origin of co2 rich natural gas in hailaer basin dawsonite 13c values are 5 3 to 1 5 average 3 4 and the calculated 13c values of co2 gas in isotopic equilibrium with dawsonite are 11 4 to 7 3 these c isotopic values are ambiguous for the dawsonite c source from the geological context the timing of events together with formation water conditions for dawsonite growth dawsonite possibly grew in meteoric derived water atmospherically derived co2 maybe or at least the dominant c source for dawsonite it seems that there are few relationships between dawsonite and the modern gas phase of co2 in the hailaer basin

the pretty hill formation as a natural analogue for co2 storage an investigation of mineralogical and isotopic changes associated with sandstones exposed to low intermediate and high co2 concentrations over geological time abstract the pretty hill formation of the otway basin australia has been studied as a natural analogue for geological storage of anthropogenic co2 in order to examine the effects that co2 concentration and reservoir heterogeneity have on co2 related reactions new petrographic data are presented which validate the use of hylogger as a tool to investigate high resolution vertical changes in reservoir mineralogy the integrated data set confirms earlier interpretations showing that chlorite has been altered to kaolinite and siderite ankerite in reservoir facies exposed to moderate and high co2 concentrations while chlorite remains the dominant clay mineral in all parts of the formation where co2 content is low differences have been observed in the degree of co2 related reaction relative to co2 concentration and reservoir heterogeneity where co2 content is very high c 98 mol and associated with high water saturations both chlorite and detrital feldspars have undergone complete reaction in the reservoir facies resulting in quartzose sandstones with a kaolinite matrix and with siderite as the dominant carbonate precipitate conversely where co2 content is moderate c 29 57 mol and within the gas leg of the reservoir chlorite has undergone significant reaction but much of the original feldspar is preserved suggesting relatively minor reaction carbonate cements from the moderate co2 gas leg comprise calcite siderite and ankerite occurring as cemented zones associated with rock heterogeneities and the present day gas water contact heterogeneities within the gas leg are likely to have associated pore fluid contacts whereby relatively high water saturations will be present in the fine grained baffles and seals the most advanced feldspar reaction occurs locally at the contact between baffles and reservoir rock while reactions have been significantly impeded in the finer grained units due to their low permeabilities stable isotope data presented for carbonate cements analysed from wells with low and moderate co2 levels show no clear distinction relatively early formed calcite has 13c values that require an organic carbon source suggesting precipitation unrelated to the reservoir co2 in the otway basin in contrast diagenetically late calcite and siderite samples display two distinct 13c groups dependent on carbonate type where the calculated fluid carbon isotope compositions are similar to documented magmatic co2 reservoired in the nearby caroline field this suggests that magma derived co2 may have been more prevalent through the pretty hill formation than previously thought although the co2 has not been contained over the long term in the low co2 sites it may have caused the local dissolution of carbonate and laumontite cement and also contributed a source of carbon for late stage calcite cements these studies illustrate the importance of understanding both the reservoir composition and vertical heterogeneity of potential storage systems fluid mineral reactions are likely to be advanced within stacked reservoir facies and impeded within siltstone layers while the distribution of carbonate cement may increase the reservoir heterogeneity by the formation of cemented siltstone sandstone layers thereby creating impermeable barriers or baffles to co2

depth characterization of shallow aquifers with seismic reflection part i the failure of nmo velocity analysis and quantitative error prediction as seismic reflection data become more prevalent as input for quantitative environmental and engineering studies there is a growing need to assess and improve the accuracy of reflection processing methodologies it is common for compressional wave velocities to increase by a factor of four or more where shallow unconsolidated sediments change from a dry or partially water saturated regime to full saturation while this degree of velocity contrast is rare in conventional seismology it is a common scenario in shallow environments and leads to significant problems when trying to record and interpret reflections within about the first 30 m below the water table the problem is compounded in shallow reflection studies where problems primarily associated with surface related noise limit the range of offsets we can use to record reflected energy for offset to depth ratios typically required to record reflections originating in this zone the assumptions of nmo velocity analysis are violated leading to very large errors in depth and layer thickness estimates if the dix equation is assumed valid for a broad range of velocity profiles saturated layer thickness will be overestimated by a minimum of 10 if the boundary of interest is 100 if the saturated layer is 10 m thick this degree of error has a significant and negative impact if quantitative interpretations of aquifer geometry are used in aquifer evaluation such as predictive groundwater flow modeling or total resource estimates

continental scale magmatic carbon dioxide seepage recorded by dawsonite in the bowen gunnedah sydney basin system eastern australia abstract dawsonite naalco3 oh 2 is widespread as a cement replacement and cavity filling in permo triassic sedimentary rocks of the bowen gunnedah sydney basin system eastern australia the origin of dawsonite in these rocks was studied by petrographic and stable isotope analysis dawsonite d13c pdb values range from 4 0 to 4 1 and are remarkably consistent throughout the bowen gunnedah sydney basin system these values indicate either a marine carbonate or magmatic source for carbon in the dawsonite a magmatic carbon source is considered more likely on the basis that 1 evidence of and the cause for widespread marine carbonate dissolution in the sedimentary successions are not apparen 2 dawsonite is widespread in both marine and nonmarine facies 3 the region has been the site of major igneous activity 4 other dawsonite deposits of similar carbon isotopic composition are linked to igneous activity and 5 magmatic co2 accumulations are known in parts of the bowen gunnedah sydney basin system the timing of igneous activity in the bowen basin constrains the timing of dawsonite formation in the bowen gunnedah sydney basin system to the tertiary consistent with textural relationships which indicate that dawsonite formed late during the burial history of the permo triassic sequences the distribution and interpreted origin of dawsonite implies magmatic co2 seepage in the bowen gunnedah sydney basin system on a continental scale

magnetic characteristics of synthetic pseudo single domain and multi domain greigite fe3s4 1 we report the magnetic properties of pure synthetic pseudo single domain psd and multi domain md greigite and the grain size dependence of the magnetic properties of greigite for the first time the dominantly psd like and md like behavior are demonstrated by hysteresis first order reversal curve diagrams low temperature cycling ltc of room temperature saturation isothermal remanent magnetization sirm and low temperature sirm warming curves variations in a range of magnetic properties clearly correlate with grain size characteristic psd md behavior is preserved at low temperatures which coupled with the small decrease in remanence during warming rule out the presence of substantial superparamagnetic behavior in the studied samples ltc sirm measurements indicate a continuous demagnetization of remanence during cooling knowledge of this expanded range of magnetic properties of greigite should be widely useful in environmental magnetic and paleomagnetic studies

magnetic hysteresis of 0 6 110 μm magnetites across the verwey transition we report saturation magnetization ms saturation remanence mrs coercive force hc and remanence coercivity hcr as a function of grain size d and temperature t for 0 6 135 m magnetites

geochemistry and origins of natural gases in the yinggehai and qiongdongnan basins offshore south china sea abstract four gas fields and a number of gas bearing structures have been discovered in the yinggehai and qiongdongnan basins offshore south china sea chemical and isotopic data indicate the presence of at least three genetic groups of gases in these basins biogenic gas thermogenic gas and gases with mixed origin thermogenic gases produced from the yacheng field are characterized by relatively high contents of benzene and toluene relatively low 13c values of toluene and high abundance of bicadinanes and oleanane in the associated condensates showing good correlation with the coal bearing sequence of the yacheng formation in the qiongdongnan basin in contrast the gases from the df1 1 and ld gas fields contain high amounts of n2 and co2 low to moderate amounts of benzene and toluene with relatively high 13c values of toluene these characteristics correlate well with the miocene neritic shales in the yinggehai basin analyses on potential source rock samples indicate excellent gas source potential for both the coal bearing sequence in the yacheng formation and the miocene neritic shales containing type iii ii2 kerogens as the result of recent rapid subsidence and sedimentation high temperature and overpressure systems are well developed in the yinggehai and qiongdongnan basins the rapid heating resulted in advanced maturation of organic matter deposited in normally pressured and overpressured strata whereas the strongly overpressured systems led to retarded organic matter maturation postponing the time when the source rocks reached peak gas generation results of 1 d modeling indicate that the coal bearing yacheng formation in the yacheng sag reached peak gas generation during mid miocene pliocene time after the deposition of upper oligocene to lower miocene reservoir rocks the lower mid miocene marine shales reached their peak gas generation stage during the pliocene quaternary in the central yinggehai basin supplying abundant gases to charge the pliocene reservoirs more importantly focused episodic gas migration significantly increased the hydrocarbon expulsion efficiencies of source rocks and thus provided favorable conditions for the accumulation of large amounts of gases in the diapiric structures in a very short geological time the gas source relationship for the yacheng field suggests dominantly short distance gas migration and thus strong source facies controls on the geographic distribution of gas fields within the qiongdongnan basin

provenance of upper miocene to quaternary sediments in the yinggehai song hong basin south china sea evidence from detrital zircon u pb ages abstract the yinggehai song hong y sh basin located on the northwestern shelf of the south china sea is one of the world s largest pull apart basins in order to identify the provenance of the upper miocene to quaternary sediments of the basin ten sediment samples were collected from drill cores in the dongfang gas field u pb dating of detrital zircon grains extracted from the ten samples using a laser ablation inductively coupled mass spectrometer la icp ms yields ages ranging from the archean to cenozoic 2898 46 ma to 26 1 ma the zircon u pb age spectra of the upper miocene to quaternary show a similar pattern with major peaks clustered at ca 32 ma ca 99 ma ca 160 ma ca 247 ma ca 422 ma and three broad age groups 600 1000 ma 1700 2100 ma and 2200 2800 ma suggestive of a wide range of igneous rock ages in the source areas the detrital zircon ages provide diagnostic criteria for identification of the various source areas and reveal that the detritus was derived from multiple sources comparison of the results with the rock types and their ages surrounding the potential source areas indicates that the clastic material was derived from three dominant age sources 1 yangtze craton 2 hainan island and 3 indochina block the yangtze craton was a major and continuous source area contribution to the basin comparing the age characteristics and its source areas since 10 ma the largely stable provenance also suggests that if the red river capture ever existed it should have happened before the late miocene

seep carbonates and preserved methane oxidizing archaea and sulfate reducing bacteria fossils suggest recent gas venting on the seafloor in the northeastern south china sea seep carbonates formed by the synergistic metabolism of methane oxidizing archaea moa and sulfate reducing bacteria srb have been found in many places worldwide but have not been found to date in the south china sea mud volcanoes and shale diapirs have been reported in the south china sea however and microscope and geochernical analysis of carbonates dredged where bottom simulating reflectors bsr suggest the presence of gas hydrate on the northeastern continental slope of the south china sea suggest that the carbonates are hydrocarbon seep related carbonates the carbonates we dredged are chimney like and preserve moa srb fossils their very light carbon isotopic compositions 51 25 to 51 76 parts per thousand suggest that their carbon was derived from microbial methane oxidization these seep carbonates and their preserved moa srb fossils imply gas venting has occurred recently on the seafloor in the northeastern continental slopes in the south china sea c 2005 elsevier ltd all rights reserved

genesis of authigene carbonate minerals in the upper cretaceous reservoir honggang anticline songliao basin a natural analog for mineral trapping of natural co2 storage abstract dawsonite naalco 3 oh 2 as a trace mineral of co 2 is abundant in the upper cretaceous reservoir of the honggang anticline southern songliao basin petrologic and isotopic geochemical characteristics have been investigated by a polarizing microscope a scanning electron microscope sem x ray diffraction xrd cathode luminescence cl fluid inclusions and carbon oxygen isotopic analysis the diagenetic sequence indicated that dawsonite late generation microcrystalline quartz late generation kaolinite late generation calcite and ankerite was formed after co 2 flooding meanwhile as a function of analysis on carbon oxygen isotope the mantle magmatic co 2 provided a carbon source for the precipitation of dawsonite late generation calcite and ankerite the three kinds of carbonate minerals mentioned earlier grew in a progressively evolved condition using a basic thermodynamic fraction equation the total quantity of natural co 2 captured by dawsonite bearing sandstones per cubic meter was calculated to be 99 51 kg

low temperature magnetic properties of greigite fe3s4 1 we provide comprehensive low temperature magnetic results for greigite fe3s4 across the spectrum from superparamagnetic sp to multidomain md behavior it is well known that greigite has no low temperature magnetic transitions but we also document that it has strong domain state dependence of magnetic properties at low temperatures blocking of sp grains and increasing thermal stability with decreasing temperature is apparent in many magnetic measurements thermally stable single domain greigite undergoes little change in magnetic properties below room temperature for pseudo single domain psd md greigite hysteresis properties and first order reversal curve diagrams exhibit minor changes at low temperatures while remanence continuously demagnetizes because of progressive domain wall unpinning the low temperature demagnetization is grain size dependent for psd md greigite with coarser grains undergoing larger remanence loss ac susceptibility measurements indicate consistent blocking temperatures tb for all synthetic and natural greigite samples which are probably associated with surficial oxidation low temperature magnetic analysis provides much more information about magnetic mineralogy and domain state than room temperature measurements and enables discrimination of individual components within mixed magnetic mineral assemblages low temperature rock magnetometry is therefore a useful tool for studying magnetic mineralogy and granulometry of greigite bearing sediments

an experimental study of co2 brine rock interaction at in situ pressure temperature reservoir conditions abstract a detailed investigation of co 2 brine rock interactions through a core flooding laboratory experiment was carried out under simulated reservoir conditions 100 c and 24 mpa changes in the ionic chemistry of the outlet solution combined with core scanning electron microscopy sem and bulk rock x ray diffraction xrd analysis of the core pre and post experiment reveal new insights into co 2 brine rock interactions minerals such as potassium k feldspar albite calcite and ankerite are variably dissolved after the experiments calcite is the mineral most affected by dissolution followed by ankerite whereas dissolution of feldspar minerals is minimal small amounts of kaolinite and solid phases were generated as a result of the experiment the solid phases are mainly comprised of c o na cl al and si and are presumed to be the transitional products in the formation of carbonate minerals the very low fluid penetration rate in the experiment resulted in significantly reduced contact area between the acidic co 2 fluid and minerals in the cores resulting in low rates of feldspar dissolution core permeability decreased substantially throughout the experiment although core porosity remained unchanged the permeability reduction is the result of precipitation of new mineral phases e g kaolinite and solid phases and potentially also the presence of clay particles released by the dissolution of carbonate cement which have then been transported in the fluid flow path and accumulated at pore throats the results provide new insights into co 2 trapping mechanisms in depleted oil and gas reservoirs and into the potential formation damage that may result from massive injections of co 2 into reservoirs during enhanced oil recovery programs

mineralogical and geochemical consequences of the long term presence of co2 in natural reservoirs an example from the springerville st johns field arizona and new mexico u s a abstract the springerville st johns co2 field in eastern arizona and western new mexico is one of more than a dozen gas fields developed within the colorado plateau and southern rocky mountain region extensive travertine caco3 deposits record a long history of co2 migration and leakage to the atmosphere the oldest travertine deposits may have formed during the initial filling of the co2 reservoir when groundwaters exsolved co2 upon reaching the surface the youngest travertine deposits are associated with springs on the floor of the little colorado river valley but travertine deposition appears to be insignificant today older deposits occur up to 325 m above the valley floor geologic relationships suggest travertine deposition began in the late pleistocene after volcanic activity ended at 0 3 ma most of the caco3 could have been derived from dissolution of the underlying limestones and dolomites interactions between the reservoir fluids and rocks were observed in core samples from one of the intervals that produced dry gas these reactions resulted in the dissolution of carbonate cements and detrital feldspars and the formation of dawsonite and kaolinite geochemical simulations suggest that the dawsonite could have been deposited when the co2 fugacity reached 20 bars and that the kaolinite formed as the co2 fugacity decreased corrosion of drill pipe by acidic waters and a pronounced hco3 anomaly above the co2 reservoir provide evidence of a continuing flux of co2 from depth co2 storage occurs primarily as dissolved carbonate species and as gas accumulations only a small percentage of the co2 was sequestered in secondary minerals

some additional parameters to estimate domain state from isothermal magnetization measurements abstract domain state and mixing analysis based on isothermal magnetization measurements is often restricted to the use of the day plot in order to make more information from simple rock magnetic measurements accessible for the quantitative analysis of domain state four additional parameters are proposed two of them a hysteresis shape parameter and a coercivity ratio can be directly calculated from standard measurements of the hysteresis loop and backfield curve the other two a transient energy dissipation ratio and the viscosity of isothermal remanent magnetization require additional measurements which however do not increase measurement time noticeably the theoretical background of the above parameters is discussed their ability to discern domain state is shown for a small set of synthetic magnetite and titanomagnetite samples and in a case study of a transect from the rim to the center of a dredged mid ocean ridge basalt

no evidence for brunhes age excursions santo antao cape verde abstract we present new 40 ar 39 ar ages and palaeomagnetic directions from two re examined sequences of basaltic lava flows on the island of santo antao cape verde tarrafal and agua nova we find no evidence for the fully reversed brunhes age excursions reported in knudsen et al earth planet sci lett 206 199 214 2003 at tarrafal complex geology including repeated gravitational collapses and or considerable erosion followed by in filling of subsequent valleys by younger lavas coupled with an insufficient number of dated reversely and normally magnetised flows led earlier researchers to suggest a series of brunhes age excursions at agua nova the use of single samples to determine palaeodirections resulted in sampling of anomalous reversed directions creating apparent excursions we report new palaeomagnetic directions from agua nova based on multiple measurements per flow determined by both alternating field and thermal demagnetisation all flows of brunhes age have virtual geomagnetic pole latitudes 60 our findings highlight the need for precise age constraints and a high sampling density of dated flows with multiple directional results per flow when using long lava sequences to reconstruct palaeofield behaviour

imaging complex structure in shallow seismic reflection data using prestack depth migration prestack depth migration psdm analysis has the potential to significantly improve the accuracy of both shallow seismicreflectionimagesandthemeasuredvelocitydistributions inastudydesignedtoimagefaultsinthealvordbasin oregon at depths from 25 1000 m psdm produced a detailed reflection image over the full target depth range in contrast poststack time migration produced significant migration artifacts in the upper 100 m that obscured reflection events and limited the structural interpretation in the shallow section additionally an abrupt increase from 2500 to 3000 m s in the psdm velocity model constrained the interpretation of the transition from sedimentary basin fill to basementvolcanicrocks psdmanalysisrevealedacomplex extensional history with at least two distinct phases of basin growthandamidbasinbasementhighthatformsthedivision betweentwomajorbasincompartments

effect of the magnetic field applied during cooling on magnetic hysteresis in the low temperature phase of magnetite first order reversal curve forc analysis the strength of a magnetic field applied during cooling through the verwey transition can control the low temperature lt magnetic hysteresis properties of magnetite this effect is investigated using the first order reversal curve forc technique lt forc distributions were measured at 20 k from four polycrystalline samples after cooling from 300 k in zero magnetic field or in the presence of a magnetic field hfc from a 0 01 to 1 5 t range the samples represented nearly single domain sd pseudo single domain psd and multidomain md magnetic hysteresis behavior the lt forc distributions measured from psd magnetite after zero field cooling are characterized by two distinct maxima located almost symmetrically with respect to the zero interaction field hu 0 axis on a forc diagram application of an intermediate magnetic field 10 mt to 50 mt during cooling results in a shift of the peaks to near symmetry with respect to the hu hfc line the manifestation of the field controlled offset of forc distribution strongly depends on the direction of the field hfc the effect is best expressed when a forc diagram is measured parallel or antiparallel to the cooling field direction for hfc 100 mt the upper peak disappears and the lt forc distribution becomes single peaked and symmetrical with respect to the hu 0 axis in contrast no correlation between hfc and features of forc distributions is observed for sd and md magnetite these experimental results provide further support to a phenomenological model which suggests that magnetic hysteresis behavior of monoclinic magnetite is controlled by an intrinsic interplay between the magnetic and crystallographic twin domain structures

wasp waisted hysteresis loops mineral magnetic characteristics and discrimination of components in mixed magnetic systems rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials hysteresis loops that are constricted in the middle section but are wider above and below the middle section are commonly observed in mixed magnetic assemblages such wasp waisted hysteresis loops have been widely documented particularly with respect to rare earth permanent magnets basaltic lava flows remagnetized paleozoic carbonate rocks and an increasingly wide range of other rocks our modelling combined with a review of previous work indicates that there are several conditions that give rise to as well as magnetic properties that are characteristic of wasp waisted hysteresis loops first at least two magnetic components with strongly contrasting coercivities must coexist this condition can arise from either mixtures of grain sizes of a single magnetic mineral or a combination of magnetic minerals with contrasting cocrcivities or a combination of these two situations second materials that give rise to wasp waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force b cr b c because b0 is controlled by the soft low coercivity component whereas bcris controlled by the hard high coercivity component third values of b cr b c 10 usually only occur for strongly wasp waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains fourth a given mixture of superparamagnetic and single domain sd grains is more likely to give rise to wasp waisted hysteresis loops than an equivalent mixture of sd and multidomain grains fifth our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component in the absence of significant magnetic interaction between particles thus to contribute significantly to wasp waisted behavior a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock as a result minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp waistedness in materials that also contain ferrimagnetic minerals we outline a method for determining the magnetic components that can give rise to wasp waisted hysteresis loops this method is based on high and low temperature magnetic measurements that are used to identify the dominant remanence bearing mineral s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states the method is illustrated with several examples from archaeological geological and synthetic materials

dawsonite and ankerite formation in the ldx 1 structure yinggehai basin south china sea an analogy for carbon mineralization in subsurface sandstone aquifers abstract the geochemistry and petrology of the ldx 1 structure of the yinggehai basin a natural analog of a sedimentary carbon storage site was investigated to understand the consequences of the charging of co2 gas in this system the rocks in this structure are dominated by subarkose and sublitharenite sandstones the authigenic minerals formed after co2 injection are dawsonite microcrystalline quartz kaolinite and ankerite dawsonite and ankerite are formed just beneath a co2 bearing anticlinal structure due to the reactions between silicate minerals feldspars and clay minerals and the fluid phase carbon and oxygen isotopic analyses indicate that the main carbon source for dawsonite and ankerite formation was mantle magmatic co2 the aqueous activities of sodium and calcium the partial pressure of co2 ph and temperature are the key factors influencing the stability of the dawsonite and ankerite the presence of the anticlinal structure maintaining a locally high co2 partial pressure in the waters beneath this structure is likely responsible for the observed long term persistence of dawsonite and ankerite in this system

molecular nitrogen in natural gas accumulations generation from sedimentary organic matter at high temperatures the occurrence of natural gas accumulations with high percentages up to 100 of molecular nitrogen in various hydrocarbon provinces represents a largely unresolved problem and a serious exploration risk in this context a geochemical and basin modeling study was performed to evaluate the potential of sedimentary organic matter to generate molecular nitrogen the masses of nitrogen present in coals if converted into molecular nitrogen are sufficient to fill commercial gas reservoirs a calculation for gas accumulations in northern germany where percentages of molecular nitrogen range from less than 5 to greater than 90 reveals that the molecular nitrogen generated in underlying coal bearing strata is sufficient to account for the nitrogen gas even in the largest fi lds in addition much of the total nitrogen in clay rich rock types such as shales and mudstones is fixed in sedimentary organic matter and may add to the nitrogen generation capacity of the coals a kinetic interpretation of the nitrogen and methane generation characteristics of humic coals during laboratory pyrolysis indicates that nitrogen is generated from organic matter in sedimentary basins at higher temperatures than methane minimum temperatures for preferential nitrogen generation are in excess of 300 c for extremely low heating rates or isothermal conditions lasting over 300 m y thus nitrogen rich gases are mainly formed in the final stage of gas generation when sedimentary rocks grade into metamorphic rocks applications of the kinetic parameters for the generation of nitrogen and methane from coaly organic matter in carboniferous sedimentary rocks are presented for part of the northwest german basin in this area immense volumes of the methane rich gas gener ted during the late paleozoic and mesozoic were lost to the atmosphere the present gas composition mainly reflects the latest cenozoic gas generation this cenozoic gas is nitrogen rich where the carboniferous source rock sequences are highly mature and deeply buried

geology of a continental slope oil seep northern gulf of mexico an oil gas seep was documented by replicate sampling with piston corer abundant high resolution and sparse multichannel seismic reflection profiling and chemical and isotopic analyses the seep occurs on the upper continental slope over a salt ridge interpreted to split and plunge eastward northeastward and northward the relatively shallow diapir over which the seepage occurs is manifested at the surface by a graben in strike section and by a half graben in dip section faulting over the crest is commonly associated with loss of reflected energy or acoustic wipeouts most cores taken in wipeouts with prolonged bottom echoes contain oil and gas the cores also commonly contain secondary carbonate derived from microbial degradation of hydrocarbons the isotopic lightne s of the carbonate and its negative correlation with porosity may be subtle indicators of seepage at sites where oil and gas are not obvious the seepage demonstrates the existence of source rocks and maturation at this site

significance of hydrocarbon seepage relative to petroleum generation and entrapment near surface indications of migrating hydrocarbons provide the petroleum systems analyst critical information about source organic matter type maturation organic maturity migration migration pathway delineation and in selected geologic settings specific prospect hydrocarbon charge all petroliferous basins exhibit some type of near surface signal but the hydrocarbon leakage to surface is not always detectable with conventional seep detection methods understanding the petroleum seepage system hence petroleum dynamics of a basin is key to understanding and using near surface geochemical methods for basin assessment and prospect evaluation the relationships between near surface hydrocarbon seepage and subsurface petroleum generation and entrapment are often complex the petroleum seepage system contain four key elements seepage activity qualitative expressions of relative leakage rates active vs passive and episodic vs continuous seepage type concentration of migrated thermogenic hydrocarbon relative to in situ material macro vs micro migration focus major direction of bulk flow leakage relative to the subsurface hydrocarbon generation and or entrapment and near surface seep disturbances near surface processes which can greatly alter or block seepage signals the rate and volume of hydrocarbon seepage to the surface greatly control near surface geological and biological responses and thus are the best method of sampling and analysis to detect hydrocarbon leakage effectively to properly predict subsurface petroleum properties interpretation of near surface geochemical data must recognize many potential problems including recent organic matter input transported hydrocarbons bacterial alteration mixing contamination and fractionation effects surface geochemical data should always be integrated with other geological data calibration datasets to determine the utility of near surface geochemical techniques within particular basinal settings are essential when evaluating prospects for hydrocarbon charge