possible pore structure deformation effects on the shale gas enrichment an example from the lower cambrian shales of the eastern upper yangtze platform south china abstract shale gas reservoir performance and canister desorption experiments of the lower cambrian organic rich shales in the eastern upper yangtze platform reveal a significant difference in shale gas content between the dabashan arc like fold thrust belt in northeastern chongqing deformed zone and the slightly folded area in southeastern chong non deformed zone integrated pore characterization methods including scanning electron microscopy sem low temperature n2 and co2 adsorption and mercury injection capillary pressure micp analyses were comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment the lower cambrian shales in both deformed zone dz and non deformed zone ndz were deposited in the deep water shelf and show similar organic matter richness and thermal maturity the majority of organic matter om hosted pores in dz samples are in nanoscale size range with the dominance of micro fractures within the om or at the interface of om and minerals in contrast om hosted meso 2 50 nm to macropores 50 nm are the dominant pore types in the ndz samples om hosted micropores the dabashan arc like fold thrust belt took place by the end of the late triassic while the lower cambrian shales have reached thermal maturity peak om hosted meso 2 50 nm to macropores 50 nm in dz samples are most probably collapsed during structural deformation related to tectonic compression while micropores due to their smaller size survived the tectonic stress the om hosted micropores are the main storage space for adsorbed gas in the dz area the dominance of micro pores in dz and lack of connection between those pores and matrix pores led to higher gas content in dz samples on the contrary the well connected om hosted pore network in ndz allows easier gas flow in the rock matrix that eventually led to significant gas leakage during uplift and exhumation and lower gas content in this zone the results of this study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in south china where had experienced complex tectonic movements

late paleozoic glacial episodes in gondwanaland reflected in transgressive regressive depositional sequences in euramerica the late paleozoic glaciation of gondwanaland comprised two short episodes in the famennian i and visean ii confined to brazil and adjacent northwest africa and a long episode that started in the namurian iiia of eastern australia and bolivia argentina expanded to cover much of gondwanaland in the stephanian asselian iiib and collapsed in the early sakmarian iiic dropstones in eastern australia indicate that small ice centers lingered to the kazanian across the belt of low latitudes north of gondwanaland short ranging fossils in widespread shallow marine and paralic deposits indicate synchronous deposition of transgressive regressive sequences in different parts of euramerica these sequences correlate with glacial events in gondwanaland at three levels a four major regressions in euramerica in the famennian 1 visean 2 namurian 3 stephanian 4 and the tastubian transgression that preceded the sterlitamakian regression 5 also recorded in gondwanaland correlate with glacial episodes i ii and iiia iiib and iiic b the time interval of cyclothemic deposition in euramerica brigantian or latest visean to sterlitamakian correlates with that of glacial episode iii and c the dominant period of the euramerican cyclothems as estimated from the middle and late pennsylvanian deposits of the mid continent of north america and of the thickest known gondwanaland glacigenic sediment the earliest permian lyons group of western australia is 0 4 ma equivalent in turn to the long orbital eccentricity period of the quaternary ice age and the dominant period of fluctuation of the late miocene antarctic ice cap the three levels of correlations confirm wanless and shepard9s 1936 hypothesis that the late paleozoic cyclothems are controlled largely by sea level fluctuations related to the gondwanaland glaciation

stratigraphical and δ13c records of permo carboniferous platform carbonates south china responses to late paleozoic icehouse climate and icehouse greenhouse transition abstract little attention has been focused on the stratigraphical imprints of the late paleozoic ice age on the equatorial eastern paleo tethys an integrated study of biostratigraphy microfacies and stable carbon isotope stratigraphy was carried out on pennsylvanian middle guadalupian carbonates of the gongchuan section in the bama platform south china it records seven cooling and two warming pulses in the pre kungurian interval comparing with ice proximal stratigraphical records and far field indirect proxies in lower latitudes a kungurian negative 13 c excursion from 4 4 at the base of the kungurian to 1 6 recorded only in the subequatorial belt is documented here it is followed by a global positive 13 c trend reaching 5 3 near the cisuralian guadalupian boundary seven third order shallowing upward cycles are reported for the first time from the kungurian succession the duration of each cycle is estimated to ca 1 myr these cycles are interpreted as being of glacioeustatic origin and mainly driven by long period modulations of obliquity 1 2 myr under major p co 2 decreasing condition this interpretation is supported by recent climate ice sheet modeling and cenozoic glaciation studies which argue that astronomical forcing will be amplified by large magnitude perturbations in the carbon cycle when only localized ice centers persist our results emphasize the need for integration of sedimentological and paleoclimatological proxies in elucidating the dynamics of the deglacial phases of the late paleozoic ice age

northern england serpukhovian early namurian farfield responses to southern hemisphere glaciation abstract during the serpukhovian early namurian icehouse conditions were initiated in the southern hemisphere however nearfield evidence is inconsistent glaciation appears to have started in limited areas of eastern australia in the earliest serpukhovian followed by a long interglacial whereas data from south america and tibet suggest glaciation throughout the serpukhovian new farfield data from the woodland throckley and rowlands gill boreholes in northern england allow this inconsistency to be addressed 18 o from well preserved late serpukhovian late pendleian to early arnsbergian woodland brachiopods vary between 3 4 and 6 3 and 13 c varies between 2 0 and 3 2 suggesting a 18 o seawater w value of around 1 8 vsmow and therefore an absence of widespread ice caps the organic carbon 13 c upward increasing trend in the throckley borehole serpukhovian to bashkirian c 24 to c 22 and the rowlands gill borehole serpukhovian c 24 to c 23 suggests large scale burial of organic material probably in burgeoning lycophyte dominated coal forest implying a fall in p co 2 p co 2 reduction appears not to have caused large scale glaciation until the early bashkirian but a scenario of coalescing upland ice caps through the serpukhovian with a background of decreasing p co 2 appears to be similar to the process that initiated cenozoic antarctic glaciation

inverse relation between ice extent and the late paleozoic glacial record of gondwana the late carboniferous earliest permian age estimate 300 280 ma for maximum ice volume during late paleozoic glaciation of gondwana is challenged past estimates assume a direct relation between extent of depositional glacial record and former ice cover this assumption cannot be sustained given that the glacial record is composed predominantly of glacially influenced marine strata that accumulated on the margins of ice covered areas we argue that gondwana ice cover expanded in the early carboniferous in response to polar position availability of moisture from a mediterranean sea and epeirogenic uplift of the gondwana interior in namurian time ca 325 ma gondwana ice cover attained a maximum extent of about 21 10 6 km 2 nearly the area of maximum pleistocene ice cover despite extensive ice cover the depositional record is meager continental glacial deposits are poorly preserved on a regional unconformity thereafter extensional subsidence of intracratonic basins promoted marine flooding fragmentation of the ice cover and the accumulation of thick glacially influenced marine deposits in stephanian asselian time ca 285 ma ice cover had decreased to about 15 10 6 km 2 but glacial marine strata were being deposited and preserved across a very large area of the gondwana supercontinent

methane leakage from evolving petroleum systems masses rates and inferences for climate feedback abstract the immense mass of organic carbon contained in sedimentary systems currently estimated at 1 56 10 10 tg des marais et al 1992 bears the potential of affecting global climate through the release of thermally or biologically generated methane to the atmosphere here we investigate the potential of naturally occurring gas leakage controlled by petroleum generation and degradation as a forcing mechanism for climate at geologic time scales we addressed the potential methane contributions to the atmosphere during the evolution of petroleum systems in two different petroliferous geological settings the western canada sedimentary basin wcsb and the central graben area of the north sea besides 3d numerical simulation different types of mass balance and theoretical approaches were applied depending on the data available and the processes taking place in each basin in the case of the wcsb we estimate maximum thermogenic methane leakage rates in the order of 10 2 10 3 tg yr and maximum biogenic methane generation rates of 10 2 tg yr in the case of the central graben maximum estimates for thermogenic methane leakage are in the order in 10 3 tg yr extrapolation of our results to a global scale suggests that at least as a single process thermal gas generation in hydrocarbon kitchen areas would not be able to influence climate although it may contribute to a positive feedback conversely only the sudden release of subsurface methane accumulations formed over geologic timescales can possibly allow for petroleum systems to exert an effect on climate

chemical diagenesis in near surface zone above oil fields in geochemical exploration abstract hydrocarbon microseepages result in chemical and mineralogical changes in the near surface zone above oil reservoirs the results of geochemical research within the three oil fields of the west siberian basin demonstrate that chemical diagenetic processes lead to an increased inhomogeneity in chemical element distribution in soils subsoil atmosphere and snow pulse cathodoluminescence pcl study of soils and transmission electron microscopy tem study of micro and nanoparticles collected from the subsoil atmosphere showed that geochemical conditions over oil reservoirs were favorable for the enrichment of silicate and aluminosilicate minerals by chemical impurities chemical composition of micro and nanoparticles of metal compounds collected from within the vertical projection of the oil field outline suggested that these compounds were formed under intense reductive conditions that are not characteristic for the hypergene zone

cessation reactivation of polygonal faulting and effects on fluid flow in the voring basin norwegian margin polygonal faults mainly oriented n50 n110 and n170 are abundant in the upper part of the mud dominated kai formation upper miocene lower pliocene of the voring basin a second less developed tier of polygonal faults oriented n20 n80 and n140 exists at the base of the overlying naust formation upper pliocene present the faults abruptly terminate upward below a thick interval of debris flows we propose a dynamic model in which 1 the development of polygonal faults discontinues temporarily as a result of a change in regional sedimentation leading to inactive polygonal faults 2 rapid emplacement of debris flows in the late pleistocene creates a new interval of polygonal faults in the lower part of the naust formation immediately beneath the debris flow and some faults penetrate into the underlying kai formation 3 some polygonal faults within the kai formation are reactivated and propagated upward into the base of the naust formation the high interconnectivity between faulted layers allows the fluids to reach shallower depths forming well expressed pipes and pockmarks on the sea floor the model of cessation reactivation of polygonal faults constrains the sealing capacity of sedimentary cover over the reservoirs and helps to reconstruct the fluid migration history through the sedimentary column

the application of sequence stratigraphy to upper carboniferous fluvio deltaic strata of the onshore uk and ireland implications for the southern north sea stratigraphical correlations and fades interpretations of upper carboniferous fluvio deltaic strata have been based traditionally on cyclothems bound by marine flooding surfaces marine bands the recent recognition of major regionally extensive erosional unconformities exxon style sequence boundaries within selected cyclothems questions their validity as units of genetically related strata u sing examples from the carboniferous of the onshore uk and ireland we present sedimentological criteria for the recognition of sequence boundaries placing particular emphasis on the regional context of these surfaces sequence boundaries comprise widespread deeply eroded surfaces at the base of major fluvial sandstone complexes and laterally equivalent palaeosols developed on interfluves at the margins of the fluvial complexes these sequence boundaries define units of genetically related strata sequences which contain other key surfaces of time stratigraphic significance including marine bands and regionally extensive coals the recognition of key surfaces enables the construction of a high resolution stratigraphic framework within which coeval facies relationships can be interpreted sequence boundaries can be correlated between individual basins in the onshore uk by reference to their position in relation to a particular marine band for example the sequence boundary at the base of the farewell rock in the south wales basin can be correlated with that at the base of the rough rock in the pennine basin northern england since both these sandstone bodies are directly overlain by the subcrenatum marine band interbasinal correlations of this nature imply that potential fluvial sandstone reservoirs within major incised valley fills in the upper carboniferous strata of the southern north sea can be predicted by correlation with the onshore uk the stratigraphical framework can be extended and tested using core and well log data particularly spectral gamma ray data which are able to identify key sequence stratigraphic surfaces

evaluation of possible gas microseepage mechanisms petroleum microseepage anomalies over petroleum accumulations are commonly explained by rapid vertical migration of colloidal gas bubbles through fracture networks this article is a theoretical anal ysis of this mechanism and of continuous gas phase flow in frac tures the gas bubble ascent mechanism is much slower than re ported microseepage velocities so it cannot account for observed microseepage in contrast continuous gas phase flow through frac tures can equal or exceed reported microseepage velocity while maintaining total flux low enough so that petroleum accumulations can exist for geological lengths of time fracture entry pressure for bubbles is more than twice that of a continuous gas phase so con tinuous gas phase migration also requires a lower pressure threshold before initiating seepage vertical microseepage is therefore best explained by the same mechanism interpreted for macroseepage although this article provides a theoretically justified mecha nism for microseepage it also shows why interpretation of surface microseepage signals is problematic fracture geometry controls seepage velocity and flux so geochemical anomalies may indicate an increase in fracture aperture as well as possible subsurface ac cumulations larger fractures require very low gas capillary entry pressures so in some settings surface seepage could result from fractures over stratal migration pathways as well as over petroleum accumulations

natural seepage of shale gas and the origin of eternal flames in the northern appalachian basin usa abstract natural hydrocarbon gas seeps are surface expressions of petroleum seepage systems whereby gas is ascending through faults from pressurized reservoirs that are typically associated with sandstones or limestones a spectacular eternal flame in western new york state marks a gas macroseep of dominantly thermogenic origin emanating directly from deep shale source rocks which makes this a rare case in contrast to most petroleum seepage systems where gas derives from conventional reservoirs the main flaming seep releases about 1 kg of methane per day and may feature the highest ethane and propane c 2 c 3 concentration ever reported for a natural gas seep 35 vol the same gas is also released to the atmosphere through nearby invisible and diffuse seepages from the ground the synopsis of our data with available gas geochemical data of reservoir gases in the region and the stratigraphy of underlying shales suggests that the thermogenic gas originates from upper devonian shales without intermediation of a conventional reservoir a similar investigation on a second eternal flame in pennsylvania suggests that gas is migrating from a conventional sandstone pool and that the seep is probably not natural but results from an undocumented and abandoned gas or oil well the large flux of the emitted shale gas in new york state implies the existence of a pressurized gas pool at depth tectonically fractured shales seem to express naturally fracked characteristics and may provide convenient targets for hydrocarbon exploration gas production from tectonically fracked systems might not require extensive artificial fracking

deep lithospheric structures along the southern central chile margin from wide angle p wave modelling p crustal and upper mantle structures of the subduction zone in south central chile between 42 degrees s and 46 degrees s are determined from seismic wide angle reflection and refraction data using the seismic ray tracing method to calculate minimum parameter models three profiles along differently aged segments of the subducting nazca plate were analysed in order to study subduction zone structure dependencies related to the age that is thermal state of the incoming plate the age of the oceanic crust at the trench ranges from 3 ma on the southernmost profile immediately north of the chile triple junction to 6 5 ma old about 100 km to the north and to 14 5 ma old another 200 km further north off the island of chiloe remarkable similarities appear in the structures of both the incoming as well as the overriding plate the oceanic nazca plate is around 5 km thick with a slightly increasing thickness northward reflecting temperature changes at the time of crustal generation the trench basin is about 2 km thick except in the south where the chile ridge is close to the deformation front and only a small 800 m thick trench infill could develop in south central chile typically three quarters 1 5 km of the trench sediments subduct below the decollement in the subduction channel to the north and south of the study area only about one quarter to one third of the sediments subducts the rest is accreted above similarities in the overriding plate are the width of the active accretionary prism 35 50 km and a strong lateral crustal velocity gradient zone about 75 80 km landward from the deformation front where landward upper crustal velocities of over 5 0 5 4 km s su 1 su decrease seaward to around 4 5 km s su 1 su within about 10 km which possibly represents a palaeo backstop this zone is also accompanied by strong intraplate seismicity differences in the subduction zone structures exist in the outer rise region where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise this plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle the southern profiles do not exhibit such a strong alteration of the lithosphere although this effect may be counteracted by plate cooling effects which are reflected in increasing rock velocities away from the spreading centre overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the m w 9 5 great chile earthquake from 1960 ruptured through all these differing age segments the rupture area however appears to coincide with a relatively thick subduction channel

pore structure of the graptolite derived om in the longmaxi shale southeastern upper yangtze region china abstract the lower silurian longmaxi shale in the southeastern upper yangtze region which has been the main target for shale gas exploration and production in china is black marine organic rich shale and rich in graptolites graptolites usually only periderms preserved in shales are important organic component of the longmaxi shale however the pore structure of graptolite periderms and its contribution to gas storage has not yet been studied before a combination of optical microscopy for identification and mark of graptolite and scanning electron microscope sem for pore observations were conducted for the longamxi shale samples results show that pores are anisotropic developed in the longmaxi graptolite periderms and greatly associated with their fine structure micrometer sized fractures and spindle shaped pores between cortical fibrils in the cortical bandage are greatly developed at section parallel to the bedding while they are rare at section perpendicular to the bedding besides numerous sapropel detritus rich in nanometer sized pores are discretely distributed in the shale though graptolite periderms are low porosity from sem image analysis microfractures and elongated pores along the graptolite periderm wall may still make the graptolite an interconnected system together with the discrete porous sapropel detritus in shale these graptolite derived organic matter om may form an interconnected organic pore system in the shale the difference of pore development observed in graptolite periderms and sapropel detritus also give us new insight for the organic pore heterogeneity study the om composition their fine structure and orientation in the rock may be important factors controlling om pore development the combination of identifying om type under optical microscopy and pores observation under sem for may be an effective method to study the om pore development especially in shale that contain more om

quantitative interpretation for gas hydrate accumulation in the eastern green canyon area gulf of mexico using seismic inversion and rock physics transform gas hydrate can be interpreted from seismic data through observation of bottom simulating reflector bsr it is a challenge to interpret gas hydrate without bsr three dimensional qualitative and quantitative seismic interpretations were used to characterize gas hydrate distribution and concentration in the eastern green canyon area of the gulf of mexico where bsr is absent the combination of qualitative and quantitative interpretation reduces ambiguities in the estimation and identification of gas hydrate sandy deposition and faults are qualitatively interpreted from amplitude data the 3d acoustic impedance volume was interpreted in terms of high p impedance hydrate zones and low p impedance free gas zones gas hydrate saturation derived from p impedance is estimated by a rock physics transform we interpreted gas hydrate in the sand prone sediments with a maximum saturation of approximately 50 of the pore space sheet like and some bright spot gas hydrate accumulations are interpreted the interpr

high resolution sequence stratigraphy of the late namurian in the widmerpool gulf east midlands uk abstract analysis of facies stacking patterns and stratal surfaces within late namurian deltas of the widmerpool gulf has revealed 10 high frequency fourth order depositional sequences sequence correlation is based on the characteristic marine band fauna of their condensed sections in many instances sequence boundaries are difficult to recognize unless there is clearly demonstrable lowstand incision consequently it is often difficult to separate late highstand from falling stage progradation with any degree of confidence sequence thickness variations and the dominance of particular stratal surfaces and systems tracts were controlled by a longer period third order cycle of relative sea level rise and fall during the marsdenian yeadonian major fluvio deltaic sand bodies in the widmerpool gulf were formed by distributary channel progradation e g chatsworth grit during the late highstand and or falling stage systems tracts and by the filling of incised valleys e g ashover grit and rough rock following marked sea level falls channel belt and valley fill orientation was initially influenced by differential compaction but this influence decreased during the late namurian the recognition of incised valley fills within the late namurian has implications for estimating the magnitudes and periodicities of carboniferous relative sea level change

characterisation and correlation of active hydrocarbon seepage using geophysical data sets an example from the tropical carbonate yampi shelf northwest australia abstract for the first time in australia active present day hydrocarbon seepage has been imaged on the tropical carbonate yampi shelf in 50 and 90 m water depth seepage is evidenced by gas plumes in the water column that are associated with seabed features such as clusters of reflective blocks hard grounds pockmark fields and mounds seepage activity and intensity seems to vary with changes in pressure related to macro tidal cycles the seabed features coincide with sub surface features such as areas of seismic signal attenuation under high amplitude reflectors seismic discontinuities and bright spots hydrocarbon migration seepage pathways appear to be controlled by the reactivation of pre existing fractures and dykes within the basement the types of seabed features and their preservation on a tropical carbonate shelf are strongly influenced by the coarse bioclastic nature of sediments and the high energy of macro tidal currents and storm reworking

abiotic methane in continental ultramafic rock systems towards a genetic model abstract methane ch4 often in substantial quantities is reported for numerous surface manifestations seeps springs and aquifers in continental serpentinized ultramafic rocks in ophiolites peridotite massifs and intrusions frequently this gas is considered to have a dominant abiotic origin with variable though generally minor components of biotic gas thermogenic or microbial abiotic ch4 production through low temperature fischer tropsch type reactions ftt between a carbon c compound and h2 is endorsed by most of scholars but direct derivation from olivine hydration serpentinization magmatic sources and fluid inclusions have also been suggested here we review the application of ftt in geology discussing the appropriateness of several c compounds co2 co formic acid formate or elemental c as ch4 precursors and of aqueous versus gas phase reactions we examine published gas geochemical and flux data that provide clear constraints on the methane origins in the analysis we add new isotopic and gas seepage data acquired in surface gas manifestations at acquasanta in the voltri ophiolite genova italy multiple lines of evidence including a stable isotope compositions of ch4 co2 and helium b radiocarbon free ch4 c isotopic disequilibrium between ch4 and h2o d low temperatures of ch4 production based on clumped isotopes and heat flow data and e methane seepage forms and intensities suggest that ch4 is not formed directly in water or from magmatic sources or fluid inclusions rather all the lines of evidence taken together are compatible with the hypothesis of low temperature

analysis of light hydrocarbons in soil gases lost river region west virginia relation to stratigraphy and geological structures analyses of 471 near surface soil gas samples for light hydrocarbons c1 c4 c2l c3l and h2 from the lost river gas field in hardy county west virginia reveal sites or clusters of sites containing anomalously high concentrations of light hydrocarbon gases which occur directly over the faulted eastern limb of the whip cove anticline compositional changes in the soil gases data clearly define major changes in the maturity and locations of potential source beds grids placed on botanically defined anomalies confirm a possible correlation between these two independent indicators statistical analysis shows that samples from 45 sites contain anomalously large concentrations of light hydrocarbons in the soil gas constituents large concentrations coupled with high saturate to olefin ratios further confirms that this active seepage is near macroseep levels variations in soil gas compositional trends separate the soil gas data into two domains with oilier compositions to the west and gassier compositions to the east although the composition of the shallow soil gases above the lost river gas field are oilier than the reservoir gases they occur directly over the eastern faulted limb of the producing anticlinal structure suggesting that the dry gases from the oriskany reservoir are probably mixed with oilier gases from organic rich strata among devonian shales the eastern anomalies are much gassier and are very similar to the oriskany gases produced by the lost river gas field the eastern anomalies directly overlie near vertical beds of devonian and older age formations that are likely conduits for deeper mature thermal gases

organic geochemistry of the vindhyan sediments implications for hydrocarbons abstract the organic geochemical methods of hydrocarbon prospecting involve the characterization of sedimentary organic matter in terms of its abundance source and thermal maturity which are essential prerequisites for a hydrocarbon source rock in the present study evaluation of organic matter in the outcrop shale samples from the semri and kaimur groups of vindhyan basin was carried out using rock eval pyrolysis also the adsorbed low molecular weight hydrocarbons methane ethane propane and butane were investigated in the near surface soils to infer the generation of hydrocarbons in the vindhyan basin the total organic carbon toc content in shales ranges between 0 04 and 1 43 the s1 thermally liberated free hydrocarbons values range between 0 01 0 09 mghc grock milligram hydrocarbon per gram of rock sample whereas the s2 hydrocarbons from cracking of kerogen show the values between 0 01 and 0 14 mghc grock based on the tmax temperature at highest yield of s2 and the hydrogen index hi correlations the organic matter is characterized by type iii kerogen the adsorbed soil gas ch4 c1 c2h6 c2 c3h8 c3 and nc4h10 nc4 concentrations measured in the soil samples from the eastern part of vindhyan basin son valley vary from 0 to 186 ppb 0 to 4 ppb 0 to 5 ppb and 0 to 1 ppb respectively the stable carbon isotope values for the desorbed methane 13c1 and ethane 13c2 range between 45 7 to 25 2 and 35 3 to 20 19 vpdb respectively suggesting a thermogenic source for these hydrocarbons high concentrations of thermogenic hydrocarbons are characteristic of areas around sagar narsinghpur katni and satna in the son valley the light hydrocarbon concentrations c1 c4 in near surface soils of the western vindhyan basin around chambal valley have been reported to vary between 1 2547 ppb 1 558 ppb 1 181 ppb 1 37 ppb and 1 32 ppb respectively with high concentrations around baran jhalawar bhanpur garot regions kumar et al 2006 the light gaseous hydrocarbon anomalies are coincident with the wrench faults kota dholpur ratlam shivpuri kannod damoh son banspur rewa wrench in the vindhyan basin which may provide conducive pathways for the migration of the hydrocarbons towards the near surface soils

identification of a large upper cretaceous polygonal fault network in the hammerfest basin implications on the reactivation of regional faulting and gas leakage dynamics sw barents sea abstract we use a high resolution 3d seismic dataset covering 970 km2 over the snohvit and albatross gas fields of the hammerfest basin in the barents sea in order to investigate the basin fault architecture and identify potential fluid pathways of deeper thermogenic gas and fluids a polygonal fault pf interval not previously published is described in the three non connected blocks within campanian sequences which could imply a permeable complex network of pathways that facilitate the vertical migration of hydrocarbons along the fault planes the dominant orientation of the pfs is similar to the main regional tectonic fault trend ew suggesting a tectonic influence upon the development of polygonal faults although some individual pfs locally seem to develop orthogonally to regional 1st order tectonic faults a lithologic control on the occurrence of pf patterns is evident as none are observed within calcareous low clay sediments the pf networks present three main orientations dominant ene wsw secondary nne ssw and a minor ne sw paleocene early eocene faults peefs prevail above the pf networks with coinciding ene wsw fault strikes interpreted as preferential reactivation of the pf systems numerous amplitude anomalies linked to the escape of thermogenic fluids from the jurassic reservoir were identified we present a conceptual model where the 1st order reactivated faults and peefs acted as pathways for fluids from the deeper hydrocarbon reservoirs the peefs are partially sealing with a lateral migration component interplay between the pfs peefs and the 1st order reactivated faults focuses the fluids vertically along the polygonal fault traces creating a wider diffuse zone which form large gas cloud anomalies in the seismic data

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

microseepage of methane to the atmosphere from the dawanqi oil gas field tarim basin china the microseepage of natural gas from subsurface hydrocarbon reservoirs is a widespread process in petroleum basins on a global scale microseepage represents an important natural source of atmospheric methane ch4 to date microseepage ch4 flux data have been obtained from 20 petroleum systems in north america europe and asia while the seasonal variations of gas flux due to soil methanotrophic activity are known the role of geological factors in controlling gas fluxes has been poorly investigated here we present new microseepage data from the dawanqi oil gas field located within the tarim basin china a petroleum system characterized by intense faulting and shallow 700 m reservoirs we measured ch4 fluxes from the ground at 51 sites along three transects by using a closed chamber connected to a portable gas sensor using off axis integrated cavity output spectroscopy our results indicate that the highest ch4 fluxes occur over faults and or shallow reservoirs especially those that were not developed and that have higher fluid pressures microseeping ch4 is thermogenic like that occurring within the dawanqi reservoirs as demonstrated by 13c enrichment 13c from 46 3 to 30 7 in the chamber mean and range microseepage values 17 mg m 2d 1 from 1 4 to 330 mg m 2d 1 are similar to those reported for other petroleum fields with active tectonics our results confirm that dry soil over petroleum fields can be a net source of atmospheric ch4 and its flux is primarily controlled by faulting and reservoir depth and pressure these factors shall be considered in global bottom up seepage emission estimates

namurian bentonites in the pennine basin uk origin and magmatic affinities nine namurian clay bands retrieved from boreholes in the northern part of the pennine basin are on the basis of their petrography mineralogy and geochemistry shown to be volcanic in origin and are therefore bentonites the bentonites which have a fragmental texture are normally graded and show rare preservation of shard textures representing vitric tuff deposits that have been altered subsequently to clay dominated horizons crystals are a minor component of the bentonites but biotite in particular is concentrated at the base of the beds a clay mineral assemblage of mixed layer illite smectite with subordinate kaolinite identifies most of the samples as k bentonites but kaolinite dominates two samples that can be classed as tonsteins temporal variation of salinity within the depositional basin is suggested to explain these different clay assemblages the major element geochemistry of the bentonites reflects their clay mineralogy and the compositions of diagenetic minerals present the latter including pyrite carbonates and hydroxyapatite enrichment of the bentonites in some trace elements including ba sr pb cu and ni can be related to the presence of the diagenetic minerals but the extent to which the elements are added from external sources as opposed to being redistributed within the ash is unclear immobile trace element systematics suggest a rhyodacite dacite composition for the original ash and derivation from the collision of plates this being supported by evidence provided by the rare earth elements ree in one group of samples however in another group of samples variations in ree concentrations may be caused by mobility of these elements during alteration the chemistry of the namurian bentonites contrasts markedly with that of the local carboniferous volcanics but is comparable in some respects with one group of westphalian tonsteins although the latter are more rhyolitic in character it is suggested that the namurian bentonites and the westphalian tonsteins of acid affinities originated from volcanic activity associated with a destructive plate margin in the variscan externides and that the observed compositional trend may reflect magma evolution possibly related to the progressive east west closure

nature and timing of late mississippian to mid pennsylvanian glacio eustatic sea level changes of the pennine basin uk discussion reply although we appreciate that the points made by bailey are relevant to any discussion of interpreting stratigraphic patterns as being astronomically paced we take this opportunity to discuss the details of the carboniferous of the pennine basin beyond simple rock thickness time concepts or models and reiterate points relevant to the conclusions of waters condon 2012 namely the nature of the expression of sea level variation within the carboniferous pennine basin and its regional and global geological context at the heart of the comment by bailey is our assertion that glacio eustatic sea level change as a response to milankovitch forcing was a first order control on the repeated marine flooding events in late mississippian to mid pennsylvanian stratigraphy of the pennine basin uk it has been proposed by numerous researchers acknowledged by waters condon 2012 that the cyclicity evident within the pennine basin

mineralogy and geochemistry of alteration induced by hydrocarbon seepage in an evaporite formation a case study from the zagros fold belt sw iran abstract leaking hydrocarbon and associated fluids produce a reduced environment which initiates diagenetic reactions in the rocks and soils overlying hydrocarbon reservoirs this article introduces mineralogical and geochemical alterations induced by hydrocarbon seepages in the gachsaran evaporite formation masjed soleiman zagros folded belt sw iran using various geochemical methods x ray diffraction xrd analysis revealed two dominant groups of mineralogical changes in the anomalous zone including 1 spotty occurrences of jarosite natroalunite and sulfur 2 simultaneous decrease of sulfate and increase of carbonate the 13c values of carbonate phases vary between 8 9 and 32 3 which support a partial contribution of hydrocarbon in these phases ph measurements of rock samples indicated that hydrocarbon seepage causes decrease of ph though not very significantly however ph in areas that show ongoing sour hydrocarbon seepage among rarely pure gypsum beds decrease to 0 09 which produces acid sulfate soil trace element concentrations indicated that cr co ni v cd u cu and zn increase in various patterns around the hydrocarbon seepages excepting the areas with very low ph

seafloor facies related to upward methane flux within a giant pockmark of the lower congo basin the origin of the cold fluid venting from a giant pockmark within the lower congo basin has been elucidated based upon results of precise mapping submersible dives gravity coring and isotopic analyses realized under a total ifremer cooperative project zaiango and biozaire projects during four dives of the ifremer rov victor 6000 the bottom was filmed hard and soft samples were lifted from the sediment and water samples were collected with a ctd rosette system the detailed dip map shows that the 800 m wide giant pockmark is a composite feature due to the coalescence of multiple 100 m wide depressions that displays a broad range of biological mineralogical and chemical features on the seafloor leading to a seafloor anomaly recorded on the multibeam imagery methane rich fluids migrating through the sedimentary column from a buried palaeochannel clearly react with the sulphate rich circulating sea water to produce hydrogen sulphide and bicarbonate ions this situation leads to a concentrical arrangement of the sedimentary facies with methane related features in the centre and sulphide related features at the periphery this organization is correlated with high levels of methane up to 20 mol l measured in the centre of the giant pockmark responsible for the crystallization of gas hydrates at the bottom in this model the concentrical organization of mineralogical and biological features reflects a geochemical partitioning related to the peripheral progressive mixing of the methane flux

on the periodicity and magnitude of late carboniferous glacio eustatie sea level changes data from sedimentary cycles in basins from northern england the appalachians and midcontinent us afford an insight into periodicities and controls of late carboniferous glacio eustatic sea level changes lithofacies with identifiable palaeobathymetry are used to assess the magnitude of sea level change in the pennine basin of northern england the method yields a minimum value of 42m walsh power spectral analysis is employed to resolve periodicities of about 120ka within the pennine basin of northern england 212 140 and 93 ka within the midcontinent us succession and 109 84 67 63 ka within the appalachian basin within the pennine basin periodicities and magnitudes are found to be independent of their position within the basin thus any tectonism can be assumed to have had a negligible effect on the transgressions a simplified modelled climatic curve is derived from milankovitch parameters and pleistocene 18 o data this is used in a basin fill model that simulates the thermally subsiding pennine basin the resulting distribution of marine deposits is seen to depend upon both the steady regional subsidence and the magnitude and extent of glacio eustatic transgressions

terrestrial methane seeps and mud volcanoes a global perspective of gas origin abstract a global database of gas composition and methane stable isotopes of 143 terrestrial mud volcanoes from 12 countries and 60 seeps independent from mud volcanism from eight countries was compiled and examined in order to provide the first worldwide statistics on the origin of methane seeping at the earth s surface sixteen seep data were coupled with their associated subsurface reservoirs the surface seepage data indicate that at least 76 of the mud volcanoes release thermogenic gas with only 4 biogenic and 20 with mixed character the average 201 data of methane concentration and methane carbon isotope ratios 13 c 1 of mud volcanoes are 90 v v and 46 4 respectively the other types of seeps which are independent from mud volcanism have an average 13 c 1 value that is slightly higher 42 9 gases from mud volcanoes are generally lighter more methane less ethane and propane than their associated reservoir gases suggesting a molecular fractionation during advective fluid migration other types of seeps especially dry seeps maintain the reservoir c 1 c 2 c 3 bernard ratio mud volcanoes behave like a natural refinery and the origin of gas more isotopically enriched than 50 and with c 1 c 2 c 3 500 should be attributed to a thermogenic source rather than partial oxidation of biogenic gas some data that appear biogenic in the bernard diagram can be explained by molecular fractionation of mixed gas consequently the bernard parameter may be misleading when applied to mud volcanoes since it does not always reflect the original gas composition the mechanisms of the molecular advective segregation should be studied quantitatively by specific models and experiments

an integrated interpretation for the origin of the patrick draw oil field sage anomaly 1 the patrick draw oil field has been the site of research into surface and remote sensing methods for hydrocarbon exploration for nearly 10 yr the oil field is a stratigraphic trap with conspicuous sage vegetation on the surface and an associated tonal anomaly within that sage visible via landsat overlying the field s gas cap this paper attempts to resolve the controversy of whether the patrick draw sage anomaly is related to oil field factors or to other independent factors geology and production history of the field show the sage die out anomaly results from upward migration of injected gases and waters used to maintain reservoir pressures in the field these gases and waters produced anoxic low eh oxidation potential high ph and high salinity soils that are toxic to the overlying sage population stunted sage associated with the anomaly commonly occurs in marginal terrains in the western united states and is not tied uniquely to oil fields a velocity for the seeping gases of 250 1000 ft yr 76 305 m yr was calculated from the reservoir to the surface failure to account for the lag time associated with movement of seepage to the surface has been a source of misinterpretation

sedimentary process control on carbon isotope composition of sedimentary organic matter in an ancient shallow water shelf succession 1 source and delivery mechanisms of organic matter are rarely considered when interpreting changing 13c through sedimentary successions even though isotope excursions are widely used to identify and correlate global perturbations in the carbon cycle combining detailed sedimentology and geochemistry we demonstrate how organic carbon abundance and 13c values from sedimentary organic matter from carboniferous aged mudstones are influenced by the proportion of terrestrial versus water column derived organic matter silt bearing clay rich shelf mudstones that were deposited by erosive density flows are characterized by 1 8 2 4 organic carbon and high 13c values averaging 22 9 0 3 n 12 typically these mudstones contain significant volumes of terrestrial plant derived material in contrast clay rich lenticular mudstones with a marine macrofauna are the products of the transport of mud fragments eroded from pre existing water rich shelfal muds when shorelines were distant and biological productivity in the water column was high higher organic carbon 2 1 5 2 and lower 13c values averaging 24 3 0 5 n 11 characterize these mudstones and are interpreted to reflect a greater contribution by isotopically more negative amorphous organic matter derived from marine algae differences in 13c between terrestrial and marine organic matter allow the changing proportions from different sources to be tracked through this succession combining 13c values with zirconium measured from whole rock here used as a proxy for detrital silt input provides a novel approach to distinguishing mudstone provenance and ultimately using 13c to identify oil prone organic matter in potential source rocks these results have important implications for using bulk organic matter to identify and characterize global c isotope excursions

nature and timing of late mississippian to mid pennsylvanian glacio eustatic sea level changes of the pennine basin uk the pennine basin of northern england contains a comparatively complete serpukhovian moscovian succession characterized by high resolution ammonoid zonation and cyclic paralic sedimentation two new isotope dilution thermal ionization mass spectrometry zircon ages from a bentonite deposited during the arnsbergian mid serpukhovian regional substage and tonstein of earliest bolsovian early moscovian regional substage have been determined the weighted mean 206 pb 238 u ages of 328 34 0 55 and 314 37 0 53 ma total uncertainty respectively require modification of the time scale for the western europe regional chronostratigraphy the areal extent of acme ammonoid facies is used as a proxy for the magnitude of 47 discrete flooding events incised valleys major sequence boundaries are used as a proxy for the magnitude of sea level falls the frequency of these events in the light of the new radiometric dating indicates the following 1 there is temporal coincidence between major glaciations in gondwana and phases of increased frequency of sequence boundaries in the pennine basin 2 high amplitude flooding surfaces have an average frequency of c 400 ka 3 average cycle durations during the pendleian early arnsbergian and chokierian bolsovian of c 111 and c 150 ka respectively reflect short duration eccentricities 4 multiple flooding surfaces with the same ammonoid assemblages may equate with sub 100 ka precession or obliquity frequencies supplementary material u pb method description and data procedure for the calculation of the areal extent of marine bands and tables showing a full listing of biostratigraphical data used in the study are available at www geolsoc org uk sup18505

cold water coral banks and submarine landslides a review this paper aims to review the relation between cold water coral bank development and submarine landslides both are common features on continental margins but so far it has not been reviewed which effect if at all they may have upon each other indirect and direct relations between coral banks and landslides are evaluated here based on four case studies the magellan mound province in the porcupine seabight where fossil coral banks appear partly on top of a buried slide deposit the sula ridge reef complex and the storegga landslide both off mid norway and the mauritania coral bank province associated with the mauritanian slide complex for each of these locations positive and negative relationships between both features are discussed based on available datasets locally submarine landslides might directly favour coral bank development by creating substratum where corals can settle on enhancing turbulence due to abrupt seabed morphological variations and in some cases causing fluid seepage in turn some of these processes may contribute to increased food availability and lower sedimentation rates landslides can also affect coral bank development by direct erosion of the coral banks and by the instantaneous increase of turbidity which may smother the corals on the other hand coral banks might have a stabilising function and delay or stop the headwall retrogradation of submarine landslides although local relationships can be deduced from these case studies no general and direct relationship exists between submarine landslides and cold water coral banks

data processing and recognition of seepage and microseepage anomalies of acid extractable hydrocarbons in the south slope of the dongying depression eastern china abstract the successful use of near surface anomalous signals exhibited in all petroliferous basins must lead to great development in petroleum exploration the elementary equations and methods for objective and accurate separation of anomalous signals from background signals are the most critical requirements in these surface geochemical surveys therefore we have worked up a series of fundamental equations and methods for eliminating surface interference and influences of geological conditions on anomaly intensities and recognizing multi type anomalies of uni and multi variates by using statistics fractal geometry wavelet analysis and artificial neural networks these equations illustrate that the threshold i e the boundary between background and micro seepage anomalies is determined by means standard deviations and prior probabilities of background and micro seepage anomalies when multi normality is met and that the traditional equation for the threshold background mean plus one or two standard deviations is not correct in the face of many new equations and methods it is important how to comprehensively use them to resolve the problems in geochemical surveys a geochemical survey in the south slope of the dongying depression eastern china provides a good opportunity to address this issue the anomalies obtained with the traditional equation do not reflect oil gas fields this study reveals three problems in the previous data processing and anomaly recognition 1 the surface interference caused by the variation of soil composition was not identified and removed 2 the influence of caprock thickness on anomaly intensities was not revealed and eliminated and 3 the micro seepage related to oil gas pools and seepage related to faults anomalies were not separated and correctly recognized furthermore the interaction of these problems makes the data processing and anomaly recognition more difficult to ensure effective elimination of the surface interference and caprock thickness influence and correct recognition of uni variate anomalies we iteratively applied the wavelet analysis based methods to eliminate the surface interference and caprock thickness influence and statistical methods with new fundamental equations to recognize uni variate anomalies of seepage and microseepage and then multi variate anomalies of seepage and microseepage were recognized with our fundamental equations and the corresponding methods in the results the seepage anomalies display a string bead shaped pattern and are distributed along faults and the micro seepage anomalies are ring shaped and coincide with oil gas pools sandbodies or traps therefore reprocessing of geochemical data with our fundamental equations and methods combined in an iterative manner can resolve the complicated problems in data processing and anomaly recognition and thus greatly improve the predictive capability of surface geochemical survey

marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal abstract detection of chemically identifiable hydrocarbon oil and gas seepage is an important tool in offshore exploration generated and or trapped oil and gas can leak to near surface sediments and ocean in detectable concentrations the seepage surface expression is controlled by hydrocarbon generation regional pressure regimes and migration pathways the migrated hydrocarbon concentration and composition varies due to seepage rates and near surface physical and biological processes that can alter the signal a seepage continuum exists from large volume oil and gas seeps known as macro seepage to lower volume seeps known as micro seepage only detectible with special sampling and analysis understanding the impact of migrated hydrocarbon concentration relative to in situ organic material and locally derived microbial hydrocarbons is critical this paper describes factors that control the near surface geochemical signal as well as the challenges in deciphering the migrated hydrocarbon signature to evaluate subsurface hydrocarbon generation and entrapment

surface sediment gases as indicators of subsurface hydrocarbons examining the record in laboratory and field studies abstract near surface sediment gases are commonly examined in surface geochemistry surveys to evaluate subsurface hydrocarbon generation and entrapment sediment gases reside in interstitial spaces bound to mineral or organic surfaces and or entrapped in carbonate inclusions characterizing migrated sediment gases with minimal compositional and isotopic fractionation is challenging and different methods are currently in use to extract and analyze surface sediment gases these methods have not been compared in controlled laboratory experiments the key to calibration experiments is to know the charge gas composition and thus have a basis to evaluate the sediment gas extraction procedure for this laboratory study marine sediments were homogenized and cleansed of in situ light hydrocarbons and bacteria replicate sediment samples were prepared and charged with two different hydrocarbon gas compositions and two concentration levels along with the requisite blank control samples the sediment gases were characterized by four laboratories using a variety of gas extraction procedures three headspace interstitial two adsorbed acid extraction and one ball mill occluded the results have been compared to the charge gases and each method evaluated the laboratory calibration experiments demonstrated that selected interstitial gas extraction methods examined do provide sediment gas compositions and compound specific isotopes similar to the charge gases however some of the results indicate that systematic sampling and or laboratory procedures associated with the adsorbed and ball mill gas extraction methods have altered the original charge gas compositions thus it is our recommendation to use a headspace interstitial gas extraction method in conjunction with a tight sealing storage container and water preparation procedure to best extract and analyze migrated sediment gases

role of diffusion in primary migration of hydrocarbons the following effective diffusion coefficients d cm2 sec were determined for the diffusion of light hydrocarbons through the water saturated pore space of shales methane 2 12 10 6 ethane 1 11 10 6 propane 5 55 10 7 iso butane 3 75 10 7 n butane 3 01 10 7 n pentane 1 57 10 7 n hexane 8 20 10 8 n heptane 4 31 10 8 and n decane 6 08 10 9 on the basis of these new data a deterministic dynamic model was set up to simulate the diffusive transport of light hydrocarbons c1 to c10 from source rocks for eight documented source rock units representing a wid range of geologic conditions maturities of 0 40 to 1 35 mean vitrinite reflectance oil to gas prone kerogens the cumulative amounts of hydrocarbons escaping with time were calculated thus it was shown that diffusion represents an effective process for primary migration of gas but not for oil the rate of mass transport for gas from source rocks with geologic time can be sufficiently high to account for the origin of commercial size gas fields for example a cumulative amount of 109 kg of methane 1 5 109 std m3 or 5 3 1010 scf has escaped by diffusion in 540 000 years from a certain volume 1 000 km2 by 200 m thick of a high mature gas prone mesozoic source rock in western canada the origin of hydrocarbon accumulations with high gas to oil ratios in low mature sediments in geologically young basins early gases and condensates can be explained by an early phase of primary migration predominantly based on diffusion during the initial stages of the accumulation history of those fields extending up to millions of years under certain conditions the reservoir gas changes with geologic time from a methane rich to a wet gas composition at low maturity levels below about 0 6 rm even oil prone source rocks yield methane rich light hydrocarbon mixtures by migration through diffusion compositional trends among reservoir gases of several multiple pay gas fields in louisiana represent evidence for diffusive transport of hydrocarbons the variation in gas composition between the individual pay zones is controlled by increasing distances of diffusive transport of the hydrocarbons from a uniform source rock at depth to their present accumulation sites in the shallow pay zones compounds of high diffusivity are enriched for example in the sligo gas field of louisiana the methane ethane and the iso butane n butane ratios increase from 10 2 to 36 0 and from 0 86 to 1 13 respectively from the deepest to the shallowest of the five productive reservoir sands which are spread over a a depth interval of 5 500 ft 1 676 m diffusion of light hydrocarbons in the subsurface can also have economically adverse effects for example existing gas accumulations can be destroyed by dissipation the rate of this destruction was calculated for the harlingen gas field holland by diffusive loss through 400 m of shale cap rock the initial amount of methane in place of 1 93 109 std m3 6 8 1010 scf is reduced by one half over a period of 4 5 million years this leads us to propose the concept that large gas accumulations can persist through extended periods of geologic time only as dynamic systems reaching some kind of steady state equilibrium between diffusive loss through the cap rock and continuous replenishment from the source rock

abiotic methane flux from the chimaera seep and tekirova ophiolites turkey understanding gas exhalation from low temperature serpentinization and implications for mars abstract the emission of abiotic methane ch 4 into the atmosphere from low temperature serpentinization in ophiolitic rocks is documented to date only in four countries the philippines oman new zealand and turkey serpentinization produces large amounts of hydrogen h 2 which in theory may react with co 2 or co to form hydrocarbons fischer tropsch type synthesis ftt similar mechanisms have been invoked to explain the ch 4 detected on mars so that understanding flux and exhalation modality of ophiolitic gas on earth may contribute to decipher the potential degassing on mars this work reports the first direct measurements of gas ch 4 co 2 flux ever done on onshore ophiolites with present day serpentinization we investigated the tekirova ophiolites at cirali in turkey hosting the chimaera seep a system of gas vents issuing from fractures in a 5000 m 2 wide ophiolite outcrop at this site at least 150 190 t of ch 4 is annually released into the atmosphere the molecular and isotopic compositions of c 1 c 5 alkanes co 2 and n 2 combined with source rock maturity data and thermogenic gas formation modelling suggested a dominant abiotic component 80 90 mixed with thermogenic gas abiotic h 2 rich gas is likely formed at temperatures below 50 c suggested by the low deuterium hydrogen isotopic ratio of h 2 d h2 720 consistent with the low geothermal gradient of the area abiotic gas synthesis must be very fast and effective in continuously producing an amount of gas equivalent to the long lasting 2 millennia emission of 100 t ch 4 yr 1 otherwise pressurised gas accumulation must exist over the same ophiolitic formation 3 km away from chimaera we detected an invisible microseepage of abiotic ch 4 with fluxes from 0 07 to 1 g m 2 d 1 on mars similar fluxes could be able to sustain the ch 4 plume apparently recognised in the northern summer 2003 10 4 or 10 5 t yr 1 over the wide olivine bedrock and outcrops of hydrated silicates in the syrtis major and nili fossae just one seep like chimaera or more realistically a weak spatially sporadic microseepage would be sufficient to maintain the atmospheric ch 4 level on mars

high precision u pb zircon age calibration of the global carboniferous time scale and milankovitch band cyclicity in the donets basin eastern ukraine 1 high precision id tims u pb zircon ages for 12 interstratified tuffs and tonsteins are used to radiometrically calibrate the detailed lithostratigraphic cyclostratigraphic and biostratigraphic framework of the carboniferous donets basin of eastern europe chemical abrasion of zircons use of the internationally calibrated earthtime mixed u pb isotope dilution tracer and improved mass spectrometry guided by detailed error analysis have resulted in an age resolution of 0 05 or 100 ka for these carboniferous volcanics this precision allows the resolution of time in the milankovitch band and confirms the long standing hypothesis that individual high frequency pennsylvanian cyclothems and bundles of cyclothems into fourth order sequences are the eustatic response to orbital eccentricity 100 and 400 ka forcing tuning of the fourth order sequences in the donets basin to the long period eccentricity cycle results in a continuous age model for the middle to late pennsylvanian moscovian kasimovian ghzelian strata of the basin and their record of biological and climatic changes through the latter portion of the late paleozoic ice age detailed fusulinid and conodont zonations allow the export of this age model to sections throughout euramerica additional ages for mississippian strata provide among the first robust radiometric calibration points within this subperiod and result in variable lowering of the base ages of its constituent stages compared to recent global time scale compilations

sand body orientation palaeoslope analysis and basin fill implications in the westphalian a c of great britain channel orientation and palaeoflow analyses across the british westphalian carboniferous coalfields show that three main inflow directions dominated the palaeogeography from the mid langsettian to the mid bolsovian in eastern scotland and ne england flow was mainly from the n and ne in the pennine basin of north central england flow was mainly from the w and nw whilst in south wales main flow from the sw was compatible with a westerly system that also branched to give the northerly inflows recorded in the culm basin of southwest england the depositional area included discrete tectonic depocentres notably the pennine basin which was traversed directly and tangentially by major channel belts these basins were essentially incidental to the main channel pathways but sand body characteristics vary according to basin location a new depositional model for the post rift pennine basin is proposed relating sand concentrations and connectivities to the depositional and tectonic setting the description of dominant sw to ne flow across south wales through much of the langsettian and duckmantian in contrast to the centripetal inflows previously proposed implies some similar channel depocentre relationships to those described here for the pennine basin

a sims zircon age for a biostratigraphically dated upper visean asbian bentonite in the central european variscides bardo unit polish sudetes a lower carboniferous platform sedimentary sequence the paprotnia beds in the bardo unit of the central sudetes ne part of the bohemian massif sw poland is biostratigraphically well dated based on rich macro and micro fossil evidence as late visean late asbian crenistria go iii zone the beds contain several bentonite layers one of which was dated using the u pb shrimp method on volcanic zircons and yielded an age of 334 3 ma this date fits well to the recently established chronostratigraphic limits of the visean and is consistent with the newest isotopic age constraints of 336 5 332 ma for the asbian boundaries

geological controls on focused fluid flow associated with seafloor seeps in the lower congo basin a synthesis of backscatter imagery coupled with a large 3d seismic dataset in the lower congo basin lcb reveals a patchy distribution of features interpreted to be associated with fluid seepage from 300 m to 2500 m water depth with the exception of one region of anomalous backscatter positive relief mounds all inferred seep sites occur in negative relief pockmarks the extensive 3d seismic dataset in the lcb offers a unique opportunity to study the plumbing system that is feeding surface cold seep systems and in general to reconstruct the relationship between tectonics and fluid flow in continental margins the fluid seeps in the lcb are associated with morphologically stratigraphically or tectonically controlled focused fluid flow the integration of the geophysical datasets backscatter imagery coupled to 3d seismic clearly indicates that fluid seeps are not randomly distributed but their seabed organization reflects 1 the location of the underlying structure reservoir or trap where the fluids are coming from 2 the geometry and morphology of the reservoir trap and 3 the discontinuities in the sedimentary column along which fluids have migrated in the lcb seafloor pockmarks are always associated with underlying tectonic structures fault zones salt diapirs polygonal faults or buried sedimentary bodies turbiditic channels erosional surfaces whereas they never occur above sub horizontal parallel stratified fine grained sediments even if triggering processes can not be clearly defined here we propose a model of seafloor fluid seep organization which represents a new tool for identifying the geometry of flow pathways and the underlying buried bodies where the fluids are originating from this qualitative 3d model provides insight into the geohydrologic processes of continental margins

restudy of acid extractable hydrocarbon data from surface geochemical survey in the yimeng uplift of the ordos basin china improvement of geochemical prospecting for hydrocarbons abstract two geochemical surveys were conducted in 1992 and 2000 respectively in the yimeng uplift of the ordos basin china the earlier survey grid had 1 5 km spacing and the later survey grid had 0 5 0 5 km spacing the acid extractable hydrocarbons of both surveys show similar geochemical trends however the anomalies obtained with traditional statistical methods do not correlate with existing oil gas fields this study reveals two problems in the data and their processing the first one is interference caused by the variation of soil composition we applied a wavelet analysis based method to eliminate this interference in the data of the later survey the second is that micro seepage anomalies did not identify existing oil gas fields and seepage anomalies related with faults had not been previously recognized we modified the logic multiplication cluster analysis and applied a multi fractal model and a back propagation artificial neural network to recognize these two types of anomalies that cannot be recognized with typical statistics in the study area the recognized seepage anomalies display a string bead shaped pattern and some are distributed along a large fault in this area the fault is the main pathway for hydrocarbon migration the micro seepage anomalies are ring shaped and are mostly distributed close to the fault they coincide with oil gas field and structure traps therefore reprocessing of existing geochemical data using these new methods can greatly improve their usefulness in hydrocarbon exploration

kilometer scale upward migration of hydrocarbons in geopressured sediments by buoyancy driven propagation of methane filled fractures several lines of evidence support kilometer scale upward migration of fluids in the gulf of mexico basin discharge of hypersaline brines at the sea floor long term natural hydrocarbon seeps and microseeps gas chimneys lead zinc mineralization in salt dome cap rocks and allochthonous brines in cenozoic sediments we explore the hypothesis that upward fluid transport in geopressured sediments is caused by buoyancy driven propagation of isolated methane filled fractures in other words instead of fluid migrating along a fixed network of interconnected pores or fractures fluid enclosed within an isolated fracture is transported upward by hydrofracturing the mechanically weak geopressured sediments thus the fluid filled fracture propagates upward through the sediments hydrofracture is driven by the pressure difference buoyancy between the enclosed methane and the surrounding sediments our results show that methane filled fractures with half lengths of a few meters should propagate upward through geopressured sediments with velocities of hundreds of meters per year or higher as methane filled fractures increase in volume and decrease in density with decreasing confining pressure they develop the potential to entrain and transport more than 1000 kg m3 of oil or brine methane filled fractures should propagate to the surface unless they are trapped beneath a layer that has high fracture toughness such as salt or are absorbed when they intersect a permeable 10 md sand layer

upper palaeozoic basins of the british isles caledonide inheritance versus hercynian plate margin processes hypotheses to explain the origin and location of upper palaeozoic sedimentary basins are presented group a basins were those old red sandstone basins whose location and subsidence were controlled by caledonide features including active and remnant fore arc basins midland valley and major transcurrent movements orcadian basin newer granite plutonic magmas in the southern uplands subduction complex post dated subduction and resulted from post collision cratonization processes as yet little understood group b basins were those whose initial positions in space were controlled by palaeo plate factors in particular the location of granitoid plutons but whose subsidence was controlled by active hercynian neoplate processes some group a basins were regenerated by this process midland valley a new hypothesis is presented to account for the origin of a carboniferous rift to sag trend similar to that proven in modern continental margin basins such as the north sea the hypothesis is an adaptation of mckenzie9s lithospheric stretching model some of whose major features are well shown in the carboniferous history of the north and central british isles group b processes were controlled by ligurian bretonic plate consumption in the s brittany massif central vosges areas group c basins in the rheno hercynian zone also owe their location and origin to back arc rifting and continental crustal oceanization processes caused by liguro bretonic subduction

kerogen maturation data in the uinta basin utah usa constrain predictions of natural hydrocarbon seepage into the atmosphere natural seepage of methane from the lithosphere to the atmosphere occurs in regions with large natural gas deposits according to some authors it accounts for roughly 5 of the global methane budget i explore a new approach to estimate methane fluxes based on the maturation of kerogen which is the hydrocarbon polymer present in petroleum source rocks and whose decomposition leads to the formation of oil and natural gas the temporal change in the atomic h c ratio of kerogen lets us estimate the total carbon mass released by it in the form of oil and natural gas then the time interval of active kerogen decomposition lets us estimate the average annual formation rate of oil and natural gas in any given petroleum system which i demonstrate here using the uinta basin of eastern utah as an example obviously this is an upper bound to the average annual rate at which natural gas seeps into the atmosphere after adjusting for biooxidation of natural gas i conclude that the average annual seepage rate in the uinta basin is not greater than 3100 900 tonne yr 1 this is 0 5 0 15 of the total flux of methane into the atmosphere over the basin as measured during aircraft flights i speculate about the difference between the regional 0 5 and the global 5 estimates

atmospheric methane from organic carbon mobilization in sedimentary basins the sleeping giant abstract the mass of organic carbon in sedimentary basins amounts to a staggering 1016 t dwarfing the mass contained in coal oil gas and all living systems by ten thousand fold the evolution of this giant mass during subsidence and uplift via chemical physical and biological processes not only controls fossil energy resource occurrence worldwide but also has the capacity for driving global climate only a tiny change in the degree of leakage particularly if focused through the hydrate cycle can result in globally significant greenhouse gas emissions to date neither climate models nor atmospheric co2 budget estimates have quantitatively included methane from thermal or microbial cracking of sedimentary organic matter deep in sedimentary basins recent estimates of average low latitude eocene surface temperatures beyond 30 c require extreme levels of atmospheric co2 methane degassing from sedimentary basins may be a mechanism to explain increases of atmospheric co2 to values as much as 20 times higher than pre industrial values increased natural gas emission could have been set in motion either by global tectonic processes such as pulses of activity in the global alpine fold belt leading to increased basin subsidence and maturation rates in the prolific jurassic and cretaceous organic rich sediments or by increased magmatic activity such as observed in the northern atlantic around the paleocene eocene boundary increased natural gas emission would have led to global warming that was accentuated by long lasting positive feedback effects through temperature transfer from the surface into sedimentary basins massive gas hydrate dissociation may have been an additional positive feedback factor during hyperthermals superimposed on long term warming such as the paleocene eocene thermal maximum petm as geologic sources may have contributed over one third of global atmospheric methane in pre industrial time variability in methane flux from sedimentary basins may have driven global climate not only at time scales of millions of years but also over geologically short periods of time earth system models linking atmospheric ocean and earth surface processes at different timescales with the sedimentary organic carbon cycle are the tools that need to be developed in order to investigate the role of methane from sedimentary basins in earth s climate

types of fluid related features controlled by sedimentary cycles and fault network in deepwater nigeria abstract the seismic characteristics of focused fluid related features such as pockmarks and carbonate build ups on the nigerian continental slope have been investigated using complementary seismic data sets reprocessed 3d exploration data and 2d very high resolution hull mounted and near bottom seismic data coupled with the results of previous studies sedimentological geotechnical and geochemical analyses the results show different types of fluid related features within the hemipelagic phase of long duration 1 myr turbidite hemipelagic cycles they are the product of the disturbance of the hosting sediments following two main distinct processes 1 no or low sediment deposition caused by fluid escape associated with seafloor settlement benthic fauna carbonate precipitation gas hydrate formation and 2 post depositional erosion caused either by fluid release dewatering from underlying features such as channels and upward migration from a deep source or by collapse destabilisation of gas hydrates the temporal evolution reveals a decrease in fluid flow during the hemipelagic deposition as witnessed by the decrease in methanogenic activity and by gas hydrate destabilisation the spatial distribution of fluid related features seems to be controlled by the type of fault small fault or structural lineament

high precision u pb calibration of carboniferous glaciation and climate history paganzo group nw argentina the duration and geographic extent of carboniferous glacial events in southern gondwana remain poorly constrained despite recent evidence for a more dynamic glacial history than previously considered we report 10 high precision 2 0 1 u pb ages for the permian carboniferous paganzo group nw argentina that redefi ne the chronostratigraphy of the late paleozoic paganzo and rio blanco basins and signifi cantly refi ne the timing of glacial events and climate shifts in the western region of southern gondwana radiometric calibration of the paganzo group indicates three pulses of carboniferous glaciation in the mid visean the late serpukhovian to earliest bashkirian and between the latest bashkirian to early moscovian an abrupt shift in depositional style from high sinuosity single storied fl uvial deposits and clay rich paleosols to low sinuosity multi storied feldspathic fluvial deposits inter calated with eolianites and calcic paleosols is constrained to the latest moscovian and earliest kasimovian these constraints indicate a relatively abrupt climate shift from humid subhumid to nonseasonal semiarid regional climate conditions that occurred signifi cantly earlier than previously inferred early permian this period of high latitude aridity was contemporaneous with a shift to dryland depositional environments and a major vegetation regime shift documented throughout the pangean paleotropics in the pennsylvanian

methane flux from miniseepage in mud volcanoes of sw taiwan comparison with the data from italy romania and azerbaijan abstract mud volcanoes mvs are considered important methane ch4 sources for the atmosphere gas is not only released from macro seepage i e from craters and visible gas bubbling manifestations but also from invisible and pervasive exhalation from the ground named miniseepage ch4 flux related to miniseepage was measured only in a few mvs in azerbaijan italy japan romania and taiwan this study examines in detail the flux data acquired in 5 mvs and 1 dry seep in sw taiwan and further compares with other 23 mvs in italy romania and azerbaijan miniseepage from the six manifestations in sw taiwan mvs and seeps annually contribute at least 110 tons of methane directly to the atmosphere and represents about 80 of total degassing during a quiescent period combining miniseepage flux and geo electrical data from the wu shan ding mv revealed a possible link between gas flux and electrical resistivity of the vadose zone this suggests that unsaturated subsoil is a preferential zone for shallow gas accumulation and seepage to the atmosphere besides miniseepage flux in chu huo everlasting fire decreases by increasing the distance from the main gas channeling zone and molecular fractionation methane ethane ratio is higher for lower flux seepage consistently with what observed in other mvs worldwide measurements from azerbaijan italy romania and taiwan converge to indicate that miniseepage is directly proportional to the vent output and it is a significant component of the total methane budget of a mv a miniseepage vs macro seepage flux equation has been statistically assessed and it can be used to estimate theoretically at least the order of magnitude of the flux of miniseepage for mvs of which only the flux from vents was evaluated or will be evaluated in future this will allow a more complete and objective quantification of gas emission in mvs thus also refining the estimate of the global methane emission from geological sources

model for hydrocarbon microseepage and related near surface alterations despite recent advances in surface and near surface geochemical methods there continues to be a need for improved understanding of the scientific basis relating these methods to petroleum deposits we have reviewed the literature pertinent to the several methods we have used and integrated that information with some 100 man years of our experience to contribute to that end evidence indicates that hydrocarbon micro seepage from reservoirs most probably involves buoyant colloidal size microbubbles of light hydrocarbons principally methane through the butanes ascending relatively rapidly through a water filled network of fractures joints and bedding planes bacterial consumption of these hydrocarbons creates carbon dioxide and hydrogen sulfide which may alter sediments producing detectable modifications in geomorphic seismic magnetic and radiometric properties we have used combinations of 1 airborne microwave sensing and laboratory analyses of soil gas hydrocarbons 2 shallow source aeromagnetic and soil magnetic susceptibility measurements 3 aerial and surface gamma ray measurements and 4 geomorphology to find productive oil and gas deposits results suggest that using these methods can substantially increase the probability of wildcat success and reduce finding costs in selected geologic settings however these surface methods cannot reveal depth size or quality of reservoirs or even if producible hydrocarbons will be found

multispectral remote sensing mapping for hydrocarbon seepage induced lithologic anomalies in the kuqa foreland basin south tian shan abstract the mineralogy of oil and gas reservoirs can be altered through the effects of hydrocarbon seepage mapping this mineral alteration is thus a potential tool for hydrocarbon exploration hydrocarbons that escape from underground reservoirs can cause oxidation reduction reactions in situ or along vertical migration pathways they can also produce anomalies in surface sediments and soils the surface changes can potentially be detected by various techniques including geochemical geophysical and remote sensing methods in this study satellite multi spectral data combined with field spectrometry geochemical and mineralogical information were evaluated for mapping areas of known hydrocarbon seepages from the qiulitage thrust and fold belt in the southern tian shan northwest china this study found that aster band ratios of 2 1 and 4 9 reveal mineral signatures related to alterations induced by hydrocarbon seepages such as bleached red bed and secondary carbonates respectively in the qiulitage thrust and fold belt these overly known hydrocarbon seepages and thus provide a targeting tool for similar styles of hydrocarbon elsewhere in addition given that hydrocarbon seepages are also one of the non negligible sources for emission of greenhouse gases multispectral remote sensing system can thus potentially be used to map and monitor emission of greenhouse gas emissions from hydrocarbon accumulations

stratigraphic imprint of the late palaeozoic ice age in eastern australia a record of alternating glacial and nonglacial climate regime stratigraphic and sedimentological data from new south wales and queensland eastern australia indicate that the late palaeozoic ice age comprised at least eight discrete glacial intervals each 1 8 ma in duration here termed glaciations separated by nonglacial intervals of comparable duration these events spanned an interval from mid carboniferous c 327 ma to the early late permian c 260 ma and illustrate a pattern of increasing climatic austerity and increasingly widespread glacial ice from initial onset until an acme in the late early permian followed by an opposite trend towards the final demise of glaciation in the late permian the alternating glacial nonglacial motif suggests that the late palaeozoic ice age was considerably more dynamic than previously thought these patterns are remarkably consistent with recent interpretations of palaeofloral change eustatic sea level fluctuations and co 2 climate glaciation relationships for this interval of time the detailed record of alternating glacial and nonglacial climate mode disclosed herein may facilitate more closely resolved evaluations of stratigraphic records elsewhere notably in far field ice distal northern hemisphere successions

warm eocene climate enhanced petroleum generation from cretaceous source rocks a potential climate feedback mechanism 1 earth surface temperatures including in the deep sea increased by 5 10 c from the late paleocene ca 58 myr ago to the early eocene climatic optimum eeco centered at about 51 myr ago a large 2 5 drop in 13c of carbonate spans much of this interval this suggests a long term increase in the net flux of13c depleted carbon to the ocean and atmosphere that is difficult to explain by changes in surficial carbon cycling alone we reveal a relationship between surface temperature increase and increased petroleum generation in sedimentary basins operating on 100 kyr to myr time scales we propose that early eocene warming has led to a synchronization of periods of maximum petroleum generation and enhanced generation in otherwise unproductive basins through extension of the volume of source rock within the oil and gas window across hundreds of sedimentary basins globally modelling the thermal evolution of four sedimentary basins in the southwest pacific predicted an up to 50 increase in petroleum generation that would have significantly increased leakage of light hydrocarbons and oil degeneration products into the atmosphere extrapolating our modelling results to hundreds of sedimentary basins worldwide suggests that globally increased leakage could have caused a climate feedback effect driving or enhancing early eocene climate warming

structural synthesis of s staffordshire uk implications for the variscan evolution of the pennine basin the presence of strata of westphalian a to possibly stephanian age in south staffordshire located at the southern margin of the pennine basin provides a rare control on the timing of late carboniferous deformation in central england during westphalian a c times sedimentation in south staffordshire was controlled by thermal subsidence with a superimposed influence of synsedimentary normal and reverse faulting alkaline basic igneous activity occurred during westphalian c times synchronous with uplift of the southern margin of the pennine basin uplift culminated during westphalian c d times in a phase of variscan inversion along basement lineaments and the development of a regional unconformity renewed subsidence and deposition of a relatively thick sedimentary sequence of westphalian d to possibly stephanian age was followed by a final phase of variscan deformation comprising possibly two pulses of uplift tilting and erosion observations presented for south staffordshire in conjunction with studies from other parts of the pennine basin indicate a complex interrelationship between rifting thermal subsidence and variscan basin inversion pulses of n s extension and e w to nw se compression are interpreted as the product of regional strike slip deformation superimposed upon a regime of thermal subsidence alkaline igneous activity is considered to be the product of mantle partial melting in response to limited decompression due to transtensional displacements on crustal thickness faults

imaging pores in sedimentary rocks foundation of porosity prediction abstract this review examines the history and current practice of technologies for imaging of pores in sedimentary rocks pores are that portion of the rock volume occupied by components of relative mobility such as water of various salinities microbial life petroleum liquids gases and supercritical fluids through a rock s history the contained pores evolve as the primary detrital assemblage responds chemically and mechanically to changing conditions in the subsurface description and classification of pores based upon their paragenesis depends upon inspection by imaging that allows the pores to be seen and interpreted in the context of historical processes responsible for formation of the pore walls since the late 1960s sample preparation and imaging methodologies that clarified our understanding of pores in their correct historical context have been at the root of important advances in prediction of porosity and related bulk properties in the subsurface these older approaches are now joined by new technologies that serve current efforts to extend predictive capabilities to nm scale pores in the fine grained low permeability rocks that dominate in the deeper parts of sedimentary basins and indeed in the sedimentary record as a whole placing the smallest pores into their proper paragenetic context presents a challenge in the quest to develop predictive models for porosity evolution in fine grained rocks

characterizing long term radon concentration changes in a geothermal area for correlation with volcanic earthquakes and reservoir temperatures a case study from mt aso southwestern japan abstract the purpose of this study is to characterize in detail the temporal changes in rn radon 222 concentration in soil gases near fumaroles and clarify its correlation with volcanic earthquakes and temperatures in two geothermal reservoirs mt aso crater in southwest japan which has two reservoirs on its western side estimated by magnetotelluric survey to be at about 2 km in depth was selected for this study for the long term survey the scintillation counter method was used weekly for 12 5 years at the three hot springs within a 2 km range rn concentrations were calculated using the cras method a calculation method that considers radioactive equilibrium or nonequilibrium state of the soil gas rn concentrations generally showed similar fluctuation patterns among the sites cras was used as a new indicator for evaluating the age of the soil gas this age corresponds to the elapsed time determined from the generation of rn based on the measurement of the numbers of atoms of rn and its daughter 218po at the start of measurement in comparing the rn data with the history of earthquakes in the aso caldera volcanic seismicity was identified as a major controlling factor in the sudden increase and decrease in rn concentration as a function of age for more precise detections of change rn concentrations were measured continuously at one site by pumping soil gas from a borehole and using an ionization chamber over 2 5 years five chemical components he h2 n2 ch4 and co2 were then measured by gas chromatography at 1 week intervals because rn concentrations are affected strongly by atmospheric temperatures the residual components were obtained by subtracting the trend of the components from the original data chemical component data were used to estimate the temperature and pressure in the reservoir at the site temperatures ranged from 229 to 280 c average 265 c average pressure 80 mpa residual rn concentrations showed a clear correlation with temporal changes in estimated temperature and to a degree with pressure considering the migration mechanism of rn by molecular diffusion and advection under rapid gas upflow conditions change in the diffusion coefficient is regarded as a possible factor to have induced temporal changes in rn concentrations in conjunction with the temperature changes in the reservoirs in addition the increase of the amount of degassed rn from the advective convective hydrothermal fluids which is associated with wall rock fracturing is another important factor earthquakes likely enhance the permeability of a reservoir by generating fractures which in turn can cause an increase in the reservoir temperature upflow fluid velocity and gas flux this provides one possible interpretation for the significant correlation of rn concentrations with earthquakes

are beds in shelf carbonates millennial scale cycles an example from the mid carboniferous of northern england abstract the mid carboniferous strata of northern england are characterised by mixed clastic carbonate cycles yoredale cyclothems attributed here to the short eccentricity milankovitch rhythm in a typical cycle transgressive normal marine shelf carbonates are succeeded by marine shales then highstand prodelta mudstones and delta front delta top sandstones with local coals a detailed study of one cycle the great limestone cyclothem of the northern pennines alston block reveals that within the transgressive carbonates the beds averaging 75 cm in thickness and defined by mm shale partings or cm mudrock layers form two thinning upward to thickening upward bed sets individual beds and the bed sets can be correlated across the region oxygen isotope and strontium trace element data also reveal patterns of increasing and decreasing values through the limestone which broadly correspond to the bed thickness cycles the beds are interpreted as millennial scale cycles the result of high frequency arid humid climatic fluctuations the bed sets are interpreted as the response to a longer term arid humid climate and sea level cycle driven by the precession rhythm it is postulated that millennial scale climatic changes which are a well known feature of the quaternary and are here inferred for the carboniferous were responsible for the deposition of the beds that are a characteristic feature of many marine sedimentary successions in the geological record the most likely over riding control is fluctuations in solar output

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