significance of late paleozoic fission track ages in volcanic rocks from the lavras do sul region southernmost brazil abstract this paper describes the application of apatite fission track thermochronology to four outcrop samples of the basaltic to andesitic volcanic rocks of the hilario formation located in the precambrian sul rio grandense shield in southernmost brazil the investigated unit was generated during the late stages of the neoproterozoic brasiliano pan african cycle 900 500 ma in a context of strike slip motion along ne trending structures and consequent basin inception the apparent fission track ft ages fall in the range 270 1 to 298 2 ma late carboniferous to early permian the mean confined fission track lengths range between 10 76 and 12 20 mm and the estimate for initial fission track recording below 110 c is positioned around 359 and 344 ma considering this the neoproterozoic volcanic rocks of the hilario formation were buried under at least 4 km of volcanic and sedimentary rocks resulting in thermal fading of all the original fission tracks and remained so until the late paleozoic then the investigated rocks and the whole region acted as the southern margin and source area for the paleozoic mesozoic intraplate parana basin a long lived cooling trend from the early carboniferous 359 344 ma to the permian triassic boundary ca 250 ma can be inferred from the analysis of the thermal history curves this tendency can be associated to the conjunction of uplift and denudation of this region during the carboniferous it probably reflects a response to the compressive events in the southwestern gondwanan margin in the context of the eo hercynian chanic orogeny this period in the studied region was also characterized by extensive ice caps related to the gondwanan glaciation and by lack of sedimentation with the collapse of the glaciers alluvial fan and glaciogenic sedimentation took place during the late carboniferous and early permian this cooling phase is probably recorded by the apparent ft ages a subsequent uplifting event is revealed by the analysis of the thermal history curves which show a strong inflection between 90 and 70 ma these data are here tentatively correlated with significant re arrangements of the brazilian southeastern coast such as the serra do mar and serra da mantiqueira uplifts

charging time of tight gas in the upper paleozoic of the ordos basin central china abstract the upper paleozoic section contains a tight gas sandstone reservoir of 2 75 10 12 m 3 in the ordos basin central china the measured porosities 13 c 1 were integrated to constrain the timing of gas charge into the tight reservoir the modeling results indicate that the natural gases in the present reservoir are similar to gases liberated from quartz inclusions in both composition and stable carbon isotope values and also similar to gas generated from upper paleozoic coal the similar geochemistry suggests that an important phase of quartz cementation must have occurred after gas emplacement in the reservoirs during regional uplift at the end of the cretaceous the latest carbonate cement postdating quartz cementation consumed most of the late co 2 generated from coal at high maturity r o 1 7 and reduced the reservoir quality dramatically on the contrary tight sandstones from non producing areas have fluid inclusions that were trapped in quartz cements much earlier these data indicate that natural gas migrated into the upper paleozoic reservoir when it still retained high porosity and permeability the reservoir continued to experience porosity and permeability reduction from continued quartz and carbonate cementation after gas charging due to low gas saturation comparison of the relative timing of gas charging with that of sandstone cementation can help to predict areas of risk during tight gas exploration and development

mantle phase changes and deep earthquake faulting in subducting lithosphere inclined zones of earthquakes are the primary expression of lithosphere subduction a distinct deep population of subduction zone earthquakes occurs at depths of 350 to 690 kilometers at those depths ordinary brittle fracture and frictional sliding the faulting processes of shallow earthquakes are not expected a fresh understanding of these deep earthquakes comes from developments in several areas of experimental and theoretical geophysics including the discovery and characterization of transformational faulting a shear instability connected with localized phase transformations under nonhydrostatic stress these developments support the hypothesis that deep earthquakes represent transformational faulting in a wedge of olivine rich peridotite that is likely to persist metastably in coldest plate interiors to depths as great as 690 km predictions based on this deep structure of mantle phase changes are consistent with the global depth distribution of deep earthquakes the maximum depths of earthquakes in individual subductions zones and key source characteristics of deep events

organic matter rich shales of china abstract organic matter rich shales are the main target rocks for unconventional oil and gas exploration and development across the world in china shale gas geological resources are estimated as approximately 110 1012 m3 with recoverable gas reserves of ca 20 1012 m3 recoverable shale oil reserves are estimated as ca 5 109 t a total 35 important organic matter rich shale units have been recognized from mesoproterozoic to cenozoic strata across the entire china these shales are categorized according to their origin under marine marine nonmarine transitional and lacustrine conditions shales of marine origin with ca 9 1012 m3 recoverable resources dominate china s potential in terms of total volume of organic carbon currently the most favorable marine shales for oil and gas exploration are found in the sichuan basin within the lower cambrian qiongzhusi formation and in the wufeng longmaxi formations of uppermost ordovician through lower silurian a fortuitous combination of of sea level variations of paleo productivity of tectonic activity causing development and migration of partially closed deep basin depocenters and of sediment accumulation rates controlled the extensive deposition and distribution of organic matter rich shales in these wufeng and longmaxi formations organic matter rich shales in marine nonmarine transitional facies associated with coal measures occur in north china within the carboniferous and permian and in south china within the permian these carboniferous permian organic matter rich shales are important source rocks for the gas fields in the ordos and sichuan basins abundant organic rich shales are also widely distributed within coal bearing clastics and coal measure shales of fluvial lacustrine and swamp facies in upper triassic to middle jurassic successions of many basins lacustrine organic rich shales were deposited during the permian through neogene in various freshwater to saline lake settings lacustrine organic matter rich shales are the main oil source rocks in the songliao bohai bay ordos and junggar basins lacustrine algae contributed to the rain of organic matter and the preservation of organic matter and distribution of organic rich shale was controlled by lake currents water depth and oxygen poor conditions with enhanced preservation when buried by turbidity currents algal blooms were partly induced by trace nutrients from volcanic ash falls in all of these lacustrine basins seawater intrusion into the freshwater lake of the songliao basin promoted some episodes of black shales saline lacustrine basins such as middle permian junggar basin contain organic rich dolomite mudstone that mainly formed during hot climate conditions when the lakes had high salinity and stratified water columns that deprived the bottom waters of oxygen thereby preserving massive amounts of organic matter laminated calcite rich mudstone in the saline lacustrine settings formed in more brackish waters under stable warm conditions and weak biological activity the modeling of the factors controlling the distribution of organic matter rich shales within china s basins is important for the exploration and development of unconventional oil and gas resources

anomalous high frequency wave propagation from the tonga kermadec seismic zone to new zealand summary bulletins of the international seismological centre isc show very large residuals up to 15 s early for arrivals from events in the tongakermadec subduction zone to the new zealand network of seismometers the very early arrivals are confined to events south of about 22 s and shallower than about 350 km the waveforms show two distinct phases an early emergent first phase with energy in the high frequency band 2 10 hz and a distinct second phase containing lower frequency energy arriving at about the time predicted by jb tables the residuals are attributed to propagation through the cold subducted lithosphere which has a seismic velocity 5 per cent faster on average than normal ray tracing shows that the ray paths lie very close to the slab for events south of 22 s but pass well beneath the slab for events further north corresponding to the change in residual pattern this characteristic of the ray paths is due to the curved shape of the seismic zone and in particular to the bend in the zone where the louisville ridge intersects the trench at 25 s the residuals can only be explained if the high velocity anomaly extends to a depth of 450 km in the region of the gap in deep seismicity from 32 to 36 s the very high frequency character of the first phase requires the path from the bottom of the slab to the stations to be of high q and to transmit 2 10 hz energy with little attenuation the absence of low frequency energy in the first phase is due to the narrowness of the high velocity slab which transmits only short wavelength waves the second phase which contains low frequencies is identified as a p wave travelling beneath the subducted slab in normal mantle there is no need to invoke any special structures such as low velocity waveguides or reflectors to explain any of the observations the s wave arrivals show similar effects

the thermal history of the bowen basin queensland australia vitrinite reflectance and clay mineralogy of late permian coal measures abstract the thermal history of the bowen basin queensland australia has been investigated using vitrinite reflectance data and clay mineralogy vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the bowen basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest middle triassic earliest late triassic as previously believed the results of the present study indicate that the development of a zone of high heat flow in the latest late triassic had a major control on the thermal history of the bowen basin high palaeogeothermal gradients estimated in the northern bowen basin are interpreted to result from convective heat transfer during a hydrothermal event variable heat distributions due to localised fracture enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures the estimated palaeogeothermal gradients in the southern bowen basin also indicate high heat flow in the lower sections of the stratigraphy sections in the southern bowen basin however are believed to reflect a rock dominated semi closed system with low water rock ratio where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction the correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite smectite i s mixed layer clays in the northern bowen basin this phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks the relationship between i s expandability and vitrinite reflectance for the bowen basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the bowen basin the effect of thin igneous intrusions on clay mineral reactions is very limited intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies this further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration in contrast to organic maturity indicators these differences between mineral and organic parameters aid in the identification of local contact metamorphic effects

thermal and tectonic history of the ordos basin china evidence from apatite fission track analysis vitrinite reflectance and k ar dating apatite fission track analysis vitrinite reflectance data and k ar dating of permian carboniferous and mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the ordos basin china apatite fission track ages of carboniferous jurassic sedimentary rocks range between 3 and 137 ma and are significantly younger than the stratigraphic ages confined fission track lengths demonstrate exclusively mixed length distribution indicating complex thermal history the data suggest that the samples must have all experienced higher paleotemperatures in the past mean vitrinite reflectance values r0 of the triassic rocks range from 0 61 to 1 06 giving a high coalification gradient of 0 36 km and suggesting a high paleothermal gradient of 57 c km permian carboniferous rocks have ro values on the order of 1 0 3 0 and locally up to 4 0 6 0 some high r0 values coincide with positive gravity and magnetic anomalies k ar dating on permian triassi samples reveals distinct illitization at 170 160 ma during which a thermal event occurred due to subsurface magmatic intrusion related to the early yanshanian movement the petroleum source rocks of the upper triassic experienced peak temperatures ranging from 90 to 160 c corresponding to the oil window and permian carboniferous source rocks were heated to more than 150 c passing through and out of the gas window due to rapid uplift and erosion in response to the rise of the qinghai tibet plateau associated with the asia india collision and the himalayan orogeny cooling has taken place at least since approximately 23 ma the difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present day s ructural pattern of the basin

paleokarst reservoirs and gas accumulation in the jingbian field ordos basin abstract the jingbian gas field in central ordos basin with a proven initial in place gas reserve of approximately 11 trillion cubic meters is the largest paleokarst carbonate gas field in china paleokarst in ordovician strata of central ordos most commonly occurs in the paleo weathering surface of the o 1 m 5 member of the majiagou formation the karst intervals are generally proximal to the sub upper paleozoic unconformity however dissolution features in strata well below that exposure surface are possibly related to intra majiagou formation unconformities the quality of gas reservoirs are initially controlled by sedimentary facies with anhydrite bearing dolomite flat facies being the most favorable zones for dissolution and dolomitization to form karst and large small dissolution cavities the gases are generally dry derived dominantly from the overlying carboniferous permian coal measures the gases are accumulated in stratigraphic traps related to karst paleo geomorphology and lithologic traps associated with late diagenetic features of carbonate rocks although the precise timings of the thermal events during the evolution of the ordos basin are still subject to considerable debate there is a general consensus that events occurring during the yenshanian orogeny 150 115 ma were the most important for the paleozoic source rocks it appears that two episodes of hydrocarbon charge have occurred in the ordovician gas reservoirs in the jingbian field

kar systematics of illite smectite in argillaceous rocks from the ordos basin china abstract conventional k ar dating x ray diffraction analyses illite crystallinity determinations and quantitative infrared spectroscopy mineral analyses were carried out on clay materials of the mesozoic and paleozoic argillaceous rocks from the ordos basin china in order to date the illite smectite diagenesis and to evaluate the influence of detrital contamination on the k ar ages a clay mineral assemblage of illite i s chlorite kaolinite was determined two different stages of illite diagenesis were distinctly identifiable which can be correlated to two thermal and or fluid flow events accompanying well known major tectonic movements in china a jurassic diagenetic event 170 160 ma is related to the early yanshanian movement whereas a silurian devonian diagenetic age 420 370 ma corresponds to the late caledonian movement no influence of the jurassic event is evident on the silurian devonian diagenetic illite attesting to the good retentivity of diagenetic illite for argon principles for estimating any influence on the k ar data of a size fraction by detrital contamination are discussed decreasing illite crystallinity and increasing or constant k ar data with increasing depth indicate that authigenic illite predominates and that the k ar data are diagenetic with little detrital influence furthermore the correlation between k ar ages and depth can reveal the temperature controlled mechanisms of the diagenetic illitization process increasing k ar ages with depth reflects burial diagenesis while constant k ar ages with depth indicates a diagenetic event caused by a short lived thermal event as shown in our study

high resolution sediment accumulation rate determined by cyclostratigraphy and its impact on the organic matter abundance of the hydrocarbon source rock in the yanchang formation ordos basin china abstract the upper triassic chang 7 unit consists of more than 110 m of lacustrine sediments including mudstone and shale and it is considered to be the most important hydrocarbon source rock in the ordos basin power spectral wavelet evolutionary fast fourier transform fft and correlation coefficient analyses of the gamma ray gr and density den logging series from well y56 in the chang 7 unit reveal strong astronomical signals of long eccentricity 405 kyr short eccentricity 100 kyr and obliquity 34 kyr cycles those provide an excellent opportunity to establish floating astronomical time scales ats and to discuss how the variations in the sediment accumulation rate sar impact organic matter abundance based on the 405 kyr tuning ats results the chang 7 unit in well y56 covers a 7 26 ma period from 239 50 ma to 232 24 ma the ages of each subunit in the chang 7 unit are estimated to be 232 24 234 25 ma 234 25 236 90 ma and 236 90 239 50 ma respectively the high resolution sars of the chang 7 unit vary from 1 18 cm kyr to 2 08 cm kyr with an average of 1 57 cm kyr according to the chemical index of alteration cia and the correlation between the sar and the al2o3 sio2 or al2o3 tio2 ratio the main factor affecting the sar of the chang 7 unit is the amount of terrestrial debris input which is the result of the warm humid paleoclimate and the intense volcanic activity during phase i of the indosinian in addition the total organic carbon per unit of paleoproductivity tocpup was applied to investigate the impact of the sar on the organic matter enrichment in order to eliminate the effect of paleoproductivity on the organic matter enrichment we conclude that an sar is sufficient to protect the organic matter and prevent oxidation but not too high to dilute the organic matter which reduces the organic matter abundance of the source rock

effect of sedimentary heterogeneities on hydrocarbon accumulations in the permian shanxi formation ordos basin china insight from an integrated stratigraphic forward and petroleum system modelling abstract sedimentary heterogeneities are ubiquitous in nature and occur over a range of scales from core reservoir to basin scales they may thus exert significant influences on hydrocarbon generation migration and accumulation the sedimentary heterogeneities of the permian shanxi formation in the ordos basin china were modelled using sedsim a stratigraphic forward modelling program the simulation results were then used to construct a 3d petroleum system model using petromod the effects of sedimentary heterogeneities on hydrocarbon accumulations were evaluated by comparing the integrated sedsim petromod model with the classic 3d basin model the sedsim simulation shows that considerable sedimentary heterogeneities are present within the shanxi formation as a result of the interplay of the initial topography tectonic subsidence base level change and sediment inputs a variety of lithologies were developed both laterally and vertically within the shanxi formation at kilometre and metre scales respectively with mudstones mainly developed in the depositional centre while sandstones developed in the southern and northern margin areas a typical source ward retrogradation is well developed within the lower shanxi formation a base case classic 3d basin model was constructed to quantify the permian petroleum system in the ordos basin the geological and thermal models were calibrated using vr and borehole temperature data the source rocks of the upper paleozoic became mature r o 0 5 and high mature r o 1 2 in the late triassic and late jurassic respectively in the central and southern areas during the early cretaceous a tectonically induced geothermal event occurred in the southern ordos basin this caused the source rocks to reach over maturity r o 2 0 quite rapidly in the early late cretaceous in the central and southern areas all the source rock transformation ratios tr at present are greater than 70 in the p 1 coal and p 1 mudstone layers with tr values approaching 100 in the central and southern areas the transformation ratios of the p 1 limestone are close to 100 over the entire interval in the base case model a large amount of hydrocarbons appear to have been expelled and migrated into the shanxi formation but only a minor amount was accumulated to form reservoirs in the model the shanxi formation sandstone layer was set to be homogeneous vertically and there was no regional seal rocks present at the top of the shanxi formation therefore hydrocarbons could not be trapped effectively with only minor accumulations in some local structural highs where hydrocarbons are trapped both at the top and in the up dip direction by the adjacent mudstone facies in contrast the integrated sedsim petromod model takes into account of the internal lithological and sedimentary facies heterogeneities within the shanxi formation forming complex contiguous sandstone mudstone stacking patterns hydrocarbons were found to have accumulated in multiple intervals of lithological traps within the shanxi formation the results indicate that lithological distinctions controlled by sedimentary heterogeneities in three dimensions can provide effective sealing in both the top and up dip directions for hydrocarbon accumulations with gas being mainly accumulated near the depocentre where lithological traps usually formed due to frequent oscillations of the lake level

the comparison of a numerical model to observations of velocity and scalar data in burrard inlet and indian arm british columbia a laterally integrated time dependent numerical model of burrard inlet and indian arm is compared to time series of cyclesonde velocity and scalar data that have 10 m vertical resolution as long as deep water renewal is not occurring the model can reproduce the observed m2 and k1 tidal response in the basin of indian arm also although the rate of freshwater inflow into indian arm and burrard inlet is not large the simulated tidal response is noticeably improved if freshwater inflow is taken into account when deepwater renewal is observed to occur the model also simulates renewal but it does so poorly and the ability of the model to accurately simulate the m2 and k1 tide degrades significantly

global mapping of topography on the 660 km discontinuity long period precursors to the ss seismic phase are used to produce global maps of topography for the discontinuity at 660 km depth in the upper mantle these maps indicate discontinuity depth variations of up to 30 km and suggest a correlation between regional depressions in the 660 km discontinuity and subduction zones a result more consistent with models in which the subducting slabs are deflected horizontally at the discontinuity than with models of unhindered slab penetration into the lower mantle

s p conversion from the transition zone beneath tonga and the nature of the 670 km discontinuity summary using more than 300 recordings of deep tonga earthquakes from the warramunga seismic array central australia we have searched for short period s p conversions from the 670 km discontinuity arriving in the p wave codas the array seismograms were stacked and plotted in sections to optimize observation of s670p yielding the following results for the northern part of tonga 17 3 18 4 s s670p phases are observed with apparent conversion depths between about 660 and 680 km for the central part 20 2 22 3 s s670p is not clearly observed for the southern part 23 1 25 7 s s670p is observed for conversion depths between about 660 and 700 km intermittent occurrence of s p conversion may result from different characteristic source radiation patterns among the three sections overall uncertainties in conversion depths resulting from unmodelled lateral velocity variations and hypocentral location errors are perhaps a few tens of kilometres the s p rays must travel either within or very close to the subducted slab because of the source receiver slab dip geometry so we conclude that large 50 km deformations of the 670 km discontinuity do not occur beneath tonga our observations appear to be inconsistent with a chemical discontinuity at 670 km but are easily explained if the 670 km discontinuity is a strongly pressure dependent and perhaps moderately endothermic phase change in both the slab and surrounding mantle

tectonic and stratigraphic controls of hydrocarbon systems in the ordos basin a multicycle cratonic basin in central china the ordos basin is the oldest and still an important hydrocarbon province in central china it is a typical cratonic basin developed on the archean granulites and lower proterozoic greenschists of the north china block the development of the ordos basin during the paleozoic mesozoic can be divided into three evolutionary stages cambrian early ordovician cratonic basin with divergent margins middle ordovician middle triassic cratonic basin with convergent margins and late triassic early cretaceous intraplate remnant cratonic basin two hydrocarbon systems are present in the basin the paleozoic gas and mesozoic oil systems in the paleozoic gas system the lower ordovician marine carbonates and pennsylvanian lower permian coal measures serve as source rocks the lower ordovician karst modified dolomites and pennsylvanian bauxitic mudstones form a significant reservoir seal association and the pennsylvanian lower permian deltaic sandstones and upper permian lacustrine mudstones form another effective reservoir seal association in the mesozoic oil system the upper triassic lacustrine mudstones are mature source rocks the upper triassic deltaic sandstones and overlying shallow lacustrine and swamp mudstones form a reservoir seal association and the lower jurassic fluvial sandstones and overlying shallow lacustrine and swamp mudstones form another reservoir seal association in both hydrocarbon systems the stratigraphic variations provide the principal traps the ordos basin is characterized by a stable tectonic setting that controlled the distribution of depositional systems and the development of erosional surfaces and ultimately governed the distribution of oil and gas fields and trap types

systematics of deep focus earthquakes along the kuril kamchatka arc and their implications on mantle dynamics subduction along the kuril kamchatka arc has been proposed as a primary example in which subducted lithosphere plunges steeply into the lower mantle with little deformation the occurrence of several unusual earthquakes along this arc during the last few years places new constraints on the configuration and the state of strain of the down going slab we systematically determined the source parameters of 27 large to moderate sized earthquakes mb 5 5 that occurred between 1963 and 1990 at depths greater than 200 km along this subduction zone to our surprise our results show that the configuration and strain due to earthquakes in subducted lithosphere are highly variable along both the strike and dip of the arc approximately 75 km farther west of the deepest earthquakes in the northern portion of the arc the earthquake of august 14 1988 occurred at a comparable depth of 636 17 km thus at least part of the slab has been displaced in a direction perpendicular to the overall strike of the wadati benioff zone earthquakes in the central part of the arc delineate shoaling of the subducted slab at a depth of approximately 570 km reminiscent of the configuration of subducted lithosphere along the tonga arc both observations indicate that the subducted slab is deforming probably in response to resistance encountered near the 670 km discontinuity for all earthquakes north of 48 n focal mechanisms of all events deeper than 200 km show a remarkably consistent pattern of downdip compression strain in the southern kuril the sakhalin island earthquake m0 8 1019n m of may 12 1990 is one of three large deep focus earthquakes that have occurred over 150 km farther west of the well defined wadati benioff zone the relationship between the source region of these events and the rest of the subducted slab is equivocal due to the lack of background seismicity nonetheless for earthquakes south of 48 n net downdip extension dominates between depths of 200 450 km before switching to downdip compression below 540 km thus the strain field within the slab is segmented both along strike and dip if the slab in southern kuril kamchatka can serve as a stress guide down to depths of 615 km the changeover from extension to compression occurs at an unusually large depth within the transition zone of the upper mantle alternatively the deep focus events west of the island of sakhalin could have occurred in a detached piece of lithosphere as a whole the configuration and state of strain of the subducted slab along the kuril kamchatka arc are highly variable close to a microcosm of variations among different deep subduction systems observed on a global scale in particular the subducted lithosphere is apparently undergoing considerable deformation as it approaches the base of the upper mantle our results do not preclude the possibility of subducted lithosphere ever reaching the lower mantle nevertheless it seems unlikely that the down going slab along the kuril kamchatka arc penetrates into the lower mantle unhindered

teleseismic travel time anomalies and crustal structure in central australia abstract teleseismic p wave travel time anomalies of more than 0 5s have been recorded in central australia between sites separated by less than 100 km late arrivals occur at sites that are located within sedimentary basins and early arrivals occur at sites located on the exposed and eroded basement early arrivals correspond to relative gravity highs and late arrivals correspond to gravity lows the differential travel times cannot be attributed to near surface crustal structure alone the most straightforward interpretation is in terms of a moho whose depth varies by about 10 km from a mean value and which is deepest below the basins this interpretation is consistent with the intracratonic basin evolution model proposed by lambeck

time and temperature in petroleum formation application of lopatin s method to petroleum exploration n v lopatin in the soviet union has developed a method for taking both time and temperature into account as factors in thermal maturation of kerogen lopatin s time temperature index of maturity tti values correlate with the thermal regimes corresponding to generation and preservation of hydrocarbons because such information is potentially of great interest for oil exploration a calibration and evaluation have been made of lopatin s method within the limitations of the data presently available the following statements can be made 1 the rate of the chemical reactions involved in thermal maturation of organic material appears to double with every 10 c 18 f rise in temperature 2 threshold values of lopatin s time temperature index of maturity tti are 15 onset of oil generation 75 peak oil generation 160 end oil generation 500 40 oil preservation deadline 1 000 50 oil preservation deadline 1 500 wet gas preservation deadline 65 000 dry gas preservation deadline 3 tti values calculated from lopatin reconstructions consistently agree with other maturation parameters commonly used by petroleum geochemists potential applications of lopatin s method for oil exploration include timing of oil generation calculation of volume of hydrocarbons generated within a basin and determination of economic deadlines

geochemical characteristics of the triassic chang 7 lacustrine source rocks ordos basin china implications for paleoenvironment petroleum potential and tight oil occurrence abstract the fact that high quality lacustrine source rocks generally shale and mudstone control the formation and distribution of conventional and unconventional oil and gas reservoirs in lacustrine basins has been well accepted in petroleum exploration and exploitation taking the chang 7 lacustrine source rocks in the ordos basin as an example several points having been reached are as follows 1 the chang 7 source rocks possess an excellent source rock potential and the shales hold a better potential than the mudstones 2 the paleoenvironments of the chang 7 source rocks were sub reducing to sub oxidizing conditions and fresh and brackish water depositional environments with a maximum water depth of 150 m the organic matter origins of the source rocks are mainly plankton algae bacteria and other aquatic microorganisms 3 the beginning time of the oil generation of the chang 7 source rocks is at 165 ma and the peak oil generation occurred during 115 95 ma the cumulative amounts of oil generation are up to 4711 103 t km2 and the ratios of peak generation amounts to cumulative generation amounts are 50 the beginning timing and peak generation timing of the shales are earlier than those of the mudstones respectively and the cumulative oil generation amount of the shales is higher than that of the mudstones indicating a better oil generation potential of the shales 4 due to the more remained oil possessed in the shales the hydrocarbon expulsion threshold of the chang 7 shales 2560 m is deeper than that of the chang 7 mudstones 2080 m 5 the occurrences of the chang 8 6 tight oils are predominantly controlled by the outer boundary of the chang 7 source rocks distribution while the transition areas between thickness toc and ro high value centers are the accumulation and enrichment zones the chang 7 shales controlled the occurrence of the chang 8 tight oil reservoirs and the chang 7 mudstones controlled the occurrence of the chang 7 and chang 6 tight oil reservoirs

reservoir property changes during co2 brine flow through experiments in tight sandstone implications for co2 enhanced oil recovery in the triassic chang 7 member tight sandstone ordos basin china abstract the effects of supercritical co2 scco2 on reservoir property changes of the chang 7 member tight sandstones of the yanchang formation ordos basin china were investigated using co2 brine flow through experiments in conjunction with detailed rock and fluid characterization pre and post experiment using x ray diffraction mineral analysis 3d computed tomography scanning high resolution scanning electron microscopy and inductively coupled plasma atomic emission spectrophotometry an in situ 3d model was developed to define porosity mineral evolution experiments on three contrasting samples show that minerals such as potassium k feldspar albite and calcite are variably dissolved during co2 brine mineral interactions the dissolution migration and re precipitation of clay minerals are common and are characterized by the dissolution of chlorite and the re precipitation of kaolinite calcium montmorillonite and sodium montmorillonite the original chlorite kaolinite framework of the host rocks was destroyed precipitated and re migrated clay particles e g kaolinites and smectites that were released by the dissolution accumulated in pore throats resulting in changes to the pore system and in the physical properties of the host rocks physical property change pathways vary among the studied samples with porosity being reduced by 4 5 and 12 8 or increased by 13 7 by the end of the experiments permeability reduction was 33 3 81 2 and is positively correlated to host rock clay mineral content the influence of the type and content of clay minerals on porosity requires further evaluation the findings of the study have implications for the large scale injection of co2 into reservoirs during enhanced oil recovery programs and for carbon capture utilization and storage programs

the interaction of tides with the sill of a tidally energetic inlet abstract the interaction of the tides with the sill of a tidally energetic inlet observatory inlet british columbia is studied because of temporal variations in the stratification of the inlet a substantial seasonal variation is observed in the power withdrawn from the barotropic tide vigorous nonlinear internal motions occur in the region of the sill but most of the withdrawn tidal power is fed into a progressive linear internal tide the first two modes which contain almost all of the energy respond very differently to changes in stratification the energy flux of the first mode is insensitive to changes in surface stratification but increases dramatically as a result of deep water renewal the energy flux of the second mode exhibits the reverse behavior being insensitive to the occurrence of deep water renewal but being a strong positive function of the surface stratification even though the inlet has a distinct surface layer in summer and appears to be a two layer system the second mode

fission track dating of phosphate minerals and the thermochronology of apatite several phosphate minerals have been investigated for their usefulness in geochronology and thermochronology by the fission track method of these apatite ca5 po4 3 f cl oh has proved pre eminently suitable for this purpose for reasons discussed below apatite was one of the first minerals amongst many others to be investigated for fission track dating by fleischer and price 1964 and has subsequently become by far the most important of all the minerals used for dating by this method the usefulness of apatites for fission track dating arises from their near universal tendency to concentrate uranium within their structure at the time of crystallization and their widespread occurrence in all of the major rock groups price and walker 1963 first recognized the possibility that the accumulation of radiation damage tracks in natural minerals from the spontaneous nuclear fission of 238u within their lattices could be used for geological dating they also demonstrated that a simple chemical etching procedure served to enlarge these fission tracks to optical dimensions so that they could be observed and measured under an ordinary optical microscope price and walker 1962b 1963 this simple procedure quickly opened the way for a wide range of nuclear particle track studies in natural minerals and glasses e g fleischer et al 1975 the most important of which was fission track dating fleischer et al 1964 also showed that spontaneous or fossil fission tracks provided an explanation for various anomalous etch pits in apatite which had puzzled crystallographers over many years the essence of a fission track age determination involves measuring the number of tracks that have accumulated over the lifetime of the mineral along with an estimate of the amount of uranium that is present knowing the rate of spontaneous fission decay a geological age can be calculated detailed

p and s wave travel time inversions for subducting slab under the island arcs of the northwest pacific we have observed slablike high p and s velocity anomalies around the wadati benioff zone under island arcs of the northwest pacific through travel time tomographic inversions nineteen years of international seismological centre travel time residuals for events and stations in this large region are used analyses of resolution and noise show that the images are generally well resolved the images illustrate that slab anomalies are continuous along strike in most parts of the upper mantle of the region and become contorted and generally broadened with depth near the bottom of the upper mantle fingering of the slabs including segmenting and spreading is indicated the fast anomalies associated with the japan izu bonin and mariana subduction zones tend to flatten to subhorizontal at depth while downward spreading may occur under parts of the mariana and kurile arcs the fast anomalies below 700 km are not in the shape of a single coherent sheet the principal compressional axes of focal mechanisms in the region consistently follow the downdip direction of the high velocity slab even when it bends to subhorizontal at depth the depth at which compression begins to dominate the downdip stress regime in the slab apparently depends on bending of the slab and its dip slab fingering and intense deep seismicity probably are the consequence of the slab encountering a barrier of some form around the 670 km discontinuity

the relationship between track shortening and fission track age reduction in apatite combined influences of inherent instability annealing anisotropy length bias and system calibration abstract the relationship between reduced track density 0 and reduced mean confined track length l l 0 during annealing has been studied for both spontaneous and induced tracks in four apatite samples with different chemical composition compositional range and origin and with spontaneous tracks giving mean confined track lengths varying from 14 5 m down to 12 3 m in all four apatites induced tracks show a relationship between l l 0 and 0 composed of two regions at higher values the two appear to be equal while at lower values l l 0 is greater than 0 this can be understood qualitatively in terms of the anisotropy of annealing and the bias against revelation of short tracks data from three apatites of uniform composition show near identical relationships in a sample showing a range of apatite compositions the point of divergence from the 1 1 relationship is somewhat higher than in the uniform composition apatites because individual grains anneal at different rates and the presence of highly annealed grains affects the track density more strongly than the mean track length spontaneous tracks show similar relationships to those observed for induced tracks in each sample when the data are renormalised to allow for the natural annealing present in the spontaneous tracks previous studies which led to the idea of length reduction without density reduction for spontaneous tracks did not renormalise density data as well as length data thereby distorting the apparent relationship data for the multi compositional apatite sample where annealing has taken place in geological conditions also agrees with the laboratory annealing data for both spontaneous and induced tracks this consistency suggests that a single process is responsible in all three cases implying that a laboratory based description of annealing can be applied to geological conditions with confidence recognition that spontaneous tracks in apatite are commonly annealed suggests that in general the fission track age should be interpreted as a reflection of both the time over which tracks have been retained and the distribution of etchable track lengths since the latter is a manifestation of the thermal history the fission track age should be interpreted in similar terms

sulige field in the ordos basin geological setting field discovery and tight gas reservoirs abstract the sulige gas field in central ordos basin is one of the largest gas fields in china with about 860 7 billion cubic meters bcm of proven gas reserve this field covers an area of about 500 km 2 with largely tight clastic reservoirs the upper carboniferous lower permian deltaic sandstones and upper permian lacustrine mudstones deposited in a stable cratonic tectonic setting form an effective petroleum system that is favorable for gas accumulation in large scale stratigraphic lithologic traps based on wire line log response drilling stem test and core data a generalized classification system was developed for the gas reservoirs of the sulige field thus type i and ii rocks are capable of gas production without natural and or artificial fracturing type iii rocks are capable of gas production after artificial fracturing and type iv rocks are too tight to produce at commercial rates even with artificial fracturing the discovery of the sulige field not only adds large gas reserve in the ordos basin but strongly indicates the overlook potential of both tight sandstone reservoir rock and relatively porous and permeable sweet spots in it

a numerical model of stratified circulation in indian arm british columbia a laterally integrated numerical model has been developed for the study of stratified circulation in inlets the model is time dependent and includes advective terms horizontal and vertical turbulent diffusion of momentum and salt and variations in width and depth initial and boundary conditions based on winter measurements in burrard inlet and indian arm were used to study the inlet s response to tidal forcing the friction term was tuned to match calculated and observed dissipation rates evidence for a standing wave pattern in the response of the m2 and k1 tidal constituents is discussed a comparison of model results with cyclesonde current meter time series at two locations in indian arm is consistent with resonance a modal decomposition of the calculated response shows that most of the energy is contained in the barotropic and first baroclinic modes with the former dominating near the sill and the latter dominating in the deep basin second mode energy was significant for the k1 at some locations

short period s p conversion in the mantle at a depth near 700 km summary data from 16 deep earthquakes originating in the tonga subduction zone and recorded at the warramunga seismic array wra in central australia show the presence of a distinct later arrival in the coda of p this phase which is not predicted by smooth velocity depth models of the earth has been called x in the paper slownesses and arrival azimuths of x are similar to those of the p wave it is unlikely that x was radiated as the result of multiple focal activity because a corresponding phase was not observed neither in the coda of scp nor on seismograms of seismic stations located at close epicentral distances relative to p the variation in arrival times with focal depth of the earthquakes indicates that x is produced by s p conversion below the source at a depth near 700 km most of the observed x phases have approximately the same signal duration as the p waves thus suggesting a transition zone near 700 km which is only a few kilometres thick synthetic seismograms calculated for a model with a first order seismic discontinuity at a depth of 700 km show reasonable agreement with the observations as demonstrated by the synthetic seismogram modelling the amplitude of x depends on the radiation pattern of the earthquake source variations in the radiation pattern offer a plausible explanation that first x is only intermittently detected at wra and secondly that x has most frequently been observed from the southern part of the tonga subduction zone

the effect of the descending lithosphere beneath the tonga island arc on p wave travel time residuals at the warramunga seismic array abstract p wave travel time residuals at the warramunga seismic array wra in the northern territory australia have been studied from 49 earthquakes with epicenters south of 19 s in the fiji tonga region focal depths are between 42 and 679 km as determined from pp p using the jeffreys bullen and the herrin travel time tables the epicentral parameters have been redetermined by considering only normal seismic stations in the location procedure these are those stations where p wave travel times are probably not affected by lateral heterogeneities caused by the lithosphere descending beneath the tonga trench epicenters of deep earthquakes below 300 km have been relocated by using stations at 25 only epicenters from shallower depth earthquakes have been recalculated without using stations between 35

diagenetic alteration pore throat network and reservoir quality of tight gas sandstone reservoirs a case study of the upper paleozoic sequence in the northern tianhuan depression in the ordos basin china the sulige gas field in the ordos basin is the largest gas field in china the studied area the northern tianhuan depression is located west of the gas field and is considered as a potential gas producing area the complex diagenetic alteration history and its effects on the pore throat network and reservoir quality of the tight gas sandstones are explained the upper paleozoic first member of the shanxi formation and eighth member of the shihezi formation tight gas reservoir rocks are composed of coarse to medium grained sandstones and are dominated by sublitharenites the most important diagenetic factor for porosity reduction was compaction followed by cementation diagenetic mineral phases include quartz overgrowth calcite cement and replacement and kaolinite cement and replacement with minor heulandite and chlorite pressure dissolution of quartz grains and quartzite fragments is the main source of silica for the early and late quartz overgrowth cementation formed mainly from 120 c to 140 c the hydrolysis and alteration of volcanic material and mica flakes released some silicon calcium and aluminum ions calcium and carbon in calcite cements could be derived from surrounding mudstones calcite replaced the quartz overgrowths the formation of kaolinite as cement and replacements was controlled by the interactions of silica aluminum ph and temperature 140 c because of deformation of detrital components mica flakes and ductile rock fragments and diagenetic replacement clear negative correlations between reservoir quality and contents of the diagenetic minerals are lacking a complex and isolated pore throat network was created by strong diagenetic transformation based on analysis of thin sections three dimensional x ray micro computed tomography and constant rate mercury injection data the samples with relatively good reservoir quality porosity 10 and permeability 0 5 md are characterized by dissolution pores and primary intergranular pores and commonly contain more and larger pore throats both in volume counts 1800 and size 0 02 per unit volume and diameter 1 20 m than the samples with poor reservoir quality the intercrystalline pores of kaolinite are very widespread and considered favorable for oil and gas accumulation because some asphalt was found within these pores this study may be used by analogy to better understand diagenetic factors controlling the formation of tight gas sandstone reservoirs and predict the development of pore throat systems

thermal history of rocks in southern san joaquin valley california evidence from fission track analysis the theory of the fission track method and its application to sedimentary basin analysis is illustrated by a case study in the southern san joaquin valley california fission tracks provide a powerful tool for studying the thermal history of sedimentary basins because the two minerals most commonly used in fission track studies apatite and zircon occur as detrital constituents in many sedimentary rocks and their annealing temperatures span the main temperature range for oil generation fission tracks also provide information on the sedimentation record and provenance of rocks in a basin we have used fission track analysis to study the thermal and depositional history of the subsurface tertiary sedimentary rocks on both sides of the active white wolf reverse fault in the southern san joaquin valley the distinctly different thermal histories of the rocks in the two structural blocks are clearly reflected in the apatite fission track data which suggest that rocks in the rapidly subsiding basin northwest of the fault have been near their present temperature for only about 1 m y compared with about 10 m y for rocks southeast of the fault these estimates of heating time agree with previous estimates for these rocks zircon fission track data indicate that the tertiary sediments were derived from parent rocks of more than one age however from at least the eocene to late miocene or pliocene the major sediment source was rocks related to the youngest sierra nevada mesozoic intrusive complexes which are presently exposed east and south of the southern san joaquin valley

seismic strain rate and deep slab deformation in tonga seismic strain rates estimated from volume averaged moment tensor data are combined with the strike dip and extent of the subducting lithosphere obtained from residual sphere analysis and seismicity to map deformation in the deep tonga slab tonga strain rate orientation tensors constructed from the harvard centroid moment tensor catalog are consistent with slab deformation models in which the lithosphere encounters resistance to penetration at the 650 km discontinuity those which sample lithospheric volumes from 18 s to 25 s at 500 700 km depth indicate that the axis of maximum shortening lies within 15 of the slab s downdip azimuth and plunge the discrepancy between shortening and slab dip may be larger in northernmost tonga 17 s 18 s where slab morphology is less well constrained but overall the slab is compressing roughly parallel to its trajectory tonga strain rate orientation tensors also indicate horizontal strain both parallel and perpendicular to the strike of the slab the proportion of along arc strain rate decreases from northern to southern tonga the scalar rate of seismic strain peaks at certain latitudes but its values averaged over northern and central tonga and southern tonga are roughly equal strain rate data may be related nonuniquely to constraints on slab structure and finite strain inferred from seismicity and travel time data assuming that aseismic and seismic flow have the same geometry and that the total rate of strain is constant along the arc the degree of cross strike advective thickening increases to the south while along arc strain decreases if along arc strain rate exclusively produces southward shear of the deeper lithosphere and the amount of shear is estimated from the offset during subduction of the northern edge of the slab and the louisville ridge thickening of at least 50 is required in central tonga this lower bound improves the resolution of minimum slab width provided by travel time observations if 50 thickening occurs an upper bound on the estimated rate of seismic moment release is large enough to account for scalar strain rate in the seismogenic core of the slab larger thickening factors or flow in the outer regions of the slab require aseismic deformation assuming that the observed travel time residuals are the result of a single coherent slab a synthesis of seismicity strain rate and travel time data indicate a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes roughly to the north at shallower depths and in the lower mantle such a contortion in the deep lithosphere is consistent with the subduction zone configuration prior to 8 m y and the tectonic history of the region

simultaneous miocene extension and shortening in the himalayan orogen the south tibetan detachment system separates the high grade metamorphic core of the himalayan orogen from its weakly metamorphosed suprastructure it is thought to have developed in response to differences in gravitational potential energy produced by crustal thickening across the mountain front geochronologic data from the rongbuk valley north of qomolangma mount everest in southern tibet demonstrate that at least one segment of the detachment system was active between 19 and 22 million years ago an interval characterized by large scale crustal thickening at lower structural levels these data suggest that decoupling between an extending upper crust and a converging lower crust was an important aspect of himalayan tectonics in miocene time

implications for organic maturation studies of evidence for a geologically rapid increase and stabilization of vitrinite reflectance at peak temperature cerro prieto geothermal system mexico discussion a short term rapid heating and cooling of the rock in well m 94 below 1300 m was caused by a pulse of hot water passing through the edge of the cerro prieto mexico geothermal system below 1300 m the peak paleotemperatures were about 225 250 degrees c but equilibrium well log temperatures indicate a decrease to 150 210 degrees c at present this hot water pulse sharply increased vitrinite reflectance to levels comparable to those measured in the central part of the system even though studies of apatite fission track annealing indicate that the duration of heating was only 10 0 10 1 yr in m 94 in contrast to 10 3 10 4 yr in the central part of the system these data indicate that sedimentary organic matter chemically reacts quickly to temperature increases of about 125 degrees c above ambient even when the higher temperature existed for only 10 0 10 1 yr the quick change of the vitrinite reflectance geothermometer indicates that thermal maturation reactions can stabilize after a geologically short period of heating to a level consistent with peak temperature under moderate to high temperature diagenesis in open fluid rich geothermal systems cerro prieto is one of the most intensively studied and well known geothermal systems in the world thus data from cerro prieto are a benchmark to compare with the predictions of published thermal maturation models such as those formulated by j karweil n v lopatin and a k burnham and j j sweeney these thermal maturation models inaccurately predict duration of heating at cerro prieto the kinetic equations used in these models explicitly allow thermal maturation to continue indefinitely at peak temperature which does not seem to be the case at cerro prieto

tibetan tectonics from 40ar 39ar analysis of a single k feldspar sample ar 39ar data on an alkali feldspar sample from the quxu pluton gangdese batholith southern tibet allow a detailed assessment of unroofing and uplift history between 35 and 18 ma the 39ar arrhenius plot for this sample shows departures from a linear relationship between the effective diffusion parameter log d r2 and reciprocal temperature which we interpret to be the result of a distribution of distinct diffusion domain sizes we use an alternative way of plotting the arrhenius data that exhibits domain size versus cumulative 39ar released during step heating the 4 ar 39ar age spectrum of the sample has features such as local age plateaus that are most easily explained in terms of the distinctive closure age of particular domains the fact that the same distribution of diffusion domain sizes explains both the arrhenius data and the age spectrum is an indication that the diffusion properties operating in the laboratory are those of the sample while it was in its natural environment modelling of the age spectrum with a distribution of domain sizes results in the recovery of a continuous cooling history segment rather than a single time temperature datum we demonstrate the robustness of the cooling curve determination by showing the large misfits to the age spectrum that arise from relatively small changes in the cooling history the best fit cooling curve for the quxu sample shows a decreasing rate of cooling in the time interval 35 18 ma followed by a very rapid cooling beginning at about 18 ma we have used a thermal model for the conductive cooling of an unroofing pluton to estimate the rate of unroofing required to explain the quxu cooling curve and find that in the 35 20 ma time interval the primary control of the thermal evolution is the conductive loss of magmatic heat with little or no unroofing unroofing rates of approximately 0 05 mm yr followed by a brief period 5 ma of very rapid unroofing with rates of order 2 mm yr

unroofing of the himalayas a view from apatite fission track analysis of bengal fan sediments apatite fission track analysis of early miocene to quaternary distal bengal fan sediments from odp leg 116 provides a unique perspective on denudation processes associated with the tertiary himalayan orogeny these sediments yield pooled apatite fission track ages that are 0 to 10 m y older than depositional ages mean track lengths range from 13 2 to 14 8 mu m these young ages and long track lengths indicate that rocks in the upper crust of bengal fan source areas cooled rapidly ge 15 degrees c m y implying denudation rates of several hundreds of meters per million years these observations support the hypothesis that nearly continuous unroofing of a large source area similar to the dimensions of the present himalayan and southern tibetan plateau deformation front and an efficient transport system similar to the present himalaya ganges brahmaputra system supplied sediment to the bengal fan since the early miocene

lithostratigraphy and vertebrate biostratigraphy of the early miocene himalayan foreland zinda pir dome pakistan abstract deposits in the sulaiman foothills and zinda pir dome in west central pakistan provide new insight into the critical early miocene record of himalayan foreland sedimentation and paleobiology the chitarwata formation which underlies the vihowa formation in the sulaiman foothills predates the siwalik deposits on the potwar plateau of north central pakistan and provides a record of mammals spanning the interval between the early miocene bugti fauna and middle miocene to pleistocene siwalik faunas siwalik deposits on the potwar plateau are no older than middle miocene they represent fluvial environments in contrast the chitarwata and lower vihowa formations in the zinda pir dome represent coastal delta plain and fluvial environments respectively biostratigraphic information from the chitarwata formation coupled with paleomagnetic data reported by friedman et al 1992 from coincident strata suggest that coastal environments persisted in the area of the sulaiman foothills until about 18 6 ma when they were replaced by fluvial environments probably representing the ancestral indus river system apparently during the early miocene when sediments of the chitarwata formation were accumulating on the western portion of the himalayan foreland basin much of the area of the potwar plateau to the north was being eroded the overlying vihowa formation along with the relatively contemporaneous kamlial formation on the potwar plateau represent the appearance of widespread terrestrial sedimentation in the himalayan foreland basin

p wave velocity anomalies between the tonga subducting slab and the overlying mantle beneath the lau basin during a 3 week period in november december 1984 nine ocean bottom seismograph obs stations were operating in the lau basin concurrently with a land based station in the tongan island of vava u vav data from two intermediate depth 200 km 400 km earthquakes in the tonga benioff zone allowed the p wave velocity contrast between the subducting slab and the overlying mantle beneath the lau basin to be determined based on relative residuals obs vav and on three dimensional ray tracing a p wave velocity contrast of 7 is inferred this is an average contrast for depths above 400 km the standard error of the estimates is of the order of 2 the data are consistent with a hot upper mantle beneath the lau basin where temperatures are close to the solidus temperature but they do not necessarily require the presence of partial melt in the asthenosphere beneath the lau basin

fractures in sandstone reservoirs with ultra low permeability a case study of the upper triassic yanchang formation in the ordos basin china the upper triassic yanchang formation in the ordos basin central china is a typical sandstone reservoir with an ultra low permeability high angle tectonic fractures and diagenetic fractures such as near horizontal bedding fractures intragranular fractures and grain boundary fractures are abundant fractures are major pathways and enhance fluid flow in sandstone reservoirs with ultra low permeability because of their weak lateral continuity and their small apertures under lithostatic pressure bedding fractures make a relatively small contribution to the overall permeability of reservoirs as they are both of small size and low permeability intragranular fractures and grain boundary fractures mainly improve the connectivity of reservoirs by connecting the matrix pores high angle tectonic fractures control the fluid movement in the ultra low permeability reservoirs under the effect of different successive tectonic stress fields four assemblages of high angle tectonic fractures developed in the sandstone reservoirs under the present day stress differently oriented fractures have different connectivities apertures and permeabilities the northeast southwest fractures parallel to the maximum principal stress have a good connectivity large apertures and a high permeability forming the dominant flow paths knowledge of these paths can be used for optimizing well placement because of their sensitivity to pressure fractures in different directions will show varying apertures as the formation pressure decreases therefore the permeability of the fractures of different orientations and its impact on reservoirs vary at different developmental stages

cooling history of the nw himalaya pakistan fission track and 40ar 39ar cooling ages indicate that the late tertiary cooling history of the himalayan ranges of northern pakistan is largely a function of uplift and erosion interpretation of cooling ages which range from under 0 5 ma to over 80 ma suggests that during the late tertiary long term uplift rates at least doubled from under 0 2 mm yr to in some cases well over 0 5 mm yr uplift rates show strong and systematic regional variations as well which reflect the greater uplift of eastern and northern regions the association of very rapid uplift and erosion with the nanga parbat haramosh massif can be explained by a locally vigorous collision of india with eurasia near a promontory of indian crust the resultant rapid uplift of the nanga parbat haramosh massif reactivated the main mantle thrust melange zone with a reversed sense of motion discontinuities in the coolingage distribution along the main mantle thrust in the southern swat hazara region may be the result of the thermal effects of overthrusting

observations of intratidal variability of flows over a sill contraction combination in a chilean fjord underway velocity measurements were carried out for the first time in a chilean fjord using an acoustic doppler velocimeter with the purpose of elucidating the intratidal variability of flows through a pass paso galvarino the pass included a sill where the bottom sloped by roughly 30 and a coastline contraction of 90 the relatively small dimensions of the pass allowed for rapid sampling of the flow evolution throughout the tidal cycle the backscattered sound signal from the velocimeter and from an echo sounder were used to describe the vertical excursions of the pycnocline throughout the domain and to identify regions of enhanced vertical mixing within the pass the spatial variability of the flow in the pass was consistent with uniform two layer flow at the narrowest section of the contraction the pycnocline dropped sharply around both maximum flood and maximum ebb while the flow accelerated downstream relative to the tidal flow the slope of the pycnocline changed sign from flood to ebb which was atypical of other fjord observations but could be explained by the transitions from subcritical to supercritical flow these transitions switched location at either side of the narrowest section of the contraction leeward of this section increased sound backscatter suggested intensified turbulence that extended over a greater area during ebb than during flood because the distance between the point of pycnocline drop and the end of the pass was longer during ebb enhanced vertical mixing within the pass was reflected in the tidally averaged fields by a three layer flow that consisted of near surface and near bottom flow converging toward the pass and flow around the pycnocline diverging away from the pass

observations of turbulence caused by a combination of tides and mean baroclinic flow over a fjord sill thisstudyinvestigates thedissipation ratesandflow conditionsatthedrobaksillinthe oslofjord thearea was transected 13 times with a free falling microstructure shear probe during 4 days in june 2011 at the same time an adcp was deployed inside the sill during most tidal cycles internal hydraulic jumps with high dissipation rates were foundon the downstreamside of the sill however the internal responsevariedstrongly betweendifferenttidal cycleswith similar barotropicforcing in the beginning of the observational period ebb tides had no hydraulic jumps and in the end one of the flood tides did not have a hydraulic jump during the same period the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer the authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation this conclusion is supported by two layer hydraulic theory the volume integrated dissipation rates within 500m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the bernoulli drop under the assumption of no energy loss in the upper layer finally the mean dissipation rateatthesillwascomparedwiththeradiationofinternaltidalenergyawayfromthesill anditwasfoundthat about 60 90 of the total energy loss was dissipated locally

a lithospheric seismic refraction profile in the western north atlantic ocean summary a study of the seismic velocity structure of the subcrustal oceanic lithosphere was conducted in the western north atlantic north of the lesser antilles where the age of the lithosphere is 80 95 ma a 1000km long north south array of 18 ocean bottom seismographs obs spaced at 60 km intervals recorded arrivals from explosions at ranges up to 800km and arrivals from earthquakes in the northern caribbean region at ranges up to 1400 km analysis of the travel time and amplitude data from the explosions provided the velocity structure down to about 65 km below sea level the earthquake data were intended to extend the travel times to ranges over 1100 km and thus sample the deeper part of the lithosphere however the geometry and velocity structure of the downdipping slab beneath the puerto rico trench deflected the ray paths between the caribbean hypocentres and the obs array such that no rays travelling through the lower lithosphere were detected by the array only rays through the upper lithosphere reached the obs array the results show that the subcrustal oceanic lithosphere consists of two main regions the uppermost region starts just below the m discontinuity and exhibits velocities of 8 2 8 4 km s 1 this region is characterized by three alternating zones of low 8 2 km s 1 high 8 4 km s 1 and low 8 2 km s 1 velocity the second region lies below 53 km and exhibits high velocities of 8 5 8 6 km s 1 a sharp boundary separates these two regions suggesting that a major change in composition or phase occurs there the velocity structure within the upper region shows broad agreement with petrological models for the generation of oceanic lithosphere starting from a pyrolite composition the alternating zones of low and high velocity correspond to zones depleted of and enriched in the basaltic fraction of the primary mantle material respectively this is a consequence of the processes of migration and differentiation of the upwelling asthenosphere near the mid ocean ridge axis however none of the petrological models based on a pyrolite or peridotite initial composition can easily explain the high velocities of over 8 5 km s 1 observed for the lower region of the seismic velocity model we speculate that this region may consist instead primarily of eclogite which is acceted from the asthenosphere as the lithosphere cools and ages the results do not permit definition of the lower boundary of the lithosphere which probably lies below the 65 km maximum depth resolved in this study s wave observations were less reliable than the p wave data but generally support the p wave velocity model no evidence was found for azimuthal anisotropy within the upper region of the subcrustal lithosphere however the existence in the western north atlantic of a weakly anisotropic or even isotropic lower lithosphere with a significant component of eclogite requires verification by future experiments

variable gas content saturation and accumulation characteristics of weibei coalbed methane pilot production field in the southeastern ordos basin china using diverse geologic and geophysical data from recent exploration and development and experimental results of analysis of gas content gas capacity and gas composition this article discusses how geologic structural and hydrological factors determine the heterogeneous distribution of gas in the weibei coalbed methane cbm field the coal rank of the pennsylvanian no 5 coal seam is mainly low volatile bituminous and semianthracite the total gas content is 2 69 to 16 15 m3 t 95 00 570 33 scf t and gas saturation is 26 0 to 93 2 burial coalification followed by tectonically driven hydrothermal activity controls not only thermal maturity but also the quality and quantity of thermogenic gas generated from the coal gas composition indicates that the cbm is dry and of dominantly thermogenic origin the thermogenic gases have been altered by fractionation that may be related to subsurface water movement in the southern part of the study area three gas accumulation models are identified 1 gas diffusion and long distance migration of thermogenic gases to no flow boundaries for sorption and minor conventional trapping 2 hydrodynamic trapping of gas in structural lows and 3 gas loss by hydrodynamic flushing the first two models are applicable for the formation of two cbm enrichment areas in blocks b3 and b4 whereas the last model explains extremely low gas content and gas saturation in block b5 the variable gas content saturation and accumulation characteristics are mainly controlled by these gas accumulation models

p wave travel times from the tonga subduction zone to stations on the oceanic side of the tonga kermadec trench p waves propagating from shallow earthquakes in the tonga kermadec seismic zone to regional stations in raratonga and niue are associated with large negative travel time residuals observed minus jeffreys bullen time of the order of 10 s for focal depths below 100 km the magnitude of residuals decreases sharply to about 2 5 s for depths near 200 km the pattern of residuals is explained by a model of oceanic lithosphere possessing an upper mantle p wave velocity of 8 3 km s and having its base at a depth near 100 km

resistance to barotropic tidal flow in straits by baroclinic wave drag energy transfer from barotropic tides to baroclinic motions may take place at the ends of straits connecting stratified basins implying generation of internal waves propagating into the basins different aspects of this have been described in the literature including quantification of the energy transfer studies of the resulting internal tides and the relationship to diapycnal mixing in the basins however the accompanying resistance to oscillatory barotropic strait flows by so called baroclinic wave drag has not been implemented earlier in models for strait flow in the present paper a formulation for the instantaneous baroclinic wave drag force is suggested this leads to a linear relationship between the strait flow and the synoptic sea level difference between the adjacent basins the model is used to study tidal response in land locked basins the response is computed for three fjords where the resistance to the barotropic flow comes essentially from baroclinic wave drag it is concluded that the strait flow model handles baroclinic wave drag in a satisfactory way since the observed responses are well predicted by the model baroclinic wave drag should provide the major resistance to surface tides in the deep ocean which indicates its importance in a wider context several mechanisms may cause resistance to barotropic flow in straits and channels friction against the sea bed is certainly the most commonly considered type of flow resistance it is often modeled with a quadratic relationship between the bottom stress and the barotropic velocity using a drag coefficient to characterize the frictional properties of the sea bed another type of flow resistance is due to large scale longitudinal variations of the vertical cross sectional area of the strait causing contraction followed by expansion of the flow this class of flow resistance appropriately termed barotropic form drag includes the resistance caused by contraction expansion induced by the ends of straits having smaller vertical cross sectional areas than the adjacent basins in the cases of frictional drag and largescale form drag barotropic energy is transferred to turbulence in the bottom boundary layer in the strait and to large scale eddies in the expanding current downstream of the strait respectively the present paper however draws attention to a third class of resistance operating on oscillating barotropic strait flow it is due to generation of baroclinic internal waves in the adjacent stratified basins by the ends of the strait and is

statistical analysis as a tool for assisting geochemical interpretation of the upper triassic yanchang formation ordos basin central china abstract the yanchang formation in the ordos basin is the most important petroleum play not only for conventional oil and gas accumulations but also for newly emerging shale oil and tight gas resources the molecular characterization of the basinwide source rocks predicts three groups of generative petroleum types paraffinic high wax oil mixed base paraffinic naphthenic aromatic low wax oil and gas and condensate supplementary to previous work 68 samples including the crude oils source bitumens and reservoir extracts from the yanchang petroleum play are analyzed the distribution of two terpane classes eight tricyclic terpanes and eight pentacyclic terpanes are determined with subsequent simultaneous rq mode factor analysis for a composite data set of these samples alongside 216 published crude oils worldwide with known facies descriptions thermal maturity has been evaluated as a consistent distribution at first using a combined method of a maturity related biomarker ts ts tm and aromatic parameters methyldibenzothiophene ratios to alleviate the maturity differences effect when discussing geochemical characterization the r mode factor analysis consists of the first two factors that are describing 45 present of the cumulative total variance in the data set and presents a sample grouping pattern in q mode factor analysis which is determined by different contributions of terpane associations i e the tricyclic c21 coupled with pentacyclic c26 c27 c28 and c30 in the same factor space three terpane associations the c26 and c28 terpanes the c21 and c30 terpanes and the c27 pentacyclic terpenes are respectively responsible for discriminating crude oil reservoir extracts and source bitumens in rq mode factor analysis molecular compositions further address more detailed interrelationships among three sample groups that crude oils and reservoir extracts are sharing close genetic relationships both in depositional environment typing and c27 c28 c29 sterane distribution samples from source rocks vary much significantly a mixing process which occurs after oils has been expelled from host source rocks into carrier units during accumulation in addition the migration contamination of c29 sterols when oils are cross through the chang 7 2 unit along migration pathways might also explain this lack of correlation between source rocks and oil reservoir system

paleoproductivity of the chang 7 unit in the ordos basin north china and its controlling factors abstract primary paleoproductivity provides basic organic matter for the formation of high quality hydrocarbon source rock a progressive method to quantitatively calculate paleoproductivity on the basis of total organic carbon toc has been proposed to evaluate the contribution of primary paleoproductivity to a hydrocarbon source rock with high organic matter abundance in this study pyrite contents organic maceral compositions and high resolution sedimentation rates srs were used to recover the amount of organic matter oxidation and to exclude the effect of terrestrial clastic input during the deposition of the chang 7 sediment multiple calculation methods were used to quantitatively reconstruct paleoproductivity and the results suggest that the ordos basin was a eutrophic lacustrine basin approximately 2 104 mg c cm 2 kyr 1 to 10 104 mg c cm 2 kyr 1 during the chang 7 stage compared with modern lake productivity the paleoclimatic analysis indicates that a warm humid paleoclimate was beneficial for increasing the weathering intensity which could have provided more nutrient inputs to the depositional environment with incoming terrestrial debris in addition the plate collision between the north china block and yangtze block generated moderate volcanic ash that spread to the study area and provided large amounts of essential nutrients for algae blooms moreover a brackish water condition promoted upward mixing of deep water which inhibited water mass stratification and then facilitated primary paleoproductivity in general the chang 7 high quality hydrocarbon source rock is a result of increased paleoproductivity resulting from suitable paleoclimate conditions brackish water masses and abundant essential nutrient inputs

upper mantle anisotropy in the s w pacific from earthquake travel time analysis abstract the negative p residuals from readings at stations on niue rarotonga and the chatham islands of shallower events along the tonga kermadec trench and below the north island new zealand exhibit a scatter too large to be accounted for solely by observation and mislocation errors the corresponding wavepaths cover a range of propagation azimuths between n30 e and n180 e travel times from a large number of events reveal a variation of p velocity with azimuth which is consistent with large scale upper mantle anisotropy in this area the p velocity varies from a maximum of 8 4 km s 1 along n62 e to a minimum of 7 8 km s 1 along a perpendicular axis perhaps indicating a fossil spreading direction along n62 e there is also a very consistent but unexplained variation of p residuals with earthquake depths between 0 and 100 km an observation warranting further study

mapping of p wave slab anomalies beneath the tonga kermadec and new hebrides arcs abstract p wave velocity anomalies of several per cent fast relative to the jeffreys bullen jb model are seen around subduction zones under the tonga kermadec and new hebrides trenches from inversion of international seismological centre isc travel time residuals from intermediate and deep focus earthquakes mantle heterogeneities outside the model are corrected using previous velocity models ray paths inside the model region are traced allowing two dimensional velocity variations the rays of similar source and receiver locations are combined into summary rays and the hypocenters are relocated as part of the inversion iterations the resulting images are interpreted in conjunction with resolution and error estimations the slab like anomalies are generally continuous in places of high seismicity and decrease in amplitude with depth most coherent slab anomalies are surrounded by large slow patches between the surface and 350 km depth the fast slab beneath the tonga arc appears to end at 550 km depth before the seismicity ends but there is a fast band at 750 1000 km depths below the deepest tongan earthquakes the fast slab like anomaly beneath the kermadec arc tends to flatten to subhorizontal near the bottom of upper mantle and it occurs aseismically near new zealand down to 500 550 km depth interpretations in the lower mantle in many places such as kermadec and new hebrides arcs however are hampered by poor resolution

thermal evolution of rifted continental margins new evidence from fission tracks in basement apatites from southeastern australia the palaeozoic granitic basement of southeastern australia shows a regular pattern of apatite fission track ages which is dominated by a rapid decrease in apparent age towards the rifted continental margin apparent ages away from the margin vary between about 230 and 360 myr but drop sharply to minimum values of around 80 myr near the coast just south of the sydney basin and 150 175 myr elsewhere along the coast fission track length information allows the age pattern to be interpreted as the result of partial to complete resetting of the older ages by a widespread thermal event at about 80 100 myr this event accompanied the rift phase of passive margin development and peaked just before the onset of sea floor spreading in the adjacent tasman sea the zone of reduced apatite ages is deeply embayed around the gippsland basin supporting the suggestion that it originated as a cretaceous failed arm of the tasman rift system the thermal event has affected apatites up to about 130 km from the margin and has produced a characteristic set of bimodal track length distributions in apatites with partially reset ages the bimodality gradually disappears towards the oldest apatite ages which have negatively skewed unimodal distributions the youngest apatites also tend towards unimodal length distributions indicating that thermal annealing of pre existing tracks in these samples went almost to completion maximum temperatures reached in samples presently exposed in the coastal area were thus probably about 110 120 c thermal activity was accompanied by considerable uplift across the region followed by gradual erosion to expose the present land surface post breakup erosion was generally greater towards the new coastline with near coastal samples being previously buried a probable 1 5 2 5 km

resource potential and core area prediction of lacustrine tight oil the triassic yanchang formation in ordos basin china unlike liquid rich hydrocarbons in marine shale systems in north america the triassic yanchang oil is preserved in tight sandstone reservoirs deposited in a lacustrine setting the tight reservoir covers an area of 30 000 km2 11 600 mi2 and is characterized by ultralow permeability 0 001 1 00 md 0 001 10 3 1 00 10 3 m2 a low subsurface pressure coefficient 0 6 0 8 and low single well yield several tons 10 40 bbl to 10 tons 70 bbl per day the reservoir also features a high content of quartz feldspar and carbonate averaging 70 80 and low crude oil density of 0 84 g cm3 36 5 api with an oil saturation of approximately 60 80 for the first time this study employs the assessment of petroleum in place resources based on the genesis and statistics predicting method gspm and estimated ultimate recovery method for the tight oil in the ordos basin the advantage of gspm over previously reported methodologies is the suitable combination of genetic and statistical methods to estimate proximally sourced tight oil the results show that the total in place resource potential of the investigated tight oil is approximately 42 26 108 t 295 82 108 bbl with an average abundance of 15 104 t km2 99 104 bbl mi2 the recoverable resources for the 50 probability of the chang 7 tight oil accumulation is 3 11 108 t 21 77 108 bbl in the ordos basin the top two grades of the chang 7 tight oil core area cover the total area of three layers comprising 12 745 km2 4921 mi2 with 22 51 108 t 157 57 108 bbl of in situ resources furthermore the four most promising zones have been identified

upper paleozoic petroleum system ordos basin china abstract the ordos basin is a typical lapped basin including three sequences of strata early paleozoic late paleozoic and mesozoic with a total thickness of 4000 6000 m impermeable sealing beds are well developed at the top and base of the upper paleozoic sequence separating it from the mesozoic and the lower paleozoic strata to form an independent petroleum system in this petroleum system the source rocks are widely distributed coals and dark mudstones occurring in the carboniferous lower permian coal measures with a thickness of 10 15 and 40 60 m respectively the reservoirs are mainly early permian tight sandstones mostly with a porosity of 4 8 and a permeability of 0 1 1 0 10 3 m2 the regional cap rock is a 100 150 m thick mudstone in the upper permian strata the structural framework of the basin is a huge asymmetric syncline dipping gently toward the east and north and steeply toward the south and west well data show that gas saturated gas water transition and water saturated zones are developed from the depositional center to the basin edges the gas saturated zone mainly lies in the gently dipping slope area of the shanbei slope toward eastern and northern up dip directions the water gas transitional zone occurs and finally the water saturated zone presenting a reverse relation of water on top of gas an abnormal negative strata pressure is developed in the gas bearing area with a pressure coefficient cp ranging from 0 83 to 0 95 fluid inclusion data indicate that the upper paleozoic gas pool began to develop around the wuqi area at about 150 ma and it extended toward the north and was largely formed at about 120 ma showing there was a regional migration of the gas water interface from south to north during the gas pool formation these characteristics appear to show that the northern and eastern margins of the petroleum system are defined by a regional hydrodynamic regime the critical moment of the petroleum system occurred during the early cretaceous the basin experienced a rapid subsidence and a significant geothermal event resulting in the source rocks being matured into the main stage of gas generation to form a huge amount of gas the gas migrated into the tight sandstone reservoirs and displaced the rock pore water to form the gas pools and developing the regional hydrodynamic trap

three dimensional imaging of fracture propagation in tight sandstones of the upper triassic chang 7 member ordos basin northern china abstract the characteristics of fracture propagation in heterogeneous tight sandstones are critical to volumetric fracturing which is the key to unlocking unconventional resources in tight sandstones quantification of the influence of pre existing pore systems and particle arrangements on the propagation of fractures is challenging due to inadequate imaging of the internal void systems in tight sandstones from three dimensional 3d aspect in this study the 3d geometry of tight sandstones from the chang 7 member in the ordos basin is continuously imaged under different loading stresses and the voxel resolution of the x ray computed tomography is 2 5 m the data set captured in this process shows the changes in the samples at the microstructural level as they approach fracturing the data are stored as a time series of 3d images the results demonstrate that i fractures propagate progressively and gradually link with pre existing pores resulting in macroscopic fractures with a maximum width of 250 m while newly generated fractures could break up particles and may not follow the line of pre existing fractures and ii three stages were identified in the failure process of tight sandstones with new fractures running at an angle of about 30 to the general direction of the stress of compression the total volumes of both the sample and pore fractures and the damage index which were extracted from the 3d images all increased when approaching fracturing the final volume of pore fracture systems could be 11 times that of the initial pore volume all of these observations provide valuable insights and design guidelines for hydraulic fracturing in unconventional tight sandstones and a quantified model of the dynamics and the morphology of fracture propagation with increasing stress approaching failure which may shed light on dynamic critical transitions in the earth s crust

p wave travel times from deep and intermediate depth earthquakes to local seismic stations and the subducted slab of oceanic lithosphere beneath the tonga island arc to investigate the seismic structure of the tonga subduction zone mobile seismic stations were temporarily operated on the four main islands of the tonga group data from intermediate depth and deep earthquakes were analyzed using p wave arrival times only to improve on single event locations as published in bulletins a joint hypocentral determination method was applied to three groups of deep earthquakes and two groups of intermediate depth earthquakes the resulting p wave travel time residuals observed in the tonga island arc are generally negative and reveal a characteristic pattern a notable feature is the observation that residuals from deep earthquakes located near 18 s are consistently less negative than residuals from deep earthquakes located south of 20 5 s it cannot completely be ruled out that part of this difference is caused by systematic depth errors that may arise as a result of slab propagation effects on the upgoing p leg of pp but only a minor part of the observed difference can be explained in this way the major part is probably caused by two factors first p waves traveling from the southern part of the tonga subduction zone to the northernmost stations located in the islands of vava u and ha apai spend more time in the high velocity slab than p waves originating in the northern part of the tonga seismic zone second ray tracing through a number of models of the subduction zone suggests that p waves from deep earthquakes located south of 20 5 s have their propagation paths close to the high velocity core of the slab whereas p waves from deep and perhaps also from intermediate depth earthquakes located near 18 s are likely to miss parts of the high velocity slab on their way up to the island arc this is most pronounced for the seismic station tga located in the island of tongatapu which is some 30 km further away from the trench than the other stations of the network the magnitude of residual differences 2 3 s at tga observed from different groups of deep earthquakes is best explained by a sharp bend in the dip of the subducting plate in the northern part of tonga near a depth of 200 km ray tracing through such a model shows that p waves from deep earthquakes near 18 s traveling to tga escape the slab at a depth of about 250 km and from there on propagate through the mantle overlying the slab if the interpretation is correct p wave velocities are indicated that are 5 7 higher in the slab above 250 km than in the overlying mantle independent of this interpretation the magnitude of negative residuals from deep earthquakes south of 20 5 s is best matched by velocities in the slab that are uniformly higher by 6 relative to the herrin model in another model where p wave velocities vary linearly over horizontal slab width from maximum values in the seismic zone to the velocities in the herrin model at the slab boundaries the data are equally well fitted if the velocity anomaly reaches a maximum value of 8 relative to the herrin model allowance for a crustal correction term of 0 5 s reduces these estimates by 1 modification of the velocity models to include the case of a velocity anomaly in the slab that decreases linearly with depth to 0 over the range from 400 to 700 km increases the estimates by 1 the data do not allow a proper assessment of the question of whether seismic velocities approach normal mantle values at depths below 400 km or remain substantially higher than in standard models of the earth

seismological study of the elevation of phase boundaries within the subducting lithospheric slabs abstract the depths of phase transformations corresponding to major velocity discontinuities differ in the subducting lithospheric slabs from those in the surrounding mantle mainly due to temperature differences we interpreted sets of p and s wave travel time observations which are anomalously late produced by 36 deep earthquakes in the tonga kermadec subduction zone as reflections from the two major velocity discontinuities in the surrounding mantle which are relatively deep and correspond to those independently obtained at the relatively shallow depths of 410 km and 600 km in the subducting slab in this region by kaila and krishna the inferred depths in the surrounding mantle for these two discontinuities respectively vary within 490 560 km and 630 680 km this implies that these discontinuities are relatively elevated by 80 150 km to 410 km depth and by 30 80 km to 600 km depth in the subducting slab in the tonga kermadec region these estimates of the elevation of the discontinuities within the subducting slab are in general agreement with those proposed by ringwood and others the present study thus provides new seismological evidence for the elevation of velocity discontinuities within the subducting slabs the velocity models considered in the present study also indicate lithospheric slab penetration well below the 600 km seismic discontinuity in the tonga kermadec region

the energetics and tidally induced reverse renewal in a two silled fjord a laterally integrated two dimensional numerical model is used to examine the influence of the m2 tide on the circulation in the saguenay fjord a two silled fjord with a large inner and a small outer basin located on the north shore of the st lawrence estuary it is found that the m2 tide is more vigorous in the outer than in the inner basin and that more vertical mixing occurs in the outer basin therefore the density at depth in the outer basin decreases faster than it does in the inner basin and the resulting horizontal pressure gradient causes a bottom flow of water from the inner to the outer basin across the inner sill this reverse renewal is evident in both the available observations and the simulation according to the model much of the m2 energy withdrawn from the surface tide is fed into the internal tide significant tidal energy is also advected by the mean flow velocity approximately 25 of the net energy flux into the fjord is dissipated within 2 km of the outer sill because of the baroclinic pressure i e the total pressure but with the influence of the surface displacement removed the subtidal circulation is associated with very large energy fluxes within the fjord these fluxes are much larger than the net rate at which tidal energy enters the fjord and they are greater than the subtidal advective energy flux they represent a large redistribution of energy within the fjord and their horizontal divergence along the fjord is almost in balance with the time rate of change of potential energy when a flux richardson number is calculated for the inner basin below sill depth a value of 0 075 is obtained which is close to values found for other fjords

a comparative study between present and palaeo heat flow in the qiangtang basin northern tibet china the qiangtang basin is a significant prospective area for hydrocarbon and gas hydrate resources in the tibetan plateau china however relatively little work has been performed to characterise heat flow in this basin which has restricted petroleum and gas hydrate exploration in this study we compare present and palaeo heat flow in the qiangtang basin to provide information on geothermal regime hydrocarbon generation and permafrost that is necessary for further petroleum and gas hydrate exploration we base our study on temperature data from a thermometer well thermal conductivity tests vitrinite reflectance data homogenisation temperature data from fluid inclusions stratigraphic information and a time independent modelling approach our results indicate that in the central qiangtang basin the present thermal gradient is approximately 15 5 c km and heat flow is approximately 46 69 mw m 2 heat flow in the qiangtang basin is not relatively stable since the early jurassic as previous research has suggested and it is generally decreasing with time additionally there is a clear difference between the hottest thermal regime of the southern and northern qiangtang depressions during cretaceous to pleistocene time in the southern qiangtang depression the palaeogeothermal gradient is approximately 32 0 c km and palaeo heat flow is approximately 70 mw m 2 however in

subtidal water flux through a multiple inlet system observations before and during a cold front event and numerical experiments 1 this paper examines the net transport through a multiple inlet bay under a combined force of strong wind and tide with observations and a model experiment the observations were made in central georgia in sapelo and altamaha sounds between 13 and 17 september 2000 wind was weak in the beginning of the survey an air pressure trough as a weak cold front passed the area on 16 september when the wind changed to the northeast and increased in magnitude this front was associated with a midlatitude cyclone in the new england area this weather event with an episode of strong northeasterly winds prompted a numerical model experiment on an idealized three inlet bay with a set of nonlinear 2 d shallow water equations on an f plane which provides some insight to the wind driven circulation under the presence of tidal forcing it is found that tidally induced currents are small compared to wind induced flows when the wind direction is not perpendicular to the alignment of the three inlets the net outward flow tends to occur at the inlet farther away in the downwind direction this is associated with a net inward transport in the inlet opposite of the downwind direction as a result the middle inlet has the minimum of the net flow when the wind is perpendicular to the barrier islands and if the three inlets have different maximum depth values the deeper inlet tends to have a net flow against the wind while the shallower inlet tends to have a net flow in the direction of the wind offshore onshore currents may develop outside of the inlet with outward inward flow as an effect of fluxes through the inlets on the coastal ocean

thermal and tectonic history of selected taranaki basin new zealand wells assessed by apatite fission track analysis apatite fission track analysis afta has been applied to samples from four hydrocarbon well sections to study the thermal and tectonic history and the hydrocarbon prospectivity of the southern part of the taranaki basin new zealand data from three of the wells 1 fresne 1 north tasman 1 surville show that the successions were exposed to higher temperatures in the past through deeper burial cooling from elevated paleotemperatures was effected by late miocene uplift and erosion of 3 0 or 0 3 km of section in 1 fresne or 2 0 or 0 5 km in 1 surville and 1 0 or 0 3 km in 1 north tasman in the fourth well 1 kupe formations are currently at their maximum temperatures since deposition afta provides unique constraints on the timing of hydrocarbon generation in relation to trap formation the proposed source rocks in 1 fresne passed through the oil formation window 100 150 degrees c and into the zone of gas production 150 220 degrees c during the middle miocene prior to the formation of potential trapping structures those in 1 north tasman passed into the oil formation zone about the same time and source rocks in 1 surville have probably never been heated enough to produce oil afta indicates considerable prospectivity remains in the region of 1 kupe where generation would have occurred after trap formation

characteristics of micropores pore throats and movable fluids in the tight sandstone oil reservoirs of the yanchang formation in the southwestern ordos basin china thin section analysis field emission scanning electron microscopy constant rate mercury injection crmi x ray computed tomography x ct and nuclear magnetic resonance nmr were used to investigate pore systems pore sizes and pore throat distributions and determine evaluation criteria for tight oil reservoirs this research shows that the average porosity of the reservoirs in the chang 63 to chang 72 members of the yanchang formation of the ordos basin ranges from 4 to 16 with a corresponding average permeability of 0 1 to 0 4 md the main pore types in these tight oil reservoirs are dissolution pores and intergranular pores and the main pore throat types are laminated throats and control shaped throats based on experimental results the pore throat distributions differ among different testing techniques the results of this study indicate that a combination of crmi derived and x ct detected pore throat distributions is most suitable for calibrating nmr derived pore sizes over the full range of pore sizes the lower pore throat radius limit of effective reservoirs is 0 24 m and the lower permeability limit for the flow of movable fluid is 0 008 md based on a cluster analysis the studied tight oil reservoirs can be divided into three types compared with type iii reservoirs porosity 11 and permeability 0 15 md and type ii 8 porosity 11 and 0 05 md permeability 0 15 md reservoirs are of relatively good quality

rapid early miocene acceleration of uplift in the gangdese belt xizang southern tibet and its bearing on accommodation mechanisms of the india asia collision abstract five samples from a biotite hornblende granodiorite phase of the 42 5 ma quxu pluton gangdese batholith southern tibet have been collected at 250 m vertical intervals biotite from these rocks yields monotonically decreasing40ar 39ar isochron ages with decreasing elevation of 26 8 0 2 23 3 0 5 19 7 0 3 18 4 0 4 and17 8 0 1ma tc 335 c coexisting k feldspars have virtually identical minimum apparent40ar 39ar ages of 17 0 0 4ma tc 285 c these data indicate parts of southern tibet experienced a pulse of uplift in the early miocene with the rate of uplift rising from 0 07 to 4 4 mm year in the interval 20 to 17 ma an apatite fission track age of 9 9 0 9ma from this locality constrains the average uplift rate at this site to about 0 81 mm year between 17 and 9 9 ma and 0 30 mm year from 9 9 ma to present k feldspar from the dagze granite 30 km to the east near lhasa yields a minimum apparent40ar 39ar age of 35 9 0 9ma tc 227 c which indicates an average uplift rate there of 0 21 mm year since then the marked pulse of uplift of the quxu granodiorite and the difference in uplift history between the dagze and quxu plutons suggests southern tibet has experienced discrete pulses of uplift variable in both space and time these data are not consistent with models which require a large proportion of uplift of the tibetan plateau to have occurred in the last 2 ma the data support the suggestion that convergence between india and asia was largely accommodated by tectonic escape during the opening of the south china sea 32 to 17 ma ago and permit distributed shortening as a mechanism for crustal thickening and uplift of this part of the tibetan plateau subsequent to 20 ma

prediction of diagenetic facies using well logs a case study from the upper triassic yanchang formation ordos basin china abstract understanding diagenetic heterogeneity in tight sandstone reservoirs is vital for hydrocarbon exploration as a typical tight sandstone reservoir the seventh unit of the upper triassic yanchang formation in the ordos basin chang 7 unit central china is an important oil producing interval results of helium porosity and permeability and petrographic assessment from thin sections x ray diffraction scanning electron microscopy and cathodoluminescence analysis demonstrate that the sandstones have encountered various diagenetic processes encompassing mechanical and chemical compaction cementation by carbonate quartz clay minerals and dissolution of feldspar and lithic fragments the sandstones comprise silt to medium grained lithic arkoses to feldspathic litharenites and litharenites which have low porosity 0 5 13 6 with an average of 6 8 and low permeability 0 009 10 3 m 2 to 1 818 10 3 m 2 with an average of 0 106 10 3 m 2 this study suggests that diagenetic facies identified from petrographic observations can be up scaled by correlation with wire line log responses which can facilitate prediction of reservoir quality at a field scale four diagenetic facies are determined based on petrographic features including intensity of compaction cement types and amounts and degree of dissolution unstable and labile components of sandstones can be identified by low bulk density and low gamma ray log values and those sandstones show the highest reservoir quality tightly compacted sandstones siltstones which tend to have high gamma ray readings and relatively high bulk density values show the poorest reservoir quality a model based on principal component analysis pca is built and show better prediction of diagenetic facies than biplots of well logs the model is validated by blind testing log predicted diagenetic facies against petrographic features from core samples of the upper triassic yanchang formation in the ordos basin which indicates it is a helpful predictive model

thermally induced phase changes lateral heterogeneity of the mantle continental roots and deep slab anomalies pressure induced solid solid phase changes are responsible for most of the increase of density and seismic velocity with depth in the upper mantle lateral variations in temperature cause a similar effect abrupt changes of density and seismic velocity due to phase changes are superposed on smaller changes associated with thermal expansion temperature induced isobaric phase changes are as important in explaining various recent geophysical data as are the more familar pressure induced phase changes in cold slabs the equilibrium mineral assemblage contains high density high velocity phases which are not stable in hotter mantle in particular the ilmenite form of mgsio 3 and the spinel form of mg 2sio 4 have broad stability fields in cold mantle which increase the density and velocity of deep slabs to values in excess of those which have been used in geoid and seismic travel time modeling recent arguments for slab penetration into the lower mantle and whole mantle convection are based on thermal models of the slab which ignore the large density and seismic velocity anomalies associated with temperature induced phase changes when these effects are taken into account the geoid and seismic anomalies associated with subducted slabs are consistent with slab confinement to the upper mantle and layered models of mantle convection the seismicity cutoff and evidence for slab thickening at 670 km also favor this style of convection mantle seismic velocities between 200 and 400 km depth in tectonic and young oceanic regions are lower than in shield regions and this is due to the presence of a melt phase and lower velocity high temperature phase assemblages deep 200 km long lived continental roots differing in chemistry from normal mantle are not required when isobaric phase changes are taken into account high velocity subshield mantle is closer to normal subsolidus mantle than is suboceanic mantle which is affected by the presence of high temperature phase assemblages the whole mantle convection thick continental root and deep slab penetration hypotheses are not supported by seismic and geoid data when isobaric phase changes are included in the analysis phase changes are more effective in changing density and seismic velocity than are lateral variations in temperature and composition the lack of correlation of the geoid with ridges shields heat flow and upper mantle velocity variations suggests a low geoid sensitivity to the upper mantle consistent with layered convection

modeling the tonga slab can travel time data resolve a metastable olivine wedge we present the results of detailed modeling of the tonga slab with the goals of determining whether high resolution travel time data 1 can be fit by simple thermal and petrological slab models and 2 can resolve a metastable olivine wedge at depths greater than the equilibrium olivine spinel phase boundary we model arrival times recorded by a 1000 km line of 23 ocean bottom seismometers obs and island broadband seismic stations extending from the lau backarc basin across the tonga trench and onto the pacific plate the data consist of 388 local p wave travel times from 17 deep and 3 intermediate earthquakes recorded during the 3 month obs deployment in late 1994 we locate the events using both local and teleseismic arrival times and apply a relocation operator to the theoretical travel times to simulate the biases introduced in the data by locating the events with a reference earth model the modeling consists of grid searches using a three dimensional finite difference algorithm to compute local first arriving travel times for equilibrium and metastable p wave velocity models constructed from thermal mineralogical and morphological constraints the travel time anomalies are well fit by standard slab thermal models and p velocity temperature derivatives of 0 4 to 0 3 ms 1 c 1 forward calculations indicate that the presence of a metastable olivine wedge has a subtle effect on the travel times due to the tendency of first arriving waves to avoid the low velocity region wedge velocity models provide a slightly better fit to the data than equilibrium models but f tests indicate the improvement is not significant at the 95 level our results suggest that providing direct seismological evidence of a wedge of metastable olivine in subduction zones will require either waveform modeling or the observation of later arriving phases created by the depressed phase boundary

organic matter enrichment of the late triassic yanchang formation ordos basin china under dysoxic to oxic conditions insights from pyrite framboid size distributions abstract pyrite framboids are spherical or ellipsoidal compact aggregates of submicron pyrite microcrystals the various lithologies in the chang 7 member of the upper triassic yanchang formation contain abundant pyrite framboids which provide an opportunity to understand the fluctuation in paleoredox conditions and relative sedimentation rates during deposition the measured diameters of the pyrite framboids in the chang 7 member are characterized by large mean diameters 9 7 16 6 m with a wide distribution range that plots in the area indicative of dysoxic to oxic paleoredox conditions during deposition of the chang 7 sediments based on correlation analysis the strong correlation between paleoredox sensitive trace elements ratios v cr u th and cu zn and the mean size of framboids indicate that the latter can serve as a reliable indicator of paleoredox conditions the results of the framboid size distribution can also be interpreted through crystal size distribution theory csdt according to csdt the relative sedimentation rate of the chang 7 member fluctuated during deposition in parallel with fluctuation in lacustrine levels furthermore the strongly positive correlation between total organic carbon toc and paleoredox conditions indicates that although relatively high sedimentation rate may have contributed to the dysoxic oxic conditions that were harmful to the preservation of organic matter but it may also have brought nutrients to increase productivity similarly the correlation between toc and relative sedimentation rates suggests that the relative sedimentation rates can affect organic matter enrichment in several ways a high sedimentation rate is favourable for the preservation of organic matter which can be prevented from oxidation but large amounts of sediments can dilute the organic matter which reduces the organic matter abundance in a potential source rock

timing and conditions of permian rotliegende sandstone diagenesis southern north sea k ar and oxygen isotopic data illitic clay ranging from pure illite to highly illitic illite smectite i s is the most abundant diagenetic phase in the eolian and sabkha facies of the rotliegende formation of the southern north sea and northeastern netherlands most k ar ages of diagenetic illite and i s in the eolian sandstones are between 100 and 175 ma illite formation is related to two major phases of tectonic activity the jurassic kimmerian orogenic movements and late cretaceous early tertiary inversion movements the burial depth of a sample at the time of diagenetic illite formation can be derived from the k ar age and the appropriate burial history curve illite formed in different localities at different times and at different depths generally diagenetic illite is more abundant in samples more deeply buried at the time of illite formation the correlation between illite formation and tectonic disturbance and the correlation between illite abundance and burial depth may be useful for predicting reservoir properties specifically we would expect reservoir permeability of the rotliegende in the study area to be least impaired by illite growth in those samples less deeply buried from the late jurassic through the late cretaceous calculated dgr18o values of most illite forming fluids lie between 0 and 4 pmil smow indicative of meteoric or marine waters modified by interaction with rock however 18o depleted fluids as low as 5 pmil indicative of a major meteoric component are inferred to have been involved in the formation of illite in the groningen gas field at a time when nearby sections of the rotliegende were eroded and exposed at the surface

attenuation of short period p pcp scp and pp waves in the earth s mantle the parameter t ratio of body wave travel time to the average quality factor q was estimated under various assumptions of the nature of the earthquake sources for short period p pcp and scp phases originating from earthquakes in the fiji tonga region and recorded at the warramunga seismic array at tennant creek northern territory australia spectral ratios were calculated for the amplitudes of pcp to p and of pp to p the data reveal a laterally varying q structure in the fiji tonga region the high q lithosphere descending beneath the tonga island arc is overlain above 350 km depth by a wedgelike zone of high attenuation with an average q between 120 and 200 at short periods the upper mantle farther to the west of the tonga island arc is less attenuating with q between 370 and 560 q is about 500 in the upper mantle on the oceanic side of the subduction zone the t estimates of this study are much smaller than estimates from the free oscillation model sl8 this can be partly explained by regional variations of q in the upper mantle if no lateral q variations occur in the lower mantle a frequency dependent q can make the pcp and sep observations consistent with model sl8 adopting the absorption band model to describe the frequency dependence of q the parameter 2 the cut off period at the high frequency end of the absorption band was determined for different source models with finite corner frequencies the average 2 for the mantle is between 0 01 and 0 10 s corresponding to frequencies between 16 and 1 6 hz as derived from the pcp data and between 0 06 and 0 12 s 2 7 and 1 3 hz as derived from the scp data

mapping the tonga slab we applied residual sphere analysis to 25 intermediate and deep focus earthquakes in the tonga subduction zone and mapped variations in slab strike and dip depth extent and deep lateral advection at all latitudes along the arc as in the northwest pacific travel time data from intermediate and deep focus events are well modeled by simple thermal slab structures and a thermal coefficient of velocity p t of 0 5 0 2 m s k i e peak velocity perturbations of about 3 7 in all regions where the seismicity extends to depths of 600 km or more the shortest thermal slab models that cannot be rejected at the 90 confidence level reach depths of 750 km northern tonga to 850 km southern tonga these models however require a value of p t 1 5 m s k 3 times the value suggested by laboratory studies if the magnitude of p t is constrained to be less than 1 0 m s k the minimum penetration depth varies from 800 km to 900 km the travel time data are equally consistent with slab structures that deform at the base of the upper mantle thickening by a factor of 3 and models that contain no advective thickening inferred slab structure varies considerably throughout the subduction zone and the nearly ubiquitous match between the observed seismicity and detailed undulations in slab strike and dip is evidence that residual spheres do reflect near source heterogeneity travel times in southern tonga 21 5 s 27 s are consistent with and in some cases require slab models that bend to a steeper dip at depth this phenomenon is also observed in parts of the northwest pacific but is not required in central 19 s 21 5 s and northern 13 s 19 s tonga the residual spheres of both intermediate and deep focus events suggest a sharp bend in slab strike at 25 s this feature coincides with a kink in strike observed at all depths in the seismicity and occurs at the intersection of the trench and the louisville ridge northern tonga is the locus of considerable complexity in deep slab structure seismicity strain rate and travel time data are consistent with although they do not uniquely require a slab structure in northern tonga which curves sharply to the west at the base of the upper mantle but which strikes to the north at shallower depths and in the lower mantle the contorted segment would have begun subducting roughly 10 m y ago during a major plate reorganization in northernmost tonga a decrease in the maximum depth of seismicity and inferred slab structure provides evidence for southward shear of deep material relative to the shallow slab compared to data from the northwest pacific where travel time observations also suggest slab penetration below the deepest earthquakes tonga seismicity and residual spheres show more complexity at depth and may indicate more deformation internal to the slab these observations argue against convective models which are rigorously stratified at 650 km depth but they are consistent with a discontinuity in mantle properties such as an increase in viscosity or a small percent variation in chemistry if it merely resists and deforms the descending slabs without acting as a complete barrier to flow

geological process of late paleozoic shale gas generation in the eastern ordos basin china revelations from geochemistry and basin modeling abstract during the meso cenozoic the eastern ordos basin eob underwent a complex tectonothermal process influenced by the destruction of north china craton significantly affecting the thermal maturation of the late paleozoic source rocks tectonothermal modeling and maturation of coal bearing shales were performed in the eob to reveal the geological process of shale gas generation organic geochemical data for shales show that toc values vary from 0 96 wt to 21 36 wt and ro values range from 1 56 to 2 47 as well as that organic matter mainly consists of type iii kerogen indicating excellent potential and sufficient maturity for gas generation thermal modeling suggests that heat flow in the eob significantly increased as a result of intense magmatic activities during the yanshanian orogeny with peak values of 108 and 128 mw m2 in the northern and southern regions respectively consequently great variations in maturation history are seen in different regions of the eob with the taiyuan and shanxi shales reaching the wet gas and dry gas stages respectively in the northern region and all shales reaching the dry gas stage in the southern region hydrocarbons were generated at 237 ma and peaked around 162 ma whereas hydrocarbon expulsion began at 230 ma and peaked at 131 ma rapid subsidence in the triassic brought the shales to the oil generation window and early cretaceous magmatic activities led to rapid maturation of shales again reaching the gas generation stage accordingly extremely high levels of thermal maturity are well documented in hydrocarbons alkane composition and stable carbon isotope ratio gas geochemical data 13c1 vs c1 c2 3 13c1 vs c2 imply that that hydrocarbons are from highly mature thermogenic gas generated mainly from humus and this offers great support for thermal maturation modeling regionally coal bearing shales have been steadily uplifted and eroded since the late cretaceous causing partial destruction of gas reservoirs the tectonothermal evolution and hydrocarbon generation of eob were controlled primarily by magmatic thermal effects resulting from the westward subduction of the pacific plate

characteristics and controlling factors of lacustrine tight oil reservoirs of the triassic yanchang formation chang 7 in the ordos basin china abstract tight sandstone oil reservoirs have received increasing attention in petroleum exploration and exploitation previous research concerning tight oil reservoirs has predominantly focused on marine basins however in china tight oil reservoirs are mostly distributed in lacustrine basins taking the tight oil sandstones of the yanchang formation chang 7 oil reservoir interval the chang 7 for short in the ordos basin as an example five factors are identified that control their formation and occurrence firstly three episodes of oil charging took place during the early cretaceous which were 142 139 ma 128 122 ma and 114 106 ma respectively continuous generation and episodic expulsion led to multiple episodes of oil accumulation secondly widely distributed source rocks toc 1 0 provide hydrocarbons to the chang 7 the distribution of tight oil was controlled by the outer boundary of the source rock distribution while the transition areas between generation expulsion centers were the accumulation and enrichment zones in the chang 7 thirdly with the favorable conditions for retained hydrocarbons massive continuous tight reservoir beds average porosity 7 6 and average permeability 0 15 md developed in the chang 7 capable of forming large scale successive tight oil reservoirs fourthly different source reservoir assemblages have different expulsion patterns and amounts which led to different charging degrees and sizes of tight oil accumulations type iii source reservoir interbeds will be the most favorable target for exploration and exploitation in the chang 7 ordos basin finally under the driving mechanism of non buoyancy migration and accumulation pressure differences between source and reservoir rocks pdbsr serve as the primary expulsion and migration force for crude oil during the main accumulation periods in the chang 7