anisotropy and crustal thickness of northern tibet new constraints for tectonic modelling in 1993 a seismological array was installed on a profile along the golmud lhasa highway from the qaidam basin to central tibet a travel time analysis for local and regional earthquakes was performed in order to better constrain the crustal structure obtained by teleseismic tomography more information about the crustal thickness and anisotropy comes from the analysis of moho converted p to sv waves the two methods supplement each other and confirm the existence of crust thinner than expected accross the northern edge of the tibetan plateau and qaidam p to sv splitting analysis yields a striking similarity between the fast polarization direction and the velocity field of present day deformation as inferred from kinematic modelling the two observations the thinning of the crust and the clockwise rotation of the fast polarization direction emphasize the leading role of eastward extrusion for the absorption of the large amount of crustal shortening occuring in central asia

continental crust composition constrained by measurements of crustal poisson s ratio deciphering the geological evolution of the earth s continental crust requires knowledge of its bulk composition and global variability the main uncertainties are associated with the composition of the lower crust seismic measurements probe the elastic properties of the crust at depth from which composition can be inferred of particular note is poisson s ratio this elastic parameter can be determined uniquely from the ratio of p to s wave seismic velocity and provides a better diagnostic of crustal composition than either p or s wave velocity alone1 previous attempts to measure have been limited by difficulties in obtaining coincident p and s wave data sampling the entire crust2 here we report 76 new estimates of crustal spanning all of the continents except antarctica we find that on average increases with the age of the crust our results strongly support the presence of a mafic lower crust beneath cratons and suggest either a uniformitarian craton formation process involving delamination of the lower crust during continental collisions followed by magmatic underplating or a model in which crust formation processes have changed since the precambrian era

sandstone diagenesis and reservoir quality prediction models myths and reality models and concepts of sandstone diagenesis developed over the past two decades are currently employed with variable success to predict reservoir quality in hydrocarbon exploration not all of these are equally supported by quantitative data observations and rigorous hypothesis testing simple plots of sandstone porosity versus extrinsic parameters such as current subsurface depth or temperature are commonly extrapolated but rarely yield accurate predictions for lithified sandstones calibrated numerical models that simulate compaction and quartz cementation when linked to basin models have proven successful in predicting sandstone porosity and permeability where sufficient analog information regarding sandstone texture composition and quartz surface area is available analysis of global regional and local data sets indicates the following regarding contemporary diagenetic models used to predict reservoir quality 1 the effectiveness of grain coatings on quartz grains e g chlorite microquartz as an inhibitor of quartz cementation is supported by abundant empirical data and recent experimental results 2 vertical effective stress although a fundamental factor in compaction cannot be used alone as an accurate predictor of porosity for lithified sandstones 3 secondary porosity related to dissolution of framework grains and or cements is most commonly volumetrically minor 2 exceptions are rare and not easily predicted with current models 4 the hypothesis and widely held belief that hydrocarbon pore fluids suppress porosity loss due to quartz cementation is not supported by detailed data and does not represent a viable predictive model 5 heat flow perturbations associated with allochthonous salt bodies can result in suppressed thermal exposure thereby slowing the rate of quartz cementation in some subsalt sands

shallow velocity structure and poisson s ratio at the tarzana california strong motion accelerometer site the 17 january 1994 northridge california earthquake produced strong ground shaking at the cedar hills nursery referred to here as the tarzana site within the city of tarzana california approximately 6 km from the epicenter of the mainshock although the tarzana site is on a hill and is a rock site accelerations of approximately 1 78 g horizontally and 1 2 g vertically at the tarzana site are among the highest ever instrumentally recorded for an earthquake to investigate possible site effects at the tarzana site we used explosive source seismic refraction data to determine the shallow 70 m p and s wave velocity structure our seismic velocity models for the tarzana site indicate that the local velocity structure may have contributed significantly to the observed shaking p wave velocities range from 0 9 to 1 65 km sec and s wave velocities range from 0 20 and 0 6 km sec for the upper 70 m we also found evidence for a local s wave low velocity zone lvz beneath the top of the hill the lvz underlies a cdmg strong motion recording site at depths between 25 and 60 m below ground surface bgs our velocity model is consistent with the near surface 30 m p and s wave velocities and poisson s ratios measured in a nearby 30 m borehole high poisson s ratios 0 477 to 0 494 and s wave attenuation within the lvz suggest that the lvz may be composed of highly saturated shales of the modelo formation because the lateral dimensions of the lvz approximately correspond to the areas of strongest shaking we suggest that the highly saturated zone may have contributed to localized strong shaking rock sites are generally considered to be ideal locations for site response in urban areas however localized highly saturated rock sites may be a hazard in urban areas that requires further investigation

origin and significance of redistributional secondary porosity abstract redistributional secondary rds porosity is created by the dissolution of framework grains and cements and the simultaneous precipitation of reaction products in the nearby pore space aluminosilicate minerals and in particular feldspars are the usual precursors of rds porosity in sandstones common product minerals are authigenic quartz and clay minerals usually kaolinite this type of secondary porosity is almost ubiquitous in sandstones rds porosity has little effect on the total porosity of reservoir rocks and contrary to popular opinion cannot give rise to higher than expected porosities at depth petrographic criteria alone are an insufficient measure that the rock has undergone leaching resulting in porosity enhancement to demonstrate porosity enhancement due to leaching it is necessary to show that the porosity of the rock is statistically higher than would otherwise be the case for a rock of similar characteristics that has not been leached the dissolution precipitation reactions that give rise to rds arise from the inherent thermodynamic instability of minerals such as the feldspars and other aluminosilicates which formed under magmatic or metamorphic conditions when these minerals dissolve they add aluminium and silica to the pore fluid however the concentration of aluminium and silica in the pore fluid is buffered by clay minerals and quartz respectively during burial diagenesis thus any increase in the concentration of these components leads to the precipitation of an authigenic clay mineral and authigenic quartz carboxylic acids are claimed to have the capacity to complex aluminium and so increase the solubility of aluminium and eliminate the precipitation of a clay phase however they are unlikely to have any major effect on aluminium solubility in natural i e complex pore fluids in fact dissolution will occur in acid neutral or alkaline conditions at a rate that is kinetically controlled reaction rates speed up exponentially with increasing temperature and therefore depth resulting in the destruction of more and more unstable components this increase in the rate of destruction of unstable components at depth can give rise to what appears to be a late leaching event what is seen in fact is the natural culmination of the destruction of thermodynamically unstable phases at an ever increasing rate during burial thus for a rock of a given composition the fraction of the total porosity that is secondary will increase with burial depth until all labile components are consumed the behaviour of mineral mixtures in the presence of a moving pore fluid is an additional contributing factor to the ubiquity of rds porosity where two or more minerals add the same ion to the pore fluid then the steady state concentration of that ion which is reached at some distance down the flow path theoretically an infinite distance is a weighted average of the equilibrium solubilities of that ion with respect to the minerals that contribute the ion thus as the pore fluid approaches a steady state concentration of the ion it will be undersaturated with respect to the least stable minerals and oversaturated with respect to the most stable mineral hence rds porosity must form as a result of the dissolution of the least stable mineral and the precipitation of the most stable mineral although kaolinite is the most common dissolution product of feldspars with increasing temperature its stability field shrinks with respect to other clay minerals such as illite thus it is possible for feldspars and other aluminosilicates to decompose at depth to give illite and authigenic quartz the growth of fibrous illite together with the change from a dominantly primary to a dominantly secondary pore structure can give rise to a rapid loss of permeability with depth where changes in the clay mineral type do not occur the impact of rds porosity on the porosity permeability behaviour of sandstones is minimal up to secondary porosities of about 10 beyond this figure there is at present too little data to make a clear statement

the crustal structure from the altai mountains to the altyn tagh fault northwest china 1 we present a new crustal section across northwest china based on a seismic refraction profile and geologic mapping the 1100 km long section crosses the southern margin of the chinese altai mountains junggar accretional belt and eastern junggar basin easternmost tianshan mountains and easternmost tarim basin the crustal velocity structure and poisson s ratio which provide a constraint on crustal composition were determined from p and s wave data despite the complex geology the crustal thickness along the entire profile is nearly uniform at 50 km the thickest crust 56 km occurs at the northern end of the profile beneath the altai mountains and the thinnest 46 km crust is beneath the junggar basin beneath surficial sediments the crust is found to have three layers with p wave velocities vp of 6 0 6 3 6 3 6 6 and 6 9 7 0 km s respectively the southern half of the profile including the eastern tianshan mountains and eastern margin of the tarim basin shows low p wave velocities and 0 25 to a depth of 30 km which suggests a quartz rich granitic upper crustal composition the northern half of the profile below the altai mountains and junggar accretional belt has a higher poisson s ratio of 0 26 0 27 to a depth of 30 km indicative of an intermediate crustal composition the entire 1100 km long profile is underlain by a 15 30 km thick high velocity 6 9 7 0 km s 0 26 0 28 lower crustal layer that we interpret to have a bulk composition of mafic granulite at the southern end of the profile a 5 km thick midcrustal low velocity layer vp 5 9 km s 0 25 underlies the tianshan and the region to the south and may be indicative of a near horizontal detachment interface pn velocities are 7 7 7 8 km s between the tianshan and the junggar basin and 7 9 8 0 km s below the altai mountains and eastern margin of the tarim basin we interpret the consistent three layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after paleozoic terrane accretion the thickness 50 km of the crust appears to be related to compression resulting from the indo asian collision

plate interactions crustal deformation and magmatism along the eastern margins of the tibetan plateau abstract in order to investigate the deep structure and dynamics of the triangular colliding system of the eastern tibetan plateau tp as well as the yangtze platform and the qinling dabie fold belt a large number of p and s wave arrival times from both local and teleseismic earthquakes were used to generate high resolution tomographic images beneath the eastern margins of tp the results of this study show that the western margin of the minjiang fault is characterized by low velocity low v anomalies in sharp contrast to the high velocity high v anomalies that are derived from the eastern part this means that the minshan uplift zone is imaged as high v and acts as a barrier to the eastward escape crustal flow which leads to obvious segmentation of the longmenshan fault lmsf zone between velocity belts that are concatenated into the minjiang fault because high v anomalies are clearly imaged in this study to extended depths of ca 450 km we consider the yangtze platform containing the sichuan basin to comprise craton like lithosphere this study reveals the presence of a low v belt extending to a depth of ca 250 km between the sichuan basin and the ordos block which probably represents the migration pathway of upwelling asthenosphere materials from tibet at the same time low v zone with a downdip depth of ca 500 km accompanied by an approximately 400 km rectangular high v body is imaged under the tengchong volcano we interpret the high v body and the low v zone as evidence for the subducted indian plate and the volcanic magma source respectively the results of this study reveal a number of new features of structural heterogeneities relative to deep coupling between the tibet and the yangtze platform as well as the relationship between the sichuan basin and the ordos block and the original source of the tengchong volcano the results presented here enable us a better understanding of the plate interaction crustal deformation and magmatism of the tengchong volcano along the eastern margins of tp

quantitative analyses of porosity evolution in tight grainstones a case study of the triassic feixianguan formation in the jiannan gas field sichuan basin china abstract tight grainstones although widespread throughout the lower triassic feixianguan formation in the sichuan basin have received little attention in part due to their lower porosity and greater heterogeneity relative to their dolostone counterparts based on data from cores and thin sections as well as petrophysical properties the feixianguan grainstones representing a major gas reservoir in the jiannan gas field were systemically analysed to better understand porosity evolution in tight carbonates that have experienced original oil accumulation and subsequent thermal cracking during progressive burial the grainstones were divided into two types according to whether pyrobitumen was present and their porosity evolutions were quantitatively reconstructed taking 40 as the original porosity the grainstones without pyrobitumen which were ineffective palaeo oil reservoirs lost 21 94 and 3 13 of their porosities through marine and burial calcite cementation respectively and 13 34 by compaction and have a current porosity of 1 59 thus allowing them to serve as major present day gas reservoirs comparatively pyrobitumen bearing grainstones which were once palaeo oil reservoirs lost 23 96 and 2 36 of their porosities through marine and burial calcite cementation respectively 11 4 by compaction and 1 44 by pyrobitumen and have a current porosity of 0 84 thus making them ineffective gas reservoirs this study provides a quantitative understanding of the close association between porosity evolution and reservoir effectiveness for the palaeo oil charge and present day gas accumulation with respect to diagenetic history which is useful for the future exploration in tight gas limestone reservoirs

variabilities of low latitude migrating and nonmigrating tides in gps tec and timed saber temperature during the sudden stratospheric warming event of 2013 the global positioning system gps deduced total electron content tec data at 15 n geomagnetic which is the crest region of equatorial ionization anomaly are used to study tidal variabilities during the 2013 sudden stratospheric warming ssw event the results from space time spectral analysis reveal that the amplitudes of migrating diurnal dw1 and semi diurnal sw2 tides are larger than those of non migrating tides after the ssw onset the amplitudes of dw1 sw2 sw1 and ds0 increase moreover they show 16 day variations similar to the periodicity of the high latitude stratospheric planetary wave pw suggesting that the non migrating tides sw1 and ds0 are possibly generated due to non linear interaction of migrating tides with pw similar spectral analysis on temperature at 10 n obtained from the sounding of atmosphere by broadband emission radiometry saber shows that the sw2 enhances at stratospheric heights and the sw2 is more dominant at 80 90 km but its amplitude decreases around 100 km the amplitudes of non migrating tides become comparable to those of sw2 around 100 km and their contribution become increasingly important at higher heights this suggests that the non linear interaction between migrating tides and pw occur at low latitude upper mesospheric heights as sw2 exhibits 16 day periodicity in saber temperature at 100 km as observed in tec besides it is observed that the eastward propagating tides are less dominant than westward propagating tides in both tec and saber temperature

high resolution regional crustal models from irregularly distributed data application to asia and adjacent areas we propose a new methodology to obtain crustal models in areas where data is sparse and data spreading is heterogeneous this new method involves both interpolating the depth to the moho discontinuity between observations and estimating a velocity depth curve for the crust at each interpolation location the moho observations are interpolated using a remove compute restore technique used in for instance geodesy observations are corrected first for airy type isostasy the residual observations show less variation than the original observations making interpolation more reliable after interpolation the applied correction is restored to the solution leading to the final estimate of moho depth the crustal velocities have been estimated by fitting a velocity depth curve through available data at each interpolation location uncertainty of the model is assessed both for the moho and the velocity model the method has been applied successfully to asia the resulting crustal model is provided in digital form and can be used in various geophysical applications for instance in assessing rheological properties and strength profiles of the lithosphere the correcting gravity for the crustal effects seismic tomography and geothermal modelling

tomographic inversion of pn travel times in china 1 pn travel times are useful for studying crustal and uppermost mantle structure and regional tectonics because they are affected by crustal velocity and thickness as well as uppermost mantle velocity and anisotropy we obtained 57 740 pn travel time picks from 5433 earthquakes and 307 stations from chinese national and provincial earthquake bulletins and the international seismological center bulletins to invert for pn velocity variation and anisotropy and station delays in china our inversion reveals significant features that correlate with surface geology the main results are as follows 1 the pn velocities show a mosaic of very fast and very slow anomalies mirroring the heterogeneous geology of china at the surface the pn velocities are high beneath the major basins in the west sichuan qaidam west tarim tulufan and junggar and low in areas of active volcanoes myanmar and western yunnan and quaternary volcanism in northern tibet in seismically active areas in north china and tien shan and in the southern part of south china the hainan plume 2 the pn anisotropy beneath the major basins in the west is generally weak strong anisotropy is found beneath high deformation regions the tibetan plateau southeastern margin of the tibetan plateau western tien shan and part of north china suggesting the anisotropy is likely related to recent large scale tectonic activity 3 a large area of north china shows prominent low pn velocity beneath archean basement with thin crust our observations are consistent with rifting lithospheric thinning and mantle upwelling in the region the pn anisotropy is consistent with a dextral simple shear in the nne direction in the lithosphere mantle during the last and ongoing major deformation period 4 the pn velocity in northern tibet is generally lower than that in the south southern tibet has significant e w structure low velocity anomalies can be traced from northern tibet across southwestern tibet and south central tibet to near the india plate anisotropy is absent beneath much of the himalaya block but consistent anisotropy with e w fast direction is present beneath the lhasa block and large anisotropy up to 4 is observed in low velocity regions of the northern and western tibet complex station delays in the eastern margin of the plateau suggest that the whole crust may be highly deformed the anisotropy pattern in the southeastern margin of the tibetan plateau suggests a mantle lithospheric deformation similar to the clockwise rotation of material observed at the surface 5 crustal thicknesses inferred from our station delays are consistent with previous models which correlate well with surface topography

crustal and upper mantle structure of northern tibet imaged with magnetotelluric data 1 magnetotelluric mt data were collected in northern tibet along the amdo to golmud highway during the 1995 and 1999 project indepth international deep profiling of tibet and the himalaya surveys broadband and long period mt data were collected and the te mode tm mode and vertical magnetic field data were inverted to yield a minimum structure two dimensional resistivity model the model obtained from inverting all responses simultaneously shows that a pervasive midcrustal conductor extends from the kunlun shan to the bangong nuijiang suture the vertically integrated conductivity conductance of this crustal layer is greatest in the northern qiangtang terrane at latitude 34 n the electrical resistivity of the upper mantle is constrained by the mt data to be in the range of 10 30 m across the songpan ganze and qiangtang terranes this is lower than would be expected if asian lithosphere underthrusts northern tibet as far as the qiangtang terrane the mt responses are more consistent with a model in which asian lithosphere extends as far south as the kunlun shan and the upper mantle beneath the songpan ganze and qiangtang terranes is sufficiently hot to contain a small fraction of interconnected partial melt

continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum the ordos basin north central china abstract global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps the characteristic features include the following a hydrocarbon source and reservoir coexist porosity and permeability are ultra low nano pore throats are widely distributed hydrocarbon bearing reservoir bodies are continuously distributed there is no obvious trap boundary buoyancy and hydrodynamics have only a minor effect and darcy s law does not apply phase separation is poor there is no uniform oil gas water interface or pressure system and oil or gas saturation varies examples of unconventional hydrocarbon accumulations are the mesozoic tight sandstone oil province and the upper paleozoic tight sandstone gas province in the ordos basin north central china generally continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources because of the great potential of unconventional petroleum resources it is believed that research on such resources will be at the forefront of the future development of petroleum geology

the thickness of seismogenic layer under the areas surrounding the ordos block northern china a seismic approach was used to calculate the thickness distribution of the seismogenic layer under the block ordos in northern china this block was chosen because of its stability the complicated tectonics around its boundaries and the completeness of the data set several strong earthquakes occurred in this area in the 20th century with the largest one haiyuan m 8 4 1920 in the southwest of the ordos most of the large faults around the ordos are strike slip ones the breaking point i e the saturation of earthquake magnitudes m c of the self similarity from small to large events based on the gutenberg richter relation is calculated under the assumption of the l model which expresses that the offset is proportional to the length of the fault scholz 1982 1990 1994 the thickness w c of the seismogenic layer is directly related to m c to display smoothly the change of the thickness of the seismogenic layer and to ensure the reliability of the results a moving cell of 150 events with a moving step of 0 1 degree was adopted to take advantage of the existing data set the mean value method which is based on the latest complete data information to make the existing data set a complete one was used to extrapolate it so that the data covers the whole time period the results show that the average thickness of the seismogenic layer in the southwest in the northwest and in the east of the ordos is around 30 9 and 17 km respectively the thickness generally decreases from the southwest to the northeast this kind of spatial variation is in agreement with the maximum depth distribution of large amounts of microearthquakes with the correlation coefficient to be about 0 88 and the moho depth

the effects of diagenesis on the reservoir characters in sandstones of the late cretaceous pab formation kirthar fold belt southern pakistan abstract the maastrichtian pab formation in the southern part of pakistan is composed of fine to very coarse grained texturally mature quartz arenite and subordinate sublitharenite varieties the sandstones have undergone intense and complex diagenetic episodes due to burial and uplift diagenetic modifications were dependent mainly on the clastic composition of sandstone burial depth and thrust tectonics diagenetic events identified include compaction precipitation of calcite quartz clay minerals and iron oxide hydroxide dissolution and alteration of unstable clastic grains as feldspar and volcaniclithic fragments as well as tectonically induced grain fracturing the unstable clastic grains like feldspar and lithic volcanic fragments suffered considerable alteration to kaolinite and chlorite dissolution and alteration of feldspar and volcanic lithic fragments and pressure solution were the main sources of quartz cements mechanical compaction and authigenic cements like calcite quartz and iron oxide hydroxide reduced the primary porosity whereas dissolution of clastic grains and cements has produced secondary porosity chlorite coatings on clastic grains have prevented quartz cementation coarse grained thick bedded packages of fluviodeltaic shelf delta lobe and submarine channels facies have higher average porosity than fine grained thin bedded and bioturbated sandstone of deeper shelf and abyssal plain environments and these facies are concluded to be possible future hydrocarbon prospects

an overview of the earth crust under china abstract we assess the results of a number of deep seismic soundings performed in china over the last few decades and study the variations in crustal structure in 18 tectonic units comprised of three platforms and 15 fold systems thickness data on 344 mesozoic cenozoic sedimentary basins as well as data on moho depth are collected in order to discuss the relationship between the thickness of the basins and the average thickness of the consolidated crust in each tectonic unit the degree of mirror image symmetry between mesozoic cenozoic sedimentary basins and the uplifting topmost parts of the mantle is herein analyzed using deep geophysical data on sedimentary basins by applying standard methods of least squares analysis to both datasets we have obtained both the average thickness of the consolidated crust and the mirror image symmetry factor for every platform and tectonic fold system thereby allowing us to explore the correlation between the depths of the bottom of the sedimentary basins and the top of the uplifting mantle the thickness of the consolidated crust in china is found to be between 20 and 63 km following a pattern of gradual thickening from east to west expressed in terms of spatial seismicity and the concentration of seismic energy and according to the sharing out of earthquake hypocenters in the top 80 km of the earth the rheology of the area does not appear to suit the widely accepted jelly sandwich model for the continents which does not seem to be entirely valid in china the findings on the mirror image symmetry factor show that this parameter varies mainly in the range 0 5 to 1 8 compared with each other tectonic unit most of the tectonic systems in china appear to be isostatically compensated tibet is an exception in that the symmetry factor is positive for the gangdise nyainqentanglha 1 0 and himalayan 0 1 fold systems implying that these tectonic zones are far from being in the isostatic equilibrium of the other regions we have also analyzed the zoning characteristics through the logarithmic relationship ln r ln h a using the average thickness of the consolidated crust and the absolute value of the symmetry factor the key finding is that regardless of geographical location all the values of r h a for those sedimentary basins in which oil gas reservoirs have to date been found fall into the narrow range of values between 19 38 and 37 40 there is some appeal in a possible relationship involving the ratio of crustal thickness to symmetry factor more so when the results obtained appear to suggest a prognostic tool for exploratory practice in relation to oil gas reservoirs

detection of widespread fluids in the tibetan crust by magnetotelluric studies magnetotelluric exploration has shown that the middle and lower crust is anomalously conductive across most of the north to south width of the tibetan plateau the integrated conductivity conductance of the tibetan crust ranges from 3000 to greater than 20 000 siemens in contrast stable continental regions typically exhibit conductances from 20 to 1000 siemens averaging 100 siemens such pervasively high conductance suggests that partial melt and or aqueous fluids are widespread within the tibetan crust in southern tibet the high conductivity layer is at a depth of 15 to 20 kilometers and is probably due to partial melt and aqueous fluids in the crust in northern tibet the conductive layer is at 30 to 40 kilometers and is due to partial melting zones of fluid may represent weaker areas that could accommodate deformation and lower crustal flow

quantitative identification of diagenetic facies and controls on reservoir quality for tight sandstones a case study of the triassic chang 9 oil layer zhenjing area ordos basin abstract the identification of diagenetic facies is essential for prospecting hydrocarbon reserves and for hydrocarbon exploration in tight oil reservoirs a comprehensive method for determining quantitative index of reservoir diagenetic facies rdi using core analysis techniques hierarchical cluster analyses hca and grey correlation analyses gca was proposed the petrological characteristics physical properties and pore structures of the chang 9 tight oil sandstones are analysed based on thin section analysis x ray diffraction xrd scanning electron microscopy sem cathode luminescence cl and mercury injection capillary pressure micp tests the properties of the chang 9 sandstones were controlled by different forms of diagenesis compaction and cementation of calcite have reduced or occluded the pore volume in the sandstones but not all cementation is deleterious to reservoir quality chlorite cementation helped preserve relict primary porosity and the direct dissolution of feldspars and unstable lithic fragments has resulted in enhanced pore connectivity following petrographic observations and quantitative analysis the sandstones were divided into four diagenetic facies calcite cementation facies intense compaction facies porosity preserved by grain coating chlorites facies and dissolution facies the intense compaction facies and calcite cementation facies are responsible for the evolution of low quality reservoirs while the reservoirs with porosity preserved by grain coating chlorites facies and dissolution facies are of better porosity and permeability the distribution of fractures improves reservoir quality by enhancing the dissolution of unstable grains and providing convenient channels and storage spaces for oil and gas migration

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

controls on reservoir heterogeneity of tight sand oil reservoirs in upper triassic yanchang formation in longdong area southwest ordos basin china implications for reservoir quality prediction and oil accumulation abstract compared to conventional reservoirs pore structure and diagenetic alterations of unconventional tight sand oil reservoirs are highly heterogeneous the upper triassic yanchang formation is a major tight oil bearing formation in the ordos basin providing an opportunity to study the factors that control reservoir heterogeneity and the heterogeneity of oil accumulation in tight oil sandstones the chang 8 tight oil sandstone in the study area is comprised of fine to medium grained moderately to well sorted lithic arkose and feldspathic litharenite the reservoir quality is extremely heterogeneous due to large heterogeneities in the depositional facies pore structures and diagenetic alterations small throat size is believed to be responsible for the ultra low permeability in tight oil reservoirs most reservoirs with good reservoir quality larger pore throat size lower pore throat radius ratio and well pore connectivity were deposited in high energy environments such as distributary channels and mouth bars for a given depositional facies reservoir quality varies with the bedding structures massive or parallel bedded sandstones are more favorable for the development of porosity and permeability sweet zones for oil charging and accumulation than cross bedded sandstones authigenic chlorite rim cementation and dissolution of unstable detrital grains are two major diagenetic processes that preserve porosity and permeability sweet zones in oil bearing intervals nevertheless chlorite rims cannot effectively preserve porosity permeability when the chlorite content is greater than a threshold value of 7 and compaction played a minor role in porosity destruction in the situation intensive cementation of pore lining chlorites significantly reduces reservoir permeability by obstructing the pore throats and reducing their connectivity stratigraphically sandstones within 1 m from adjacent sandstone mudstone contacts are usually tightly cemented carbonate cement 10 with low porosity and permeability lower than 10 and 0 1 md respectively the carbonate cement most likely originates from external sources probably derived from the surrounding mudstone most late carbonate cements filled the previously dissolved intra feldspar pores and the residual intergranular pores and finally formed the tight reservoirs the petrophysical properties significantly control the fluid flow capability and the oil charging accumulation capability of the chang 8 tight sandstones oil layers usually have oil saturation greater than 40 a pore throat radius of less than 0 4 m is not effective for producible oil to flow and the cut off of porosity and permeability for the net pay are 7 and 0 1 md respectively

the origin and charging directions of neogene biodegraded oils a geochemical study of large oil fields in the middle of the shijiutuo uplift bohai sea china abstract two large oil fields qhd32 6 and qhd33 1 located in the middle part of the shijiutuo uplift have generally suffered mild biodegradation based on multivariate statistical analysis of the biomarker parameters this study discussed the origin and charging directions for these two oil fields in contrast to ed 3 derived oil all available oil samples from these two large oil fields displayed low c 19 c 23 c 24 c 26 and high g h and 4 msi which are attributed to the mixtures of oils derived from the shahejie es 1 and es 3 source rocks oils in qhd32 6 which contain relatively more es 3 derived oil are called group i oils and most oils in qhd33 1 which share relatively more es 1 derived oil are called group ii oils our mixed oil experiments reveal the predominant es 3 and es 1 derived oil contribution for group i and group ii oil groups respectively however the selection of end member oils warrants further research based on comparisons of biomarker parameters the qhd32 6 oil field was mainly charged in the north by oil generated from shahejie formation source rocks in the bozhong depression however oils from the north of qhd32 6 field display a remarkable difference to the oils in the south of this field which may indicate that a charging pathway exists from the qhd33 1 field considering the variations in biomarker compositions in the west to east and northwest to southeast sections across the qhd33 1 and qhd32 6 oil fields it can be deduced that es 3 sourced oil migrated westward to the qhd32 6 traps and then charging by es 1 oil from the bozhong sag resulted in the qhd33 1 oil field being characterized by the mixture of es 3 and es 1 sourced oil moreover migration of es 1 derived oil from the qinnan sag was not identified implying that the qhd33 1 oil field is mainly charged from the northeast of the bozhong sag

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

moho offset across the northern margin of the tibetan plateau anomalous double pulse teleseismic p wave arrivals were observed at one station near the northern margin of the tibetan plateau the azimuthal dependence of the waveform distortion and its absence at nearby stations indicated that the distortion was produced by receiver side crustal heterogeneity modeling of the three component data revealed a 15 to 20 kilometer moho offset that occurs over a narrow lateral range of less than 5 kilometers this east west striking offset separates the thick tibetan plateau crust from the qaidam basin crust such a sharp crustal thickness change implies a weak tibetan plateau crust that thickens vertically in response to penetration by india from the south and to blockage caused by a strong qaidam basin crust to the north

the crustal and upper mantle structures beneath the northeastern margin of tibet abstract teleseismic receiver functions are used to estimate the crustal thickness v p and v p v s or poisson s ratio beneath the northeastern margin of the tibetan plateau surface wave dispersion measurements are combined with teleseismic receiver functions to improve estimates of model parameters a migration method based on a local velocity model is used to image the upper mantle discontinuities according to the location of pierce points of the direct converted phases on the observed receiver functions we estimate the topography of upper mantle discontinuities the thicker crust in the altyn tagh fault hexi corridor basin may indicate the superposition of crusts during the collision between the indian and the eurasian plates beneath tianshui and its neighboring regions in the eastern margin of tibet high v p and high v p v s values close to uplifts of magma and hot springs might result from the upwelling of mantle materials induced by the collision the depressed 410 km discontinuity might be related to the warm temperature below the northeastern margin of tibet we speculate that the mantle flow beneath the eastern margin of tibet may be turning downward and subsiding into the mantle transition zone

distribution of the crustal low velocity zones beneath the central and northeastern tibetan plateau insights from joint analysis of receiver functions and surface wave dispersion observations abstract a crustal and uppermost mantle shear wave velocity model beneath the central and northeastern tibetan plateau is obtained from the joint analysis of rayleigh wave phase dispersions and teleseismic p wave receiver functions at 188 broadband seismic stations deployed in this region this newly presented model reveals wide spread low velocity zones in the middle crust and these zones are mainly confined to the region south of the kunlun fault the velocities in the low velocity zones are extremely low

anomalously high porosity and permeability in deeply buried sandstone reservoirs origin and predictability porosity and permeability generally decrease with increasing depth thermal exposure and effective pressure however a significant number of deep 4 km approximately 13 000 ft sandstone reservoirs worldwide are characterized by anomalously high porosity and permeability anomalous porosity and permeability can be defined as being statistically higher than the porosity and permeability values occurring in typical sandstone reservoirs of a given lithology composition and texture age and burial temperature history in sandstones containing anomalously high porosities such porosities exceed the maximum porosity of the typical sandstone subpopulation major causes of anomalous porosity and permeability were identified decades ago however quantification of the effect of processes responsible for anomalous porosity and permeability and the assessment of the predictability of anomalous porosity and permeability occurrence in subsurface sandstones have rarely been addressed in published literature the focus of this article is on quantification and predictability of three major causes of anomalously high porosity 1 grain coats and grain rims 2 early emplacement of hydrocarbons and 3 shallow development of fluid overpressure grain coats and grain rims retard quartz cementation and concomitant porosity and permeability reduction by inhibiting precipitation of quartz overgrowths on detrital quartz grains currently prediction of anomalous porosity associated with grain coats and grain rims is dependent on the availability of empirical data sets in the absence of adequate empirical data sedimentologic and diagenetic models can be helpful in assessing risk due to reservoir quality such models provide a means to evaluate the effect of geologic constraints on coating occurrence and coating completeness required to preserve economically viable porosity and permeability begin page 302 in a given play or prospect these constraints include thermal history and sandstone grain size and composition the overall effect of hydrocarbon emplacement on reservoir quality is controversial it appears that at least some cements quartz and illite may continue to precipitate following emplacement of hydrocarbons into the reservoir our work indicates that integration of basin modeling with reservoir quality modeling can be used to quantify prior to drilling the potential impact of hydrocarbon emplacement on porosity and permeability the best case scenario for significant reservoir quality preservation due to fluid overpressure development is in rapidly deposited tertiary or quaternary sandstones our models suggest that significant porosity can be preserved in sandstones that have experienced continuous high fluid overpressures from shallow burial depths the models also indicate that the potential for porosity preservation is greatest in ductile grain rich sandstones because compaction tends to be the dominant control on reservoir quality in such rocks the case for significant porosity preservation associated with fluid overpressures in pre tertiary basins however is more problematic because of the complexities in the history of fluid overpressure and the greater significance of quartz cementation as a potential mechanism of porosity loss

how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones observations and experiments observations and hydrothermal experiments were used to derive new information about how clay grain coats inhibit quartz cement and preserve porosity in deeply buried sandstones samples of deeply buried porous sandstones with different types of clay coats were split in two coats removed from one of each pair of splits and grain surfaces inspected with scanning electron microscopy quartz grains in a fluvial deltaic sandstone buried to 115c had no visible authigenic quartz on grain surfaces cleaned of diagenetic chlorite coats though well developed overgrowths occurred on nearby naturally uncoated grains however in similar sandstones buried to 164c quartz grain surfaces exposed by chlorite coat removal were covered with small 5 m mainly anhedral syntaxial quartz overgrowths similar overgrowths were observed under various detrital and diagenetic clay coats in porous eolian sandstones buried to temperatures up to 215c we conclude that clay coats may retard quartz nucleation at moderate temperatures but at high temperatures many coats permit quartz nucleation and preserve porosity by limiting cement growth to investigate cement growth limitation mechanisms samples with coats removed were subjected to quartz cementing conditions in a hydrothermal reactor during experiments the naturally occurring small overgrowths on clay cleaned grains coalesced and grew suggesting that clay particles in coats inhibit cement growth by forming barriers to early overgrowth coalescence although the fraction of grain surface coverage is the primary control on cement inhibition by coats cement growth interference textures vary with coat type providing a mechanism by which coat composition may be a secondary control on inhibitory effectiveness in deeply buried sandstones quartz cement can fill significant microporosity within diagenetic chlorite coats potentially affecting mechanical and petrophysical rock properties

seismic evidence for stepwise thickening of the crust across the ne tibetan plateau we performed a receiver function analysis on teleseismic data recorded along two 550 km long profiles crossing the northeastern tibetan plateau results from time to depth migration grid search vp vs determination and simulated annealing inversion of waveforms reveal that the crust thickens from 50 km near the northern edge of the plateau to 80 km south of the jinsha suture in the qiang tang block crustal thickening occurs in staircase fashion with steps located beneath the main reactivated sutures the vp vs ratio close to the global continental average does not suggest widespread partial melting but rather a more usual separation between an upper felsic and a lower mafic part within the northeastern tibetan crust

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

adaptive moving window method for 3d p velocity tomography and its application in china a new tomography method the adaptive moving window is introduced and applied to construct the velocity structure of the crust and upper mantle of china and surrounding areas more than 345 000 high quality compressional body wave phase data extracted from the annual bulletin of chinese earthquakes spanning from 1990 to 1998 are used the area of interest is represented horizontally by 2338 points with 1 intervals each point is assigned a window a cell or a region centered at each point whose size is varied depending upon the ray path density a five layer 1d model from surface down to uppermost mantle is then determined at each point by performing a monte carlo random search where earthquake locations are held constant combining and smoothing the obtained 1d models an equivalent 3d model is achieved the predicted travel times through the 3d model match very well with the observed ones from local to regional distances the model has a good correlation with tectonic features and is generally consistent with the existing models constructed by other researchers our model gives detailed information about structure and is feasible for application to high quality earthquake location problems manuscript received 24 june 2003

northeastward growth and uplift of the tibetan plateau magnetostratigraphic insights from the guide basin 1 most of the evidence for the uplift of the tibetan plateau comes from its southern and central parts although the northern rim has been less studied it may greatly contribute to the understanding of the mechanism and timing of the uplift recent studies on the northeastern part of the plateau suggest that the uplift can largely be explained by cenozoic thrusting and folding linked to the movement of the altyn tagh and kunlun faults and that diffuse shortening might be a realistic way to explain the thickening of the continental interior nevertheless the age of sediments that record the tectonic processes is poorly known the terrigenous formations in the guide gonghe basin ne tibet show several phases of deformation and include thick deposits of conglomerates interbedded with sandstones and siltstones that are thought to be the result of an increased sediment flux dating of these gravel deposits with magnetostratigraphic and faunal studies has permitted the precise timing of the basin infilling and its tectonic evolution to be established our results indicate that the onset of conglomerate accumulation occurs in the pliocene an observation that is in agreement with those of the linxia basin just east of the plateau if the high sedimentation rates 22 cm kyr in the guide basin are mostly due to tectonism then significant uplift occurred as late as pliocene

composition of the crust in the grenville and appalachian provinces of north america inferred from v p v s ratios we use the ratios between p and s wave velocities vp vs derived from seismic refraction data to infer the composition of the crust in the grenville and the appalachian provinces of north america the crust exhibits vp vs increasing with depth from 1 64 to 1 84 there is a clear distinction between the grenville province average vp vs 1 81 and the appalachian province average vp vs 1 73 which persists at all depths the boundary between these provinces is east dipping extending for 100 km east of the champlain thrust in the appalachian province the increase in vp vs ratios with depth from 1 67 to 1 74 0 02 may reflect a normal decrease of silica content in the continental crust in the grenville province beneath the central granulite terrane an anomalous vp vs ratio of 1 82 0 02 is observed extending to a depth of 10 km this correlates with the abundance of ca plagioclase in the marcy anorthosite at greater depth 15 20 km where seismic lamination and high electrical conductivity is observed vp vs is 1 84 0 02 and correlates with the tahawus complex a layered mafic intrusion within the 25 km thick lower crust of the grenville province the vp vs is 1 84 0 02 and p velocity is 7 0 0 1 km s which are typical for plagioclase bearing rocks gabbro norite the high vp vs ratio in the grenville province has not been reported in crust of any other age since the grenville province contains 75 of the world s known anorthosites high vp vs ratio is related to high plagioclase we suggest that the composition of the grenville lower crust was significantly modified by the emplacement of the anorthosites in the mid proterozoic

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

crustal structure of the altiplano from broadband regional waveform modeling implications for the composition of thick continental crust we have modeled the full waveforms from six intermediate depth and two shallow earthquakes recorded at regional distances by the banjo broadband andean joint experiment banjo and seismic exploration of the deep altiplano seda portable seismic networks in the central andes in this study we utilize data from those banjo and seda stations located within the altiplano and eastern cordillera we used reflectivity synthetic seismograms and a grid search to constrain four parameters of the altiplano eastern cordillera lithosphere crustal thickness average crustal velocity vp and crustal and upper mantle poisson s ratios crust and mantle using our grid search we investigated the crustal and upper mantle structure along 36 individual event station paths and applied forward modeling to 56 event station paths robust models for the altiplano that provide the best overall fit between the data and synthetic seismograms are characterized by an average vp of 5 75 6 25 km s crustal thicknesses of 60 65 km crust 0 25 and mantle 0 27 0 29 we find a north south variation in the structure of the altiplano with the crust south of the banjo transect characterized by either lower than average crustal p wave velocities or a slightly higher crust relative to crust north of the banjo transect these results are consistent with a model of crustal thickening caused predominantly by tectonic shortening of felsic crust rather than by underplating or magmatic intrusion from the mantle

crustal structure of china from deep seismic sounding profiles abstract more than 36 000 km of deep seismic sounding dss profiles have been collected in china since 1958 however the results of these profiles are not well known in the west due to the language barrier in this paper we summarize the crustal structure of china with a new contour map of crustal thickness nine representative crustal columns and maps showing profile locations average crustal velocity and pn velocity the most remarkable aspect of the crustal structure of china is the well known 70 km thickness of the crust of the tibetan plateau the thick 45 70 km crust of western china is separated from the thinner 30 45 km crust of eastern china by the north south trending seismic belt 105 e the average crustal velocity of china ranges from 6 15 to 6 45 km s indicating a felsic to intermediate bulk crustal composition upper mantle pn velocities are 8 0 0 2 km s equal to the global continental average we interpret these results in terms of the most recent thermo tectonic events that have modified the crust in much of eastern china cenozoic crustal extension has produced a thin crust with a low average crustal velocity similar to western europe and the basin and range province western usa in western china mesozoic and cenozoic arc continent and continent continent collisions have led to crustal growth and thickening inferences on the process of crustal thickening are provided by the deep crustal velocity structure as determined by dss profiles and other seismological studies a high velocity 7 0 7 4 km s lower crustal layer has been reported in western china only beneath the southernmost tibetan plateau we identify this high velocity layer as the cold lower crust of the subducting indian plate as the indian crust is injected northward into the tibetan lower crust it heats and assimilates by partial melting a process that results in a reduction in the seismic velocity of the lower crust in the central and northern tibetan plateau

the oleaginous botryococcus from the triassic yanchang formation in ordos basin northwestern china morphology and its paleoenvironmental significance high abundance but rather low diversity algal fossils were found in the hydrocarbon source rocks of the ch 7 2 ch 7 3 section triassic yanchang formation in the xifeng area of southwest ordos basin which are mainly composed of prolific leiosphaeridia and botryococcus botryococcus colonies are of various forms the majority is nubbly with some of cluster and cotton shape the nubbly colonies appear globular cordiform ternate petal obtuse triangle chrysanthemum shape and so on most botryococcus are saffron or brown and are frequently covered with clay under transmission microscope and shows strong yellow and light brown under fluorescence microscope botryococcus could live in freshwater and brackish water the botryococcus colonies that lived in fresh water are small with small single cells arranged radially with undulant or indented edges the botryococcus colonies that lived in brackish water are bigger with larger single cells arranged irregularly with slippery contours the most of botryococcus are discovered from the organic rich argillaceous sediment with abundant pyrites in the semi and deep lake facies and shows they were preserved in low energy reducing environments taphonomic characteristics of various microfossils and the present of pediastrum in the phytoplankton flora indicate that they are in situ or near burial the lake area of the ordos basin was gradually expanding and reaching its most extensive flood surface in the ch 7 of yanchang formation interval during the middle and late triassic with warm climate plentiful rainfall and luxuriant vegetation as determined by the environmental analysis with botryococcus in xifeng area the presence of two ecological types of botryococcus indicates that the salinity of lake water was fluctuating in the ch 7 interval the occurrence of symbiotic acritarchs and geochemical salinity indices show that the ordos lake was a typical fresh water lake which was gradually desalted and its salinity fluctuation was narrow during the mid later triassic the ecological type of the palynological flora discovered from the ch 7 to ch 8 in xifeng area is similar to that from the fuxian lake with abundant botryococcus in the yungui plateau of china these findings imply that the ordos basin was in a lower latitude area of temperate to subtropical climate during the middle and late triassic

investigation of fluid inclusion and oil geochemistry to delineate the charging history of upper triassic chang 6 chang 8 and chang 9 tight oil reservoirs southeastern ordos basin china abstract the chang 6 chang 8 and chang 9 reservoirs of the upper triassic yanchang formation in the zhenjing area of the ordos basin are characterized by low porosity low permeability high oil saturation and large oil reserves which indicate huge tight oil potential to comprehensively investigate the hydrocarbon contributors and charging history fluid inclusions were systemically studied coupled with basin modeling and oil geochemical analysis according to the petrographic characteristics two episodes of fluid inclusions occurred in each of the three reservoirs microthermometry analysis suggests that the hydrocarbon inclusions of the first episode in each reservoir are distributed mainly along overgrowth quartz or secondary microcracks and exhibit light yellow or light brown fluorescence the coeval aqueous inclusions display low homogenization temperature th peaks with 80 100 c the hydrocarbon inclusions of the second episode in each reservoir are mainly distributed in microcracks that cut across or within quartz grains and exhibit light green or bluish green fluorescence the coeval aqueous inclusions are characterized by high th peaks with 110 120 c the different characteristics of the two episodes of the fluid inclusions imply the existence of two episodes of hydrocarbon charging events the burial and geothermal histories of the various reservoirs were reconstructed using one dimensional basin modeling and pressure volume temperature composition pvtx simulation coupled with the authigenic illite k ar dating the reconstruction implied the time of the two hydrocarbon charging episodes to be early cretaceous and late cretaceous oil source correlation studies illustrate that the chang 6 chang 8 and chang 9 crude oils all originate from the chang 7 source rock the migration characteristics and directions were comprehensively analyzed by using maturity parameters this explanation of the charging history and oil migration pathway of the upper triassic yanchang formation in the zhenjing area provides a theoretical basis for further exploration

uppermost mantle velocities beneath china and surrounding regions 1 seismic p wave velocity variations in the uppermost mantle beneath china are tomographically imaged using pn wave travel time data the average velocity of the mantle lid beneath china is 8 05 km s with a range from 0 30 km s to 0 35 km s eastern china has slower mantle velocities and a thinner crust than western china different inversions performed for shorter ray path and longer ray path data sets separated the shallow and deep structure within the mantle lid velocities in the lower lid are no more than 0 2 km s faster than at the top the fastest velocities are beneath cratons which remained relatively undeformed despite orogenic events around their margins the pn velocity beneath southern tibet is faster than that beneath northern tibet suggesting that this contrast delineates the northern edge of the indian lithosphere on the southeastern margin of the tibetan plateau the panxi rift slow pn velocity exists in the lithosphere due to the transextensional tectonic environment in eastern china extension of the north china block produced a series of nne ssw trending rifts that cause slow mantle lid velocities these slow velocities extend to the base of the mantle lid suggesting that late cenozoic pacific subduction drove the rifting both high temperature and hydration may contribute to these low uppermost mantle velocities the shanxi rift along the east margin of the ordos platform is characterized by a slow mantle velocity in contrast the yinchuan rift on the west margin does not show any uppermost mantle velocity anomaly

fracture detection in oil based drilling mud using a combination of borehole image and sonic logs abstract the deeply buried 5 5 7 1 km lower cretaceous bashijiqike sandstones are important fractured reservoirs in the kuqa depression in these high temperature high pressure drilling environments oil based muds are used to limit drilling risks and improve efficiency making facture detection using resistivity based image logs difficult the maximum horizontal stress sh max trends close to north south 0 10 based on orientations of the drilling induced fractures and borehole breakouts core descriptions thin sections and image logs used to describe the fractures in the bashijiqike sandstone reservoirs show that opening mode natural fractures are common and the fractures range from fully open to sealed by calcite and clay minerals the open fractures are dominantly steeply dipping or vertical and primarily strike north south locally horizontal fractures or fractures with low angle dip are detected we compare fracture visibility on image logs acquired in the same ks 2 2 12 key well in oil based mud and water based mud fmi hd logs successfully detect large 0 1 mm aperture vertical fractures in oil based mud better than obmi image logs and the two imaging tools have a similar ability in detecting the high dip angle fractures since the ubi tool is not affected by oil based muds most of the fractures can be picked out on ubi images minor negative deviations of bulk density could be observed in fractured zones and the presence of fractures obviously causes an increase in sonic transit time full waveforms from dsi tools combined with electrical image logs detect fractured zones both the velocity and amplitude of the sonic waves are sensitive to fractures and the fractured zones could be identified according to the attenuation of stoneley waves the combination of cores image logs and full wave sonic logs offers comprehensive detection of fractures in oil based mud environments

crustal structure of the northeastern margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin abstract the 1000 km long darlag lanzhou jingbian seismic refraction profile is located in the ne margin of the tibetan plateau this profile crosses the northern songpan ganzi terrane the qinling qilian fold system the haiyuan arcuate tectonic region and the stable ordos basin the p wave and s wave velocity structure and poisson s ratios reveal many significant characteristics in the profile the crustal thickness increases from northeast to southwest the average crustal thickness observed increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane the crust becomes obviously thicker south of the haiyuan fault and beneath the west qinlin shan the crustal velocities have significant variations along the profile the average p wave velocities for the crystalline crust vary between 6 3 and 6 4 km s beneath the songpan ganzi terrane west qinling shan and haiyuan arcuate tectonic region p wave velocities of 6 3 km s are 0 15 km s lower than the worldwide average of 6 45 km s north of the kunlun fault with exclusion of the haiyuan arcuate tectonic region the average p wave velocity is 6 4 km s and only 0 5 km s lower than the worldwide average a combination of the p wave velocity and poisson s ratio suggests that the crust is dominantly felsic in composition with an intermediate composition at the base a mafic lower crust is absent in the ne margin of the tibetan plateau from the songpan ganzi terrane to the ordos basin there are low velocity zones in the west qinling shan and the haiyuan arcuate tectonic region the low velocity zones have low s wave velocities and high poisson s ratios so it is possible these zones are due to partial melting the crust is divided into two layers the upper and the lower crust with crustal thickening mainly in the lower crust as the ne tibetan plateau is approached the results in the study show that the thickness of the lower crust increases from 22 to 38 km as the crustal thickness increases from 42 km in the ordos basin to 63 km in the songpan ganzi terrane south of the kunlun fault both the conrad discontinuity and moho in the west qinling shan and in the haiyuan arcuate tectonic region are laminated interfaces implying intense tectonic activity the arcuate faults and large earthquakes in the haiyuan arcuate tectonic region are the result of interaction between the tibetan plateau and the sino korean and gobi ala shan platforms

charging of carboniferous volcanic reservoirs in the eastern chepaizi uplift junggar basin northwestern china constrained by oil geochemistry and fluid inclusion substantial amounts of petroleum were recently discovered in the carboniferous andesite tuff breccia and basalt reservoirs of the chepaizi uplift in the western junggar basin however the charging history of the carboniferous petroleum reservoir is poorly understood oil oil correlation studies indicate that all of the oils were mainly derived from the middle permian wuerhe formation source rocks possibly mixed with a small contribution from carboniferous baogutu formation source rocks in the neighboring changji sag based on the petrographic and microthermometry of fluid inclusions two hydrocarbon charging episodes are defined these episodes were characterized by a low peak range homogenization temperature th distribution 80 c 90 c and high salinity 13 22 13 42 wt nacl and a high peak range th distribution 120 c 130 c and low salinity 4 89 11 72 wt nacl respectively through one dimensional basin modeling and pressure volume temperature composition simulation the burial thermal histories for wells p61 p66 p668 and p663 were reconstructed and their trapping temperatures of the hydrocarbon inclusions were calculated to be higher than their corresponding highest paleotemperature i e 56 8 c 53 7 c 60 9 c and 58 1 c respectively implying fast hydrocarbon charging processes promoted by deep hydrothermal fluids associated with the hydrocarbon generation history sealing process of the hongche fault and regional tectonic evolution these two hydrocarbon charging events were deduced as the adjustments of oils previously accumulated along the hongche fault zone because of the tectonic extension in the paleogene and regional tilting in the neogene respectively the general direction of oil charging was traced from south to north and from east to west as indicated by the molecular parameters of nitrogen bearing compounds and c20 c21 triaromatic steroids c20 c21 c26 c28 triaromatic steroids ta i ta i ii which roughly coincided with the active fracturing

lithospheric structure of the ordos block and its boundary areas inferred from rayleigh wave dispersion abstract we have measured rayleigh wave phase velocities of the ordos block and its southern and western boundary areas and inverted these data to describe the upper mantle structure the ordos block was found to be characterized by distinctly high seismic velocities at depths commonly associated with a low velocity asthenosphere to the south abnormally slow velocity anomalies appeared in the weihe rift probably indicating subsolidus grain boundary weakening effect under high temperatures near solidus instead of partial melt in the asthenosphere an intermediate seismic velocity structure was observed beneath the northern qinling orogen nql to the south of the weihe rift a thin high velocity lid which was probably a cratonic relic was found under the haiyuan arcuate tectonic region hatr west of the ordos block a thick lithosphere was present beneath the ordos block 200 km and the nql 140 km while a thinner lithosphere 100 km was found beneath the hatr and the weihe rift the observed structural differences were likely associated with different lithospheric tectonics in the study region whereas the weihe rift and the hatr had been strongly affected by cenozoic lithospheric modification and thinning the ordos block and the nql appeared to have been much less influenced combined with previous studies our results suggested that the far field effect of the indo asia collision may have played an important role in the unique lithospheric process in the study region such as the extension of the weihe rift the preservation of a cratonic relic under the hatr and the localized intense lithospheric modification of these two regions the substantial variations of deep structure and thus contrasting rheological properties between the ordos block and the surrounding notable seismic belts the hatr and weihe rift may be a candidate mechanism for the contrasting seismicity of the region the lateral variations in the lithospheric structure estimated here generally agreed with changes in upper mantle deformation patterns previously derived from sks splitting measurements

p and s wave velocity model along crustal scale refraction and wide angle reflection profile in the southern korean peninsula abstract the onshore seismic experiment kcrt2004 explored the crustal velocity structure of the southern korean peninsula we present an interpretation of seismic data along this 340 km long nnw sse trending profile the crust was found to consist of three layers the upper middle and lower crust with p and s wave velocities ranging 5 50 to 6 95 km s and 2 82 to 3 91 km s respectively the average p wave velocity 6 26 km s and pn velocity 7 82 7 88 km s are lower than the worldwide average of continental crust moho depths are 29 0 34 9 km gradually thickening toward south the vp vs ratio of crustal material is estimated to be 1 73 0 249 for the upper and middle crust and the ratio increases with the depth in the lower crust the gyeonggi massif in the north of the profile has a lower vp vs ratio than other tectonic units the average crustal vp vs ratio of 1 74 0 253 is remarkably lower than the average value 1 78 0 27 for the bulk continental crust the low average crustal vp vs ratio is similar to that measured in eastern china the empirical analysis using both p wave velocity and vp vs ratio shows that the upper and middle crust is dominantly felsic and the lower crust is intermediate in composition the absence of the mafic material in the lower crust that is also found in eastern china contrasts with the generally accepted global model of the mafic lower crust

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

control of reservoir properties of tensleep sandstone by depositional and diagenetic facies lost soldier field wyoming the pennsylvanian tensleep sandstone in the lost soldier field of sweetwater county wyoming is composed of texturally homogeneous fine grained sandstones and interbedded dolomite beds geologic setting and sedimentary features indicate that the tensleep was deposited in supratidal eolian environments detailed petrographic cathodoluminescence and sem analyses of core samples from a well in the lost soldier field show authigenic features which resulted from nine diagenetic events in the tensleep 1 pervasive anhydrite cementation 2 precipitation of feldspar overgrowths 3 poikilotopic nodular calcite cementation 4 nodular anhydrite cementation 5 fabric selective anhydrite cementation 6 precipitation of silica overgrowths and silicification of carbonates 7 alteration of feldspar 8 dolomite cementation and 9 hydrocarbon accumulation four diagenetic facies with their own specific patterns of alterations were identified 1 early carbonate precipitation facies 2 early anhydrite cementation facies 3 primary carbonate grain facies and 4 nodular cementation facies the carbonate rich diagenetic facies are closely associated with the supratidal sebkha depositional environment the anhydrite rich facies is related to the interdune sebkha depositional environment and the nodular cementation facies is found within rocks deposited in the eolian dune environment reservoir qualities i e porosity and permeability of the tensleep sandstone in the lost soldier field vary considerably both vertically and laterally porosity and permeability are primarily controlled by compaction and cementation good correlation exists between reservoir quality and volume of cements for samples from nodular cementation and primary carbonate grain diagenetic facies samples from other diagenetic facies can also be grouped into distinct areas on a cross plot of porosity versus volume of cements therefore the cross plot can be used as a tool to differentiate different diagenetic facies at least for the evaporitic eolian rock system

impact of sedimentology diagenesis and solid bitumen on the development of a tight gas grainstone reservoir in the feixianguan formation jiannan area china implications for gas exploration in tight carbonate reservoirs abstract tight gas grainstone reservoirs in the third member of the feixianguan formation jiannan area evolved from a paleo oil accumulation as evidenced from abundant solid reservoir bitumen porosity evolution of the grainstones was studied by evaluating relative influences of sedimentology diagenesis and solid bitumen formed during cracking of accumulated oils grainstones exhibited regional distinct effectiveness for paleo oil and present gas accumulations during oil window and subsequent gas window diagenesis in the southern zone where grainstones were not subjected to subaerial exposure and meteoric diagenesis in the early diagenetic stage paleoporosity at the time of oil charge was mainly controlled by sedimentologic factors e g grain size sorting and grain type and paleo oil reservoirs only occurred in the ooid dominated grainstones with good sorting and coarse grain size in contrast in the northern zone meteoric diagenesis was responsible for paleoporosity preservation due to the early mineral stabilization of grains and meteoric calcite cementation which caused grainstones greater resistance to compaction hence most of the grainstones in the northern zone regardless of textural variables formed effective reservoirs for paleo oil accumulation as the oil cracked to gas with increasing depth and temperature during the late oil window and initial gas window solid bitumen occluded reservoir pores to varying degrees and caused paleo oil reservoirs to be significantly heterogeneous or completely ineffective for gas accumulation in contrast most grainstones that were once ineffective oil reservoirs transformed into effective gas reservoirs due to no or minor influence of solid bitumen precipitation the model of reservoir transformation development of tight grainstones provides a plausible explanation for key observations concerning the diagenetic and distribution differences between paleo oil and present gas reservoirs it is useful in predicting the distribution of potential reservoirs in carbonate strata in future exploration

depositional facies diagenesis and their impact on the reservoir quality of silurian sandstones from tazhong area in central tarim basin western china abstract the silurian sandstones of the tazhong area are an important oil exploration target in the tarim basin western china this study deals with the lower silurian of the kalpingtag formation which was deposited in a braided delta system four major depositional microfacies are identified consisting of distributary channel interdistributary bay front bar and sand sheet in spite of their large lateral extent exploration of these sandstones is complicated by variation of sediment facies and intense diagenesis which strongly affected reservoir quality depositional environment exerted an essential control on the reservoir quality higher energy well sorted and relatively coarser grained sandstones have better quality than low energy fine grained clay rich sandstones diagenesis was mainly composed of mechanical and chemical compaction and cementation by quartz overgrowths carbonates and various clay minerals mechanical compaction was a significant porosity reducing agent in the silurian sandstones the presence of carbonate cement exerts a dominant influence on porosity and permeability in tazhong area other cements are less important but quartz cement is significant the third significant element is clay minerals the porosity reduction by compaction and cementation and the relative timing of them indicates that compaction was much more important than cementation in reducing porosity

quantitative impact of diagenesis on reservoir quality of the triassic chang 6 tight oil sandstones zhenjing area ordos basin china abstract the upper triassic chang 6 sandstone an important exploration target in the ordos basin is a typical tight oil reservoir reservoir quality is a critical factor for tight oil exploration based on thin sections scanning electron microscopy sem x ray diffraction xrd stable isotopes and fluid inclusions the diagenetic processes and their impact on the reservoir quality of the chang 6 sandstones in the zhenjing area were quantitatively analysed the initial porosity of the chang 6 sandstones is 39 2 as calculated from point counting and grain size analysis mechanical and chemical compaction are the dominant processes for the destruction of pore spaces leading to a porosity reduction of 14 2 20 2 during progressive burial the porosity continually decreased from 4 3 to 12 4 due to carbonate cementation quartz overgrowth and clay mineral precipitation diagenetic processes were influenced by grain size sorting and mineral compositions evaluation of petrographic observations indicates that different extents of compaction and calcite cementation are responsible for the formation of high porosity and low porosity reservoirs secondary porosity formed due to the burial dissolution of feldspar rock fragments and laumontite in the chang 6 sandstones however in a relatively closed geochemical system products of dissolution cannot be transported away over a long distance as a result they precipitated in nearby pores and pore throats in addition quantitative calculations showed that the dissolution and associated precipitation of products of dissolution were nearly balanced consequently the total porosity of the chang 6 sandstones increased slightly due to burial dissolution but the permeability decreased significantly because of the occlusion of pore throats by the dissolution associated precipitation of authigenic minerals therefore the limited increase in net porosity from dissolution combined with intense compaction and cementation account for the low permeability and strong heterogeneity in the chang 6 sandstones in the zhenjing area

depositional process and astronomical forcing model of lacustrine fine grained sedimentary rocks a case study of the early paleogene in the dongying sag bohai bay basin abstract the paleogene strata of the bohai bay basin include multiple sets of lacustrine fine grained sediments which provide a detailed record of the climate changes that occurred in the lake basin early paleogene fine grained sedimentary rocks are the main target layers of continental shale oil exploration in eastern china therefore studies of the geological responses to astronomical factors have important theoretical and practical significance for understanding the depositional process of lacustrine fine grained sedimentary rocks in this paper we focus on the lower es3 es3l upper es4 es4u units in four shale oil wells in the dongying sag of the bohai bay basin based on the previous establishment of a high resolution astronomical time scale in this area we have further studied the petrological characteristics and lithofacies of the fine grained sedimentary rocks then the geological responses to astronomical cycles on different scales are investigated and the models for the depositional process of fine grained sedimentary rocks under the control of astronomical factors are revealed the results indicate that the cyclic changes in mineral content and lithofacies in fine grained sedimentary rocks in the dongying sag are controlled by short eccentricity and precession cycles periods near the maximum short eccentricity range had large precession amplitudes humid climates and the development of organic rich laminated calcareous mudstone lithofacies in contrast periods near the minimum short eccentricity range had small precession amplitudes a dry climates and the development of organic containing layered muddy limestone lithofacies this study explores the depositional process of fine grained sedimentary rocks from the perspective of astronomical cycles which is conducive to the comprehensive prediction of favorable shale oil layers

3d coseismic deformations and source parameters of the 2010 yushu earthquake china inferred from dinsar and multiple aperture insar measurements abstract the coseismic deformation field related to the 2010 mw6 9 yushu earthquake qinghai china has been investigated by previous studies using the satellite differential interferometric synthetic aperture radar dinsar method however one of the limiting factors in the conventional dinsar is that it can measure only one dimensional 1d ground motion along the radar line of sight los direction thus resulting in anamorphic coseismic displacements and biased source parameters in this paper the three dimensional 3d deformation maps for this earthquake were reconstructed over an area of about 100 70 km2 for the first time by a combined use of the dinsar method for detecting los motion and the multiple aperture interferometry mai method for detecting along track at motion the fault slip map for this earthquake was generated using the 3d deformation maps based on the fault traces on the fault slip map we constructed a fault model with five segments and preformed a two step inversion using the 3d deformation data as the constraint our results show that the maximum coseismic displacements in up down north south and east west directions are 40 4 113 8 and 65 7 cm respectively and the peak to peak slip offset between the southern and northern fault wall is 1 82 m our model reveals a shallow dislocation on a nw se trending fault extending about 74 km along which the four fault segments are nearly vertical 84 88 and the one fault segment dips 66 2 to the southwest the estimated seismic moment is 2 43 1019 nm mw6 92 which is close to the solution provided by the global centroid moment tensor gcmt project the slip distribution derived for each fault patch shows that the slip occurrence extends down to 12 km beneath the ground surface and a peak slip of 2 23 m appears at the depth of about 5 km right near the epicenter comparison with the results derived from the seismic waves field investigation gps and conventional dinsar observations indicates that the source parameters of the yushu earthquake can be improved due to the combined use of dinsar and mai measurements

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

outcrop architecture of a fluvio lacustrine succession upper triassic yanchang formation ordos basin china abstract although several studies have considered the sedimentary facies of the upper triassic yanchang formation in terms of their hydrocarbon potential in the petroliferous ordos basin such studies have mostly interpreted subsurface data and few have systematically examined the detailed outcrop based anatomy of this succession this study characterizes a series of well exposed outcrop sections along an 80 km long e w oriented series of road cuttings in the southeast ordos basin to reconstruct the sedimentary architecture of a fluvio lacustrine succession and determine the principal controls that governed the style of accumulation and preservation the yanchang formation is divided into 10 depositional units chang 10 base to chang 1 top that record the sedimentary evolution of a series of large scale fluvial systems that constructed delta front and pro delta bodies as they entered a large interior draining lake detailed analysis of the internal architectures of the delta front bodies records the mechanism of growth of a large scale shallow water delta succession lacustrine subaqueous distributary channels are composed of 3 15 m thick sandstone fills such channelized bodies are highly elongate and many can be traced several kilometres such bodies can be shown to be commonly associated with thin bedded mouth bar and subaqueous interdistributary bay deposition the progradational architecture of the delta front elements is difficult to recognize on outcrop scale deposits of the chang 10 to 1 units record 4 major lacustrine transgressive regressive cycles during the late triassic transgressive events are signified by the widespread occurrence of 10 20 m thick oil shale prone intervals regressive events are signified by the progradational stacking towards the basin centre of sandstone prone facies associations mainly accumulated in fluvial channel delta plain and upper delta front environments the uppermost chang 1 interval records late stage filling of the lacustrine basin the transgressive regressive cycles equate to sequences that can be correlated across the region development of which was controlled by a combination of changes in the rate of tectonic subsidence the rate of sediment delivery from the basin margin hinterland and climate factors that were themselves influenced by intracontinental growth of the neighbouring qinling orogen

impact of diagenesis on reservoir quality and heterogeneity of the upper triassic chang 8 tight oil sandstones in the zhenjing area ordos basin china abstract reservoir quality and heterogeneity are critical risk factors in tight oil exploration the integrated analysis of the petrographic characteristics and the types and distribution of diagenetic alterations in the chang 8 sandstones from the zhenjing area using core log thin section sem petrophysical and stable isotopic data provides insight into the factors responsible for variations in porosity and permeability in tight sandstones the results indicate that the chang 8 sandstones mainly from subaqueous distributary channel facies are mostly moderately well to well sorted fine grained feldspathic litharenites and lithic arkose the sandstones have ultra low permeabilities that are commonly less than 1 md a wide range of porosities from 0 3 to 18 1 and two distinct porosity permeability trends with a boundary of approximately 10 porosity these petrophysical features are closely related to the types and distribution of the diagenetic alterations compaction is a regional porosity reducing process that was responsible for a loss of more than half of the original porosity in nearly all of the samples the wide range of porosity is attributed to variations in calcite cementation and chlorite coatings the relatively high porosity reservoirs formed due to preservation of the primary intergranular pores by chlorite coatings rather than burial dissolution however the chlorites also obstruct pore throats which lead to the development of reservoirs with high porosity but low permeability in contrast calcite cementation is the dominant factor in the formation of low porosity ultra low permeability reservoirs by filling both the primary pores and the pore throats in the sandstones the eogenetic calcites are commonly concentrated in tightly cemented concretions or layers adjacent to sandstone mudstone contacts while the mesogenetic calcites were deposited in all of the intervals and led to further heterogeneity this study can be used as an analogue to understand the variations in the pathways of diagenetic evolution and their impacts on the reservoir quality and heterogeneity of sandstones and is useful for predicting the distribution of potential high quality reservoirs in similar geological settings

modes of raising northeastern tibet probed by explosion seismology abstract new wide angle reflection and refraction seismic data provide constraints on the structure of the upper lithosphere and test models of its evolution to raise the northeastern part of the tibetan plateau amplitudes observed for reflections from the crust mantle boundary are sufficiently large to suggest that there is no significant partial melt in the deep crust the data show an increase of the crustal thickness between terranes from north of the kun lun fault into the qang tang of central tibet and a contrast among their intracrustal images and compositions in the north p and s velocities are consistent with a dominantly felsic composition and show that only the upper crust thickened south of the kun lun fault a thicker crust made of two layers could result from the superposition of the originally thin crust of the bayan har terrane on the lower part of the crust of the domain to the north which upper crust it shoved and thickened different modes of crustal thickening either by thickening of individual layers or superpositions and imbrication among them appear to work jointly to raise the topography

identification of sedimentary diagenetic facies and reservoir porosity and permeability prediction an example from the eocene beach bar sandstone in the dongying depression china abstract accurate prediction of reservoir porosity and permeability is essential for prospecting hydrocarbon reserves and petroleum production capacity we propose an innovative reservoir porosity and permeability prediction method through identifying sedimentary diagenetic facies determining the porosity permeability trends using core measurement data extrapolating the spatial distribution of the sedimentary diagenetic facies using log data through the bayes discriminant analysis and predicting the reservoir porosity and permeability the essence of the method was illustrated and its effectiveness was demonstrated using the eocene beach bar sandstones in the dongying depression bohai bay basin eastern china the eocene beach bar sandstones are classified into fine sandstone siltstone and argillaceous siltstone based on grain sizes sorting and matrix contents the major diagenetic processes that influence the porosity and permeability of the beach bar sandstones are compaction carbonate cementation and feldspar dissolution seven sedimentary diagenetic facies were identified in the beach bar sandstones based on the lithological types and their corresponding diagenesis and influence extent on reservoir properties the variation ranges of porosity and permeability log k of these sedimentary diagenetic facies are typically less than 6 and 1 2 respectively both the porosity and permeability have well defined functional relationships with depth the sedimentary diagenetic facies can be identified effectively from logging data through the use of the bayes discriminant analysis and corresponding cross plots the porosity and permeability log k of the well evaluated in the study were predicted with errors in the range of 3 and 0 6 respectively based on the distribution and the fitting equation of the trend lines for the seven sedimentary diagenetic facies the predicted porosities and permeability of sedimentary diagenetic facies match the measured porosities and permeability well