earth stress and reservoir quality evaluation in high and steep structure the lower cretaceous in the kuqa depression tarim basin china abstract knowledge of the state of the in situ stress field provides important insights into the petroleum engineering and geology the in situ stress orientation and magnitudes in the high and steep structure of lower cretaceous bashijiqike formation in kuqa depression were analyzed based on well log calculations and the impacts of in situ stress field on reservoir quality macroscopic reservoir properties and microscopic pore structure as well as fracture aperture were investigated the maximum horizontal stress shmax orientation was determined as nearly north south according to the image log derived drilling induced fractures and borehole breakouts the vertical stress sv shmax and minimum horizontal stress shmin magnitudes were calculated via well logs the result reveals a dominant strike slip stress regime shmax sv shmin in the bashijiqike formation in the kuqa depression the sv at the depth ranging from 5000 to 7100 m is 150 180 mpa and horizontal stresses shmax and shmin vary significantly from 130 to 210 mpa the horizontal stress difference is in the range from 40 to 70 mpa and fluid overpressures are commonly encountered the effective stress e has a positive relationship with the when the is less than 50 mpa intergranular pores can be preserved in layers with low values the microscopic pore structure will be complex and macroscopic reservoir quality will be poor with the increasing values in addition fractures are mainly corresponding with the layers where the value is low and the fracture aperture and porosity decreased significantly with the effective stress e by determining the magnitude and orientation of in situ stresses around the wellbore insights can be provided into the wellbore instability optimum drilling trajectories and reservoir quality evaluation

phase based dispersion analysis for acoustic array borehole logging data a phase based dispersion analysis method for velocity slowness extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented the technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component the approach is nonparametric and completely data driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes results are free of the aliases and spurious modes which are characteristic of some prior approaches examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools results are compared with those from the weighted spectral semblance wss and amplitude and phase slowness estimation apes methods to demonstrate the effectiveness and utility of the proposed method

the tectonics and petroleum system of the qiulitagh fold and thrust belt northern tarim basin nw china the timing of hydrocarbon charging relative to trap formation is a key factor for evaluating the exploration potential of the qiulitagh fold and thrust belt qftb on the north flank of the tarim basin data obtained by this study indicate that structural traps were filled by hydrocarbons in two episodes in this area at least four major deformation events have been identified for the period of late cretaceous to pliocene the first event occurred in late cretaceous and led to the formation of paleo uplift structures the second event was characterized by thrust features and occurred in latest oligocene the third and fourth events were represented by compressional structures that were developed in late miocene and late pliocene respectively based on oil and gas source rock correlations and 1 d basin modeling two episodes of hydrocarbon generation and migration are proposed to be responsible for oil and gas accumulations in the qftb the two episodes of hydrocarbon charging from a source kitchen north of the study area took place in early miocene and pliocene respectively since the qftb was uplifted in late oligocene and late miocene the hydrocarbon generated during all two periods could migrate toward this area to form oil and gas accumulations in suitable structures

present day stress field rotations associated with evaporites in the offshore nile delta evaporitic horizons are routinely interpreted to act as mechanical detachment sequences and thus signifi cantly infl uence the structural evolution of sedimentary basins and fold thrust belts however over 30 years of global in situ stress analysis have provided only poor evidence to support this widespread assumption this study examines present day stress orientations inferred from borehole breakout and drilling induced fractures in 44 boreholes in the offshore nile delta a total of 446 breakouts and 19 drilling induced fractures from 37 wells reveal sharply contrasting present day maximum horizontal stress shmax orientations in sequences above and below the extensive messinian evaporites of the eastern nile delta a typical deltaic margin parallel shmax e w in the central and ese wnw in the eastern nile delta is observed in parts of the nile delta that are below or do not contain evaporites however a scattered but largely margin normal nne ssw shmax is observed in sequences underlain by evaporites the 90 variation in present day shmax orientation above and below the messinian salts provides the fi rst convincing in situ evidence that evaporite sequences can act as major mechanical detachment horizons in addition the margin normal shmax orientation is subperpendicular to the strike of nearby active extensional faults indicating the existence of non andersonian faulting in the suprasalt region furthermore the evidence that the messinian evaporites act as an effective mechanical detachment suggests that suprasalt faulting in the eastern nile delta is not the result of basement related deformation and thus raises doubts about the often postulated extension of the suez fault zone into the eastern mediterranean

global crustal stress pattern based on the world stress map database release 2008 the world stress map wsm project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators the wsm database release 2008 contains 21 750 stress data records that are quality ranked using an updated and refined quality ranking scheme almost 17 000 of these data records have a c quality and are considered to record the orientation of maximum horizontal compressional stress sh to within 25 as this is almost a triplication of data records compared with the first wsm database release in 1992 we reinvestigate the spatial wave length of the stress patterns with a statistical analysis on a global 0 5 grid the resulting smoothed global stress map displays both the mean sh orientation that follows from the maximum smoothing radius for which the standard deviation is b25 and a countour map that displays the wave length of the stress pattern this smoothed global map confirms that long wave length stress patterns n2000 km exist for example in north america and ne asia these have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region however our analysis reveals that rather short wave length of the stress pattern b200 km are quite frequent too particularly in western europe alaska and the aleutians the southern rocky mountains basin and range province scandinavia caucasus most of the himalayas and indonesia this implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern

fractured tight sandstone oil and gas reservoirs a new play type in the dongpu depression bohai bay basin china although conventional reservoirs dominate the bohai basin china a new type of sandstone reservoir also exists in the dongpu depression that has a low matrix porosity tight in which natural fractures govern both permeability and porosity these fractured sandstones are located on a structurally modified buried hill underlying paleogene mudstones and are truncated along an angular unconformity the fractured sandstone oils of the triassic liujiagou heshanggou and ermaying formations are derived from the paleogene shahejie formation which reached peak oil generation and expulsion during the oligocene to early miocene 32 8 15 6 ma gas was generated primarily during the paleogene from carboniferous and permian coals petrographic evidence suggests that oil and gas emplacement followed the compaction and cementation of the triassic sandstone reservoirs fluid inclusion evidence and burial history analysis suggest that fractures developed before oil emplacement but may have coincided with peak gas generation which suggests that oil and gas mainly migrated and accumulated in fractures

fracture apertures from electrical borehole scans three dimensional finite element modeling was performed to investigate the response to fractures of the formation microscanner mark of schlumberger which records high resolution electrical scans of the borehole wall it is found that the equation w carmbrx0l describes over two orders of magnitude of resistivity contrasts between borehole mud and the formation the relationship between fracture width w in mm formation sensitivity rx0 mud resistivity rm and the additional current flow a caused by the presence of the fracture a is the additional current which can be injected into the formation divided by the voltage integrated along a line perpendicular across the fracture trace coefficient c and exponent b are obtained numerically from forward modeling tool standoffs of up to 2 5 mm and fracture dips in the range from 0 to 40 were found to have an insignificant effect on the above relation a three step approach to detect trace and quantify fractures is used potential fractures in formation

a preliminary study of the present day in situ stress state in the ahe tight gas reservoir dibei gasfield kuqa depression abstract knowledge of the present day in situ stress state has significant applications in the exploration and development of tight gas reservoirs the ahe formation is an important tight gas reservoir in the dibei gasfield of kuqa depression however prior to this study little attention has been paid to the present day in situ stress field within the formation in the present study the in situ stress orientation and magnitudes were investigated based on well log calculations and geomechanical modeling the horizontal maximum principal stress shmax orientation was determined from interpretations of drilling induced tensile fractures ditfs and borehole breakouts in imaging logs which showed variations between nnw sse trending and nne ssw trending in the dibei gasfield the in situ stress magnitudes were calculated in four wells based on well logs the results indicated a normal faulting stress regime within the ahe tight gas reservoir numerical simulation of the present day in situ stresses showed that the magnitudes of vertical stress sv shmax and horizontal minimum principal stress shmin were 105 5 mpa 191 0 mpa 88 9 mpa 142 9 mpa and 79 1 mpa 127 7 mpa within the ahe formation respectively in addition considering the present day in situ stress state in the ahe formation of dibei gasfield natural fractures in directions parallel sub parallel to the shmax orientation with high fracture angles showed great contributions to subsurface fluid flow borehole instability may become a potentially significant problem when drilling vertical wells and horizontal wells deviated toward the shmax orientation in the ahe tight gas reservoir of dibei gasfield

crustal stress determination from boreholes and rock cores fundamental principles abstract obtaining quantitative information on the state of stress in the crust can only be accomplished by accessing the zone of interest by drilling the borehole cavity however disrupts the virgin stress state by concentrating stresses in predictable patterns with a number of consequences at sufficiently low stress magnitudes the concentrated stresses amplify the elastic anisotropy azimuthally around the borehole because the elastic properties of most rocks depend nonlinearly on stress at higher levels the stresses damage then fracture the rock near the wellbore wall or within a growing core stub indeed almost all of the borehole techniques indirectly measure stress through these manifestations of the concentrated stresses this contribution reviews at a fundamental level the concentration of stresses by the borehole the effects on the materials in the borehole s vicinity and how these are used to infer stress states stress concentrations applied to rocks which are generally nonlinear elastic materials induce both radial and azimuthal variations in elastic wave speeds near the borehole that can be used to infer stress directions from advanced acoustic logging methods hydraulic fractures initiate propagate and remain propped open by measured pressures that can be interpreted with knowledge of the stress concentrations to obtain quantitative stress magnitudes at higher stress levels the rock fails producing borehole breakouts or drilling induced tensile fractures both of which are indicative of stress directions and can be used to constrain stress magnitudes similarly the various styles of drilling induced core fractures indicate both stress directions and the faulting environment encountered unfortunately no method currently allows for complete determination of the stress tensor using a number of different but complementary techniques is necessary to best constrain in situ stress states

the case for dynamic subsidence of the u s east coast since the eocene the dynamic subsidence of the united states east coast is addressed using the discrepancy between regional and global estimates of sea level elevation of paleoshorelines and adjoint models of mantle convection that assimilate plate motions and seismic tomography the positions of eocene and miocene paleoshorelines are lower than predicted by global sea levels suggesting at least 50 m and possibly as much as 200 m of subsidence since the end of the eocene dynamic models predict subsidence of the east coast since the end of eocene although the exact magnitude is uncertain this subsidence has been occurring during an overall global sea level fall with the eustatic change being larger than the dynamic subsidence this results in a regional sea level fall in the absence of land subsidence dynamic subsidence is consistent with the difference between eustasy and regional sea level at the new jersey coastal plain

seismic seafloor observations a study of anomalous intraplate seismicity in the northeastern indian ocean abstract present day intraplate tectonic activity in the northeast indian ocean which manifests itself in high seismicity is unusual for oceanic lithosphere plates the contradictory points of the various tectonic hypotheses proposed to explain this anomalous seismicity may be due to sparsity of ocean intraplate earthquakes of m 4 5 which are recorded by means of networks of land based permanent seismic stations seismological observations on the ocean floor by means of ocean bottom seismographs may provide additional and important data some results of such observations carried out in the central indian basin simultaneously with deep seismic sounding surveys are analysed in this paper about 118 seismic events were recorded at a site near 3 45 s and 80 e for a period of 7 3 days and 24 events were recorded at a site near 0 50 s and 83 53 e for a period of 3 3 days magnitudes of these events range approximately from 2 8 to 4 0 estimates of epicentre locations based on the jeffreys bullen travel time table show that over 90 of the earthquakes recorded at the first site were related to the chagos trough during a period when no event was recorded there by land based seismic stations the most probable reason for these events is the chagos bank earthquake of 30 november 1983 m s 7 6 which caused many aftershocks extending eastward from the main shock thus the high aftershock activity east of the chagos bank earthquake revealed by ocean bottom seismic observations complements the data on epicentres of larger earthquakes which in turn provide the scope to extend the known seismicity of the chagos bank further east in general our results are in accordance with the most popular tectonic model of the central indian ocean in which a diffuse plate boundary divides the australian plate from the indo arabian or the indian plate initiating a new subduction zone

the present day state of tectonic stress in the darling basin australia implications for exploration and production knowledge of the full present day stress tensor and pore pressure has significant applications in the exploration and production of conventional and unconventional hydrocarbon reservoirs the darling basin of new south wales australia is an old sedimentary basin late cambrian silurian to early carboniferous in which there was limited information about the present day stress field prior to this study in this paper we evaluate the contemporary stress field of the darling basin using a dataset from recent exploration wells and perform a geomechanical risk assessment with respect to borehole stability fracture fault generation and reactivation our interpretations of borehole failures in borehole image logs reveal a prevailing east west orientation of the maximum horizontal stress throughout the darling basin the estimates of the magnitudes for the vertical minimum and maximum horizontal stress in the studied wells indicate a transition between thrust and strike slip faulting stress regime at 600 700 m depths where the magnitude of vertical stress and minimum horizontal stress are close to each other however the presence of borehole breakouts and drilling induced tensile fractures that we observe in the image logs at greater depths 900 2100 m indicate a transition into a strike slip tectonic stress regime below a depth range of approximately 700 900 m these findings are in agreement with overcoring stress measurements east and west of the investigated wells furthermore there are several neogene to recent geological structures in the study area that indicate thrust faulting with an east west oriented maximum horizontal stress orientation around this old sedimentary basin the consistency between the orientation of maximum horizontal stress determined from wellbore data and neotectonic structures is significant and implies that horizontal stress orientations derived from very recent geological features may be valuable inputs to geomechanical models in the absence of wellbore or other data however the recent surface geological structures suggest a thrust faulting stress regime that is in slight contrast to the transition between thrust and strike slip stress regime sh sh similar to sv indicated by petroleum data and highlights a potential pitfall of using neotectonic structures in geomechanical models in particular careful attention and verification should be made when using neotectonic structures for input calibration or confirmation of geomechanical models especially in intraplate tectonic settings such as australia c 2016 elsevier ltd all rights reserved

on the temporal evolution of long wavelength mantle structure of the earth since the early paleozoic the seismic structure of the earth s lower mantle is characterized by a dominantly degree 2 pattern with the african and pacific large low shear velocity provinces i e llsvp that are separated by circum pacific seismically fast anomalies it is important to understand the origin of such a degree 2 mantle structure and its temporal evolution in this study we investigated the effects of plate motion history and mantle viscosity on the temporal evolution of the lower mantle structure since the early paleozoic by formulating 3 d spherical shell models of thermochemical convection for convection models with realistic mantle viscosity and no initial structure it takes about 50 myr to develop dominantly degree 2 lower mantle structure using the published plate motion models for the last either 120 ma or 250 ma however it takes longer time to develop the mantle structure for more viscous mantle while the circum pangea subduction in plate motion history models promotes the formation of degree 2 mantle structure the published pre pangea plate motions before 330 ma produce relatively cold lower mantle in the african hemisphere and significant degree 1 structure in the early pangea 300 ma or later times even if the lower mantle has an initially degree 2 structure and a viscosity as high as 1023 pas this suggests that the african llsvp may not be stationary since the early paleozoic with the published plate motion models and lower mantle viscosity of 1022 pas our mantle convection models suggest that the present day degree 2 mantle structure may have largely been formed by 200 ma

deep burial diagenesis and reservoir quality evolution of high temperature high pressure sandstones examples from lower cretaceous bashijiqike formation in keshen area kuqa depression tarim basin of china the deep high temperature high pressure lower cretaceous bashijiqike sandstone buried to depths as great as 6 5 7 1km is an important natural gas reservoir in keshen gas field kuqa depression of the tarim basin reservoir quality is a critical risk factor in the development of these ultradeep reservoirs integrated approaches incorporating routine core analyses mineralogical petrographic and geochemical analyses have been used to investigate the diagenetic history of these rocks and their impact on reservoir quality with the aim to unravel the mechanisms for maintaining anomalously high porosities in sandstones that are buried to such a great depth these sandstones are dominantly fine to medium grained moderately to good sorted lithic arkoses and feldspathic litharenite most primary pores have been lost by mechanical compaction or carbonate cementation and the reduction of porosity by mechanical compaction was more significant than by cementation dissolution of framework grains contributed to the enhancement of reservoir quality eogenetic diagenetic alterations mainly include mechanical compaction precipitation of calcite cements and grain coating clays mesogenetic diagenesis is characterized by dissolution of framework grain by organic acids and subsequent precipitation of clay minerals and quartz infiltration of meteoric water related to teleodiagenesis would result in dissolution of the framework grains the meteoric leaching events during teleodiagenesis are of great importance for the bashijiqike sandstones grain coating clay minerals mixed layer illite smectite clays help to preserve porosity at depth by retarding quartz cementation and pressure solution the unique burial regime as early stage shallow burial with late stage rapid deep burial contributes to porosity preservation in eodiagenesis fluid overpressure caused by intense structural compression in the middle himalayan movement retarded compaction and helped preserve porosity in the late rapid deep burial stage anomalously high porosities are mainly found in medium grained good sorted sandstones with grain coating clays but with low clay and carbonate cement content of which the porosity is preserved primarily and enhanced secondarily the lowest porosities are associated with sandstones that are tightly compacted or cemented with carbonates or rich in detrital matrix porosity depth trends may vary significantly with lithofacies due to its differences in textural and compositional attributes five lithofacies are defined in terms of detrital composition and texture type and degree of diagenesis the reservoir quality prediction models of various lithofacies are constructed and the results of this study provide insights into mechanisms for maintaining anomalously high porosity and permeability in high temperature high pressure sandstone reservoirs and may help explain hydrocarbon distribution

fracture zones constrained by neutral surfaces in a fault related fold insights from the kelasu tectonic zone kuqa depression abstract stress variation associated with folding is one of the controlling factors in the development of tectonic fractures however little attention has been paid to the influence of neutral surfaces during folding on fracture distribution in a fault related fold in this study we take the cretaceous bashijiqike formation in the kuqa depression as an example and analyze the distribution of tectonic fractures in fault related folds by core observation and logging data analysis three fracture zones are identified in a fault related fold a tensile zone a transition zone and a compressive zone which may be constrained by two neutral surfaces of fold well correlation reveals that the tensile zone and the transition zone reach the maximum thickness at the fold hinge and get thinner in the fold limbs a 2d viscoelastic stress field model of a fault related fold was constructed to further investigate the mechanism of fracturing statistical and numerical analysis reveal that the tensile zone and the transition zone become thicker with decreasing interlimb angle stress variation associated with folding is the first level of control over the general pattern of fracture distribution while faulting is a secondary control over the development of local fractures in a fault related fold

fracture characterization at multiple scales using borehole images sonic logs and walkaround vertical seismic profile we present a quantitative forward modeling methodology to link and interpret several measurements relevant to mechanical properties of fractures such as borehole images sonic anisotropy logs and borehole seismic anisotropy the analysis is applied to a case study from a north african tight gas field using data from a vertical well two studies are conducted independently using the same geological fracture data to model fracture induced anisotropy in the first study we use the orientation of the natural and drilling induced fractures interpreted on the image log to model the azimuthal fracture induced anisotropy at the sonic scale the mechanical effects of natural and drilling induced fractures are treated using different compliance parameters for each fracture type we show that modeled sonic fast shear azimuths could be biased by the presence of noncompliant fractures in each fracture type and we propose an empirical selection criterion to reject noncompliant fractures prior to compliance estimation then we estimate the fracture compliances and confirm that natural open fractures have larger compliances than drilling induced fractures in the second study we apply interpreted borehole images toward modeling of the azimuthal vertical seismic profile vsp attributes as a function of source azimuthal position natural fractures inside a window of height h and located at depth d are included and several volume sizes and positions i e h and d are considered we find a good agreement between modeled and observed transverse over radial displacement trends using natural fractures within windows located at the depth of the vsp receiver and having window heights on the order of one to two vsp shear wavelengths

3d spatial variation in vertical stress in on and offshore netherlands integration of density log measurements and basin modeling results abstract the principal vertical stress at a certain depth is equivalent to the weight of the overburden rocks and pore fluids at that depth vertical stress trends will be similar across areas with a simple layer cake lithostratigraphic sequence and similar burial history these conditions are not fulfilled for the lithostratigraphy and burial history of the onshore and offshore netherlands burial histories vary over short distances and resulted in highs platforms and inverted basins with different lithostratigraphic sequences and porosity distributions density log measurements and basin modeling results were used to assess the spatial variation of vertical stress in the dutch subsurface this paper presents the approach and assessed spatial variation in vertical stress the 3d basin modeling proved to be a good tool for correcting and extending density log derived vertical stress depth relations results show that the vertical stress varies spatially and that the vertical stress gradient is less than the standard gradient in large part of the area the largest local deviations from the standard gradient occur in areas with a thick package of low density cenozoic sediments and large low density salt structures i e in the northern offshore relatively old and more dense stratigraphic units occur at relatively shallow depth on the highs and in strongly inverted parts of the mesozoic basins in these areas the vertical stress reaches and follows the standard vertical stress gradient at shallower depths than in other areas

downhole well log and core montages from the mount elbert gas hydrate stratigraphic test well alaska north slope abstract the bpxa doe usgs mount elbert gas hydrate stratigraphic test well was an integral part of an ongoing project to determine the future energy resource potential of gas hydrates on the alaska north slope as part of this effort the mount elbert well included an advanced downhole geophysical logging program because gas hydrate is unstable at ground surface pressure and temperature conditions a major emphasis was placed on the downhole logging program to determine the occurrence of gas hydrates and the in situ physical properties of the sediments in support of this effort well log and core data montages have been compiled which include downhole log and core data obtained from the gas hydrate bearing sedimentary section in the mount elbert well also shown are numerous reservoir parameters including gas hydrate saturation and sediment porosity log traces calculated from available downhole well log and core data

thermoelasticity of a mineral composite and a reconsideration of lower mantle properties a re examination of thermal and elastic properties of a mineral composite emphasising derivative properties especially pressure and temperature dependences of incompressibility resolves residual doubts about the application to the lower mantle of thermodynamic estimates of these properties the only counter intuitive conclusion is that ks p s 4 21 at p 0 which is significantly higher than the values for component minerals this leads to a revised lower mantle equation of state and also to a new one which requires as input only k0 4 21 and k 1 425 k k0 k k0 k p k these equations and three other finite strain theories fit the lower mantle prem tabulation very well and agree on the extrapolation to p 0 k0 203 1 1 6 gpa 0 128 2 1 2 gpa 0 3972 5 4 0 kg m 3 a further six finite strain equations are rejected as incompatible with homogeneity of the lower mantle application of the theory to thermal properties gives s 1 ks ks t p decreasing with depth from 2 7 to 1 6 over the lower mantle depth range conversely e 1 t p increases with depth from 7 1 to 8 7 corresponding values of lnvs lnvp p vary from 1 7 to 2 3 this conclusion is not consistent with a simple thermal interpretation of tomographic observations of lower mantle velocity anomalies compositional variations must be invoked

on the deep mantle origin of the deccan traps the deccan traps in west central india constitute one of earth s largest continental flood basalt provinces whose eruption played a role in the cretaceous paleogene extinction event the unknown mantle structure under the indian ocean at the start of the cenozoic presents a challenge for connecting the event to a deep mantle origin we used a back and forth iterative method for time reversed convection modeling which incorporates tomography based present day mantle heterogeneity to reconstruct mantle structure at the start of the cenozoic we show a very low density deep seated upwelling that ascends beneath the reunion hot spot at the time of the deccan eruptions we found a second active upwelling below the comores hot spot that likely contributed to the region of partial melt feeding the massive eruption

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

the use of artificial neural network analysis and multiple regression for trap quality evaluation a case study of the northern kuqa depression of tarim basin in western china abstract artificial neural network analysis is found to be far superior to multiple regression when applied to the evaluation of trap quality in the northern kuqa depression a gas rich depression of tarim basin in western china this is because this technique can correlate the complex and non linear relationship between trap quality and related geological factors whereas multiple regression can only describe a linear relationship however multiple regression can work as an auxiliary tool as it is suited to high speed calculations and can indicate the degree of dependence between the trap quality and its related geological factors which artificial neural network analysis cannot for illustration we have investigated 30 traps in the northern kuqa depression for each of the traps the values of 14 selected geological factors were all known while geologists were also able to assign individual trap quality values to 27 traps they were less certain about the values for the other three traps multiple regression and artificial neural network analysis were therefore respectively used to ascertain these values data for the 27 traps were used as known sample data while the three traps were used as prediction candidates predictions from artificial neural network analysis are found to agree with exploration results where simulation predicted high trap quality commercial quality flows were afterwards found and where low trap quality is indicated no such discoveries have yet been made on the other hand multiple regression results indicate the order of dependence of the trap quality on geological factors which reconciles with what geologists have commonly recognized we can conclude therefore that the application of artificial neural network analysis with the aid of multiple regression to trap evaluation in the northern kuqa depression has been quite successful to ensure the precision of the above mentioned geological factors and their related parameters for each trap a study of the petroleum system in kuqa depression was conducted which included the partitioning and mechanisms of the kuqa petroleum system three migration models are presented

fracture responses of conventional logs in tight oil sandstones a case study of the upper triassic yanchang formation in southwest ordos basin china fractures are the main fluid flow pathways in tight oil sandstones and they have a significant influence on tight oil distribution exploration and development cores and image logs are commonly unavailable because of their high costs so employing conventional logs for fracture detection is imperative for tight oil sandstones we compared the fracture response characteristics of conventional logs based on two data sets one from 8 cored wells with fracture intensities greater than 1 m 1 3 3 ft 1 and the other from 11 cored wells with fracture intensities less than 0 5 m 1 1 6 ft 1 with a case study of the upper triassic yanchang formation in southwest ordos basin china the results indicate that when tight oil sandstones are more intensely fractured the caliper log acoustic log compensated neutron log density log dual induction logs and laterolog 8 present fracture responses to some extent however it is difficult to make a distinction between fractured and nonfractured zones using conventional logs in sandstones with smaller fracture intensities the fracture response intensities of conventional logs are weak and they are influenced by fracture abundance fracture occurrence fracture scale and mineral filling degree moreover lithology fluids and rock physical properties can cause fracturelike responses hence some ambiguity exists when using conventional logs to directly identify fractures accompanying fracture sensitive conventional logs with some methods to enhance fracture response intensity and eliminate nonfracture influence could enable fracture identification in tight oil sandstones

how does the pore throat size control the reservoir quality and oiliness of tight sandstones the case of the lower cretaceous quantou formation in the southern songliao basin china abstract pore throat size is a very crucial factor controlling the reservoir quality and oiliness of tight sandstones which primarily affects rock properties such as permeability and drainage capillary pressure however the wide range of size makes it difficult to understand their distribution characteristics as well as the specific controls on reservoir quality and oiliness in order to better understand about pore throat size distribution petrographic scanning electron microscopy sem pressure controlled mercury injection pmi rate controlled mercury injection rmi quantitative grain fluorescence qgf and environmental scanning electron microscopy esem investigations under laboratory pressure conditions were performed on a suite of tight reservoir from the fourth member of the lower cretaceous quantou formation k 1 q 4 in the southern songliao basin china the sandstones in this study showed different types of pore structures intergranular pores dissolution pores pores within clay aggregates and even some pores related to micro fractures the pore throat sizes vary from nano to micro scale the pmi technique views the pore throat size ranging from 0 001 m to 63 m and revealed that the pore throats with radius larger than 1 0 m are rare and the pore throat size distribution curves show evident fluctuations rmi measurements indicated that the pore size distribution characteristics of the samples with different porosity and permeability values look similar the throat size and pore throat radius ratio distribution curves had however significant differences the overall pore throat size distribution of the k 1 q 4 tight sandstones was obtained with the combination of the pmi and rmi methods the permeability is mainly contributed by a small part of larger pore throats less than 30 and the ratio of the smaller pore throats in the samples increases with decreasing permeability although smaller pore throats have negligible contribution on reservoir flow potential they are very significant for the reservoir storage capacity the pore throats with average radius larger than 1 0 m mainly exist in reservoirs with permeability higher than 0 1md when the permeability is lower than 0 1md the sandstones are mainly dominated by pore throats with average radius from 0 1 m to 1 0 m the ratio of different sized pore throats controls the permeability of the tight sandstone reservoirs in different ways we suggest that splitting or organizing key parameters defining permeability systematically into different classes or functions can enhance the ability of formulating predictive models about permeability in tight sandstone reservoirs the pmi combined with qgf analyses indicate that oil emplacement mainly occurred in the pore throats with radius larger than about 0 25 0 3 m this result is supported by the remnant oil micro occurrence evidence observed by sem and esem

diverse fracture properties and their impact on performance in conventional and tight gas reservoirs saudi arabia the unayzah south haradh case study the upper permian carboniferous unayzah formation in south haradh saudi arabia includes two major mechanical and petrophysical layers that are separated by shale rich zones open tectonic fracture clusters are rare and not essential for fluid flow in the unayzah a zone which has high porosity and permeability however such fracture clusters are essential to and impact the production performance in the unayzah b c tight gas reservoir the occurrence of the tectonic fractures in the unayzah formation is linked to the rock mechanical properties which vary with porosity shale volume cement type and texture the b c unit is more fractured than the a unit but its layers vary in the degree of fracturing the variation in fracture development within the b c unit results in differences in fracture enhanced permeability based on production profiles where flow is restricted to preferentially fractured mechanical layers that lack effective vertical fluid communication with other layers we identify two tectonic fracture systems an older subordinate fully mineralized system and a younger primary mostly open system early extensional fractures including joints and faults developed parallel to the basement faults during the opening of the neotethys these are fully mineralized and have little or no function as fluid conduits the younger system includes open fracture clusters that are predominantly parallel or nearly parallel to the regional east northeast west southwest maximum horizontal stress of the zagros that has been active since the late cretaceous and is independent of local structures therefore these fractures are controlled by remote stresses instead of the basement rooted forced folds and faults in this article we demonstrate that in the unayzah b c natural fractures are essential to permeability and in some areas to porosity and thence to reservoir performance the results of this study are being implemented in well placement and completion design to optimize the intersection of open fracture clusters with positive preliminary results

impacts of the tectonic stress field on natural gas migration and accumulation a case study of the kuqa depression in the tarim basin china stress fluid and temperature are three of the major factors that impact natural gas migration and accumulation in order to study the influences of tectonic stress field on natural gas migration and accumulation in low permeability rocks we take the kuqa depression as an example and analyze the evolution of the structure and tectonic stress field at first then we study the influences of tectonic stress field at different tectonic episodes on fractures and fluid potentials through the numerical simulation method on the section across the kl2 gas field we summarize two aspects of the impact of the tectonic stress field on natural gas migration and accumulation firstly under the effects of the tectonic stress field the rock dilation increases with the added stress and strain and when the shear stress of rock exceeds its shear strength the shear fractures are well developed on one hand the faults which communicate with the hydrocarbon source rocks become the main pathways for natural gas migration on the other hand these positions where fractures are well developed near faults can become good reservoirs for natural gas accumulation secondly because fluid potentials decrease in these places near the faults where fractures are well developed natural gas can migrate rapidly along the faults and accumulates the impact of tectonic stress fields on natural gas migration and accumulation allows for hydrocarbon migration and accumulation in the low permeability rocks in an active tectonic compressive setting

quantitative prediction of fractures using the finite element method a case study of the lower silurian longmaxi formation in northern guizhou south china abstract natural fractures have long been considered important factors in the production of gas from shale reservoirs because they can connect pore spaces and enlarge transport channels thereby influencing the migration accumulation and preservation of shale gas industrial level shale gas production has been initiated in the lower silurian longmaxi formation in northern guizhou south china however it is important to quantitatively predict the distribution of natural fractures in the lower silurian shale reservoirs to locate additional sweet spots in northern guizhou in this study data obtained from outcrops cores thin sections field emission scanning electron microscope fe sem images and x ray diffraction xrd were used to determine the developmental characteristics and controlling factors of these fractures correlation analysis indicated that the mechanical parameters of the longmaxi shale are mainly related to the total organic carbon toc quartz clay calcite and dolomite contents the spatial variations in the mechanical parameters of the longmaxi shale were determined based on the spatial variations in the toc and mineral contents then a heterogeneous geomechanical model of the study area was established based on interpretations of the fault systems derived from seismic data and acoustic emission ae experiments performed on samples of the relevant rocks the paleotectonic stress fields during the yanshanian period were obtained using the finite element method fem finally a fracture density calculation model was established to analyze the quantitative development of fractures and the effects of faults and mechanical parameters on the development of fractures were determined the results suggest that the main developmental period of tectonic fractures in the longmaxi formation was the early yanshanian period during this time the horizontal principal stress conditions were dominated by a se nw trending 135 315 compressional stress field and the longmaxi formation experienced a maximum tectonic stress of 110 120 mpa this simulated paleotectonic stress field was mainly controlled by faults and the contents of toc quartz clay calcite and dolomite at different positions along the same fault the degree of fracture development varies significantly overall the distribution of fractures in the longmaxi formation can be used to optimize well deployment and provides a basis for the future exploration of shale gas

time dependent convection models of mantle thermal structure constrained by seismic tomography and geodynamics implications for mantle plume dynamics and cmb heat flux summary one of the outstanding problems in modern geodynamics is the development of thermal convection models that are consistent with the present day flow dynamics in the earth s mantle in accord with seismic tomographic images of 3 d earth structure and that are also capable of providing a time dependent evolution of the mantle thermal structure that is as realistic earth like as possible a successful realization of this objective would provide a realistic model of 3 d mantle convection that has optimal consistency with a wide suite of seismic geodynamic and mineral physical constraints on mantle structure and thermodynamic properties to address this challenge we have constructed a time dependent compressible convection model in 3 d spherical geometry that is consistent with tomography based instantaneous flow dynamics using an updated and revised pseudo spectral numerical method the novel feature of our numerical solutions is that the equations of conservation of mass and momentum are solved only once in terms of spectral green s functions we initially focus on the theory and numerical methods employed to solve the equation of thermal energy conservation using the green s function solutions for the equation of motion with special attention placed on the numerical accuracy and stability of the convection solutions a particular concern is the verification of the global energy balance in the dissipative compressible mantle formulation we adopt such validation is essential because we then present geodynamically constrained convection solutions over billion year timescales starting from present day seismically constrained thermal images of the mantle the use of geodynamically constrained spectral green s functions facilitates the modelling of the dynamic impact on the mantle evolution of 1 depth dependent thermal conductivity profiles 2 extreme variations of viscosity over depth and 3 different surface boundary conditions in this case mobile surface plates and a rigid surface the thermal interpretation of seismic tomography models does not provide a radial profile of the horizontally averaged temperature i e the geotherm in the mantle one important goal of this study is to obtain a steady state geotherm with boundary layers which satisfies energy balance of the system and provides the starting point for more realistic numerical simulations of the earth s evolution we obtain surface heat flux in the range of earth like values 37 tw for a rigid surface and 44 tw for a surface with tectonic plates coupled to the mantle flow also our convection simulations deliver cmb heat flux that is on the high end of previously estimated values namely 13 tw and 20 tw for rigid and plate like surface boundary conditions respectively we finally employ these two end member surface boundary conditions to explore the very long time scale evolution of convection over billion year time windows these billion year scale simulations will allow us to determine the extent to which a memory of the starting tomography based thermal structure is preserved and hence to explore the longevity of the structures in the present day mantle the two surface boundary conditions along with the geodynamically inferred radial viscosity profiles yield steady state convective flows that are dominated by long wavelengths throughout the lower mantle the rigid surface condition yields a spectrum of mantle heterogeneity dominated by spherical harmonic degree 3 and 4 and the plate like surface condition yields a pattern dominated by degree 1 our exploration of the time dependence of the spatial heterogeneity shows that for both types of surface boundary condition deep mantle hot upwellings resolved in the present day tomography model are durable and stable features these deeply rooted mantle plumes show remarkable longevity over very long geological time spans mainly owing to the geodynamically inferred high viscosity in the lower mantle

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

a review on the applications of image logs in structural analysis and sedimentary characterization abstract image logs provide important information on lithology sedimentary textures paleoflow directions fractures and in situ stress analysis lacking of criteria used for establishing the image log facies limits the applications of image logs in sedimentary reservoir interpretation this paper provides a thoroughgoing review focusing on the recent applications of borehole image logs for sedimentological and structural description and interpretation and aims to establish image log facies which can provide guidelines in sedimentary reservoir interpretation this paper firstly summarizes the principles and basic characteristics of various imaging logging tools and then briefly introduces the pre processing workflow of the image log data then the generated images are used for depth and orientation shifts of cores by calibrating individual sedimentary and structural features descriptive and concise image log facies are established based on combinations of image textures including dip type dip pattern and color scheme and the characteristics as well as physical criteria for each individual image facies are summarized the established image log facies are then interpreted in terms of structural and sedimentological features such as lithology sedimentary structures vugs fractures and faults the image log facies and its stacking patterns are then used to interpret the lithofacies associations by calibrating with cores and conventional logs natural fractures and induced fractures are recognized by image logs and the principles of breakouts and drilling induced fractures for in situ stress analysis are reviewed then the applications of image logs in investigation of fracture attitudes and states as well as in computation of fracture parameters are summarized the procedures to evaluate fracture effectiveness through image logs are discussed at last the application of image logs for structural dip analysis is reviewed and image logs are used for recognizing faults fracture sets and attitudes of stratum through the dip patterns the basic procedures for paleocurrent reconstruction which includes dip picking of cross beddings structural dip determination and structural dip removal are reviewed then the paleocurrent directions of the lower cretaceous bashijiqike formation in the kuqa depression were reconstructed which could help further understanding of the depositional systems this review will help extend the utility of image logs in interpreting small to large scale sedimentary and structural features and bridges the gaps between well log analysis and sedimentary and structural interpretation

lithology dependent minimum horizontal stress and in situ stress estimate abstract based on the generalized hooke s law with coupling stresses and pore pressure the minimum horizontal stress is solved with assumption that the vertical minimum and maximum horizontal stresses are in equilibrium in the subsurface formations from this derivation we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress using anderson s faulting theory and this lower bound of the minimum horizontal stress the coefficient of friction of the fault is derived it shows that the coefficient of friction may have a much smaller value than what it is commonly assumed e g f 0 6 0 7 for in situ stress estimate using the derived coefficient of friction an improved stress polygon is drawn which can reduce the uncertainty of in situ stress calculation by narrowing the area of the conventional stress polygon it also shows that the coefficient of friction of the fault is dependent on lithology for example if the formation in the fault is composed of weak shales then the coefficient of friction of the fault may be small as low as f 0 2 this implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones to avoid the weak fault from shear sliding it needs to have a higher minimum stress and a lower shear stress that is the critically stressed weak fault maintains a higher minimum stress which explains why a low shear stress appears in the frictionally weak fault

effect of basement structure and salt tectonics on deformation styles along strike an example from the kuqa fold thrust belt west china abstract the kuqa fold thrust belt kftb has a complex thrust system geometry and comprises basement involved thrusts decollement thrusts triangle zones strike slip faults transpressional faults and pop up structures these structures combined with the effects of paleogene salt tectonics and paleozoic basement uplift form a complex structural zone trending e w interpretation and comprehensive analysis of recent high quality seismic data field observations boreholes and gravity data covering the kftb has been performed to understand the characteristics and mechanisms of the deformation styles along strike regional sections fold thrust system maps of the surface and the sub salt layer salt and basement structure distribution maps have been created and a comprehensive analysis of thrust systems performed the results indicate that the thrust fold system in paleogene salt range can be divided into five segments from east to west the kela 3 keshen dabei bozi and awate segments in the easternmost and westernmost parts of the paleogene salt range strike slip faulting and basement involved thrusting are the dominant deformation styles as basement uplift and the limits of the cenozoic evaporite deposit are the main controls on deformation salt core detachment fold thrust systems coincide with areas of salt tectonics and pop up imbricate and duplex structures are associated with the main thrust faults in the sub salt layer distribution maps of thrust systems basement structures and salt tectonics show that paleozoic basement uplift controlled the paleozoic foreland basin morphology and the distribution of cenozoic salt in the kftb and thus had a strong influence on the segmented structural deformation and evolution of the fold thrust belt three types of transfer zone are identified based on the characteristics of the salt layer and basement uplift and the effects of these zones on the fault systems are evaluated basement uplift and the boundary of the salt deposit generated strike slip faults in the sub salt layer and supra salt layers at the basin boundary model a when changes in the basement occurred within the salt basin strike slip faults controlled the deformation styles in the sub salt layer and shear zone dominated in the supra salt layer model b a homogeneous basement and discontinues salt layer formed different accommodation zones in the sub and supra salt layers model c in the sub salt layer the thrusts form imbricate structures on the basal decollement whereas the supra salt layer shows overlapping discontinuous faults and folds with kinds of salt tectonics and has greater structural variation than the sub salt layer

characterization of braided river delta facies in the tarim basin lower cretaceous application of borehole image logs with comparative outcrops and cores abstract borehole electrical image log with core like presentation is an efficient way to conduct sedimentary description in the subsurface the key is to set up a series of geological diagnostic criteria the subsurface lower cretaceous bashijiqike formation in the kelasu belt kuqa depression described as a sedimentary facies of braided river delta front has measured a mass of borehole image logs and open hole logs at depths more than 6000 m combined with comparative outcrops limited cores and thin sections high quality borehole images were fine calibrated to reveal their sedimentary details with the aim of subtle sedimentary interpretation in the uncored intervals the image elements all have corresponding geological indications the lithologies are interpreted including mudstones siltstones fine and medium sandstones and conglomerates and the sedimentary structures are interpreted in terms of various bedding types scoured surface and stacking modes thus interdistributary bay sandy or conglomerate subaqueous distributary channel and channel mouth bar are interpreted based on the lithologies sedimentary structures and textures basal and top contacts and succession variations furthermore determination of paleo current direction using borehole image logs is proposed for some time but is far from mature and need to consider tectonic deformation settings of studied stratigraphy based on this the paleo current in the kelasu belt is interpreted to flow southward from the southern tianshan to the depression during the early cretaceous which is consistent with the results derived from the heavy mineral analysis however the subaqueous distributary channels could frequently migrate locally and the paleo current direction changed at localities with an offset angle varied between 30 and 150 regionally the scale of single sand body decreases lakeward and the sand body may stretch at a maximum distance of 685 1 m along the flow direction the diagnostic criteria and the workflow for borehole image interpretation in this study will have potential application to subsurface sedimentary description and guide sedimentary architecture where cores are not available

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

in situ stress prediction using differential strain analysis and ultrasonic shear wave splitting abstract this paper presents a comparison study between two in situ stress measurement techniques based upon stress relief microcracks differential strain analysis dsa and ultrasonic shear wave splitting usws this study is based on a fact that the importance of in situ stress information has been increasingly realized in recent years various subsurface engineering activities such as those in petroleum and geothermal exploitation are dependent on a proper knowledge of this parameter therefore there is always a continuing need to find and develop techniques for determining this important parameter a contribution to the development of the relatively new concept of usws laboratory technique has been made by comparing usws measurement results from ten rock samples taken from deep boreholes with results from the more established dsa technique using the same rock samples as well as with results from sleeve fracturing obtained from the same area the comparison results have proved usws as a reliable technique for determining trends of in situ stresses and combined utilisation of the two techniques have also proved useful for examining rock microstructural features

two stage subduction history under north america inferred from multiple frequency tomography eastward subduction of oceanic tectonic plates has shaped the geologic history of western north america over the past 150 million years the mountain building and volcanism that brought forth the spectacular landscapes of the west are credited to the vast ancient farallon plate which interacted mechanically and chemically with the overlying continent as it plunged back into the mantle here we use finite frequency travel time and amplitude measurements of teleseismic p waves in seven frequency bands to obtain a high resolution tomographic image to approx1 800 km depth we discover several large previously unknown pieces of the plate which show that two distinct stages of whole mantle subduction are present under north america the currently active one descends from the pacific northwest coast to 1 500 km depth beneath the great plains whereas its stalled predecessor occupies the transition zone and lower mantle beneath the eastern half of the continent we argue that the separation between them is linked to the laramide era 70 50 myr ago a time of unusual volcanism and mountain building far inland generally explained by an episode of extremely flat subduction

unreliable determination of in situ stress orientation by borehole breakouts in fractured tight reservoirs a case study of the upper eocene hetaoyuan formation in the anpeng field nanxiang basin china elliptic borehole breakouts are usually used to determine the orientation of in situ stress in deep sedimentary basins the long axes of borehole breakouts are generally perpendicular to the maximum horizontal principal compression stress shmax however the azimuth of borehole breakouts is found perpendicular to the chief strike but not to shmax of natural fractures in tight reservoirs anpeng field of nanxiang basin china based on the core data and acoustic and resistivity borehole image logs the natural fractures are intensively striking at east west orientation where borehole breakouts occurred in north south if the borehole breakouts are induced only by in situ stresses surrounding the well and the borehole breakouts are known at north south shmax should be perpendicular at east west but according to analyses of the earthquake focal mechanism in circumjacent regions hydraulic fracturing data drilling induced fracture data and the production performance data shmax is in the northeast southwest direction this contradiction indicates that the influence of natural fractures may cause a serious deviation of the azimuth of borehole breakouts therefore in this case it is unreliable to determine the orientation of in situ stress only by borehole breakouts in fractured tight reservoirs in addition the main fluid flow direction is not parallel to the dominant natural fracture which is controlled by in situ stress in tight reservoirs

geochemistry of palaeozoic marine petroleum from the tarim basin nw china part 1 oil family classification abstract a series of palaeozoic marine oils ranging from condensates to very heavy oils from the cratonic region of the tarim basin nw china have been examined to delineate oil families crude oils from the mesozoic foreland kuqa depression were also analyzed for comparison the genetic family was assessed using bulk hydrocarbon isotopic composition alkylation pattern of polycyclic aromatic hydrocarbons pahs and biomarker distributions detailed oil oil correlation suggests two distinct families occur in the cratonic region despite the diversity of the reservoir rocks and some variability in oil physical properties the tabei and tazhong oils are classified in general into a single family with a possible indication for secondary subdivision these oils are characterized by low pristane phytane pr ph ratio high dibenzothiophene phenanthrene dbt p ratio light bulk isotopic composition high degree of pah alkylation high proportion of c23 and c24 tricyclic terpanes and low abundance of c24 tetracyclic terpane and triaromatic dinosteroids this family is inferred to have a middle upper ordovician source rock the tadong family distinctly different from the tabei and tazhong families is characterized by relatively high pr ph ratio low dbt p ratio slightly heavier bulk isotopic compositions and low degree of pah alkylation it also has a unique biomarker signature with a high proportion of triaromatic dinosteroids possibly indicating cambrian lower ordovician source rocks the oils from the kuqa depression are easily distinguished from the other two families due to their terrigenous origin the major characteristics of the kuqa oils are high pr ph ratio very low dbt p ratio heavy bulk isotopic composition low pah alkylation degree high proportion of c20 and c21 tricyclic terpanes and c24 tetracylic terpane and abundance of triaromatic dinosteranes and 4 methyl 24 ethyltriaromatic cholestanes these characteristics are consistent with terrestrial organic source rocks deposited under sub oxic conditions

polyaxial strength criteria and their use in estimating in situ stress magnitudes from borehole breakout dimensions abstract we conducted laboratory simulation tests of borehole breakouts and investigated their potential use as an indicator of in situ stress magnitudes in westerly granite and berea sandstone we also carried out simple triaxial tests and used the results to derive several strength criteria for these rocks using the assumption that the state of stress at the breakout boundary on the borehole wall is in a state of limit equilibrium with the rock strength we attempted to determine the appropriate strength criterion for each of the two rocks we concluded that the well known mohr coulomb criterion is not compatible with the condition of stress at breakout failure on the other hand truly triaxial polyaxial strength criteria which incorporate the effect of the intermediate principal stress on failure are much more in agreement with the stress at the breakout boundary one such criterion due to nadai and another due to mogi appear suitable for determining breakout failure in the sandstone and the granite respectively thin section analysis suggests that breakout failure mechanism may play an important role in determining the appropriate strength criterion for a given rock type utilizing the appropriate strength criterion for the corresponding rock type and having knowledge of the breakout span at the borehole wall and the minimum horizontal and vertical principal stresses we were able to estimate the maximum far field principal stress within an average of 14 error

plate tectonics and aspherical earth structure the importance of poloidal toroidal coupling we derive poloidal flow internal loading green functions for incompressible fluid models of the mantle consisting either of a single constant viscosity spherical shell or of two adjacent spherical shells having different viscosities from the green function we obtain kernels connecting the surface divergence the geoid and the surface topography fields to the lateral density heterogeneity inferred on the basis of the application of seismic tomographic imaging techniques and with these kernels we argue that both the surface divergence and geoid fields consisting of harmonic degrees 2 5 may be reasonably fit with only a factor of 8 viscosity increase at a depth of 1200 km we point out however that the coupling from poloidal to toroidal flow which is required to understand surface velocity spectra may allow these observations to be understood in terms of a viscosity increase at depth which is smaller than required by nonhydrostatic geoid data in the context of pure poloidal models using the observed kinetic energy in the surface plate motions as a constraint allows us to infer a value for the steady state upper mantle viscosity of 2 0 0 5 1021 pa s we comment on the implication of the difference between this number and the somewhat lower value 1 1021 pa s which has been derived on the basis of analyses of signatures of the glacial isostatic adjustment process

petroleum geology of kela 2 the most productive gas field in china abstract the kela 2 gas field located in the kuqa depression of tarim basin is the most productive gas field in china based on per unit surface acreage the gas trap is a subsurface anticline formed along an e w trending thrust belt in front of the southern tianshan mountains the gas pay zones are located in fine grained sandstones of a fan delta front facies in the lower cretaceous bashijiqik formation k l bs and eogene evaporitic rocks serve as regional cap rocks the gases were derived primarily from over mature jurassic coal measures peak gas generation and final shaping of the kela 2 structure occurred relatively late in the late himalayan orogeny 5 0 ma thrust faulting appears to have played a significant role in the hydrocarbon generation migration accumulation and preservation in this foreland basin detailed delineation of the kela 2 gas field indicates that high quality regional cap rocks are essential for the formation of the super rich abnormally pressured giant gas fields while buried anticlines under the overthrust zone are the favorable gas habitat late gas accumulation is considered to be decisive for the conservation of the giant gas fields

quasi reversibility method for data assimilation in models of mantle dynamics summary rapid progress in imaging deep earth structures using seismic tomography and in studies of physical and chemical properties of mantle rocks facilitates research in assimilation of data related to mantle dynamics in this paper we present a new numerical approach for data assimilation which allows for incorporating observations at present and unknown initial conditions in the past for mantle temperature and flow into a 3 d dynamic model in order to determine the initial conditions the dynamic model is described by the backward heat motion and continuity equations the use of the quasi reversibility qrv method implies the introduction into the backward heat equation of the additional term involving the product of a small regularization parameter and a higher order temperature derivative the data assimilation in this case is based on a search of the best fit between the forecast model state and the observations by minimizing the regularization parameter we apply the qrv data assimilation method to restore the evolution of i mantle plumes a synthetic case study and ii the lithospheric slab imaged by teleseismic body wave tomography in the southeastern carpathians for both models the present temperature and mantle flow are assimilated to the geological past and the prominent features of mantle structures are recovered we then model the evolution of the mantle structures forward in time starting from the restored state to the present state and estimate the accuracy of the model predictions the results of the qrv data assimilation are compared to that obtained by the variational var and backward advection data assimilation although the accuracy of the qrv data assimilation is lower than that of the var data assimilation the qrv method does not require any additional smoothing of the input data or filtering of temperature noise as the var method does based on the results and the comparison of the methods we consider the qrv method to be a highly promising approach to assimilation of data related to mantle dynamics

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

in situ stress variations associated with regional changes in tectonic setting northeastern brooks range and eastern north slope of alaska abstract the northeastern brooks range of northern alaska is an active north directed fold and thrust belt that is advancing on the barrow arch and the north facing passive margin of the arctic basin density logs leak off tests and mud weight profiles from 57 wells from the northeastern north slope were used to determine the magnitude of the present day in situ stresses and document significant regional lateral and vertical variations in relative stress magnitude preliminary analysis of the in situ stress magnitudes indicates two distinct stress regimes across this region of alaska areas adjacent to the eastern barrow arch exhibit both strike slip and normal stress regimes this in situ stress regime is consistent with fault patterns in the subsurface and with north south extension along the barrow arch and the northern alaska margin to the south in and near the northeastern brooks range thrust front in situ stress magnitudes indicate that an active thrust fault regime is present at depths up to 6000 ft 1829 m this is consistent with the fold and thrust structures in surface exposures and in the subsurface however at depths greater than 6000 ft 1829 m the relative in situ stress magnitudes indicate a change to a strike slip regime this observation is consistent with the few earthquake focal mechanisms in the area and suggests deep north northeast oriented strike slip faults may underlie the western margin of the northeastern brooks range

investigation of pore structure and petrophysical property in tight sandstones abstract laboratory measurements include porosity permeability high pressure mercury intrusion hpmi nuclear magnetic resonance nmr measurements and microscopic analysis of thin sections and scanning electron microscope sem were used to gain insight into the nature of the pore throat structure including pore geometry pore size distribution and pore connectivity of bashijiqike tight gas sandstones in keshen 5 keshen 6 keshen 8 keshen 9 and keshen 13 wellblock of kuqa depression the relationships between microscopic pore structure parameters and macroscopic petrophysical behaviors were investigated by regression analysis the results show that various types of pores with wide ranges of pore radius are observed the pore throats are very small commonly rapex controls permeability in tight gas sandstones the nmr pore size distribution is broader than the hpmi pore throat distribution in the nmr analysis only the pore systems connected by the relatively large pore throats should be accounted as movable and some portion of large pores which are not connected by effective large pore throats are not effectively movable the effective movable porosity is calculated by excluding those pores restricted by the tiny pore throats the content of effective movable porosity from nmr measurements shows good correlation relationship with the porosity connected by large pore throats in hpmi analysis three typical types of pore structures are recognized through thin section sem analysis combined with the capillary curve and nmr t2 spectrum and the microscopic and macroscopic characteristics of the three pore structures are investigated the pore structure were comprehensively evaluated and characterized by linking nmr t2 spectrum with hpmi analysis

dynamic topography and long term sea level variations there is no such thing as a stable continental platform abstract globally uniform i e eustatic sea level trends with time scales approaching 100 myr have been inferred from both seismic and backstripping stratigraphic analysis at a small set of geographic sites that are presumed to lie on stable continental platforms and passive continental margins characterized by simple thermal subsidence histories e g the new jersey margin the western african margin we demonstrate using mantle flow simulations based on high resolution seismic tomography that both the new jersey margin and the conjugate western african margin have been subject to orders of 100 m of dynamic i e flow induced topography change over the last 30 myr we also show that the changing pattern of downwelling mantle flow associated with plate subduction is a significant contributor to the background eustatic sea level trend which is also of order 100 m during the 30 myr time window therefore late cenozoic variations of dynamic topography on these passive margin sites are comparable to the eustatic sea level changes and can partially mask the latter furthermore even if the observed trend could be accurately corrected for local dynamic topography variation the residual eustatic signal does not merely reflect changes in mean spreading rates at mid ocean ridges we conclude that the observed long term sea level variations at so called stable sites cannot be interpreted as eustatic moreover previous analyses that have used long term sea level trends as a proxy for spreading rates and geochemical fluxes must be revisited

variations in grain size and viscosity based on vacancy diffusion in minerals seismic tomography and geodynamically inferred mantle rheology the effect of grain size on mantle viscosity is comparable to that of temperature and pressure the current 3 d distribution of grain size in the mantle is however unknown to explore the possible variability of grain size we use the following geodynamic inferences of effective viscosity vacancy diffusion rates in upper mantle minerals and perovskite in the lower mantle lateral variations in temperature derived from seismic images and different geotherms an important outcome of this modeling is a new mapping of lateral viscosity variations throughout the mantle the corresponding 3 d variations in grain size are characterized by 2 orders of magnitude changes we find a correlation between grain size variability in the mantle and absolute viscosity changes with depth our findings suggest that the traditional assumption of arrhenius temperature dependence for vacancy diffusion in the lower mantle is not sufficient to constrain the deformation mechanisms that determine its effective bulk viscosity

developing a model discrete fracture network drilling and enhanced oil recovery strategy in an unconventional naturally fractured reservoir using integrated field image log and three dimensional seismic data in this study we develop a model discrete fracture network dfn for the unconventional naturally fractured tensleep sandstone oil reservoir at teapot dome wyoming reservoir characterization is based on three dimensional 3d seismic data fracture image logs from teapot dome and field observations of the tensleep exposure in the alcova anticline and fremont canyon areas image logs reveal that the dominant reservoir fracture set trends parallel to the present day maximum horizontal compressive stress inferred from drilling induced fractures analog field studies of the alcova anticline and fremont canyon suggest fracture heights and lengths are power law distributed while the fracture spacing distribution is best described as log normal image log derived fracture apertures are also log normally distributed these properties are incorporated into a model dfn we assume subseismic folds faults and fracture zones control fracture intensity distribution and use composite 3d seismic attributes to locate subtle changes in seismic response interpreted to result from subseismic structure directional curvature defines aperture opening strain normal to the dominant reservoir fracture set seismic attributes are scaled and combined to control fracture intensity variations in the model grid cell porosity and permeability distributions derived from the dfn suggest the presence of northeast southwest trending reservoir compartments we suggest that enhanced oil recovery operations may be optimized using lateral injection and production wells oriented along interpreted compartment boundaries at high angles to this combination of injection and production laterals could help maximize storage and hydrocarbon recovery in depleted reservoirs and in down dip residual oil zones

reconstructing the cenozoic evolution of the mantle implications for mantle plume dynamics under the pacific and indian plates article history the lack of knowledge of the initial thermal state of the mantle in the geological past is an outstanding problem in mantle convection the resolution of this problem also requires the modelling of 3 d mantle evolution that yields maximum consistency with a wide suite of geophysical constraints quantifying the robustness of the reconstructed thermal evolution is another major concern to solve and estimate the robustness of the time reversed inverse problem of mantle convection we analyse two different numerical techniques the quasi reversible qrv and the backward advection bad methods our investigation extends over the 65 myr interval encompassing the cenozoic era using a pseudo spectral solution for compressible flow thermal convection in 3 d spherical geometry we find that the two dominant issues for solving the inverse problem of mantle convection are the choice of horizontally averaged temperature i e geotherm and mechanical surface boundary conditions we find in particular that the inclusion of thermal boundary layers that yield earth like heat flux at the top and bottom of the mantle has a critical impact on the reconstruction of mantle evolution we have developed a new regularisation scheme for the qrv method using a time dependent regularisation function this revised implementation of the qrv method delivers time dependent reconstructions of mantle heterogeneity that reveal 1 the stability of pacific and african large low shear velocity provinces llsvp over the last 65 myr 2 strong upward deflections of the cmb topography at 65 ma beneath the north atlantic the south central pacific the east pacific rise epr and the eastern antarctica 3 an anchored deep mantle plume ascending directly under the epr easter and pitcairn hotspots throughout the cenozoic era and 4 the appearance of the transient reunion plume head beneath the western edge of the deccan plateau at 65 ma our reconstructions of cenozoic mantle evolution thus suggest that mantle plumes play a key role in driving surface tectonic processes and large scale volcanism 2014 elsevier b v all rights reserved

subsurface fracture analysis and determination of in situ stress direction using fmi logs an example from the santonian carbonates ilam formation in the abadan plain iran abstract the relationship between the present day stress field and natural fractures can have significant implications for subsurface fluid flow in particular fractures that are aligned in orientations favourable for reactivation by either shear or tensile failure in the in situ stress field often exhibit higher hydraulic conductivities the ilam formation of southwestern iran is an important hydrocarbon reservoir containing numerous natural fractures however little is known about the state of stress in this region or any of iran s petroleum provinces we conducted analysis of the present day maximum horizontal stress orientation and the density orientation and hydraulic conductivity of natural fractures in the ilam carbonates using high resolution formation micro imager resistivity logs in two wells a total of 51 breakouts with an overall length of 215 m were observed in the two wells indicating a maximum horizontal stress orientation of 68 n 7 6 in well a and 58 n 6 3 in well b furthermore the wellbore derived stress orientations determined herein are consistent with those inferred from nearby earthquake focal mechanism solutions indicating that stresses in the sedimentary cover are linked to the resistance forces generated by arabia eurasia collision furthermore the correlation between stress orientations estimated from earthquake focal mechanism solutions and breakouts indicates that focal mechanism solution data which is often considered to be unreliable for stress field analysis near transform margins may provide reliable information on the stress orientation near continental collision zones the image log data also reveals three sets of open and presumably hydraulically conductive fractures with strikes of i 160 170 n ii 110 140 n and iii 070 080 n fracture set iii is consistent with being formed and open in the present day stress field however fracture sets i and ii strike at a high angle to the present day maximum horizontal stress and are interpreted herein to be the result of either pre or syn folding related forces the observation that different sets of open fractures in the field can be either sensitive or insensitive to the present day stress is critical for improving hydrocarbon recovery

3d geomechanical modeling and numerical simulation of in situ stress fields in shale reservoirs a case study of the lower cambrian niutitang formation in the cen gong block south china abstract an analysis of the in situ state of stress in a shale reservoir was performed based on comprehensive information about the subsurface properties from wellbores established during the development of an oil and gas field industrial level shale gas production has occurred in the niutitang formation of the lower cambrian cen gong block south china in this study data obtained from hydraulic fracturing drilling induced fractures borehole breakout global positioning system gps and well deviation statistics have been used to determine the orientation of the maximum horizontal principal stress additionally hydraulic fracturing and multi pole array acoustic logging xmac were used to determine the vertical variations in the in situ stress magnitude based on logging interpretation and mechanical experiments the spatial distributions of mechanical parameters were obtained by seismic inversion and a 3d heterogeneous geomechanical model was established using a finite element stress analysis approach to simulate the in situ stress fields the effects of depth faults rock mechanics and layer variations on the principal stresses horizontal stress difference horizontal stress difference coefficient k h and stress type coefficient s p were determined the results show that the direction of the maximum principal stress is ese 120 additionally the development zones of natural fractures appear to correlate with regions with high principal stress differences at depths shallower than 375 m the stress type is mainly a thrust faulting stress regime at depths ranging from 375 to 950 m the stress type is mainly a strike slip faulting stress regime when the depth is 950 m the stress type is mainly a normal faulting stress regime depth fault orientation and rock mechanics all affect the type of stress the knowledge regarding the cen gong block is reliable and can improve borehole stability casing set point determination well deployment optimization and fracturing area selection

extrapolating oceanic age distributions lessons from the pacific region abstractextrapolation of the age distribution of oceanic lithosphere has played a significant role in assessments of variations in global mean spreading rate global mean ocean basin depth and implications for global mean sea level subduction has already removed 50 of oceanic lithosphere younger than 55 7 ma making some level of extrapolation a necessary part of global plate reconstructions an area equal in size to the pacific basin oceanic lithosphere must be extrapolated for ages older than 29 1 ma three modes of extrapolation are identified mode 1 extrapolation uses the preserved history as recorded on one plate to infer the history of the previously adjacent plate this mode of extrapolation is exemplified by the inferred history of the farallon vancouver nazca and cocos plates relative to the pacific plate on which this record is preserved mode 2 involves extrapolation beyond the preserved age extent of a given ridge system no observable data exist that directly constrain the motions bey

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

well bore breakouts and in situ stress the detailed cross sectional shape of stress induced well bore breakouts has been studied using specially processed ultrasonic borehole televiewer data we show breakout shapes for a variety of rock types and introduce a simple elastic failure model which explains many features of the observations both the observations and calculations indicate that the breakouts define relatively broad and flat curvilinear surfaces which enlarge the borehole in the direction of minimum horizontal compression this work supports the hypothesis that breakouts result from shear failure of the rock where the compressive stress concentration around the well bore is greatest and that breakouts can be used to determine the orientation of the horizontal principal stresses in situ

effect of normal faulting on in situ stress a case study from mandapeta field krishna godavari basin india abstract the horizontal compressive stress orientations for mandapeta field in krishna godavari k g basin had been calculated from borehole breakout data using four arm dipmeter caliper logs the minimum horizontal stress magnitude for mandapeta field was calculated from pore pressure data using poroelastic equation three dimensional 3 d stress modeling using finite element analyses has been carried out for layered sediments and faulted layered sediments in the mandapeta sub basin the length and width of the models are chosen as 5 km and 2 km respectively vertical depths of models are considered for sediments at depth 2 km to 5 km this paper presents a numerical study of stress distributions contours and trajectories around fault models of layered geometries submitted to overburden and unequal horizontal compressive loads reorientation and magnitude variation of in situ stresses near faults and stress discontinuities across faults are observed changes in the stress patterns are observed at the interfaces between sediments due to faulting and sediment basement layers due to contrast in elastic constants remarkable stress gradients are observed between the sediment and basement magnitudes of normal component of stress decrease near the faulting whereas shear stress magnitudes increase at the faulting site there is increase of horizontal principal stresses in the downthrown faulted layers the maximum horizontal principal stress rotates about 90 just adjacent to the fault fluid flow at depth of oil reservoir could be indirectly influenced by the stress contours and by geometries of the stress trajectories fluid migration in low permeability reservoir may occur in response to mean stress gradients induced by the tectonic loading the areas of high mean stress gradient are located at the interface between sediment and basement whereas the areas of low mean stress gradient is exactly coinciding with the areas of upper sediments

methodology for predicting reservoir breakdown pressure and fracture opening pressure in low permeability reservoirs based on an in situ stress simulation abstract using numerical simulations of the paleostress field during the time of fracture formation we predict the occurrence of fractures in the reservoirs of the second member of the funing formation in the tongcheng fault zone subei basin east china we use sonic velocity measurements from rock cores and microseismic monitoring to determine the in situ stress which is combined with fracture data to calculate the magnitude of the in situ stress at the well site by determining the mechanical parameters of the rock we use the finite element method fem to develop a geomechanical model and conduct numerical simulations of the in situ stress field based on the occurrence of fractures and the numerical simulation results of the in situ stress field we predict the opening pressure and opening sequence of natural fractures in the reservoirs the results indicate that the maximum horizontal principal stress in the second member of the funing formation in the tongcheng fault zone is oriented ene during the injection process the ene trending fractures open first and the ese trending fractures open later the opening pressure of the fractures increases as the angle between the fracture strike and the maximum horizontal principal stress increases and the burial depth of the fractures also has a relatively positive correlation with the opening pressure at the structurally high site 1850 m 2350 m the opening pressure of the fractures is in the range of 25 mpa 50 mpa and at the structurally low site 3300 m 4000 m the opening pressure of the fractures is in the range of 45 mpa 75 mpa by calculating the actual fracture pressure of the reservoirs we propose using different injection pressures in different blocks to ensure the high and stable production of oil from wells

characteristics and origin of natural gases in the kuqa depression of tarim basin nw china abstract significant gas condensate as well as some black oils have been discovered in the kuqa depression of tarim basin nw china dry gases with high 13 c values occur in the kelasu structural belt wet isotopically light gases in the yiqikelik belt whereas condensates are distributed mainly in the front uplift area kinetic modeling results show that the variation of methane and ethane isotopes with increasing vitrinite reflectance is independent of heating rates for a given source rock two maturity trends have been observed on a c 2 c 1 13 c 1 plot one for thermogenic gases associated with coal another for oil associated gases with minor contribution from biogenic gas it is most likely that the gases in the kelasu belt and front uplift area were derived from the jurassic coal measures and triassic lacustrine shales respectively with those in the yiqikelik belt being the mixtures of gases from the two sources both jurassic coals and triassic lacustrine shales appear to have contributed to the gases in the kela 2 kl2 and dina 2 dn2 the two largest gas fields in the tarim basin

a revised crustal stress orientation database for canada abstract the canadian database on contemporary crustal stress has not been revised systematically in the past two decades here we present the results of our new compilation that contains 514 new data records for the orientation data of maximum compressive horizontal stress and 188 data records that were re assessed in total the canadian stress database has now 1667 data records which is an increase of about 45 from these data a new canadian stress map as well as one for the province of alberta is presented to analyse the stress pattern we use the quasi median on the circle as a smoothing algorithm that generates a smoothed stress map of the maximum compressive horizontal stress orientation on a regular grid the newly introduced quasi interquartile range on the circle estimates the spreading of the data and is used as a measure for the wave length of the stress pattern the result of the hybrid wavelength analysis confirms that long spatial wavelength stress patterns 1000 km exist in large areas in canada the observed stress pattern is transmitted through the intra plate regions the results reveal that shorter spatial wave length variation of the maximum compressive horizontal stress orientation of less than 200 km prevails particularly in south eastern and western canada regional stress sources such as density contrasts active fault systems crustal structures etc might have a significant impact in these regions in contrast to these variations the observed stress pattern in the alberta basin is very homogeneous and mainly controlled by plate boundary forces and body forces the influence of curvature of the rocky mountains salient in southern alberta is minimal the present day horizontal stress orientations determined herein have important implications for the production of hydrocarbons and geothermal energy in the alberta basin

plate motions and stresses from global dynamic models delineating the driving forces behind plate motions is important for understanding the processes that have shaped earth throughout its history however the accurate prediction of plate motions boundary zone deformation rigidity and stresses remains a difficult frontier in numerical modeling we present a global dynamic model that produces a good fit to such parameters by accounting for lateral viscosity variations in the top 200 kilometers of earth together with forces associated with topography and lithosphere structure as well as coupling with mantle flow the relative importance of shallow structure versus deeper mantle flow varies over earth s surface our model reveals where mantle flow contributes toward driving or resisting plate motions furthermore subducted slabs need not act as strong stress guides to satisfy global observations of plate motions and stress

the angular velocities of the plates and the velocity of earth s centre from space geodesy summary using space geodetic observations from four techniques gps vlbi slr and doris we simultaneously estimate the angular velocities of 11 major plates and the velocity of earth s centre we call this set of relative plate angular velocities geodvel for geodesy velocity plate angular velocities depend on the estimate of the velocity of earth s centre and on the assignment of sites to plates most geodetic estimates of the angular velocities of the plates are determined assuming that earth s centre is fixed in an international terrestrial reference frame itrf and are therefore subject to errors in the estimate of the velocity of earth s centre in itrf2005 and itrf2000 earth s centre is the centre of mass of earth oceans and atmosphere cm the velocity of cm is estimated by slr observation of lageos s orbit herein we define earth s centre to be the centre of mass of solid earth ce we determine the velocity of ce by assuming that the portions of plate interiors not near the late pleistocene ice sheets move laterally as if they were part of a rigid spherical cap the geodvel estimate of the velocity of ce is likely nearer the true velocity of cm than are the itrf2005 and itrf2000 estimates because 1 no phenomena can sustain a significant velocity between cm and ce 2 the plates are indeed nearly rigid aside from vertical motion and 3 the velocity of cm differs between itrf2005 and itrf2000 by an unacceptably large speed of 1 8 mm yr 1 the velocity of earth s centre in geodvel lies between that of itrf2000 and that of itrf2005 with the distance from itrf2005 being about twice that from itrf2000 because the geodvel estimates of uncertainties in plate angular velocities account for uncertainty in the velocity of earth s centre they are more realistic than prior estimates of uncertainties geodvel differs significantly from all prior global sets of relative plate angular velocities determined from space geodesy for example the 95 per cent confidence limits for the angular velocities of geodvel exclude those of revel sella et al for 34 of the 36 plate pairs that can be formed between any two of the nine plates with the best constrained motion the median angular velocity vector difference between geodvel and revel is 0 028 myr 1 which is up to 3 1 mm yr 1 on earth s surface geodvel differs the least from the geodetic angular velocities that altamimi et al determine from itrf2005 geodvel s 95 per cent confidence limits exclude 11 of 36 angular velocities of altamimi et al and the median difference is 0 015 myr 1 geodvel differs significantly from nearly all relative plate angular velocities averaged over the past few million years including those of nuvel 1a the difference of geodvel from updated 3 2 myr angular velocities is statistically significant for all but two of 36 angular velocities with a median difference of 0 063 myr 1 across spreading centres eight have slowed down while only two have sped up we conclude that plate angular velocities over the past few decades differ significantly from the corresponding angular velocity averaged over the past 3 2 myr

forward modeling of fracture induced sonic anisotropy using a combination of borehole image and sonic logs we develop a methodology to model and interpret borehole dipole sonic anisotropy related to the effect of geologic fractures using a forward modeling approach we use a classical excess compliance fracture model that relies on the orientation of the individual fractures the elastic properties of the host rock and the normal and tangential fracture compliance parameters orientations of individual fractures are extracted from borehole image log analysis the model is validated using borehole resistivity image and sonic logs in a gas sand reservoir over a 160 ft 50 m vertical interval of a well significant amounts of sonic anisotropy are observed at three zones with a fast shear azimuth fsa exhibiting 60 of variation and slowness difference between 2 and 16 numerous quasivertical fractures with varying dip azimuths are identified on the image log at the locations of strong sonic anisotropy the maximum horizontal stress direction given by breakouts and drilling induced fractures is shown not

origin of fracture porosity example from altamont field utah the occurrence of natural fracture systems in subsurface rock can be predicted if careful evaluation is made of the geologic processes that affect sedimentary strata during their cycle of burial diagenesis uplift and erosional unloading variations in the state of stress within rock arise for example from changes in temperature pore pressure weight of overburden or tectonic loading hence geologic processes acting on a sedimentary unit should be analyzed for their several contributions to the state of stress and this information used to compute a stress history from this stress history predictions may be made as to when in the burial cycle to expect fracture joint formation what type of fractures extension or shear may occur and which geologic factors are most favorable to development of fractures a stress history is computed for strata of the naturally fractured altamont oil field in utah s uinta basin calculations suggest that fractures formed in extension that the well cemented rocks are those most likely to be fractured that fractures began to develop only after strata were buried to great depth and that the fracture system continued to develop as strata were uplifted and denuded of overburden geologic evidence on fracture genesis and development is in accord with the stress history prediction stress history can be useful in evaluating a sedimentary basin for naturally fractured reservoir exploration plays

an overview of rock stress measurement methods this paper presents an overview of methods that have been used to estimate the state of stress in rock masses with the emphasis on methods applicable to hard rocks and scandinavia rock stress is a difficult quantity to estimate because the rock stress measuring techniques consist of perturbing the rock measuring displacements or hydraulic parameters and converting the measured quantities into rock stresses there are two main types of method those that disturb the in situ rock conditions i e by inducing strains deformations or crack opening pressures and those that are based on observation of rock behaviour without any major influence from the measuring method the most common methods are briefly described including their application areas and limiting factors

incompatible stress regimes from geological and geomechanical datasets can they be reconciled an example from the carnarvon basin western australia abstract knowledge of the in situ stress field is essential in petroleum basins as it exerts a prime control over seal integrity fracture stimulation wellbore stability and fluid flow previous geomechanical analysis of australia s carnarvon basin the continent s premier hydrocarbon region highlights a normal to strike slip fault stress regime however neotectonic evidence and contemporary seismicity suggest the possibility of a strike slip to reverse fault stress regime we attempt to reconcile these conflicting datasets through new analysis of the in situ stresses principal stress magnitudes and orientations were defined with data from 76 previously unanalysed petroleum wells wellbore image logs yield a maximum horizontal stress orientation of 113 n consistent with modelling of the stress field throughout the indo australian plate vertical stress magnitudes are estimated to range from 20 mpa km 1 to 22 4 mpa km 1 leak off tests lot from 42 wells are analysed using two methodologies one assuming tensile failure as traditionally accepted the other assuming shear failure as outlined by couzens schultz and chan 2010 and result in two different sets of horizontal stress magnitude estimates traditional interpretation of lots results in a minimum horizontal stress gradient of 16 8 mpa km 1 and a maximum horizontal stress gradient of 21 8 mpa km 1 the new method for lot interpretation results in a minimum horizontal stress gradient of 18 1 mpa km 1 and a maximum horizontal stress gradient of 25 4 mpa km 1 inclined natural fractures observed at lot depths supports application of the new method of lot interpretation traditional interpretation of stress magnitudes implies a normal to strike slip fault stress regime while the new method implies a strike slip fault stress regime this latter interpretation is favoured by the authors as it allows for a reconciliation of the geological and geomechanical datasets the assessment undertaken herein allows for reinterpretation of stress magnitudes in seismically active basins hosting differing geomechanical and neotectonic regimes potentially altering existing understanding of seal integrity fracture stimulation wellbore stability and fluid flow

in situ stress state in the linxing region eastern ordos basin china implications for unconventional gas exploration and production abstract the carboniferous and permian sedimentary rocks mainly the shanxi and taiyuan formations in the linxing region eastern ordos basin china host a significant volume of unconventional gas resources coalbed methane shale gas and tight sandstone gas currently the in situ stress state is poorly understood but knowledge of this is extremely important for a range of applications such as gas exploration and production fracture stimulation and wellbore stability the maximum horizontal stress s hmax minimum horizontal stress s hmin and vertical stress s v magnitudes and the s hmax orientation in the linxing region were systematically analyzed for the first time in the present study which can provide a reference for subsequent numerical simulation and hydraulic fracturing design based on borehole breakouts and drilling induced tensile fractures interpreted from borehole imaging logs the s hmax orientation rotates from nee sww trending in the southern part to nww see trending in the northern part of the linxing region both conventional logs and extended leak off tests were used for stress magnitude determination the results revealed three types of in situ stress fields s v s hmax s hmin s hmax s v s hmin and s hmax s v s hmin and a dominant strike slip stress regime s hmax s v s hmin was found for the entire well section in the target shanxi formation and taiyuan formation in the linxing region in addition differential stress increased with depth in the linxing region which indicates that wellbore instability might be a potentially significant problem when drilling wells that are vertical or n s trending

grain size variations in the earth s mantle and the evolution of primordial chemical heterogeneities 1 chemical heterogeneities with relatively high viscosity are known to be poorly mixed by mantle convection one of the factors which can cause viscosity variations is the grain size analysis of various physical processes in the earth s mantle suggests that variation in the grain size can be extremely large even outside of subducting slabs the largest grains are formed in the early molten earth where ostwald ripening is controlled by liquid state diffusion the smallest grains are formed due to recrystallization by the spinel perovskite phase transformation in downwelling flow and subsequent ostwald ripening the viscosity contrast between primordial large grain size material and recrystallized material is likely to be many orders of magnitude preliminary numerical simulations show that strongly time dependent mantle convection can process primordial large grain size heterogeneities through the spinel perovskite phase boundary only after a few overturns this reduces the viscosity contrasts and leads to rapid mixing by mantle convection a more steady flow in the lower mantle may lead to the formation of stagnant poorly mixed regions which may never have had a chance to cross the 660 km discontinuity and be recrystallized by the spinel perovskite phase transformation heterogeneities can also avoid grain size reduction and mixing if their density is slightly larger than the surrounding mantle in this case they accumulate at the core mantle boundary

descent of the ancient farallon slab drives localized mantle flow below the new madrid seismic zone 1 the great earthquake sequence that occurred in the central mississippi river valley during the winter of 1811 1812 is unprecedented in the historical record of seismicity within stable continental plate interiors we show using viscous flow models based on high resolution seismic tomography that the descent of the ancient farallon slab into the deep mantle beneath central north america induces a highly localized flow directly below the new madrid seismic zone nmsz this localization arises because of structural variability in the farallon slab and the low viscosity of the sub lithospheric upper mantle and it represents a heretofore unrecognized and possibly significant driving mechanism for the enigmatic intraplate seismicity

a new back and forth iterative method for time reversed convection modeling implications for the cenozoic evolution of 3 d structure and dynamics of the mantle

responses of two lithosomes of lower cretaceous coarse clastic rocks to tectonism in kuqa foreland sub basin northern tarim basin northwest china abstract coarse clastic rocks developed widely at foredeep adjacent to orogene were usually regarded as the mark of renewed thrusting action however a good example from the lower cretaceous in kuqa foreland sub basin illustrated that the coarse clastic rocks developed not only in initial thrusting stage but in early quiescence stage the kuqa foreland sub basin adjacent to the south tianshan orogene lies to the north of tarim basin northwest china the lower cretaceous outcrop in the front of south tianshan orogene was identified as a filling sequence of foredeep which includes two depositional cycles of kapushaliang group and bashijiqike formation each of them presents fining upwards and then coarsening upwards sequences two lithosomes of coarse clastic rocks developed in the lower of yageliemu formation of kapushaliang group and bashijiqike formation respectively both of them are mainly composed of thick bedded massive and chaotic conglomerate with debris flow microfacies of fan delta plain the analyses on stratigraphic and depositional records for tectonism revealed abrupt sedimentary change from delta facies to fan delta facies stratal geometry transformation and reflection termination migration of truncation and top lap between the kapushaliang group and the bashijiqike formation these records indicated that foredeep subsided rapidly while forebulge uplifted in the development of kapushaliang group which coincided perfectly with lithosphere deformation during thrusting stage foredeep uplifted rapidly while forebulge subsided in the later development of bashijiqike formation which coincided with lithosphere deformation during quiescence stage of foreland basin development therefore the lower cretaceous recorded a whole episode of foreland basin evolution from earlier thrusting to later quiescence the underfilled cycle of kapushaliang group corresponding to the thrusting stage is much thick with mainly muddy and silty sediments the overfilled cycle of bashijiqike formation corresponding to the quiescence stage is thin with mainly sandy sediments the two lithosomes of coarse clastic rocks were very similar with each other in the lithology and sedimentology however the one from lower yageliemu formation is responded to the renewed thrusting but the other from the lower bashijiqike formation is responded to the lithosphere rebounding during primary quiescence stage therefore cannot only coarse clastic rocks indicated a renewed thrusting

role of dynamic topography in sustaining the nile river over 30 million years the nile is the longest river on earth and has persisted for millions of years it has been suggested that the nile in its present path is 6 million years old whereas others argue that it may have formed much earlier in geological history here we present geological evidence and geodynamic model results that suggest that the nile drainage has been stable for 30 million years we suggest that the nile s longevity in essentially the same path is sustained by the persistence of a stable topographic gradient which in turn is controlled by deeper mantle processes we propose that a large mantle convection cell beneath the nile region has controlled topography over the last 30 million years inducing uplift in the ethiopian yemen dome and subsidence in the levant sea and northern egypt we conclude that the drainage system of large rivers and their evolution over time can be sustained by a dynamic topographic gradient the path of the river nile has been stable for as long as 30 million years sustained by mantle convection according to geophysical and geological evidence and geodynamic model simulations

drilling induced tensile wall fractures implications for determination of in situ stress orientation and magnitude abstract detailed investigation of failure of the borehole wall in two scientific drilling projects the german ktb kontinentales tiefbohrprogramm der bundesrepublik deutschland and the european geothermal research project at soultz sous forets france has lead to new insight in the phenomena of tensile fractures induced in the wellbore wall during drilling comparison of the orientation of the fractures with the orientation of the horizontal principal stress known from breakout and hydraulic fracturing analysis demonstrates that these fractures are reliable indicators of the orientation of the maximum horizontal principal stress s h a model for the initiation of the fractures is presented which points out the important influences of a the tectonic stress state b increased mud pressures during drilling operation and c thermal stresses induced by circulation of relatively cold drilling mud analysis of drilling induced fractures in the gpk1 borehole at soultz sous forets where the magnitude of s h is known from hydraulic fracturing experiments demonstrates the validity of this model for the initiation of the fractures further a new method is proposed to estimate the magnitude of s h from the occurrence of drilling induced fractures and knowledge of thermally induced stress and pumping pressure during drilling the method is successfully applied to both ktb boreholes an independent method to estimate the magnitude of s h based on the analytical calculation of the stress intensity factor for drilling induced fractures taking into consideration both increased mud pressure and thermal stress is also presented application of this method confirms the results derived with the analysis described above additionally the evaluation of the orientation of the fractures with respect to the wellbore axis indicates that over major depth sections of the investigated wells the vertical stress is a principal stress

two deep mantle sources for paleocene doming and volcanism in the north atlantic the north atlantic igneous province naip erupted in two major pulses that coincide with the continental breakup and the opening of the north atlantic ocean over a period from 62 to 54 ma the unknown mantle structure under the north atlantic during the paleocene represents a major missing link in deciphering the geodynamic causes of this event to address this outstanding challenge we use a back and forth iterative method for time reversed global convection modeling over the cenozoic era which incorporates models of present day tomography based mantle heterogeneity we find that the paleocene mantle under the north atlantic is characterized by two major low density plumes in the lower mantle one beneath greenland and another beneath the azores these strong lower mantle upwellings generate small scale hot upwellings and cold downwellings in the upper mantle the upwellings are dispersed sources of magmatism and topographic uplift that were active on the rifted margins of the north atlantic during the formation of the naip while most studies of the paleocene evolution of the north atlantic have focused on the proto icelandic plume our cenozoic reconstructions reveal the equally important dynamics of a hot buoyant mantle wide upwelling below the azores