modern analogs for paleocave sediment fills and their importance in identifying paleocave reservoirs abstract coalesced collapsed paleocave reservoirs occur in lower ordovician ellenburger arbuckle and knox carbonates and less commonly in other paleozoic strata one of the most distinctive features of these paleocave systems on wireline logs and in core data is sediment fill modern analogs of paleocave sediment fills from several gulf of mexico cave systems characterize the texture fabric and associated sedimentary structures of these fills the sediment fill facies can consist of carbonate and or siliciclastic debris clastic cave sediment ranging from clay to cobble sized material commonly contains variable amounts of breakdown derived breccia sections of passages can be completely plugged by sediment depositional processes include suspension traction and mass flow mechanisms sedimentary structures such as parallel laminae chaotic bedding cross bedding graded bedding and scour are common reservoir quality of the sediment fill facies depends on the texture and mineralogy of the material sediment fills are distinctive in core and can be found more than 100 m beneath the unconformity that produced the cave system on wireline logs they are characterized by high positive spontaneous potential and high gamma ray responses that reflect the terrigenous content in an otherwise carbonate system sediment fills distinctive eatures of paleocave reservoir systems are important in substantiating the origin of these reservoirs

stacked pile solution for axial load settlement analysis of driven piles in multi layered soils the load settlement q s response of deep foundations is influenced by the soil stiffness one of the most common methods of installing these foundations is the process of driving which changes t

structural control on sweet spot distribution in a carbonate reservoir concepts and 3 d models cogollo group lower cretaceous venezuela numerous major hydrocarbon accumulations are related to burial diagenesis in carbonate reservoirs however three dimensional 3 d modeling of such reservoirs is a major challenge the geometries of diagenetic fronts are poorly understood and few quantitative data on field and outcrop examples for these diagenetic zones exist the early cretaceous cogollo carbonates urdaneta west field northwest venezuela are an example of deeply buried 5000 m 1600 ft diagenetically altered limestones producing from a few high porosity streaks and fracture corridors reservoir sweet spots formed because of the interplay of structural evolution and diagenesis structural deformation not only generated a fracture network but also influenced primary facies distribution circulation of diagenetic fluids and distribution of early charged paleohighs in the reservoir the prediction of reservoir sweet spots is of key importance for economic success in the cogollo carbonates but constitutes a major challenge this article outlines the genetic relationships between structural evolution and sweet spot distribution as well as the workflow developed to transfer these concepts into 3 d geocellular models these 3 d models include a novel approach to fracture permeability prediction based on a geomechanical approach the reservoir model predicts sweet spots along leached faults which are best preserved in early charged paleohighs the cogollo and other reservoirs suggest that favorable structural development can generate and retain significant matrix porosity even at great depth the workflow established and applied might help to maximize field potential in similar settings

approximate inversion of airborne em data from a multilayered ground1 a complex transfer function c or generalized skin depth can be derived from data for the secondary magnetic field measured by a dipole system with small coil spacing at height h above the ground this function has a useful property for a uniform or layered ground the real part of c yields the centroid depth z of the in phase current system as a function of frequency this parameter can be combined with the apparent resistivity a derived by conventional methods the function a z if known over a broad frequency range yields a smoothed approximation of the true distribution z without an initial model the relations between a z and z are studied for a number of multilayer models an example of the application of the a algorithm to data from a groundwater survey is given

impact of pore structure and fractal characteristics on the sealing capacity of ordovician carbonate cap rock in the tarim basin china abstract evaluating the heterogeneity and sealing capacity of carbonate cap rocks is of great significance for hydrocarbon production however the main controls on pore size distribution in carbonate cap rocks are not well constrained in this contribution we analyze drill core samples of carbonate cap rocks from the ordovician yingshan formation tarim basin to study their pore properties and investigate the controls of pore throat structure and fractal dimension on the sealing capacity of the cap rock results from petrography and a combination of mercury injection capillary pressure and nitrogen gas adsorption tests reveal that there are four lithological cap rock types with micrite being the most prevalent one followed by calcarenite six types of pore throat structures and fractal dimensions from d1 to d6 are classified based on the pore size distributions including macropore i macropore ii mesopore i mesopore ii transitional pore i and transitional pore ii the fractal dimensions show that there is a decreasing trend from d1 to d6 with decreasing pore size diameter suggesting that the macropore and mesopore types present more complex pore throat structures than the transitional type the total fractal dimension dtotal indicates that the pore throat structures are complex and heterogeneous the sealing heights of the gas column have a positive correlation with cement content in calcarenite and a negative correlation with microfracture density mesopore ii and transitional pore types make a significant contribution to enhancing the breakthrough pressure the fractal variations mainly result from macropores and mesopores while transitional pores and micropores weaken the impact of the whole pore system on the dtotal value the heterogeneity and anisotropy of macropores is the main control on the sealing capacity of the cap rock the increasing of d1 considerably contributes to the pore shape heterogeneity and enhances the capillary resistance force to hydrocarbon migration therefore the fractal dimension is an effective approach for investigating the pore throat structure and sealing capacity of carbonate cap rocks

three dimensional seismic geomorphology and analysis of the ordovician paleokarst drainage system in the central tabei uplift northern tarim basin western china high quality three dimensional seismic data acquired in the central tabei uplift tarim basin western china provide a rare opportunity to characterize in exceptional detail the three dimensional geomorphology of a deeply buried 5500 6500 m 18 045 21 325 ft ordovician unconformity and the related paleokarst drainage system an integrated approach was applied that emphasized integration of seismic data with available conventional core wireline logs and age equivalent outcrops the exceptional quality of the seismic data allowed a seismic detection limit of karstified features of less than 75 75 m 246 246 ft horizontally and 6 m 20 ft vertically interpreted geomorphologic and depositional elements include fluvial channels and canyons fluvial valleys sinkholes and tower karsts and hills the modern tower karst drainage system in guilin china is very similar to the mapped ordovician karst drainage system and is used as a modern analog the interaction between the surface karst drainage system and the shallow subsurface cave passage system is evidenced by the observation that surface canyons appear to initiate in areas associated with intense sinkhole development also surface river valleys tend to correspond to dip oriented surface depressions partly related to near surface cave collapse during burial into the deeper subsurface the combination of intrastratal collapse karstified strata and suprastratal collapse postkarst deposited strata created large damage zones hundreds of meters thick and kilometers wide coalesced collapsed paleocave systems can be interpreted from the unique circular pattern of faults observed in map view that are associated with seismic bright spots

interpretation of airborne electromagnetic data using the modified image method the modified image method is used to invert active electromagnetic aem data from a 1984 u s navy survey of cape cod bay the high frequency data 7200 hz give a robust value for the altitude of the helicopter towed aem bird and for the first layer skin depth and hence for the first layer conductivity the inversion of low frequency 385 hz bottom probing signals produces more noise sensitive estimates for the water depth and for the conductivity contrast k the ratio of the bottom to water conductivities the results show good agreement with ground truth radar altimeter sea conductivity and sonar depth data to demonstrate the accuracy of the modified image method of inversion we incorporate ground truth measurements along a flight line and the experimental frequencies in a forward sommerfeld calculation to generate synthetic data which then are inverted using this technique

hydrogeomorphologic architecture of epikarst reservoirs in the middle lower ordovician tazhong uplift tarim basin china abstract paleokarst vugs and fissures in the middle lower ordovician from the tazhong uplift have been a productive unit in the tarim basin for more than two decades a new approach was applied to delineate the characteristics of these reservoirs by integration of 3d seismic data with thin sections cores and imaging logs as well as related geochemical analysis reconstructed hydrogeomorphologic architecture was defined to illustrate the origin of the paleokarst reservoirs in this study fissures and fissure vugs of epikarst reservoirs are the predominant pore type in the central uplift in the tazhong area which pervasively distributed as planes and steps in shallow depth of 0 30 m below the regional unconformity t74 the favorable position for reservoir development was intimately related to fissures and faults in ne and nww directions surface topography of the middle lower ordovician was flat initially with karren as the most widespread landform category hence surface water tended to be sheet flow and concentrated in places where karrens and cracks developed well and thus the infiltration strength and exhumation capacity of surface water suffered from extensive constrains due to limited hydraulic gradient which resulted in immature drainage system and incomplete karst sequence therefore the hydrogeomorphologic architecture in the middle lower ordovician was likely lack of surface drainages and a fissure seepage karst reservoir development model was established to interpret the influence of hydrogeomorpholoic architecture on paleokarst reservoirs distribution this study confirms that paleokarst fissure vug system developed in karst initiation period can still be potential petroleum bearing target in deeply buried subsurface

estimation of dispersion in orientations of natural fractures from seismic data application to dfn modeling and flow simulation this article describes a statistical methodology to estimate dominant fracture orientation and dispersion from seismically calculated 3d structural attributes the orientation dispersion parameter is a modified version of the circular variance that is independent of direction and can be used to compute the fisher coefficient a key parameter in the probability density function used in discrete fracture network dfn modeling to stochastically generate fracture orientations we show examples of applications to seismic constrained dfn modeling and perform flow simulations on fractured models built using different dispersion parameter models to discuss possible implications for drainage of naturally fractured unconventional reservoirs

geocellular rendering of cave surveys in paleokarst reservoir models abstract infilled and collapsed cave systems are an important component of many paleokarst reservoirs incorporating these features into industrial reservoir models commonly relies on geostatistical modelling methods that often fail to capture key aspects of connectivity geometry and volume of the paleokarst features realistically the present work investigates the implementation of realistic cave geometries in geocellular models using survey data from an active karst cave as a starting point the proposed method utilizes cave survey data to generate a dense equally spaced point cloud representing the cave system the point clouds are used for geometric modelling and subsequent geocellular discretization of the karst system the volumetric and geometric accuracy of this novel reservoir modelling method is compared to that from two established methods by benchmarking against the cave survey data additionally the interlinkage between grid cell resolution applied filter cut off and geocellular rendering are evaluated this study demonstrates that our proposed novel methodology can provide an excellent geometric and volumetric geocellular rendering of karst systems using cave survey data as input employing a combination of cave network maps and forward modelling of collapse and infill may enable model rendering of these features that more closely echoes processes controlling cave and karst breccia formation and geometric characteristics in turn this could offer better constraints to forecast paleokarst reservoirs architecture and properties

geological and industrial implications of extensive dilatancy anisotropy extensive dilatancy anisotropy is the hypothesized distribution of stress aligned fluid filled microcracks pervading most rocks in the earth s crust the geometry of the cracks and the aligning stress field can be monitored by analysing the waveforms of shear waves propagating through the rockmass the ability to estimate some of the parameters of the crack and stress geometry by analysing shear wave particle displacements has widespread implications

paleocave carbonate reservoirs origins burial depth modifications spatial complexity and reservoir implications paleocave systems form an important class of carbonate reservoirs that are products of near surface karst processes and later burial compaction and diagenesis features and origins of fractures breccias and sediment fills associated with paleocave reservoirs have been studied in modern and ancient cave systems information about such cave systems is used in this paper to reconstruct the general evolution of paleocave reservoirs and their associated scale pore networks and spatial complexities spatial complexities in paleocave reservoirs result from near surface and burial processes near surface processes include dissolutional excavation clastic sedimentation chemical precipitation and localized fracturing brecciation and collapse of cave walls and ceilings burial processes begin as cave systems subside into the subsurface remaining cave passages commonly collapse and early formed breccia clasts are rebrecciated differential compaction of strata around and over collapsed passages produces fractures crackle breccias and mosaic breccias near surface and burial processes combine to produce typically complex reservoirs with several scales of heterogeneity hydrocarbon reservoirs of paleocave origin are commonly the product of coalesced collapsed paleocave systems the coalescing of passages in a cave system into larger connected porosity zones results from a combination of multiple cave forming episodes at composite unconformities and from the collapse of cave systems during burial where surrounding host strata are brecciated and fractured this combination of processes creates spatially complex reservoirs that can be hundreds to several thousands of meters across commonly forming large exploration targets final size pore network types and spatial complexities of coalesced collapsed paleocave systems are products of their evolution from near surface development through burial into the deeper subsurface the coalesced collapsed paleocave reservoir hypothesis explains the scale of reservoirs observed and the spatial complexities involved

paleo heat flow evolution of the tabei uplift in tarim basin northwest china abstract the paleo heat flow evolution of the tabei uplift in the tarim basin is investigated based on burial and thermal history reconstruction of 14 wells and using basin modeling numerous geological parameters such as temperature data and missing sediment thickness by erosion were used in the modeling the basin model was calibrated using 460 measured vitrinite reflectance vr o and vitrinite like maceral reflectance vlmr o values to constrain the validity of the maturity model the heat flow history of the tabei uplift tarim basin shows the following characteristics 1 the highest paleo heat flow was predicted to have occurred in the early ordovician as 65 5 mw m 2 and gradually decreased to 55 5 mw m 2 during the late carboniferous 2 a thermal kick was modeled to have occurred in the permian as suggested by an abrupt rise in the heat flow 3 the heat flow gradually decreased since the triassic 4 the present day heat flow was predicted to be as low as 38 mw m 2 this heat flow history honors the geologic and tectonic evolution history of the tabei uplift and is suggested as the best case heat flow model

from fracture analysis to flow simulations in fractured carbonates the case study of the roman valley quarry majella mountain italy abstract this work assesses the impact of the stratigraphic and structural heterogeneities on the fluid distribution and migration within a deformed carbonate reservoir analogue the study was carried out in the abandoned roman valley quarry majella mountain central italy well known for its historical bitumen extraction the studied rocks consist of ramp carbonates of the lower member of the bolognano formation oligocene miocene in age mainly composed of grainstones and secondary packstones and wackestones in the quarry the exposed rocks are crosscut by two high angle oblique slip faults striking wnw ese with up to 40 m of throw with the aim of deciphering the contribution of both matrix and fracture pore space on the fluid storage and migration both laboratory measurements and field based discrete fracture network dfn models cubes with 4 m per side have been carried out the obtained dual porosity permeability model which includes the lithological and the structural heterogeneities observed at the location was used to run flow simulations under different scenarios of well locations the fluid distribution in the matrix resulting from these flow simulations is consistent with field observations where bitumen localizes within the most permeable lithofacies grainstones in the fault zones the fracture network causes an important fluid flow anisotropy enhancing the flow in a direction subparallel to the faults the flow orthogonal to the fault zones is controlled by type and lateral continuity of fault rocks where fault breccias represent conduits and cataclasites localized barriers

sand grain crushing and interface shearing during displacement pile installation in sand particle crushing shear banding interface abrasion and migration of crushing products all have the potential to influence the behaviour of displacement piles in sands this paper considers these particulate processes reporting experiments with model displacement piles installed in uniform pressurised sand and parallel interface ring shear tests the findings offer new insights into the mechanics of displacement piles in sands

early paleozoic tropical paleokarst geomorphology predating terrestrial plant growth in the tahe oilfield northwest china abstract a deeply buried tropical paleokarst landscape that predates terrestrial plant growth was discovered in west district 10 of the tahe oilfield located on the northern margin of the tarim basin in the xinjiang uygur autonomous region of northwestern china the paleokarst unconformity surface at depths deeper than 6000 m had been subjected to subaerial karstification under humid tropical climatic conditions during the first hiatus period in the early paleozoic a high quality full azimuthal 3d seismic survey was conducted in this study area area of 10 10 km2 in order to visualize the buried paleokarst landscape with high resolution seismic data with this data and previous geological geochemical and paleomagnetic data we reconstructed the large scale paleokarst landform assemblage compared it to a modern humid tropical subtropical karst and investigated the different controlling factors for the formation of the paleokarst and the modern karst results show that 1 the original landform assemblages of the tropical paleokarst landscape are distinct from those of the modern humid tropical subtropical karst geomorphology 2 the paleokarst denudation surface that formed in the early paleozoic was relatively smooth with shallow depressions and the relative relief was less than that of the modern tropical subtropical cockpit karst area 3 during the time of epigenetic paleo karstification lithological hydrogeological and climate conditions were optimal for the formation of cockpit karst the tropical paleokarst landscape demonstrates that extensive terrestrial plant growth plays an important role in creating large scale karst landform assemblages of peak clusters and depressions cockpit karst

effect of friction fatigue on pile capacity in dense sand the paper presents the results of a series of field tests performed to study the causes of friction fatigue experienced by displacement piles four instrumented model piles were installed at a dense sand test bed site the test series was designed to impose different levels of cyclic loading during pile installation static and cyclic load tests were subsequently performed to study the differences in the axial capacities developed for ostensibly monotonic and cyclic installations the test results indicated that the mobilized horizontal effective stress regime that controls pile side friction primarily depends on the in situ sand state as reflected by the cone penetration test cpt qc resistance a zone of highly stressed sand that produced a concentration of high shear resistance was mobilized in the vicinity of the pile base the horizontal effective stress that acted on the pile shaft reduced in response to cyclic loading with the largest reductions occurring for high intensity cyclic loading or when the pile had experienced only a few load cycles during installation although cyclic loading caused a reduction in the horizontal effective stress that acted on the pile shaft the elevated stress built up in the vicinity of the pile base during installation remained higher than that remote from the base the elevated stress in the vicinity of the pile base only dissipated after cyclic tension loading had been applied

static response of monopile to lateral load in overconsolidated dense sand abstractlarge diameter rigid monopile foundations have been extensively used in the fast growing offshore wind energy industry over the last two decades in view of the offshore environment later

evaluation of installation effects on set up of field displacement piles in sand the paper presents results from a new series of tests on displacement piles in sand involving different installation modes and combines these with results from previous tests at the same site as

time lapsed shear wave velocity vs tomographic images and stress in sand surrounding displacement pile during setup this study reports model pile tests designed to characterize the underlying mechanisms of driven pile setup in dry sand by means of stress measurement with the tactile pressure sensors and spatio t

on the interpretation of airborne electromagnetic data for the two layer case for helicopter borne electromagnetic systems the distance between the transmitting and the receiving coils is small compared with the altitude above ground for this case a major simplification can be made for the calculation of model curves some two layer curves for the interpretation of frequency measurements are presented a very simple procedure is demonstrated for the conversion of the relative secondary field into apparent resistivity and apparent distance for the mapping of airborne electromagnetic data furthermore an approximation is described for the determination of the thickness and the resistivity of a layer lying on a perfectly conducting half space

integrated geological geophysical characterizations of deeply buried fractured vuggy carbonate reservoirs in ordovician strata tarim basin abstract the central uplift region of the tarim basin contains a series of thick ordovician carbonates that form fractured vuggy hydrocarbon rich reservoirs along an unconformity these fractured vuggy reservoirs are difficult to characterize using traditional well logging and seismic interpretation methods because these carbonates feature complex and heterogeneous lithologies and variations in scale geometry and spatial distribution in this paper a series of deeply buried fractured vuggy carbonate reservoirs are characterized on micro macro and mega scales these characterizations are based on previous studies detailing the carbonate sedimentation diagenesis and tectonic processes in the study area and abundant geological and geophysical data including core samples thin sections and well logging and seismic datasets which have been obtained and upscaled using the choosing classification evaluation generalizing upscaling ccegu method in this area different facies can be identified at different scales 1 detailed core and thin section observations performed based on the dunham classification demonstrate that 7 lithofacies and 19 rock types are present at the micro scale 0 01 m 2 logging data and assorted reservoir characteristics indicate that these 19 different rock types consist of 9 different electro facies at the macro scale 0 1 m 1 m and the presence of electro facies in individual wells can be recognized and 3 these 9 different electro facies comprise 6 distinct seismic facies at the mega scale 10 m and can be used to delineate the spatial outlines of potential reservoirs and evaluate their properties the targets of particular interest in the study area are a series of partially dissolved dolomites located near large faults and a series of vuggy soluble limestones located near ne trending strike slip faults this study combines geological and geophysical response data from fractured vuggy carbonate reservoirs to achieve seamless connections among micro scale macro scale and mega scale observations to effectively characterize a series of strongly heterogeneous fractured vuggy reservoirs this method can also be applied to characterize other deeply buried paleokarst reservoirs in the tarim basin as well as other strongly heterogeneous reservoirs

diagenesis of an evaporite related carbonate reservoir in deeply buried cambrian strata tarim basin northwest china many hydrocarbon shows have been reported from potential evaporite related carbonate reservoirs in the lower to middle cambrian units in the tarim basin petrography facies and geochemical analyses from outcrop and core samples were integrated to document the effect of diagenetic evolution on these evaporite related carbonate reservoirs a simplified depositional model has been developed that reveals a restricted carbonate platform dominated by an evaporatic inner platform lagoon with shoal and reef facies developed around the platform margins grainstones and packstones were deposited at the platform margin and on inner platform shoals whereas carbonate mudstones and wackestones were deposited in the lagoon associated with evaporites early dolomitization and dissolution occurred related to the reflux of evaporated seawater breccia associated fracture porosity caused by a short period of meteoric water induced dissolution is the predominant type of effective porosity in the anhydrite bearing dolomudstone reservoir thermochemical sulfate reduction tsr occurred in the deep subsurface diagenetic environment characterized by high temperature 100 c 160 c and high salinity 16 26 wt aqueous fluid inclusions in tsr calcite that has relatively negative 13c values as low as 12 vienna peedee belemnite and elevated 87sr 86sr values the porous dolomites contain solution enlarged pores caused by dissolution of anhydrite and carbonate as a secondary consequence of tsr this study shows that deeply buried evaporite related ancient carbonate rocks may be effective reservoirs because of a combination of fracture porosity and tsr induced secondary porosity evaporitic cambrian carbonates buried to over 5000 m 16 500 ft can thus be considered as targets for future exploration

piles under cyclic axial loading study of the friction fatigue and its importance in pile behavior the main point of this paper is to identify the key features and load transfer mechanisms controlling friction fatigue of piles under cyclic loading numerical modeling makes use of an advanced ela

multiscale geological geophysical characterization of the epigenic origin and deeply buried paleokarst system in tahe oilfield tarim basin abstract the irregular structures of paleokarst systems are among the major factors resulting in carbonate reservoir heterogeneity which restricts the further exploration of paleokarst reservoirs using multiscale geological geophysical g g data including thin section core sample and well log data the caves cave fillings and fractures in individual wells in the northern tahe oilfield were characterized guided by seismic datasets the interwell paleokarst structures were interpreted in profile and plane view the paleokarst system in the study area was divided into four vertical zones and the relative genetic structures were classified in the upper three zones the epikarst zone includes paleo canyons karst towers dolines paleo soils and fractures and the water flows horizontally the vadose zone includes vadose caves and fractures and the water flows vertically the epiphreatic zone includes chambers main channels branch channels and fractures and the water flows primarily horizontally along the water table for each genetic type the total hydrocarbon production and monthly production were analyzed furthermore a genetic classification model was built for the paleokarst reservoirs of the study area which increased our understanding of the structure of the paleokarst system the results of the structure classification and production analysis can effectively guide hydrocarbon exploration and this approach can be applied to similar paleokarst reservoirs

seismic inversion for reservoir properties combining statistical rock physics and geostatistics a review there are various approaches for quantitative estimation of reservoir properties from seismic inversion a general bayesian formulation for the inverse problem can be implemented in two different work flows in the sequential approach first seismic data are inverted deterministically or stochastically into elastic properties then rock physics models transform those elastic properties to the reservoir property of interest the joint or simultaneous work flow accounts for the elastic parameters and the reservoir properties often in a bayesian formulation guaranteeing consistency between the elastic and reservoir properties rock physics plays the important role of linking elastic parameters such as impedances and velocities to reservoir properties of interest such as lithologies porosity and pore fluids geostatistical methods help add constraints of spatial correlation conditioning to different kinds of data and incorporating subseismic scales of heterogeneities

multi layered ordovician paleokarst reservoir detection and spatial delineation a case study in the tahe oilfield tarim basin western china abstract paleokarst systems are major carbonate reservoirs that can form significant and very large oilfields the ordovician carbonate reservoirs of the tahe oilfield represent a special type of hydrocarbon reservoir consisting primarily of paleokarst caves and fractures detailed characterization of paleokarst reservoirs in the tahe area is challenging because of their heterogeneous distribution and in particular because these reservoirs are at depths exceeding 5500 m this paper describes a detailed approach for integrating data from cores imaging and conventional well logs and high quality seismic datasets to delineate the complex paleokarst systems in the tahe oilfield particularly the paleocaves in the run off zone based on the hydrologic genetic relationships between the karst and paleocaves observed in outcrops we divided the paleocaves in the run off zone into chambers main channels and branch channels the chambers which are generally taller than 15 m are generally collapsed the main channels mostly 5 15 m tall were the main passages for underground water flow and display good connectivity the branch channels typically less than 5 m tall are relatively dispersed and still have a considerable volume this method was applied to the area around well t615 and two layers of paleocaves in the run off zone were interpreted seismic mapping and three dimensional modeling of the paleocaves were performed and the lengths widths areas and volumes of these three types of paleocaves were statistically analyzed although most of the chambers have been penetrated by existing wells the areas where the main channels and a dense network of branch channels developed are favorable targets for future drilling in the tahe oilfield our method of detecting and analyzing the paleokarst reservoir structure is applicable to similar paleokarst reservoirs

3d seismic analysis of karstified interbedded carbonates and evaporites lower permian gipsdalen group loppa high southwestern barents sea abstract this paper presents a detailed seismic analysis of a hitherto undescribed permian succession interpreted to consist of interbedded anhydrite and carbonate from the northern flank of the loppa high western barents sea the fafner succession forms a locally restricted transgressive unit down dip on the rotated loppa high it is subdivided into four seismic sequences each interpreted to be composed of a basal transgressive anhydrite overlain by highstand carbonates fafner seismic sequences 2 and 3 include well developed buildup complexes which in map view form polygonal patterns analysis of attribute maps indicates that the fafner succession is overprinted by two dissolution systems related to two prominent events of subaerial exposure creating numerous paleo sinkholes breccia pipes and collapsed composite cavern systems the stratigraphical position of the fafner succession below the mid sakmarian artinskian bjarmeland group and above the top orn unconformity resembles that of the lower permian gipshuken formation on spitsbergen and the fafner succession is thus regarded as its offshore equivalent

alteration and multi stage accumulation of oil and gas in the ordovician of the tabei uplift tarim basin nw china implications for genetic origin of the diverse hydrocarbons abstract the ordovician is the most important exploration target in the tabei uplift of the tarim basin which contains a range of petroleum types including solid bitumen heavy oil light oil condensate wet gas and dry gas the density of the black oils ranges from 0 81 g cm 3 to 1 01 g cm 3 20 c and gas oil ratio gor ranges from 4 m 3 m 3 to 9300 m 3 m 3 oil source correlations established that most of the oils were derived from the mid upper ordovician marine shale and carbonate and that the difference in oil properties is mainly attributed to hydrocarbon alteration and multi stage accumulation in the tabei uplift there were three main periods of hydrocarbon accumulation in the late caledonian stage ca 450 430 ma late hercynian stage ca 293 255 ma and the late himalayan stage ca 12 2 ma the oil charging events mainly occurred in the late caledonian and late hercynian stage while gas charging occurred in the late hercynian stage during the late caledonian stage petroleum charged the reservoirs lying east of the uplift however due to a crustal uplifting episode in the early hercynian ca 386 372 ma most of the hydrocarbons were transformed by processes such as biodegradation resulting in residual solid bitumen in the fractures of the reservoirs during the late hercynian stage a major episode of oil charging into ordovician reservoirs took place subsequent crustal uplift and severe alteration by biodegradation in the west central basin resulted in heavy oil formation since the late himalayan stage when rapid subsidence of the crust occurred the oil residing in reservoirs was exposed to high temperature cracking conditions resulting in the production of gas and charged from the southeast further altering the pre existing oils in the eastern reservoirs a suite of representative samples of various crude oils including condensates lights oils and heavy oils have been collected for detailed analysis to investigate the mechanism of formation based on the research it was concluded that the diversity of hydrocarbon physical and chemical properties in the tabei uplift was mainly attributable to the processes of biodegradation and gas washing the understanding of the processes is very helpful to predict the spatial distribution of hydrocarbon in the tabei uplift and provides a reference case study for other areas

characterizing seismic bright spots in deeply buried ordovician paleokarst strata central tabei uplift tarim basin western china anomalous seismic amplitude bright spots are a common feature in deeply buried 5500 to 6500 m ordovician limestone strata in the central tabei uplift area of the tarim basin in northwest china those anomalies have proven to be useful indicators of reservoir quality the bright spots as seen on seismic data are tied to high gamma ray low velocity zones in wireline logs and correspond to clastic cave sediment fills in the host limestone in core synthetic seismic models confirm this relationship between seismic bright spots and cave sediment fills a seismic traveltime map of the top ordovician unconformity illustrates erosional topography and seismic geomorphologic patterns associated with the unconformity with numerous sinuous fluvial channels and canyons fluvial valleys sinkholes and tower karsts and hills a mature surface drainage system interacted with a near surface karst system and allowed terrigenous sediments to enter an underground cave system karst related bright spots probably correspo

field experiments at three sites to investigate the effects of age on steel piles driven in sand this paper investigates the influences that steel type in situ soil properties water table depth pile diameter roughness and driving procedures have on the ageing behaviour of piles driven in s

karst controlled reservoir heterogeneity in ellenburger group carbonates of west texas petroleum production from restricted shelf carbonates of the lower ordovician ellenburger group is commonly considered to have been a result of a pervasive relatively homogeneous tectonic fracture system within the reservoir rock however regional facies and diagenetic paleokarst studies of ellenburger strata based on cores and wireline logs have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit ellenburger group carbonates which attain a thickness of over 1 700 ft 520 m record sedimentation on a shallow water restricted shelf occupying most of west texas logging of over 10 000 ft 3 050 m of core from 63 wells has allowed recognition of six facies assemblages 1 lithic arenite 2 mixed siliciclastic carbonate packstone grainstone 3 ooid peloid grainstone 4 mottled mudstone 5 laminated mudstone and 6 gastropod intraclast packstone grainstone these facies assemblages record initial transgression and subsequent progradation and aggradation paleoslope was generally south and east from the texas arch toward the ouachita and marathon orogenic belts all facies assemblages with the local exception of the ooid peloid grainstone assemblage are characterized y very low intergranular and intercrystalline porosity porosity development in ellenburger group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a middle ordovician eustatic lowstand prior to transgression of simpson group siliciclastics during this episode a widespread system of caves sinkholes joint controlled solution features and collapse breccias developed of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft 30 and 90 m beneath the exposed ellenburger surface infill of this cave system by simpson group sand and clay segmented the upper ellenburger into three karst facies which are in descending order 1 cave roof dolomites fracture and mosaic breccias 2 laterally persistent cave fill facies siliciclastic matrix supported and carbonate matrix supported breccias and 3 lower collapse facies chaotic clast supported breccias of the cave floor pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the emma andector martin block 13 and several other major ellenburger reservoirs lateral reservoir heterogeneities formed by localized laterally extensive collapse structures such as in the shafter lake reservoir also contribute to compartmentalization of producing zones within the upper ellenburger group secondary and tertiary recovery programs in these ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model

permeability distribution and scaling in multi stages carbonate damage zones insight from strike slip fault zones in the tarim basin nw china abstract architecture and permeability of fault damage zones are important for understanding faulting mechanisms and fluid flow in fractured rocks nevertheless this understanding is often hindered by limited availability of data especially in the subsurface a comprehensive suite of cores logs and production data is here presented to unravel petrophysical properties and fracture characteristics of deep 6000 m ordovician limestones within strike slip fault zones of the tarim intracratonic basin nw china the results show that 1 the carbonate porosity is mainly secondary dissolution porosity and comprises low porosity 5 md and high porosity 8 2 fault damage zones are generally tight with fracture aperture

the nature and origins of decametre scale porosity in ordovician carbonate rocks halahatang oilfield tarim basin china at 7 km depths in the tarim basin hydrocarbon reservoirs in ordovician rocks of the yijianfang formation contain large cavities c 10 m or more vugs fractures and porous fault rocks although some yijianfang formation outcrops contain shallow formed near surface palaeokarst features cores from the halahatang oilfield lack penetrative palaeokarst evidence outcrop palaeokarst cavities and opening mode fractures are mostly mineral filled but some show evidence of secondary dissolution and fault rocks are locally highly c 30 porous cores contain textural evidence of repeated formation of dissolution cavities and subsequent filling by cement calcite isotopic analyses indicate depths between c 220 and 2000 m correlation of core and image logs shows abundant cement filled vugs associated with decametre scale fractured zones with open cavities that host hydrocarbons a sm nd isochron age of 400 37 ma for fracture filling fluorite indicates that cavities in core formed and were partially cemented prior to the carboniferous predating permian oil emplacement repeated creation and filling of vugs timing constraints and the association of vugs with large cavities suggest dissolution related to fractures and faults in the current high strain rate regime corroborated by velocity gradient tensor analysis of global positioning system gps data rapid horizontal extension could promote connection of porous and or solution enlarged fault rock fractures and cavities supplementary material stable isotopic analyses and the velocity gradient tensor and principal direction and magnitude calculation are available at https doi org 10 6084 m9 figshare c 4946046 thematic collection this article is part of the the geology of fractured reservoirs collection available at https www lyellcollection org cc the geology of fractured reservoirs

fracture characterization and modeling of karsted carbonate reservoirs a case study in tahe oilfield tarim basin western china abstract the characterization and modeling of fractures in karsted carbonate reservoirs are challenging due to the mechanical interactions between fractures and irregular karst elements this study proposes an integrated approach for characterizing and modeling large scale fractures within deeply buried fractured karst carbonates of the yingshan formation ordovician in age located at the tahe oilfield tarim basin western china this investigation implements a promising workflow to stochastically model the fracture sets interpreted from borehole image logs and compute both fracture porosity and permeability tensor in karsted carbonates for this case study we have constructed a discrete fracture network dfn by combining two structural seismic attributes i e local structural dip and ant tracking we have also applied the automatic fault extraction afe algorithm the results indicate that this method detects a greater number of discontinuities associated with fractures compared to the conventional methods employed in this kind of reservoir e g seismic coherence and curvature the fracture set with higher values of fracture porosity and permeability coincided with the orientation of maximum horizontal stress wsw ene in the model the highest values of fracture intensity correspond to the most productive oil wells finally this study highlights the importance of fracture modeling for evaluating the distribution of the most favorable production areas so called sweet spots in karsted carbonates reservoirs

permeability tensor for discontinuous rock masses rock masses which commonly contain a large number of discontinuities are treated as homogeneous anisotropic porous media to formulate the corresponding permeability tensor this has been successfully achieved by introducing a symmetric tensor crack tensor which depends only on the geometry of the related cracks aperture size and orientation the principal directions associated with the symmetric crack tensor are coaxial with those of the permeability tensor the first invariant of the crack tensor is proportional to the mean permeability while the deviatoric part is related to the anisotropic permeability these results are well supported by numerical experiments on the permeability of cracked media by long remer wilson and witherspoon an actual rock mass moderately jointed granite was studied to see whether the crack tensor can be determined in situ stereology based on geometrical statistics provides a sound basis for determining the crack tensor in situ the crack tensor is obtained by

three dimensional architecture of a coalesced collapsed paleocave system in the lower ordovician ellenburger group central texas the three dimensional interwell scale architecture of a lower ordovician ellenburger coalesced collapsed paleocave system was constructed through the integration of ground penetrating radar gpr shallow core and outcrop data the data were collected near marble falls in central texas over an area 800 1000 m 2600 3300 ft that could cover several oil well locations 160 ac 0 65 km2 typical of a region such as west texas integration of core based facies descriptions with gpr reflection response identified several paleocave facies that can be recognized and mapped with gpr data alone 1 continuous reflections image the undisturbed strata 2 relatively continuous reflections over tens of meters characterized by faults and folds image the disturbed strata and 3 chaotic reflections having little to no perceptible continuity image heterogeneous cave related brecciated facies recognized in core that cannot be individually resolved with the gpr data these latter facies include the highly disturbed strata coarse clast chaotic breccia fine clast chaotic breccia and sediment fill the three dimensional architecture of the coalesced collapsed paleocave system based on core and gpr data indicates that there are trends of brecciated bodies that are as much as 350 m 1100 ft wide greater than 1000 m 3300 ft long and tens of meters high these brecciated bodies are coalesced collapsed paleocaves between the brecciated bodies are areas of disturbed and undisturbed host rock that are jointly as much as 200 m 660 ft wide as a cave system is buried many structural features form by mechanical compaction these features include folds sags and faults the folds and sags measure from a few meters to several hundred meters wide the collapse related faults are numerous and can have several meters of displacement most are normal faults but reverse faults also occur

inverting airborne electromagnetic aem data with zohdy s method zohdy s method for the inversion of dc resistivity data has been adapted to the inversion of airborne electromagnetic aem data aem responses are first transformed into apparent conductivity depth profiles followed by an iterative adjustment of layer thicknesses and interval conductivities the start model including the number of layers is determined from the data this approach optimizes model flexibility without the need for parameter regularization results from zohdy s inversion applied to tempest geotem and dighem v data acquired in a range of conductivity scenarios including the bull creek prospect in queensland australia the boteti area botswana and the reid mahaffy test site in ontario canada show well delineated target zones a comparison with occam s inversion shows good agreement between the conductivity depth models recovered by the two methods with zohdy s inversion being 25 to 80 times faster

the occurrence of ultra deep heavy oils in the tabei uplift of the tarim basin nw china abstract deeply buried heavy oils from the tabei uplift of the tarim basin have been investigated for their source origin charge and accumulation time biodegradation mixing and thermal cracking using biomarkers carbon isotopic compositions of individual alkanes fluid inclusion homogenization temperatures and authigenic illite k ar radiometric ages oil source correlation suggests that these oils mainly originated from middle upper ordovician source rocks burial history coupled with fluid inclusion temperatures and k ar radiometric ages suggests that these oils were generated and accumulated in the late permian biodegradation is the main control on the formation of these heavy oils when they were elevated to shallow depths during the late hercynian orogeny a pronounced unresolved complex mixture ucm in the gas chromatograms together with the presence of both 25 norhopanes and demethylated tricyclic terpanes in the oils are obvious evidence of biodegradation the mixing of biodegraded oil with non biodegraded oil components was indicated by the coexistence of n alkanes with demethylated terpanes such mixing is most likely from the same phase of generation but with accumulation at slightly different burial depths as evidenced by overall similar oil maturities regardless of biodegradation level and or amount of n alkanes although these ordovician carbonate reservoirs are currently buried to over 6000 m with reservoir temperatures above 160 c no significant secondary hydrocarbon generation from source rocks or thermal cracking of reservoired heavy oil occur in the study area this is because the deep burial occurred only within the last 5 ma of the neogene and there has not been enough heating time for additional reactions within the middle upper ordovician source rocks and reservoired heavy oils

advanced inversion methods for airborne electromagnetic exploration airborne electromagnetic aem surveys can contribute substantially to geologic mapping and target identification if good quality multifrequency data are produced properly evaluated and displayed a set of multifrequency em data is transformed into a set of apparent resistivity a and centroid depth zp values which then are plotted as a sounding curve these a zp curves commonly provide a smoothed picture of the vertical resistivity distribution at the sounding site we have developed and checked methods to enhance the sensitivity of sounding curves to vertical resistivity changes by using new definitions for apparent resistivity and centroid depth one of these new sounding curves with enhanced sensitivity to vertical resistivity contrasts is plotted from nb zs values derived from differentiation of the a f curve with respect to the frequency f this approach is similar to the niblett bostick transform used in magnetotellurics it not only enhances vertical changes in resistivity but also