quartz cementation in late cretaceous mudstones northern north sea changes in rock properties due to dissolution of smectite and precipitation of micro quartz crystals abstract late cretaceous mudstones from two wells located in the northern north sea and the norwegian sea have been examined with respect to quartz cement two different types of quartz cement type 1 and type 2 have been identified using sem eds cl analysis of drill bit cuttings at depths 2370 2670 m 80 85 c type 1 appears as relatively large aggregates 30 100 m of depth temperature related crypto or microcrystalline to macrocrystalline irregular quartz cement formed by local re crystallization of biogenic silica the cl responses of type 1 quartz cement give a clear indication of an authigenic origin type 2 quartz cement represents relatively high amounts of extremely fine grained micro sized 1 3 m crystals embedded as discrete short chains or small clusters nests within the illitized clay matrix the cl responses of micro quartz crystals indicate an authigenic origin the micro quartz is most probably sourced from silica released during the smectite to illite dissolution precipitation reaction the petrographic evidence indicates that most of the silica released by the smectite to illite reaction has not been exported out of the mudstones the silica released produce a subtle inter connected micro quartz network interlocked with aggregates of micro quartz and authigenic clay crystals this micro quartz cementation process causes a significant and sharp change in the mudstone stiffness at the onset of the chemical compaction regime this is indicated by an abrupt increase in well log velocity vp and change in seismic facies close to 2500 m 80 85 c

petroleum geology of the puguang sour gas field in the sichuan basin sw china abstract the puguang sour gas field in northeastern sichuan basin is the largest gas discovery in the paleozoic marine strata in china with at least 9 trillion cubic feet tcf of proven gas reserves it is one of the many anticlinal structures formed during the indo sinian orogeny with gas being trapped in a composite structure lithology pool the presence of the upper permian coal measures lower silurian and lower cambrian marine shales provided excellent hydrocarbon source rocks the lower triassic and upper permian oolitic dolomites serve as high quality reservoir rocks with 10 25 porosity and 1 1000 md permeability largely due to diagenetically produced secondary porosity the lower and middle triassic evaporate beds have acted as the direct top seal for the large lower triassic upper permian gas accumulation the thick upper triassic and jurassic mudstone beds regionally cap the underlying marine carbonate strata the deep seated basement faults acted as vertical hydrocarbon migration conduits during faulting and became effective flow barriers when the faulting ceased the deep burial of a paleo oil accumulation in reservoirs rich in sulfate resulted in extensive thermochemical sulfate reduction tsr this together with the new gas injection and structural reconfiguration transformed a paleo oil pool to a large sour gas pool

north sea rift system abstract the mesozoic north sea rift forms an integral part of the arctic north atlantic rift system rifting activity in the north sea commenced during the earliest triassic during the mid jurassic the central north sea underwent a temporary doming stage crustal extension peaked during the late jurassic earliest cretaceous abated subsequently and terminated altogether during the paleocene the saucer shaped cenozoic post rift basin straddles the axis of the viking and central grabens this speaks for the applicability of a depth dependent pure shear model rather than a simple shear model geophysical data indicate an important discrepancy between upper and lower crustal attenuation across the north sea rift suggesting that during rifting processes the mono discontinuity was seriously destabilized this requires a basic modification of currently favoured crustal stretching models

ultrasonic velocity and anisotropy of hydrocarbon source rocks an experimental study of the physical properties of black kerogen rich shales also including maturation analysis scanning electron microscope sem observations and physical modeling revealed fairly peculiar petrophysical parameters specifically these rocks have very low porosity and density but most importantly both p and s ultrasonic velocities normal to bedding are extremely low whereas they are much higher parallel to bedding giving rise to a strong anisotropy even at high confining pressures we found that these parameters primarily reflect kerogen content microstructure and maturation level of these rocks we found also that microcracks inferred from ultrasonic velocity measurements occur only in mature shales these microcracks are parallel to the bedding plane and further enhance strong intrinsic anisotropy notably at low effective pressure our results show that on a small scale kerogen rich shales are transversely isotropic rocks and can be effectively modeled using the thin laye

changes in physical properties of a reservoir sandstone as a function of burial depth the etive formation northern north sea abstract rock physical properties like velocity and bulk density change as a response to compaction processes in sedimentary basins in this study it is shown that the velocity and density in a well defined lithology the shallow marine etive formation from the northern north sea increase with depth as a function of mechanical compaction and quartz cementation physical properties from well logs combined with experimental compaction and petrographic analysis of core samples shows that mechanical compaction is the dominant process at shallow depth while quartz cementation dominates as temperatures are increased during burial at shallow depths

plio pleistocene palaeogeography of the southern north sea basin 3 75 0 60 ma abstract palaeogeographical maps are presented for the southern north sea basin from approximately 1 w to 7 e and 51 55 n for the period from approximately 3 75 0 60 ma reconstructions centre on 3 75 ma coralline cragbrunssumian 2 55 ma red crag brielle ground reuverian 2 40 ma red crag westkapelle ground praetiglian early tiglian 2 00 ma norwich crag smith s knoll and ijmuiden ground middle tiglian 1 75 1 70 ma cromer forest bed paston winterton shoal late tiglian eburonian 1 40 ma kesgrave yarmouth roads waalian 1 15 ma kesgrave yarmouth roads menapian and 0 60 ma cromer forest bed west runton yarmouth roads cromerian complex from 3 75 2 40 ma the basin is mainly a marine area fringed to the east and south by fluviatile deposition and broadly open via the southwest to the atlantic following marked marine regression and cooling at around 2 40 ma the period from 2 00 1 70 ma is mainly one of renewed marine transgression and warmer conditions during which a great european delta built out fed mainly from the east and south into the southern north sea from 1 40 0 60 ma marine waters were almost entirely excluded from the area and temperate intervals alternated with increasingly glacial episodes whilst the great european delta continued to expand northwards into the northern north sea

overpressure and mudrock compaction in the lower kutai basin indonesia a radical reappraisal lateral drainage and high temperatures in the shelfal area of the lower kutai basin provide an exceptional opportunity to study compaction of miocene mudrocks and overpressure generation previous workers agreed that the principal mechanism of overpressure generation is disequilibrium compaction but sonic and resistivity logs in several fields display reversals at a transition zone into high overpressure indicating that overpressure is generated by unloading processes the transition zone coincides with the vitrinite reflectance threshold for gas generation extreme overpressures in some wells are associated with reversals on density logs too interpreted to result from opening cracks the density depth trends through the mudrocks are similar in all wells and independent of overpressure until extreme overpressures are encountered this observation strongly suggests that porosity reduction is controlled by chemical compaction and that cementation has caused the mudrocks to become overcompacted relative to the prevailing effective stress at burial depths of approximately 3 km 1 9 mi where the top of overpressure is encountered hence the lower kutai basin contains a unique reported example to date of a neogene succession in which high overpressures are generated by unloading processes with no contribution from disequilibrium compaction density logs from the peciko field have been used to derive the empirical porosity depth trend 0 434 e 0 164 z for mudrocks in the depth range 6000 to 15 000 ft 1800 to 4600 m where z is depth in thousands of feet the corresponding temperature range is 85 to 170 c so this compaction curve applies for mudrocks in the chemical compaction regime where no discrete smectite is present

compaction and rock properties of mesozoic and cenozoic mudstones and shales northern north sea abstract in this study rock physical properties and their evolution resulting from compaction processes are investigated for mesozoic and cenozoic mudstones and shales located in southern viking graben and adjacent areas within the norwegian north sea the studied sediments are deposited within a progressively subsided sedimentary basin with no major experience of exhumation events a suite of well log data from 43 exploration wells was utilized to study the compaction behaviour of the mesozoic and cenozoic mudstone and shale intervals the gamma ray log derived shale volume vsh was used to define different lithofacies and discriminate between the studied mudstones and shales the rock properties as a function of burial depth were plotted for the identified mudstone and shale intervals the trends could be divided into a mechanical compaction part and a chemical compaction part depending on the prevalent processes controlling the rock properties with burial depth the transition from mechanical compaction domain to the zone of dominant chemical compaction takes place between 70 and 90 c corresponding to a depth of 2 2 5 km the onset of chemical compaction and cementation occurs in the same sediment found at the same depth range almost throughout the study area in spite of variable geothermal gradient indicating a lithological control on the development of chemical compaction the degree of chemical compaction and cementation reflects the initial smectite content and the availability of potassium for the smectite to illite and quartz reaction to take place this study contributes to the understanding of compaction processes in fine grained siliciclastic sediments delineating the controlling factors in a region which can be regarded a natural laboratory to study compaction mechanisms due to being a subsiding basin with the extensive availability of pertrophysical data generated by hydrocarbon exploration and production activities in the area

sequential development of interfering metamorphic core complexes numerical experiments and comparison with the cyclades greece the mechanics of metamorphic core complex mcc development and the associated process of lower crustal flow have been the topic of several modelling studies the model setup usually includes a local heterogeneity forcing deformation to localize at a given site enabling only one mcc to develop this paper presents numerical lithospheric scale experiments in whichdeformationis not a priorilocalizedinaspecificplace inordertoexaminewhethermultiple mccs could develop during extension at which conditions and how configurations with either a singlemccorseveralfar distantmccsalignedinthesectionparalleltoextensionare obtainedfor arelativelywiderangeofinitialconditions theonlyfirmrequirementbeingthatthelowercrustand the sub moho mantle both have very low strengths in contrast only a narrow range of conditions leads to the development of closely spaced mccs in this case the mccs interfere with one another the domes are partly superimposed or and share a shear zone in common and develop in sequence this configuration is compared with the cyclades archipelago where closely spaced chains of mccs have been described in the literature a review of available data on the islandsdocumentsa goodagreement with the experiments intermsoffinal depthofthe moho geo metry and kinematic pattern of the mccs and timing of exhumation of the metamorphic rocks based on this agreement we tentatively deduce from the numerical results some of the conditions that prevailed at the initiation of and during post orogenic mcc type extension in the cyclades themostlikely initial thicknessofthecrust is between40 and44 km a thermallithospheric thick ness of only c 60 km is also likely which might be a condition at the onset of extension or may have been obtained during early stages of extension while the lithosphere was warmed up either a backarc subduction setting or a process of mantle delamination may account for this situ ation the numerical results also suggest a boundary velocity of 2 0 2 3 cm a which should basi cally reflect the rate at which the south hellenic subduction zone retreated considering c 500 km as an upper bound for the amount of retreat balanced by aegean extension and assuming that this retreat mostly occurred during mcc type extension in the cyclades we find that the boundary velocity could have been as high as 2 1 cm a if mcc type extension lasted 24 ma starting at c 30 ma and finishing at c 6 ma as suggested by available geochronological data a velocity of 2 1 cm a agrees well with the numerical results

quantification of tertiary exhumation in the united kingdom southern north sea using sonic velocity data sonic velocities from the upper and middle chalk upper cretaceous the bunter sandstone and the bunter shale both lower triassic were used to independently quantify apparent exhumation height above maximum burial depth in the united kingdom uk southern north sea apparent exhumation is the displacement on the depth axis of a given velocity depth trend from the normal unaffected by exhumation trend apparent exhumation results derived from the upper and middle chalk the bunter sandstone and the bunter shale are statistically similar the consistency of results from carbonate and clastic units suggests that at a formational and regional scale overcompaction i e anomalously high sonic velocity in all three units analyzed reflects previously greater buria depth rather than sedimentological and or diagenetic processes and validates the use of lithologies other than shale in maximum burial depth studies the consistency of results from units of early triassic to late cretaceous age suggests that tertiary exhumation was of sufficiently great magnitude to mask any earlier mesozoic periods of exhumation and that maximum mesozoic cenozoic burial depth in the southern north sea was attained prior to tertiary exhumation the proposed magnitudes of exhumation are generally greater than those previously published for the southern north sea but they are consistent with recent estimates from apatite fission track analysis the amount of exhumation of the rock column is greatest on the recognized inversion axes i e sole pit cleveland where it reaches approximately 2 5 km however there was a regional component of 1 0 1 5 km exhumation during the tertiary that affected structurally uninverted areas and on which the more localized inversion related exhumation was superimposed cretaceous tertiary burial prior to exhumation must have been of greater magnitude and more rapid than suggested by the preserved stratigraphy the effect of this extra burial and subsequent exhumation on sedimentary rock decompaction procedure and thermal maturation modeling is illustrated for the cleethorpes 1 and 44 7 1 wells and must also be incorporated in modeling reservoir diagenesis the regional tertiary tectonic uplift associated with exhumation must have had a thick skinned origin

physical properties of cenozoic mudstones from the northern north sea impact of clay mineralogy on compaction trends vertical and lateral changes in physical properties in cenozoic mudstones from the northern north sea basin reflect differences in the primary mineralogical composition and burial history which provides information about sedimentary facies and provenance integration of well log data with mineralogical information shows the effect of varying clay mineralogy on compaction curves in mudstones the main controlling factor for the compaction of eocene to early miocene mudstones within the north sea is the smectite content which is derived from volcanic sources located northwest of the north sea mudstones with high smectite content are characterized by low p wave velocities and bulk densities compared to mudstones with other clay mineral assemblages at the same burial depths smectitic clays are important during mechanical compaction because they are less compressible than other types of clay minerals a comparison between well log data and experimental work also shows that smectite may be a controlling factor for overpressure generation in the smectite rich eocene and oligocene sediments at greater burial depths and temperatures 70 80c the dissolution of smectite and precipitation of illite and quartz significantly increases velocities and densities miocene and younger mudstones from the northern north sea have generally low smectite contents and as a result have higher velocities and densities than eocene and oligocene mudstones lateral differences in the compaction trends between the north and south for these sediments also exist which may be related to two different source areas in the pliocene the log derived petrophysical data from the northern north sea basin show that mudstone lithologies have very different compaction trends depending on the primary composition simplified compaction curves may therefore affect the outcomes from basin modeling the amplitude versus offset response of hydrocarbon sands and the seismic signature on seismic sections are also dependent on the petrophysical properties of mudstones and will vary depending on the mineralogical composition

the lack of relationship between overpressure and porosity in north sea and haltenbanken shales abstract disequilibrium compaction undercompaction is probably the most accepted mechanism for explaining overpressured formations this mechanism relies on the hypothesis that overpressured shales have higher porosity than normally pressured shales for a given depth converted to porosity units log response data from two jurassic shales at haltenbanken and nine cretaceous and tertiary shales in the norwegian sector of the north sea have been investigated to see whether such a relationship exists in these areas the depth of the studied shale formations varies between 0 5 5 km the degree of overpressuring within one formation typically varies by 20 mpa the log response data show that neither the porosity of massive north sea shales derived from sonic and density logs nor the porosity of haltenbanken intra reservoir shales derived from density and neutron logs differ significantly between the overpressured and normally pressured stratigraphic units distinct differences between the pressure regimes are however apparent in the resistivity and sonic log data from deeply buried shales at haltenbanken these differences do not reflect porosity variations and are interpreted to reflect textural changes resulting from the overpressuring these observations demonstrate that the haltenbanken and north sea shales do not conform to an undercompaction mechanism for overpressuring as the overpressured wells fail to show elevated porosity on the contrary each formation seems to have been individually compacted according to burial depth independent of pressure regimes

velocity depth trends in mesozoic and cenozoic sediments from the norwegian shelf sonic velocity density and resistivity log data from 60 wells on the norwegian shelf have been used to investigate velocity depth trends in sedimentary rocks as a function of sediment composition porosity pore pressure burial history and compaction processes a first order linear velocity depth trend line has been estimated from published velocity data the data analyzed in this study however show significant variations from this trend line indicating that no general velocity depth function can be used when performing more accurate analyses like depth conversion of seismic data pore pressure prediction or basin modeling lower tertiary smectitic sediments from the northern north sea and haltenbanken are characterized by relatively low velocities compared to the overlying pliocene and pleistocene sediments causing a distinct velocity inversion a significant velocity increase at a burial depth corresponding to 70 100c was found and may reflect the alteration of smectite to illite and the initial precipitation of quartz cement in both sandstones and shales overpressured jurassic sediments from haltenbanken have lower velocities than equivalent hydrostatically pressured sequences but no significant porosity difference the reduced velocities may be a direct response to lower effective stresses and thus reduced elastic compaction low velocities in source rocks are mainly attributed to the relatively soft kerogen and resulting velocity anisotropy the high velocity depth ratio of barents sea sediments after correcting for tertiary exhumation is explained by the burial history of the area the subsequent thermal exposure of the sediments over time and thus the amount of quartz cementation

elastic anisotropy of source rocks implications for hydrocarbon generation and primary migration we combined acoustic velocities and anisotropy measurements with x ray diffraction mineralogy rock eval organic geochemical analyses vitrinite reflectance and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins the velocity data indicate the lamellar distribution of solid organic matter kerogen in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness the experimental results are consistent with the two component anisotropic model of black shales with a continuous kerogen network in both bedding parallel and bedding normal directions beginning from relatively low toc values the strong elastic anisotropy of tight thermally mature source rocks i further enhanced by overpressure induced bedding subparallel microcrack development during the main stage of hydrocarbon hc generation and primary migration at r0 greater than 0 75 in agreement with published petrographic observations these microcracks significantly aid primary hc migration particularly horizontally elastic anisotropy of medium to high porosity illite smectite shales is weaker than in lower porosity predominantly illite shales however the overall positive trend of elastic anisotropy with kerogen content remains unchanged

a physical model for shear wave velocity prediction the clay sand mixture model of xu and white is shown to simulate observed relationships between s wave velocity or transit time porosity and clay content in general neither s wave velocity nor s wave transit time is a linear function of porosity and clay content for practical purposes clay content is approximated by shale volume in well log applications in principle the model can predict s wave velocity from lithology and any pair of p wave velocity porosity and shale volume although the predictions should be the same if all measurements are error free comparison of predictions with laboratory and logging measurements show that predictions using p wave velocity are the most reliable the robust relationship between s and p wave velocities is due to the fact that both are similarly affected by porosity clay content and lithology moreover errors in the measured p wave velocity are normally smaller than those in porosity and shale volume both of which are subject to errors introduced by imperfect models and imperfect parameters when estimated from logs because the model evaluates the bulk and shear moduli of the dry rock frame by a combination of kuster and toksoz theory and differential effective medium theory using pore aspect ratios to characterize the compliances of the sand and clay components the relationship between p and s wave velocities is explicit and consistent consequently the model sidesteps problems and assumptions that arise from the lack of knowledge of these moduli when applying gassmann s theory to this relationship making it a very flexible tool for investigating how the v p v s relationship is affected by lithology porosity clay content and water saturation numerical results from the model are confirmed by laboratory and logging data and demonstrate for example how the presence of gas has a more pronounced effect on p wave velocity in shaly sands than in less compliant cleaner sandstones

porosity and pore structure from acoustic well logging data1 wyllie s time average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional p wave velocity or its reciprocal transit time recorded on a sonic log this model while simple needs to be more convincingly explained in theory and improved in practice particularly by making use of shear s wave velocity one of the most important although often ignored factors affecting elastic velocities in a rock is pore structure which is also a controlling factor for transport properties of a rock now that s wave information can be obtained from the sonic log it may be used with p waves to provide a better understanding of pore structure a new acoustic velocities to porosity transform based on an elastic velocity model developed by kuster and toksoz is proposed employing an approximation to an equivalent pore aspect ratio spectrum pore structure for reservoir rocks is taken into account in addition to total pore volume equidimensional pores are approximated by spheres and rounded spheroids while grain boundary pores and flat pores are approximated by low aspect ratio cracks an equivalent pore aspect ratio spectrum is characterized by a power function which is determined by compressional and shear wave velocities as well as by matrix and inclusion properties as a result of this more sophisticated elastic model of porous rocks and a stricter theory of elastic wave propagation the new method leads to a more satisfactory interpretation and fuller use of seismic and sonic log data calculations using the new transform on data for sedimentary rocks obtained from published literature and laboratory measurements are presented and compared at atmospheric pressure with those estimated from the time average equation results demonstrate that to compensate for additional complexity the new method provides more detailed information on pore volume and pore structure of reservoir rocks examples are presented using a realistic self consistent averaging scheme to consider interactions between pores and the possibility of extending the method to complex lithologies and shaly rocks is discussed

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

the effect of kerogen on the elastic anisotropy of organic rich shales abstractthe elastic properties of reservoir rocks are important for geomechanics applications the most important of which are analysis of stress changes due to production analysis of rock deformation and failure wellbore trajectory optimization and the design of hydraulic fractures organic rich shales are often observed to be strongly anisotropic due to the partial alignment of anisotropic clay minerals and the bedding parallel lamination of organic material within the shale neglecting shale anisotropy may lead to incorrect estimates of the in situ stress or stress changes resulting from production as a result isotropic models may fail to describe geomechanical behavior correctly the distribution of the organic phase plays an important role in determining the elastic properties of organic rich shales and this has a significant effect on production induced stress changes the presence of kerogen leads to a decrease in all of the elastic moduli and has a significant effect on the geomechanical b

a practical model for organic richness from porosity and resistivity logs a practical method the delta log r technique for identifying and calculating total organic carbon in organic rich rocks has been developed using well logs the method employs the overlaying of a properly scaled porosity log generally the sonic transit time curve on a resistivity curve preferably from a deep reading tool in water saturated organic lean rocks the two curves parallel each other and can be overlain since both curves respond to variations in formation porosity however in either hydrocarbon reservoir rocks or organic rich non reservoir rocks a separation between the curves occurs using the gamma ray curve reservoir intervals can be identified and eliminated from the analysis the separation in organic rich intervals results from two effects the orosity curve responds to the presence of low density low velocity kerogen and the resistivity curve responds to the formation fluid in an immature organic rich rock where no hydrocarbons have been generated the observed curve separation is due solely to the porosity curve response in mature source rocks in addition to the porosity curve response the resistivity increases because of the presence of generated hydrocarbons the magnitude of the curve separation in non reservoirs is calibrated to total organic carbon and maturity and allows for depth profiling of organic richness in the absence of sample data this method allows organic richness to be accurately assessed in a wide variety of lithologies and maturities using common well logs

mechanical compaction and ultrasonic velocity of sands with different texture and mineralogical composition this study presents the results of experimental compaction while measuring ultrasonic velocities of sands with different grain size shape sorting and mineralogy uniaxial mechanical compaction tests up to a maximum of 50 mpa effective stress were performed on 29 dry sand aggregates derived from eight different sands to measure the rock properties a good agreement was found between the gassmann saturated bulk moduli of dry and brine saturated tests of selected sands sand samples with poor sorting showed low initial porosity while sands with high grain angularity had high initial porosity the sand compaction tests showed that at a given stress well sorted coarse grained sands were more compressible and had higher velocities vp and vs than fine grained sands when the mineralogy was similar this can be attributed to grain crushing where coarser grains lead to high compressibility and large grain to grain contact areas result in high velocities at medium to high stresses the angular coarse to medium grained sands both sorted sands and un sorted whole sands showed high compaction and velocities vp and vs the small grain to grain contact areas promote higher deformation at grain contacts more crushing and increased porosity loss resulting in high velocities compaction and velocities vp and vs increased with decreasing sorting in sands however at the same porosity the velocities in whole sands were slightly lower than in the well sorted sands indicating the presence of loose smaller grains in between the framework grains quartz poor sands containing less than 55 quartz showed higher velocities vp and vs compared to that of quartz rich sands this could be the result of sintering and enlargement of grain contacts of ductile mineral grains in the quartz poor sands increasing the effective bulk and shear stiffness tests both from wet measurements and gassmann brine substitution showed a decreasing vp vs ratio with increasing effective stress the quartz rich sands separated out towards the higher side of the vp vs range the gassmann brine substituted vp and vs plotted against effective stress provide a measure of the expected velocity range to be found in these and similar sands during mechanical compaction deviations of actual well log data from experimental data may indicate uplift the presence of hydrocarbon overpressure and or cementation data from this study may help to model velocity depth trends and to improve the characterization of reservoir sands from well log data in a low temperature 80 100o c zone where compaction of sands is mostly mechanical

source rock quality and hydrocarbon migration pathways within the greater utsira high area viking graben norwegian north sea the greater utsira high area is located within the southern part of quadrants 24 and 25 and the northern part of quadrants 15 and 16 in the norwegian north sea in this part of the viking graben the main exploration play is the submarine fan sands of paleocene and eocene age these sands balder heimdal and ty formations pinch out to the east in blocks 25 8 jotun field and 25 11 balder and grane fields and along the western margin of the utsira high to form a combination of stratigraphic and structural traps marine sands of middle and late jurassic age typically present in rotated fault blocks constitute another important play geochemical analyses show that the upper jurassic draupne formation has a good potential for oil generation along the entire western margin and northern nose of the utsira high both upper and lower draupne source intervals along the western graben margin however contain more terrigenous kerogen than in the eastern part of the graben such change in organic facies within the draupne source interval naturally results in a higher proportion of gas generation and the possibility for generating a more waxy crude than typically encountered in the viking graben detection and characterization of oil and gas shows within the tertiary section permit mapping of migration entry points from the jurassic source rocks and help delineate secondary and tertiary migration pathways within the paleocene eocene play

compaction processes and rock properties in uplifted clay dominated units the egersund basin norwegian north sea abstract a thick succession of fine grained sediments in any sedimentary basin can serve as source if organic rich and top seals both sealing and petroleum generation capacity of any fine grained succession is dependent on compaction history mechanical and chemical compactions of the sediments in the egersund basin a thick succession of fine grained sediments of upper jurassic lower cretaceous age overly middle jurassic reservoir sands the egersund basin however has experienced a complex depositional and tectonic history particularly late exhumation neogene which complicates the overall burial thermal and pressure history in this study petrophysical and acoustic properties bulk density and p wave velocity of 10 exploration wells from two neighboring blocks in the egersund basin were utilized to investigate compaction processes in the area in addition x ray diffraction xrd and scanning electron microscopy sem analyses from selected wells and available total organic carbon toc data were used as guide to interpret the compaction processes and resulting rock properties the results show that the transition from mechanical to chemical compaction is not sharp the onset of chemical compaction imposes a measurable deviation in the burial depth trends of the petrophysical properties expected from purely mechanical compaction the continuation of the depth related trends however indicates that chemical compaction may occur in concurrent with continuing mechanical compaction the observed compaction varies between the different clay dominated stratigraphic units indicating that the major control on both mechanical and chemical compaction is the primary sediment composition

experimental mechanical compaction of clay mineral aggregates changes in physical properties of mudstones during burial dry and brine saturated clay aggregates ranging from pure smectite to pure kaolinite were compacted in the laboratory experiments were conducted by increasing vertical effective stress up to 50 mpa to study the changes in physical properties of mudstones during burial the results suggest that the physical properties porosity density acoustic velocity etc of mudstones vary greatly with increasing effective stress clay mineralogy and fluid content kaolinite aggregates are much more compressible than those composed of smectite brine saturated clay mixtures are much more compressible than dry clay mixtures brine may soften and lubricate the clay matrix compared to dry clays resulting in higher compressibility in the brine saturated state the lesser degree of compaction of dry smectite compared to dry kaolinite aggregates can be explained by the differences in grain size as kaolinitic clays have much larger grains than smectitic clays the extremely fine grained nature of smectite implies that the imposed vertical stress is distributed over a very large number of grain contacts so that the force per contact area is very low in smectite compared to that in coarse grained kaolinite at 20 mpa effective stress corresponding to about 2 km burial depth with hydrostatic pore pressure brine saturated pure kaolinite compacted to 20 porosity while pure smectite retained 41 porosity brinesaturated clay mixtures show higher vp and lower vs than dry clay mixtures a pronounced difference in vs was observed among the brinesaturated clay mixtures the lowest vs as a function of vertical effective stress was found for pure smectite for a given porosity value smectitic clays have higher velocities and elastic moduli than the kaolinitic clays the vp vs ratio is significantly higher in smectite than in kaolinite aggregates treating mudstones as a uniform class of rocks introduces significant errors in estimates of physical properties which are strongly affected by the clay mineralogy grain size the amount of other minerals present and fluid content our results have implications for well log interpretation and mudstone and shale property determinations from seismic data at shallow burial depth o 2k m 801c above depths where significant chemical compaction takes place r 2007 elsevier ltd all rights reserved

time depth relationships for multilayer depth conversion the conversion of seismic time to depth through the use of analytical functions has been a common procedure in seismic work for many decades with the exception of recent examples dealing with the linear function none of the published time depth relationships corresponding to these functions is applicable to multilayer depth conversion the present work redresses this situation it presents formulae applicable to multilayer depth conversion for a large number of analytical functions the derivation is based on a procedure generally similar to that presented by japsen 1993 most of the functions considered date back to a publication by kaufman in 1953 and earlier publications a number of other functions hitherto not known in the industry are also presented

de convoluting mixed crude oil in prudhoe bay field north slope alaska abstract seventy four crude oil samples from the barrow arch on the north slope of alaska were studied to assess the relative volumetric contributions from different source rocks to the giant prudhoe bay field we applied alternating least squares to concentration data als c for 46 biomarkers in the range c19 c35 to de convolute mixtures of oil generated from carbonate rich triassic shublik formation and clay rich jurassic kingak shale and cretaceous hue shale gamma ray zone hue grz source rocks als c results for 23 oil samples from the prolific ivishak formation reservoir of the prudhoe bay field indicate approximately equal contributions from shublik formation and hue grz source rocks 37 each less from the kingak shale 26 and little or no contribution from other source rocks these results differ from published interpretations that most oil in the prudhoe bay field originated from the shublik formation source rock with few exceptions the relative contribution of oil from the shublik formation decreases while that from the hue grz increases in reservoirs along the barrow arch from point barrow in the northwest to point thomson in the southeast 250 miles or 400 km the shublik contribution also decreases to a lesser degree between fault blocks within the ivishak pool from west to east across the prudhoe bay field als c provides a robust means to calculate the relative amounts of two or more oil types in a mixture furthermore als c does not require that pure end member oils be identified prior to analysis or that laboratory mixtures of these oils be prepared to evaluate mixing als c of biomarkers reliably de convolutes mixtures because the concentrations of compounds in mixtures vary as linear functions of the amount of each oil type als of biomarker ratios als r cannot be used to de convolute mixtures because compound ratios vary as nonlinear functions of the amount of each oil type

effects of source thermal maturity and biodegradation on the distribution and isomerization of homohopanes in petroleum abstract the distributions of c31 c35 17 21 h homohopanes in marine petroleums can be used to describe redox conditions in the source rock depositional environment for example high c35 homohopane indices c35 c31 c35 are typical of petroleums generated from source rocks deposited under anoxic marine conditions where the c35 backbone of the precursor bacteriohopanetetrol is preferentially preserved homohopane distributions have been used to distinguish petroleums from different organic facies of the same source rock homohopane distributions are altered by secondary processes for example the c35 homohopane index decreases with increasing api gravity and thermal maturity of related oils from the monterey formation offshore california similar decreases in the homohopane index are observed for expelled oils from monterey source rock during hydrous pyrolysis biodegradation can result in selective loss of high or low molecular weight homologs apparently depending on the bacterial population in the reservoir bacterial demethylation of homohopanes appears necessary to explain 25 norhopanes in at least one biodegraded oil hydrous pyrolysis studies of the phosphatic and siliceous members in the monterey formation show that identical heating conditions result in different homohopane c 22 epimer ratios furthermore a reversal in the trend of the epimer ratio at high experimental temperatures is observed for the phosphatic but not for the siliceous lithology this could be caused by the combined effects of isomerization and differential thermal destruction of epimers at high temperatures possibly mediated by rock mineralogy

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

compressional velocity and porosity in sand clay mixtures laboratory measurements of porosity and compressional velocity were conducted on unconsolidated brine saturated clean ottawa sand pure kaolinite and their mixtures at various confining pressures a peak in p velocity versus clay content in unconsolidated sand clay mixtures at 40 percent clay by weight was found the peak in velocity is 20 30 percent higher than for either pure clay or clean sand a minimum in porosity versus clay content at 20 40 percent clay by weight is also observed such behavior is explained using a micro geometrical model for mixtures of sand and clay in which two classes of sediments are considered 1 sands and shaley sands in which clay is dispersed in the pore space of load bearing sand and thus reduces porosity and increases the elastic moduli of the pore filling material and 2 shales and sandy shales in which sand grains are dispersed in a clay matrix for these sediments the model reproduces the extrema in velocity and porosity and accounts for much of the scatter in