permeability prediction from geologic models permeability is a critical parameter for the petroleum geologist by simulating the processes of compaction and cementation in a model porous medium we have gained a new understanding of how permeability is controlled in reservoir sandstones this understanding can be used predictively for simple sandstones the geometry of the model pore system is completely defined so permeability can be calculated directly using a network model for flow the calculation is based on first principles and is physically rigorous in contrast to many previous efforts to predict permeability there are no adjustable parameters in the calculations and no additional measurements or correlations e g capillary pressure data or pore system data from thin sections are required the model derived porosity permeability trend for a compacted or quartz cemented sandstone or a sandstone having a combination of these processes closely matches measurements on fontainebleau sandstone samples whose permeabilities span nearly five orders of magnitude the model also correctly predicts mercury injection measurements of pore throat size distribution for the fontainebleau sandstone pore scale geometric features of the model are found to be spatially correlated and this departure from randomness significantly affects macroscopic properties such as permeability the agreement between predictions and measurements suggests that spatial correlation is inherent in granular porous media and consequently uncorrelated or arbitrarily correlated models of transport in such media are unlikely to be physically representative we also discuss extending the model to predict properties of more complicated rocks

diagenesis in north sea hpht clastic reservoirs consequences for porosity and overpressure prediction upper jurassic clastic reservoirs of the fulmar formation in the central north sea possess anomalously high porosities for their present day depth of burial reservoirs with the highest overpressures have the highest porosities possess less macroquartz cement and have significant secondary porosity quartz cementation in hpht high pressure high temperature reservoirs has been inhibited by a combination of factors high overpressure limited fluid movement presence of early grain coating cements high pore fluid salinity and possibly petroleum migration secondary porosity has contributed to reservoir quality with an average of 4vol extra porosity created quantitative prediction of porosity would require an improved depositional model for the fulmar accurate thermal and pressure modelling and detailed knowledge of field filling and leakage histories theoretical calculations indicate that diagenetic reactions occurring in the fulmar formation smectite illitisation and quartz cementation did not generate significant overpressure because seal permeabilities were too high and the rate of volume increase associated with the reactions too small therefore diagenetic reactions can effectively be ignored when modelling overpressure generation in the central north sea although cementation will affect rock permeability and rates of fluid dissipation

reservoir characteristics and their effects on hydrocarbon accumulation in lacustrine turbidites in the jiyang super depression bohai bay basin china abstract a great difference exists between the hydrocarbon charging characteristics of different tertiary lacustrine turbidites in the jiyang super depression of the bohai bay basin east china based on wireline log data core observation and thin section analyses this study presents detailed reservoir property data and their controlling effects from several case studies and discusses the geological factors that govern the hydrocarbon accumulation in turbidite reservoirs the lacustrine fluxoturbidite bodies investigated are typically distributed in an area of 0 5 10 km2 with a thickness of 5 20 m the sandstones of the tertiary turbidites in the jiyang super depression have been strongly altered diagenetically by mechanical compaction cementation and mineral dissolution the effect of compaction caused the porosity to decrease drastically with the burial depths especially during the early diagenesis when the porosity was reduced by over 15 the effect of cementation and mineral dissolution during the late stage diagenesis is dominated by carbonate cementation in sandstones high carbonate cement content is usually associated with low porosity and permeability carbonate dissolution secondary porosity zone and primary calcite dissolution is believed to be related to thermal maturation of organic matter and clay mineral reactions in the surrounding shales and mudstone two stages of carbonate cementation were identified the precipitation from pore water during sedimentation and secondary precipitation in sandstones from the organic acid dissolved carbonate minerals from source rocks petrophysical properties have controlled hydrocarbon accumulation in turbidite sandstones high porosity and permeability sandstones have high oil saturation and are excellent producing reservoirs it is also noticed that interstitial matter content affects the oil bearing property to some degree there are three essential elements for high oil bearing turbidite reservoirs excellent pore types low carbonate cement 15 and average permeability 10 md

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

massive recrystallization of low mg calcite at high temperatures in hydrocarbon source rocks implications for organic acids as factors in diagenesis we document massive recrystallization of low mg calcite lime mudstone source rocks at moderately high temperatures in the smackover formation of mississippi this process was driven by organic acids produced during kerogen maturation and holds the implications for studies of primary migration of hydrocarbons secondary porosity generation in reservoirs and global environmental change this interpretation is based on contrasting the following petrographic and bulk rock geochemical characteristics of organic poor vs organic rich lime mudstones

late archean sm nd isochron age for mafic ultramafic supracrustal amphibolites from the northeastern sino korean craton china isotopic data published by previous workers document the presence of early archean 3 5 ga metabasaltic enclaves in grey gneisses west of bohai bay eastern hebei province p r c metabasaltic and metakomatiitic amphibolites crop out as lower supracrustal members of the archean section northeast of bohai bay in southern jilin eastern liaoning provinces six analyzed specimens of the latter unit contain more magnesium on analysis averaging approximately 13 vs 7 wt percent mgo but although they exhibit a wider range of ree concentrations they average a similar range of ree abundance patterns la lu 24 7x chondrite as the former 24 9 chondrite new sm nd isotopic and ree analyses combined with available petrologic geologic information indicate that metakomatiitic amphibolites from the sino korean craton northeast of bohai bay are late archean in emplacement age a wholerock sm nd isochron age of 2660 75 2 ma was obtained with nd t 4 4 0 5 as in many parts of the sino korean craton the highly positive nd t value suggests a prior extended history 1 0 ga of depletion of subcontinental mantle sources in some incompatible elements and in lree relative to hree a seventh analyzed sample a mafic inclusion in tonalitic gneiss of unknown age has an unusual chemical composition with strong enrichment in lree sr and zr and severe depletion in ni and cr judging from isotopic data which yield a much younger depleted mantle model age of 1 67 ga it could represent a cognate xenolith in a mid proterozoic tonalitic intrusion shown on present maps as archean in age the protolith might have been an evolved alkali basalt

distribution chemistry isotopic composition and origin of diagenetic carbonates magnus sandstone north sea abstract diagenetic ferroan carbonates grew in the upper jurassic reservoir sandstones of the magnus oilfield in porewaters which differed in composition across the field these porewaters remained compositionally different and stratified for at least 35 m y variations in carbonate chemistry across the field are attributable to these porewater variations which resulted from displacement of marine depositional water from the crest of the field by meteoric water during late cimmerian subaerial exposure original depositional facies and detrital mineralogy strongly influenced diagenetic carbonate distribution rare diagenetic calcite occurs as discrete rhombic crystals diagenetically late magnesian siderites have developed throughout the reservoir sandstone and are commonly intimately associated with altered detrital biotite grains poikilotopic ankerite cement postdates calcite and siderite and occurs only adjacent to mudstones and in thin sandstones within mudstones three compositional growth zones in siderite crystals are observed across the field from crest to flank in all three wells studied a similar trend of compositional evolution through time is observed in both two and three zoned rhombs first formed siderite is relatively magnesian intermediate zones are more ferroan and outer zones are at least as magnesian as the first stage these individual grain variations overprint a fieldwide variation where siderite is more ferroan in the crestal samples up to 87 mol fe mn and more magnesian downdip up to 58 mol mg this reflects the greater influence of relatively fe rich meteoric derived water in the crest and the greater influence of marine derived mg rich porewater downdip ankerite shows a similar variation in fe and mg abunda ces across the field crest and flank maximum 25 and 17 mol fe mn respectively 27 and 43 tool mg and developed due to release of mg fe ca and hco3 ions from mudstones into adjacent sandstones following dissolution of detrital minerals and organic decarboxylation reactions both siderite and ankerite have lower 18o at the crest of the oilfield than downdip respectively siderite 16 0 and 17 6 smow ankerite 17 7 and 21 4 these differences in 18o reflect the retention during burial diagenesis of a larger component of meteoric water in the crest of the field below the unconformity whereas downdip porefluid contained a larger marine derived component strong organic influence on 13c 8 0 to 14 6 pdb for magnesian siderite 7 7 to 13 6 for ankerite closed system 34s values up to 15 7 for late cubic pyrite and stratified 18o from crest to flank of the field argue against large scale porewater movements diagenetic porewater stratification is strongly supported by the parallel but distinct geochemical fingerprints of siderite and ankerite cements from the crest to the flank of the field

diagenetic effects on porosity permeability relationships in red beds of the lower triassic bunter sandstone formation in the north german basin abstract carbonate and anhydrite cement clay clasts and inter granular clay are the main components that reduce reservoir quality in the studied bunter sandstone formation the impacts of these parameters on porosity and permeability are determined by combining petrographic mineral quantification with conventional core analysis of samples from the danish part of the north german basin the depositional environments are considered because they largely control the distribution of cements clays and grain sizes the lateral variability of depositional environments is defined by the position in the basin and the proximity to the source areas the stratigraphic distribution of depositional environments is related both to local topography and to climate because high aridity promoted aeolian deposition the bunter sandstone formation has high porosity and permeability in most of the sandstone intervals in the northern north german basin the reservoir quality is good as long as the cements and clays are present as confined bodies that leave the remaining pore spaces available for flow in contrast inter granular clay and pervasive cementation hinder virtually all flow through the sandstone the ephemeral fluvial deposits have an average porosity and permeability of 20 3 and 810 md respectively and the values are 24 6 and 807 md for the aeolian sandstones excluding the unconsolidated aeolian sands which presumably have higher porosity and permeability the aeolian sandstones of the volpriehausen member have very good reservoir quality since they have a thickness of about 25 m are laterally continuous are largely clay free and the cement occurs in small amounts the sandstones of the solling member consist mainly of ephemeral fluvial deposits which generally have good reservoir quality however some intervals have high contents of inter granular clays or pervasive carbonate anhydrite or halite cement and these components reduce the permeability significantly the lateral distribution of the ephemeral fluvial sandstones is variable and therefore difficult to predict when planning a geothermal exploration well thus the volpriehausen member is the preferred target

diagenetic controls on reservoir quality in middle to upper jurassic sandstones in the south viking graben north sea the deeply buried synrift play of the south viking graben is characterized by highly variable reservoir quality an integrated approach incorporating petrophysics petrography and one dimensional basin modeling methods was applied to investigate these variations analysis shows that average porosities below 4000 m 13 123 ft vertical depth below sea floor range from approximately 5 to as much as about 25 in comparable quartz arenitic sandstones from porosity depth trends three porosity categories can be recognized normal low and high porosity sandstones normal porosity sandstones fall along the regional average porosity depth trend low porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity depth trend high porosity sandstones plot above the regional porosity depth trend here quartz cementation has been inhibited by grain coating microquartz and thus porosity has been preserved hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area but this study concludes that the reservoirs are mainly water wet allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids predicting the distribution of microquartz coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play this study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area

carbonate cementation patterns and diagenetic reservoir facies in the campos basin cretaceous turbidites offshore eastern brazil massive turbidite arkoses contain more than 80 of the oil reserves of the campos basin the main petroleum province of brazil the porosity and permeability distribution in the namorado albian cenomanian and carapebus turonian santonian sandstones is

storm frequency and magnitude in response to holocene climate variability lake tutira north eastern new zealand abstract a mid to late holocene record of storm events is preserved in the sediments of lake tutira located on the eastern north island of new zealand previous studies of a 6 m sediment core established a storm chronology for the last 2250 years eden d n page m j 1998 palaeoclimatic implications of a storm erosion record from late holocene lake sediments north island new zealand palaeogeogr palaeoclimatol palaeoecol 139 37 58 here we extend the record using a 27 m sediment core to lake formation 7200 cal yr ago outside the last 500 yr of human influence the core records landscape response to natural climate variability and to tectonism volcanism associated with the active collisional plate boundary setting based on sedimentological and biogeochemical analyses 1400 layers of dominantly terrigenous storm derived sediment have been identified a storm event chronology supported by twelve tephra and three 14c ages indicates that storm magnitude and frequency has varied over the last 7200 yr twenty five periods with an increased frequency of large storms occur typically of decadal to centennial duration periods of major storm activity occur at about 500 700 1100 1250 1850 2100 2850 3200 3600 4000 4300 4500 4700 4900 5700 5900 and 6850 6900 cal yr bp several other local and regional climate proxies record conditions that are consistent with the timing and periodicity of major storms in the tutira record a period centred on 2000 yr ago has the highest incidence of storms with a recurrence of 1 storm 2 9 yr this is identified as a time of major environmental change by several other records by comparison average storm frequency throughout the record is 1 storm 5 yr and while this is consistent with modern el nino southern oscillation enso periodicity the record shows no increase in frequency around 4000 years at the time of enso intensification this likely reflects new zealand s location between tropical and polar latitudes where climate shifts are driven by the interplay between enso interdecadal pacific oscillation ipo and the southern annular mode sam the lake tutira record provides support for the growing evidence that climate variability during the holocene has involved periods of rapid change correlation of this detailed storm event and sediment flux record with other geomorphic and sedimentary records in the nearby waipaoa source to sink sedimentary system improves understanding of the timing and rates of climatically induced landscape change and its downstream effects

oil generation as the dominant overpressure mechanism in the cenozoic dongying depression bohai bay basin china the dongying depression in the bohai bay basin is a young prolific oil producing basin in china the gray to black mudstones calcareous mudstones and oil shales of the eocene shahejie formation es3 and es4 are the major source rocks that are primarily dominated by type i kerogens with a high total organic carbon of up to 18 6 the es3 interval is characterized by a sedimentation rate of up to 500 m m y widespread overpressures are present in the eocene es3 and es4 intervals in the depression with pressure coefficients up to 1 99 from drillstem tests among the sonic resistivity and density logs only the sonic log displays an obvious response to the overpressure from which the top of the overpressure can be clearly identified acoustic traveltime versus effective vertical stress analysis of more than 300 wells in the dongying depression suggests that the acoustic traveltime of the normally pressured and overpressured mudstones is reduced with increasing vertical effective stress pore pressures are accurately predicted in the dongying depression using an eaton 1972 exponent of 2 0 by comparing the predicted pressure coefficients with measured pressure coefficients disequilibrium compaction has been previously proposed as the sole cause of the high magnitude overpressures in the eocene strata of the dongying depression because of rapid deposition of fine grained sediments however our data indicate that the overpressures are caused by oil generation from the source rocks within the es3 and es4 intervals the overpressured sediments display a normal compaction as evidenced from the overpressured mudstones exhibiting no anomalously low density the apparent lack of correlation between mudstone densities and effective vertical stress and the overpressured reservoir sandstones showing no anomalous high matrix porosities or anomalous geothermal gradient the depths to the top of the overpressure intervals range from 2000 to 3000 m 6562 9843 ft following closely source rock depths all the overpressured reservoirs and source rocks have a minimum temperature of approximately 87c and overpressured source rocks generally have vitrinite reflectance ro values of 0 6 or higher overpressures are not found in the strata within which the ro values are less than 0 5 the overpressured reservoirs in the es3 and es4 intervals are predominantly oil saturated or oil bearing organic rich source rocks with overpressures are capable of generating hydrocarbons and thus can maintain an abnormally high pressure the calcite precipitation in the calcareous mudstones may have caused a significant reduction in porosity and permeability to form an effective pressure seal the presence of widespread microfractures in the source rocks may relate to episodic expulsion of hydrocarbons or overpressure dissipation overpressures in the reservoir rocks are generated by the fluid transmission from the overpressured source rocks through active faulting and fracturing

constraints on the timing of uplift of the yanshan fold and thrust belt north china this paper reports in situ u pb ages and hf isotopic data on detrital zircons from paleozoic to late mesozoic strata in the xishan area near beijing that provide important constraints on the evolution and paleogeography of the northern part of the north china craton ncc zircons from carboniferous to late mesozoic sandstones are characterized by three major groups of u pb ages 2 5 2 3 ga 1 9 1 7 ga and late phanerozoic ages the neoarchean and paleoproterozoic zircons and some phanerozoic zircons have negative ehf t values similar to igneous zircons from intrusive rocks known from the ncc indicating that the source of the sandstones was locally derived however late triassic to late jurassic 205 158 ma sandstones contain a group of phanerozoic zircons with positive ehf t values distinct from known igneous zircons in the ncc suggesting an exotic source the nearest potential source is the xing meng orogenic belt xmob to the north of the ncc where considerable amounts of juvenile crustal materials exist it is concluded that there are two sudden shifts in source provenances of the xishan sediments from the ncc before the late triassic to a mixed source from the ncc and xmob during the late triassic to late jurassic and then from the ncc again during the late jurassic 158 ma the first shift in source resulted from the exhumation and denudation of rocks due to uplift of the xmob which coincided with post collisional extension in the late triassic and intraplate extension of the ncc in the middle to late jurassic the second source shift resulted from the uplift of the yanshan fold and thrust belt thus constraining the timing of this to the late jurassic

prediction of permeability in quartz rich sandstones examples from the norwegian continental shelf and the fontainebleau sandstone abstract correctly predicting sandstone permeability from variables such as porosity composition and texture is one of the major problems of petroleum geology and hydrology using data sets comprising composition texture porosity and permeability for sandstone samples from the norwegian continental shelf and the fontainebleau sandstone we have tested several previously suggested methods for permeability prediction and also suggest a new method for sandstones containing significant volumes of clay our results indicate that permeability is successfully predicted from porosity and mean grain size by the kozeny equation for most sandstones with clay contents less than 3 when porosities exceed 6 14 with the lowest threshold values of porosity tending to occur in the sandstones containing least clay the kozeny equation also seems to enable calculation of the probable maximum possible permeabilities for sandstones with given porosities and grain sizes for clay contents above 3 the kozeny equation typically overpredicts permeability including a percolation threshold in the kozeny equation using estimated macroporosity rather than total porosity or applying a form of the equation where internal surface area is used as input instead of grain size does not provide generally acceptable permeability estimates however a modified kozeny equation which includes a parameter reflecting the type of pore system often provides useful permeability estimates even for clay rich samples this parameter can often be regarded as a constant for a specific formation or sandstone unit when porosity variations are not excessive mercury injection data suggest that its value is controlled by the width and connectivity of the dominant flow paths through the system which in turn are probably determined by factors such as volume and distribution of clay

diagenetic processes influencing porosity in sandstones from the triassic buntsandstein of the iberian range spain abstract sandstones of the triassic buntsandstein of spain were studied in an attempt to quantify effects of diagenetic processes such as compaction cementation and leaching on reservoir properties samples were taken from shell well siguenza 44 3 some 110 km northeast of madrid drilled on the siguenza high in the iberian range the sandstones are arkoses to sublitharenites of a typical triassic redbed facies characteristic of the whole region the present day depth of the base buntsandstein is 538 m whilst a maximum burial depth of ca 2600 m was estimated from vitrinite reflectance 0 94 to 1 14 kaolinite temperature 76 c and illite crystallinity kuebler index 4 9 from an estimated average initial porosity of 38 porosity was reduced to the present value of 13 the strongest process to reduce porosity was found to be mechanical compaction causing a loss of 15 followed by chemical compaction pressure solution with a loss of about 6 the cement 11 4 r v apparently reduced the initial porosity by about only 5 early carbonate and sulphate cements are interpreted to have stabili sed the rock framework working against mechanical compaction and pressure solution this is supported by an increase in sutured grain contacts where early cements are absent or rare the later quartz cement interpreted to be largely the product from pressure solution has reduced porosity in a ratio of 0 5 1 this suggests that compaction continued after the formation of the late quartz cement although leaching was observed especially in feldspar grains clay and k feldspar neomorphosis is interpreted to have counteracted the formation of substantial secondary porosity permeability in the buntsandstein is generally low average 17 4 md and has been strongly affected by the alteration of feldspars to clay causing a wide range of permeability from 0 11 to 73 0 md

hummocky cross stratification like structures and combined flow ripples in the punta negra formation lower middle devonian argentine precordillera a turbiditic deep water or storm dominated prodelta inner shelf system abstract turbidity current and storm induced deposits may exhibit similarities in particularly when the latter is laid down by a combination of oscillatory and unidirectional flows recent progress in facies analysis helps to discriminate the sedimentary effects of oscillatory from unidirectional components of the flow on the basis of detailed analysis of sedimentary facies strata geometry and palaeocurrent data the present study reinterprets the punta negra formation pnf lower middle devonian argentine precordillera previously considered as a depositional system of deep water as a storm dominated prodeltaic shelf depositional system in the sandstone beds of the pnf planar low angle and undulating laminations with weakly asymmetric hummocky and swaley bedforms combined flow ripples accretionary hummocky cross stratification like hcs like and anisotropic hcs like suggest the action of oscillatory currents combined with unidirectional currents in forming the deposits different hypotheses on the origin of the oscillatory currents have been examined the most convincing interpretation is that the oscillatory component of the velocity is attributed to storm induced waves the palaeocurrent data indicate offshore current directions suggesting that the unidirectional flow was a gravity induced bottom current inverse grading at the base and overlying normally graded divisions of the sandstone beds testify to waxing waning behaviour of the depositional flows interbedding of sedimentary structures undulating laminations low angle and parallel laminations and combined flow ripples in the lower and intermediate divisions of the beds indicate fluctuations of flow velocity this organisation of the sedimentary structures permits association of the unidirectional component with hyperpycnal bottom currents the terrestrial origin of the hyperpycnal flows is suggested by the abundance of terrestrial plant remains the mineralogical and textural immaturity of the sandstone composition and the relative scarcity of bioturbation which was likely controlled by fresh water input and a high rate of sedimentation storm influenced hyperpycnal flows generated subaqueous channelised forms at the mouth of the river deltas which later filled with sand at the distal end of the channelised forms lobe shaped sandstone beds were deposited evolving distally into thin sandstone beds alternating with sandy mudstone

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

the influence of calcite dissolution on reservoir porosity in miocene sandstones picaroon field offshore texas gulf coast abstract middle miocene sandstones occur in picaroon field corsair trend offshore texas gulf coast at depths of approximately 4 9 to 5 2 km 16 17 000 ft these deltaic sandstones contain evidence of the following sequence of diagenetic events 1 formation of chlorite coatings on detrital grains 2 partial dissolution of detrital feldspar 3 quartz cementation 4 calcite cementation 5 dissolution of calcite cement and grains 6 ankerite cementation the reservoir quality of the sandstones is largely a function of porosity enhancement by calcite dissolution calcite cement was emplaced at depths of approximately 1 8 to 2 6 kin the calcite has 87 86sr values of 0 7083 0 7086 eliminating coeval 15 myr seawater and marine carbonate 0 70873 0 70885 as the primary source of strontium the 87 86sr composition of calcite cement implies that mass transfer of calcite from older marine sources to younger sediments has occurred fluid inclusion measurements indicate that ankerite cement formed at minimum temperatures of 120 188 c 3 0 5 1 km oxygen isotope modeling predicts that at these depths shales would expel waters with 18osmow of 5 to 9 during smectite illite conversion ankerite 18opdb 7 8 would be in isotopic equilibrium with these waters at temperatures similar to those derived from fluid inclusions ankerite cements have relatively radiogenic 87 86sr ratios 0 7097 which are consistent with their formation from shale derived fluids calcite dissolution occurs between the precipitation of calcite and ankerite it is therefore concluded that calcite cement dissolution occurred at burial depths of 2 6 to 3 0 km 107 120 c

the effect of ductile lithic sand grains and quartz cement on porosity and permeability in oligocene and lower miocene clastics south china sea prediction of reservoir quality ductile clay rich sand grains control porosity and permeability in the fluvio deltaic oligocene and lower miocene sandstones of the south china sea ductile grains account for between approximately 5 and 50 of the original sand grain population there is a pronounced loss of porosity with increasing burial depth in the basin at depths of less than 3000 m this is due solely to ductile grain compaction where the rate of porosity loss with depth increases with increasing ductile grain content at depths greater than 3000 m the steep porosity loss with depth is due to combined ductile grain compaction and quartz cementation the amount of quartz cement increases with increasing burial depth however cleaner sandstones tend to have greater amounts of quartz cement at any given depth below 3000 m this leads to convergence of porosity evolution for the clean and ductile rich sandstones below 3000 m there is a rapid loss of permeability with decreasing porosity because compaction of ductile grains smears them between rigid quartzose grains leading to blocked pore throats a consequence of this process is that the lowest permeabilities are found in sandstones with the highest ductile grain contents quartz cement does not have a clear and discernible control on permeability the pronounced loss of porosity with increasing depth and permeability with decreasing porosity leads to low permeability at relatively shallow burial depths reservoir quality is thus controlled by the nature of the primary sand and depth of burial the sediment supply system led to systematic changes in ductile grain content across the basin with ductile content increasing into the more distal part of the sediment system the consequence is that depth of economic basement in terms of porosity or permeability can be predicted as a function of ductile grain content and burial depth for prospects across the basin

compaction of lithic sands experimental results and applications compaction experiments showed that the degree of physical compaction of a lithic sand is related to the amount and type of lithic material present these experiments produced a product that is texturally indistinguishable from natural lithic sandstones generally sedimentary lithic fragments are more ductile than metamorphic lithic fragments volcanic lithic fragments when altered to phyllosilicate minerals by weathering or diagenesis are extremely ductile mineralogy and intragranular microporosity influence ductility ratios based on weight percent of soft ductile minerals clay and micas to hard brittle minerals quartz and feldspar within the lithic fragments provides a good indication of ductility a ratio greater than 1 indicates the lithic material will be h ghly ductile microporous lithic fragments are more ductile than dense lithic fragments physical compaction models were developed based on experimental data from well sorted lithic sandstones for which the percentage of preserved porosities were functions of the effective stress the type of lithic material and the quantity of lithic material these models are derived from laboratory sandstones with at least 25 lithic grains in the following categories moderately ductile metamorphic lithic fragments highly ductile shale lithic fragments and extremely ductile altered volcanic lithic fragments application of the compaction models of the experimental compaction techniques to modern subsurface sand samples or outcrop equivalents can provide valuable estimates of the preserved porosities i e reservoir potential for lithic sandstones that may be exploratory objecti es in frontier basins examples using the models and experimental compaction techniques for exploration are given experimental data showed that precompaction cement and overpressure may be beneficial to porosity preservation an early formed partial cement can retard compaction and preserve porosity by stabilizing the sand pack a later cement does not have this beneficial effect overpressure when developed early retards compaction by reducing the effective stress of course a late developing overpressure is not effective at preserving porosity because compaction is an irreversible process

qualitative and quantitative prediction of diagenetic facies via well logs abstract drilled cores thin section petrography x ray diffraction xrd scanning electron microscopy sem routine core analysis rca and mercury injection capillary pressure micp tests were used to establish a predictable model for diagenetic facies and reservoir quality using well logs in sandstones of the paleogene dongying formation in nanpu sag bohai bay basin eastern china the results show that the sandstones are texturally and compositionally immature and have primary intergranular pores secondary intergranular and intragranular pores moldic pores and micro pores associated with diagenetic clay minerals the sandstones have undergone low to moderate mechanical compaction but a high degree of framework grain dissolution diagenetic minerals are dominantly of carbonates calcite dolomite and fe dolomite and clay minerals illite mixed layer illite smectite and kaolinite four types of diagentic facies are recognized tightly compacted carbonate cemented clay mineral filling and dissolution facies porosity permeability and oil saturation ranges among these diagenetic facies were determined from routine core analyses the diagenetic facies were then translated to conventional well logs by core log calibration and the well log expressions in terms of natural gamma ray neutron porosity bulk density sonic transit time and resistivity logs of various diagenetic facies were summarized the mineralogical composition obtained from the elemental capture spectroscopy ecs logs was then used for the diagenetic facies prediction the diagenetic facies predicted with the comprehensive conventional log analyses were combined with the ecs log profile and were calibrated with the xrd sem analysis and logging interpretation results for wells without ecs log the diagenetic facies can be quantitatively predicted by the mathematical calculations of reservoir quality index rqi via the gr ac and den log curves reservoir quality or oil bearing properties can be evaluated and predicted by translating the diagenetic facies to well logs

quantitative evaluation of the reservoir potential and controlling factors of semi deep lacustrine tempestites in the eocene lijin sag of the bohai bay basin east china abstract ancient lacustrine storm deposits that act as petroleum reservoirs are seldom reported the lijin sag which is located in the southeastern corner of the bohai bay basin in east china is a ne sw trending cenozoic half graben basin some of its eocene deposits bindong deposits were interpreted as lacustrine tempestites the bindong tempestites which developed in the lower part of the upper fourth member of the shahejie formation es41 constitute a new kind of petroleum reservoir and are novel petroleum exploration targets in the bindong area however the characteristics of the es41 bindong tempestite reservoirs and their controlling factors are poorly understood point count analyses of thin sections scanning electron microscope image analyses x ray diffractometry data and the petrophysical parameters of the bindong tempestite reservoirs were utilized to estimate the reservoir quality the reservoirs have undergone significant diagenetic alteration which can be divided into negative and positive aspects the negative alteration includes compaction authigenic minerals and cementation such as carbonates clay minerals and overgrowths of quartz and feldspar the uneven distribution of carbonate cement increased the reservoir s heterogeneity with carbonate cement commonly precipitating along the sandstone mudstone contacts the primary porosity was severely reduced because of compaction and extensive carbonate cementation positive alteration includes dissolution carbonate cementation undercompaction and fractures carbonate cementation exhibited both positive and negative effects on the reservoir quality overall the objective reservoir quality is quite poor a quantitative evaluation of the reservoirs potential was conducted the cutoff values of several of the reservoir s parameters were calculated the lower limits of the porosity and permeability are 8 35 5 85 and 1 2587 0 2753 10 3 m2 respectively depending on the depth the upper limits of the carbonate and mud content are approximately 18 5 and 9 10 respectively a fundamental understanding of these characteristics will provide necessary information for extracting hydrocarbons from analogous subsurface reservoirs

diagenesis and reservoir quality evolution of palaeocene deep water marine sandstones the shetland faroes basin british continental shelf abstract the palaeocene deep water marine sandstones recovered from six wells in the shetland faroes basin represent lowstand transgressive and highstand systems tract turbiditic sediments mineralogic petrographic and geochemical analyses of these siliciclastics are used to decipher and discuss the diagenetic alterations and subsequent reservoir quality evolution the middle upper palaeocene sandstones subarkoses to arkoses from the shetland faroes basin british continental shelf are submarine turbiditic deposits that are cemented predominantly by carbonates quartz and clay minerals carbonate cements intergranular and grain replacive calcite siderite ferroan dolomite and ankerite are of eogenetic and mesogenetic origins the eogenetic alterations have been mediated by marine meteoric and mixed marine meteoric porewaters and resulted mainly in the precipitation of calcite 18 o v pdb 10 9 and 3 8 trace amounts of non ferroan dolomite siderite 18 o v pdb 14 4 to 0 6 as well as smectite and kaolinite in the lowstand systems tract lst and highstand systems tract hst turbiditic sandstone below the sequence boundary minor eogenetic siderite has precipitated between expanded and kaolinitized micas primarily biotite the mesogenetic alterations are interpreted to have been mediated by evolved marine porewaters and resulted in the precipitation of calcite 18 o v pdb 12 9 to 7 8 and fe dolomite ankerite 18 o v pdb 12 1 to 6 3 at temperatures of 50 140 and 60 140 c respectively quartz overgrowths and outgrowth which post and pre date the mesogenetic carbonate cements is more common in the lst and tst of distal turbiditic sandstone discrete quartz cement which is closely associated with illite and chlorite is the final diagenetic phase the clay minerals include intergranular and grain replacive eogenetic kaolinite smectite and mesogenetic illite and chlorite kaolinite has been subjected to mesogenetic replacement by dickite the k feldspar and plagioclase grains have been albitized dissolution of calcite cement and of framework grain feldspar volcanic fragments and mud intraclasts has resulted in a considerable enhancement of reservoir quality

porosity permeability and textural heterogeneity of reservoir sandstones from the lower cretaceous putaohua member of yaojia formation weixing oilfield songliao basin northeast china abstract with the granulometric analyses mineral composition x ray diffraction xrd analyses porosity and permeability measurements scanning electron microscopy sem observations and mercury porosimetry measurements of the reservoir sandstones from four wells in the weixing oilfield songliao basin ne china the porosity permeability and pore structural heterogeneity features of the putaohua member sandstone of the lower cretaceous yaojia formation have been investigated in this paper the sandstones are mostly characterized by a similar deposition in continental detrital environments with quartz and feldspar taking the majority 97 and argillaceous minerals illite kaolinite and chlorite the minority 3 however their granule size sorting index porosity and permeability vary significantly in contrast to the generality that high porosity permeability often corresponds to large grain sizes and sorting indexes the porosity and permeability here show no good correlations to the grain size and sorting index suggesting the presence of a remarkable textural heterogeneity in terms of sequential change in permeability along the well depth cyclic sedimentation modes have been identified in some sub layers despite the porosity data showing little response to those scenarios this may be ascribed to the sandstone s peculiar intergranular pore structures in which the cementation occurred in pore lining and filling manners to different extents depending on their depositional environments the distributary channel facies tends to have lowest detrital clay content and rather open packed texture in pore lining cementation manner whereas sand sheet facies has pore filling manner the hg porosimetry analytical data are in fairly good agreement with the heterogeneous pore structures observed by sem large sized pores and channels connecting them though only accounting for a low percentage of the total porosity are believed to contribute mostly to the effective permeability in the sandstone reservoir such textural heterogeneities should be closely associated with the dynamic changing sedimentary facies of sandstones in the continental environment

factors controlling reservoir properties and hydrocarbon accumulation of lacustrine deep water turbidites in the huimin depression bohai bay basin east china abstract lacustrine deep water turbidite plays are a novel area for exploration in the huimin depression bohai bay basin turbidites in the shang 847 block a typical turbidite play in the huimin depression provide an opportunity to study the factors controlling the reservoir properties and hydrocarbon accumulation in lacustrine turbidite sandstones the reservoir quality of turbidite sandstones very fine grained moderately to well sorted mainly lithic arkose in this study area are mainly controlled by the distribution patterns of carbonate cements and pseudomatrix significant inverse relationships exist between the volume of carbonate cement and both porosity and permeability of the turbidite sandstones carbonate cement is located preferentially near the margins of the sandstone bodies sandstones with distance from the sandstone mudstone contact surface less than 0 7 m or with thickness less than 1 2 m are commonly tightly cemented carbonate cement 15 with low porosity and permeability lower than 10 and 0 1 md respectively the source of carbonate cement was most likely external probably derived from the surrounding mudstone most pore filling carbonate cements occurred during late diagenesis at burial depths greater than 2200 m the petrophysical properties of turbidites have a positive relationship with the content of kaolinite and chlorite but have a negative relationship with the content of illite 2 d and 3 d reconstructions of non oil bearing and oil bearing layers indicate that dissolution of carbonate cement feldspars and unstable rock fragments was more developed in oil bearing layers than in non oil bearing layers and hance oil bearing layers have higher porosity and larger pore sizes petrophysical property appears to have a significant effect on the hydrocarbon accumulation in the turbidite sandstones sandstones with porosities lower than 9 and or permeabilities lower than 0 78 md are not prone to contain oil

combined flow model for vertical stratification sequences in shallow marine storm deposited beds abstract we develop a model to help explain the wide variety of vertical stratification sequences in shallow marine sandstones this model embodied in a set of vertical sequence cartoons predicts or accounts for vertical stratification sequences in beds of free sandstone deposited from a full range of decelerating combined flows as well as from purely unidirectional and oscillatory flows according to the model vertical stratification sequences in storm deposited sandstone beds are the outcome of the time history of the bed configuration as deposition proceeds the idealized sequences are based on the initial velocities and subsequent deceleration histories of the unidirectional and oscillatory flow components twenty qualitatively different sequences starting with plane bed are generated and an additional 26 sequences result from omitting one or more of the lowermost intervals in succession the model thus accounts for 46 distinct kinds of sequences five major simplifying assumptions are built into the model bidirectional flows linear deceleration of both unidirectional and oscillatory flow components time independent deposition rate time independent sediment size and equilibrium bed configurations owing to these assumptions the model is likely to be applicable to only a small subset of vertical stratification sequences observed in shallow marine sandstones as more information on combined flow bed configurations becomes available these assumptions could be relaxed to provide a more refined and complete representation of vertical stratification sequences since the model generates vertical sequences formed from any kind of decelerating flow the bouma sequence for deposits from purely unidirectional turbidity currents and also sequences resulting from decelerating purely oscillatory flows similar in appearance to many proposed generalized tempestite sequences are end member cases in the model the model is also extended slightly to explain the geometry of the uppermost bedding surfaces of storm generated sandstones by consideration of disequilibrium effects

paleo pacific subduction accretion evidence from geochemical and u pb zircon dating of the nadanhada accretionary complex ne china the nadanhada terrane located along the eastern margin of eurasia contains a typical accretionary complex related to paleo pacific plate subduction accretion the yuejinshan complex is the first stage accretion complex that consists of meta clastic rocks and metamafic ultramafic rocks whereas the raohe complex forms the main parts of the terrane and consists of limestone bedded chert and mafic ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta sedimentary rocks geochemical data indicate that the yuejinshan metabasalts have normal mid ocean ridge basalt n morb affinity whereas the raohe basaltic pillow lavas have an affinity to ocean island basalts oib sensitive high resolution ion microprobe shrimp u pb zircon analyses of gabbro in the raohe complex yield a weighted mean 206pb 238u zircon age of 216 5 ma whereas two samples of granite intruded into the complex yield weighted mean 206pb 238u zircon ages of 128 2 and 129 2 ma laser ablation inductively coupled plasma mass spectrometry la icpms u pb zircon analyses of basaltic pillow lava in the raohe complex define a weighted mean age of 167 1 ma two sandstone samples in the raohe complex record younger concordant zircon weighted mean ages of 167 17 and 137 3 ma these new data support the view that accretion of the raohe complex was between 170 and 137 ma and that final emplacement of the raohe complex took place at 137 130 ma the accretion of the yuejinshan complex probably occurred between the 210 and 180 ma suggesting that paleo pacific plate subduction was initiated in the late triassic to early jurassic

estimation of single phase permeability from parameters of particle size distribution the single phase permeability of a permeable medium is determined by both the bulk physical properties of the interconnected pore system e g porosity and tortuosity and the statistics of its particle size distribution psd an expression of the relation between permeability and pore level properties is the carman kozeny ck equation this equation has historically been used to explain the fundamental causes of permeability because it provides a link between media attributes and flow resistance because the ck equation has not been applied to media consisting of mixed particle sizes we modify the ck equation to express the single phase permeability of unconsolidated media in terms of both the psd statistics and the bulk physical properties we find that we can relate permeability to the parameters of a psd for unconsolidated media this relationship matches published data it generally fails for permeabilities less than 1 m2 1 darcy probably because not all of the pore space is supporting flow in this region with the model partially verified by experimental data we use it to investigate the nature of permeability porosity relationships and of the origin of variability in permeability the latter capability will explain many of the permeability differences observed in outcrop studies

dolomite from clay in argillaceous or shale associated marine carbonates abstract postcompactional late stage ferroan dolomite commonly occurs as 1 finely disseminated rhombic microspar in argillaceous lime mudstone 2 sparry cement within voids in argillaceous carbonates and 3 sparry cement within carbonates adjacent to marine shales the distribution and abundance of these dolomites correspond with that of clay deposited in the marine environment and cannot easily be explained by alternative models of dolomite formation conversion of smectite to illite during burial is a common diagenetic process which is capable of releasing large amounts of fe mg ca na and si sandstone petrologists have emphasized the significance of this process in generating quartz cements in sandstone shale sequences however the potential of this conversion for producing postcompactional dolomite cement in limestone shale sequences or in argillaceous limestones has been neglected smectite illite conversion is here invoked as the source of ions that formed late stage postcompactional dolomites in examples of the pennsylvanian of oklahoma kansas missouri and iowa the devonian of iowa and the ordovician of oklahoma the smectite illite conversion model for generating dolomite may be widely applicable to many other ex mples of late stage dolomitization it should prove useful in predicting the distribution of late stage dolomite cement that reduces reservoir quality and may prove useful for estimating the timing of hydrocarbon migration relative to carbonate cementation

intergranular pressure solution in four quartzose sandstones abstract cathodoluminescence petrography and scanning electron microscopy reveal that several geologic variables significantly influenced intergranular pressure solution and quartz cementation in the bromide ordovician oklahoma upper minnelusa permian wyoming nugget jurassic utah and tuscarora silurian pennsylvania sandstones a linear relationship exists between mean grain size and intergranular pressure solution with finer grained samples having experienced more intergranular pressure solution than coarser grained ones samples that display pervasive illite grain coatings have also experienced more intergranular pressure solution than those with no or poorly developed grain coatings sorting and limited framework grain compositional variability have exerted little influence o intergranular pressure solution silica budgets indicate that in the bromide upper minnelusa and nugget sandstones more quartz has been dissolved by intergranular pressure solution than is present as cement these sandstones have exported silica during burial diagenesis in the tuscarora the amount of quartz dissolved by intergranular pressure solution is about equal to the amount of quartz cement present implying approximate mass balance however petrographic evidence suggests that most of the quartz cement in at least three of these sandstones predates most intergranular pressure solution implying that they acted as silica importers during shallow diagenesis and silica exporters during deeper diagenesis intergranular pressure solution has played varying roles in determining the ultimate porosity of these four sandstones in the nugget intergranular pressure solution was the single most important diagenetic process that influenced porosity evolution in the bromide intergranular pressure solution was slightly more important than mechanical compaction and considerably more important than cementation in the tuscarora intergranular pressure solution was slightly more important than mechanical compaction and of about equal importance to quartz cementation in the upper minnelusa intergranular pressure solution was less important than both mechanical compaction and gypsum anhydrite cementation

the pre devonian tectonic framework of xing an mongolia orogenic belt xmob in north china abstract a new tectonic division of the xing an mongolia orogenic belt xmob in north china has been presented according to our research and a lot of new data of tectonics geochronology and geochemistry four blocks and four sutures have been recognized in the xmob including the erguna eb xing an airgin sum xab songliao hunshandake shb and jiamusi jb and xinlin xiguitu xxs xilinhot heihe xhs mudanjiang ms and ondor sum yongji sutures oys the framework of the xmob is characterized by a tectonic collage of the blocks and orogenic belts between them different precambrian basements have been found in the blocks including the neoproterozoic metamorphic rocks and plutons in the eb the neoproterozoic metamorphic rocks in western and eastern of segments of the xab mesoproterozoic and neoproterozoic metamorphic rocks in middle segments of the xab respectively the neoproterozoic metamorphic rocks and mesoproterozoic volcanic rocks and plutons in the shb and neoproterozoic metamorphic rocks in the jb the xxs resulted from a northwestward subduction of the xab beneath the eb during the cambrian which was followed by the forming of the xhs and oys in the northwest and south margins of the shb in the silurian respectively the ms was caused by a westward subduction of the jb beneath the east margin of the shb during the middle devonian the three cambrian silurian and middle devonian events indicate that the xmob belongs to a pre middle devonian multiple orogenic belt in the central asian orogenic belt caob forming of the xmob suggests that the southeast part of the paleo asian ocean closed before the middle devonian

sediment accumulation in a modern epicontinental shelf setting the yellow sea sediment accumulation in the yellow sea epicontinental shelf environment is investigated on 100 yr and 1000 yr time scales using 210pb and 14c geochronologies the distribution of modern 210pb accumulation rates in the yellow sea reveals that the loci of modern huanghe sediment accumulation are the topset 1 2 mm yr foreset 4 9 mm yr and proximal bottomset deposits 2 4 mm yr of the shandong subaqueous delta which extends south from the shandong peninsula 210pb rates in the distal bottomset deposits of the subaqueous delta in the central and southern yellow sea are generally low 0 3 0 9 mm yr a sediment budget demonstrates that between 9 15 of the annual huanghe discharge is accumulating in the yellow sea about two thirds of this sediment is accumulating in the topset foreset and proximal bottomset deposits of the shandong subaqueous delta with the remaining third accumulating as widespread distal bottomset deposits 14c age dates of subaqueous delta sediments indicate that the thick 40m clinoform structure formed predominantly between 6200 and 4060 yrs b p at rates approaching 20 mm yr observations in the yellow sea as well as on the amazon and ganges brahmaputra shelves demonstrate that subaqueous deltaic stratigraphy is the general rule where major rivers enter energetic shelves whether epicontinental or pericontinental the development of extensive bottomset deposits may be restricted to epicontinental shelf environments and may be diagnostic of sedimentation in this type of setting

process controls on the development of stratigraphic trap potential on the margins of confined turbidite systems and aids to reservoir evaluation stratigraphic trapping at pinch out margins is a key feature of many turbidite hosted hydrocarbon reservoirs in systems confined by lateral or oblique frontal slopes outcrop studies show that there is a continuum between two geometries of pinch out configuration in type a turbidites thin onto the confining surface although the final sandstone pinch out is commonly abrupt and individual beds tend not to erode into earlier deposits in type b turbidite sandstones commonly thicken toward the confining slope and beds may incise into earlier deposits these two types may occur in combination to give a wide spectrum of pinch out characteristics our analysis suggests the principal control in determining pinch out character is flow magnitude with smaller flows producing type a and larger flows producing type b in areas of poor seismic control it can be difficult to assess either pinch out character or the proximity of wells to confining slopes because estimates of paleoflow magnitude can be made from core or high quality log image data however it is possible to make reasonable estimates of pinch out character even from wells such as exploration wells which may be placed conservatively away from the field margins furthermore systematic paleoflow variations and thickness trends are commonly seen in individual turbidite sandstones as they approach confining slopes for example dispersal directions indicate flow deflection parallel with the strike of confining topography beds thin toward type a onlaps and thicken toward type b onlaps these relationships can be exploited via analysis of vertical successions to constrain well position with respect to the slope similarly the presence location and frequency of locally derived debrites can provide information on the presence and proximity of confining slopes begin page 972

evolution of the solonker suture zone constraints from zircon u pb ages hf isotopic ratios and whole rock nd sr isotope compositions of subduction and collision related magmas and forearc sediments abstract the solonker zone in northern inner mongolia china is considered as the suture between the north china craton and the south mongolian microcontinent two magmatic belts are recognized along the suture zone a subduction related magmatic belt represented by the baolidao arc rocks and a younger collision related granite belt represented by the halatu granites we use zircon u pb ages zircon in situ hf isotopic analyses and whole rock nd sr isotopic data of the two magmatic belts and related forearc sediments the xilinhot metamorphic complex to constrain timing of the suturing and to discuss the petrogenesis of the magmatic rocks a gabbroic diorite bld 1 of the baolidao arc was dated at 310 5 ma by shrimp this sample shows an e nd t value of 2 5 and i sr of 0 7052 hf isotopic analyses on 25 zircons from the same sample show e hf t 5 4 to 11 5 another diorite sample xh 2 of the same arc from south of xilinhot displays even more depleted isotopic compositions with e nd t 5 6 and i sr 0 7037 the main population of zircons from this sample have highly variable and depleted hf isotopic compositions e hf t 0 18 3 the large variation in hf isotopic composition of zircons with largely the same crystallization age from a single pluton is explained by a mixing process between depleted mantle derived magma and continental crust in an active continental arc setting the halatu granite hlt 2 was dated at 234 7 ma by shrimp zircons from the granite also show a large variation of e hf t values 9 1 to 26 despite most samples having whole rock e hf t 2 the large variation in e hf t values suggests that the granite formed probably by partial melting of two source regions a dominant juvenile crust and a subordinate old continental crust most zircons from the xilinhot metamorphic complex show ages comparable with those of the baolidao arc rocks suggesting that the protolith of the metamorphic complex was probably deposited during or after arc magmatism some zircons however show precambrian ages that fall into two groups one with ages of 780 900 ma resembling those from the south mongolian microcontinent and the other with ages of 1524 2900 ma similar to those of the north china craton thus the protolith of the metamorphic complex probably formed in a forearc basin during convergence of the two continents and metamorphosed subsequently during collision in the late paleozoic our zircon age data thus constrain timing of collision between the south mongolian microcontinent and the north china craton to have been between 296 and 234 ma

horizontal and vertical variations in sedimentation and resuspension rates in a stratifying lake effects of internal seiches the temporal and spatial variations in the rate of sedimentation and sediment resuspension in lake rehtijarvi southern finland were studied by sediment traps deployed in the stratifying and non stratifying regions of the lake both the gross sedimentation rate and the resuspension rate recorded by near bottom traps were significantly higher in the stratifying region 20 m depth than in the shallow part of the lake 2 m depth these rate changes were caused by temporarily elevated hypolimnetic resuspension rates exceeding 40 g dry weight m 2 day 1 in july 2005 whereas in the shallow station no such peak was observed and the resuspension rate remained below 30 g dry weight m 2 day 1 throughout the study the elevated resuspension rate in the hypolimnion could not be attributed to surface wave action or sediment disturbance by fish but was due to the occurrence of internal seiches the seiche amplitude was on average the highest in july although the wind velocities were low this was because winds temporarily were blowing along the longitudinal axis of the lake due to the steep slopes the sediments of lake rehtijarvi are prone to redistribution and sediment focusing towards the accumulation areas also affected trap catches in the deep stations

calcite cement in permian deep water sandstones delaware basin west texas origin distribution and effect on reservoir properties calcite cement is the dominant control on reservoir quality in turbidite sandstones of the upper permian bell canyon formation delaware basin these well sorted very fine grained arkoses were deposited in a basin floor setting by channel levee systems terminating in broad lobes calcite cement distribution in the east ford and geraldine ford fields was mapped using core log and thin section data calcite is concentrated in tightly cemented zones that are mostly less than 1 ft 0 3 m thick areas that have high percentages of calcite cemented sandstone 20 occur along the margins of the sandstone bodies in overbank and lobe deposits where sandstone pinches out into siltstone areas that have the lowest percentage of calcite cemented sandstone 10 occur where the sandstone is thickest in the channel facies isotopic composition of the calcite 13c 1 8 to 3 0 relative to the peedee belemnite pdb 18o 4 6 to 6 3 pdb is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris because internal sources of calcite were apparently insufficient to account for the cement volume cement components are interpreted as having been transported into the sandstones from organic rich basinal siltstones and limestones feldspars buffered acidic formation waters near where they entered the sandstone resulting in calcite concentrated near the sandstone margins the calcite formed near maximum burial depths of 4800 ft 1 5 km and temperatures of 104f 40c from marine pore waters with 18o of approximately 0 relative to standard mean ocean water most of the calcite cemented zones are interpreted as being concretions that extend no more than a few meters laterally production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft 300 m and cause vertical reservoir compartmentalization laterally extensive calcite layers may be associated with the base of turbidite deposits

diagenesis and reservoir quality evolution of the eocene sandstones in the northern dongying sag bohai bay basin east china abstract the eocene sandstones in the northern dongying sag bohai bay basin china are reservoirs for large accumulations of hydrocarbons the sandstones are mainly lithic arkoses and feldspathic litharenites texturally and compositionally immature these sandstones have a wide range of porosity 0 4 37 and permeability 0 004 6969 md and show an overall decrease in reservoir quality from 1500 m to 5000 m below sea level the reduction in reservoir quality is a product of several digenetic processes these include compaction precipitation of dolomite and calcite in eodiagenetic stage compaction feldspar dissolution precipitation of quartz cements and clays kaolin and illite and precipitation of ferrocalcite and ankerite in mesodiagenetic stage mineral distribution pattern and isotopic composition suggest carbonate cements in sandstones originate from sources outside the sandstones carbonate cementation together with compaction reduced the sandstones porosity and permeability significantly in a sandstone bed marginal sandstones with distance to sandstone mudstone interface less than one meter always have lower porosity than central sandstones as burial depth exceeds 4000 m marginal sandstones have very low porosities 2 m can be potential effective reservoirs feldspar dissolution and precipitation of clays and quartz cements have little impact on absolute porosity mineral distribution pattern and quantitative data show that leached feldspars are the internal source of authigenic quartz and clays in sandstones and the volume difference between feldspar secondary porosity and related authigenic cements is generally less than 0 25 however although there is little or no net import of matter to the sandstones the pore architecture changes dramatically primary macropores are lost as clays and quartz precipitate while the proportion of microporosity increases occurring mainly between clay crystals the overall result is that permeability is significantly degraded

relationship of diagenetic chlorite rims to depositional facies in lower cretaceous reservoir sandstones of the scotian basin the relationship between diagenetic chlorite rims and depositional facies in deltaic strata of the lower cretaceous missisauga formation was investigated using a combination of electron microprobe bulk geochemistry and x ray diffraction data the succession studied comprises several stacked parasequences the delta progradational facies association includes i fluvial or distributary channel sandstones some with tidal influence ii thick bedded delta front graded beds of sandstone interpreted as resulting from fluvial hyperpycnal flow during floods and storms and iii more distal muddier delta front and prodeltaic facies the transgressive facies association includes lag conglomerate siderite cemented muddy sandstone and mudstone and bioclastic sandy limestone chlorite rims are absent in the fluvial facies and best developed in thick sandstones lacking mudstone baffles good quality chlorite rims are well correlated with ti in bulk geochemistry ti is a proxy for fe availability principally from the breakdown of abundant detrital ilmenite fetio3 under conditions of sea floor diagenesis the abrupt decrease in sedimentation rate at transgressive surfaces caused progressive shallowing of the sulphate depletion level and of the overlying eh controlled diagenetic zones resulting in conditions suitable for diagenetic formation of berthierine to migrate upwards through the packet of reservoir sandstones this early diagenetic berthierine suppressed silica cementation and later recrystallized to chlorite thick euhedral outer chlorite rims were precipitated from formation water in sandstone lacking muddy baffles on this chlorite substrate and inhibited late carbonate cementation this study thus shows that the preservation of porosity by chlorite rims is a two stage process rapidly deposited delta front turbidite facies create early diagenetic conditions that eventually lead to the formation of chlorite rims but the best quality chlorite rims are restricted to sandstones with high permeability during burial diagenesis

the impact of diagenesis on the heterogeneity of sandstone reservoirs a review of the role of depositional facies and sequence stratigraphy diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability linking the types and distribution of diagenetic processes to the depositional facies and sequence stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity the heterogeneity patterns of sandstone reservoirs which determine the volumes flow rates and recovery of hydrocarbons are controlled by geometry and internal structures of sand bodies grain size sorting degree of bioturbation provenance and by the types volumes and distribution of diagenetic alterations variations in the pathways of diagenetic evolution are linked to 1 depositional facies hence pore water chemistry depositional porosity and permeability types and amounts of intrabasinal grains and extent of bioturbation 2 detrital sand composition 3 rate of deposition controlling residence time of sediments at specific near surface geochemical conditions and 4 burial thermal history of the basin the amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and therefore can be predicted in the context of sequence stratigraphy particularly in paralic and shallow marine environments changes in the relative sea level exert significant control on the types and extent of near surface shallow burial diagenetic alterations which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs carbonate cementation is more extensive in transgressive systems tract tst sandstones particularly below parasequence boundaries transgressive surface and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter bioturbation and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates which also enhance the formation of glaucony eogenetic grain coating berthierine odinite and smectite formed mostly in tst and early highstand systems tract deltaic and estuarine sandstones are transformed into ferrous chlorite during mesodiagenesis helping preserve reservoir quality through the inhibition of quartz cementation the infiltration of grain coating smectitic clays is more extensive in braided than in meandering fluvial sandstones forming flow barriers in braided amalgamated reservoirs and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars micas and mud intraclasts under wet and warm climates whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls as well as along layers of concentration of carbonate bioclasts and intraclasts commonly hybrid carbonate turbidite arenites are pervasively cemented proximal massive turbidites normally show only scattered spherical or ovoid carbonate concretions improved geologic models based on the connections among diagenesis depositional facies and sequence stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies as well as for co2 geologic sequestration but also support more effective hydrocarbon exploration through reservoir quality prediction

a physical model of cementation and its effects on single phase permeability in this paper we present a mathematical model to quantify the effects of cements on the single phase permeability estimate of clastic rocks the model represents pore filling pore lining and pore bridging cements by treating the cement particles as rectangular crystals attached radially to detrital grains these clusters influence permeability through an increase in effective specific surface area and tortuosity because the model is based on physical principles it is largely devoid of empiricism and should be applicable to a wide range of rock types permeability estimates by our model match experimental data from two sets of sandstone samples of five hydrocarbon bearing formations in the north sea the model also confirms the well known result that the effect of pore bridging cement is a factor of ten greater in reducing the single phase permeability compared to pore lining and pore filling cements

the timing of final closure along the changchun yanji suture zone constraints from detrital zircon u pb dating of the triassic dajianggang formation ne china abstract the dajianggang formation is located in the changchun yanji suture zone of central jilin province and unconformably overlies the changchun yanji accretionary complex cyac which is a melange resulting from subduction of the jiamusi khanka block jkb beneath the north china craton ncc la mc icp ms u pb dating of detrital zircon from four samples of the formation yields ages of 2516 to 216 ma zircons with u pb ages at 2516 2501 ma and 1897 1832 ma indicate a provenance from precambrian basement rocks of the ncc the 525 482 ma ages indicate a provenance from metamorphic rocks of late pan african age in the jkb that have a tectonic affinity to the central asian orogenic belt caob zircon grains with ages of 383 314 ma and 275 250 ma were likely derived from the underlying cyac the youngest population has a peak age of ca 225 ma which together with late triassic fossils suggests that deposition of the dajianggang formation was late triassic or younger this result supports the view that the final collision of the jkb and ncc along the changchun yanji suture took place before the late triassic furthermore this closure time is at least 10 20 ma later than closure along the solonker xar moron changchun suture in the late permian we thus establish that the changchun yanji suture is not related to the collision between the siberia craton sc and the ncc but was instead related to the paleo pacific plate subduction consequently the changchun yanji suture is not the eastward extension of the solonker xar moron changchun suture as previously considered but the southern margin of the jilin heilongjiang high pressure metamorphic belt ji hei hp belt and resulted from westward subduction of the paleo pacific ocean thus the late triassic marked the switch in subduction from the paleo asian ocean to the paleo pacific ocean in ne china

bypass zone tempestite facies model and proximality trends for an ancient muddy shoreline and shelf member 2 of the late precambrian to lower cambrian chapel island formation of southeast newfoundland was primarily deposited in storm influenced nearshore and shelf environments along a fine grained coastline deposition took place within or adjacent to a deltaic system that supplied abundant silts and clays thin bedded graded sandstone beds hummocky cross stratified beds and a variety of very thin conglomerate units contain evidence for deposition by storm currents sedimentological and stratigraphic evidence is used to reconstruct the relative paleobathymetric position of the three major facies in member 2 the gutter cast facies is a shallow subtidal deposit characterized by very thin sandstone laminae and abundant pot and gutter casts the siltstone dominated facies contains more laterally extensive thin bedded sandstones and fewer erosional features and was deposited in the inner shelf the sandstone dominated facies consists of thin to medium graded and hummocky cross stratified sandstone beds deposited in a more distal shelf setting but above storm wave base the facies model and set of proximality trends developed for member 2 deposits may be applicable to other storm influenced fine grained shorelines in this model the shallow subtidal is a zone of throughput with high velocity sediment laden flows eroding deep narrow scours gutter casts and depositing very little sand outside of these scours as the storm generated flows move into deeper water they decelerate resulting in less erosion of the sea floor and depositing thicker and more continuous sand beds siltstone dominated facies further from shore bed thickness reaches a maximum sandstone dominated facies and hummocky crossstratification is abundant even more distally bed thickness decreases again represented by thinly laminated siltstones of the overlying member 3 analysis of sedimentary structures and paleocurrent data suggests deposition by storm currents that transported sediment nearly perpendicular to shore

shrimp u pb zircon dating of the wulian complex defining the boundary between the north and south china cratons in the sulu orogenic belt china abstract neoproterozoic metasedimentary and metavolcanic rocks of the wulian group are associated with gneissic granitoids at the northern margin of the sulu uhp belt adjacent to the wulian yantai fault and are collectively referred to as the wulian complex shrimp u pb dating of detrital zircon from the metasedimentary rocks of the wulian group indicates a range in age from 3478 to 625 ma based on zircons 3000 ma with the oldest age being 3339 ma 2700 2400 ma with a peak at 2436 ma 2200 1800 ma with a peak at 2156 ma and at 770 ma these results indicate that deposition of the wulian group was after 770 ma and that some detrital material most likely came from neoproterozoic sources in the south china craton scc shrimp u pb zircon dating of a metavolcanic rock defines a weighted mean 206 pb 238 u age of 761 8 ma which is consistent with other rocks of south china affinity and can be correlated with syn rift magmatism during the breakup of the supercontinent rodinia the wulian group together with neoproterozoic granites of the wulian complex constrains the location of the triassic suture zone between the north china and south china cratons based on our new data the suture must lie north of the wulian complex and is marked by the baichihe yantai fault

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral

depositional and diagenetic controls on the reservoir quality of lower cretaceous pendencia sandstones potiguar rift basin brazil the quality of the reservoirs of the lower cretaceous pendencia formation of the potiguar basin northeastern brazil is directly controlled by depositional facies related carbonate cementation and compaction the study of the interplay of these processes in the reservoirs offers an opportunity to unravel the diagenetic patterns of clastic sequences in interior rifts and in particular the role of carbonate cementation in poorly understood continental systems the pendencia formation is a thick sequence of fan deltaic fluvial deltaic turbiditic and lacustrine sandstones conglomerates and shales deposited during the rift stage of the basin the sandstones are predominantly arkoses average q49f40l11 with subordinate plutonic and volcaniclastic feldspathic litharenites compaction and cementation had similar importance in the destruction of porosity with a dominance of cementation in the turbidites and of compaction in the fluvial deposits carbonate cementation in pendencia reservoirs increases progressively with depth and is facies controlled eodiagenetic nonferroan calcite i d18opdb 10 7 to 4 0 d13cpdb 17 5 to 8 5 mesodiagenetic rhombohedral ferroan dolomite ankerite d18opdb 9 3 to 3 9 d13cpdb 1 7 to 1 1 and ferroan calcite ii d18opdb 17 2 to 6 8 d13cpdb 13 6 to 2 3 were precipitated at three distinct temperature intervals calculated from the d18o values 21 to 58 c 70 to 79 c and 85 to 150 c respectively according to the d13c values dissolved carbonate for calcite i was derived from oxidation and methanogenic fermentation of organic matter and from methane oxidation ferroan mesogenetic cements were derived from thermal decarboxylation of organic matter the shales were a major source of dissolved begin page 1720 carbonate as indicated by the isotopic similarity between their calcite d13cpdb 0 2 to 1 8 87sr 86sr 0 719 and most cements in the sandstones and by the peripheral cementation along the contacts of interbedded sandstones as a result of this cementation pattern thin turbiditic and deltaic sandstone beds are pervasively cemented the best reservoir quality potential is encountered in the partially cemented fluvial sandstones at moderate depths deltaic and turbiditic sandstones are more pervasively cemented by carbonate derived from the interbedded shales alluvial fan conglomerates and sandstones were flushed by telogenetic meteoric waters close to the borders of the basin and to the proximity to the postrift unconformity however porosity enhancement was very limited due to the precipitation of kaolinite and the intense compaction related to the compositionally immature detrital framework

porosity depth trends in reservoir sandstones theoretical models related to jurassic sandstones offshore norway abstract theoretical models of compaction and porosity modification in sandstones during burial are discussed theory predicts that mechanical compaction is the dominant porosity reducing mechanism during burial from 0 to 2 5 3 km porosity depth trends are at this stage mainly controlled by the stability of the grain framework and the effective stress time and temperature also influence the porosity depth gradients at shallow to moderate depths but are probably less important than effective stress and framework grain composition at greater burial depths below 2 5 3 km chemical compaction including pressure solution can explain most of the porosity reduction in well sorted clean arenites theoretical modelling of the intergranular pressure solution suggests that sandstones may lose porosity rapidly by this mechanism below 2 5 3 km and that factors capable of retarding pressure solution and quartz cementation are required to maintain prospective porosities below 4 km this theory and comparisons with empirical data from jurassic sandstones from offshore norway indicate that high porosity is favoured by 1 overpressures approaching the lithostatic pressure gradient 2 a short residence time below 3 km burial 3 coatings on grains of clay or microcrystalline quartz which prevent quartz dissolution and precipitation and 4 a high content of feldspar and early diagenetic secondary pores early hydrocarbon emplacement may retard quartz cementation and preserve porosity but chemical compaction probably continues after oil emplacement except at very high hydrocarbon saturations