the coupling mechanism of basin and orogen in the western ordos basin and adjacent regions of china abstract during the late triassic and jurassic three structural belts developed in the western ordos basin and adjacent regions of china the alxa compressive wedge the helanshan structural belt and the liupanshan fold and thrust belt three types of basin also developed a late triassic composite basin with a northwestern ordos rift sub basin and a southwestern ordos foreland sub basin an early middle jurassic intracratonic basin and a late jurassic foreland type molasse wedge it is suggested that these basins resulted from late triassic unconstrained lateral extrusion and jurassic constrained lateral extrusion the alxa region is interpreted as an extrusional wedge bounded by strike slip deformation which escaped laterally from the indentation of the liupanshan thrust belt from the south during the late triassic the sino korean block to the east of helanshan was controlled by a stress state with nearly e w trending extension at this stage extrusion was unconstrained the helanshan developed as a rift sub basin due to transtensional tectonics at the lateral margin of the wedge at this time a foreland sub basin was formed in front of the liupanshan thrust belt during the jurassic the sino korean block to the east of the helanshan was gradually compressed westwards and the extrusion structure became constrained the early middle jurassic shows a transitional stage from unconstrained to constrained extrusion and the ordos basin developed as an intracratonic basin from the latest middle jurassic to the late jurassic intense lateral escape and opposite compression led to the formation of the helanshan fold and thrust belt and its frontal foreland molasse wedge at the same time another foreland molasse wedge was developed in front of the liupanshan thrust belt the switch from rifting to thrusting in the helanshan is a result of the changing stress field at the circum pacific plate boundary the clockwise rotation of the sino korean block to the east of the helanshan and the anticlockwise rotation of the alxa block

spectrum of pore types and networks in mudrocks and a descriptive classification for matrix related mudrock pores matrix related pore networks in mudrocks are composed of nanometer to micrometer size pores in shale gas systems these pores along with natural fractures form the flow path permeability network that allows flow of gas from the mudrock to induced fractures during production a pore classification consisting of three major matrix related pore types is presented that can be used to quantify matrix related pores and relate them to pore networks two pore types are associated with the mineral matrix the third pore type is associated with organic matter om fracture pores are not controlled by individual matrix particles and are not part of this classification pores associated with mineral particles can be subdivided into interparticle interp pores that are found between particles and crystals and intraparticle intrap pores that are located within particles organic matter pores are intrap pores located within om interparticle mineral pores have a higher probability of being part of an effective pore network than intrap mineral pores because they are more likely to be interconnected although they are intrap om pores are also likely to be part of an interconnected network because of the interconnectivity of om particles in unlithifed near surface muds pores consist of interp and intrap pores and as the muds are buried they compact and lithify during the compaction process a large number of interp and intrap pores are destroyed especially in ductile grain rich muds compaction can decrease the pore volume up to 88 by several kilometers of burial at the onset of hydrocarbon thermal maturation om pores are created in kerogen at depth dissolution of chemically unstable particles can create additional moldic intrap pores

how porosity and permeability vary spatially with grain size sorting cement volume and mineral dissolution in fluvial triassic sandstones the value of geostatistics and local regression although it is well known that sandstone porosity and permeability are controlled by a range of parameters such as grain size and sorting amount type and location of diagenetic cements extent and type of compaction and the generation of intergranular and intragranular secondary porosity it is less constrained how these controlling parameters link up in rock volumes within and between beds and how they spatially interact to determine porosity and permeability to address these unknowns this study examined triassic fluvial sandstone outcrops from the uk using field logging probe permeametry of 200 points and sampling at 100 points on a gridded rock surface these field observations were supplemented by laser particle size analysis thin section point count analysis of primary and diagenetic mineralogy quantitative xrd mineral analysis and sem edax analysis of all 100 samples these data were analyzed using global regression variography kriging conditional simulation and geographically weighted regression to examine the spatial relationships between porosity and permeability and their potential controls the results of bivariate analysis global regression of the entire outcrop dataset indicate only a weak correlation between both permeability porosity and their diagenetic and depositional controls and provide very limited information on the role of primary textural structures such as grain size and sorting subdividing the dataset further by bedding unit revealed details of more local controls on porosity and permeability an alternative geostatistical approach combined with a local modeling technique geographically weighted regression gwr subsequently was used to examine the spatial variability of porosity and permeability and their controls the use of gwr does not require prior knowledge of divisions between bedding units but the results from gwr broadly concur with results of regression analysis by bedding unit and provide much greater clarity of how porosity and permeability and their controls vary laterally and vertically the close relationship between depositional lithofacies in each bed diagenesis permeability and porosity demonstrates that each influences the other and in turn how understanding of reservoir properties is enhanced by integration of paleoenvironmental reconstruction stratigraphy mineralogy and geostatistics

diagenetic history of tight sandstones and gas entrapment in the yulin gas field in the central area of the ordos basin china abstract the ordos basin is a large cratonic basin with an area of 250 000 km2 in central china upper paleozoic coals and shales serve as gas source rocks with peak generation and migration at the end of the early cretaceous recent exploration has verified the huge gas potential in the basin centered gas accumulation system bcgas however the mechanism for the gas accumulation is controversial with an integrated approach of thin section petrography ultra violet fluorescence microscopy fluid inclusion microthermometry raman microspectrometry scanning electron microscopy and x ray diffractometry we identified diagenetic trapping and evaluated the diagenetic history of sandstone reservoirs in the yulin gas field in the central area where structural stratigraphic and or sedimentary lithologic traps have not been found it was revealed that three phases of diagenesis and hydrocarbon charging occurred respectively in the late triassic late jurassic and at the end of the early cretaceous in the first two phases acidic water entered the reservoir and caused dissolution and cementation resulting in porosity increase however further subsidence and diagenesis including compaction and cementation markedly reduced the pore space at the end of the early cretaceous the bulk of the gas migrated into the tight reservoirs and the bcgas trap was formed in the updip portion of this system cementation continued to occur due to low gas saturation and has provided effective seals to retain gas for a longer period of time than water block in the bcgas the mechanism for the gas entrapment was changed from water block by capillary pressure in the bcgas to diagenetic sealing the diagenetic seals in the updip portion of the sand body were formed after gas charging which indicates that there is a large hydrocarbon exploration potential at the basin centered area

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

grain size trends associated with net sediment transport patterns an example from the belgian continental shelf grain size trends in relation to net sediment transport pathways are examined using some commonly used grain size parameters the results of the trend analysis for 15 types of the trends are compared with a known net sediment transport pathways established on the basis of the sandbank hydro and sediment dynamics i e general water and sediment movement patterns and bedform asymmetry it is shown that 1 the results for grain size trends associated with a worsening in sorting along the transport pathways have little similarity to transport pathways 2 the results for grain size trends associated with an improvement in sorting along the transport pathways have relatively high degree of similarity to the identified pathways and 3 the residual pattern derived on the basis of a combined grain size trend used elsewhere gao and collins 1992 appears to be most similar and suitable for defining transport pathways

sediment transport patterns in the san francisco bay coastal system from cross validation of bedform asymmetry and modeled residual flux abstract the morphology of 45 000 bedforms from 13 multibeam bathymetry surveys was used as a proxy for identifying net bedload sediment transport directions and pathways throughout the san francisco bay estuary and adjacent outer coast the spatially averaged shape asymmetry of the bedforms reveals distinct pathways of ebb and flood transport additionally the region wide ebb oriented asymmetry of 5 suggests net seaward directed transport within the estuarine coastal system with significant seaward asymmetry at the mouth of san francisco bay 11 through the northern reaches of the bay 7 8 and among the largest bedforms 21 for 50 m this general indication for the net transport of sand to the open coast strongly suggests that anthropogenic removal of sediment from the estuary particularly along clearly defined seaward transport pathways will limit the supply of sand to chronically eroding open coast beaches the bedform asymmetry measurements significantly agree up to 76 with modeled annual residual transport directions derived from a hydrodynamically calibrated numerical model and the orientation of adjacent flow sculpted seafloor features such as mega flute structures providing a comprehensive validation of the technique the methods described in this paper to determine well defined cross validated sediment transport pathways can be applied to estuarine coastal systems globally where bedforms are present the results can inform and improve regional sediment management practices to more efficiently utilize often limited sediment resources and mitigate current and future sediment supply related impacts

morphodynamic modelling for a tidal inlet in the wadden sea abstract a dynamic model for morphological development in a tidal inlet het friesche zeegat in the wadden sea along the northern coast of the netherlands is set up to obtain a better understanding of the behaviour of the morphological system around tidal inlets the model is based on delmor a program package for morphological changes developed at delft hydraulics computations of the flow the sediment transport the initial bed level change rate as well as long term morphological development have been carried out the model has proven to be a useful research tool even though it does not yet reproduce reality in detail

origin distribution and significance of carbonate cements in the albert formation reservoir sandstones new brunswick canada abstract carbonate cements in the fluvio lacustrine reservoir sandstones of the albert formation mainly occur in three phases pre compaction calcite post compaction calcite and ankerite the precompaction calcite cement 25 to 40 is characterised by bright luminescence high concentrations of fe mn and sr means of 1 56 wt 1 42 wt and 0 25 wt respectively slightly heavy 13c and 18o values mean 1 79 pdb mean 14 99 pdb respectively and its selective occurrence in lacustrine sandstones suggestive of cementation at shallow depths from lacustrine connate water dull luminescence variable concentrations of fe mn and sr variable but slightly heavy 13c and 18o values mean of 2 97 pdb and 11 05 pdb respectively of the post compaction calcite as well as its occurrence in alluvial fan sandstones exhibiting high pressure solution of detrital grains suggest cementation from pore waters modified by water rock interactions we suggest that the ankerite cement was formed from upwardly migrating formation water based on the following evidence i ankerite cement filling the secondary pores created in the deep subsurface ii an absence of unconformity during ankerite cementation precluding involvement of meteoric water iii ankerite cementation in brecciated veins synchronous with ankerite precipitation in host sandstones iv occurrences of significant volumes of ankerite 20 to 30 in the shales indicating that the fe and mg ions expelled during smectite to illite transformation reactions were consumed within the shales and v the heavy 13c and 18o isotope values of the ankerite cements means of 2 2090 pdb and 11 05 pdb respectively

sand waves and sediment transport around the end of a tidal sand bank two detailed surveys have been made of the north western end of the haisborough sand off the norfolk coast using echosounder 3 5 khz reflection profiler and side scan sonar asymmetrical sand waves indicate north westerly sand movement on the southern side and south easterly sand movement on the steeper northern side of the bank secondary superimposed megaripples which are probably better indicators of sediment movement give evidence of a cross bank component between the north westerly and south easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves these are usually taken to indicate zero net transport but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests this suggests the route by which sand travels around the end of the bank to form a roughly closed circulation sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing though that could allow more than 0 25 km shift to the north east in 100 years to pass undetected a box model is drawn up for the estimated sediment fluxes around the end of the bank and implications for residence times and circulation rates are drawn from it

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

variations in sediment wave dimensions across the tidally dominated irish sea nw europe abstract new high resolution multibeam echosounder data have been collected in the irish sea in the last decade sediment waves oriented transverse to tidal flow dominate many parts of the irish sea and exhibit an almost continuous spectrum of heights and spacings sediment wave growth remains poorly quantified and from the irish sea swath bathymetry data many bedform dimensions have been measured and examined statistically a strong positive power correlation between sediment wave height and spacing was observed s 0 0692 0 8020 n 935 r 0 853 while height and spacing only weakly vary with water depth to assess the main factors influencing sediment wave height and spacing deviations from the typical hydrodynamically analogue sediment wave geometry in equilibrium are qualitatively and semi quantitatively investigated in the irish sea in conjunction with seabed characteristics coarser sediment generally forms larger sediment waves while suppressed sediment growth is found likely due to the limitations in mobile sediment supply and inefficiency of very coarse bedload transport

origin and growth mechanism of authigenic chlorite in sandstones of the lower vicksburg formation south texas abstract authigenic chlorite occurring as grain coatings pore fillings and rosettes is common in deltaic sandstones of the lower vicksburg formation in south texas chlorite rosettes are isolated and occur late in the diagenetic history whereas grain coating and pore filling chlorite occur early predating the development of secondary porosity and the precipitation of quartz overgrowths and fe poor calcite grain coating and pore filling chlorite which are interstratified with approximately 15 percent 7 a layers are fe rich with a mean fe fe mg ratio of 0 68 stable isotope data 18o averages 13 5o oo smow indicate that the grain coating and pore filling chlorite began precipitating between 20 c to 40 c within the upper 1 8 km 6000 ft of burial sandstones of the lower vicksburg formation were deposited in a relatively shallow deltaic environment as part of a large fluvial system draining the volcanics of sierra madre occidental mexico and trans pecos texas weathering of these volcanics brought large amounts of particulate fe to the river mouth an increase in salinity in the distributary channel complex caused fe flocculation and deposition during periods of high river discharge or when coastal currents acted to concentrate particulate fe large amounts of particulate fe were deposited in the delta system this resulted in thick layers of fe rich clay odinite around grains during periods of low river discharge or when coastal currents caused wide distribution of particulate fe less particulate fe was deposited it is proposed that the grain coating and pore filling chlorite began forming through the transformation of an fe rich clay precursor in the formation water hydropressure zone following ostwald processes the presence of this synsedimentary fe rich clay and its recrystallization to fe chlorite strongly influenced fluid flow and played a large role in developing the diagenetic heterogeneity seen in lower vicksburg sandstones today

an investigation of seawater and sediment depth using a prototype airborne electromagnetic instrumentation system a case study in broken bay australia previous studies using commercial airborne electromagnetic equipment that is not optimized for marine surveying have demonstrated the use of airborne electromagnetic methods for measuring water depth and estimating sediment thickness a new prototype helicopter time domain airborne electromagnetic system seatem 0 is now under development for bathymetric surveying the first sea trial of the seatem 0 system took place over broken bay new south wales australia in shallow water up to 30 m in depth broken bay was chosen because the separate paleodrainage systems for the hawkesbury river brisbane water and pittwater which join in broken bay give rise to paleovalleys infilled with unconsolidated sediments ranging in thickness between 0 m bedrock outcrop and 200 m the survey area also included a tombolo with a beach either side which provided the opportunity to measure water depth through a surf zone sediment thickness and water depth is predicted from stitched layered earth inversion of data based on a simplified two layer model that represents seawater and sediment overlying a resistive half space basement bedrock the resulting bathymetric profiles show agreement typically to within 1 m and 0 5 m with known water depths in areas less than 20 and 6 m deep respectively the inverted depth profile of the second sediment layer is noisy however the profiles reveal coarse topographic features of paleovalleys to depth limits of 60 to 80 m below sea level in 20 to 30 m water depth as well as resolving bedrock ridges and exposed reefs in shallow waters

the morphodynamic modelling of tidal sand waves on the shoreface an artificial sand wave on the dutch shoreface of the north sea has been studied in conditions with relatively strong tidal currents in the range of 0 5 to 1 m s and sediments in the medium sand size range of 0 2 to 0 5 mm the sand wave is perpendicular to the tidal current and has a maximum height and length of the order of 5 m and 1 km respectively the sand wave is dynamically active and shows migration rates of the order of a few metres per year a numerical morphodynamic model delft3d model has been used to simulate the morphological behaviour of the sand wave in the north sea this model approach is based on the numerical solution of the three dimensional shallow water equations in combination with a surface wave propagation model wind waves and the advection diffusion equation for the sediment particles with online bed updating after each time step the model results show that the sand wave grows in the case of dominant bed load transport weak tidal currents relatively coarse sediment small roughness height low waves and that the sand wave decays in the case of dominant suspended transport strong currents relatively fine sediment large roughness height storm waves

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

morphology and dynamics of large subtidal dunes in bahia blanca estuary argentina abstract the purpose of this paper is to carry out a detailed analysis of subtidal dune morphology and temporal variability and to estimate dune migration rate to improve knowledge of this topic and so enhance the existing data on different marine environments in the world and especially in south america where this information is limited two swath bathymetry surveys were conducted across a dune field in the bahia blanca estuary argentina morphometric parameters and migration rate according to the dune type were analyzed the field is composed of large dunes exhibiting two morphological configurations which are differentiated into sinuous and barchan dunes the dunes studied are the largest of the estuary with heights and wavelengths greater than 5 m and 130 m respectively the crests of the large dunes are arranged with an orientation perpendicular to the axis of the channel from geometrical analysis of the parameters the dunes show a weakly positive correlation between dune height and wavelength as too between dune height and water depth no clear relationship was observed between maximum height and wavelength parameters with water depth across the estuary the bedforms migrate in the ebb direction with mean rate of 43 m year 1 comparison of our results with previous data shows that during three decades the western boundary of dune field has been displaced 900 m towards the outer estuary however the dune field configuration and distribution of diverse types of bedform appear to be relatively stable

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

potassium leached zones at the contacts of the garn formation mid norwegian continental shelf evidence for the role of shale in sandstone diagenesis abstract

flow resistance and bed form geometry in a wide alluvial channel 1 this paper explores the underlying mechanism of flow resistance in a wide alluvial channel with bed forms on the basis of published data it is shown that the grain roughness can be taken as equal to 2 times the median diameter of the bed sediment an empirical equation for the bed form roughness has been proposed and it depends on the bed form height and bed form steepness the influence of the bed form length and height on the total bed shear stress and energy slope is deliberated and empirical expressions for the length of the separation zone behind the bed forms are also proposed the study proposed an equation to compute the total bed shear stress as a function of the grain and bed form roughness as well as the important role of the bed form geometry in the overall flow resistance in alluvial channels the model is tested and verified against 670 flume measurements and 1540 field observations the computed and measured energy slopes are in good agreement with close to 71 of all data sets within the 20 error band

modification to the hardisty equation regarding the relationship between sediment transport rate and particle size bagnold type bed load equations are widely used for the determination of sediment transport rate in marine environments the accuracy of these equations depends upon the definition of the coefficient k 1 in the equations which is a function of particle size hardisty 1983 has attempted to establish the relationship between k 1 and particle size but there is an error in his analytical result our reanalysis of the original flume data results in new formulae for the coefficient furthermore we found that the k 1 values should be derived using u 1 and u 1cr data the use of the vertical mean velocity in flumes to replace u 1 will lead to considerably higher k 1 values and overestimation of sediment transport rates

tectonic and stratigraphic controls of hydrocarbon systems in the ordos basin a multicycle cratonic basin in central china the ordos basin is the oldest and still an important hydrocarbon province in central china it is a typical cratonic basin developed on the archean granulites and lower proterozoic greenschists of the north china block the development of the ordos basin during the paleozoic mesozoic can be divided into three evolutionary stages cambrian early ordovician cratonic basin with divergent margins middle ordovician middle triassic cratonic basin with convergent margins and late triassic early cretaceous intraplate remnant cratonic basin two hydrocarbon systems are present in the basin the paleozoic gas and mesozoic oil systems in the paleozoic gas system the lower ordovician marine carbonates and pennsylvanian lower permian coal measures serve as source rocks the lower ordovician karst modified dolomites and pennsylvanian bauxitic mudstones form a significant reservoir seal association and the pennsylvanian lower permian deltaic sandstones and upper permian lacustrine mudstones form another effective reservoir seal association in the mesozoic oil system the upper triassic lacustrine mudstones are mature source rocks the upper triassic deltaic sandstones and overlying shallow lacustrine and swamp mudstones form a reservoir seal association and the lower jurassic fluvial sandstones and overlying shallow lacustrine and swamp mudstones form another reservoir seal association in both hydrocarbon systems the stratigraphic variations provide the principal traps the ordos basin is characterized by a stable tectonic setting that controlled the distribution of depositional systems and the development of erosional surfaces and ultimately governed the distribution of oil and gas fields and trap types

unconventional shale gas systems the mississippian barnett shale of north central texas as one model for thermogenic shale gas assessment shale gas resource plays can be distinguished by gas type and system characteristics the newark east gas field located in the fort worth basin texas is defined by thermogenic gas production from low porosity and low permeability barnett shale the barnett shale gas system a self contained source reservoir system has generated large amounts of gas in the key productive areas because of various characteristics and processes including 1 excellent original organic richness and generation potential 2 primary and secondary cracking of kerogen and retained oil respectively 3 retention of oil for cracking to gas by adsorption 4 porosity resulting from organic matter decomposition and 5 brittle mineralogical composition the calculated total gas in place gip based on estimated ultimate recovery that is based on production profiles and operator estimates is about 204 bcf section 5 78 10 9 m 3 1 73 10 4 m 3 we estimate that the barnett shale has a total generation potential of about 609 bbl of oil equivalent ac ft or the equivalent of 3657 mcf ac ft 84 0 m 3 m 3 assuming a thickness of 350 ft 107 m and only sufficient hydrogen for partial cracking of retained oil to gas a total generation potential of 820 bcf section is estimated of this potential approximately 60 was expelled and the balance was retained for secondary cracking of oil to gas if sufficient thermal maturity was reached gas storage capacity of the barnett shale at typical reservoir pressure volume and temperature conditions and 6 porosity shows a maximum storage capacity of 540 mcf ac ft or 159 scf ton

stability of bedforms in a tidal environment abstract large scale bedforms developed in a tide dominated salt estuary near sapelo island georgia are morphologically separated into two distinct groups megaripples include bedforms 4 to 15 m in wavelength and 10 to 75 cm in height sand waves are 15 to 45 m in wavelength and 85 to 130 cm in height and can be further subdivided into megarippled forms and smooth crested forms ebb oriented geometry of large scale bedforms is maintained through both ebb and flood portions of the tidal cycle bedforms are in equilibrium with peak tidal flow and ebb orientation is the direct result of large pressure gradients and related bed shear values that develop during the ebb bedform heights can be statistically related to flow power shear velocity water depth and sediment grain size mean flow velocity values used in many previous studies of bedform stability did not produce significant f statistic values in a multiple linear regression procedure used to discern relationships among variables stability fields of bedform types can be empirically defined using parameters statistically significant in the regression analysis flow power shear velocity water depth and sediment grain size were used in various combinations to construct bivariate plots showing separate stability fields of sand waves and megaripples

on mapping seafloor mineral deposits with central loop transient electromagnetics electromagnetic methods are commonly employed in exploration for land based mineral deposits a suite of airborne land and borehole electromagnetic techniques consisting of different coil and dipole configurations have been developed over the last few decades for this purpose in contrast although the commercial value of marine mineral deposits has been recognized for decades the development of suitable marine electromagnetic methods for mineral exploration at sea is still in its infancy one particularly interesting electromagnetic method which could be used to image a mineral deposit on the ocean floor is the central loop configuration central loop systems consist of concentric transmitting and receiving loops of wire while these types of systems are frequently used in land based or airborne surveys to our knowledge neither system has been used for marine mineral exploration the advantages of using central loop systems at sea are twofold 1 simplified navigation because the transmitter and receiver are concentric and 2 simplified operation because only one compact unit must be deployed we produced layered seafloor type curves for two particular types of central loop methods the in loop and coincident loop configurations in particular we consider models inspired by real marine mineral exploration scenarios consisting of overburdens 0 to 5 m thick overlying a conductive ore body 5 to 30 m thick modeling and resolution analyses showed that using a 50 m 2 transmitting loop with 20 a of current these two configurations are useful tools to determine the overburden depth to a conductive ore deposit and its thickness in the most extreme case absolute voltage errors on the order of 10 nv are required to resolve the base of a 30 m thick ore deposit whether such noise floors can be achieved in real marine environments remains to be seen

diagenesis of a tight gas sand reservoir upper cretaceous mesaverde group piceance basin colorado abstract the upper cretaceous mesaverde group in the piceance basin western colorado contains thick sections of low porosity low permeability sandstones that are reservoirs for large accumulations of hydrocarbon gas the mesaverde sandstones are lithic arkoses and feldspathic litharenites containing quartz plagioclase variable k feldspar chert and volcanic rock fragments important diagenetic processes that have affected the sandstones include compaction dissolution of feldspar and or alteration to clay precipitation of carbonate and quartz cements precipitation of illite and alteration of detrital clays mixed layer illite smectite porosity is relatively constant a decrease in primary porosity downward is approximately balanced by an increase in secondary porosity with significant microporosity in authigenic and diagenetic clays k feldspar is almost completely absent below 5500 feet 1675 m fibrous illite is relatively abundant above this depth and variable in abundance below the k ar ages of the clays increase with depth from 40 m y at the top of the sampled section to 55 m y at the base indicating illitization of a precursor smectite at approximately the 100 c isotherm samples with abundant fibrous illite have relatively smaller age values mass balance calculations indicate that dissolution of k feldspar illitization of smectite and precipitation of fibrous illite were coupled these reactions suggest the transport of dissolved potassium upward hundreds of feet 100 300 m in the section possibly associated with water driven vertically by the migration of gas

porosity preserving chlorite cements in shallow marine volcaniclastic sandstones evidence from cretaceous sandstones of the sawan gas field pakistan sandstones that have high porosity and permeability at great burial depth and high temperatures are of economic importance because a significant amount of hydrocarbons have been discovered in such reservoirs the sawan gas field with an expected ultimate recovery of more than 1 tcf lies in the middle indus basin the reservoir rocks cretaceous volcaniclastic sandstones of the lower goru formation show very high porosities at a reservoir temperature of 175c and depths of 3000 to 3500 m 9842 to 11 483 ft the sandstones are mostly feldspathic litharenites strongly altered volcanic rock fragments are the most important lithic component the clay fraction consists of fe rich chlorite chamosite and illite diagenetic features such as compaction quartz overgrowths carbonate cements and feldspar dissolution are observed the most distinguishing feature is a double layer of authigenic chlorite lining the pores of the sandstones chlorite additionally occurs as a pore filling cement and as chloritized detrital components all having similar chemical composition the pore lining cement clearly developed in two stages an earlier poorly crystallized and a later better crystallized growth missing rims at grain contacts show that precipitation occurred after an initial stage of compaction but early relative to other diagenetic phases both chlorite rims grew by direct precipitation from pore waters using products derived from volcanic rock fragments in areas with no thin or discontinuous chlorite rims quartz cementation is common well developed chlorite rims inhibited quartz cementation preserved porosities of up to 20 and good permeabilities porosity preserving chlorite cementation in sawan is restricted to sediments of a shallow marine environment

diagenesis and secondary porosity in vicksburg sandstones mcallen ranch field hidalgo county texas abstract lower vicksburg sandstones oligocene at mcallen ranch field in hidalgo county texas consist of thin sandstones interbedded with shales the sands were deposited by turbidity currents as channel and overbank deposits the sandstones produce gas from depths of 9300 to 15 000 ft 2835 to 4572 m depositional patterns were controlled by a diapiric shale uplift and related faults petrographic analyses show that primary porosity was reduced during early diagenesis by calcite cementation however dissolution of calcite cement feldspar and volcanic rock fragments which took place after deep burial led to secondary porosity development dissolution is evidenced by the formation of intergranular porosity oversized pores grain molds and by microporosity within individual grains dissolution was followed by precipitation of quartz overgrowths formation of authigenic clay minerals kaolinite chlorite illite smectite and vermiculite and by precipitation of iron rich calcite cement scanning electron microscopy confirms that clay minerals are primarily authigenic and uniformly distributed chlorite grain coatings and pore lining probably aided in preservation of porosity by inhibiting cementation highest porosites and permeabilities are found where the sandstones have the highest secondary porosities as determined by petrographic study porosity increases from about 15 percent to as much as 20 percent in the section from 9642 to 12 586 ft 2939 to 3836 m this increase is related to the abnormally high pressure gradient of about 0 92 psi ft 20 8 kpa m and to an elevated geothermal gradient of about 2 f 100 ft optimum reservoir properties are present where late stage cementation by clays and iron rich calcite has not been extensive

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

variation in ebb delta morphologies along the west and east frisian islands the netherlands and germany abstract ebb delta morphologies in the west and east frisian islands differ mainly in terms of the direction and the degree of asymmetry of the ebb deltas in the east frisian islands the ebb tidal deltas and their main ebb channels are asymmetrically directed downdrift to the east in the west frisian islands they are asymmetrically directed updrift to the west except for a few of the small ebb tidal deltas which are asymmetrically directed downdrift earlier theories for the east frisian islands considered wind driven waves coming dominantly from the west to be the main cause of the downdrift asymmetry of the ebb tidal deltas theories for the west frisian islands related the updrift directed ebb tidal deltas to tidal currents none of these theories can explain the morphological variation in the ebb tidal deltas for both the west and east frisian islands observation of the ebb delta morphologies of the west and east frisian islands in relation to tidal and wave regimes shows that the above mentioned difference in the ebb delta morphologies is due to the varying importance of the tidal prism and the wave induced longshore drift for the different inlets the difference is especially pronounced between the large and the small ebb tidal deltas because their tidal prisms are different the inlets with a large tidal prism have large ebb tidal deltas and they are dominated by tidal currents interaction of marine shore parallel tidal currents with onshore offshore inlet tidal currents which is enhanced by the ebb delta shoals causes the large ebb tidal deltas and their main ebb channels to be asymmetrically directed updrift to the west the inlets with a small tidal prism have small ebb tidal deltas and they are dominated by waves eastward moving deep water wave and wave induced longshore drift drive the small ebb tidal deltas asymmetrically downdrift to the east the general model proposed not only explains the varying morphologies of the ebb tidal deltas of the west and east frisian islands but is also applicable to other ebb tidal deltas

grain size control of large compound flow transverse bedforms in a tidal inlet of the danish wadden sea abstract flow transverse bedforms were recorded along a 2 km reach of the northernmost tidal inlet of the danish wadden sea the bed of the inlet channel is covered with large to very large compound dunes and superimposed medium sized dunes composed of medium to coarse sands the mean grain size of the bed material along the centre line of the inlet throat decreases from 0 56 mm in the inner part to 0 3 mm in the outer part both dune populations are essentially two dimensional with straight to slightly curved crest lines the ebb current is very similar in magnitude along the entire channel axis whereas the flood current is of similar magnitude in the inner half but reaches only about half of the maximum ebb current velocity in the outer section sediment transport occurs during both the ebb and flood periods the latter being less vigorous and of shorter duration as a result the inlet as a whole is ebb dominated dominant ebb current 1 01 m s 1 dominant flood current 0 79 m s 1 in agreement with the tidal dynamics a net ebb oriented bedform migration rate of 32 m yr 1 was determined from the survey data the large to very large dunes display distinct flood caps during the flooding tide whereas the medium sized dunes reverse their direction during each tide the downstream dimensional evolution of the large dunes is complex switching from initial growth to progressive decline followed by renewed growth in length but progressive decline in height this latter degradational phase begins in the middle of the survey reach and continues seawards until the large forms disappear completely at the same time the outer channel section becomes dominated by medium sized dunes which grow in size with decreasing grain size surprisingly there is no correlation between dune dimensions and water depth as would be expected at such shallow depths the complex evolution of dune dimensions as a function of grain size is discussed in terms of recent tidal bedform stability theory it is clear however that more site specific observations are required to fully explain the phenomenon

impact of sea level rise on the morphological equilibrium state of tidal inlets abstract this study addresses the question whether the geomorphology of a tidal inlet i e the coastal inlet and associated tidal basin can maintain equilibrium under a rising relative sea level when a tidal inlet system is exposed to a constant rate of sea level rise slr the system will be permanently in a state that deviates from the equilibrium state corresponding to zero slr this is a physical requirement to create a permanent incentive to import sediment into the system in case the rate of sediment import matches the rate of sea level rise a new state of dynamic morphological equilibrium is reached if the actual import rate is less than this the system s morphological state will deviate increasingly from its equilibrium and finally degenerate in this case slr has exceeded an upper boundary named state limit denoted as slrlimit and the system will not be able to reach a state of dynamic morphological equilibrium any longer and drown because the slrlimit predictions are sensitive to a variation of internal sediment exchange rate and availability of sediment at the seaward boundary of the system we assess the slrlimit predictions quasi probabilistically to gain quantitative insight into the inherent uncertainties this technique leads to an estimate of the probability distribution for the slrlimit of two selected tidal inlet systems in the dutch wadden sea the calculated probabilities comply with the available fragmentary geological data which suggest that the former tidal inlets in the western region of the netherlands were drowned under the influence of a sea level rise of 80 cm to a few metres per century

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

testing the limits of aem bathymetry with a floating tem system a floating transient electromagnetic tem system sea ring simulating a low altitude helicopter airborne electromagnetic aem system was constructed to test the accuracy of the aem method for measuring water depth and estimating sediment thickness in shallow coastal waters a square transmitter loop 10 10 m plus concentric inner and outer receiver loops was strung from masts supported by the circular sea ring base data were stacked over periods from 1 to approximately 60 s and with loop heights ranging between approximately 5 5 and 12 m above sea level the towed sea ring provides a stable platform at a known fixed altitude in calm waters we have undertaken modeling to investigate the effect of vertical and horizontal displacements of the loops and to compare circular and square loopgeometries in proximity to the sea surface with relatively long stacking times as long as approximately 60 s the uncertainty in altitude can be reduced to very low levels the sea ring has been deployed near

pore characteristic responses to categories of depositional microfacies of delta lacustrine tight reservoirs in the upper triassic yanchang formation ordos basin nw china abstract primary depositional variations interact with diagenetic modifications to exert strong control on pore networks in tight hydrocarbon reservoirs however the linkage between pore characteristics and depositional microfacies remains enigmatic which restricts the prediction of high quality reservoir distributions the continental tight reservoirs of the yanchang formation in the ordos basin are targeted to clarify and estimate the sensitivity of pore characteristics to depositional microfacies a multi methodical approach utilizing petrographic assessments logging evaluations digital imaging analyses and high pressure mercury intrusion experiments enables us to reveal the high microheterogeneity of pore types pore size distribution and pore fractal features the results identified the microfacies including distributary channels dcs mouth bars mbs interdistributary bays ibs sheet sands sss turbidite fans tfs shallow lacustrine mudstones slms and semi deep lacustrine mudstones sdlms from the chang 8 to chang 6 members dominated by residual intergranular pores and microfractures the main storage spaces of the dc and mb microfacies are within mesopores which provide 62 92 and 81 of the total pore volume respectively however the micropores that are mainly composed of clay intercrystalline pores and feldspar dissolved pores provide 55 70 52 62 and 53 80 of the total pore volume for the ib slm and sdlm microfacies respectively in addition the pore fractal dimension results indicate that the mesoporous structure of the argillaceous microfacies ib slm and sdlm is more heterogeneous than that of the arenaceous microfacies dc mb and tf therefore the dominant reservoir spaces of the different depositional microfacies are distributed over diverse pore radius ranges and the varying degrees of mesoporous development are a crucial factor affecting the reservoir capacity moreover the fractal dimensions have a significant negative correlation with pore volume in the meso micropore range characterized by lower pore sorting coefficients broader skewness and more abundant rigid siliceous grains the arenaceous microfacies with lower fractal dimensions and higher pore volume have better reservoir capacity than argillaceous microfacies that are rich in ductile minerals microscopic observations show that the muddier reservoirs of interdistributary bays and lacustrine mudstones are more susceptible to pore filling chlorite and illite which can deteriorate the reservoir quality these results indicate that the controlling effect of depositional microfacies on pore characteristics is fundamentally caused by differences in mineralogical composition and depositional texture and these results could provide new insights into the mechanisms of microfacies controls on reservoir quality

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

diagenesis in porosity evolution of opening mode fractures middle triassic to lower jurassic la boca formation ne mexico abstract opening mode fractures joints in middle triassic to lower jurassic la boca formation sandstones northeastern mexico have patterns of fracture porosity mineral fill structures and size distributions not previously described from outcrop patterns match those found in cores from many basins we used aperture measurements along lines of observation scanlines fracture trace maps petrography high resolution scanning electron microscope sem based cathodoluminescence and fluid inclusions to characterize fracture populations open fractures are lined by quartz that precipitated while fractures were opening whereas sealed fractures additionally contain calcite deposited after fractures ceased opening large fractures and arrays of contemporaneous microfractures have consistent power law aperture size scaling over approximately three orders of magnitude our results imply that open fractures and fracture sizes depend on diagenetic state the interplay of fracture mechanics and diagenetic history is a determinant on effective porosity within fractures and thus open fracture persistence connectivity and fluid flow

improved seawater depth determination using corrected helicopter time domain electromagnetic data abstract two areas within sydney harbour were surveyed in 2002 with a helicopter borne time domain electromagnetic system to test its potential for bathymetric mapping in shallow seawater as delivered the data were improperly calibrated therefore a re calibration was performed to reconcile the measured data with ground truth synthetic electromagnetic transients were computed for two layer models representing the seawater and the sediment overlying bedrock at a number of locations within the survey area the seawater depth in the models varied between 3 m and 70 m the measured and calculated data were compared at each delay time and were found to be linearly related the slope and intercept of the line of best fit were used to correct all the measured data inversion of the corrected time domain electromagnetic data generally resolved the bathymetry to submetre accuracy down to depths of about 55 m

validation of a coupled wave flow model in a high energy setting the mouth of the columbia river 1 a monthlong time series of wave current salinity and suspended sediment measurements was made at five sites on a transect across the mouth of columbia river mcr these data were used to calibrate and evaluate the performance of a coupled hydrodynamic and wave model for the mcr based on the delft3d modeling system the mcr is a dynamic estuary inlet in which tidal currents river discharge and wave driven currents are all important model tuning consisted primarily of spatial adjustments to bottom drag coefficients in combination with near default parameter settings the mcr model application is able to simulate the dominant features in the tidal flow salinity and wavefields observed in field measurements the wave orbital averaged method for representing the current velocity profile in the wave model is considered the most realistic for the mcr the hydrodynamic model is particularly effective in reproducing the observed vertical residual and temporal variations in current structure density gradients introduce the observed and modeled reversal of the mean flow at the bed and augment mean and peak flow in the upper half of the water column this implies that sediment transport during calmer summer conditions is controlled by density stratification and is likely net landward due to the reversal of flow near the bed the correspondence between observed and modeled hydrodynamics makes this application a tool to investigate hydrodynamics and associated sediment transport

pore throat sizes in sandstones tight sandstones and shales discussion wayne k camp nelson 2009 prepared a nice compilation describing the wide range of pore throat sizes that span a continuum from conventional sandstone reservoirs to unconventional tight gas siliciclastic reservoirs and shale seals in this article nelson 2009 proposes a new and rather unique definition for unconventional reservoirs including tight gas sands that lacks sufficient supporting evidence or references to support this definition nelson 2009 p 329 defines a conventional reservoir as one in which evidence that buoyant force has formed and maintained the disposition of oil and gas is present and thus for unconventional reservoirs it follows that nelson concludes p 330 that in these systems evidence for buoyancy as a dominant force in the disposition of oil and gas is lacking it appears that the sole argument for nonbuoyant or buoyancy subordinate unconventional reservoir systems is the perceived lack of evidence of buoyancy which is generally not good science absence of evidence is not evidence of absence it would be much better to present a clear and concise hypothesis followed by objective observations that support the conclusions i appreciate that such a thesis may be beyond the scope of a geologic note article but until such evidence is presented it is difficult to have intelligent discussions and design experiments to test the hypothesis of nonbuoyant gas systems although it is not clear from the definition of nelson 2009 the reader is left to conclude that the natural laws of buoyancy are not dominant factors during the evolution of tight gas sand accumulations from hydrocarbon migration to the present day observed distribution of hydrocarbons and water this conclusion is misleading and ignores a significant body of recent published research shanley et al 2004 cluff et al 2005 fassett and boyce 2005 camp 2008 nelson 2009 p 339 further concludes

3d porosity and mineralogy characterization in tight gas sandstones tight gas reservoirs exhibit storage and flow characteristics that are intimately tied to depositional and diagenetic processes as a result exploitation of these resources requires a comprehensive reservoir description and characterization program to identify properties which control production in particular tight gas reservoirs have significant primary and secondary porosity and pore connectivity dominated by clays and slot like pores this makes them particularly susceptible to the effects of overburden stress and variable water saturation this paper describes an integrated approach to describe a tight gas sandstone at the pore scale in 3d in particular the primary and secondary porosity of a tight gas sandstone are identified and quantified in three dimensions using 3d x ray micro ct imaging and visualization of core material at the pore scale 3d images allow one to map in detail the pore and grain structure and interconnectivity of primary and secondary porosity once the tomographic images ar

stable oxygen and carbon isotopes of carbonate concretions of the miocene yeonil group in the pohang basin korea types of concretions and formation condition abstract carbonate concretions in the miocene sedimentary rocks of the yeonil group in the pohang basin korea were investigated in terms of stable oxygen and carbon isotope composition to delineate the origin and associated diagenetic environment for their formation carbonate concretions are widely distributed in all the sedimentary rocks in the pohang basin showing that the calcitic concretions are preserved within the mass flow deposits and the dolomitic ones mostly in the hemipelagic siliceous rocks diatomites concretions can be classified into four different types on the basis of the stable isotopic signatures each of which represents its own geochemical range type i concretions are calcitic and are composed of micrite to microspar they occur in the conglomerates and sandstones which were deposited by mass flows debris flow to turbidity current it shows relatively lower 18 o 14 0 to 9 3 and 13 c 19 6 to 8 4 values these concretions grew in a sulfate reducing zone under the influence of residual ambient seawater which had been significantly modified by volcanogenic sediments type ii concretions are also calcitic composed mostly of micrite with minor microspar and found in the sandstones these concretions are characterized by relatively high 18 o 1 8 to 2 4 and variable 13 c 17 3 to 0 4 values these isotopic signatures reflect that type ii concretions formed from just beneath the sediment water interface down to the sulfate reducing zone through the early stage of methanogenesis type iii concretions are also calcitic and composed largely of micrite with a minor contribution of microspar they are observed in hemipelagic mudrocks which were deposited under the influence of mass flows they are characterized by intermediate to high 18 o 4 6 to 1 6 and high 13 c 1 3 to 8 8 values these concretions grew in a methanogenic zone by residual ambient seawater and or seawater slightly modified by reaction with volcanogenic sediments type iv concretions are dolomite with calcite inclusion and occur in hemipelagic siliceous rocks these concretions are mostly composed of micrite and characterized by variable 18 o 9 1 to 0 7 and high 13 c 3 1 to 17 9 values suggesting formation in the methanogenic zone although the residual ambient seawater is slightly modified by volcanogenic sediments the same type of the concretions is widely distributed throughout the basin and always shows its own distinctive stable isotopic signature this means that the formation of the given type depends upon the lithology and composition of host sediments that are closely related to the depositional process of the fan delta systems regardless of their localities further the different types of concretions are also found at the different but closely spaced stratigraphic levels in the same locality displaying the distinctive diagenetic conditions for each type such preservation of the unique diagenetic signatures in individual type of concretion suggests that the concretions formed in a completely closed diagenetic system therefore caution should be made to simplify and generalize the diagenetic condition for the formation of any concretions in a large sedimentary basin

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

upper paleozoic petroleum system ordos basin china abstract the ordos basin is a typical lapped basin including three sequences of strata early paleozoic late paleozoic and mesozoic with a total thickness of 4000 6000 m impermeable sealing beds are well developed at the top and base of the upper paleozoic sequence separating it from the mesozoic and the lower paleozoic strata to form an independent petroleum system in this petroleum system the source rocks are widely distributed coals and dark mudstones occurring in the carboniferous lower permian coal measures with a thickness of 10 15 and 40 60 m respectively the reservoirs are mainly early permian tight sandstones mostly with a porosity of 4 8 and a permeability of 0 1 1 0 10 3 m2 the regional cap rock is a 100 150 m thick mudstone in the upper permian strata the structural framework of the basin is a huge asymmetric syncline dipping gently toward the east and north and steeply toward the south and west well data show that gas saturated gas water transition and water saturated zones are developed from the depositional center to the basin edges the gas saturated zone mainly lies in the gently dipping slope area of the shanbei slope toward eastern and northern up dip directions the water gas transitional zone occurs and finally the water saturated zone presenting a reverse relation of water on top of gas an abnormal negative strata pressure is developed in the gas bearing area with a pressure coefficient cp ranging from 0 83 to 0 95 fluid inclusion data indicate that the upper paleozoic gas pool began to develop around the wuqi area at about 150 ma and it extended toward the north and was largely formed at about 120 ma showing there was a regional migration of the gas water interface from south to north during the gas pool formation these characteristics appear to show that the northern and eastern margins of the petroleum system are defined by a regional hydrodynamic regime the critical moment of the petroleum system occurred during the early cretaceous the basin experienced a rapid subsidence and a significant geothermal event resulting in the source rocks being matured into the main stage of gas generation to form a huge amount of gas the gas migrated into the tight sandstone reservoirs and displaced the rock pore water to form the gas pools and developing the regional hydrodynamic trap

modelling an anthropogenic effect of a tidal basin evolution applying tidal and wave boundary forcings ley bay east frisian wadden sea abstract potential physical impacts of an anthropogenic effect on a tidal basin evolution are investigated applying the delft3d model suite under both tidal and wave boundary forcings study area is based on a peninsula construction of the ley bay in the east frisian wadden sea model simulation spans from 1975 to 1990 in two stages of which the second stage begins with the implemented peninsula on the 1984 predicted morphology the model bed consists of initially distributed three sediment fractions sensitivity of the ley bay evolution is analysed under three different sediment transport formulations 1 van rijn 1993 vr93 2 soulsby 1997 svr and 3 van rijn et al 2004 vr04 offshore tides and waves are transformed up to the model boundaries via a nested modelling approach and a statistically derived highly schematised wave climate is adopted in the simulations predicted morphologies indicate lower agreement with the measured morphology due to including very sparse data despite this discrepancy they reproduce the major changes in the ley bay caused by the peninsula construction while each formula results in a slightly different channel shoal pattern predicted evolution under the svr shows the strongest sediment exporting system and therefore the lowest agreement with the 1990 measured morphology both vr93 and vr04 formulas resulted in marginal exporting systems and more or less similar morphologies in fact only the vr04 prediction indicates a fair agreement with the 1990 data temporal evolution under the vr04 shows concentrated velocity patterns at the bay entrance and in the eastward bay channel resulting in the development of this channel and sedimentation in the southern part of the bay as found in the data

wave driven along channel subtidal flows in a well mixed ocean inlet observations of waves flows and water levels collected for a month in and near a long narrow shallow 3000 m long 1000 m wide and 5 m deep well mixed ocean inlet are used to evaluate the subtidal periods 30 h along inlet momentum balance maximum tidal flows in the inlet were about 1 5 m s and offshore significant wave heights ranged from about 0 5 to 2 5 m the dominant terms in the local across the km wide ebb shoal along inlet momentum balance are the along inlet pressure gradient the bottom stress and the wave radiation stress gradient estimated nonlinear advective acceleration terms roughly balance in the channel onshore radiation stress gradients owing to breaking waves enhance the flood flows into the inlet especially during storms

selective sorting and abrasion of river gravel i theory most gravel rivers show a tendency for characteristic grain size to decrease in the downstream direction over scales of tens or hundreds of kilometers it has been surmised that this downstream fining is due to some combination of selective sorting by which finer grains are preferentially transported downstream and abrasion by which individual particles are reduced in size here a framework for the simultaneous treatment of both phenomena is developed the analysis is restricted to bed load transport of gravel only abrasion due to binary collisions between moving bed load and stationary bed particles is considered the formulation includes the possibility of differential basin uplift and subsidence due to tectonism and mixtures of rock types with differing coefficients of abrasion the formulation of selective sorting is based on the concept of hiding as applied to a surface based bed load transport relation abrasion is first treated in terms of a lagrangian formulation for individual grains and t

controls on river delta formation insights from numerical modelling abstract primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology based on many field observations however detailed mechanistic models were restricted to the river dominated delta class analogous to the concept of morphological conditioning here we show that antecedent stratigraphy controls morphometrics of prograding river dominated delta distributary networks under steady sea level further we use coupled hydrodynamic morphodynamic stratigraphic detailed numerical modelling to assess the influence of wind generated waves and tides on clastic river delta formation our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth bar induced flow bifurcation and upstream channel shifting windwave action suppresses sequestration of fine sediments on the developing delta plain entailing relatively smooth shorelines perturbed by a limited number of distributary channels in contrast tide influenced river deltas are found to prograde mainly via lengthening of initially formed relatively stable distributaries as well as being characterized by cyclicity in deposits interbedding of sands and silts these results provide a framework for physics based river delta modelling under various environmental conditions particularly our findings suggest that relatively low energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas

a method to detect natural fracture strike in sandstones in siliciclastic hydrocarbon reservoir rocks economic gas and oil production may depend on the attributes of natural fractures and with the advent of horizontal drilling fractures are increasingly exploration and development targets yet reliable information on such key fracture attributes as orientation strike is sparse because few fractures intersect vertical well bores this paper describes how fracture strike can be documented on a bed by bed basis even in well bores where few or no visible fractures are directly sampled quartz lined opening mode microfractures lengths of microns to millimeters in quartz cemented sandstones commonly are not visible using standard petrographic methods but systematic mapping of these microfractures is possible using photomultiplier based electron beam induced luminescence scanned cathodoluminescence imaging as shown by observations primarily from three natural gas plays and one oil play in the united states microfracture strike is a good guide to the strike of large fractures macrofractures that formed concurrently because microfractures are widespread and small specimens can be used to get accurate fracture strike data this approach can be applied to samples obtained from wireline conveyed rotary drilled sidewall coring devices as well as to samples from full diameter core

application of back scattered electron microscopy to the quantification of clay mineral microporosity in sandstones abstract computer assisted image analysis of back scattered electron bse micrographs of petrographic sections is useful for the quantification of clay mineral microporosity in sandstones authigenic clay minerals from north sea clastic reservoirs generally have higher microporosities than detrital clay minerals diagenetic kaolinite has microporosities varying from 25 to 50 diagenetic chlorite has a generally uniform grain coating texture and microporosities of about 50 dispersed diagenetic illitic clays are difficult to characterize using this method because their delicate morphology is susceptible to damage during sample preparation despite this limitation the analytical data indicate a minimum microporosity of 63 for dispersed illitic clays analytical data for clay rich detrital lasts have much lower microporosity values of about 10 interpretation of images from detrital clasts is complicated by the effects of sample preparation

unified view of sediment transport by currents and waves i initiation of motion bed roughness and bed load transport attention is given to the properties of sediment beds over the full range of conditions silts to gravel in particular the effect of fine silt on the bed composition and on initiation of motion critical conditions is discussed high quality bed load transport data sets are identified and analyzed showing that the bed load transport in the sand range is related to velocity to power 2 5 the bed load transport is not much affected by particle size the prediction of bed roughness is addressed and the prediction of bed load transport in steady river flow is extended to coastal flow applying an intrawave approach simplified bed load transport formulas are presented which can be used to obtain a quick estimate of bed load transport in river and coastal flows it is shown that the sediment transport of fine silts to coarse sand can be described in a unified model framework using fairly simple expressions the proposed model is fully predictive in the sense that only the basic hydrodynamic parameters dep

airborne electromagnetic bathymetry and estimation of bedrock topography in broken bay australia abstracta helicopter transient electromagnetic tem survey was flown over shallow coastal waters in broken bay australia overlying several palaeovalleys and exposed reef sections the infilled palaeovalleys contain unconsolidated sediments with variable thicknesses exceeding 100 m previous marine seismic reflection and vibrocore studies provide an estimate of sediment thickness and surficial sediment conductivity respectively which combined with known bathymetry sonar soundings and measured seawater conductivity can be used to generate a crude 1d geoelectric ground truth model consisting of two layers seawater sediment overlying a relatively resistive basement the primary focus was to examine the accuracy of interpreted water depths obtained by 1d inversion of airborne tem data assuming a two layer over resistive basement model by comparing these depths with known water depths the secondary focus was to compare the interpreted sediment thickness i e the thickness of the second geoelectr

effect of pore structure on the producibility of tight gas sandstones the estimated ultimate recovery eur is one of the most significant properties of tight gas sandstone reservoirs but it remains difficult to predict estimated ultimate recovery is dependent on the success of stimulation by hydraulic fracturing the existence and connectivity of natural fractures and as illustrated in this article the pore structure of the matrix here we analyze the lab measurements that are indicative of the pore structure and then we predict the effect of pore structure on producibility we develop a relationship between the eur of tight gas sandstones and their petrophysical properties measured by drainage and imbibition tests mercury intrusion withdrawal and porous plate and by resistivity analyses we use the ratio of residual mercury saturation after mercury withdrawal s gr to initial mercury saturation s gi which is the saturation at the start of withdrawal as a measure of gas likely to be trapped in the matrix during production and hence a proxy for eur a multitype pore space model is required to explain mercury intrusion capillary pressures in these rocks implications of this model are supported by other available laboratory measurements the model comprises a conventional network model and a treelike pore structure an acyclic network that mimic the intergranular and intragranular void spaces respectively the notion of the treelike pore structure is introduced here for the first time in the context of tight gas sandstones applying the multitype model to porous plate data we classify the pore spaces of rocks into intergranular dominant intermediate and intragranular dominant this pore space classification is topological and is not based on scale or size these classes have progressively less drainage and imbibition hysteresis which leads to the prediction that significantly more hydrocarbon is recoverable from intragranular porosity than intergranular porosity available field data production logs corroborate the higher producibility of intervals with intragranular porosity although the data are not sufficient to eliminate the possible contribution of other factors such as size and shape of the volume contacted by hydraulic fractures or the presence and attributes of natural fractures the superior recovery of hydrocarbon from intragranular dominant pore structures is despite its inferior initial production rate

petroleum geology of the giant elm coulee field williston basin the elm coulee field of the williston basin is a giant oil discovery in the middle bakken formation devonian mississippian discovered in 2000 horizontal drilling began in the field in 2000 and to date more than 600 wells have been drilled the estimated ultimate recovery for the field is more than 200 million bbl of oil 31 8 million m3 the bakken formation in the field area consists of three members 1 upper shale 2 middle silty dolostone and 3 lower siltstone the total bakken interval ranges in thickness from 10 to 50 ft 3 1 to 15 3 m over the field area the upper shale is dark gray to black hard siliceous slightly calcareous pyritic and fissile the shale consists of dark organic kerogen minor clay silt size quartz and some calcite and dolomite the kerogen consists mainly of amorphous material and the organic material is distributed evenly throughout the shale interval not concentrated in laminations or lenses the upper shale ranges in thickness from 6 to 10 ft 1 8 to 3 1 m over the field area the middle member consists of a silty dolostone and ranges in thickness from 10 to 40 ft 3 1 to 12 2 m the lower member in the elm coulee field consists of brownish gray argillaceous organic rich siltstone burrowing and brachiopod fragments are common in the lower member this facies is equivalent to the lower bakken black shale facies on the northern side of the field and is interpreted to be an updip landward equivalent to the deeper water black shale facies the lower member ranges in thickness from 2 to 6 ft 0 61 to 1 8 m based on the abundance of fossil content and amount of burrowing the members of the bakken formation are interpreted to have been deposited under aerobic middle member common burrows and rare fossils dsyaerobic lower member common fossils and lesser amount of burrows and anaerobic conditions upper member rare fossils and burrows the main reservoir in elm coulee field is the middle member which has low matrix porosity and permeability and is found at depths of 8500 to 10 500 ft 2593 to 3203 m the current field limits cover approximately 450 mi2 1165 km2 the middle bakken porosities range from 3 to 9 and permeabilities average 0 04 md overall the reservoir quality in the middle bakken improves upward as the amount of clay matrix decreases the middle bakken is interpreted to be a dolomitized carbonate shoal deposit based on subsurface mapping and dolomite lithology the main production is interpreted to come from matrix permeability in the field area occasional vertical and horizontal fractures are noted in cores the vertical pay ranges in thickness from 8 to 14 ft 2 4 to 4 3 m the bakken is slightly overpressured with a pressure gradient of 0 53 psi ft 0 02 kpa m horizontal wells are drilled on 640 to 1280 ac 259 to 518 4 ha spacing units long single laterals dual laterals and trilaterals have all been drilled in the field the horizontal intervals are sand gel and water fracture stimulated initial production ranges from 200 to 1900 bopd 31 8 to 302 1 m3 day initial potential rates for vertical wells are generally less than 100 bopd 15 9 m3 day the upper bakken shale probably also contributes to the overall production in the field the exact contribution is unknown but estimated to be less than 20 of the total production the elm coulee field illustrates that the bakken petroleum system has enormous potential for future oil discoveries in the williston basin

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea 1 high resolution bathymetry at centimeter scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid shaped large i e length 10 100 m to very large i e length 100 m dunes in the gradyb tidal inlet channel in the danish wadden sea the development of these dunes is due to an increase in dune celerity from 12 m yr in the center of the channel to around 30 m yr at the sides this increase in dune celerity can be explained by the fact that dune heights decrease from 3 1 m in the center of the channel to 1 4 m at the sides as a smaller sediment volume has to be moved per unit time for equal dune celerity water depth is uniform across the channel likewise high resolution acoustic doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow thus no correlation between dune dimensions and water depth or flow velocity was established instead high accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0 63 mm in the center to 0 36 mm at the sides of the channel the decrease in dune height is ascribed to this decrease in grain size given that flow depth and flow velocity are uniform across the channel the lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough lee side region of the barchanoid shaped dunes

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

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

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

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea development of barchanoid shaped dunes

sedimentary processes of fine grained material and the effect of seawall construction in the daeho macrotidal flat nearshore area northern west coast of korea abstract sedimentary processes on the lower tidal flat of the reclaimed seosan bay daeho tidal flat at the west coast of korea have undergone significant changes since 1984 when seawalls over 8 km long were constructed at the bay mouth comparison of pre and post construction tidal flow models indicates that in general tidal currents have increased considerably in front of the daeho tidal flat with tidal flow directions having changed from shore normal to shore parallel the 12 h measurements of currents and suspended sediment concentrations at four anchored hydrographic stations show nearshore suspended sediments drifting southward throughout the year bypassing the daeho tidal flat cyclic seasonal variations occur prominently in the grain texture of the tidal flat sediments and associated sedimentation rates in summer fine grained sediments deposit at a rate of 2 6 mm month particularly in sheltered places during the winter monsoon erosional processes are greatly enhanced by the interplay of tidal currents and waves up to 30 mm month this sedimentary seasonality is reflected in the variations in the concentration of nearshore suspended matter the concentration of suspended matter is much higher in winter 60 80 mg l because of vigorous resuspension activities compared with the summer concentration 10 30 mg l the direct effect of seawall construction coupled with strong natural seasonality can be seen from the continuous monitoring of sedimentation rates on the tidal flat the 2 year 1993 1995 measurements indicate that the entire daeho tidal flat was eroded during that period at the abnormally high average rate of 10 cm year the causal blocking of bayward sediment and water movements by any artificial structure might lead to similar erosional conditions to many other bays facing the yellow sea on the both chinese and korean coastlines subjected to energetic winter wave attack

a study of the dynamics of a marine sandwave the movement of the crest of a sandwave was studied using cross sectional profiles obtained from lines of sea bed reference stakes measurements were made over a six month period before and after flood and ebb tides in relation to both spring and neap tides and surface wave conditions additional observations were obtained on a daily basis over an equinoctial neap to spring to neap tidal period in conjunction with boundary layer flow measurements tracer experiments were conducted to study the dispersion of sediment from the sandwave crest the results showed that the sandwave was relatively stable at neap tides whilst at higher tidal ranges the crest position oscillated with successive flood and ebb tides net flank erosion occurred on the less steep upstream slope during the dominant ebb tide this together with increased deposition on the lee slope caused the crest to advance it was not possible to extrapolate sandwave migration over long periods as the tidal dynamic trends were interrupted by wind stress and surface wave activity high particle orbital velocities generated at the sea bed by storm waves caused major reductions in crestal heights calculated volumes of sediment eroded and accreted were used with boundary layer flow measurements to calculate threshold velocities for the movement of the sediment and sediment transport rates

coupling bedform roughness and sediment grain size sorting in modelling of tidal inlet incision abstract a key problem in the decadal morphodynamic modelling of tidal inlet system is on the evolution of the inlet channels especially the unrealistic channel incision the present study attempts to couple two processes including the bedform roughness and sediment grain size sorting in the two dimensional depth averaged process based model to solve the problem based on the comparison of the modelling results and relevant field observations in the dutch wadden sea it is suggested that bedform roughness height predictor is a useful tool for determining spatial and temporal heterogeneous bed frictions within tidal channels bedform roughness is dominated by dune roughness in order to represent the memory of old bedforms a relaxation time method was implemented the corresponding large relaxation time 2 m2 tidal periods smooths the tidal variations of dune roughness and makes it respond to hydrodynamic changes on a longer time scale when decadal morphological modelling is carried out the model performance can be significantly improved by introducing either bedform dune roughness predictor or sediment sorting processes the mechanisms are twofold first in the deep channels where flow velocities are high large dunes develop and cause large bed drag coefficients second coarser bed sediments within the channel area induced by sorting prevent the channel incision if both effects are coupled in the morphodynamic modelling the inlet channel incision is further controlled which is not just a result of the linear superposition of both effects because sorting can further promote the development of dunes within the deep channels

modeling of gas generation from the barnett shale fort worth basin texas the generative gas potential of the mississippian barnett shale in the fort worth basin texas was quantitatively evaluated by sealed gold tube pyrolysis kinetic parameters for gas generation and vitrinite reflectance ro changes were calculated from pyrolysis data and the results used to estimate the amount of gas generated from the barnett shale at geologic heating rates using derived kinetics for ro evolution and gas generation quantities of hydrocarbon gas generated at ro 1 1 are about 230 l t 7 4 scf t and increase to more that 5800 l t 186 scf t at ro 2 0 for a sample with an initial total organic carbon content of 5 5 and ro 0 44 the volume of shale gas generated will depend on the organic richness thickness and thermal maturity of the shale and also the amount of petroleum that is retained in the shale during migration gas that is reservoired in shales appears to be generated from the cracking of kerogen and petroleum that is retained in shales and that cracking of the retained petroleum starts by ro 1 1 this result suggests that the cracking of petroleum retained in source rocks occurs at rates that are faster than what is predicted for conventional siliciclastic and carbonate reservoirs and that contact of retained petroleum with kerogen and shale mineralogy may be a critical factor in shale gas generation shale gas systems together with overburden can be considered complete petroleum systems although the processes of petroleum migration accumulation and trap formation are different from what is defined for conventional petroleum systems

fractionation of carbon and oxygen isotopes in 13c rich palaeoproterozoic dolostones in the transition from medium grade to high grade greenschist facies a case study from the kola superdeep drillhole samples from the kola superdeep drillhole 12 262 m a deep drillhole 1060 m and from the surface seaprated by only around 10 km provided a unique opportunity for direct tracing of 13 c and 18 o changes through a low to high grade greenschist facies transition within impure 13 c rich palaeoproterozoic dolostones the least altered dolostones have 13 c of 9 and 18 o of 22 the metamorphic transition is expressed by dolomite calcite 1 quartz k feldspar tremolite calcite 2 dolomite calcite 1 and defined by 13 c depletion of calcite 2 c 3 0 calcite 1 1 0 2 0 and dolomite 18 o shows a considerable resetting in all carbonate components by around 6 caused by a rayleigh distillation process coupled with isotopic exchange between the carbonates and fluids with an external source of oxygen the retrograde alteration is expressed by the formation of quartz chlorite veinlets within tectonically bound zones of brecciated and sheared dolostones the maximum 18 o depletion in dolomite 9 and calcite 1 c 4 were probably controlled by infiltration into permeable zones of external fluids associated with retrograde alteration 13 c remains largely unaffected

geochemical study of crude oils from the xifeng oilfield of the ordos basin china abstract the xifeng oilfield is the largest newly discovered oilfield in the ordos basin in order to determine the possible source crude oils collected systematically from the oilfield and an adjacent oilfield have been examined isotopically and molecularly the predominance of long chain n alkanes high abundance of c29 sterane lower ratios of c25 c26 tricyclic terpane and c25 tricyclic terpane c24 tetracyclic terpane and high c24 tetracyclic terpane c24 tetracyclic terpane c26 tricyclic terpanes ratio in the studied oils suggest generation from a source with mixed terrigenous and algal bacterial organic matter the presence of diterpenoid hydrocarbon with abietane skeletons is characteristic of the main contribution of higher land plants to the oils the biomarker distributions in the oils show that they were formed under a weakly reducing freshwater environment molecular maturity parameters indicate that the crude oils are mature the oil source rock correlation and oil migration investigation suggest that the oils in the xifeng oilfield originated from the source rocks of the yanchang formation deposited in a shallow to deep freshwater lacustrine environment especially chang 7 source rocks the data from the distribution of pyrrolic nitrogen compounds indicate that the charging direction of the chang 8 crude oils is mainly from the zhuang 12 well northeast of the oilfield toward the southwest this direction of oil migration is consistent with that indicated by regional geological data

predicting ripple roughness and sand resuspension under combined flows in a shoreface environment ripple measurements and flow and sediment dynamical data obtained from the shoreface of the middle atlantic bight using instrumented tripods were analyzed to evaluate various predictors of ripple geometry and roughness ripple roughness controls on sand resuspension and suspended sediment concentration profiles under combined waves and currents were also evaluated the limited observation of sand ripples in the field indicates that the grant and madsen 1982 method overestimates ripple roughness while the nielsen 1981 method tends to under predict ripple roughness a modified ripple predictor is thus proposed based on the grant and madsen method and it is shown to give reasonable predictions under the present experiment conditions this modified ripple prediction along with wave current and suspended sediment concentration data recorded by the tripods were then brought into the combined flow bottom boundary layer model of grant and madsen 1986 and the modified rouse equation of glenn and grant 1987 to calculate sand resuspension coefficient 0 and to predict suspended sediment concentration profiles it was found that under low energy fair weather conditions sand ripples are in the equilibrium range and ripple roughness increases with the bed shear stress this causes strong vortex activity close to seabed and thus higher resuspension coefficient reference concentrations are moderate due to this high resuspension coefficient even though bed shear stresses are low under moderate storm conditions ripple break off occurs and ripple roughness will decrease with bed shear stress this reduces the vortex activity and hence sand resuspension coefficient 0 the combination of this moderately high bed shear stress and reduced but still moderate ripple roughness favours sand suspension and produces the highest reference concentration for the encountered experimental conditions as bed shear stress is further increased ripples are nearly washed out and sand resuspension coefficient is further decreased approaching the previously suggested constant value of 1 3 10 4 this corresponds with the lowest reference concentration despite the high bed shear stress suspended sediment concentrations predicted by the modified rouse equation using this time variable resuspension coefficient and properly calculated bottom boundary layer parameters are reasonable compared to the field measured concentration profiles

microtextures in mudrocks as revealed by backscattered electron imaging abstract this report is one of the first attempts to examine polished mudrock sections with backscattered scanning electron microscopy bse in order to image characterize and understand their microtextures as well as to help reconstruct sedimentary and diagenetic environments over 180 polished thin sections of mudrocks from precambrian to early middle tertiary age were examined in this study using bse seven common microtextures have been identified in the samples examined with bse microlaminations fossil forms pyrite clay minerals and micas chemical and physical alteration glauconization and porosity microlaminations have been examined at the single grain level foraminifera in the gulf coast tertiary reflect a sequence of preservation relations from shallow depths to a few thousand meters fecal pellets in a black mudrock from spitsbergen of upper jurassic age contain what appear to be well preserved fossil vertebrates replaced by pyrite the chemical and physical variations within micas and clay minerals seem to be the result of selective alteration along cracks and defects in the minerals and these have been imaged mineral growth and dissolution at the submicron level can be imaged particularly in the clay minerals quartz and feldspar glauconite growth conditions geochemical complexities of carbonate formation and the size shape and orientation of micron sized pore spaces in shales have been obtained thus the microtextural analysis of mudrocks not only is possible but can be accomplished in great detail with existing techniques

sediment connectivity a framework for analyzing coastal sediment transport pathways connectivity provides a framework for analyzing coastal sediment transport pathways building on conceptual advances in graph theory from other scientific disciplines connectivity schematizes sedi