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

anisotropy of schists contribution of crustal anisotropy to active source seismic experiments and shear wave splitting observations we have made sets of five independent compressional and shear wave velocity measurements which with density allow us to completely characterize the transverse isotropy of samples from five metamorphic belts the haast schist terrane south island new zealand poultney slate chugach phyllite coldfoot schist and pelona schist united states these velocity measurements include compressional wave velocities for propagation parallel perpendicular and at 45 to the symmetry axis shear wave velocity for propagation and particle motion perpendicular to the symmetry axis and shear wave velocity for propagation parallel to the symmetry axis velocity measurements were made up to pressures of 1 gpa 35 km depth where microcracks are closed and anisotropy is due to preferred mineral orientation our samples exhibit compressional wave anisotropy of 9 20 as well as significant shear wave splitting metamorphic terranes that are anisotropic to ultrasonic waves may also be anisotropic at the scale of active and passive seismic experiments our data suggest that a significant thickness 10 20 km of appropriately oriented steeply dipping foliation schist in the crust could contribute as much as 45 of observed shear wave splitting our data set can also be used to model the effects of crustal anisotropy for active source seismic experiments in order to determine if the anisotropy of the terrane is significant and needs to be taken into account during processing and modeling of the data

shear stress and hydrodynamic recovery over bedforms of different lengths in a straight channel abstractpools and riffles are common morphological features in rivers that are frequently used but poorly specified analogs in restoration design here straight two dimensional 2d bedforms are conceptualized as perturbations and flow recovery is measured in a laboratory flume with an array of ultrasonic doppler velocity profilers udvps the objectives are to 1 assess the variation of skin friction turbulent stresses and total stress 2 assess the role of topographical feedback on flow recovery and 3 compare flow recovery in isolated and bedforms in series the results show that the total shear stress and near bed turbulence greatly exceed the skin friction in decelerating flow and the pool and that hydrodynamic recovery tends to occur at length scales similar to geophysical scales despite potential negative feedback from the bed repeating short bedforms can push the flow to a more turbulent and laterally concentrated equilibrium condition implications for sediment entrainment thresholds e

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

anomalous diffusion of electromagnetic eddy currents in geological formations controlled source electromagnetic em induction in some geological formations isshown here to be compactly described by an anomalous subdiffusion process such aprocess which is not universal is governed by a fractional diffusion equation oralternatively the convolutional form of ohm s law a subdiffusing eddy current vortex or electromagnetic smoke ring propagates in such a way that its position of medianintensity overruns its position of peak intensity this behavior is not allowed in classicaldiffusion but is a simple consequence of diffusion within a stationary fractal medium a similar analysis has been applied to understand heavy tailed traveltime distributions thatappear in certain hydrological time series the tell tale signature of anomalouselectromagnetic diffusion is a slope b of the magnetic zero crossing moveout curve that isconstant with transmitter receiver rx offset and significantly different from unity neither lateral heterogeneity nor unixial anisotropy can generate such a constant slopemoveout curve with an economy of model parameters controlled source em data fromtwo sites in texas and one in new mexico are used in this study to test the eddy currentsubdiffusion hypothesis

carbon isotope reversal and its relationship with natural gas origins in the jingbian gas field ordos basin china abstract the geochemical characteristics of natural gases from lower paleozoic jingbian gas field are abnormal and the carbon isotope of most natural gases has reversed which led to a dispute over the source of natural gas in the lower paleozoic the upper paleozoic yulin gas field is adjacent to the jingbian gas field but the geochemical characteristics of the yulin gas field did not appear abnormal besides the ethane carbon isotope of upper paleozoic yulin gas field is much higher than that of lower paleozoic jingbian gas field in order to identify the reasons for the carbon isotope reversal of natural gas in lower paleozoic jingbian gas field and its relationship with natural gas source we conducted the components and stable carbon isotopes of natural gas from upper paleozoic yulin gas field and lower paleozoic jingbian gas field on the basis of these experiments we compared the natural gas components and carbon isotope of 100 samples from both upper paleozoic yulin gas field and lower paleozoic jingbian gas field the results indicate that higher maturity will break the conventional rule when maturity reaches a certain threshold 13cethane value will decrease with maturity although 13cmethane value will also increase with maturity the extent will be relatively smaller this leads to unusual 13cmethane 13cethane in the region with higher maturity in the process of downward migration of coal derived gas from carboniferous permian coal measures component fractionation and carbon isotope fractionation might occur resulting the ethane components and carbon isotope value of lower paleozoic less than those of upper paleozoic which will lead to the carbon isotope reversal the mixing of oil derived gas and coal derived gas at different ratios may also lead to the reversal of carbon isotopes the natural gas types of jingbian gas field are mainly coal derived gas generated from carboniferous permian coal measure strata with a small amount of oil derived gas generated from carboniferous permian marine terrigenous limestone

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

occurrence characteristics and influential factors of movable fluids in pores with different structures of chang 63 reservoir huaqing oilfield ordos basin china abstract exploration and development of chang 63 reservoir in the huaqing oilfield ordos basin is challenging because of the complex geological features including strong heterogeneity complicated microscopic pore structures low saturation of movable fluids and distinct fluid distribution characteristics confronted with this challenge this study used the nuclear magnetic resonance nmr technique to quantitatively evaluate the movable fluids in pores with different structures moreover microscopic experiments were conducted to analyze the occurrence characteristics and influential factors of movable fluids in pores with different structures including conventional physical property analysis image porosity measurement casting thin section observation sem analysis high pressure mercury intrusion testing and constant pressure mercury intrusion testing the results showed that gray and grayish brown lithic feldspar sandstones were the dominant types of sandstone residual intergranular pores and feldspar dissolved pores were the main pore types with unevenly distributed pore throat radii among which small pores and micropores were primary four types of pore structures could be divided according to comprehensive analysis of capillary pressure curves namely types i ii iii and iv and they were featured by movable fluids with distinct occurrence characteristics in them reservoir permeability was well correlated with movable fluid saturation multiple factors were demonstrated to influence the movable fluid saturation in pores with different structures including pore throat radius pore throat radius ratio sorting coefficient effective pore and throat volume maximum mercury saturation in pores and throats and interstitial clay minerals generally good physical properties led to larger variation of movable fluid saturation among them effective pore throat radius and pore throat radius ratio were the dominant factors effective pore and throat volume as well as sorting coefficient had some degree of influence and maximum mercury saturation and physical properties had little effect on the movable fluid saturation

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

two episodes of structural fractures numerical simulation of yanchang oilfield in the ordos basin northern china abstract the tight oil sand reservoirs of the yanchang oilfield in the upper triassic yanchang 6 7 and 8 formations t3yc6 7 8 fms in the ordos basin are in the exploration stage two episodes of fractures during the yanshanian and himalayan periods observed in outcrops and cores of the t3yc6 7 8 fms are thought to be the controlling factors for the tight oil sweet spot finite element numerical simulation fes technology is used to perform a three dimensional 3d fracture simulation for a large region during the exploration stage to serve in tight oil reconnaissance for the yanchang oilfield the innovations of our work are mainly shown in the following simulation process 1 three controlling factors including the paleotopography layer thickness and rock mechanical faces are used to construct the 3d geologic model in the simulation 2 the rock mechanical environment characterized by the an gou qiliha zhangjiatan and dongjiahe outcrops the y1011 well cores and the corresponding mechanical experimental parameters is divided into three types as follows the delta front tractive current sand faces df tr s the lacustrine turbidity current sand faces l tu s and the mud faces m 3 the spatial boundary stress conditions are calculated based on the newberry formula for tensional and compressional stress fields considering the synthetical effects of the overburden pressure the pore fluid pressure and the tectonic stress 4 the modified griffith s criterion is used to fit the fracture index by simultaneously taking into consideration the tension and shear fracture mechanisms for the simulation results the predicted fracture index distribution map agrees well with the measured fracture density the yanshanian fractures mainly propagated in the dingbian and an bian areas adjacent to the western thrust belt while the himalayan fractures mainly propagated in the zhidan fuxian and huangling areas adjacent to the weihe graben the simulated fracture index has a positive correlation with the stratigraphic dip and a negative correlation with the layer thickness additionally structural fractures are prone to propagate near the mechanical interface in either the upward or downward penetration direction and are assumed to produce a fracture type sweet spot section in the vertical direction the reservoir oiliness also increases with the simulation fracture index indicating the fracture type tight oil sweet spot of the yanchang oilfield moreover the critical components used in the 3d fracture fes including the 3d geologic model the spatial boundary stress conditions and the fracture index fitted by the modified griffith criterion can be of great significance to produce more accurate numerical simulations during the oil and gas exploration stage in large regions

lacustrine paleoshoreline determination under established sequence stratigraphic framework and its controls on sand bodies and hydrocarbon accumulations abstract the lacustrine palaeoshoreline is the intersection between lake level and ancient land in geological time and it marks the boundary between onshore deposits and subaqueous deposits identifying lacustrine paleoshorelines is the process of reconstructing past depositional environments the study establishes the sequence stratigraphic framework of the ordos yanchang formation based on a comprehensive analysis of seismic and well log data the hierarchical lithological cycles is determined through fourier transform and band pass filtering analysis of gramma ray data subsequently the lacustrine paleoshoreline is determined using the shoreline hierarchical control method under the established sequence stratigraphic framework with additional evidence from cores well logs and cutting and seismic sections lacustrine paleoshorelines has controlling effects on the sand bodies and hydrocarbon accumulations for one thing different types of paleoshoreline control the genesis of sand bodies and the location of paleoshoreline in sag basin controls sand body distribution furthermore the migration of lacustrine paleoshoreline in sag basin indicates stacking patterns of sand bodies and hydrocarbon accumulations

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

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

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

major factors controlling lamina induced fractures in the upper triassic yanchang formation tight oil reservoir ordos basin china abstract lamina induced fractures lf are abundant in lacustrine formations playing an important role for the tight oil resource exploration studies of controlling factors of the lf are still lacking especially for the heterogeneous formations characterized with complex combinations of the lamina and surrounding rock therefore 28 long column samples for triaxial compression tests 37 short column samples for brazil split tests and 12 powder samples for mineral component analysis and isotopes analysis carbon and oxygen are made from 6 cores in 4 petrophysical faces the experiments are designed to study the lf propagation process considering the controlling factors in terms of mineral component the lamina angle and the petrophysical faces our results show that clay content and dolomite cement degree more significantly control the lf propagation than the other minerals the lf propagation is not only dependent on the mineral component in the lamina but even more on the mineral component difference between the lamina and the surrounding rock i e the l s values where l represents the lamina and s represents the surrounding rock adjacent to the lamina the lf tended to propagate in a middle lamina angle range approximately 10 60 in shear properties and characterized with lower compression strength values all the lf observed in the thin sections were associated with oil although there was no obvious oil shown in the study cores the 13c and 18o values of lamina samples varied with the lamina mineral component for the complex organic matter oxidation and degradation reactions additionally the hydrocarbon flow parameter phf has been proposed on the carbon and oxidation isotope trace lines to indicate the oil rock reactions during the lf open period

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

origin of calcite cements and their impact on reservoir heterogeneity in the triassic yanchang formation ordos basin china a combined petrological and geochemical study abstract calcite cement is a typical product of fluid rock interaction in petroleum reservoirs and its migration has a major impact on reservoir heterogeneity a case study of a closed diagenetic system was carried out on the upper triassic yanchang formation of ordos basin china the most successful exploration region for tight sandstone oil in china the investigated sandstones are mainly lithic arkoses q31f42l27 three occurrences of calcite cement are the most important diagenetic minerals affecting reservoir quality early calcite completely filling intergranular volumes late calcite filling pores after chlorite coating distributed in oil layers or locally replacing feldspar and crystal stock calcite typically distributed in dry layers 13c values of the sandstones are in the range 9 69 to 2 05 indicating that calcite is derived mainly from thermodynamic decarboxylation of organic matter dark mudstone interbedded with sandstone and hydrocarbon emplacement are the two crucial sources of organic fluid providing sufficient ca2 mn2 fe2 and mg2 for calcite precipitation through interaction with feldspar and other silicates such as dissolution and replacement together with transformation of smectite to illite manganese was continuously enriched in calcite during burial diagenesis driven by compaction of interbedded mudstone and fluid overpressure calcite migrated widely into calcite cemented zones cczs where fluid rock interaction was strong such as in the margins of sandstone bodies intrastratal sandstones with original high permeability zones and thin sandstones thereby forming an important organic co2 sink potential high quality reservoirs are distributed mainly in the central part of the closed system restricted by calcite cemented zones and are an exploration target our data suggest that frequently alternating sandstone and mudstone sequences comprise a complete organic co2 source sink system controlling the migration of calcite cement in a closed diagenetic system and reservoir heterogeneity by fluid rock interaction

sedimentary and diagenetic controls on reservoir quality of low porosity and low permeability sandstone reservoirs in chang101 upper triassic yanchang formation in the shanbei area ordos basin china abstract there are many reservoirs with low porosity and low permeability in the world and many large oilfields have been found in these reservoirs unlike conventional reservoirs the genesis of low porosity and low permeability reservoirs and the controls of relatively high porosity and high permeability zones are still unclear this study takes the chang101 reservoir sandstones of the shanbei area in the ordos basin china a typical low porosity and low permeability as an example chang101 sandstones are fine grained to medium grained moderately to well sorted arkose and lithic arkose deposited in a delta plain environment it is a typical low porosity and low permeability reservoir with an average porosity of 10 21 and an average permeability of 2 88 md pore types mainly include primary intergranular pores and secondary dissolved pores the main controls on the reservoirs are the sedimentary environment and diagenesis the sedimentary environment determines the initial properties of the reservoir sandstones including the material composition and texture that determine the primary porosity and permeability diagenesis further transforms the reservoir quality to develop heterogeneity in the porosity and permeability the reservoir sandstones have undergone complex diagenetic processes and are currently in stage a of mesodiagenesis compaction accounts for the major reduction of the primary porosity of the sandstones different types of cementation have both positive and negative influences on reservoir quality carbonate cementation quartz overgrowths and some authigenic clay cementation i e i s and illite are major pore filling components that reduce the reservoir quality an appropriate amount of chlorite resists compaction and prevents quartz overgrowth but excessive chlorite coatings fill the pores and block the pore throats the laumontite cement is not only resistant to compaction but also easily dissolved to form a large number of effective secondary pores the dissolution of feldspar and rock fragments plays a role in contributing to connecting the isolated intergranular pores and increasing the secondary pores relatively high porosity and high permeability zones are deposited in high energy environment mainly delta plain distributary channel sandstones with more detrital quartz fewer rock fragments and coarser grains than other sandstones were deposited in high energy environment there is weaker compaction and less or no carbonate cement with the preservation of the primary pores by chlorite coatings and an increase in the pores by the dissolution of laumontite cement

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

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

the detection of lateral anisotropy of the ocean floor by electromagnetic methods summary in a typical marine controlled source electromagnetic csem experiment one or more self contained receivers capable of measuring small variations in the electromagnetic field are located in an array on the sea floor a remote horizontal electric dipole is towed close to the sea floor by a surface vessel variations in the current through the dipole cause correlated variations in the electric and magnetic fields at the receivers the variations contain information about the distribution of electrical conductivity of the subsurface rocks the electrical conductivity is related to such critical physical parameters as porosity temperature and fluid content published studies of the response of the sea floor to electromagnetic excitation have not included any systematic variation in the sea floor conductivity with horizontal direction a horizontally isotropic oceanic lithosphere is not an appropriate assumption particularly in the vicinity of the ridge here for example in the near surface layers basalts are extruded onto the sea floor within the very narrow fissured neo volcanic zone the fissures may be filled with sea water and sediment which are better conductors than typical basalt alternatively as time progresses they may become sealed with hydrothermal mineral deposits which can be less conductive than the host rocks in both instances the electrical conductivity of the composite material in the direction oriented along the fisures parallel to the ridge will be greater than that in the transverse direction measures of anisotropy on sea floor of varying age are an indication of the history of the tectonic activity at the ridge the electromagnetic effects of lateral anisotropy are examined through the study of the response of a simple double half space model an upper half space representing sea water is connected to a lower half space representing anisotropic crustal material a general theory describing the excitation of this model by any transient electromagnetic source is developed and the step on responses for systems composed of a horizontal electric dipole transmitter and horizontal magnetic and electric field receivers are computed the results of previous work indicate that following a change in current in the transmitter two distinct changes in horizontal electric field strength separated in time are observed at the receiver the first is caused by the diffusion of the electromagnetic field through the relatively resistive sea floor the second is caused by diffusion through the relatively conductive sea water the times at which the two events occur are robust measures of the isotropic conductivity of the sea floor and the sea water respectively the results of this study show that shape and time of the initial change in the electric field varies systematically with both the average conductivity and the anisotropy of the sea floor for some orientations of the transmitter and the receiver the effect of increasing the anisotropy is similar to the effect of increasing the average conductivity the initial event is delayed however there are orientations for which increasing the anisotropy actually results in an earlier arrival natural science and engineering research council in canada

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

quantitative identification of diagenetic facies and controls on reservoir quality for tight sandstones a case study of the triassic chang 9 oil layer zhenjing area ordos basin abstract the identification of diagenetic facies is essential for prospecting hydrocarbon reserves and for hydrocarbon exploration in tight oil reservoirs a comprehensive method for determining quantitative index of reservoir diagenetic facies rdi using core analysis techniques hierarchical cluster analyses hca and grey correlation analyses gca was proposed the petrological characteristics physical properties and pore structures of the chang 9 tight oil sandstones are analysed based on thin section analysis x ray diffraction xrd scanning electron microscopy sem cathode luminescence cl and mercury injection capillary pressure micp tests the properties of the chang 9 sandstones were controlled by different forms of diagenesis compaction and cementation of calcite have reduced or occluded the pore volume in the sandstones but not all cementation is deleterious to reservoir quality chlorite cementation helped preserve relict primary porosity and the direct dissolution of feldspars and unstable lithic fragments has resulted in enhanced pore connectivity following petrographic observations and quantitative analysis the sandstones were divided into four diagenetic facies calcite cementation facies intense compaction facies porosity preserved by grain coating chlorites facies and dissolution facies the intense compaction facies and calcite cementation facies are responsible for the evolution of low quality reservoirs while the reservoirs with porosity preserved by grain coating chlorites facies and dissolution facies are of better porosity and permeability the distribution of fractures improves reservoir quality by enhancing the dissolution of unstable grains and providing convenient channels and storage spaces for oil and gas migration

coupling relationship between tight sandstone reservoir and gas charging an example from lower permian taiyuan formation in kangning field northeastern ordos basin china abstract the lower permian taiyuan formation is a lithic abundant tight gas sandstone reservoir in the kangning field northeastern ordos basin to clarify the gas accumulation process in tight gas sands the relationship between the densification of taiyuan formation sandstones and gas charging is analyzed core porosities of the taiyuan formation are mostly below 10 and air permeabilities are below 1 md the sandstone reservoirs are characterized by strong heterogeneity dissolution pores and large amounts of micropores in clay minerals dominate the storage space this porosity is the result of the impact of diagenesis porosity evolution modeling confirmed well the prediction of the period of sandstone densification 225 215 ma based on the basin analysis as well as diagenetic calibration strong compaction dominated the physical property porosity and permeability reduction with cementation as a subordinate factor the integration of burial history and fluid inclusion data suggested that large quantities of gas migrated into the taiyuan sandstone reservoirs from 176 ma to 80 ma when the reservoirs were already tight porosity

diagenetic alteration pore throat network and reservoir quality of tight gas sandstone reservoirs a case study of the upper paleozoic sequence in the northern tianhuan depression in the ordos basin china the sulige gas field in the ordos basin is the largest gas field in china the studied area the northern tianhuan depression is located west of the gas field and is considered as a potential gas producing area the complex diagenetic alteration history and its effects on the pore throat network and reservoir quality of the tight gas sandstones are explained the upper paleozoic first member of the shanxi formation and eighth member of the shihezi formation tight gas reservoir rocks are composed of coarse to medium grained sandstones and are dominated by sublitharenites the most important diagenetic factor for porosity reduction was compaction followed by cementation diagenetic mineral phases include quartz overgrowth calcite cement and replacement and kaolinite cement and replacement with minor heulandite and chlorite pressure dissolution of quartz grains and quartzite fragments is the main source of silica for the early and late quartz overgrowth cementation formed mainly from 120 c to 140 c the hydrolysis and alteration of volcanic material and mica flakes released some silicon calcium and aluminum ions calcium and carbon in calcite cements could be derived from surrounding mudstones calcite replaced the quartz overgrowths the formation of kaolinite as cement and replacements was controlled by the interactions of silica aluminum ph and temperature 140 c because of deformation of detrital components mica flakes and ductile rock fragments and diagenetic replacement clear negative correlations between reservoir quality and contents of the diagenetic minerals are lacking a complex and isolated pore throat network was created by strong diagenetic transformation based on analysis of thin sections three dimensional x ray micro computed tomography and constant rate mercury injection data the samples with relatively good reservoir quality porosity 10 and permeability 0 5 md are characterized by dissolution pores and primary intergranular pores and commonly contain more and larger pore throats both in volume counts 1800 and size 0 02 per unit volume and diameter 1 20 m than the samples with poor reservoir quality the intercrystalline pores of kaolinite are very widespread and considered favorable for oil and gas accumulation because some asphalt was found within these pores this study may be used by analogy to better understand diagenetic factors controlling the formation of tight gas sandstone reservoirs and predict the development of pore throat systems

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

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

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

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

differentiating anisotropy and lateral effects using azimuthal resistivity offset wenner soundings azimuthal resistivity surveys increasingly are being used by hydrogeologists in the identification and characterization of fractured rocks in these investigations electrical resistivity is measured as a function of azimuth about a fixed central point in most recent published examples any observed change in apparent resistivity with azimuth is interpreted as being indicative of fracture anisotropy however interpretation of rotational sounding data is actually more complicated as azimuthal variations in apparent resistivity are also produced by the presence of dipping stratigraphy and other lateral changes in formation resistivity such effects are generally overlooked because the field techniques normally employed are incapable of detecting them consequently it is quite probable that the results of many published surveys have been wrongly interpreted an alternative field procedure and interpretation methodology has been developed to differentiate anisotropy dipping layers and lateral effects this approach makes use of the offset wenner technique and examines the different responses of the individual wenner resistances above different geological structures analysis of data obtained using the azimuthal offset wenner technique from sites in britain and ireland have successfully identified subsurface structures and determined the anisotropy where it is present

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

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

controls of diagenetic alteration on the reservoir quality of tight sandstone reservoirs in the triassic yanchang formation of the ordos basin china abstract the ordos basin is rich in tight oil resources and the upper triassic yanchang formation tight oil reservoir is one of the most important exploration targets in the study area the chang 63 tight sandstone reservoir exhibits lacustrine deep water lobe shaped delta deposits with low porosity low permeability and a complex diagenetic alteration history the reservoir quality is extremely heterogeneous because of the heterogeneity of the depositional lithofacies and diagenetic alterations this research integrates multiple techniques to investigate the depositional lithofacies the characteristics and distributions of various diagenetic alterations and their controls on reservoir quality four lithofacies are identified and the various lithofacies have different diagenetic characteristics and diverse reservoir qualities the chang 63 tight sandstone reservoir has experienced significant diagenetic alteration mechanical compaction and authigenic illite are the most important causes of the decrease in reservoir quality carbonate cementation preferentially occurs in sandstones of lithofacies i and ii near the mudstone sandstone contact surfaces sandstones within 1 m of adjacent mudstone sandstone contact surfaces are usually tightly cemented by carbonate cement illite can be divided into two types according to its morphology both types of illite lead to a decrease in reservoir quality ultimately by combining the relationship between depositional facies and lithofacies with the distribution of different diagenetic processes five evolutionary patterns are identified for reservoir quality in the lacustrine deep water lobe shaped delta deposits can be summarized by ratings of best moderate relatively poor poor and worst

characteristics of micropores pore throats and movable fluids in the tight sandstone oil reservoirs of the yanchang formation in the southwestern ordos basin china thin section analysis field emission scanning electron microscopy constant rate mercury injection crmi x ray computed tomography x ct and nuclear magnetic resonance nmr were used to investigate pore systems pore sizes and pore throat distributions and determine evaluation criteria for tight oil reservoirs this research shows that the average porosity of the reservoirs in the chang 63 to chang 72 members of the yanchang formation of the ordos basin ranges from 4 to 16 with a corresponding average permeability of 0 1 to 0 4 md the main pore types in these tight oil reservoirs are dissolution pores and intergranular pores and the main pore throat types are laminated throats and control shaped throats based on experimental results the pore throat distributions differ among different testing techniques the results of this study indicate that a combination of crmi derived and x ct detected pore throat distributions is most suitable for calibrating nmr derived pore sizes over the full range of pore sizes the lower pore throat radius limit of effective reservoirs is 0 24 m and the lower permeability limit for the flow of movable fluid is 0 008 md based on a cluster analysis the studied tight oil reservoirs can be divided into three types compared with type iii reservoirs porosity 11 and permeability 0 15 md and type ii 8 porosity 11 and 0 05 md permeability 0 15 md reservoirs are of relatively good quality

tectonic fabric of the subcontinental lithosphere evidence from seismic magnetotelluric and mechanical anisotropy abstract knowledge of anisotropy is key to our understanding of tectonic fabrics in the lithosphere and sublithospheric mantle anisotropy provides a unique constraint on the character of past and present deformation and thus is critical to our understanding of dynamics of the plate tectonic system in particular how continents with their thick lithospheric mantle roots form stabilize and interact with underlying mantle regions improved observational methods and rapid developments of geophysical inversion techniques are contributing to more realistic earth models that include various types of anisotropy and heterogeneity a more robust characterization of anisotropic parameters can be achieved by integrating complementary datasets incorporating constraints from seismology magnetotellurics geodynamics and mineral physics valid comparisons between different measures of anisotropy however require a solid understanding of the principles and limitations of the various techniques to this end we briefly summarize a new compilation of original articles with a common focus on anisotropy of continental lithosphere including theory laboratory experiments and observations that span a range of different geophysical techniques

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

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

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

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

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

the diagenetic controls of the reservoir heterogeneity in the tight sand gas reservoirs of the zizhou area in china s east ordos basin implications for reservoir quality predictions abstract the shan 23 sandstone formations are important exploratory targets for tight gas resources in the zizhou area of the ordos basin in this study we investigated the effects of diagenesis on the heterogeneity of tight sandstone deposits in similar sedimentary facies and identified the main controlling factors of reservoir quality a total of 169 samples were collected from 50 wells for a series of analyses the heterogeneity of the sandstones is mainly controlled by cement and pore throat characteristics which showed regional differences in the study area consistent with the regional variations in the detrital components these results indicate that the diagenesis influenced by the sediment composition is the main controlling factor of the reservoir heterogeneity the compaction dominated by ductile grains is the major cause of the primary porosity losses in the sandstones followed by the quartz and clay kaolinite and illite cementation residual intergranular pores rarely occur in the sandstones with a ductile lithic grain content greater than 2 altered kaolinite and recrystallized illite commonly occur in the sandstones with high ductile grain content residual intergranular pores quartz overgrowths and authigenic kaolinite are the most common phases in the sandstones rich in detrital quartz the dissolution formed secondary pores and provided a minor contribution to reservoirs permeability secondary pores predominantly occur in the quartz arenite due to the preservation of the primary pores the main controlling factor for the quality of the sandstones is the pore type which has pronounced effects on both the pore structure and the pore throat size distribution the residual intergranular pores preserved in sandstones will lead to larger radii and better sorting characteristics of the pore throats which subsequently contribute to the higher permeability therefore the sandstones with the higher content of primary intergranular pores are considered high quality reservoirs they are characterized by quartz richness and mainly located in the zizhou mizhi area the inter crystalline micropores occur in the clay mineral resulting in small sized and poor sorted pore throats they give subordinate contributions to the reservoir quality and are the key factor of the extremely low permeability in sandstones the dry layers of the study area are mainly composed of sandstones dominated by inter crystalline micropores these sandstones are characterized by the high content of ductile grains and are mainly located in the qingjian area

qualitative and quantitative characterization of multiple factors that influence movable fluid saturation in lacustrine deep water gravity flow tight sandstones from the yanchang formation southern ordos basin china abstract fluid mobility is one of the most important factors in evaluating the potential for recovering tight oil however quantitative effects of multiple factors that influence movable fluid saturation in different pore throat combinations and their relationships with lithofacies in tight sandstones especially for lacustrine deep water gravity flow deposits remain controversial due to the strong heterogeneity and complex pore structure of tight reservoirs core samples obtained from the upper triassic yanchang formation in southern ordos basin were evaluated by using a variety of techniques including nuclear magnetic resonance nmr scanning electron microscopy sem x ray diffraction xrd pressure controlled porosimetry pcp rate controlled porosimetry rcp impregnated thin sections and helium porosity and nitrogen permeability measurements based on nmr t2 distributions under water saturated and centrifugal conditions four types of pore throat combinations are identified by using the relaxation time thresholds ts1 and ts2 from nmr large intergranular pore dominated pore throat combinations lip small intergranular pore dominated pore throat combinations sip intragranular pore dominated pore throat combinations iap and micropore dominated pore throat combinations mp among them sip is the dominant pore space favorable for fluid flow movable fluid saturation in sip pore space varies among different lithofacies the best fluid mobility commonly occurs in fine grained cross bedded sandstones sc whereas the worst fluid mobility usually appears in siltstones to very fine grained sandstones ss the quantitative effects of a total of 41 factors that influence movable fluid saturation of different pore throat combinations were investigated through the analysis of the pearson correlation matrix the results demonstrate that porosity and permeability mainly affect the movable fluid saturation in the pore space of sip and mp in addition maximum mercury intrusion saturation from pcp smax and tortuosity are the critical pore structure parameters affecting the movable fluid saturation in the pore space of sip and mp whereas the average pore throat radius ratio is the critical factor affecting the movable fluid saturation in lip pore space overall movable fluid saturation in different pore throat combinations is characterized mainly by different microscopic pore structure parameters a general pore network model for different lithofacies with different fluid mobility is established to facilitate assessment of the heterogeneity of tight sandstones and to further guide hydrocarbon exploration and development in similar lacustrine deep water depositional settings

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

clay mineral content type and their effects on pore throat structure and reservoir properties insight from the permian tight sandstones in the hangjinqi area north ordos basin china abstract clay mineral content and pore throat structure including type morphology size distribution and porosity and permeability are very important in factors that control gas storage in tight sandstone reservoirs in this study integrated thin section and field emission scanning electron microscope fe sem observation x ray diffraction xrd nuclear magnetic resonance nmr and rate controlled mercury porosimetry rcp analysis were used to investigate pore throat structure and the effects of clay minerals on pore throat connectivity size distribution and reservoir properties of tight sandstones in the middle permian successions of the hangjinqi area north ordos basin china results show that the tight sandstones have very low porosity and permeability averaged 9 76 and 0 968 md with diverse clay mineral compositions and grain sizes the main pore types are intergranular and intragranular dissolution pores intercrystalline and interstitial micropores most pores and throats are infilled with authigenic clay minerals such as acicular chlorite booklet kaolin and scaly and filamentous illite with minor silica and calcareous cements the pore and throat sizes in tight sandstones exhibit large variation with 147 648 m and 1 051 m in sandy conglomerates 160 998 m and 1 285 m in pebbly coarse grained sandstones 155 016 m and 0 868 m in coarse grained sandstones 158 134 m and 1 178 m in medium grained sandstones and 154 m and 0 379 m in fine grained sandstones respectively the results indicate that clay minerals affected the pore throat connectivity size distribution and reservoir properties of different lithofacies the properties of tight sandstone reservoirs are heavily influenced by illite content and throat size played a more important role influencing porosity and permeability this study helps to clarify the relationship of clay minerals pore throat structure and reservoir properties of the permian tight sandstones in the hangjinqi area and can guide associated work on tight sandstone reservoirs in the ordos basin and other basins worldwide

detection of near surface horizontal anisotropy in a weathered metamorphic schist at llano uplift texas by transient electromagnetic induction abstract the use of transient controlled source electromagnetic prospecting to detect buried steeply dipping foliation in metamorphic rocks is illustrated with data acquired over the precambrian packsaddle schist in the llano uplift of central texas the azimuthal variation of the transient voltage at a given transmitter receiver separation about a fixed central point is consistent with the forward model response of a homogeneous halfspace exhibiting horizontal electrical anisotropy the loop loop exploration configuration is ideally suited to probe horizontal anisotropy a quantitative match of the forward response to the observed data produces reasonable electrical conductivity values and coefficient of anisotropy for resistive crystalline geological materials the most conductive direction consistently lies within a few degrees of the geologically mapped foliation strike direction the electrical anisotropy is strongest below the near surface weathered layer within the more competent bedrock the agent responsible for generating the anistropy cannot be definitively determined because it is likely to be a combination of geological factors such as weathering compositional banding and microcracking all of which enhance electrical conductivity parallel to the plane of foliation the transient electromagnetics is supplemented by dc resistivity and seismic surveys the elastic anisotropy is evident in the near surface weathered layer but it may not persist very deep into the underlying competent schist the exposure of foliated schist at the surface is not sufficient to rule out a possible role for systematically aligned macrofracture sets as a secondary cause of the observed anisotropy

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