rate of dune reddening in a humid tropical climate the formation of red beds has long interested geologists and pedologists1 3 it has been suggested that the nature and degree of reddening in dune sands can be a useful indicator of the relative age and environmental history of the sands several authors have concluded that dune sands undergo progressive reddening with time4 7 and james and stanley8 considered that coastal dunes might actually be mapped on the basis of colour however quantitative data regarding the rate of reddening in different environmental conditions are generally lacking i present here radiocarbon evidence from north east australia which suggests that red weathering profiles have developed in stabilized dunes of holocene age in humid tropical conditions within the past 7 500 yr

distribution and significance of novel low molecular weight sterenes in an immature evaporitic sediment from the jinxian sag north china abstract an unusual series of c 22 c 27 monounsaturated sterenes and c 24 c 30 tetracyclic terpanes 17 21 seco hopanes were detected in relatively high concentrations in an immature evaporitic marl sediment of the jinxian sag bohai bay basin north china the site of unsaturation in these novel sterenes is assigned tentatively to the d ring on the basis of mass spectral interpretation which also distinguishes them from reported unsaturated sterenes other hydrocarbon biomarker or stable isotope characteristics are indicative of microbial e g methyl hopanes phytoplankton or higher plant depleted 13 c values of isoprenoids and hopanes inputs and an anoxic carbonate depositional environment hexacyclic hopanes tetracyclic terpanes the hydrocarbon composition showed no obvious biodegradation and the relatively high concentration of unsaturated terpenoids e g gammacerene and low values of other established maturity parameters t s t m 0 23 r o 0 44 t max 417 c are consistent with sediments of low maturity the novel low molecular weight sterenes and the tetracyclic terpanes may be early diagenetic products of microbial sources in a carbonate environment

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

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

secondary porosity generation during deep burial associated with overpressure leak off fulmar formation united kingdom central graben the overpressure history of a sandstone can be estimated using a numerical model if the burial curve and geological setting are known from the resulting effective stress the maximum potential porosity mpp can be calculated the mpp is the maximum porosity the rock could theoretically hold open at the modeled burial depth and pore pressure measured rock porosities should be at or below the mpp we have determined the mpp history for the fulmar formation sandstones upper jurassic of the central graben north sea and have compared the predictions to measured core data we conclude that for the majority of the fulmar formation sandstones the porosity evolution is a simple pattern of reduction during burial caused by compaction and cementation however in wells sited close to regional overpressure leak off points the porosity has been significantly increased from an end of oligocene low mean 21 to the present day values mean 31 this porosity increase occurred by feldspar dissolution with the reaction products being removed from the sandstones secondary porosity generation and the export of solute occurred while the sandstone was highly overpressured although still part of an open hydrogeological system the generation of porosity within deeply buried sandstones is of commercial importance and potentially can be predicted

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

geochemical characteristics of crude oil in fluvial deposits from maling oilfield of ordos basin china abstract maling oilfield is one of the earliest discovered oilfields in the ordos basin it is also a typical representative of oil reservoirs in fluvial deposits however due to the deficiency of geochemical study on the crude oils and source rocks the interpretation of the oil origin is largely based on the geological situation maling crude oils 21 and source rocks 14 from the basin have been systematically analyzed the distribution of the biomarkers indicates that the crude oils mainly originated from source rocks deposited under a weakly reducing to sub oxic freshwater lacustrine environment besides algae higher land plants have also made an important contribution to the source organic matter comparison of the distributions of the biomarkers suggests that there is a correlation between the maling oilfield oils and the source rocks of the yanchang formation

isotopic geochemistry of the saratoga springs implications for the origin of solutes and source of carbon dioxide we report the results of an isotopic study designed to determine the source of solutes and carbon dioxide in the famed saratoga springs new york mineral waters these waters have thousands of milligrams per liter total dissolved solid concentrations and are highly charged with carbon dioxide gas the spring waters are cold 12 c and there is no local deep seated thermal anomaly they emerge through thick shale caprock along the surface expression of normal faults the 13c 5 8 to 0 8 vienna peedee belemnite of the dissolved inorganic carbon and elevated 3he 4he ratios suggest that the source of the co2 is the mantle or an ancient deep crystallized igneous melt the stable isotopic content of the spring waters defines a mixing line between modern local meteoric waters 70 and a component with heavier d but similar 18o values this trend and that of 87sr 86sr of dissolved strontium versus 1 sr are consistent with the hypothesis that canadian shield type brines contribute salinity to the springs these brines plausibly migrate from the adirondack mountains to the topographically low mcgregor fault system in the hudson river lowlands where the saratoga springs discharge

continuous hydrocarbon accumulation over a large area as a distinguishing characteristic of unconventional petroleum the ordos basin north central china abstract global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps the characteristic features include the following a hydrocarbon source and reservoir coexist porosity and permeability are ultra low nano pore throats are widely distributed hydrocarbon bearing reservoir bodies are continuously distributed there is no obvious trap boundary buoyancy and hydrodynamics have only a minor effect and darcy s law does not apply phase separation is poor there is no uniform oil gas water interface or pressure system and oil or gas saturation varies examples of unconventional hydrocarbon accumulations are the mesozoic tight sandstone oil province and the upper paleozoic tight sandstone gas province in the ordos basin north central china generally continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources because of the great potential of unconventional petroleum resources it is believed that research on such resources will be at the forefront of the future development of petroleum geology

closed system burial diagenesis in reservoir sandstones examples from the garn formation at haltenbanken area offshore mid norway abstract the middle jurassic garn formation of the haltenbanken area has been studied using mineralogical and geochemical data from 21 wells ranging in burial depths from 2 0 to 4 1 km relative to seafloor rsf k feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth the content of pore filling authigenic illite increases sharply and the content of k feldspar and kaolinite decreases in garn sandstones presently at depths greater than 3 6 3 7 km rsf 120 130 c the depletion in k feldspar below 3 7 km rsf is not accompanied by lower potassium values in the bulk chemical composition wt k2o this suggests that the potassium released during k feldspar dissolution is retained in the sandstones and is precipitated as illite the variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy the shallower sandstones 1 3 have a k feldspar kaolinite ratio greater than one the deeply buried 3 7 km rsf sandstones with similar potassium contents contain excess k feldspar and most of the kaolinite is illitized however deeply buried sandstones containing an average of 0 38 wt k2o k al molar ratio 1 4 contain a significant amount of kaolinite but negligible k feldspar this suggests that the k feldspar kaolinite ratio before the onset of illitization was less than one and hence that the kaolinite illite reaction has been restricted by an insufficient supply of potassium absence of k feldspar this illustrates how illitization of kaolinite depends upon k feldspar as a local source of potassium prediction of illitization in sandstones therefore must be based on integration of models for provenance facies and early diagenesis in addition to burial and thermal history the formation of pore filling authigenic illite in these sandstones is an important influence on the total reservoir quality

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

distribution of carbonate cements within depositional facies and sequence stratigraphic framework of shoreface and deltaic arenites lower miocene the gulf of suez rift egypt abstract this study aims to unravel the spatial and temporal distribution of diagenetic alterations of the mheiherrat member the rudeis formation lower miocene of the gulf of suez rift egypt within depositional facies and sequence stratigraphy the mheiherrat member is represented by shoreface calcarenites and hybrid arenites transgressive and highstand systems tracts tst and hst respectively and deltaic rudites and coarse grained calcarenites and hybrid arenites lowstand systems tracts lst petrographic stable o and c isotopes mineral chemical and geochemical analyses have revealed that the arenites are pervasively cemented by eogenetic carbonates and to small extent by zeolite and pyrite as well as by telogenetic palygorskite the shoreface tst and hst calcarenites and hybrid arenites were dominantly cemented by microcrystalline grain rimming and inter and intragranular pore filling calcite 18 o v pdb 3 6 to 0 3 and 13 c v pdb 2 3 to 0 7 and rhombic dolomite 18 o v pdb 3 9 to 0 9 and 13 c v pdb 2 5 to 0 7 these cements are interpreted to be formed by marine to brackish waters with 18 o v smow 1 2 to 3 2 at temperature of 20 55 c the deltaic lst coarse grained calcarenites and hybrid arenites were dominantly cemented by coarse crystalline inter and intragranular pore filling calcite 18 o v pdb 4 4 to 2 3 and 13 c v pdb 2 8 to 1 3 which are interpreted to have precipitated from pore waters with 18 o v smow 3 5 to 5 5 at temperatures of greater than 55 c such pervasive cementation by carbonates is attributed to the occurrence of abundant intrabasinal carbonate grains the carbonate cement texture is suggested to be controlled by changes in pore water chemistry owing to transgression and regression events this case study revealed that better understanding of factors controlling the extent and textural habits of carbonate cements can be achieved when it is linked with depositional facies and sequence stratigraphy

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

geochemical impact of injection of eagle ford brine on hosston sandstone formation observations of autoclave water rock interaction experiments abstract this study investigates water rock interactions induced by brine injection into the cretaceous clastic hosston formation in south central texas a potential host formation for disposal of produced water from the eagle ford shale during oil and gas extraction the primary objectives of this experimental study were to 1 predict the geochemical reactions between the injected brine and the host formation during disposal operation and 2 assess potential impact on flow properties of the host rock the results provide important information of potential impact to injectivity and available methods to alleviate the problems if they occur during brine disposal five batch autoclave experiments were conducted to react an argillaceous sandstone of the hosston formation with a series of synthetic brines and the eagle ford brine for 3 weeks at 193 bar and 80 c to better identify mineral dissolution and precipitation rock samples were ion milled and examined using scanning electron microscopy sem before and after the experiments for each experiment approximately 15 aqueous samples were taken with increasing time intervals from hours to days dissolution of anhydrite which results in a rapid increase of so4 and ca concentrations in the solution of 500 ppm and 200 ppm respectively is the most significant reaction small amounts of dolomite and k feldspar were also dissolved to release mg ca k and si small amounts of iron hydroxides and kaolinite precipitated geochemical modeling is able to reproduce the behavior of the major aqueous components and of the observed mineral dissolution and precipitation overall change in flow properties from the water rock interaction is insignificant mercury intrusion capillary pressure micp data show little difference in porosity permeability and pore throat size distribution between the pre and post experiment samples this study suggests that injection of eagle ford brine into the hosston formation will pose minimum risks for the brine disposal operation

modal size classification of sands an example from the northern portugal continental shelf abstract modal analyses of the sand sized fraction of 200 surface samples provide significant information on the origin and distribution of continental shelf sediments of northern portugal about 80 of the samples are polymodal and the highest detected polymodal character number of modes of the sample was heptamodality 7 modes the areal distribution of principal modes reveals the general depositional pattern on the shelf coarsest modes delineate the presence and general surficial geometry of relict deposits and the finest modes reflect accumulation zones or areas where the sediments are in adjustment to shelf processes the latter type sediments tend toward unimodality finer modes are more affected by distributor processes while coarser modes develop during regressions and persist during transgressive periods relict modes are thought to persist where bottom energy is low and sediment supply is low or where sediment bypassing occurs the grouping of all modes leads to the definition of six modal classes the genesis of which is related to either ancient or recent processes areal distribution of these classes of modes delineates two paleo littoral zones and suggests that some fine sand from the nearshore is bypassing the shelf to accumulate near the shelf break although not as powerful as the decomposition of multimodal curves results indicate the utility and efficiency of modal analysis in deducing relevant aspects of ancient and modern processes

clay mineral diagenesis and quartz cementation in mudstones the effects of smectite to illite reaction on rock properties abstract the late cretaceous to early tertiary sequence of the voring and more basins from the norwegian sea has been examined with respect to mineralogy based on 319 cutting samples from five wells a clear relationship between mineralogy and well log data is demonstrated a significant change with respect to velocity porosity and density occurs within the depth interval corresponding to 80 90 c at shallow depths temperatures

comparative study on micro pore structure of marine terrestrial and transitional shales in key areas china abstract the purpose of this article is to make a comparative study on the pore structure of different sedimentary facies shales developed in typical areas china results show that the surface area sa and pore volume pv of the over mature marine shales are in the range of 17 83 29 49 m 2 g and 3 65 18 26 ml kg respectively which are much higher than that of the highly mature transitional and less mature terrestrial shales in the range of 0 9 1 9 m 2 g and 2 5 10 9 ml kg respectively in the view of the pore size distribution psd marine shales concentrate at the fine mesopores

integrated analysis for constraining palaeoclimatic and volcanic influences on clay mineral assemblages in orogenic basins palaeogene andean foreland northwestern argentina abstract variations in clay mineral assemblages in ancient continental deposits are frequently used to reconstruct past climate changes in active settings volcanic events can supply highly labile volcaniclastic material which can easily be transformed into smectite via diagenesis which can produce a noticeable footprint in clay mineral assemblages southern central andean foreland deposits are appropriate case studies to ascertain whether the climatic signal was preserved in the clay assemblages of their fine grained sediments as tectonic uplift volcanism and sedimentation have been interacting since the cretaceous we have studied a 1400 m thick coarsening upward palaeogene succession of the tin tin basin northern calchaqui valley argentina applying x ray diffraction xrd electron microscopy and detailed sedimentary facies analysis with the aim of comparing tendencies in the vertical fluctuations of clay minerals with evidence from sedimentological facies illite muscovite plus smectite account for 78 to 100 of the clay minerals in the fine fraction with kaolinite and chlorite in subordinate amounts the vertical variation of sedimentary settings from an overbank lacustrine domain to fluvial braided plains and an aeolian dune field suggests a gradual increase in aridity upsection however smectite abundances do not show a gradual decreasing trend compatible with progressively lower hydrolyzing conditions their relative abundances vary widely throughout the section depicting pulse like abrupt fluctuations despite the absence of field evidence for volcanic influence several indications of volcanic and volcaniclastic material have been found under scanning electron microscopy sem in levels with high smectite abundances from the middle to the top of the succession they include quartz crystals showing embayments and skeletal forms with smectite filling the voids microcrystalline silica as well as heulandite crystals in close association with authigenic smectite the xrd analyses of these levels evidence well crystallized smectite which is characteristic of a volcaniclastic origin therefore the increase in smectite abundance in these beds reflects a significant volcaniclastic contribution which is also evidenced by a centimetre thick ash layer topward in the sequence the only smectite rich level near the base of the tin tin section also contains well crystallized smectite associated with heulandite thus probably evidencing volcaniclastic input we infer that most of the smectite in these sediments formed during early diagenesis probably through the dissolution of labile tuffaceous material textural and morphological analysis by sem is essential to determine whether clay mineral assemblages could be interpreted in terms of palaeoclimate

diagenesis of carbonate cements in permo triassic sandstones from the iberian range spain evidence from chemical composition and stable isotopes petrographic chemical and stable isotope investigations have been made on calcite dolomite and fe dolomite ankerite from permian triassic clastic sequences from the iberian range spain calcite and dolomite cements have precipitated before the fe dolomite ankerite calcite however has also formed by calcitization of dolomite due to uplift the negative 13cpdb values 9 51 to 2 13 in these carbonates suggest derivation of carbon from oxidation of organic matter in the early carbonates and from decarboxylation of organic matter in the late fe dolomite ankerite early diagenetic dolomitic cements have heavier 13c 3 54 higher mn mn fe and mn mn mg and lower fe fe mg ratios than later diagenetic dolomitic cements the latters are also characterized by lighter 18opdb 10 64 than the early cements due to increase in temperature and to pore water mineral reactions

diagenetic evolution and porosity destruction of turbiditic hybrid arenites and siliciclastic sandstones of foreland basins evidence from the eocene hecho group pyrenees spain abstract this study aims to unravel the impact of diagenetic alterations on porosity loss of foreland basin turbiditic hybrid arenites and associated siliciclastic sandstones of the eocene hecho group south central pyrenees spain in this succession hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement 18ovpdb 10 0 to 5 8 th mode 80 c salinity mode 18 8 wt eq nacl fe dolomite 18ovpdb 8 5 to 6 3 and trace amounts of siderite the extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains which provided nuclei and sources for the precipitation and growth of carbonate cements other diagenetic alterations such as pyrite and albitization had no impact on reservoir quality scarce early diagenetic cements coupled with abundant ductile carbonate and siliciclastic framework grains have led to rapid porosity loss owing to compaction conversely abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction reservoir quality was affected by mesogenetic cementation by quartz overgrowths calcite and dolomite intergranular pressure dissolution of quartz grains and formation of fracture filling calcite cement 18ov pdb values from 10 4 to 7 8 th temperatures of 150 c which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism the results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition particularly the proportion and types of extrabasinal noncarbonates extrabasinal carbonates and intrabasinal carbonate grains these insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones degradation of reservoir quality makes these hybrid arenites calclithites and sandstones suitable as tight gas reservoirs but only if fracture porosity and permeability develop during tectonic deformation

pore fluid evolution distribution and water rock interactions of carbonate cements in red bed sandstone reservoirs in the dongying depression china the compositions distribution and its interaction with rocks of the evolving pore fluids controls the distribution of carbonate cements and reservoir storage spaces the reservoir quality of the red bed sandstone reservoirs in the dongying depression was investigated by an integrated and systematic analysis including carbonate cement petrology mineralogy carbon and oxygen isotope ratios and fluid inclusions the investigation was also facilitated by probing the mineral origins precipitation mechanisms pore fluid evolution and distribution and water rock interaction of carbonate cements and their influences on reservoir quality diagenetic evolving fluids in the interbedded mudstones are the main source for the precipitation of calcite cements that completely fill the intergranular volume cfiv calcite with heavier oxygen and carbon isotopes the ferro carbonate cements in the reservoir sandstone are enriched in lighter carbon and oxygen isotopes in addition to the cations released by the conversion of clay minerals in reservoirs products of organic acid decarboxylation and the associated feldspar dissolution process provide important sources for such carbonate cementation the carbon isotopes of co2 and the oxygen isotopic composition of fluids equilibrated with the cfiv calcite ferro calcite dolomite and ankerite cements indicate that the pore in the red bed reservoirs experienced high salinity fluids which evolved from the early formed interbedded mudstones through organic acid input and to organic acid decarboxylation pore fluids from nearby mudstones migrated from the edge to the centre of sandbodies causing strong calcite cementation along the sandbody boundaries and forming tight cementation zones pore fluids associated with organic co2 and acids and organic acid decarboxylation are mainly distributed in the internal portion of sandbodies causing feldspar dissolution and precipitation of ferro carbonate cements the distribution of pore fluids caused the zonal distribution of carbonate cements in sandbodies during different periods this may be advantageous to preserve the porosity of reservoirs as exemplified by the distribution of high quality reservoirs in the red bed sandbodies

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

shale characteristics in the southeastern ordos basin china implications for hydrocarbon accumulation conditions and the potential of continental shales abstract continental shales in china are generally characterised by low thermal maturity and high clay content characteristics that are significantly different from those of marine shales documented in the usa and elsewhere whether such continental shales have commercial hydrocarbon potential as marine shale in the usa is a question for the china petroleum industry recently vertical drills in the lacustrine shales in the yanchang formation produced an average of two to five tons of oil and 1000 3000 m3 of gas per day by reservoir hydraulic fracturing which demonstrates that the continental shales can also achieve viable hydrocarbon production shale and gas samples collected from the chang 7 c7 and chang 9 c9 members yanchang formation in the southeastern ordos basin were examined for geochemical petrologic and gas content analysis the results show that the c7 and c9 members developed a huge volume of organic rich shales with 2 4 wt toc in the deep or semideep lacustrine characterised primarily by type ii kerogen with a relatively low thermal maturity and a vitrinite reflectance ranging from 0 5 to 1 5 ro which decreases from west to east the hydrocarbon product varies with the thermal maturity in the southwestern corner the gas content measured by canister desorption equals 1 15 3 49 m3 t rock and the gas absorption capacity ranges from 3 to 5 m3 t rock whereas in the eastern part the gas content is low and oil production is 1 4 m3 day on average nanometre scale pores and micro fractures are well developed all of this indicates that the continental shales in the southeastern ordos basin might have huge shale hydrocarbon resources however the clay content of the continental shales of c7 and c9 ranging from 40 to 60 of the bulk mineral content are much higher than for gas produced in marine shales which might lead to significant challenges for successful development

carbon isotopic compositions of 28 30 bisnorhopanes and other biological markers in a monterey crude oil abstract an immature tar like oil api 3 13 c 23 6 vs pdb from the miocene monterey formation offshore california was selected for a study of carbon isotopic signatures of individual biomarkers the three principal stereoisomers of 28 30 bisnorhopane c 28 have within analytical precision identical carbon isotopic compositions average 13 c 32 3 0 4 and are considerably depleted in 13 c compared to the whole oil these 28 30 bisnorhopanes bnh differ isotopically from c 29 and c 30 17 h hopanes 25 8 and 26 1 and c 31 c 35 extended hopanes 13 c 27 7 and suggest different precursors for the c 28 hopanes than for c 29 c 35 hopanes the relative depletion of bnh of almost 9 compared to the isotopic composition of the whole oil suggests that these hopanes derive from chemoautotrophic bacteria possibly not yet identified h 2 s oxidizers which utilize 13 c depleted substrates the c 29 and c 30 hopanes are within analytical precision isotopically identical 26 and similar to algal derived compounds e g c 27 steranes 25 9 which is consistent with a cyanobacterial source for these hopanes an archaebacterial biomarker 1 1 biphytane 13 c 25 5 likely derived from methanogens is also isotopically similar to c 27 sterane norpristane pristane and phytane liberated by desulfurization of the aromatic and polar maltene fractions show isotopic compositions similar to the same isoprenoids in the free lipids of the bitumen total range from 24 5 to 27 5 this isotopic similarity supports a common origin for the free and sulfur bound forms of these isoprenoids this origin could be algal and or archaebacterial lipids which both show isotopic compositions within the range of the c 18 c 20 isoprenoids like other marine derived organic matter this monterey oil does not show the strong 13 c depletion typical for methylotroph derived compounds characteristically found in organic matter of lacustrine origin this may indicate fundamental differences of methane recycling processes in marine sulfate dominated as compared to lacustrine environments

the role played by carbonate cementation in controlling reservoir quality of the triassic skagerrak formation norway anomalously high porosities up to 30 at burial depth of 3000 m along with varying amounts and types of carbonate cements occur in the fluvial channel sandstone facies of the triassic skagerrak formation central graben norway however porosities of the skagerrak formation are lower in the norwegian sector than in the uk sector in this study petrographic analysis core examination scanning electron microscopy elemental mapping carbon and oxygen isotope fluid inclusion and microgeometry analysis are performed to determine the diagenesis and direct influence on reservoir quality with particular focus on the role played by carbonate cementation the sandstones are mainly fine grained lithic arkosic to sub arkosic arenites and display a wide range of intergranular volumes 2 3 43 7 with an average of 23 6 porosity loss is mainly due to compaction av 26 6 with minor contribution from cementation av 12 1 the carbonate cements are patchy in distribution from trace to 20 7 and appear as various types e g calcretes i e calcareous concreted gravels poikilitic sparite and sparry ferroan dolomite and euhedral or and aggregated ankerite ferroan dolomite crystals this study highlights the association of carbonate precipitation with the remobilisation of carbonate from intra skagerrak calcretes during early burial stage i e 500 m during deeper burial compaction is inhibited by carbonate cements resulting high intergranular volume of up to 32 and 29 for fine and medium grained sandstones respectively carbonate cement dissolution probably results from both meteoric water flow with co2 during shallow burial and organic co2 and carboxylic acid during deep burial the maximum intergranular volume enhanced by dissolution of early carbonate cements is calculated to 8 and 5 for fine and medium grained sandstones respectively compaction continues to exert influence after dissolution of carbonate cements which results in a loss of 6 intergranular volume for fine and medium grained sandstones reservoir quality of the norwegian sector is poorer than that of the uk sector due to a lower coverage of clay mineral coats e g chlorite later and deeper onset of pore fluid overpressure lower solubility of carbonate compared to halite and a higher matrix content

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

geochemical characterization of tarballs on beaches along the california coast part i shallow seepage impacting the santa barbara channel islands santa cruz santa rosa and san miguel tarballs are common along the southern california coastline this study investigates tarballs from beaches along this coastline with a focus on santa cruz santa rosa and san miquel islands in the santa barbara channel the tarballs were fingerprinted using biomarker and stable carbon isotope parameters and then grouped according to genetic similarities the data show that the tarballs are of natural and not anthropogenic origin and that all originate from source rock within the miocene monterey formation via shallow seeps offshore sterane biomarker parameters were found to vary widely in the sample set biodegradation especially of the regular steranes is the primary process impacting the biomarker distributions in a large group of samples the most common tarball occurrences appear to come from offshore seepage near the west end of santa cruz island another major group most likely was transported north from near santa monica bay several individual occurrences of some of these tarball groups also were found on beaches as far north as pt reyes and as far south as san diego indicating significant long distance dispersal by ocean currents this study begins a library of tarball fingerprints to be used as a database to help distinguish between natural and anthropogenic tar occurrences all along the california coast and to compare shallow seepage with future samples of deeper production oils from the same area

genesis and distribution pattern of carbonate cements in lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation in the dongying sag jiyang depression eastern china abstract the lacustrine deep water gravity flow sandstone reservoirs in the third member of the shahejie formation are the main exploration target for hydrocarbons in the dongying sag eastern china carbonate cementation is responsible for much of the porosity and permeability reduction in the lacustrine deep water gravity flow sandstone reservoirs the sandstones are mainly lithic arkose with an average framework composition of q43f33l24 the carbonate cements are dominated by calcite ferroan calcite ankerite and a small amount of dolomite the calcite and ferroan calcite are mainly poikilotopic blocky crystals while the dolomite and ankerite are mainly euhedral rhombohedra crystals filling intergranular and intragranular pores the relatively positive 13c values 2 to 3 9 of the carbonate cements in the sandstone reflect a mainly inorganically sourced carbon from 32 ma to 25 ma the pore water was rich in bicarbonate and ca2 due to carbonate dissolution in mudstone and which were transported with the pore water from mudstone to sandstone via advection and precipitated calcite cementation in thinly bedded sandstones and some high permeability zones in the middle of medium to thick sandstone beds from 12 ma to present abundant ca2 fe3 fe2 mg2 and bicarbonate had been transported from mudstone to sandstone via diffusion to form tight ferroan calcite cementation in the upper and lower parts of the medium to thick bedded sandstones ankerite is mainly distributed in the reservoirs associated with oil migration or charge because change of fe3 to fe2 from oil charge may supply sufficient fe2 for ankerite precipitation the center of sandstone beds 0 6 m is with potential of high quality reservoirs in the research area carbonate cementation appears to be an important factor that controls the accumulation of oil in deep water gravity flow sandstone reservoirs in the study area

norcholestane in miocene onnagawa siliceous sediments japan abstract a significant amount of 24 norcholestane with 20r 5 h 14 h 17 h stereochemistry is often found in marine siliceous sediments of the middle miocene onnagawa formation japan some onnagawa siliceous sediments are abnormally abundant in 24 norcholestane the primary production in the northeastern japan sea during the middle miocene was markedly increased due to the invasion of nutrient rich cold seawater from the north accompanied by the tectonic opening of the japan sea 24 norcholestane in high relative abundance is derived mainly from the marine diatoms which were enhanced during this event the drastic ecological change caused by rapid tectonic opening of the japan sea and global climatic deterioration during middle miocene time is suggested to be influential in the formation of 24 norcholestane rich onnagawa siliceous sediments

a novel sterane 27 nor 24 methyl 5α cholestane in sediments a novel sterane 27 nor 24 methyl 5 alpha cholestane has been identified in sedimentary hydrocarbon mixtures by coinjection with an authentic standard the c sub 27 sterane occurs in silica rich sediments from the monterey formation usa the onnagawa formation japan and the menilite shale poland and in bituminous marls from the vena del gesso basin italy the compound seems to be biosynthetically related to 24 norcholestanes which co occur in sometimes relatively high amounts the sup 13 c content of the sterane in the monterey formation indicates an algal source similar to other steranes present in this sediment a diatom or a dinoflagellate source is suggested for this compound

paleoenvironmental implications of novel c30 steranes in precambrian to cenozoic age petroleum and bitumen petroleums and bitumens from early proterozoic 1800 ma to miocene 15 ma age marine strata contain 24 isopropylcholestanes a novel group of c30 steroids the abundance of these compounds relative to 24 n propylcholestanes varies with source rock age late proterozoic vendian and early cambrian oils and or bitumens from siberia the urals oman australia and india have a high ratio of 24 isopropylcholestanes to 24 n propylcholestanes 1 while younger and older samples have a lower ratio 0 4 temporal changes in this parameter may reflect the relative abundance of certain porifera sponges and certain marine algae through time geochemical indicators such as this which can constrain the source rock age of a migrated oil are useful in source rock identification during petroleum exploration

open or closed geochemical systems during diagenesis in sedimentary basins constraints on mass transfer during diagenesis and the prediction of porosity in sandstone and carbonate reservoirs descriptions of mineralogy and textural relationships in sandstones and limestones have been used to establish a sequence of diagenetic events epigenesis involving mineral dissolution and precipitation which have been interpreted to have occurred during the burial history published epigenetic sequences commonly imply a geochemically open system with very significant changes in the bulk chemical composition of the sediments during burial near surface diagenetic reactions may be open involving significant changes in the sediment composition and formation of secondary porosity caused by high pore water flow rates of meteoric water or reactions with sea water near the sea floor calculations show that the bulk chemical composition of the sediments below the reach of high pore water flow rates of meteoric water or hydrothermal convection should remain nearly constant during progressive burial because of limited pore water flow mass transport between shales and sandstones is also limited because the pore water is in most cases buffered by the same minerals so that the concentration gradients are low recent studies show that silica released from clay mineral reactions in mudstones has been precipitated locally as small quartz crystals and not exported to adjacent sandstones if the geochemical constraints for mass transfer during burial diagenetic reactions are accepted the chemical reactions involved in diagenesis can be written as balanced equations this offers the possibility to make predictions about reservoir quality based on assumptions about primary sediment composition related to facies and provenance large scale changes in the bulk composition of sandstones and mudstones during burial diagenesis have been suggested but because such changes cannot be explained chemically and physically no predictions can be made burial diagenetic processes are in most cases not episodic but occur as slow adjustments to increased stress and temperature driving the sediments toward increased mechanical and thermodynamic stability as a result the porosity of a single lithology must decrease during progressive burial but each lithology has a different porosity curve this article discusses quantitative calculations and estimates that show clearly that burial diagenesis must represent geochemically nearly closed systems where mineral dissolution and precipitation must be balanced this provides a theoretical basis for the modeling and prediction of reservoir quality

berthierine formation in reservoir rocks from the siri oilfield danish north sea as result of fluid rock interactions part i characterization abstract berthierine as the only authigenic fe rich clay mineral occurs in an oil bearing glauconite rich sandstone paleocene heimdal member of the lista formation danish north sea based on the results of thin section transmission and scanning electron microscopy five different morphological types of berthierine were identified 1 mainly as grain coatings partly together with authigenic quartz covering detrital grains 2 as pore filling clay in open pores 3 small crystals in transformed glauconite grains 4 small crystals associated with siderite in layers of mica micaceous glauconite and 5 as elongated crystals in siderite concretions although different in occurrence all berthierine types investigated by microprobe analysis are of similar chemical composition suggesting similar precipitation conditions investigations by transmission electron microscopy reveal that fine berthierine crystals display straight parallel layers with a typical lattice fringe spacing of 0 7 nm and that their occurrence is intimately connected to oil phases in pore space partly berthierine textures resemble mineralized oil water emulsions berthierine is more abundant in the oil leg than in the water leg but occurs especially abundant at the oil water contact the coexistence of berthierine together with quartz overgrowth but also its appearance in glauconite indicates that berthierine formed in situ and that it is a product of glauconite dissolution berthierine formation is coupled to the reduction of aqueous fe 3 into fe 2 derived from glauconite dissolution and is related to fluid rock interactions triggered by oil degradation which occurred in the oil filled zone and at the oil water contact the biomarker parameters pristane n c 17 and phytane n c 18 indicate an increased extent of oil degradation at the oil water contact on the one hand soluble oil degradation products methane carbon dioxide and hydrogen changed the hydrogeochemical conditions such as ph pe alkalinity etc and controlled dissolution of glauconite and feldspar on the other hand the changed hydrogeochemical conditions led to berthierine and quartz precipitation in water wet reservoir intervals

a molecular stable carbon isotope study of organic matter in immature miocene monterey sediments pismo basin the 300 m section of the miocene monterey formation outcropping at shell beach pismo basin ca 15 11 ma is composed of calcareous phosphatic 15 1 14 5 ma and siliceous facies 14 5 11 o ma an objective of this paper is to document lateral paleoenvironmental changes in the miocene monterey formation by comparing the shell beach sb profile with the naples beach nb section in the santa barbara ventura basin schouten et al 1997 which is 80 km to the south eight samples one sample representing on average a time period of ca 2000 y from this section were analyzed for variations of extractable biomarkers and their carbon isotopic signatures as indicators for paleoenviron mental change during the miocene saturated hydrocarbons present include 28 30dinorhopane phytane n alkanes c c3r lycopane and 17 21p h homohopane the biomarkers released after desulfur ization of the polar fractions predominantly consist of phytane 2 6 10 14 tetramethyl 7 3 methylpen tyl pentadecane c c3 n alkanes regular 5a and sd steranes dinosteranes and 22r 17 21 h pentakishomohopane steranes have similar carbon isotopic compositions 25 to 27 0 throughout the section and are isotopically similar at both sites indicating laterally similar and vertically stable environmental conditions for algae living in the upper part of the photic zone free and s bound n alkanes at sb mainly originate from marine organisms and not from terrestrial sources as in the nb section s bound pentakishomohopane is ca l 4 0 depleted compared to the steranes and is thought to be derived from the deeper water dwelling cyanobacteria these findings are consistent with stable carbon isotopic data obtained for these compounds from middle miocene monterey sediments at naples beach and indicates similar environmental conditions in the depositional environments of the santa barbara ventura and the pismo basin s bound highly branched isoprenoids have at both sites different isotopic compositions indicating the presence of different diatom species special growth conditions or different bloom periods in the pismo basin the carbon isotopic composition of 28 30dinorhopane shifts to more depleted values up section suggesting that the dinorhopane synthesizing organism or organisms live on c02 which is isotopically influenced by methane production and oxidation the cs hopane is enriched by l 4 0 in 13c compared to other hopanes and steranes specific bacteria possibly heterotrophs may have been the organisms producing this compound copyright 0 1997 elsevier science ltd

diagenesis porosity evolution and petroleum emplacement in tight gas reservoirs taranaki basin new zealand abstract tight gas reservoirs have become popular targets in petroleum exploration in recent years due largely to the increasing market demand for gas and also to technology advances used in extraction reservoir quality is typically poor due to deep burial diagenesis resulting in significant compaction cementation and illitization however analysis of tight reservoirs using integrated techniques can improve our understanding about the controls on reservoir quality and these results can potentially be used to help predict reservoir quality at other sites the use of integrated analysis of diagenetic and burial history is here shown for the eocene k3e kapuni group reservoir in cardiff 1 taranaki basin new zealand the k3e belongs to a deep gas play that was considered uncommercial in the early 1990 s due to poor reservoir quality associated with extensive authigenic illite potassium argon dating has shown that destruction of the reservoir through illitization occurred in the pliocene associated with a late stage heating and or fluid flow event timing of illite authigenesis in the k3e reservoir postdates the likely circulation of co2 rich fluids that caused reaction of feldspar to form kaolinite and quartz homogenization temperatures from two phase aqueous fluid inclusions within quartz cements together with 1d basin models of co2 expulsion from coaly source rocks indicate that feldspar reactions may have started in the mid miocene the co2 rich fluids are thought to have been generated by thermal decarboxylation of intraformational paleocene eocene coals at the same time oil was being expelled from older cretaceous coals and migrated into the mid late eocene reservoirs evidence for oil migration is demonstrated by the presence of residual oil and by the local occurrence of abundant oil bearing fluid inclusions the products of feldspar dissolution appear to be in reasonable balance with the amount of dissolved feldspar suggesting a relatively closed diagenetic system with little improvement in reservoir quality despite the large volume of feldspar dissolved petrographic evidence of reservoir sandstones at cardiff 1 demonstrates the presence of some illite free secondary macropores lined by residual oil this local preservation of porosity may be replicating the original distribution of oil saturated pores in the pliocene at the time when authigenic kaolinite underwent reaction to form illite remobilization of the oil probably occurred by gas flushing after the main phase of illitization at c 5 ma the data presented in this study are consistent with redistributed liquid petroleum into up dip traps and represent the potential for a new exploration play the eocene reservoir at cardiff 1 is currently gas charged with the pore system composed of approximately one third large macropores connected by a tortuous microporous network however it is possible that with the currently buoyant gas market and modern hydraulic fracturing techniques it may now be economic to flow gas from the locally large pores identified in the k3e reservoir

dinosterane and other steroidal hydrocarbons of dinoflagellate origin in sediments and petroleum abstract the steroidal alkanes of a selection of sediments and oils have been examined by gc ms with multiple metastable reaction monitoring specific 4 methyl sterane isomers have been identified by comparison with isomers synthesized from sterols isolated from dinoflagellates an immature marine oil shale and two mature marine oils of triassic to early cretaceous age contained high concentrations of c30 steranes comprising desmethyl 24 ethyl 4 methylcholestane and 4 23 24 trimethylcholestane dinosterane isomers an immature non marine oil shale and two non marine oils of cretaceous to eocene age contained stereoisomers of 24 ethyl 4 methylcholestane as the dominant c30 steranes reaction monitoring analyses in gc ms are particularly suited to unravelling complex distributions of homologous and stereoisomeric steroids encountered in oils and their source rocks

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

biomarker geochemistry of a foreland basin the oligocene menilite formation in the flysch carpathians of southeast poland black shales of the menilite formation the source rock for oils in the carpathian overthrust belt display a large variability in their bulk and molecular geochemical parameters biomarker and stable carbon isotope analyses indicate a variable contribution from different algae particularly dinoflagellates and diatoms and cyanobacteria this is reflected by specific predominantly sulfurised biomarkers e g c35 homohopanoids c25 highly branched isoprenoids and marine n alkanes and by particular distributions of steranes and 4 methylsteranes comprising 24 nor and 24 methyl 27 norcholestanes and related novel steranes with a methylation at c 23 the presence of hopanoids of methanotrophic origin 13c up to 57 implies a temporarily enhanced full methane cycle in a marine environment which affected the isotopic composition of organisms dwelling in the upper photic zone the presence of isorenieratene derivatives indicate periods of euxinic conditions within the photic zone in all investigated sub basins

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 effect of oil emplacement on quartz cementation in a deeply buried sandstone reservoir quartz is an important porosity occluding cement in sandstone reservoirs that have been subjected to elevated temperature 80 c to 100 c for a substantial period of time the effect of oil emplacement on quartz cementation in reservoir sandstones is controversial some studies have concluded that early oil emplacement can inhibit quartz cementation leading to the preservation of porosity whereas other studies have concluded that quartz cementation appears largely unaffected by oil emplacement here we have studied shallow marine upper jurassic sandstones from ula field norwegian north sea with reservoir temperatures of approximately 150 c to determine whether oil emplacement had a significant impact on diagenesis with particular attention to quartz cementation following sedimentological description of cores samples above and below the oil water contact have been collected adjacent to core analysis plug points these samples then underwent a series of studies including petrographic point counting with a transmitted light microscope scanning electron microscopy sem backscattered electron microscopy sem cathodoluminescence microscopy and fluid inclusion studies these data were integrated with routine core analysis and petrophysical log data density and resistivity log data have been used to determine the precise oil saturation of each sample studied the distributions of all potential controls on porosity and permeability such as grain size sorting matrix clay content and degree of bioturbation the presence of grain coatings and dolomite cement and the amount of quartz cement have been assessed the presence of primary oil inclusions within quartz cement shows that oil ingress into the ula reservoir commenced prior to the onset of quartz cementation very fine grained matrix rich bioturbated microquartz cemented sandstones have uniformly low quartz cement contents irrespective of oil saturation medium grained graded matrix poor microquartz poor sandstones have quartz cement ranging from 1 to greater than 17 associated with core porosities of approximately 22 and 7 respectively higher oil saturations equate to higher porosities and permeabilities in the medium grained graded matrix poor microquartz poor sandstones which cannot be explained by any control other than the amount of quartz cement as a function of pore fluid type oil emplacement therefore appears to have inhibited quartz cementation at high oil saturations and can be viewed as an important control on reservoir quality the significance of this study is that the presence of oil in a sandstone reservoir at the time that quartz cement was growing can have a considerable impact on reservoir quality models that seek to predict quartz cement and reservoir quality in sandstones need to account for the timing of oil emplacement compared with other diagenetic processes

feldspar dissolution authigenic clays and quartz cements in open and closed sandstone geochemical systems during diagenesis typical examples from two sags in bohai bay basin east china feldspar dissolution and precipitation of clays and quartz cements are important diagenetic reactions affecting reservoir quality evolution in sandstones with detrital feldspars we examined two sets of sandstone reservoirs to determine whether the sandstone diagenetic systems were open or closed to the mass transfer of products from feldspar dissolution and its impact on reservoir quality one of the reservoirs is the eocene fan delta sandstone buried 2 5 4 0 km 1 5 2 5 mi below sea level bsl in the gaoliu gl area of the nanpu sag and the other is the eocene subaqueous fan sandstone buried 1 5 4 5 km 1 2 8 mi bsl in the shengtuo st area of the dongying sag both sandstones consist mainly of lithic arkoses and feldspathic litharenites and have secondary porosity formed by dissolution of feldspars in the gl sandstones the absolute amounts of authigenic clays and quartz cements generally 125 c 257 f the low abundance of authigenic clays and quartz cements and low pore water salinity indicate that much of the and released from leached k feldspars were exported from the gl sandstone system and the extensive feldspar dissolution enhanced much porosity and permeability in contrast the st sandstones with secondary pores formed by feldspar dissolution generally contain authigenic clays kaolinite and illite and quartz cements with almost identical volume of secondary pores kaolinite dominates in the st sandstones at shallower depth 3 1 km 2 mi bsl where temperature exceeds 125 c 257 f the presence of abundant clays and quartz cements indicates that and released from leached feldspars were retained in the st sandstone system the dominance of authigenic illite at greater depth indicates that sufficient should have been retained within the sandstones for occurrence of illitization of kaolinite and feldspars secondary porosity in thin sections can be up to 3 but little porosity the diagenetic difference between the gl and the st sandstones can be interpreted by assessing pore water evolution in these two areas the current pore waters with low salinity and negative hydrogen isotopic compositions in the gl sandstone system indicate the significant impact of meteoric water whereas the current pore waters with high salinity and the paleofluids with positive oxygen isotopic compositions in the st sandstone system indicate little trace of meteoric water access of meteoric freshwater to the gl area probably occurred during the late oligocene to neogene through widely developed faults in the paleogene and neogene strata the low salinity water could have been responsible for flushing of solutes derived from feldspar dissolution as such diagenesis in the gl sandstones is considered to have occurred in an open geochemical system whereas with limited faults and high water salinity the st sandstones acted as a closed geochemical system where precipitation of kaolinite illite and quartz cements occurred following dissolution of feldspars

upper permian junggar and upper triassic ordos lacustrine source rocks in northwest and central china organic geochemistry petroleum potential and predicted organofacies abstract two sets of lacustrine source rocks in the junggar basin and ordos basin have been analysed by organic geochemical approach and organic petrographic technique one of these source rocks the junggar shales lucaogou formation and hongyanchi formation was deposited in a hydrologically closed brackish lake under a sub humid climate in higher latitude 39 43 n and consisted of organic rich profundal lake shales lucaogou formation and lean fluvial lacustrine lake margin mudstones the other one the chang 7 shale was deposited in a hydrologically open freshwater lake under a humid climate in a 31 n palaeolatitude and was characterized by high concentration of alginite rich organic matter the profundal lucaogou shale and chang 7 3 shale possess abundant total organic carbon high generative potential s2 and high hydrogen indices which rank as the best prolific source rocks in both basins followed by chang 7 1 shale hongyanchi formation and chang 7 2 are much leaner than their counterparts and contain relatively small portion of alginite but more terrigenous macerals this nature of organic matter makes them less prolific and becoming gas prone potential these differences in petroleum potential mirror depositional environment changes in source rocks formation potential source rocks in both basins have the potential for generating high wax oil but the brackish system clearly contains more pna oil the kerogen structures revealed by pyrolysis gas chromatography allowexplaining this trend the lucaogou chang 7 3 and chang 7 1 shales enriching in long and moderate alkyl chains share a nature of high wax oil generation trend in contrast the hongyanchi and chang 7 2 shales appears to be dominated by short to moderated chain length distribution and higher overall aromaticity to generate more pna oil even gas and condensate thermal liability was shown to be linked to kerogen structure in particular the presence of alicyclic and oxygen containing moieties and these in turn are correlate with total organic carbon content