origin and diagenetic evolution of ca mg fe carbonates in some coalfields of japan carbonate concretions lenses and bands in the pleistocene palaeogene and upper triassic coalfields of japan consist of various carbonate minerals with varied chemical compositions authigenic carbonates in freshwater sediments are siderite calcite ankerite dolomite ferroan magnesite in brackish water to marine sediments in the coal measures calcite dolomite ankerite siderite ferroan magnesite and in the overlying marine deposits calcite dolomite siderite most carbonates were formed progressively during burial within a range of depths between the sediment water interface and approximately 3 km the mineral species and the chemical composition of the carbonates are controlled primarily by the initial sedimentary facies of the host sediments and secondarily by the diagenetic evolution of pore water during burial based on the regular sequence and burial depth of precipitation of authigenic carbonates in a specific sedimentary facies three diagenetic stages of carbonates are proposed carbonates formed during stage i 2000 m are fe calcite and extremely high ca mg siderite the latter is exclusively in marine mudstones the supply of ca is partly from the alteration of silicates in the sediments at elevated burial temperatures after uplift calcite with low mg content precipitates from percolating groundwater and fills extensional cracks

analysis of factors influencing excess heads in shales during burial abstract excess fluid pressure in compacting shales is analysed with the help of an approximate solution of the fluid flow equation excess head depends on hydraulic diffusivity thickness of the shale bulk rock density and rate of sedimentation both storage coefficient and hydraulic conductivity of shales decrease with compaction hydraulic diffusivity of shales increases up to a maximum and then decreases with decreasing porosity in case the rate of permeability decline with compaction is high then significant excess heads may develop in shales during burial shales which include sand lenses acquire higher excess fluid pressures than pure shales because the storage coefficient of sands decreases at a far lower rate than that of shales during burial

clay mineral formation and transformation in rocks and soils three mechanisms for clay mineral formation inheritance neoformation and transformation operating in three geological environments weathering sedimentary and diagenetic hydrothermal yield nine possibilities for the origin of clay minerals in nature several of these possibilities are discussed in terms of the rock cycle the mineralogy of clays neoformed in the weathering environment is a function of solution chemistry with the most dilute solutions favouring formation of the least soluble clays after erosion and transportation these clays may be deposited on the ocean floor in a lateral sequence that depends on floccule size clays undergo little reaction in the ocean except for ion exchange and the neoformation of smectite therefore most clays found on the ocean floor are inherited from adjacent continents upon burial and heating however dioctahedral smectite reacts in the diagenetic environment to yield mixed layer illite smectite and finally illite with uplift and weathering the cycle begins again

extraction of whole versus ground source rocks fundamental petroleum geochemical implications including oil source rock correlation abstract in petroleum geochemistry extractable hydrocarbons hcs in source rocks have typically been studied by grinding the rock to a fine powder 100 mesh and then extracting the hcs from the rock with a solvent this procedure carries the implicit assumption that the hcs are homogeneously distributed throughout the rock however sequential soxhlet extractions of whole unpowdered source rocks have shown that progressive extracts from the same rock can be quite different and may not even correlate with each other a crude oil like material clearly has been fractionated from indigenous bitumen in these rocks has moved to cracks and parting laminae in the rocks is ready for expulsion from the rocks and is thus most accessible to the first extracting solvents this process which we believe is largely due to hc gases and carbon dioxide generated over all maturation ranks in source rocks carries petroleum geochemical implications of a fundamental nature for oil source rock correlations and gives insight into primary migration mechanisms origin of oil deposits and use of maturity and organic facies indices

understanding of hc migration in sedimentary basins present state of knowledge abstract great advances were made during the decade of 1975 1985 in the understanding of hc migration by establishing that the main mechanisms were polyphasic flow whatever its stage and that migration in solution in water and or by molecular diffusion was insignificant except for methane ethane and benzene research is now focusing on points which still have not been satisfactorily answered such as 1 1 behaviour of source rocks during hc expulsion the main driving force for fluid expulsion has been recognized to be excess fluid pressure within the source rock with respect to its surrounding environment of permeable beds the pressure build up is due mainly to accommodation by the fluids of part of the mechanical stresses which are exerted on the rock mainly sedimentary load but also stresses developed by tectonics to thermal expansion of water and to hc formation from kerogen in response to increasing temperatures however it is not clear whether or not petroleum expulsion proceeds preferentially along oil wet networks kerogen or oil wet mineral surfaces nor it is not clear whether or not microfracturing provoked by an internal fluid pressure higher than allowed by the mechanical strength of source rocks is necessary for hc expulsion 2 2 modifications of the compositions of hc phases e g oil versus gas or the distribution of biomarkers during migration and losses of hydrocarbons along the migration pathways although careful observations of natural sample series and also experimentation have been done no clear picture has emerged for the moment however compositional effects seem to be due more to changes in thermodynamic conditions along migration pathways which provoke inter alia hydrocarbon phase separation retrograde condensation than to differential retention by mineral surfaces it is likely that while careful observations on a large variety of source rock samples are needed to more clearly understand source rocks functioning progress in understanding hc migration will come increasingly from modeling and experimentation models which mathematically simulate natural processes are necessary to help the human mind in handling such complex phenomena and therefore are of precious help to focus research on the most important points they replace migration in its geological and historical dimensions with numerical or conceptual tools while observation only describes the present state models are also the tools by which knowledge on migration can be applied to exploration problems however quantitative applications of models require numerous and precise geological data that are difficult to gather at early stages of exploration in the field of experimentation it will be easier to design crucial experiments now that hc migration mechanisms are better understood thanks to experimentation it should also be possible to validate models however so as to reach such aims substantial progress has to be made in the technology of experimental systems

structural evolution metamorphism and restoration of the arabian continental margin saih hatat region oman mountains abstract 170 m y of relatively stable passive margin sedimentation along the northern continental margin of arabia was abruptly terminated during the cenomanian turonian 95 ma when the oman continental margin collapsed and subsided rapidly aruma basin to accommodate obduction of the semail ophiolite complex and underlying thrust sheets haybi and hawasina complexes in the oman mountains the ophiolite was emplaced at least 200 km over the passive margin probably over 450 km in total from ne to sw during the later stages of obduction the leading edge of the continental margin was subducted to depths where carpholite bearing rocks 6 8 kbar blueschist 12 15 kbar and eclogite ca 20 kbar facies metamorphism formed in a ductile deforming ne dipping subduction zone along the northeastern margin of the saih hatat culmination in ne oman the mesozoic shelf carbonates are deformed by recumbent folding on all scales the present day structure is dominated by four sw vergent thrust slices in carpholite grade rocks at higher structural levels overlying lower units of crossite bearing blueschist hulw unit garnet glaucophane chloritoid blueschists and garnet clinopyroxene phengite glaucophane eclogites 15 20 kbar the largest shear zone separates 7 8 kbar hulw unit blueschists above from 12 15 kbar retrogressed eclogites and ca 20 kbar eclogites of the as sifah unit below further inboard sw a major shear zone separates the lower plate hulw blueschist facies rocks from the upper plate sedimentary rocks upper plate lower plate discontinuity which also show carpholite grade hp metamorphism towards the ne restoration of the entire saih hatat continental margin shows that the upper plate lower plate discontinuity cuts down section to the sw as far as the proterozoic hatat schists ne directed extensional crenulation schistosity and nne oriented stretching lineations in the eclogite and blueschist facies rocks are consistent with sw directed exhumation of footwall hp rocks ne facing folds and spectacular sheath folds with greatly attenuated limbs in the upper plate sediments are interpreted as antithetic backfolds with shortening in the upper plate balanced by the subduction of the lower plate consistent with a ne directed subduction of the continental margin rocks beneath the sw obducting ophiolite haybi and hawasina thrust sheets recent suggestions of a nascent sw directed subduction beneath the oman margin are not consistent with the sedimentary evolution of the shelf and slope carbonates or the geological structure of saih hatat two main phases of tectonic uplift of saih hatat and jebel akhdar are firstly the latest cretaceous ramp culmination associated with foreland propagating thrusts in the sw and backfolds in the ne and secondly post late eocene oligocene uplift by ca 2000 m

distribution of hydrocarbon source potential in the jurassic smackover formation abstract there are regionally consistent differences in source potential between the upper and lower members of the smackover formation measurements of total organic carbon and visual kerogen assessments indicate that the lower smackover has source potential for crude oil from texas to florida source potential is largely restricted to the laminated lime mudstone facies of the lower smackover where deposition occurred in a subtidal environment that was anoxic and probably hypersaline algal derived amorphous kerogen is concentrated in laminations of depositional origin carbonate diagenesis during burial enhanced source potential by further concentrating kerogen along stylolites we suggest that the efficiency of hydrocarbon expulsion has been enhanced by depositional and diagenetic factors that concentrate source rock kerogen along conduits for fluid movement the upper smackover serves mainly as a reservoir rock and other jurassic formations lack source potential

hydrocarbon generation and migration from jurassic source rocks in the e shetland basin and viking graben of the northern north sea in the e shetland basin oil generation began 65 ma ago peak oil generation maturity occurs today at 3250 m 0 7 r 0 and was first reached 40 50 ma ago the oil generation threshold is at 2500 m highest oil saturations in the kimmeridge clay occur at 0 8 r 0 oil expulsion efficiencies are 20 30 oil phase migration has probably occurred through oil wet kerogen laminae and through interconnected large pores aided by low oil water interfacial tensions oil migrated along strong lateral fluid pressure gradients from overpressured source rocks in half grabens to jurassic reservoirs in tilted fault blocks in the viking graben the kimmeridge clay is at oil floor maturity below 4500 m oil and peak oil generation began 7c 80 and 55 65 ma ago respectively 40 ma ago the kimmeridge clay passed through peak generation and gas generation by cracking of oil had begun peak dry gas generation from brent coals occurs today below 5000 m and began 40 ma ago the frigg field gas probably generated from late jurassic source rocks migrated through microfractures in overpressured mudstones below 3500 m above 3500 m methane probably migrated in aqueous solution and was exsolved in the early tertiary aquifer

geochemical effects of petroleum migration and expulsion from toarcian source rocks in the hils syncline area nw germany for mature oil prone source rocks of toarcian age in the hils syncline area nw germany the geochemical effects of primary migration of petroleum are shown expulsion has occurred at high efficiencies primarily via a network of macro and micro fractures the effects of this fracture controlled expulsion with respect to gross and molecular composition of c15 soluble organic matter are documented samples from fractured intervals have expelled total c15 soluble organic matter more efficiently than adjacent unfractured intervals a systematic comparison of samples taken at cm intervals away from macro fractures reveals compositional gradients which are interpreted to reflect preferential depletion of saturated and aromatic hydrocarbons from the source rock shale into the fractures extracts of shale samples directly at fracture walls are enriched in nso compounds and highly polar asphaltene like fractions the composition of residual petroleum fluids extracted from the fracture fill materials is variable in some cases there is a close match with that of the shale at some distance away from the fractures in other cases there is geochemical evidence to suggest that petroleum fluids were chemically altered during lateral flow along the fractures based on available geological and geochemical evidence a conceptual model is proposed to explain fracture development and fluid flow processes rapid heating by a nearby intrusive body at shallow depths when the toarcian sediments still contained about 15 20 pore water has played an important role overpressuring of aqueous pore fluids with abundant dissolved carbonate caused intense fracturing of the rich intervals in the mature toarcian sections finally the closely interdependent relationships between petroleum generation and petroleum migration processes are documented especially the evolution of these relationships with maturity progress from immature 0 48 rm to mature 0 68 and 0 88 rm and overmature 1 45 rm stages is shown for example in the maturity interval from 0 68 to 0 88 rm most of the measured shale compaction and porosity decrease is achieved by expulsion of petroleum for some samples there is an inverse relationship between kerogen quality and extractable hydrocarbon yields suggesting that expulsion efficiencies were higher from the better quality intervals because the petroleum achieved higher saturations and hence higher relative permeabilities

a simple analysis of oil induced fracturing in sedimentary rocks abstract geometric form of kerogen patches control oil induced fracturing in impermeable source rocks thin elongate kerogen flakes cause lateral fracturing kerogen has to occur in spherical or cylindrical forms to induce vertical fractures if length to width ratio of kerogen flakes is sufficiently large then lateral fractures are definitely initiated during oil generation in impermeable source rock oil and rock compressibilities and lateral to vertical stress ratio are decisive factors in vertical fracturing in impermeable rocks lateral to vertical stress ratio is generally higher than the maximum for oil induced vertical fracturing in tectonically relaxed sedimentary basins oil generation cannot initiate vertical fractures except when the lateral to vertical stress ratio is reduced below critical level either by tectonic tension or erosion of overburden when the source rock is permeable then the rate of oil seepage from kerogen into the surrounding rock is crucial in fracture initiation oil induced fractures cannot form unless permeability and hence rate of oil seepage from kerogen into the surrounding rock is non existent or negligible most source rocks have some permeability therefore oil induced fracture initiation should be a rare phenomenon

state of the art of research in kinetic modelling of oil formation and expulsion abstract the last ten years have seen a rapid development in the understanding of petroleum generation and expulsion by a combined use of natural series and experimental simulation this knowledge is now used to develop mathematical models which reproduce the kinetics of petroleum generation and migration in the purpose of basin evaluation by a combination of observation of homogeneous geochemical series experimental simulation and mathematical modelling it has been now established that parameters governing the quantity and composition of petroleum are temperature type of organic matter time or heating rate catalysis by clay minerals which would produce important amounts of isoalkanes is not necessary to produce petroleum the role of minerals although still controversial is rather to retain the heavy fraction of petroleum pressure seems to have a limited influence within the range for effective source rocks kerogen is a tremendously complex macromolecule which is now insufficiently characterized to develop fundamental predictive models of thermal cracking i e involving intermediates such as radicals and olefins such models presently apply only to the cracking of simple molecules hexane hexadecane at limited degrees of cracking for this reason kinetic models are still empirically calibrated on pyrolysis experiments and or on natural maturation trends the most popular models involve a series or a continuous distribution of parallel reactions to describe primary cracking i e direct formation of mobile compounds from kerogen these are generally calibrated against open system pyrolysis with various temperature programs consideration of natural maturation trends i e decrease in petroleum potential may be used also for calibration but discrepancies of organic matter properties availability of deep samples and uncertainties on thermal history limit the effectiveness of such methods whatever the method main activation energies are found in the range 45 60 kcal mol secondary cracking which refers generally to cracking of mobile compounds cannot be modelled from open system pyrolysis nor from natural series unknown residence times but only from closed system pyrolysis although models of primary and secondary cracking still need experimental improvements and more realistic kinetic schemes their ability to predict quantities and composition of petroleum depends primarily on migration which is poorly known indeed migration controls the residence time of petroleum in hot zones of sedimentary basins and thus determines the extent of oil conversion into gas as reproduced by recent numerical models preliminary studies documenting the interactions between thermal cracking and expulsion are shown from haltenbanken area and paris basin in the last few years a kind of agreement has been reached that primary migration was most likely occuring in a separate phase from water and that a sufficient concentration of petroleum must be generated for expulsion to start indeed high expulsion efficiencies over 50 are systematically found for rich and mature source rocks a further understanding of primary migration requires observations of natural series petrophysical studies of source rocks porosities under 10 permeabilities in the nanodardy range thermodynamic investigation of unconventional multiphase organic systems kerogen liquid s gas and experimental simulation under stress this on going research should enable models to track correctly migration paths and thermal history of petroleum fluids as necessary to model their composition

intraplate stresses a new tectonic mechanism for fluctuations of relative sea level stress variations in the lithosphere of a few hundred bars can explain the major part of the seismostratigraphic record at passive margins and intracratonic basins however to induce apparent sea level fluctuations with magnitudes greater than 50 m changes in stress level of more than 1 kbar are required these must be related to major reorganizations at convergent plate boundaries fragmentation of plates or collision processes specific rapid falls in the vail et al curves can be associated quantitatively with particular plate tectonic reorganizations of lithospheric stress fields the seismostratigraphic record may provide a new source of information on paleo stress fields to be correlated with results of independent numerical modeling of intraplate stresses

organic inclusions and their relations to oil and gas occurrences abstract 154 core and outcrop samples of proterozoic and palaeozoic reservoir and non reservoir strata were collected from renqui subei guangxi sichuan etc organic inclusions contained in the samples were studied by means of polarized light microscope fluorescence microscope micro cooling heating stage and compositional analysis according to hc phase colour refractive index and fluorescent properties organic inclusions can be classified as two braod types hc inclusions and hc containing inclusions which can be further subdivided into eight subtypes also discussed in this paper are the types and peculiarities of organic inclusions as well as the composition of gases parameters for organic molecules and paleotemperature which are considered to be useful indicators for oil and gas migration evolution and prospective evaluation some typical examples are also given in this study

secondary porosity creation of enhanced porosities in the subsurface from the dissolution of carbonate cements as a result of cooling formation waters abstract theoretical calculations indicate that the cooling of pore waters initially in equilibrium with a carbonate mineral can result in the water becoming undersaturated with respect to that mineral if the formation water has an initial ph below 6 9 then cooling will result in a water with a considerable potential to leach calcite the temperature of formation waters may be forced to decrease where faults act as major vertical migration pathways for waters escaping from a compacting basin where faults cease to act as permeable pathways the escaping formation water may be forced through an aquifer in a partly carbonate cemented aquifer the cooled formation waters are capable of dissolving the carbonate minerals and creating porosities that are higher than would otherwise be expected on the basis of burial depth

stratigraphic modelling of epicontinental basins two applications abstract this paper highlights some recent results of computer simulations of sedimentary basin fills two types of applications of the simulation program are illustrated from epicontinental basins 1 1 conceptual modelling computer simulations are ideal for establishing and quantitatively testing geological models here we investigate hierarchical levels of stratigraphic cyclicity and record some criteria to distinguish cycles caused by custatic variations and by fluctuations in clastic input or in intraplate stress these possibilities may be differentiated by details in stratal patterns time lines 2 2 evaluation of real basin stratigraphy this application is demonstrated with a calibration exercise from the paris basin data from only one well proved sufficient to simulate adequately the gross features of the basin fill with more well data being incorporated more acurate simulations are obtained stratigraphic modelling is useful as a learning tool and as a predictive tool and is best used in conjunction with conventional basin analysis techniques constraining the input parameters e g by more empirical data and the model sensitivity will remain a major task for future work

the pristane n heptadecane ratio as an indicator for recognition of hydrocarbon migration effects a simple scheme is proposed for type iii kerogen source rocks which uses pristane n heptadecane concentration ratios for recognition of hydrocarbon expulsion effects within the maturity interval 0 82 1 03 rm this scheme can be applied from gas chromatography data routinely obtained by most petroleum geochemistry studies in exploration exploration of this scheme to oil prone type ii source rocks appears very promising

geochemical effects of primary migration of petroleum in kimmeridge source rocks from brae field area north sea ii molecular composition of alkylated naphthalenes phenanthrenes benzo and dibenzothiophenes abstract variations with depth in the quantitative distributions of di and tricyclic alkylaromatic hydrocarbons and sulfur heterocycles determined for a total of thirty two core samples from two thick shale intervals and thirteen thin interbedded shale layers of the upper jurassic age kimmeridge clay formation brae field area north sea reveal in part the effects of depletion with hydrocarbon expulsion trends of increasing depletion with approach to the nearest shale sandstone contacts as documented by a decrease in carbon normalized yields are more distinctive for the alkylnaphthalenes as compared to the alkylphenanthrenes and alkyldibenzothiophenes but less drastic than previously observed for long chain alkanes cf part i leythaeuser et al 1988 lowest yields consistently were encountered at the outermost 1 2 metres of the thick shale intervals and for the thin shale layers where the relative expulsion efficiencies were much higher for c15 to c30n alkanes 80 95 than for c2 and c3 alkylnaphthalenes 30 40 in contrast c2 and c3 alkylbenzothiophene yields were higher in these shale portions than further inside the thick shale which is interpreted to result from a post expulsion selective replenishment from the adjacent oil accumulation expulsion was not associated with any detectable fractionation among c1 and c3 alkylnaphthalenes or c1 and c2 alkylphenanthrenes as evidenced by a mass balance approach and concentration ratios of individual alkylaromatic isomers commonly used as maturity indicators based on a reconstruction of the quantitative distributions of aromatic hydrocarbons and sulfur aromatics expelled from thin shale layers from early mature through to mature stages of thermal evolution it was verified that the methylphenanthrene index mpi 1 and methyldibenzothiophene ratio mdr of the expelled oil reflects the average of the maturity levels during which the source rock has expelled petroleum

microscopic and sedimentologic evidence for the generation and migration of hydrocarbons in toarcian source rocks of different maturities abstract this study is part of a comprehensive research program on the generation and migration of petroleum from the organic rich posidonia shale lower toarcian jurassic of the hils syncline northern germany four cores of the complete posidonia shale with varying amounts of underlying pliensbachian and overlying aalenian mudstones provided the sample base depending upon their distance from a presumed subjacent heat source the vlotho massif the cores display progressive levels of increasing thermal maturation with r 0 values 0 48 0 68 0 88 and 1 45 which span the limits of the oil generation window primary stratigraphic lithologic and petrographic attributes of the four sections are all very similar each includes a lower marlstone member about 5 m thick overlain by 11 31 m of calcareous clay shale clay minerals calcite pyrite kerogen and quartz are the principal constituents thus there is no evidence that depositional conditions of the posidonia shale varied significantly over the sampled area the omnipresence of framboidal pyrite clearly formed from the activity of sulphate reducing bacteria at or near the depositional interface is strong evidence that uniform reducing anoxic conditions were persistent at all localities this conclusion supports the assumption that initially each section was identical in overall properties and that significant differences observed in geochemistry organic macerals and other properties are directly related to the variable levels of thermal maturation kerogen from the least mature section r o 0 48 is type ii and composed of alginite a alginite b liptodetrinite bituminite and some terrestrial sporinite vitrinite and inertinite macerals with increasing maturation the alginite macerals exhibit a progressive change in morphology and fluorescence properties alginite of the least mature section displays a strong green fluorescence at 0 68 r o alginite macerals exhibit yellow orange fluorescence and are structurally well preserved although geochemical data indicate generation and loss of large amounts of oil at 0 88 r o hydrocarbon generation is almost complete and few primary alginites remain they have been transformed into secondary liptinites designated as metaalginite which have similar fluorescence colours at the highest maturation level 1 45 r o metaalginites are only weakly fluorescent with a shift toward red colours calcite and or bitumen filled bedding plane micro and macrofractures occur exclusively in the organic rich posidonia shale at maturity levels of 0 68 r o and above such features are totally absent in the immature posidonia section and in the adjacent organic poor mudstones this observation indicates that the fractures developed in direct response to overpressuring i e pore pressure exceeding lithologic load related to oil and gas generation in the source rock the fractures undoubtedly provided important expulsion conduits for generated products