anatomy of a world class source rock distribution and depositional model of silurian organic rich shales in jordan and implications for hydrocarbon potential silurian organic rich hot shales were the source of large amounts of hydrocarbons in northern gondwana with supergiant and giant fields in the middle east and north africa a study of these black shales has been conducted in jordan where they represent the source for the risha gas field two organically enriched horizons occur in the silurian in jordan termed the lower hot shale and the upper hot shale deposition of the transgressive lower hot shale occurred during the early llandoverian and was restricted to the earliest silurian paleodepressions three lower hot shale depocenters have been identified in jordan they are located in the western risha eastern wadi sirhan and jafr areas the eastern risha area was part of a larger scale paleohigh covering northeast jordan most of syria and iraq and north central saudi arabia qusaiba area at least in jordan the high coincides with the depocenter of the latest ordovician glaciation sedimentation of the upper hot shale occurred around peak sea level during the late llandoverian and or early wenlockian and was limited to the distal parts of the shelf risha area in front of the prograding silty sandy deltaic front thermal maturity increases from immature in the southern desert outcrops to late or postmature in northern jordan organic richness and pyrolysis data deteriorate significantly with increasing thermal maturity because of hydrocarbon generation prior to maturation maximum organic richness is interpreted to have considerably exceeded 10 with good s2 remaining generative potential yields as reflected in the values of the immature lower hot shale in the shallow borehole bg 14 in the southern desert outcrop area and exploration well hunt jf 1 in the jafr area identification and isopach mapping of the lower hot shale is based on its characteristic uranium enrichment the gamma ray total organic carbon correlation coefficient increases with increasing maturity suggesting that uranium is less vulnerable to maturity effects than the organic matter itself the gamma ray log therefore can be used as a proxy for the prematuration organic richness representing a suitable basis for cross basin comparisons of original organic richness trends a two phase generation of hydrocarbons is interpreted for both the risha and wadi sirhan areas with the first as a pre hercynian phase in the carboniferous and the second as a less intense generation phase during the mesozoic to early tertiary in contrast gas generation of the silurian sourced giant fields in the central part of the arabian peninsula and iran was restricted to the post hercynian in jordan both structural and stratigraphic play concepts are envisaged including fault paleohigh and channel plays key elements that need attention in future exploration of the silurian plays in jordan are ordovician reservoir quality and timing of maturation poor permeabilities have been reported from wells in both the risha and wadi sirhan areas

petrology of graptolites and their utility as indices of thermal maturity in lower paleozoic strata in northern yukon canada abstract the optical characteristics and utility of graptolites as a measure of thermal maturity have been evaluated by measuring maximum minimum and random reflectance of specimens from the lower paleozoic road river group in northern yukon the progressive changes in optical properties of graptolites produced by increasing depth of burial are corrected to the optical properties of other organic matter conodonts and bitumen in the same sample measurement of maximum reflectance ro max is the most reliable method of assessing the level of thermal maturity of gratolites because they have biaxial optical properties at high levels of maturation the reflectance of graptolites is greater than that of vitrinite whereas at low levels of maturation the reflectance of vitrinite is greater in the road river group the mean maximum reflectance of graptolites varies laterally from 4 0 to 6 5 ro max rrmomax f graptolites increasing depth of burial and the relationship between reflectance and depth fits a log reflectance linear depth relationship the vertical increase of maximum reflectance of graptolites through an 800 m section ranges from 4 2 ro max at the top to 6 5 ro max at the bottom yielding a maturation gradientof 0 39 log romaxkm 1 a correlation of graptolite reflectance calibrated by color changes of conodonts cai indicates that road river strata are overmature cai 5 4 0 6 5 ro max graptolite reflectance with respect to hydrocarbon generation

chitinozoan graptolite and scolecodont reflectance as an alternative to vitrinite and pyrobitumen reflectance in ordovician and silurian strata anticosti island quebec canada because of the absence of vitrinite in lower paleozoic sediments the reflectance of pyrobitumen is used as an indicator of thermal maturation a new technique of preparing organic matter permits discrimination among types of zooclasts and solid bitumen including pyrobitumen and leads to more precise reflectance histograms studies of organic matter from ordovician silurian strata of anticosti island show that reflectance of zooclasts is different from that of solid bitumen and that each group of the following zooclasts chitinozoans graptolites and scolecodonts has a specific reflectance evolution path with increasing depth on a logarithmic scale the paths of the three groups of zooclasts are linear highly correlated and converging however reflectance of solid bitumen is not linear throughout the range studied solid bitumen becomes linear only where the reflectance is greater than 0 75 comparing the relations between zooclasts and solid bitumen with the vitrinite solid bitumen correlation in the literature suggests that a hypothetical vitrinite evolution line in the anticosti island basin should be subparallel with that of chitinozoans and graptolites but 0 4 0 8 lower in the 1 2 range however scolecodonts and vitrinite should have the same reflectance at 1 but scolecodont reflectance should be slightly 0 2 higher for a vitrinite reflectance of 2

cambrian ordovician depositional sequences in the middle east a perspective from turkey abstract in southern turkey taurus chain or taurides and southeastern turkey border folds of the arabian plate nearly complete cambrian to ordovician successions are preserved four major sedimentary sequences are defined according to the main transgressive events and the subsequent shelf progradations these sequences allow us to link the turkish mostly distal lower palaeozoic detritals to the more proximal arabian formations in spite of contrasting facies the lower palaeozoic development is outlined through four successive steps defined in southern turkey 1 initiation of a cratonic platform regime terreneuvian i e earliest cambrian depositional sequence 1 with fluviatile conglomerates and red bed sediments associated with volcanics 2 development of a stable marine platform late terreneuvian to early ordovician depositional sequences 2 and 3 including widespread but diachronous middle cambrian carbonates and subsequent storm dominated detritals 3 tectonic instability middle ordovician leading to a new palaeogeography on the gondwana platform with the differentiation of a sag basin bedinan qasim depocenter lower part of depositional sequence 4 and 4 glaciation latest ordovician upper part of depositional sequence 4 identified by glaciomarine deposits forming the hirnantian glacial record this thick 2000 m detrital succession records a first order long term subsidence driven cambrian to ordovician transgressive trend and includes second order sequences reflecting interactions between global eustasy and tectonic instability on a larger scale the identification of recurrent sequence stratigraphic features with similar trends in the cambrian ordovician successions on the arabian platform implies that a former location of the taurus domain cannot be situated in the present day eastern mediterranean area but should lie directly north of the arabian platform this location places southern turkey at the junction between the north african segment of the gondwana siliciclastic shelf to the west present day coordinates and the eastern arabian segment in good agreement with mixed faunal signatures from both areas primary control of the tectonostratigraphic development of southern turkey during the lower palaeozoic may be a forced evolution from arc platform to rift drift transitions cordilleran model that has migrated progressively eastwards from the avalonian domain along the northern gondwana margin to the arabian domain throughout the lower palaeozoic

temporal spatial reconstruction of the early frasnian late devonian anoxia in nw africa new field data from the ahnet basin algeria anoxic conditions were widespread in nw africa during the early frasnian late devonian that resulted in deposition of organic rich shales and limestones with total organic carbon toc values of up to 14 organic richness and thickness of these sediments vary laterally and organic rich vs organic poor facies boundaries are likely to have been diachronous a precise temporal spatial reconstruction of this anoxic phase in nw africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts this contribution is based on eight outcrop sections at the margin of the central algerian ahnet basin where detailed spectral gammaray measurements were carried out using a handheld instrument the pre weathering organic richness in frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that based on well studies is positively correlated with the total organic carbon content conodont biostratigraphic results from these sections suggest that the uraniumenriched interval the anoxic interval at the basin margin is most common in the basal frasnian conodont biozones 1 2 confirming previous results from the anti atlas in morocco in three of the eight localities studied the basal frasnian has not been deposited and the frasnian here commences with distinctly younger uranium enriched intervals including zones 4 11 well data from the eastern algerian berkine basin is interpreted to indicate a significantly longer anoxic phase there million yearscale diachroneities of the frasnian anoxia therefore clearly exist across the north africa shelf it is assumed that the palaeorelief might have been a major factor in controlling the onset duration and intensity of anoxia in the region d 2003 elsevier b v all rights reserved

drilling mud contamination of samples in x ray diffraction and atomic absorption analyses drilling mud contamination in well cuttings may complicate determinations of minerals and elements commonly analyzed na k ba cr pb sr co ag ni in addition to caving and overrepresentation of the more consolidated fragments drilling mud contamination may cause ambiguity in sedimentologic studies utilizing cuttings analyses

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

source rock characteristics and biostratigraphy of the lower silurian telychian organic rich shales at akyaka central taurus region turkey the akyaka section in the central taurus region in the southern part of turkey includes the organic matter and graptolite rich black shales which were deposited under dysoxic to anoxic marine conditions in the early silurian a biostratigraphical analysis based on graptolite assemblages indicates that the sediments studied may well be referable to the querichi biozone and early telychian llandovery a total of 15 samples have been subjected to leco and rock eval pyrolysis and graptolite reflectance measurements for determination of their source rock characteristics and thermal maturity the total organic carbon content of the graptolite bearing shales varies from 1 75 to 3 52 wt with an average value of 2 86 wt the present rock eval pyrolytic yields and calculated values of hydrogen and oxygen indexes imply that the recent organic matter type is inert kerogen the measured maximum graptolite reflectance grmax values are between 5 04 and 6 75 corresponding to thermally over maturity this high maturity suggests a deep burial of the lower silurian sediments resulting from overburden rocks of upper paleozoic to mesozoic upper cretaceous and middle upper eocene thrusts occurred in the region

geochemistry of some volcanic rocks from south eastern sicily rare earth and other trace element distribution some trace element data for volcanic rocks found at different levels from tertiary to holocene in south eastern sicily iblean plateau and mt etna are presented and discussed in the present paper in order to better the information about the origin and relationships of the various rock types

guidelines for evaluating petroleum source rock using programmed pyrolysis rock eval pyrolysis is used to rapidly evaluate the petroleum generative potential and thermal maturity of rocks accurate conclusions require programs every 30 60 ft 9 18 m understanding of interpretive pitfalls and supporting data such as visual kerogen vitrinite reflectance and elemental analyses the generative potential of coals is commonly overestimated by pyrolysis and is best determined by elemental analysis and organic petrography most coals show high s2 s3 5 and low hi values 300 mg hc g toc migrated oil and mud additives which alter rock eval data can sometimes be removed by special processing for immature rocks bimodal s2 peaks and pi values over 0 2 indicate contamination pyrolysis downgrades organic poor clay rich rocks which show lower hi and higher tmax values than isolated kerogen because of adsorption of pyrolyzate on the clays tmax values for small s2 peaks 0 2 mg hc g toc are unreliable tmax is affected by maturation organic matter type contamination and the mineral matrix s3 is sensitive to inorganic and adsorbed carbon dioxide and to instrumentation problems acidification of carbonate rich samples and proper maintenance improves s3 measurement constant sample weights 100 mg are recommended below a threshold weight tmax increases by up to 10 c and other parameters decrease organic rich samples which overload the detector can be diluted with carbonate detector linearity is determined by pyrolyzing various weights of an organic rich rock

sequence stratigraphical interpretation of organic facies variations in marine siliciclastic systems general principles and application to the onshore kimmeridge clay formation uk

lower silurian hot shales in north africa and arabia regional distribution and depositional model abstract lowermost silurian organic rich hot shales are the origin of 80 90 of palaeozoic sourced hydrocarbons in north africa and also played a major role in petroleum generation on the arabian peninsula in most cases the shales were deposited directly above upper ordovician peri glacial sandstones during the initial early silurian transgression that was a result of the melting of the late ordovician icecap deposition of the main organic rich shale unit in the north african arabian region was restricted to the earliest silurian rhuddanian stage acuminatus atavus and probably early cyphus graptolite biozones during this short period 1 2 m y a favourable combination of factors existed which led to the development of exceptionally strong oxygen deficiency in the area in most countries of the study area the post rhuddanian silurian shales are organically lean and have not contributed to petroleum generation the distribution and thickness of the basal silurian hot shales have been mapped in detail for the whole north african region using logs from some 300 exploration wells in libya tunisia algeria and morocco in addition all relevant accessible published and unpublished surface and subsurface data of the lower silurian shales in north africa and arabia have been reviewed including sedimentological biostratigraphic and organic geochemical data the lowermost silurian hot shales of northern gondwana are laterally discontinuous and their distribution and thickness were controlled by the early silurian palaeorelief which was shaped mainly by glacial processes of the late ordovician ice age and by pan african and infracambrian compressional and extensional tectonism the thickest and areally most extensive basal silurian organic rich shales in north africa occur in algeria tunisia and western libya while on the arabian peninsula they are most prolific in saudi arabia oman jordan and iraq the hot shales were not deposited in egypt which was a large palaeohigh at that time the depositional model presented may help in better understanding the source potential of the basal silurian shales in less explored regions of north africa and arabia including morocco northern niger and the kufra basin in southeast libya

diagenesis and anchimetamorphism in an overthrust belt external domain of the taconian orogen southern canadian applachians ii paleogeothermal gradients derived from maturation of different types of organic matter abstract 3hermal maturation patterns have been established for the external domain of taconian orogen of the southern quebec appalachians between quebec city and the u s border reflectance of three types of organic matter om measured in the same sample shows that graptolite reflectance r g is higher than pyrobitumen reflectance r b which in turn is higher than reflectance of amorphous organic matter r a r g r b and r a are converted to vitrinite equivalent reflectance from which maximum paleotemperatures were calculated inverted diagenesis is preserved in northwest part of the chaudiere nappe to the southeast original burial diagenetic grade was upgraded by later synorogenic heating in the st lawrence lowlands paleogeothermal gradients at maximum burial were 33 37 c km a gradual southwestward increase in maturation level suggests increasing post taconian burial depth from 4 6 to 6 7 km indicating an increasing thickness of eroded rocks from northeast to southwest along logan s line one of the most important contributions of this study is the proof that the acadian orogeny has affected the successor basin of the taconian orogeny in the st lawrence lowlands the thermal effect of the mid cretaceous intrusion of the monteregian hills on their country rocks is limited to one diameter or less of the intrusive body as shown by the narrow

late ordovician glaciation in southern turkey abstract we present a new survey of several palaeozoic sections in both the taurus range and the border folds that documents typical glacial features including a glacial pavement and striated dropstones halevikdere formation and demonstrates the former presence of an ice sheet in southern and south eastern turkey evidence for the late ashgill hirnantian age of this episode is provided by macro and microfossils found within the glacial formation the extension of ice related deposits into the northernmost part of the arabian platform mardin region implies a much wider distribution of the ordovician ice sheet than was previously believed and strongly suggests that southern turkey lay close to egypt during the lower palaeozoic

correlations among the reflectances of vitrinite chitinozoans graptolites and scolecodonts abstract paleozoic marine sequences sampled in outcrops and wells of northeastern gaspe peninsula quebec appalachians yield kerogen composed in part of vitrinite and zooclasts chitinozoans scolecodonts and graptolites on which reflectance measurements were made the comparison between means of random measurements show that reflectance of chitinozoans is similar to that of telinite vitrinite with a cellular texture and that the reflectance of scolecodonts is significantly lower than that of telinite in the lower range of studied values at higher reflectivities differences decrease but at the end of the condensate gas zone reflectancea are not yet identical as shown by previous studies reflectance of graptolites is slightly less than that of telinite pyrobitumen reflectance is related to the lithology of the host rock and no generalized correlation can be established between the reflectance of pyrobitumen and that of vitrinite

dispersed organic matter from silurian shales of the barrandian basin czech republic optical properties chemical composition and thermal maturity abstract samples of lower silurian liten and kopanina formations dark shales from the barrandian basin czech republic were examined using reflected light microscopy fourier transform infrared spectroscopy ft ir and gas chromatography mass spectrometry gc ms the samples contained uniform assemblage of organic material that included dominant graptolites chitinozoa bitumens and minor anthracite to meta anthracite rank particle of unknown origin two types of graptolite morphologies were recognized a nongranular blocky type and a granular type bitumen is also common in these rocks and forms discrete oval bodies resembling droplets or angular particles with granular internal fabrics the thermal maturity of the sediments was examined by measuring random reflectance of organic particles the variations in graptolite reflectance in between three localities in the western part of the basin were relatively minor suggesting a similar level of diagenetic transformation the values range from 0 78 to 1 53 rr the reflectance of graptolites was correlated with the reflectance of chitinozoa and bitumen and also indirectly with illite crystallinity data which indicates thermal maturation levels in the high temperature part of the oil window to the onset of wet gas condensate zone mature petroleum that commonly impregnates veins crosscutting the sequence further supports a maturation level within the oil window graptolite and bitumen reflectance values markedly elevated above the regional diagenetic background were found in several centimeter wide contact zones immediately adjacent to basalt sills that locally penetrate the silurian strata as shown by computer modeling heating in the range of 600 800 c which lasted only several years was sufficient to promote an increase in reflectance up to 2 2 5 rr in the contact samples with increasing thermal stress the graptolite periderm undergoes progressive optical and structural changes being gradually converted into a highly condensed aromatic residuum structurally and chemically similar to highly matured kerogen on a basin wide scale however the overall impact of basalt intrusions on organic maturity of enclosing shales was minimal

effective petroleum source rocks of the world stratigraphic distribution and controlling depositional factors six stratigraphic intervals representing one third of phanerozoic time contain petroleum source rocks that have provided more than 90 of the world s discovered original reserves of oil and gas in barrels of oil equivalent the six intervals are 1 silurian generated 9 of the world s reserves 2 upper devonian tournaisian 8 of reserves 3 pennsylvanian lower permian 8 of reserves 4 upper jurassic 25 of reserves 5 middle cretaceous 29 of reserves and 6 oligocene miocene 12 5 of reserves this uneven distribution of source rocks in time displays no obvious cyclicity and the factors that controlled the formation of source rocks vary from interval to interval maps that show facies structural forms and petroleum source rocks were prepared for this study analysis of the maps indicates that several primary factors controlled the areal distribution of source rocks their geochemical type and their effectiveness i e the amounts of discovered original conventionally recoverable reserves of oil and gas generated by these rocks these factors are geologic age paleolatitude of the depositional areas structural forms in which the deposition of source rocks occurred and the evolution of biota the maturation time of these source rocks demonstrates that the majority of discovered oil and gas is very young almost 70 of the world s original reserves of oil and gas has been generated since the coniacian and nearly 50 of the world s petrole m has been generated and trapped since the oligocene the term petroleum as used in this study includes oil gas condensate and gas but does not include bitumen and other solid and semisolid oil derivatives

tectonic and depositional model of the arabian and adjoining plates during the silurian devonian during the late ordovician and early silurian the western part of the arabian peninsula was covered by polar glaciers that advanced from the south pole in african gondwana during this period nondeposition erosion or marginal marine conditions prevailed in eastern and northern arabia when the glaciers melted in the early silurian sea level rose sharply and the paleo tethys ocean transgressed the arabian and adjoining plates depositing a thick organic rich shale directly over the glaciogenic and periglacial rocks and related unconformities the post glacial sequence coarsens upward reflecting the passage of a coastline prograding northward from african and arabian gondwana to northern arabia a sea level drop in the late silurian placed the study area in a terrestrial environment however as sea level recovered in the early devonian a carbonate sequence blanketed most of the area the transgression however was interrupted by regional uplift and local orogenic movements in the middle and late devonian these movements constitute the onset of hercynian tectonism which resulted in erosion of the older sequences depositional hiatuses and regional facies changes

re evaluation of the petroleum potential of the kufra basin se libya ne chad does the source rock barrier fall abstract the kufra basin is a large underexplored palaeozoic intracratonic sag basin in se libya and ne chad with extensions into nw sudan and sw egypt the basin fill consists of shallow marine to fluvial deposits ranging in age from infracambrian to cretaceous geologically the basin is very similar to the murzuq basin in sw libya which recently presented libya with its largest oil discovery for over a decade most of the hydrocarbon play elements known from the murzuq basin also occur in the kufra basin thick porous cambro ordovician sandstones are present and would form good reservoirs lower silurian shales may act as effective seals and there are potential structural traps in seismically defined fault blocks however the source rock availability in the kufra basin is currently unclear one of the two main source rock candidates in the basin is a lower silurian shale unit tanezzuft formation the tanezzuft shales have been described as being up to 130 m thick in outcrops at the basin margins but the shales were found to be replaced by siltstones and sandstones in two dry exploration wells drilled in the northern part of the basin by agip between 1978 and 1981 hot shales developed at the base of this widespread silurian shale unit form important source rocks in many areas of north africa and arabia these hot shales are interpreted to have been deposited in palaeodepressions such as incised valleys of the preceding lowstand or intrashelf basins during the initial transgression after the melting of the late ordovician ice cap the areal distribution of the organic rich unit is therefore discontinuous fieldwork in the kufra basin has shown that the basal tanezzuft horizon is not exposed on the northern and eastern margins of the basin deep infracambrian rift grabens have been interpreted on seismic lines from the kufra basin and in analogy to oman and algeria could contain organic rich infracambrian deposits the infracambrian succession in the kufra basin may contain a second major potential source rock and warrants further investigation

organic petrography of graptolite fragments from turkey abstract the reflectance and bireflectance of graptolite fragments silurian ordovician from southeast turkey were examined and the morphology was described using reflected light microscopy the dispersion of maximum and minimum reflectance of graptolites sectioned parallel and perpendicular to the bedding was also determined the graptolites were anisotropic and the anisotropy was stronger for sections perpendicular to the bedding the graptolite fragments show two types of surface morphology granular and non granular the granular type had a rough surface and granular anisotropy the non granular fragments showed a smooth surface and basic anisotropy the fine internal structure of graptolite was evident under crossed polars the reflectance of graptolite increased with depth and was higher than other organic material e g bitumen found in the same sediment the present study indicates that the reflectance of graptolite in sediment can be used to determine the maturity of the dediment

precambrian to jurassic rocks of arabian gulf and adjacent areas their facies depositional setting and hydrocarbon habitat the first sediments to onlap the metamorphosed precambrian arabian shield were infracambrian proterozoic to middle cambrian carbonates clastics and evaporites the oldest arabian reservoir rocks occur in the precambrian to lower paleozoic ara salt of the huqf group which forms the birba field of oman the middle cambrian sequence was followed by late cambrian through early permian marine sandstones and continental to littoral siltstones and variegated shales the first commercial oil discovered in the arabian gulf region occurs in fluvial sands of the ordovician to permian haima and haushi groups of the marmul field in south oman these strata are also productive in other fields and are sealed by unconformable contact with the al khlata formation or beneath shale of he albian nahr umr formation the deeply buried kerogen sediments of the huqf group to the southeast are believed to be the source rocks for these fields of south oman the late permian to triassic deposits of the arabian peninsula are mainly widespread carbonates and evaporites that were deposited during a period of relative tectonic stability their deposition on an epeiric shelf was punctuated by a series of transgressions and regressions significant gas reserves have been proven in deep wells in the arabian gulf these wells penetrate large deep structures in the permian khuff shelf carbonates these carbonates have developed secondary porosity and lie beneath interbedded shale and dolomites of the sudair or suwei formation the source of gas in the khuff is unknown but could lie in more deeply buried formations the large deep structures of the khuff are considered to be among the most attractive for gas potential in the region today the stable shelf depositional environment established during the permian and triassic continued through the mesozoic jurassic rock units of the arabian gulf formed a sedimentary megacycle of epeiric shelf carbonates these include relatively deep water mudstones and wackestones to shallow water grainstones and packstones with the greatest facies variations occurring in the post dhruma to arab d rocks some of the best and most extensive reservoirs of the area occur in the primary and secondary porosity of the upper jurassic arab formation and in the middle jurassic araej and dhruma formations of saudi arabia qatar and abu dhabi the sources for hydrocarbons in these jurassic sediments are thought to be in the dhruma and lower hanifa formation diyab equivalent of the gulf area n general the jubailah is a source in qatar and some of the offshore gulf the most common seals are the argillaceous carbonate at the top of the dhruma and the hith anhydrite the latter also marking the termination of jurassic deposition across the arabian platform the upper jurassic cycles of marine limestones and anhydrites of the arab and hith formations accumulated in response to changes of sea level caused either by regional epeirogenic movements or by eustasy

the lower silurian qalibah formation of saudi arabia an important hydrocarbon source rock 1 the lower silurian qalibah formation represents a distinct lithostratigraphic time transgressive sequence it is disconformably bounded below by the upper ordovician to possibly lower silurian sarah formation and above by the continental to marginal marine lower devonian and possibly upper silurian tawil sandstone formation this paper redefines the qalibah formation within these boundaries in its reference section near the town of al qalibah in northwest saudi arabia the qalibah is 499 m thick whereas in a subsurface composite section it reaches 955 m the qalibah formation is a coarsening upward progradational sequence consisting of a lower qusaiba shale member and an upper sharawra sandstone member the basal hot shale of the qusaiba is a black euxinic bottomset shale up to 70 m thick which was deposited during the early silurian sea level rise following the deglaciation of gondwana this paleozoic shale is organic rich and the most likely source rock of the low sulfur high gravity oil condensate and gas discovered in recent years in the paleozoic rocks of central and eastern saudi arabia lithologically similar silurian sequences were deposited over most of the broad stable shelf of gondwana from the middle east to the african sahara

variation of graptolite reflectance with depth of burial abstract changes in reflectance and bireflectance of graptolite with increasing depths of burial were studied in four sequences of samples two from boreholes in poland and sweden and from two outcrop sections in western and northwestern canada graptolites show similar optical changes in reflectance and bireflectance with increasing depths of burial to those of vitrinite maximum reflectance of low maturity graptolites r o max 1 1 1 7 increases gradually with depth of burial at 0 14 per 100 m for the polish samples and at a similar but more variable rate at 0 12 0 43 per 100 m for the canadian samples for high maturity graptolites r o max 8 0 9 0 in the chlorite zone of metamorphism indirect correlation between the optical properties of graptolites and those of vitrinite has been established using conodont alteration indices in general graptolites show higher reflectance levels than vitrinite for the same conodont indices of thermal maturity for immature rocks r o vitrinite 0 2 0 5 cai 1 5 graptolite maximum reflectances range from 0 6 to 1 2 for the oil generation zone r o vitrinite 0 5 1 30 cai 1 5 2 5 from 1 2 to 2 2

revised stratigraphy and relationships of lower palaeozoic rocks eastern taurus mountains south central turkey lithostratigraphic terminology is revised for rocks of ordovician age in the eastern taurus the seydisehir formation of late middle cambrian to early ordovician age is shown to be applicable in the eastern and western taurus as well as in south eastern turkey near deg irmentas northeast of adana the seydisehir formation is overlain with inferred unconformity by clastic strata of ashgill age referred to the ort tepe formation first defined south of hakk ri in the border folds of southeastern turkey palaeontological evidence is cited for the early silurian llandovery age of the unconformably overlying halityayla formation farther south on the coast near ovacik where the ort tepe formation is as yet unrecorded a general model is proposed showing relationships of cambrian and ordovician rocks and faunas in southern and southeastern turkey