carbonate source rock with low total organic carbon content and high maturity as effective source rock in china a review abstract carbonate rocks with high maturity and low total organic carbon content are widely distributed in china whether they can be effective source rocks remains controversial but they have great significance for the evaluation of hydrocarbon generation potential and exploration prospects of carbonate rocks dataset reveals that as the depth or maturity increases total organic carbon toc content atomic h c ratio and hydrocarbon generation potential of carbonate source rocks with low total organic carbon csrltoc all decrease because of hydrocarbon expulsion moreover some pyrolysis experiments also indicate that the csrltoc are able to generate and expel a quantity of hydrocarbons the above studies indicate that csrltoc can be effective source rocks for oil gas pools the unique features of carbonate rocks and csrltoc demonstrate that they are easier to expel hydrocarbons and contribute to oil gas reservoirs than shale and thus the lower limit of toc lltoc of effective carbonate source rocks could be smaller than shale 0 5 the lltocs of effective carbonate source rocks with type i ii1 kerogen mainly sapropelic group at the levels of immaturity and low maturity maturity high maturity and over maturity are toc 0 5 0 5 0 3 0 3 0 2 and 0 2 0 1 respectively if the carbonate source rocks are at the high over mature stage we could take 0 2 as the lltoc the present organic poor source rocks with high maturity are possibly moderate or good source rocks at the low mature level in the geological periods especially carbonate source rocks if type i ii1 kerogen some typical oil and gas fields in china derive partly or mostly from the csrltoc proving that csrltoc could be effective source rock in china

stratigraphy and source potential of the smackover formation in the northern manila embayment southwest alabama abstract petrographic studies of conventional cores and correlation of electric logs indicate that upper smackover hydrocarbon reservoir facies are more widely distributed in the northern manila embayment than previously recognized the embayment does not appear to have been a separate basin during early smackover deposition but rather was an extension of the mississippi salt basin a carbonate shelf and starved basin topography developed in the manila embayment during deposition of the smackover formation and is reflected in the distribution of smackover lithofacies fine grained clastics are abundant in shelf and backshelf environments source rock geochemistry of the manila embayment appears to be atypical of the eastern smackover trend because of a strong terrestrial influence although the upper smackover is organic rich potential for generation of crude oil is limited by abundant terrestrially derived kerogen and thermal immaturity the updip lower smackover nearest the paleoshoreline also appears to be characterized by terrestrial kerogen thermal maturity and source potential for crude oil in the lower smackover increase southward this contributes to explaining the clustered distribution of crude oil discoveries downdip in proximity to the thermally mature lower smackover source facies

oolitic ironstones and contrasting ordovician and jurassic paleogeography distribution of abundant ordovician and jurassic oolitic ironstones provides a test for current ideas about factors controlling their origin these include warm climate and deep weathering dispersed continents and highstand of global sea level most of the jurassic ironstones developed in middle northern latitudes on the unstable european part of assembled laurasia at a time of low global sea level mild moist climate and abundant vegetation most of the ordovician ironstones accumulated around the northwestern margin of assembled gondwana and armorica during highstand of sea level and an absence of land plants and in high southern latitudes that supported an ice cap by the end of the period these contrasts demonstrate that neither dispersed continents nor major highstand of sea level was a necessary factor moreover requisite weathering in ordovician time may have been induced by adequate soil air co 2 pressure maintained by elevated atmospheric p co 2 in tne absence of land plants

geochemical relationships of petroleum in mesozoic reservoirs to carbonate source rocks of jurassic smackover formation southwestern alabama algal carbonate mudstones of the jurassic smackover formation are the main source rocks for oil and condensate in mesozoic reservoir rocks in southwestern alabama this interpretation is based on geochemical analyses of oils condensates and organic matter in selected samples of shale norphlet formation haynesville formation trinity group tuscaloosa group and carbonate smackover formation rocks potential and probable oil source rocks are present in the tuscaloosa group and smackover formation respectively extractable organic matter from smackover carbonates has molecular and isotopic similarities to jurassic oil although the jurassic oils and condensates in southwestern alabama have genetic similarities they show significant compositional variations due to differences in thermal maturity and organic facies lithofacies organic facies reflect different depositional conditions for source rocks in the various basins the mississippi interior salt basin was characterized by more continuous marine to hypersaline conditions whereas the manila and conecuh embayments periodically had lower salinity and greater input of clastic debris and terrestrial organic matter petroleum and organic matter in jurassic rocks of southwestern alabama show a range of thermal transformations the gas content of hydrocarbons in reservoirs increases with increasing depth and temperature in some reservoirs where the temperature is above 266 f 130 c gas condensate is enriched in isotopically heavy sulfur apparently derived from thermochemical reduction of jurassic evaporite sulfate this process also results in increased h2s and co2 in the gas and depletion of saturated hydrocarbons in the condensate liquids thermochemical sulfate reduction probably depends on the mineralogic composition of the reservoir rock as well as temperature because some deep 18 000 ft or 5 5 km and hot 320 f or 160 c smackover and norphlet reservoirs contain low sulfur petroleum

armoured sponge spicules from panarea island italy implications for their fossil preservation abstract a variety of calcareous and siliceous skeletal components are associated with exceptional still active deposits of modern iron ooids off panarea one of the volcanic islands of the aeolian arc tyrrhenian sea italy both ooids and skeletal components occur as loose sediments influenced by submarine hydrothermal fluid vents whereas iron ooids are exclusively made of goethite laminae primarily nucleated on volcaniclastic material sponge spicules that represent most of the siliceous skeletal component develop laminated coatings concretions of varying sizes this partial or total coating consists of regularly banded fe rich layers exhibiting the same textural features and mineralogical composition goethite of the ooid cortex developed around inorganic cores spicules did not reveal any obvious attaching structure or interfingering with the surrounding coating and did not undergo bioerosion or any other evidence of biological intervention during their development a hydrothermal origin compatible with the general setting is therefore proposed for these armoured sponge spicules we believe that this unique modern case of iron concretions produced on siliceous spicules can contribute to explain other known fossil cases as well as to understand the taphonomy of this type of biogenic silica that seems rarely to have been preserved in such an extreme habitat

synsedimentary formation of ooidal ironstone an example from the jurassic deposits of se central iran abstract the study of the upper lower to middle jurassic badamu formation in se of central iran represents the first report of jurassic ooidal ironstone in iran and exhibits a record of ferruginous radial ooid dominated ironstone the studies showed that this chamosite or ferric oxide chamosite type ooidal ironstone is similar to minette type ironstones a shoreface zone in an epeiric or epicontinental sea is proposed for formation and accumulation of the ferruginous ooids and the ooidal ironstone fe derived from lateritic weathering has been transported into the shoreface zone via a fluvial current during the synsedimentary formation of the ooidal ironstone during the periods with moderate to lower energy or weakly agitated and quiet environmental conditions suspended ferruginous small radial ooids and fe were increased in the lower shoreface subzone higher environmental energy resulted in the development of large concentric ooids and limited distribution of fe in the upper shoreface subzone higher environmental energy also prevented the development of radial ooids and restricted fe precipitation on carbonate ooids variations in environmental energy in the middle shoreface subzone were unfavorable for increasing ferruginous ooids and fe in the environment although short term periods with weakly agitated and quiet or moderate to lower energy conditions could had been existed in the middle shoreface subzone but precipitation of fe on a few small radial ooids was allowed environmental energy in the shoreface zone was the major factor controlling precipitation of fe especially suspended river fe loads on the ooids however precipitation of dissolved fe could have been related to oxidizing conditions in the environment also the probable presence of fe oxidizing bacteria in the cortexes of the ferruginous small radial ooids indicates that bacteria could have possibly contributed to precipitation of fe on the ooids in a calm environment

regional trends in water chemistry smackover formation southwest arkansas geochemical and physical controls we present results on regional geochemical analyses of formation waters from jurassic carbonate strata of the upper smackover formation from the southwest arkansas shelf gulf coast basin these formation waters have been noted in earlier studies for their exceptional salinities and abnormally br rich composition our detailed and regionally extensive database shows that chemical heterogeneities occur on a relatively small scale and have spatial relations to faults which may act as pathways for fluid migration from deeper and hotter portions of the stratigraphic section one of the most pronounced heterogeneities in water chemistry is an h 2 s rich anomaly in the center of the shelf enrichments in h 2 s are accompanied by large increases in alkali elements li k and b and b which could be produced by higher temperature diagenesis of clay minerals and feldspars in deeper seated siliciclastic strata bromide enrichments in all smackover formation waters are apparent from relations between br and cl contents as well as br and divalent cation carpenter function values as suggested by earlier studies of these waters evaporatively concentrated seawater expelled from associated thick monomineralic evaporite sections may be an important source of br cl and na whereas ca mg and sr concentrations have been modified by carbonate mineral recrystallization and dolomitization mineral equilibria exert significant controls on concentrations of ca mg sr and so 4 in these brines br enrichment likely was produced by halite recrystallization in fluids already enriched in br from seawater evaporation elevated cl br ratios occur in regional association with lower salinity water near t e south arkansas fault system consistent with halite dissolution during meteoric water recharge thus despite relatively uniform reservoir lithology and simple structural setting processes on small spatial scales have resulted in significant heterogeneity in water chemistry within a small area of the gulf coast sedimentary basin although movement of fluids along these fault systems is not demonstrated in our study the regional trends in geochemistry are strongly suggestive of their important role in controlling chemical heterogeneities in smackover formation waters

the highest latitude foliomena fauna upper ordovician portugal and its palaeogeographical and palaeoecological significance abstract here we report and describe for the first time the foliomena fauna fossil record from portugal kralodvorian upper katian this deep water brachiopod association originates from the uppermost beds of the porto de santa anna formation uppermost katian cropping out in several localities of the portuguese bucaco penacova region central iberian zone the three studied localities have yielded different associations composed of 15 species of brachiopods and two trilobites including both key elements of the deep water foliomena fauna and taxa typically from shallower benthic biotas e g the nicolella fauna the mixed character of the associations indicates that they are probably representatives of the atypical foliomena fauna characteristic of relatively shallow benthic assemblages the portuguese associations represent the highest latitude occurrences of the foliomena fauna so far and one of the youngest in the analyses carried out the two portuguese associations are grouped together with the other peri gondwanan associations sardinia and carnic alps and are included within a major grouping including associations previously interpreted as inhabiting deep water environments and also predominantly late katian in age according to these results the association of locality 2 may have inhabited a slightly deeper environment than the one present in locality 3 this agrees with the presence in the former of a greater number of deep water genera despite of the occurrence of a great number of deep water taxa in the association of locality 2 the co occurrence of several elements related to the shallower water nicolella fauna probably transported offshore as a result of the slope of the inclined shelf suggest that this association probably inhabited slightly shallower environments than those expected for a typical foliomena fauna ba5 6 with no shallow water elements represented this inference is also supported by the absence from the association of cyclopygid trilobites

origin of ooidal ironstones in relation to warming events cretaceous eocene bakchar deposit south east western siberia abstract an integrated study involving petrographical mineralogical and geochemical investigations proposes the origin of the bakchar ironstone deposit by upward migration of a mixture of fe rich brine and hydrothermal fluid through marine sediments trace element concentrations of the ironstone deposits rule out continental derivation of fe while high ree concentrations indicate mixed hydrogeneous and hydrothermal source three main types of ironstone within the bakchar deposit are characterized by distinct mineralogical assemblage of authigenic minerals corresponding to oxic methanic and weakly sulfidic dysoxic conditions changing redox conditions of the depositional environment methane seepage in warm seawater and availability of iron influx possibly determined the final mineralogy of three varieties of ironstones the association of siderite with ferrimagnetic sulfides like greigite and pyrrhotite in type 1 ore indicate anaerobic oxidation of methane with limited bacterial sulfate reduction the ironstone deposits of narym kolpashevo and bakchar are contemporaneous with geological events like ocean anoxic event 3 oae 3 cretaceous paleogene k pg boundary and palaeocene eocene thermal maximum petm respectively all corresponding to warm seawater conditions

early telychian silurian marine siliciclastic red beds in the eastern yangtze platform south china distribution pattern and controlling factors the distribution pattern of early telychian turriculatus crispus graptolite biozone red beds in the eastern yangtze platform of south china is reconstructed based on regional geologic data the red beds are developed in three areas which are separated by regions without red deposition the distribution pattern indicates that the cathaysian oldland was the provenance of sediment rich in ferric oxides which are essential for the formation of red beds silurian marine siliciclastic red beds both in china and worldwide tended to develop during times of relatively low sea level coeval hematitic oolites that formed far from the coast may record a change from reducing to oxidizing conditions in the ocean furthermore it is likely that a fall in global sea level a transition from reducing to oxidizing conditions in the ocean and a cooling climate all of which were closely related to the early telychian valgu event promoted the global development of marine red beds during this period

arenig sedimentation and basin tectonics in the harlech dome area dolgellau basin north wales arenig ordovician clastic sediments crop out in the harlech dome region north wales and are placed in a single stratigraphic unit the allt lwyd formation this unit records a marine transgression onto an erosion surface produced during late tremadoc arc volcanicity four discrete petrofacies are denned and reflect differing proportions of detritus derived from tremadoc type basic intermediate igneous rocks and the local sedimentary basement initial shallow marine siliciclastic sandstones and conglomerates are overlain by extensive deep water mud rich units these generally shallow up into a complex arc apron deposit with sediments derived from the eroding tremadoc arc as well as from similar synchronous volcanics predominantly epiclastic sandstones and conglomerates were deposited in deltaic and tidal environments in an arc apron complex and capped by condensed mudstones and an ironstone deposited as sea level rose across these systems sediments were ponded in north south orientated troughs and derived from uplifted blocks facies and petrofacies distribution were controlled by syn sedimentary north south and northeast southwest faults the allt lwyd formation was ponded in a fault controlled basin the dolgellau basin one of a series of interconnected sub basins flooded by the arenig transgression the sediments preserved reflect deposition during the transgression of a volcanic arc prior to the extrusion of marginal basin type volcanics

impact of communal irrigation on the 2018 palu earthquake triggered landslides anthropogenic changes to the environment can enhance earthquake triggered landslides yet their role in earthquake disasters is often overlooked co seismic landslides frequently involve liquefaction of granular materials a process that reduces shear strength and facilitates downslope motion even on gentle slopes irrigation systems can increase liquefaction susceptibility and compromise otherwise stable slopes here we investigate devastating landslides that affected palu indonesia during the 28 september 2018 earthquake of moment magnitude 7 5 we document fields and buildings translated over 1 km down slopes of 2 and show that landslides were limited to irrigated ground a liquefied detachment was rooted upslope in a conveyance canal that supplied water to the irrigation network a strong correlation between landslide displacement irrigation infrastructure and the highest slopes 1 5 suggests a causative mechanism that should provoke urgent assessment of gently sloping irrigated terrain elsewhere in sulawesi and in tectonically active areas worldwide landslides triggered during the palu 2018 earthquake correlate spatially with the presence of irrigation systems according to satellite analyses suggesting that liquefaction of alluvial fans played a role

ironstones and green marine clays in the dongjeom formation early ordovician of korea abstract in the sino korean north china block the occurrence of ironstone is reported for the first time from the lower ordovician dongjeom formation korea the dongjeom formation comprises much quartzose sediment deposited on the extensive carbonate platform and is composed of three coarsening upward sequences ironstone beds are distributed on the tops of the lower two sequences suggesting that they mark approximately maximum flooding events during the relative lowstand of sea level when the dongjeom formation was deposited extensive weathering in the source areas coupled with higher order sea level rises resulted in the formation of the dongjeom ironstones the dongjeom ironstones include much green marine clays ironstones of the lowest sequence are characterized by glaucony facies and those of the middle sequence by chamosite facies chemical compositions of dongjeom glauconies reveal that they were originally nascent to slightly evolved 3 to 5 k 2 o wt and that they were affected by illitization after burial their immature compositions suggest that glauconies were formed in about 10 3 to 10 4 years or less on the seafloor the substrates for glauconitization are interpreted as detrital mineral grains regressions after glauconitization introduced iron oxides to the green grains eventually forming the iron oxide rich band under the most oxidizing conditions the distribution of glauconies in limited horizons suggests that the dongjeom glauconies are autochthonous in origin chamosites have granular shapes and are mixed with quartzose sand grains suggestive of reworking during storms the dongjeom chamosites were transformed from the precursor berthierines the occurrence of glauconies in correlative horizons in korea and north china may indicate that glauconitization in the host sediments which were deposited in far northern gondwana occurred under the influence of a coeval transgressive event

time specific aspects of facies state of the art examples and possible causes abstract the term time specific facies tsf was introduced to the scientific community by the late otto h walliser to refer to unique facies typical of particular narrow intervals some of which were related to bioevents in some senses however the concept was recognized much earlier and is even engrained in the very names of some geologic periods the concept of time specific facies is expanded slightly herein to include distinctive or unique regional to global characteristics of the sedimentary record that characterize particular intervals of geologic time the recognition of time specific and widespread processes in the sedimentary record is a critical step in unraveling the interplay between processes of differing scale these range from very short term but widespread facies that overlap with event deposits to general facies types that may persist for intervals up to 10s of millions of years in duration this paper briefly summarizes the history of development of the tsf concept provides examples to illustrate a few of the key aspects of time specific facies and offers a few tentative explanations for this phenomenon among the factors that control tsfs abrupt changes in redox conditions and early diagenetic mineralization sedimentary condensation often associated with abrupt sea level change altered climate and paleoceanography and biotic evolution and extinction seem to be most critical and permit a preliminary genetic classification of tsfs inevitably however many tsfs reflect multiple effects and some remain largely unexplained

marginal faunal belts and their structural implications with examples from the lower palaeozoic fossils provide an independent criterion for testing structural hypotheses generated from field evidence or geochemistry as well as functioning in their traditional role in the dating of rocks or determining biogeographic realms on a global scale the identification of exterior oceanic biofacies may be of use in discriminating the course of former continental margins the isograptid graptolite biofacies early to middle ordovician can be distinguished from contemporary epicratonic graptolite biofacies its worldwide occurrence corresponds with continent margins reconstructed from other criteria hence a new isograptid record from irian jaya e new guinea although based on a small sample suggests that there may have been an ordovician oceanic site there on a continental scale the tectonic juxtaposition of different biofacies or of different stratigraphical packages contributes to an understanding of tectonics and can serve to quantify displacement the accretionary prism hypothesis for the southern uplands of scotland is supported and contrasted with the rearrangement of biofacies in the cambro ordovician cow head group of western newfoundland which reveals not only the original shelf margin slope topography now vanished but also provides an estimate of the movement involved in the emplacement of one taconic allochthon on a more local scale faunal evidence is used in assessing the original position of 9suspect9 marginal areas using the simplest possible model this shows for example that anglesey was not greatly separated from gondwana in the early ordovician but strike slip movement cannot be disproved local changes in relative sea level are determinable from sediments and faunas in marginal belts these may differ from the global eustatic curve and when this happens it is a measure of local tectonic activity particular sea level curves may serve to define structurally coherent units within mobile belts new sea level curves are contrasted for a segment of n wales north of the bala fault with that for central sw wales arenig llandovery the s welsh area mostly responded passively to the eustatic curve which can adequately account for shifts in facies and faunas while n wales had a curve often at odds with eustasy and reflecting its structural history

sources of environmental sulfur in the groundwater system southern new zealand abstract sulfide minerals commonly occur in sediments and basement rocks in southern new zealand as authigenic precipitates from groundwater below the oxygenated surface zone there are two principal potential sources for sulfur in the groundwater system weathering of sulfide minerals in the metamorphic basement and rainwater derived marine aerosols we present data for these two key sulfur sources metamorphic sulfide and associated hydrothermal au bearing veins within the otago schist average 34s 1 8 2 4 and an inland saline lake s derived entirely from rainwater 34s 21 4 0 8 we use these two end member 34s values to estimate the contributions of these sources of sulfur in authigenic groundwater sulfide minerals and in waters derived from oxidation of these sulfide minerals across a range of environments we show that authigenic groundwater pyrite along joints in the otago schist is derived primarily from metamorphic basement sulfur in contrast authigenic groundwater pyrite cementing miocene recent aquifers shows a substantial marine aerosol component and represents a distinct hydrogeological system we suggest that marine aerosols represent a significant flux to the terrestrial sulfur cycle that has been present through the groundwater system in otago over the past 20 million years

the ordovician iron ore of the anti atlas morocco environment and dynamics of depositional process abstract in morocco iron deposits of sedimentary origin are mainly developed in the ordovician where they are interbedded at different levels of the stratigraphic column a detailed analysis of the sedimentary facies shows that the iron ores are not randomly distributed but are concentrated at omission surfaces of the sequences thus raising the question of their sedimentological significance within the sequences that shelter them this study determines three morphological types of ore which provide a possible avenue for economic exploitation these morphological types are firstly from metric to decametric bodies of regional extension examples are caid rami and tachilla consisting of a set of detrital facies organized in two stratigraphic stages that cyclically recur to form genetic sequences within each sequence the maximum mineralization is at the top of the prograding basal stage and is expressed by diagenetic iron cement and at the base of the regressive stage in the form of strata of iron oolites or in massive beds secondly lenses with a thickness up to 30 m over a kilometer width example of imi n tourza field corresponding to incisions cut along the ravinement surfaces the maximum mineralization of this second type is within the median sandstone stage in connection with a large transgressive event thirdly there is a type of reworked material consisting of discontinuous layers of ferruginous conglomerates and bioclastics lining the ravinement surface that separates the higher order sequences it is this third type that has generated the most economic interest the iron bearing mineral phases identified are magnetite hematite iron chlorite maghemite iron hydroxides goethite lepidocrocite and siderite the ultimate source of iron would probably be the large reserves of banded iron formation of the west africa craton mauritania in particular intense tropical weathering would have leached the silica and left a highly enriched iron residuum that was carried as particulate iron to shallow marine shelf environments where fe3 was converted to fe2 during early diagenesis this reduced iron was recycled into the water column by tidal currents or storm waves where it precipitated as ferric oxide or ferrous silicate ooliths this depositional mechanism accounts for the positioning of ironstone beds at times of stasis and reworking

genetic origin of sour gas condensates in the paleozoic dolomite reservoirs of the tazhong uplift tarim basin abstract twenty gas condensates six oils and their associated formation water from the tazhong uplift in the tarim basin nw china were characterized for their molecular and isotopic compositions and water chemistry to elucidate the origins of the sour condensates thermochemical sulfate reduction tsr is the main secondary alteration process relevant to the generation of sour gas condensates those condensates are slightly heavier than regular condensates with a wide range of physical property and chemical composition variations the hydrogen sulfide h 2 s concentrations in gas phase show a strong positive correlation to gas oil ratio organosulfur compound osc concentrations in the liquid phase while methane isotopic values are mainly controlled by thermal maturity wet gas components show unusual 13 c enrichment at a low degree of thermal cracking geochemical anomalies occur mainly in the lower ordovician yingshan fm where more h 2 s has been discovered while the upper ordovician lianglitage fm shows less irregularity and h 2 s concentration is much lower generation of sour gas condensate is most likely caused by tsr alteration which could oxidize liquid hydrocarbons and form a large quantity of oscs isotopic enrichment of 34 s in formation water sulfate and elevated osc concentrations provide supplemental evidence of tsr alteration both burial history reconstructions based on fluid inclusion analysis and pvt simulation indicate that in situ tsr reactions mainly occurred around last 10 ma during the late himalayan orogeny and overall tsr extent is relatively low in the tazhong uplift

condensation and phosphatization of the middle and upper ordovician limestones on the malopolska block poland response to paleoceanographic conditions abstract the growth fault activity in the late middle ordovician resulted in the establishment of a horst like submarine paleohigh in the northern malopolska block this topographic submarine swell was the site of carbonate deposition characterized by a low overall net sediment accumulation rate the sedimentary and ecological conditions on this paleohigh were controlled by a paleocurrent that flowed along the northern margin of the malopolska block however seasonal intense storms associated with the mid latitude location of this terrane cannot be excluded the paleocurrent induced seasonal upwelling of nutrient rich waters from deeper parts of the current system that shifted onto the paleohigh and caused phosphatization of the sediment the winnowing and reworking of sediment together with slow carbonate production by a temperate water biota and relatively long periods of omission resulted in low accumulation rates and sediment condensation the cessation of the paleocurrent and associated phosphatization coincides with changes in the oceanic circulation system due to the late ordovician collision of avalonia with baltica and gradual closure of the tornquist sea

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

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

the lithostratigraphy of the upper ordovician of central portugal a lithostratigraphic scheme for the upper ordovician of central portugal is described incorporating previously erected local lithostratigraphic terms and new units thirteen formations 10 new including 11 members eight new with one named bed are arranged into four groups three new a description of each unit is given together with comments on its age and correlation deposition in a storm dominated shallow shelf environment through llandeilo and caradoc time is indicated differential subsidence occurred to the ne and sw of the dornes mac o rise particularly during the caradoc lower to mid ashgill sequences in the north are dominated by volcanic rocks with subordinate carbonate deposition farther south a hiatus preceded the regressive deposits associated with the onset of the upper ordovician glaciation hirnantian glaciomarine deposits occur across most of the area except for the regions of carbonate and volcanic build ups and appear to represent the period of waning of the glaciation the ordovician silurian boundary is difficult to place but probably lies close to or within a sheet sand body which underlies a condensed llandovery sequence lithostratigraphic correlation of the study area with w brittany france is generally good the relationship of some of the upper ordovician units in central portugal aids the interpretation of the relationships of the equivalent deposits in brittany

lower palaeozoic iron oolites and volcanism from a baltoscandian perspective the ordovician iron oolites in baltoscandia are generally geographically extensive but lean 30 ooids sensu bhattacharyya d p 1989 concentrated and lean oolites examples from nubia formation at aswan egypt and significance of the oolite types in ironstone genesis in young t p taylor w e g eds phaneorozoic ironstones geological society special publication 46 93 103 and have undergone minimal reworking and diagenetic alteration the ooids were formed in a normal shallow marine environment around a nucleus of a calcite skeletal grain the oolites were formed during the period of the highest global iron oolite production ever most of the major oolites from this period have been altered in various ways which has obscured their origin the presence of volcanic matter in the baltoscandian and some other european oolites points to a volcanic origin a conclusion further supported by the ree distributions in iron ooids which are close to those in associated volcanic tuffs the global 87sr 86sr curve for phanerozoic seawater in general and for the palaeozoic in particular shows that negative excursions in the sr isotope ratio often coincide with periods of high global iron oolite productivity this temporal coincidence points to a possible causal link iron ooids volcanic ashes and bentonite beds occur in the ordovician of baltoscandia during a negative excursion in the 87sr 86sr curve the variation in the 87sr 86sr ratio in seawater mirrors the balance between the two major sources of sr mid ocean ridge volcanism and continental weathering and indicate that volcanism could be the general cause of most palaeozoic iron oolites two cambrian iron oolites occur in baltoscandia and at least one of them is suggested to be of volcanic origin

chemical and carbon isotopic fractionations of gaseous hydrocarbons during abiogenic oxidation abstract the variations of chemical and carbon isotopic compositions in gaseous hydrocarbons upon abiogenic oxidation by sulfates hematite and both in laboratory studies were clearly demonstrated through a unique two step experimental approach in the first step a large number of small gold capsules containing gaseous hydrocarbons were prepared by kerogen pyrolysis experiments which were conducted under the same condition 450 c and 50 mpa for 72 h using the same kerogen the analytical results demonstrated that the chemical and carbon isotopic compositions of gases were almost identical among the capsules in the second step each of these gaseous hydrocarbon bearing capsules along with mineral oxidant hematite or magnesium sulfate heptahydrate or both and deionized water 15 of the amount of the oxidant was placed into a large gold capsule after welding sealing the small gaseous hydrocarbon bearing capsule was forced to leak while the large capsule remained undamaged by compressing the large capsule from the outside at the position where this small capsule was located then these large capsules containing gas and mineral reactants were heated isothermally at 350 c and 50 mpa for 72 144 216 and 288 h respectively the results of the oxidation experiments with increasing heating time can be outlined as follows 1 the amount of methane remained almost unchanged in the experiments using hematite and the mixed oxidants hematite mgso4 while it increased substantially in the experiment using mgso4 after 72 h indicating methane was one of the final products of c2 oxidation 2 the amounts of c2 c5 hydrocarbons decreased consistently and more rapidly in the experiment using mgso4 than that using hematite and the mixed oxidants 3 the oxidation rates of gas hydrocarbons increased with increasing carbon number of hydrocarbons 4 the oxidation rates of i butane and i pentane were substantially higher than those of the corresponding n butane and n pentane 5 the amount of h2s increased substantially in the experiment using mgso4 whereas it was below the detection level in the experiments using hematite and the mixed oxidants 6 the 13c values of c1 c5 hydrocarbons became less negative and the isotopic fractionation extent increased with increasing carbon number of hydrocarbons and oxidation extent 7 carbon isotopic fractionation factor k12 k13 decreased with increasing oxidation rate of gas hydrocarbons and 8 the rate of thermochemical sulfate reduction tsr was strongly dependent upon the presence and concentrations of h2s

geochemical studies of crude oil generation migration and destruction in the mississippi salt basin abstract the main source for crude oil in the mississippi salt basin is the laminated lime mudstone facies of the lower smackover crude oil generation and migration commenced at a level of thermal maturity equivalent to about 0 55 vitrinite reflectance short range lateral migration of crude oil was focused to upper smackover and norphlet reservoirs but vertical migration also charged some overlying cotton valley rodessa lower tuscaloosa and eutaw reservoirs following migration from the lower smackover thermal maturity history of reservoir rocks controls the preservation of crude oil gas condensate and methane slow thermal cracking of crude oil occurred in deep upper smackover reservoirs resulting in formation of gas condensate and precipitation of solid bitumen the maximum thermal maturity for preservation of condensate is equivalent to about 1 3 vitrinite reflectance only methane pyrobitumen and nonhydrocarbon gases including hydrogen sulfide persist at higher levels of thermal maturity early destruction of methane in deep upper smackover reservoirs near the wiggins arch is driven by thermochemical sulfate reduction lesser availability of sulfate in norphlet reservoirs in lower mobile bay could account for methane preservation at higher levels of thermal maturity one basic geochemical strategy for further exploration of the mississippi salt basin is to focus exploration effort on traps with reservoirs in the thermal maturity window for hydrocarbon preservation another strategy is to avoid drilling traps with overmature reservoir rocks

lower ordovician iron ooids and associated oolitic clays in russia and estonia a clue to the origin of iron oolites abstract on the baltic platform a lower llanvirn ordovician iron oolite can be traced for a distance of 1200 km from norway to the east of lake ladoga in russia this oolite is usually thin seldom exceeding 0 5 m and is dominated by goethite limonite type ooids the easternmost part of the oolite from tallinn to ladoga is examined here the oolitic limestone is intercalated with oolitic clay beds the mineralogical chemical and isotopic composition and other indicators point to volcanic ash being the source for the clay similarities in ree distribution patterns and immobile element contents between ooids and the oolitic clay suggest that the ooids were also formed from volcanic ash

ooidal ironstones from ordovician gondwana a review abstract ooidal ironstones occur widely in the ordovician shelf sequences of sw europe the western european platform the avalonian terranes and north africa they form the most important group of deposits of the earlier of the two major periods of phanerozoic ooidal ironstone generation ordovician devonian and jurassic paleogene individual ooidal ironstone horizons are generally thin less than 2 m but may have a very wide lateral extent the ooidal ironstones represent sediments formed under extremely low sedimentation rates and were most commonly developed as the initial deposit above a disconformity ooidal ironstone producing events occurred almost synchronously over wide areas of the gondwanan shelf reflecting the uniform sedimentary conditions across large areas of the shallow low gradient shelf clastic sediments deposited during periods of normal sedimentation show similar remarkable uniformity over large areas this geometry together with the abundant fauna both micro and macro in the ironstone facies means that these thin ironstone beds form important marker horizons for correlation within the region the stratigraphic distribution of the ironstones shows strong correlation with periods of high relative sea level sedimentological study of examples in sw europe indicate that they separate transgressive systems tracts with parasequences bounded by phosphorites from the early part of the following highstand systems tracts without significant parasequences and are therefore interpreted to lie on maximum flooding surfaces such maximum flooding surfaces occur in the early llavirn the late llanvirn the early caradoc and the early ashgill of sw europe outside sw europe the ooidal ironstones may have a different relationship to the sequence stratigraphy possibly reflecting the influence of absolute water depth

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

diamictite and oolitic ironstones a sedimentary association at ordovician silurian transition in the north gondwana margin new evidence from the inner nappe of sardinia variscides italy abstract the contact between the silurian black phyllite and the cambro ordovician underlying rocks has been investigated over different tectonic units affected by green schist facies metamorphism in the inner nappe zone of the sardinia variscides in spite of strain and metamorphism the field work highlighted the occurrence of diamictic sediments in the canaglia tectonic unit the diamictite consists of dark massive metamorphic claystone bearing chamositic ooliths chamositic nodules and millimetre to centimetre sized clasts dispersed or gathered in clusters within the muddy matrix in the argentiera tectonic unit the diamictite consists of angular clasts ranging in size from few millimetres to several decimetres scattered within a finely laminated black sericitic meta argillite field data textural and compositional analyses suggest a glacio marine environment for the formation of the diamictites the canaglia diamictite deposited in a protected glacial influenced shore compositionally it can be defined as ironstone in the upper part it hosts a horizon of clast supported conglomeratic hard ironstone mostly made of magnetite which testifies for sub aerial reworking the source of the iron is to be related to local glacio eustatic driven emergence of upper ordovician alkaline mafic volcanics these are widespread in the uppermost ordovician of the canaglia unit possibly linked to the rifting stage that invested the north gondwana margin before the uppermost ordovician early silurian sea level rise the argentiera diamictite deposited beyond the iron rich diamictite in the outer euxinic shelf that was reached by rain out of rafted debris

evaluation of the rhenium osmium geochronometer in the phosphoria petroleum system bighorn basin of wyoming and montana usa rhenium osmium re os geochronometry is applied to crude oils derived from the permian phosphoria formation of the bighorn basin in wyoming and montana to determine whether the radiogenic age reflects the timing of petroleum generation timing of migration age of the source rock or the timing of thermochemical sulfate reduction tsr the oils selected for this study are interpreted to be derived from the meade peak phosphatic shale and retort phosphatic shale members of the phosphoria formation based on oil oil and oil source rock correlations utilizing bulk properties elemental composition 13c and 34s values and biomarker distributions the 34s values of the oils range from 6 2 to 5 7 with oils heavier than 2 interpreted to be indicative of tsr the re and os isotope data of the phosphoria oils plot in two general trends 1 the main trend n 15 oils yielding a triassic age 239 43 ma with an initial 187os 188os value of 0 85 0 42 and a mean square weighted deviation mswd of 1596 and 2 the torchlight trend n 4 oils yielding a miocene age 9 24 0 39 ma with an initial 187os 188os value of 1 88 0 01 and a mswd of 0 05 the scatter high mswd in the main trend regression is due in part to tsr in reservoirs along the eastern margin of the basin excluding oils that have experienced tsr the regression is significantly improved yielding an age of 211 21 ma with a mswd of 148 this revised age is consistent with some studies that have proposed late triassic as the beginning of phosphoria oil generation and migration and does not seem to reflect the source rock age permian or the timing of re migration late cretaceous to eocene associated with the laramide orogeny the low precision of the revised regression 21 ma is not unexpected for this oil family given the long duration of generation from a large geographic area of mature phosphoria source rock and the possible range in the initial 187os 188os values of the meade peak and retort source units effects of re migration may have contributed to the scatter but thermal cracking and biodegradation likely have had minimal or no effect on the main trend regression the four phosphoria sourced oils from torchlight and lamb fields yield a precise miocene age re os isochron that may reflect the end of tsr in the reservoir due to cooling below a threshold temperature in the last 10 m y from uplift and erosion of overlying rocks the mechanism for the formation of a re os isotopic relationship in a family of crude oils may involve multiple steps in the petroleum generation process bitumen generation from the source rock kerogen may provide a reset of the isotopic chronometer and incremental expulsion of oil over the duration of the oil window may provide some of the variation seen in 187re 188os values from an oil family

late ordovician ice marginal processes and sea level change from the north gondwana platform evidence from the valongo anticline northern portugal abstract in the valongo anticline northern portugal the upper ordovician age sobrido formation records glacioeustatic sea level changes and glacially influenced depositional environments on the north gondwana platform associated with the hirnantian glaciation new stratigraphic data from the sobrido formation taking into account lateral facies variations not exposed in the type section allow for a better understanding of hirnantian glacial sedimentary processes sea level changes and palaeogeography the lower member of the sobrido formation is composed of quartzites alternating with siltstones and mudstones the transition to the upper member is most commonly an erosional contact marked by a ferruginous horizon the lowermost part of the upper member contains a thinly laminated and rhythmically bedded mudstone that is capped by a thin mm scale shelly bed and overlain by massive diamictites laterally conglomerate beds that have erosion bases are rhythmically interbedded with slates and diamictites that sometimes contain outsized mudstone clasts the uppermost part of the upper member is very variable in thickness laterally and contains massive to laminated diamictite beds intercalated with quartzites conglomerates and slates massive diamictites commonly exhibit thin horizons with fe oxides and occasionally manganese phosphate and oolitic chamosite the lower member of the sobrido formation is interpreted as ice proximal outwash that is transitional laterally to subaqueous ice contact fans formed by debris and mass flows erosion followed by chemical weathering of the lower member and variably within the lowermost part of the upper member is interpreted as a response to sea level fluctuations and development of spatially variable anoxic conditions and groundwater migration within isolated marine basins facies within the upper member of the sobrido formation are interpreted as ice distal laminated diamictites and ice proximal deposits massive diamictites quartzites and conglomerates groove casts and hummocky cross stratification within quartzite beds suggest deposition in a shallow shelf setting the upper ordovician lower silurian transition is locally marked by a ferruginous horizon black basal quartzites are interpreted to record development of an erosional unconformity during silurian sea level rise in other areas a chloritoid layer indicates proximity to dry land these palaeoenvironmental interpretations suggest dynamic changes in sedimentary and geochemical processes in response to sea level and related palaeogeographic changes during the upper ordovician and transitional to the lower silurian on the north gondwana platform

stable carbon isotope compositions of crude oils application to source depositional environments and petroleum alteration the carbon isotopic composition of the c15 saturate and aromatic hydrocarbon fractions and the gas chromatographs of the c15 saturate fraction of 339 oils were geochemically and statistically evaluated results of this study show that the average isotopic compositions of the c15 aromatic hydrocarbon fractions of oils sourced from terrigenous or marine organic matter are identical the average isotopic composition of the c15 saturate fraction of oils derived from terrigenous organic sources is slightly more negative 0 9 pmil than the average for marine oils but this difference is insufficient to be used as a reliable source indicator reports in the literature stating that marine oils are isotopically more positive than te rigenous oils and that the difference can be utilized to distinguish between the two are therefore not supported by this study the isotopic differences between oils sourced from terrigenous and marine organic matter do not manifest themselves in the range of absolute values of one oil fraction but rather in the isotopic relationship between the saturate and aromatic hydrocarbon fractions the isotopic relationship is the following oils of terrigenous organic source dgr13caro 1 12 dgr13csat 5 45 oils of marine organic source dgr13caro 1 10 dgr13csat 3 75 the difference between the two equations was evaluated statistically and a statistical parameter cv the canonical variable cv 2 53 dgr13csat 2 22 dgr13caro 11 65 was used to distinguish between marine and terrigenous oils cv values larger than 0 47 indicate predominantly a terrigenous organic source for the oil whereas cv values smaller than 0 47 indicate mostly a marine organic source thermal maturity differences in oils and isotopic variations in the source beds can cause as much as 2 pmil variations in the isotopic composition of one family of oils finally stable carbon isotopes can in some situations help correlate biodegraded oils to their nondegraded counterparts

wiggins arch southern mississippi new exploratory data from 3 d seismic newly released data from a 65 mi2 168 km2 3 d three dimensional seismic survey on the south flank of the wiggins arch reopens the question of exploratory opportunity in this sparsely drilled region although surrounded by significant production from deep jurassic and more shallow cretaceous reservoirs the wiggins has not yet been the site of successful exploration only nine deep tests however exist within an area over 2000 mi2 5180 km2 in size thus previous interpretations of low potential on the wiggins suffer from limited data the recently completed 3 d survey in conjunction with systematic evaluations of existing sample well log and 2 d two dimensional seismic information shifts the focus of attention from the smackover formation to overlying haynesville and cotton valley intervals data suggest that large scale oolite shoals exist in the upper haynesville immediately below the bossier shale potential shoals coincide in occurrence with a major northeast southwest trending anticlinal structure changes in the character of the haynesville reflector imply significant porosity development while time structure mapping and isochron thinning in the haynesville cotton valley interval indicate an important degree of potential entrapment in cotton valley sandstones a well planned for an updip portion of the anticline should yield extremely valuable information for future analyses of wiggins arch geology

modern and palaeozoic iron ooids a similar volcanic origin abstract the discovery of an unconsolidated deposit of iron ooids of a kind not previously described and in the vicinity of a volcanic island in indonesia has shed new light on the genesis of many ancient iron oolites the discovery shows that iron ooid formation is geologically a rapid process the ooids form by chemical precipitation of cryptocrystalline iron oxyhydroxides on available grains on the seafloor from seawater enriched with fe al and si the enrichment can be a result of hydrothermal fluids volcanic ash falls into shallow basins or rapid weathering of fresh volcanic rocks ordovician iron ooids from northern europe are strikingly similar to the modern ooids in chemical composition ree distributions and internal structures the differences in mineralogy can be explained as diagenetic transformation reactions under different environments a model for the formation of the ordovician iron ooids in northern europe from volcaniclastic material is presented which could be applicable to other ancient oolites in the world

norphlet formation upper jurassic of southwestern and offshore alabama environments of deposition and petroleum geology upper jurassic norphlet sediments in southwestern and offshore alabama accumulated under arid climatic conditions the appalachian mountains of the eastern united states extended into southwestern alabama to provide a barrier for air and water circulation during the deposition of the norphlet formation these mountains produced topographic conditions that contributed to the arid climate and they affected sedimentation norphlet paleogeography in southwestern alabama was dominated by a broad desert plain rimmed to the north and east by the appalachians and to the south by a developing shallow sea the desert plain extended westward into eastern and central mississippi norphlet sedimentation initiated as a result of basin subsidence accompanied by erosion of the southern appalachians norphlet conglomerates were deposited in coalescing alluvial fans in proximity to an appalachian source the conglomeratic sandstones grade downdip into red beds that accumulated in distal portions of alluvial fan and wadi systems quartz rich sandstones were deposited as dune and interdune sediments on a broad desert plain the principal source of the sand was updip alluvial fan and plain and wadi deposits wadi and playa lake sediments also accumulated in the interdune areas a marine transgression was initiated during the late phase of deposition of the norphlet formation resulting in the reworking of previously deposited norphlet sediments norphlet hydrocarbon potential in southwestern and offshore alabama is excellent six oil and gas fields already have been established petroleum traps discovered to date are primarily structural traps involving salt anticlines faulted salt anticlines and extensional fault traps associated with salt movement reservoir rocks consist primarily of quartz rich sandstones that are eolian wadi and marine in origin porosity is principally secondary dissolution with some intergranular porosity smackover algal carbonate mudstones were probably the source for the norphlet hydrocarbons jurassic oil generation and migration probably were initiated in the early cretaceous

phanerozoic ironstones an introduction and review s u m m a r y ironstones particularly ooidal ironstones have long fascinated sedimentary geologists and have generated an enormous variety of interpretations but have remained poorly understood a recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences the first part of this introduction examines recent advances and attempts to produce a synthesis of them with particular reference to the genesis of marine ooidal ironstones current models for the formation of marine oolitic ironstone formations are discussed and a working model based on various lines of recent research is proposed this model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis the ooids may be mechanically modified and mineralogically transformed by reworking as well as by later diagenesis goethite ooids may be oxidized berthierine ooids reworked lateritic ooids or primary grains the formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply continued supply of iron and physical reworking many sedimentary environments may supply these parameters but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply in the second part of this introduction the terminology and classification are discussed and conventions proposed the term berthieroid is introduced as a non specific term for material of undetermined berthierine or chamosite composition the term ironstone is proposed as a petrological term and as an informal lithostratigraphic term a deposit composed of ironstones may be termed an ironstone formation it is recommended that a dunham style of classification is employed petrographically with ironstone replacing the stone of the original carbonate classification the nomenclature of allochems in ironstones is also discussed a glossary of ironstone terminology is provided

diagenetic controls on reservoir development and quality smackover formation of southwest alabama abstract abstract smackover reservoirs in southwest alabama are much more complex than those encountered in the central and western portions of the gulf coast basin this heterogeneity is a product of a complex history of diagenetic modification the diagenetic history can be divided into five separate stages 1 marine stage extensive grain micritization and precipitation of minor amounts of fibrous cement 2 meteoric vadose stage extensive dissolution of aragonitic grains 3 meteoric phreatic stage precipitation of granular and blocky cements 4 brine reflux stage extensive dolomitization and sulfate emplacement associated with refluxing of buckner brines and 5 burial stage grain to grain compaction of lithologies not stabilized by early cements stylolitization precipitation of poikilotopic calcite and dolomite cements minor dissolution of calcite associated with the introduction into the formation of undersaturated fluids hydrocarbon migration and sulfate replacement and cementation the diagenetic sequence varies dramatically over short distances in southwest alabama reflecting variation in paleotopography the products of this complex diagenetic history are heterogeneous reservoirs which consist of one or some combination of primary interparticulate moldic dolomitic intercrystalline and vuggy porosity primary interparticulate porosity is less common in southwest alabama than in the central and western portions of the gulf coast basin largely because of extensive compaction or early cementation where present however interparticulate reservoirs are characterized by moderate to high porosity and good permeability moldic porosity is produced by early fabric selective dissolution of aragonitic allochems and is associated with areas of subaerial exposure moldic reservoirs commonly have high porosity but low permeability and are generally productive only when permeability is enhanced by other pore types dolomitic intercrystalline porosity is common in the smackover of southwest alabama intercrystalline porosity alone however is seldom of reservoir quality and hydrocarbon production from dolomitized lithologies is generally dependent upon the coexistence of moldic or vuggy porosity vuggy porosity is the product of late non fabric selective dissolution of calcite vuggy pores are produced by solution enlargement of earlier formed interparticulate moldic or intercrystalline pores vuggy reservoirs are by nature composite and are characterized by moderate to high porosity and good permeability

origin of deep sour natural gas in the ordovician carbonate reservoir of the tazhong uplift tarim basin northwest china insights from gas geochemistry and formation water abstract deep sour gases are found in the ordovician carbonate reservoir of the tazhong uplift in the tarim basin northwest china all sour gases in this field are dominated by gas hydrocarbons c1 c4 with varying amounts of h2s most of the gases in the eastern tazhong are characterized by high gas dryness coefficients 0 95 13c enriched methane 13c1 ranging from 44 6 to 35 8 and small difference between values of the 13c2 and 13c1 13c2 1 in contrast the gases in the western tazhong are characterized by relatively low gas dryness coefficients mostly

relationship between phosphogenesis and sequence architecture sequence stratigraphy and biostratigraphy in the middle ordovician of the armorican massif nw france abstract in the armorican massif nw france the middle ordovician formations contain abundant phosphatic p beds associated with terrigenous sediments laid down on a storm dominated shelf facies analysis and sequence stratigraphy carried out on sections located in the crozon peninsula w armorican massif allow us to distinguish four p facies 1 beds with p nodules and crusts contained within sediments laid down on the distal and median parts of the inner shelf 2 amalgamated beds with p nodules and crusts and 3 beds with lingulid brachiopods heavy minerals and p clasts associated with retrogradation phases 4 p clast bearing conglomerates associated with progradation phases the genesis of these facies and their location in the depositional sequences are controlled by the fluctuation of terrigenous inputs during sea level changes sediment volume partitioning we propose a genetic model integrating the superposition of very high frequency vhf and high frequency hf eustatic signals in this model the amalgamated beds and beds with lingulid brachiopods heavy minerals and p clasts result from the superposition of several sea level rise episodes of vhf cycles occurring within the retrogradation phase of a hf cycle the lateral extension of p facies was investigated in the eastern part of the armorican massif benioc domfront les tanneries and st aubin du cormier sections several early darriwilian transgressive events associated with phosphogenesis episodes are dated through high resolution chitinozoan studies some of these transgressive events are at least of regional scale e g ornensis and protocalix transgressive events the topmost calix transgressive event is clearly identified over the northern gondwanan and avalonian domains and may reflect a global eustatic signature

stratigraphic significance of ooidal ironstones from the cretaceous western interior seaway the peace river formation alberta canada and the castlegate sandstone utah u s a ooidal ironstones form prominent units and an appreciation of their sedimentological and stratigraphic significance is important for the interpretation of sedimentary successions ooidal ironstones from the albian paddy member of the peace river formation alberta canada and the campanian castlegate sandstone of the mesaverde group in colorado have been analyzed the ooidal ironstones are associated with a major sequence boundary and subsequent transgressive surface both are interpreted to have been deposited under conditions of low net sediment accumulation in well oxygenated bottom water conditions with episodic storm events reworking the sediments the paddy member ironstone is interpreted to have been deposited in an estuarine environment and the castlegate ironstone is interpreted to have been deposited in a shallow marine environment both ironstones are dominated by berthierine ooids and grain rimming and pore filling siderite with later ferroan dolomite and calcite in the castlegate ooidal ironstone petrographic geochemical and isotopic evidence coupled with thermodynamic considerations indicates that the ooidal ironstone mineralogy formed during suboxic diagenesis the conditions required for suboxic diagenesis were extensive sediment reworking and slow net sediment accumulation rates we propose that these conditions arose as a result of marine transgression during the initial relative sea level rise following the development of a sequence boundary ooidal ironstone formation ceased once sedimentation rates increased and transgression deepened water depths considerably these results illustrate the association between ooidal ironstones and major stratal surfaces in sedimentary successions and highlights the complexity that early diagenesis at such surfaces can take

origin and depositional environment of ordovician stratiform iron mineralization from zamora nw iberian peninsula the ordovician stratiform iron deposits at zamora nw iberia are arranged in several levels ranging between 0 2 and 1 5 m in thickness which are interstratified in the upper member of the pielgo quartzites formation arenig the sandy nature and trace fossils corresponding to the ichnogenus cruziana and daedalus together with major and trace element contents of this formation suggest an inter and subtidal shallow marine depositional environment which on a global scale formed part of a broad shelf situated in the northern margin of the gondwana continent the iron mineralization displays a foliated and banded structure due to the alternation of quartzitic phosphatic apatite chamositic chamositic biotitic and ferriferous magnetite and hematite beds they have high tio2 ta sc v nb co zn and y contents the magnetite contains unusually large amounts of tio2 v cr and ni there is also a clear depletion in eu and the eu sm cn ratio is 1 the chamosite contains high concentrations of cr and v these results suggest that iron was supplied from the weathering of a continental source in combination with volcanic activity such as within basin basic volcanism or the presence of basic volcanic rocks in the exposed land the physicochemical conditions of iron mineral crystallization calculated from chamosite compositions are the following log f o2 38 8 to 30 7 log f s2 13 2 to 9 5 and t 200 to 330 c these results together with the 18o value 2 of the magnetite suggest that chamosite and magnetite were crystallized during later diagenesis and early low grade metamorphism under redox conditions below the magnetite hematite buffer

evolution of a late jurassic carbonate evaporite platform conecuh embayment northeastern gulf coast u s a successive time slice lithofacies maps of the smackover haynesville depositional sequence constrained by marker beds an interpreted maximum flooding surface and interpreted sequence boundaries show an onlapping and retrograding sequence of lithologies characteristic of a transgressive systems tract tst the tst is followed by several prograding sequences characteristic of a highstand systems tract hst and stacked shelf margin wedges smw comparison of stratigraphic cross sections lithofacies maps and a paleotopographic map of the sub smackover transgressive surface shows that the distribution of lithofacies within the tst was strongly controlled by two types of sub smackover paleotopographic highs 1 norphlet dune fields and 2 eroded basement rock as th tst became progressively buried during deposition distribution of lithofacies within the hst and the smws was controlled less by these early paleotopographic features and more by 1 differential compaction around shoal water facies in the tst 2 early louann salt movement 3 basement related faulting and 4 freshwater influx along the basin rim