Home About us Contact
|
Home About us Contact | ||
|
|
||
Ordovician
Kinds of Ordovician Terms modified by Ordovician Selected AbstractsVARIATIONS IN COMPOSITION, PETROLEUM POTENTIAL AND KINETICS OF ORDOVICIAN , MIOCENE TYPE I AND TYPE I-II SOURCE ROCKS (OIL SHALES): IMPLICATIONS FOR HYDROCARBON GENERATION CHARACTERISTICSJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2010H. I. Petersen Lacustrine and marine oil shales with Type I and Type I-II kerogen constitute significant petroleum source rocks around the world. Contrary to common belief, such rocks show considerable compositional variability which influences their hydrocarbon generation characteristics. A global set of 23 Ordovician , Miocene freshwater and brackish water lacustrine and marine oil shales has been studied with regard to their organic composition, petroleum potential and generation kinetics. In addition their petroleum generation characteristics have been modelled. The oil shales can be classified as lacosite, torbanite, tasmanite and kukersite. They are thermally immature. Most of the shales contain >10 wt% TOC and the highest sulphur contents are recorded in the brackish water and marine oil shales. The kerogen is sapropelic and is principally composed of a complex of algal-derived organic matter in the form of: (i) telalginite (Botryococcus-, Prasinophyte- (Tasmanites?) or Gloeocapsomorpha-type); (ii) lamalginite (laminated, filamentous or network structure derived from Pediastrum- or Tetraedron-type algae, from dinoflagellate/acritarch cysts or from thin-walled Prasinophyte-type algae); (iii) fluorescing amorphous organic matter (AOM) and (iv) liptodetrinite. High atomic H/C ratios reflect the hydrogen-rich Type I and Type I-II kerogen, and Hydrogen Index values generally >300 mg HC/g TOC and reaching nearly 800 mg HC/g TOC emphasise the oil-prone nature of the oil shales. The kerogen type and source rock quality appear not to be related to age, depositional environment or oil shale type. Therefore, a unique, global activation energy (Ea) distribution and frequency factor (A) for these source rocks cannot be expected. The differences in kerogen composition result in considerable variations in Ea -distributions and A-factors. Generation modelling using custom kinetics and the known subsidence history of the Malay-Cho Thu Basin (Gulf of Thailand/South China Sea), combined with established and hypothetical temperature histories, show that the oil shales decompose at different rates during maturation. At a maximum temperature of ,120°C reached during burial, only limited kerogen conversion has taken place. However, oil shales characterised by broader Ea -distributions with low Ea -values (and a single approximated A-factor) show increased decomposition rates. Where more deeply buried (maximum temperature ,150°C), some of the brackish water and marine oil shales have realised the major part of their generation potential, whereas the freshwater oil shales and other brackish water oil shales are only ,30,40% converted. At still higher temperatures between ,165°C and 180°C all oil shales reach 90% conversion. Most hydrocarbons from these source rocks will be generated within narrow oil windows (,20,80% kerogen conversion). Although the brackish water and marine oil shales appear to decompose faster than the freshwater oil shales, this suggests that with increasing heatflow the influence of kerogen heterogeneity on modelling of hydrocarbon generation declines. It may thus be critical to understand the organic facies of Type I and Type I-II source rocks, particularly in basins with moderate heatflows and restricted burial depths. Measurement of custom kinetics is recommended, if possible, to increase the accuracy of any computed hydrocarbon generation models. [source] AN INTEGRATED STUDY OF DIAGENESIS AND DEPOSITIONAL FACIES IN TIDAL SANDSTONES: HAWAZ FORMATION (MIDDLE ORDOVICIAN), MURZUQ BASIN, LIBYAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2009A. Abouessa Studies of the impact of diagenesis on reservoir quality in tidal sandstones can be of great importance in successful hydrocarbon exploration. The study reported here shows that diagenetic alterations and bioturbation have induced considerable deterioration and heterogeneity in the reservoir quality of the sand-dominated tidal deposits of the Middle Ordovician Hawaz Formation in the Muruq Basin, Libya. Comparison is made between the diagenetic evolution of samples from the subsurface (present-day depth 1500 m) and from surface outcrops in order to study the impact of burial and uplift on the spatial and temporal distribution of reservoir quality in the Hawaz Formation sandstones. Eogenetic alterations, which were mediated by meteoric water circulation, included kaolinitization and dissolution of framework silicates and mechanical compaction. Mesogenetic alterations (T > 70°C; depth > 2 km) included pressure dissolution of quartz grains and concomitant quartz cementation, conversion of kaolinite into dickite, illitization of kaolinite and of grain-coating clays, and the precipitation of Mg-rich siderite cement. Reduction of intergranular porosity was due more to compaction than to cementation, yet quartz overgrowths are up to 16% in some of the sandstones. Bioturbation has resulted in a greater reduction in sandstone permeability in the lower part of the formation than the upper part. A higher ratio of dickite to kaolinite in subsurface samples than in outcrop samples is attributed to the longer residence time of the former sandstones under mesogenetic conditions. Telodiagenesis has not resulted in enhancement of reservoir quality of the Hawaz Formation Sandstones but in pseudomorphic calcitization of siderite and oxidation of pyrite to goethite. This study shows that the reservoir-quality evolution of tidal sandstones can best be elucidated when linked to depositional facies and distribution of diagenetic alterations. [source] HYDROTHERMALLY FLUORITIZED ORDOVICIAN CARBONATES AS RESERVOIR ROCKS IN THE TAZHONG AREA, CENTRALTARIM BASIN, NW CHINAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2006Zhijun Jin Reservoir rocks at the Tazhong 45 oil pool, central Tarim Basin, consist of fluoritized carbonate strata of Middle - Late Ordovician age. Petrological observations indicate that the fluorite replaces calcite. Several other hydrothermal minerals including pyrite, quartz, sphalerite and chlorite accompany the fluorite. Two generations of fluid inclusions are present in the fluorite. Homogenization temperatures (Th) for primary inclusions are mostly between 260°C and 310°C and represent the temperature of the hydrothermal fluid responsible for fluorite precipitation. Th for secondary inclusions range from 100°C to 130°C, and represent the hydrocarbon charging temperature as shown by the presence of hydrocarbons trapped in some secondary inclusions. The mineral assemblage and the homogenization temperatures of the primary fluid inclusions indicate that the precipitation of fluorite is related to hydrothermal activity in the Tazhong area. Strontium isotope analyses imply that the hydrothermal fluids responsible for fluorite precipitation are related to late-stage magmatic activity, and felsic magmas were generated by mixing of mafic magma and crustal materials during the Permian. Theoretical calculations show that the molecular volume of a carbonate rock decreases by 33.5% when calcite is replaced by fluorite, and the volume shrinkage can greatly enhance reservoir porosity by the formation of abundant intercrystalline pores. Fluoritization has thus greatly enhanced the reservoir quality of Ordovician carbonates in the Tazhong 45 area, so that the fluorite and limestone host rocks have become an efficient hydrocarbon reservoir. According to the modelled burial and thermal history of the Tazhong 45 well, and the homogenization temperatures of secondary fluid inclusions in the fluorite, hydrocarbon charging at the Tazhong 45 reservoir took place in the Tertiary. [source] ORDOVICIAN,PERMIAN PALAEOGEOGRAPHY OF CENTRAL EURASIA: DEVELOPMENT OF PALAEOZOIC PETROLEUM-BEARING BASINSJOURNAL OF PETROLEUM GEOLOGY, Issue 3 2003V. A. Bykadorov In this paper, we discuss three petroleum-bearing basins of Palaeozoic age in Central Eurasia,the Precaspian, Tarim and Chu-Sarysu Basins. We make use of recently-published palaeogeographic maps of the Central Eurasian region, six of which are presented here (Late Ordovician, Early-Middle Devonian, Late Devonian, Early Carboniferous, Early Permian and Late Permian). The maps illustrate the development through the Palaeozoic of the Palaeoasian and Palaeotethys Oceans; of the East European, Siberian and Tarim cratons; and of the Kazakhstan and other microcontinental blocks. The Kazakhstan block formed during the Late Ordovician and is a collage of Precambrian and Early Palaeozoic microcontinents and island arcs. It is surrounded by collisional foldbelts (Ob-Zaisan, Ural-Tianshan and Junggar-Balkhash) which formed in the Late Carboniferous , Permian. We believe that the formation of a stable Kazakhstan block is not consistent with the existence of the previously-identified "Kipchak arc" within the Palaeoasian ocean, or (as has previously been proposed) with activity on this arc up to the end of the Palaeozoic. The oil and gas potential of the Precaspian, Tarim and Chu-Sarysu Basins depends to a large extent on their tectonic stability during the Palaeozoic and subsequent time. The Precaspian Basin has been stable since the Cadomian orogeny (Early Cambrian) and is known to have major hydrocarbon potential. The Tarim Basin (NW China) has somewhat lower potential because the margins of the Tarim continental block have been affected by a series of collisional events; that margin with the Palaeotethys Ocean, for example, was active during the Late Palaeozoic. The Chu-Sarysu Basin on the Kazakhstan block is the least stable of the three and contains only minor gas accumulations. [source] EARLY ORDOVICIAN (ARENIG) TRILOBITE PALAEOECOLOGY AND PALAEOBIOGEOGRAPHY OF THE SOUTH CHINA PLATEPALAEONTOLOGY, Issue 3 2005SAMUEL T. TURVEY Abstract:, Faunal composition of South Chinese Arenig benthic trilobite associations is investigated using the multivariate techniques of TWINSPAN, DCA and seriation. Eight fairly distinct benthic associations can be differentiated, organized primarily along a palaeobathymetrical gradient across the Yangtze Platform in southern Shaanxi and western Hubei and the Jiangnan Transitional Belt in northern Hunan: the Taihungshania, Neseuretus, Trinucleid and Asaphid- Pseudocalymene associations (shallow shelf), the Asaphid-Raphiophorid and Nileid-Asaphid associations (< 100 m outer shelf), the Nileid-Illaenid Association (deep outer shelf carbonates) and the Pseudopetigurus Association (deep outer shelf clastics). The highest levels of diversity are displayed by the Asaphid-Raphiophorid Association of western Hubei. Investigation of the biogeographical affinities of this fauna indicates that South China is biogeographically closest to the other Chinese geotectonic units, and displays strong faunal connections to other central and eastern Asian regions as well as to Tethyan and South American regions. Subdivision of the South Chinese trilobite fauna into different depth zones shows a statistically significant correlation between increasing water depth and increasing faunal cosmopolitanism, although taxa endemic to China or Gondwana also occur even in slope faunas. The results of this biogeographical analysis are consistent with a tropical or subtropical peri-Gondwanan association of South China with other Asian terranes. [source] HIRNANTIAN (LATEST ORDOVICIAN) GRAPTOLITES FROM THE UPPER YANGTZE REGION, CHINAPALAEONTOLOGY, Issue 2 2005CHEN XU Abstract:, The Upper Yangtze region yields a Hirnantian (latest Ordovician) graptolite fauna that includes 41 species assigned to 13 genera. This fauna is particularly important for understanding the Late Ordovician mass extinction event because it is the most diverse known from this interval. In addition, it records the survival, well into the Hirnantian, of many taxa of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which was previously regarded as having gone extinct at the beginning of the Hirnantian. Taxa exhibiting six different astogenetic patterns, including taxa with reclined stipes, scandent, biserial, full-periderm and ,archiretiolitid' rhabdosome forms occur in the lower Normalograptus extraordinarius-N. ojsuensis Biozone. In contrast, in the upper N. persculptus Biozone only four genera remain, all but one of which are Normalograptidae: scandent and biserial taxa with Pattern H astogeny. Normalograptids are the dominant form of the succeeding, lower Rhuddanian, faunas. The Yangtze faunas also document the early expansion of normalograptids coeval with the decline of the DDO fauna. Many previously identified species considered endemic to China have been synonymized; 24 of the 41 species recorded here have been recognized elsewhere. No new taxa are described. [source] Origin of planktotrophy,evidence from early molluscsEVOLUTION AND DEVELOPMENT, Issue 4 2006Alexander Nützel SUMMARY The size of early ontogenetic shells (protoconchs) of ancient benthic molluscs suggests that feeding larvae occurred at about 490 myr (approximately, transition from Cambrian to Ordovician). Most studied Ordovician protoconchs were smaller than Cambrian ones, indicating smaller Ordovician eggs and hatchlings. This suggests substitution of nutritious reserve matter such as yolk by plankton as an energy source for larvae. The observed size change represents the first direct empiric evidence for a late Cambrian to Ordovician switch to planktotrophy in invertebrate larvae. It corroborates previous hypotheses about a possible polyphyly of planktotrophy. These hypotheses were primarily based on molecular clock data of extant clades with different types of larva, change in the overall body size, as well as increasing predation pressure on Early Paleozoic sea floors. The Early Ordovician is characterized by an explosive radiation of benthic suspension feeders and it was suggested that planktotrophy would prolongate escape from benthic predation on hatchlings. This biological escalation hypothesis does not fully explain why planktotrophy and suspension feeding became important at the same time, during a major biodiversification. An additional factor that probably included availability of nutrients must have played a role. We speculate that an increasing nutrient supply and availability of photoautotrophic plankton in world oceans have facilitated both planktotrophy and suspension feeding, which does not exclude a contemporaneous predation-driven escalation. It is very likely that the evolution of planktotrophy as well as increasing predation contributed to the Ordovician radiation. [source] Fossilized embryos are widespread but the record is temporally and taxonomically biasedEVOLUTION AND DEVELOPMENT, Issue 2 2006Philip C. J. Donoghue SUMMARY We report new discoveries of embryos and egg capsules from the Lower Cambrian of Siberia, Middle Cambrian of Australia and Lower Ordovician of North America. Together with existing records, embryos have now been recorded from four of the seven continents. However, the new discoveries highlight secular and systematic biases in the fossil record of embryonic stages. The temporal window within which the embryos and egg capsules are found is of relatively short duration; it ends in the Early Ordovician and is roughly coincident with that of typical "Orsten"-type faunas. The reduced occurrence of such fossils has been attributed to reducing levels of phosphate in marine waters during the early Paleozoic, but may also be owing to the increasing depth of sediment mixing by infaunal metazoans. Furthermore, most records younger than the earliest Cambrian are of a single kind,large eggs and embryos of the priapulid-like scalidophoran Markuelia. We explore alternative explanations for the low taxonomic diversity of embryos recovered thus far, including sampling, size, anatomy, ecology, and environment, concluding that the preponderance of Markuelia embryos is due to its precocious development of cuticle at an embryonic stage, predisposing it to preservation through action as a substrate on which microbially mediated precipitation of authigenic calcium phosphate may occur. The fossil record of embryos may be limited to a late Neoproterozoic to early Ordovician snapshot that is subject to dramatic systematic bias. Together, these biases must be considered seriously in attempts to use the fossil record to arbitrate between hypotheses of developmental and life history evolution implicated in the origin of metazoan clades. [source] Darriwilian (Middle Ordovician) graptolite faunas of the Sandia Region, southern PeruGEOLOGICAL JOURNAL, Issue 4 2010Jörg Maletz Abstract Ordovician graptolite faunas of Peru are restricted to a short interval in the Middle to basal Upper Ordovician, found in three regions of the country. All Peruvian graptolite faunas are strongly dominated by shallow water elements of the Atlantic Faunal Realm, represented largely by Didymograptus s. str. and Aulograptus, but a number of faunal elements of the pandemic isograptid biofacies have recently been discovered in the Sandia Region of SE Peru. Peruvian graptolite faunas are reviewed and the new records from the Sandia Region are discussed in detail. The faunas from the Purumpata and Iparo members of the San José Formation range in age approximately from the Undulograptus austrodentatus Biozone to the Holmograptus lentus Biozone (early to middle Darriwilian). The faunas provide a better understanding of faunal composition and diversity in this region and help to correlate shallow water and deeper water graptolite faunas from this time interval. Biserial graptolites are rare in most samples and usually indeterminable, but a single identifiable specimen of Undulograptus austrodentatus was found, indicating a level close to the base of the Darriwilian. A number of specimens of the genera Isograptus and Arienigraptus from the Sandia Region represent pandemic graptolite faunas of the isograptid biofacies, described for the first time from this region. Copyright © 2009 John Wiley & Sons, Ltd. [source] Sedimentary and faunal events revealed by a revised correlation of post-glacial Hirnantian (Late Ordovician) strata in the Welsh Basin, UKGEOLOGICAL JOURNAL, Issue 3 2009Jeremy R. Davies Abstract The discovery of a previously unrecognized unconformity and of new faunas in the type Llandovery area underpins a revised correlation of Hirnantian strata in mid Wales. This has revealed the sedimentary and faunal events which affected the Lower Palaeozoic Welsh Basin during the global rise in sea level that followed the end-Ordovician glacial maximum and has allowed their interpretation in the context of local and global influences. In peri-basinal shelfal settings the onset of post-glacial deepening is recorded by an unfossiliferous, transgressive shoreface sequence (Cwm Clyd Sandstone and Garth House formations) which rests unconformably on Rawtheyan rocks, deformed during an episode of pre-Hirnantian tectonism. In the deep water facies of the basin centre, this same sequence boundary is now recognized as the contact between fine-grained, re-sedimented mudstones and an underlying regressive sequence of turbidite sandstones and conglomerates; it is at a level lower than previously cited and calls into question the established lithostratigraphy. In younger Hirnantian strata, graptolites associated with the newly recognized Ystradwalter Member (Chwefri Formation) demonstrate that this distal shelf unit correlates with the persculptus graptolite-bearing Mottled Mudstone Member of the basinal succession. Together these members record an important macrofaunal recolonization of the Welsh Basin and mark a key event in the post-glacial transgression. Further deepening saw the establishment of a stratified water column and the imposition of anoxic bottom water conditions across the basin floor. These post-glacial Hirnantian events are consistent with the re-establishment of connections between a silled Welsh Basin and the open Iapetus Ocean. However, a comparison with other areas suggests that each event records a separate deepening episode within a pulsed glacio-eustatic transgression, while also reflecting changes in post-glacial climate and patterns of oceanic circulation and associated biotic flux. British Geological Survey © NERC 2009. All rights reserved. [source] Enigmatic sedimentary,volcanic successions in the central European Variscides: a Cambrian/Early Ordovician age for the Wojcieszów Limestone (Kaczawa Mountains, SW Poland) indicated by SHRIMP dating of volcanic zirconsGEOLOGICAL JOURNAL, Issue 4 2008Ryszard Kryza Abstract Metamorphosed volcanic and sedimentary successions in the central European Variscides are, in many areas, poorly biostratigraphically constrained, making palaeotectonic interpretations uncertain. In such instances, geochronological data are crucial. Sensitive high resolution ion microprobe (SHRIMP) dating of volcanic zircons from a quartz,white mica schist (interpreted as deformed metavolcaniclastic/epiclastic rock) within the stratigraphically controversial Wojcieszów Limestone of the Kaczawa Mountains (Sudetes, SW Poland), near to the eastern termination of the European Variscides, has yielded an age of 498,±,5,Ma (2, error), corresponding to late Cambrian to early Ordovician magmatism in that area and constraining the depositional age of the limestones. The new SHRIMP data are not consistent with the recent revision of the age of the Wojcieszów Limestone based on Foraminifera findings that ascribed them to a Late Ordovician,Silurian or even younger interval. They are though, consistent with sparse macrofossil data and strongly support earlier interpretations of the lower part of the Kaczawa Mountains succession as a Cambrian,Early Ordovician extensional basin-fill with associated initial rift volcanic rocks, likely emplaced during the breakup of Gondwana. Copyright © 2008 John Wiley & Sons, Ltd. [source] ,-diversity fluctuations in Early,Mid Ordovician brachiopod communities of South ChinaGEOLOGICAL JOURNAL, Issue 3-4 2006R.-B. Zhan Abstract Lower to Middle Ordovician transitional strata (Acrograptus filiformis Biozone to Exigraptus clavus Biozone) of the Upper Yangtze Platform contain rich and diverse brachiopods, particularly in the Daguanshan Formation of Sichuan Province, the Meitan Formation of Guizhou Province, and the Yingpan Formation of Houping, Chongqing City. On the basis of multivariate analyses of 207 samples (about 15,000 brachiopod specimens) and 61 species from these strata, 23 brachiopod-dominated communities and associations are delineated through six graptolitic biozones (Acrograptus filiformis to Exigraptus clavus biozones). The disparity of brachiopod associations or ,-diversity, as measured by the number of contemporaneous associations across variable ecological settings, was low in the A. filiformis Biozone and increased through the overlying graptolitic biozones to reach an acme in the Azygograptus suecisus and Expansograptus hirundo biozones, where as many as nine different brachiopod associations thrived in mid-shelf and also established in inner- and outer-shelf environments. Despite a sharp increase in ,-diversity within the Didymograptellus eobifidus Biozone, ,-diversity remained relatively low, with only three distinct brachiopod communities. The diversity of brachiopod associations suffered an abrupt drop in the E. clavus Biozone, as a result of a major faunal turnover event, with only a single Metorthis Community present in an upper mid-shelf setting. Temporally, brachiopod associations appear to be most stable in mid-shelf settings, with the Paralenorthis, Sinorthis, Desmorthis, and Euorthisina communities having a relatively high ,-diversity and lasting through two to five graptolitic biozones. In both species composition and temporal duration, the brachiopod associations at the shallow- (BA1 to upper BA2) and deep-shelf (BA4) settings are unstable, characterized by low ,-diversity and rapid vertical changes. This study implies that, during the Early and Mid Ordovician, brachiopod communities already attained a high level of organization in mid-shelf settings, but remained poorly established in high-stress environments of very shallow or very deep shelf settings. Copyright © 2006 John Wiley & Sons, Ltd. [source] Ordovician graptolite evolutionary radiation: a reviewGEOLOGICAL JOURNAL, Issue 3-4 2006Chen Xu Abstract Based on the global graptolite genera and higher rank taxa, we propose three radiation stages through the Ordovician. The isograptid type of development is present within anisograptids predominating in the Tremadocian. Thus, the evolutionary radiation of the Anisograptid fauna from Tremadocian is proposed as the beginning of the Ordovician graptolite radiation. The second graptolite radiation event is the radiation of the Dichograptid fauna, which began from the T. fruticosus Biozone. The third radiation event, the radiation of Diplograptid fauna began immediately after that of the dichograptids. This radiation includes the peak in total diversity of all the Ordovician graptoloids in the Nemagraptus gracilis Biozone. The radiation extended through the Sandbian and Katian and then was interrupted by a major extinction during the early Hirnantian. Thus, the Ordovician graptolite radiation events coincide with those of the three graptolite faunas proposed by Bulman. The distribution and expansion of the Ordovician graptolites in South China may exemplify the graptolite origination pattern, which begins from the slope belt and expanded into both the shelf and oceanic zones. Copyright © 2006 John Wiley & Sons, Ltd. [source] Facies development, depositional settings and sequence stratigraphy across the Ordovician,Silurian boundary: a new perspective from the Barrandian area of the Czech RepublicGEOLOGICAL JOURNAL, Issue 2 2006torch Abstract The Hirnantian and Llandovery sedimentary succession of the Barrandian area has been assigned to middle and outer clastic-shelf depositional settings, respectively. Deposition was influenced by the remote Gondwanan glaciation and subsequent, long-persisting, post-glacial anoxia triggered by a current-driven upwelling system. High-resolution graptolite stratigraphy, based upon 19 formally defined biozones,largely interval zones,and five subzones, enabled a detailed correlation between 42 surface sections and boreholes, and enabled linking of the sedimentary record, graptoloid fauna dynamics, organic-content fluctuations and spectral gamma-ray curves. The Hirnantian and Llandovery succession has been subdivided into four biostratigraphically dated third-order sequences (units 1,4). Time,spatial facies distribution recorded early and late Hirnantian glacio-eustatic sea-level lowstands separated by a remarkable mid-Hirnantian rise in sea-level. A major part of the post-glacial sea-level rise took place within the late Hirnantian. The highstand of Unit 2 is apparently at the base of the Silurian succession. Short-term relative sea-level drawdown and a third-order sequence boundary followed in the early Rhuddanian upper acuminatus Zone. Early Aeronian and late Telychian sea-level highstands and late Aeronian drawdown of likely eustatic origin belong to units 3 and 4. Sea-level rise culminated in the late Telychian, which may also be considered as a highstand episode of a second-order Hirnantian,early Silurian cycle. Facies and sequence-stratigraphic analysis supports recent interpretations on nappe structures in the core part of the Ordovician,Middle Devonian Prague Synform of the Barrandian. Copyright © 2006 John Wiley & Sons, Ltd. [source] Parablastoid holdfasts from the Lower Ordovician of South WalesGEOLOGICAL JOURNAL, Issue 3 2005John C. W. Cope Abstract Problematical discoid fossils from the Lower Arenig (Moridunian Stage) of the Llangynog Inlier, South Wales, are interpreted as parablastoid holdfasts. They constitute the first record of holdfasts in this rare echinoderm class. The lower surface has a radial ribbed structure, in contrast to the concentric, reticulate structure of the upper surface. The substrate for attachment was not lithified and may instead have been sediment stabilized by microbial mats. An associated pluricolumnal indicates that the parablastoid column was homeomorphic. Copyright © 2005 John Wiley & Sons, Ltd. [source] Fluid evolution in base-metal sulphide mineral deposits in the metamorphic basement rocks of southwest Scotland and Northern IrelandGEOLOGICAL JOURNAL, Issue 1 2005Martin Baron Abstract The Dalradian and Ordovician,Silurian metamorphic basement rocks of southwest Scotland and Northern Ireland host a number of base-metal sulphide-bearing vein deposits associated with kilometre-scale fracture systems. Fluid inclusion microthermometric analysis reveals two distinct fluid types are present at more than half of these deposits. The first is an H2O,CO2,salt fluid, which was probably derived from devolatilization reactions during Caledonian metamorphism. This stage of mineralization in Dalradian rocks was associated with base-metal deposition and occurred at temperatures between 220 and 360°C and pressures of between 1.6 and 1.9,kbar. Caledonian mineralization in Ordovician,Silurian metamorphic rocks occurred at temperatures between 300 and 360°C and pressures between 0.6 and 1.9,kbar. A later, probably Carboniferous, stage of mineralization was associated with base-metal sulphide deposition and involved a low to moderate temperature (Th 70 to 240°C), low to moderate salinity (0 to 20,wt% NaCl eq.), H2O,salt fluid. The presence of both fluids at many of the deposits shows that the fractures hosting the deposits acted as long-term controls for fluid migration and the location of Caledonian metalliferous fluids as well as Carboniferous metalliferous fluids. Copyright © 2004 John Wiley & Sons, Ltd. [source] Magnetostratigraphic constraints on the Gondwanan origin of North China: Cambrian/Ordovician boundary resultsGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002Zhenyu Yang Summary A significant gap in the middle Palaeozoic apparent polar wander (APW) path precludes polarity definition of the early Palaeozoic palaeopole for North China. This problem can in part be resolved by the intercontinental correlation of magnetic polarity patterns across small time intervals. A magnetostratigraphic study was carried out on upper Cambrian to lower Ordovician sediments near Zhaogezhuang (long. 118.5°E, lat. 39.7°N), North China. After stepwise thermal or thermal and alternating field demagnetizations, a characteristic magnetic component with normal and antipodal reversed directions was identified. These data, drawn from 49 samples, yield a north palaeopole at long. 294.6°E, lat. 32.9°N (dp = 3.0°, dm = 5.3°). A concordant magnetic polarity pattern around the Cambrian,Ordovician boundary and lowest Ordovician obtained from different continents favours a Southern Hemisphere origin (,17°) formation site. Using the Cambrian,Ordovician APW paths between North China and Gondwana, we suggest that the North China block (NCB) was part of Gondwana during the Cambro,lowest Ordovician, and started breaking away from Gondwana in the lower Ordovician. This finding is contrary to some palaeomagnetic models where the NCB was quite separate from Gondwana in the late Proterozoic, and was attached to the ,Pacific' side of Antarctica. [source] Modeling of a Deep-Seated Geothermal System Near Tianjin, ChinaGROUND WATER, Issue 3 2001Zhou Xun A geothermal field is located in deep-seated basement aquifers in the northeastern part of the North China Plain near Tianjin, China. Carbonate rocks of Ordovician and Middle and Upper Proterozoic age on the Cangxian Uplift are capable of yielding 960 to 4200 m3/d of 57°C to 96°C water to wells from a depth of more than 1000 m. A three-dimensional nonisothermal numerical model was used to simulate and predict the spatial and temporal evolution of pressure and temperature in the geothermal system. The density of the geothermal water, which appears in the governing equations, can be expressed as a linear function of pressure, temperature, and total dissolved solids. A term describing the exchange of heat between water and rock is incorporated in the governing heat transport equation. Conductive heat flow from surrounding formations can be considered among the boundary conditions. Recent data of geothermal water production from the system were used for a first calibration of the numerical model. The calibrated model was used to predict the future changes in pressure and temperature of the geothermal water caused by two pumping schemes. The modeling results indicate that both pressure and temperature have a tendency to decrease with time and pumping. The current withdrawal rates and a pumping period of five months followed by a shut-off period of seven months are helpful in minimizing the degradation of the geothermal resource potential in the area. [source] Robust support for tardigrade clades and their ages from three protein-coding nuclear genesINVERTEBRATE BIOLOGY, Issue 2 2004Jerome C. Regier Abstract. Coding sequences (5,334 nt total) from elongation factor-1,, elongation factor-2, and the largest subunit of RNA polymerase II were determined for 6 species of Tardigrada, 2 of Arthropoda, and 2 of Onychophora. Parsimony and likelihood analyses of nucleotides and amino acids yielded strong support for Tardigrada and all internal nodes (i.e., 100% bootstrap support for Tardigrada, Eutardigrada, Parachela, Hypsibiidae, and Macrobiotidae). Results are in agreement with morphology and an earlier molecular study based on analysis of 18S ribosomal sequences. Divergence times have been estimated from amino acid sequence data using an empirical Bayesian statistical approach, which does not assume a strict molecular clock. Divergence time estimates are pre-Vendian for Tardigrada/Arthropoda, Vendian or earlier for Eutardigrada/Heterotardigrada, Silurian to Ordovician for Parachela/Apochela, Permian to Carboniferous for Hypsibiidae and Macrobiotidae, and Mesozoic for Isohypsibius/Thulinia (both within Hypsibiidae) and Macrobiotus/Richtersius (both within Macrobiotidae). [source] Tectonic and stratigraphic significance of the Middle Ordovician carbonate breccias in the Ogcheon Belt, South KoreaISLAND ARC, Issue 3 2002In-Chang Ryu Abstract Carbonate breccias occur sporadically in the Lower,Middle Ordovician Maggol Limestone exposed in the Taebacksan Basin in the northeastern part of the northeast,southwest-trending Ogcheon Belt, South Korea. These carbonate breccias have been previously interpreted as intraformational or fault-related breccias. Thus, little attention has been focused on tectonic and stratigraphic significance of these carbonate breccias. The present study, however, indicates that the majority of these carbonate breccias are solution,collapse breccias, which are causally linked to paleokarstification. Carbonate facies analysis in conjunction with conodont biostratigraphy suggests that an overall regression toward the top of the Maggol Limestone probably culminated in subaerial exposure of platform carbonates during the early Middle Ordovician (earliest Darriwilian). Extensive subaerial exposure of platform carbonates resulted in paleokarst-related solution,collapse breccias in the upper Maggol Limestone. This subaerial exposure event is manifested as a major paleokarst unconformity at the Sauk,Tippecanoe sequence boundary elsewhere beneath the Middle Ordovician succession and its equivalents, most notably North America and North China. Due to its global extent, this paleokarst unconformity has been viewed as a product of second- or third-order eustatic sealevel drop during the early Middle Ordovician. Although a paleokarst breccia zone is recognized beneath the Middle Ordovician succession in South Korea, the Sauk,Tippecanoe sequence boundary appears to be a conformable transgressive surface on the top of the paleokarst breccia zone in the upper Maggol Limestone. The paleokarst breccia zone beneath the conformable transgressive surface is represented by a thinning-upward stack of exposure-capped tidal flat-dominated cycles that are closely associated with multiple occurrences of paleokarst-related solution,collapse breccias. This paleokarst breccia zone was a likely consequence of repeated fourth- and fifth-order sealevel fluctuations. It suggests that second- and third-order eustatic sealevel drop may have been significantly tempered by substantial tectonic subsidence near the end of the Maggol deposition. The tectonic subsidence in the basin is also evidenced by the occurrence of coeval off-platform lowstand siliciclastic quartzite lenses as well as debris flow carbonate breccias (i.e. the Yemi Breccia). With the continued tectonic subsidence, subsequent rise in the eustatic cycle caused drowning and deep flooding of carbonate platform, forming a transgressive surface on the top of the paleokarst breccia zone. This tectonic implication contrasts notably with the slowly subsiding carbonate platform model for the basin as has been previously interpreted. Thus, it is proposed that the Taebacksan Basin in the northeastern part of the Ogcheon Belt evolved from a slowly subsiding carbonate platform to a rapidly subsiding intracontinental rift basin during the early Middle Ordovician. The proposed tectonic model in the basin gives much better insight to unravel the stratigraphic response to tectonic evolution of the Ogcheon Belt, which remains an enigmatic feature in formulating a tectonic framework of the Korean peninsula. The present study also provides a good example that the falling part of the eustatic sealevel cycle may not produce a significant event in a rapidly subsiding basin where the rate of eustatic fall always remained lower than the rate of subsidence. [source] Cladistic biogeography and the art of discoveryJOURNAL OF BIOGEOGRAPHY, Issue 4 2002Malte C. Ebach Aims Cladistic biogeography is about discovering geographical congruence. The agreement of several taxon-area cladograms (TACs) rarely yields a perfect result. Areas may overlap, taxa may not be evenly distributed, and thus, ambiguity may be prevalent in the data. Ambiguity is incongruence and may be resolved by reducing paralogy and resolving potential information. Recently, several new approaches in cladistic biogeography [i.e. Brooks parsimony analysis (BPA), Assumption 0] interpret ambiguity as congruence. These methods are problematic, as they are generational. Methods constructed under the generation paradigm are flawed concepts that are immunized from falsifying evidence. A critique of modified BPA reveals that taking an evolutionary stance in biogeography leads to flaws in implementation. Methods Area cladistics is a new development in cladistic biogeography. Area cladistics adopts paralogy-free subtree analysis using Assumption 2, to discover the relative positions of continents through time. Results Geographical congruence is the result of allopatric (geographical) speciation. Vicariance, dispersal and combinations of both, are recognized causes for allopatric speciation. Area cladistics highlights the concept that all these events occur in response to geological changes (e.g. continental drift) either directly, by geographical boundaries, or indirectly, at the level of ocean currents. Samples of chosen examples all respond to the geological process. The examples include Ordovician,Silurian and Lower Devonian trilobites to yield a general areagram which is a representational branching diagram that depicts the relationships of areas. Main conclusion Finding one common biogeographical pattern from several unrelated groups is a qualitative approach to interpret the positions of continental margins through time. Area cladistics is not a substitute for palaeomaps that are derived from palaeomagnetic data, but general areagrams adding to the body of knowledge that yields more precise interpretations of the earth's past. [source] A comparative U,Th,Pb (zircon,monazite) and 40Ar,39Ar (muscovite,biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross OrogenJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2007G. DI VINCENZO Abstract Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U,Th,Pb system in zircon and monazite and of the 40Ar,39Ar system in micas during ductile deformation. Meso- and micro-structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top-to-the-SW and -NE, respectively) and developed under upper greenschist facies conditions. In situ U,Pb dating by laser-ablation inductively coupled plasma-mass spectrometry of zircon areas with well-preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross,Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U,Th,Pb ages of c. 440 Ma, in agreement with in-situ Ar laserprobe ages of syn-shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal-chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U,Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ,ghost' structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U,Pb ages of c. 450 and 410 Ma. The c. 450-Ma date matches within errors most monazite U,Th,Pb ages and in-situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician,Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear-zone system formed in response to ,NE,SW-directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician,Late Devonian Lachlan Fold Belt. [source] Timing of high-grade metamorphism: Early Palaeozoic U,Pb formation ages of titanite indicate long-standing high- T conditions at the western margin of Gondwana (Argentina, 26,29°S)JOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2003F. Lucassen Abstract Concordant U,Pb ages of c. 530,510 Ma and c. 470,420 Ma on titanite from calcsilicate, orthogneiss and amphibolite rocks constrain the age of high- T metamorphism in the Early Palaeozoic mobile belt at the western margin of Proterozoic Gondwana (Argentina, 26,29°S). The U,Pb ages document the time of titanite formation at high- T conditions according to the stable mineral paragenesis and occurrence of titanite in the metamorphic fabric. The presence of migmatite at all sample sites indicates temperatures were > c. 650 °C. Titanite formed at similar metamorphic conditions at different times on the regional and on the outcrop scale. The titanite crystals preserved their U,Pb isotopic signatures and chemical composition under ongoing upper amphibolite to granulite facies temperatures. Different thermal peaks or deformations are only detected by the different U,Pb ages and not by changes in the mineral paragenesis or metamorphic fabric of the samples. The range of U,Pb ages, e.g. in the Ordovician and Silurian (c. 470, 460, 440, 430, 420 Ma), is interpreted as the effect polyphase deformation with deformation-enhanced recrystallization of titanite and/or different thermal peaks during a long-standing, geographically fixed, high- T regime in the mid-crust of a continental magmatic arc. A clear correlation of the different ages with distinct tectonic events, e.g. collision of terranes, is not possible based on the present knowledge of the region. [source] Ordovician high-grade metamorphism of a newly recognised late Neoproterozoic terrane in the northern Harts Range,central AustraliaJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2001I. S. Buick Abstract Granulite facies rocks from the northernmost Harts Range Complex (Arunta Inlier, central Australia) have previously been interpreted as recording a single clockwise cycle of presumed Palaeoproterozoic metamorphism (800,875 °C and >9,10 kbar) and subsequent decompression in a kilometre-scale, E-W striking zone of noncoaxial, high-grade (c. 700,735 °C and 5.8,6.4 kbar) deformation. However, new SHRIMP U-Pb age determinations of zircon, monazite and titanite from partially melted metabasites and metapelites indicate that granulite facies metamorphism occurred not in the Proterozoic, but in the Ordovician (c. 470 Ma). The youngest metamorphic zircon overgrowths from two metabasites (probably meta-volcaniclastics) yield 206Pb/238U ages of 478±4 Ma and 471±7 Ma, whereas those from two metapelites yield ages of 463±5 Ma and 461±4 Ma. Monazite from the two metapelites gave ages equal within error to those from metamorphic zircon rims in the same rock (457±5 Ma and 462±5 Ma, respectively). Zircon, and possibly monazite ages are interpreted as dating precipitation of these minerals from crystallizing melt within leucosomes. In contrast, titanite from the two metabasites yield 206Pb/238U ages that are much younger (411±5 Ma & 417±7 Ma, respectively) than those of coexisting zircon, which might indicate that the terrane cooled slowly following final melt crystallization. One metabasite has a second titanite population with an age of 384±7 Ma, which reflects titanite growth and/or recrystallization during the 400,300 Ma Alice Springs Orogeny. The c. 380 Ma titanite age is indistinguishable from the age of magmatic zircon from a small, late and weakly deformed plug of biotite granite that intruded the granulites at 387±4 Ma. These data suggest that the northern Harts Range has been subject to at least two periods of reworking (475,460 Ma & 400,300 Ma) during the Palaeozoic. Detrital zircon from the metapelites and metabasites, and inherited zircon from the granite, yield similar ranges of Proterozoic ages, with distinct age clusters at c. 1300,1000 and c. 650 Ma. These data imply that the deposition ages of the protoliths to the Harts Range Complex are late Neoproterozoic or early Palaeozoic, not Palaeoproterozoic as previously assumed. [source] VARIATIONS IN COMPOSITION, PETROLEUM POTENTIAL AND KINETICS OF ORDOVICIAN , MIOCENE TYPE I AND TYPE I-II SOURCE ROCKS (OIL SHALES): IMPLICATIONS FOR HYDROCARBON GENERATION CHARACTERISTICSJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2010H. I. Petersen Lacustrine and marine oil shales with Type I and Type I-II kerogen constitute significant petroleum source rocks around the world. Contrary to common belief, such rocks show considerable compositional variability which influences their hydrocarbon generation characteristics. A global set of 23 Ordovician , Miocene freshwater and brackish water lacustrine and marine oil shales has been studied with regard to their organic composition, petroleum potential and generation kinetics. In addition their petroleum generation characteristics have been modelled. The oil shales can be classified as lacosite, torbanite, tasmanite and kukersite. They are thermally immature. Most of the shales contain >10 wt% TOC and the highest sulphur contents are recorded in the brackish water and marine oil shales. The kerogen is sapropelic and is principally composed of a complex of algal-derived organic matter in the form of: (i) telalginite (Botryococcus-, Prasinophyte- (Tasmanites?) or Gloeocapsomorpha-type); (ii) lamalginite (laminated, filamentous or network structure derived from Pediastrum- or Tetraedron-type algae, from dinoflagellate/acritarch cysts or from thin-walled Prasinophyte-type algae); (iii) fluorescing amorphous organic matter (AOM) and (iv) liptodetrinite. High atomic H/C ratios reflect the hydrogen-rich Type I and Type I-II kerogen, and Hydrogen Index values generally >300 mg HC/g TOC and reaching nearly 800 mg HC/g TOC emphasise the oil-prone nature of the oil shales. The kerogen type and source rock quality appear not to be related to age, depositional environment or oil shale type. Therefore, a unique, global activation energy (Ea) distribution and frequency factor (A) for these source rocks cannot be expected. The differences in kerogen composition result in considerable variations in Ea -distributions and A-factors. Generation modelling using custom kinetics and the known subsidence history of the Malay-Cho Thu Basin (Gulf of Thailand/South China Sea), combined with established and hypothetical temperature histories, show that the oil shales decompose at different rates during maturation. At a maximum temperature of ,120°C reached during burial, only limited kerogen conversion has taken place. However, oil shales characterised by broader Ea -distributions with low Ea -values (and a single approximated A-factor) show increased decomposition rates. Where more deeply buried (maximum temperature ,150°C), some of the brackish water and marine oil shales have realised the major part of their generation potential, whereas the freshwater oil shales and other brackish water oil shales are only ,30,40% converted. At still higher temperatures between ,165°C and 180°C all oil shales reach 90% conversion. Most hydrocarbons from these source rocks will be generated within narrow oil windows (,20,80% kerogen conversion). Although the brackish water and marine oil shales appear to decompose faster than the freshwater oil shales, this suggests that with increasing heatflow the influence of kerogen heterogeneity on modelling of hydrocarbon generation declines. It may thus be critical to understand the organic facies of Type I and Type I-II source rocks, particularly in basins with moderate heatflows and restricted burial depths. Measurement of custom kinetics is recommended, if possible, to increase the accuracy of any computed hydrocarbon generation models. [source] LOWER SILURIAN "HOT SHALES" IN JORDAN: A NEW DEPOSITIONAL MODELJOURNAL OF PETROLEUM GEOLOGY, Issue 3 2009D. K. Loydell Data are presented from the Batra Formation (also known as the Mudawwara Shale Formation) of a core from well BG-14 in the Batna el Ghoul area, southern Jordan, which enable a new depositional model to be proposed for the middle Rhuddanian (lower Llandovery, Silurian) "hot shale" which may be applicable to other Arabian and North African "hot shales" of similar stratigraphical age. This "hot shale" probably results from rapid early burial of organic carbon associated with a minor regression during which anoxic bottom conditions were maintained for most, but not all, of the time. Evidence for regression comes from (1) increased sediment grain size within the "hot shale" by comparison with underlying shales; (2) palynological changes including a decrease in acritarch species diversity; an increase in the relative abundance of sphaeromorphs, veryhachiids with three processes and acritarchs with short, simple processes; and a decrease in the relative abundance of acanthomorphs; (3) a positive ,13Corg excursion (other Late Ordovician and Silurian positive ,13Corg excursions occur during regressions); and (4) very brief intervals of oxygenation (associated with sediment influx) reflected in the preservation of graptolites as three-dimensional pyrite internal moulds, rather than as flattened periderm. The minor regression reflects a eustatic sea-level fall, evidence for which has recently been presented from several regions, including Arctic Canada, Bohemia and Scotland. The BG-14 "hot shale" is shown to be thicker than estimated in previous studies. Previous TOC measurements from the upper part of the "hot shale" were affected by the weathering of overlying strata in the BG-14 core. ICP-MS measurements show that uranium content is high in these weathered levels, extending the stratigraphical extent of the "hot shale" interval into the middle Rhuddanian. Depositional models such as that presented here rely on a robust biostratigraphical framework; in the Ordovician and Silurian of Arabia and North Africa, this can be provided by graptolites and chitinozoans. [source] HYDROTHERMALLY FLUORITIZED ORDOVICIAN CARBONATES AS RESERVOIR ROCKS IN THE TAZHONG AREA, CENTRALTARIM BASIN, NW CHINAJOURNAL OF PETROLEUM GEOLOGY, Issue 1 2006Zhijun Jin Reservoir rocks at the Tazhong 45 oil pool, central Tarim Basin, consist of fluoritized carbonate strata of Middle - Late Ordovician age. Petrological observations indicate that the fluorite replaces calcite. Several other hydrothermal minerals including pyrite, quartz, sphalerite and chlorite accompany the fluorite. Two generations of fluid inclusions are present in the fluorite. Homogenization temperatures (Th) for primary inclusions are mostly between 260°C and 310°C and represent the temperature of the hydrothermal fluid responsible for fluorite precipitation. Th for secondary inclusions range from 100°C to 130°C, and represent the hydrocarbon charging temperature as shown by the presence of hydrocarbons trapped in some secondary inclusions. The mineral assemblage and the homogenization temperatures of the primary fluid inclusions indicate that the precipitation of fluorite is related to hydrothermal activity in the Tazhong area. Strontium isotope analyses imply that the hydrothermal fluids responsible for fluorite precipitation are related to late-stage magmatic activity, and felsic magmas were generated by mixing of mafic magma and crustal materials during the Permian. Theoretical calculations show that the molecular volume of a carbonate rock decreases by 33.5% when calcite is replaced by fluorite, and the volume shrinkage can greatly enhance reservoir porosity by the formation of abundant intercrystalline pores. Fluoritization has thus greatly enhanced the reservoir quality of Ordovician carbonates in the Tazhong 45 area, so that the fluorite and limestone host rocks have become an efficient hydrocarbon reservoir. According to the modelled burial and thermal history of the Tazhong 45 well, and the homogenization temperatures of secondary fluid inclusions in the fluorite, hydrocarbon charging at the Tazhong 45 reservoir took place in the Tertiary. [source] BURIAL HISTORY RECONSTRUCTION AND THERMAL MODELLING AT KUH-E MOND, SW IRANJOURNAL OF PETROLEUM GEOLOGY, Issue 4 2003M. R. Kamali At the Kuh-e Mond anticline (Fars Province, SW Iran) and in nearby offshore structures, large volumes of natural gas are reservoired in the Permian , Early Triassic Dehram Group while heavy oil has been discovered in the Cretaceous Sarvak and Eocene Jahrum Formations. In this paper, we use data from six exploration wells and from nearby surface exposures to reconstruct the burial history at Kuh-e Mond. Regional observations show that the thick sedimentary fill in this part of the Zagros Basin was subjected to intense tectonism during the Zagros Orogeny, with a paroxysmal phase during the late Miocene and Pliocene. Thermal modelling and geochemical data from Kuh-e Mond and adjacent fields allows possible hydrocarbon generation and migration mechanisms to be identified. Maturities predicted using Lopatin's TTI model are in accordance with maturities obtained from vitrinite reflectance measurements. We show that formations which have source potential in the nearby Dezful Embayment (including the Pabdeh, Gurpi, Gadvan and Kazhdumi Formations) have not reached the oil window in the Mond wells. Moreover, their organic carbon content is very low as they were deposited in oxic, shallow-water settings. Underlying units (including the Ordovician and Cambrian) could have reached the gas window but contain little organic matter. Silurian shales (Sarchahan Formation), which generate gas at Kuh-e Gahkum and Kuh-e Faraghan (north of Bandar Abbas) and in Saudi Arabia and elsewhere in the Middle East, are absent from the Mond structure. The absence of source rocks suggests that the gas and heavy oil accumulations at Kuh-e Mond and at nearby fields have most probably undergone long-distance lateral migration from distant source kitchens. [source] ORDOVICIAN,PERMIAN PALAEOGEOGRAPHY OF CENTRAL EURASIA: DEVELOPMENT OF PALAEOZOIC PETROLEUM-BEARING BASINSJOURNAL OF PETROLEUM GEOLOGY, Issue 3 2003V. A. Bykadorov In this paper, we discuss three petroleum-bearing basins of Palaeozoic age in Central Eurasia,the Precaspian, Tarim and Chu-Sarysu Basins. We make use of recently-published palaeogeographic maps of the Central Eurasian region, six of which are presented here (Late Ordovician, Early-Middle Devonian, Late Devonian, Early Carboniferous, Early Permian and Late Permian). The maps illustrate the development through the Palaeozoic of the Palaeoasian and Palaeotethys Oceans; of the East European, Siberian and Tarim cratons; and of the Kazakhstan and other microcontinental blocks. The Kazakhstan block formed during the Late Ordovician and is a collage of Precambrian and Early Palaeozoic microcontinents and island arcs. It is surrounded by collisional foldbelts (Ob-Zaisan, Ural-Tianshan and Junggar-Balkhash) which formed in the Late Carboniferous , Permian. We believe that the formation of a stable Kazakhstan block is not consistent with the existence of the previously-identified "Kipchak arc" within the Palaeoasian ocean, or (as has previously been proposed) with activity on this arc up to the end of the Palaeozoic. The oil and gas potential of the Precaspian, Tarim and Chu-Sarysu Basins depends to a large extent on their tectonic stability during the Palaeozoic and subsequent time. The Precaspian Basin has been stable since the Cadomian orogeny (Early Cambrian) and is known to have major hydrocarbon potential. The Tarim Basin (NW China) has somewhat lower potential because the margins of the Tarim continental block have been affected by a series of collisional events; that margin with the Palaeotethys Ocean, for example, was active during the Late Palaeozoic. The Chu-Sarysu Basin on the Kazakhstan block is the least stable of the three and contains only minor gas accumulations. [source] Trilobite and xiphosuran affinities for putative aglaspidid arthropods Caryon and Drabovaspis, Upper Ordovician, Czech RepublicLETHAIA, Issue 3 2010JAVIER ORTEGA-HERNÁNDEZ First page of article [source] | |||||||||||||||||||||||||||||||