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The calyx of a new genus and species, Kukrusecrinus stellatus from the Kukruse Regional Stage (Sandbian Upper Ordovician) of Northern Estonia, close to Colpodecrinus from the Bromide Formation (Sandbian) of North America, is described in detail. A fragment of Kukrusecrinus sp. is also described from the Ukhaku Regional Stage (Darriwilian). These echinoderm finds indicate the presence of biogeographical connections between Baltica and Laurentia in the second half of the Middle Ordovician (Late Darriwilian). The composition of the family Colpodecrinidae Sprinkle et Kolata, 1982 and its possible phylogenetic relationships with other Ordovician crinoids are considered, based on the reconstruction of the morphogenesis of unusual key features of the family. [ABSTRACT FROM AUTHOR]

The multielement apparatuses of conodonts from the upper part of the Dzherome Formation (Baksanian and Dolborian regional stages) of the Upper Ordovician strata exposed along the Moyero and Moyerokan rivers on the Siberian Platform have been reconstructed for the first time. Multielement apparatuses include 32 formal species represented by large hyaline coniform conodont elements, described by T.A. Moskalenko in the 1970s–1980s. A new genus Acanthodistacodus gen. nov. has been described. The apparatus of the Acanthodistacodus, Dolborodina, and Acanthodina tentatively consists of 17 elements that can be classified into four morphotypes: dolaborate, quadricostate, quinquicostate, and bipennate elements. The apparatus of Ac. festus Moskalenko includes at least 13 dolaborate and bipennate elements. The species of Acanthodistacodus and Acanthodina are probably endemic to the Siberian Platform; Ac. festus Moskalenko, 1973, Dol. juktaliensis (Moskalenko, 1973), and Dol. striata Moskalenko, 2003 have also been found in northeastern Russia, which indicates that this region is biogeographically similar to the Siberian Platform. [ABSTRACT FROM AUTHOR]

The "short-great-appendage" arthropods (Megacheira), such as Leanchoilia , have featured heavily in discussions of arthropod evolution, particularly related to the head and its appendages. 1,2,3,4 Megacheirans are subject to competing interpretations, either as a clade 4 or a grade, 5 in the stem group of Euarthropoda 6 or, alternatively, Chelicerata. 4 They are most diverse in Cambrian Burgess-Shale-type deposits, where the family Leanchoiliidae is represented by six genera, 7,8,9,10,11,12 characterized by the presence of three distal flagella on the great appendage with a presumed sensory function. We describe the first post-Cambrian member of this family, Lomankus edgecombei gen. et sp. nov, from the Upper Ordovician (Katian) Beecher's Trilobite Bed site of New York State—the first post-Cambrian megacheiran with the exception of the Silurian and Devonian Enaliktidae. Micro-computed tomography (micro-CT) scanning reveals the morphology of the short great appendage with elongate flagella, four biramous cephalic limbs, 11 trunk segments with biramous limbs and dorsal tergites, and an elongate telson unique within Leanchoiliidae. The great appendage is also unique: the long endites that bear the flagella in other leanchoiliids are absent (or at least greatly reduced) and each flagellum appears to attach directly to an individual podomere, suggesting a sensory rather than a raptorial function. The remarkable preservation of a well-developed ventral plate (epistome-labrum complex) anterior of the mouth reinforces a deutocerebral origin 2,13 of the short great appendages. Lomankus edgecombei unveils the three-dimensional (3D) head morphology of leanchoiliids in unparalleled detail and demonstrates that these iconic fossil arthropods ranged into dysaerobic environments in the Ordovician, where Lomankus occupied a deposit-feeding niche. • The youngest leanchoiliid arthropod described from the Late Ordovician of the USA • Iron pyrite preservation allows high-resolution 3D reconstruction by CT scanning • The head anatomy of leanchoiliids is resolved in unprecedented detail Parry et al. describe a new leanchoiliid arthropod from the Ordovician Period that is preserved in pyrite, allowing it to be reconstructed in 3D, revealing new anatomical details. This group of arthropods is well known from the preceding Cambrian, but this new discovery reveals that they survived long after the Cambrian explosion. [ABSTRACT FROM AUTHOR]

Caryocaridids exhibited a wide geographical distribution and high diversity during the Great Ordovician Biodiversification Event. The global geographical distribution, limited taxonomic examination and lack of insight into preservational conditions have collectively contributed to an inadequate systematic assessment of Ordovician caryocaridids. To address this situation, we re-examined Ordovician caryocaridid specimens from Australia and New Zealand. Considering these findings, we establish Rhinopterocaris fordycei sp. nov. This new species is characterized by the presence of a well-developed anterior carapace horn, a rounded, truncated posteroventral margin, and furcal rami that lack triangular expansion and spines. In doing so, we confirm the validity of Rhinopterocaris. We reclassify specimens of Lingulocaris maccoyi Etheridge, 1892 into Rhinopterocaris maccoyi (Etheridge, 1892) and highlight that specimens previously reported as Rhinopterocaris maccoyi belong in Jellicaris stewarti. A phylogenetic analysis of 44 Paleozoic phyllocarids reveals that Rhinopterocaris is basal within caryocaridids and is a sister group to younger caryocaridids. Finally, we present a hypothetical evolutionary lineage of caryocaridids and explore the evolutionary history of the group. [ABSTRACT FROM AUTHOR]

With the continuous increase in exploration efforts in new zones and new strata, significant breakthroughs have been made in the natural gas exploration of the O¹m56 to O1m4 formations in the Ordos Basin. Thus, the origin and exploration potential of subsalt natural gas have attracted much attention and urgently need to be addressed. On the basis of certain geochemical characteristics, genetic types, and sources of natural gas, a comprehensive study on the sedimentary environment, organic geochemical characteristics, and spatial distribution scale of source rocks are conducted in this paper by using geological and geochemical methods. The study shows that: (1) The Ordovician subsalt natural gas is mainly "pyrolysis dry gas," among which the δ13C1 of Ordovician subsalt low sulfur (sulfur-free) natural gas is lighter, with an average value of -39.6‰; the δ13C2 ranges more largely from -35.6‰ to -25.8‰. In contrast, both δ13C1 and δ13C2 values are heavier in high-sulfur natural gas, revealing that different Thermochemical Sulfate Reduction (TSR) reaction stages have different degrees of influence on natural gas components and carbon isotope composition. (2) Subsalt natural gas is classified as "oil-type gas," which is self-generated and self-accumulated, whose source rocks are mainly Ordovician subsalt marine deposits. (3) Three types of marine source rocks are developed in Ordovician subsalt, including black argillaceous rock, dark argillaceous dolomite (dolomitic mudstone), and dark micrite (bioclastic) limestone. In addition to micrite limestone, these rocks were mainly formed in a confined lagoon sedimentary environment with high salinity and anoxia. Sedimentary water was significantly stratified and the environment was highly reduced. The organic matter content of the source rocks is relatively high, with an average TOC value of 0.45%. The hydrocarbon-generating parent materials are mainly composed of bacteria and algae, and the organic matter evolution reaches highover maturity stage. The total gas generation amount of the marine source rocks in Ordovician subsalt is approximately 43.8 × 1012 m³, which can provide hydrocarbons and accumulate for the subsalt favorable reservoir facies located far from Upper Paleozoic gas sources. [ABSTRACT FROM AUTHOR]

The recent record of extremely rare rhabdopleurid graptolites with a supposed epibiontic life style in the Fezouata biota has to be rejected, as the specimens indicate an epibenthic growth on a shell fragment lying on the sea bottom. In absence of morphological data that support a truly colonial development on these encrusters, it cannot be ruled out that the specimens may alternatively represent the pseudo-colonial tubaria of cephalodiscid-like pterobranchs. The interpretation of the presence of benthic graptolites (class Pterobranchia; subclass Graptolithina) from the Fezouata Shale biota of Morocco provides us with some serious problems. Their life style as benthic or epibenthic organisms living on firm substrates and hardgrounds makes it difficult for them to be preserved in these highly fossiliferous, originally 'soupy' soft sediments of the Fezouata Shale, unless they are transported and covered by sediment subsequently. The graptolite record of the Fezouata biota appears to be restricted to planktic forms of Graptoloidea: the review of the few benthic dendroids so far cited for the Lagerstätte resulted in the identification of the rare planktic dendroid genus Calyxdendrum. The problematic species Webbyites felix may represent a hydrozoan (Cnidaria) rather than a benthic graptolite. [ABSTRACT FROM AUTHOR]

Over the last two decades, there has been an intensive research effort focused on studying the climate fluctuations during the Ordovician period. In this study, we provide brachiopod and bulk carbonate rock stable isotopic data for oxygen (O) and carbon (C) from the Middle Ordovician succession from northern Estonia for evaluating the Middle Ordovician palaeotemperature and palaeoenvironmental history. We present δ18O curves from eight northern Estonian outcrops. This new data, together with the published results, enable compiling the first complete stable oxygen isotopic curve for Middle Ordovician to early Silurian for the eastern Baltoscandian Palaeobasin. Combining the published and new δ13Cbrac and δ18Obrac data allows us to address chemostratigraphic correlation in the Middle Ordovician. The Baltoscandian δ18O data generally confirm a cooling trend from Early to Middle Ordovician documented by previous studies in different palaeobasins. [ABSTRACT FROM AUTHOR]

The tectonic of the Middle and Late Ordovician in the western margin of the north China Platform is complex, and the accumulation models of organic matter of the Wulalike Formation formed during this period are still unclear. Total organic carbon (TOC) content, mineral composition, organic carbon isotope composition, as well as the major and trace elements in the shale samples were all measured in this study. The Wulalike Formation was formed during a tectonic transition from a passive continental margin to an active continental margin. Hydrocarbon-forming organisms are mainly algae. Primary productivity, terrigenous input, and sedimentation rates influence input, dilution, and degradation of organic matter. The degree of degradation, redox, and basin restriction control the preservation of organic matter. The TOC of the Wulalike Formation is determined by the input and preservation degree of organic matter at different sedimentary periods. The organic matter accumulation model in the Wulalike Formation is controlled by regional structures, dominated by sedimentation rate and preservation conditions. [ABSTRACT FROM AUTHOR]

The Early-Mid Ordovician Kheneg el Aatène Formation of the Ougarta Range is here divided into two members: (i) the Aatène Micaceous Sandstone Member and (ii) the Aatène Quartzite Member. A low diversity trace fossil assemblage is present in the upper part of the second member with abundant Skolithos linearis, Skolithos isp. and Rosselia erecta, common Arenicolites isp. and Diplocraterion isp. and rare Palaeophycus isp. and ?Rusophycus isp. This association is characteristic to the Skolithos ichnofacies and indicates high-energy conditions and opportunistic colonisation within a siliciclastic shallow marine environment. The ichnoassemblage is dominated by vertical traces which occur in great densities forming typical Skolithos pipe-rocks. The pipe-rock ichnofabric is characterised by burrows that are closely spaced and occur at 20 distinct levels. Possible predation has been detected in the same Member due to the presence of ?Rusophycus intersecting worm burrows. High densities of Trypanites are present in two hardgrounds composed of calcareous lithified sandstones developed during transgressive pulses that can be related to the brevis event. Trypanites borings are evidenced for the first time from inorganic hard substrates of the Gondwanan Ordovician. These borings can be related to beginning of the Ordovician Bioerosion Revolution in Gondwana. [ABSTRACT FROM AUTHOR]

Some circular fossils showing radial structures concentrated in a small area have been recorded from the Criadero Quartzite, 1 km north from El Centenillo village (Iberian Central Zone, SE Spain). The fossils resembling cnidarians have been studied in detail, both at the field and in the laboratory. The specimens have been preserved in thick-grained quartz sandstones. They were analysed in thin sections and rock samples using optical microscopy, micro-reflectance spectroscopy, Fourier transform infrared–attenuated total reflectance spectroscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy and epifluorescence microscopy. The radial structure is composed of ridges and grooves, which correspond to clear and dark ochre bands filled with middle-sized, angular-elongated quartz grains without tractive structures. Mineralogical analyses reveal that composition is based on quartz and illite in both areas. The morphological and compositional features of the fossils have been compared with a set of ichnofossils of multiple taxonomic origins and ages without conclusive results. The preservation is probably produced by the existence of microbial mats on the sand floor and the impression of moulds by authigenic cementation by an unidentified organism. To elucidate the nature of the producer, potentially soft-bodied cnidarian, including fossil medusae and sea anemone, further investigations are needed. [ABSTRACT FROM AUTHOR]

Abstract The tectonic of the Middle and Late Ordovician in the western margin of the north China Platform is complex, and the accumulation models of organic matter of the Wulalike Formation formed during this period are still unclear. Total organic carbon (TOC) content, mineral composition, organic carbon isotope composition, as well as the major and trace elements in the shale samples were all measured in this study. The Wulalike Formation was formed during a tectonic transition from a passive continental margin to an active continental margin. Hydrocarbon-forming organisms are mainly algae. Primary productivity, terrigenous input, and sedimentation rates influence input, dilution, and degradation of organic matter. The degree of degradation, redox, and basin restriction control the preservation of organic matter. The TOC of the Wulalike Formation is determined by the input and preservation degree of organic matter at different sedimentary periods. The organic matter accumulation model in the Wulalike Formation is controlled by regional structures, dominated by sedimentation rate and preservation conditions.

Based on logging, core, thin section and geochemical analysis, the tectonic-lithofacies paleogeographic pattern of first member to third member of Ordovician Majiagou Formation (O1m1–O1m3 for short) in Ordos Basin is reconstructed, and the tectono-sedimentary evolution characteristics and oil-gas geological significance are discussed. The results are obtained in four aspects. First, a set of marginal argillaceous dolomites with high gamma ray value developed steadily and diachronously at the bottom of Majiagou Formation, which distributed over the Huaiyuan Movement unconformity, with δ13C values positive drift characteristics comparable to global transgression of the Early Ordovician Floplian. Second, the global sea level rose and the ancient land was submerged into the underwater uplift in O1m1 to O1m2, and the central uplift was deposited for the first time in the Ordovician, forming a tectonic pattern of “one uplift and two depressions”. Subsequently, the subduction and extrusion outside the basin and the differentiation of uplift and depression in the basin of O1m3 resulted in the activation of the Wushenqi–Jingbian bulge. Third, the evolution of the tectonic pattern had a significant impact on the sedimentary paleoenvironment. The O1m1 overlaps westward, and saline lagoon is formed in eastern depression and influenced by the transgression. The transgression continued in O1m2 and resulted in communication with the wide sea, and the large-scale grain shoal developed around eastern depression, and the late dry shrinkage formed a small scale evaporite lagoon in upper part. Under the influence of highland sealing in O1m3, the water body gradually differentiated into dolomitic gypsum and saline lagoons to the east, and the grain shoal spread along the highs around sag. Fourth, the source rocks developed diachronously at the bottom of Majiagou Formation form a favorable source-reservoir assemblage with the shoal facies reservoir distributed around the slope of O1m2–O1m3, and they have certain exploration potential for natural gas.

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the OrdovicianâSilurian continentâcontinent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the Jämtland County, central Sweden, provides the base for detailed sedimentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66â1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27â1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin preserved in a window beneath the Caledonian thrust sheets.

Based on logging, core, thin section and geochemical analysis, the tectonic-lithofacies paleogeographic pattern of first member to third member of Ordovician Majiagou Formation (O1m1-O1m3 for short) in Ordos Basin is reconstructed, and the tectono-sedimentary evolution characteristics and oil-gas geological significance are discussed. The results are obtained in four aspects. First, a set of marginal argillaceous dolomites with high gamma ray value developed steadily and diachronously at the bottom of Majiagou Formation, which distributed over the Huaiyuan Movement unconformity, with d13C values positive drift characteristics comparable to global transgression of the Early Ordovician Floplian. Second, the global sea level rose and the ancient land was submerged into the underwater uplift in O1m1 to O1m2, and the central uplift was deposited for the first time in the Ordovician, forming a tectonic pattern of "one uplift and two depressions". Subsequently, the subduction and extrusion outside the basin and the differentiation of uplift and depression in the basin of O1m3 resulted in the activation of the Wushenqi-Jingbian bulge. Third, the evolution of the tectonic pattern had a significant impact on the sedimentary paleoenvironment. The O1m1 overlaps westward, and saline lagoon is formed in eastern depression and influenced by the transgression. The transgression continued in O1m2 and resulted in communication with the wide sea, and the large-scale grain shoal developed around eastern depression, and the late dry shrinkage formed a small scale evaporite lagoon in upper part. Under the influence of highland sealing in O1m3, the water body gradually differentiated into dolomitic gypsum and saline lagoons to the east, and the grain shoal spread along the highs around sag. Fourth, the source rocks developed diachronously at the bottom of Majiagou Formation form a favorable source-reservoir assemblage with the shoal facies reservoir distributed around the slope of O1m2-O1m3, and they have certain exploration potential for natural gas. [ABSTRACT FROM AUTHOR]

The oxygen isotope ratio 18O/16O (expressed as a δ18OVSMOW value) in marine sedimentary rocks has increased by ~8‰ from the early Paleozoic to modern times. Interpretation of this trend is hindered by ambiguities in the temperature of formation of the carbonate, the δ18Oseawater, and the effects of postdepositional diagenesis. Carbonate clumped isotope measurements, a temperature proxy, offer constraints on this problem. This thermometer is thermodynamically controlled in cases where carbonate achieves an equilibrium internal distribution of isotopes and is independent of the δ18O of the water from which the carbonate grew; therefore, it has a relatively rigorous chemical-physics foundation and can be applied to settings where the δ18O of the water is not known. We apply this technique to an exceptionally well-preserved Ordovician carbonate record from the Baltic Basin and present a framework for interpreting clumped isotope results and for reconstructing past δ18Oseawater. We find that the seawater in the Ordovician had lower δ18Oseawater values than previously estimated, highlighting the need to reassess climate records based on oxygen-isotopes, particularly where interpretations are based on assumptions regarding either the δ18Oseawater or the temperature of deposition or diagenesis. We argue that an increase in δ18Oseawater contributed to the long-term rise in the δ18O of marine sedimentary rocks since the early Paleozoic. This rise might have been driven by a change in the proportion of high- versus low-temperature water-rock interaction in the earth's hydrosphere as a whole. [ABSTRACT FROM AUTHOR]

The marine carbonates in the Ordovician Majiagou Formation in the Ordos Basin have significant exploration potential. Research has focused on their thermal history and hydrocarbon accumulation stages, as these are essential for guiding the exploration and development of hydrocarbons. In this paper, we study the thermal evolution history of the carbonate reservoirs of the Ordovician Majiagou Formation in the east-central Ordos Basin. Furthermore, petrographic and homogenization temperature studies of fluid inclusions were carried out to further reveal the hydrocarbon accumulation stages. The results demonstrate that the degree of thermal evolution of the Ordovician carbonate reservoirs is predominantly influenced by the deep thermal structure, exhibiting a trend of high to low values from south to north in the central region of the basin. The Fuxian area is located in the center of the Early Cretaceous thermal anomalies, with the maturity degree of the organic matter ranging from 1 to 3.2%, with a maximum value of 3.2%. The present geothermal gradient of the Ordovician Formation exhibits the characteristics of east–high and west–low, with an average of 28.6 °C/km. The average paleo-geotemperature gradient is 54.2 °C/km, the paleoheat flux is 55 mW/m2, and the maximum paleo-geotemperature reaches up to 270 °C. The thermal history recovery indicates that the Ordovician in the central part of the basin underwent three thermal evolution stages: (i) a slow warming stage before the Late Permian; (ii) a rapid warming stage from the end of the Late Permian to the end of the Early Cretaceous; (iii) a cooling stage after the Early Cretaceous, with the hydrocarbon production of hydrocarbon source rocks weakening. In the central part of the basin, the carbonate rock strata of the Majiagou Formation mainly developed asphalt inclusions, natural gas inclusions, and aqueous inclusions. The fluid inclusions can be classified into two stages. The early-stage fluid inclusions are mainly present in dissolution holes. The homogenization temperature is 110–130 °C; this coincides with the hydrocarbon charging period of 210–165 Ma, which corresponds to the end of the Triassic to the end of the Middle Jurassic. The late-stage fluid inclusions are in the dolomite vein or late calcite that filled the gypsum-model pores. The homogenization temperature is 160–170 °C; this coincides with the hydrocarbon charging period of 123–97 Ma, which corresponds to the late Early Cretaceous. Both hydrocarbon charging periods are in the rapid stratigraphic warming stage. [ABSTRACT FROM AUTHOR]

New Floian and early middle Darriwilian brachiopod assemblages of the San José Formation of the Eastern Cordillera of Peru are presented. A new genus and species, Apurimella santiagoi, and two new species, Phragmorthis henrylunae and Nocturnellia ashaninka, are described. The assemblages also contain additional characteristic taxa demonstrating links with other proto‐Andean localities (Peruvian Altiplano, Argentinian Eastern Cordillera, Famatina) as well as with some localities of Baltica (Estonia, Ingria, Norway), Ganderia (Anglesey, Tramore, Indian Bay, Summerford, Miramichi), peri‐Laurentia (Mayo, Svalbard) and Laurentia (Klamath Mountains). Those shared genera indicate dynamic faunal exchanges between the Peruvian Eastern Cordillera and these terranes during the Early–Middle Ordovician transition, suggesting active brachiopod dispersal mechanisms across the Rheic and Iapetus oceans. These connections seem to have still been active by the middle Darriwilian, as suggested by the presence of Phragmorthis in the Darriwilian assemblages of Cuyania and Laurentia. Similarly, the new Peruvian Nocturnellia species is of particular interest, given that this genus is likely to have originated in South China during the Floian, but extended its distribution to Avalonia and to high‐latitude peri‐Gondwana by the Dapingian. It thrived there until the middle Darriwilian, but by that time had also reached the proto‐Andean margin either via Avalonia across the Rheic Ocean or along the Gondwanan coast from high‐latitude settings. All of these occurrences improve our knowledge of Early–Middle Ordovician brachiopod faunas of the northern Central Andean Basin and their relationships with contemporaneous faunas worldwide. [ABSTRACT FROM AUTHOR]

A new species and genus of colonial coral, Lindaphylon solovjevi, from the Upper Ordovician of Estonia is described in detail. The colony has a zonation similar to the zonation structure of the mesogleal skeleton of octocorals, since it consists of a narrow axis and a surrounding wide cortex of small calcite sclerites and a system of large solenia. Corallites present 12 sclerosepta in a wide shallow cup. A new family Lindaphylonidae and an order Lyndaphylonaceae are based on these features, and a new order placed in the subphylum Anthozoa as incertae sedis. A comparative study of skeleton formation and mesenteric apparatus symmetry in different groups of coral polyps is considered in detail. [ABSTRACT FROM AUTHOR]

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The protective structures of bryozoans representing four orders—Trepostomata, Cryptostomata, Fenestrata, and Cheilostomata—were analyzed and classified for the first time based on functional criteria. The proposed classification includes five groups of protective structures responsible for: 1) reducing water pressure; 2) increasing the feeding area by expanding the distance between the autozooecia apertures, as well as strengthening the colony’s stability and integrity; 3) physically protecting the polypide and providing attachment points for muscles; 4) scaring off predators; and 5) enabling physical protection against predators while also cleaning the colony from sediments, larvae, and small invertebrates. The study of the development of heterozooids, stylets, and avicularia in fossil bryozoans revealed the emergence of persistent, effective polymorphs in the Early Ordovician, Late Devonian, and Early Cretaceous. The evolutionary changes in bryozoans were primarily driven by fluctuations in the levels of the World Ocean, climate, phytoplankton biomass, and predator composition, among other complex interplays between abiotic and biotic factors. To shield themselves from predation threats, bryozoans evolved acanthostyles protruding above colony surfaces and various types of heterozooids physically protecting them from being eaten (caverno-, para-, cyclo-, lepto-, and allozooids in the old groups, along with avicularia in cheilostomates). Avicularia and similar structures occurred repeatedly in different orders: in fenestrates during the Early Ordovician and Early Devonian, in cryptostomates during the Middle Carboniferous, and in cheilostomates during the Early Cretaceous.

To clarify the genetic mechanism for phase change of the hydrocarbon in the ultra-deep reservoirs, a case study from the Ordovician hydrocarbon in the Halahatang–Shunbei area (HSA), Tarim Basin, NW China, was conducted. The results show that the Ordovician reservoirs in the HSA are characterized as multi-phase reservoirs with a lateral co-existence of condensates, volatile-oil reservoirs, normal oil reservoirs, and heavy oil reservoirs. From north to south, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon in different blocks of the HSA, showing an increasing trend in GOR, dryness coefficients, methane contents, methane carbon isotope values, and ethane carbon isotope values, while a decreasing trend in oil densities and wax contents. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed and the kerogen-cracking gas is dominated in the HSA, the regular variations of the hydrocarbon phases and geochemical characteristics can be interpreted as records of biodegradation and multistage oil–gas filling rather than controlled by the source rock organofacies, oil cracking, and gas invasion. The formation mechanism of the Ordovician multi-phase reservoirs in the HSA suggests that the deep strata of the Tarim Basin hold potential for the exploration of natural gas resources.

In recent years, exploration breakthroughs have been successively achieved in the strata deeper than the fifth member of the Lower Ordovician Majiagou Formation in the Ordos Basin, revealing its tremendous exploration potential. With the deployment of high-precision 3D seismic data and the deepening of oil and gas exploration, the control exerted by faults on the gas reservoirs in the deeper layers of the fifth member has become increasingly prominent. Based on high-precision 3D seismic data, this study systematically characterized the geometric features of faults in the Gaojiabao area, dissected typical fault structural patterns, established fault formation and evolution models, and, in conjunction with drilling and testing data, discussed the control of faults on the differential enrichment of gas reservoirs in the deeper layers of the Majiagou Formation. The study yielded the following insights: Faults in the Lower Paleozoic of the Gaojiabao area in the Ordos Basin exhibit distinct stratified deformation characteristics, with the two sets of gypsum-salt rocks in the third and fifth members being major contributors to fault stratification. The Lower Paleozoic in the Gaojiabao area mainly develops strike-slip faults with compressional and torsional properties and thrust faults. Through studying the structural morphology and fault patterns of the Gaojiabao area, it is inferred that the Yanshan tectonic movement was the most significant phase of reformation affecting the faults formed during the Caledonian tectonic movement. The coupling of the structural uplift period with the oil-gas charging period in the Gaojiabao area means that faults play multiple roles, including improving reservoirs, connecting source rocks, and controlling traps.

The evolution of brachiopod symbiosis is closely tied to the evolution of brachiopod faunas and their partner groups during the early Palaeozoic. Brachiopod groups with a larger number of taxa had more symbiotic associations, and there was no specific group that was more prone to symbiosis during this time interval. The first symbiotic associations appeared soon after the emergence of certain brachiopod groups, with Cambrian brachiopods partnering with typical representatives of the Cambrian fauna. Bryozoans and tentaculitoid tubeworms, which became important partners during the Ordovician and Silurian, first diversified in the Ordovician. The gradual decrease in the number of brachiopod partner groups from the Cambrian to the Silurian was likely due to specialisation. However, the number of symbiotic associations did not increase faster than the number of brachiopod taxa. The GOBE-induced diversification of brachiopod taxa did not lead to an escalation in symbiotic relationships. Symbiotic associations involving brachiopods continued after the end-Ordovician mass extinction. Although early Palaeozoic brachiopods were vulnerable to kleptoparasites, the harm caused by these parasites was not enough to drive their associated brachiopods to extinction. [ABSTRACT FROM AUTHOR]

Because their resistant, sporopolleninous walls preserve a record of morphogenetic change during spore formation, fossil cryptospores provide a direct physical record of the evolution of sporogenesis during the algal–plant transition. That transition itself is a story of the evolution of development—it is not about phylogeny. Here, we review the fossil record of terrestrially derived spore/cryptospore assemblages and attempt to place these microfossils in their evolutionary context with respect to the origin of complex multicellularity in plants. Cambrian cryptospores show features related to karyokinesis seen in extant charophytes, but they also possess ultrastructure similar to that seen in liverworts today. Dyadospora, a cryptospore dyad recovered from sporangia of Devonian embryophytes, first occurs in the earliest Ordovician. Tetrahedraletes, a likely precursor to the trilete spore, first occurs in the Middle Ordovician. These fossils correspond to evolutionary novelties that were acquired during a period of genome assembly prior to the existence of upright, axial sporophytes. The cryptospore/spore fossil record provides a temporal scaffold for the acquisition of novel characters relating to the evolution of plant sporogenesis during the Cambrian–Silurian interval. [ABSTRACT FROM AUTHOR]

There are some persistent basic questions pertaining to the bedrock schist of New York City (NYC). How many mappable schist formations are exposed in NYC, and what was the sedimentary protolith of the Manhattan schists? Our proposed answers are based in part on a blending of published paleontological and radiometric dating results that constrain the timing of Taconic subduction and the best choice of a pelitic protolith for the schists of NYC. We have chemically analyzed some samples of schist and shales at key locations to evaluate the plausibility of our proposals. The compelling published evidence indicates that the Taconic Orogeny began about 475 Ma, when peri-Laurentian plates began the process of east-dipping subduction under the Moretown Terrane, resulting in a magmatic flareup of the Shelburne Falls arc that carried the Moretown Terrane west across NYC. East-dipping subduction accounts for early Ordovician metamorphism until an oceanic slab break-off event at about 466 Ma. Our review of the biostratigraphic data indicates a continuation of subduction and the deposition of pelitic sediments until about 455 Ma, during the transition to deep-water turbiditic sediment deposition. This disqualifies all post-455 Ma turbidites as viable protoliths for the NYC Manhattan schists but does include the Late Cambrian to lowermost Late Ordovician pelites of the Jutland Sequence that are exposed directly west of NYC in New Jersey. Our new chemical analyses of Jutland sediments and each of the three named schists from the NYC plot as a single geochemical population. We, therefore, propose that the schists of NYC could collectively be referred to as the Manhattan schist of the Late Cambrian to lower Late Ordovician. [ABSTRACT FROM AUTHOR]

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The Ordovician thick dolostone in Shuntogol area of the Tarim Basin has the potential to form a large-scale reservoir, but its genesis and reservoir development model are still unclear. Starting from a sedimentary sequence, this study takes a batch of dolostone samples obtained from new drilling cores in recent years as the research object. On the basis of core observation and thin section identification, trace elements, cathodoluminescence, carbon and oxygen isotopes, rare earth elements, and X-ray diffraction order degree tests were carried out to discuss the origin of the dolomite and summarize the development model of the dolostone reservoir. The analysis results show that the Ordovician dolomite in the study area had a good crystalline shape, large thickness, high Fe and Mn values, and mostly showed bright red light or bright orange–red light under cathode rays. The ratio of δ18O values to seawater values at the same time showed a negative bias; the δCe values were negative anomalies, the δEu values were positive anomalies, and the order degree was high. This indicates that the dolomitization process occurred in a relatively closed diagenetic environment. The Ordovician carbonate rocks in the study area were low-lying during the sedimentary period, and with the rise of sea level, the open platform facies continued to develop. When the Middle and Lower Ordovician series entered the burial stage, the main hydrocarbon source rocks of the lower Cambrian Series entered the oil generation peak, and the resulting formation overpressure provided the dynamic source for the upward migration of the lower magnesium-rich fluid, and the dolomitization fluid entered the karst pore system in the target layer to produce all the dolomitization. This set of dolostone reservoirs is large in scale and can be used as a favorable substitute area for deep carbonate exploration for continuous study. [ABSTRACT FROM AUTHOR]

The distribution of fractures in fractured-vuggy reservoirs of Ordovician carbonate rock in the Tahe oilfield is one of the main controlling factors for oil and gas enrichment in this area. Determining the spatial distribution of fractures is the key to stable production in this area. First, this article uses cores and thin sections to analyse the characteristics of fractures. The area is dominated by structural fractures, with relatively few dissolution fractures and diagenetic fractures. The average length of the fractures is 5.25 cm, the average opening is 0.21 cm, the average angle is 40.33°, and the average line density is 3.05 lines/m. Second, rock mechanical parameters were calculated using dipole shear wave logging curves and conventional logging curves, and fracture development index (FI) curves were constructed. Using the variations in the performance characteristics of the FI curve the spatial distribution patterns of fractures in different fracture development segments are qualitatively analyse. The predicted results are then verified by combining core, thin section, and imaging logging data, and the coincidence rate reached 83%. The following conclusions are drawn: Vertically, fractures are mostly concentrated in the Yijianfang Formation, with the characteristics of “small scale and scattered”. On the plane, the development scale of fractures in the denudation area is good, with great development potential. Combined with the production of each well, it is verified that the fracture development segment identified by the fracture FI curve has guiding significance for oil and gas exploration. [ABSTRACT FROM AUTHOR]

Documentation of cryptic trilobite behavior has presented important insights into the paleoecology of this fully extinct arthropod group. One such example is the preservation of trilobites inside the remains of larger animals. To date, evidence for trilobites within cephalopods, gastropods, hyoliths, and other trilobites has been presented. Importantly, most of these interactions show trilobite molts, suggesting that trilobites used larger animals for protection during molting. To expand the record of molted trilobites within cephalopods, we present a unique case of a Toxochasmops vormsiensis trilobite within the body chamber of a Gorbyoceras textumaraneum nautiloid from the Upper Ordovician Kõrgessaare Formation of Estonia. By examining this material, we present new insights into the ecology of pterygometopid trilobites, highlighting how these forms used large cephalopods as areas to successfully molt. [ABSTRACT FROM AUTHOR]

A large and taxonomically diverse collection of Ordovician Asterozoa from France and Morocco enables new insights into the early evolution of the subphylum. Available specimens of both Asteroidea and Stenuroidea are comparatively few, the collection dominated by Ophiuroidea and to a lesser extent, Somasteroidea. Nearly all the asteroid specimens are assigned to Petraster Billings, 1858; P. caidramiensis n. sp. is differentiated based on the complexity of the extraxial skeleton. The genus Petraster is reviewed. Another asteroid fragment, Euaxosida sp. A, is too incomplete to assign below the ordinal level. Among species of Petraster, the extraxial skeleton suggests homoplastic emergence of ossicular series definition whereas evolution of ambulacral axial-adaxial series expressions was conservative. The stemward asteroid axial/adaxial configuration is considered largely a plesiomorphic derivative of the somasteroid condition. Ossicular details of all series, however, differ among Petraster species. Euaxosida sp. A also offers oral-surface expressions suggestive of a transitional status. Differences among ossicular series expressions exemplify concerns surrounding interpretation of early asterozoan phylogeny and taxonomy. [ABSTRACT FROM AUTHOR]

Detrital zircon dating allows us to elucidate the age and provenance of siliciclastic strata of the Sarydjaz and Ichkebash formations in the Middle Tianshan of eastern Kyrgyzstan, previously thought to be of the Middle or Late Ordovician age. New data indicate that the upper part of the Ichkebash Formation contains Lower Silurian sediments, previously unknown in this region. The Kanachu Formation, which lies stratigraphically above, is also not older than the Silurian in age. The absence of marine fauna at these levels apparently reflects the predominant development of deltaic facies. The paleocurrent indicators and the ages of detrital zircons point to its provenance in the North Tianshan (NTSh). Grains with ages from the Late Cambrian to the Silurian, synchronous with the main stage of the Early Paleozoic magmatism in the NTSh, predominate in the Ichkebash Formation. The occurrence of Precambrian detrital zircons in all dated samples and especially in the Sarydjaz Formation indicates the erosion of Precambrian rocks. The provenance most likely located in the Chu-Kendyktas Massif, where the Precambrian detrital zircons in the Neoproterozoic and Early Paleozoic metasedimentary rocks demonstrate a similar distribution of the ages. The foredeep in the Middle Tianshan was formed in the Late Ordovician and Early Silurian generally subsynchronously with the main orogenic event in the NTSh. [ABSTRACT FROM AUTHOR]

Copyright of Petroleum Geology & Experiment is the property of Petroleum Geology & Experiment Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The study of the ancient subterranean river system in the Tahe Oilfield in the Tarim Basin is still in its initial stage. Subterranean river caves are mainly classified based on the characteristics of the depth distribution and structural patterns of the subterranean rivers. However, there is a lack of comprehensive analysis regarding the spatial development patterns of complex subterranean river systems from geological perspectives such as structure, fault, paleogeomorphology, and water table. This hinders the understanding of primary and secondary relationships of subterranean rivers, spatial superposition patterns, and original connectivity relationships, thereby impeding comprehensive management research in the later stages of Tahe Oilfield development. To address this gap, an investigation was carried out in the S67 well block to clarify the developmental characteristics of the Ordovician ancient subterranean river system in the thrust anticline area of the main Tahe Oilfield. The study utilized methods like structural fault analysis, paleogeomorphology restoration, seismic attribute characterization, and vertical section interpretation to identify river types, classify systems, and examine geological origins. Noteworthy findings include the identification and analysis of a phreatic loop river for the first time. Results show that the S67 well block is located in a low-lying region at the southern edge of the karst platform within the main Tahe Oilfield area. This region features peak cluster depressions and karst hill depressions with minimal amplitude differences and shallow incision depths of surface water systems. The low-angle thrust structural style of the thrust anticline, combined with a network of faults, provides favorable dissolution channels for the multi-layered subterranean river system. In the study area, the Ordovician period is characterized by the emergence of distinct water-table subterranean river systems and phreatic loop river systems, all of which display a dendritic structure. The water-table subterranean rivers are divided into main, branch, and abandoned types, while the phreatic loop rivers are classified as ascending or symmetrical. The formation of ancient subterranean rivers in the thrust anticline area is primarily influenced by factors such as paleogeomorphology, water table levels, thrust anticline structure, and secondary fault networks.

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project focuses on processes related to the closure of the Iapetus Ocean, causing the Ordovician-Silurian continent-continent collision between Baltica and Laurentia. The rock succession in the second drill core (COSC-2) from the Jämtland County, central Sweden, provides the base for detailed sedi - mentological, stratigraphic, geophysical, geochemical, geothermal and structural studies. The basement, comprising 1.66-1.65 Ga Transscandinavian Igneous Belt porphyries intruded by 1.47 Ga and 1.27-1.26 Ga mafic dykes and sills, is heavily weathered towards the top. Here it grades into typical saprock and saprolite (including immature soil reflecting the sub-Cambrian peneplain). The overlying sedimentary sequence starts with basal conglomerates and heterogeneous sediments with shell fragments, indicating an early Cambrian rather than a Neoproterozoic age for the marine transgression in the area. The developing early Cambrian basin was rapidly filled, initially by mostly coarse-grained sediment gravity flows. These strata are covered by sandstone turbidites that show an upward transition into the Alum Shale Formation, representing a tectonically quieter period (mid-Cambrian/Maolingian to Early Ordovician/Tremadocian). The upper part of the Alum Shale Formation is overlain by a late Early Ordovician turbidite succession. Local sources of sediments below the Alum Shale Formation and the extended deposition period may indicate continuous sedimentation in a pull-apart basin pre - served in a window beneath the Caledonian thrust sheets. [ABSTRACT FROM AUTHOR]

The Ordovician rocks on the Qinghai-Tibetan Plateau represent the oldest non-metamorphic strata, and are critical to understanding the history of regional geology and biotic evolution of the entire plateau. Strata of Floian, Darriwilian, Sandbian, Katian and Hirnantian are represented in the plateau with a hiatus of variable duration occurring underneath the basal Ordovician across the area. Five stratigraphical regions, including the Himalaya, Gangdise-Zayu, Qiangtang-Qamdo, Songpan-Garze, and Karakoram-Kunlun-Altun, are differentiated for the Ordovician strata, which are correlated with their equivalents in the Sibumasu, Indochina, Qaidam-Qilian, Tarim-Tianshan, and the Yangtze (western margin) stratigraphical regions. On the Qinghai-Tibetan Plateau, graptolites, conodonts, and cephalopods are the most common and useful fossils for the Ordovician biostratigraphy. The Ordovician biotas of the Qinghai-Tibetan Plateau bear some distinguishable palaeobiogeographical signatures, among which the cephalopods are characterized by the flourishing actinocerids of North China affinity in the Early-Middle Ordovician, and by the thriving lituitids and orthocerids of South China affinity in the Middle-Late Ordovician. Fossil occurrences and their palaeobiogeographical evolution provide critical evidence bearing on the reconstruction of the geological history of the Qinghai-Tibetan Plateau and surrounding terranes in northeastern peri-Gondwana. The stratigraphical successions of the Cambrian-Ordovician transition in the Himalaya and Lhasa and nearby Sibumasu terranes were significantly affected by the Kurgiakh Orogeny, which resulted in the extensive unconformity between the Ordovician and the underlying rocks in most areas of the Qinghai-Tibetan Plateau. In southern Xizang, a warm-water biota of Middle Ordovician age was recovered from oolitic limestones, suggesting a likely palaeogeographical location in low-latitudes near the equator. In the Himalaya and Sibumasu regions, the Upper Ordovician was typified by the occurrence of red carbonates with distinctive reticulate structures, which are correlative to their equivalents in the Yangtze region of South China, and might be deposited under similar geological conditions. The global end-Ordovician glaciation and sea-level drop likely caused the wide absence of late Katian strata in western Yunnan of China and the Shan State of Myanma, and may have also affected deposition in the Xainza and Nyalam areas of the Qinghai-Tibetan Plateau during this time interval. [ABSTRACT FROM AUTHOR]

[Display omitted] • Detrital zircon U-Pb-Hf and rutile U-Pb-trace element from Ordovician strata in Iran. • Ordovician sandstones in Iran are locally sourced from 600—500 Ma magmatic rocks. • In early Paleozoic this segment of Gondwana margin formed a wide extensional zone. The Paleozoic tectonic evolution of the northern margin of Gondwana is inferred to comprise several episodes of continental breakup and ocean opening. However, the timing and number of rifting events, and the identity and paleogeography of the continental blocks involved in these inferred breakup events remain poorly understood. This study examines the early Paleozoic tectonic evolution of northern Gondwana by analyzing the paleogeography and provenance of Ordovician strata exposed in the Iranian blocks. New detrital zircon U–Pb–Hf measurements from Ordovician siliciclastic strata in Iran define two major age fractions at 600—500 Ma (41 %; ɛHf (t) = −11.9 to +9.6) and 1100—600 Ma (37 %; ɛHf (t) = −21.6 to +10.6), and two minor fractions at > 1100 Ma (13 %; ɛHf (t) = −13.9 to +5.9) and 500—444 Ma (10 %; ɛHf (t) = −9.1 to +4.4). Detrital rutile U–Pb ages from these units similarly include two major age fractions at 1100—600 Ma (49 %) and 600—500 Ma (33 %), and two minor fractions at > 1100 Ma (14 %) and 500—444 Ma (4 %). Zr-in-rutile temperatures and Cr–Nb compositions indicate that most detrital rutiles are sourced from amphibolite facies metapelitic or metafelsic rocks. The new data suggest sediment was ultimately sourced from the Arabian-Nubian Shield (∼54 %) and 600—500 Ma (∼39 %) and Ordovician (∼7%) magmatic rocks in local basement in Iran. The widespread exposure of 600—500 Ma magmatic rocks in Iran indicates significant changes in basin configuration from a stable platform in the Ediacaran—Cambrian to a series of fault-bound extensional basins in the Ordovician. We explore two possible scenarios for the Ordovician extension in Iran. The first involves detachment of a continental block from northern Iran leading to opening of the Paleo-Tethys Ocean. However, because the identity of this hypothesized rifted continental block remains unclear, we interpret that early to middle Paleozoic extension across the region occurred within an already established continental margin. [ABSTRACT FROM AUTHOR]

Two new species of cornutan stylophora of the genus Phyllocystis are described from the Volkhov Regional Stage (Middle Ordovician, Dapingian) of Baltica. This indicates a biogeographic connection between the Afro-European part of Gondwana and Baltica since the very beginning of the Middle Ordovician. Both species were confined to cold-water, shallow basins with low water mobility. Analysis of the axial symmetry of stylophorans and the location of the hydropore indicates the absence of torsion in their ontogeny and the location of the ambulacral system to the right of their anteroposterior axis. To explain this morphology, two alternative hypotheses are proposed: (1) inversion in the development of right and left coeloms; (2) inverted state of stylophorans compared to other invertebrates, in which respect they resemble chordates in terms of the position of the dorsal and ventral sides. [ABSTRACT FROM AUTHOR]

Strophomenoid brachiopods had thin, concavo‐convex shells, were ubiquitous colonizers of Palaeozoic muddy seafloors, and are hypothesized to have filter‐fed in a concave‐upward orientation. This orientation would elevate their line of commissure out of potentially lethal lophophore‐clogging mud. The paradox is that epibiont distributions on strophomenoids support a convex‐upward life position, as do studies of strophomenoid stability and trace fossils formed by strophomenoid sediment‐clearing. A premise of the concave‐upward orientation hypothesis is a narrow gape, which causes narrow, high velocity inhalant currents, leaving strophomenoids vulnerable to sediment entrainment. Herein we investigate the gape angle of Rafinesquina using serial thin sections and peels, silicified specimens, computer modelling, SEM analysis, x‐ray microCT, and 3D printing. Hinge line structure suggests that, conservatively, Rafinesquina could gape 40–45°. Such a gape occurred when diductor muscle contraction could not cause any further rotation, hinge teeth and crenulations were disengaged, and interareas interlocked. In contrast, when closed, hinge teeth were locked in hinge sockets. This wide gape eliminates constraints on feeding orientation. In either convex‐up or concave‐up orientation, Rafinesquina could feed with slow, diffuse inhalant currents incapable of disturbing sediment, and could snap valves shut to forcefully expel enough water to clear sediment from the mantle cavity, explaining previously described moat‐shaped trace fossils associated with shells. Our findings demonstrate that Rafinesquina gaped at an angle approximately equal to the angle between the two interareas when the valves are closed. Our analyses hint that other strophomenoids with similar interarea angles also lived with their shells widely agape. [ABSTRACT FROM AUTHOR]

The abundant shells and hardgrounds in the Cincinnatian Group (Upper Ordovician, Katian) of the upper midwestern United States were commonly encrusted and bored by a variety of organisms. Numerous studies of these sclerobiont communities have provided valuable data for models of ecological succession, symbiosis, space and food resource competition, and taphonomy. An underlying assumption of this work is that most of the skeletal encrusters have remained in place, firmly attached to their hard substrates. This is especially the case with the most common encrusters, thick trepostome bryozoan and cystoporate skeletons, on their most common substrates, flat strophomenide brachiopods. We present evidence here, though, that these bryozoans were often dislodged from their brachiopod hosts, leaving no evidence of their attachment other than horizontal borings in semi-relief from organisms that excavated tunnels (Trypanites and Palaeosabella) at the interface of the brachiopod shell and attaching bryozoan. Similar borings are found on the bases of dislodged bryozoans and in bryoimmured mollusc external moulds. These borings along the bryozoan attachment surfaces caution us that there are significant numbers of missing skeletal encrusters on these hard substrates. [ABSTRACT FROM AUTHOR]

The distribution characteristics and main controlling factors of hydrocarbon phases in deep strata from petroliferous basins are important for the evaluation of oil–gas resources and decision-making regarding exploration. The distribution characteristics and controlling factors of the Ordovician hydrocarbon phases are systematically analyzed in the Tazhong I block, Tarim Basin, NW China. The results show that the Ordovician reservoirs in the Tazhong I block are characterized as multi-phase reservoirs with a lateral co-existence of condensates, normal oil reservoirs, and heavy oil reservoirs. From east to west, gas-rich in the fault belt and oil-rich in the platform area are presented. Meanwhile, there are regular variations in the geochemical characteristics of the Ordovician hydrocarbon, showing decreasing trends in the gas/oil ratio (GOR), wax contents, dryness coefficients, methane contents, and methane carbonate isotope values (δ13C1) and an increasing trend in oil densities. Because the same Cambrian–Lower Ordovician source for the Ordovician hydrocarbon is observed in the Tazhong I block, the regular variations in the hydrocarbon phases and geochemical characteristics can be interpreted as records of gas invasion, biodegradation, multi-stage filling, thermal cracking, and thermochemical sulfate reduction (TSR) rather than controlled by the source rock organofacies. This indicated that different kinds of secondary processes for a diversity of the hydrocarbon phase can appear in one region. Our re-construction of the Ordovician hydrocarbon accumulation model in the Tazhong I block encourages future exploration to target gas reservoirs in the fault belt and oil reservoirs in the platform area. [ABSTRACT FROM AUTHOR]

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Natural Gas Industry is the property of Natural Gas Industry Journal Agency and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Paired petrography and acid maceration has shown that preferential silicification of shelly faunas can bias recovery based on taxon and body size. Here, silicified fossils from the Upper Ordovician Edinburg Formation, Strasburg Junction, Virginia, USA, were analyzed using X-ray tomographic microscopy (μCT) in conjunction with recovered residues from acid maceration of the same materials to further examine sources of potential bias. Results reveal that very small (<~1 mm) fossils are poorly resolved in μCT when scanning at lower resolutions (~30 µm), underestimating abundance of taxa including ostracods and bryozoans. Acid maceration, meanwhile, fails to recover poorly silicified fossils prone to disarticulation and/or fragmentation during digestion. Tests for patterns of breakage, however, indicate no significant size or taxonomic bias during extraction. Comparisons of individual fossils from 3-D fossil renders and maceration residues reveal patterns of fragmentation that are taxon-specific and allow the differentiation of biostratinomic and preparational breakage. Multivariate ordinations and cluster analyses of μCT and residue data in general produce concordant results but indicate that the variation in taxonomic composition of our samples is compromised by the resolvability of small size classes in μCT imaging, limiting the utility of this method for addressing paleoecological questions in these specific samples. We suggest that comparability of results will depend strongly on the sample size, taphonomic history, textural, and compositional characteristics of the samples in question, as well as μCT scan parameters. Additionally, applying these methods to different deposits will test the general applicability of the conclusions drawn on the relative strengths and weaknesses of the methods. [ABSTRACT FROM AUTHOR]

The La Cantera Formation is a siliciclastic unit with restricted distribution that crops out at the eastern edge of the Villicum range (Eastern Precordillera), province of San Juan, Argentina. Its age has previously been recorded based on index graptolites. Several conglomerate, sandstone, and mudstone samples were collected from the lower and middle members of the La Cantera Formation, which provided conodonts and other microfossils such as ostracoderm plates, gastropods, bryozoans, and fragmented brachiopod shells. The retrieved conodont fauna is poorly diverse and just appears as casts in the mudstone bedding planes. It is composed mostly of elements of the genus Eoplacognathus that were recovered from sandstone units and elements of the genus Pygodus that appear as casts in the mudstone units. In addition, elements of the genera Baltoniodus, Pygodus, Periodon, and Erismodus among others, were retrieved from sandstone units. The conodont associations allow proposing, for the first time, a Middle-Upper Ordovician biostratigraphy for the Eastern Precordillera which includes three conodont zones and subzones, in ascending order: Lenodus suecicus Zone (Pygodus anitae Subzone), Pygodus serra Zone (Eoplacognathus robustus Subzone), and Pygodus anserinus Zone (Upper Subzone). This new biostratigraphic information provides an accurate correlation of this classic unit from the Eastern Precordillera with other units from the Central Precordillera. Also, the recognition of several hiatuses in different time intervals reveals the instability of the basin during the late Darriwilian–early Sandbian.

Sweet spot evaluation is of great significance for efficient exploration and development of shale gas reservoirs. Taking the shale reservoir in Ordovician Wulalike Formation on the western margin of the Ordos Basin as the research object, eight experiments including rock section analysis, X-ray diffraction, SEM scanning electron microscopy, low-temperature nitrogen adsorption, isothermal adsorption, total organic carbon (TOC) content test, organic matter vitrinite reflectance (Ro) test and triaxial rock mechanics test were carried out in this study. It is found that the rock type in the target area is gray brown mud shale, with the pore size in the ranges of 2-4 nm and 35-61 nm, and intergranular pores, clay mineral interlayer pores and intragranular pores are developed. The TOC content is 1.01%, the average Ro value is 1.75%, and the brittleness index is 47.8%. By analyzing the influence of different factors on the selection and evaluation of shale reservoir sweet spots, it is concluded that the content of siliceous minerals, clay minerals, pore specific surface area, TOC content and Ro value play a decisive role on the adsorption perfor-mance of the reservoir, the pore size and the number of pore types control the reservoir property, the content of brittle minerals and rock mechanics parameters affect the compressibility of the reservoir. According to the two indexes of adsorption property and reservoir property evaluated by geological sweet spots and the compressibility index of engineering sweet spots, the parameter indexes corresponding to different characteristics are finely classified, and a classification and evaluation scheme of geological sweet spots and compressibility sweet spots of the marine shale gas reservoir in Ordovician Wulalike Formation in the Ordos Basin is preliminarily established. The results show that all characteristic para-meters in the target area meet the grade Ⅱ standard. It could be a sweet spot for shale gas development.