Ordovician Research Articles

Discovery of organic-rich black shale xenolith from kimberlite on the Hall Peninsula, Nunavut and its implication for petroleum potential in Cumberland Sound

The Chidliak kimberlite province covers an area of 40 × 70 km on the eastern Hall Peninsula, southeastern Baffin Island, Nunavut, between Frobisher Bay and the Cumberland Sound (Fig. 1). The eastern Hall Peninsula is an Archean gneissic terrain, into which all the Late Jurassic–Early Cretaceous aged kimberlites (156.7–139.1 Ma; Heaman et al., 2012) intruded. At present, there is no consolidated Phanerozoic sedimentary cover on the Hall Peninsula; however, a great number of sedimentary rock xenoliths yielding conodont microfossils were entrained by the kimberlites. These xenoliths suggest that Upper Ordovician and Lower Silurian strata with a total thickness about 270–305 m were present on the Hall Peninsula at least until Early Cretaceous time (Zhang and Pell, 2013; 2014). Figure 1 A). Simplified geologic map of the Foxe Basin and vicinity (modified from Wheeler et al., 1997), showing the location of the Chidliak project area (red polygon), the drill hole CHI-482-10-DD01 with the black shale xenolith (red star), suspect natural hydrocarbon seep (yellow dots; location data from Budkewitsch et al., 2013) and Paleozoic (black cross) and Mesozoic (red cross) cores collected by shallow drillings during 1970s and 1980s (location data from Zhang, 2013a). B). Enlargement of the area enclosed by black rectangle in (A). Cumberland Sound, to the northeast, is partially underlain by Cretaceous clastic and Paleozoic carbonate sedimentary rocks (Fig. 1; MacLean et al., 1986). On the sea surface of Cumberland Sound, 28 suspect natural hydrocarbon seep occurrences were identified (Fig. 1; Budkewitsch et al., 2013), indicating an active petroleum system may occur in the sound; however, the source rocks remain uncertain. A rare black shale xenolith (Fig. 2A) was discovered from a kimberlite on the Hall Peninsula. Given the geologic and geographic position of the peninsula, it provides valuable evidence about the possible existence of an active …

Records of the Moon‐forming impact and the 470 Ma disruption of the L chondrite parent body in the asteroid belt from U‐Pb apatite ages of Novato (L6)

AbstractNovato, a newly observed fall in the San Francisco Bay area, is a shocked and brecciated L6 ordinary chondrite containing dark and light lithologies. We have investigated the U‐Pb isotope systematics of coarse Cl‐apatite grains of metamorphic origin in Novato with a large geometry ion microprobe. The U‐Pb systematics of Novato apatite reveals an upper intercept age of 4472 ± 31 Ma and lower intercept age of 473 ± 38 Ma. The upper intercept age is within error identical to the U‐Pb apatite age of 4452 ± 21 Ma measured in the Chelyabinsk LL5 chondrite. This age is interpreted to reflect a massive collisional resetting event due to a large impact associated with the peak arrival time at the primordial asteroid belt of ejecta debris from the Moon‐forming giant impact on Earth. The lower intercept age is consistent with the most precisely dated Ar‐Ar ages of 470 ± 6 Ma of shocked L chondrites, and the fossil meteorites and extraterrestrial chromite relicts found in Ordovician limestones with an age of 467.3 ± 1.6 Ma in Sweden and China. The lower intercept age reflects a major disturbance related to the catastrophic disruption of the L chondrite parent body most likely associated with the Gefion asteroid family, which produced an initially intense meteorite bombardment of the Earth in Ordovician period and reset and degassed at least approximately 35% of the L chondrite falls today. We predict that the 470 Ma impact event is likely to be found on the Moon and Mars, if not Mercury.

When and how do species achieve niche stability over long time scales?

Niche conservation is not ubiquitous among all species or all environmental changes. Identifying the underlying basis for variable species responses is important for conservation efforts and macroevolutionary theory and requires comparing multiple clades and types of environmental change. To examine this issue, niche stability was analyzed for a phylogenetically diverse suite of invertebrates that inhabited the epicontinental seas of eastern North America during the Late Ordovician Period (∼450 Ma). These taxa experienced fluctuating sea level and a regional biotic invasion event, the Richmondian invasion, during nine time slices spanning approximately three million years. Niche models were generated with Maxent and niche stability was assessed via geographic overlap, niche similarity, and environmental parameters.These taxa exhibited high fidelity habitat tracking and stable ecological niches during the pre‐invasion interval characterized by gradual sea‐level changes, whereas taxa exhibited statistically lower niche stability and more niche evolution during and after the Richmondian invasion. Conversely, environmental parameter similarity between niche models of adjacent time slices was lowest during the pre‐invasion interval and increased following the Richmondian invasion. Declining niche similarity and geographic stability coincident with increasing environmental parameter similarity indicates ecological niche change resulted from contracting species' realized niches to a subset of the original fundamental niche dimensions rather than shifting to new ecospace. Niche contraction is interpreted as a response to increased competition from invasive species, which may have arisen from adaptation, sorting (differential birth and death processes) and/or ecological fitting. These results indicate that consideration of multiple hierarchical levels, temporal scales and ecological contexts will yield new insights into the processes that generate both macroevolutionary and ecological patterns. They also suggest the widely employed assumption of niche conservatism through time and space may be more frequently valid for intervals of gradual abiotic change than for biotic changes caused by invasive species.

The Ordovician Mukteian Horizon of the Siberian Platform: paleontological substantiation, distribution, and correlation of sections

The new-generation regional stratigraphic chart of the Siberian Platform was accepted by the Interdepartmental Stratigraphic Committee of Russia in April 2013. Two main supplements to the chart of 1979 were made: (1) the new scale of stages, accepted by the International Commission on Stratigraphy in 2008, was presented for the Ordovician System along with the previous British standard and (2) the nomenclature of the regional stratigraphic units was supplemented by the Mukteian Horizon, which occupies an intermediate position between the Vikhorevian and Volginian Horizons. The first substantiation of the necessity of viewing this stratigraphic interval as an independent horizon was made in 1980, after it had become possible to use well cores for a more precise definition of the paleontologic characteristics and boundaries of the Vikhorevian Horizon, originally detected from several isolated sections in the Irkut amphitheater with an indistinct boundary with the overlying Volginian Horizon. Recent data, particularly those from drilling, permit a more complete paleontological substantiation of the Mukteian Horizon, consideration of its distribution in different structure-facies zones, and correlation of type sections.

Environmental conditions of the mass extinction of marine biota at the end of the Ordovician

During the Phanerozoic (since 542 Ma) overall progressive development of biota occurred, which was disturbed by more or less significant episodes of biodi� versity decrease. Especially catastrophic extinctions on the Earth when more than 75% of the species dis� appeared within geologically short intervals are referred to the great mass extinctions. During the Phanerozoic five great extinctions are recognized: —the Ordovician, at the boundary of the Ordovi� cian and Silurian; —the Devonian at the boundary of the Devonian

Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian orogenic belt

Recent results show that evolution of the huge Central Asian orogenic belt can be explained in terms of southwest Pacific–style accretion of arcs and microcontinents. A better understanding of the origin and evolution of arcs and microcontinents will enhance our knowledge on the evolution of the Central Asian orogenic belt. As one of the most important early Paleozoic arc systems south to the Solonker suture zone, the origin and evolution of the Bainaimiao arc belt are still not well constrained. New zircon U-Pb geochronological and geochemical results on magmatic rocks in the Bainaimiao arc belt indicate that the arc was active from 0.52 Ga to 0.42 Ga and can extend to east Siping in NE China. Zircon U-Pb geochronological results of metasedimentary rocks in the Bainaimiao arc belt indicate that they are early Paleozoic in age, not Precambrian as previously regarded. Detrital zircon analysis of metasedimentary rocks and Sr-Nd-Hf geochemical results of magmatic rocks indicate the existence of some Proterozoic basement rocks beneath the Bainaimiao arc, and it was built upon a Precambrian microcontinent that has a tectonic affinity to the Tarim or Yangtze cratons. The Bainaimiao arc is an ensialic island arc characterized by different evolution history and basement compositions from the northern North China craton. It was separated by a wide ocean from the northern North China craton during the Cambrian–Ordovician period. Successive northward subduction resulted in contraction of the ocean and final accretion of the Bainaimiao arc to the northern North China craton during the Late Silurian–earliest Devonian by arc-continent collision. Arc-continent collision could be an important mechanism for continental crustal growth and formation of the huge Central Asian orogenic belt.

Exceptionally Preserved 450-Million-Year-Old Ordovician Ostracods with Brood Care

Ostracod crustaceans are the most abundant fossil arthropods and are characterized by a long stratigraphic range. However, their soft parts are very rarely preserved, and the presence of ostracods in rocks older than the Silurian period [1-5] was hitherto based on the occurrence of their supposed shells. Pyritized ostracods that preserve limbs and in situ embryos, including an egg within an ovary and possible hatched individuals, are here described from rocks of the Upper Ordovician Katian Stage Lorraine Group of New York State, including examples from the famous Beecher's Trilobite Bed [6, 7]. This discovery extends our knowledge of the paleobiology of ostracods by some 25 million years and provides the first unequivocal demonstration of ostracods in the Ordovician period, including the oldest known myodocope, Luprisca incuba gen. et sp. nov. It also provides conclusive evidence of a developmental brood-care strategy conserved within Ostracoda for at least 450 million years.

A suspension-feeding anomalocarid from the Early Cambrian

Large, actively swimming suspension feeders evolved several times in Earth's history, arising independently from groups as diverse as sharks, rays and stem teleost fishes, and in mysticete whales. However, animals occupying this niche have not been identified from the early Palaeozoic era. Anomalocarids, a group of stem arthropods that were the largest nektonic animals of the Cambrian and Ordovician periods, are generally thought to have been apex predators. Here we describe new material from Tamisiocaris borealis, an anomalocarid from the Early Cambrian (Series 2) Sirius Passet Fauna of North Greenland, and propose that its frontal appendage is specialized for suspension feeding. The appendage bears long, slender and equally spaced ventral spines furnished with dense rows of long and fine auxiliary spines. This suggests that T. borealis was a microphagous suspension feeder, using its appendages for sweep-net capture of food items down to 0.5 mm, within the size range of mesozooplankton such as copepods. Our observations demonstrate that large, nektonic suspension feeders first evolved during the Cambrian explosion, as part of an adaptive radiation of anomalocarids. The presence of nektonic suspension feeders in the Early Cambrian, together with evidence for a diverse pelagic community containing phytoplankton and mesozooplankton, indicate the existence of a complex pelagic ecosystem supported by high primary productivity and nutrient flux. Cambrian pelagic ecosystems seem to have been more modern than previously believed.

The conodont Iapetognathus and its value for defining the base of the Ordovician System

Nicoll et al. (1999, Brigham Young University Geology Studies 44, 27–101) published the taxonomy of species of the ramiform conodont Iapetognathus Landing in Fortey et al. (1982, The Cambrian–Ordovician boundary: sections, fossil distributions, and correlations, National Museum of Wales, Geological Series No. 3, Cardiff, 95–129) and its ancestor Iapetonudus Nicoll et al., 1999. Cooper et al. (2001, Episodes 24, 19–28) used the First Appearance Datum of Iapetognathus fluctivagus Nicoll et al., 1999 to mark the base of the Ordovician System at Green Point, Newfoundland. Terfelt et al. (2012, Lethaia 45, 227–237) re-evaluated Iapetognathus at Green Point and made several taxonomic and stratigraphic conclusions, nearly all of which we refute herein.

Investigation on the great Ordovician biodiversification event (GOBE): Review and prospect

The great ordovician biodiversification event (GOBE) is one of the major biotic revolutions in Early Paleozoic marine ecosystems. Through three internationally coordinated IGCP projects (IGCP 410, 503 and 591) in the past two decades, an enormous amount of paleontological, stratigraphical, paleoecological, and paleobiogeographical data have been accumulated, leading to a number of significant discoveries and common understandings regarding the GOBE. First, the multiple radiation pulses spanned nearly the entire Ordovician Period (more than 40 Ma), with several diversity acmes. Second, the biotic radiations were highly diachronous, characterized by different timing and rates in different regions. Third, the radiations were not associated with fundamental changes in the marine environments, but were rather controlled by both intrinsic factors (macroevolutionary regularities) and extrinsic forces (gradual environmental changes). Case studies conducted in South China have contributed greatly to the understanding of GOBE at the global scale. Future work on the GOBE includes more in-depth case studies to reveal the exact process and pattern of the radiations by combining systematic paleontology, high-resolution biostratigraphy, sedimentology, plate tectonics and geochemistry. This will help improve our understanding of the dynamics of the radiation and co-evolution between the organic and inorganic realms of Ordovician marine ecosystems.

The Maximum Burial Depth in Hydrocarbon Source Rocks on Kongquehe Slope in the Tarim Basin

The strata of the Kongquehe slope in the Tarim basin have suffered from long-term denudation, causing the determination of maximum ancient buried depth in mainly hydrocarbon source rocks to be very difficult. The denudation restricts the conduct of research into the hydrocarbon generation history of the source rocks. Through the use of drilling, seismic, logging, and geochemical data, calculation of the amount of denudation of the main unconformable surface, consideration of the present residual thicknesses of major hydrocarbon source rocks and other data, the authors determine maximum ancient buried depth in four major hydrocarbon source rocks and determines the time required to reach the maximum ancient buried depth. The authors have ascertained four main results through comprehensive analysis. (a) A majority of the Jurassic hydrocarbon source rock passed the hydrocarbon-generating threshold during the Neogene period and are now in the early-mature to mature stage. (b) Carboniferous hydrocarbon source rock is only distributed in the Caohu depression, having crossed the hydrocarbon-generating threshold in the Cretaceous period, and is now in the mature to late-mature stage. (c) Most of the middle-upper Ordovician hydrocarbon source rock crossed the hydrocarbon-generating threshold in the late Ordovician period and is now in the late-mature to over-mature stage. There are two blocks in the east research area that appear to contain the maximum ancient buried depth which may have experienced secondary hydrocarbon generation in the late Silurian. (d) Cambrian-Lower Ordovician hydrocarbon source rock is in the late-mature to over-mature stage and may have experienced secondary hydrocarbon generation in the Cenozoic period in most of Kongquehe area.

Geochemistry, provenance and tectonic setting of the Late Cambrian–Early Ordovician Seydişehir Formation in the Çaltepe and Fele areas, SE Turkey

The major, trace and rare earth element (REE) contents of metapelite (MPL), metapsammite (MPS) and metamarl (MM) samples from the Cambro-Ordovician Seydişehir Formation were analyzed to investigate their provenance and tectonic setting. The MPS, MPL, and MM samples have variable SiO2 concentrations, with average values of 72.36, 55.54, and 20.95wt%, moderate SiO2/Al2O3 ratios (means of 6.88, 3.23, and 3.80), moderate to high Fe2O3+MgO contents (means of 5.14, 9.55, 3.56wt%), and high K2O/Na2O ratios (means of 3.26, 3.64, 2.90), respectively. On average, the chemical index of alteration (CIA) values of the MPS and the MPL are 65.87 and 71.96, respectively, while the chemical index of weathering (CIW) values are 74.54 and 85.09, respectively. These data record an intermediate to high degree of alteration (weathering) of plagioclase to illite/kaolinite in the samples’ provenance. The chondrite-normalized REE patterns of all the sample groups are similar and are characterized by subparallel light rare earth elements (LREE)-enriched, relatively flat heavy rare earth elements (HREE) patterns with pronounced Eu anomalies (mean of 0.69) and moderate fractionation [average (La/Yb)N=8.7]. Plots of sediments in ternary diagrams of La, Th, Sc and elemental ratios (La/Sc, Th/Sc, Cr/Th, Eu/Eu*, La/Lu, Co/Th, La/Sc and Sc/Th), which are critical for determining provenance, and REE patterns indicate that the metaclastic units of the Seydişehir Formation were derived dominantly from felsic to intermediate magmatic rocks and not from a mafic source. The La–Sc–Th and Th–Sc–Zr/10 ternary diagrams of the Seydişehir Formation are typical of continental island arc/active continental margin tectonic settings. The geologic location and geochemistry of the Seydişehir Formation suggest that it was deposited in an Andean-type retroarc foreland basin during the Late Cambrian–Early Ordovician period. The Neoproterozoic intermediate to felsic magmatic rocks and metaclastic sediments with felsic origins of the Sandıklı–Afyon Basement Complex (SBC) and their equivalent units, which are thought to be overlain by the younger units in the study area, may be the dominant source rocks for the Seydişehir Formation.

A CONOP9 composite taxon range chart for Ordovician conodonts from Baltoscandia: a framework for biostratigraphic correlation and maximum-likelihood biodiversity analyses

The Middle and Upper Ordovician rocks of Baltoscandia have been divided into spatially distinct, composite litho- and biofacies units called confacies belts. The precise regional correlation of outcrops and boreholes, which is necessary for biodiversity analyses, has always been problematic due to the pronounced biogeographical differentiation of macrofossils and horizontal lithofacies changes. We used a computer-assisted numerical sequencing program (CONOP9) to construct a correlation model and composite range chart from the stratigraphic range data of 159 conodont species in 24 boreholes and outcrops in Baltoscandia. We converted the composite section into a timescale in which to calculate the biodiversity, extinction, origination and fossil sampling probabilities through the Ordovician Period. Rates of extinction and origination were calculated using both simple approaches which do not incorporate estimates of sampling probability, and also more complex maximum-likelihood approaches based on Capture–Mark–Recapture (CMR) models. Our data show that overall biodiversity increases steadily from the base of the Paltodusdeltifer Zone to the uppermost Baltoniodusnorrlandicus Zone and then maintains an uneven diversity plateau until the earliest Sandbian. Diversity then declines dramatically throughout the remainder of the Ordovician. CMR analyses suggest that extinction rates remain constant throughout most of the Ordovician indicating that the dramatic late Middle and Late Ordovician decline in conodont diversity in Baltoscandia is attributable to depressed origination.

Optimizing Fracture Stimulations in Low-Permeability Oil Reservoirs in the Ordos Basin

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 158562, ’Optimizing Fracture Stimulations in Low-Permeability Oil Reservoirs in the Ordos Basin,’ by Xinghui Liu, SPE, Pinnacle—A Halliburton Service; Yanrong Chang, Hongjun Lu, Baochun Chen, Jianshan Li, Yin Qi, and Chengwang Wang, PetroChina Changqing Oilfield Company, prepared for the 2012 SPE Asia Pacific Oil and Gas Conference and Exhibition, Perth, Australia, 22-24 October. The paper has not been peer reviewed. Huge low-permeability oil reserves are found in the Ordos basin in China. Key characteristics are low reservoir quality (10 to 13% porosity), low permeability (0.3 to 2.0 md), and low reservoir pressures (pressure gradients of 0.0075 to 0.009 MPa/m). Fracture stimulation is essential for commercial production in these reservoirs. To understand fracture-growth behavior better and to optimize treatment designs, microseismic monitoring and fracture modeling were performed for pilot wells in key areas. In some reservoirs in the basin, fracture-closure stress sometimes could not be obtained from commonly used pump-in/shut-in tests because pressure decline is extremely slow. To obtain closure stress in these reservoirs, conventional pump-in/flowback (PI/FB) tests were modified and used. Fracture-modeling analysis then was conducted with a calibrated fracture model, which must match both the observed net fracture pressures and the fracture dimensions from microseismic mapping. This approach led to significant improvements in fracture-treatment designs in these tight oil reservoirs. Introduction The Ordos basin is a lacustrine/fluvial and sedimentary basin in northern China and covers approximately 370 000 km2 across the Shaanxi, Gansu, and Shanxi provinces, and the Ningxia and Inner Mongolia autonomous regions, as shown in Fig. 1. Oil reservoirs are found at depths ranging from 800 to 2800 m in the south, and natural-gas reservoirs are found at depths ranging from 2700 to 3600 m in the north. Oil reservoirs are mainly Triassic and Jurassic systems, while natural-gas reservoirs are mainly Permian and Ordovician systems. The Ordos basin also has abundant coal and coalbed methane. Fracture stimulation of the low-permeability oil reservoirs in the Changqing oil field of the Ordos basin was studied. Changqing oil field is a general term that represents many individual oil fields across a large area. Oil production in the Changqing oil field started in 1971. During early development, oil was produced only from reservoirs with relatively good properties (permeability of 10 to 100 md) in the Jurassic formations, and small-scale fracturing treatments were necessary to remove near-wellbore formation damage. In recent years, these Jurassic reservoirs with relatively good properties have been depleted. Currently, the Triassic reservoirs with poorer reservoir properties account for more than 85% of the proved oil reserves in the Changqing oil field. The Triassic reservoirs are characterized by low reservoir quality (10 to 13% porosity), low reservoir permeability (0.3 to 2.0 md), and low reservoir pressures (pressure gradients of 0.0075 to 0.009 MPa/m). Development of the low- permeability reservoirs in the Changqing oil field began in the 1990s, while the large-scale development of ultralow-permeability reservoirs began in 2008. Production from these reservoirs requires the use of fracture stimulation.

Proposed evolutionary changes in the role of myelin.

Myelin is the multi-layered lipid sheet periodically wrapped around neuronal axons. It is most frequently found in vertebrates. Myelin allows for saltatory action potential (AP) conduction along axons. During this form of conduction, the AP travels passively along the myelin-covered part of the axon, and is recharged at the intermittent nodes of Ranvier. Thus, myelin can reduce the energy load needed and/or increase the speed of AP conduction. Myelin first evolved during the Ordovician period. We hypothesize that myelin's first role was mainly energy conservation. During the later “Mesozoic marine revolution,” marine ecosystems changed toward an increase in marine predation pressure. We hypothesize that the main purpose of myelin changed from energy conservation to conduction speed increase during this Mesozoic marine revolution. To test this hypothesis, we optimized models of myelinated axons for a combination of AP conduction velocity and energy efficiency. We demonstrate that there is a trade-off between these objectives. We then compared the simulation results to empirical data and conclude that while the data are consistent with the theory, additional measurements are necessary for a complete evaluation of the proposed hypothesis.

Evidence for an impact origin of the Late Ordovician Bear Swamp structure in upstate New York, USA

A 3D seismic survey has imaged a complex crater of possible impact origin in the Finger Lakes region of upstate New York near Bear Swamp State Forest. The Bear Swamp crater is uppermost Ordovician (∼444 Ma) in age and is completely buried in the subsurface at a depth of approximately 1220 m (4000 ft). The nearly circular crater is about 3.5 km (2.2 mi) in diameter and contains a central rebound structure with a diameter of about 1 km (0.6 mi). Two exploration wells were drilled into the crater and core and image-log data were obtained. The first well tested the central rebound which consisted of steeply dipping beds and heavily brecciated zones as seen on the image logs. The second well tested the flank of the central uplift in the roughly 300 m (1000 ft) thick annular basin. Thin sections taken from whole core recovered in the second well revealed planar deformation features (PDF) in quartz grains within the reworked crater fill sediments. Minor gas shows were encountered at the base of the crater fill section in the second well, and both wells encountered significant gas shows in the fractured target rock beneath the crater. Both macroscopic and microscopic evidence from the seismic and well data suggest this could have been the site of a meteor impact in a shallow marine to transition zone environment at the end of the Ordovician Period.

Dehydration stress‐induced oscillations in LEA protein transcripts involves abscisic acid in the moss, Physcomitrella patens

• Physcomitrella patens is a bryophyte belonging to early diverging lineages of land plants following colonization of land in the Ordovician period. Mosses are typically found in refugial habitats and can experience rapidly fluctuating environmental conditions. The acquisition of dehydration tolerance by bryophytes is of fundamental importance as they lack water-conducting tissues and are generally one cell layer thick. • Here, we show that dehydration induced oscillations in the steady-state transcript abundances of two group 3 late embryogenesis abundant (LEA) protein genes in P.patens protonemata, and that the amplitudes of these oscillations are reflective of the severity of dehydration stress. • Dehydration stress also induced elevations in the concentrations of abscisic acid (ABA), and ABA alone can also induce dosage-dependent oscillatory increases in the steady-state abundance of LEA protein transcripts. Additionally, removal of ABA resulted in rapid attenuation of these oscillatory increases. • Our data demonstrate that dehydration stress-regulated expression of LEA protein genes is temporally dynamic and highlight the importance of oscillations as a robust mechanism for optimal responses. Our results suggest that dehydration stress-induced oscillations in the steady-state abundance of LEA protein transcripts may constitute an important cellular strategy for adaptation to life in a constantly changing environment.

Analyzing Seismic Destructive Effect to the Submarine Rock and Soil Layers According to Submarine Seismic Records

By means of survey and study, various submarine seismic records were identified in the marine facies strata from Sinian Period to Ordovician Period in Tancheng-Lujiang seismic fault zone and its vicinity. These submarine seismic records are divided into two classes. According to the mode of seismic action, every class of submarine seismic records is divided into three to four types. On the basis of classification, this paper studied and summarized seismic destructive effects to the submarine rock and soil layers and diagrammatized mechanism and process of seismic destructive effects. Seismic destructive effects to soft sedimentary soil strata mainly include seismic vibratory liquefaction, vibratory thixotropic effect, seismic fold deformation, sliding deformation, tensile deformation and combined action of shaking and weight. Seismic destructive effects to submarine soil strata of semi-consolidated to un-completely consolidated sediments mainly include syn-sedimentary fault effect, crack and smashing effect, filling effect of seismic crack, etc.

Graptolite biostratigraphy of the E1-NC174 core, Rhuddanian (lower Llandovery, Silurian), Murzuq Basin (Libya)

The graptolite biostratigraphy of the Rhuddanian (lower Llandovery, Silurian) of the E1-NC174 core, Murzuq Basin, Libya is described. The generally low diversity graptolite assemblages include both North African endemic taxa and cosmopolitan species. The lower part of the cored interval, below the ‘hot’ shale, is dominated by Normalograptus tilokensis, a species previously considered to be confined to the Upper Ordovician, but which almost certainly ranges into (or perhaps occurs only in) the lower Silurian. As in the BG-14 core, Jordan, the base of the ‘hot’ shale lies just above the first appearances of Neodiplograptus africanus and Normalograptus rectangularis. The latter species dominates throughout the mid Rhuddanian ‘hot’ shale interval, with the highest strata in the core, of late Rhuddanian age, yielding abundant Neodiplograptus fezzanensis.

Quantitative assessment and significance of gas washing of oil in Block 9 of the Tahe Oilfield, Tarim Basin, NW China

Some Ordovician and Triassic oils in Block 9 are characterized by light oils, which have distinctly differentiated from heavy oils in other blocks in the Tahe Oilfield, Tarim Basin. Based on the whole oil gas chromatograms, this paper estimates the effect of oil migration and fractionation and the amount of depletion (Q) in terms of the n-alkanes depletion model. The results showed that the amount of depletion in the Ordovician reservoir is highest in the east of this block, e.g. the depletion is 97% in Well T904. The amount of Q gets lower to the west, e.g. the depletion is 53.4% in Well T115 and there is no sign of depletion in Well S69. It is suggested that the direction of gas washing is from the east to the west. The compositions and isotopic characteristics of associated gas in Ordovician oils indicated that the gas might be derived from Cambrian source rocks of the Caohu Depression which lies to the east of Block 9. In contrast, no obvious depletion of n-alkanes in Triassic oils was found, suggesting that the migration pathway of natural gas has been limited to the Ordovician karst fracture system formed in the Early Hercynian Orogeny. Different depletions of the Ordovician and Triassic oils can reveal fault activities in this region.