Jurassic Series Research Articles

AUTOMATIC ETCHECOPAR MICROTECTONIC ANALYSIS OF THE DAJY SECTOR (AGADEZ REGION, NORTHERN NIGER)

This study, based on microtectonic analysis using Echecopar's automatic method combined with field work in the Dajy sector, highlighted that the direction of minimum stress Ϭ3 is not homogeneous in this study area. It was thus highlighted : 1. An extensive tensors linked to a direction of Ϭ3 of approximately N90° ; 2. An extensive tensors linked to a subhorizontal Ϭ3 direction N120° E to N160° E, 3. An extensive tensors with Ϭ3 directions parallel or subparallel to the N70° E faults. This category of tensors appears to be associated with strike slip fault (Fig. 6). 4. And an average N156° extension for the Jurassic series

COMPLEX BIOSTRATIGRAPHIC SCHEME OF THE JURASSIC DEPOSITS OF THE PIENNINE ZONE OF THE UKRAINIAN CARPATHIANS

The article summarizes the results of the study of paleontology and biostratigraphy of the Jurassic of the Piennine Zone of the Ukrainian Carpathians by domestic and foreign researchers. For the first time, a summary paleontological description of these sediments is presented, taking into account numerous published data on identified macro- and mictofossils. A correlation biozonal scale has been created, as well as complex biostratigraphic schemes for each Jurassic series of its region in accordance with the International Stratigraphic Scale (2020). Because the stratigraphic sequence was disturbed by disjunctive dislocations, these schemes were developed by analysis of fragmentary sections. At present, three series of the Jurassic are determined by ammonites, belemnites, pelecypods, brachiopods, the Lower and Upper series are also dated by dinocysts, the Upper also by foraminifera and tintinnids. The stages and standard zones of the modern international scale are determined by ammonites: Liasicus of the Gettangian; Bucklandi, Obtusum, Raricostatum of the Sinemurian; Jamesoni, Emaciatum of the Pliensbachian; Tenuicostatum, Serpentinum, Bifrons, Thouarcense of the Toarcien; Opalinum, Murchisonae, Bradfordensis, Concavum of the Aalenian; Propinquans, Humpriesianum, Parkinsoni of the Bajocian; Zigzag, Aurigerus, Subcontractus, Bremeri, Retrocostatum of the Bathonian; Athletа of the Callovian; Plicatilis і Bifurcates of the Oxfordian; Divisum, Acanticum і Cavouri of the Kimmeridgian. Continuous dinocyst zonation is traced in the Upper Jurassic – Fibrata Аcme (Upper Oxfordian), Parvula Acme, Moluccana і Borzai (Kimmerigian), Pulla, Tithonica, Malmica, Semiradiata (Lower Tithonian) and all standard tintinnide zones of Tithonian — Chitinoidella (підзони Dobeni і Boneti), Praetintinnopsella, Crassicollaria. The boundaries of the stages from the Gettangiean to the Oxfordian are not determined. The boundaries between the Lower and Upper Oxfordian as well as between the Kimmeridgian substages are determined by ammonites as well as the boundary between the Kimmeridgian and Tithonian is determined by dinocysts, and the boundary between the Jurassic and Cretaceous is determined by tintinnids (corresponds to the boundary between the zones Crassicollaria of Tithonian and Calpionella of Berriassian). Stratigraphic unconformities were revealed by the macro- and mictofossils: the regional erosion on the boundary between the Aalenian and Bajocian as well as in Early Callovian, and short gap on the boundary between the Lower and Upper Bajocian.

Hydrogen sulfide occurrence states in China's coal seams

The occurrence states of hydrogen sulfide in coal seams are crucial in preventing and controlling hydrogen sulfide emission in coal mines and the safe development of coal bed methane. In this study, the research status of the occurrence states of free-state, adsorbed-state, and water-soluble hydrogen sulfide in coal seams was systematically analyzed. H2S anomaly areas in China's coal seams are mainly located in the Carboniferous-Permian and Jurassic series of northern, eastern, central, and northwest regions of China. Bacterial sulfate reduction accounts for most of the hydrogen sulfide anomalies of low-rank coal, while thermochemical decomposition thermal desorption spectroscopy and thermochemical sulfate reduction may also result in hydrogen sulfide anomaly in medium- and high-rank coal. In contrast, magmatism-induced hydrogen sulfide anomalies are rarely found. Absorbed-state hydrogen sulfide anomalies are prevailing, while water-soluble and free-state hydrogen sulfide anomalies are relatively scarce. Coal seam's porosity mainly controls the hydrogen sulfide adsorption, pressure, coalification degree, pore volume, and specific area, while water-soluble hydrogen sulfide is influenced by pressure, sulfate-reducing bacteria, burn, porosity, fractures, water temperature, and hydrodynamic conditions. The fractures in coal seams, their burial depth, coal quality, coal rank, roof, and floor lithology are the main factors controlling the free-state hydrogen sulfide preservation. The absorbed-state hydrogen sulfide in coal seams is mainly mitigated by varying the ventilation mode, increasing the ventilation capacity, spraying alkali fog into the air, and injecting alkali liquid into coal seams for governance.

Reconstruction of Mesozoic paleostress associated with the tectono-sedimentary evolution of the basins bordering the Aouli inlier (Upper Moulouya, Morocco)

During the Mesozoic, the basins bordering the Aouli inlier (BBAI) experienced a geodynamic evolution marked by five successive stages: (i) during the Upper Triassic-Middle Liassic (Middle Carixian?), several sedimentary basins occurred with detrital, volcanic, and carbonate filling. Sedimentation and volcanism are controlled by NE-SW to E-W normal faults developed under a NW-SE to NNW-SSE extensional tectonic regime related to the Atlas rifting stage; (ii) during the Middle Liassic (Upper Carixian?-Domerian)-Toarcian, a pure extensive tectonic regime with NE-SW trending σ3 axis is responsible for the appearance NW-SE synsedimentary normal faults. Lateral variations of facies and thicknesses of layers and basin subsidence are controlled by these faults; (iii) during the Aalenian-Bajocian, the tectonic regime is extensive with NNW-SSE σ3 axis responsible for southward deepening of the sedimentary basins bounded by ENE-WSW normal faults. Sedimentary series are represented by bioclastic limestones and a thick succession of marls; (iv) during the Infracenomanian, the tectonic regime becomes transpressive with a sub-horizontal NNW-SSE trending σ1 axis. This is responsible for the development of ENE-WSW reverse faults, NE-SW and NW-SE strike-slip faults, the folding of Jurassic series, and the deposit of ante-Cenomanian continental succession unconformably overlying the underlying series; (v) during the Cenomano-Turonian, the setting up of carbonate platform marks a marine transgression on the entire Upper Moulouya domain. This is related to an extensive tectonic regime with sub-horizontal NW-SE σ3 axis, responsible for the development of NE-SW trending normal faults that control marine transgression and the subsidence of Cretaceous basins.

Hydrocarbon Potential of the Middle–Late Jurassic Series of Northwestern Iraq: A Case Study in the Shaikhan Oil Field

The Middle–Late Jurassic Sargelu, Naokelekan, and Barsarin formations of northwestern Iraq have been investigated in the Shaikhan oilfield (well Shaikhan-8) to assess their potential for hydrocarbon generation.The results of total organic carbon analysis and rock-eval pyrolysis revealed a good-to-excellent hydrocarbon content and suggest that the depositional conditions were suitable for the production and preservation of organic matter. The thermal maturity proxy indicates that the studied formations were at the start of the hydrocarbon generation period. Most of the samples from the Sargelu and Barsarin formations belong to kerogen type II, whereas those of the Naokelekan Formation belong to kerogen type II/III. The Pr/Ph, Pr/n-C17, and Ph/n-C18 ratios of the extracted bitumen indicated that the organic matter originated from marine sources under reducing conditions. The stable carbon isotope composition of the saturated and aromatic hydrocarbon fractions ranged from –28.3 to –27.7 ‰ and –28.0 to –27.7 ‰, respectively. The biomarker results show a high contribution of marine organic matter that was preserved under relatively anoxic conditions. The profiles of the burial and thermal maturity history show that the simulated generation zones, based on the calculated vitrinite reflectance, indicate immature (0.44%–0.6%)-to-early oil generating (0.6%–0.75%) source rock. The low thermal maturity of the studied formations relative to the depth may be attributed to the low geothermal gradient and heat flow.

Early Jurassic palaeopolar marine reptiles of Siberia

AbstractMarine reptile occurrences are rare in the Lower Jurassic Series outside of Europe. Here we describe diverse marine reptile faunas from the Lower Jurassic Series (Pliensbachian and Toarcian stages, including the Toarcian–Aalenian boundary interval) of Eastern Siberia. The taxonomic composition of Toarcian marine reptile assemblages of Siberia highlight their cosmopolitan nature, with the presence of taxa previously known nearly exclusively from coeval strata of Europe, such as ichthyosaurians Temnodontosaurus and Stenopterygius, microcleidid plesiosaurians (including the genus Microcleidus), rhomaleosaurids and basal pliosaurids. The palaeogeographic reconstruction places these faunas to the palaeopolar region, north of the 80th northern parallel and up to the palaeo north pole (upper value within the 95% confidence interval for some of the localities). The materials include remains of both mature and juvenile (or even infant, judging by their very small size and poor ossification) animals, indicating a possibility that these polar seas may serve as a breeding area. The diversity and abundance of plesiosaurians and ichthyosaurians, along with a lack of thalattosuchian remains (considering their wide distribution elsewhere at low latitudes), is an additional argument that plesiosaurians and neoichthyosaurians were able to live and reproduce in a polar environment. There is no certainty whether these animals lived in polar seas permanently, or whether they were taking seasonal migrations. However, given the polar night conditions at high latitudes, the latter seems more plausible, and both these scenarios are further indirect evidence that these groups likely had a high metabolism.

Hydrogeological systems of landslides in central part of the Volga-Ural region

The study of landslide massifs made it possible to identify two types of hydrogeological systems that differ by feeding area structure and transit zone. In landslide bodies of Urzhuminan stage, composed by dolomites and marls, hydrogeological systems are simple stratal. The aquifers are localized in fractured marls, which, under the influence of infiltration water seeping through them, have turned into a poorly structured gruss-rock cover, which lies on dense dolomites - water-resistant. The inclined bedding of dolomite layers creates the prerequisites for directed filtration of groundwater towards natural lowering. Here forming areas of ground water discharge as low-water springs. In landslides composed of the Upper Jurassic series clays and marls, hydrogeological systems are distinguished by the prevailing vertical groundwater infiltration in transit zone. Theirs’s water source is shallow lakes formed on the surfaces of landslide terraces. Under water pressure influence of in the lake basins and gravity, infiltration groundwaters leaking through the systems of cracks in clays penetrate to a more dense clay layers which are fluid trap. Here, directly under the lake basin, a dome of groundwater is formed. The flow of groundwater from the dome towards inclined depressions leads to creation of unloading areas. They are presented by permanently wet land (flarks) or low-water infiltration springs that come to the surface at the same hypsometric level.

New sedimentological and palynological data from the Yarkand-Fergana Basin (Kyrgyz Tian Shan): Insights on its Mesozoic paleogeographic and tectonic evolution

The Talas Fergana/Karatau Fault, is a major tectonic boundary separating the Kazakh-Turan domain to the west from the Tian Shan domain to the east. During the Jurassic, movements along the fault led to the opening of several basins. Still, the Mesozoic kinematics of the fault and the geodynamic mechanism that led to the opening of these basins are largely unconstrained. Located at its southwestern termination, the Yarkand-Fergana Basin is certainly the best exposed and however still poorly understood. In this study, we provide new sedimentological description of the Jurassic series from the northern part of the Yarkand-Fergana Basin as well as new palynological data. Following a Middle–Late Triassic period dominated by regional erosion, the onset of sedimentation in the Yarkand-Fergana Basin occurred during the Sinemurian(?)–Pliensbachian. The basin opened as a half graben controlled by the Talas Fergana/Karatau Fault and separated from the Fergana Basin by basement highs. Extension persisted during the late Pliensbachian–Middle Jurassic, leading to a general widening of the Yarkand-Fergana Basin. Finally, Late Jurassic–Early Cretaceous renewed tectonic activity in the area led to the inversion of the north Yarkand-Fergana Basin. The Early to Middle Jurassic timing of development of the Yarkand-Fergana Basin suggests that the coeval movements along the Talas Fergana/Karatau Fault are not associated to the collision of the Qiangtang block along the southern margin of Eurasia. We favor the hypothesis of an opening controlled by transtension related to far field effects of back-arc extension along the Neo-Tethys subduction zone to the west.

Contribution of well logging and seismic reflection to the Jurassic petroleum system characterization in the Chotts zone (Southern Tunisia)

In the Chotts zone central part (southern Tunisia), the Jurassic series are interesting for hydrocarbon exploration. However, these series’ composition and geometry are little studied to propose prospection programs. Using boreholes and seismic reflection data, the present study aims a better characterization of the Jurassic series in the Chotts zone central part, in view of petroleum prospects identification. The analysis of the lithological columns and their corresponding well logs highlights an interesting petroleum system: The Tithonian, Kimmeridgian and Oxfordian carbonated deposits are the main reservoirs formations, whereas the Kimmeridgian and Callovian clays constitute the source rocks. The seal rocks are composed of the Jurassic clayey intercalations. Forty-four seismic reflection profiles calibrated using the time–depth relation in petroleum wells are interpreted in this study. The reflectors corresponding to the Malm, Dogger and Lias tops are identified and picked on the different seismic profiles. The isochrone, isobath and isopach maps subsequently deduced disclose important tectonic features that affect the Jurassic series in the Chotts zone and control their geometry. They highlight two sunken zones separated by a raised structure. This structure corresponds to an anticline, overturned toward the south, limited by E–W major faults and truncated by other oriented NW–SE. It constitutes an excellent trap structure where five interesting prospects are precisely delimited. Furthermore, the determined tectonic accidents may facilitate the migration of oil from Jurassic source rocks. The findings of this study can help for better understanding of the Jurassic petroleum system in the Chotts area and guide enormously its exploration.

Provenance analysis and tectonic setting of the Upper Jurassic Series in Mohe Basin, northeast China: Implication for the closure of Mongol–Okhotsk Ocean

The Mohe Basin, to the south of Mongol–Okhotsk suture belt (MOSB) closely, is a good window into investigations of the evolution of the Mongol–Okhotsk Ocean during the Late Mesozoic. Sandstone samples of the Upper Jurassic Series in the Mohe Basin, Mohe, and Kaikukang formations are studied in detail in terms of petrography, geochronology, and geochemistry. All samples have low compositional and textural maturity, which are located in the areas of dissected arc and basement uplift in Dickinson's discrimination diagrams. LA‐ICP‐MS U–Pb zircon dating yields the youngest concordant ages of 173 ± 4 Ma and 157 ± 3 Ma for the Mohe and Kaikukang formations, respectively, which constrain the maximum sedimentary periods of them. The peak ages of all concordant ages correspond identically to the regional magmatic activities on both the Siberian Craton and Erguna Block. All samples demonstrate geochemical characteristics not so different from the upper continental crust, whose provenance is felsic in lithology and is situated in active continental margins. Thus, a Mohe foreland basin and its binary provenance are strongly suggested, with the Erguna Block – a southern source and the Siberian Craton – a northern one. Based on all the evidence above, a simple tectonic model of the evolution of the Mongol–Okhotsk Ocean is raised, that is, from the Triassic to the Middle Jurassic; the Mongol–Okhotsk oceanic plate continued to subduct under the Siberian Craton and the Erguna Block, triggering intense magmatism in these areas. In the Late Jurassic, the rapid closure of the Mongol–Okhotsk Ocean caused the basement uplift and intense denudation which provided abundant sources of dissected arc and basement materials for the Mohe Basin.

Закономерности изменения фильтрационно-емкостных свойств региональных резервуаров в зависимости от глубины их залегания в юрских отложениях арктических районов Западно-Сибирской и Хатангско-Вилюйской нефтегазоносных провинций

Закономерности изменения фильтрационно-емкостных свойств региональных резервуаров в зависимости от глубины их залегания в юрских отложениях арктических районов Западно-Сибирской и Хатангско-Вилюйской нефтегазоносных провинций

Widespread nearshore and shallow marine deposition within the Lower Jurassic Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia

Abstract In the Surat Basin of eastern Australia, the Lower Jurassic Precipice Sandstone and Evergreen Formation are a highly prospective reservoir-seal pair for notional future carbon capture and storage. However, the succession remains poorly constrained from a paleodepositional standpoint and this has impacted the capacity to construct predictive reservoir models. Here we integrate sedimentological, ichnological, and palynological data from ten cores located across a large region of the northern and central basin to produce conceptual depositional models for the interval. Our analysis shows that the Lower Jurassic Series consists of fifteen recurring sedimentary facies that are arranged into six facies associations – braidplain, lower delta plain, subaqueous delta, delta-influenced shoreface, tidally-influenced shoreline, and restricted marine shoals. The facies associations occur in the context of large scale fluvio-deltaic systems that occupied the basin. These results are supported by ichnological indications of marine and brackish-water, a coastal suite of palynomorphs, and marine palynomorphs including rare dinocysts, acritarchs, and copepod fragments. The very low abundance of marine palynomorphs are confined to the upper portion of the Evergreen Formation, and in combination with the sedimentology and ichnology suggest that marine influence increased through time as the basin was filled. The elucidation of marine-influenced deposition contravenes all but the most recent facies interpretations of the Precipice Sandstone and Evergreen Formation, and suggests that the paleogeography of the Mesozoic of eastern Australia needs to be reconsidered. Importantly, the nearshore and shallow marine depositional affinity has important implications for the size, orientation, and distribution of geobodies when building geologically-realistic static reservoir models for dynamic flow simulation.

Выявление структурно-литологических ловушек в юрских и нижнемеловых отложениях Западной Сибири на основе палеогеографических реконструкций

Выявление структурно-литологических ловушек в юрских и нижнемеловых отложениях Западной Сибири на основе палеогеографических реконструкций

New sedimentological and palynological data from the Permian and Triassic series of the Sancerre-Couy core, Paris Basin, France

A new description and sampling campaign of the Sancerre-Couy core has been achieved to reconstruct the evolution of the depositional environments of the Permian and Triassic series and the palynological evolution of the Permian to Lower Jurassic series. High-resolution sequence stratigraphy of the Permian and Triassic series of the Paris Basin, based on well-log analysis, is used to determine seven progradational-retrogradational cycles in the Permian and twelve cycles in the Triassic. Out of the 54 samples collected from the Sancerre-Couy-1 core, only 23 were palynologically productive. Based on the taxonomic composition, four palynological assemblages are distinguished, three of which have been described in previous studies. These three classical palynological assemblages of the Sancerre-Couy-1 core are documented in detail for the first time in this present study. The description of the new assemblage and its correlation can be used to reassess the previously assigned ages: assemblage SC-1 is late Ladinian in age, SC-2 is of the Ladinian–Carnian transition, SC-3 is early Carnian in age, and SC-4 is of late Rhaetian age, whereas it had previously been assigned a Hettangian age.

Coupling accumulation model with gas-bearing features to evaluate low-rank coalbed methane resource potential in the southern Junggar Basin, China

To reveal the resource potential of low-rank coalbed methane (LRCBM) and determine the favorable area from six different zones in the southern Junggar Basin in China, a new LRCBM assessment methodology was developed. This methodology comprises three steps: developing the regional accumulation model, determining the zone with gas-bearing features, and exploring for the favorable targets. First, the coalbed methane (CBM)–generating characteristics, the sealing properties of the over- and underlying rocks in the CBM reservoir, and the hydrogeological characteristics preserved in the reservoir were analyzed. By combining these characteristics with the tectonic burial history, geothermal history, hydrocarbon evolution, and CBM gas-saturation changes, the LRCBM regional accumulation model that illustrates the LRCBM-enrichment characteristics of the Middle–Lower Jurassic series in the southern Junggar Basin was derived. Second, the LRCBM gas-bearing features of each zone were comprehensively analyzed using a large data set acquired from a coalfield; the LRCBM-bearing features and the variation of their related parameters in six different zones of the southern Junggar Basin were presented. These parameters relate to features such as coal-seam distribution, petrology, reservoir property, coal thermal evolution, and coal and gas content characteristics. Third, the favorable targets are explored by calculating the LRCBM resources and intensively evaluating each zone; the favorable targets are candidates for further exploration in the southern Junggar Basin. At depths shallower than 2000 m (∼7000 ft), the LRCBM resources in the southern Junggar Basin are 3054.7 × 108 m3 (10.8 × 1012 ft3), with resource abundance of (0.42–11.65) × 108 m3/km2 ([0.06–1.66] × 108 ft3/ac), thereby demonstrating a significantly good exploitation prospect. Moreover, the favorable and relatively favorable areas for further exploration in the southern Junggar Basin are zone 4 and 5, respectively.

The Tunisian Jurassic aquifer in the North African Sahara aquifer system: information derived from two-dimensional seismic reflection and well logs

Southern Tunisia is an arid area where socio-economic activities are dependent on groundwater resources. The presented study aims to better characterize the Jurassic aquifer based on geological and geophysical data, with a view to develop a rational exploitation program. Well logs are used to precisely determine the position and composition of the known Jurassic aquifer layers and to identify others able to produce good quality water. The logs show that limestones, sandstones and dolomites of the Krachoua, Techout and Foum Tataouine formations are the main Jurassic aquifers. Sixty-eight seismic-reflection sections are integrated within this study. The interpolation between the interpreted sections leads to the construction of isochronous isopach maps and geoseismic sections, and their analysis finds that compressive and extensive tectonic deformations have influenced the Jurassic aquifer geometry. The Hercynian orogeny phase manifestation is remarkable in that there are several stratigraphic gaps in the Jurassic sequence. The E–W, NW–SE, and NNW–SSE accidents, reactivated in normal faults since the Permian to Lower Cretaceous epochs, have generated the structures found in the Jurassic series, such as subsided and raised blocks. Their syn-sedimentary activity has controlled the thickness and facies of these series. The Cretaceous, Tortonian and Post-Villafranchian compressions are responsible for the Jurassic-deposits folding in some localities. The highlighted tectonic and sedimentary events have an important impact on the Jurassic aquifer function by favoring the Jurassic aquifer interconnections and their connections with the Triassic and Cretaceous permeable series.

Chemometric classification and geochemistry of oils in the Iranian sector of the southern Persian Gulf Basin

Crude oil samples collected from several oilfields across the southern Persian Gulf Basin from Jurassic to Cretaceous reservoirs were chemometrically classified into two genetically distinct oil families. Family A oils, occurring in the western areas, are restricted to the Jurassic series. Family B oils, mostly occurring in the eastern areas, were generated from a younger, homogeneous marine carbonate source rock. An improved geochemical understanding of Family A oils is achieved in this study, and a refined paleo-depositional model for their source rock is presented. These high-maturity oils were generated from a Middle Jurassic carbonate source rock formed within a shallow, anoxic intra-shelf basin that was connected to the Neo-Tethys marine environment. An initial phase of biodegradation is suggested to have affected family A oils during their initial accumulation phases, but the imprint has likely been masked with mixing of multiple charges or phase fractionation. Similarly, Family B oils appear to be controlled by mixing of at least two separate charges or phase-fractionation processes, although the trap charging dynamics of the Cretaceous accumulations agrees with a stronger probability for secondary alteration processes. Based on these findings, we conclude that the present-day geochemistry of both oil families is the result of a complicated sequence of events, including mixing of at least two separate charges with varying maturities or phase separation. These new insights can have practical implications for assessing volumetric mass balance relationships in advanced hydrocarbon generation models, identifying low-risk plays, and ranking prospects.

EARLY JURASSIC (SINEMURIAN TO BASAL TOARCIAN) AMMONITES OF THE BRESCIAN PREALPS (SOUTHERN ALPS, ITALY)

This work provides a more precise up-to-date biostratigraphical framework of the Early Jurassic ammonite succession of the Brescian Prealps with more than seventy taxa for the Sinemurian, Pliensbachian and basal Toarcian corresponding to a about 30 horizons or faunal assemblages rather well correlable with the NW European standard zonation. These results are supported with already published data, new data from recent field investigations, revised contributions of different authors and with the study of the historical collection of the Museum of Natural Sciences of Brescia (Northern Italy). The biohorizons are referred to the Lower Jurassic carbonate series of the Brescian Prealps, cropping out between the eastern surroundings of Brescia (Botticino-Serle), to the East, and the Lake Iseo, to the West. This area was located in the eastern border of the wide Lombardian Basin, a part of the southern continental passive margin subjected to the Jurassic rifting, preceding the Neo-Tethys opening.

Sinemurian–Pliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe

The biostratigraphy of Sinemurian to lower Toarcian calcareous nannofossils has been investigated in the Sancerre-Couy core (Paris Basin), which contains a mixed assemblage of species with affinities to the northern and southern areas of the peri-tethyan realm, thus allowing for the use and calibration of the Mediterranean Province (Italy/S France) and NW Europe (UK) biozonation schemes. This study is based on semi-quantitative analyses of the calcareous nannofossil assemblage performed on 145 samples and the recorded bioevents are calibrated to the NW European Ammonite Zonation and to a new organic carbon isotope curve based on 385 data points. The main bioevents, i.e. the first occurrences of Parhabdolithus liasicus, Crepidolithus pliensbachensis, Crepidolithus crassus, Mitrolithus lenticularis, Similiscutum cruciulus sensu lato, Lotharingius hauffii, Crepidolithus cavus and Lotharingius sigillatus as well as the last occurrence of Parhabdolithus robustus, have been identified. However, we show that a large number of standard biostratigraphic markers show inconsistent occurrences at the base and top of their range, possibly accounting for some of the significant discrepancies observed between the different domains. In addition to the nine main bioevents used for the biozonation of the core, we document an additional 50 distinct bioevents, evaluate their reliability and discuss their potential significance by comparison to previous studies. A total of five significant negative organic carbon isotope excursions are identified and defined in the Paris Basin including the well-documented Sinemurian–Pliensbachian boundary event. One positive excursion is further defined in the Pliensbachian interval. Our calibration of high-resolution calcareous nannofossil biostratigraphy to ammonite biostratigraphy and organic carbon isotopes represents a new stratigraphic reference for the Lower Jurassic series.

LOWER JURASSIC DEPOSITS STRATIFICATION IN THE SECTION OF TOTA-YAKHINSKAYA 25 WELL AND OIL AND GAS PROSPECTS ON GYDAN PENINSULA

A new option of stratigraphic disjunction of the Lower Jurassic deposits in the section of Tota-Yakhinskaya 25 well is offered. A substantiation of the proposed stratification option is presented and its comparison with other researchers’ works is made. A conclusion is drawn about a significant influence of the Kiterbyuit horizon position variation upon the estimation of oil-and-gas content in the entire Lower Jurassic series within the Gydan peninsula.