Devonian Deposits Research Articles

Microstructure of the Valves of the Genus Praeoehlertella Mergl (Family Discinidae) from the Upper Devonian Deposits of Kazakhstan

The shell microstructure of rarely discovered lingulate genus Praeoehlertella Mergl, 2001 was studied in detail. It was obtained in Famennian stage of Kazakhstan. The peculiarity of the microstructure the secondary granular layer was discovered. The prints of the cells of the inner epithelium of mantle were observed.

Deciphering Permian wetland deposits of the Parnaíba Basin through an integrated study of lithofacies and palynofacies in Western Gondwana

During the Permian, the northwestern portion of the supercontinent Gondwana experienced significant climate changes, among which the increasing aridity in the Parnaíba Basin, located between Brazil's northern and northeastern regions. The marine influence associated with the Panthalassa Ocean, evident in the Silurian and Devonian deposits, was progressively attenuated in the late Carboniferous, culminating in extensive continental deposits during the Permian and the Early Triassic in this region. In this paper, we present the first palynofacies analysis of the Permian Pedra de Fogo Formation, conducted concurrently with the description and association of sedimentary facies from outcrops located in the state of Maranhão, southwest of the Parnaíba Basin. A total of 15 m of sedimentary succession was described, and 28 samples were selected for palynofacies analysis. As a result, we identified six sedimentary facies, which were grouped into two distinct facies associations: Fa1 – Central Lake and Fa2 – Playa Lake. The palynofacies analysis considered the sedimentary organic matter, classifying it into amorphous organic matter, phytoclasts (subdivided into eight subgroups), and palynomorphs (with six subgroups). Integrating the results enabled an understanding of the depositional context, which is closely related to water dynamics and climate seasonality. The shallow lake system interpreted during the wet phases recorded interruptions in the normal sedimentation of the central lake due to turbidite deposits associated with ephemeral rivers. Conversely, during dry phases, salt precipitation occurred with subsequent substitution and aerial exposure of the surface when the water balance was negative. Eodiagenetic events were inferred and are considered throughout the article. The alternation of the hydrological regime allowed the deposition in proximal, transitional, and distal environments, creating a distinctive mosaic of wetland landscapes. The data presented indicate humid conditions within a context of increasing aridity in the studied location, particularly in the upper part of the Pedra de Fogo Formation.

The Frasnian stage of the Upper Devonian in Nakhichevan Autonomous Republic (Southern Transcaucasia)

Marine carbonate-terrigenous deposits of the Lower (partially), Middle, and Upper Devonian, rich in coral-brachiopod and other fauna, are widespread across the territory of the Nakhichevan Autonomous Republic. Various ore and non-metallic minerals are associated with these deposits: the Gyumushlug and Yaydzhinsky polymetallic deposits, etc. Further ex-pansion of the mineral resources of the study area, as well as the challenges associated with the establishment of large-scale geological mapping in this region, make the development of a detailed stratigraphic scheme crucial. An indispensable condition for the creation of such a scheme is the study of the most characteristic groups of ancient organisms, among which brachiopods play a dominant role in the organic world. Devonian deposits in the Southern Transcaucasus are distributed over a limited area – in the western part of the Na- khchivan Autonomous Republic – but they are widespread here and are represented by all three well-characterized faunal divisions. For the first time, a diagram has been compiled, reflecting the sequence of appearance and distribution over time of Devonian brachiopods. On this basis, a scheme for the zonal division of the Devonian deposits in the region has been developed, identifying 20 biostratigraphic zones and 16 stratigraphic units (formations). This study summarizes the scattered information contained in various sources on the paleontology and stratigraphy of the Devonian period in the region. The identified brachiopod zones are linked to standard conodont zones. The great importance of brachiopods is determined by the fact that they are one of the most effective groups of fauna not only for dating sediments in natural outcrops, but they also provide good results for broad interregional correlations. Based on the macrofaunal complex, the Frasnian age of the sediments within the formation in question is firmly established. A detailed biostratigraphic scheme with zonal division of the Devonian deposits of Transcaucasia was developed, based on the most important stages in the development of the brachiopod fauna. Biostratigraphic zones, formations, and subformations identified in the Devonian deposits of the Nakhchivan Autonomous Republic and adjacent areas, based on the study of the entire complex of organic remains, mainly brachiopods and cono- donts, can be compared with corresponding deposits of typical sections of Western Europe (Ardennes and Eifel Mountains) and many regions of Eurasia. The obtained data are of practical importance for theoretical geologists, prospectors and developers.

FEATURES OF THE LITHOLOGICAL STRUCTURE AND DEVELOPMENT OF RESERVOIR ROCKS AND HYDROCARBON RESERVOIRS IN THE MIDDLE DEVONIAN SEDIMENTS OF THE SOUTHERN PART OF THE TUZLIV DEPRESSION (DOBRUDJA FOREDEEP)

The aim of the work was to clarify the features of the lithological structure of the Middle Devonian sediments of the Zhovtoyarska-Tuzlivska and Biloliska-Zarichnenska prospective areas of the Dobrudja Foredeep and to determine its influence on the formation of reservoir rocks, traps and hydrocarbon reservoirs. The research was based on the results of geophysical research of wells (radioactive methods) in combination with fragmentary geological (lithological) data. As a result, 9 lithocycles of regressive nature were identified in the section for the first time (e-1 – e-5; g-1 – g-4). Each lithocycle is characterized by a two-membered structure and represents a separate productive horizon (PH). The lower parts of the latter are composed of porous and fractured reservoir rocks (limestones, dolomites, siltstones, sandstones), and the upper parts are composed of fluid-resistant packs (marls, anhydrites). It has been shown that cyclites are grouped into larger units (mesocyclites) of re- or progressive nature, which in different wells (in separate intervals) exhibit a certain similarity or difference in structure. Such spatial and age variability is caused, on the one hand, by the placement of wells in different facies zones of the sulfate-carbonate shelf, and on the other, by the processes of interaction of synsedimentary tectonic movements and paleoceanographic (sea level changes, etc.) factors. The consequence of this is the lateral lithological heterogeneity, which in the petrogeological aspect is manifested in the morphological features of the reservoirs and the character of the development of various types of reservoirs and ultimately determines, taking into account secondary rock changes, the prospects of specific productive horizons. Thus, for the Zhovtoyarska-Tuzlivska area, the vaulted trap is localized near borehole Zhovtoyarska-1 (e-1, e-2 productive horizons), according to other horizons it is fixed near borehole Zhovtoyarska-2. The development of lithological traps is predicted in e-1, e-3 and e-4 productive horizons, which is associated with the wedging of pore reservoirs in the direction from borehole Tuzlivska-2 to borehole Zhovtoyarska -2 and -1. For most productive horizons, the quality of reservoir rocks is expected to deteriorate in the same direction, which suggests the possibility of catagenetic screening of fluids. For the Biloliska-Zarichnenska area, along all productive horizons, trap vaults are localized near borehole Zarichnenska-1 or further south. The most capacious are the traps of horizons e-1, e-4 and e-5, composed mainly of pore-type collectors, the quality of which is low due to anhydritization. The best reservoir rocks are predicted in the Givetian sediments (g-2 – g-4 productive horizons) of borehole Zarichnenska -1.

Middle and Upper Devonian depositional settings of the Moesian Terrane (northern Bulgaria)

The Devonian sediments from the Moesian Terrane (Moesian Platform, northern Bulgaria) are known only from oil and gas prospecting wells. Middle and Upper Devonian deposits are interpreted as formed in great variety of depositional settings. Thus, the Eifelian carbonates were regarded as inner- and mid ramp deposits developed in a shallowing-upward sequence. The shallowing tendency continued during the Givetian and Frasnian, when arid tidal-flat and lagoon settings predominated. At the beginning of the Famennian, a climate change occurred when the Moesian Terrane was removed from the Subtropical Arid to the Warm Temperate Climatic Zone. Then, carbonate deposition took place in various shallow- to deepermarine environments.

Berthierine/Chamosite ooids and associated phases in Devonian deposits of the Parnaíba Basin, Brazil: Genesis and diagenetic evolution

The Parnaíba Basin Devonian deposits comprise ironstones and organic-rich sediments deposited in an epeiric sea margin under greenhouse conditions, enhanced weathering, reduction of oceanic circulation, and black shale deposition. This study aims to contribute to the understanding of the interrelation between depositional and shallow diagenetic processes of the Devonian units of the Parnaíba Basin, specifically regarding the genesis of iron minerals that constitute the ooidal ironstones of the Itaim and Pimenteiras formations. These units comprise bioturbated heterolites, ooidal ironstones, intraclastic conglomerates, and shales, deposited in a deltaic to neritic, storm-dominated platform. This research investigates drill cores from two wells in the southern part of the Parnaiba Basin, focusing on depositional and diagenetic processes. The proposed model suggests deposition of sediments originally rich in organic matter, under low siliciclastic input, and initially anoxic, transgressive conditions. Under these conditions, ferroan precursor clays were deposited, alternating with periods of reworking by storm currents, generating concentric ooids of berthierine. Bioturbation processes resulted in the concentration of previously formed berthierine ooids during intervals of low sedimentation rates. Eodiagenetic microcrystalline siderite formed along the redox boundary between oxic/bioturbated and anoxic zones. Chamosite formed by partial transformation of berthierine, upon incipient mesodiagenetic evolution of the ooidal ironstones. This study contributes to the understanding of depositional and diagenetic conditions in the Devonian section of the Parnaíba Basin, and of similar deposits, offering insights into the formation and development of berthierine ooids under the influence of sea level fluctuations, oxic/anoxic conditions, sedimentation rate, and bioturbation in sedimentary environments, providing key insights into the paleoenvironmental conditions along the northern Gondwana margin during this period.

Magnetic Stratigraphy of Lower Devonian Sediments from Spitsbergen (Frænkelryggen Formation)

Magnetic Stratigraphy of Lower Devonian Sediments from Spitsbergen (Frænkelryggen Formation)

Early–Middle Devonian paleomagnetic results from the Zhongba microterrane, Tibetan Plateau: Evidence for its origin from the northern margin of Greater India

Abstract The initial disintegration of Gondwana during the Paleozoic laid the foundation for the formation of the Tibetan Plateau in the Cenozoic. Determining the relative positions of the microterranes in Gondwana during the Paleozoic not only informs the subsequent drift and accretion processes of these microterranes but is also crucial to the paleogeographic reconstruction of Gondwana. However, the lack of paleomagnetic constraints on the Devonian paleogeography of the microterranes in the northern part of Gondwana makes this effort challenging. Here, we report paleomagnetic results for the first time from the Early–Middle Devonian sediments of the Zhongba microterrane. The site-mean direction is declination (Ds) = 310.7°, inclination (Is) = −67.2°, ks = 31.2, α95 = 8.3°, and n = 11, in stratigraphic coordinates. Positive fold and reversal tests, together with rock magnetism results and microscopic observations, strongly suggest that the remanence carriers are of depositional origin. The paleomagnetic results meet the paleomagnetic reliability criteria and therefore can be used for tectonic reconstructions. Our results constrain the paleolatitude of the Zhongba microterrane to be 50.0°S ± 11.7°S in the Early–Middle Devonian. Combined with published detrital zircon ages as well as paleomagnetic results and geological data, our data indicate that the Zhongba microterrane, which had no tectonic affinity with the Lhasa terrane, was part of the northern margin of Greater India during 408–388 Ma and coupled with the South Qiangtang terrane, Tethyan Himalayas, and other terranes to form the continuous northern continental margin of East Gondwana.

Insight into Oligocene–Early Miocene palaeogeography of the Carpathians in Poland: first cycle and recycled detrital zircon provenance in the Menilite and Krosno formations

A set of 6368 U–Pb isotope ages of detrital zircons from 53 samples of the Oligocene to Early Miocene Menilite and Krosno formations (Outer Western Carpathians) was obtained and interpreted in terms of their sources. In both units the most frequent are mid-Variscan and Cadomian zircons that were derived from the Variscan Internides underlying the studied basin, and from the Brunovistulia Terrane, respectively. The coincidence of detrital zircon ages and clast ages indicates that zircons of that age can be at least partly of first cycle. Sveconorwegian and Calymmian–Statherian recycled zircons of Baltica origin, highly frequent in the Menilite Formation, are most probably sourced from Lower Devonian sediments of the Małopolska Massif. The Oligocene basin of the Menilite Formation was supplied in detrital material largely from the north where the Meta-Carpathian Swell was elevated in the area of the Małopolska Massif and Brunovistulia Terrane. During sedimentation of the Krosno Formation this source expired. A considerable input to the Menilite Formation of Baltica derived zircons and a limited number of zircons from sources located SE of the study area suggests that they were deposited in a relatively shallow and isolated basin. Supplementary material : A list of samples collected, lithological features of sampled rocks and lithostratigraphic sub-units, methods of zircon separation and U–Pb dating, and detailed results of the dating of detrital zircons, with concordia plots and probability density diagrams are available at https://doi.org/10.6084/m9.figshare.c.7100071

Heavy mineral indicators of ridge subduction beneath the Bainaimiao arc in the southeastern Central Asian Orogenic Belt

As one of the major subduction-accretionary systems in the southeastern Central Asian Orogenic Belt, the Bainaimiao arc presents a valuable opportunity for investigating the subduction–accretion process of accretionary orogens. It is located adjacent to the northern margin of the North China Craton and developed arc-related deposits during the Silurian–Devonian period. In this paper, we employed heavy mineral geochemistry and U–Pb ages of zircon and rutile, to determine the provenance of the Silurian–Devonian deposits and reconstruct the paleogeography of the source area. The stratigraphic units involved include the Xuniwusu Formation of Early Silurian and the Xibiehe Formation of Early Devonian.Both the Xuniwusu and Xibiehe formations exhibit heavy mineral assemblages dominated by zircons and tourmalines. Nevertheless, the Xibiehe Formation contains Cr-spinels and garnets which are absent in the Xuniwusu Formation. Geochemical analyses of the detrital Cr-spinels exhibit characteristics of a supra-subduction zone ophiolite. The garnets originated from middle- to high-grade metamorphic rocks, as well as mafic and felsic rocks. The tourmalines in both formations primarily indicate a metasedimentary rock source, with a small percentage of Li-poor granitoid species in the Xuniwusu Formation. The U–Pb age spectra of both detrital zircon and rutile in the Xuniwusu Formation show multiple peaks. The age spectra consist of Precambrian ages, indicative of a recycled origin, as well as Paleozoic ages, suggesting a magmatic origin. In contrast, the U–Pb age spectra of zircon and rutile in the Xibiehe Formation display a unimodal age peak at ca. 455 and 490 Ma, respectively. These differences suggest a provenance shift from the Xuniwusu to Xibiehe formations. The Xuniwusu Formation was sourced from the Bainaimiao arc itself, whereas the Xibiehe Formation has a mixed origin of the Bainaimiao arc and Ondor Sum accretionary complex. The provenance change correlates with the unconformity between the two formations, suggesting a forearc uplift event. In combination with the previous research on the Tulinkai ophiolite, it can be inferred that the forearc uplift was triggered by ridge subduction of the Paleo-Asian Ocean plate at ca. 450–430 Ma. This research provided a new perspective on the ridge subduction of the oceanic slab in fossil orogens.

Detrital zircon U–Pb and Hf analyses of Silurian–Devonian sediments in the Sukhothai Terrane, northern Thailand: implications for the Mid-Paleozoic arc belt

The Sukhothai Terrane in northern Thailand consists of continental basement and a Permo-Triassic magmatic arc related to the subduction of the main Palaeo-Tethys Ocean. The Donchai Group represents the oldest sedimentary sequence of the Sukhothai Terrane and consists mainly of meta-sandstone, quartzo-feldspathic schist, phyllite and silty slate. We present new detrital zircon U–Pb geochronology, Lu–Hf isotope data and geochemical data for the sedimentary rocks of the Donchai Group to elucidate the depositional age, provenance and nature of the group. The youngest detrital zircon ages (433–403 Ma) suggest that the Donchai Group accumulated between 433 and 368 Ma, revealing Silurian–Devonian arc magmatic rocks on the western margin of the Sukhothai Terrane. Sediments of the Donchai Group were sourced from both the continental basement and a Silurian–Early Devonian magmatic arc, suggesting a depositional setting on the continental slope of a back-arc basin along the western flank of the Sukhothai Terrane. The Silurian–Devonian arc belt in SW China probably extends to the Chiang Rai region to the west of the Sukhothai Terrane, northern Thailand, revealing the northwards subduction of the Proto-Tethys Ocean along the western Simao and Sukhothai margin during the mid-Paleozoic. The inferred arc and back-arc configuration of the Proto-Tethys Ocean in northern Thailand is comparable with that recently established in Yunnan, SW China. Supplementary material: Detrital zircon U–Pb ages and trace element analyses, zircon in-situ Lu–Hf isotopic compositions and whole-rock geochemical data are available at https://doi.org/10.6084/m9.figshare.c.6858373

Некоторые детали строения северной центриклинали Косью-Уньинской синклинали (р. Илыч, Северный Урал)

The latest most detailed geological map of the Upper Pechora transverse subsidence, compiled by V. V. Yudin, shows an undisturbed western wing passing into the centricline of the Kosyu-Un’ya syncline, composed of Middle Ordovician and Silurian deposits falling to the southeast in the area of the middle course of the Ilych River (from the mouth of the Shezhimyu River to Anna-Di Island). Detailed studies of the sections uncovered in this area, as well as analysis of the literary and field materials of the predecessors, allowed updating ideas about the stratigraphic confinement of some strata and the structure of the area as in general. According to our data, deposits from the Middle Ordovician to the Ludlov unit of the Silurian are revealed within the northern centricline of the Kos’yu-Un’ya syncline, which are divided into three block scales (Shezhimdisk, Mikolael and Ust’Bol’shekosyu), bounded by faults with a plane of incidence inclined to the east-southeast at angles of 30–40°. Tectonic confinement of the complexly dislocated Lower Devonian sediments of the Valgan formation observed at the northern centricline of the fold to the east of the mouth of the Kos’yu River and previously referred to the eastern wing of the Kos’yu-Un’ya syncline, is currently being discussed. According to their lithological composition, they belong to the Shantym facies complex and are probably already part of another tectonic structure — the Valgan «monocline» of the eastern subzone of the Upper Pechora transverse subsidence.

Palaeoecology and palaeobiogeographic relationships of Lower Devonian bryozoans from the Guadámez and Peñón Cortado Sections of Sierra Morena (SW Spain)

Bryozoan fauna from the Lower Devonian (Pragian – Emsian) deposits of the Ossa-Morena Zone (SW Spain) comprises twenty-eight species: one cyclostome, two cystoporates, sixteen trepostomes, five cryptostomes, and four fenestrates. Three new genera with one new species, respectively, are described: cyclostome Diploclemella serenensis n. gen. n. sp., trepostome Cordobella tenuis n. gen. n. sp., and cryptostome (rhabdomesine) Serenella dubia n. gen. n. sp. Three trepostome species are new: Leptotrypa parva n. sp., L. modesta n. sp., and Boardmanella spinigera n. sp. Ten species are described in open nomenclature. The studied bryozoan fauna shows high morphological and taxonomical diversity, comprising mostly species of moderate size. The assemblage is clearly dominated by branched ramose and encrusting growth forms. The studied bryozoan fauna shows some distinct palaeobiogeographic relations to the bryozoans from the Lower Devonian of NW Spain, Morocco, and Czech Republic.

Crude Oils of Domanik Genotype from Middle Devonian Deposits of the Timan–Pechora Oil-and-Gas Province

The composition of biomarker hydrocarbons in crude oil samples from Middle Devonian deposits of the Western Tebuk, Dzh”er, and Michayu oil fields of the Omra–Lyzha saddle in the Timan–Pechora oil-and-gas province was studied. Significant effect of the Upper Devonian Domanik source rocks on the formation of the composition of these crude oils was revealed. The main parameters demonstrating the genetic relationship of the oils from Middle Devonian reservoirs with this source rocks are distribution of steranes С27 : С28 : С29, ratio of isoprenoid to normal alkanes, and distribution of normal alkanes. All these characteristics sharply distinguish the crude oils under consideration from paraffinous oils syngenetic to Middle Devonian deposits, characteristic of deposits in the southern part of the Pechora–Kozhva megarampart.

A New Fossil Plant from the Late Devonian Strata of Northern Timan (Russia)

Remains of a new fossil plant Petrosjania salarina gen. et sp. nov. were found in the Upper Devonian deposits of the Northern Timan (eastern coast of the Chesha Bay of the Barents Sea). This plant has a number of features characteristic of different groups of higher plants: Barrandeinoid stems (characteristic of the Barrandeinales), flabelloid leaves on stems (Devonian groups incertae sedis) and axilar leaves (characteristic of many plants, including horsetails), fibrous root system (typical for Pteridophyta, Monocotyledones). The systematic position of the new fossil plant, its morphology and possible relationships are discussed.

A new species of possible archipolypodan millipedefrom the Carboniferous of the Netherlandswith unusually long tergites

Millipedes have a long evolutionary history, with the oldest presumed fossils of Diplopoda de Blainville in Gervais, 1844 being from the Silurian and the first definite fossil record originating from Devonian deposits. The phylogeny of Diplopoda is not fully resolved yet, especially not concerning fossil representatives. At the same time, already in the Palaeozoic millipedes showed quite a morphological and presumably also ecological variety. We describe here a new species of a Carboniferous millipede from the Westphalian A of the Netherlands, Lauravolsella willemeni n. gen., n. sp., a possible representative of Archipolypoda (†Archipolypoda Scudder, 1882). The species is based on a single specimen, preserved with part and counterpart, which both show a three-dimensional preservation. The specimen has unusually long tergites, in normal life position covering most of the following segment. These long tergites might have been beneficial when performing defensive enrolling. In extant millipedes, enrolling is usually facilitated by softer areas between the sternites, allowing for a certain degree of ventral compression. In the new fossil, the sclerotic sternites occupy the entire length of the ventral side of the segment, not allowing for any type of compression. The new fossil therefore demonstrates another solution for the mechanical challenges during enrolment and increases the morphological diversity of Carboniferous millipedes.

Detrital zircon U‐Pb geochronology and provenance of Silurian, Devonian and Permian sedimentary rocks, South Qinling: Constraints on late Palaeozoic tectonic evolution history of the Qinling Orogenic Belt

The Qinling Orogenic Belt (QOB), formed by the subduction and collision between the North China Block (NCB) and the South China Block (SCB), is a key area for exploring the tectonic process and timing of integration between the two blocks. A range of tectonic models have been proposed with regard to the subduction and collision of the QOB, but controversy in the timing and integration process still remain debatable, especially in the late Palaeozoic orogeny. The South Qinling Belt (SQB) is located between the Shangdan and Mianlue sutures, widely dispersed sediments outcropped in the SQB provide natural ground for the detrital zircon study, that could trace the sedimentary provenance and continental crustal evolution history, thus provide important evidence for the late Palaeozoic evolutionary processes in the QOB. In this study, we applied a detailed geochronological U‐Pb zircon dating of the Silurian, Devonian and Permian sedimentary rocks from the SQB. The result of the U‐Pb isotopic data shows distinct age populations at 439–474 Ma of the Silurian samples, ca. 400–490 Ma of the Devonian samples and 260–310 Ma of the Permian samples, respectively. By comparing the regional geochronological data, we sum up that the Silurian detrital zircons are mainly derived from the SQB and the northern margin of SCB (N‐SCB), the Devonian detrital zircon age populations are dominated by the North Qinling Belt (NQB) and SQB and the Permian sediments mainly stemmed from the southern margin of the North China Block (S‐NCB) and the NQB. A mutual age population of the Devonian sediments at ca. 400–490 Ma from the NQB constrains the closure of the Shangdan Ocean between the NCB and the SCB before the middle Devonian times. Ca. 260 ~ 380 Ma age population of the Permian sediments show similarities with the age data from the northern margin of the NCB, indicating an uplift event of NQB before the late Permian, and further constrains the closure of Mianlue ocean later than Permian times.

Scientific and applied significance of late devonian paleontological thin sections collection (algoflora) stored in the Institute of Geological Sciences of the National Academy of Sciences of Ukraine

Information on the thin sections collection from Late Devonian deposits of Donets Basin, the Dnipro-Donets Depression, the Pre-Dobrudzhian and Lviv Paleozoic Toughs is provided. This collection is kept at the Institute of Geological Sciences of the National Academy of Sciences of Ukraine and represented by thin sections of rocks from 13 boreholes and one outcrop. The collection data is a valuable material for studying the development of the organic world of past eras and includes important information for the study of oil-bearing sulfate-carbonate deposits of the Middle and Upper Devonian of Ukraine.The monograph “The Atlas of Late Devonian сalcareous algae of Ukraine” has been based on this collection. The atlas is the result of many years of research by O. I. Berchenko on the study of calcareous algae and stratigraphy of the Late Devonian in Ukraine. “The Atlas of Late Devonian сalcareous algae of Ukraine” is the first, and so far the only, work including all known monographically described fossil calcareous algae of the Late Devonian in Ukraine at that time and containing images of 123 species (7 of them are new) wich belong to 30 genera of Late Devonian calcareous algae genera of the Donets Basin, the Dnipro-Donets Depression, the Pre-Dobrudzhian and Lviv Paleozoic Toughs. The images of holotypes of new species, as well as the lists of paleoalgoflora have been represented in our paper.

Geochemical properties of the upper devonian deposits (Semiluk, Mendym and Kynovian horizons) of the Volga-Ural province for improving the Romashkinskoye oil field development efficiency

This paper presents the results of geochemical and petrophysical studies of the Semiluk-Mendym (Domanik) and Kynovian deposits of the Frasnian age of the Devonian system at the Romashkinskoye giant oil field of the Republic of Tatarstan. The biomarker composition of the organic matter of the rocks is shown, its catagenetic maturity and facies conditions of its formation are assessed. In addition, the composition of saturated biomarkers (alkanes, steranes, and triterpanes) of Devonian oils were studied. The correlation «oil-organic matter of rocks» was carried out for oils of the Domanik and Kynovian horizons. It is shown that the differences in the biomarker composition of oils from the Domanik and Kynovian horizons are of practical importance in the field development process, since this allows optimizing the oil production process by evaluating the contributions of oil inflows from different reservoirs. Keywords: Domanik deposits; oil geochemical research; biomarkers; alkanes; terpanes.

RHEOLOGICAL PARAMETERS OPTIMIZATION OF HYDROCARBON-BASED DRILLING FLUID DEPENDING ON THE MINING AND GEOLOGICAL CONDITIONS OF THEIR APPLICATION

Hydrocarbon-based solutions (HCBS) today are capable ofperforming tasks that water-based muds cannot do. First of all — theability to keep the well walls at a lower density, their insensitivity to clayand salt deposits, the similar nature of the hydrocarbon base and reservoirfluids. These advantages make it possible to use HCBS in the constructionof wells characterized by intensive caving, with high zenithangles and extended horizontal sections. HCBS is successfully appliedat the fields located beyond the Arctic Circle when opening, for example,Devonian deposits. But in addition to the indisputable positive aspects,HCBS also have disadvantages. Among their main disadvantages, highcost is distinguished, which increases the requirements for the quality ofthe solution for trouble-free and fast well drilling, the impossibility ofusing HCBS in absorbing horizons. In addition to the high cost, hydrocarbon-based fluids, due to their nature, are characterized by compressibilityand a non-linear dependence of viscosity on temperature. Simplyput, under the influence of high temperatures, the rheological parametersof the solution decrease, and under the influence of low temperatures,they increase. And if solutions are constantly being found for the firstproblem, improving and developing new viscosity modifiers, emulsifiersand additives, then the second problem has long gone unnoticed. But forthe deposits of the Arctic, characterized by vast zones of permafrost, thesolution of this problem is more relevant than ever. An increase in plasticviscosity when the solution is cooled (for example, in permafrost zones)is possible during long-term circulation stops: during tripping operationsor running casing, etc. This increase in rheology can cause circulationloss, fluid loss, increased timewell construction, and hydraulic fracturing.In this work, we decided to investigate the dependence of the HCBSrheology on temperature and to choose the concentration of reagents thatwill allow leveling this dependence, since, due to the nature of theHCBS, it is impossible to eliminate it completely.