Borehole Sections Research Articles

Oil well compressive strength analysis from sonic log; a case study

Appropriate selection of bits for different bore-hole sections is the key to achieve superior drilling performance. This is done with the intention to maximize the rate of penetration while maintaining bit integrity and drilling safety, which plays an important role in maintaining well economies. An accurate selection of drilling bit is dependent on the physical characteristics of formation and the compressive strength of rocks. The acquisition of rock strength along the wellbore can be obtained from various sources such as logs, cutting and rock mechanical test or drilling data. This paper posed a trial to obtain compressive strength profile of oilfield’s formation from a sonic log. According to the results, the formations have been divided into several groups from very soft to very hard formation to optimize bit selection. The acquisition of rock strength information in different conditions is made possible by the generation of similar rock strength logs by different sources. Nevertheless, the best prediction will be given by meter-by-meter based logs from different references. Hence, log based or drilling based methods remains the most preferred methods used to obtain rock strength logs. In this paper, it is desired to predict the compressive strength of wellbore by using empirical correlation based on well logging data and then investigate the confidence of results by data obtained from drilling data. Later, this method is used to predict uniaxial compressive strength in the entire of oilfield.

THE DESIGN OF THE TRAJECTORY OF A SPATIALLY DEVIATED WELL

Natural spatial deviation is due to the non-conformity of the type of a BHA and geotechnical drilling conditions at a certain interval, as well as mistakes in conducting geological exploration. The article presents the method which benefits the most from the natural deviation and the use of non-directional bottom hole assemblies. The main idea of this method lies in the fact that the design of the well trajectory is directed from the center of the target area to the wellhead. The article describes the main sections which constitute the trajectory of a spatially deviated well including the section of big diameter in the upper intervals. The input data necessary for designing the profile are given. The authors present the method of calculating the drilling lot using non-directional BHAs. The method consists of several cyclic repetitive operations and each subsequent interval of the wellbore is designed taking into account the change of the inclination and azimuthal angles in the previous interval. The authors conduct the survey of devices for drilling the borehole section which is made using a directional BHA. In rotor drilling, the technical devices and directional drilling technology are quite complex, therefore whipstocks with mud motors are used more often. As a result of calculating the trajectory of the well with different transition points between directional andnon-directional BHAs on the axis parallel to the azimuth direction, an array of transition points from two-axis to single-axis directional BHAs is formed. When drawing these points on the well plan, the ellipse of intermediate depths of BHA change is formed. This ellipse makes it possible to choose the optimal two-axis BHA using the necessary inclination angle values at the final stage of drilling a big diameter wellbore and using the final depth of this interval. On the basis of the this method the authors create software that allows automating the process of designing the well trajectory, taking into account the change of geology in the log, technical parameters of wells construction and technological parameters of drilling. This software was used while designing a well trajectory, referring to certain geological drilling conditions.

Application of Geophysical Borehole Logs For Aquifer Characterization in Coastal Environment of Lagos, Southwestern Nigeria

Well correlation and aquifer characterization were successfully carried out across a part of Lekki, Lagos state, Nigeria. The formation stratification sequence comprises predominantly of sand and clay with intercalations of both occurring as clayey sand and sandy clay. Three distinct fluid types (saline water, brackish water and fresh water) were delineated. The bulk resistivity (formation and fluid resistivity) ranged from 0 - 80Ωm for saline water sand and clay, 80 – 120 Ωm for the brackish water sand and clay and 120 – 620 Ωm for fresh water. Salinity occurrence was observed throughout the entire study area with depth of occurrence ranging between 14 – 152m. The depth to saline/fresh water interface range from 123 – 167m. Two aquifer types (unconfined and leaky confined aquifers) were observed in the study area. A maximum of five aquifers were delineated across the borehole sections. The depths to the delineated aquifers ranged from 163 – 249m with the aquifer thickness ranging from 10 to 56m. The maximum saline water depth extent was at about 152m (BH7 and BH5). Brackish water (the saline water interface) ranges from about 123.5 – 167m while the minimum depth to freshwater was about 163.5m (BH4) and maximum depth was about 175m (BH8) for all observed logs. The hydrogeological importance of the Coastal Plain Sand (a good aquifer unit) in the study area is under severe threat of continued seawater (saline water) intrusion on its coastal flank as observed from the vulnerability analysis of the study area. The study area is under intense stress from probably heavy abstraction as more than 80% of the observed boreholes were characterized as highly vulnerable and all of the observed boreholes already exceeded the standard range of values for some of the vulnerability parameters particularly.

Quantitative risk assessment for Optimum Mud weight window design: A case study

Deterministic approach in selecting mud weight window at pre-drill condition has uncertainties for wellbore stability analysis. Quantitative Risk Assessment (QRA) technique has been successfully used in pre-drill condition addressing the variability of input parameters like, principal stresses and rock properties. The output of this process recommends the mud weight window with probability of drilling success which can avoid the wellbore collapse and lost circulation events. This kind of stochastic approach to predict safe mud weight window can assure stable wellbore with significant cost effectiveness related to drilling success.A workflow to perform QRAis demonstrated which starts with derivation of stresses in deviated wellbore. Seismic velocity data has been used to estimate pore pressure and validated with formation pressure obtained from reservoir characterization instrument data for a study well located in east coast of India. Mogi-Coulomb failure criteria have been used to estimate required mud weights to avoid shear failure. Mud weights required to avoid lost circulation has also been calculated. Then QRA has been accomplished to predict safe mud weight window sat specific depth within 14 ¾” hole and borehole section (12 ¼”). The results have been validated with real data set and based on that exercise, mechanical earth model and uncertainty associated with input parameters for study well has been established. A feasibility study has been performed to check the viability of drilling a near horizontal (75ο inclination) well along the reservoir unit with azimuth of 250ο. The wellbore collapse and lost circulation curves intersect near 35% probability of success point. The results of QRA indicate that the inclination and azimuth would not be favorable for successful drilling in this scenario. Based on these QRA, recommendations have been made in data acquisition plan for future drilling to reduce the variability in input parameters and successful completion of well.

Simulation of steady-state cuttings transport through a horizontal annulus channel

The current study is devoted to simulating cuttings transport by drilling fluid through a horizontal section of borehole with an annular cross section. Drill pipe rotates in fixed eccentric position. Steady-state flow is considered. Cuttings are rigid spheres with equal diameters. The carrying fluid is drilling mud with Herschel-Bulkley rheology. Suspension rheology depends on local shear rate and particles concentration. Continuous mixture model with algebraic equation for particles slipping velocity is used. Two hydrodynamic regimes are considered: axial flow without drill pipe rotation and with drill pipe rotation. In the case of axial flow was shown that increasing of power index n and consistency factor k increases pressure gradient and decreases cuttings concentration. Increasing of yield stress leads to increasing of pressure gradient and cuttings concentration. Cuttings concentration achieves constant value for high yield stress and not depends on it. Rotation of the drill pipe significantly changes the flow structure: pressure loss occurs and particles concentration decreases in the cross section. Two basic regimes of rotational flow are observed: domination of primary vortex around drill pipe and domination secondary vorticity structures. Transition between regimes leads to significant changes of flow integral parameters.

ГЕОЛОГИЧЕСКОЕ СТРОЕНИЕ ПРОНИЦАЕМЫХ ПЛАСТОВ ГРУППЫ СГ СИГОВСКОЙ СВИТЫ УСТЬ-ПОРТОВСКОГО МЕГАВЫСТУПА

The article deals with issues of indexing the Upper Jurassic productive layers developed in the northeastern part of the West Siberian petroleum province. Based on lithological division and detailed correlation of borehole sections, sandy horizons were identified and indexed in the Upper Jurassic section of the western part of the Ust-Port uplift.

Graptolite biostratigraphy and dating of the Ordovician–Silurian shale succession of the SW slope of the East European Craton

This paper deals with the graptolite biostratigraphy and age determination of the Ordovician and Silurian lithological successions of the Baltic, Podlasie and Lublin basins that existed during the early Palaeozoic on the SW slope of the East European Craton. The biostratigraphic research described was conducted on core material coming from old boreholes and cores from several new wells. Graptolite zones were identified and the chronostratigraphic succession was constrained, with the depths to the stratigraphic units, especially those considered prospective for petroleum, being determined in the individual borehole sections. Old local stratigraphic schemes of the Silurian used for many years in the Polish geological literature are correlated with the standard schemes. The most complete succession of graptolite zones, both in the Ordovician and the Silurian, is observed in the Baltic region. The number of stratigraphic gaps increases towards the east and southeast of the regions. The stratigraphic range of the Sasino Shale Formation decreases in this direction; in the Podlasie and Lublin regions, it comprises only the Katian Stage. The stratigraphic range of the Jantar Formation in the western part of the area spans not only the Rhuddanian but also part or the whole of the Aeronian. In the Podlasie and especially the Lublin regions, sedimentation of the Jantar Formation began in the latest Rhuddanian–Aeronian. A large stratigraphic gap, spanning part or the whole of the Llandovery and increasing eastwards, was documented in the Podlasie-Lublin region. The biostratigraphic research allowed a more precise constraint on the temporal and spatial extent of erosion of Pridoli deposits and the beginning of coarse-grained, siliciclastic sedimentation (Kociewie Formation) in the Baltic Basin.

Hydrogeological investigation of landslides Urbas and Čikla above the settlement of Koroška Bela (NW Slovenia)

The area above the settlement of Koroška Bela is highly prone to slope mass movements and poses a high risk for the safety of the settlement. To get an insight into the hydrogeological conditions and processes which can affect mass movements in this area, hydrogeological investigations, including hydrogeological mapping, discharge measurements of springs, performance of infiltrometer and slug tests were performed. The results of these investigations show complex and heterogeneous hydrogeological conditions, predisposed by geological and tectonic setting and active mass movements which cannot be uniformly described. Observed large fluctuations in the rate of discharge of springs and groundwater level in observation wells are highly dependent on meteorological conditions. Estimated hydraulic conductivity of the soil is relatively high (2×10-4 m/s) and reflects the loose structure and high content of organic matter in the upper part of the forest soil. Hydraulic conductivity of more permeable sections of boreholes is in general higher in the upper parts, in predominantly gravel layers (in range from 2×10-3 to 1×10-5 m/s), than in the deeper clayey gravel parts (3×10-5 to 1×10-7 m/s). In the area of the Čikla landslide the average hydraulic conductivity is estimated at 8.99×10-4 m/s and is higher than in the area of the Urbas landslide (3.05×10-4 m/s).

Design and interpretation of packer permeability tests for geotechnical purposes

Packer permeability tests are used routinely in geotechnical investigations to allow estimation of hydraulic conductivity by analysis of pressure/flow rate response during controlled injection of water into a section of borehole, isolated by packers. This paper is a review of the hydraulic fundamentals of the packer permeability test methods and analyses used routinely in geotechnical investigations and discusses the usefulness and limitations of the test. Guidance is given on design of tests, including the maximum hydraulic conductivity that can be measured by the method. Interpretation of tests must recognize that responses are influenced by the entire test system – the host rock, the borehole and any associated zone of disturbance, water quality (injected water and water in the host rock), the packers or isolation system and the head/flow rate measurement system. It is proposed that, for geotechnical projects, presenting test results as a Q–H diagram (plotting injection flow rate v. applied excess head) is useful and allows results to be classified against seven conventional and three unconventional test responses. Guidance is given on the selection of values of hydraulic conductivity, for geotechnical design purposes, from various types of test responses.

Новые данные об арсенатах зоны гипергенеза Шерловогорского рудного района (Восточное Забайкалье)

The similarity in systematic composition of the new foraminiferal assemblages and litological affinity of such stages as Aptian, Al-bian, Cenomanian and Turonian have given opportunity to establish the Middle order of Cretaceous system. The first time these forami-niferal assemblages have been found in borehole sections of Samotlorskaya, Van-Eganskaya, South-Russkaya, Parusovaya and several areas of Yamal peninsula, which keep to Northern paleobiogeographical district of West Siberian province. The related connecting between species of foraminiferal assemblages of four stages have been united in one association of these taxons. Holding rocks of Vicu-lian, Chanty-Mansian, Uvatskian horizons consisting of similar on litology argilit-alevrolit-silks have been united in Pocurskaya series of deposits, without opoka and carbonate infusions. Due to similarity in foraminiferal assemblages Kusnetsovskian horizon also offer to Middle Cretaceous. New data on Foraminifera and Biostratigraphy of each stage have been received during study kern specimens from borehole sections of these areas. Some species, subspecies and vicariant in foraminiferal assemblages have known in such age formations of Canadian province. On this base West Siberian and Canadian provinces belong to Arctic paleobiogeographical Realm of same name circum pole Belt.

Обоснование безопасных режимов эксплуатации водозаборных скважин Ваделыпского месторождения

The sand production problem for the water sources wellbores at the Vadelypskoye oilfield that is located in Hanty-Mansy region of Russia is considered. The serious issue during field operations is the open wellbore wall destruction even if low drawdown rates (4-6 bar) are applied to the water-bearing pay at one kilometer depth. Petrophysical analysis of high permeable (0.6-2.5 D) sandstones of water-bearing pay is presented. The effectiveness of the sands prevention methods used at the deposit is discussed and the practical recommendations for drilling and operations are given. To identify the conditions of safe fluid extraction the three-dimensional poroelastic software was used for modeling of the vertical borehole section located in the water saturated sandstone. We consider the mudcake is stripped and disappeared due to the wellbore technological operations but the plugging zone persisted. The colmatage zone was taken into account in the modeling as the radial area with reduced relative to formation permeability. Along with this, the axial sandstone permeability is non-uniform and assigned according to the core data. Different horizontal stresses ratios and values are considered in the simulations. The medium-clogged and high‑clogged damage zone influence on the formation effective stresses was studied for minimal and maximal drawdown values. It was established that the most stable borehole relative to the draw-down is located in the homogeneous horizontal stress field within the medium-clogged damage zone. Special attention was given to the damage zone permeability and thickness influence on the wellbore wall failure while pumping. It was determined that rock failure is more possible when the thin and low-permeable colmatage zone is built in formation. The near-wellbore zone colmatage effect on rock destruction can be definitely seen for conditions of equal horizontal stresses but decreases when horizontal loads are unequal. The analysis of different in-situ conditions showed that the difference between horizontal stresses has more effect on destruction than colmatage zone parameters changes. The water-pumping admissible drawdown pressure ranges are specified for each horizontal stresses regime and the recommendations were given for sanding screen characteristics. For wellbore planning at the Vadelypskoye deposit the preferable drill mud composition was described that will contribute to subsequent safe water pumping.

Cross-Association Analysis of Barren Measures Lithologies and Microfacies, Talcher Coalfield, Odisha

Cross-association analysis of lithologies and microfacies of two borehole sections of the Barren Measures Formation, Talcher coalfield situated 12 km apart show strikingly dissimilar results. In case of lithologies, one-to-one correlation becomes significant, which suggests continuity of the formation throughout the Barren Measures Formation and prevalence of braided stream depositional environment. However, lack of significant correlation of microfacies suggests existence of different sub-environments at different parts of the Barren Measures Formation during each stage of sedimentation. When channel facies were laid down at one place, homotaxial leeve, floodplain and swamp facies were deposited elsewhere.

Automatic drawing technique for horizontal projection diagrams of exploration borehole deviations

The objective of this paper is to propose an automatic drawing technique for horizontal projection diagrams of exploration borehole deviation. In this study, a computer program was developed so as to achieve AutoCAD automatic creation of horizontal projection diagrams with low computational complexity. The main procedures include: the original borehole deviation data are used, the coordinates of the measuring points are calculated by using the average zenith angle and azimuth angle, the coordinates of the middle points are determined using the average borehole section length, and the final coordinates and the respective AutoCAD drawing exchange files are outputted. The conclusions are: the drawing exchange files can be called directly by the AutoCAD software, thereby achieving the automatic drawing of the horizontal projection diagrams of the borehole deviations. An example is provided to demonstrate that the proposed automatic drawing method is correct, concise, and efficient. And this automatic drawing technology provides significant advantages, avoiding the shortcomings of inefficient and error-prone method in the past.

Roveacrinid microfacial assemblages (Roveacrinida, Crinoidea) from the Lower-Middle Cenomanian of the Adıyaman area (SE Turkey)

In the northwestern part of the Arabian Platform, the Adiyaman district (SE Turkey) displays some Lower-Middle Cenomanian deep-marine sediments of the Derdere Formation. These sediments are yielding abundant echinodermal remains, among which some thecal and brachial plates are assignable to roveacrinids. Routine microfacies analysis of the Lower-Middle Cenomanian part of the Derdere Formation revealed unexpected early Cenomanian assemblages of roveacrinoidal ossicles, comparable with those formerly reported further south in the Cenomanian-Turonian of the Arabian Platform. For the first time, genuine and undisputable Roveacrinidae are illustrated for SE Turkey. Seven borehole sections were scrutinized in search of microcrinoidal sections, most especially within carbonate microfacies. Within the scope of better constraining the position of the Lower-Middle Cenomanian, we had been compiling the successive occurrence of respective identified roveacrinid remains. These roveacrinidal assemblages are consisting in: Roveacrinus communis Douglas (1908) (= R. derdereensis Farinacci & Manni); Roveacrinus cf. alatus Douglas (1908); Roveacrinus spinosus Peck (1943); Roveacrinus sp.; Roveacrinidae indet.; Applinocrinus sp.; and Saccocomidae indet. These specimens provide significant clues to constrain the palaeogeographic reconstruction of Tethyan seaways, and represent potential fossil index candidates for the Lower Cenomanian stratigraphy of the Adiyaman district.

Sequence paleogeography and coal accumulation of the Early–Middle Jurassic in central Qilian Mountain belt (Muli Basin), Qinghai Province, northwestern China

The Muli Basin in the Qinghai Province, located at the northeastern margin of the Tibetan Plateau, is a Jurassic coal-bearing basin, and is an important area for coal production in China. Recent explorations have revealed the presence of unconventional types of gases such as coalbed methane and gas hydrates. We have investigated the sedimentology, sequence stratigraphy, and lithofacies paleogeography of this basin based on data from outcrop and borehole sections. The coal-bearing strata include the Lower Jurassic Reshui and the Middle Jurassic Muli and Jiangcang Formations. Lithofacies including conglomerates, sandstones, mudstones, and coal were identified and have been interpreted to be formed in environments ranging from braided fluvial, braided fluvial delta, to lacustrine. The preferred sites of coal accumulation are the interdistributary bays of lower delta plain and upper delta plain environments. Three types of sequence boundaries are recognized, including regional unconformities between the Lower Jurassic and underlying Triassic, the basal surface of incised valley fills of thick-bedded sandstones in the Muli and Jiangcang Formations, and the surfaces that mark abrupt depositional facies shifts. Four third-order sequences were subdivided, namely S1, S2, S3, and S4 in ascending order, which correspond to the Reshui Formation, the Muli Formation, the lower member of the Jiangcang Formation, and the upper member of the Jiangcang Formation, respectively. Each sequence consists of a lowstand systems tract, a transgressive systems tract (TST), and a highstand systems tract. A series of sequence-specific basin-wide paleogeographic maps have been constructed based on contour maps of several lithological parameters including stratal thickness, sandstone-to-mudstone ratio, and mudstone thickness. From S1 to S4, the Muli Basin experienced an evolution from a braided fluvial system to braided fluvial delta, then to a lacustrine system, showing a gradual deepening process. Thick coal seams with thickness greater than 3.5 m (11.5 ft) were primarily developed in the delta plain environment where the rate of peat accumulation can be easily balanced by the rate of accommodation generation. A five-stage coal-accumulation model has been summarized, and the major coal seams mainly developed in the early and late TST of sequences S1 and S2.

3D lithostratigraphic model of the Paleogene of the onshore part of the Moesian Platform (Northeast Bulgaria)

The Paleogene sedimentary rocks in the north-easternmost part of the territory of Bulgaria have been penetrated by numerous boreholes. In terms of regional tectonic zonation, the study area is a part of the onshore sector of the Moesian Platform, which partly includes the South Dobrogea Unit and the easternmost part of the North Bulgarian Dome with its eastern slope. The lithostratigraphy of the Paleogene successions consists of six formal units (the Komarevo, Beloslav, Dikilitash, Aladan, Avren, and Ruslar formations) and one informal unit (glauconitic marker). For compiling an overall conception of the regional aspects (lithology, thickness, spatial distribution, and relationships) of the individual lithostratigraphic units and for illustration of their spatial distribution, a 3D lithostratigraphic model based on reinterpretation of individual borehole sections has been created. The model database was compiled by integration of the original lithological data from 338 borehole sections.

Sequence stratigraphy, palaeogeography, and coal accumulation in a gently sloping paralic basin: A case study from the Carboniferous–Early Permian Wuwei Basin, northwestern China

The Wuwei Basin is one of the major coal‐ and hydrocarbon‐bearing basins in northwestern China. Depositional environments, sequence stratigraphy, and palaeogeography of the Carboniferous to Early Permian coal measures in the Wuwei Basin were studied based on the data from outcrop sections and boreholes cores. The coal accumulation model is established within the sequence‐stratigraphic context. Fourteen lithofacies were recognized with the lithologies ranging from conglomerates, sandstones, siltstones, mudstones, carbonates, to coals, which are arranged into five facies associations representing delta, tidal flats, barrier and lagoon, littoral and associated strand‐plain, estuary, and carbonate ramp environments. These facies associations collectively revealed an overall marine‐continental transitional basin setting. The key sequence bounding surfaces, such as the regional unconformity, the basal surface of incised valley fills, and the surface that mark the facies reversal from regression to transgression, have been identified. Based on these surfaces, the Carboniferous to Early Permian in the Wuwei Basin were subdivided into seven third‐order sequences. The distribution of lithofacies and depositional facies in borehole sections, outcrop sections, and three correlation cross sections was investigated, and the sequence‐based palaeogeographic maps were reconstructed. It is inferred from the coal thickness and palaeogeographic maps that the most favourable coal accumulation environments were tidal flat and barrier–lagoon, followed by delta. The sequence stratigraphic framework controls the vertical horizons of the coal seams, whereas the palaeogeographic units determine the lateral development of the coal seams. Tidal flat, barrier–lagoon, and delta in the late transgressive systems tract and early highstand systems tract are the most favourable coal accumulation palaeogeographic units. The main coal accumulation centres were mainly distributed in the Yingpan Sag in the south of the basin, followed by the Ermahu Sag in the north of the basin.

AN INTEGRATED PLANKTONIC (FORAMINIFERA, CALCAREOUS NANNOFOSSILS, DINOCYSTS) BIOSTRATIGRAPHY OF THE PALEOGENE SEDIMENTS FROM THE OIL AND GAS BEARING PROVINCES OF UKRAINE

In Ukraine, the perspective oil and gas bearing regions are located within the Carpathian-Black Sea segment of the Tethys, and the southern slopes of the East European Platform. We examined the plankton microfossil content from numerous Paleogene samples obtained from outcrop and exploration borehole sections. The interpretation of the large dataset concerning the spatial-temporal distribution of Foraminifera, Calcareous nannofossils and Dinocysts is provided. The characteristics of the stratigraphic distribution of planktonic fossil assemblages in the Paleogene sections, their cyclicity in the formations and the specifics of the zonal subdivision based on the defined correlation levels are given. As a result of an integrated study the biozonal subdivision of Paleogene sediments of Ukraine is substantiated, and an interregional correlation of various facies of the Paleogene sections with the International Stratigraphic Scheme is undertaken.

Biostratigraphy and foraminiferal Assemblages of the West Siberian Albian

New data of the Biostratigraphy and Albian foraminiferal assemblages from the sections of newly drilled wells in the Yamal peninsula have been obtained. This territory is assigned by the author to the Northern palaeobiogeographical district, the southern border of which goes along the latitudinal flow of the Ob River. The most diverse Albian foraminiferal assemblages have been discovered in the section of borehole 50 of the Malyginskaya area as well as in the sections of borehole 124 of the West-Tambeiskaya, boreholes, 201, 205 of the North-Tambeiskaya areas. Somewhat similar Albian foraminiferal assemblages were earlier studied by the author in 10 sections of the boreholes of the Samatlor area in the latitudinal Ob River Region. The Middle Albian assemblage is the most stable and diverse in its specific respect. It was discovered not only in the identified sections in the Yamal wells but also southwards in the earlier explored areas. The Albian foraminiferal species of the Yamal peninsula are illustrated in 5 photos and 1 drawing.

Jurassic continental deposits in the sections of the Irkutsk Coal Basin stratoregion

This paper presents bed-by-bed description and paleofloristic characterization (on the basis of plant macroremains) of the Jurassic continental deposits of the Irkutsk Coal Basin, in the sections of the stratotype region where the lithostrata now widely used for the geological mapping works were established for the first time. The characteristics of these units are refined along with their age and occurrences. Newly obtained taphofloras are comparatively analyzed and established to be synchronous. The spatial distribution of Jurassic deposits from different outcrops is correlated with sections of the reference boreholes. Two successive phytostratigraphic assemblages, Cheremkhovo and Prisayan, are recognized. These assemblages are correlated based on the taxonomic composition of the dominant plant groups, with the assemblages of the end of the Early Jurassic and the beginning of the Middle Jurassic of Western Siberia, which are dated using coastal marine fauna.