Secondary Reservoir Research Articles

Evaluation and regulation methods for the utilization of Type-II oil reservoirs using ASP flooding

Abstract Existing methods for calculating oil reservoir utilization in chemical flooding rely on the instantaneous water absorption profile, which fails to provide an accurate description of the actual utilization conditions of the reservoir, making it imperative to find a method that accurately reflects the reservoir utilization. Type-II oil reservoirs often exhibit significant vertical heterogeneity. During chemical flooding, the impacts of high-permeability layer utilization, low-permeability layer utilization, and the simultaneous utilization of both layers on recovery are to be explored. Therefore, it is important to investigate the recovery requirements for different types of reservoirs. This study explores the solutions for improved reservoir utilization as well as the evaluation and regulation methods for the utilization of secondary oil reservoirs using the Alkaline-Surfactant-Polymer (ASP) flooding, which is of great significance for the adjustment of ASP flooding, injection, and production of secondary oil reservoirs.

Morphofunctional characterization of the three main adipose tissue depots in rainbow trout (Oncorhynchus mykiss)

Visceral adipose tissue (VAT) is the primary fat reservoir and energy source in fish. Other relevant fat depots include subcutaneous adipose tissue (SAT), located under epithelial layers, and intramuscular adipose tissue (IMAT), found between the myotomes. The present study investigates the morphological, gene expression and functional characteristics of these different depots in rainbow trout (Oncorhynchus mykiss). Commercial rainbow trout of two different average weights were sampled for histology, lipid quantification and fatty acids profile. Mature adipocytes were isolated for gene expression analyses of lipid metabolic markers. Both VAT and SAT showed large adipocytes, and high total lipid content, suggesting hypertrophic growth. Adipocytes in IMAT were consistently smaller regardless of fish size. While fatty acid composition was similar across depots, SAT had lower levels of palmitic acid and higher levels of polyunsaturated fatty acids that act as precursors of phospholipids and eicosanoids such as eicosapentaenoic acid, compared to VAT and IMAT. Gene expression analyses revealed higher levels of fatty acid transporters, lipolysis and β-oxidation markers in VAT and SAT compared to IMAT, suggesting a more active lipid metabolism. These data support the role of VAT as the main energy depot, while SAT may act as a secondary reservoir, and IMAT potentially serves as an occasional energy source for muscles. This study provides valuable insights into the distinct properties of the different fat depots in fish, which may help to optimize strategies to modulate adiposity for improved health, metabolism, and product quality.

VOC emission rates from an indoor surface using a flux chamber and PTR-MS

Arising from the Chemical Assessment of Surfaces and Air (CASA) 2022 study at the National Institute of Standards and Technology (NIST) Net-Zero Energy Residential Test Facility (NZERTF), this paper presents the first evaluation of indoor surface emissions to a house measured with a surface flux chamber coupled to an online, non-targeted volatile organic compound (VOC) mass spectrometric detection. These surface emissions are compared to those assessed using ambient, whole house indoor VOC measurements and the outdoor air change rate. Chamber emission rates varied by almost four orders of magnitude across 35 quantified VOCs. The whole house emissions measured by campaign-long ambient measurements and the flux chamber emissions (when scaled to the painted surface area of the house) are similar, with an average ratio between the two of 1.3 ± 1.0. The general agreement between these two approaches indicates that the flux chamber was not solely measuring primary emissions from building materials located below the chamber. Rather, the results suggest that over the 12-year house lifetime, VOCs have been widely distributed around the house, migrating from their primary sources to secondary surface reservoirs. With the house in a quasi-steady state, the thermodynamic activities (i.e., the vapor pressures) of the VOCs within the different reservoirs become similar. Emissions of aromatics and monoterpenes have declined since the house was built, whereas aldehyde emissions have remained relatively constant.

Seroepidemiology of bovine herpesvirus-1 in goats in south-western Iran.

Widely regarded as one of the chief causes of diseases in cattle population, bovine herpesvirus-1 (BoHV-1) has the potential to infect sheep and goat, making them potential reservoirs or hosts for this virus. Thus, preventive measures against BoHV-1 in cattle should not overlook the ability of this virus to infect other animals. Therefore, the focal point of this study was to ascertain the seroprevalence of BoHV-1 in 300 healthy goats, the relationship between host and the environmental determinants of infection, and the contributing role of goats in the epidemiology of the BoHV-1. In order to pinpoint the existing antibodies to BoHV-1, the obtained sera were analyzed by Virus Neutralization test. According to this test, the seroprevalence of BoHV-1 appeared to be 64.33% in southwestern Iran. What logistic regression disclosed was that the odds ratio between age and infection with BoHV-1 was 0.83 (p=0.01), representing a decrease of 17% as goats grew one year older. In addition, females manifested a higher relative frequency of infection compared to males, with the odds of infection in female goats being registered at 1.88, compared to those in males (p=0.2). Moreover, contrasted with goats lacking any history of abortion, those with a history of abortion featured 1.1 as the odds ratio (p=0.87). The seroprevalence in Hendijan, Ahvaz, Shushtar and Dasht e Azadegan was detected to stand at 73.24, 71.30, 55.56 and 47.06 percent, respectively, with 6% of fluctuation in the infection rates being attributed to various geographical locations under the scrutiny of this study (p=0.003). Having attested the marked seroprevalence of BoHV-1, the definitive role of goats in the epidemiology of this virus as a secondary host or reservoir was confirmed by the present study, necessitating the strict monitoring of BoHV-1 in goats by animal health authorities in areas where BoHV-1 abounds in cattle.

Oil-and-gas potential of the northeast Predpatom trough

This article discusses the geological structure and prospects of the oil and gas potential in the northeastern part of the Predpatom trough. The materials of the study included geological findings from deep drilling and data from direct geochemical studies conducted in recent years. It describes the specific features of the structural plans of the upper versus the lower part of the geological cross-section. It is assumed that deep drilling in the area of the Ulugur structure has not reached the target productive horizons in the autochthonous part of the section. After acquiring more recent seismic data, there is a proposal to drill a deeper exploratory well in the area. As another priority target for oil and gas exploration, a potential zone of Vendian clastic deposits wedging on the southwestern slope of the Suntar uplift has been identified. A forecast has been made for the distribution of lithological traps stacked with well-sorted granular Vendian reservoirs in the northern parts of the Ergedzheysky and Ulugursky license areas. In the allochthonous part of the section, the potential for oil and gas content in carbonate deposits from the Vendian and Lower Cambrian periods is highlighted in reservoirs formed by secondary processes. It is assumed that thrust deformations could lead to the circulation of aggressive solutions, resulting in the formation of secondary fractured cavernous reservoirs. Subsequently, hydrocarbon accumulations may migrate and accumulate within these reservoirs. Good gas prospects were also noted in the upper part of the section, where gas deposits were confirmed in newly discovered fields. The results of direct geochemical testing of the Ulugur and Ergedzheysky license areas are presented, enabling the preliminary delineation of promising zones and areas in the studied territory. At the same time, narrow elongated geochemical anomalies are associated with the prospects of the allochthonous part of the section, while isometric anomalies are linked to the autochthonous part of the section. The need to consolidate all available geological and geophysical information on the Pre-Atomic deflection within the framework of case studies is noted.

Deep basin conductor characterization using machine learning-assisted magnetotelluric Bayesian inversion in the SW Barents Sea

SUMMARY In this paper, we use a new workflow to substantiate the characterization of a prominent, deep sediment conductor in the hyperextended Bjørnøya Basin (SW Barents Sea) previously identified in smooth resistivity models from 3-D deterministic inversion of magnetotelluric data. In low-dimensionality environments like layered sedimentary basin, 1-D Bayesian inversion can be advantageous for a thorough exploration of the solution space, but the violation of the 1-D assumption has to be efficiently handled. The primary geological objectives of this work is therefore preceded by a secondary task: the application of a new machine learning approach for handling the 1-D violation assumption for 21 MT field stations in the Barents Sea. We find that a decision tree can adequately learn the relationship between MT dimensionality parameters and the 1-D–3-D residual response for a training set of synthetic models, mimicking typical resistivity structures of the SW Barents Sea. The machine learning model is then used to predict the dimensionality compensation error for MT signal periods ranging of 1–3000 s for 21 receivers located over the Bjørnøya Basin and Veslemøy High. After running 1-D Bayesian inversion, we generated a posterior resistivity distribution for an ensemble of 6000 1-D models fitting the compensated MT data for each 21 field stations. The proportion of 1-D models showing ρ < 1 Ω·m is consistently beyond 80 per cent and systemically reaches a maximum of 100 per cent in the Early Aptian–Albian interval in the Bjørnøya Basin. In hyperextended basins of the SW Barents Sea, the dimensionality compensation workflow has permitted to refine the characterization of the deep basin conductor by leveraging the increased vertical resolution and optimal used of MT data. In comparison, the smooth 3-D deterministic models only poorly constrained depth and lateral extent of the basin anomaly. The highest probability of finding ρ < 1 Ω·m is robustly assigned to the syn-tectonic Early Aptian–Albian marine shales, now buried at 6–8 km depth. Based on a theoretical two phase fluid-rock model, we show that the pore fluid of these marine shales must have a higher salinity than seawater to explain the anomaly ρ < 1 Ω·m. Therefore, the primary pore fluid underwent mixing with a secondary brine during rifting. Using analogue rift systems in palaeomargins, we argue that two possible secondary brine reservoir may contribute to deep saline fluid circulation in the hyperextended basin: (1) Permian salt-derived fluid and, (2) mantle-reacted fluid from serpentinization.

Impact and implications of hydrothermal fluids migration in the Frolov hydrocarbon province in West Siberia

Both the prolific source rocks of the Bazhenov Formation and the reservoir-prone units of the Abalak Formation cover most of the West Siberian region. Hydrothermal fluids circulation has been suggested to affect hydrocarbon potential and reservoir properties at some localities of this vast hydrocarbon province. However, integrated studies are currently lacking. Here we present an extensive multidisciplinary study combining regional studies with core logging properties and sample analyses (i.e. geochemistry, petrography, organic geochemistry) to reveal processes related to the hydrothermal activity that affected both formations. We also focus on the factors and implications for secondary unconventional reservoir formation within the deposits from the Kamennaya summit of the Frolov oil and gas region. Three boreholes corresponding to type sections located in different parts of the study area (elevated, transitional and deep) were investigated. The elevated and transitional parts of the basin are adjacent to the weakened contact zones of a granite massif. These tectonic discontinuities acted as conductors for hydrothermal fluids circulation on the area. Lithological and stable isotope studies revealed the presence of intensive hydrothermal secondary alterations in various types in rocks, including massive metasomatic carbonatization, sulfatization, sulfide mineralization and leaching processes. This hydrothermal fluid activity resulted in reservoir formation within siliceous radiolarite layers, while in the deep part of the basin potential reservoirs were completely healed. Organic matter maturity varies in different parts of the basin. We identify several mechanisms related to the various hydrothermal processes that influenced rocks composition, structure and organic matter maturity: (1) A weakened zone located on the periphery of the granite massif in the basin basement is characterized by extensive fault damage zones facilitating the migration of high temperature fluids along deposits during post-sedimentation stages; (2) The thickness, mineral composition of the rocks overlying the basement control fault attenuation and fluid workflow; (3) The fluids composition, their acidity/alkalinity, prompted various changes in rock structure, especially in secondary porosity formation. The identification of various hydrothermal processes and their influence on pore space allowed to determine the approach for the spatial distribution prediction of unconventional reservoirs in the studied formations, and subsequently improve the efficiency and effectivity of the exploration process not only at the Kamennaya summit, but also on the other similar West Siberian basin areas.

Detecção de anticorpos anti Leishmania spp. utilizando ensaio de imunoabsorção enzimática em gatos na fronteira Oeste do Brasil

ABSTRACT: Feline leishmaniosis is infrequent worldwide, and cats have been suggested as secondary reservoirs for the parasite. However, specific diagnostic techniques for feline samples are scarce. In this study, we standardized an in-house indirect enzyme-linked immunosorbent assay (ELISA) using crude Leishmaniainfantum antigen to detect antibodies in feline samples from an endemic canine visceral leishmaniosis (CVL) area in the western border of Brazil. The results were compared with those of an indirect immunofluorescence assay (IFA). We tested semi-domiciled felines residing in Uruguaiana and Barra do Quaraí, Rio Grande do Sul. Among the 41 samples, 25 (61%) were positive using ELISA and 24 (58%) were positive using IFA (1:40). Our findings demonstrated a high seropositivity of feline samples from the endemic CVL area in the western border of Brazil, and we proposed the use of an in-house ELISA with crude antigen for population screening. This is the first serological survey on felines in a region where CVL is well established.

Evolution of the magma system at Makushin volcano, Alaska, from 2004 to 2021

Located in the northeast of Unalaska Island in the central part of the Aleutian, Alaska, Makushin volcano is one of the more active volcanoes in the United States, with the most recent eruption occurred in 1995. Pre-eruptive inflation and post-eruptive deflation were identified from 1993 to 1995 and 1996 to 2000, respectively, and were interpreted as volume changes associated with magma movement within the plumbing system. In this study, we investigate the surface deformation of Makushin from 2004 to 2021 to evaluate its present-day activity, using Synthetic Aperture Radar (SAR) interferometry technique. SAR images acquired from C-band Envisat and Sentinel-1, X-band TerraSAR-X, and L-band ALOS-2 satellites are analyzed using an advanced multiple temporal Interferometric SAR (InSAR) algorithm. The produced deformation time series are then used to track the temporal evolution of the volcanic system with Ensemble Kalman Filter (EnKF) assuming Mogi sources. The results show that Makushin is characterized by time-varying deformation episodes including quiescence, and inflation/deflation cycles at varied rates since 2004. The derived deformation histories can be well reproduced by a spatially stationary pressure source located to the east-northeast of the summit, at a depth about 6 km beneath the sea level (BSL). The inflation/deflation cycles with distinct temporal patterns are interpreted as results from volatile-dominated volume accumulation/loss in the magmatic reservoir. A secondary reservoir connected to the main magma chamber is revealed from the deformation time series. No surface displacement associated with the large earthquake swarm in 2020 is identified from the InSAR-derived deformation. The cumulative volume change in the magma reservoir is about −6×106m3 from 2004 to 2021, indicating a volume/pressure deficit in the magma plumbing system.

Spatio-temporal spread of Lassa virus and a new rodent host in the Mano River Union area, West Africa

The spread of Lassa virus (LASV) in Guinea, Liberia and Sierra Leone, which together are named the Mano River Union (MRU) area, was examined phylogeographically. To provide a reliable evolutionary scenario, new rodent-derived, whole LASV sequences were included. These were generated by metatranscriptomic next-generation sequencing from rodents sampled between 2003 and 2020 in 21 localities of Guinea and Sierra Leone. An analysis was performed using BEAST to perform continuous phylogeographic inference and EvoLaps v36 to visualize spatio-temporal spread. LASV was identified as expected in its primary host reservoir, the Natal multimammate mouse (Mastomys natalensis), and also in two Guinean multimammate mice (Mastomys erythroleucus) in northern Sierra Leone and two rusty-bellied brush-furred mice (Lophuromys sikapusi) in southern Sierra Leone. This finding is consistent with the latter two species being secondary host reservoirs. The strains in these three species were very closely related in LASV lineage IV. Phylogenetic analysis indicated that the most recent common ancestor of lineage IV existed 316-374 years ago and revealed distinct, well-supported clades from Sierra Leone (Bo, Kabala and Kenema), Guinea (Faranah, Kissidougou-Guekedou and Macenta) and Liberia (Phebe-Ganta). The phylogeographic scenario suggests southern Guinea as the point of origin of LASV in the MRU area, with subsequent spread to towards Mali, Liberia and Sierra Leone at a mean speed of 1.6 to 1.1 km/year.

Origin of gas condensate reservoir in Fuman Oilfield, Tarim Basin, NW China

To understand the reservoir property and hydrocarbon accumulation conditions of the Middle and Upper Ordovician intraplatform shoal between ultra-deep main strike-slip faults in Fuman Oilfield of the Tarim Basin, China, the main strike-slip faults in and around well FD1 in the basin were analyzed in terms of sedimentary facies, sequence stratigraphy, intraplatform shoal reservoir property, and oil and gas origins, based on drilling data. The Yingshan Formation intraplatform shoal between the main strike-slip faults is superimposed with low-order faults to form a new type of hydrocarbon play. Firstly, hydrocarbons generated from the Lower Cambrian Yuertusi Formation source rocks vertically migrated into the second member of Yingshan Formation through the main strike-slip faults, and then migrated laterally until they were accumulated. A small amount of oil from Well FD1 came from the Yuertusi Formation source rocks in the mature stage, and a large amount of gas originated from oil cracking in the ultra-deep reservoirs. Therefore, the secondary gas condensate reservoir in Well FD1 is characterized by high gas to oil ratio, dry gas (dryness coefficient being 0.970) and hybrid origin. This new type of hydrocarbon play characterized by intraplatform shoal and low-order fault suggests a prospect of continuous hydrocarbon-bearing area in Fuman Oilfield, which will expand the ultrap-deep oil and gas exploration in the oilfield.

Features of Gold Placer Formation in the Ukrainian Crystalline Shield And Southern Ukraine

The presence of numerous native gold deposits of the Ukrainian Crystalline Shield (UCS), as well as the spread of placer gold manifestations, determines the high probability of the formation here of gold placers with industrial parameters. The main factors and geological conditions that contribute to the formation of gold placers are manifested at the UCS. An important reason for the failures of previous geological works is the outdated scientific and methodological platform, which involved the search for typical gold-bearing deposits with metal of gravity size classes. They did not take into account the possibility of significant movement of placer gold in the geologically clay water flow and the formation of long-distance placers. The traditional Schlich method used in the study did not make it possible to determine the actual gold content when particles with a size smaller than 0.25 mm predominate in the sample, which is precisely the particle size of the primary sources of the UCS. The applied means of research could not be effective in the absence of the necessary analytical base for determining the gold content, modern methods of studying the geological structure and tectonics of the territories. The negative results did not provide any real grounds for further specialized large-scale research and prospecting for placer gold. Today, it is getting obvious that placer gold of small and fine classes can easily move over considerable distances in the water flow and form accumulations not only in boulder-pebble material, but also in pelitic sediments. Due to repeated block movements, erosion and redepositing of gold-bearing accumulations in secondary reservoirs could occur. In this aspect, neotectonic studies (using materials from space surveys) become the main predictive and prospecting method for identifying promising areas of placer gold concentration in the region. When carrying out fieldwork, drilling and modern methods of laboratory analysis of gold samples should be used. The results of many years of research give hope for the discovery of real placer gold deposits in the Northern Black Sea region in the near future.

Diagenesis of the Marinoan cap dolostone, Southern Amazon Craton: An unconventional petroleum system in the evolution of the Araras-Alto Paraguai Basin

The post-snowball Earth world favors the development of hydrocarbon source rock intimately linked to periods of post-glacial marine transgression generating important potential petroleum plays. In the Southern Amazon Craton, hydrocarbons occur in one of the most spectacular examples of post-Marinoan cap carbonate, related to the mid-Ediacaran to Early Cambrian (c. Post 635 Ma) interval associated with the early stabilization of the Gondwana Supercontinent. The Puga cap carbonate consists of the basal portion of the Araras Group exposed in the Araras Alto-Paraguai Basin inserted as a reservoir in an unconventional petroleum system. The cap dolostone overlies diamictites and is overlaid by bituminous limestone and shales (source-rock), and dolostone in the top of a 700 m-thick depositional sequence. The cap dolostone has a diachronic relation with the limestone beds typifying as a secondary reservoir characterized by bitumen-filled fenestral and interpeloidal porosity. The upper dolostone has a closed framework and acts as sealing rocks. This study aims to establish paragenesis sequence and decipher its control on the quality of the cap dolostone as a microbialites-dominated reservoir. Drill core and outcrop samples were analyzed using microfacies, scanning electron microscopy, and cathodoluminescence to unravel the diagenetic history. Aftermath, the Marinoan events favored dolomuds precipitation in the shallow platforms, preserving primary porosity and permeability reduced by early gypsum cementation during oversaturation events. Other destructive processes, such as mechanical and chemical compaction (e.g., stylolites) and authigenesis of pyrite and low-Mg ferroan calcite cementation, occurred during burial and mesodiagenesis. The low permeability restricts hydrocarbon migration concentrated in the upper cap dolostone, and the early cementation increases the potential for preserving these carbonate deposits. Hydrocarbon maturation was triggered during the burial and uplift of the Araras-Alto Paraguai basin. The cementation phases described here enhance our understanding of the mechanisms behind syndepositional diagenesis in microbially-induced reservoirs. This study uses proxies associated with diagenetic studies to quantify the quality of microbially induced dolostone reservoir linked to the evolution of the Neoproterozoic unconventional petroleum system. In addition, this conception will promote a guide for future exploration and understanding of petroleum plays and suggests a possible Precambrian source rocks component of traditional Phanerozoic Petroleum systems in the central part of Brazil.

Magma Transport and Storage Along Rift Zones Through Volcanic Conduits

AbstractRecent geodetic data has clearly imaged significant contraction of rift zones during effusive eruptions, which is attributed to deformation of active, elongated feeders, challenging the rigid conduit paradigm. In this work we develop a physical model to understand the impact of conduit deformation on the eruptive dynamics. Numerical calculations show that magma overpressure increases the width of the conduit, resulting in higher discharge rates than would be expected from the rigid case. At the same time, conduits with high aspect ratio, have larger compressibility and can store significant amount of magma, thereby acting as secondary reservoirs. The net result is that rift zones can maintain high fluxes over prolonged periods of time, leading to large volume eruptions and biasing magma compressibility estimates. We apply our findings to the 2018 Kīlauea eruption where episodic collapse of the summit led to pressure pulses that propagated down‐rift and that were recorded by both tiltmeters and peaks in the effusion rates. Inversion of the data indicates a conduit with a height of 700–800 m and a maximum opening of 4 m, located at a depth of 2.5 km in the Upper East Rift Zone, becoming shallower in the Puʻu ʻŌʻō region and propagating sub‐horizontally in the Middle and Lower East Rift Zone. Based on these properties we infer that up to 30% of the erupted volume can be attributed to magma stored in the rift zone. In agreement with recent studies, we find that magma over‐pressure was low at the end of the 2018 eruption.

Preferred Selection of Fractured Sweet Spot Section for Horizontal Shale Oil Well in West 233 Area

Six parameters, which can effectively reflect the reservoir quality and the completion quality, have been selected, including the sand structure index, the physical parameters, the oil content index, the brittleness index reflecting, the stress difference coefficient and the fracture pressure. The evaluation index of fracturing sweet spots in shale oil horizontal wells was constructed, and the fracturing sweet spot selection criteria for shale oil horizontal wells in the study area were established. Combined with dynamic production data, the fracturing sweet spots were classified into Class I (high quality reservoir), Class II (secondary reservoir) and Class II (non-fracturing sweet spot), which provided technical support for the preferential selection of fracturing formations in shale oil horizontal well.

A laboratory observation for gases transport in shale nanochannels: Helium, nitrogen, methane, and helium-methane mixture

Mudstones are widely distributed in sedimentary basins with abundant nanopores and has very low fluid mobility. It is widely accepted that gas migration in micro-nanochannels no longer follows Darcy's law and the Hagen-Poiseuille equation and that molecular kinetic features profoundly influence gas migration in micro-nanochannels. Here, we characterize the nanochannel sizes of Longmaxi Shale cores under different overburden pressure and pore pressure conditions. And the apparent permeabilities of individual component gases (helium, nitrogen, and methane) in different size nanochannels are compared. Then the variations for components and methane carbon isotope values of the overflowing helium-methane mixture are analyzed. The results show that the molecular slippage effect contributes significantly to the transport of the three gases, especially at low pore pressures; helium has the largest apparent permeability because of its largest molecular mean free path (or Knudsen number). Methane has the smallest molecular mean free path, and the adsorption effect reduces channel size and increases surface roughness, which inhibits the molecular slip effect and therefore results in methane having the smallest apparent permeability. Helium has a transport advantage in the helium-methane mixture due to its Knudsen number advantage, and the larger Knudsen number of 12CH4 likewise leads to a transport advantage over 13CH4. This study suggests that the molecular slippage effect may be an important molecular dynamics mechanism for the efficient development of shale gas. Pressure gradient-driven transport of helium-bearing gas in nanochannels is an essential mechanism for helium enrichment, and high-grade helium reservoirs should occur in secondary low-pressure reservoirs in the basin.

Productivity of unconventional Senon reservoirs in the north of West Siberia

At the current stage of geological exploration, hydrocarbon deposits have been discovered in complex reservoirs that are "unconventional" in nature. In particular, at the Medvezhye field, according to the interpretation of seismic surveys and the drilling of exploratory wells, a gas deposit was discovered in the Senonian deposits of the Nizhneberezovskaya suite of Upper Cretaceous deposits. The gas deposit of the NB formation of the Medvezhye field is characterised by productive reservoirs, mostly represented by mudstones and opoka-like clays. These rocks are characterized by low filtration parameters. Commercial gas inflows from secondary reservoir openings indicate the presence of fractured rock, as confirmed by core testing, mechanical logging. A comprehensive analysis of hydrodynamic studies of field parameters, such as reservoir pressure, drawdown, flow rate, allows calculating the filtration characteristics of a productive formation. The performance of fractured and mixed pore-fractured reservoirs is determined using induction diagrams and pressure recovery curves, the reservoir hydrodynamic model and methods for improving development in complex "unconventional" reservoirs are refined.

Geological Structure of Deep-Submerged Complexes of Sedimentary Basins: Hydrogeological Anomalies and Oil and Gas Potential as a Result of Implementation of Deep-Seated Fluids (on Example of the South Mangyshlak Fields)

The authors of the article substantiate the induced nature of hydrogeological anomalies in deep-submrged complexes of sedimentary basins. As a result of significant catagenetic transformations, the rocks of the lower hydrogeological floor have practically lost their primary capacitance-filtration properties. The water saturation of the rock matrix, the focal nature of the development of secondary reservoirs against the background of extremely low permeability of the surrounding strata makes it impossible to develop elision flows. This causes the high sensitivity of the lower floor to various compression processes, including those caused by the intrusion of the deep high-energy fluid flows. The injection of these fluid flows into low-permeability strata leads to the formation of centers of desalinated waters of various hydrochemical types, from hydrocarbonate-sodium to calcium chloride, and also to the formation of hydrodynamic anomalies. When moving away from the intrusion channels, the hydrodynamic and hydrochemical parameters gradually level off, approaching the background value. It is shown that when fluids are difficult to move upward, hydraulic fracturing occurs in the layers into which the oil-water mixture enters under high pressure. The injection of fluids into the formation is accompanied by the decompression of low-permeability strata, the formation of additional fractures, and the formation of secondary voids of metasomatic origin. As a result, secondary reservoirs of complex morphology are formed, filled with hydrocarbons. The coincidence in terms of hydrogeochemical and hydrodynamic anomalies, areas of secondary reservoirs with distinct traces of metasomatosis and associated accumulations of oil and gas indicate their genetic relationship. The oil and gas reservoirs and their accompanying hydrogeological anomalies are considered on the example of the fields of the South-Mangyshlak oil and gas region, which is part of the North Caucasian-Mangyshlak oil and gas province.

Hydrothermal silicification in Ediacaran Dengying Formation fourth member deep dolomite reservoir, Central Sichuan Basin, China: Implications for reservoir quality

The Ediacaran Dengying Formation fourth member in Central Sichuan Basin is the deep dolomite gas reservoir with the largest natural gas reserves in China, providing an excellent example for understanding the effect of hydrothermal silicification on deep dolomite reservoir quality. By integrating petrology, geochemistry and physical property measurement, this study aims to reveal the genesis of different quartz phase and their corresponding diagenetic events, and to discuss the temporal relationship of diagenetic events and their effects on the quality of deep dolomite reservoir in Dengying fourth member. Three phases of quartz cement are identified in Ediacaran Dengying Formation deep dolomite reservoirs of Central Sichuan Basin as follows: (1) first‐stage quartz filled in primary reservoir spaces, (2) second‐stage quartz mainly filled in secondary reservoir spaces, and (3) third‐stage quartz filled in secondary reservoir spaces with Mississippi Valley‐Type minerals. By analysing petrographic and temporal relationships between these fabrics, it is suggested that the first‐stage quartz precipitated during the syndiagenetic stage, the second‐stage quartz precipitated during middle diagenetic stage, and the third‐stage quartz precipitated during middle diagenetic stage as well. Silicification during syndiagenetic stage is characterized by replacement of cryptocrystalline quartz and cementation of first‐stage quartz within matrix dolomite (MD). After syndiagenetic silicification, the mineral composition of the MD changed from magnesium calcium carbonate to silica, and the later constructive diagenetic alterations were almost ineffective. The distribution of syndiagenetic silicification during syndiagenetic stage gradually decreases from the platform‐interior to the platform‐margin. The second‐stage and third‐stage quartz cements, derived from two‐stage hydrothermal silicification in the middle stage, are both destructive for reservoir preservation. This study, combined with coeval vapour–liquid inclusions Th data and thermal‐burial models, constrains the time of the second‐stage quartz to ca. 403–408 Ma, suggesting an hydrothermal silicification related to the Caledonian movement. The time of the third‐stage quartz cement is constrained to ca. 259.4 Ma, associated with Late Permian eruption of Emeishan flood basalts.

Pore Distribution Characteristics of Different Lithofacies Shales: Evidence from Scanning Electron Microscopy

To disclose the pore distribution characteristics of different lithofacies lacustrine shales, ten samples collected from the Shahejie Formation, Dongying Sag, Bohai Bay Basin, China, were examined using argon ion beam milling–scanning electron microscopy (SEM). A quantitative method was adopted to characterize shale pore distributions based on the SEM images. Mercury intrusion capillary pressure was employed to determine the pore throat size distributions of the shales. The SEM images indicated that in shale reservoirs, interparticle pores at the edges of brittle particles and intraparticle pores in clay mineral aggregates primarily contribute to the reservoir spaces and that in calcite-rich shales, dissolution pores provide secondary reservoir space. Among the morphologies of dissolution, intraparticle, and interparticle pores, the morphology of the dissolution pores is the simplest, followed by those of intraparticle and interparticle pores in that order. Clay and felsic minerals primarily control the shale pore sizes and the larger the clay mineral content in the shales, the smaller their pore size; the felsic minerals demonstrate the opposite behavior. The image-based point counting data indicate that shale pore sizes are mostly between 20 nm and 2 μm. In contrast, most pore throats are less than 20 nm in diameter, implying that the pores in the nanometer to micrometer scales are connected by tiny throats. Among the four lithofacies shales, felsic-rich shales are favorable for shale oil accumulation and seepage because of their large pore sizes and throats their ability to form intercalated shale oil adjacent to organic-rich shales. Calcareous shales with a large number of dissolution pores and a large content of organic matter could produce self-generation and self-storage matrix shale oil and would typically develop fractures, thereby creating a seepage channel for shale oil. This study presents the micro-distributions of different lithofacies of shale pores, which would help in understanding the occurrence and seepage of oil in shale reservoirs.