Gas Hydrate Zone Research Articles

The role of mobile muds in basin form and fill in the tectonic and sedimentologically complex region of the Southeastern Margin of the Caribbean Plate, Trinidad and Tobago, West Indies

The southern portions of the Barbados Accretionary Prism and the eastern deepwater region of Trinidad and Tobago are a seafloor terrain marked by scattered mud volcanoes, mud walls, and mud canopies that rise several hundred meters above the modern seafloor. We use 3D seismic and recently drilled wells to examine this area, dividing it into three tectonomorphic provinces based on the nature of the mud architecture, the structuration, basin types developed, and sedimentary systems occupying the basins. Mud architectures become more massive as one moves northward toward the active tectonic prism. The mud wall in the far northern Mud Wall Province is classified as a mud canopy system, similar to those documented by workers in the North Sabah Pagasa Wedge, Borneo. The Trinidad mud canopy exhibits sutures in the mud mass margins (akin to salt sutures) that separate submasses likely composed of different materials and exhibiting different pressures and properties. These submasses and associated sutures are imageable using the sweetness attributes on the near-stack seismic volume. Anticlines in the isolated mud and distributive provinces, similar to those that are the dominant hydrocarbon-producing structures onshore and offshore Trinidad, show mobile mud cores and are associated with extensional and compressional faulting that provide pathways for hydrocarbon migration into the surrounding lutokinetic stratigraphy. A bottom simulating reflector associated with the base of the gas hydrate zone, extensive across the study area, may be a good indicator of fluid flux activity and subsurface pressure states in mud masses. It is important to use all the seismic data volumes (full, near, mid, and far seismic data stacks) to assess different aspects of these mud diapiric systems.

Identification and Characterization of Gas Hydrate using Marine Seismic and Well Data: South Makassar Strait case study

Gas hydrate has the potential as an alternative unconventional resource to current oil and gas. Identification of gas hydrate is made possible by applying the seismic method with the main indicator of the bottom simulating reflector (BSR). BSR is characterized on the seismic section by the presence of high amplitudes, reverse polarity reflections, and cutting through the stratigraphy. In this study, the presence and thickness of the gas hydrate zone have been identified using five 2D seismic tracks, namely CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11, and the SULTAN-1 well. The gas hydrate zone was identified at a depth of about 2300 m. The existence of the gas hydrate zone is followed by the presence of a gas chimney indicator as a free gas migration path. The gas hydrate zones are estimated to be 15 m, 28 m, 33 m, 23 m, and 30 m thick from the respective seismic tracks CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11.

Seismic stratigraphy and attenuation of gas-hydrate zones within Hikurangi and Gondwana margins, eastern New Zealand

SUMMARY Gas hydrates that occur on many continental margins have received global attention. In reflection seismic imaging, the bottom-simulating reflector (BSR) is a common indicator of gas hydrates. However, it is difficult to identify gas hydrates and quantify their amounts through the BSR alone. For gas-hydrate characterization, it is therefore useful to measure seismic stratigraphic and attenuation attributes. Short-scale patterns of layering that contain information about the amount and mechanism of gas hydrates can be identified through stratigraphic and attenuation attributes. We measure the complete time-variant spectra by using sparse strongest peaks, and the spectral differences at different times through attenuation parameters Q–1 and γ. The traditional Q–1 is associated with the attenuation of the frequency-dependent part of wavefield, and the γ characterizes the frequency-independent attenuation. The measurement approach is straightforward and requires no sophisticated inverse algorithm and is applied to surface seismic data acquired over the Hikurangi and Gondwana margins, eastern New Zealand. High-quality spectral and attenuation images are obtained. Spectral attributes correlate with BSRs and large positive Q–1 and negative γ-values are below and above the BSRs, which are interpreted as being related to free-gas and gas-hydrate accumulations. These results will aid the quantification of gas hydrates and the assessment of their roles as an energy resource, as a potential geological hazard, and in climate change and ocean warming.

The majority of microorganisms in gas hydrate-bearing subseafloor sediments ferment macromolecules

BackgroundGas hydrate-bearing subseafloor sediments harbor a large number of microorganisms. Within these sediments, organic matter and upward-migrating methane are important carbon and energy sources fueling a light-independent biosphere. However, the type of metabolism that dominates the deep subseafloor of the gas hydrate zone is poorly constrained. Here we studied the microbial communities in gas hydrate-rich sediments up to 49 m below the seafloor recovered by drilling in the South China Sea. We focused on distinct geochemical conditions and performed metagenomic and metatranscriptomic analyses to characterize microbial communities and their role in carbon mineralization.ResultsComparative microbial community analysis revealed that samples above and in sulfate-methane interface (SMI) zones were clearly distinguished from those below the SMI. Chloroflexota were most abundant above the SMI, whereas Caldatribacteriota dominated below the SMI. Verrucomicrobiota, Bathyarchaeia, and Hadarchaeota were similarly present in both types of sediment. The genomic inventory and transcriptional activity suggest an important role in the fermentation of macromolecules. In contrast, sulfate reducers and methanogens that catalyze the consumption or production of commonly observed chemical compounds in sediments are rare. Methanotrophs and alkanotrophs that anaerobically grow on alkanes were also identified to be at low abundances. The ANME-1 group actively thrived in or slightly below the current SMI. Members from Heimdallarchaeia were found to encode the potential for anaerobic oxidation of short-chain hydrocarbons.ConclusionsThese findings indicate that the fermentation of macromolecules is the predominant energy source for microorganisms in deep subseafloor sediments that are experiencing upward methane fluxes.84_GYpCEs37AndHmKLQLsZVideo

Pore-Water Geochemical Gradients of Sulfate, Calcium, Magnesium, and Iodide Correlated With Underlying Gas Hydrate Potential: A Case Study of the Shenhu Area, South China Sea

Geochemical profiles in pore water of marine sediments have been considered as, important indicators of gas hydrate occurrence. In the gas hydrate area around the world, the decrease of sulfate, calcium, and magnesium concentrations with depth mainly results directly or indirectly from the anaerobic oxidation of methane (AOM). The ubiquitous abnormally high concentration gradients of iodide in the research area reflect the large methane-generating potential of the area. Thus, we explore the feasibility of using gradients of sulfate, iodide, and authigenic carbonate precipitation as indicators for gas hydrate in the regional exploration of gas hydrate formation. We test the criterion in the gas hydrate zone in the South China Sea (SCS) for the recognition of gas hydrate by using the gradients of sulfate, calcium plus magnesium, and iodide. Contour maps of pore-water gradients from expeditions in the study area are used to correlate the key gradients to underlying gas hydrate occurrence. The results show that the largest potential gas hydrate indicated by the contour maps of the indicators are well consistent with the discovery of GMGS1 and GMGS3 expedition. It implies the possible applicability of this geochemical method in gas hydrate exploration. Also, we identify a promising area in the South China Sea for future gas hydrate investigations. It is the first collective application of the gradients of sulfate, calcium plus magnesium, and iodide to a gas hydrate terrane, especially to a large area of the SCS. We believe that the result of this research will benefit the future exploration of gas hydrate and will arouse a lot of interest from other researchers.

Petro-physics Analysis and Rock Physics Modeling for Estimation of Gas Hydrate Saturation: A Case Study in the Mahanadi Offshore Basin

Abstract Gas hydrate saturation (Sh) is the most challenging key parameter of petrophysical evaluations for reservoir characterization. In total three wells were used (namely, NGHP-01-19, NGHP-01-09, and NGHP-01-08) to compute the petrophysical parameters in the gas hydrate-bearing sediments of the Mahanadi offshore basin. Initially, effective porosity and volume of shale are computed using conventional log data varying from 11-36% and 55-75% respectively. The most prominent technique responding to the physical property has been used to compute the Sh by using Archie’s empirical electrical resistivity method. The presence of gas hydrate, free gas, and other lithology affects the sonic velocity which is widely used to quantify the hydrate saturation. Therefore,an attempt is made to quantify the Sh from the wellbore sonic velocity using a rock physics model at higher porosity (~62%) for unconsolidated marine sediments below the seafloor. The model is best suited for gas hydrate zones where velocity increases with the hydrate saturation but underestimates the saturation for free gas zones below the bottom simulating reflector. In the study area,Sh ranges from 5-13% in the depth interval of 175-200 mbsf for well NGHP-01-19 whereas, small amount of 3-10%and 2-8% for well NGHP-01-09 and NGHP-01-08 respectively. The Sh obtained from the rock physics model is insignificantly mismatched with the saturation obtained from temperature and resistivity data due to the presence of an isotropic layer with fracture filling sediments having anisotropic properties.

Preparation and investigation of microencapsulated thermal control material used for the cementing of gas hydrate formations

Deep-water cementing operations may encounter gas hydrate formations, and the cement slurry hydration exotherm can easily cause their decomposition and pose a threat to cementing operations. Therefore, the cement slurry with low hydration heat is needed for cementing in deep water gas hydrate zones. In this paper, a microencapsulated temperature control material (MEPCM) was synthesized. When MEPCM was incorporated into the oil well cement slurry, its temperature control effect on cement slurry was evaluated. Firstly, the chemical structure, microstructure, and phase transformation characteristics of MEPCM were researched by FTIR, SEM-EDS, DSC, etc. The results show that the thermal evolution of the cement slurry can be controlled by MEPCM. Secondly, MEPCM was introduced into oil well cement, and the temperature change of the cement slurry hydration process was detected by a self-made semi-insulating device to study the temperature control ability of MEPCM in cement slurry. The results show that the thermal development of the cement slurry is regulated by the MEPCM. At the same time, the effect and mechanism of MEPCM on the early mechanical performance of cement slurry were studied, and found that the right amount of MEPCM can promote the cement stone’s early strength development. As a result, a low hydration heat cement slurry system suitable for cementing deep-water gas hydrate formations was successfully prepared.

The in-situ NMR evidence of gas hydrate forming in micro-pores in the Shenhu area, South China Sea

Nuclear magnetic resonance (NMR) measurement could provide information during the formation and dissociation of gas hydrate on the pore structure properties which are of great important to understand the gas production behavior. In this study, the in–situ NMR and other well logging data achieved during the natural gas hydrate drilling program were used to study the pore size and hydrate formation properties in the Shenhu area, South China Sea by comparing transverse magnetic relaxation time (T2) distributions between the gas hydrate zone and the upper water–saturated zone whose properties represent the virgin status. Compared with those of the upper water–saturated layer, increasing resistivity, higher–than–baseline shear modulus, less–than–density–porosity NMR porosity, shortening T2 peak time, reducing amplitude and covered area of T2 distributions and decreasing water–filling pore size denote the association of hydrate. Based on eight bin porosities distribution derived from user-defined time intervals of the T2 distribution for researching the hydrate formation characteristic in different pore size, macro–pores (T2 ≥ 10 ms, rc≥ 0.9 μm), meso–pores (3 ms ≤ T2 < 10 ms, 0.27 μm ≤rc<0.9μm) and micro–pores (T2 < 3 ms, rc< 0.27 μm) were divided. It was found that the meso–pores are the dominant pore size in the Shenhu area, but hydrate form more easily in the macro–pores. Just as the previous experience, gas hydrate forms primarily in macro–pores and meso–pores resulting in an increase of micro–pores. In addition, another mode was achieved that gas hydrate forms in the three types of pore spaces leading to a decrease of micro–pores. Both forming modes can develop reservoirs with high hydrate saturation that can exceed 30%.

Identification of gas hydrate on bottom-simulating reflector characteristics with 2D seismic pre-stack time migration of North Bali Waters

Gas hydrate is a physical compound composed of gas molecules that are formed in a seabed layer characterised by high pressure and low temperature. It is known as one of the alternative non-conventional hydrocarbons besides petroleum and natural gas. One of the identified areas of gas hydrate stability zone is in the North Bali Waters. The North Bali Waters is part of the North East Java Basin, which has oil and gas exploration and production, both conventional and non-conventional. One method of identifying the content of gas hydrates is by looking at the appearance of the Bottom Simulating Reflector (BSR) as shown on the Pre-Stack Time Migrated seismic sections. The detection of gas hydrate zone is determined by the presence of high amplitude, reversed polarity reflection and cross-cut reflection of sedimentary layer. This study aims to determine the existence of a BSR in the waters of North Bali. The procedures for analysing the existence of Bottom Simulating Reflector in this study are pre-processing, processing, and interpretation of 2D marine seismic data. The result shows gas hydrates found with indicated Bottom Simulating Reflector on CDP 35-812 at TWT depth of 1526-1582 ms, characterised by high amplitude-reverse polarity.

Seismic response of gas-hydrate zones: a case study from the Oman Sea

An offshore vintage 2D seismic dataset acquired in the Oman Sea recently has been reprocessed to achieve two objectives: mapping gas-hydrate zones and structural imaging beneath the bottom-simulating reflector (BSR). We present results of the analysis of the seismic data associated with three seismic lines in the project area. The sea-bottom water-saturated sediment column (WSZ) has an average thickness of 100 m and an interval velocity range of 1550–1700 m/s. The underlying gas-hydrate stability zone (GHSZ) has an average thickness of 180 m and an interval velocity range of 2000–2200 m/s. The BSR with its low-frequency, large-amplitude character is very prominent in the seismic images derived from prestack time migration (PSTM). The BSR defines the base of the GHSZ and thus is a crucial indicator of the presence of a gas-hydrate zone. Velocities close to the top of the GHSZ interval are slightly lower at some locations along the line traverses – indicative of a transitional boundary between the WSZ and GHSZ. The overthrust tectonics during the Mid-Miocene to Pliocene caused by the counterclockwise rotation of the Arabian Plate is evident in the seismic images. The resulting thrust faults may facilitate upward migration of fluids and gas, thus forming the free-gas zone beneath the BSRs.

Multi-attribute machine learning analysis for weak BSR detection in the Pegasus Basin, Offshore New Zealand

Gas hydrates that exist in the subsurface are often difficult to detect with reflection seismic data if the seismic data lacks a strong bottom simulating reflection (BSR). In these cases, the imaging and detection of the gas hydrate stability zone (GHSZ) becomes particularly difficult, as hydrate detection relies heavily on the BSR, gas chimneys, or pockmarks on the seafloor. To address and improve upon these imaging complications, an unsupervised machine learning multi-attribute analysis is performed on 2D seismic data in the Pegasus Basin in New Zealand where the BSR is not continuously or clearly imaged. Rock physics analysis has demonstrated that the inclusion of methane gas hydrates in the pore space results in a slightly increasing amplitude at the base of the gas hydrate zone, regardless of the fluid (brine or gas) in the pore space below the hydrates. This increasing amplitude is quite weak and can be masked by noise. In the scenarios where a strong seismic impedance difference is lacking, a BSR is not typically observed in the seismic data, even though gas hydrates do exist in the subsurface. To enhance the detection of the presence of gas hydrates, a multi-attribute analysis is performed with a series of seismic attributes that can detect the minute changes in the seismic waveform due to the presence of gas hydrates. The successful attributes are those that are sensitive to attenuation, frequency, and small amplitude anomalies.

Microbial diversity in fracture and pore filling gas hydrate-bearing sediments at Site GMGS2-16 in the Pearl River Mouth Basin, the South China Sea

Gas hydrate systems are unique habitats for microorganisms in deep marine sediments, and gas hydrate reservoirs can be divided into focused high-flux and distributed low-flux gas hydrate systems. Little is known about the microbial distribution patterns in these systems, especially in the South China Sea (SCS). In this study, a macroscopic fracture filling gas hydrate zone (GHZ) and a microscopic pore filling GHZ, used as analogs to focused high-flux gas hydrate systems and distributed low-flux gas hydrate systems, were sampled and microbial diversity was investigated at Site GMGS2-16 in the northern SCS during the GMGS2 gas hydrate expedition based on high-throughput 16S rRNA gene sequencing. Compared to previous studies of the gas hydrate-bearing sites, high microbial diversity was observed in the sulfate methane transition zone (SMTZ) at Site GMGS2-16, and ANME-1b, a clade of anaerobic methanotrophic archaea (ANME), may mainly perform anaerobic oxidation of methane (AOM) in partnership with Desulfarculaceae and Desulfobacteraceae. For the GHZ (fracture-fill GHZ and pore-fill GHZ) and non-GHZ (SMTZ and Other, Other refers to the depth of 31.0-179.5mbsf), the bacterial richness indices (Shannon and inverse Simpson) revealed higher diversity in the GHZ than in the non-GHZ, while the archaeal richness indices showed a contrary pattern. Non-metric multidimensional scaling ordination revealed no significant differences in archaeal communities between the GHZ and non-GHZ, while differences between these zones were discovered in bacterial communities (P < .05), as the relative abundances of Bacilli and Clostridia (Phylum Firmicutes), Bacteroidia (Phylum Bacteroidetes), Alphaproteobacteria, Acidobacteria were significantly higher in the GHZ samples than in the non-GHZ samples. For the fracture-fill GHZ and pore-fill GHZ, the archaeal richness indices showed a higher diversity in the fracture-fill GHZ than in the pore-fill GHZ. High relative abundance of Hadesarchaea were found in the GHZ, especially in the pore-fill GHZ (accounting for 92.0–94.3% of the archaeal sequences), which might indicate Hadesarchaea play a vital role in the biogeochemical processes. For bacteria, the diversity indices were similar between the fracture-fill GHZ and the pore-fill GHZ, and no significant differences in bacterial communities were discovered between the two zones. Our study has provided new information for understanding the microbial diversity of the gas hydrate systems.

A rock magnetic perspective of gas hydrate occurrences in a high-energy depositional system in the Krishna-Godavari basin, Bay of Bengal

We conducted a detailed rock magnetic study complemented by sedimentological and mineralogical methods on a 177.2-m-long sediment core of Hole NGHP-01-14A to constrain the influence of high-energy depositional environment on the magnetic mineral diagenesis and formation of gas hydrates in the Krishna-Godavari (K-G) basin, Bay of Bengal. Five sediment magnetic zones were identified based on the downcore variation in rock magnetic parameters. The magnetic mineralogies comprised of detrital titanomagnetite, diagenetically formed magnetic iron sulfides and their mixture. A distinct magnetically enhanced zone (Z-3) close to a bottom simulating reflector (BSR) is dominated by higher magnetite content, sediment bulk density, and grain size and probably represents several sand-rich sediment layers formed as a result of short-duration intense sedimentation events and got rapidly buried and provided conducive environment for the formation of gas hydrate deposits at the studied site. The concurrence of gas hydrates and high gas saturations within the sand-rich layers (Z-3) suggests that the high-energy depositional environment had a larger control over hydrate formation at Hole NGHP-01-14A. Two magnetic iron sulfide bearing sediment intervals in Z-1b and Z-1c below sulfate-methane transition zone (SMTZ) suggest that their formation is controlled by microbially mediated diagenetic reactions fuelled by presence of gas hydrates and probably represents the fossil gas hydrate zones. We propose that the rock magnetic variations in the studied sediment core at Hole NGHP-01-14A is controlled by both differential loading of detrital magnetic minerals as well as hydrate-induced late diagenetic magnetic iron sulfide formation.

Unique Authigenic Mineral Assemblages and Planktonic Foraminifera Reveal Dynamic Cold Seepage in the Southern South China Sea

Many cold seeps and gas hydrate areas have not been discovered beside the Beikan basin in the southern South China Sea (SCS), and their characteristics and histories also remain poorly known. Here we describe authigenic minerals and the carbon and oxygen isotopic composition of planktonic foraminifera Globigerinoides ruber from sediment core 2PC, recovered from the gas hydrate zone of the Nansha Trough, southern SCS, to elucidate its history of dynamic cold seepage. We infer that the occurrence of authigenic gypsum crystals and pyrite concretions, and anomalously negative δ13C values of Globigerinoides ruber, reflect paleo-methane seepage. Two major methane release events were identified, based on remarkable excursions in foraminifera δ13C at depths of 150–250 cm and 350–370 cm. Euhedral gypsum crystals and tubular pyrite concretions co-occur with extremely negative planktonic foraminifera δ13C values, indicating a shift in the sulfate methane transition zone and a change in the methane flux. Our data suggest that authigenic mineral assemblages and δ13C values of planktonic foraminifera provide a valuable tool in elucidating the characteristics of dynamic methane seepage in a marine environment.

Characteristics of gas hydrate reservoir from resistivity image in the Shenhu area, China

In 2016, logging-while-drilling (LWD) was conducted at 11 sites in the Shenhu area of the South China Sea to characterise the gas hydrate reservoir. The authors used LWD resistivity image from these sites to analyse the hydrate occurrence mode, near-well structure, sedimentary structure, fracture and in situ stress orientation of the gas hydrate reservoir. The analysis revealed the following. (a) Three categories of gas hydrate occurrence mode were identified: thick-bedded, dispersed and thin-bedded. (b) The dip angle of the regional stratigraphic structure is concentrated between 2 and 4°, and the dip direction is mainly south-west. (c) The typical sedimentary structure is deformed bedding and primarily developed in the gas hydrate intervals. It is inferred that the deformed bedding is affected by gas hydrate formation and the drilling process. (d) Based on the borehole breakout, the minimum horizontal principal stress direction was determined to be north-east–south-west, which provides valuable information for the design of the gas hydrate production. (e) Two categories of fracture were identified: conductive fracture and resistive fracture. Conductive fracture developed in the gas hydrate zone and contributed to higher gas hydrate saturation. High-resolution resistivity images provide extensive visual geological information and are particularly important in future exploration plans.

Research Progress of Seafloor Pockmarks in Spatio-Temporal Distribution and Classification

Seafloor pockmarks are important indicators of submarine methane seepages and slope instabilities. In order to promote the understanding of submarine pockmarks and their relationship with sediment instabilities and climate changes, here we summarize the research results of pockmarks in the spatio-temporal distributions and shaping factors. Most of pockmarks occur along active or passive continental margins during the last 25 kyr B.P.. Circular and ellipse are the most common forms of pockmarks, whereas pockmarks in a special crescent or elongated shape are indicators of slope instabilities, and ring-shape pockmarks are endemic to the gas hydrate zones. Further researches should be focused on the trigger mechanism of climate changes based on the pockmarks in the high latitudes formed during the deglaciation periods, and the role of gas hydrates in the seafloor evolution should be elucidated. In addition, the feature of pockmarks at their early stage (e.g., developing gas chimneys and gas driving sedimentary doming) and the relations between pockmarks and mass movements, mud diapirs could be further studied to clarify the influences of rapid methane release from submarine sediments on the atmospheric carbon contents.

Experimental Research on Seafloor Mapping and Vertical Deformation Monitoring for Gas Hydrate Zone Using Nine-Axis MEMS Sensor Tapes

Seafloor mapping and vertical deformation monitoring are important for a gas hydrate exploitation. Due to the complexity of the marine environment and the difficulty of the seafloor operation, an in situ seafloor mapping and high-resolution vertical deformation monitoring simultaneously are big challenges. This paper describes an experimental research on an in situ system for a seafloor mapping and vertical deformation monitoring based on an elastic steel tape, along which an array of nine-axis microelectromechanical system (MEMS) sensors is arranged. The tape can measure both the motion of the bend and the twist. The MEMS sensor consists of an accelerometer, a gyroscope, and a magnetometer. The Euler angles obtained from the nine-axis MEMS sensor can be converted to a relative rotation matrix, showing the amount of the bend and the twist, and the curved shape of the tape can be calculated. Based on this concept, multitapes are used for the seafloor mapping and vertical deformation monitoring. To assess the monitoring system, a lab scale experiment in a water tank is carried out. The results demonstrate the feasibility of the proposed system for the seafloor mapping and deformation monitoring.

ИССЛЕДОВАНИЕ РАСПРЕДЕЛЕНИЯ ВЫХОДОВ УГЛЕВОДОРОДНЫХ ГАЗОВ В ЗАВИСИМОСТИ ОТ ГЕОЛОГИЧЕСКОГО СТРОЕНИЯ ЮЖНОГО КАСПИЯ ПО ДАННЫМ ГАЗОВОЙ СЪЕМКИ

Актуальность исследования заключается в изучении газов верхней части разреза Южного Каспия, в особенности ее глубоководной части, представляющей значимый практический интерес при поисках и оценках углеводородного потенциала зон газогидратов. Анализ ранее проведенных работ в пределах Южного Каспия показал, что все исследования в основном сконцентрированы в пределах разбуренной зоны Абшеронского, Бакинского и Нижнекуринского нефтегазоносных районов и не охватывают глубоководную зону Южного Каспия. Цель исследования заключается в изучении распределения интенсивности проявления углеводородных газов по площади, выявлении зон газогенерации, а также проведении работ по определению и анализу путей миграции углеводородов и типов формирования газогидратов в пределах глубоководной зоны Южного Каспия. Объектом исследования послужили 1282 пробы газа, отобранные в Южном Каспии на месторождениях Шах-дениз, Бахар, Пираллахи, Хали, в глубоководной части Южного Каспия и др., а также данные геолого-геофизических исследований. Использованы данные изучения химического и изотопного состава углеводородных газов Южно-Каспийской впадины по 147 пробам газа, отобранным из нефтегазовых месторождений, 153 пробам из грязевых вулканов, 29 пробам донных осадков и 4 пробам газогидратов. Эти данные позволили более обосновано подойти к анализу генетических вопросов происхождения газов в верхней части разреза. Методы исследования основаны на интерпретации данных газовой съемки и изучении изотопного и химического состава углеводородных газов Южного Каспия. Наличие точных значений координат, а также данных по структурным картам в координатной сетке позволили сопоставить имеющиеся данные с геологическим строением. В результате исследования построены карты изменения содержания метана, этана, пропана, бутана, изо-, нормального пентана в пределах верхней части разреза по изучаемым месторождениям. Сопоставлены данные результатов анализа газовой съемки с данными структурных карт и других геолого-геофизических материалов. Установлено, что структура Бахар характеризуется относительно пониженной интенсивностью газопровялений по метану от 5,37•10–4 до 152944•10–4 % (в среднем 9109,0354•10–4 %), в то время как месторождение Шах-дениз характеризуется максимальными значениями от 4,6200•10–4 до 367840•10–4 %, в среднем составляя 85572,3•10–4 %. Небольшие значения интенсивности проявления газа, установленные в зоне структур Пираллахи, Хали, свидетельствуют о незначительном газовом потенциале глубокопогруженных отложений изучаемой зоны. Детально рассмотрены результаты газовой съемки в пределах глубоководной части Южного Каспия и установлено, что для этой зоны характерна газогенерация с преобладанием двух компонентов – метана и этана. Исходя из анализа изменения интенсивности проявления газа по площади и в зависимости от геологических условий установлено, что грязевые вулканы и разломы являются прекрасными путями для миграции углеводородов и отражают геологическую обстановку, генерационный потенциал погруженных отложений Южного Каспия. Установлено, что углеводородные газы в донных осадках и отложениях верхней части разреза Южной части Каспийского моря находятся в тесной зависимости от источников формирования углеводородов, миграции и других процессов, протекающих в глубокопогруженных отложениях, а также в верхней части разреза. Изучены данные химического и изотопного состава газов газогидратов, нефтяных и газовых месторождений Южного Каспия. На базе данных изотопно-геохимических исследований описаны механизмы образования гигантских месторождений нефти Балаханы-Сабунчи-Романы и газоконденсата Шах-дениз, расположенных на одной антиклинальной зоне Абшеронского нефтегазоносного района, а также аномалии, выявленные по данным газовой съемки в пределах изучаемых структур Южного Каспия.

Microbial diversity of two cold seep systems in gas hydrate-bearing sediments in the South China Sea

Cold seep is a unique habitat for microorganisms in deep marine sediments, and microbial communities and biogeochemical processes are still poorly understood, especially in relation to hydrate-bearing geo-systems. In this study, two cold seep systems were sampled and microbial diversity was studied at Site GMGS2-08 in the northern part of the South China Sea (SCS) during the GMGS2 gas hydrate expedition. The current cold seep system was composed of a sulfate methane transition zone (SMTZ) and an upper gas hydrate zone (UGHZ). The buried cold seep system was composed of an authigenic carbonate zone (ACZ) and a lower gas hydrate zone (LGHZ). These drill core samples provided an excellent opportunity for analyzing the microbial abundance and diversity based on quantitative polymerase chain reaction (qPCR) and high-throughput 16S rRNA gene sequencing. Compared to previous studies, the high relative abundance of ANME-1b, a clade of anaerobic methanotrophic archaea (ANME), may perform anaerobic oxidation of methane (AOM) in collaboration with ANME-2c and Desulfobacteraceae in the SMTZ, and the high relative abundances of Hadesarchaea, ANME-1b archaea and Aerophobetes bacteria were found in the gas hydrate zone (GHZ) at Site GMGS2-08. ANME-1b, detected in the GHZ, might mainly mediate the AOM process, and the process might occur in a wide depth range within the LGHZ. Moreover, bacterial communities were significantly different between the GHZ and non-GHZ sediments. In the ACZ, archaeal communities were different between the two samples from the upper and the lower layers, while bacterial communities shared similarities. Overall, this new record of cold seep microbial diversity at Site GMGS2-08 showed the complexity of the interaction between biogeochemical reactions and environmental conditions.

ГАЗОГИДРАТЫ АЗЕРБАЙДЖАНСКОГО СЕКТОРА ЮЖНОГО КАСПИЯ: ТЕРМОДИНАМИКА, СЕЙСМОАКУСТИКА И ГАЗОПРОЯВЛЕНИЯ

Актуальность. Геологическое строение и батиметрия морского дна глубоководной части Каспийского моря указывают на возможность образования значительных объемов газогидратов. Анализ опубликованных и неопубликованных материалов по газогидратам Каспийского моря показал, что в основном все работы базируются лишь на изучении скопления газогидратов Элм, Боздаг, площади Абшерон, а также термодинамических факторах региона (без учета реальных значений температуры и давления) с целью определения возможной зоны их скопления. Цель исследования заключается в подсчете объема углеводородных газов газогидратов по термодинамическим данным, а также в изучении и прогнозировании их поступления в верхнюю часть разреза. Объектом исследования являются отложения верхней части разреза. Несмотря на большой фактический материал, имеющийся в различных организациях, верхняя часть разреза в связи с оценкой газоносности является наименее изученной и описанной в литературе зоной. Это связано с тем, что основной интервал исследований в Южном Каспии сосредоточен в участках с высоким потенциалом нефти и газа. Этим интервалом является продуктивная толща и подстилающие отложения, вошедшие в зону интенсивной генерации нефти и газа. Более молодые отложения, накопившиеся до абшеронского региояруса, в подавляющем случае состоящие из глинистых осадков, не представляли интереса. Во многих работах эта зона описывается как нерасчлененная толща четвертичных отложений. Методы исследования основаны на моделировании термодинамического равновесия углеводородных газов с целью установления зоны формирования газогидратов. В работе использованы данные температур и давления, замеренные в скважинах Южного Каспия, а также данные 160 сейсмоакустических профилей, разрезы интервальных скоростей и данные по нефтегазопроявлениям в пределах шельфовой зоны Южного Каспия. В результате исследования термодинамического равновесия рассчитаны объемы зон газогидратов. Исходя из расчетов установлено, что суммарные объемы газа для изучаемой площади отложений составляют 1,151013 м3газа. В результате интерпретации сейсмоакустических данных установлено, что интенсивность проявлений газа изменяется как по площади, так и с глубиной. В зонах, прилегающих к глубоководной части Южного Каспия, присутствие аномалий, характерных для газовых зон, увеличивается по сравнению с прибортовыми зонами. В результате анализа данных по газопроявлениям можно прийти к выводу, что верхняя часть разреза Южного Каспия сильно обогащена газом, причем интенсивность газопроявлений в нижней части наиболее высокая. Нефтепроявления на структурах Хали, Чилов адасы, Палчыг Пильпиля, Чираг указывают на то, что эти структуры более благоприятны для поисков нефтяных залежей. Пластовая вода зафиксирована в широком интервале глубин. Полученные выводы дают основание прогнозировать поступление значительных объемов флюидов в верхнюю часть разреза глубоководной части Южного Каспия.