Diagenesis Processes Research Articles

Reservoir characteristics and controlling factors of Silurian Lower Kepingtage Formation in Tahe area, Tarim Basin, NW China

With the breakthrough of exploration in Well TP16-1, the lower Kepingtage Formation becomes a key target for petroleum exploration of deep clastic reservoir in Tahe area. In this paper we focused on the research of the reservoir characteristics and its controlling factors in two sub-member formations (S1k 1 1 and S1k 1 3 ). Based on X-ray diffraction, conventional physical properties data (porosity and permeability) and reservoir storage space data (casting thin section and scanning electron microscope), we determined that the S1k1 Formation belongs to extra-low porosity and permeability reservoir, although the upper S1k 1 3 Formation shows relative better physical characteristic than the lower S1k 1 1 Formation. The development of storage space in the study area is controlled by sedimentary microfacies, diagenesis process. Reservoirs in S1k1 Formation are mainly located in channel (S1k 1 1 sandstones) and sand flat (S1k 1 3 sandstones). The sand flat sediments with a more coarse grain size compared with the channel. In diagenesis, compaction is the major controlling factor for reducing the porosity, followed by cementation. Dissolution of diagenesis is the major controlling factor in enhancing the reservoir porosities. Compared with channel (S1k 1 1 ) sandstones, sand flat sandstones (S1k 1 3 ) have better reservoir quality for its weaker compaction, cementation and stronger dissolution. On the basis of sedimentary characteristics (grain size and subfacies), physical property (porosity and permeability) and reservoir storage space, we divide the S1k1 reservoir into three categories (I, II and III). Type I reservoir is high quality reservoir. It is mainly distributed in the south area of S1k 1 1 and S1k 1 3 reservoir. Type II is moderate reservoir. It is located in the middle of S1k 1 1 reservoir and in the north of S1k 1 3 reservoir. Type III is the poor reservoir. It is only located in the north of S1k 1 1 reservoir.

Distribution of n-alkanes in the ferromanganese nodule–sediment–pore water system (Clarion–Clipperton Fracture Zone)

The paper presents the first results of a comparative study of the composition and distribution of organic matter (OM) (Corg, n-alkanes, Pr, Phy) in samples of the ferromanganese nodule–sediment–pore water system taken at a single site in the Clarion–Clipperton Fracture Zone, Pacific Ocean. Samples were taken during Cruise 120 of R/V James Cook in 2015. The results of the comparison of organogeochemical parameters in the lipid fraction of OM in ferromanganese nodules of different size and morphology with the underlying sediment (0–1 cm) and pore water revealed both principle genetic affinity of OM therein and significant discrepancies between the processes of diagenesis during the formation and growth of each individual ore sample.

Aliphatic Biomakerof Crude Oil from Juwata, Tarakan, North Kalimantan

Characteristic organic geochemistry of crude oil Juwata, Tarakan, North Kalimantan provide information about the depositional sedimental environment and maturity of the oil. Crude oil was extracted with n -hexane (50: 2) for 24 hours to separate the crude oil with asphaltene, and if precipitation does not separate perfectly then centrifuged. Extracts were then refractionated using column chromatography according to the methods McCharty base. Fractionation results are obtained in the form of the neutral fraction, the acid fraction and polar fraction. Neutral fraction separated with KLTP method and aliphatic fraction and aromatic fractions. Aliphatic fraction subsequently analyzed by GC-MS. GC-MS analysis aliphatic fraction of n-alkanes in the form of compounds, and hopane, bikadinancompounds. The pattern of distribution of n -alkanes which consists of a range of C- 17 to C- 33 . Carbon distribution with a range of C- 21 to C- 33 is dominating compared to the C- 17 to C- 20 . This distribution pattern indicates the source of organic matter from terrestrial land wax component derived from higher plants. Hopane presence of compounds in crude oil aliphatic fractionsJuwata Tarakan, North Kalimantan analyzed by fragmentogram m/z = 191. Hopane biomarker provide that information during the sedimentation stage diagenesis these compounds are degraded and stereochemical changes towards a stable isomer form, configuration changes 17α (H), 21β (H) during the process of diagenesis to a stable isomer which is 17 α (H), 21β (H) or 21β (H), 17 α (H) indicates that oil comes from the old sediments. Biomarka bikadina identified according fragmentogram m/z = 191 multiple ion fragment m/z = 109.191, 369, 397 and ion molecular m/z = 412 showed the characteristic fragment bikadinna compound. Biomarka bikadina give information influx of organic material terrestrial region. The compounds showed crude oils biomarker Juwata Tarakan, North Kalimantan derived from terrestrial depositional environments.

Predicting the effects of sediment based internal nutrient loads on eutrophication in Küçükçekmece Lagoon for rehabilitation planning

In deep stratified coastal lagoons, hypoxic waters that result from phytoplankton decomposition in the stratified bottom waters are often associated with eutrophication. Decomposing biomass reaches the bottom sediments and enriches them with nutrients and organic matter. Nutrients trapped in sediments are released with time and promote excessive phytoplankton growth in the surface water. Because eutrophication in lentic ecosystems progresses in a self-fuelling cycle, outflow is the only available process for exporting excess nutrients to recover from eutrophication. Thus, rehabilitation of eutrophic coastal lagoons that have limited seawater interactions is a long term process. The importance of nutrient release from sediments on eutrophication and the delay effect of internal nutrient loading on the rehabilitation of a eutrophic coastal lagoon with limited seawater exchange were analysed in this study. An ecological model that couples the water column and the sediment diagenesis processes, was developed for water quality management purposes. Our findings indicate that the recovery of the Lagoon from eutrophication will be taken decades even in the absence of external nutrient loading. Therefore, we suggest applying rehabilitation strategies that control the nutrient fluxes from sediments for a faster recovery from heavily eutrophic conditions. Land-based nutrient sources must also be controlled because they feed water column and the bottom sediments with nutrients.

Lower Paleozoic reservoir zonation into different flow units using turbulence factor and their relations to diagenesis

Surface Lower Paleozoic sandstone samples were collected from southeast central Sinai. These samples subjected to petrophysical measurements which are porosity and permeability. It is known that turbulence factor (β) is a good tool to describe turbulent flow level, hence it was used to detect the flow properties distribution that corresponds to the nature of the pore system, so turbulence factor (β) was correlated with porosity, permeability, and reservoir quality index using many equations, where porosity correlation shows the presence of three groups having same porosity ranges but correspond to different values of turbulence factor, this refers to the presence of different flow zones. Porosity–permeability relation confirmed also the presence of similar three groups, where there are nearly same porosity ranges but with different permeability values. Consequently, samples with nearly same porosity values and representing the three groups were selected for capillary pressure by mercury injection to reveal the flow properties that characterize each group. Capillary pressure results confirmed that each group has their own pore system and flow regime. Petrography was done by optical polarizing microscope (OPM) through thin sections. The results indicated that the studied sandstone is composed mainly of quartz arenite microfacies of fine, angular to subrounded, moderately well sorted monocrystalline quartz grains. There are several features indicating that the investigated sandstones have been subjected to diagenesis processes. Diagenetic events identified in these sandstones include considerable compaction during burial diagenesis at higher temperatures and low flow rates, cementation by clay minerals and iron oxides, dissolution and alteration of unstable clastic grains, and tectonically induced grain fracturing. Unstable clastic grains like feldspars suffered considerable alteration to kaolinite when exposed to meteoric water of low ionic strength near the surface. The distribution of this kaolinite rather than the post-depositional iron oxides in pores of the studied sandstone leaded to differentiating these sandstones into three different flow units. This zonation indicates that the turbulence properties are proportional to the diagenetic effects inside the pores.

Insights into post-magmatic metasomatism and Li circulation in granitic systems from phosphate minerals of the Nanping No. 31 pegmatite (SE China)

Phosphate minerals are key indicators for understanding the processes of diagenesis and mineralization in granitic systems. More importantly, these minerals provide constraints on the post-magmatic behaviour of Li in pegmatite systems that remain to be explored, although Li from early-crystallized Li phosphates is known to leach into hydrothermal fluids. In this study, we performed a detailed petrographic and compositional analysis of phosphate minerals from the Nanping No. 31 pegmatite in southeastern of China, which provides new evidence of Li circulation in granitic systems. Primary phosphate minerals evolved from xenotime and monazite to Mn-rich fluorapatite and then to triphylite and montebrasite, with large amounts of montebrasite in the intermediate zones of the pegmatite, reflecting the increasing activities of both Li and P in peraluminous granitic melts. Montebrasite experienced stages of both high-temperature (360–273°C) and low-temperature (273–100°C) hydrothermal alteration. The high-temperature hydrothermal alteration of montebrasite by Fe- and Mg-rich fluids resulted in the formation of a series of Fe-, Mg-bearing phosphates; the replacement of montebrasite under low-temperature alteration generated amounts of Ca-, Sr-, Ba-bearing phosphate minerals, muscovite and akdalaite (Al2O3)4·H2O. The formation of micro-networks of akdalaite from montebrasite indicate the low mobility of Al in hydrothermal fluids. A four-stage scenario of post-magmatic Li transport in the Nanping No. 31 pegmatite is proposed: (1) Li derived from the breakdown of primary montebrasite was locally recrystallized and produced secondary montebrasite (Mtb-1); (2) hydrothermal alteration by Fe- and Mg-bearing fluids leached Li from montebrasite to form secondary triphylite and simferite; (3) Li derived from the alteration of montebrasite by Sr-, Ca-rich fluids was involved in the formation of secondary palermoite and bertossaite; and (4) the replacement of secondary triphylite, montebrasite, palermoite and bertossaite by later phases (apatites, ludlamite, anapaite, augelite and fine-grained muscovite) reflect the leaching of Li back into hydrothermal fluids. The re-enrichment of Li during the post-magmatic stage most likely increased the solubility of Ta in the hydrothermal fluids of the granitic pegmatite.

Phase transformation processes in karst-type bauxite deposit from Yunnan area, China

Bauxite is the only resource of aluminum metal and forms through weathering, leaching, and deposition (i.e., bauxitization processes). Here we characterize the bauxite deposit in Yunnan area, China, through mineralogical and textural characteristics using XRD, SEM-EDX, and micro-Raman analyses. The Al-oxyhydroxide polymorphs (AlO(OH)) such as diaspore and boehmite are the dominant phases in most of the samples. Gibbsite (Al(OH)3) is the main mineral in two samples from Quaternary bauxite, and these three Al-rich minerals mixed with goethite, hematite, kaolinite, and small amounts of TiO2 polymorphs (anatase and rutile). Considering the textural characteristics and the stratigraphy of the bauxite deposit, some of the Al-oxyhydroxides and Ti-oxides are inferred to be products of the bauxitization in the lateritic soil during late Permian weathering. In the subsequent transgression stage, the earlier formed (Al-rich minerals and anatase) phases were transformed to diaspore or boehmite as spherulitic form and to rutile, respectively, with the processes of diagenesis, burial metamorphism, tectonic movements and magmatic activity. Following crustal uplift in the Yunnan Province after the late Triassic, diaspore was transformed into boehmite and the bauxite deposit formed earlier was exposed again to weathering resulting in the formation of gibbsite, hematite, goethite, kaolinite, and anatase, with diaspore, boehmite, and rutile remaining as relics. Our study provides insights into phase transformation during bauxitization and subsequent processes.

Numerical modeling of early diagenetic processes in Haiyang 4 Area in the northern slope of the South China Sea

Early diagenesis affects the distribution of solutes and minerals in unconsolidated sediments. The investigation of diagenesis is critical to understanding the geochemical transformation and benthic fluxes of elements. During the cruise mission SO-177 in 2004, gravity coring samples were recovered in the Haiyang 4 Area of the northern slope of the South China Sea (SCS). The geochemical concentrations in interstitial water were determined onboard. The 1D C.CANDI reactive transport software was used to model the early diagenesis processes at four sites: 56-GC-3, 70-GC-9, 94-GC-11, and 118-GC-13. All of the simulations reproduced concentration profiles that matched the measurements with the implemented geochemical reactions. The degradation of organic carbon and anaerobic oxidation of methane (AOM) primarily determine the distribution of solutes in the working area. The degradation is active in the top 150 cm, and AOM is vigorous at depths below 200 cm. The local advective flux, sediment rate, and kinetic reaction constants of organic matter, methane and sulfate were calibrated based on the existing concentrations of pore water solutes. Geochemical reactions in this area occur in considerably deeper layers compared to depths cited in the literature. The model results provide evidence for the existence of deep hydrocarbon reservoirs that provide methane to the upper sediments.

Soil characteristics and new formation model of loess on the Chinese Loess Plateau

In this paper characteristics of loess are discussed from a pedological perspective. A new model of loess formation is presented, developed in different soil formation processes according to its pedogenic characteristics, including soil structure, organic matter content and CaCO3 content. Loess has all the characteristics of typical soils and all five soil-forming factors have important roles in loess formation. In this regard, loess is a type soil and the previous concept of loess actually comprises various kinds of palaeosols developed in cold and arid climatic conditions. The loess-forming process is, in fact, equivalent to a pedogenic process and loess layers with different characteristics represent different pedogenic processes indicative of different environments, such as desert-steppe, steppe or forest-steppe climatic conditions. In contrast to red-brown palaeosol layers, which develop in warm and wet climates, loesses–more accurately called ‘loessial palaeosols’–are grey-yellow palaeosols and can be regarded as a reliable indicator of cold and arid climate. The model of loess formation suggests that aeolian dust transfers to grey-yellow palaeosols via pedogenesis and the characteristics of loess are mainly a result of climatic conditions and diagenesis processes after its sedimentation. This new model of loess formation also suggests that traditional pedological theory has its limitations when explaining soil formation processes.

Effects of complexation between organic matter (OM) and clay mineral on OM pyrolysis

The stability and persistence of organic matter (OM) in source rocks are of great significance for hydrocarbon generation and the global carbon cycle. Clay-OM associations commonly occur in sedimentation and diagenesis processes and can influence the pyrolytic behaviors of OM. In this study, clay-OM complexes, i.e., interlayer clay-OM complexes and clay-OM mixture, were prepared and exposed to high-pressure pyrolysis conditions in confined gold capsule reactors to assess variations in OM pyrolysis products in the presence of clay minerals. Three model organic compounds, octadecanoic acid (OA), octadecy trimethyl ammonium bromide (OTAB), and octadecylamine (ODA), were employed and montmorillonite (Mt) was selected as the representative clay mineral. The solid acidity of Mt plays a key role in affecting the amount and composition of the pyrolysis gases generated by the clay-OM complexes. The Brønsted acid sites significantly promote the cracking of hydrocarbons through a carbocation mechanism and the isomerization of normal hydrocarbons. The Lewis acid sites are primarily involved in the decarboxylation reaction during pyrolysis and are responsible for CO2 generation. Mt exhibits either a catalysis effect or pyrolysis-inhibiting during pyrolysis of a given OM depending on the nature of the model organic compound and the nature of the clay-OM complexation. The amounts of C1–5 hydrocarbons and CO2 that are released from the Mt-OA and Mt-ODA complexes were higher than those of the parent OA and ODA, respectively, indicating a catalysis effect of Mt. In contrast, the amount of C1–5 hydrocarbons produced from the pyrolysis of Mt-OTAB complexes was lower than that of OTAB, which we attribute to an inhibiting effect of Mt. This pyrolysis-inhibiting effect works through the Hoffmann elimination that is promoted by the catalysis of the Brønsted acid sites of Mt, therefore releasing smaller amounts of gas hydrocarbons than the nucleophilic reaction that is induced by the halide ions in OTAB. In particular, the interlayer space of Mt acts as an ‘amplifier’ that magnifies the above-mentioned catalysis or pyrolysis-inhibiting effect, due to the greater number of Brønsted acid sites with high acidity in the interlayer space. These findings are potentially important for understanding the storage and transfer mechanisms of natural OM in sedimentation and diagenesis processes.

Diagenesis and its controls on reservoir properties and hydrocarbon potential in tight sandstone: a case study from the Upper Triassic Chang 7 oil group of Yanchang Formation, Ordos Basin, China

The sandstones in the Chang 7 oil group are typically recognized as unconventional tight clastic rock reservoirs. These sandstones are fine-grained and have low permeability and porosity and low textural and compositional maturity. In the present study, optical microscopy, SEM, cathodoluminescence, and wireline log, core analysis permeability and porosity, and production test data are applied to analyze the diagenesis and its controls on hydrocarbon potential in these tight sandstones. The results indicate that the sedimentary environment and diagenesis controlled the physical properties of the reservoir. Three different diagenesis processes were likely occurred in these sandstones. Early strong carbonate cementation is a key factor for other subsequent alterations and the final reservoir properties. The sandstones dominated by dissolution and compaction have more capacity for hydrocarbon preservation.

Prediction of diagenesis and reservoir quality using wireline logs: evidence from the Upper Triassic (Raethian) fluvial reservoir tags - Toual field, Gassi Touil Area, SE Algeria

Reservoir quality is mainly controlled by environment deposit type and diagenesis processes. To investigate such subject we usually proceed to microscopic techniques. Absence of outcrops and missing of core samples let us use conventional wireline logs and core lab measurements as primary data. Direct lecture of well logs calibrated by core photos and wellsite description, well logs correlation and cross plots were our main methodology axis. Target of this study is the fluvial Shaly Sandstones of Upper Triassic (Rhaetian) reservoir (abreviated in French; TAGS) located in Toual field – SE of Algeria. The present investigation confirms the absence of clean sandstone levels in the studied reservoir. It detects also the presence of halite and clays (Illite) as cements in sandstone. The non-influence of illite on permeability was confirmed whereas porosity is less affected.

Introduction to the carbonate microfacies of the Farasan Islands, southern Red Sea

Microfacies analysis of the Farasan Islands sediments was carried out to unravel the depositional environment and evolution of the area. Thin section petrographic studies were carried out to expand on the field identifications, to study the composition and to classify the Farasan Islands rock units. Sedimentological characteristics and depositional settings were also established. Several fossil assemblages have been identified, including Forams, Algae, Corals, Mollusks, Ostrocods, and Ooids. The most common diagenesis processes observed were dissolution and replacement by gypsum and anhydrite. The carbonates of Farasan Islands show normal shallow marine processes. Shale and anhydrite sequences were deposited most likely in subtidal to open marine environments. Progressive shallowing of the depositional environment is illustrated by the evolution of sedimentary facies in the Farasan Islands from low energy subtidal to open marine and finally to high energy at the top of the sequence. Field observation of palaeoshorelines may reflect the control of sea-level changes on reef formations during the Plio-Pleistocene time in the Red Sea.

Karstifikasi dan Pola Struktur Kuarter Berdasarkan Pemetaan Lapangan dan Citra SRTM Pada Formasi Wapulaka, Pasar Wajo, Buton, Sulawesi Tenggara.

Karstification process occurs as a result of dissolution on limestone lithology formed in typical morphology of caves, rounded or elongated pits and conical hills. This also related with structural pattern that also intesively occurs at western part of Buton Island. Research area located at lattitude 5027’0” S – 5032’0’’ S and longitude 122049’30” E - 122052’30” E. Purpose of this research was to identified karst landscape and karstificatiom process on Wapulaka Formation and recognize the structural pattern on this formation. By using field observation and SRTM data interpretation we considered that karst formations formed on Wapulaka Formation was Quarter limestone which diagenesis process of dissolution forming karst landform also related with uplifted event at pliocene – pleistocene. This formed related with complex interaction between the geology, climate, hydrology and biological factors that created various landscape morphology such as caves, underground river, stalagtite, stalagmite and others cave ornament.

Early diagenesis and authigenic mineral formation in mobile muds of the Changjiang Estuary and adjacent shelf

Abstract Large-river delta-front estuaries (LDEs) and their adjacent shelf margins are sites of dynamic diagenetic processes that play a significant role in coastal biogeochemical cycling. In this study, we used dissolved inorganic carbon (DIC), redox sensitive elements (Fe 2 + and Mn 2 + ), dissolved inorganic nitrogen (DIN) nutrients (NH 4 + , NO 3 − , and NO 2 − ), major cations and anions (K + , Ca 2 + , Mg 2 + , SO 4 2 − , and Cl − ) in bottom-water and sediment pore-waters, to investigate the early chemical diagenesis and authigenic mineral formation in mobile-mud deposits of the Changjiang Estuary and adjacent inner shelf of the East China Sea (ECS). Vertical profiles of DIC and NH 4 + in pore-waters had similar trends at most sites, showing a significant increase with depth near the Changjiang Estuary and being relatively constant at offshore sites. Higher pore-water DIC and NH 4 + concentrations were observed in nearshore sites in winter, which were likely attributed to exposure of deeper deposits by winter coastal erosion. Nitrification was observed at most sites, and AOB (ammonia-oxidizing bacteria) played a leading role in ammonia oxidation in the study areas. The nitrification-denitrification was likely important in contributing to the loss of DIN in offshore sites during summer. Large inputs of organic carbon (OC) and terrestrial materials from Changjiang River resulted in intense sulfate reduction and Fe and Mn reduction in nearshore sites. Lower C/N and C/S ratios coupled with an apparent decrease in pore-water Ca 2 + and Mg 2+ concentrations with depth near the Changjiang Estuary, which indicated that authigenic carbonate formation occurs in these sediments. Decreases in K + and Mg 2 + with depth reflected that reverse weathering was an important process of authigenic mineral formation in these sediments. We conclude that adsorption process, seasonal erosion-redeposition, and summer hypoxic conditions of bottom-waters may play an important role in early diagenesis processes and remineralization of SOC in the Changjiang LDE.

Limestone Facies And Diagenesis On Tondo Formation At Kaisabu Village Bau-Bau City Southeast Sulawesi Province

This study aims to determine the limestone facies and diagenesis on Tondo Formation. The method used was petrographic method bypolarizing microscope. Based on the physical characteristics and biota contained, the carbonate rocks on tondo formation (Tmtl) can be grouped into two facies, namely: Wackestone and Packestone. The diagenesis process that occurred in Tondo Formation research area is microbial micritization, cementation and neomorphism which indicate that the Tondo Formation has existed on diagenetic environment of marine phreatic, meteoric phreatic and meteoric vadose.

Diagenesis of unconformity and the influence on reservoir physical properties: a case study of the lower Jurassic in Xiazijie area, Junggar Basin, NW China

Diagenesis process of unconformity is very important in understanding the physical properties of the reservoirs. In this paper, based on a comprehensive analysis of well cores, thin sections, casting thin sections, scanning electron microscopy and other data, we discussed the diagenetic characteristics and the influences on reservoir physical properties of unconformity of the lower Jurassic in Xiazijie area, Junggar Basin. The results show that the semi-weathered rock layers of the unconformity had undergone a series of diagenesis, such as compaction, cementation, dissolution and disruption during burial diagenetic evolution. However, the diagenetic evolution is mainly in the stage A of middle diagenesis, and its alteration degree has become gradually weak as the distance increased from the unconformity surface. The quantitative analysis of diagenesis shows that diagenetic characteristics of semi-weathered rock layer of the unconformity are as follows: strong compaction (average optic compaction is 67.28%), middle cementation (average optic cementation rate is 42.97%) and strong dissolution (average optic dissolution porosity is 65.00%). According to their influence on reservoir physical properties, the intensity of diagenesis is sequenced in the following order: compaction > dissolution > cementation > disruption. Among these processes, compaction, dissolution and cementation can change the porosity by −26.1, 6.0 and −4.3%, respectively. Dissolution is the main controlling factor on reservoir properties of unconformity semi-weathered rock layers.

黄河口凹陷古近系砂岩储层岩石学及成岩作用特征研究

黄河口凹陷古近系储集砂体类型和成岩过程复杂，为储层优选提供依据，需要全面总结黄河口凹陷古近系砂岩储层的岩石学特征和成岩作用特征。通过大量的基础分析测试工作以及钻井薄片观察，将杂砂岩和净砂岩的特征进行区分，明确了净砂岩是古近系的主要储层，并将其划分至3类8种成分-成因储层类型。黄河口凹陷古近系砂岩储层以长石砂岩类为主，地层单元由下至上石英砂岩类比例呈增加趋势，岩屑砂岩类呈减少趋势。砂岩成岩过程中经历了压实压溶、胶结、交代、蚀变、溶解、黏土矿物转化等6种成岩作用，储层岩石学特征的差异影响了不同层位储层的成岩作用及其储层物性。 Because the sand body type and diagenetic process were complex in the Palaeogene reservoir of Huanghegou Depression, for the purpose of providing the basis for the reservoir optimization, it was necessary to sum up the Palaeogene petrologic and diagenetic characteristics in the depres-sion. Through plenty of basic analyses and drilling slice observation, the characteristics of grey-wacke and pure sandstone were distinguished and it was defined that the pure sandstone was the major reservoir in Palaeogene. And the pure sand reservoirs were divided into three types and eight kinds. In the Palaeogene of Huanghegou Depression, arkose was dominant. Meanwhile, from the bottom up, the proportion of quartz sandstone type showed increasing trend and lithic sandstone type was in decreasing trend. During the diagenetic process, sand reservoir has experienced compaction pressolution, cementation, metasomatism, alteration, dissolution and clay mineral transformation. The whole diagenesis process and reservoir physical property in different zones are influenced by the differences of petrologic characteristics.

REE Distribution During Sedimentation in Soda Lake Doroninskoye (Eastern Transbaikalia)

Data are presented on detailed sampling of the surface and pore waters of soda Lake Doroninskoye (Eastern Transbaikalia, Russia). Particular attention has been paid to the possible REE precipitation species from both water types. Thermodynamic modeling at 25°C and 1 bar total pressure was performed with the “HCh” code using a free-energy minimization algorithm. We modeled the heterophase 43-component system; both individual minerals and ideal solid solutions of the REE and numerous dissolved species were incorporated into the model. According to the calculations, redox conditions in the lake waters are established by the sulfur species. The main result is that the surface water is slightly supersaturated with respect to CaCO3-REE (65%) and F-apatite-REE (35%) solid solutions, as well as bastnäsite Ce0,5La0,25Nd0,2Pr0,05CO3F. A trend of the HREE predominance in the solution during the process of early diagenesis has been confirmed. Nevertheless, the thermodynamic model does not entirely explain the REE fractionation in pore waters (pore solutions are enriched in the LREEs in situ but are drastically depleted according to the thermodynamic model). The results of this study suggest that further considerations about solid solutions should be taken into account to model and understand exact REE fractionation during water-rock interaction in salt lakes.

Contribution of thermogenic organic matter to the formation of biogenic gas hydrate: Evidence from geochemical and microbial characteristics of hydrate-containing sediments in the Taixinan Basin, South China Sea

High-saturation (40–100%), microbial gas hydrates have been acquired by expedition GMGS2 from the Taixinan Basin. In this study, geochemical and microbial features of hydrate-containing sediments from the drilling cores (GMGS2-09 and GMGS2-16) were characterized to explore their relationships with gas hydrate formation. Results showed that the average TOC content of GMGS2-09 and GMGS2-16 were 0.45% and 0.63%, respectively. They could meet the threshold for in situ gas hydrate formation, but were not available for the formation of high-saturation gas hydrates. The dominant members of Bacteria at the class taxonomic level were Alphaproteobacteria, Bacilli, Bacteroidia, Epsilonproteobacteria and Gammaproteobacteria, and those in Archaea were Marine_Benthic_Group_B (MBGB), Miscellaneous_Crenarchaeotic_Group (MCG), Group C3, Methanomicrobia and Methanobacteria. Indicators of microbes associated with thermogenic organic matter were measured. These include: (1) most of the dominant microbes had been found dominant in other gas hydrates bearing sediments, mud volcanos as well as oil/coal deposits; (2) hydrogenotrophic methanogens and an oilfield-origin thermophilic, methylotrophic methanogen were found dominant the methanogen community; (3) hydrocarbon-assimilating bacteria and other hyperthermophiles were frequently detected. Therefore, thermogenic signatures were inferred existed in the sediments. This deduction is consistent with the interpretation from the seismic reflection profiles. Owing to the inconsistency between low TOC content and gas hydrates with high saturation, secondary microbial methane generated from the bioconversion of thermogenic organic matters (oil or coal) was speculated to serve as enhanced gas flux for the formation of high-saturation gas hydrates. A preliminary formation model of high-saturation biogenic gas hydrates was proposed, in which diagenesis processes, tectonic movements and microbial activities were all emphasized regarding to their contribution to gas hydrates formation. In short, this research helps explain how microbial act and what kind of organic matter they use in forming biogenic gas hydrates with high saturations.