Thermal anomalies associated with shallow gas hydrates in the K-2 mud volcano, Lake Baikal

Thermal measurements and hydrate mapping in the vicinity of the K-2 mud volcano in Lake Baikal have revealed a particular type of association of thermal anomalies (29–121?mW?m^–2) near hydrate-forming layers. Detailed coring within K-2 showed that hydrates are restricted to two distinct zones at sub-bottom depths exceeding 70–300?cm. Temperature data from stations with hydrate recovery and degassing features all display low thermal gradients. Otherwise, the thermal gradients within the mud volcano are generally increased. These findings imply a more complicated thermal regime than often assumed for mud volcanoes, with important roles for both fluids and hydrates. The coexistence of neighbouring low and high thermal anomalies is interpreted to result from discharging and recharging fluid activity, rather than hydrate thermodynamics. It is suggested that hydrates play a key role in controlling the fluid circulation pattern at an early stage. At a later stage, the inflow of undersaturated lake water would favour the dissolution of structure I hydrates and the formation of structure II hydrates, the latter having been observed on top of structure I hydrates in the K-2 mud volcano.     

Stratigraphic and structural control on the distribution of gas hydrates and active gas seeps on the Posolsky Bank, Lake Baikal

The distribution and origin of shallow gas seeps in the vicinity of the Posolsky Bank in Lake Baikal were studied based on the integration of detailed seismic, multibeam, and hydro-acoustic water-column investigations. In all, 65 acoustic flares have been detected on the Posolsky Fault scarp near the crest of the bank and in a similar, nearby setting at water depths of ?43 to ?332?m. The seismic data reveal BSRs (bottom-simulating reflectors) occurring up to water depths of ?300?m. Calculations involving hydrate stability, heat flow, and topographic modulation based on BSR occurrence and multibeam bathymetry enabled prediction of a methane–ethane gas mixture and heat-flow values that would account for gas hydrate stability in the lake sediments under prevailing ambient conditions. These predictions are supported by ground truth data. The findings suggest that seeps concentrated along the crest of the Posolsky Bank are fed mainly by gas coming from below the base of the gas hydrate stability zone, which would migrate updip via permeable stratigraphic pathways beneath the bank. Gas would ultimately be released into the water column where these pathways are cut off by faults.

Microstructures of structure I and II gas hydrates from the Gulf of Mexico

Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities.     

Effects of natural organic matter from sediments on the growth of marine gas hydrates

Sediments recovered from 0 to 27 + meters below the seafloor (mbsf) of a gas-hydrate and gas-venting active area in the Gulf of Mexico were added to a hydrate growth test cell to determine the influence of the organic and inorganic sedimentary components on hydrate induction times and formation rates. Induction times were sixteen times shorter in the presence of sediment from approximately 18 mbsf (relative to sediment from 1 mbsf), and remained stable in the presence of sediment from 18 to 27 mbsf. Formation rates increased by a factor of 2.5 in the presence of sediments from approximately 18 mbsf and decreased somewhat in the presence of sediment from 18 to 27 mbsf. Selected samples (surface, 18 and 27 mbsf) were density fractionated and subjected to bulk elemental and X-ray photoelectron spectroscopy (XPS) analysis. XPS revealed the presence of iron in various chemical environments at depths of 18 and 27 mbsf. High Resolution Magic Angle Spinning Nuclear Magnetic Resonance (HR-MAS NMR) was used to characterize the organic component of sediments from selected depths. The discovery of intact proteinaceous material in the surface sediment was surprising due to the labile nature of these biopolymers, and potentially reflects microbial activity in these surface layers. This material was less abundant in sediment from increasing depths, where more lipid-like compounds were prominent. The results suggest that hydrate growth is inhibited by the presence of proteinaceous material but enhanced by lipid-like compounds associated with iron-bearing mineral surfaces.     

Molecular and isotopic composition of hydrate-bound and dissolved gases in the southern basin of Lake Baikal, based on an improved headspace gas method

Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67?mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.     

南海北部气烟囱成因及其与油气及水合物运聚成藏关系

南海北部大陆边缘盆地油气勘探及天然气水合物调查与勘查评价中,地球物理资料尤其是二维/三维地震剖面上常见不同类型、不同特征且与油气藏及天然气水合物藏密切相关的地震反射模糊带等地震地质异常体,即“气烟囱”或“流体底辟”或“含气陷阱”。本文拟重点研究“气烟囱”成因及其与油气及天然气水合物运聚成藏的关系。油气勘探实践表明,气烟囱往往与油气藏尤其是天然气水合物藏伴生,且油气及水合物多处于其上覆或两侧位置附近。通过大量油气及天然气水合物勘探实践、地质地球物理资料综合分析及油气地球化学分析等,充分证实了气烟囱与其上覆或两侧附近的油气及天然气水合物藏具有密切的成因联系。研究表明,气烟囱作为连接和沟通烃源/气源供给系统与油气藏及水合物藏之间的重要桥梁和纽带,是油气（水合物）勘探中判识追踪油气藏及水合物藏成因、确定其烃源/气源供给系统活动特征的重要依据和指示。因此,深入分析研究气烟囱系统成因及类型,不仅能够追踪探索和判识确定油气及水合物成因,阐明其运聚成藏规律及控制因素,而且能够指导油气及水合物勘探部署与综合评价工作。同时,通过油气及水合物成因的地球化学分析,亦可判识确定其气烟囱成因及其对油气和水合物运聚成藏的控制影响作用。     

Sequentially sampled gas hydrate water,coupled with pore water and bottom water isotopic and ionic signatures at the Kukuy mud volcano,Lake Baikal: ambiguous deep-rooted source of hydrate-forming water

The isotopic and ionic composition of pure gas hydrate (GH) water was examined for GHs recovered in three gravity cores (165–193 cm length) from the Kukuy K-9 mud volcano (MV) in Lake Baikal. A massive GH sample from core St6GC4 (143–165 cm core depth interval) was dissociated progressively over 6 h in a closed glass chamber, and 11 sequentially collected fractions of dissociated GH water analyzed. Their hydrogen and oxygen isotopic compositions, and the concentrations of Cl^– and HCO₃ ^– remained essentially constant over time, except that the fraction collected during the first 50 minutes deviated partly from this pattern. Fraction #1 had a substantially higher Cl^– concentration, similar to that of pore water sampled immediately above (135–142 cm core depth) the main GH-bearing interval in that core. Like the subsequent fractions, however, the HCO₃ ^– concentration was markedly lower than that of pore water. For the GH water fractions #2 to #11, an essentially constant HCO₃ ^–/Cl^– ratio of 305 differed markedly from downcore pore water HCO₃ ^–/Cl^– ratios of 63–99. Evidently, contamination of the extracted GH water by ambient pore water probably adhered to the massive GH sample was satisfactorily restricted to the initial phase of GH dissociation. The hydrogen and oxygen isotopic composition of hydrate-forming water was estimated using the measured isotopic composition of extracted GH water combined with known isotopic fractionation factors between GH and GH-forming water. Estimated δD of ?126 to ?133‰ and δ¹⁸O of ?15.7 to ?16.7‰ differed partly from the corresponding signatures of ambient pore water (δD of ?123‰, δ¹⁸O of ?15.6‰) and of lake bottom water (δD of ?121‰, δ¹⁸O of ?15.8‰) at the St6GC4 coring site, suggesting that the GH was not formed from those waters. Observations of breccias in that core point to a possible deep-rooted water source, consistent with published thermal measurements for the neighboring Kukuy K-2 MV. By contrast, the pore waters of core St6GC4 and also of the neighboring cores GC2 and GC3 from the Kukuy K-9 MV show neither isotopic nor ionic evidence of such a source (e.g., elevated sulfate concentration). These findings constrain GH formation to earlier times, but a deep-rooted source of hydrate-forming water remains ambiguous. A possible long-term dampening of key deep-water source signatures deserves further attention, notably in terms of diffusion and/or advection, as well as anaerobic oxidation of methane.     

Studies in seawater and lake water on interactions of trace metals with humic substances isolated from marine and estuarine sediments: I. Characterisation of humic substances

Two humic acids, HAL and HAN, isolated from marine sediments (Adriatic and Norwegian Sea) and humic (HAM) and fulvic (FAM) acids isolated from an estuarine sediment (Mahakam, Borneo) were characterized.The following characteristics: carbon content, hydrophobicity, degree of condensation and the content of aliphatic groups, are most pronounced for HAN, less pronounced for HAL and the least pronounced for HAM. The abundance of trace metals in the humic acids decreases in the following order: Al > Cu > Zn > Pb > Cd.Fulvic acid (FAM) has a lower carbon and a higher oxygen content than humic acids. It is hydrophilic and hardly any aliphatic groups were recorded. At pH 7 the predominant fraction of FAM has the apparent molecular weight of 20 000 daltons while the humic acids are of higher molecular weight.     

Crustorhabditis chitwoodi sp. nov. (Nematoda: Rhabditidae): An intertidal species from the coast of New South Wales,Australia, with observations on its ecology and life history

A new species, Crustorhabditis chitwoodi, was discovered in nutrient agar plates inoculated with beach sand collected from an ocean beach in New South Wales, Australia. The three other known species of the genus have all been found as commensal inhabitants of ocypodid shore crabs. It is suggested that the sand inoculum was seeded with dauer larvae of the nematode that had been dispersed by scavenging crabs. The new species is distinguished from the three previously described species by possessing a smaller number of differently arranged bursal papillae, eight rather than 10. The species is amphimictic, oviparous, and produces abundant dauer larvae in culture, which can be sustained indefinitely on sea water nutrient agar in which it feeds on associated bacteria.     

Studies in seawater and lake water on interactions of trace metals with humic substances isolated from marine and estuarine sediments: II. Voltammetric investigations on trace metal complex formation in the dissolved phase

In seawater and lake water the complexation of several heavy metals, present in the dissolved state at relevant trace levels, with humic substances isolated from two marine and an estuarine sediment has been studied by differential pulse anodic stripping voltammetry (DPASV) at a hanging mercury drop electrode (HMDE). Taking Zn(II) as an example, the aim of this study was to gain direct information on the general level of importance of humic substances for the speciation of certain heavy metals in fresh and saline waters.In seawater humic acids originating from different sediments have very similar complexing properties for Zn(II), decreasing slightly in the order HAN > HAL > HAM. In lake water HAL is most efficient for complexing Zn(II), followed by HAN and HAM.In both types of natural waters, the sedimentary fulvic acid is less efficient in complexing Zn(II) than the respective humic acid from the same site. In general, the complexing efficiency decreases in the order EDTA > NTA > humic acids > fulvic acid.The problem of humic acid adsorption at the electrode, which somewhat limits the investigation of Pb(II) and Cd(II), and the impact of pH on the amount of complexed Zn(II) are also discussed.The findings provide further direct evidence for the conclusion previously drawn from existing complexation data that because of the rather low levels of dissolved humics in large parts of the oceans, the complexation efficiency of humics for Cd, Pb and Zn is too low to affect the speciation pattern of these three heavy metals.     

Influence of detrital composition on the diagenetic history of tight sandstones with implications for reservoir quality: Examples from the Permian Xiashihezi Formation and Carboniferous Taiyuan Formation,Daniudi gas field,Ordos Basin,China

Tight-gas reservoirs, characterized by low porosity and low permeability, are widely considered to be the product of post-depositional, diagenetic processes associated with progressive burial. This study utilizes a combination of thin section petrography, scanning electron microscopy, microprobe and back scatter electron analysis, stable isotope geochemistry and fluid inclusion analysis to compare the diagenetic history, including porosity formation, within sandstones of the second member of Carboniferous Taiyuan Formation (C₃t₂) and the first member of Permian Xiashihezi Formation (P₁x₁) in the Ordos Basin in central China.In the P₁x₁ member, relatively high abundances of metamorphic rock fragments coupled with a braided river and lacustrine delta environment of deposition, produced more smectite for transformation to illite (50–120 °C). This reaction was driven by dissolution of unstable minerals (K-feldspar and rock fragments) during the early to middle stages of mesodiagenesis and consumed all K-feldspar. Abundant intragranular porosity (average values of 2.8%) and microporosity in kaolinite (average values of 1.5%) formed at these burial depths with chlorite and calcite developed as by-products.In the C₃t₂ member, relatively low abundances of metamorphic rock fragments coupled with an incised valley-coastal plain environment of deposition resulted in less smectite for transformation to illite. High K⁺/H⁺ ratios in the early pore waters related to a marine sedimentary environment of deposition promoted this reaction. Under these conditions, K-feldspar was partially preserved. During the middle to late stages of mesodiagenesis, K-feldspar breakdown produced secondary intragranular (average values of 1.4%) and intergranular pores (average values of 1.2%). Release of K⁺ ions promoted illitization of kaolinite with quartz overgrowths and ferrous carbonates developed as by-products.This study has demonstrated that whereas both members are typical tight-gas sandstones, they are characterized by quite different diagenetic histories controlled by the primary detrital composition, especially during mesodiagenesis. Types of secondary porosity vary between the two members and developed at different stages of progressive burial. The content of unstable detrital components, notably feldspar, was the key factor that determined the abundance of secondary porosity.