论文快速发表网

社科类论文科技类论文医学类论文管理类论文教育类论文农林类论文新闻类论文建筑类论文文艺类论文法学类论文

EI Compendex Source List（2022年1月） EI Compendex Source List（2020年1月） EI Compendex Source List（2019年5月） EI Compendex Source List（2018年9月） EI Compendex Source List（2018年5月） EI Compendex Source List（2018年1月）中国科学引文数据库来源期刊列 CSSCI(2017-2018)及扩展期刊目录 2017年4月7日EI检索目录（最新） 2017年3月EI检索目录最新公布北大中文核心期刊目录 SCI期刊（含影响因子）

本站服务项目浅谈青年教师如何开展教科研大学青年教师如何处理教学和科

论文范文

Improvements in Drill-Core Headspace Gas Analysis for Samples from Microbially Active Depths 时间:2018-10-17 22:00 来源:未知作者:admin 点击: 次 Abstract:The IsoJar™ container is widely used in headspace gas analysis for gases adsorbed on cuttings or bore cores from oil and gas fields. However, large variations in the carbon isotopic ratios of CH4 and CO2 are often reported, especially for data obtained from depths of <1000 m. The IsoJar™ method leaves air in the headspace that allows microbial oxidation of CH4 to CO2, meaning that isotopic fractionation occurs during storage. This study employed the IsoJar™ method to investigate the causes of differences in δ13C data reported by previous studies in the Horonobe area of Japan. It was found that after 80 d storage, δ13CCO2 values decreased by ~2‰, while δ13CCH4 values increased by >30‰, whereas samples analyzed within a week of collection showed no such fluctuations. The conventional amount of microbial suppressant (~0.5 ml of 10% benzalkonium chloride (BKC) solution) is insufficient to suppress microbial activity if groundwater is used as filling water. The significant variations in carbon isotopic compositions previously reported were caused by microbial methane oxidation after sampling and contamination by groundwater from different depths. To avoid these problems, we recommend the following: (1) if long-term sample storage is necessary, >10 ml of 10% BKC solution should be added or >0.3% BKC concentration is required to suppress microbial activity; (2) analyses should be performed within one week of sampling; and (3) for CO2 analyses, it is important that samples are not contaminated by groundwater from different depths.
1. Introduction
Investigation of the origin of deep hydrocarbons is an important aspect of resource exploration and may lead to an improved understanding of geological environments. In this investigation, gases adsorbed on rock fragments or bore cores were studied by headspace gas analysis (e.g., [1–5]), which provides information on the generation and migration of light hydrocarbons and gases. The IsoJar™ (Isotech Laboratories and Humble Instruments, USA) container, which is widely used in such analyses (e.g., [6, 7]) comprises a plastic container of ~600 ml volume with an aluminum screw cap on which there is a rubber septum through which headspace gas can be taken by syringe. The analysis procedure (e.g., [8]) normally involves storage of wet cuttings or cores in the jar with water and an air headspace for several days or weeks, the addition of a microbicide such as benzalkonium chloride (BKC) to minimize bacterial activity, the partitioning of gas into the headspace during storage, and analysis of these gases (e.g., [9–11]). The use of distilled or tap water avoids contamination from dissolved gases. The δ13CCH4 values of gases from depths of <1000 m, in the biogenic region, are usually in the range of –70‰ to −60‰, with isotopic compositions becoming heavier as depth increases towards the thermogenic region (e.g., [12, 13]). Large variations in carbon isotopic ratios in CH4 and CO2 are often reported for depths of <1000 m, with δ13CCH4 values sometimes reaching −20‰ (e.g., [14–17]). These variations are associated with the effects of microbial activity on methane production or oxidation in underground environments [18]. There are a number of factors that control the rate of methanogenesis [19], including temperature [20], groundwater salinity [21], pH [22], and pore space [23]. Peak microbial activity occurs at 35–45°C, which corresponds to depths of <1000 m [19, 24]. At greater depths, microbial action decreases as thermogenic production increases with the onset of catagenesis (subsequent to diagenesis at shallower depths [8]). More importantly, pore diameters of at least 1 μm are necessary for in situ methanogenesis, as microbes are in the 1–10 μm size range [25], which suggests that active methane production occurs at depths of <1500 m [24]. At shallow depths (less than several meters) below the ocean floor, where the concentration of dissolved gas is relatively low, considerable care was taken to avoid contamination and microbial activity (e.g., [26]). Hachikubo et al. [27] adjusted the concentration of BKC in samples to ~2.5% using 25 ml vials to obtain precise depth profiles of gases relative to hydrates. While the concentration and/or amount of microbicide normally added to IsoJar™ vessels are often omitted in reports, it is considered that the final concentration in IsoJar™ containers should be of the order of 0.01%, which is two orders of magnitude less than that reported by Hachikubo et al. [27]. It is speculated that another possible cause of variations in carbon isotopic composition may be microbial activity in the headspace after sampling, as the amount of microbicide commonly used with samples from microbially active depths might be insufficient to suppress microbial activity.