生物气成因的特殊性,使得对生物气源岩的评价也有异于常规烃源岩。有利于生物气生成的物质与环境条件包括源岩快速大量沉积、烃源岩处于低温低演化阶段、原始生源中富含甲烷生物潜量高的草本植物、封闭的缺氧环境等。促使有机质转化为生物气的动力则包括产甲烷菌等微生物菌群的联合作用。反映微生物活动及其强度的指标,如地层中的微生物计数、厌氧菌发酵产气模拟试验以及与微生物作用有关的生物标志化合物的存在及其丰度等,是生物气源岩评价的有效地球化学参数,也是未来需要加强研究的领域。

南海北部边缘莺―琼盆地及珠江口盆地白云凹陷天然气资源丰富,天然气成因类型多,是一个具有多种勘探领域和颇大资源潜力的大气区。由于勘探及研究程度尚低,在该区浅层及中深层勘探中所发现的天然气藏多以成熟―高成熟的热成因腐殖型气(煤型气)为主,但也发现了一些生物气及亚生物气(生物―低熟过渡带气)气藏且具较高的产量和一定的储量规模、资源潜力及勘探前景广阔。认为这是一个非常值得重视与关注的天然气勘探新领域,只要加大勘探投入力度和天然气地质综合研究的深度,在该区生物气勘探领域获得突破是指日可待的.

Since 2004, several gas fields (Shwe, Shwe Phyu, May, Thalin etc.) have been discovered from shelf edge to deep-water (more than 1 000 m) in the Rakhine Basin, northeast Bay of Bengal. The gas is likely to be biogenic based on the methane content more than 97， and δ13C ranging from -70‰ to -60‰. The biogenic gas was trapped in turbidity sandstones (lobes, channels, levees and sandy MTCs) of NW-SE anticlines from Lower Pleistocene to Pliocene. The reservoir quality is moderate to good with the porosity of 20，-30， and the permeability from tens of millidarcy to hundreds of millidarcy. This paper uses high-resolution seismic data covering from shelf edge to the slope and basin, well data and gas play data to analyze the biogas generation and accumulation in deep-water turbidites, in the hope of providing some experiences for the biogas exploration in offshore Rakhine and other areas with the similar depositional environment. The research reveals the following results：(1) The gas bearing sandstones are buried at depth of 1 200-3 000 m below the sea level, where the reservoir temperature ranges from 30℃ to 90℃. This is the suitable environment for methanobacteria. Low geothermal gradient, high sediment accumulation rate and rapid temperature rise provide good conditions for biogas generation. (2) The discovered gas fields are located at the Pliocene lower slope. The thick mudstones in the lower slope can generate a large amount biogas which can migrate in short distance and accumulate, and the high sedimentation rate in the slope is also good for preservation of porosity and suitable for methanobacteria to live. (3) The stacked large-scale channel-levee systems transported rich terrestrial organic materials. The biogas generated from these organic material can migrate into adjacent channel sandstones and lobe sandstones, forming the favorable source-reservoir-cap assemblages. (4) The anticlines in the gentle folded belt can trap a large quantity of biogas. (5) The organic matter in offshore Rakhine Basin mainly derived from the terrestrial plants. The experimental analysis of drilling in this area proved that the source rocks of the Pliocene-Lower Pleistocene is much better than the Miocene. In addition, the isotopic data shows that organic matters in Pliocene-Lower Pleistocene are dominated by C4 biomass which is more favorable for biogas generation than C3 biomass based on a series of experimental analysis. Thus, the Lower Pleistocene and Pliocene turbidite sandstones are the main targets for biogas exploration in the offshore Rakhine Basin.

From analysis of resources distribution types and controlling factors of Quaternary biogas in Sanhu region in Qaidam Basin, three occurrence waysof biogas are pointed out: the low-rate structural traps, the lithologic traps, and the widely distributed water-soluble form. The exploration focus is the NorthSlope area, the Yanhu-Yabaer area, the low-rate structural traps in the west and sags. The lithologic pinchout traps in the S-N slopes, the low uplifts and slopesin the basin, and the lens in the lake basin are the future exploration targets.Water-soluble natural gas exploration is heuristic exploration direction in the area

生物气形成于有机质演化的未成熟―低成熟阶段,形成过程非常复杂和多元化;主要通过H2+CO2生物化学还原反应和乙酸发酵两条途径完成。通过研究柴达木盆地生物气认为:生物气气源岩的地球化学特征均与细菌的产甲烷活动有关,有机质中存留被细菌改造的遗迹;生气母质多为混合型,有机质含碳量低,IHC+IH和IO指数主要分布在大于100mg/g?TOC值域范围内;氯仿沥青\"A\"族组成中非烃含量较高,多数样品的正构烷烃分布模式与脂肪酸化合物相似;植烷类、甾萜烷类化合物主要被细菌生源烃以及其它被细菌改造后的生源烃所表征;有机母质沉积在强还原环境中。

<p>Biogas has been formed in the immature and/or lowe-mature stage of organic matter, which generating process is very complex and divers, mainly including two ways of H-2+CO-2 biochemical reduce and the acetate fermentation. According to biogas forming machine, the geochemical characteristics of its source rocks are relation to the action of bacteria-generating methane.;The source materirals mainly are mixed type. The content of organic carbon is low, and I&nbsp; {HC}+IH>100 mg/g&middot;TOC, but also IO>100 mg/g&middot;TOC. The abundance of non-hydrocarbon is higher in chloroform extracts. The distributional pattern of n-alkane is similar to that of fatty acid compound. The phytane, terpane and sterane compounds have been characterized by bacterial source and other source transformed by bacteria. Some terpane compositions of indicating environment suggest that organic matter has been deposited in strong reducing conditions.</p>

生物气的主要组分甲烷是不同微生物菌群协同降解复杂有机质的终端产物,研究生物气生成过程中微生物对沉积有机质降解的生物标志化合物特征,对柴达木盆地东部生物气气源层识别具有重要意义。柴达木盆地三湖坳陷涩北一号气区第四系沉积物饱和烃生物标志化合物的研究发现,一些泥岩层内饱和烃组分遭受微生物降解,使部分正构烷烃和无环类异戊二烯烷烃消失,色谱不可分辨的复杂有机混合物显著升高。微生物降解作用明显的泥岩,降解参数升高,可溶有机质含量与烃转化率也显著升高,反映第四系部分泥岩层中高的可溶有机质含量与烃含量,是微生物活动和降解原始沉积有机质的产物。在微生物降解作用明显的泥岩中,检测到了丰富的产甲烷菌特殊生物标志化合物2,6,10,15,19-五甲基二十烷等,表明降解程度高的泥岩中,不仅厌氧降解细菌活动强烈,产甲烷菌活动也很强烈,是生物气的优质烃源岩。因此可以得出:可溶有机质含量,特别是其中烃含量以及微生物降解参数,是评价生物气气源岩的重要依据。

The major component of biogenic gas, methane, is an important end product of biogeochemical processes associated with degradation of complex sedimentary organic matter by communities of microbes. Efforts to better understand the characteristics of biomarkers as a result of biodegradation of sedimentary organic matter during gas generation are therefore valuable in assessing and constraining the potential of mudstone as source rocks for biogenic gas in the eastern Qaidam Basin. Based on the study of biomarkers in saturated hydrocarbons of the Quaternary sediments from the Sebei 1 gas field in the Sanhu depression of the Qaidam Basin, it is found that saturated hydrocarbons in some mudstone samples were biodegraded and resulted in a partial removal of n-alkanes and acyclic isoprenoids and a significant increase in the "unresolved complex mixture". Mudstone samples undergone obvious biodegradation processes show increased biodegradation index and remarkable increase in the contents of soluble organic matter and hydrocarbon conversion ratios, suggesting that the high contents of soluble organic matter and hydrocarbons in some of the Quaternary mudstone layers are the results of microbial activities and biodegradation of original sedimentary organic matter. Abundant particular biomarkers for methanogens, such as 2, 6, 10, 15, 19-pentamethylicosane, were detected in mudstone samples undergone obvious biodegradation processes, indicating that both methanogenic archaea and anaerobic bacteria intensely developed in these sediments and they are therefore proposed to be excellent biogenic gas source rocks. It is the contents of soluble organic matter and hydrocarbons and biodegradation index that significant for assessing and constraining the potential of mudstone as source rocks for biogenic gas.

综合论述了生物气的特征及其研究进展,着重探讨了生物气成因、地球化学特征、勘探研究现状、生物气形成机制和控制因素,并介绍了生物气系统概念和特征,最后叙述了生物气藏的分布和我国生物气藏的远景。生物气是在还原环境的生物化学作用带内有机质为厌氧微生物所分解的最终产物,它以甲烷为主,并合部分二氧化碳及少量氦气和其它微量气体组分,生物甲烷气 &#x003b4;¹³C₁值一般小于-55&#x02030;。生物甲烷气的形成途径主要有乙酸发酵和 CO₂还原2种类型。生物气生成与其所处的沉积环境、古气候、有机质的类型和丰度、水介质性质、地质作用、沉积时间等众多因素密切相关。生物气埋藏浅,分布广泛,一般存在于三角洲、大陆架和部分陆相沉积环境中,储层时代主要为白垩纪、古近纪、新近纪和第四纪。白垩纪的储量最丰富,古近纪、新近纪次之,第四纪生物气藏的规模一般较小。我国生物气勘探研究历史虽然不长,但生物气资源量丰富,具有良好的勘探前景。

This paper summarizes the characteristics and research progress of biogenic gas, with an emphasis on the genesis, geochemical characteristics, exploration and research status, formation mechanism and controlling factors of biogenic gas. It introduces the concept and characteristics of the biogenic gas system, and finally describes the distribution of biogenic gas pools and prospectivity of biogenic gas in China. Biogenic gas is the final product of the degradation of organic matter by anaerobic microorganisms in the biochemical process zone of deoxidization environment. It mainly consists of methane, and also contains some carbon dioxide and tiny amounts of nitrogen and other trace gases. The &#x003b4;¹³C₁ values for the biogenic methane gas are generally lower than -55&#x02030;. Biogenic methane gas mainly results from acetate fermentation and carbon dioxide reduction. Formation of the biogenic gas is affinitive with depositional environment, paleoclimate, types and abundance of organic matter, water medium&#x02019;s characteristics, geological process and depositional time Biogenic gas is shallowly buried and widely distributed. Biogenic gas pools are developed in delta, continental shelf and continental depositional environments. The geological ages of reservoirs are Cretaceous, Paleogene, Neogene and Quaternary. In terms of reserves, the Cretaceous is the most important and followed by the Paleogene and the Neogene. The Quaternary biogenic gas pools are generally of small sizes Exploration and study history of biogenic gas in our country is not long, but biogenic gas resources are abundant and their exploration potential is encouraging.

为了进一步研究源-汇系统模式及相关因素，在双层盐岩结构广泛发育的墨西哥Burgos盆地Perdido构造带，以Wilcox组为研究对象，在有限井控条件下，基于测井、岩心、岩石薄片及三维地震资料，研究了板块活动历史与主要水系分布。通过地震沉积充填结构、岩屑组成明确物源方向，依据单井相、地震反射与属性特征分析刻画了深水沉积体系特征，综合分析了Wilcox组物源及其控制下的深水扇沉积体系，探讨了源-汇各要素间相关性并建立了耦合模式。研究结果表明，Wilcox组物源主要来自于西侧落基山脉，经Rio Grand、Colorado两大水系搬运至研究区，具有双向物源的特征；主要发育水道化朵叶、近端朵叶和远端朵叶，其中水道化朵叶粒度更粗，储集物性更好，是最为有利的沉积相带；Wilcox组上覆巨厚的盐岩沉积密度较低，其导热率较高可降低地层温度，减缓压实和胶结作用，更有利于盐下储层原生孔隙的保存；充足的沉积物供给量和高效的陆架搬运体系为Perdido构造带深水扇沉积创造了极为有利的条件，源-汇模式表现为落基山脉&mdash;Rio Grand、Colorado水系&mdash;陆架边缘三角洲&mdash;大型峡谷&mdash;深水扇模式；由于Perdido构造带勘探程度较低，烃源条件优越，砂体分布广泛，双层盐岩构造为油气成藏创造了诸多有利条件，因此具备广阔的勘探空间与巨大的潜力。

Aiming at Wilcox Formation in the Perdido fold belt of the Burgos Basin, Mexico, where double-layered salt rock structures are widely developed, this study explores the history of plate activity and the distribution of major river systems based on logging, core, thin-section and 3D seismic data under limited well control conditions. The provenance direction was determined by the seismic sedimentary filling structure and debris composition; the characteristics of the deepwater sedimentary system were analyzed based on the single well facies, seismic reflections and attributes. The provenance of the Wilcox Formation and the deep-water fan deposition system under its control were comprehensively analyzed, the correlation between source-to-sink elements was explored, and the coupling model was established. The results show that the provenance of Wilcox Formation is mainly from the western Rocky Mountains and is transported to the study area by the two major river systems of Rio Grand and Colorado, characterized by bidirectional provenance. Channelized lobe, proximal lobe and distal lobe are mainly developed in this area. Among them, the channelized lobe is characterized by coarser particles and better reservoir property, and is the most favorable sedimentary facies belt. The overburdened hugely-thick salt rock of Wilcox Formation has low density and high thermal conductivity, and thus can reduce formation temperature, slow down compaction and cementation, which is conducive to the preservation of primary pores in subsalt reservoirs. Extremely favorable conditions for deposition of deep-water fan in the Perdido fold belt are created by abundant sediment supply and an efficient shelf-transportation system. The source-to-sink model is represented by the Rocky Mountains-Rio Grand, Colorado drainage system-shelf edge delta-canyons-deep-water fan mode. In addition to the low degree of exploration, superior source conditions, and extensive sand body distribution in the Perdido fold belt, many favorable conditions are provided by the double-layered salt rock structure for oil and gas accumulation, presenting a broad exploration space and huge potential.