| Biogas is an important sustainable energy with advantages such as shallow burial depth, wide distribution, large reserves and purity. Biogas is widely distributed in all over the world, and is the important component of natural gas. According to statistics, biogas resource is about14.5%of the total content of natural gas in the world, and the prototyped reserves is about15.5thousand billion m3, which is about20%of the prototyped reserves of natural gas. If the shallow biogas was to be prototyped, the biogas will be25～30%of the natural gas. Therefore, biogas has been one of the important component of natural gas resource.Sanhu district, Chaidamu basin is the famous Quaternary biogas production area in both China and all the world. Up till now there are six equipped biogas field, natural gas in the third-level geology is about3202.71×108m3. The basin is a large inland sedimentary basin from the middle Cenozoic age. Three surfaces of the basin is bounded by mountains, that is, Altun Mountains lie in the northwest, Qilian Mountains lie in northeast, and Kunlun Mountains is in the southwest, the altitude is about2650-3000m. Sanhu district lie in Nabei-Luoyan shan-Hongsanhan No.4at its west, and the southern and northern Hibsan Lake at its east, and there is a crack layer bounded by Lingjian-Xinan-Ainan at its north, and boundary fault at the front of Kunlun Mountains at its south. The area is about50000km2.From1958, investigation and exploration of the biogas in Chaidamu basin has been carried on for about60years, authors made a lot of investigations on geochemical characteristics, identification mark, genetic mechanism, conditions of producing gas, accumulation condition and prospective reserves from the view of microbiology and the geochemical characteristics, however, there are still some unsolved problem in evaluation method and system of biogas, the original evaluation method and system of petroleum geochemistry cannot satisfy and catch the exploration of biogas resource which has both microbiological mechanism and key factors. As to how the biogas formed, the type and abundance of sedimentary organic matter is not important, and the abundance of sedimentary organic matter has nothing with the formation of biogas, so a new evaluation method of biogas source rock should be built.Based on these, this paper used unconventional analysis with analysis method of organic matter in shallow sediment, focused on the distribution law of sedimentary organic matter in the district, discussed the source mechanism of the nutrition of the biogas, and presented sustainable producing gas mode of biogas. The main part is listed below:1. Interaction of thermal force during weak diagenesis process and microbes is the main mechanism of the formation of biogas in the basin. The formation of biogas in Chaidong is not only dependent on inheritance in vivo organic matter such as protein buried in the sediment, but also concerns with the abundance of secondary in vivo organic matter which was produced by the thermal force. The content of both protein and organic acids in the sediment from Sanhu district, Chaidamu basin are both low, and it is not sensitive to the buried depth. However, as the depth increases, both the temperature and pressure are changed, the original inherited in vivo organic matters such as protein and organic acids are consuming and reducing, and at the same time, the secondary in vivo organic matters have further supplement. The corresponding mechanism is mainly because of the effect of temperature on the transformation and recombinant of organic matter. Meanwhile, labile organic matter in the shallow sediment which can be exhausted by microbe is more than that in the deep sediment.2. TOC can be regarded as a good indicator to estimate the gas generating capacity of biogas source rock. Experiments indicated that TOC content has an explicitly linear relationship with ROC content, meanwhile, labile organic matter in the alm is much higher than that in the dark mudstone. ROC content variation during the increase of temperature has also an explicitly positive relationship, which suggests that under the action of thermal force, the richer the labile organic matter in sediment is, the larger the increase of the labile organic carbon is. TOC has a preferable relationship with mass loss during heating, which also indicate that the higher the TOC content is, the lever of labile organic matter which can be used by microbe is much higher. Therefore, TOC has a positive relationship with nutrition of microbes, which shows that TOC can be used as a good indicator to estimate the gas generating capacity of biogas source rock. The existence of organic matter layer with high abundance is the foundation and premise of biogenetic gas reservoir.3. The accumulation model of biogas in Qaidam is continuous gas generating and dynamic reservoiring. In Sebei No.1well located between Sebei No.1and No.2constructures, microbe was observed in different depths, which indicates that the existence and reproduction of methanogens provide source of power to the continuous generation of biogas. Simulation Experimental data of methanogenesis shows that under the action of methanogens and other anaerobic bacteriums, sedimentary organic matter in Quaternary system and upper Tertiary can produce biological methane. Meanwhile, biogas source rock in Sanhu district (lacustrine dark mudstone) is developed well. Developing of biogas source rock observed from shallow to deep layer establishes the continuity of biogas formation in this district, and is a physical foundation for biogas gas pool. Therefore, rich microbe community, developing of source rocks and continous high-speed sediment and burial of organic matter maintain the continuous gas generating and dynamic reservoir of biogas in Qaidam basin.Questions to be solved are listed as belows:First, it was proved that the interaction of thermal force during weak diagenesis process and microbes is the main mechanism of the formation of biogas in the basin, however, questions such as how the abundance of organic matter controls and affect the formation of biogas should be further investigated.Second, there is a loss of quantitative indicator on the transformation degree of organic matter. My study suggests that soluble organic matter in biogas source rock can greatly improve the abundance of organic matter. It was indicated that the plenty nutrition can be used by microbes. However, the existence of organic matter can only show that it has large potential in the formation of biogas in the future, and there is a loss of index connected with the organic matter.Third, there is still no clear understanding of geochemical condition and main controlling factors of biogas formation in Qaidam basin. Accumulation condition of biogas is very complex. The formation has not only plenty condition of gas resource, but also effective allocation of trap, reservoir cap, migration-accumulation and preservation and so on. Enrichment and reservoiring laws of biogas and controlling factors of accumulation as well as control effect of hydrocarbon generation center at different layer and generation intensity on gas reservoir should be studied further. Formation mechanism of biogas (is it dominated by carbon dioxide reduction or decomposition of acetic acid) and formation condition in formation (Eh value, temperature, PH value, salinity, pressure, composition of organic matter, diagenesis, et al.) should also be investigated deeply. Evaluation criteria of biogas resource rock and evaluation parameter should be studied to determine the field of effective gas resource rock, hydrocarbon-generating scale and resource extent. |
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