Geochemical Characteristics And Formation Mechanism Of Natural Gas In The Sanhu-Yiliping Area,Qaidam Basin

The Sanhu-Yiliping area of Qaidam Basin is a famous biogenic gas generating area in China.With the improvement of biogenic gas exploration level,it becomes difficult for biogenic gas exploration.The abundant thermogenic gas resources discovered in the Neogene strata of this area is an important alternative area for natural gas exploration.In this paper,Base on the evaluation of biogenic gas source rocks,the detection of microbial species and proportions in biogenic gas source rocks,the study of biogenic gas generation mechanism,the exploration of gas-generating layers of main biogenic gas source rocks,the simulation of gold tube hydrocarbonin Neogene source rocks,and the study on Neogene thermogenic gas generation mechanism and the history of hydrocarbon generation,it has been achieved the following main research conclusions:?1?The formation mechanism of biogenic gas and thermogenic gas in Sanhu area is different.The abundance of soluble organic matter in source rocks can evaluate the Quaternary-Neogene biogenic gas source rocks in Sanhu-Yiliping area.The abundance of soluble organic matter in the Quaternary-Neogene source rocks is much larger than the traditional TOC value,which is of great significance for the evaluation of biogenic gas resources.The low-temperature thermal degradation of organic matter in the Neogene strata of the Sanhu-Yiliping area is combined with the degradation of the microbial population in the strata to provide a rich nutrient substrate for methanogens in the formation.?2?High-throughput sequencing technology is used to establish the distribution characteristics of the flora related to methanogenesis in the Quaternary-Neogene source rock profile.In the depth of 0-1,000 m,the water is with high content chloride,large salinity and high content sulfate.High content salinity and sulfate strongly inhibit the activity of microflora related to methanogenesis;in the depth of 1,000m-1,750m,as the increase of depth,the water salinity decreases,resulting in the sulfate content reducing significantly with the proportion of halophilic bacteria declining significantly,and the microorganisms involved in the methanogenesis process become active.Cellulose-proteolytic bacteria,acidophilic bacteria and methylophilic bacteria are active in the depth of 1,750-2,300m,and the degradation of cellulose-proteolytic bacteria will provide a rich source of nutrients for methanogenic activities;?3?Base on the research of biomarker,high-throughput sequencing,organic matter abundance and type,sedimentary environment and maturity evolution in source rock,the biogenic gas generation and evolution profile in Sanhu area is established to discuss the mechanism of the formation of Quaternary-Neogene biogenic gas.In the depth of1,250-1,750m,cellulose-proteolytic bacteria,acidophilic bacteria,methylophilic bacteria and methanogens are obviously active,indicating that this layer has strong methanogenesis and is the main gas-generating layer of biogenic gas;methanogens are active in the depth of 2,300m-3,300m.The organic matter abundance of the source rock in this layer is stable,and the organic matter type is obviously improved.The low temperature of the layer helps the organic matter in the source rock to release small molecular compounds,providing a nutrient substrate for the methanogens bacteria,and is the main gas generating layer of the methanogenic bacteria;?4?Through the dynamic simulation of gold tube hydrocarbon,combined with the sedimentary history and thermal history in this area,a hydrocarbon history model of Neogene source rocks in this area was established.The N₂³ source rock has entered the threshold of generating gas at 1.0Ma,with the gas conversion rate of only 1%.The N₂²source rock has started to generate gas at 2.8Ma,with the gas conversion rate of 7%.The N₂¹ source rock has started to generate gas at 5.3Ma,with the gas conversion rate of25%;?5?The aggregation period of Neogene thermogenic gas was discussed by carbon isotope kinetics.There is a mixture of biogenic gas and thermogenic gas in the Neogene strata.The natural gas N₂² generated at the bottom of Taishen-1 well is mainly composed of natural gas generated at 0.4Ma and above,and the corresponding methane conversion rate is about 20%,which belongs to the stage accumulation entrapment?...