Influence Factors And Formation Of Quaternary Biogenic Gas Of Nuobei Area In Sanhu Depression,Qaidm Basin

Biogenic gas is generated by organic matters through biochemical by anaerobic microorganism under low temperature condition.The component is mainly methane,containing carbon dioxide,nitrogen and other trace gases,with characteristics of high efficiency,high energy and low pollution when compared with conventional natural gas.Therefore,research on the biogenic gas has become a key point of oil and gas research exploration.Biogenic gas resources are widely distributed in China,and biogenic gas pools have been discovered in various basins,such as the Qaidam Basin,Jiangzhe coastal area,Bohai Bay Basin,Songliao Basin and so on in China.The biogenic gas pool of the Sanhu Depression in the Qaidam Basin is main area to develop biogenic gas resource.It is almost 60 years of explorations and development since 1958.During this period,predecessors have systenatically studied the characteristics of sedimentary environment,the characteristics of gas source rocks,migration and accumulation in the west of the Sanhu Depression.However,the research about the formation and influencing factors of biogenic gas in the eastern part of the depression still needs further investigation.In this thesis,based on basic geological data,combined a vast amount of data from lithology paleontology,elemental geochemistry,organic geochemistry and other data.We studied the sedimentary environment,the characteristic of gas source rocks and the living environment of methanogen form Quaternary Qigequan Formation of Nuobei area.Based on it,we analyze the influence of them to form biogenic gas.This study has great significance to provide geological theoretical basis for the exploration and development of Quaternary biogenic gas in Nuobei area.The sedimentary facies of Quaternary Qigequan Formation in Nuobei area are mainly lakes,including alluvial fans,rivers and fan deltas.The sedimentary evolution of Quaternary Qigequan Formation can be divide into 5 stages.There are the formation period of lakes in the Early Pleistocene(K9?K13),the peak period of lakes in the Middle Pleistocene(K5?K9),the stable period of lakes in the Middle-Late Pleistocene(K2?K5),the shrinkage period of lakes in the Late Pleistocene(K0?K2)and the died period of lakes in the Early Holocene as followed.The sedimentary facies in the study area is in a ring-shaped distribution.From the center to both sides,the sedimentary facies gradually transition from lacustrine facies to alluvial fans and fan deltas.The formation mechanism of biogenic gas is completely different from kerogen pyrolysis,the assessment of source rock for biogenic gas should consider the influence of soluble organic matter.Reference to the standard about the abundance of biogenic gas source rocks of the Quaternary Qigequan Formation in the Qaidam Basin,we consider that the abundance of biogenic gas source rocks from the Nuol well is low to medium.The volume of biogenic gas source rock will directly determines the potential and scale of biogenic gas pool.The proportion of dark argillaceous rock in the Nuobei area can be as high as 45%or more,this can make up for the low organic matter abundance.It is still in the immature stage and the type of biogenic gas source rocks is III.It is rich in cellulose,hemicellulose and carbohydrates,which is easily used by anaerobic bacteria and can maximize the generation of biogenic gas.The flourish and methanogenesis of methanogens are strictly controlled by environmental factors,such as temperature,salinity,formation water properties and redox properties.The result shows,the environment of Nuobei area is brackish,neutral-alkaline with strong anoxic condition.The sedimentary temperature was low,the current temperature is mainly affected by geothermal gradiene.The surface water of Sanhu Depression is mainly from the Kunlun Mountains in the south,forming the characteristics of groundwater runoff from south to north.During the migration process,the flow of water gradually decreases and the salinity gradually increases with evaporation from south to north.The fundamental factor to form biogenic gas is not only the organic matter that can be used by methanogens,but also the geological conditions that are suitable for the flourish of methanogens.The development of gas source rock is constrained by various factors,the sedimentary environment is the most important factor controlling the organic matter abundance,quality and distribution characteristics of gas source rock,indirectly affecting the generation of biogenic gas.The lake are more development in the 3-5 member of Qigequan Formation,and the biogenic potential of it is better than 1?2 member.The geochemical environment affects the flourish of Methanogens,neutral-alkaline and strong reducibility environment is suitable for Methanogens to growth.Low temperature not only save the organic matter,but also inhibit the activity of Methanogens,avoid excessive decompostion of organic matter when it is shallow burial.High salinity is good for the preservation of organic matter,and would inhibit the activity of Methanogens.So we should considered the effects of regional flow field,for the high salinity that can form biogenic gas reservoirs.Biogenic gas is dominated by near source vertical migration,and source rock is the basis for the large-scale production of biogenic gas.The extensively developed lacustrine mudstone,the large reservoir space,the thick sedimentary and the deposition of gypsum rock increase the sealing capacity of the caprock,forming a good combination of raw,reservoir and cover.Among them,the condition of gas source rock in the Central Sag is best,and developed a large-scale Quaternary sedimentary anticline,which is the most favorable block for the biogenic gas to formation.The geological conditions of the North Slope are similar to those biogenic gas pool discovered previously,and there may be a long-distance migration and accumulation of biogas,which should be the next key exploration area.The salinity in the South Slope is low,has no inhibitory effect on the methanogenic bacteria,and a large amount of biogas is generated and then dissipated,making it less likely to form biogas accumulation.