The Study Of Evidence And Gas Source Of Natural Gas Hydrate In Wuli Area,Qinghai Province

Wuli area is located in the Tuotuo river basin,Tibet Plateau.There are a lot of well-developed stratas which are from the Upper Permian?P₃n?Nayixiong Formation and Upper Triassic?T3bg?Bagong Formation.The sedimentary system of Wuli area consists of the barrier coast system and the delta depositional system,which is further divided into two parts:braided river delta system and meandering river delta system.In the survey,the author found a lot of evidences to prove natural gas hydrate that exists in the Wuli area,such as the geological Evidence,gas content,core's Evidence etc.The gas in the Wuli area mainly consists of carbon dioxide,a small amount of non-hydrocarbon gas and the gas accumulated from volcanic activities.Carbon dioxide is mainly from the inorganic mantle-magma source.Methane is composed of coal-derived gas from Nayixiong Formation and the inorganic gas from the mantle.1.The sedimentary evolution process of Permian strata mainly developed the barrier coast sedimentary system and delta sedimentary system in Wuli area.The delta sedimentary system mainly exists in the west of the research area which is divided into the braided river delta and meandering river delta based on Profile's structure and sediments'composition.2.There are plenty of evidences of the gas hydrates:frost heaving mounds,abnormal halo of soil acid,the spring's abnormal chemical properties,cores'extreme cold and laser Raman spectrum,gas content anomalies etc.The samples of the nature gas hydrate were found in Wuli area,which are mainly composed of carbon dioxide hydrate and water.3.The compositions of the gas imdiate that carbon dioxide makes up 92%⁹9%and methane makes up 1%⁹%in Wuli area.The source of carbon dioxide mainly consists of volcano-mantle origins and a flot organic origins.The source of methane is composed of coal-derived gas and mantle-derived inorganic gas.Magma or volcanic activities accelerated the sedimentary organic matter thermal cracking process into methane.The process also transforms part of the methane or organic matter into carbon dioxide.At the same time,magma and volcanic activities carry a large amount of carbon gathered in the rock and lithoclase.During the movement of the glaciers and permafrost,those preserved gas transformed into the nature gas hydrate.