The Gas Kitchen And The Origion Of Natural Gas In The Daniudi Area And Surrounding Area Of Ordos Basin

The origin of the Daniudi gas field and its surrounding Ordovician gas reservoirs in the Ordos Basin is controversial.In view of this scientific problem,this paper applies petroleum geology and geochemistry,as well as quantitative analysis techniques of petroleum system to analyze the organic facies and thickness of the Ordovician source rocks(Majiagou Formation Marine source rocks)and carboniferous Permian source rocks in and around Daniudi gas field in the Ordos Basin.Based on the quantitative characterization of organic maturity and gas source range,the relationship between gas range and reservoir and the origin of gas were analyzed according to the characteristics and origin analysis of natural gas in the Ordovician Jingbian and Daniudi gas fields,and the following understandings were obtained:(1)There are two sets of source rocks in and around Danudi,namely,carboniferous and Permian coal measure source rocks and Ordovician Marine source rocks.Among them,the source rocks of the Ordovician Majiagou Formation,with a thickness of 5m～ 60 m,are mostly dolomite,argillaceous dolomite and dolomitic limestone,and the organic carbon content is generally lower than 0.3%.The source rocks have poor organic matter types and are mainly F organic facies.The source rocks of the carboniferous and Permian coal measures include coal and dark mudstone,of which the thickness of the coal seam is 8m ～ 20 m,with an average of 15.2m,and the thickness of the dark mudstone is 40 m ～ 100 m,with an average of 80.1m.The source rocks of Carboniferous and Permian coal measures are mainly F organic facies.(2)The source rocks of the Carboniferous and Permian coal measures are in the mature and high mature stage,and the source rocks of the Ordovician Majiagou Formation are in the high mature and over mature stage.The exhaust intensity of Ordovician Marine source rocks is between 0 and 20 million square meters /km2,and the contribution to reservoir formation should be small.The exhaust intensity of carboniferous and Permian coal measure source rocks is 0 ～ 5 billion square meters/km2,which is the main gas source in this area.The Ordovician natural gas is humus gas,mainly derived from carboniferous and Permian coal measure source rocks,which denies that the Ordovician natural gas is self-generated and self-stored oil gas.The high degree of thermal evolution leads to the inversion of ethane carbon isotope,which is the reason for the light carbon isotope of ethane in natural gas.According to the analysis of accumulation conditions,the exhaust intensity of Ordovician gas source is low,and the gas reservoir of Majiagou formation is mainly from carboniferous and Permian coal,and the natural gas generated from carboniferous and Permian coal measure source rocks migrates downward and laterally to Majiagou Formation through the solution gully and paleokarst weathering crust.