Coal-Formed Gas of Deep Sequence and Its Prospects in Xujiaweizi of Songliao Basin

Key words: 
• Xujiaweizi /
• Jurassic /
• Denglouku Formation /
• coal-fomed gas /
• source-reservoi rcover system

Abstract: The deep sequence of Xujiaweizi includes the first number and second number of Quntou Formation,Denglouku Formation,and Jurassic.The deep sequence has its own particular source rock and reservoir,and the gas in it has the special character different from the oil type gas from shallow horizon.We investigated the carbon isotope of methane,and the value of which ranges from-15‰ to -34‰,indicating that the deep gas is relative to deep mudstone.According to addtional data,the author believes that the deep gas is formed by coal.We also investigated the age of the deep gas,which is similar to deep source rocks,but is different from source rock of shallow sequence.The source rock of deep sequence has the value of Pr/Ph above 1.This is the character of coal system.And it is formed in late diagenesis with the Ro ranging from 1.25% to 2.25%,the organic matter is cracked under the temperature above 500℃ during the stage of wet gas and dry gas. The source rocks are controlled by sedimentary facies.The deep scquence has the following sedimentary facies:the fluvial fan,river channel,delta system and lake system,but the source rocks(mudstone)mainly belong to lake system.During the stage of Hushiling,the mudstone only developed in Yingshan subsidence.During the Shahezi stage,the lake system extended to Xingshan sag and Miaotaizi.During the period of Yingchen Formation,sediments extended further,the lake facies produced in the centre of the subsidence. The reservoir rocks and cover rocks are also controlled by sedimentary facies.The reservoir rocks developed in Denglouku Formation,Quantou Formation and Jurassic.The gas reservoirs are formed in sandstones,erosion crust and fracture zone of volcanic rocks.The reservoir is mainly controlled by the sedimentary facies and volcanic fracture zone.The sandstone of Denglouku Formation,Quantou Formation and Jurassic can be good reservoir of gas,and the conglomerate of Jurassic and the volcanic zone can also be good reservoir.The reservoir developed in the region where both the tectonic unit and the sedimentary facies are suitable for gas preservation.The porosity of deep sequence is not good,but some revervoirs have the good condition.For example,in Denglouku Formation,the porosity increases with the depth,sandstone percentage also increased.The grains became coarser.In the third number of Dengluolu Fm.good reservoir developed there. As for the cover rocks,there are three series of cover rocks developed there.The mudstone and siltstone developed there,with the porosity of 1% and permeability of 0.05×10 ^-3 μm 2.In the Xujiaweizi subsidence,the cover rocks developed in Quantou Formation,the second number of Denglouku Formation and the Jurassic. We also dealt with the trap type of reservoir and the prospects of gas exploration in this region. There are seven types of trap in deep sequence according to the shape of trap,the ratio of length to width, the style of tectonic and fracture aswellas combination type.These trap types included anticline,fracture nose like rise,fracture step,fracture platform and fracture block. The favorable regions include the Zaozhou-Anda anticline belt,Zaodong-Chaoyanggou anticline belt,Chang wu-Zao dong nose-like structure,we expect to find a big gas trap there. The Anda-Zaozhou anticline belt developed in the northern part of central rise.It is 114km long along the North-south direction,and 21-41 km along the west-east direction with an area of 3 040km 2,there four boreholes which produce gas,these drilling wells includes the Fangsheng-1,Fangsheng-2,Fangsheng-4 and Zao sheng-1,The gas is found in the Denglouku Formation,base erosion crust.Cover rocks are the first and second number of Quantou Formation,and the mudsone of Denglouku Formation. Zaodong-Chaoyanggou anticline belt located in the middle part of the Xujiaweizi subsidence,extending there north-east,with an area of 2 590km 2.There are seven boreholes,only one of them produce gas.This anticline belt locates between Xujiaweizi subsidence and Yingshan subsidence.There are sandy conglomera.