FORMATION AND DISTRIBUTION OF LARGE-MEDIUM SCALED VOLCANITE GAS FIELDS IN EASTERN CHANGLING FUALT DEPRESSION OF THE SONGLIAO BASIN

A series of large-medium volcanite(oil) gas fields have been found in the east of Changling Fault Depression in southern Songliao Basin,such as Songnan,Dabei,Dongyingtai,and Dongling gas fields.These volcanite(oil) gas fields have the following accumulation and distribution features.1) These fields are located in the hydrocarbon bearing depression,such as Changling,Qian'an,Chaganhua depressions,which have been proved by drilling to be bearing high quality source rocks of lacustrine and eulimnetic facies of Shahezi-Yingcheng Group.2) A lot of large inherited uplift structure-volcanite combined traps were developed near the fault belts controlling sags with source rocks and in the breaks of the gentle slopes.3) Major gas pools were distributed in massive acidic volcanite reservoir rocks in upper Yingcheng Group,which were covered with the upper sections of Denglouku Group and Quantou Group,as a favorable sealing layer.4) The sands and the fracture as well as volcanic rocks were interlaced to form complicated grid-and blanket-like hydrocarbon migration system and establish a solid foundation for large scale hydrocarbon migration and accumulation.5) In the course of late tectonic evolution since late depositional stage of Denglouku Group,the eastern part of Changling Depression have been remaining favorable accumulation and preserving conditions.Next-step work should focus on the sag-controlling fault belts and breaks of the gentle slopes in Changling Depression,exploring inherited nose-like uplift structure-volcanite combined traps in Yingcheng Group.