Distribution Characteristics Of Carboniferous Volcanic Rocks And Identification Of Favorable Traps In Mahu Sag, Junggar Basin

With the increasing demand for petroleum and natural gas worldwide,volcanic reservoir,one of unconventional oil and gas resources,has become a hot area of oil and gas exploration and development in various countries.However,due to the complex lithology of volcanic rocks,the diversity of lithologic assemblages and interior structures,the rapid lateral change of lithologic lithofacies,the multiple factors controlling hydrocarbon accumulation,and the large burial depth,the quality of seismic data is uneven.All these restrict the progress of Carboniferous oil and gas exploration.Taking the Mahu sag and its surrounding Carboniferous volcanic rocks in Junggar Basin as the research object,this paper has carried out the research work of Carboniferous target strata structure,palaeogeomorphology and evolution characteristics,volcanic seismic facies characteristics,volcanic lithofacies spatial distribution characteristics,identification of volcanic traps and reservoir prediction,analysis of main controlling factors of hydrocarbon accumulation and favorable exploration areas.Comprehensive analysis considers that:(1)The Carboniferous fault system in the slope zone of Mahu sag develops on a large scale.The fault system develops continuously upward,which is a good oil and gas transmission channel in the middle and shallow layers of Mahu sag,and is also the main path of hydrocarbon generation and upward migration of Carboniferous source rocks.There are two types of faults in the Carboniferous system:the first type is a deep and large thrust fault controlling the boundary of the uplift structure,which is widely distributed in the study area,mainly along NE-SW trending,and the second type is a small-scale reverse fault with coordinated stress,and the fault spacing is generally small,basically along NW-SE trending.The number of type ? faults is large,mainly distributed in Dabason uplift area,Xiayan uplift area and northwest slope area.(2)The Carboniferous volcanic activity is intense,and there are three stages of volcanic eruption.They are lower submember of Lower Carboniferous Songkarsu Formation(C1sa),upper submember of Lower Carboniferous Songkarsu Formation(C1sb)and upper Carboniferous Batamayineshan Formation(C2b),and there are many volcanic island arc belts,mainly distributed along NE-SW direction in Dabason uplift,NNE-SSW direction in Mahu anticline and NW-SE direction in Zhongguai uplift.Around the volcanic island arc zone,a large range of multi-stage volcanic rock body development areas are developed.Dense paleovolcanic craters and adjacent areas are favorable reservoir development areas for volcanic rocks of eruptive and overflow facies.(3)There are 18 fault lithologic traps of volcanic rocks and 7 lithologic traps of volcanic rocks in the Carboniferous of Mazhong area.The total area of the traps is 861.89km2.There are 7 traps in the Upper Carboniferous Bashan Formation,mainly between MZ4-MZ1 and DT1-YT1,with large scale traps;6 traps in the Upper Submember of Lower Carboniferous Songkarsu Formation,mainly between MZ4 and MZ1 wells and southwest DT1 wells;12 traps in the Lower Submember of Lower Carboniferous Songkarsu Formation,mainly in the southeast of MZ4 well area and northeast of DT1 well area.The trap scale is small.(4)Through the analysis of hydrocarbon accumulation conditions in the study area,combined with reservoir prediction and trap optimization,it is considered that the Carboniferous is a low-amplitude uplift area west of Well Datan 1 in the central part of Mahu Depression,near the sedimentary center of hydrocarbon source rocks of the Fengcheng Formation of Permian,and at the same time,deep faults of the Carboniferous are developed,and they are located in the intermediate-acid volcanic facies belt,with volcanic rocks of explosive and overflow facies and volcanic The development of traps is a favorable area for hydrocarbon exploration in the Carboniferous system.And volcanic lithologic traps No.13 and No.14 in the west of Well Datan 1,located in the low-amplitude uplift area,are the most favorable exploration targets.