Source Rock Evaluation And Resource Potential Of Fengcheng Formation In Mahu Sag, Junggar Basin

In order to deeply study the characteristics of source rocks and the potential of shale oil resources in the Source Formation of the Fengcheng formation of the Mahu Depression in the Junggar Basin,on the basis of the research results of predecessors,taking the northwestern Fengnan region with a high degree of exploration as an example,the data and methods such as centimeter-level core observation and description,logging curve,rock flake identification,and organic geochemical testing were used to study the rock type,geological characteristics,distribution characteristics and oil and gas resource potential of the source rocks of the Fengcheng Formation.The main results and recognitions achieved are as follows:Source rock mineral composition of the Fengcheng Formation includes quartz,feldspar,dolomite,calcite,clay minerals and alkaline minerals,with the main rock types including mudstone,carbonate rock,volcaniclastic rock and alkaline mineral rocks;The source rocks are complex in lithology and change rapidly in lithofacies,and four types of lithofacies are mainly developed in the Fengnan area:feldspar-quartz mudstone facies,dolomite calcite mudstone facies and clay mudstone facies;the first Fengcheng section mainly develops clay mudstone facies;the second Fengcheng section is dominated by cloud mudstone deposition,and the frequent interlayering of the three phases of mudstone is the main feature of this period;the mudstone decreases during the third period of Fengcheng and the increase of mud shale is increased.The main features on the plane are that the carbonate lithofacy is more developed near the center of the southern depression,and the long-term euphemistic mudstone facies are more developed to the northwest.The organic matter abundance of the source rocks of the Fengcheng Formation was generally higher,with the TOC distribution of 0.06%～3.19%,with an average of 0.81%;S₁+S₂was distributed in 0.02～24.05mg/g,with an average of 4.36mg/g;the"A"content of chloroform asphalt was distributed in 0.005%～1.88%,with an average of 0.25%;the organic matter type was good,mainly type II₁ and II₂;the maturity was moderate,and the Ro average was 0.80%.There are differences in the potential of various lithological hydrocarbons in each stratum,which are generally manifested as:P₁f₂>P₁f₃>P₁f₁,clay mudstone>gray mudstone>cloud mudstone>long-british mudstone.The TOC of the first stage of the Fengcheng is distributed in 0.02%～3.18%,with an average of 0.63%,and the content is gradually decreased from northeast to southwest;the TOC of the second stage of the Fengcheng is distributed in 0.13%～2.88%,with an average of 1.02%,which gradually decreases from the middle to the edge;the toc distribution of the third stage of the Fengcheng is 0.17%～2.18%,with an average of 0.75%,decreasing from the middle to the north and south.The source rocks of the Fengcheng Formation as a whole have entered the mature stage,and the Ro distribution of the third section of the wind is 0.69%～0.95%and the average is 0.79%;the ro distribution of the second stage of the wind is 0.73%～1.04%,with an average of 0.86%;and the distribution range of the first section of the wind is 0.75%～1.11%,with an average of 0.95%.The three layer segments show consistent variation characteristics on the plane,that is,the maturity gradually increases from northwest to southeast.Fengcheng Formation in Fengnan area is rich in oil and gas resources.The Fengcheng Formation in fengnan area is rich in oil and gas resources,and the genesis method shows that the total oil production of the source rocks of the Fengcheng formation in the study area is 27357.85×10⁴t,and the gas gas is 17731.94×10⁴ m3;the calculation of the small surface element volume method shows that the total amount of shale oil geological resources of the Fengcheng Formation in the Fengnan area is 14882.10×10⁴t,and the average resource abundance is 49.74×10⁴t/km2;The grading evaluation results of shale oil resources show that:the invalid（decentralized）resources of the Fengcheng Formation were 1337.50×10⁴t,and the amount of inefficient and enriched resources was 13544.60×10⁴t.