Study On The Characteristics And Resource Potential Of Source Rocks In Shahezi Formation In Changling Fault Depression

With the continuous development and improvement of oil and gas exploration and development technologies,the oil and gas potential of deep rifted layers in continental rifted basins has attracted more and more attention of researchers.The Changling fault depression is located in the south of the fault depression area in the middle of the Songliao Basin,with an area of about 1.3×10⁴Km²,including the Da'an fault depression,Gudian fault depression,Heidimiao fault depression and the original Changling fault depression.It is divided into three oil and gas belts,the western marginal oil and gas fault depression belt,the central oil and gas fault depression belt,and the eastern near-uplift oil and gas fault depression belt,with a total of 8 fault depression/fault depression structural units.The Mesozoic strata are mainly developed in the study area,which are Huoshiling Formation-Shahezi Formation-Yingcheng Formation-Denglouku Formation from bottom to top.Mudstone is more widely distributed and thicker,and has better potential for oil and gas exploration.Through detailed core observation,analysis and arrangement of logging and seismic data,23 types of seismic facies were identified,the corresponding transformation relationship between seismic facies and sedimentary facies of Shahezi Formation was established,and the distribution of sedimentary facies belt of Shahezi Formation in Changling fault depression was clarified.The Shahezi Formation in the study area mainly developed four types of sedimentary facies:braided river delta facies,fan delta facies,lacustrine facies and sublacustrine fan facies,and divided into 8sedimentary subfacies and 19 sedimentary microfacies.Braided river delta facies and fan delta facies are mostly distributed in the edge areas of each tectonic unit,mainly composed of coarse clastic deposits.It gradually transitioned to the middle of the fault depression/fault depression to the shore-shallow lake sediments,with the largest distribution area.The semi-deep lake subfacies are mostly developed in the middle of the fault depression,where thick dark mudstone is deposited,which is the main source rock development area.Sublacustrine fan facies is sporadically distributed in the semi-deep lake subfacies area.The source rocks of the Shahezi Formation belong to the mudstone type,and are mainly dark gray-gray-black-black mudstones,with a large thickness,and the maximum thickness can reach 450m.The high-quality source rocks are mainly distributed in the Fulongquan fault depression and the Heidimiao fault depression,with a maximum thickness of 48.80m.The abundance of organic matter is high,with an average value of1.42%,reaching the standard of medium source rock-best source rock.The types of organic matter are mainly type II₁,type II₂ and type III,which have a good foundation for hydrocarbon generation.Except for the Ganbei fault sag,the thermal evolution of organic matter has reached the stage of maturity and above,which is favorable for oil and gas generation.Using the genetic method to estimate the total resources of each fault depression/depression in the study area,the Heidimiao fault depression contains the largest amount of resources,about 126 million t,and the overall average resource is about 56 million t.Based on the distribution characteristics of sedimentary facies,the geochemical characteristics of source rocks and the oil and gas storage conditions,it is determined that the Chaganhua fault sag,the Fulongquan fault sag and the Heidimiao fault depression are the most favorable target areas of Class I.