A Study On The Thermal Evolution History Of Mesozoic In The Guxi Area,Jiyang Depression

Jiyang depression is a typical Meso-Cenozoic superimposed basin with abundant oil and gas resources.At present,the research on Jiyang depression is mainly focused on the Cenozoic,but less on the Mesozoic,especially on the thermal history of the Mesozoic basin and the characteristics of subsidence uplift in Jiyang Depression.By using apatite fission track thermochronology and taking Guxi area of Jiyang depression as an example,15 test samples were collected from three tectonic units of Gubei sag,Gudao salient and Gunan sag in Jiyang depression,respectively.The fission track experiments are carried out and the main conclusions are as follows.(1)The result of fission track test shows that the fission track age of the samples in Guxi area is between 3.8 ±0.9(Ma)-116.8 ±9.2(Ma),the fission track ages of most samples are smaller than their formation ages,and the length of the fission tracks is between 7.07um-11.85 um,and the fission track ages of the samples are within the range of 3.8 ±0.9(Ma)-116.8 ±9.2(Ma).The initial length of the sample is significantly smaller than that of the track,which indicates that all the samples have undergone different annealing processes.The distribution of fission track age and length also reveals the tectonic thermal events experienced by the samples in sedimentary burial.(2)Based on the analysis of the subsidence history of three different tectonic units in Guxi area,it is concluded that the burial history of the three different tectonic units since the Cenozoic is consistent.Both uplift and denudation occurred in 40 Ma and 28 Ma,with the characteristics of small settlement amplitude and fast subsidence rate,but the burial history of the three tectonic units in the Mesozoic has obvious differences.In Mesozoic,the uplift and denudation occurred between 94-110(Ma),the subsidence rate was slow,the settlement amplitude was large,the maximum buried depth of Gubei sag was only about 800 m,and the Gudao salient was the second.The maximum buried depth of Gunan sag is 3000 m,and the buried depth increases gradually from north to south.This may be related to the activity intensity of the Guxi fault in the north-south direction.(3)The results of thermal history simulation show that the geothermal gradient of Jiyang depression has gradually decreased from old to new since Mesozoic and Cenozoic.The geothermal gradient of Mesozoic is between 5.2-5.5 ? / 100 m,while that of Cenozoic is between 3.6-5 ? / 100 m.The distribution of paleogeothermal temperature of three different tectonic units is different,and the maximum paleogeothermal temperature of Gubei depression is about 60 ? in Mesozoic.The Gudao salient is the second,and the Gunan sag is the largest,reaching 160 ?.It is considered that the higher geothermal gradient is mainly due to the internal magmatic activity caused by the Tanlu fault in the vicinity.The uplift and denudation of different tectonic units and the difference of thermal history may be due to the collision between different blocks and the difference of heat flux properties in different regions.