Analysis Of Natural Gas Accumulation Process Of Shahezi Formation In Dehui Fault Sag,Songliao Basin

In recent years,Shahezi Formation is one of the key layers for deep natural gas exploration in Dehui fault sag.Based on the comprehensive analysis of burial history-thermal history recovery,hydrocarbon generation dynamics,fluid inclusions and migration pathway systems,the accumulation process of natural gas of Shahezi Formation was studied in this thesis.The results of burial history-heat history recovery show that the source rocks of Dehui fault sag experienced similar thermal evolution processes.In the late sedimentation period of Huoshiling Formation,Shahezi Formation and Yingcheng Formation,uplift and denudation with different degrees occurred.Main strata reached the maximum burial depth in the Late Cretaceous,and then entered a long stage of large-scale regional uplift and intense denudation,resulting in extensive erosion of the Paleogene-Neogene formation.The results of thermal simulation test of hydrocarbon generation in Golden Tube show that the average activation energies of C_1-5and C₆₊components generated from source rocks of Shahezi Formation are 63 kcal/mol and 56 kcal/mol,respectively.Based on these results,it is deduced that hydrocarbon generation of Shahezi Formation source rocks started at 120 Ma,and there were two peaks of gas generation,which were the sedimentation period of K₁d-early stage of K₁q and K₁q-K₂n.At present,the source rocks in Huajia-Baojia area are generally under the hydrocarbon expulsion threshold,and have undergone the large-scale hydrocarbon expulsion stage,which belong to the effective source rocks.According to the study of fluid inclusions,it is considered that there are three periods of natural gas accumulation in Shahezi Formation,which are the sedimentation period of K₁d,K₁q-K₁qn and K₁y-K₂n.The latter two periods of gas accumulation match the main hydrocarbon generation period of source rocks,of which K₁q-K₁qn Formation is the main natural gas accumulation period with large-scale gas charging and a long duration.The migration pathway systems of Shahezi Formation are dominated by its top unconformity surface and internal sand body,which controls the lateral migration process of natural gas.Natural gas migrated through dominant migration pathway systems after generation,forming fault block type,anticline-fault block and unconformity-fault block complex type gas reservoirs at high structural positions.