Present Geothermal Fields And The Cenozoic Tectono-thermal Evolution Of Dongpu Sag, Bohai Bay Basin

The Dongpu Sag is located in the south of the Bohai Bay Basin,which has the greatest potential for oil and gas exploration.The study of the present geothermal field and tectonic-thermal evolution history in the Dongpu Sag is weak and it restricts the maturity evolution of source rocks and the resource potential assessment of the Shahejie 3 Formation.In this paper,192 rock thermal conductivity data and 673temperature data were used to calculate the present geothermal gradient in the Dongpu Sag.Meanwhile,18 fission-trace and 287 vitrinite reflectance data were also used to restore the tectonic-thermal evolution history of the Dongpu Sag during Cenozoic era.Based on all above,combined with geochemical parameters and advanced Basin Modeling techniques,this paper restored the history of source rock maturation.The results show that:?1?The present geothermal gradient of the Dongpu Sag is between 28.9 and 40.2°C/km,with an average geothermal gradient of31.6°C/km,The terrestrial heat flow in the Dongpu Sag is between 55.8 and73.1mW/m²,with an average value of 64.7mW/m².Therefore,the Dongpu Sag has the characteristic of medium-temperature geothermal field.The geothermal gradient of the uplift in the center and the slope in the west is high,while the geothermal gradient of the eastern sub-sag and western sub-sag is relatively lower;?2?The Dongou Sag has experienced 4 tectonic-thermal evolution stages Cenozoic:?1?50⁴2Ma:Initial rifting stage,the geothermal gradient rapidly increased from42°C/km to 48°C/km,and the first peak of geothermal gradient was reached at this time;?2?42³3Ma:First thermal settlement stage,the temperature gradient ranged from 48°C/km to 40°C/km;?3?33²7Ma:Rapid rifting stage,the geothermal gradient rose continuely from 40°C/km to 43°C/km and the geothermal gradient reached the second peak;?4?27⁰Ma:Second thermal settlement stage,and the geothermal gradient continuously decreased from 43°C/km to 32°C/km approximately.?3?The maturity evolutions of source rock in the Dongpu Sag is significantly different,which show that there are twice hydrocarbon generations in the source rocks of Shahejie 3Formation in the Changyuandong area of Menggangji and Haitongji sub-sag of western secondary Sag.There are twice hydrocarbon generations in the Nanhejia area of Haitongji,the third part of the Shahejie 3 Formation?Es₃³?in the Xuji uplift of the central uplift,the first part of the Shahejie 3 Formation?Es₃¹?in the Weicheng-Wenmingzhai uplift of the central uplift,and the second part of the Shahejie 3 Formation?Es₃²?in the Qiaokou uplift of central uplift.The maturity evolutions of source rocks in these regions were controlled by present geothermal field.In addition,at the end of the depositional period of the Dongying Formation,the third and forth part of the Shahejie 3 Formation reached the high-over mature stage in eastern sub-sag,western sub-sag and central uplift;The Es₃¹ and Es₃² only reached the high-mature stage in the eastern,and western sub-sags?the Menggang sub-sag,the Haitongji sub-sag and the Qianliyuan sub-sag?.The maturity of the source rock in the central uplift zone only reached the low-middle maturity stage.Additionally,the maturity evolutions of source rock in the absence of the twice hydrocarbon generation were controlled by the paleogeothermal field at the end of the deposition of the Dongying Formation.According to the research,the results show that,oil and gas exploration should focus on the source rocks of the third and forth part of the Shahejie3 Formation,And the main exploration targets are two connected areas,that one is the connected area between eastern sub-sag?Gegangji sub-sag?,western sub-sag?Menggangji sub-sag and Haitongji sub-sag?and the central uplift?Qiaokou-Baimiao uplift,Xuji uplift and Wenliu uplift?,and the other connected area between Western secondary Sag?Menggangji sub-sag and Haitongji sub-sag?and the western slope.The results of this paper can provide the basis for oil and gas exploration decision-making in the Dongpu Sag.