Thermal History Reconstruction and Hydrocarbon Accumulation Period Discrimination of Jinhu Depression in Subei Basin

Key words: 
• Jinhu depression /
• geothermal gradient /
• ancient temperature scale /
• thermal history reconstruction /
• hydrocarbon accumulation period

Abstract: Based on the analysis of the vitrinite reflectance and apatite fission track inclusions system testing, we carried out the calculation of paleotemperature gradient and reconstruction of thermal history , and then identified the paleotemperature gradient of west slope and Bianminyang tectonic zone of Jinhu depression. According to the vitrinite reflectance, we calculated that the range of the paleotemperature being between 45.6~128.4℃ and the paleotemperature gradient was
45.5 ℃/km in the west slope, the paleotemperature in Bianminyang tectonic zone was 26.4~120.3℃ and the paleotemperature gradient was 42.7℃/km. According to the apatite fission track, we calculated that the paleotemperature gradient in the west slope was 40.7℃/km, and in Bianminyang tectonic zone was 45.8℃/km. From the comparative analysis with different tectonic zones of Jinhu depression, we concluded a law that the paleo temperature gradient was higher than presentday geothermal gradient, specifically as follows: in the west slope, paleotemperature was 10.4~15.2℃/km higher than the current, and in Bianminyang tectonic zone paleotemperature was 12.4~15.3℃/km higher than the present.By the thermal history modeling of typical wells in west slope and Bianminyang tectonic zone, it could be seen that the paleogeothermal gradient became lower with the stratigraphical time changed for the new. It shows that the geothermal gradient of K2t~E1f was higher than E2d~Ny. Before the uplift and erosion caused by the Sanduo tectonic events, the paleotemperature of depression had reached the maximum. The maturity history of depression reflected that the Ro was 0.4% in the depth of 1 000 m of Jinhu depression. The source rock was at the lowmature stage.The Ro was 0.65% in the depth of 1 900 m and the temperature reached 90℃, the hydrocarbon source rocks entered the peak phase. The homogenization temperature of fluid inclusion samples was between 62~93℃ of Well Cui 2 in the west slope. Through the comprehensive analysis of the burial historythermal history of typical singlewell and the homogenization temperature of fluid inclusions, it can be identified that the accumulation period of Jinhu depression was between 47~41.5 Ma.Then it could be judged that the Sanduo period(E2s)was the main hydrocarbon accumulation period of Jinhu depression.