Thermal Evolution Of Marine Strata In The Central And Western Sichuan Basin

As a major gas produced area,the deep marine strata in the Sichuan Basin is important for increasing natural gas production.Because of the old age and the complex evolution,the exploration of Sinian^Paleozoic marine strata are required further research.The thermal evolution plays a crucial role in the formation of high-quality reservoirs and oil and gas accumulations.In this study,the present geothermal field characteristics of the central paleo-uplift and the northwest Sichuan Basin are analyzed based on various temperature data.Combined with thermal dynamic model of different geothermometer,we test the geothermometer of core and outcrop samples and reconstructed the Sinian to Present thermal history of the central paleo-uplift and the Late Paleozoic to Present thermal history of the northwest Sichuan Basin.Based on the reconstructed thermal history,clumped isotope geothermometer was explorded.On the basis of reconstructed sedimentary burial history and thermal history,the influence of thermal evolution on hydrocarbon accumulation in deep marine strata in the central and western Sichuan basin is analyzed.The current thermal system in the central and western Sichuan Basin is mainly controlled by the buried depth of basement.With a shallow depth of basement,the central paleo-uplift has a high terrestrial heat flow value,which is about 60 mW/m²⁶5 mW/m²and the heat flow value of the northwest Sichuan Basin is about 48 mW/m²⁵7 mW/m²because of the deeper basement depth.There are three phases of heat flow evolution in the central and western Sichuan basin:?1?The stable low heat flow stage during Cambrian-Early Permian.The heat flow is low and stable during this period,which is about 55 mW/m².?2?The rapidly raised stage during Early Permian-Early Triassic.The heat flow raised rapidly during this period and the maximum is 100 mW/m².?3?The decline stage since Early Triassic.The Dengying Formation and Longwangmiao Formation reservoir in the central paleo-uplift experienced a process of warming?80?¹10??-cooling-fast heating?180?²20??-fast cooling.In the late Jurassic,the temperature was higher than 160?,which is the crude oil cracking gas production peak.The Qixia Formation and Maokou Formation reservoir in the northwest Sichuan Basin experienced a process of rapid heating?130?¹50??-slow heating?160?²10??-cooling.In the late Triassic,the temperature was generally higher than 140?,and to the late Jurassic,the temperature rose even higher and had the characteristics that the temperature is gradually decreasing from the interior of the depression to the orogenic belt.In addition,the carbonate clumped isotope technique is used to reconstructed thermal history of sedimentary basins based on the series carbonate samples of the Sichuan Basin and Tarim Basin.The results show that the"closed temperature"of solid state rearrangement of natural evolution calcite clumped isotope ¹³C-¹⁸O is not higher than 120?and the thermodynamic"equilibrium temperature"not less than 160?,which is to sensitive for high temperature thermal history?>120??.It is potential for reconstructed thermal history in carbonate reservoir.Controlling by the thermal evolution,the time for oil and gas generation and accumulation is relatively late,which is beneficial to the preservation of giant gas fields although the age of source rocks and reservoirs are old.