Tectono-thermal Evolution Of The Eastern Sichuan Basin Since The Late Paleozoic

The Eastern Sichuan Basin,located in the east of the Sichuan Basin,is one of the most important exploration areas.The Jialingjiang Formation is one of the oil and gas bearing formation with the longest exploration and development history in the Eastern Sichuan Basin,but the main exploration and development are restricted by lots of controlling factors such as the output and main controlling factors of gas reservoir.Therefore,the study of thermal history and the thermal evolution of source rocks in research area can provide an important basis for reservoir forming of the Jialingjia Formation.In this paper,the present geothermal gradient and terrestrial heat flow in Eastern Sichuan Basin are calculated based on the oil production test temperature data of 24 wells,systematic steady-state temperature data of 5 wells and the rock thermal conductivities.The tectonic-thermal evolution history of Eastern Sichuan Basin since the Late Paleozoic is restored based on the vitrinite reflectance data of 18 wells.Combined the tectonic-thermal evolution history and the gas-source correlation of 48wells,this paper cleared that the source rocks.Then the thermal maturity history of source rocks of the Eastern Sichuan Basin is reconstructed according to the basin modeling.The results show that:the present geothermal gradient in Eastern Sichuan Basin is 16.0?21.3°C/km,with an average of 18.3±1.59°C/km.and the terrestrial heat flow is 44.3?67.7m W/m~2,with an average of 55.5±6.0 m W/m~2.The Eastern Sichuan Basin is characterized by a low-temperature geothermal field in the structurally stable region.The terrestrial heat flow of the Eastern Sichuan Basin is much lower than that of the central and western Sichuan Basin and the distribution is characterized by a higher value in the west and a lower value in the east.The terrestrial heat flow of the Eastern Sichuan Basin can be divided into three stages since the Late Paleozoic:(1)the peak of the terrestrial heat flow from the Early Permian to the Early Triassic.At the beginning of the Early Permian,the crust was uplifted and the mantle plume upwelling,and the terrestrial heat flow rose sharply.At this time,the heat flow value was67?77m W/m~2;(2)the decrease of the terrestrial heat flow from the Early Triassic to the Late Triassic.During this stage,the magmatic activity and the deep thermal process of the Eastern Sichuan Basin were gradually stable,and the thermal disturbance caused by lithosphere thinning began to decline,making the heat flow of the basement continue to decrease.At the end of the Late Triassic,the terrestrial heat flow value ranged from54m W/m~2 to 65m W/m~2;(3)from the Late Triassic to present day,the eastern Sichuan Basin was influenced by the Dabashan overthrusting,flexure of lithosphere,and sediment thickening,plus the Himalayan Movement caused the Pacific plate to subduct westward to form a thin-skinned structure.The basement burial depth began to thicken,and the terrestrial heat flow value is 44?68m W/m~2.Through gas source comparison,it is clear that the natural gas of the Jialingjiang Formation in the Eastern Sichuan Basin is from the source rocks in the Upper Permian Longtan Formation and Changxing Formation.The process of Upper Permian maturity evolution history is as follows:At the end of the Jialingjiang Formation depositional period,the oil threshold was entered.During the Xujiahe Formation depositional period,the marine sediments transformed into continental sediments,and the peak of heat flow in the Early Permian basically disappeared.The Xujiahe Formation was deposited slowly and the fill the eroded thickness of the Leikoupo Formation.Therefore,during this stage,the degree of thermal evolution becomes slow.The maturity of the source rocks continues to increase during the Jurassic and the Cretaceous,but the increase trend is significantly less than that of the Early Triassic.Then,the Yanshan movement and Himalayan movement made the Eastern Sichuan Basin uplift,the stratum suffered huge erosion,and the thermal maturity no longer increased.On the whole,the thermal evolution degree in the north was higher than that in the south.The thermal maturity is controlled by paleotemperature field.