Mesozoic-Cenozoic Tectono-thermal Evolution In Southwest Depression Of Tarim Basin

As a secondary tectonic unit of the Tarim Basin,the Southwest Depression is rich in oil and gas resources.However,the hydrocarbon exploration level of the Southwest Depression is still low due to the later complex basin-mountain tectonic setting.Temperature is the most critical factor on governing the hydrocarbon generation and preservation.This thesis calculates and analyzes the present-day geothermal heat flow,geothermal gradient,and fixed-depth temperature distribution characteristics of the Southwest Depression based on the detailed measured temperature data and the one-dimensional thermal steady-state conduction equation.We reconstructed the thermal history of the typical samples from the Southwest Depression by apatite fission track and(U-Th)/He data and clarified the tectonic uplift events and their denudation amounts since the Late Paleozoic.Finally,the paleo-heat flow evolution history and the maturation process of the primary source rocks in the Southwest Depression were revealed by combining the vitrinite reflectance data and basin simulation technique.The average present-day geothermal heat flow in the Southwest Depression is41.9±7.6m W/m~2 and its average geothermal gradient at a uniform depth of 0-5000m is20.6±3.1°C/Km,which are lower than other tectonic units in the Tarim Basin.The temperature at the depth of 6000 m in the Southwest Depression ranges from 99.6°C to161.3°C.The temperature gradually increases from the piedmont of the thrust belts to the Maigaiti Slope.The temperature in the northern Maigaiati Slope is about 20°C higher than the surrounding areas.The thermal anomaly could be caused by the upwelling of the hydrothermal fluid(such as the oil and gas)from the deeper strata through the strike-slip faults.The temperatures at the depths of 7000～10000m in the Southwest Depression increases with depth,their horizonal distribution characteristics are similar to that at the depth of 6000m.According to the thermal modeling results and previous studies,this study revealed that the Southwest Depression and the West Kunlun Mountain experienced seven tectonic uplift events,which occurred in the Late Permian-Triassic,the Late Triassic-Early Jurassic,the Late Jurassic-Early Cretaceous,the Late Cretaceous,the Paleocene-Early Eocene,the Late Oligocene-Early Miocene and since the Late Miocene.These tectonic uplift events were related to the collisions of the Songpan-Ganzi terrane and Tianshuihai terrane,the Qiangtang and Lhasa Plate with the southern margin of the Eurasian Plate,the Karakoram and Kohistan-Ladakh arcs,and the Indian plate with the south margin of the Eurasian plate.The paleo-heat flow in the Southwest Depression was 54m W/m~2 during the Carboniferous and gradually decreased to～40 m W/m~2at present.Due to the thermal effect of the Early Permian large igneous rock province,the central and eastern Southwest Depression had a heat flow peak of～60m W/m~2.The Carboniferous and Permian source rocks in the Southwest Depression entered the maturation stage and generated oil in the Late Permian,while the Jurassic source rock entered the maturation stage and generated oil at the end of the Neogene.This study enriches thermal regime results of the Southwest Depression and has crucial scientific significance for regional tectonic evolution and hydrocarbon resources exploration and evaluation.