地质科技通报 (Sep 2023)
Characteristics and controlling factors of the present geothermal field in the Songliao Basin
Abstract
Objective The geothermal field in the present sedimentary basin is a result of the energy balance between various dynamic processes on the Earth. It serves as an important tool for understanding continental dynamics, such as tectonic deformation and the evolution of the continental lithosphere. Additionally, it provides the basis for evaluating regional geothermal resources. Methods A comprehensive study of the geothermal field in the entire Songliao Basin was conducted using oil-test temperatures from 826 wells and thermal conductivities measured by the optical scanning method. Formation temperatures at depths of 1 000 m, 2 000 m, 3 000 m, and 4 000 m were estimated using deep temperature prediction technology. Results The results indicate that the present geothermal gradient in the Songliao Basin ranges from 22.5 to 69.0℃/km, with an average of 44.0℃/km. The thermal conductivity values of rocks in the central depression area are relatively concentrated, mostly ranging from 1.60 W/(m·K) to 2.40 W/(m·K), with an average of 1.84 W/(m·K). Among these, mudstone has the lowest thermal conductivity, with an average of 1.77 W/(m·K); siltstone has a middle range with an average of 1.87 W/(m·K); and fine sandstone has the highest thermal conductivity, with an average of 2.12 W/(m·K). The heat flow ranges from 35.0 to 98.8 mW/m2, with an average of 76.9 mW/m2. The basin exhibits a typical "hot basin" characteristic, with higher heat flow in the central depression and lower heat flow in the slope and uplift areas, forming an annular distribution pattern. Geothermal anomaly areas are distributed in the northeast of the central depression and the northwest of the southeastern uplifted region. The formation temperature at a depth of 1, 000 m ranges from 26.9 to 72.3℃, with an average of 47.9℃; at 2 000 m, it ranges from 49.4 to 141.3℃, with an average of 91.9℃; at 3 000 m, it ranges from 71.8 to 167.5℃, with an average of 135.8℃; and at 4 000 m, it ranges from 94.3 to 210.9℃, with an average of 179.8℃. Conclusion These findings suggest that the subduction of the Pacific Plate beneath the Eurasian Plate has caused upwelling of the asthenosphere and rapid thinning of the regional lithosphere, resulting in a significant increase in heat from the mantle. Simultaneously, the thinned crust facilitates the upward conduction of mantle heat. The widespread NNE and NW fault systems in the basin provide channels for the rise of mantle material and heat flow. Some mantle material remains in the middle and lower crust along deep faults, forming high-conductivity and low-velocity bodies that continuously heat the crust. A portion of the volcanic activity in the Songliao Basin is attributed to the eruption of Cenozoic volcanoes through faults. The presence of granite in the basin's basement plays a significant role in generating heat through the decay of radioactive elements, thereby serving as an important heat source. The heterogeneity of the current geothermal field can be attributed to variations in crustal structure among different tectonic units. The difference in thermal conductivity between the basement and sedimentary layers leads to a "refraction" effect on heat flow, resulting in the redistribution of heat in the shallow part of the basin and the formation of a distinct heat flow distribution pattern between concave and convex areas. The favorable combination of reservoir capacity in the Songliao Basin provides ideal conditions for heat storage, making it conducive for the development of low- and medium-temperature geothermal resources.
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