Prediction of Formation Temperature from 4402.0m to 7018.0m in the Second Scientific Drilling Borehole (CCSD SK-2), Songliao Basin, NE China

The second continental scientific drilling (SK-2) borehole in the Songliao Basin has been drilled through the Cretaceous continental strata under the framework of the International Continental Scientific Drilling Program (ICDP). It is also the deepest drilling with the most complete strata in the Songliao Basin. The depth of steady-state temperature logging of SK-2 is 6.4~4402.0m, and the steady-state formation temperature at 4402.0m is 157.6°C. However, the steady-state temperature measurement has not been carried out from 4402.0m to 7018.0m until now, and there is only transient temperature which cannot reflect the in situ temperature. Therefore, we try to predict the formation temperature from 4402.0m to the bottom of SK-2 (7018.0m). Using the geophysical log data of SK-2, such as elemental capture spectroscopy(ECS), gamma ray spectra, array acoustic, density and temperature logs, thermophysical parameters (including thermal conductivity, heat production and heat flow) of the whole strata of SK-2 are estimated. In addition, the formation temperature from 4402.0m to 7018.0m is predicted by one-dimensional steady-state heat conductive equation. The main results are as follows: (1) The average heat production is between 1.5~2.0μW/m³, and the thermal conductivity is between 2.4~2.6W/(m•K) which is significantly affected by lithology. The average heat flow is 87.0mW/m², which is higher than the average heat flow in the Songliao Basin (about 70.0mW/m²) and is the direct evidence of the high heat flow background value in the Songliao Basin. (2) The strata of SK-2 from 4402.0m to 7018.0m are characterized by high formation temperature. The formation temperature at 7018.0m of SK-2 is between 242.3~263.2°C, which is 1.3~22.2°C higher than the transient temperature. The predicted range of temperature is close to the average formation temperature in the corresponding interval depth of the central part of the Songliao Basin. The prediction results of the formation temperature provide strong support for further study of the deep geothermal field and thermal structure of the Songliao Basin.