Temperature measurement used for 4000-m depth drilling and geothermal characteristics in Jiaodong Peninsula, China

Twice-repeated stationary-state temperature measurements have been carried out for prospecting drilling at a 4000-m depth (119°59′42.7″E, 37°24′11.6″N; starting on September 18, 2010 and ending on May 29, 2013; borehole depth is 4006.2 m) using the geophysical well logging method and the new DS2000 temperature continuous data acquisition system. Temperature measurements were obtained on June 25, 2013, and December 6, 2013, at 28 and 192 days after drilling had ended, respectively. The first measurement used the traditional logging method after the drilling was completed; and the second measurement was taken upon completion of drilling using the DS2000 temperature continuous data acquisition system started. Both results are highly consistent, and the change in trends with depth are linear. The results show that the temperature reaches 107.8 °C at a depth of 4004 m. The average geothermal gradient is 2.3 °C/100 m. The geothermal gradient is controlled by the lithology distribution: the geothermal gradients of monzonite granite are 2.0 °C/100 m at a depth range of 30.0–1753.0 m and 2.5 °C/100 m below 1753 m for granitic protomylonite. An intense fluctuation of geothermal gradients is occurred at the fracture zone at a depth of 3190.0–3530.0 m (the fracture zone will not cause the fluctuation, here I think it should be the fluid flow in the fracture zone). Thermal conductivity was tested for 42 core samples at depths of 0–4003.6 m. The results show that the average thermal conductivity is 2.1 W/(m K). Combined with the average gradient, the average terrestrial heat flow is calculated at 48.1 mW/m². These measurements have provided basic and reliable data for further studies of the deep geothermal field and geodynamics that are characteristic of the study area as well as direct geological information for deep mineral resource potential prediction and resource exploitation.