The subsurface temperature readings obtained from maximum temperature of electric logs (T
ES) generally do not indicate true subsurface temperature, and are lower than true subsur-face temperature because thermal equilibrium between bore hole mud and formation has not atta fined at the logging time.
On the other hand, the temperature reading from bottom hole pressure measerment (T
BHP) indicates almost true subsurface temperature. The subsurface temperature and geothermal gradient are important to consider terrestrial heat flow, generation and maturation of hydrocarbons, well cementing and casing programs, and reservoir fluid.
The writers obtained two kinds of geothermal gradient, GG
BHP from temperature of bottom hole pressure measerment and GG
ES from logging temperature, in several oil and gas fields of Niigata basin(Fig. 1). Average surface temperature was assumed to be 15°C.
Although the gradients are different from every other fields, the ratio of GG
BHP/GG
ES shows only small difference among the fields and is 1.31 on the average.
Therefore, the following formula is derived
T
BHP=1.31(T
ES-15°C)+15°C More accurate subsurface temperature estimation will be made from T
ES by using above mentioned formula.
Dowdle and Cobb (1975) recommended a method similar to the conventional pressure build up method for estimating true formation temperature from T
ES.
The estimated formation temperature using their method as shown in their field examples and ours (Figs. 5, 6) is an excellent approximation to T
BHP (almost true formation temperature).
Therefore, in the future, this method should be used to estimate true subsurface temperature.
The higher geothermal gradient obtained from our method assuming constant surface temperature merely means that certain higher temperature block such as volcanic bodies or basement rocks is present at shallow depth.
The geothermal gradients obtained in the studied area, however, are well reflected in the underground structure and indicate high value at uplifted part and low value at subsided part(Fig. 9).
It is necessary to accumulate the numbre of measurement of thermal conductivity in cores, especially sedimentary rocks in order to obtain terrestrial heat flow from geothermal gradient.
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