Formation temperature is one of the most critical in many drilling and workover operation and it varies by true vertical depth of wellbore. Therefore, it is very critical to determine correct formation temperature.
The following formula show relationship between formation temperature and true vertical depth of well.
Formation temperature in °F = (ambient surface temperature in °F) + (temperature increase in °F per TVD depth x TVD in ft)
Example: If the temperature increase in a specific area is 0.015 °F/ft of depth (temperature gradient) and the ambient surface temperature is 90 °F.
Determine the estimated formation temperature at a TVD of 12,000 ft:
Formation Temperature, °F = 90 °F + (0.015 °F/ft x 12,000 ft)
Formation Temperature, °F = 90 °F + 180 °F
Formation Temperature = 270 °F (estimated formation temperature)
Please find the Excel sheet used for estimating formation temperature.
Ref book: 
Formulas and Calculations for Drilling, Production and Workover, Second Edition
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