A regional study of geothermal gradients in the northern North Sea (59 to 62°N) has been carried out. True formation temperatures (T_f) are estimated by extrapolation of maximum temperatures (BHT) recorded during logging. The method also requires data about mud circulation time (t) and the time since cessation of circulation (^Dgrt). The estimation of T_f is made difficult because of the following factors: the frequent use of only one thermometer (tool failure happens); the recording of BHT on one log run only while more logs have been run to the same depth; inaccurate or lack of ^Dgrt values; and the circumstantial work which is necessary to find t values in old well data. Because of the latter factor, an average value of t is sometimes used. No reat errors normally result. If extrapolation of BHT's is precluded (only one value exists), T_f may be estimated from a mean correction line based on nearby wells.

Both total and interval geothermal gradients are calculated. The magnitude of the total gradients is largely dependent on the formation in which the wells have terminated. Differences of more than 5°C/km (0.3°F/100 ft) due to varying interval gradients have been observed. A general pattern of interval geothermal gradients is recognized: high Tertiary gradients (30 to 40°C/km = 1.7 to 2.2°F/100 ft); low Cretaceous gradients (10 to 30°C/km = 0.5 to 1.6°F/100 ft); and high Jurassic gradients (40 to 100°C/km = 2.2 to 5.5°F/100 ft).

End_of_Article - Last_Page 430------------