The cooling of subsurface formations by circu- lating drilling mud causes the formation temper- ature, shown on openhole well log headings, to Since early as 1947(2), numerous attempts have consistently be lower than the true or static been made to better understand and analytically formation temperature. Numerous investigations describe the effects that several parameters have been carried out to better understand and have on the temperature distribution in well- analytically describe the effects that several bores. Parameters studied include fluid temper- parameters have on this temperature distribution ature b havior in the borehole during well in wellbores. These concepts are reviewed and tripping~3) bottomhole temperature during field examples are presented illustrating the drilling(4), temperature profile over the entire principles and effects. borehole(5), and determination of static (true) formation temperature from geophysical wireline INTRODUCTION logging devices.