Well-log temperatures have been recorded in more than 400 wells drilled so far by Oil India Ltd. in its concessions in the Upper Assam Tertiary sedimentary basin at the northeastern corner of the Indian subcontinent. Subsurface temperatures presented in the form of isogeothermal maps show (1) a general increase with the rise of basement configuration; and (2) maximum values over the crests of local highs with gradual decrease toward the flanks, the difference being on the order of 15°C.

Geothermal gradients in the areas studied decrease with depth within argillaceous sediments but increase in the arenaceous group. The geothermal gradients obtained for different areas of this sedimentary basin can be used in the "hydrocarbon-liquid window concept" for oil exploration. The study reveals that in the eastern part of this basin, theoretically a considerable thickness of potentially hydrocarbon-bearing section could be down to a depth of about 9,150 m which is yet to be explored by deep drilling.