Geothermal gradients in the Chad Basin, Nigeria, from bottom hole temperature logs

Abstract

Bottom Hole Temperature (BHT) data from 21 oil wells in the Nigerian sector of the Chad Basin were analyzed and interpreted to investigate the thermal structure of the basin. A regional average geothermal gradient of 3.4°C/100 m geothermal gradients in the range of 3.0 to 4.4 °C/100 m were obtained. The gradients are relatively lower at the Northeast and Southwest axis and maximum at the North-centre. These differences in geothermal gradients may reflect changes in thermal conductivity of rocks, groundwater movement and endothermic reactions during diagenesis. Also the presence of Tertiary intrusive that is prevalent in the basin may be connected with the variations in the computed geothermal gradient values. The study also reveals that sediments with relatively higher geothermal gradients (3.5 to 4.4°C/100 m) mature earlier (low oil window) than those with low gradient values. Thus, under normal circumstances a high geothermal gradient enhances the early formation of oil at relatively shallow burial depths, but it causes the depth range of the oil window to be quite narrow, while low geothermal gradient causes the first formation of oil to begin at fairly deep subsurface levels, but makes the oil window to be quite broad. Key words: Chad basin, well logs, bottom hole temperature, geothermal gradient, oil window.