Geothermal Gradients in Selected Wells by Using Borehole Temperatures

The temperature statistics from 48 oil and water boreholes are exploited to infer the geothermal inclination of Iraq. The recorded BHT of open hole logs is regularly lower than the accurate temperature. Consequently, the Horner scheme, was utilized in this study to correct BHT in which the reported temperature at a selected depth will be plotted versus rewarm time. The geothermal gradients (G.G.) vary noticeably horizontally (borehole to borehole) and vertically (depth). The inconsistencies in geothermal gradients throughout Iraq are a result of sediments’ thermal conductivity variation, heat generation, distribution and geometry of faults, and groundwater effects. In Northern Iraq, the maximum, minimum, and mean geothermal gradients (G.G.) are 29°C/km in Butma-2, 15°C/km in Ziwe-1, and 21°C/km, respectively. In Western Desert, the lowermost gradient within the layers above Paleozoic units is 12°C/km in KH-9/7 water borehole within Rutbah Subzone and the maximum is 31°C/km for Akkas-1 oil borehole in the north western portion in the region within Jezira Subzone. However, the maximum G.G. (61°C/km) in the Silurian Akkas is verified within 2100-2350 meters’ interval below the ground level. The average G.G. of 26°C/km in Western Iraq noticed in sections above the Paleozoic; however, G.G. value increases to 40°C/km in Paleozoic formations owing to presence of Akkas shale and rising heat from the basement unit. In Central Iraq, the average geothermal incline is about 20°C/km built on surrounding areas. In southern Iraq, the maximum G.G. (30°C/km) was reported in Nahr Umr-7 oil borehole in the Zubair Subzone. The lowest geothermal gradient (17°C/km) was detected in Luhays-12 oil borehole in the Zubair Subzone. The mean regional G.G. in southern Iraq is 22°C/km.