A study of the effects of lithologic formations on geothermal gradients is carried out in the south-east Niger Delta, Nigeria, using continuous temperature and lithologic log data from closely-spaced petroleum wells. The gradient profiles obtained for the deep wells, logged to depths between 6500 ft (1981m) and 8500ft (2591m), were observed to have markedly different geothermal gradients with corresponding vertical lithologic units. For the upper sandy lithology, the Benin Formation, the temperature gradient ranges between 0.80 oF/100 ft (1.456 oC/100m) and 1.22 oF/100 ft (2.220 oC/100m) with an average of 1.03 oF/100 ft (1.875 oC/100m). For the paralic shaly Agbada Formation segment, the temperature gradient ranges between 0.92 oF/100 ft (1.674 oC/100m) and 1.87 oF/100ft (3.403 oC/100m) with an average of 1.40 oF/100ft (2.548 oC/100m). The depths of Benin Formation were observed to vary between 4200ft (1280 m) and 5400ft (1879 m). Geothermal gradients are thus observed to increase with depth and shale units.
Key Words: Subsurface temperature, lithology, temperature gradient, thermal.
(Global Journal of Pure and Applied Sciences: 2002 8(3): 325-338)