The Geothermal Regime in Sirt Basin, Libya: The Geological Role and Heat Flow Studies

Bottom hole temperatures (BHTs) and static formation temperatures (DSTs) of 70 deep exploratory wells are used to evaluate the geothermal regime in the northeastern part of Sirt Basin. A linear regression was derived between the BHT’s and the DST’s, for correcting the bottom hole temperatures from the drilling factors that lower them from the true formation temperatures. The geothermal gradients were calculated using the corrected BHT’s and the heat flow has been estimated. Interpretation of the geothermal data, utilizing subsurface maps and isothermal geologic cross section, revealed a shallow, local semi-thermal reservoir of Oligo-Miocene age (at depths 60°C/Km and from 80 up to >130 mW/m2, respectively. At deeper depths (>1000 m until the maximum depth of investigation), the area has more or less moderate to low geothermal gradients that range from 40 to <20°C/Km and heat flow <80 to <20 mW/m2. The study indicates that the vertical and the lateral variations of the formation temperatures, geothermal gradients and the heat flow values are controlled by the structural, groundwater movements as well as lithological and thermal characteristics of the subsurface sequence.