Source rocks thermal maturity reconstruction on high and deep structures in the Sahel region (central-eastern Tunisia)

This study evaluates the thermal maturity of the Albian Lower Fahdene, the Late Cenomanian-Early Turonian Bahloul and the Ypresian Bou Dabbous source rocks within the Sahel area (Central-Eastern Tunisia). Several data including well logs, geochemical parameters (vitrinite reflectance, Hydrogen Index), seismic data and Bottom Hole Temperatures (BHT) from 42 wells (1309–4003m in depth), were used to assess source rock's thermal maturity and aid to identify promising areas for oil and gas exploration. To calibrate the present-day heat flow, BHTs were corrected using empirical methods, on which only Harrison et al. (1983) and Bouaziz et al. (2015) equations showed the best fit with Drill Stem Test (DST) temperatures. The 1D basin modelling results of drilled wells show that the thermal maturity stages of the studied source rocks are recorded earlier when correction is applied. Consequently, temperature correction increased oil production by around 15 % in the Lower Fahdene and Bahloul and nearly doubled it in the Bou Dabbous. Meanwhile, gas output rose by around 42 % and 35 % in the Lower Fahdene and Bahloul, respectively. This study emphasises the importance of temperature corrections for accurate geothermal gradient and heat flow assessments. However, burial history and subsidence are the primary drivers of hydrocarbon generation and expulsion. Indeed, the modelling of pseudo-wells in deep areas, derived from seismic interpretations, indicated higher source rock maturity due to more significant subsidence and deeper structures. The reconstructed maturity maps showed that the Lower Fahdene and Bahloul source rocks have reached advanced maturity in basinal zones, whereas the Bou Dabbous source rock remains immature to marginally mature. In addition, modelling results reveal that significant amounts of hydrocarbons are still retained within the sourcing rocks of the Sahel area. These constitute unconventional resources to be explored in the future.