Diagenetic history vs. thermal evolution of Paleozoic and Triassic reservoir rocks in the Ghadames-Illizi Basin (Algeria-Tunisia-Libya)

Unravelling the time-space distribution of diagenetic events modifying the pore network of reservoir rocks is fundamental for hydrocarbon research, but it needs to constraint a number of variables driving that distribution along the geological history of sedimentary basins. Here, we present the results of an integrated study performed on Paleozoic and Triassic reservoir sandstones coming from the Ghadames-Illizi Basin (North Africa), where it is still debated the possible thermal effect on the petroleum system of the Cenozoic magmatic activity occurred in neighbouring regions of the studied basin. The study aims to contribute in solving that problem by combining: the reconstruction of the relative timing of diagenetic events obtained by petrography; the detection of precipitation temperature of cements from micro thermometric analyses of fluid inclusions; the collection of thermal constraints from low-T thermochronology (Fission tracks and (U?Th)/He analyses) on detrital apatite grains; the maturity profile of organic matter obtained through vitrinite reflectance and spore analyses. This integrated approach was applied to samples coming from a basin region where Hercynian erosion was minor, in order to discuss different thermal scenarios, with or without a Tertiary thermal overprint. The results point out that a simple thermal scenario with heating only due to increasing burial depth is not able to account for the observed experimental data (both organic matter and fluid inclusions data sets); conversely, a Tertiary heating overprint well explain the data collected. Between the tested scenarios considering a Tertiary overheating, the one with an Early Tertiary thermal event seems to fit better data, even though a younger thermal peak is possible. Whatever the case, the choice of the thermal scenario significantly changes the calculated age of cements precipitation in the pore system of the studied reservoir rocks.

Automated summary

By integrating various methods in a study of Paleozoic and Triassic reservoir sandstones from the Ghadames-Illizi Basin, potential thermal effects on the pore network modifying diagenetic events were explored. Results suggest that a Tertiary heating overprint, particularly an Early Tertiary thermal event, better explains the data collected, significantly impacting the calculated age of cements precipitation in the studied reservoir rocks.