Diagenesis- and thermal maturity-evolution of the Silurian unconventional hydrocarbon deposits (Tassili n’Ajjer plateau, Algeria): Clay mineralogy, graptolite reflectance, and K–Ar dating

The need for determining the thermal maturity of Lower Paleozoic rocks has increased with the intensified unconventional shale oil/gas resources exploration within North Africa, Arabian Peninsula, and the adjoining regions. Indeed, Lower Paleozoic strata represent an increasingly important resource that is related to the globally widespread occurrence of graptolite-bearing marine sediments in an expansive shelf area of the Gondwana. However, some crucial information for basin analysis remains poorly understood even across the largest hydrocarbon-producing province in Algeria.
The present study provides a solid understanding of the diagenesis- and thermal maturity-evolution from the Tassili n’Ajjer plateau while involving multiple and independent approaches, such as clay mineralogy, petrography, graptolite reflectance, source rock maturity, as well as illite K−Ar geochronology. The combined use of X-ray analysis and field emission scanning electron microscope evidenced kaolinite, illite, and iron-rich chlorite as the main authigenic mineral phases. K−Ar data indicate that episodic in situ illite crystallization occurred at different times, the oldest illite at about 335 Ma and the youngest illite between 238 to 179 Ma, under diagenetic-to-hydrothermal conditions. Paleotemperature estimates (~165–232°C) derived from illite crystallinity (0.37–1.58 Δ°2θ), and graptolite reflectance (VReqv, 1.09–1.84%) values indicate deep diagenetic-low anchizone boundary conditions. This is broadly suggestive of late oil-to-dry gas zones of hydrocarbon generation and destruction, notably in the western sector of the study area.
At least two heating events and diagenetic fluid flow processes are identified mainly in response to various tectonic events. They are largely due to fault reactivations, and migration of hot, potassium-rich, fluids throughout the Phanerozoic. Additionally, these events and processes had a later influence on the hydrocarbon maturation, migration, and/or entrapment, especially along the inherited N-S lineaments and Hoggar Massif mega-shear zones in the westernmost part of the Tassili n’Ajjer plateau.
Lastly, it is important to highlight that the promising areas offering the highest potential for future unconventional hydrocarbon resource exploration could be most likely those bordering major lineaments, coupled with subsequent igneous activity, where unrestricted hydrothermal fluids are frequently reported, as well as the maturation, being much more advanced.