Application of the dating by fission tracks to determine thermicity of basins within petroleum potentialities: example of Sbaa and Ahnet-North basins located in Western Saharan platform, Algeria

The interpretation of old fundamental research works and the results obtained by this study based on: the analysis of apatite fission tracks (AFT), the burial evolution curves and the sequential analysis of outcrop section, as well as the curves of signal of gamma ray (GR) analysis from drilling wells, allow to precise that the geodynamic evolution of the studied Algerian sedimentary basins (Ahnet and Sbaa) have been occurred principally in the Paleozoic era. The analysis of burial evolution curves shows that during the Paleozoic period, the sedimentation is controlled by the tectonic subsidence, particularly at the end of Carboniferous in the favor of Ougarta folding. The curves indicate an uplift from the Permian to middle Jurassic, expressed by an important erosion of sedimentary series and is considered as a response of thermal convection of the Saharan platform followed by thermal subsidence. This can be explained by the halt motion of rift extension caused by the opening of the Atlantic Ocean. To bring the sequential analysis out, it appears that Ahnet and Sbaa basins present contrasted stratigraphic recordings, so much in time than in space, indicated the different effects from subsidence and erosions according to their sectors. In thermal point of view, temperatures are still moderate in Sbaa depression and favorite the preservation of organic matter and might generate hydrocarbons until now. However, these conditions are not similar to ones in the case of Ahnet basin. In the North part of Ahnet basin (MRS-1, MSL-1), the ages are around 50 Ma, attested that sedimentary layers have been sustained a post-Hercynian thermal phase. This phase could be estimated probably more than 100 C. This is also produced during the regional extension of the Triassic-Jurassic rifting. This episode could also be responsible of disappearance (total?) of pre-existence fission tracks in the Devonian layers of the well MSR-1, which exhibits at the depth 505 m under the Hercynian unconformity an age FT of 100 my. In the south part, ages are respectively 37 and 26 Ma at the depths: 1030 m (MKRN-1) and 1532 m (BH-5). These suggest that Cenozoic thermal alteration of tracks is more intense than in the north of the basin and may reflect important erosion, eventually accompanied with high geothermal gradients. In conclusion, three thermal episodes in the studied area have influenced the formation and the migration of hydrocarbons and consequently their nature; Varisc orogenesis, the Atlantic opening to Jurassic and the thermal convection of the Hoggar at the Eocene age. Clearly, the latest is the most event which affects the hydrocarbons of Ahnet reservoirs as shown by the increased temperatures from the north to south of the studied area. A secondary cracking occurred certainly during Eocene transforming oil to gas. Also, the outcrop studies allow defining the deposits influenced by storms (shallow depth ≤ 100-150 m) in the sedimentary series of the Ougarta ridge and the evident deposits of Jokulhlaups in the unit IV in Kheneg Etlaia, locality (Ougarta). This can be observed distally at the Cambrian-ordovician of the well MSR-1 (Ahnet). (author)