Silurian to early Devonian overmature hot shales in Eastern Taurus: Source rock characteristics and links to North African petroleum systems

Abstract

The study area is located in the Eastern Taurus region and is characterized by the Paleozoic autochthonous Geyik Dağı Unit (GDU) in the northern Paleo-Tethys back-arc basin. The Eastern Taurus encompasses Silurian-Early Devonian units, which may have the characteristics to be called hot shales. In the study area, the Silurian black and organic lean shales and Orthoceras-bearing limestone and shale alternation have directly deposited on the Late Ordovician (Hirnantian) peri-glacial clastics. However, while Hot shale-I unit was deposited over the ice cover rebound-related paleo-depression areas in the southern Paleo-Tethys (North Africa) during the early Silurian (Rhuddanian- Early Telychian), turbiditic black lydite successions were deposited due to the radiolarian bloom in the northern Paleo-Tethys (Taurus Belt). Therefore, the studied black shales overlying the lydite successions correspond to the late Telychian hot shale II (Sheinwoodian and Pridoli- Lochkovian), which has only been defined in the Moroccan basins. On the other hand, both lydite occurrence and late Telychian-Sheinwoodian, and Pridoli-Lochkovian black shales (equivalents of Akakus and/or Tadrart formations) indicate that the Taurus basin under distinct paleoclimate conditions witnessed different bathymetry and nutrients source (e.g., silica). In addition, pyrolysis results indicate that the studied black shales belonging to GDU have lost 85–98% of their HC potential due to the overburden resulted from the allochthonous units. However, isotopic composition of the oils from the Adana basin and from the surface seepage samples show that these oils were expelled from Devonian shales. According to the pyrolysis and isotope data, we suggest that there may be a Pridoli-Lochkovian source rock unit in the Eastern Taurus, unlike the North Africa basins (except for Tadla basin). These data indicate that the basins developed over the Silurian-Early Devonian succession, which are believed not to have been affected by the nappe events, could have yielded an economic petroleum system over the onshore and/or offshore of eastern Mediterranean basins.