Geochemical Analysis of Shemshak Shale Formation in Gushfil Mine (Iran): Paleo-Depositional Environment and Organic Matter Thermal Maturity

The present study investigated the geochemical characteristics of Shemshak shales as a probable oil source rock in the Gushfil mine located in the Sanandaj-Sirjan Zone (SSZ), Iran. Trace elements such as nickel, vanadium, chrome, molybdenum, and cobalt are used as paleoenvironmental indicators. Moreover, the ratio of these elements shows that oxic to disoxic conditions prevailed during the sedimentation period. The interrelation of these elements indicates that the upper part of Shemshak Formation of the Jurassic age was deposited in a terrestrial to the marine-terrestrial influenced environment. The solid bitumen reflectance (BR) documents that the black shales presently are overmature. Conjugation of BR and the insolubility of organic matter in carbon disulfide illustrates the presence of pyrobitumen and its subgroup epi- to meso-impsonite, which is also characterized by the absence of any fluorescence under ultraviolet light. The ratio of light to heavy hydrocarbons proves that the type of solid bitumen before pyro-bituminization has been a primary-oil solid bitumen, which could migrate through fractures and coarse pores. The primary-oil solid bitumen might be derived from Kerogen types II and III as documented by fibrous plant fragments, translucent phytoclasts and pollens. Presently, due to intense degradation, kerogen type IV dominates. The modeling confirms that a thermal degradation had probably occurred after the deposition of Lower Cretaceous carbonates when the shales were able to produce bitumen. Ultimately, intense hydrothermal degradation led the solid bitumen to evolve into pyrobitumen and caused the shales to evolve into a dry gas window.