The Mahogany Zone, Uinta Basin, USA: insights from the organic matter evolution of oil shale, Green River Formation source, maturity, and deposition

In this work, the organic matter content present in core samples from the Mahogany Zone of the Green River Formation was examined in order to better understand the sources of organic matter, thermal maturity, and environmental conditions of deposition. The preserved organic matter is rich in n -alkanes, especially long odd carbon chain n -alkanes (≥ C 25 ), which is derived from terrestrial sources and some algae. Contributions from an aquatic source were also identified during certain periods. This is aligned with records that showed that Green River Formation was formed during climate changes during the Eocene, with alternating periods of intense rain fall and drought periods, resulting in different organic matter sources. The n -alkanes detected ranged from n -C 14 to n -C 37 , with n -C 31 and n -C 29 prevailing. Typical proxies, such as carbon preference index (CPI) and n -C 17 / n -C 31 , were applied in order to assure that the main sources of organic matter were terrestrial and aquatic sources alternated. Types of kerogen were identified using the occurrence of pristane and phytane, which indicates that kerogen type III predominates. The same compounds indicate alternating periods of anoxic and oxic deposition of organic matter. The maturity was assessed by proxies involving selected individual hopanes and steranes. The results indicate that most of sections of core are composed of mature organic matter. Finally, the deposition occurred under mostly anoxic conditions. The stages of lake formation may have controlled deposition as well as the organic matter source.