Depositional environment and source rock characteristics in the Eastern Paratethys – Interpretation of organic matter in the Kura Basin (Azerbaijan)

Abstract

The Paratethys and its predecessor, the Eocene Peri-Tethys, underwent several phases of oxygen-depletion, which led to the deposition of organic-rich sediments. These sediments serve as hydrocarbon source rocks in the Paratethys region, but also provide a record of the Paratethys´ evolution during the Cenozoic. The Cenozoic succession in Azerbaijan, located in the Eastern Paratethys, comprises three organic-rich units: (i) the Eocene Middle Koun Member, (ii) the Oligo-Miocene Maikop Group, and (iii) Upper Miocene paper shales in the Diatom Formation. This paper investigates the depositional environment of these units, the origin of their organic matter, the type of oil generated, and the thermal stability of the organic matter using maceral and biomarker analysis, pyrolysis-gas chromatography, and kinetic studies. The Middle Koun Member was deposited during the Middle Eocene Kuma anoxia, which affected large parts of the Caucasus region. It contains very high amounts of organic matter. Locally, the total organic carbon [TOC] content exceeds 10 % by weight. The organic matter is derived mainly from marine diatoms, Tasmanites algae and dinoflagellates. Evidence of methanogenic/ methanotrophic archaea and sulphate-reducing bacteria proves strictly anoxic conditions. Although the Middle Koun Member was deposited at greater water depths than formations of the same age in the western and northern Caucasus region, the characteristics of the organic matter are similar. The Upper Maikop Formation in Azerbaijan generally contains moderate amounts of TOC (~2–3 %) and a high proportion of terrigenous organic matter that has been deposited in a marine basin with reduced oxygen availability (Maikop anoxia). Within this succession, there are several distinct layers with higher TOC contents (≤5.12 %), which contain either marine or lacustrine algal material. A layer with organic matter of lacustrine origin in the upper part of the formation reflects the onset of the late Early Miocene (“Kozakhurian”) low-salinity event and provides the first evidence of an oil-prone source rock associated with this event outside the Black Sea. The paper shale interval in the Diatom Formation has been deposited at the shallow western margin of the South Caspian Basin during a late Miocene lowstand that caused fragmentation of the Paratethys and reduced salinity. Changes in biomarker composition reflect the decline in salinity. The very high amount of organic matter in the paper shales (locally >15 % TOC) originates mainly from algae (including the freshwater algae Pediastrum ), diatoms, dinoflagellates, and microbial mats. Water column stratification during the early stages of paper shale accumulation contributed to oxygen depletion. The organic matter at the study sites onshore Azerbaijan is thermally immature. At oil window maturity, the Middle Koun Member and the paper shales will generate a paraffinic-naphthenic-aromatic oil. However, the wax content of paper shale oil will be slightly higher. The Maikop sediments will generate different oil types. A high wax paraffinic oil is predicted for the lacustrine sample. The organic matter in the paper shale is thermally more stable than that in the Middle Koun Member. Except for the lacustrine sample, hydrocarbon generation will occur over a wide temperature interval (40–60 °C).