High-quality source rocks in an underexplored basin: The upper Carboniferous–Permian succession in the Zaysan Basin (Kazakhstan)

The Zaysan Basin is a petroliferous basin in eastern Kazakhstan. Its upper Carboniferous–Permian sedimentary succession outcrops in the Kenderlyk Trough and includes, from base to top, the Kenderlyk, Karaungur, Tarancha, and Maychat formations, which contain 5 to 65 m-thick oil shale deposits—the principal subject of this study. Results from 49 outcrop samples show high total organic carbon (TOC) contents (1.2–21%, mean 7.8%), extract yields (1.2–15 mg HC/g rock, mean 6.8 mg HC/g rock), and ultimate expulsion potential (UEP; up to 23 mmboe/km²), making these oil shale deposits among the best source rocks in the world. Sedimentological, organic geochemical and organic petrographic analyses suggest an overall evolution of the basin from a deep/semi-deep lake during deposition of the Kenderlyk, Karaungur, and Tarancha formations to a deltaic setting during deposition of the Maychat Formation. The organic material in the Kenderlyk, Karaungur, and Tarancha formations is predominately composed of Type I and mixed I/III kerogens with high hydrocarbon generation potential (S₁ + S₂; up to 172.8 mg HC/g rock), high hydrogen index (HI; up to 838 mg HC/g TOC), and low oxygen index (OI; < 60 mg CO₂/g TOC). The sporadic influx of terrigenous organic matter resulted in layers with lower HI and higher OI index, and increased inertinite and vitrinite contents. The Maychat Formation includes hydrogen-poor Type III kerogen with low HI (< 200 mg HC/g TOC) and high OI index (> 60 mg CO₂/g TOC) in the Kenderlyk Trough. While a highly oxidizing environment during deposition of the Maychat Formation is postulated for the Kenderlyk Trough, it is likely that oxygen-depleted conditions in the depocenter of the Zaysan Basin favored accumulation of Type I kerogen. The oil-source correlation shows that the produced oils are chemically distinct from the source rock extracts of the Kenderlyk, Karaungur, and Tarancha formations. We propose that these source rocks are “massive”, where the retention of generated oil is too high, causing the hydrocarbons to “bleed” from the source rock's edge only. The expelled oil has probably charged the lower Permian deposits, which have not yet been explored. Regional geological cross-sections and seismic lines allow for selecting sweet spots, characterized by high TOC, yields, and temperatures needed for oil generation and unconventional hydrocarbon resource development. This study provides the play concept and a risk assessment analogue for tectonically and magmatically dynamic settings and basins with multiple organic rich strata. Furthermore, the results and proposed concepts may play a significant role in future petroleum exploration and development activities in the Zaysan Basin. In addition, this highly multidisciplinary study emphasizes the significance of integrating several data sources and weighing contradictory information to get the most reasonable conclusion.