Multi-stage hydrocarbon migration and accumulation of Permian petroleum system in the Zaysan Basin, NE Kazakhstan

The Zaysan Basin in northeastern Kazakhstan is a petroliferous basin that closely correlates with the Central Asian Orogenic Belt (CAOB). The basin is characterized by multi-stage hydrocarbon migration and accumulation, while the details are poorly understood. Hydrocarbon accumulation processes were investigated within the constraints of the regional tectonic evolution and the timing of trap formation. Along with assessment of geochemical characteristics, hydrocarbon generation and accumulation models were established. Light blue-green fluorescence colours of intergranular oil were identified for the deep Permian and Jurassic reservoirs, but they were absent within the shallow Paleogene Severozagsan reservoirs. Geochemical results indicate that source rocks for the oils were deposited in anoxic brackish-saline environments, and mainly contain type I and type II kerogen. Oil and extracted bitumen from the Permian and Jurassic reservoirs originate from the same Permian source rocks and show low maturity. The biodegradation of oils within the Permian and Jurassic reservoirs, however, indicates different alteration processes. Paleogene gas is the biodegradation product of the oils. In the study area, two stages of oil charge and one stage of gas charge were identified. The first oil charge began about 207 Ma (Late Triassic) when the source rock entered the oil window. The oil migrated into the Permian reservoirs and subsequently underwent biodegradation. The second oil charge took place about 70–100 Ma and was controlled by Late Cretaceous cooling and exhumation events. Oil in the Jurassic reservoir originated by oil remigration from the Permian reservoir. The gas charge occurred during the Late Cenozoic. The gas was mainly the biodegradation of oils and accumulated in Paleogene reservoirs.