Multi-stage primary and secondary hydrocarbon migration and accumulation in lacustrine Jurassic petroleum systems in the northern Qaidam Basin, NW China

• Primary and secondary hydrocarbon accumulations in the northern Qaidam Basin, China.
• Two-stage oil and one-stage gas charges.
• Integrated oil-bearing fluid inclusion analyses.
• Lacustrine Lower and Middle Jurassic petroleum systems.
• Hydrocarbon remigration from deep Paleogene to shallow Neogene reservoirs.

To improve the understanding on lacustrine Jurassic petroleum migration and accumulation in the northern Qaidam Basin (NW China), we conduct an integrated analysis of the petrography and geochemistry of oil-bearing fluid inclusion, including inclusion petrography, homogenization temperature combined with reservoir burial and thermal history, and reservoir sequential extraction. Results show that during the first stage of late Oligocene to early Miocene, the deep Paleogene reservoir was first charged by oils sourced from Middle Jurassic rocks (petroleum inclusion). This charge was accompanied by some oil alteration. During the second stage of early to middle Miocene, the deep Paleogene reservoir was charged by Lower-Jurassic-sourced oils (intergranular free oil) and this charging event is different from the first one in that it witnessed little hydrocarbon alteration. During the third stage in the Pliocene, widespread gas sourced from Lower and Middle Jurassic rocks occurred in both the deep Paleogene and shallow Neogene reservoirs and primary oil accumulations in deep Paleogene reservoirs migrated vertically to shallow Neogene reservoirs along faults, and formed secondary accumulations. Little alteration took place, favorable for the remigration. Thus, the lacustrine Lower and Middle Jurassic petroleum systems in the northern Qaidam Basin represent multi-stage primary and secondary hydrocarbon accumulations and mixing of hydrocarbons was very common.