The depositional model of the Lower Cretaceous Nantun Formation and its implications for oil accumulation in the northern Wuerxun Sag of Hailar Basin, NE China

The northern Wuerxun Sag is a lacustrine rift basin which experienced multiple faulting and subsidence during the Lower Cretaceous. The tectonic evolution with uplifted regions as the provenances caused the complex sedimentary fill of the Lower Cretaceous Nantun Formation. In this paper, we use well log, core and 3D seismic data to characterise the sedimentary facies and depositional environment of the Nantun Formation, and consider how this affects the distribution of hydrocarbons within the basin. Four depositional systems are recognised in this study: fan deltas located along the steep slopes; braided deltas along the gentle slopes; sublacustrine fans and lacustrine in the centre of the basin. Fan deltas are mainly composed of coarser sediments with lower sediment maturity than braided deltas, and are mainly located in steep slopes with rift border faults. Sublacustrine fans form lenticular depositional bodies in the basin centre, with higher sediment maturity as a result of reworking. Three sediment-transport pathways were recognised which governed the sediment deposition in this basin, which are (1) the steep rift border fault in the northwest, (2) the NE–SW Surennuoer strike-slip fault zone (SSFZ), and (3) the major faults in the south-eastern gentle slope. Most oil is found in sublacustrine fan sandstones which were transported along the SSFZ mainly from shallower fan deltas, with underlying mudstones (SQN1) as effective source rocks and overlying thick and laterally continuous lacustrine mudstones (SQD1) as seal rocks. The good source–reservoir–seal assemblages enable prolific near-source oil distribution in this area, suggesting the sedimentary controls on oil accumulation, which may guide future oil exploration and production activities in this area.