The northern offshore part of the Cenozoic Song Hong Basin in the Gulf of Tonkin is at an early stage of exploration with only few wells drilled. A proven petroleum system exists in a narrow, structurally inverted zone characterised by a thick Miocene predominantly deltaic succession between the Song Chay and Vinh Ninh/Son Lo fault zones. These faults are separate splays belonging to the offshore extension of the Red River Fault Zone. Source rock to oil correlation indicates that Miocene deltaic coals are responsible for the sub-commercial oil-gas accumulations and shows in sandstones in two of the four wells drilled on faulted anticlines and flower structures. The coals are confined to an upper and lower interval. Seismic facies analysis suggests, however, the upper interval to be of limited areal extent. Extensive maturity modelling indicates that the lower coal-bearing interval presently is in the oil and condensate generating zone in large parts of the area, but in the dry gas window in the deep synclines between the inversion ridges. In the synclines the source rock interval entered the main oil window prior to formation of the traps formed as a result of Late Miocene inversion movements. Thermally immature, oil-prone Oligocene lacustrine mudstones with subordinate coals are exposed in a Cenozoic outlier at Dong Ho at the margin of the onshore Hanoi Trough immediately north of the study area. Based on mapping of seismic reflection characteristics – distinct continuous, high reflection amplitudes – similar lacustrine shale intervals seem to be preserved in a system of narrow Palaeogene halfgrabens in the offshore part of the basin north-east of the Song Lo Fault Zone. Large parts of these so far undrilled intervals are modelled to be in the oil and condensate window at present, suggesting that additional active Palaeogene source kitchens exist in the northern part of the Gulf of Tonkin.