Rifting and regional subsidence characterize the Cuu Long and Nam Con Son basins, offshore southern Vietnam. Initial rifting began in the Eocene-early Oligocene, followed by the uplift and rotation of the crustal blocks in the late Oligocene. The erosion of the uplifted blocks marked the transition from rifting to regional subsidence in the Cuu Long Basin. A second phase of rifting began in the Nam Con Son Basin, lasting until the late Miocene. Parts of the Nam Con Son Basin experienced inversion in the middle to late Miocene.
The synrift and postrift units in the Cuu Long Basin consist of nonmarine deposits and paralic to shallow-marine sediments, respectively. The synrift deposits in the Nam Con Son Basin can be divided into the early synrift unit, corresponding to the initial rifting phase, and the late synrift unit, deposited during the second rifting phase. The early synrift and late synrift units consist of nonmarine sediments and nonmarine to shallow-marine sediments, respectively; the postrift unit is composed of shelf and deeper marine deposits.
The data compiled from the published reports suggest that the Cuu Long Basin is oil-prone, with the oil reservoired mainly in fractured basement highs, whereas the Nam Con Son Basin is generally gas-prone, with the gas trapped in Miocene sands and late Miocene carbonates. These distinct trends may be attributed to differences in timing of trap formation and the disruption of trap integrity caused by prolonged rifting and inversion in the Nam Con Son Basin.