Abstract

The Nam Con Son Basin is an extensional Tertiary basin formed as a result of Indochina's extrusion and East Vietnam Sea spreading. The basin has been under an extensive exploration programme since 1989, and a number of oil and gas discoveries are being developed.

The sedimentary section of the Nam Con Son Basin can be divided into distinct syn-rift and post-rift structural units, which are separated by the Mid-Miocene unconformity. The Middle Miocene syn-rift section is affected by NE-SW faults and comprises interbedded sandstones, siltstones and mudstones; the net to gross ratio for sandstone is quite high. The Late Miocene post-rift section is much less affected by faults and consists of progradational sequences that have rapidly filled the basin from west to east and progressively eliminated large carbonate build-ups of Upper Miocene to Recent age.

The central area of the Nam Con Son Basin is overpressured. The vertical distribution of over-pressure is a function of post-rift undercompaction and syn-rift inflation. Sedimentary processes control the post-rift compaction, and the Mid-Miocene unconformity plays an important role in determining the syn-rift pressure. Higher inflation from syn-rift horst blocks may have resulted in more undercompaction of the overlying post-rift section, and this facilitated processes for migration and accumulation of hydrocarbons.

Once the factors controlling overpressure have been identified with some degree of confidence, then future exploration targets may be better defined and well pressure prognoses can be constructed.