Abstract

The northern part of the Nam Con Son Basin of Vietnam is a functioning petroleum system that has not lived-up to initial industry expectations. Analysis of data from Block 04-2, in the northern sub-basin, provides reasons for the limited exploration success.

Structuring in the Nam Con Son Basin follows a pattern similar to other rift basins in Southeast Asia. Rifting commenced in the Eocene-Oligocene, culminated at the end of the Middle Miocene and was followed by a thermal sag phase up to the Present. Inversion at the end of the Middle Miocene is not present in Block 04-2.

The main clastic reservoir targets in the basin are Middle Miocene shallow marine and deltaic sandstones. They contain a high concentration of ductile grains that were squeezed into the pore spaces during compaction, limiting the reservoir effectiveness to approximately 3500m burial depth.

Source rocks are present in the Lower Miocene Dua Formation and are suspected to be present in the Eocene-Oligocene. Vitrinite suppression makes assessment of thermal maturity using Ro data alone, difficult. Apatite fission track analysis demonstrates that the sediments are presently at maximum temperature, although a late heat pulse possibly occurred over the last 2 Ma. This is supported by the identification of Plio-Pleistocene subaqueous volcanoes.

The prospective source rocks are located towards the base of the syn-rift sequence. They were deeply buried early in the basin history and expelled most of their oil during the Middle Miocene. Trap formation and deposition of a regional seal was only achieved during the post-rift, thermal sag phase of the basin (Late Miocene). By this time the syn-rift sequence was generating dominantly gas.

The relationship between hydrocarbon generation, trap formation, reservoir quality and sealing capabilities results in a narrow, gas prone, play fairway in the northern sub-basin. A four-dimensional petroleum systems chart is an effective illustration of the system.