Abstract

The Tertiary sedimentary basins of Nam Con Son and Cuu Long lie off the southeast coast of Vietnam and have become important producers of oil and gas for Vietnam. Source rocks are considered to be Oligocene carbonaceous claystones in the Cuu Long Basin and Oligocene coal measures in the Nam Con Son Basin. The present-day thermal regime in the two basins is analyzed from bottom hole temperatures and thermal properties of the sediments for 35 exploration wells. The average steady-state surface heat flow is estimated to be 70 ± 5 m W/m² for the Cuu Long Basin and 80 ± 8 m W/m² for the Nam Con Son Basin about 100 km towards the southeast. There is evidence to suggest a slight increase in heat flow towards the south in the Nam Con Son Basin.

Thermal modelling of basin evolution, using a 1-D finite element code that accounts for transient thermal effects, is used to predict the timing of oil and gas generation. High recent sedimentation rates have contributed to masking the higher basal heat flow in the Nam Con Son Basin, relative to basal heat flow in the Cuu Long Basin. The effects of this high basal heat flow towards the southeast, and the kinetic parameters used for the differing source rocks in the two basins, however, combine to create similar hydrocarbon generation with depth relations in the two basins. Their depth range for the oil expulsion window is about 3 to 4 km. In addition to similar oil generation histories, the Nam Con Son Basin is modelled to have also generated significant quantities of gas, due to the greater depth of sediments (up to 10 km) and the prevalence of terrestrial type III source rocks.