Abstract

The early history of the West Natuna Basin is similar to that of many Sundaland basins, with Late Eocene to Early Oligocene extension creating a complex system of rift basins. From earliest Miocene times these basins were progressively inverted to form folds as a result of a right-lateral stress regime. Wrench zones also developed by reactivation of northwest-southeast orientated faults within the earlier rift system. The displacement across these faults is relatively small. For the example in the Cumi-Cumi PSC the displacement is of the order of 1 to 2 km.

The inversion histories of graben within the Cumi-Cumi PSC are examined in detail to show how the inversion of the West Natuna Basin developed. Restored and balanced cross sections, generated using the GEOSEC modelling software from Cogniseis Development Inc., have been used to help determine the original rift geometry, quantify extension and shortening, and to forward model the inversion process.

The magnitude of inversion depends on the initial size and orientation of the original half graben. The position of the graben within the basin may also be significant. Each graben has a unique inversion history within the overall framework of Miocene inversion.