Abstract

A joint study between LASMO and PERTAMINA was carried out during 1996-7 to evaluate the Palaeogene hydrocarbon potential of the Northern Kutei Basin. As a result of this study an updated structural and stratigraphic framework was developed for the area which has implications for the Neogene section of the basin as well.

The structural model breaks the basin evolution into 4 discrete phases: 1) Middle to Late Eocene extension. 2) Late Eocene to Oligocene sag phase subsidence. 3) Late Oligocene to Lower Miocene renewed extension and subsidence. 4) Early Miocene to recent delta progradation and basin in-filling coincident with inversion of older depocentres with axis of deformation moving progressively eastward with time.

Restoration of regional geological cross-sections has enabled recognition of the topographic complexity associated with the development of the Middle Eocene extensional fabric and has enabled an understanding of the rapid facies variations that occur with small extensional depocentres. Individual rift half grabens are approximately 20 km across and up to 70 km long. Those proximal to the eroding hinterland received large amounts of coarse clastic sediment of alluvial fan to deltaic facies, but more distal depocentres accumulated marine shale. Intrabasinal highs accumulated either thin clastic intervals or limestones.

A more regional depocentre developed in response to regional subsidence in the post rift phase beginning towards the end of the Upper Eocene, and resulted in a more continuous facies distribution, which exists up to the present day.

The inversion process created two distinct deformation styles: 1) Classic Sunda fold inversion anticlines within the Palaeogene section. 2) Detached tight anticlines within the thick Neogene section. Structural modeling using GEOSEC has shown that within the error range of the technique and interpretation, the overlying detached section has approximately the same amount of shortening (10-15%) as the deeper inverted section. This type of model, developed on the basin margin provides a good analogue for the Samarinda Anticlinorium to the south where the Palaeogene section is deeper than can be resolved on present seismic.