The nature and significance of regional unconformities in the hydrocarbon-bearing Neogene sequence offshore West Sabah


Author : B.K. LevellPublication : Bulletin of the Geological Society of MalaysiaVolume : 21Page : 55-90Year : 1987


Description

Bulletin of the Geological Society of Malaysia, Volume 21, Dec. 1987;  pp. 55 – 90

 

The nature and significance of regional unconformities in the hydrocarbon-bearing Neogene sequence offshore West Sabah

B.K. LEVELL

Sabah Shell Petroleum Co. Ltd.

 

Abstract: The Tertiary West Sabah basin is a trench-associated sedimentary basin containing up to 12 km of predominantly siliciclastic sediments. The basin history can be divided into two phases:

1. Pre-Middle Miocene deposition of deep marine deposits with tectonic imbrication related to south southeastward convergence along the fore-runners of the Palawan Trough/NW Borneo Trench.

2. Middle Miocene and later deposition, after the cessation of subduction, by a series of northwestward-prograding shelf/slope sequences associated with important wrench-faulting in the basement. 

Although a small amount of oil and gas has been discovered in the Pre-Middle Miocene deep water deposits, all commercial accumulations discovered to date are in the Middle Miocene and younger deposits.

The boundary between the two sequences is an unconformity at the landward margin of the basin, where deformation was the most intense, with terrestrial or coastal deposits directly overlying deep marine sediments. This major unconformity, the "Deep Regional Unconformity" is possibly related to the end of active subduction along the Palawan Trough/N.W. Borneo Trench in the Early Middle Miocene.

Deposition of the Middle Miocene and younger sequence was characterised by syn-depositional tectonic deformation. However, accelerated rates of deformation at certain times resulted in the formation of five regional unconformities which provide the correlation framework of the basin. These unconformities are: the Lower and Upper Intermediate Unconformities (LIU and UIU) in the late Middle Miocene, the Shallow Regional Unconformity (SRU) in the middle Late Miocene and Horizons II and I in the Pliocene and Pleistocene respectively. Each of these unconformities was the product of both local structure formation and a regional tilting down towards the northwest. Typically each unconformity passes from an erosion surface to an onlap surface towards the NW.

From the lateral extent of the unconformities it is argued that the inner part of the NW Sabah basin is underlain by at least six separate basement blocks which were internally deformed only at certain times and remain undeformed at other times.

The style of deformation also varied temporally. Erosion of the UIU and Horizon II were both being preceded by open flexural folding and tilting whereas the LIU and SRU were associated with tight folds and reverse or strike slip faulting.

The unconformity maps demonstrate, despite the local structural complexity, the seaward migration of successive unconformities as the landward basin margin was progressively uplifted. This pattern is familiar from other trench-related/fore-arc basins.