OVERPRESSURE MECHANISMS IN COMPRESSIONAL TECTONIC OF BORNEO DEEPWATER FOLD-THRUST BELT

摘要

Borneo is a significant petroleum producer for Indonesia, Malaysia, and Brunei Darussalam. It is comprised of a regionally extensive deltaic depositional system with good reservoirs located primarily in the eastern and northwestern part. Easily accessed onshore and shelf plays have been exploited since the late 1800's. Recently, oil and gas exploration focus has been extended to the deepwater plays, which are geologically characterized by fold and thrust belts. Drilling deepwater Borneo prospects, particularly in the northwest, has been challenging due to subsurface drilling hazards related to pore pressure (PP) and wellbore stability, to the point that it often exceeds initial budget costs and hinders reaching geological targets. This study reviews the mechanisms that caused high overpressure in deepwater Borneo and revisits the common PP prediction methods. The current established empirical effective stress-based PP prediction is dissected to reveal its pitfalls, particularly with respect to application of the common compaction mechanism assumption against the compressional tectonic nature of the area. This study confirms that active compressional shortening of the mega fold/thrust belt, accompanied by continuous rapid deltaic sedimentation, significantly affects the pore pressure (PP) regime of the area, and the lack of understanding of the subregional geology can lead to PP underestimation. Dual compaction trend solutions are recommended to help avoid PP prediction pitfalls. Kaeng et al. 2014) because of subsurface hazards related to PP and wellbore stability. It is believed that this is attributed to the distinct overpressure characteristic of Borneo's deepwater compressional fold and thrust belt system.