Unexpected Form of Oil Accumulation under Abnormally High Pressure Condition

Abstract

Oil and gas column heights and accumulation status within the reservoir can be understood from the relationship between top seal capacity and height of trap (level of spill point) . In previous studies, such evaluations were limited to hydrostatic conditions. This paper presents a study on abnormally high pressure conditions where pressure increases stepwise in sandstone layers toward the core of a folded structure, as in the case study offshore Sabah, Malaysia. First, the stepwise convex-upward shaped oil water contact (OWC) is investigated in the field, from which it is concluded that abnormally high pressures caused the conditions. A gradually progressive pressure increase toward the core axis within the folded structure contributed to the reduction in the column height of oil accumulated at the central part, showing a stepwise upwardly convex OWC. Second, an attempt is made to quantitatively predict conditions by applying data from only the exploration well. Estimated oil reserves based on this form of accumulation were found to be more than double that assumed by the conventional flat OWC. This concept will widen the scope of applications for future oil exploration.