Abstract
As part of our effort to improve pore-pressure estimation in diagenetically altered mudstones, we have used wireline logs to estimate disequilibrium compaction and unloading contributions to the hard overpressure encountered in the Bekapai Field, Lower Kutai Basin. The maximum vertical effective stress that the overpressured mudstones have experienced is estimated from the density log using Dutta9s relationship between vertical effective stress and void ratio. The sonic–density cross-plot is then used to estimate the sonic reference trend: that is, the expected sonic response if the mudstones were currently at maximum vertical effective stress. Finally, comparison of the sonic log with the sonic reference trend gives the unloading contribution to overpressure using Bowers’ unloading relationship between the vertical effective stress and velocity. In spite of poor data quality, fair results were obtained showing a steady increase in disequilibrium compaction overpressure below the top of the sharp pressure ramp. Immediately below the pressure ramp, the unloading contribution to overpressure dominates, with gas generation being the most likely cause. Our interpretation explains the pressure and wireline log data in this deltaic setting satisfactorily, resolving a debate on overpressure-generation mechanisms in the shelfal area of the basin that has been ongoing for 25 years.
Highlights
The Lower Kutai Basin is located in eastern Kalimantan (Fig. 1) and is the largest basin in Indonesia for gas production
We suggested that the density reversal observed in the deepest well below the sharp pressure ramp was a consequence of ‘chemical undercompaction’, a process in which porosity was preserved by very high pore pressure holding pores open while the mudstone matrix was being cemented by the products of clay diagenesis (Goulty et al 2012)
Most of the exploration wells drilled in the shelfal area of the Lower Kutai Basin terminated above the sharp pressure ramp, and others terminated within it
Summary
The Lower Kutai Basin is located in eastern Kalimantan (Fig. 1) and is the largest basin in Indonesia for gas production. Doubts had been expressed about the quality of the density logs, the low mudstone porosities observed in the zone of hard overpressure are inconsistent with the disequilibrium compaction mechanism. These low porosities prompted Schneider et al (1993) and Burrus (1998) to suggest that compaction depends on the Biot effective stress. We claim that our interpretation makes sense of the pressure and wireline log data in this deltaic setting, resolving conflicting suggestions about overpressure generation mechanisms in the shelfal area of the Lower Kutai Basin
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