Abstract: Evaluation and Detection of Overpressures in a Deltaic Basin: The Sisi Field Case History, Offshore Mahakam, Kutei Basin, Indonesia

Y. Grosjean, M. Bois, L. De Pazzis, J. Burrus

Widespread overpressure occurrences in the Mahakam delta area have caused a number of kicks and several blow-outs during earlier exploration. A review of pore pressure indications was conducted in conjunction with an extensive reinterpretation of seismic and well data basin wide. The iso-pressure lines were found to be broadly parallel to the facies change from sandy, delta-front deposits to outer shelf and slope shales, and are not associated with organic matter maturation or clay diagenesis. Overpressures are reached at depths ranging from 1000 m to more than 4000 m, depending on the facies. A 2-D numerical basin model calibrated on observed pressure profiles at wells indicated that the excess pressures were a direct function of the variable drainage efficiency of the formations by nterbedded sands.

The Sisi discovery located at the eastern, distal extremity of the upper Miocene deltaic sands, provided a unique opportunity for a more detailed analysis. A hypothesis of widespread decoupling between reservoir and shale pore pressures was tested against well data during appraisal drilling. D-exponent plots and gas shows were carefully monitored to assess the excess pressures, and quantitative estimates of the shale pore pressures were computed from sonic logs. A regionally consistent calibration was achieved, which confirmed very large discrepancies compared to the pressures measured in interbedded permeable reservoirs.

These conclusions have since been generalized to other areas of the basin, where they allowed safer drilling practices to be established as demonstrated by the observed reduction of lost time.

AAPG Search and Discovery Article #90982©1994 AAPG International Conference and Exhibition, Kuala Lumpur, Malaysia, August 21-24, 1994