Publications

South Mahakam is an offshore area consisting of three gas fields which are structurally controlled by the strike-slip Sepinggan fault. The fields are separated by normal faults that have curved out in an en-echelon style from the main fault as a consequence of its right lateral movement. The normal faults have throws varying from tens to hundreds of meters and play an important role in creating intra-field compartmentalization. Fault seal analysis was performed on the normal faults in the area, focusing on the Tertiary clastic, Sepinggan Deltaic Sequence. The sequence ranges from delta-plain to pro-delta environments, with pore-pressure increasing from hydrostatic to highly overpressured towards distal areas. Analysis was performed using log and grid data, and calibrated with available dynamic information. Results show the high sealing nature for all faults, confirming their role in the trapping mechanism. Fault planes were dominated by sand-shale juxtapositions, high Shale Gouge Ratio (SGR > 0.6), and very low fault permeability (kf < 0.01 mD). Geomechanics parameters also show that faults tend to dilate (tendency of 0.8) than to slip (tendency of 0.3). This dilatation tendency increases with depth, and towards distal areas, along with an increase in pore pressure, as illustrated in the Mohr diagram. This study is a case where shale domination in a deltaic sequence can control the sealing behavior of faults, exhibited by strong correlation between sealing parameters and Vshale. High shale content that has been equally distributed vertically and laterally in the area, is proven to be the main factor for the sealing parameter regardless of the throws. Comparison to published literature also shows that the sealing nature is likely to be originated by shale-smear mechanism.