THE CHARACTERISTICS OF THE SEPFNGGAN STRIKE SLIP FAULT ZONE AND ITS ROLE IN FORMING STRUCTURAL TRAPS THE SOUTHEAST KUTEI BASIN

摘要

The Sepinggan Fault is a right-lateral strike-slip fault trending SE-NW extending more than 70 km from offshore southeast Balikpapan to onshore southern Penajam area. It is part of the bigger Adang fault zone with an oblique strike direction against the main Adang fault's strike line in Southern Kutei Basin. Several significant hydrocarbon traps have formed along Sepinggan Fault's principle deformation zone through transtension and transpression mechanisms. Sepinggan, Mahoni, and Yakin Fields are among the main fields formed by Sepinggan Fault deformation. The Sepinggan fault exhibits near vertical fault dip and shows indication of semi helical pattern commonly observed in strike slip fault. In map view, Sepinggan fault has deformed pre existing N-S faults, offsetting them in a right lateral sense. Seismic data indicates the pre existing N-S faults have been offset more than two kilometers by the Sepinggan Fault. The right lateral slip assessment on Sepinggan fault can be done with what we called iso-depth fault kinematic analysis. Composite maps were developed from stratigraphic horizons of different ages which exist at the same depth across the fault, and are thus within the same mechanical force regime along Sepinggan Fault Zone. This analysis shows consistent transpression folds on the left bending fault segments and transtension pull a part structures on the right bending fault segments, which confirm the right lateral sense of movement on Sepinggan fault zone. These variations in fault bends either left or right and the relationship between Sepinggan Fault to other faults are the factors to the formation of structural traps. Compressional anticline associated with reverse faulting style traps formed at left bending segments of the Sepinggan Fault while rotated fault block trap will be the typical style at the right bending segment. A consistent fault behavior also serves as a predictive tool in assessing potential traps along the fault.