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ABSTRACT: Seismo-geological modelling explains intrabasement reflections: a case study from upper Assam oilfields, India

Sibal, V. K., K. K. Nath, S. N. Singh, and B. N. Singh, Oil India Limited, Duliajan, Assam, India

The deepest mappable seismic horizon in southern part of the operational area of Oil India Limited in Upper Assam, India, is encountered around 3.0 to 4.5 Sec. (TWT) which corresponds to the possible granitic basement. During analysis of the seismic data recorded in the area, a well-defined reflector was noticed below the possible basement, which was thought to be either due to the possible presence of Pre-rift phase sediments, or as non-genuine seismic artifacts.

The authors have tried to explain the above reflector through seismic modelling studies in the light of the established geological information. It is envisaged that the upheaval of Brahmaputra Arch in the region in mid-Miocene period, might have reactivated the pre-existing normal listric faults formed in the basement during rifting of Indian plate. Considering that these seismic signatures could have been due to the possible extension of these listric normal faults into the basement, the modelling studies were carried out through seismic ray tracing. The effect of "tuning" and the relative shift of the said reflections in relation to the location of the listric faults and the Brahmaputra Arch could be brought out. The well drilled in the vicinity encountered basement rocks prior to the penetration upto the questionable reflectors. This paper emphasizes in the importance of seismo-geological modelling through intelligent integration of geology, geophysics and the interpreter's acumen in providing definite leads in explanation of seismic signatures where the possible causes could be non-unique.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia