Abstract

Post Miocene tectonics is believed to have caused final structurization that has resulted in entrapment of hydrocarbons across major fields of North Assam shelf. Two major fault trends are identified in the region; the earlier Permian age rifting associated down to basement sub-longitudinal faults and the later cross trends associated with the main Himalayan orogeny. During the later phases of deformation the earlier faults were reactivated and oblique movement along them often resulted in renewed movement and modification of earlier fault geometry (commonly referred to as inversion), formation of en-echelon oblique shears and associated folds. These features associated with both the major fault trends have been mapped in a small sized oil field of Upper Assam. It was also remarkable that such features are best developed in incompetent lithologies, such as interbedded sand/shale of Oligocene sequences. The variation in intensity of deformation from older to younger sequences has also been brought out. The study also identifies new prospective structures, associated with shear faults, in the deeper Palaeozoic sequences, on the rising flanks of the regional low