Abstract

In the Changmaigaon area of Upper Assam Shelf, Tipam sandstone of Miocene age is main oil bearing reservoir. The sandstones are massive in nature and were deposited by fluvial processes of braided and meandering rivers in continental environments. The fluvial depositional cycles of 10 to 20 m thickness are separated by thin shale bands and within cycles thin hard streaks of 1 to 2 m thickness and of variable composition occur intermittently. The thin hard streaks have very high velocity and density and due to this impedance contrasts between hard streaks and sands or shales are very high. The seismic response is composite of reflections from sands, thin shales and thin hard streaks and other non lithological changes such as fluid saturation. The subtle variations in reflections which may be caused by saturating fluids are often masked by lithological changes, mostly by hard streaks because of very high acoustic properties.