Structural Geometry Across India's Oldest and Still Producing Digboi Oil Field, North-East India

Balanced cross section is a technique that allows derivation of geometrically and kinematically valid structural geometry. We constructed five balanced cross section across Digboi oil field, which is the oldest and still producing oil field in India, as well as across adjacent Kusijan oil field. Both the oil fields are present in the hangingwall of the Naga Thrust, which is the frontal most thrust of the Naga Schuppen Belt in NE India. Naga Schuppen belt formed as a consequence of Tertiary collision between Indian plate and west Burma plate. Apart from these two oil fields no major hydrocarbon discoveries have been made in the hanging wall of Naga Thrust or in the hinterland direction of Naga Schuppen Belt. However, several hydrocarbon fields have been discovered in the footwall of Naga Thrust. All the five cross sections show correlatable ramp anticline above the Naga Thrust. The ramp-anticline can be modelled as fault-propagation fold with high angle breakthrough. In the Kusijan area, the sections show blind splay fault branching out from Naga Thrust leading to anticline-syncline pair. All the five cross sections have detachment at the same stratigraphic level (Tikak Parbat Fm.) but the depth to detachment varies from 4.1 km at Kusijan field to 5.2 km at Digboi field. A comparison among the five cross sections shows variations in parameters such as shortening, displacement along the detachment and dimensions of fault ramp. In Digboi oil field, breakthrough is at deeper stratigraphic horizon compared to Kusijan field. The shortening amount in Kusijan field is about 38.2% to 42.7% while in Digboi field it is 37.3% to 37.6%. Thus, shortening in Kusijan area is somewhat more than Digboi area. This differential slip may be responsible for the "Assam Railroad Tear Fault" separating the two areas. The ongoing section-balancing project across the entire Naga Schuppen Belt will help us understand structural evolution of this collision belt as well as hydrocarbon prospectivity in the hinterland of the Naga Thrust.