Balanced Cross Sections and Kinematic Evolution Across Digboi Oil Field, Northeast India

Abstract

Digboi oil field, the oldest continuously producing oil field in India, is known to occur in an anticlinal trap (Digboi anticline) in the hanging wall of the frontal most thrust (Naga thrust) of Assam Arakan fold thrust belt. Since its discovery in 1890, about a thousand wells have been drilled in the Digboi anticline. The kinematic evolution of the Digboi anticline, however, is not well understood. In this article a geometrically valid balanced cross section and derive an internally consistent evolutionary model for the Naga thrust and related Digboi anticline are presented. It is shown that the Digboi anticline is a ramp anticline and can be best modelled as a fault-propagation fold with a high-angle breakthrough. There are two possible solutions (Model 1 and Model 2) to model the seismic events (sub-thrust high) in the footwall of Naga thrust. Model 1 considers the sub-thrust high to be real and conjectures a buried ramp anticline (Kusijan Anticline). Model 2 considers sub-thrust high to be a velocity pull up and the footwall rocks are undeformed. Both the models are geometrically and kinematically valid. Model 1 accommodates a total shortening of 32.9% while Model 2 accommodates a total shortening of 30.1%. From the present structural work and interpretation of seismic profiles, possible loci of branch line of Naga thrust along Digboi oil field is also traced.