Abstract

The first hydrocarbon discovery from the basement rocks in India was encountered in the year 1970 in the Borholla-Changpang field of Upper Assam Shelf. The fractured/weathered Basement and the Basal Sandstone Formation constitute the primary reservoir in this field. Although the presence of hydrocarbon is established in the basement, a detailed analysis of their reservoir nature using well logs and cores has not been carried out so far. In this study, petrophysical core studies were carried out and integrated with well-logs to evaluate the reservoir nature of the basement of the Borholla-Changpang field. The study shows that although the basement is mostly massive and devoid of any reservoir properties, there are zones of weathering and fractures that have provided the essential porosity and permeability in an unconventional basement. The study identifies four facies based on their capability to hold and flow hydrocarbon within the drilled basement and helps in reducing the uncertainties in the formation evaluation of the basement in the study area.