Abstract

The recent findings of oil occurrences in Gondwana sediments in the wells of East Lakhibari and Furkating area in Assam Shelf have enhanced the hope of discovering hydrocarbon pools in these sediments. Few wells have encountered Gondwana sediments in Dhansiri valley whose thickness varies from a maximum of 525m in north in Dergaon area to a minimum of 25m in Yampa area in the south. Based on lithological, electrolog and biostratigraphic data the Gondwana sediments have been divided into two lithostratigraphic units: i) Early Permian (Lower Gondwana) consisting of coarser clastics and ii) Early Cretaceous (Upper Gondwana) consisting of alternating sandstone and carbonaceous-coaly-shale. Trap (basalt/dolerite) with intertrappeans are present in a few wells towards the top of the Upper Gondwana unit and are of the same age. The Early Cretaceous sequence shows high gamma character with good quality potential source rocks. The source rocks are in early maturation stage, and are likely to have been matured and generated hydrocarbon at deeper burial depth. The Lower Gondwana unit was deposited in the grabens formed along pre-existing weak planes / lineaments whereas the Upper Gondwana unit was deposited in the NE-SW trending grabens generated by crustal thining and development of fault systems prior to breakup of Indian plate from Antarctica and Australia. The Gondwana grabens are oriented ENE-WSW in the Daldali area, NE-SW in Barpathar-Jamuguri-East Lakhibari area, and E-W in Dergaon area. Northwest of Borholla field, Jamuguri-East Lakhibari graben and south of Dergaon area have been identified as prospective areas for hydrocarbon exploration in Gondwana sediments. In addition to established Tertiary petroleum system, it is envisaged that the Gondwana sediments can act as an independent petroleum system where source, reservoir, cap and entrapment exists in the deeper part