Petroleum System of Dhansiri Valley: From Example from Shelf and Foreland Setting, Assam and Assam Arakan Basin, India
Ram K. Singh
ONGC, KDMIPE, Dehradun, India
Dhansiri valley (South Assam Shelf) forms part of Assam and Assam-Arakan Basin located in the northeastern part of India is a well-established hydrocarbon producing area. Success in hydrocarbon exploration in South Assam Shelf (SAS) initiated with the discovery of hydrocarbon from Borholla structure in the year 1970. In the structure, oil has been tested from the Precambrian fractured basement, Basal sandstone (Paleocene), Sylhet and Kopili Formations of Eocene age. Since than the Borholla structure is on regular production, besides nearly half a dozen of later commercial discoveries from the area.
The Assam Shelf typically represents poly-tectonic history where hydrocarbon migration is different at different time in a complex structural setting. The NE-SW trending major faults are dissected by number of cross faults, which have divided the area into number horst and graven features. Most of the hydrocarbon migration in the area is through the faults and the majority of the known hydrocarbon bearing structures of Dhansiri valley are located in the horst blocks or up thrown/ down thrown part of hanging blocks.
The main producing reservoir in Dhansiri valley is distributed from Paleocene to Miocene, besides fractured reservoir of Pre-cambrian age from Borholla-Changpang field. However, hydrocarbon shows have also been observed in few wells drilled through Gondwana sediments in the area.
The hydrocarbon bearing reservoirs at different stratigraphic levels are mainly represented by clastic reservoir of sandstone and hydrocarbons entrapment is mainly controlled by either structural or strati-structural type of traps.
The Kopili Formation of Eocene age represents thick shale section with thin intervening sandstone and forms major source facies in the entire area.
Six petroleum systems have been identified in the area, which are: (1) Kopili-Fractured Basement and Basal sandstone, (2) Kopili-Sylhet, (3) Kopili-Kopili, (4) Kopili-Bokabil, (5) Kopili-Tipam/Namsang are envisaged for the Tertiary and Gondwana-Gondwana for Pre-Tertiary sediments in Dhansiri valley.
Presentation GEO India Expo XXI, Noida, New Delhi, India 2008©AAPG Search and Discovery