Abstract: Yet-To-Find Hydrocarbons in Assam Foreland, India - A Two Phase Structural Inversion Model

VASUDEVAN, K., K. PALAKSHI

Exploration in Assam Foreland has been based on a east-west dextral strike-slip structural model where entrapment condition and hydrocarbon migration paths are controlled by transtentional and transpressional forces.

A two-phase structural inversion model related to episodic reactivation of 'inherited' normal faults trending ENE-WSW is proposed. The early inversion corresponds to Late Oligocene - Early Miocene epeirogenic readjustment of the Indian craton as a response to the onset of collision of Indian and Eurasian plates. This resulted in basin-wide positive movements marked by a tectonic unconformity. The second phase, a weak foreland inversion, affecting only the proximal foreland towards southeast, is related to Late Pliocene - Pleistocene reactivation concomitant with the imbricate thrust-fold propagation from southeast and subsequent basin tilt towards northwest.

Consequently, two petroleum systems appear to be present; a Paleogene system consisting of Eocene source and Paleogene reservoirs charged after formation of structures affected during early inversion; and a Late Oligocene-Miocene system consisting of Oligocene source and Miocene reservoirs charged after Late Pliocene - Pliestocene inversion.

The source parameters indicate a higher generation potential for older system as it holds much thicker and more mature source rocks. On the contrary only 33% of the established reserves are from the older system. Major part of yet-to-find resources prognosticated as 1050 MMt in this basin are expected to be in combination traps of Paleogene system at depths of 3000 m - 5000 m.

AAPG Search and Discovery Article #90942©1997 AAPG International Conference and Exhibition, Vienna, Austria