Petroleum Systems of Cachar-Tripura of Assam -Arakan Basin, India
Cachar-Tripura is foredeep part of Assam-Arakan Basin, comprises of a series of N-S oriented anticlinal structures which developed during Late Pliocene to Pleistocene uplift, folding and thrusting. The present study aims to identify the elements and processes making up the petroleum systems of Cachar-Tripura to assist in prioritizing future exploration effort. The shales within Paleocene to Oligocene sediments, which are main source in foreland part and the Surma basin of Bangladesh, are thought to be source rocks in Tripura-Cachar. In addition, shales within shallow horizons of Midddle Miocene sediments are also considered to be potential source rocks. Hydrocarbons are primarily reservoired in Oligocene, Miocene and Pliocene sandstone. Lenticular sandstones reservoirs are likely to be sealed vertically and laterally by intercalated shales. The thick marine shale and finer fluvial high accommodation stand clastics of foredeep sequences of Mio-Pleistocene age might act as good cap rock. Overpresurred zones in Miocene shales due to compaction disequilibrium might have prevented the vertical escape of fluids during burial. Migration is vertical and occurs through basement-rooted faults.
Thermal maturity and hydrocarbon generation modeling indicate that hydrocarbon generation and expulsion has continued in the synclinal parts of fold belt throughout the main Late Pliocene to Pleistocene (~1.8Ma) phase of trap formation and might have allowed remigration of hydrocarbons from earlier hydrocarbon charged structures. Generation and migration of hydrocarbon was started in the area since about 20-28 Ma. Critical moment for Oligocene and Paleocene -Eocene source rocks are 2-8 Ma and 8-12 Ma respectively.
C29 Sterane biomarker ratios for oils and condensates suggest an early mature to moderately mature source. Gases are thermogenic, enriched in methane (> 90%) and δ13C1 ranging from -46.3 to -39.2 per mil with maturity 1.2 % VRo. The study suggests the presence of Paleocene to Oligocene - Miocene to early Pliocene (!) total petroleum system with two assessment units (AU’S): one in Paleocene to Oligocene - Miocene to early Pliocene (!) and another Paleocene to Oligocene - Paleocene to Oligocene (.). Under filled reservoirs in Tripura-Cachar also indicate that migration to the traps is recent or that the major generation and expulsion preceded trap formation. Best targets for future exploration for oil are pre-folding, extensional structures.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009