Stress Studies in the Mahanadi Offshore Basin, India, Determined to 4.0 km from Wellbore Breakout Data

摘要

Stress direction has been determined to about 4 km depth under the Mahanadi Offshore basin using four-arm dipmeter caliper logs for six wells spanning an area of about 18,000 square kilometers between the Mahanadi coast and the Eocene "hinge zone." The hinge zone is a regional feature delimiting the seaward extent of the Mahanadi pull-apart basin. The hinge zone supposedly marks the continent-ocean transition against the Bengal Fan. Breakout orientation deduced from data of the six wells shows a prevalent modal class represented in the histogram plots between azimuth versus resultant of cumulative zone length of breakout interval as well as between azimuth and the frequency of occurrence of breakouts. The minimum horizontal compressive stress direction varies between N40 deg -60 deg E for the Eocene-Pliocene sediments lying at depths between 500-2300 m, but the direction changes to N40 deg -80 deg W for deeper litho-horizons (below 3000 m) while approaching the Eocene hinge zone and further oceanward. This interpretation is corroborated by stress analysis results determined from a single well oceanward of the hinge zone by previous researchers. Breakouts are clearly distinguished by the symmetric elongations in caliper difference plots. These features are consistent with known history of formation of the Mahanadi pull-apart basin and general northward movement of continental India.