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3D pore pressure modeling and overpressure zone prediction in the upper Assam Shelf, India

Rai Neha, Singha Dip Kumar, Chatterjee Rima

Acta Geophysica

2022

Vol. 70, no 3

1203--1221

Quantitative variation of pore pressure within the formations is essential for the selection of drilling mud, and to avoid catastrophic incidents such as blowouts. In this paper, we estimate to detect in-situ overpressure zone (OPZ) and to establish the adequate spatial distribution of PP from a 3D seismic data containing three wells in the foreland basin of the upper Assam shelf. We applied the fact that the porosity of Shale decreases monotonically as the effective stress increases, so we delimited the Shale volume up to 70% for PP estimation. The OPZ has been identified in the wells by comparing two methods: first, deviation of sonic transit time from normal compaction trend, and second, the separation between sonic-density porosity. The predicted PP is validated by repeat formation test and mud weight data. The 3D pore pressure model that obtained by velocity-effective stress transformation method matches with pore pressure in the wells with excellent goodness of ft. The PP gradient varies from 14.22 to 15.50 MPa/km in OPZ, and the top of OPZ ranges from 1225 to 2182 m, respectively. The spatial distribution of pore pressure is found to be mostly normal pressure for Barail, Sylhet and fracture basement except in the Kopili formation in which OPZ is spatially distributed, and higher pressure is observed in the locations toward the S-SE direction. Our results reveal the occurrence of overpressure zone in Barail and Kopili formations of Oligocene to Eocene, which can be attributed to the disequilibrium compaction phenomenon.