Abstract

Occurrence and evaluation of overpressured subsurface formations are critical to exploration concepts, drilling and production operation, involving hydrocarbon resources. It is therefore necessary that inter-disciplinary technical approach is required to optimise exploration efforts in such hostile subsurface environments. In the search for hydrocarbon resources in many areas both onshore and offshore, abnormal formation pressures have been encountered. This paper is aimed on the analytical studies on such overpressures in the Bombay Offshore basin, which is a major oil/gas province in the country.

Bombay Offshore basin which is the southern extension of Cambay basin, came into existence during Upper Cretaceous time as a result of faulting in the Deccan trap basement. Two prominent structural trends, one aligned N-5, while the other E-W, are present in the Offshore, similar to land part of the basin.

The sedimentary section of this basin comprises mostly of shale and limestone, ranging from Paleocene/Cretaceous to Recent. Considerable facies changes has been observed in the different parts from north to south and east to west in this basin which has played a significant role in the generation of overpressure, specially in the North, North West and North East region, besides contributing to commercial/non-commercial hydrocarbon accumulations in different areas. Overpressured zones in this basin have been encountered in the structures, which are mostly aligned in east-west direction and are of late origin. They are also devoid of major commercial accumul tions of hydrocarbons.

The possible reason for the occurrence of such horizons which occur in the sediments of Eocene to Miocene in the Bombay Offshore basin falls in two broad categories.

Primarily Compaction and/or Diagenesis of limestone which might have created permeability barrier in the porous sequences, thus inhibiting expulsion of fluids from the rock.

As a result of work which has been done on the prediction of such overpressured horizons in the sediments with the help of DXC, Shale density, flow-line temperature and conductivity studies, etc, it has been possible to forewarn the occurrence of such overpressured horizons, in different areas in Offshore before they were uncovered by the bit and by devising Mud and Casing policies suitably, it has been thus possible to avoid many well hazards and there by achieve the objectives.

Field case histories from the area will highlight the presentation.