Geopressure Analysis In the Subsalt Knotty Head Field: Deepwater Gulf of Mexico

The discovery well for the Nexen Knotty Head Field, GC 512 No.1 Bp2, was drilled in November 2005, to a total depth of 34,189 ft TVD in 3,557 ft of water, and is one of the deepest wells to be drilled in the offshore Gulf of Mexico basin. The well encountered multiple hydrocarbon bearing pay intervals in a pore pressure regression below the salt. This pressure regression has been observed in a number of other wells in this area of the basin including the nearby K2 and Tahiti fields. Long hydrocarbon columns in excess of 2,000 feet have been seen in association with this pressure regression.

Petrophysical analysis of the log response to overpressures indicates that the resistivity log shows the best match to the pressure regression measured in the sands; better than the sonic and density data. Salinity normalization of the resistivity response indicates that it may be due to a salinity change to fresher waters within the shales (and a higher resistivity reading) adjacent to the sands that show the pressure regression. The opposite response is observed in the more saline shales immediately below the salt. Several alternate scenarios for this response were examined.

This salinity response is due to the loss of more saline waters in the pore space of the shales, whereas the fresher bound water surrounding the shale platelets remains. The result is an increased resistivity in the shales adjacent to the sands where the pressure has been reduced by bleed-off of formation waters to shallower intervals.

The recognition of a salinity response on resistivity data using a normalization procedure in a wet well may be an indicator of the possible presence of a long hydrocarbon column in a nearby undrilled structure due to the increased effective stress that results from this lowered pressure.