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The AAPG/Datapages Combined Publications Database
GCAGS Transactions
Abstract
Geopressure
Analysis
in the Subsalt Knotty Head Field, Deepwater Gulf of Mexico
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Kenneth E. Williams1, Richard Redhead2, and William Standifird1
1Knowledge Systems, Inc. / A Landmark Graphics Company, 2107 CityWest Blvd., Bldg. 2, Houston, Texas 77242
2Nexen Petroleum, 12790 Merit Dr., No. 800, Dallas, Texas 75251
ABSTRACT
The discovery well
for the Nexen Knotty Head Field, Green Canyon Block 512
No. 1 BP 2, was drilled in November 2005 to a total depth of 34,189 ft (10,421
m) true vertical depth in 3557 ft (1084 m) of water. This
well
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 2000
ft (610 m) 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 movement of formation waters to shallower intervals.
The recognition of a salinity response on resistivity data using a normalization
procedure in a wet well
may indicate 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.
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