Downloaded - Ikon Science

Africa
A more challenging predrill prediction environment occurs
at depths at which temperatures exceed a threshold where
thermal processes can create additional overpressure as well
as change sediment properties used in routine approaches to
pressure prediction. The deeply buried Jurassic/Cretaceous
source rocks along the West African margin, in particular at
temperatures in excess of 160°C, are prone to generate gas.
Gas requires a higher volume than oil or water and hence increases
the pressure on generation. Increases in pressure reduce
the effective stress (the difference between the pore fluid pressure
and the minimum stress required to fracture the rock), a
sediment property used in pore-pressure prediction. There is
a tendency to underpredict pore pressures if this effect is not
recognized. Similarly, at temperatures in excess of 100–120°C,
there are many processes, for typical burial rates, which modify
shale properties (such as clay-mineral transformation of
smectite to illite) that once again challenge the premises used
to estimate pore pressures ahead of the bit. In areas, such as
offshore Angola, where thick and sometimes mobile salt is
present, pore pressures can be extremely difficult to predict,
because seismic imaging is poor, both for structure and the
velocities used in predrill prediction techniques. The amount
of pressure found beneath salt largely depends on the extent
of the salt as a rock unit and the ability of the salt to trap fluid
beneath. Fluids can escape where salt is absent if there are sufficient
volumes of connected reservoirs to allow high-pressure
fluids to flow around the salt bodies.
Summary
Continental margins are characterized by geological conditions
that favor development of high-pressure reservoirs, except
where there is an overabundance of reservoirs, such as
in the Niger Delta. Even here, deeply buried reservoirs can
have extremely high pressures. There are multiple overpressure
mechanisms operating simultaneously both in the shallow
section (inability of water to escape from rapidly buried, finegrained
sediments) and in the deep section (gas generation and
changes in sediment fabric because of diagenesis). In all cases,
the pressures found in reservoirs are closely related to the connectivity
of reservoirs both vertically and laterally along and
down the margin. High connectivity permits fluids to escape,
while isolated reservoirs will maintain the pressures being generated
in the surrounding fine-grained rocks. Regional mapping
of overpressures along the West African margin, as being
done in the current Niger Delta pressure study, has the potential
to reduce drilling risk by illustrating pressure distributions
as well as reducing the exploration risk associated with trapping,
both conventional and hydrodynamic.
References
Bowers, G. L., 1994, Pore pressure estimation from velocity data: accounting
for overpressure mechanisms besides undercompaction:
SPE paper 27488.
Bowers, G. L., 1995, Pore pressure estimation from velocity data:
Accounting for overpressure mechanisms besides undercompaction:
SPE Drilling and Completion, 10, no. 2, 89–95,
doi:10.2118/27488-PA.
Chopra, S. and A. Huffman, 2006, Velocity determination for pore
pressure prediction: CSEG Recorder, April 2006, 28–46.
Dickenson, G., 1953, Geological aspects of abnormal reservoir pressures
in Gulf Coast Louisiana: AAPG Bulletin, 37, 410–432.
Ellenor, D. W., 1984, quoted in The geology and hydrocarbon resources
of Negara Brunei Darussalam: Brunei Shell Petroleum
Company.
Pressure study for Nigeria, Offshore, 2010, http://www.offshore247.
com/news/art.aspx?Id=16868.
Swarbrick, R. E., M. J. Osborne and G. S. Yardley, 2002, The magnitude
of overpressure from generating mechanisms under realistic
basin conditions; in: Pressure regimes in sedimentary basins and
their prediction, A. R. Huffman and G. L. Bowers, eds. AAPG
Memoir 76, 1–12.
Swarbrick, R. E., R. Lahann, S. O’Connor, and A. Mallon, 2010, Role
of the chalk in development of deep overpressure in the Central
North Sea, in B. A. Vining and S. C. Pickering, eds., Petroleum
geology from mature basins to new frontiers: Proceedings of the
7th Petroleum Geology Conference, volume 1, 493–507.
Acknowledgments: GeoPressure Technology (an Ikon Science company)
and its Nigerian partner Sonar Limited are currently engaged
with the Nigerian federal authorities, DPR and Napims, and with
a group of six sponsor companies, in mapping pressures across the
Niger Delta. Phase 1, covering the deepwater and ultra-deepwater
areas was due to be completed in April 2011. Phase 2 (continental
shelf and near-shore areas) and Phase 3 (onshore and swamp) will
be completed between 2011 and 2014.
Corresponding author: r.e.swarbrick@geopressure.co.uk
June 2011 The Leading Edge 687
Downloaded 08 Jul 2011 to 195.99.165.130. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/