Risk, Uncertainty and Priorities - Quantitative Models
4
An Illustration of Relative Importance: Lateral Variations of Sub-Salt
Overpressure buildup
435
The presence of laterally extensive sheets of salt in the Gulf of Mexico has been
well documented from both seismic and direct drilling through salt sheets (see
e.g. many of the references in Lerche and Petersen 1995).
In addition, the lateral variations with depth of different stratigraphic formation thicknesses in relation to current salt sheet positions, and the juxtapositioning of the same formations both above and below salt at different laterallocations, have been used to infer rapid lateral motion of salt sheets, with speeds of
order 1-20 cm a-I (West 1989; Lowrie 1994).
Because salt has an extremely low permeability, a salt sheet provides an effective seal after emplacement at a given location, trapping sub-salt fluids (including hydrocarbons) and retaining porosity in sub-salt formations. Because of the
weight of the salt and of formations above the salt, the fluid pressure in sub-salt
formations is enhanced by the salt emplacement, as has been detailed elsewhere
from simple model considerations as well as by observational information,
where fluid pressure increases at sub-salt breakout are about 100-300 atm/
km- 1 thickness of salt (O'Brien and Lerche 1994).
And yet the evolution of sub-salt overpressure development with both time
and spatial location in relation to the motion of a salt sheet remains an outstanding concern. The point here is that there is a degree of uncertainty on the assessment of the lateral speed of salt, and there is also a degree of uncertainty on the
lateral variations of thickness of a salt sheet; accordingly, both the rate at which
sub-salt overpressure builds up with time at any given location, as well as the
spatial pattern of variation of sub-salt overpressure at any instance of time, have
corresponding degrees of uncertainty.
The ability to bracket the spatial and temporal structure of sub-salt overpressure
development is of use in a variety of contexts. First, both the earliest and latest times
for the sealing of sub-salt formations by emplaced salt, with respect to lateral position in the basin, could be bracketed, thereby providing both maximum and minimum estimates for the retention of hydrocarbons in formations underlying salt.
Second, the deeper the emplacement of salt at a given location, the more the
formations are compacted prior to the appearance of salt. In this case the porosity available for hydrocarbon storage in sub-salt sediments is reduced relative to
early salt emplacement. Further, the more compacted the sediments are, the
greater is the effective stress (and so the lower the excess fluid pressure) and the
greater is the sedimentary consolidation strength, so that salt speed and direction of motion can be influenced more significantly by the corresponding higher
sedimentary resistance.
Third, early development of excess pressure implies a higher porosity retention and, accordingly, a higher geothermal gradient below salt because of the
lower formation thermal conductivity. The maturation of hydrocarbons entrapped below salt can be hastened by this increase in temperature.
4
An Illustration of Relative Importance: Lateral Variations of Sub-Salt
Overpressure buildup
435
The presence of laterally extensive sheets of salt in the Gulf of Mexico has been
well documented from both seismic and direct drilling through salt sheets (see
e.g. many of the references in Lerche and Petersen 1995).
In addition, the lateral variations with depth of different stratigraphic formation thicknesses in relation to current salt sheet positions, and the juxtapositioning of the same formations both above and below salt at different laterallocations, have been used to infer rapid lateral motion of salt sheets, with speeds of
order 1-20 cm a-I (West 1989; Lowrie 1994).
Because salt has an extremely low permeability, a salt sheet provides an effective seal after emplacement at a given location, trapping sub-salt fluids (including hydrocarbons) and retaining porosity in sub-salt formations. Because of the
weight of the salt and of formations above the salt, the fluid pressure in sub-salt
formations is enhanced by the salt emplacement, as has been detailed elsewhere
from simple model considerations as well as by observational information,
where fluid pressure increases at sub-salt breakout are about 100-300 atm/
km- 1 thickness of salt (O'Brien and Lerche 1994).
And yet the evolution of sub-salt overpressure development with both time
and spatial location in relation to the motion of a salt sheet remains an outstanding concern. The point here is that there is a degree of uncertainty on the assessment of the lateral speed of salt, and there is also a degree of uncertainty on the
lateral variations of thickness of a salt sheet; accordingly, both the rate at which
sub-salt overpressure builds up with time at any given location, as well as the
spatial pattern of variation of sub-salt overpressure at any instance of time, have
corresponding degrees of uncertainty.
The ability to bracket the spatial and temporal structure of sub-salt overpressure
development is of use in a variety of contexts. First, both the earliest and latest times
for the sealing of sub-salt formations by emplaced salt, with respect to lateral position in the basin, could be bracketed, thereby providing both maximum and minimum estimates for the retention of hydrocarbons in formations underlying salt.
Second, the deeper the emplacement of salt at a given location, the more the
formations are compacted prior to the appearance of salt. In this case the porosity available for hydrocarbon storage in sub-salt sediments is reduced relative to
early salt emplacement. Further, the more compacted the sediments are, the
greater is the effective stress (and so the lower the excess fluid pressure) and the
greater is the sedimentary consolidation strength, so that salt speed and direction of motion can be influenced more significantly by the corresponding higher
sedimentary resistance.
Third, early development of excess pressure implies a higher porosity retention and, accordingly, a higher geothermal gradient below salt because of the
lower formation thermal conductivity. The maturation of hydrocarbons entrapped below salt can be hastened by this increase in temperature.