444
I. Lerche· S. Cao . S. Mallory· K. Petersen· A. Lowrie
4.2
Inclusion of Sub-Salt lithology and Salt Burial Depth
For particular and specified parameter values for moving salt sheets, calculations have been given (O'Brien and Lerche 1994) which illuminate the excess fluid pressure (i.e. fluid pressure in excess of hydrostatic pressure) values to be expected. Four factors confound the use of the calculations in assessing drilling
risk. First, the lithology under a salt sheet is often imperfectly known, so that the
permeability of formations underlying a salt sheet is unclear. Because of the
many orders of magnitude variation in permeability between sands and shales
(which are the dominant lithologies in the Gulf of Mexico), the overpressure beneath a salt mass could then vary considerably depending on lithology variations. Second, it is often difficult to be precise about the salt thickness ahead of
drilling, due to the problem of determining the base of salt from seismic information. The thicker the moving salt, the greater the load produced by the salt
and so the greater the underlying fluid pressure needs to be in order to support
the load. Thus the uncertainty in salt thickness also influences the degree of
overpressure in a sub-salt formation. Third, the timing of salt emplacement at a
given location in a basin is also not too well determined; a high speed salt mass
presumably overshadows a given location much earlier geologically than a slower speed salt, thereby trapping fluids (including hydrocarbons) at that location
for a longer period of time. Thus a more rapid, and earlier, emplacement of salt
is expected to be responsible for a higher degree of overpressure. Fourth, the
sub-sea bed depth of the salt top also influences the overpressure of sub-salt formations. For a given salt sheet thickness, a shallow salt top means that the salt
sheet has thinner sediments of higher porosity above it than a more deeply buried salt sheet. Thus the weight of the sediments overlying the salt is less in the
case of a shallow salt top. Because the weight of such sediments is transferred
through the salt to be balanced by overpressure in the sub-salt formations, a
shallower buried salt sheet will contribute less sub-salt overpressure, all other
factors being equal.
The influence of uncertainties in the above factors on the sub-salt overpressure is by no means equal, due to the non-linearity of response of compaction
and fluid-flow in sediments to geological conditions, and their variations: (a) to
parameters in equations of state controlling the intrinsic evolution of sediments
under applied loads; and (b) to the basic non -linear dynamical response of the
geologic system. Thus it is important to determine the relative importance (RI)
of uncertainties in each factor (salt thickness, sub-salt lithology, salt speed or
emplacement time, salt top depth) in contributing to estimates of sub-salt overpressure.
West (l989) and Lowrie (l994) have argued that the salt sheet displayed on the
cross-section has moved to its currently observed location at a rate of between
land 20 cm a~l • At location A, the emplacement of the salt, which is interpreted
to be currently situated in the midst of Quaternary sediments, is treated as a
wedge of inserted salt, currently of thickness 1700-2300 m. The top-of-salt depth
I. Lerche· S. Cao . S. Mallory· K. Petersen· A. Lowrie
4.2
Inclusion of Sub-Salt lithology and Salt Burial Depth
For particular and specified parameter values for moving salt sheets, calculations have been given (O'Brien and Lerche 1994) which illuminate the excess fluid pressure (i.e. fluid pressure in excess of hydrostatic pressure) values to be expected. Four factors confound the use of the calculations in assessing drilling
risk. First, the lithology under a salt sheet is often imperfectly known, so that the
permeability of formations underlying a salt sheet is unclear. Because of the
many orders of magnitude variation in permeability between sands and shales
(which are the dominant lithologies in the Gulf of Mexico), the overpressure beneath a salt mass could then vary considerably depending on lithology variations. Second, it is often difficult to be precise about the salt thickness ahead of
drilling, due to the problem of determining the base of salt from seismic information. The thicker the moving salt, the greater the load produced by the salt
and so the greater the underlying fluid pressure needs to be in order to support
the load. Thus the uncertainty in salt thickness also influences the degree of
overpressure in a sub-salt formation. Third, the timing of salt emplacement at a
given location in a basin is also not too well determined; a high speed salt mass
presumably overshadows a given location much earlier geologically than a slower speed salt, thereby trapping fluids (including hydrocarbons) at that location
for a longer period of time. Thus a more rapid, and earlier, emplacement of salt
is expected to be responsible for a higher degree of overpressure. Fourth, the
sub-sea bed depth of the salt top also influences the overpressure of sub-salt formations. For a given salt sheet thickness, a shallow salt top means that the salt
sheet has thinner sediments of higher porosity above it than a more deeply buried salt sheet. Thus the weight of the sediments overlying the salt is less in the
case of a shallow salt top. Because the weight of such sediments is transferred
through the salt to be balanced by overpressure in the sub-salt formations, a
shallower buried salt sheet will contribute less sub-salt overpressure, all other
factors being equal.
The influence of uncertainties in the above factors on the sub-salt overpressure is by no means equal, due to the non-linearity of response of compaction
and fluid-flow in sediments to geological conditions, and their variations: (a) to
parameters in equations of state controlling the intrinsic evolution of sediments
under applied loads; and (b) to the basic non -linear dynamical response of the
geologic system. Thus it is important to determine the relative importance (RI)
of uncertainties in each factor (salt thickness, sub-salt lithology, salt speed or
emplacement time, salt top depth) in contributing to estimates of sub-salt overpressure.
West (l989) and Lowrie (l994) have argued that the salt sheet displayed on the
cross-section has moved to its currently observed location at a rate of between
land 20 cm a~l • At location A, the emplacement of the salt, which is interpreted
to be currently situated in the midst of Quaternary sediments, is treated as a
wedge of inserted salt, currently of thickness 1700-2300 m. The top-of-salt depth