BAKER HUGHES - Drilling Fluids Reference Manual

RESERVOIR APPLICATION FLUIDS
Combination Trap
The geometry of a combination trap is the result of a combination of tectonic processes and
changes in lithology. A common trap that would be an example of a combination trap is a salt
dome. A salt dome is a mass of NaCl (sodium chloride) generally of a somewhat cylindrical shape
and with a diameter of about 2 km near the surface, though the size and shape of the dome can
vary. This mass of salt has been pushed upward from below through the surrounding rock and
sediments into its present position. The source of the salt lies in a deeply buried layer that was
formed in the geologic past.
Salt is an evaporite. It exhibits no inherent rock structure. It is subject to deformation resulting
from applied pressure and temperature. Salt beds are formed by the natural evaporation of sea
water from an enclosed basin. In the North Sea area this occurred during the Triassic period.
Subsequently, the precipitated salt layer is buried by successive layers of sediments over geologic
time until segments of it begin to flow upward due to increasing overburden pressure toward the
surface of the earth. The origin of salt domes is best explained by the plastic-flow theory.
Salt has a density of 2.2 SG under standard conditions. At a depth of about 12,000 feet, the mass of
the overlying sediments exerts a compressive, downward force. The combination of this applied
pressure (overburden) and accompanying increase in temperature (geothermal) causes a density
decrease and salt begins to flow like a plastic substance. A small fracture in the overlying, higher
density sediments or a slightly elevated mass of salt above its surroundings would trigger the
upward movement. Once this upward salt movement begins, salt from elsewhere in the salt bed
moves into the region surrounding the salt plug to replace the salt that is flowing upward to form
the salt plug. The upward movement of the salt plug, or dome, continues as long as there is
sufficient source of salt “feeding” the dome or until the upward movement is halted by a more rigid
formation. Once equilibrium is reached, upward movement of the salt dome ceases, but may begin
again if sufficient sediments are added to the weight of the overburden which again increases the
load pressure on the parent salt mass. Salt domes come in all sizes and shapes, but all have certain
characteristics. For example, they have a cap rock. The cap rock is composed of limestone located
at the top of the dome followed by, in descending order, gypsum, anhydrite, and finally rock salt.
The average cap rock is between 300 to 400 feet thick, but cap rocks up to 1,000 feet thick are
known to exist. Cap rock is created as the salt dome ascends through the overlying materials. The
top of the dome dissolves as it rises through the sediments and the residual constituents of the salt
become concentrated at the top of the rising plug becoming three layers: limestone, gypsum, and
anhydrite. The anhydrite zone represents the less soluble components of the salt that accumulated
as the halite was dissolved by pore fluids. The anhydrite zone locally contains abundant celestite
that probably formed as the result of interaction with external Sr-bearing brine. The calcite zone
formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone
contains significant amounts of sulfide minerals and oil. The sulfide-rich calcite zone is dominated
by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep
heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool
ground waters.
The cap rock of some salt domes, and sometimes the rock salt itself, is found to overhang, or drape
down, the side of the main salt mass. It is commonly found to overhang on one or two sides. The
reason for the overhang probably is dependent upon the nature of circulating waters in the
sediments through which the dome is ascending. Salt is soluble and dissolves, but the cap rock is
not very soluble and is left more intact. Thus the appearance of the cap rock “overhanging” the
main salt body is the result of the dissolving of the salt body within the subsurface, but not
dissolving the cap rock. Cap rock has all the characteristics of a petroleum reservoir and often
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 6-15