Page 246 - Origin and Prediction of Abnormal Formation Pressures

218 V.I. ZILBERMAN, V.A. SEREBRYAKOV, M.V. GORFUNKEL, G.V. CHILINGAR AND J.O. ROBERTSON JR.

Vertical gas migration has played a leading role in the formation of overpressured
gas accumulations (secondary traps) in the evaporite seals. The recognition of this fact
is a starting point of the AHFP quantitative forecast. A substantiation of this theory is
found in the large DDB fields with massive gas-condensate accumulations of significant
vertical extent. The following zones may be identified between GWC and the top of the
accumulation: (1) zone of overpressure (pressure greater than the hydrostatic) caused by
the height of the accumulation and the density difference between the reservoir water
and gas; (2) zone of overpressure; (3) zone of normal hydrostatic pressure. These three
zones are respectively associated with the sub-salt, salt and post-salt deposits (Fig. 9-5).
-- AHFP in the evaporite sealing sequence
Thus, in Fig. 9-5, Px -- (P + q Ah), where Px
at point x (Zone II), p -- initial gas pressure in the massive gas reservoir (Zone I), Ah
= depth difference between the gas-water contact (GWC) of the reservoir and point x,
and q - initial reservoir pressure gradient in massive accumulation (Zone I).
The pressure in Zones I and III (Fig. 9-5) increases with depth due to the weight of
reservoir fluid column (gas or water). There is no regularity in the overpressure changes
within Zone II. Apparently, high-pressure gas accumulations in the sealing sequence
(secondary traps, Zone II) have been derived from the massive accumulations of the
lower zone (Zone I), as a result of vertical gas migration. They could not have formed
independently from the massive accumulation located underneath, or simultaneously
with it.
During the vertical gas migration, the major reservoir trap (Zone I) is filled up first.
Then, the gas breaks through (probably along faults) into the sealing sequence filling
up secondary reservoir traps (lenses) in the sealing evaporites (Zone II) along the way.
Some evidences of vertical gas migration include the following.
(1) Association of the AHFP with fault zones, especially those adjacent to the salt plugs
and to the areas above the crests of massive accumulations.
(2) Almost identical gas composition in the massive accumulations (Zone I, Fig. 9-5)
and in the accompanying secondary traps in the sealing evaporite sequences (Zone
II, Fig. 9-5).
(3) Association of the AHFP with local, isolated reservoirs with no reservoir water.
(4) Absence of any data suggesting that hydrocarbon generation occurred in situ within
the evaporite sequence.
(5) The deepest penetration of gas into the sealing evaporite sequence occurs over the
highest portions of the anticlines, where the height of massive accumulations and,
hence, reservoir pressure are maximum.
Thus, thick massive accumulations have caused surplus gas pressure in the lower
zone (Zone l, Fig. 9-5) and the gas penetration into the overlying low-permeability
rocks (Zone II, Fig. 9-5). In the sealing evaporite sequences (Zone II), the overpressured
gas accumulations occur in the fractured (due to tectonic movements) and/or cavernous
zones, and in the sand lenses. The evaporite sequence occupies an intermediate
position in the aforementioned vertical zonation. It serves as a transition between the
overpressures and the normal hydrostatic pressure. Thus, the pressure in that sequence
is within a range whose upper limit is set by the pressure at the top of the massive
accumulations (Zone I), whereas the lower limit is determined by the hydrostatic
pressure at the base of the upper zone (Zone IID.