Page 239 - Origin and Prediction of Abnormal Formation Pressures

PREDICTION OF ABNORMALLY HIGH PRESSURES IN PETROLIFEROUS SALT-BEARING SECTIONS 211

complications with AHFP-associated brine shows have occurred while drilling the
salt-anhydrite sequence in the Bukhara-Khiva region of Uzbekistan (Kushnirov et al.,
1972). Similar complications have occurred in West Uzbekistan, East Turkmenistan,
Tadzhikistan, over the Astrakhan Arch in Russia and in some other regions.

INDICATORS OF APPROACHING THE OVERPRESSURED ZONES
The absence of transition zones sealing the high-pressure hydrocarbon accumulations
in evaporite sequences makes the forecasting of AHFP very difficult. This makes
the development of precursor indicators for AHFP very important. An interesting
example may be found among the gas condensate fields in the central graben of the
Dnepr-Donets Basin (southeastern Ukraine). The Permian and Upper Carboniferous
section there includes commercial gas accumulations and an evaporite sequence (Lower
Permian Bakhmut Series), which is a regional seal. These gas-condensate fields are
located within a complex geologic environment, with a massive accumulation height of
up to 1500 m, total gas-saturated column of up to 1800 m, reservoir pressure of up to
40 MPa, and abnormality coefficient (Pa/Ph, where Pa is the abnormally high pressure
and ph is the normal hydrostatic pressure) of up to 1.9. The sealing evaporite sequence
includes 'high-pressured-low-volume' and 'high-pressured-low-permeability' local gas
accumulations (Melik-Pashayev, 1973). These small accumulations (Zone II, Fig. 9-5)
are satellites of the massive-bedded large gas accumulations (Zone I) and sometimes lie
500 to 600 m above them.
AHFP zones in the sealing sequence are associated mostly with faults and the
crestal portions of the accumulation structures, which experience the greatest excess
gas pressure. It was found that the AHFP occurs mostly within the structural elements
that experienced the most active neotectonic evolution. Evaporite sequences have better
sealing capacity than clastic sequences. As a result, predictive precursors of the AHFP
begin to appear closer to the accumulations. For instance, an indication of the approach
to the massive gas condensate accumulation in the Shebelinka Field is a drastic decrease
in drilling rate in a very firm anhydrite bed, 4 to 12 m thick. This bed is a part of the
Svyatogor Rhythm underneath the salt, and serves as a seal for the massive gas accumu-
lation (Fig. 9-2). It is a lithologic barrier similar to clayey seals established over many
accumulations in thick sand-shale sequences (Anikiyev, 1971; Durmishyan, 1973). The
hard anhydrite bed is the upper limit for small high-pressured gas accumulations and
of decompacted clays (as a result of elevated pore pressure and decrease in effective
stress). The only gas occurrence above that bed is found in well No. 80 located within
a fractured zone over an uplifted fault-block of the Shebelinka anticline. This well
penetrated the gas accumulation 50 m above the main accumulation of gas.
A plastic clay (greenish-gray and brown) is encountered in the Shebelinka Field
directly above the lithologic barrier. This clay is observed in the drilling mud returns
as a bubbly flaky mass with a pronounced smell of condensate. The drillers caI1 it 'gas
clay'. Fig. 9-2 shows the lithologic barrier and 'gas clay' for a typical stratigraphic
section of the Svyatogor Rhythm of the Shebelinka Field. The 'gas clay' appearance
in the circulating mud is a signal to prepare to drill into the massive overpressured gas