Page 241 - Origin and Prediction of Abnormal Formation Pressures

PREDICTION OF ABNORMALLY HIGH PRESSURES IN PETROLIFEROUS SALT-BEARING SECTIONS 213

Kagichev fields) and in other regions where gas-condensate accumulations, similar to
those of the southeastern Dnepr-Donets Basin, are sealed with evaporites. Gas shows in
the upper portion of the Philippov Horizon (directly underneath a thick salt sequence)
are an indication of approaching the AHFP in the Orenburg Field (Meshcheryakov et al.,
1966). The scattered nature of the local gas accumulations in the Orenburg Field sealing
sequence is believed to indicate their association with fractured zones. These fractures
are associated with faults used by gas to migrate up from the major hydrocarbon
accumulation.

LOCATING THE AREAL POSITIONS OF AHFP ZONES
Evaporite sequences possess very peculiar physicochemical properties; therefore, all
indirect techniques should be utilized in the AHFP forecast that could help establish
patterns of geologic evolution of regional or local structures and associated hydrocarbon
accumulations. Possible ways to forecast AHFP in the Paleozoic salt-bearing sequence
of the Dnepr-Donets Basin (DDB) are as follows. Two salt-beating formations, a Lower
Permian and a Devonian, are developed in the DDB. The Lower Permian evaporites are
encountered at a depth of 1000 to 2000 m and are 500 to 1500 m thick. They form a seal
for large gas accumulations, but also contain small overpressured gas accumulations.
The Devonian salt deposits, which are found at a depth ranging from 3500 to 4000 m in
the northwestern part of the region, are too deep to be reached by drilling in the central
and southeastern parts of the region. The gas reserves controlled by the Lower Permian
salt seals and by the buried Paleozoic highs with Devonian salt plugs are concentrated
within the southeastern DDB.
The Devonian salt domes pierced the overlying Carboniferous rocks along the large
faults mainly over the peripheral areas of the anticlines. The portion of the sediment
cover where piercement had occurred are fractured (Fig. 9-3, III). These zones are
called 'zones of weakness'. Such structurally weakened areas are present around each
salt plug, i.e., within each salt plug-associated structure. Gas accumulations in the
sealing salt sequence in a number of the southeastern DDB gas fields are spatially
associated with the salt plugs (among such fields are the West Krestishchenskoye,
Efremov, Melikhov, Medvedov and Kagichev). A study of the available logs and drilling
information demonstrates a clear correlation between the AHFP manifestations in the
Lower Permian evaporites and the structurally weakened near-plug areas. Faults, which
are very common here, have served as conduits for gas overflows up from the sub-salt
(under-salt) deposits (Zone I, Fig. 9-5). This resulted in the formation of AHFP within
the salt seals. An inference is that the identification of a zone of weakness makes a
forecast of possible AHFP zones possible.
Currently, available techniques for the identification of overpressured zone are based
on well logs obtained in the process of drilling (MWD). This means that an AHFP
zone is discovered vertically; usually, its areal extent remains unknown. This prevents
undertaking of protective measures in the nearby wells drilled subsequently. As a result,
AHFP may be encountered unexpectedly at different stages of oilfield exploration and
development.