Geology Reference
In-Depth Information
discharge zones. The diagnostics of the fluid migration direction in such a
case are performed based on careful consideration of geologic specifics of
areas and prospects.
Wells often penetrate sections with a more complex vertical pressure
gradient distribution. In particular, they may display a step-like change -
they may increase to a certain depth (interval), then drastically decline,
and then increase again (southeastern plunge of the East Kuban trough,
the Chernoles depression). Such distribution may be caused by the pres-
ence in the section of two foci (zones) of AHFP generation with substantial
vertical separation between each other. Obviously, with greater distance
between them up or down the section, abnormal pressure intensity will
weaken. A typical picture of the described distribution is observed in the
central Fore-Caucasus. There, the first focus of the abnormal pressure gen-
eration is a thick mostly clayey Maikopian Fm., and the second focus is the
Upper Jurassic chemical sequence. In evaluating the potential of oil and
gas occurrences in such a case, special attention should be devoted to the
intermediate interval. Potentially it is capable of accumulating hydrocar-
bons from both descending and ascending migration.
To this point, we reviewed oil and gas occurrence forecast as a func-
tion of the presence in the section of thick uncompacted clay sequences
with syngenetic AHPP. In addition, an important exploration value
belongs to epigenetic AHPP within relatively thin clayey seals. As early
as 1959-1961, the SW Apsheron fields showed that clay seals overlying
gas, gas-condensate and gas-oil accumulations with AHFP inherit excess
pressure from the underlying accumulations and themselves become car-
riers of super-high pressure. That is why in drilling, often long before
the penetration of high-pressure accumulations, strong manifestations of
abnormal pressure are recorded in the overlying clays. In some cases, pore
pressure in clays is so great that 2.0-2.2 g/cm
3
drilling mud is too light to
create the required counter-pressure necessaryto continue normal drill-
ing. Such cases are usually accompanied by significant gas shows (similar
AHPP manifestations are recorded in the seals of Karadag, Shebelinka,
Vuktyl, Orenburg and other fields). Thus, findings tell us: If while drilling
through relatively thin clay seals epigenetic AHPP is discovered, it may
be associated with large hydrocarbon accumulations of a great height in
the not yet penetrated section. Based on this, epigenetic AHPP in the
clay seals was considered to be an important exploration criterion, indi-
cating the possibility of discovering underneath it thick mostly gas and
gas-condensate accumulations (Durmishyan, Muradian and Rachinsky,
1973, 1976, 1980).
