Geology Reference
In-Depth Information
The occurrence of syngenetic AHFP feeding the overlying complexes
(at the current stage of region's knowledge) appears most likely within the
Silurian-Lower Cretaceous stratigraphic range (some scientists believe
that this interval includes oil and gas source rocks). The suggested prefer-
able mechanism of their generation is baroeffects. These baroeffects are
caused by the difference in the heat expansion factors between fluids and
the enclosing rocks in the high-temperature zone (at great depths) and by
the secondary cementation of the reservoir pore space in the stylolite-for-
mation (as suggested by Dunnington, 1967).
It is believed that the dominant trend of the fluid mass transfer in the
region is through vertical migration from lower to upper structural stages.
The reasons for this conclusion are:
t
The identified patterns in the AHFP spatial distribution in
the drilled Upper Jurassic-Neogene section of the Zagros
Trough.
t
Matching of their epigenetic formation model with forma-
tion pressure abnormality.
t
Commonality of the waterhead system vertical discharge
onto the surface (surface shows of oil, gas and underground
thermal water, salt lakes, kir sinters, etc.).
Relatively unobstructed migration facilitated by the optimal ratios of
the media factors determines generally low state of stress of the geo-fluid-
dynamic system within local highs (average excess of formation pressure
over the hydrostatic is 9.3 MPa, the abnormality factor 1.29).
It was shown that in foredeep troughs of the Alpine mobile belts a direct
correlation exists between impermeable (low-permeability)/permeable
rocks ratio in the sedimentary sections (parameter
ะก
) and formation pres-
sure abnormality factors (see Figure 3.8). That, in turn, indicates a substan-
tial role of syngenetic component in AHFP formation.
A review of the Tables included in this Chapter identifies individual
deviations of the formation pressure abnormality factor values from the
stated general trend of their consistent decrease up the section in all regions
(South Caspian Depression - Pliocene, Padan Basin - Pliocene, Indolo-
Kuban Trough - Miocene, Tersk-Caspian Trough - Oligocene-Lower
Miocene (Maikopian Formation), Carpathian Trough - Miocene). The
causes of these phenomena identified in the analysis of hydrogeological
environment in the specific areas and prospects are mostly the superposi-
tion of the additional factors on the main mechanism through-the-sedi-
ment-cover vertical migration of high-pressure fluids.
