Geology Reference
In-Depth Information
Taking the aforementioned into account, it is obvious that the determi-
nation of prevailing type of fluid migration, formation, distribution condi-
tions and oil and gas forecast based on abnormally high formation pressure
is grounded in finding the direction and realization extent of the present-
day normalized AHFP gradients causing and controlling ground water and
hydrocarbon migration, formation and preservation of their accumulations.
The precondition for solving the stated issues is a review of theoretical
models of spatial formation pressure gradient distribution as a function
of the status of geologic medium and implemented (being implemented)
migration processes
and associated pressure redistribution. The match of
actual (by the regions) correlations to a model should be interpreted as
objective manifestation of certain migration mechanism.
In some cases horizontal and vertical fluid migration is absent (or sig-
nificantly restricted). Then the current geobaric regime in the waterhead
complexes in general and individual local structures is mostly determined
by AHFP relaxation conditions. AHFP are generated in such geologic envi-
ronment due to elastic compression of the natural reservoirs as a result of
neotectonic stresses. They are also generated by baric effects associated with
differences in fluid and rock heat expansion in the process of subsidence
and at secondary cementing of the reservoir pore space in the processes of
mineralogical new-formation. The diagnostic feature of this model is prac-
tically omnipresent equi-gradient AHFP in reservoirs independent of the
structure-tectonic and facies-lithological conditions. The absence of local
contrasting hydrodynamical (piezometric) anomalies is also a diagnostic
feature. If the factors generating AHFP are absent, the formation pressure
all over the area of the water complexes will be hydrostatic.
At dominant lateral fluid flow under the elision water-exchange model,
the flow occurs over large areas up the regional dip. The baric exchange
between the migrating fluids and fluids in the enclosing geologic medium
(if they are piezo-conductive) occurs along the entire migration paths and
continues for a long time. The quantitative incompatibility of the reacting
masses and long duration of their contact in real geologic environments of
this model cause pressure and pressure gradient leveling in the reservoirs
to the value normal for a given area. In other words, under the lateral filtra-
tion model the baric exchange between the flowing fluids and the enclos-
ing medium cannot form noticeable piezo-anomalies and preserve them in
time. The diagnostic specifics of this model are:
t
Sequential over the area of the regions decrease in normalized
formation pressure gradients from the deepest central parts
of the depressions and troughs toward their flank frameworks
(from charge zones to the hydrodynamical discharge zones);
