Geology Reference
In-Depth Information
the conditions of their contact with compacted clays incapable of releasing
the squeezed-out fluids under the present-day geologic environment.
Important for the regional forecast is the genetic association between
AHFP and gas-mud volcanism caused by a decisive role played by abnor-
mal pressures in the mechanism of mud volcano formation (Kalinko, 1964,
1969, 1987). First, the existence of mud volcanoes spewing large amounts
of gas positively characterizes stratigraphic intervals where their roots are
positioned. Besides, the preservation of high excess pressures in zones
modified by the volcanoes is an indication of continuing communication
between the reservoirs, which
their channels cut, and deep zones of the
sediment cover containing high-pressure fluids including hydrocarbons.
As mentioned, the formation of commercial oil and gas accumulations
was caused by hydrocarbon migration from oil and gas generation zones
into the oil and gas accumulation zones. An important role in this process
belongs to the pressure gradients pointed from the generation zones to the
accumulation zones and areas. The real geologic environment is defined by
the specifics of geologic structure of a specific region, its geologic evolution,
belonging to a tectonic mega-element, etc. Apparently, in the real geologic
environment the values of these gradients and their directions may widely
vary. Thus, it is believed necessary to review, as theoretically possible under
natural conditions, options of hydrodynamical regime caused by the ori-
gin, distribution nature and changes in AHPP and AHFP gradients.
On ancient platforms, abnormally high pressures and their present-day
gradients are in most cases minimal. Recorded there is also the absence of
positive hydrodynamical anomalies. Under these conditions, the forma-
tion of natural hydrocarbon accumulations is believed to have been a result
of the implementation of AHPP and AHFP paleo-gradients in the process
of migration.
In the regions belonging to young (Epi-Hercynian) platforms and
Alpine folded systems, the accumulations formed mostly in Paleogene-
Neogene (Burshtar and Mashkov, 1963; Mekhtiyev and Rachinsky, 1967;
Mekhtiyev, 1969) during time intervals of major phases of the Alpine and
neo-tectogenesis. That is why together with the effect of the paleo-gradients
a substantial role belongs also to the present-day gradients. They provide
both for the formation of the newest accumulations and for the hydrocar-
bon “replenishment” into the earlier formed accumulations. Under such
conditions, the hydrocarbon accumulation is usually accompanied by pos-
itive hydrodynamic anomalies.
Under the reviewed geologic environments, the decisive role belongs to
vertical gradients. They support the flow of fluids (including hydrocarbons)
