Geology Reference
In-Depth Information
t
Its spatial fit with the step-fault-block structure of the area,
region.
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Close direct correlation of abnormality indications with the
section's clay content (shale-out of permeable intervals),
temperature conditions of hydrocarbon aggregations, real-
ization extent of hydrocarbon thermal conversion and sec-
ondary mineral-formation.
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In the process of drilling, typical features are frequent
blowouts of drilling fluid, circulation loss with application
of super-weighted drilling fluid, intense gas-water shows,
“interjection” of gas into drilling mud, dilution and pulveri-
zation of cuttings, absence of swelling/blowout in boreholes,
overpull and sticking of drill-string, and decreasing hook
load.
In the most general case the spatial commonality and long preserva-
tion of high syngenetic AHPP and AHFP indicates a hydrodynamic
closeness of the sections and reservoirs. This means the lack (or extreme
difficulty) of their hydraulic communication (drainage) with the reservoir
series of the sedimentary sequences, their quasi-stagnant hydrogeological
regime (Rachinsky, 1981; Rachinsky, 1983; Rachinsky and Kerimov, 1980;
Rachinsky, 1990).
Epigenetic AHPP'
is are typical of the areas where the section is com-
posed of alternating compact (sometimes, fractured) clays and permeable
beds. It is materialized mostly through penetration into the clays of fluids
from the underlying hydrocarbon accumulations in case these accumula-
tions have great height and thus significant excess pressure caused by den-
sity differential between oil, gas and formation water. In particular that is
how the so-called “invasion halos” in clay seals over the accumulations are
formed (Anikeyev, 1977; Durmishyan, Muradian and Sporadic; Rachinsky,
1982). This process is accompanied by sporadic decompacting of overly-
ing clayey formations. It occurs locally, and the development zones of this
type AHPP correspond in map view with the outlines of the crestal areas of
“feeding” reservoirs. Epigenetic “invasion halos” are important exploration
indicators of high-pressured accumulations of natural hydrocarbons in the
underlying reservoirs.
The formation of epigenetic AHPP is associated with the penetration
into clays of high-pressure fluids possibly including injected water. It is,
therefore, possible that they contain halos of invaded water areally associ-
ated with “invasion halos” of hydrocarbons. Their presence may be identi-
fied from results of studying the ion-salt composition distribution patterns