Geology Reference
In-Depth Information
t
They appear only within individual highs against the back-
ground of normal pressures in the reservoir or subregional
geo-hydrodynamic systems.
t
They are clearly associated with invaded agents (hydro-
chemical, temperature, isotope and other anomalies of the
general background).
t
Decrease on the local structures of formation pressure
abnormality with the distance from conducting faults,
faulted zones and areas of elevated fracturing;
t
Regional association of maximum abnormality values with
the least faulted structures, which, therefore, have limited
opportunity for vertical fluid cross-flows (hydrodynamic
discharge) into the upper intervals of the section or onto the
surface.
The diagnostic attribute of epigenetic AHFP
in the case of osmosis is the
broad areal development not associated with structural-tectonic condi-
tions in the area. The sedimentary section must be composed of alternating
permeable and impermeable rocks, which must be saturated with ground
waters of drastically different salinity.
The values of abnormal formation pressure in the reservoirs are con-
trolled by the ratio of inflow and outflow volumes of the invading fluids
per unit of time. AHFP formed in the reservoirs because of injection may
redistribute as the reservoirs have some piezo-conductivity. The possibility
and preservation duration of abnormal pressure depend on several factors.
They include the absolute value of the injected component, integral pres-
sure in the reservoir, amount of invaded fluids, thickness, permeability,
reservoir's lithofacies uniformity and extent of their hydraulic communi-
cation with the enclosing hydrodynamic system.
Very peculiar are conditions of the generation and transfer of abnormal
pressure into reservoirs underlain and overlain by thick salt sequences.
In this case, the salt behaves as a plastic rock of significantfluidity. The
compaction of salt formations is so far poorly studied. However, by
analogy with clay sequences (which have about identical physic-rheo-
logical properties), one may suggest that qualitatively this compaction
is identical with known consolidation schemes of incompetent forma-
tions. Quantitatively, salts usually exceed clays in their plasticity, fluidity,
compressibility, etc. This is evidenced by numerous facts of definitive salt
influence on the formation of structural-tectonic scenarios and faults in
many regions and the presence there of very mobile salt masses (plugs,
domes, etc.)
