Geology Reference
In-Depth Information
from stratigraphic complexes to other ones by way of ascending or descend-
ing contact or fault migration.
Penetration through this way into the reservoirs of high-pressure fluids
leads to their subsequent lateral migration within a regionally developed
bed. This in turn signifies the transformation of vertical gradients into hor-
izontal ones and change from earlier prevailing vertical migration to the
fluid flow from replenishment zones into discharge zones. If the latter are
clearly expressed, it results in the appearance of formation pressure gradi-
ents within the entire reservoir interval. Sometimes the discharge zones
are absent or the discharge is obstructed. In such cases, if the reservoir
beds are not regionally developed, AHFP is formed over their entire extent
without the spatial differentiation into the areas of piezometric maxima
and minima.
Usually major hydrocarbon accumulations are associated with the dis-
charge zones. Exceptions occur in cases when these zones are clearly unfa-
vorable for the preservation of oil and gas accumulations (no anticlinal
traps, specifics of the litho-reservoir properties, etc.). It is believed that the
entire band of horizontal AHFP gradients in a regionally continuous reser-
voir is favorable for the hydrocarbon concentration into commercial accu-
mulations. This is so because long-term fluid migration occurred through
this band. That action must have provided for the hydrocarbon concentra-
tion within anticlinal, fault-restricted and other traps.
It is noteworthy that the “replenishment” zones in most cases are dis-
tinct in their regional oil and gas shows (residual oil in the reservoirs),
although not always do they contain more or less significant commercial
hydrocarbon accumulations.
When discussing a possibility of oil and gas forecasting based on data
about AHFP, it is necessary to keep the following in mind: The formation
and placement of the accumulations by the hydrocarbon type (oil, gas-oil,
gas and gas-condensate) is controlled not only by the ratio of competent
and incompetent intervals and by the size and direction of AHPP and
AHFP gradients but is a complex function of various geologic, geochemi-
cal, hydrodynamical, thermodynamic and other factors.
The reviewed examples and data do not exhaust the significance of the
abnormal pressure for the forecast of oil and gas occurrences. Their most
important role in the formation of accumulations, local structures, mud
volcanoes and tectonic structure of individual areas and regions makes this
factor into a multifaceted medium enabling the use of AHPP and AHFP
for the solution of a broad complex of other general geologic and explora-
tion tasks.
