Geology Reference
In-Depth Information
and subsequently their outflow into adjacent reservoirs. That is horizontal
or vertical gradients of syngenetic AHPP are realized. It occurs at relatively
shallow depths (up to 1,500-2,000 m) in the process of clay compaction or
over long geologic time at great depths.
In some situations the upper and lower hydrogeological stages of the
sediment cover communicate. The pore solutions may be released from the
compacting clays at depths of 1,500 to 2,000 m or deeper where the major
fluid-mass-transfer occurs through diffusion, film and sinilar mechanisms.
In such a hydrogeological environment, it is possible to determine the for-
mation and placement conditions of commercial oil and gas occurrences
through finding the directions and implementation extent of the paleo-
and present-day AHFP and AHPP gradients.
This is known from practice of the petroleum exploration and produc-
tion: when the sediment cover includes thick continuous clay series with
syngenetic AHPP caused by lagging of the pore fluids flow-off in com-
paction, usually there are within these series favorable conditions for the
formation and preservation of oil and gas accumulations and the corre-
sponding oil and gas accumulation zones. The best potential is associated
with the areas where these clay series are in contact with thick reservoir
members.
There are three options for the reservoirs/clayey sequences combina-
tion: the reservoirs overlie the clays; they are positioned within the clayey
sequence; and they are overlain by the clays. It was established in numer-
ous regions that the formation and resources of natural hydrocarbons in
oil and gas accumulation zones significantly depend on mutual spatial
relationships between competent and incompetent intervals of the sedi-
mentary section.
In some cases, the reservoir sequence is positioned above a
clayey sequence believed to be the source rock. Then it may serve in
a long geologic time as a probable discharge area for the underlying
complex, which releases upward along the contact zone the fluids
expelled in clay compaction. These fluids include hydrocarbons. The
result of this mechanism may be:
t
Primary vertical fluid penetration from the clayey sequences
into reservoirs.
t
Subsequent oil and gas migration into them.
t
Hydrocarbon accumulation in the traps.
t
Regular liquid and gas phase distribution subject to the
known schematics of increased gas-saturation of the section
