Geology Reference
In-Depth Information
In other words, the water emigration from reservoirs (the main pre-
condition for the formation of commercial hydrocarbon accumulations) is
progressively limited with depth. That, in turn, limits the probability and
frequency of occurrence at great depths (over 4,000 m) of large oil and gas
accumulations. In specific areas, the interval of maximum hydrocarbon
concentration may vary with depth. It clearly correlates with local ground
water discharge, oil and gas accumulation conditions. It follows from the
above that large hydrocarbon accumulations at depths over 3,500-4,000 m
may be the result of two occurrences:
1. Present-day large-scale interformational ground water
migration (emigration) under conditions of active hydraulic
communication between deeply-buried oil and gas intervals
with near-surface section zones (the surface).
2. Paleo discharge of a geo-fluid-dynamical system realized at
shallower depths during the former periods when the accu-
mulations were positioned at shallower depths.
Actual patterns of oil and gas occurrences spatial distribution and the
aforementioned considerations enable us to suggest the following concep-
tually important conclusion: The formation of large natural hydrocarbon
accumulations occur only (mostly) within the drainage zones of regional
geo-fluid-dynamic systems and within the depth range (commonly to a
depth of 2,000-2,500 m), where it is possible to realize a section's paleo- or
present-day hydraulic communication with local ground water discharge
areas and intervals.
Of great interest in this connection is the following view: It is sug-
gested that in the sections of oil and gas basins exists a so-called “optimum
reservoir properties zone” at the same depth as the major commercial
hydrocarbon resources (Minsky, 1975). This concept is based on the zone
is identified from a complex of different attributes. Its application to the
described aspects and mechanisms of oil and gas occurrences' forma-
tion and their logical tie-in suggests that within the sediment cover of
most regions is a clearly depth-restricted interval with optimum oil and
gas accumulation conditions. The depth boundaries of this interval are
defined: from the top, by the necessary combination of factors providing
for the preservation of hydrocarbon accumulations; from the bottom, by
the presence of sufficient conditions for ground water discharge (outflow)
from reservoirs.
A confirmation of legitimacy of the proposed concepts may be found
globally. It is the exclusive association between the largest oil and gas
