Mass Properties of Sedimentary Rocks and Gravimetric Effects of ...

GRAVIMETRIC EFFECTS OF PETROLEUM AND NATURAL-GAS RESERVOIRS A25
· The subsurface gravimetric effects produced in the
regions below a low-density mass are rather like inverse,
or mirror, images across a level surface through
the middle of the mass of those in the regions above it.
At a point immediately beneath the base of the lowdensity
mass, the absence of that component of the upwardly
directed gravitational attraction of an equal
volume of denser rock (for example, water-saturated
rather than petroleum-bearing sandstones) is sensed
by the borehole gravimeter, and the measured value
of gravity is excessive in proportion to the thickness,
volume, and negative density contrast of the mass
above. In other words, if a gravity anomaly can be
perceived by surface and (or) borehole gravity surveys
above an anomalous density mass, that mass will
produce an anomaly of opposite sign in the regions beneath
it unless its effect is compensated by a geologic
correction. The practical significance of this fact for
exploration can be visualized by considering qualitatively
the effect of an undiscovered deeper or flanking
pool on borehole gravity surveys in and beneath a
known and exploited reservoir. If a positive anomaly
does not occur beneath the known reservoir, or if its
amplitudeis less and its shape different from that expected,
a deeper, or flanking, undiscovered reservoir
-can be suspected. Such techniques of exploration for
deeper pools and undetected small extensions of known
pools in oil fields where numerous production wells
provide the access for fairly extensive underground
surveys at several levels are probably the most commercially
significant type of inquiry for the borehole
gravimeter in the immediate future.
With such problems in mind, let us examine again
the possible prospecting utility of departures from normal
of the underground vertical gravity gradient corrected
for AT and u. We have seen that in a vertical
borehole drilled above a reservoir, this quantity is relatively
negative and is more and more negative as the
low-density mass is approached vertically from above.
. As the reservoir is penetrated, the steep gravity gradient
associated with its lower density compensates for
the. accumulative departure from normal and, in fact,
overcompensates for it so that beneath the reservoir
the gradient is again abnormally low, becoming more
and more normal as measurements are made at still
deeper and deeper levels. If these departures of the
underground gravity gradients along a vertical line,
or a vertical or horizontal profile, could be measured
with a borehole gravimeter or gravity gradiometer, a
very powerful and depth-sensitive technique would be
available for prospecting in general, but particularly
for prospecting for deeper pools and extensions of
partly exploited oil fields. Therefore, the factors that
limit ability to apply this technique should be examined.
Figure 13 illustrates graphically that smaller real
values of Ag I AZ signify denser rocks than larger values
do. A variation in Ag I tl.Z of 0.01 mgal per 1,000 feet
is equivalent to an apparent variation in density of
0.0004 g per ems over that interval. If a departure
from normal of the underground gradient of 0.01
1ngal per 1,000 feet is to be detected, therefore, the
density term of equation 14 must be known with an
accuracy greater than 0.0004 g per ems; such accurate
measurements of density in situ are probably not possible
(except by borehole gravimeter measurements
where AOg, the quantity being sought in effect, can be
evaluated with an equally high accuracy). The author
(McCulloh, 1965) has shown that core-sample density
profiles of compact rocks can be constructed that appear
to be accurate on the average over large depth
ranges to about 0.001 g per ems. This suggests that
departurP.s from normal of the underground gravity
gradient begin to enter the range of detectability, by
comparison of accurate core-sample density profiles
with profiles calculated from the abnormal gravity
gradient, when the departure equals or exceeds 0.03
mgal per 1,000 feet. As will be seen in following pages,
a departure from normal calculated for a fairly large
reservoir in the interval from sea level to minus 2,600
feet is only half the value that is the present practical
limiting value, so one can see that this technique has
only limited applications in regions close to small disturbing
masses or more distant from masses of large
volume or very large density contrast. Probably the
greatest utility of the method again is to be found in
searching for deeper pool extensions or lateral extensions
of known traps in already drilled oil fields, and
in checking the significance in shallow core holes of
small local surface gravity, minimums that might be
related to shallow reservoirs or to other shallow masses
of low density .
HYPOTHETICAL GRAVITY PROFILES CALCULATED
FOR SELECTED WELL-DRILLED OIL FIELDS IN
VENTURA COUNTY, CALIF.
The recognition, almost by chance, that some productive
petroleum and natural-gas reservoirs in California
and South Dagestan, Azerbaijan, U.S.S.R., are
marked by local small negative gravity anomalies
should probably lead to suitable statistical studies of
gravity maps of those and other petroliferous regions
to establish empirical correlations between known reservoirs
and observed gravity anomalies. Such studies
. would provide guides to further exploration. Of greater
importance to scientific understanding, however,