Page 80 - Petroleum Geology

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We would expect, therefore, that the state of compaction at 3250 m under
these conditions will be comparable to an identical, but normally compacted
rock at about 768 m. The common occurrence of drilling breaks at the top
of abnormal pressures lends support to this approach.
There is a real geological meaning to be attached to the equilibrium com-
paction depth, ze. It is an estimate of the maximum thickness of overburden
at the time compaction equilibrium was lost in the sedimentary rock now at
depth z, and this overburden was approximately the thickness ze on top of
z. We shall return to this in Chapter 14 when we consider the time of genera-
tion of abnormal pressures.
These results can now be incorporated into our various compaction curve
formulae, eqs. 3.5a, 3.8 and 3.9. These equationsrelate strictly to equilibrium
compaction depths, ze in the notation now developed, so we write the dimen-
sionless depth more generally as 6z/b. The formulae so modified to take pore
pressure into account, with others used with them, are shown in the box on
p. 63; and the values of the dimensionless depth for the range of At from 56
to 165 ps/ft are tabulated in Table 3-3. If the value of At and any two com-
ponents of dimensionless depth are known, the third can be simply calculated.
Note also that the effective compaction depth, ze, is a function of the sonic
transit time, At, and can therefore be estimated directly from the sonic log.
Practical hints on the construction of At plots and the use of these formulae
will be found in Appendix I.
The formulae enable us to estimate pore-fluid pressures in mudstones
from the sonic log through the parameter 6. Given a plot of mudstone or

TABLE 3-3

Values of dimensionless depth, &/b, for Atsh. Read tens in lines and units in columns (e.g.,
the value of 6zlb for At,h = 117 p/ft is 0.573)

0 1 2 3 4 5 6 7 8 9
-
5 4.700 4.007 3.602 3.314
6 3.091 2.909 2.755 2.621 2.503 2.398 2.303 2.216 2.136 2.061
I 1.992 1.928 1.867 1.810 1.756 1.705 1.656 1.609 1.565 1.522
8 1.482 1.442 1.405 1.368 1.333 1.299 1.266 1.235 1.204 1.174
9 1.145 1.117 1.090 1.063 1.037 1.012 0.987 0.963 0.939 0.916
10 0.894 0.872 0.850 0.829 0.809 0.788 0.769 0.749 0.730 0.711
11 0.693 0.675 0.657 0.640 0.623 0.606 0.590 0.573 0.557 0.542
12 0.526 0.511 0.496 0.481 0.466 0.452 0.438 0.424 0.410 0.396
13 0.383 0.370 0.357 0.344 0.331 0.318 0.306 0.294 0.282 0.270
14 0.258 0.246 0.235 0.223 0.212 0.201 0.190 0.179 0.168 0.157
15 0.147 0.136 0.126 0.116 0.105 0.095 0.085 0.076 0.066 0.056
16 0.047 0.03 7 0.028 0.018 0.009 (At,)