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tension (below the line) than for an open fracture to occur. Because fractures
exist in this field that are closed, we infer that the tensile strength of the
rocks is close to zero.
This inference is not inconsistent with Secor’s conclusions. Our conclusion
is that in young rocks with normal faulting, or incipient normal faulting, the
differential stress (u 1-u3) is too large and the tensile strength of the rocks too
small for faults to open and form conduits, or for open faults to be created.
If this seems a sweeping conclusion from a single case study, it must be re-
membered that the many known fault traps also support this conclusion.
Figure 11-9 shows the depths to which open fractures could occur in ma-
terials of tensile strengths to 1 MPa (1@ bars). It is a matter of scale. Figure
11-10 shows the critical region of the Mohr diagram plotted in units of uw
When uo is very small, the value of u1 required for open fracture is also very
small.
Open fractures, then, appear to be restricted to areas in which the sediments
are consolidated, the stress field is close to hydrostatic (in the structural sense
of u = u3), and the pore-fluid pressures are close to the overburden pressures.
We cannot invoke this process in primary or secondary migration without
evidence of these criteria. The commonest geological context of the first two
criteria is probably stratigraphic traps in transgressive sequences; but the geol-
ogical context of the last criterion is generally structural traps in regressive
sequences. That the tensile strength of the rocks is a critical parameter is in-
dicated by Hull and Warman’s observation that whereas the Asmari limestone
has open fractures, sandstones that occur with it do not appear to be frac-
tured (Hull and Warman, 1970, p. 431).
Returning to the problem of source rocks in the Niger delta, we see that
there are serious geological and physical objections to a deep source, strati-
graphically removed from the accumulations. It seems geologically implaus-
ible that the source rocks are far removed stratigraphically from the accumu-
lations, and physically impossible for petroleum to migrate in fault planes

Fig. 11-10, Critical region of the Mohr diagram, plotted in units of 00.