Page 52 - Petroleum Geology

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about the same time as that of the maximum fault growth in nearby faults.
South, towards the Gulf of Mexico, is the direction of regression.
The pattern of growth-fault movement in growth faults caused by base-
ment faults is related to the sequence of basement faults. This too can be
determined from thickness contrasts, but no detailed study has been publish-
ed on these.
Growth faults in terminal regressive sequences are characteristically curved
in section and in plan, concave to the direction of regression; and they com-
monly occur en echelon. In section, the dip of the fault plane reduces from
a maximum of perhaps 70" at the top, to 40" or even less at the bottom. The
simplest, but not the only explanation of this curvature lies in compaction.
The growth fault cuts unconsolidated sediments that recently accumulated
into the stratigraphic record, so its attitude is determined by the mechanical
properties of such material, and its dip will be about 60". Subsequent com-
paction reduces the dip of the fault: the greater the compaction, the greater
the flattening. There are two influences here. First, the greater compaction
with increasing depth, increasing overburden load, tends to flatten the fault
with depth. Secondly, such faults in regressive sequences have a decreasing
proportion of compactible mudstone with time, so an increasing proportion
of mudstone with depth, so accentuating the flattening.
But compaction cannot be the only cause because reduction of dip from
60" to 40" implies about 50% Compaction, which would be achieved by reduc-
tion of mudstone porosity from 50% to zero. This is much more than is ob-
served: compaction of mudstone from 50 to 20% porosity would reduce the
dip from 60" to about 47". We shall see in Part 3 on regressive sequences that
mass flow of relatively imcompetent mudstones at depth could cause the
extra flattening of the fault plane. The formation of "roll-over" anticlines in
the downthrown block of some growth faults (Fig. 2-2) is probably due to
both movement on the curved fault plane and any mass flow at depth.
Once a growth fault has developed and existed during the accumulation of
a considerable thickness of sedimentary rocks, differential compaction pro-
cesses alone must tend to extend its life to some extent. Such processes may
also lead to some irregularity in the fault plane because sandstone is less com-
pactible than mudstone. Any irregularities that develop during fault move-
ment will be sheared out by the movement, and a zone of disturbance is to
be expected.
It will be appreciated that, by their very nature, growth faults are tilted
and folded with the sedimentary sequence that they cut, so that the original
dip is measured relative to the dip of the strata (see Fig. 15-15 of Miri field,
Sarawak). Any sinuosity introduced into the fault plane during movement
may lead to satellite faults that will not have growth characters except where
they influenced the rate of accumulation of sediment. If there is sufficient
information, much of the history of growth faults and their associated struc-
tures can be elucidated.