Page 54 - Petroleum Geology

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tion in the downthrowing block. In both cases, the stratigraphic sequence of
the downthrowing block is more complete than the other. But in an area
with two or more active growth faults, each with different rates of subsidence,
the same lowering of baselevel may have different stratigraphic expressions.
The difficulty is not so much in the concept as in the recognition of such
hiatus.
A stratigraphic hiatus associated with a growth fault may easily be mistaken
for a fault in a borehole - a misinterpretation that will be plausible because
of the association with known faults. Spurious complexity will be introduced
into the geological interpretation that will not only confuse the geologist but
also, in a producing field, the reservoir engineer. It will affect reserve estimates
and production planning because reservoirs thought to exist in the upthrown
block may not exist at all.
The association of growth faults with antithetic and synthetic faults, and
stratigraphic hiatus, may therefore present formidable problems of interpreta-
tion. There is no certain way of distinguishing hiatus and faults in a borehole,
but awareness of the possibilities is a good beginning. It is essentially a geo-
logical problem, the solution of which lies in part in the degree of association
between “gaps” in the sequence in the borehole and their stratigraphic posi-
tions. It is in the geological nature of the phenomena that there is a causal asso-
ciation between hiatus and stratigraphic position, but a chance relationship
between faults and their stratigraphic position in a borehole. A hint may also
be found in those areas where rock units in the sequence are of comparable
thickness in the same block, because reduction in the rate of sediment accu-
mulation may show as a hiatus in the upthrown block but as thinner beds
than normal in the downthrown block (see Chapman, 1973, pp. 233-254).
Reverse growth faults would seem to be unlikely, but most of the difficul-
ties that spring to mind at first are found not to be real difficulties. These ap-
parent difficulties stem from the common misconception (aided by the ter-
minology) that a fault separates a block that has been raised from one that
has been lowered. As we have seen, the movement is relative, and both may
have been downwards relative to baselevel. Topographic expression of growth
faults is not a necessary consequence of fault movement provided there is an
adequate supply of sediment.
Compaction processes would deform the fault plane of a reverse growth
fault in the same way that they deform the fault plane of a normal growth
fault. The dip of the fault plane would decrease with depth; but whereas the
opposing blocks of a normal fault tend to be separated at the surface and at
shallow depth by the movement of the fault, the movement of a reverse
growth fault would tend to separate them at depth. This component of hori-
zontal compressive stress induced in the relatively shallow incompetent beds
would presumably lead to a zone of disturbance adjacent to the fault.
In general, it is clear that all faulting that takes place at the surface has a
potential to influence the deposition of sediment and its accumulation.