Page 53 - Petroleum Geology

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Fig. 2-5. Diagrammatic cross-section through complex growth fault. Fault movement be-
gan after accumulation of Marker 12, and ceased after accumulation of Marker 3.

There are several points of detail in the nature of growth faults that are
worth examination. Reservoir sandstones not only show volumetric differences
due to the different thicknesses but may also have different porosities and
permeabilities across a growth fault. The tendency is for the thinner sand-
stone in the upthrown block to have smaller porosity but larger permeability
than the same sandstone in the downthrown block. This is also consistent
with the conclusions drawn earlier concerning the accumulation of sediment,
because smaller porosity in cleaner sands is to be expected where the accu-
mulation rate was slower.
The stratigraphic-structural relationships about a growth fault may be very
complex. In contrast with the simple case in which correlative sequences on
either side of the fault differ only in thickness, there are growth faults of
which the upthrown, or both, blocks may contain stratigraphic hiatus (Fig.
2-5). These are but variations on the theme of contrasting capacity to accu-
mulate sediment. An eustatic fall of sea level, causing a fall of baselevel that
equals the rate of subsidence of the upthrowing block leads to non-accumu-
lation of sediment in the upthrowing block, but continued accumulation in
the downthrowing block at a reduced rate. Such is the hiatus A in Fig. 2-5.
Similarly, a fall in baselevel that equals the rate of subsidence of the down-
throwing block leads to erosion of the upthrowing block and non-accumula-