Reservoir properties and petrophysical modelling of carbonate sand ...

Geological Society, London, Special Publications published online June 27, 2012 as doi: 10.1144/SP370.6
D. PALERMO ET AL.
Fig. 12. Rock-columns from the central part of a major shoal body were derived for a 2.1× 1.1 m sized slab which was
previously mapped for sedimentary structures. Permeability shows a patchy distribution but also a clear upward
increasing trend in all four rock columns.
facies types. The well-sorted pack and pack- to
grainstones (A) of the bottom part have about
10% porosity and low permeability, which can be
explained by comparably high mud content, occluding
parts of potential primary interparticle porespace.
In contrast, the upper well-sorted grainstone
sheet (C) has lower mud content and higher
amounts of interparticle porosity, resulting in better
porosity and permeability values. The poorly sorted
bioclastic pack- to grainstones also show textural
differences, but are additionally controlled by the
amount of biomodic porosity of leached shells.
Generally, the porosities in the investigated poorly
sorted pack- to grainstone beds (B, D) range
between 11.3 and 18.2%. The moderate permeability
in bed B ranges between 7.6 and 32.9 mD, while
the average permeability in the upper bed (D)
varies between 90.1 and 202.0 mD. Thus, lateral
changes in porosity and permeability within an individual
facies body remain in the same order of
magnitude within a similar stratigraphic position.
These changes seem to correspond generally to the
vertical textural changes that seem to follow the
observed stratigraphic cycles. Moreover, also the
lateral differences in porosity and permeability
within the individual layers seem to be accompanied
by slight lateral variations in the Dunham texture.
The general occurrence of laterally continuous
reservoir bodies is commonly controlled by
medium-scale cycles, since porous units occur
preferentially around their regressive maxima. In
contrast, internal porosity and permeability variations
are often controlled by small-scale cycles.
The best reservoir properties are found around
small-scale regressive maximum. This effect is
most likely the result of stratigraphically controlled
variations in mud content, and additional complex
diagenetic overprints of cementation and selective
leaching (see previous section).
Regarding the entire flow unit, the lateral pattern
of the permeable layers along the same stratigraphic
cycles indicates that the observed centimetre-scale
heterogeneities owing to sedimentary structures
play a subordinate role.