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250 micrometres
Fig. 23.
Secondary porosity produced by
Rhaxella
sponge spicule dissolution and lining of pore
spaces with microcrystalline quartz which inhibits
later quartz overgrowth.
Sponge spicule mouldic porosity
et al
., 2010). However, spicule dissolution
occurred relatively early in the burial process,
before hydrocarbon charging and makes the result-
ing secondary porosity an important contribution
to the overall porosity. Spicule dissolution poros-
ity, due to its secondary nature and limited pore
connectivity, does not enhance permeability.
Hence, hydrocarbon-bearing sandstones with
spicule porosity tend to contribute more signifi-
cantly to production in gas condensate fields than
oil fields.
Rhaxella
sponge spicules have been documented
in many fields, for example the Gyda Field (Aase
et al
., 1996; discussed in more detail later), the
Clyde Field (Stevens & Wallis, 1991) and the
Fulmar Field (Johnson
et al
., 1986; Kuhn
et al
., 2003)
where it occurs in two distinct heterogeneous
intervals (Lower Usk and Clyde Sands facies of
Unit 3 Ribble Sands). Spicules can be found
within a large range of different rock types from
Sinemurian to Kimmeridgian in age (Vagle
et al
.,
1994). They are found in the Moray Firth in silty
or fine-grained sandstones of middle Oxfordian
age and are abundant in the Alness Spiculite
Formation (Johnson
et al
., 2005).
Early spicule dissolution and re-precipitation
occurs due to the inherent instability of biogenic
opal-A silica, of which spicules are formed
(Hendry & Trewin, 1995). Together, sponge spic-
ule dissolution and feldspar dissolution can
account for up to half the point-counted total
porosity. Spicule-derived porosity can be easily
observed in petrographic sections due to the
circular or oval form of the pores (Fig. 23).
Microquartz coating of pore spaces is thought to
inhibit the growth of quartz cement (Bloch
et al
.,
2002; Walgenwitz & Wonham, 2003; Taylor
et al
.,
2010) and microquartz crystals may resist pressure
solution by solidifying contacts between quartz
grains (Osborne & Swarbrick, 1999).
Significantly, secondary porosity created by
sponge spicule dissolution is mainly developed in
the upper, transgressive unit characterised by ret-
rogradational stacking of shoreface parasequences
(Wonham
et al
., 2002; Fig. 24). The significance of
such observations for mapping reservoir quality
can be seen on phi-K cross-plots comparing the
characteristics of (1) retrogradational, spicule-
bearing Upper Franklin B Sandstone and (2) pro-
gradational, Lower Franklin B Sandstone with no
spicules (Fig. 25). This cross-plot emphasises a
marked change in the relationship of porosity
and permeability between the Lower and Upper
Franklin B Sandstone.
Abundant spicules within the Upper Franklin
B Sandstone may reflect slower rates of sedimen-
tation primarily within Facies S3 lower shoreface
deposits during the retrogradational phase of
deposition. Ajdukiewicz
et al
. (2001) concluded
that spicules are most common in the relatively
distal reaches of transgressive systems tracts,
where clear water and limited volumes of detrital
fines provided favourable environmental condi-
tions. The development of lower permeability
within the retrogradational succession has been
recognised earlier (Cannon & Gowland, 1996) but
the link to spicule presence was not made.
Gowland (1996) speculatively associated abun-
dant spicule development to a shelf depositional
environment. However, the observation of the
retrogradational nature of the spicule-bearing
sandstones is more useful for predictive pur-
poses. Gowland (1996) also suggested that
palynofacies data demonstrated that spicule-rich
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