Geoscience Reference
In-Depth Information
A number of the models proposed by Robinson
appear to be unsupported, based on the knowl-
edge that there is little evidence to suggest struc-
tural compartmentalisation between the two wells
concerned (30/16-A36 and 30/16-A31). In addi-
tion, there is no strong reason to suppose that
the Ribble Sandstone was sourced directly from
the Auk Horst. Mineralogically, the sands of the
Ribble Sandstone Member are arkosic or subarko-
sic and quite similar in composition to the sand-
stones in the shoreface sandstone members of the
Fulmar Formation below. The Ribble Sandstone
might just as easily have been sourced from the
underlying shoreface sandstone uplifted to the
north-east of the Ribble Sandstone depositional
area, as from the Auk Horst.
It is suggested here that the observed facies change
could reflect a rapid loss of depositional energy
within turbidite sandstones whose flows had a very
short run out. A lower energy setting and an increase
in mud content of the sediment could have been
sufficient for this facies to be attractive for biotur-
bating organisms giving Facies T5 a very different
textural appearance to the massive or parallel lami-
nated sandstones of Facies T4. A facies change of
this kind is interpreted to have resulted from a
depositional model such as that shown in Fig. 19 in
which shoreface sandstone is reworked to produce
turbidite mass flow sands by the combined influ-
ence of local tectonic uplift and transgressive shore-
face erosion and retreat. Correlations of Kuhn
et al
.
(2003) suggest that shoreface sandstones underlying
the Ribble Sandstone Member are eroded on the
crest of the Fulmar Field structure (Fig. 20) and it is
here suggested that this eroded shoreface sandstone
material could be the main source of the Ribble
Sandstone Member, rather than the Auk Horst to the
west. This idea is supported by the observed facies
change between Well 30/16-A31 and well 30/16-
A36 which suggest a lower energy system to the
south-west than to the north-east (Fig. 21).
Elgin-Franklin and Fulmar fields share a com-
mon tectono-stratigraphic setting (Type B reacti-
vated pod setting) and also appear to show a
similar stratigraphic and sedimentological devel-
opment, notably: (1) initial shoreface deposition
within reactivated pod structure and (2) turbidite
sandstone deposition over erosive unconformity
following pod grounding onto the Rotliegend.
Faulting during pod collapse may have assisted
and focused the distribution of reworked shore-
face sandstones as turbidite deposits away from
the pod crest as illustrated in Fig. 10. For both
fields, a structural inversion and repositioning of
sediment source and sink may have occurred.
Case Study 5: Jacqui turbidites:
Type B - Reactivated pod setting
The Freshney Sandstone Member of the Jacqui
area presents a further example of turbidite sand-
stone deposition, based on the results of wells
30c/13-3, 30c/13-4, 30c/13-6 and 30c/13-7). These
wells have proven a transition from sandy shore-
face to distal silty offshore deposits passing from
the south-east towards the north-west (Fig. 5).
However, the overlying turbidite sandstones show
a different trend and are scattered broadly across
the area. Correlation of the turbidite sandstone
units is not straightforward and indicates either
the lobate geometry of sandstone units or, perhaps
more probably, the influence on their deposition
of marked tectonic uplift on the Joanne Horst to
the west. Seismic lines (e.g. Fig. 22) show signifi-
cant uplift and steepening of strata over the under-
lying Rotliegend footwall in such a way that
previously deposited shoreface sandstones and
Triassic sediments could have been eroded and
transported the short distance to the site where
turbidite deposition occurred. These turbidite
sandstones could, therefore, have been the result
of deposition from high density but low energy
turbiditic flows that rapidly deposited their sedi-
ment load, a scenario similar to that envisaged for
the Elgin-Franklin-Shearwater and Fulmar Field
turbidite deposits. Unlike these fields, pod ground-
ing in the Jacqui area appears to have occurred
very late so that the turbidites are ponded within
the pod, rather than shedding to the sides of the
pod as seen in the other examples.
OCCURRENCE OF
RHAXELLA
SPONGE
SPICULES AND THEIR INFLUENCE ON
RESERVOIR QUALITY
The Fulmar sandstones of Elgin and Franklin
fields are typically subarkosic arenites which
show: (1) field-wide average porosity/permeability
of 17%, 25 mD and (2) local maximum porosity/
permeability of 30% and several Darcies.
Secondary porosity evolution has been enhanced
through the dissolution of feldspar and sponge
spicules. Early compactional effects have been
lessened as a result of the growth of early authi-
genic minerals (microquartz and dolomite) which
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