Geoscience Reference
In-Depth Information
Case Study 3. The Elgin-Franklin Field:
Type B - Reactivated pod setting
GR
DT
POR (%)
4110
0 00 140400
20 30
The Elgin and Franklin fields are structural traps,
the main reservoir being the shallow marine
Franklin Sandstone (equivalent to the Puffin
Formation of Price
et al
., 1993). The sandstones
are sealed by the overlying Heather and
Kimmeridge shales and, ultimately, by the Cromer
Knoll Group of Lower Cretaceous age. Minor pro-
duction also comes from the underlying Pentland
Formation, a fluvial reservoir of Middle Jurassic age.
The structural context of the Elgin Field can be
demonstrated with a W-E geoseismic section
across the area (Fig. 15). The deepest observable
seismic event is the Top Rotliegend, which is at
approximately 7 km depth (over 5s two-way-time).
The predominant basement-linked faults trend
NNW-SSE with secondary faults trending ENE-
WSW. The Top Zechstein salt pick indicates that
there is a marked detachment between the sub-
salt and supra-salt successions. Salt movement
has created sediment 'pods' infilled by Triassic
and Jurassic strata. Both salt movement (swelling
and withdrawal) and grounding of pods on the
underlying Rotliegend fault blocks have influenced
the distribution of Upper Jurassic reservoir sand-
stone thickness. Hence, the Elgin-Franklin Field is
an example of a Type B tectono-sedimentary set-
ting. The structure also reflects a phase of late
Jurassic extension, when thick Kimmeridge Clay
was deposited in the hangingwall basins.
The Franklin Sands net reservoir thickness
averages 200m and consists of a succession of
fine- to medium-grained sandstones that are
mainly strongly bioturbated. The succession is
divided into three major lithostratigraphic units
separated by minor shale intervals: Franklin A, B
and C Sandstone (Lasocki
et al
., 1999). Each sand-
stone unit can be more finely divided into minor
progradational-retrogradational sequences on the
basis of changes in sandstone clay content
detected by the GR log (Fig. 16).
Reduced vertical seismic resolution at more
than 5km depth (18Hz to 20Hz dominant fre-
quency) makes intra-reservoir surfaces difficult
to pick in a consistent manner. In addition, the
homogenous character of the reservoir has led to a
number of different sedimentology-based correla-
tions, each with significant uncertainty. A genetic
sequence stratigraphic approach was used to rec-
ognise multiple phases of progradation and retro-
gradation within the Franklin Sands (Lasocki
4150
4200
4250
4300
4350
Fig. 13.
Enhanced porosity due to secondary porosity devel-
opment (yellow shading) in zones where sponge spicule
occurrence has been demonstrated by Aase
et al
. (1996).
Extensive dissolution of sponge spicules can generate second-
ary porosity as shown in Fig. 18. In this example (and several
others) it is observed that sponge spicules are much more
abundant during phases of shoreface retrogradation. Red tri-
angle: long term progradational trend; green triangle: long
term retrogradational trend. GR: Gamma Ray log; DT: Delta
Time (Sonic) log; POR: total porosity (%) measured on core.
Wells situated on the flanks of the Gyda struc-
ture, for example the 1/3-3 Tambar well (Fig. 14),
have several thin beds (20 cm to 2 m thick) of
sandstone that are present in the basal 50m of the
shale-dominated J71 Sequence (Farsund Formation)
immediately overlying the Ula Formation. The
best examples of these sandstones are not cored in
the 1/3-3 well, but those which are show highly
bioturbated, fine-grained sandstone beds corre-
sponding to previously described Facies T3. It is
suggested that these sandstone units are turbidite
deposits resulting from erosion on the crest of the
Gyda structure. This erosion would have reworked
the pre-J71 TEMFS shoreface sandstones and
redistributed them as thin sandstone stringers
within the overlying shales.
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