Exploration Insights December 2019/ January 2020 | Page 20

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Exploration Insights | 21
Figure 1> Location map of the United Kingdom, highlighting the Southern North Sea and the Central North Sea basins, with well
locations of the undeveloped discoveries investigated. The Mid North Sea High is also shown, for reference.
2. Is reservoir quality dominated by primary
or secondary controls?
3. What do these reservoir quality analyses
mean for the undeveloped discoveries
and further exploration within the basin?
SOUTHERN NORTH SEA BASIN
In the Southern North Sea Basin, significant
reservoir risks were identified in the Permian
(Rotliegend Group) and Carboniferous
stratigraphy. Discoveries with poor permeabilities
were common, and reservoir quality was noted as
a risk both in the montage data and from existing
commercial discoveries.
Within each period, various reservoir units were
studied. Within the Permian, the Rotliegend
Group Leman Sandstone Formation was
evaluated, and within the Carboniferous, the
Visean Fell Sandstone Formation, the Namurian
Millstone Grit Formation, and the Westphalian
Flora and Boulton formations were assessed
(Figure 3), as these provide the most common
commercial reservoirs.
Figure 2> Map of the Southern North Sea Basin illustrating the commercial fields of the Permian Rotliegend Group, and the
Carboniferous Visean, Namurian, and Westphalian intervals.
Porosity and permeability values are key factors
when determining reservoir quality. Both
parameters can be affected by primary (e.g.
depositional environment) or secondary (e.g.
diagenetic) controls. In this study, both primary
and secondary controls were analysed for the
Permian reservoir interval, but only primary
controls were analyzed for the Carboniferous
intervals, as insufficient data were available on
secondary controls. heterogeneity may have created the changes in
the distribution of the reservoir quality that we
observed. For example, one well may penetrate
good quality channel sand bodies, while a nearby
well may penetrate the floodplain, which would
typically have a poorer quality. In the Permian,
this pattern was observed when penetrating wet
interdune or fluvial deposits (often poorer quality
facies), compared with the clean aeolian sands
(often better quality facies).
Primary Controls Secondary Controls
Depositional environment provides the first
indicator of reservoir quality. It creates the
initial reservoir quality, which can control how
diagenesis later affects the rock. The depositional
environments may have caused variability in the
porosity and permeability across the Southern
North Sea Basin (figures 4 and 5). Secondary effects contribute significantly to the
reservoir quality in the Southern North Sea Basin.
The common cementing minerals in the Permian
interval, identified through well report analyses,
include quartz, siderite, dolomite, anhydrite, illite,
and kaolinite. These cements, however, do not
show any clear spatial trends (Figure 5). This is
likely due to a complex burial history, which is
supported by Green et al. (2018), who state that
the Southern North Sea Basin has undergone
three main phases of exhumation.
The paleogeographies of the Carboniferous
interval were largely fluvio-deltaic. Sand bodies
are likely to have formed stringers in medial to
distal areas of the system, and laterally extensive
areas where channels amalgamated. This lateral
Faulting can increase diagenetic effects by
allowing an influx of meteoric fluids, which
precipitate authigenic minerals. As there is
significant faulting within the Permian interval,
this may be a significant contributing factor
to the varied diagenetic mineral distributions.
Additionally, the clean aeolian sands may have
aided the flow of meteoric waters, resulting in
diagenetic alteration of the rock.
When comparing the porosity and permeability
of both reservoir intervals, it is clear that the
Permian is superior to the Carboniferous (Figure
6). Though this may be true in some areas,
the data from the Carboniferous interval are
likely to be skewed to show poorer reservoir
qualities because the primary targets of the
wells were likely Permian. These target wells
usually reach total depth in the top few meters
of the Carboniferous. As the Carboniferous was
not the main exploration objective, wells often
sampled non-reservoir sediments. Therefore,
the Carboniferous porosity and permeability
results may be more representative of the poorer
quality facies/depositional environment. As the
Carboniferous is also deeper, negative diagenetic
effects are likely to be more prevalent.