Exploration Insights December 2019/ January 2020 | Page 24

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Exploration Insights | 25
a
b
c
for gas production; however, the reservoir quality
values indicate that the sandstones may have
been affected by illitization, which can reduce
permeability without critically affecting porosity.
When comparing the undeveloped discoveries
with the diagenetic effects within the Leman
Sandstone, the Glein discovery well report
does support this prediction, as illite is present,
alongside quartz, siderite, and dolomite. As
the Eos discovery well is just south of the Glein
discovery, it is no surprise that they show similar
diagenetic reservoir patterns.
The measured and predicted porosities and
permeabilities of the Carboniferous correspond
reasonably well. Marjan is the only exception,
having significantly poorer porosities and
permeabilities than predicted. The Marjan
discovery targeted stacked Westphalian
channels, and poor porosity and permeability
values were not anticipated in this facies.
Assuming the sands initially had good reservoir
potential, consequent high fluid flow may
have resulted in the precipitation of diagenetic
minerals, ultimately decreasing the reservoir
quality.
Broadly speaking, the prediction of reservoir
quality from the project analysis is consistent
with the data from the undeveloped discoveries.
Where reservoir quality is lower than predicted,
diagenetic effects are the most likely cause.
More detailed analysis of these effects is
proposed to allow for better prediction of
reservoir quality.
CONCLUSION
Figure 5> (a) Map of horizontal permeability data for the Permian
Rotliegend Group; (b) Map of the diagenetic mineral distribution
in the Rotliegend Group Leman Sandstone Formation. Minerals
include illite, kaolinite, quartz, siderite, dolomite, and anhydrite; (c)
Map of horizontal permeability data for the Carboniferous. The Base
Rotliegend Group structure depth map is also shown, for reference.
Figure 6> (a) Porosity, and (b) permeability versus depth for the Permian Rotliegend Group and the Carboniferous interval in the Southern
North Sea Basin. P10, P50, and P90 cases are shown.
completed on the distribution of primary reservoir
facies deposits in the basin. More detailed
depositional environment maps would aid
future exploration and appraisal, reducing risks
associated with uncertainty, as well as offering
possible secondary objectives to Permian targets.
When trying to predict the reservoir quality of
a rock unit, burial history, facies distribution,
and diagenesis need to be considered. In
basins with reservoirs as old as the Permian
and Carboniferous, burial histories can be very
complex, in turn affecting compaction and
diagenetic processes. Therefore, additional
work on holistic diagenetic prediction across
the Southern North Sea Basin for Permian and
Carboniferous deposits is suggested, taking into
account the burial history of the basin. ACKNOWLEDGMENTS
Additionally, when exploring the reservoir
potential of the Carboniferous in the Southern
North Sea Basin, further analysis should be Green, P., Duddy, I. & Japsen, P., (2018). Multiple episodes
of regional exhumation and inversion identified in the UK
Southern North Sea based on integration of palaeothermal
and palaeoburial indicators. In: Bowman, M. & Levell, B.
(eds), Petroleum Geology of NW Europe: 50 Years of Learning
The author acknowledges the help and support of
STEPS mentors Lawrie Cowliff, Matthew Booker,
and Sarah Laird, and academic mentor, Professor
John Howell.
REFERENCES
Besly, B., (2018). Exploration and development in the
Carboniferous of the Southern North Sea: a 30-year
retrospective. In: Monaghan, A. A., Underhill, J.R., Hewett,
A.J. & Marshall, J.E.A. (eds), Paleozoic Plays of NW Europe.
Geological Society, London, Special Publications, 471.
– Proceedings of the 8th Petroleum Geology Conference.
Geological Society, London, 8, 47–65.
Ramm, M., (1992). Porosity-depth trends in reservoir
sandstones: theoretical models related to Jurassic
sandstones offshore Norway. Marine and Petroleum
Geology, 9, 553-567.
Oil and Gas Authority, Open Data, https://data-ogauthority.
opendata.arcgis.com/
Oil and Gas Authority, National Data Repository, https://ndr.
ogauthority.co.uk/
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