Poster I produced for the Synthesis of a core drilled by ConocoPhillips in the Southern North Sea. I achieved a 1st class grade (A4) for the Poster and accompanying report.

1. Mud
Silt
Vf.S
Height
(m)
Lithology
BoxNumber
Timing of Events
Mineral Early Late
Anhydrite
Gypsum
Calcite
Dolomite
Pyrite
Quartz (Chert)
Stylolite

2. Accompanying Information Leaflet for Well 41/15-1
Geological Context:
The core represents Well 41/15-1 drilled by ConocoPhilips in the Southern North Sea off the Yorkshire
coast. This section of core can be correlated to the Zechstein group which is of Permian age.
Diagenetic Processes: The relative timing of these events is shown on Figure 3.
Pyrite formation requires the presence of organic matter in the sediment, sulfate dissolved in aqueous
solution in the pore water and, locally anaerobic (reducing) conditions. It is the presence of decaying
organic matter in the sediment that creates the reducing chemical environment (Bacterial Sulfate
Reduction). The bacteria reduce the sulfate ions (SO42-) in pore water to sulfide ions (S2-) and if iron is
present it will react with it to form pyrite (FeS2). CH2O + SO4
2- -> H2S + CO2. These sulfate reducing
bacteria require other nutrients to survive which is provided by the organic matter (sugars). The
pyrite-sulfur content of the rock correlates directly with TOC content of the rock (a measure of source
rock richness).
Stylolites are formed due to the compaction of the micrite. It involves a large-scale loss of material.
During compaction, the micrite dissolves along horizontal planes on which insoluble residue
accumulates to form stylolites. The insoluble residue, consists largely of clay minerals, accumulates on
these planes and produces a dark foliation/band. They form by pressure dissolution, which is a
dissolution process that reduces pore space under pressure during diagenesis (Klein and Philpotts,
2017). This results in the micrite having a very low porosity and this has a knock-on effect on
permeability, reducing it also. This nature of the rock makes it a good seal. Stylolites are present
throughout the whole core, highlighting the fact that, the whole section previously consisted of Micrite.
Quartz nodules suspected to be Chert were present in the core. Chert consists almost entirely of Silica
that forms when the Siliceous skeletons of marine plankton are dissolved during diagenesis, with Silica
being precipitated from pore water into fractures/pores within the rock.
Dolomite forms from the post-depositional alteration of the lime mud by magnesium rich groundwater.
The groundwater has a meteoric origin. The available Mg facilitates the conversion of calcite to
dolomite. Only the upper core (Box 1-9) has experienced significant dolomitisation. Evidence for this is
through the weak reaction with dilute HCl. This is most likely the extent to which Mg rich groundwater
could penetrate and had enough Mg2+ to replace Ca2+.
Anhydrite was identified throughout the core, SEM and thin section images (Figures 4-7). It was present
in the section of dolomitic lime mud. It is an evaporite deposit that likely precipitated as gypsum but
upon burial and compaction, has been dehydrated to Anhydrite. The gypsum was likely deposited on
the edges of the lagoon (the intertidal zone).
Depositional Environment:
The depositional environment of the whole unit is a stratified lagoonal environment. The upper core
consists of dolomitised lime mud with thin sections of anhydrite and traces of pyrite and chert. This
represents a depositional environment with partial terrestrial exposure whilst also having anoxic,
reducing conditions. The micrite was deposited in a lagoon. A regression likely occurred after, allowing
for evaporite precipitation in the upper core. Block diagram illustrating this (Figure 1.) A basinal
environment was also considered but due to being unable to explain the presence of evaporites in a
short transition (i.e. going from basinal facies to intertidal facies in 20m) this option was ruled out.
In Figures 4-5 the rocks are stained with Alizarin Red solution (a surface precipitate) that reacts with
carbonates to highlight calcite (pink) and doesn’t react with dolomite (unstained). (J. A. D. Dickson,
1966).

3. References:
Cameron, T. (1992). The Geology of the Southern North Sea. HMSO, London.
Data-ogauthority.opendata.arcgis.com. (2020). OGA - Oil & Gas Activity - SNS. [online] Available at:
https://data-ogauthority.opendata.arcgis.com/datasets/1e557a10142a467ca53d8b664d9c5a65
[Accessed 26 Jan. 2020].
J. A. D. Dickson (1966). Carbonate Identification and Genesis as Revealed by Staining. Journal of
Sedimentary Research, 36, 491-505.
Klein, C. and Philpotts, A. (2017). Earth materials. 2nd ed. Cambridge University Press, St Ives.
Underhill, J. and Hunter, K. (2008). Effect of Zechstein Supergroup (Z1 cycle) Werrahalit pods on
prospectivity in the southern North Sea. AAPG Bulletin, 92(7), 827-851.