Palaeogene glauconitic reservoir sandstones in the produced Nini West oilfield in the Siri Canyon, Danish North Sea, are chosen as the first pilot CO2 storage site in Denmark. Since glauconitic ...clasts comprise up to 35 vol% of the sandstones, their stability is crucial for the reservoir permanence. The likely reactivity of glauconitic clasts during CO2 injection is evaluated from their changes during burial diagenesis (long-term reactions) as a contribution to experimental testing (short-term reactions) of sandstone resilience towards CO2. Cored wells penetrate the Palaeogene gravity-flow sandstone units in depth intervals of 1700 m (<52 °C) to 3100 m (107 °C). The depth range allows a detailed microscopic and geochemical investigation of diagenetic changes in mineralogical composition with increasing burial depth. Although the glauconitic clasts show a large variation in chemical composition, clear average trends of increasing K2O and decreasing Fe2O3 in glauconitic clasts are recorded with increasing burial depth down to 2600 m. This trend is caused by the gradually reduced number of expandable layers (smectite) in the mixed-layer glauconitic mica/Fe-smectite. This shift in smectitic content is accompanied by a change in authigenic minerals from berthierine, siderite, microquartz and opal coatings at shallow burial depth to chlorite, calcite and quartz overgrowths at burial > 2200 m. Chloritisation of glauconitic clasts is intensive in the central parts of the gravity-flow sandstone units in the deepest (2800–3000 m) and most distal settings. Consequently, authigenic chlorite precipitation in pores occurs at shallower burial depth than chloritisation of glauconitic clasts. The microporous structure of the glauconitic clasts controls the slow gradual illitisation during increased burial. Abrupt lowering of pore-water salinity is considered the main reason for expansion of smectite components in the mixed-layer glauconitic mica/Fe-smectite, opening of the micropores and thereby intense chloritisation of glauconitic clasts in the deep and distal gravity-flow units. Hence, glauconitic clasts are potentially highly reactive, although slow to react due to their microporous structure unless the water composition distinctly changed.
•Glauconitic clasts consisting of mixed-layer glauconitic mica/Fe-smectite•Illitisation of the smectitic part of the glauconitic clasts with increased burial depth•Chloritisation of glauconitic clasts when in contact with low-salinity formation water•Slow reactivity of glauconitic clasts due to internal nano-size microporosity•Glauconitic clasts are expected to show delayed reactions in CO2 storage reservoirs.
The Siri Canyon has proved to be a perfect area for investigating various morphologies of diagenetic silica in sandstones. The development in silica morphologies can be observed from very shallow (∼
...1700
m) to increased burial depth (∼
3000
m) and increased proximity to the Central Graben (distance from 0 to 65
km). Hydrocarbons and pore fluids, now found in the Siri Fairway, have (at least partly) originated from the Central Graben.
The Siri Canyon is a submarine canyon system eroded into the uppermost chalk deposits and filled with Palaeogene hemipelagic and turbiditic marls and mudstones interbedded with sandstone units deposited from sandy mass-flows and sandy turbidites, which originated on the Stavanger Platform. Several hydrocarbon exploration and production wells have been drilled in the Siri Canyon, seven of which are included in this study (Nini-3, Nini-1, NA-2P, Sofie-1, Siri-4, Celilie-1A and Augusta-1). The reservoir sandstones in these wells all contain authigenic silica of various morphologies identified with a combination of traditional optical microscopy and scanning electron microscopy.
The silica morphologies in some places are classic and well-documented in the literature, whereas others, at least to our knowledge, have never previously been described. Some of the silica morphologies presented here show gradual transition from one to another, and others are stand-alone forms without clear relationships to other forms.
The silica morphologies can be expressed in the following way:
1.
Opal rims; characteristic of the initial phase of the silica diagenesis in most sandstone units in the Siri Canyon. Thick opal rims characterise the sandstone parts adjacent to the mudstone units in the Stine segment of the Siri Field.
2.
Microquartz (quartz crystals with a size of 1–5
µm); seen as coatings on the opal rims, both ordered and random.
3.
Cavity overgrowth; found as quartz outgrowths in circular and angular cavities formed by dissolution of early authigenic phases. Angular cavities in the microquartz coatings origin from dissolution of clinoptilolite, possibly with a source in volcanic ash, whereas dissolution of lepispheres probably resulted in circular cavities.
4.
Quartz ridges (syntaxial quartz overgrowths, a few micro millimetre in width, in parallel lines — defined in this paper).
5.
Quartz mountains (syntaxial, irregular 5–10
µm large quartz crystals — defined in this paper); developed in the water-filled sandstone intervals where fluids from the Central Graben were introduced, possibly together with the hydrocarbon, and where the continued growth was not retarded by the presence of hydrocarbons in the pore fluids.
6.
Macroquartz (syntaxial quartz overgrowths >
20
µm); seen as continued growths from quartz mountains. This takes place in the deeper reservoir sandstones, promoted by fluids originating from the Central Graben and increased burial depth.
7.
Quartz with sutured grain contacts; occurring only in the deepest well (3000
m).
The Hermod sand of the Stine Segments, Siri Field, Danish North Sea, displays large permeability variations (1–600
mD). These permeability variations represent horizontal layering in the reservoir, ...largely formed by diagenetic processes. They have a significant impact on reservoir performance, because horizontal producers in fairly thin reservoirs (∼30
m) are protected against bottom-water influx and can produce water-free for long periods of time.
This paper presents the results of a detailed multidisciplinary study where the observed variation in permeability can be explained by two main diagenetic phases: silica, as opal cement and microquartz, and berthierine, a serpentine mineral that is closely related to chlorite.
Opal/microquartz and berthierine cements dominate in separate parts of the reservoir bodies. Opal/microquartz cemented sands have high permeability (typically 500–600
mD). Berthierine cement has a different growth pattern in the oil and water zones, resulting in intermediate permeability in the oil zone (typically 50–100
mD) and very low permeability in the water zone (typically 1–10
mD).
