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Diagenetic Variations between Upper Cretaceous Outcrop and Deeply Buried Reservoir Chalks of the North Sea Area*
By
Morten L. Hjuler1 and Ida L. Fabricius1
Search and Discovery Article #50056 (2007)
Posted November 1, 2007
*Adapted from extended abstract prepared for presentation at AAPG Annual convention, Long Beach, California, April 1-4, 2007
1Institute of Environment & Resources, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark
In the central North Sea Basin hydrocarbon-bearing chalks are deeply buried (2-3 km) whereas chalks in the rim areas are cropping out in the surrounding countries. The differing diagenetic histories between buried and outcrop chalk result in different rock properties, which are of great importance when simulating reservoir conditions using outcrop chalks as models.
In general, deeply buried reservoir chalks show significant overgrowth as witnessed by reshaping of particles together with strengthening of particle contacts. Most outcrop chalks are moderately affected with looser interparticle connections and less altered particle shapes. The non-carbonate mineralogy of outcrop chalks is dominated by quartz, occasionally opal-CT and clinoptilolite, and the clay mineral smectite. In offshore chalks quartz still dominates; opal-CT has recrystallized into submicron-size quartz crystals, and smectite has been replaced by kaolinite. These diagenetic variations are explained by higher temperatures and pressures in the deeply buried reservoir chalks.
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Chalk deposition of north-western Europe was controlled by the Late Cretaceous sea-level highstand (Figure 1) together with the regional subsidence of the entire North Sea area basin. Subsidence of the central North Sea continued during the Cenozoic whereas rim area inversions caused removal of Cenozoic overburden as well as chalk layers (Hillis, 1995; Japsen, 1998). As a consequence the North Sea chalk constitutes a coherent, saucer-shaped body with the rim cropping out in several countries surrounding the North Sea Basin while the central part is buried beneath more than three km of Cenozoic sediments (Japsen, 1998). Chalk is a porous, very fine-grained pelagic sediment (particle size ~ 1 μm) composed primarily of skeletal debris from calcareous nannofossils, mainly coccoliths, with minor contributions from microfossils and macrofossil fragments. During burial diagenesis the contact points between calcite particles are strengthened due to porosity-preserving recrystallization and, at greater burial (higher stress), porosity-reducing cementation due to pressure dissolution at calcite-silicate contacts (stylolitization) (Fabricius, 2003; Fabricius and Borre, 2007). Deeply buried chalks in the Central Graben are overpressured. Pore fluids support part of the overburden load thus relaxing the effective stress at calcite particle contacts and delaying mechanical compaction. As a result porosities are generally high and often comparable with porosities of outcrop chalks. However, offshore chalks are deeply buried (~3 km) and maximum burial depths of outcrop chalks are estimated to 1-1.5 km in Yorkshire (Hillis, 1995) and 0.5-1 km in eastern and northern Denmark (Japsen, 1998). This difference in burial depth impacts stress and temperature conditions, for example, and most likely type and degree of diagenetic alterations. To reveal how different diagenetic histories are reflected in petrographical, petrophysical and mineralogical properties of chalk a number of offshore samples and outcrop samples encircling the present-day North Sea were investigated.
The investigated chalk samples span the stages Santonian, Campanian, and Maastrichtian and were selected from 17 outcrop localities covering 6 countries and from 4 offshore localities within the Central Graben (Figure 1). Apart from offshore chalks all samples were cored and crushed, respectively. Cores were used to determine porosity and permeability. Crushed samples were used for measurements of carbonate content, oxygen isotope values (δ18O), specific surface area, loss on ignition (LOI), and for mineral identification by X-ray diffraction (XRD). In addition crushed chalk powder was dissolved with HCl and chemical analysis performed on the insoluble residue to establish quantitative relations between silica, silicates, and other non-carbonate phases. Scanning electron microscopy (SEM) investigations were carried out on a Zeiss Supra 35VP field emission microscope equipped with a SE detector, and Backscatter electron microscopy (BSE) investigations were carried out using a Jeol JSM-5900 instrument equipped with EDS detector.
Based on petrographical, petrophysical, and mineralogical data (Tables 1, 2 and 3, respectively), a number of diagenetic trends within chalks can be described.
Characterization of Outcrop and Offshore Chalk As outcrop chalks have been subjected to limited maximum burial (e.g., Hillis, 1995; Japsen, 1998) and pressures and temperatures accordingly remained low, incipient stylolites and porosity-reducing cementation are only observed at few relatively deeply buried or tectonically affected localities. High porosities are preserved, and δ18O values remain relatively high. Due to recrystallization chalk particles have formed loose-firm contacts (Figure 2A, B), as witnessed by varying degrees of contact cementation. In general original particle shapes are preserved, although overgrowth is always present and some reshaping into rhombs occurs. Non-calcite mineralogy is mainly dominated by quartz and smectite, but occasionally opal-CT, clinoptilolite, and apatite occur in vast quantities. Where smectite and especially opal-CT dominates, the specific surface area of the non-carbonate phase is at maximum; in combination with low carbonate content the specific surface area of chalk increases.
Some pronounced diagenetic variations occur within outcrop chalks:
Deeper burial (2400-3600 m) and thus increased pressure and temperature of offshore chalks have generally reduced porosities, and stylolites, cementation features, and lower δ18O values occur commonly. However, overpressuring of the chalks has created a special diagenetic environment where porosities are much higher (up to 48%) than dictated by burial depth. Even in high-porosity samples where little or no cementation has taken place, recrystallization has mostly reshaped chalk particles into rhombs (Figure 2C, D) and strengthened the cohesion between particles as seen from conspicuous contact cements. In low-porosity samples cementation has reshaped nearly all particles, and even microfossils may be hard to recognize. Only coccoliths show remarkable resistance against alterations, although their numbers diminish. In contrast to outcrop chalks non-calcite mineralogy is unvaried and consists mostly of quartz, kaolinite, and mica (Table 3). Opal-CT and clinoptilolite were not observed in the studied samples, and smectite in just one sample. Quartz is the dominant mineral and occurs in larger amounts offshore than onshore, indicating that opal-CT and clinoptilolite have been transformed into quartz. Possibly submicron-size quartz crystallites arranged in aggregates have recrystallized from opal-CT lepispheres. The specific surface area of chalk is very unvaried despite variations in carbonate content. This is probably an effect of equalization of particle sizes during recrystallization and due to the absence of smectite and opal- CT, minerals with large specific surface areas.
Outcrop Chalks as Substitutes for Reservoir Chalks Judged from petrographical and petrophysical evidence, English chalks resemble offshore chalks the most. Recrystallization and cementation features in Queensgate and Whitecliff Bay chalks are nearly as well-developed as observed in offshore chalks. Chalks from Stevns, Rørdal, and Hallembaye are often used as substitutes for reservoir chalks. However, these outcrops chalks are less diagenetically altered than chalks from the Dan, South Arne, and Ekofisk fields. Recrystallization features are clearly visible, but severe reshaping, porosity-reducing cementation, and strengthened particle contacts are either absent or less developed; these expectedly, from a matrix point of view, would make them mechanically weaker than reservoir chalks. One exception is the Valhall field, where overpressure and hydrocarbon presence has preserved very high porosities in some sections and impeded cementation, as witnessed by loose chalk particle contacts, less pronounced particle reshaping, and high δ18O values.
Some chalks differ significantly from reservoir chalks and will probably constitute poor substitutes for the latter. These chalks include:
Burial depth is a main diagenesis-controlling factor. Recrystallization and porosity-reducing cementation of chalk particles, coccoliths, and microfossils are clearly evident in onshore samples subjected to shallow maximum burial, but much more pronounced in deeply buried offshore samples. In addition, mineralogy changes and δ18O values are lowered in response to burial.
The significant diagenetic alterations of English chalks make these rather similar to, and possibly acceptable substitutes for, offshore chalks from the Dan, South Arne, and Ekofisk fields. In contrast less diagenetically altered chalks from Stevns, Rørdal, and Hallembaye, for example, share characteristics with Valhall field chalks.
BP generously provided the funding for this study.
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