Precipitation and flocculation of spherical nano-silica in North Sea chalk

Clay Minerals ◽  
2000 ◽  
Vol 35 (1) ◽  
pp. 175-184 ◽  
Author(s):  
F. Jakobsen ◽  
H. Lindgreen ◽  
N. Springer
Keyword(s):  
North Sea ◽  
Water Environment ◽  
Free Water ◽  
Water Phase ◽  
Spherical Particles ◽  
Force Microscopy ◽  
The North ◽  
Atomic Force ◽  
The North Sea ◽  
Nano Size

AbstractIn the Maastrichtian-Danian chalk in the North Sea, discrete intervals, appearing as normal white chalk, contain up to 60% α-quartz <2 μm in size. Atomic force microscopy (AFM) reveals that the particles are of nm size, appearing as spherical particles and aggregates. Similar particles consisting of opal-CT were found in surface exposures of chalk in Denmark. Two new abiogenic pathways of silica formation in chalk are proposed. The first model proposes that SiO2 nano-size particles and aggregates precipitated and flocculated in the free-water phase as opal and were diagenetically transformed from opal-CT at low temperature to α-quartz at elevated temperature. In the second model, the dominance of radiolarians in the deep-water environment of the North Sea resulted in low dissolution supply with subsequent precipitation and flocculation of nano-size α-quartz particles. In the shallower water of the shelf environment of the present onshore chalk, the abundance of sponges and their dissolution supplied enough Si to precipitate opal-CT in the free-water phase.

Nano-size quartz accumulation in reservoir chalk, Ekofisk Formation, South Arne Field, North Sea

Clay Minerals ◽  
2010 ◽  
Vol 45 (2) ◽  
pp. 171-182 ◽  
Author(s):  
H. Lindgreen ◽  
F. Jakobsen ◽  
N. Springer
Keyword(s):  
North Sea ◽  
Regional Distribution ◽  
Free Water ◽  
Water Phase ◽  
Reservoir Properties ◽  
Force Microscopy ◽  
Hydrocarbon Reservoir ◽  
The North ◽  
Clay Layers ◽  
Nano Size

AbstractIn the oil fields in the Central Graben of the North Sea, Maastrichtian chalk is an important hydrocarbon reservoir, but oil may also be found in the Danian chalk, consisting of white chalk interbedded with clay layers. Within the chalk section, discrete intervals appear like chalk but contain large amounts of quartz (up to 100%). The aim of the present investigation is to reveal the mechanism for formation of the quartz in these intervals and to discuss their regional distribution and importance for the reservoir properties. Samples of chalk, including quartz-rich intervals, and clay layers from three wells SA-1, Rigs-1 and Rigs-2 in the South Arne Field have been investigated. Calcite-free residues obtained using a buffered dissolution of calcite were investigated using X-ray diffraction, scanning electron microscopy and atomic force microscopy. The main silica phase in the Upper Cretaceous–Danian chalk in the three wells is a nano-size α-quartz. This has probably formed from Si dissolved from radiolarians in the free-water phase. The 600 Å diameter α-quartz spheres precipitated in the free-water phase with subsequent flocculation of the spheres and sedimentation of the flocs. Variations in the proportion of quartz in the chalk are attributed to variations in the amount of radiolarians in combination with variations in CO2 concentrations in the water; increased CO2 causes dissolution of coccoliths and thus a relative enrichment in quartz. This formation mechanism is regional and makes it probable that the layers rich in nano-quartz may be found over large areas provided that the chalk is authigenic. Quartz-rich layers are generally of low permeability and in areas with authigenic chalk these layers may act as internal seals in chalk reservoirs.

Formation of flint horizons in North Sea chalk through marine sedimentation of nano-quartz

Clay Minerals ◽  
2011 ◽  
Vol 46 (4) ◽  
pp. 525-537 ◽  
Author(s):  
H. Lindgreen ◽  
V. A. Drits ◽  
A. L. Salyn ◽  
F. Jakobsen ◽  
N. Springer
Keyword(s):  
North Sea ◽  
Volcanic Eruptions ◽  
High Concentration ◽  
Cell Parameters ◽  
Force Microscopy ◽  
The North ◽  
Atomic Force ◽  
Marine Sedimentation ◽  
The North Sea ◽  
Diffraction Patterns

AbstractIn the Upper Cretaceous-Danian North Sea chalk, silica composed of nano-size quartz spheres is dispersed in the chalk matrix, and quartz is present in bands and nodules of flint. In the present investigation of the North Sea Danian chalk the nano-quartz in the chalk matrix is compared with the silica in the flint. Samples of chalk and flint layers from four North Sea wells have been investigated. Atomic Force Microscopy (AFM) has been applied to image the quartz in the chalk and in the flint. X-ray diffraction (XRD), including analysis of the positions and profiles of hkl reflections in powder diffraction patterns, has been applied to characterize the lattice of the quartz in both the chalk matrix and in the flint. The quartz in the chalk matrix and in the flint is composed of nano-quartz spheres having identical cell parameters. Based on the results we propose a new model for formation of flint in North Sea chalk: (1) The nano-quartz in the flint, like the nano-quartz in the chalk matrix, has crystallized in the marine Chalk Sea environment. The colloidal quartz particles flocculated and were deposited on the sea floor mixed with calcitic bioclastic material. (2) Regional variations in the concentration of nano-quartz particles in the sediment reflect different degrees of acidification of the Chalk Sea. (3) This resulted in areas where practically all the calcite bioclasts were dissolved leaving a high concentration of nano-quartz particles to form flint layers; where there was less dissolution, indurated chalk with abundant nano-quartz particles is now preserved. (4) The acidification could have been caused by the effects of enhanced atmospheric CO2 linked to massive short-lived volcanic eruptions in the British Tertiary Igneous Province.

scholarly journals The Eemian stratotype locality at Amersfoort in the central Netherlands: a re-evaluation of old and new data

2000 ◽  
Vol 79 (2-3) ◽  
pp. 197-216 ◽  
Author(s):  
P. Cleveringa ◽  
T. Meijer ◽  
R.J.W. van Leeuwen ◽  
H. de Wolf ◽  
R. Pouwer ◽  
...  
Keyword(s):  
North Sea ◽  
Water Environment ◽  
Type Locality ◽  
Local Conditions ◽  
Sedimentary Environments ◽  
The North ◽  
Sea Bed ◽  
New Information ◽  
Clastic Sediments ◽  
The North Sea

AbstractIn order to obtain a better understanding of the infilling of the Saalian glacial basins during the Eemian, particularly following the recent research in the Amsterdam Basin (Terminal borehole), it was necessary to re-investigate the type locality of the Eemian at Amersfoort. Both published and unpublished data from various biota (diatoms, foraminifers, molluscs, ostracods, pollen) provide new information on the changing sedimentary environments during the Eemian. Although the organic and clastic sediments of the infilling represent nearly all the pollen zones, the sedimentary sequence at Amersfoort is discontinuous: four breaks at least are recognised at the type locality.The successive sedimentary environments and the breaks in the record are linked with the transgression of the Eemian sea, the topographic position at the margin of an ice-pushed ridge, and the changes in hydrodynamic conditions. Local conditions, such as a sandy sea bed, shallow water and a reduced water exchange near the North Sea margin, influenced the salinity of the basin. Rib counts of Cerastoderma edule shells indicate a higher salinity at the end of the Taxus (E4b) and the beginning of the Carpinus (E5) zones than that present in the modern North Sea. Local conditions were responsible for the higher salinity following the climate optimum.During the Abies phase (the later part of regional pollen zone E5), the sea level had already fallen. The change from eu-trophic peat growth (with Alnus and Salix) to an oligotrophic Ericaceae/Sphagnum community at the end of the Eemian resulted from the change from a marine to a fresh-water environment, probably coherent with a deterioration of the climate.

scholarly journals Nano-quartz in North Sea Danian chalk

1969 ◽  
Vol 26 ◽  
pp. 9-12
Author(s):  
Holger Lindgreen ◽  
Finn Jakobsen
Keyword(s):  
North Sea ◽  
Clay Fraction ◽  
High Porosity ◽  
Quartz Crystals ◽  
Force Microscopy ◽  
Variable Porosity ◽  
The North ◽  
Porosity And Permeability ◽  
The North Sea ◽  
Structural Methods

The main oil reservoir in the Central Graben in the North Sea is chalk of the Maastrichtian Tor Formation, which has high porosity and relatively high permeability. The chalk of the Danian Ekofisk Formation is an additional reservoir, but with highly variable porosity and permeability. Whereas the Tor Formation is almost pure calcite primarily consisting of coccolith debris, the Ekofisk Formation also comprises significant proportions of phyllosilicates (clay minerals) and quartz in addition to coccolith debris. For decades the quartz was assumed to be a normal crystalline α-quartz such as is present in quartz sand, and the clay fraction was assumed to consist predominantly of phyllosilicates. However, Maliva & Dickson (1992) reported the presence of presumably authigenic submicron-size quartz crystals arranged in clusters, and suggested that these clusters were transformed opal-CT lepispheres. Investigations by nano-structural methods (Xray diffraction and atomic force microscopy (AFM)) revealed that the prevailing quartz component in the North Sea chalk comprises α-quartz appearing as nano-size quartz spheres (Jakobsen et al. 2000; Lindgreen et al. 2010). Nano-quartz spheres were first observed in indurated chalk in the Ekofisk Formation in the Ekofisk Field and later in the South Arne Field. Subsequent analyses of the Ekofisk Formation in different chalk fields showed that the content of nano-quartz varies throughout the chalk succession and to some degree reflects the cyclic development of the chalk. The proportion of dispersed nano-quartz in the chalk is highly variable, from 10% to more than 80% in the Lower Danian (Lindgreen et al. 2010). This paper describes the nano-quartz, its formation and structure and presents a model for the formation of flint from nano-quartz in the North Sea Ekofisk chalk.

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