Grain size distributions of chalk from image analysis of electron micrographs

In the chalk of the Ekofisk formation in the Chalk Group of the North Sea, substantial depth-related variations in porosity are observed. With the aim of obtaining a textural interpretation of these porosity data, we have dev eloped a method to assess the grain size distribution of the chalk from ima ge analysis. The chalk is composed of a fine-grained matrix of nannofossils and predominantly calcitic fossil debris with larger microfossil grains, b ut the chalk may also contain significant amounts of silica and siliciclast ic clay. For image analysis, we used backscatter electron images of epoxy-i mpregnated, polished samples from the Ekofisk Formation. Tyra field, Danish North Sea. On backscatter images the calcite phase will appear light as op posed to the dark pore space. The procedure involves the combination of dat a acquired at two magnifications and by this method analysis of only four i mages per sample is sufficient. The two applied magnifications result in im ages measuring 300 mum x 400 mum and 30 mum x 40 mum, respectively. The met hod does not build on image analysis alone because the siliciclastic clay i s not resolved at these magnifications but will appear grey; and because th e total porosity is difficult to assess from image analysis due to rim effe cts inherent in backscatter images at high magnification. Thus, in order to obtain a consistent interpretation, we use total (He) porosity and insolub le residue as measured in the laboratory. We find that the volume density o f larger grains (cross section larger than 316 mum(2)) relative to solids ( one minus porosity) primarily controls the porosity of the chalk samples. ( C) 2001 Elsevier Science Ltd. All rights reserved.