Porosity Variation in Cenozoic and Upper Chalk from the Ontong Java Pleateau

Porosity was obtained from matrix- and intraparticle porosity assessed from image analysis of backscattered electron micrographs of 3000x and 300x magnification. Comparing porosity assessed from image analysis with porosity measured by index properties, it was seen that image analysis data at 3000x magnification overestimates the dominating phase where images are resolved well, because of transparency in the upper few microns of the sample.

Thus image analysis porosity in the upper ooze interval (porosities more than 65%) show a good correspondence, because of the combined effect of porosity being the domination phase and insufficient resolution leading to porosity underestimation. In chalk samples (porosities around 50% or more) the lowest estimate of image analysis porosity corresponded because of the transparency, whereas in limestone samples (porosities less than 25%) highest image porosity estimate corresponded also because of the transparency.