Abstract
The diagenetic processes (dissolution and/or precipitation of minerals), which modify the micro-geometry at the scale of pore, result in the evolution of permeability in reservoir rocks. The aim of this study is to point out relationships between micro-geometry, permeability and illite. The 27 studied samples (sub-arkosic sandstones), originating from the Brent of the Alwyn area (North Sea), contain clay (kaolinite and illite) formed during late diagenesis. They display highly contrasting permeability values from 0.1 to 3000 mD. This study is based upon the correlation by multi-linear regression procedures of image analysis data with petrophysical data. Images of thin sections were acquired in the SEM/BSE and binary mosaic (2048X2048 pixels) of the porous networks were realized. Pores appear as elemental surfaces named "porels", which are the complex 2D signature of a simpler 3D geometry. The pore types (PT) present in this set of samples are obtained by polytopic factor analysis. Each PT is characterized by its distribution of shapes and sizes. Five PT were obtained and interpreted according to petrography. The validity of the obtained solution is discussed. This classification is of statistic and local nature because it takes into account only the morphology of "porels". It contains no information about the 3D-connectivity. The structural information is recovered by the capillary pressure curves coming from the same samples. By analogy with Laplace's equation, the parametres of the model are the relative amount of each PT connected by a throat size representative of each pressure bin. Pores of a same type tend to be connected by similar and exclusive throats. This relation is used to express permeability variations as an indirect function of the relative abundance of the PT showing the largest pore throats.
