Tight Chalks - How Does Microtexture Affect Petrophysical and Geomechanical Properties?

High porosity chalks have been extensively studied over the past decades. Only recently, there has been an increasing interest in the understanding of low permeability chalks, forming potential unconventional reservoirs or intra-reservoir seals. In order to better understand the properties of those tight chalks, an
integrated petrographical, petrophysical and geomechanical study was carried out on a set of 35 carefully selected outcrop samples, covering a wide range of lithotypes. The dataset gathered covers a broad spectrum of values with regards to determined petrophysical (e.g.porosities from 15 to 45%, pore throat diameters from 25nm to 1080nm) and geomechanical properties (e.g.: strengths from 3 to 50 MPa). The samples have also been characterized in terms of microtextures by integrating both geological and sedimentpetrological data. Tight chalks encompass different lithotypes, but the main factors controlling the microtexture are: (1) the non-carbonate content, either related to sedimentological settings (e.g.
argillaceous chalks) or burial diagenesis (e.g. marl-seam chalks); (2) the degree of cementation, either eogenetic (e.g. hardgrounds) or mesogenetic. Those parameters strongly modify chalk microtexture and thus its porous network, reducing pore-throat and pore body sizes hence altering poroperm properties. Beyond petrophysical properties, cementation appears to be the main parameter controlling the strength of chalks.