Relations between lithology and geomechanical properties of chalks from NW Europe

High porosity pure white chalks have been extensively studied for more than forty years in relation with oil and gas production. Recently, there has been an increasing interest for unconventional reservoirs, among which other chalk formations, leading to the need for investigating a wider range of materials.
For a better understanding of their mechanical behaviour, a relation can be made with the diagenesis of chalk formations. Several classification systems exist for carbonated rocks, based on their mineralogical content, their granulometry. Faÿ (2014) developed a diagenesis index based on the evaluation of textural and diagenetic parameters: the micritic matrix texture, grains contact, coccolithe disintegration, cemented zone, automorphous cement crystals, coccolithe grain overgrowth and intraparticle cementation. This tool, especially developed for chalk, aims to quantify the global porosity reducing diagenesis affecting chalk microtexture.
In this study, a wide variety of chalk samples from NW Europe have been investigated: classical high porosity chalk, clayey chalk, cemented chalk, coarse white chalk,... On one hand, a petrographic study has been performed to determine their density, porosity, permeability and sonic velocity. On the other hand, uniaxial compression experiments have been performed. The dataset gathered on more than 30 samples covers a wide range of values for the determined properties: porosities between 15 and 45%, strength between 3 and 50 MPa, just to give two examples. Additional data from literature are also considered in the study.
The relations between petrographic and mechanical properties are investigated. The porosity-permeability relationship is found to be non-linear. Porosity is linked to P-wave velocity, strength and Young's modulus: high porosity chalk have lower P-wave velocities and lower strength. In addition, the pore size shows an interesting relationship with Young's modulus. Finally, when assessing those relationships, chalk families based on their lithology can also be emphasized.