Relationships between geomechanical properties and lithotypes in NW European chalks

Abstract

Abstract As a result of increasing interest in unconventional reservoirs, a wide range of sedimentary systems are now being investigated with regard to petroleum applications, including various tight chalk formations. We examined a wide variety of chalk samples from NW Europe (micritic, grainy, argillaceous, marl seam, cemented and silicified chalks) and investigated the relationships between their petrophysical properties, mechanical properties and associated microtextures and how diagenesis can affect these properties. A diagenesis index based on an evaluation of textural and diagenetic parameters was used to quantify the effect of global porosity-reducing diagenesis on the microtexture of chalks. We used petrographic and petrophysical measurements to determine the petrography, density, porosity, permeability and sonic velocity of the chalk samples and uniaxial compression experiments to assess their mechanical behaviour. Our dataset of >30 samples covers a wide range of values for these properties. We determined a linear porosity–permeability relationship controlled by the diagenesis index. Porosity influences the unconfined compressive strength and Young's modulus, but our analyses suggest that the diagenesis of the studied lithologies provides us with a further understanding of the mechanical behaviour of chalks. Micritic and grainy chalks are associated with the lowest diagenesis index and exhibit the lowest strength, whereas the higher diagenesis indices observed for other microtextures correspond to higher compressive strengths.