Tight chalk: How does microtexture control petrophysical and geomechanical properties?: The role of clay flakes and calcite cements

Since the discovery of the Ekofisk chalk hydrocarbon reservoir in the North Sea in the 1960s, porous reservoir chalk has been extensively studied. Recently, with the maturation of the existing hydrocarbon fields, focus has been on maximizing the production and as such, increasing attention is payed to understanding low reservoir-quality chalks or tight chalks. Defined by a permeability lower than 0.2mD, these lithologies may act as seals or form potential unconventional reservoirs. In order to unravel their characteristics, an integrated petrographical, petrophysical and geomechanical study was carried out on 65 samples from outcrops in NW-Europe. The key parameter defining intrinsic properties of chalk is the microtexture. Tight chalks encompass different lithotypes, but our study shows that the main factors controlling microtexture are the non-carbonate content and degree of cementation. The dataset gathered covers a broad spectrum of petrophysical (e.g. porosities from 9 to 45%) and geomechanical properties (e.g.: strengths from 3 to 50 MPa). SEM and FIB-SEM observations reveal how clay flakes and calcite cement control microtextural properties and shape the porous network. Clay flakes reduce both pore body and throat sizes, which in turn reduce porosity and permeability. Matrix interparticle cementation also reduces pore size, and has inherently a similar effect on porosity and permeability. Petrographic observations reveal that cementation strengthens grain contacts, resulting in higher UCS strength. Statistical analyses support the conclusion that petrophysical and geomechanical properties are controlled by the type of microtexture.