AAPG Bulletin, Vol.93, No.2, 271-290, 2009
Dependence of joint spacing on rock properties in carbonate strata
Joint density is studied in relation to petrographic and petro-physical parameters in two sedimentary carbonate formations characterized by different diagenetic histories: the Kimmeridgian limestones of the Chay Peninsula (western France) with a mean joint density of 6.37 fractures per meter (fr/m) (1.94 fr/ft), and the Bathonian limestones of the Bouye outcrop (western France) with a mean joint density of 1.9 fr/m (0.58 fr/ft). The Chay carbonates are characterized by a lower CaCO3 content, a higher average porosity, and a lower sound velocity than values recorded in the Bouye limestones. The compressive strength and Young's modulus of the Bouye carbonates are, respectively, 10 and 3 times higher than in the Chay carbonates. A statistical analysis was used to identify relationships between joint density and carbonate rock properties. When facies variations are marked, the joint density at outcrop scale is related to the mean bed thickness, the facies descriptors, and the Young's modulus. When textural variations are more limited, the joint density is controlled by the porosity. At the scale of a sedimentary basin, 63.8% of the variation in joint density may be accounted for by Young's modulus and the sparite/micrite ratio. The decrease in the sparite/micrite ratio reflects an increased number of grain boundaries in the carbonate rock, which limits grain deformation and enhances joint density. The variations in Young's modulus depend essentially on the porosity and mineralogy of the studied rocks. Any increase in CaCO3 content or decrease in porosity is associated with an increase in the elastic properties of the rock and a reduction of joint density.