Mechanical and petrophysical behavior of organic-rich chalk from the Judea Plains, Israel

The geomechanical and petrophysical properties of the Late Cretaceous organic-rich chalk from the Shefela basin in central Israel were studied using laboratory tests performed on solid cylindrical samples from core retrieved from the resource. The investigated section includes two formations, the Ghareb and the Mishash of central Israel. The Shefela chalk is dark brown and rich in organic matter, with up to 20% TOC at the middle of the Ghareb formation. Both uniaxial compressive strength and Young's modulus of the chalk display stress dependency with the mean values increasing with depth. The tensile strength parallel to the bedding is found to be higher by 1.7 on average for Ghareb and by 2.2 on average for Mishash compared to the tensile strength normal to the bedding. Unconfined compressive strength appears to increase with total organic carbon (TOC) whereas the effect of TOC on tensile strength and elasticity is less conclusive. A linear compression-dilation boundary is defined for the Shefela chalk based on analysis of uniaxial and triaxial test results. The porosity is high, approx. 37% at the Ghareb formation, decreasing to 32% at the Mishash formation. The permeability is very low, ranging between 0.001mD and 1mD, and exhibits strong anisotropy with a vertical to horizontal ratio of 0.2. The porosity and permeability are found to be independent of the variation in organic content, confirming co-precipitation of the calcite minerals and organic matter during deposition.