Influence of laboratory-induced maturation on rock-physics of Organic-Rich Chalks

We investigated the rock physics of immature organic-rich chalk from the Late Cretaceous Ghareb and Mishash formations in the Shefela basin, central Israel, and its variation upon pyrolysis-induced maturation. The study was carried out on core samples from a ∼280 meters sequence of immature organic-rich chalk in the Zoharim well. We measured acoustic velocities using water-saturated core plugs in the bedding-normal direction, under simulated field pressures. Porosity and density data were obtained for oven-dried core plugs using nitrogen gas. Results measured for the immature chalk were compared with properties obtained at two different induced maturity levels: early-mature and over-mature. In the early-mature stage, where bitumen filled the newly generated and some of the pre-existing pore space, we observed an increase in S wave velocity by ∼11%, and a slight decrease of P wave velocity, which was identical to the decrease in porosity and density. In the over-mature stage, where the pore space increased by ∼29%, and the solid skeleton included kerogen residue and unaltered minerals, S wave velocity increased by ∼19% and P wave velocity decreased by ∼16%. We concluded that the kerogen experiences stiffening upon thermal maturation that results in faster shear waves. P wave velocity decreased upon maturation due to significant porosity enhancement. P wave - S wave velocities ratio (Vp/Vs) starts at ∼2.2 in the immature stage, then reduces to ∼2 in the early-mature stage, and further down to ∼1.6 in the over-mature stage.