Abstract:
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.
Introduction
Organic-rich sediments are widely known as source rocks due to their high potential within the organic matter to produce oil and gas. These rocks are typically fine-grained sediments that include organic matter in solid and fluid states. Kerogen is solid, and is the most abundant organic phase in thermally immature source rocks. The physical properties of source rocks are strongly influenced by kerogen properties such as density, chemical composition, elastic moduli and acoustic velocities. Thermal maturation causes significant changes in kerogen properties and thus in the entire rock properties. The dynamics of source rocks during thermal maturation is complex due to interaction of processes within the organic matter and field conditions (e.g. stress field, pore pressure, host rock properties, temperature). In addition to the importance for conventional oil systems, advanced technologies allows now to produce oil and gas from immature source rocks via in situ methods for inducing maturation. These aspects, among others, motivate the study of rock physics of organic-rich rocks and its variation upon maturation.
