Geophysical prospecting of organic-rich rocks usually combines petrophysical data with elastic properties, in order to estimate the acoustic velocities. We propose here a model designed for an organic-rich chalk based on porosity, density and TOC data. The model includes an extension for estimating bitumen and kerogen contents based on bitumen extraction measurements. Using the densities and volume fractions of each phase, the elastic moduli are calculated using the “HS kerogen” model which simulates a kerogen-supported rock and found suitable for the organic-rich chalk at its immature state. The model is used to create a rock physics template of the organic-rich chalk. We find that where the Vp/Vs ratio is plotted against the acoustic impedance (typical layout), the effects of organic content and maturity are unclear. By using a matrix-focused layout, in which the acoustic impedance is replaced by the product of solids density and S-wave velocity (ρs∙Vs), the changes associated with organic content and maturity level are much better pronounced. We believe this layout can be useful for all organic-rich rock types. As for the Ghareb-Mishash chalk studied here, we find that its elastic properties are strongly influenced by porosity and organic-content, while their dependence on maturity is somewhat smaller.


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