Source rocks possess complex heterogeneous matrices with soft organic matter, consisting mainly of kerogen, interspersed within a stiff inorganic mineral framework that varies in composition. There is not a clear understanding nor adequate knowledge of how geochemical properties influence the rock physics, especially when predicting a seismic response. While many attempts have been made to use seismic to empirically quantify these properties for the purpose of exploration, those attempts have often failed due to the complexity of the elastic properties of kerogen and the laminated geometry of the rock. This is due primarily to uncertainty over how these properties change with maturity as a result of burial and subsequent uplift. Therefore, knowledge of (1) the elastic properties of kerogen, (2) the amount and geometric distribution of organic matter within the rock matrix, and (3) the impact of kerogen maturity on its elastic properties is needed to predict a seismic response. An elastic property modeling method has been developed to address this challenge based on the integration of high-resolution microscopy, geochemical analysis, and velocity measurements. Using this approach, endmembers are obtained that allow for building rock-physics models that can predict elastic uncertainty from mineral heterogeneity and estimate the elastic properties of organic matter. Digital images, geochemical data, and velocity measurements coupled with maturity modeling suggest that bulk and shear softening of kerogen can help distinguish between maturity-induced seismic responses.