Elastic Anisotropy of Source Rocks: Implications for Hydrocarbon Generation and Primary Migration¹

We combined acoustic velocities and anisotropy measurements with x-ray diffraction mineralogy, Rock-Eval organic geochemical analyses, vitrinite reflectance, and scanning electron microscope observations on 69 cores of hydrocarbon source rocks collected from eight sedimentary basins. The velocity data indicate the lamellar distribution of solid organic matter (kerogen) in compacted black shales and show a strong correlation between their elastic anisotropy and organic richness. The experimental results are consistent with the two-component anisotropic model of black shales with a continuous kerogen network in both bedding-parallel and bedding-normal directions beginning from relatively low TOC values. The strong elastic anisotropy of tight, thermally mature source rocks is further enhanced by overpressure-induced bedding-subparallel microcrack development during the main stage of hydrocarbon (HC) generation and primary migration at R_o greater than 0.75%. In agreement with published petrographic observations, these microcracks significantly aid primary HC migration, particularly horizontally. Elastic anisotropy of medium- to high-porosity illite/smectite shales is weaker than in lower porosity, predominantly illite shales; however, the overall positive trend of elastic anisotropy with kerogen content remains unchanged.