Acoustic Signatures and Impedance Microstructure, Textural Scales, and Anisotropy of Kerogen-Rich Shale

A big challenge in studying organic rich shale is to predict its kerogen content and maturity from indirect observations. Kerogen maturation changes shale texture; for example, it generates microcracks and fractures in the matrix. Assessment of maturity from indirect measurements can be greatly enhanced by exploiting any existing correlations between physical properties, microstructure, and kerogen content. This paper shows how impedance microstructure of organic rich shale can be related to its maturity and elastic wave velocity. Microstructural variations significantly affect seismic wave propagation. Traditional techniques for studying microstructure either give surface information or are limited in resolution. We have used results of scanning acoustic microscopy to analyze and map impedance microstructures in organic rich shale. We have also analyzed the acoustic images to quantify textural anisotropy and heterogeneity and relate these textural properties to acoustic wave propagation parameters of impedance at micrometer and centimeter scales. The acoustic and microstructural differences in shale from various stages of kerogen maturation (diagenesis, catagenesis, and metagenesis) show that: 1. Acoustic impedance of the shale matrix is related to its total organic content and to hydrogen index 2. Pyrite, a common accessory mineral, increases impedance of the altered areas as compared to the unaltered kerogen material 3. In high porosity shale, velocity is directly related to porosity. In low porosity shale, velocity is dependent on kerogen content 4. Textural heterogeneity, elastic impedance, velocity, and density increase with increasing shale maturity. [Note: parts of this paper were published in two parts in the Acoustical Imaging Proceedings, 2005]