M. Prasad and A. Nur

Geophysics Department, Stanford University, CA

ABSTRACT: Impedance Microstructure of Kerogen in Organic Rich Shales

A big challenge in studying organic rich shales is to predict their kerogen content and maturity from indirect observations. Kerogen maturity changes shale texture, for example, it generates microcracks and fractures in the matrix. Assessment of maturity from indirect measurements can be greatly enhanced by correlation between physical properties, microstructure, and kerogen content. This paper relates impedance microstructure of organic rich shales to their 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 use a unique method of scanning acoustic microscopy to analyze and map impedance microstructures in organic rich shales. Its main advantage is that the microstructural maps, made from reflected acoustic waves, can be quantified in terms of acoustic wave propagation parameters of impedance (equals acoustic velocity times density). Since acoustic waves can penetrate below the surface, both surface and subsurface textures can be imaged. The acoustic and microstructural differences in shales 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 shales, velocity is directly related to porosity. In low porosity shales, velocity is dependent on kerogen content.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado