Maturity and Impedance Analysis of Organic-Rich Shales

Organic-rich shales (ORSs) need to be studied in detail to understand the provenance and the generation of hydrocarbons from source rocks. In recent years, ORSs have gained importance as hydrocarbon resources as well. Successful exploration and production programs for ORSs need reliable identification of their kerogen content as well as maturity through indirect seismic methods. However, the properties of kerogen are poorly understood, so predictions about maturity and rock-kerogen systems remain a challenge. Assessment of maturity from indirect measurements can be greatly enhanced by establishing and exploiting correlations between physical properties, microstructure, and kerogen content. We show correlations between the impedance microstructure of ORSs and their maturity and elastic properties. We have used scanning acoustic microscopy to analyze and map the impedance microstructure in ORSs. We quantified textural properties in the images and related these textural properties to maturity and to impedance from acoustic-wave propagation measured at centimeter scales. This combined study of acoustic properties and microstructures of ORSs gives important insight into changes resulting from kerogen maturation. We introduce a modified porosity term and find that (1) there is a significant correlation between velocity and modified porosity of all ORSs; (2) imaging and quantifying microscale impedance texture and contrast in the images allow us to correlate them with ultrasonic measurements on a centimeter scale; and (3) textural heterogeneity, elastic impedance, velocity, and density increase with increasing shale maturity. We also discuss possible methods to predict maturity from impedance on the basis of understanding the changes resulting from maturity in well-log response, core measurements, and microstructure of ORSs. Our work has important bearing on developing successful production and stimulation methodologies.