Maturity and Impedance Analyses of Organic-Rich Shales

Manika Prasad and Kenechukwu C. Mba, SPE; and T. Elizabeth McEvoy and Michael L. Batzle, SPE, Colorado School of MinesSummaryOrganic-rich shales (ORSs) need to be studied in detail to under-stand 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 pro-duction programs for ORSs need reliable identification of their kerogen content as well as maturity through indirect seismic meth-ods. 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 centime-ter scales. This combined study of acoustic properties and micro-structures 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 micro-structure of ORSs. Our work has important bearing on developing successful production and stimulation methodologies.Introduction ORSs and oil shales increasingly are being studied in detail to understand the provenance and the generation of hydrocarbon from source rocks. This recent interest stems from the recogni-tion that ORSs are important self-contained hydrocarbon systems because they provide the source material and act as reservoir and seal formations. Because of our limited understanding of their hydrocarbon-generation and -storage capacity, it is challenging to assess reserves in ORS reservoir systems accurately. Furthermore, because shales are the most common sedimentary formation, reserves estimates of ORS reservoir systems have been updated dramatically in the past few years. Successful exploration and production programs for ORSs must rely on an accurate identification of the kerogen content and its maturity through indirect seismic methods. The seismic properties of kerogen are poorly understood. Consequently, predictions of the seismic response of a rock/kerogen system and the kerogen matu-rity remain a big challenge. Kerogen maturity changes shale tex-ture. For example, overpressure because of hydrocarbon generation can lead to microcracks and fractures in the matrix (Lempp et al. 1994). Assessment of maturity from indirect measurements can be enhanced greatly by exploiting any existing correlations between physical properties, microstructure, and kerogen content. One way of looking at ORSs is through acoustic analysis. Numerous authors have shown that kerogen content, and with it maturity, can be related to