Organic-rich shales (ORS) are common source rocks for most clastic reservoirs. More recently, they have gained importance as reservoirs. However, the processes of kerogen maturation, hydrocarbon storage, and hydrocarbon transport are still poorly understood. Empirical relations have been developed to relate the increase in acoustic velocity and elastic modulus with increasing maturity. The reason for this increase in velocity also remains poorly understood. We conducted experiments on ORS samples with a range of maturities from the Bakken formation. Our study focuses on investigating methods of predicting maturity of ORS by evaluation of their impedance micro-structure. Our Young's modulus measurements on a nanometer scale help understand variations of Young's modulus of minerals, clay particles, and kerogen matter in naturally matured shales. The samples were re-measured after subjecting them to hydrous pyrolysis. This step helped us investigate the cause for change in modulus with maturity and the mobility of the pyrolysed organic matter. In the naturally matured samples, we find direct qualitative relationships between the Young's modulus of shale samples and its maturity indicators, such as TOC and Transformation Ratio. After hydrous pyrolysis, there is a significant lowering of the Young's modulus in some immature samples. We will present results of elastic property changes before and after hydrous pyrolysis in shales of various maturities. This study improves our current understanding of maturity-related variations by using analysis from nanoindentation. We integrate these measurements with geochemical analysis, and observations from downhole sonic measurements to develop relationships of elastic impedance to shale maturity. These results are critical to help us understand how shales evolve with burial and maturation and how hydrocarbons are stored and transported to sustain large storage even at high overburden stresses.

Organic-rich shales (ORS) need to be studied in detail to understand the provenance and the generation of oil from source rocks. In recent years, ORS have become interesting as important hydrocarbon resources as well. Successful exploration and production programs for ORS need reliable identification of the kerogen content and the maturity through indirect seismic methods. However, the seismic properties of kerogen are poorly understood and 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. In this paper we show correlations between the impedance microstructure of ORS and their maturity and elastic properties. We have used scanning acoustic microscopy to analyze and map the impedance microstructure in ORS. 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 and microstructures of ORS give important insight in changes due to kerogen maturation. We introduce a modified porosity term and find that (i) there is a significant correlation between velocity and modified porosity of all ORS; (ii) Imaging and quantifying microscale impedance texture and contrast in the images allows us to correlate them with ultrasonic measurements on a cm-scale; and (iii) textural heterogeneity, elastic impedance, velocity, and density increase with increasing shale maturity. In this paper, we show typical acoustic images of ORS and discuss possible methods to predict maturity from impedance based on understanding the changes due to maturity in well log response, core measurements, and microstructure of organic-rich shales. Our work has important bearing on developing successful production and stimulation methodologies. Introduction Organic-rich shales (ORS) and oil shales (OS) are increasingly being studied in detail to understand the provenance and the generation of oil from source rocks. In recent years, they have gained importance as hydrocarbon resource rocks. Proven and recoverable oil reserves from ORS and OS account for about 33 trillion tons of shale and 68 billion tons of oil, respectively. Of these, the US alone has 3.3 trillion tons of shale (10% of proven reserves from oil shales) and 60 billion tons of oil (90% of total recoverable oil from shales) with the estimated U.S. OS and ORS reserves totaling 1.5 trillion barrels of oil (Hepbasli, 2004). Successful exploration and production programs for ORS must rely on reliable 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 maturity remain a big challenge. Kerogen maturity changes shale texture. For example, overpressure due to hydrocarbon generation can lead to microcracks and fractures in the matrix (Lempp et al., 1994). Assessment of maturity from indirect measurements can be greatly enhanced by exploiting any existing correlations between physical properties, microstructure, and kerogen content.