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.
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.
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