Elsevier

Marine and Petroleum Geology

Volume 70, February 2016, Pages 175-184
Marine and Petroleum Geology

Research paper
Impact of thermal maturation on nano-scale elastic properties of organic matter in shales

https://doi.org/10.1016/j.marpetgeo.2015.12.001Get rights and content

Highlights

  • Elastic properties of bitumen and kerogen in shales were mapped at the nanometer scale.

  • Samples from different levels of thermal maturity show that kerogen stiffens with increased maturation.

  • Some kerogen macerals contain soft bitumen inclusions.

  • Specific types of bitumen can be linked to parent kerogen macerals.

Abstract

The properties of organic matter change during diagenesis and catagenesis, potentially altering the way shales deform and fracture. Although kerogen in mudrocks is thought to become stiffer during thermal maturation, few studies have been able to confirm this by direct measurement, as standard mechanical testing techniques cannot easily be used to measure the micrometer sized organic components in shales. Here, we use a new non-destructive atomic force microscopy technique to map the elastic modulus of organic and inorganic components at the nanometer scale in shales containing Type II kerogen from three different levels of thermal maturation. We found that when vitrinite reflectance increases from 0.40 to 0.82, the average Young modulus of kerogen increases from 6.1 GPa to 16.0 GPa. However, as %Ro increases further from 0.82 to 1.25, the modulus values for kerogen do not change significantly. A high degree of variance is registered in the elastic moduli, particularly at higher levels of thermal maturation, probably reflecting the inherent heterogeneity in the depositional organic matter present in the shales. The mean modulus for bitumen in the same samples – identified as void filling organic matter that was present only at intermediate and higher levels of maturation - was relatively constant with mean values of 7.5 GPa and 8.5 GPa, respectively. In the samples that experienced catagenesis, the modulus maps reveal that individual kerogen macerals possess soft regions - interpreted as exuded bitumen - which act to soften the overall structure of the kerogen. As well as providing high resolution mechanical data, this technique could be used to track the way bitumen and other compounds are generated from kerogen during catagenesis.

Introduction

In both conventional and unconventional reservoirs, the mechanical behavior of shales is often poorly understood. As the elastic properties of both the organic and inorganic components that make up shales strongly influence their mechanical behavior, a comprehensive characterization of the constituent components in shales could improve the accuracy of rock mechanics models. However, although the mechanical properties of many of the mineral phases present in shales are reasonably well known, the characteristics of the more compliant (softer) organic components remain enigmatic. During thermal maturation new hydrocarbon phases - such as bitumen - form from kerogen. In addition to altering the kerogen's chemical composition, the cracking process also changes the physical and mechanical properties of the organic matter (e.g., Ujiie, 1978, Okiongbo et al., 2005). However, directly measuring changes to the organic matter within shales has proved challenging because much of the organic matter comprises micrometer-scale kerogen grains, or is present as bitumen occupying nano-scale voids between grains (Fig. 1). Standard techniques, such as nanoindentation, have helped to shed critical light on the problem (e.g., Zeszotarski et al., 2004, Ulm and Abousleiman, 2006, Bobko and Ulm, 2008, Ahmadov et al., 2009, Ahmadov, 2011, Kumar et al., 2012, Shukla, 2013, Zargari et al., 2013), but lack the necessary high resolution to fully characterize the organic components which often have dimensions in the micrometer and even nanometer range. Recently, however, new analytical advances have provided an opportunity to probe directly the elastic properties of shales at the nanometer scale (e.g., Wilkinson et al., 2015; Eliyahu et al., 2015).

In this study, we use a new non-destructive technique, based on atomic force microscopy (AFM), to map the elastic moduli of the organic and inorganic components in shale samples that have experienced different levels of natural thermal maturation. Using the method we are able to obtain high resolution maps of mechanical properties for both kerogen and bitumen, and track the evolution of elastic modulus during catagenesis.

Section snippets

Sample preparation and characterization

In this study we selected six organic-rich shale samples from three different levels of thermal maturation (increasing burial depth of the same formation). The samples are from a carbonate-rich Cretaceous source rock in the southern United States. At low thermal maturities, organic petrology point count data show that the organic component in the samples is dominated by Type II marine (alginite macerals) with a minor Type III (vitrinite and inertinite) fraction. Thermal maturation expelled

Evolution of kerogen during maturation

In our analysis we distinguish between kerogen (i.e., depositional organic matter) and bitumen (i.e., migrated organic matter) according to the morphological criteria described by Loucks and Reed (2014). If the organic matter filled voids that had been partially occluded by mineral cementation, or if the organic matter was present within fossil body-cavities, we defined it as bitumen. Other organic matter was defined as kerogen. The different morphologies of the two organic matter types are

Concluding remarks

In this paper we used a new atomic force microscopy technique to measure in situ the elastic modulus of organic matter in fine grained shales. We found that kerogen in low maturity shales was significantly softer than kerogen from the oil and gas maturation windows. The stiffness of bitumen on the whole increased only slightly with increasing maturation. Our nano-scale observations reveal that some kerogen macerals contain softer regions that probably represent bitumen inclusions or extruded

Acknowledgments

This work was supported by the Israeli Ministry of National Infrastructures, Energy, and Water Resources (213-17-012 3-10643). Shell International E&P Inc. are thanked for releasing samples and their support of the AFM research.

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