Application of Raman Spectroscopy and Transmission Spectroscopy for Shale Reservoir Characterization: Thermal Maturity Correlation and Organic Matter Type Differentiation
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Organic rich shale formations have historically been of interest principally as source rocks for hydrocarbon (HC) generation. As such the prime areas of interest in their study have included organic matter type, as evaluated using geochemical techniques, and thermal maturity. The standard measures of thermal maturity include vitrinite reflectance and various other visual (e.g., TAI, solid bitumen reflectance, etc.) and geochemical measurements (e.g., Rock-Eval T-Max) all reported as %Ro or an equivalent %Ro. Recently, with advances in hydraulic fracturing and horizontal drilling, shale formations have become of economic importance as unconventional reservoirs. As a result, in addition to thermal maturity, other aspects of shale formations related to their reservoir properties have received increasing attention. These include: 1) the development of porosity in the OM with increasing thermal maturity; 2) the permeability of shale formations and techniques for its measurement; 3) the distribution of OM in the shale formation; and 4) the OM type in the shale formation, particularly in terms of porosity generation. With the increasing need for estimating these factors, and because shale reservoirs are heterogeneous on many scales, new tools and methods have been developed and integrated together in order to build models for shale reservoir properties. These techniques include but are not limited to: thin section imaging in transmitted, reflected, and fluorescent light, broad area scanning electron microscopy (SEM) imaging of argon ion milled surfaces, focused ion beam (FIB) imaging, medical-CT, micro-CT, nano-CT, helium ion imaging, and development of low permeability measurement systems, etc.ix In this work, the original TCI system is modified and a new calibration with thermal maturity is obtained. Raman spectroscopy is investigated using intact, ion-milled, and isolated kerogen samples for OM type differentiation and thermal maturity determination. For OM type differentiation, particular Raman spectral characteristics are recommended. Different thermal maturity relationships are obtained for different OM types. The spectroscopic techniques are combined with Rock-Eval pyrolysis, vitrinite reflectance, and TAI analysis. Finally, geomechanical properties of cores from the shale formations at different thermal maturity levels are investigated using acoustic velocity measurements. The acoustic and geomechanical properties will be the subject of future work.