ELASTIC, MICROSTRUCTURAL AND GEOCHEMICAL CHARACTERIZATION OF KEROGEN MATURITY FOR SHALES
This dissertation focuses on how the kerogen content and its maturity affect the seismic properties for organic-rich shale reservoirs. The dissertation links geochemistry, seismic and petrophysics. The experimental part of the dissertation involves rock-eval pyrolysis, TOC estimation, and anhydrous pyrolysis for artificial maturation, kerogen isolation, porosity and ultrasonic measurements on shales and kerogen. It also involves working with SEM and micro-CT scan for microstructural characterization. Porosity and seismic velocities are measured on an immature Green River shale sample. The sample is then subjected to anhydrous pyrolysis for artificial maturation by cooking the sample at 3500C for three days. Hydrocarbon induced cracks are visible on the sample. A weight loss of 9 grams is observed after maturation. The matured sample is compacted, reshaped and ultrasonic measurements are repeated. A significant change in the P-wave anisotropy (ε) is observed after maturation, all of which indicates that hydrocarbon induced cracks have no significant effect on seismic velocities. The ultrasonic velocities of isolated Kerogen are also measured. The measurements are carried out on the immature (Green River shale) and matured (Irati shale) kerogen samples. The bulk modulus of kerogen is found to be between 4-5 GPa. The theoretical part of this dissertation involves kerogen maturity modeling, as well as micro structural characterization using GSA. Kerogen maturity modeling is carried out on Bakken shale published data. Hydrocarbons that are generated during maturation develop over pressure. Immature source rocks are under the normal pressure regime and mature source rocks are under the overpressure regime. The properties of matured shale are expressed in terms of excess pore pressure. P and S wave velocities decrease and anisotropy increases with the rise of kerogen maturity. The wave propagation of elastic waves (especially P and Sv waves) is affected by kerogen maturity. Maturity also affects AVO (Amplitude Variation with Offset). GSA method is used for microstructural characterization of Green River shale samples. One of the major advantages of GSA over other methods is that we can estimate the connectivity between pores/cracks (friability). The inversion results show that the pores have spherical shape and are well connected.