Abstract
Recovery of hydrocarbons from organic-rich shales has played a significant role in changing the distribution of reserves worldwide and has also impacted on carbon dioxide emissions where extracted gas has been used to replace coal to power electricity grids. Such extraction is predicated on a good understanding of local and regional geological history as well as close examination of the rocks involved from seismic to nano-scale. This study looks at the impact of thermal maturity on the organic and diagenetic mineral fabrics observed in gas shales from different parts of the world, highlighting similarities and differences in their impacts on rock properties. Organic fabrics can present as pore filling migrated bitumen visualized in scanning and transmission elctron microsopy and the degree of thermal maturity directly impacts for example on the electrical properties, shown by contrasting examples from the Marcellus (ultra-high maturity) and Utica (moderately high maturity) shales; the former has extremely low resitivity while the latter extremely high. Dielectric properties are shown to be useful for rock typing in the Utica shale where standard resistivity logs are off the scale as the material is so resistive. Such properties have also been shown to be useful for estimating water saturation in the Roseneath-Epsilon-Murteree Formations of the Cooper Basin. Mineral diagenesis and its timing are also shown to be important for quartz cementation and pore structure modification in the Marcellus, Bongabinni and Goldwyer formations, with the latter two contrasted in terms of elastic and strength properties. Overall, micro-structural, laboratory and wireline log studies combined have given significant insights into the interplay between organic and diagenetic fabrics and resultant rock properties.
