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Developing a Rock Physics Template for Improved Seismic Mapping of New Zealand Coaly Source Rocks


Coal and coaly mudstones are considered the main source rocks of the petroleum systems in New Zealand. Our ability to predict their nature in the subsurface from seismic mapping is constrained by a lack of calibration of geophysical attributes (e.g. acoustic impedance and seismic amplitudes) to the physical properties of these rocks. Few studies have evaluated the relationships between geophysical properties (e.g., P- and S- wave velocities) and coaly source rock properties (e.g., TOC, rank, and organic constituents). New Zealand presents a unique opportunity to refine this link and apply the results towards future exploration. The North and South Islands of New Zealand host a variety of coal ranks spanning from low-grade lignites to higher ranking bituminous coals. We analyze a suite of coaly rock samples from boreholes and outcrops throughout the country and prepare a rock physics template. Laboratory measurements of density and velocity under confining pressures and with non-contact ultrasonic lasers are integrated with comprehensive coal quality analysis. The laboratory study and source rock characterization are extended to predict geophysical signatures (forward seismic modeling) by combining experimental data and well logs. Previous work has demonstrated the influence of coal maturity on dynamic elastic properties. Morcote et al. (2010) characterized a relationship of increasing dry bulk and dry shear moduli with increasing coal rank. This study builds upon their empirical findings through a targeted approach evaluating entire coal seam profiles. The transitional lithologies surrounding coal seams, including shaly coals and coaly mudstones, contribute to the source rock potential of New Zealand petroleum systems. The integration of laboratory-based rock physics with the signatures and profiles of coaly source rocks in well logs and seismic will be a powerful tool towards enhancing our understanding of their distribution and quality in the offshore basins. These rocks are regionally significant. In addition to New Zealand, they contribute to petroleum systems in Australia and other southeast Asian basins. This project is part of a multiyear nationwide study to characterize New Zealand petroleum source rocks, fluids, and plumbing systems and is funded by the Ministry of Business, Innovation, and Employment. Our aim is to contribute to their overall objective of unlocking the potential indigenous energy resources of New Zealand.