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Mudrocks are the dominant rocks in the Earth crust and are as well noted for their heterogeneity at several orders magnitude of scales. This implies a significant challenge in relating observations at varying scales to one another. A unique attribute that controls petrophysical properties of mudrocks is their microstructure which also controls fluid movement within them. Due to the fine-grained size of mudrocks, high-resolution measurement is required to reveal their microstructural characteristics. High-resolution scanning electron microscopy has advanced knowledge about microstructure of mudrocks but further development of fast and reliable methods to accurately determine the micron-submicron features of mudrocks is still on-going.
In this paper we present several information on mudrocks properties derived from scanning electron microscopy including grain size, grain-orientation, mineralogy and porosity, pore size distribution. The method involves multiple large-area, high-resolution scanning electron microscopy through automated acquisition and stitching of backscattered images (BSE) from polished thin-sections in combination with machine learning segmentation and energy dispersive X-ray analysis (EDX). Preliminary results obtained from the methodology was applied to seven mudrocks from deep-water identified as hemipelagites from the New Zealand Continental Slope (IODP Expedition 317) and Iberian Peninsula (IODP Expedition 339).
Grain size analysis shows that all the samples are within silt-mud class size. Orientation analysis indicates that randomly representative areas within each sample are heterogeneous; displaying a combination of preferred orientation direction and random orientation. The samples were differentiated into clay dominant and calcite dominant based on EDX. Porosity on representative areas for individual samples are heterogeneous reflecting areas that are tightly porous, partially porous and highly porous area. Interestingly, calcite dominated samples showed tighter porosity compared to clay dominated samples. However, there is no significant difference among representative areas per sample in terms of pore size distribution.
Automated image analysis of large area, high-resolution montages presented herein, is fundamental to revealing heterogeneity and deriving plethora of information on mudrock microstructure. The process minimises human subjectivity and bias but the limitations to the workflow are the time involved for individual runs and large amount of computer memory required. In addition, cracks in the sediment samples resulting from drying, preparation of polished thin-section as well as stress relaxation during coring, restrict the area available for high-resolution large-area imaging to that between the cracks. This method is very significant for improved understanding of subsurface mudrocks and their capacity for fluid movement and storage.
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