Geometry and distribution of microcracks and kerogen in shales from the Phitsanulok Basin

Abstract

Shale is a fine-grained sedimentary rock, comprising of clay-sized particles, organic matter or kerogen, micropores, and microcracks. An assessment of quality in shale is difficult to observe under general optical instruments due to the micro-scale features. Microcrack in shale is potentially related to fluid overpressure by the expulsion of hydrocarbon from organic matter during maturation. The microcrack causes a significant increase in permeability. Therefore, a good understanding of shale characteristics is needed to develop petroleum industry and waste storage technology. Three shale samples were collected from the same borehole in Chumsaeng formation, the Phitsanulok Basin, X-ray computed tomography with synchrotron light source produced high-resolution 3D images for investigation of micro-scale materials such as pores, microcracks, and kerogen in shale samples. Results from this research suggest that various pore types including intraparticle pores, organic-matter pores, and fracture pores (microcracks) were found in shale samples. Organic-matter pores often coexist with perpendicular cracks and kerogen edge cracks. Porosities in shale samples are ranging from 1.12-1.98% mainly calculated from microcracks. Other minor pore types such as interparticle pore and intraparticle pore were found and accounted for very small volume. Kerogen values have a wide range of 1.93-12.97% by volume. Major pores in samples have crack-like geometry (average aspect ratio = 0.27), while kerogen has patch-like geometry (average aspect ratio = 0.34). Microcracks and kerogen have obvious orientation parallel to each other and bedding plane with inclination less than 7%. Moreover, these cracks significantly influence fluid transport properties of shale.