Texture, porosity and diagenesis - a Marcellus Shale case study

Abstract

Marcellus Shale samples were used to quantify mineralogy and texture, evaluate the abundance and thermal maturity of organic matter, describe porosity and interpret the diagenetic history of this post-mature shale-gas reservoir. A multidisciplinary approach was adopted comprising X-ray diffraction to quantify the mineralogy, neutron diffraction to quantify the texture of the rock-forming minerals, electron microscopy to visualise porosity in the shale and distinguish between detrital and diagenetic phases and Raman spectroscopy to quantify thermal transformation in the organic matter. Results indicate that the samples are composed of quartz, illite, calcite, chlorite, albite, and pyrite with a total organic content ranging between 3 and 7 wt %. There is a significant crystallographic preferred orientation in the diagenetic illite and calcite that can be well modelled assuming transverse isotropy; quartz shows random texture. Nano sized pores are observed within the organic matter as well as at mineral junctions. Raman geothermometry indicate that the sediment witnessed maximum temperatures of approximately 250°C commensurate with the high optical reflectance (R0 > 4.5%) reported on the same material. This and the analysis of illite cristallinity indicate that the Marcellus Shale has been exposed to prehnite-pumpellyite metamorphic facies and a maximum burial depth of 6-8 km. © EAGE Publications BV