Clay diagenesis and porosity analysis of shales

Abstract

The origin of overpressure in shale has long been a debate as multiple factors are able to induce high pressure anomalies. Apart from the other overpressure-contributing factors including disequilibrium compaction and hydrocarbon generation, a rapid-increase of pore fluid volume through clay minerals diagenetic process is the most complex factor. The proportion of clay minerals vary greatly among heterogeneous shale. In addition, the extremely tiny grain size of clay minerals make quantitative analysis challenging. This study thus aim to investigate a relationship between overpressure, degree of clay diagenesis and the porosity of shale. A suite of 8 samples were first used in synchrotron X-ray diffraction (SXRD) experiment for mineral compositions identification. SXRD data show a broad range of clay mineral content (15-60 vol.%). The main minerals include quartz, siderite, kaolinite, illite-mica and illite-smectite while chlorite, pyrite, albite, calcite and dolomite serve as minor components. Moreover, synchrotron X-ray microtomography (SXRT) experiments were performed for 3D microstructure and porosity analysis. According to the SXRT data, most shale samples have very low porosity (0.4-2%) and are dominated by microfracture (0.2-1.2%). The fractured shale sample has significantly higher porosity (~7.4%). Porosity in sandstone samples range from 7 to 13% and can be categorized as an intergranular pore. Results from these experiments show that the clay diagenesis has minor impact on the shale pore pressure. However, discrete shale pore networks can enable pore pressure build up