Mineralogical characteristics of continental shale: a case study in Yan-Chang Formation, Ordos Basin
The Chang 7 Member of the Yan-Chang Formation (Yan-Chang #7 Member), which is located in the central south of the Ordos Basin (China), is assessed for its potential as a shale gas resource. The characteristics and spatial variability of mineral components in this continental shale formation play a crucial role in evaluating and characterising the shale reservoirs. We collected 64 shale core samples from 30 representative sampling sites located in the central south of the Ordos Basin using X-ray diffraction and field emission scanning electron microscopy to study the mineral compositions, vertical/planar variations of minerals, and the major controlling factors that result in such variations. Based on the relative fractions of the dominant minerals, the shale rocks can be classified into four categories: quartz-rich (type #1), illite/chlorite-rich (type #2), illite–smectite mixed-layer-rich (type #3) and feldspar-rich (type #4). In general, type #1 is mainly located in the northwest of the study area, type #4 is mainly located in the south of the study area, and types #2 and #3 are sandwiched between types #1 and #4. In the centre of the basin, the illite content increases with burial depth and the conversion from smectite to illite, which is experimentally confirmed in this study, enhances the surface porosity of shale. The major factors influencing the properties and spatial variability of the mineral components include sedimentary environment, provenance and diagenesis. Compared with marine shales in China (e.g. Longmaxi marine shales), the Yan-Chang #7 Member continental shale has a higher clay content, but lower calcite, dolomite and pyrite contents. The brittleness indexes of type #1 shale in Wuqi and its surrounding areas are marginally higher than that of Longmaxi marine shales, which makes the type #1 shale in the Wuqi and its surrounding areas slightly easier to fracture than the Longmaxi marine shales.
