Sequential mineralogy characterization of organic-rich shale: Example from the hanadir shale, northwest Saudi Arabia

The Middle Ordovician (Middle to Late Cardocian) Hanadir Shale member of the Qasim Formation is a potential shale gas exploration target. This paper quantitatively characterizes the mineralogy of the Hanadir Shale using an applied methodology to better understand the interrelationship between mineralogy and reservoir properties of shale gas formations. A 9-m thick sequence of the Hanadir Shale within the Tabuk quadrangle in northwest Saudi Arabia was investigated using sedimentological, geochemical, and mineralogical analyses. Sequential analyses were performed on shale samples: X-ray diffraction (XRD), X-ray fluorescence (XRF), cations exchange capacity (CEC), and linear swelling meter (LSM). This shale sequence is dominantly comprised of laminated clay-rich shale interbedded with thin siltstone to very fine sandstone beds. It is overlying a 5-m thick bearing with massive-To-low-Angle crossbedding in the uppermost intervals of the Sajir member of the Saq Formation and underlying 30-m thick tigillites in the Kahfah member. The XRD results showed the Hanadir Shale is characterized by similar mineralogical composition of different proportions with all the samples containing high silica content (~50%), K-feldspar (~30%), and Na-feldspar (3%). Compared to the bulk mineralogy of the shale gas, clay content in the samples (10 to 20%) is relatively low. The identified clay types include kaolinite, illite, and smectite. Carbonate minerals (calcite and dolomite) also occur as traces (~1%) in all the samples. Swelling tests were performed to understand the swelling properties of the clay content and better understand reservoir properties, performance, and fluid properties in the Qasim Shale's reservoir equivalents. The study contributes to the existing database of shale gas exploration information. Clay mineral distribution patterns and their effects on the reservoir quality of the unconventional shale gas are not well understood; therefore, this study contributes to the understanding of the reservoir behavior of not only the Hanadir Shale but also other regional potential shale formations.