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Petrophysical Characterization of the Qusaiba Formation, Tabuk Basin, North West Saudi Arabia: Implications for Reservoir Dynamics in a Shale Play


 The Qusaiba formation is an emerging unconventional play in Saudi Arabia. A detailed study was conducted on acquired representative samples which covered all the units of the Qusaiba formation from three locations in North Western Saudi Arabia. This study was conducted to fully characterized the formation to understand the governing petrophysical factors, their variability in the basin and implications for reservoir fluid dynamics in this unconventional reservoir.
 This is a field to laboratory study. A field trip was conducted to the location to collect representative samples at three locations in the Tabuk basin. The formation outcrops were sampled at intervals of about one meter, to ensure a detailed coverage of the sampled Qusaiba formation. The samples were then subjected to detailed mineralogical analysis comprising of XRD, SEM and petrophysical characterization to determine the porosity, permeability, pore size distribution and capillary pressure profile of the sampled formation. Detailed elemental and mineralogical analysis was necessary to understand paleo-depositional environments and infer any diagenesis in the samples.
 Results show that the Qusaiba formation is divided into five main units. The basal Unit 1 overlies the Sarah sandstone, it is mainly thinly laminated dark grey shales, with some dark brown, grading into clayey siltstone at the top. Unit 2 is a mix of shales, siltstone and sandstone, Unit 3 is composed of thinly laminated dark-gray black shales, Unit 4 is composed of thinly laminated gray-green shales, with some mixture of siltstone. Unit 5 is a mixture of thinly laminated gray-green shale in the lower parts, thinly laminated gray-beige silty claystone and laminated beige-brown siltstone at the topmost part. Four lithofacies were identified, these are shale, claystone, siltstone and sandstones. The average porosity, permeability and pore radii in the sandstone facies was 14.13%, 8.3 md and 0.1323 microns respectively; in the siltstone it was 14.85%, 7.12 md and 0.1047 microns, in the claystone, it was 9.8%, 11.40 md and 0.029 micros, while in the shale facies it was 18.31%, 8.21 md and 0.0167 microns. It was observed from the pore size distribution plot that the shale samples exhibit fractures sized pore radii in the macro-pore range. These data demonstrate the high heterogeneities present within the Qusaiba formation, especially in the interbedded layers.
 The results from this study provide an understanding of the pore structures in the Qusaiba formation and the main lithostratigraphic facies governing reservoir fluid flow. The pore structures of the main Qusaiba lithofacies were characterized extensively, this would enhance the understanding of how to unlock the potentials of this unconventional reservoir. It shows that fractures in the fractured shale units could have high impact on fluid flow.