Sedimentology, geochemistry and reservoir potential of the organic-rich Qusaiba Shale, Tabuk Basin, NW Saudi Arabia

Abstract

Abstract The Qusaiba Shale Formation of the Qalibah Group is the most prolific source rock for the Paleozoic petroleum system in Saudi Arabia and is recently considered as a potential unconventional shale gas reservoir. Detailed sedimentological, petrographical, and geochemical analyses were undertaken on an ~31 m thick outcrop of this organic-rich shale exposed in the northwest of Tayma City, NW of Saudi Arabia to provide a better understanding of the vertical and lateral lithofacies variations and the effects of such variations on reservoir properties. X-ray diffraction analysis reveals that clay minerals (average: 50%) and quartz (average: 28%) dominate the bulk mineralogical composition while K-feldspar, plagioclase, and pyrite occur as common to minor components in the analyzed samples. An integration of the sedimentological description, petrography and mineralogical composition allows the subdivision of the investigated outcrop section into six lithofacies. These are: Laminated organic-rich shale, Graptolitic concretion, Massive organic-rich shale, Organic-lean shale, Siltstone-shale interbedded, and Cross-stratified siltstone/sandstone lithofacies. TOC content in the laminated and massive organic-rich shale lithofacies ranges between 0.5 and 6.1 wt% (average 2.52 wt%) then it decreases upward (0.04–0.35 wt%, average 0.18 wt%) in the overlying organic-lean shale and siltstone lithofacies. Trace elements (Cu, Mo, As, Ni, Co, U and V) show positive excursions in the lowermost 20 m of the organic-rich, dark-grey shale interval and display abrupt depletions at the contact with overlying organic-lean shale. The enrichment factors of these trace elements are characterized by similar distribution patterns as their measured concentrations. Like the elements, these factors also exhibit a significant depletion at the contact between the organic-rich shale and the overlying organic-lean shale. Out of all the redox indices, V/(V + Cr), V/(V + Ni), Th/U and [Mo]s/TOC were found to best distinguish between the organic-rich and the overlying organic-lean intervals. The laminated organic-rich shale lithofacies indicate deposition of hemipelagic sediments by a relatively low energy quiet anoxic bottom water, most likely in a sulfidic conditions that were associated with a strongly restricted bottom water mass circulation at the time of its deposition. This restricted hydrographic condition was likely enhanced in the study area due to the paleotopography that was created by the deglaciation of the Early Silurian time. The TOC richness, common occurrence of pyrite and high enrichment of redox elements Mo, U and V in the organic-rich shale intervals support the existence of anoxic bottom water at the time of deposition. The abrupt decrease of the TOC and the redox sensitive elements and their associated indices from the underlying organic-rich lithofacies to the overlying organic-lean shale lithofacies is likely due to a gradual increase in oxygen level. This improved bottom water oxygenation condition might have led to a reduction in organic productivity and limited organic matter preservation in the organic-lean shale lithofacies. The graptolitic concretion lithofacies found in the sequence, refers to an in-situ, syngenetic to early diagenetic origin formed close to the sediment-water layer when there was little or no deposition or at periods characterized by low sedimentation rates. The siltstone - fine sandstone lithofacies in the topmost part of the studied section, is interpreted to indicate the dominance of high energy and highly oxygenated shallow bottom water conditions during deposition, probably in a middle to inner shelf depositional settings. The kerogen in QS is a mixture of type III (gas prone) and type IV (inert kerogen), Tmax values with an average of 430 °C, and VRo values (average 0.58) calculated from the Tmax, assigned the studied section as immature. Qualitative evaluation of porosity allows the identification of three different types of porosity including inorganic porosity, organic porosity and natural fracture porosity. Although the ductile clay minerals are the most abundant minerals in the lower part of the studied QS (average 56%), the brittle minerals (Quartz, Feldspar, and Pyrite) form an average of 42% and exhibit a uniform distribution of brittleness in the organic-rich interval.