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Digital Outcrop Modeling of the Lower Silurian Qusaiba Shale Member; Implications for Reservoir Quality and Architecture, Central Saudi Arabia

Abstract

The hot shale (organic-rich) of the Lower Silurian Qusaiba Member of the Qalibah Formation is one of the most prolific source rocks in the Arabian Peninsula. Almost 90 % of the Paleozoic light oil and gas accumulations are well-known to be sourced by Qusaiba Shale unit. Also, recently this unit is believed to act as a potential unconventional shale gas reservoir. The understanding of Qusaiba as tight gas reservoir is critical and mysterious. This study aims to characterize and model the reservoir quality and architecture using outcrop analog for Qusaiba Shale Member. Traditional geological field data such as; vertical stratigraphic sections, structural measurements, sampling and photographs were acquired. In addition to that, Terrestrial Laser Scanning (LiDAR) was the main tool utilized to model Qusaiba outcrop digitally. The scanner operated in this study manufactured by Optech company and it has a range reach up to 1000 m. External high resolution digital camera was mounted with the scanner to obtain better photos. Three scan positions were selected to scan the outcrop of Qusaiba Member with resolution about 30 mm. VRGS and GIS softwares were utilized to process and interpret the digital model and integrate it with the traditional geological data from the field. The lithofacies of the studied section can be described as interbedding and alternation between fine laminated fissile grey and black shales with few cycles of sandstone and siltstone. The lithofacies and their associated sedimentary structures indicated shallow marine depositional environment with high energy. One of the important results of this study and revealed by the digital model is the percentage of the fractures within Qusaiba outcrop. The intensity of the fractures are higher in the fissile shale and mudstone lithofacies than sandstones lithofacies. The average spacing between fractures is smaller in the fissile shale and mudstone lithofacies than sandstones lithofacies. The continuity of the units within the Qusaiba shale model is highly affected by the fractures network as reservoir architecture point of view. Generally, the interested results of this work may have a significant contribution to tight shale exploration plans in the subsurface by providing information about the fractures network of Qusaiba shale member of Qalibah Formation.