2D spatial analysis of the natural fractures in the organic-rich Qusaiba Shale outcrop, NW Saudi Arabia

摘要

Natural fracture is the main control on production from unconventional shale reservoirs as it serves as a drainage conduit and promotes the efficiency of the hydraulic fracturing technique. The Qusaiba Shale (QS), particularly its basal unit was recently identified as a potential unconventional shale gas reservoir in Saudi Arabia. This study presents a comprehensive and systematic description as well as statistical analysis of the natural fractures that cut through the QS outcrop in the Tabuk Basin, NW Saudi Arabia. These analyses were used to identify the factors that control the development of a fracture system in this organic-rich Shale. Three main approaches including areal sampling, scanline and topology analyses were utilized to collect fracture measurements. A total of 495 fracture traces were measured. The fracture length ranges between 7 cm and 8.2 m. The measured fracture traces are classified into four groups; subvertical to vertical, high-angle, low angle and bedding to subparallel fracture traces. Using twenty-one scan lines, the total number of fracture traces that cut through are 374, with average fracture intensity ranging from 0.9 to 3.7 fracture/meter. Four main fracture sets trending E-W, NE-SW, NW-SE and NNW-SSE were identified. The measured fractures vary from cemented (59%), partially filled (38%) to open-mode (3%). The cement in fractures is either gypsum or calcite. Applying the topology method analysis, the total number of nodes is 811, where the isolated nodes (I-type) account for 522, the termination nodes (Y-type) accounts for 222, and the cross-cutting nodes (X-type) are only 83. Microfractures are sharp with high-angle, closely spaced, open-mode fractures with lithology-induced terminations. The small vertical to sub-vertical fracture traces are terminated against the more resistant silty laminae. Pyrite shows positive correlation with the linear fracture density. The high amount of pyrite in association with high quartz and subordinate feldspars contents in the studied interval might have enhanced the brittleness of the lithologies and facilitated fracture propagation. Tectonically, two main fracture types exist in the studied section; shear and extensional fractures. The strike of the shear fractures in the study area is NE-SW and NW-SE directions. The open-mode, vertical long fractures cutting through the entire outcrop indicate an extension stress regime, which has already shaped the Tabuk Basin with multiple NW-SE rift structures.