Fracture Characteristics and Timing in Organic-Rich Unconventional Hydrocarbon Source Rocks: Implications for Fluid Storage and Migration

Unconventional hydrocarbon source rocks represent significant potential worldwide. This study aims to examine the characteristics of natural fractures within unconventional hydrocarbon source rocks from the Eastern Mediterranean, focusing on their stratigraphic distribution, timing, and formation mechanisms. Understanding these factors is vital for the broader context of hydrocarbon exploration and resource management on a global scale. This research employs a multi-faceted approach, integrating core samples, and geochemical data to analyze natural fractures in organic-rich Upper Cretaceous source rocks, specifically in Jordan, which has relevance to similar geological settings worldwide. The study includes detailed fracture core analysis, assessing the intensity, orientation, and filling materials of main fracture networks. Systematic sampling was conducted for both fracture geochemistry (inorganic and organic), isotopic analysis, and petrography, employing advanced microscopic techniques, including optical and scanning electron microscopy (SEM). The study reveals a diverse set of natural fractures within these source rocks, consisting of calcite-filled, silica-filled, and bitumen-filled fractures, displaying both vertical and horizontal (bedding-parallel) orientations. Notably, the lithological characteristics significantly influence the distribution and formation of fractures; for instance, calcite-filled veins are primarily observed in calcareous mudstone and carbonate strata, while silica-filled veins are prevalent in chert-rich intervals. Mechanical stratigraphic factors, such as compaction and early cementation, along with the presence of bitumen within fractures, indicate that fracture development has been influenced by various geological processes over distinct timeframes. The mechanisms behind fracture formation can be categorized into three primary processes: fluid pressurization (bedding-parallel veins), early diagenetic folding (calcite and silica veins), and late tectonic activity (planar calcite veins and joints). Petrographic and geochemical analyses suggest two main stages of fracture development: (1) early diagenesis (calcite veins dated at 40-30 Ma), followed by a phase of fracturing during burial diagenesis, and (2) subsequent late fractures (25-20 Ma) associated with uplift, extensional tectonics, or fault reactivations. This study contributes valuable insights into the characteristics and timing of natural fractures in unconventional hydrocarbon source rocks, with broader implications for fluid storage and migration processes. The findings underscore the critical importance of understanding fracture systems for enhancing hydrocarbon exploration and production strategies across numerous geological contexts around the globe.