Stratigraphic Significance of Mud-Rich Ephemeral Fluvial Deposits: Implications of Lateral Seals for Petroleum System Evaluation

Summary Mud-rich ephemeral fluvial deposits, shaped by episodic downstream discharge variations, form distinctive sedimentary successions in terminal fluvial systems where channels terminate distally into sheet-like, laterally extensive mudstone units. These deposits, predominantly formed under dry climatic regimes, undergo modification via deflation and eolian reworking, further complicating their sedimentary signatures and preservation. The understanding of their formation and preservation mechanisms is essential for evaluating their role in hydrocarbon systems, especially where structural closure is limited. This study provides a comprehensive stratigraphic assessment of such deposits within the Early Permian successions of Saudi Arabia. Through subsurface core analysis and high-resolution seismic interpretation, geomorphological attributes and depositional styles of confined and unconfined channel networks were characterized. The integration of lithofacies analysis, seismic geomorphology, and regional stratigraphy facilitated the depositional transitions and the identification of extensive laterally continuous muddy facies. These fine-grained deposits are interpreted as forming in unconfined terminal settings where suspended sediment settles under low-energy conditions. Results indicate that laterally extensive muddy sheetflood deposits represent distal expressions of low-magnitude flood events, occurring as amalgamating bodies of mudstones deposited during rapid fluvial flow cessation. These are interbedded with more localized, muddy-sandstone channel-fill successions linked to periodic high-energy discharge pulses. Together, these facies create vertically stacked, laterally amalgamated units with strong sealing capacity due to their fine-grained matrix and lateral continuity. The mudstones demonstrate sealing effectiveness not only as lateral barriers but also as top and basal seals that cap or underlie reservoir-quality sands. The clay-rich nature of these deposits further enhances their sealing efficiency. The predominance of clay minerals contributes to their low permeability and high capillary entry pressures, effectively impeding fluid flow. This mineralogical signature, along with the spatial persistence of the deposits, makes these mudstones critical components of the petroleum system framework. This study presents new insights into their lateral continuity, and sealing behavior emphasizing their value in de-risking exploration targets in fluvially dominated, structurally subtle settings.