Depositional facies, sequence stratigraphy and diagenesis of the Cambrian-Middle Ordovician siliciclastic outcrops, Tayma Region, NW Saudi Arabia: An integrated approach for reservoir characterization

This study explores how integrating depositional facies, sequence stratigraphy, and diagenetic attributes from outcropped siliciclastic rocks can enhance the characterization of reservoirs in a siliciclastic system. The Cambrian–Middle Ordovician siliciclastic outcrops in the Tayma region of northwestern Saudi Arabia are used as a case study to address this integrative approach. This represents the first integrated study of its kind for the Tayma region. The rationale for this choice is addressed herein. This work has been achieved through detailed field observations accompanied by petrographic, X-ray diffraction, and petrophysical analyses. The studied stratigraphic successions comprise the Saq Formation (the Risha and Sajir members) and the lower members of the Qassim Formation (the Hanadir and Kahfah members). Based on field observation, sixteen lithofacies were identified and grouped into six facies associations. These lithofacies and their related facies associations reflect deposition in braided fluvial, tidally influenced fluvial (upper estuarine), foreshore-upper shoreface, lower shoreface, lower shoreface-offshore transition, and offshore environments. The stacking pattern of the inferred facies and their facies associations led to the subdivision of the studied successions into three superimposed depositional sequences, including highstand-, lowstand-, and transgressive-systems tracts. Petrographic analyses indicate that the studied sandstones are primarily quartz arenites, subarkose arenites, and quartz wackes. These sandstones contain detrital components with different grain textures ranging from poorly to well sorted, subrounded to rounded, and fine- to coarse-grained. The sandstones underwent both shallow and deep burial diagenesis, including compaction, carbonate and iron cementation, clay authigenesis (kaolinite, dickite, and illite), quartz overgrowth, and dissolution of unstable feldspar and mica grains.

The integration of the above findings reveals that fluvial-related sandstones linked to lowstand systems tracts (LSTs), along with diagenetic features such as dissolution, mechanical compaction and partial overgrowths) display characteristic features of superior reservoir quality. In contrast, marine-related sandstones (foreshore-shoreface–offshore transition and offshore/shelf environments), typically linked to transgressive (TSTs) and highstand systems tracts (HSTs), along with diagenetic features such as cementation, illite-smectite authigenesis, and complete silica overgrowths exhibit relatively lower reservoir quality at various scales. The reasons behind these findings are discussed herein. In this context, the offshore facies of the Hanadir Shale, which are part of the transgressive systems tract (TSTs), can be recognized as potential source rocks. As a result, this study highlights that reservoir quality in siliciclastic systems is primarily controlled by depositional facies, sequence stratigraphic context, and diagenetic processes. Additionally, this outcrop-based case study provides a valuable analog for characterization of similar subsurface reservoirs elsewhere, especially in areas where core and well log data are limited or uncertain.