Shelf tidal sand ridges as stratigraphic traps and reservoir units: characterization through an integrated analysis of modern seas and Quaternary and ancient deposits

Shelf tidal sand ridges are large (up to over 200 km long and over 50 m high) depositional landforms underlain by co-genetic sedimentary bodies. In relation to their size and internal characteristics, sand ridges may form... [ view full abstract ]

Shelf tidal sand ridges are large (up to over 200 km long and over 50 m high) depositional landforms underlain by co-genetic sedimentary bodies. In relation to their size and internal characteristics, sand ridges may form excellent hydrocarbon reservoir units. Observations from Quaternary and modern depositional systems indicate that the development of tidal sand ridges is common in tide-dominated shallow seas under transgressive conditions, and that the resulting ridges may be buried by mud-prone deposits (e.g. prodelta mudstones) during the subsequent sea-level highstand. In this context, and depending on co-occurring factors, sand-ridge architectural elements may form elongated stratigraphically-trapped reservoirs of significant size. Sedimentary facies models provide constraints to guide interpretations of tidal sand-ridge deposits in the subsurface rock record. However, predictions of their reservoir characteristics or trapping potential are largely qualitative and based on individual, well-documented examples.
This study integrates sedimentological data from ancient and Quaternary deposits interpreted as preserved tidal sand ridges with geomorphological data from modern seas. A composite dataset has been compiled from a number of published case studies to offer a quantitative characterization of the sedimentary architecture of tidal sand-ridge deposits. All data have been included in the Shallow Marine Architectural Knowledge Store (SMAKS), a relational database designed to accommodate data on the architecture of shallow-marine and paralic depositional systems, coded to a common standard. Qualitative information on the studied examples is considered together with quantitative SMAKS database output that describes the geometries and facies properties of tidal sand-ridge architectural elements and geomorphic units, and their relationships with associated units.
This work assesses the value of modern sand-ridge landforms as analogues for stratigraphic traps and reservoir units, by assessing the roles of morphodynamic evolution and preservation potential in controlling the architecture of preserved stratigraphic products. The results of the analysis highlight the uncertainty in considering modern forms as representative analogues to depositional products, in particular with respect to their preserved morphometry and notably in relation to the compound depositional-erosional nature of forms whose height is only partially related to build-up of depositional relief.
This study demonstrates the sensitivity of sand-ridge morphometry to plausible controlling factors, thereby enabling predictions of likely reservoir continuity or trapping potential from knowledge of depositional boundary conditions including shelf-edge depth, tidal range and transgression rate.