Transgressive Reworking of Deltaic Headlands and the Formation of Isolated Shelf Sandstone Reservoirs

Pronounced deltaic headlands and other geomorphic features along a coastline can dramatically impact the provenance, genesis, placement and preservation of isolated sandstone bodies in the nearshore to shelf setting, especially during transgressions that follow a significant lowstand.

Much of the sand in the Upper Mississippian Springer Fm. in the northern Anadarko basin of Oklahoma was deliverd via incised valleys and lowstand deltas that pumped clastic sediment into a low accommodation setting with strong longshore currents. Subsequent transgressive erosion and longshore transport produced progressive migration of elongate and isolated “bar” sands (stranded shoreline deposits and shelf shoals) on the Springer shelf. This process placed these reservoirs far from their sediment source and encased them within sealing, shelfal mudstones. The preserved Springer gas reservoirs bear no apparent genetic relationship to the initial incised valley or deltaic source of the sediment.

A similar relationship exists for numerous isolated sandstones in the mid to upper Cretaceous of Wyoming. Significant structural uplift and erosion in western Wyoming fed at least two major deltaic progradations across much of the state (Turonian Frontier Fm. and Campanian Ericson Fm.). Subsequent preservation of transgressively reworked sand derived from these lowstand-deltaic headlands wthin tectonically mediated lows and further transgressive reworking created a series of highly productive, stratigraphically trapped oil reservoirs.

The greatest erosion, reworking and longshore transport of sand is likely to occur during the transgression following a significant eustatic lowstand, when the deltaic headland is most pronounced into a basin. Substantive along-strike transport of sediment during transgression greatly complicates sequence stratigraphic and source to sink sediment-budget analysis within both the eroded source area and in the final preservation area. However, very real exploration opportunities likely exist down longshore drift from many deltaic headlands deposited in low accommodation settings.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California