ABSTRACT
Established models for landward barrier-island migration focus primarily on the preservation pattern of transgressive facies in the shoreface, which are typically thin and buried beneath finer grained marine lower shoreface-to-offshore sediments. In contrast, transgressive barrier-island sandstones in the Triassic Halfway Formation of Wembley field in Alberta are preserved interbedded with, and overlain by, backbarrier and nonmarine sediments. These transgressive barrier sandstones formed from coalescing washover fans during shoreface retreat and were subsequently “abandoned” as the shoreline stabilized and resumed progradation. The abandoned transgressive barrier sandstones were subsequently blanketed by backbarrier and nonmarine sediments as the coastline continued to prograde.
Abandoned transgressive barrier island sandstones in the Halfway Formation are 2–6 m thick, up to 2 km wide, and form paleocoastline-parallel trends tens of kilometers in length. The trends define the paleolandward limit of transgressive events. The updip pinch-out of these sandstones in backbarrier mudstones forms a stratigraphic trap for hydrocarbons in Wembley field. Top seal is provided by nonmarine mudstones and evaporites which buried the abandoned transgressive barrier island. The sandstone has porosities and permeabilities averaging 11% and 63 md, respectively.
By using well logs and cores to correlate individual parasequences in the Halfway Formation to their updip termination, it is possible to define the extent of associated marine flooding events and therefore identify exploration targets for abandoned transgressive barrier-island sandstones.
