Sequence Stratigraphic and Synsedimentary Tectonic Controls on Reservoir Compartmentalization in a Transgressive Sequence Set: Almond Formation, Southwest Wyoming
Lee F. Krystinik and Richard H. Mead
The Campanian upper Almond Formation in southwestern Wyoming contains at least 15 aggradational to backstepping microtidal to low mesotidal barrier/shoreline complexes laid down during a period of net transgression from 72 to 70.5 million years ago. Reservoir compartmentalization in the upper Almond occurs at several scales, including an aggradational to retrogradational sequence set composed of 3 retrogradational parasequence sets; numerous parasequences, and diverse barrier sub-facies units. The lowstand shorelines of these sequence sets stack aggradationally prior to transgression by areally extensive, marine mudstone horizons which separate the sequences. Highstand systems tracts are poorly preserved, often completely removed below fourth of fifth order sequence bound ries which cause seaward jumps of facies in excess of 30 Km and place fluvial sediment, coal and lagoonal deposits abruptly over marine mudstone. Each sequence in the upper Almond is composed of several parasequences (sanding-upward, storm-dominated barrier shorefaces) which intercalate with marine mudstone to the east and grade into oyster-bearing, organic-rich lagoonal mudstone to the west. Compartmentalization in the barrier complexes occurs at most parasequence boundaries and in association with major sub-facies boundaries (barrier margins, tidal inlets, flood-tidal deltas, washover fans).
Further reservoir compartmentalization is induced by synsedimentary faulting and subsidence which locally preserve isolated reservoir-quality barrier/shoreline sandstone bodies by dropping them below the depth of ravinement (5-30 m). The recognition of synsedimentary faulting and subsequent ravinement is critical to accurate sequence stratigraphic analysis and for prediction of reservoir compartments.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California