Architecture of Inclined Heterolitic Strata of Nelslen Fluvial-Tidal Channel Deposits: Lessons on Reservoir Compartmentalization

The architecture of stacked inclined heterolitic strata (IHS) in the Upper Cretaceous Neslen Formation in Book Cliffs area northeast of Green River in Utah has been studied using sedimentological observations combined with three-dimensional LIDAR outcrop data. The channelized deposits of Neslen Fm. are fluvial-tidal deposits and are known to have fed the tidal and wave dominated marine shorelines of the Sego, Corcoran and Cozette to the east.

Single IHS units can reach 10 meters in height but at any location there are three, four or five IHS units that are typically 5 to 6 meter thick. In the lower part of the IHS succession there are highly organic mudstones and silty-coals. The contact between each IHS unit is sharp with the overlying unit typically truncating the beds of the underlying IHS units. The point-bar inclined strata (accretion surfaces) have variable dips (3 to 8 degrees) with variable orientations. The accretion surfaces dip in multiple directions suggesting highly meandering channels.

The accreting point bars are formed of beds of ripple-laminated fine sandstones with thicknesses from a few cm to 50 cm. Sandstone bed thickness varies laterally over relative short (meters to tens of meters) distances. The rippled beds have variable amounts of mud from some that are completely sandy to flaser or wavy-laminated beds. Between the sandstone beds there are rippled siltstones with cm thin sandstones, or flat laminated siltstone beds with thickness up to 30 cm. The ripples are asymmetrical current ripples and migrate at low angle up or down the accretion surfaces. The tidal influence is suggested by the overall heterogeneous character of the channel deposits, by bi-directional paleocurrents in places by tidal rhythmites. The low azimuth difference between ripple migration and accretion surface orientation also suggest tidal influence based on analogy with some modern IHS.

Such IHS reservoirs will have flow units that are disconnected, with different flow units represented by flow compartments with different accretion-surface orientation, even in cases where one IHS unit cuts into the adjacent underlying unit.

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