Abstract

North Hollingsworth field in Fall River County, South Dakota produces from a preserved dune sandstone in the second bench of the Leo Formation. It was discovered in 1984 by Citation Oil and Gas using high-resolution seismic data. The field was fully developed on the basis of seismic data and consists of five wells, all of which encountered the dune sandstone. Four of the wells produce oil and one is used to re-inject produced water.

Both the Converse and Leo Formations consist of interbedded sandstones, shales, carbonates and anhydrites with occasional sandstones that preserve the geomorphic shape of dunes. Hydrocarbon traps are formed by the preserved topographic relief over the upper surfaces of dune sandstones or by isolated sandstones encased in impermeable sediments. The sandstone isopach anomalies which occur in several benches of the Converse and Leo Formations can be seen in outcrops and along subsurface cross sections developed from well logs. High-resolution seismic data can be used to locate these anomalies efficiently and economically. Seismic models, synthetic seismograms and seismic field data are used to demonstrate a method of identifying preserved dunes.