TILLMAN, RODERICK W., Consulting Sedimentologist-Stratigrapher, Tulsa, OK

ABSTRACT: Valley-Fill Sandstone Reservoirs, Southwest Stockholm Field, Kansas

Valley-fill reservoirs are internally complex, differ from superficially similar deltaic deposits, contain geographically and vertically limited reservoirs and can only be effectively drained if the internal reservoir architecture of individual reservoirs is understood. SW Stockholm valley-fill reservoirs in the Morrowan (Pennsylvanian) age Stateline Trend along the Kansas-Colorado border produce from the Stockholm Sandstone. The valleys in which the reservoirs occur were cut by rivers during several drops in sea level.

Reservoir production properties vary greatly between the two major producingfacies within the up to 150-ft thick valley-fill sequence in the

field. A variety of heterogeneities control production rates. Topographic highs on the valley bottom, covering an area as large as 1/4 section, locally reduce the depositional thickness of the reservoir sandstones. Impermeable limestones and shales commonly form lateral barriers to flow at valley margins and locally within the valleys. Erosion of portions of potentially productive sandstones within the valleys is also important in isolating reservoirs (flow units).

Within the field, tidally deposited (estuarine) sands are finer grained andare more clay prone than the fluvial sandstones and as a result have poorer reservoir properties. Fluvial deposits have average porosities of 15% and average permeabilities of 784 md. Tidal channel sandstones have average porosities of 10% and permeabilities of 80 md. Where tidal sandstones interfinger with fluvial sandstones, vertical permeability is diminished and flow units are fragmented. Understanding the differences in heterogeneity between fluvial and tidal fills of valleys allows more efficient development and reservoir management.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.