Abstract: Reservoir Modeling of Sandy Fluvial Systems: Salt Wash Member (Morrison Formation), Southern Utah

John W. Robinson

Sandy fluvial systems are important reservoirs in fields around the world. A clear understanding of lateral and vertical reservoir connectivity can provide a framework for increased recovery from these heterogeneous reservoirs. Such analysis is justified when a mere 1% increase in recovery from Prudhoe Bay Field yields 100 MMBO.

The Salt Wash Member in southern Utah is up to 160 m thick, is composed of 65-90% sandstone, and was deposited by braided streams flowing northeast. The Salt Wash is composed of single-storey and multistorey channel sandstone units separated by floodplain/overbank heterolithic units. Storeys average 7.4 m thick and amalgamate to form multistorey intervals 20-33 m thick. Storeys are 33-1400 m wide and multistoreys are laterally continuous for several kilometers.

When considered as a reservoir analogue, the Salt Wash has three scales of reservoir baffle/barrier: (1) Overbank/floodplain heterolithics, 1 to 15 m thick, have sufficient lateral continuity to compartmentalize multistorey intervals, (2) Abandoned channel-fill mudstones and basal mudstone intraclast lags are 1-5 m thick and up to 300 m wide, (3) Intra-storey mudstone lags or drapes average 2 cm thick and 10 m wide.

A three-dimensional reservoir model that includes each reservoir element and flow barrier was constructed using a major oil company computer program. Flow simulations on the model indicate overbank/floodplain heterolithics cause significant reservoir compartmentalization. Reserve additions from sandy fluvial reservoirs will come from recognition and correlation of flow barriers such that supplemental recovery programs can minimize irregular waterflood distribution and maximize sweep efficiency.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994