Abstract
The Cherry Canyon Formation consists of a 925-ft (280-m) thick section of up to 25 different sandstone and siltstone units that were deposited in deep water in the Delaware basin. Lowstand sedimentation by fluid density currents with periodic turbidity currents resulted in a broad migrating channelized slope fan/basin-floor fan complex exhibiting a complex depositional architecture of reservoir sandstones. Original depositional fabric modified by diagenetic cements and authigenic clays create a range of petrophysical rock types. Type I reservoirs are found in channel sandstones; beds of lesser reservoir quality (type II) are present in laminated overbank/interchannel sandstones. A practical pore geometry classification scheme based on pore/grain shapes and pore throat rad i is used to understand fluid saturations and predict well performance within the context of the depositional framework and structural relief. Pore geometry factors combine with insufficient structural closure to create high water saturations throughout the oil column. Type I (macroporous) reservoirs exhibit oil-water transition zones, and are interbedded with type II (mesoporous to microporous) reservoirs which contain all or mostly water due to high capillarity associated with small pore throat size. Accurate reservoir water saturations can be derived using Archie's equation; when combined with a movable oil analysis and drainage relative-permeability/fractional-flow curves, initial watercuts can be predicted to maximize deliverability from optimal rock types. High original water saturations in the transition zone decrease average water saturations behind the waterflood front, resulting in inefficient secondary recovery. A combined application of sedimentological, petrophysical, logging, and reservoir engineering technologies is necessary to properly evaluate the Cherry Canyon reservoirs.
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