ABSTRACT: Reservoir Characterization of a Permian Slope Fan/Basin-Floor Fan Complex: Cherry Canyon Formation, Ward County, Texas

David R. Spain

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 a deep-water environment 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 interpreted to exhibit an intricate reservoir geometry. Thirteen lithofacies are identified. Primary reservoirs are found in massive channel sandstones, and beds of lesser reservoir quality are present in laminated overbank/interchannel sandstones. Original depositional fabric modified by diagenetic cements and authigenic clays creates three petrophysical rock types. Type I reservoirs contain intergranular macroporosity relat vely free of carbonate cement and authigenic clay. Types II and III reservoirs contain mesoporosity and abundant microporosity created by moderate to abundant carbonate cementation and plugging of pore throats by authigenic grain-coating chlorite and pore-bridging fibrous illite.

Depositional and diagenetic factors combine with insufficient oil column height to yield low initial oil saturations that decrease with depth in a hydrocarbon-water transition zone. Mercury injection capillary pressure measurements illustrate the vertical stratification of petrophysical rock types that exist in the section; reservoirs which contain all water are interbedded with reservoirs containing mostly oil. Subsequently, a slight change in height above free water can drive production from all water to all oil. Hydrocarbon column heights greater than 60 ft are required to establish water-free oil production. Accurate reservoir water saturations can be derived using Archie's equation; when combined with a movable oil analysis and drainage relative permeability/fractional flow curve , initial water cuts can be predicted to maximize deliverability. High initial water saturations decrease average saturations behind the waterflood front, resulting in inefficient secondary recovery. Proper drilling, completion and simulation design are necessary to prevent formation damage within the sandstone section.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990