Slice-Mapping: Reservoir Characterization Technique--West Yucca Butte Field, Pecos County, Texas

R. R. Casavant

The West Yucca Butte field, located in Pecos County, Texas, is a structural-stratigraphic field that lies within the geologic province known as the Sheffield Channel-Val Verde basin area of west Texas. It is one of several producing fields situated in an en echelon pattern along several major northwest-southeast-trending fault systems. These fields are anticlinal features producing gas, condensate, and high-gravity oils from Lower Ordovician dolomites and sandstones, Devonian cherts and cherty limestones, and Pennsylvanian-Permian limestones.

Production in the West Yucca Butte field is from the Lower Ordovician Ellenburger Group and the Pennsylvanian Strawn, Canyon, and Cisco Groups. In general, wells located highest on the structure, with adequate porosity, are the better wells. Production is also obtained from downdip wells where reservoirs stratigraphically pinch out along the flanks of the structure. However, in all reservoirs, internal stratigraphic heterogeneities significantly complicated development strategies.

The Pennsylvanian reservoirs are very heterogeneous. The carbonates have undergone considerable diagenesis, resulting in the occlusion of primary porosities. Secondary spicular-moldic and microvuggy brecciated porosities are the principal porosity types. Petrographic studies were useful in describing the physical properties of this massive carbonate section and the depositional and diagenetic implications; however, little was known about the distribution and quality of the reservoirs.

A slice-mapping method was used to define the distribution, size, and geometries of the discrete carbonate reservoirs. The Pennsylvanian-Permian carbonate section was sliced into genetic stratigraphic units observed from log character throughout the study area. Using gamma-ray and porosity cutoffs, a net porosity value was calculated for each well per slice and isopach maps were constructed. The distribution, geometries, and lateral continuity of porosity within each slice were now illustrated. By stacking these maps, a three-dimensional assessment of the reservoir was realized, which addressed aspects like vertical continuity. Finally, each isopach was superimposed on a structure map constructed from a regional marker within the carbonate beds. This characterized reservoir limits. Ea h unit's individual contribution to the overall reservoir performance was determined. This structural-stratigraphic analysis, tested against surveillance data, delineated the reservoirs quite accurately. The advantage of this technique is that it broke down a confusing three-dimensional package of sediments and associated properties into several "digestible" two-dimensional units (slices). Engineering studies and reservoir management decisions were significantly enhanced by the characterization.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.