Slice-Mapping: Management of a Carbonate Reservoir, West Yucca Butte Field, West 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 region of west Texas. It is one of several producing fields situated in an en echelon pattern along several major northwest-southeast-trending strike-slip(?) fault systems. These fields are situated on anticlinal features and produce gas, condensate, and high-gravity oils from Lower Ordovician dolomites and sands, Devonian cherts and cherty limestones, and Permian-Pennsylvanian limestones.

Production in the field is from the Lower Ordovician Ellenburger formation and the Pennsylvanian Strawn, Canyon, and Cisco formations. Generally, 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 cases internal stratigraphic heterogeneities significantly complicated development strategies.

The Pennsylvanian carbonate reservoirs, the focus of this paper, are extremely heterogeneous. They 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 their depositional and diagenetic implications; however, little was known about the areal distribution and quality of the reservoirs.

A slice-mapping method was used to help define the distribution, size, and geometries of the discrete carbonate reservoirs. The Permian-Pennsylvanian carbonate section was sliced apart 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 illustrated. By stacking these maps, a three-dimensional assessment of the reservoir was realized which addressed aspects such as vertical continuity. Finally, each isopach was superimposed on a structure map constructed from a regional marker within the carbonates. This characterized reservoir limit . Each unit's individual contribution to the overall reservoir performance was determined. The 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 #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.