Horizontal wells are rarely horizontal. Instead, operators commonly try to drill such wells into particular rock layers, or target zones, which may or may not be truly horizontal. Thicknesses of target zones commonly range from a few feet to a few tens of feet (1-10 m). Target-zone evaluation concerns whether a horizontal well was successfully located and drilled in a given rock layer. Borehole-imaging logs provide a powerful tool for stratigraphic interpretation and target-zone evaluation in the Austin Chalk, Niobrara Formation, San Andres Formation, and other units.

This study uses borehole images generated by Schlumberger's Formation MicroScanner (FMS), a microconductivity logging device. Open fractures and clay-rich interbeds appear as dark, high-conductivity traces on the FMS log. These traces can be fit with sinusoidal curves and oriented on a computer workstation. The shape of the sinusoidal curve that fits a particular bedding plane tells the interpreter whether the borehole was moving upward or downward through the strata. STRATLOG (trademark of Sierra Geophysics, Inc., a Halliburton Company) software has been used to display borehole profiles by combining FMS data on fracture intensities and bedding-plane intersections with gamma-ray logs, mud logs, and borehole-deviation surveys. To aid in planning future wells, multiple penetrations of he same horizon can be detected and used to calculate highly accurate bedding-plane dips. Fault interpretation, including the detection of rollover beds, is also possible. Finally, stratigraphic interpretation can be combined with observed fractures to determine which rock layers are most highly fractured, and, therefore, should be target zones.