Abstract

New techniques that use microstructures to predict orientation and fill of macrofracture sets have been successfully applied using rotary-drilled sidewall cores. The sidewall cores were drilled from open-hole intervals in two 45-year-old wells in the Ellenburger dolomite in West Texas.

The new techniques allowed measurement of 17 fractures in one well, grouped into two distinct, steeply dipping sets striking NE-SW and NW-SE. These data give a much more robust indication of dominant fracture orientations than the four moderately dipping possible fractures discernible on 116 ft of image logs, which in isolation could give no clear indication of dominant fracture orientation, and which alone might have been interpreted as artifacts. Fractures observed in core and image logs from a recently drilled vertical well are consistent with the two orthogonal fracture sets identified using side-wall cores. The NW-SE-trending set is dominant in most horizons, although fracture orientation varies with depth. Petrographic analysis and SEM-based cathodoluminescence observations of horizontal and vertical thin sections from side-wall cores indicate a history of dolomite and later calcite precipitation in fractures. Hydrocarbons are also present in and around fractures, with migration probably postdating earlier cements. This information is not obtainable from image logs.