PRELIMINARY FRACTURE ANALYSIS OF MISSISSIPPIAN BARNETT SHALE SAMPLES, FORT WORTH BASIN, TEXAS

The T.P. Sims #2 well from southeast Wise Co., Texas, has a cored interval in the lower Barnett Shale. Previous workers noted WNW-trending, calcite-lined macrofractures in the core. A set of NE-trending open fractures were interpreted as coring induced. Two new oriented samples, a typical mudrock and an adjacent dolomitic layer, have been examined using light microscopy, secondary- and backscattered-electron imaging, energy-dispersive spectroscopy, and both cold-cathode and SEM-based cathodoluminescence microscopy. Both samples contain multiple sets of fractures in a range of sizes.

The dolomitic sample is mostly Fe-dolomite and calcite, along with phosphatic material, pyrite, and clay minerals. The sample has N-S-, and WNW-trending fracture sets. Calcite lines the WNW-trending fracture set, and dolomite + calcite ± pyrite line the N-S-trending set. Crack-seal texture was observed in the N-S-trending set. Two sets of thin, straight, dark microfractures (NE- and N-S-trending) are also present. N-S-trending macrofractures are cut by WNW-trending fractures and both are cut by NE-trending microfractures.

The mudrock sample is composed primarily of clay-sized quartz and feldspar with subsidiary dolomite, clay minerals, organics, pyrite, and microfossils. Fracture sets trend NE, WNW, and (minor) N-S. The WNW-trending macrofracture set has an atypically large assemblage of fracture-lining minerals: calcite, quartz, albite, pyrite, barite, and dolomite. Albite and quartz occur at the fracture tips; calcite is dominant elsewhere. Paragenetic sequence is complex: quartz and albite are synchronous and early, calcite formed after at least some quartz, barite is late, and dolomite and possibly pyrite appear to be replacing calcite rather than filling fracture porosity.

A prominent NE-trending en echelon macrofracture set occurs within the mudrock. Previous workers interpreted fractures in this orientation to be induced. Most of these fractures are completely open. However, one of them grades into a narrow planar zone of opaque material. Similar microfractures in the same NE-trending orientation are found in the dolomitic layer. The “induced” fractures may result from coring-induced damage being localized along previously existing microfractures that control orientation.