Natural Fractures in the Barnett Shale in the Delaware Basin, Pecos Co. West Texas: Comparison with the Barnett Shale in the Fort worth Basin

Gale, Julia F.¹; Reed, Robert M.¹; Becker, Stephen P.¹; Ali, Walaa ^2
1 Bureau of Economic Geology, Jackson School of Geological Sciences, The University of Texas at Austin, Austin, TX.
² Department of Geological Sciences, Jackson School of Geological Sciences, The University of Texas at Austin, Austin, TX.

This study describes several sets of natural fractures in a Barnett Shale core from Pecos County, including partly open fractures, fractures associated with chert layers and early, deformed fractures. These are compared with fractures previously described in the Barnett Shale in the Fort Worth Basin. The steep, narrow, calcite-sealed fractures that are present in many Barnett cores in the Fort Worth Basin are important because of their likely tendency to reactivate during hydraulic fracture treatments. In the core studied here from the Delaware Basin there are many different fracture types, including open fractures with cement bridges. The importance of natural fractures for completions in the Delaware Basin is therefore different from that in the Fort Worth Basin.

The range of fracture types is also of potential use in documenting chemical and mechanical processes that were operative during basin development. Early, sediment-filled fractures that were folded during compaction are present. Later fractures contain quartz and dolomite sealing cements. Fluid inclusions and fracture sealing cement patterns can provide information on temperature, pressure and composition of fluids at the time of fracturing. For example, fibrous, bedding-parallel quartz veins contain petroleum inclusions with gas bubbles, indicating this fracture set must have developed under conditions of hydrocarbon cracking, and is probably due to overpressuring.

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009