Unconventional Wisdom—It’s More than Just Technology

Shale reservoirs are continuous accumulations in which the same formation often serves as the source, reservoir and seal for commercial accumulations of natural gas.  Intrabasinal differences within continuous accumulations account for the indistinctly bound areas of better gas production termed “sweet spots” by operators.  Generally similar sets of facies have been recognized in the Barnett Shale in the Fort Worth basin by all recent workers.  Dark mudstone to claystone with a matrix of clay minerals and cryptocrystalline quartz is the most common depositional facies in the Barnett.  Two predominantly calcareous depositional facies are next in abundance—argillaceous lime mudstone and skeletal, argillaceous lime packstone.  A variety of minor depositional and diagenetic facies are also present.  The abundance and distribution of facies changes with geographic location within the basin and stratigraphic position within the Barnett.  The most obvious example of this is the relative abundance of calcareous depositional facies in the northern portion of the basin compared to their relative scarcity in the central portion of the basin.  All of the major facies recognized in the Barnett have high concentrations of organic matter.  Variation in facies is greater than variation in organic matter content.  The location of “sweet spots” with higher production rates within the Barnett may ultimately be explained by the distribution of facies that respond differently to various completion procedures.  As the play matures it is likely that a detailed understanding of the geology, especially the distribution of facies, will become increasingly important in selecting well locations, intervals in which to land laterals, and which fracture stimulation techniques to employ.