Natural fractures exert a strong influence over Spraberry Trend Area reservoirs in the Permian Basin of west Texas. Little detailed study, however, has been made of the fracture system until recently. A horizontal Spraberry well was cored as part of a DOE Class III Project. Fractures in this and other vertical cores were analyzed for information on both large scale features including orientation and spacing and small-scale features such as the relationship between fracture mineralization and reservoir diagenesis.

At least four distinct sets of fractures are found within upper and middle Spraberry cores. These sets have differing orientations, spacing, mineralization, surface characteristics, and distribution with respect to lithology. Fractures found in the 1U zone of the upper Spraberry have a NE strike, and tend to be partly mineralized with barite, quartz, and dolomite. Distribution of these mineral phases can greatly affect conductivity between the fractures and the rock matrix. The 5U zone of the upper Spraberry contains fractures with NNE and ENE orientations. Both sets are unmineralized. One set of fractures appears to have a shear origin while the other set appears to be tensional. Natural fractures in black shales overlying both the 1U and the 5U have an ENE orientation similar to unmineralized fractures in the 5U. No fractures were encountered in similar shales underlying reservoir zones. A fourth set of fractures was found in vertical cores taken from more calcareous, non-reservoir parts of Spraberry. Most of these fractures are hairline width and completely healed with calcite cement. The detailed examination of cores, particularly the horizontal cores, has provided invaluable information about fracture spacing, orientation, mineralization and lithologic affinity.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas