Fractures of the Weber Formation from Split Mountain Anticline, Utah: a Database to Investigate Structural Characterizations of Folded Fractured Reservoirs
The organization of the structural discontinuities affecting the sandstones of the Permian Weber Formation at Split Mountain Anticline can be studied using detailed description of the sedimentology and the natural fractures encountered across nine vertical profiles, and over the stratigraphic surface of the Upper Weber at 36 locations evenly distributed. Maps of the magnetic susceptibility and of the sandstone matrix density as well as 109 thin sections could be produced from these measurement sites. These data allow a detailed description of a complex structure to be used as a field analogue of fractured reservoir in order to investigate geomechanical models.
A classification of the principal families of fractures is presented based of relative chronology and on possible correlations about the sedimentary facies or about the local or regional structural position. At the onset of Sevier orogeny, populations of fractures and joints developed parallel to an WNW-ESE bulge related normal faults system. Then during the layer parallel shortening (LPS) loading conditions, the Sevier compression led to the creation of NE-SW joints as conjugate elements to the earlier WNW-ESE fractures. Finally, N-S to NE-SW fracture families, kinematically compatible with the Laramide tectonic phase, could have been generated very early during the LPS build up before the growth of the Split Mountain. The proportion of reactivated fractures or of fold-related generated fractures is clearly difficult to assess from outcrop observations only, thin sections analyses can nevertheless produce some diagnostic to discuss these issues. Along the vertical direction, the distribution of the fractures families is very different from one bed to and other and several scales of discontinuities were identified. The micro-structures revealing shear displacement with cataclasis, pressure-solution, distinct carbonate cements between the fractures and the matrix, are suggesting that the WNW-ESE and NE-SW fracture systems were reactivated several time in opening mode and in shear mode and played a role on vertical migrations of fluids.
This scenario may be validated provided additional evidences concerning the diagenetic and cementation episodes will become available/inferred because today such record was probably erased by post-Laramide circulation of acid fluids (CO2) as suggested by the area distribution of calcite cements in the different units of sandstones and their fractured zone.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009