Abstract

Previous studies of fractured sedimentary rocks have shown that the fracture architecture is directly controlled by the mechanical stratigraphy. This is defined by (1) the thickness and “brittleness” of stratigraphic units and (2) the nature of interfaces between beds. Using fracture distribution through the sedimentary column to deduce the mechanical differences between intervals is problematical because of the possibility that these properties may change between fracturing events or after deformation has stopped. Observation of both through-going and bed-contained fracture sets in the West Texas Permian basin may reflect changing mechanical properties (i.e., via diagenesis) during the folding process. Using accoustic logs, 3D seismic, and formation image logs, we assess the mechanical contrast between the individual units identified from logs and show that the present-day mechanical contrast does not explain the fracture distribution. The result is an interpretation that late-stage mechanical changes resulted in through-going fractures formed as a response to deformation in strata in the present day mechanical conditions and that the early bed-contained fractures formed when the brittleness contrast between beds was significantly greater than that of the present day. We interpret that these changes reflect a diagenetic evolution across layers of differing lithology and grain size, producing a mechanical stratigraphy that changed through time. These changes in mechanical stratigraphy may have previously been unrecognized by other workers and that these changes affect fracture network architecture and fluid-flow pathways in low permeability rocks.