Sedimentological Controls on Natural Fracture Networks in the Turonian Second White Specks Formation, Highwood River, Southwestern Alberta

Abstract

Detailed analysis of natural fracture network geometry is an important step in the geomechanical modelling and characterization of unconventional tight reservoirs as fractures provide flow pathways for hydrocarbons in the subsurface as well as influence hydraulically induced fracture development. Characterization of subsurface fractures is challenging since boreholes provide a limited view, but outcrops provide useful 3D subsurface analogs. Outcrops of the Second White Specks Formation along the Highwood River in southwestern Alberta were divided into three major facies: 1) the Jumping Pound Sandstone; 2) interbedded finely laminated siltstones and mudstones; and 3) black organic-rich mudstone. Fracture parameters were recorded from each facies interval at five structurally distinct outcrops using the scanline and circular estimator methods. Results were used to examine the differences in natural fracture characteristics between sedimentary facies in the Second White Specks Formation. The Jumping Pound Sandstone contains compressional conjugate shear fractures that occur at intensities of 4.2–7.4 fractures per meter with average heights of 0.41–0.86 meters. The interbedded finely laminated siltstones and mudstones contain extensional fractures that occur at higher intensities of 24-30 fractures per meter with much shorter average heights of 0.04–0.12 meters, the latter being related to the finely interlaminated siltstone-mudstone fabric. The black organic-rich mudstone contains fractures that are conjugate to the underlying thrust fault in addition to extensional fractures that both occur at intensities of 5.2–8.5 fractures per meter with average heights of 0.44–0.76 meters. Elevated fluid pressures resulting from oil generation from Type II kerogen within the two mudstone facies could have increased pore pressure to the point that promoted the formation of extensional fractures compared to the compressional shear fractures that occur in the overlying Jumping Pound Sandstone. The results from this study suggest the anisotropy and heterogeneity of sedimentary facies characteristics such as lithology, organic content and type, sedimentary fabric and mechanical bed thickness have strong influences on fracture characteristics in the Second White Specks Formation outcrops along the Highwood River. These observed relationships give valuable insight into the influence of sedimentary facies on natural fracture networks in unconventional type reservoir targets.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016