Comparative Fracture Characterization of the Turonian Second White Specks Formation, 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 and other fluids in the subsurface as well as influence hydraulically induced fracture development. Characterizing subsurface fractures is challenging since boreholes provide a limited view, but outcrops provide useful 3D subsurface analogs. Exposed 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. Natural fracture parameters were recorded from each facies interval using scanlines and additionally using the circular estimator method on the bedding plane of the Jumping Pound Sandstone. Results from the sampling methods were used to compare the relative 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 relatively low intensity (2.56–4.7 fractures per meter) with relatively tall heights (0.79–3.38 meters). The interbedded finely laminated siltstones and mudstones contain extensional fractures that occur at relatively high intensity (29.2 fractures per meter) with relatively short heights (0.18 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 relatively low intensity (4.88–7.4 fractures per meter) with relatively tall heights (1.18–1.25 meters). Elevated fluid pressures resulting from hydrocarbon generation within the two mudstone facies could have altered the stress field in such a way 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 analysis suggest that sedimentary facies characteristics such as lithology, heterogeneity and mechanical bed thickness have a strong influence on fracture generation and propagation in the Second White Specks Formation outcrops along the Highwood River that are also likely to be present in the subsurface.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015