Natural Fractures Within the Souris Valley Marker Bed ‘B’, Lodgepole Formation, Southern Saskatchewan, Canada

Abstract

The recent increased attention given to “self sourced unconventional reservoirs” has resulted in a number of publications that seek to understand and characterize natural fractures within argillaceous and fine-grained organic rich lithologies. It has been suggested that the existence of nano- and micro-scale natural fractures act as fluid-conduits in unconventional ‘shale-gas plays’. However, the role of such fine-scale natural fractures within ‘shale-oil plays’ is not so clearly understood, although the presence of such natural fractures is generally believed as having a role in facilitating the movement of oil within argillaceous and fine-grained lithologies (Hang, 2011; Larter et al, 2012). The goal of this study is to evaluate the occurrence of natural fractures within the Mississippian Souris Valley Marker Bed ‘B’ in relation to the pervaisive ‘varve-like’ laminae and highly visible bitumen content. The core at Silver Hawk Hardy Borehole 01/07-9-4-21W2 was oriented relative to a datum (north) at the time of drilling. Lamination analysis, petrographic analysis, autofluorescence microscopy, fractional bitumen analysis, and fracture analyses were used to analyze Marker Bed ‘B’. The pervasive alternation of light and dark laminae are due to the relative percentage of amorphous organic matter versus the percentage of carbonate mud within each lamination. The typical sedimentary assemblage of silt-sized quartz, chert, dolomite, carbonate mud, bioclastic material also includes anhydrite, occurring as cannibalized grains of calcite or dolomite. Marker Bed ‘B’ contains a very high proportion of bitumen in-filled natural fractures, characterized here as nano- micro- meso- and macro-scale fractures. Fracture frequency, height, termination style, fill type, angle from bedding and orientation were all recorded. Fractures occur as both cross-cutting the rock fabric and within the plane of lamination. The mud-rich laminae are more ductile than the generally brittle, silty-rich lamina. The Marker Bed ‘B’ contains a very high fracture saturation ratio that ranges from 0.2 to 2.4, with the majority of micro- and meso-fractures propagating within the more brittle carbonate laminae. The high fracture saturation ratio and fracture orientation suggest high pore-pressure and lower differential stress. The high proportion of existing natural fractures perhaps indicate that this zone may be highly responsive to hydraulic fracturing for enhanced oil recovery.

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