Geology Seminar Series - Naveed Iqbal - Sedimentological properties and reservoir characterization of the Late Ordovician Black Island Member of the Winnipeg Formation, Southeastern Saskatchewan

Abstract: The Late Ordovician (Trentonian) Winnipeg Formation is a siliciclastic unit that occurs in the subsurface of southern Saskatchewan, Manitoba, and North Dakota. The formation unconformably overlies shale-dominated Late Cambrian upper Deadwood Formation and unconformably underlies the carbonates of Late Ordovician (Edenian) Yeoman Formation. In southeastern Saskatchewan, the Winnipeg Formation consists of two members: lower sandstone-dominated Black Island and upper shale-dominated Icebox members. The Black Island Member is a major oil-producing unit in North Dakota but less productive in Saskatchewan and non-productive in Manitoba.

Sedimentologic and diagenetic data from cores drilled in SE Saskatchewan, along with their wireline curves, have been collected and analyzed. This study investigates petrographic properties, depositional environment, and reservoir characteristics of the Black Island Member.

Petrographic analysis indicates that the member consists of ten lithologic units;  (F1) cross-bedded clean quartz arenite with zero bioturbation index (BI= 0), (F2) ferruginous lithic arenite (BI=0-1), (F3) slightly bioturbated cross- to planar-laminated quartz arenite (BI = 1), (F4) medium-grained, light grey quartz arenite with vertical (Skolithos) bioturbation (BI = 1-2),  (F5) variably bioturbated quartz arenite with floating mudrock pebbles (BI =1-3),  (F6) moderate to well bioturbated quartz wacke (BI = 3-4), (F7) siltstone (BI = 3), (F8) extensively bioturbated feldspathic greywacke (BI = 5-6), (F9) shale, and (F10) fine-grained, green quartz wacke with discrete patches of light grey bioturbated zones (BI = 0-2). The textural and structural properties of these lithofacies suggest depositional setting that ranges from intertidal (foreshore) through subtidal (shoreface) to lower offshore.

The reservoir properties of sedimentary rock is defined by the volume of pore spaces and their interconnectivity. The different types of the sandstone lithofacies (i.e., the quartz arenites and the wackes) of the Black Island Member are deemed to have fair to excellent reservoir properties. Among the different well cores studied, the combined quartz arenite lithofacies constitute about 47% of the member whereas the wacke and the mudrock lithofacies constitute 44% and 9%, respectively. The porosity of the quartz arenite strata ranges between ~22% and ~29% whereas that of the wacke facies varies from ~8% and ~11%. The pore spaces of the sandstone facies are partially filled by subordinate mud matrix and cements, such as clays, quartz overgrowth, and localized carbonate cement. The pore spaces of the various sandstone lithofacies are well to moderately interconnected and expected to provide excellent permeability. The muddy facies (siltstone facies and the shale) have very low porosity. Thus, the sandstone lithofacies (the arenites and the wackes), which form a total thickness of about 91% of the member, are envisaged to constitute fair to excellent reservoirs.