Depositional Setting and Oil Sands Reservoir Characterization of Giant Longitudinal Sandbars at Ells River: Marginal Marine Facies of the McMurray Formation, Northern Alberta Basin, Canada
Paul L. Broughton, 2013. "Depositional Setting and Oil Sands Reservoir Characterization of Giant Longitudinal Sandbars at Ells River: Marginal Marine Facies of the McMurray Formation, Northern Alberta Basin, Canada", Heavy-oil and Oil-sand Petroleum Systems in Alberta and Beyond, Frances J. Hein, Dale Leckie, Steve Larter, John R. Suter
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The McMurray Formation (Aptian) Ells River bitumen deposit is hosted by two to three superimposed wave-dominated shoreface sands distributed as giant longitudinal bars northwest of Fort McMurray, Athabasca oil sands deposit, northeastern Alberta. The multibillion-barrel Ells River steam-assisted gravity drainage (SAGD) bitumen reservoir is 15 to 40m (49–131 ft) thick, with up to 12 to 14% bitumen by weight. Significant bitumen deposits have been discovered in recent years, where thickened McMurray clastics accumulated at the junctions of secondary paleovalley tributaries with the main northward-flowing trunk system and within offset, detached, secondary paleovalleys such as at Ells River. The middle and upper McMurray bitumen sands at Ells River accumulated as wave-dominated shore-face sands that formed giant longitudinal bars at the mouth of a north-trending paleovalley. This developed as an embayment on the Devonian paleosurface to the west of the main Athabasca bitumen fairway. The Ells River clastics were sourced from the east by long shore drift emanating from the tide-dominated delta that fronted the Cretaceous seaway, north of the Fort McMurray mining area. These middle and upper McMurray shoreface sands transgressed westward along the shelf margin to the Ells River embayment and covered the lower McMurray bay-fill clastics and paleosols. The Ells River depositional model contrasts with the main Athabasca bitumen deposits that accumulated as fluvial and estuarine tidal-and point-bar channel sands along themain bitumen fairway and infilled the paleovalley trunk system. The northward-flowing trunk system was entrenched into the underlying Devonian paleotopographic surface and broadened into a tide dominated deltaic complex at the confluence with the Cretaceous seaway during the middle McMurray. These fluvioestuarine sands nowhost extensive minable oil sand deposits in the Fort McMurray area of northeastern Alberta. The Ells River reservoir-quality sands were deposited as tidal sand-wave complexes with large-scale cross bedding. Reservoir-quality sands are the upper shoreface sands, which are relatively clean and have low volumes of shale (Vsh ã 5%). Lower quality sands are the lower shoreface sands that have been locally modified by waves, as shown by reactivation structures and thin, discontinuous, clay-draped wave-ripples. At Ells River, a partially muddy maximum flooding surface (MFS) interval, strongly burrowed by Thalassinoides, separates the middle McMurray lower SAGD reservoir chamber from the overlying upper McMurray reservoir chamber. This horizon may impair vertical steam chamber growth between the lower and upper reservoir levels for some areas of the Ells River bitumen deposits. Additional operational challenges are thin to patchy lean bitumen zones (bulk oil weight, <6%) and/or water saturation (Sw ã 40%); potential influx of mobile water from zones overlying the midreservoir MFS baffle; the shallow overburden (<200 m; <656 ft) that necessitates use of low-pressure operations; and the presence of partially depleted top gas sand that may locally be a thief zone.