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The two largest tar sand provinces of the world are those of the Orinoco petroleum belt in Venezuela (estimated in-place reserves of 1,000 to 2,000 × 109 bbl) and the Mannville Group oil sand deposits of Alberta (1,350 × 109 bbl estimated inplace reserves). Possible additions to these ranks are the Russian deposits of the Melekess depression and the Olenek area (reserve figures very erratic), and the bitumen-bearing Paleozoic rocks of the Carbonate Triangle in Alberta (reserves tentatively estimated to be more than 1,000 × 109 bbl). Other significant accumulations including those in the United States (principally Utah and California), Madagascar, Albania, Romania, the Caribbean, and the Canadian Arctic are all markedly small r (reserves two or more orders of magnitude less).
Elements common to most of the major accumulations include: stable tectonic settings, usually in large foreland basins; thermally immature and only moderately compacted reservoir sediments, commonly uncemented, with very high porosities and permeabilities; far-reaching hydrocarbon migration networks with access to organic-rich source beds; near-surface settings, within the realm of pervasive oil degradation by water washing and bacterial action; and complicated, often enigmatic trapping configurations, involving both stratigraphic and structural factors, with bitumen plugs perhaps serving locally as updip seals. The deposits in which the reservoirs are dominantly hydrophyllic (water wet), including those of both Venezuela and Alberta, are the most amenable to exploitation in that they can be processed by the inexpensive and relatively efficient hot-water extraction methods.
Athabasca is the largest of Alberta's oil sand deposits. It encompasses a total area of 32,000 sq km and is estimated to contain 870 billion bbl of oil in place. The Lower Cretaceous McMurray Formation reservoir is 35 to 70 m thick of uncemented, very fine to fine-grained, moderately sorted quartz sand and associated shale. The most important control on the grade of the oil sands is the distribution of primary porosity and permeability in the McMurray Formation sediments. Achieving a cogent understanding of the facies patterns thus leads directly to a predictive capability regarding the geometry and character of the oil-bearing zones.
Initial infilling of the McMurray depression appears to have developed in a wide variety of fluvial-deltaic environments, many of which are not yet fully understood. Subsequently, there developed a regime marked by the presence of deep channels, trending north and northwest, which locally incised the preexisting sedimentary sequences and deposited a characteristic, fining-upward cycle in many areas, particularly in the northern half of the deposit: trough cross-bedded channel-bottom sands at the base; giving way upward to solitary sets of epsilon cross-strata deposited on the sloping
flanks of channel-margin bars; passing upward into argillaceous sands of flood-plain origin. The channels which produced this sequence were up to 45 m deep and many hundreds of meters wide. Where they eroded and reworked the preexisting sedimentary pile, apparently along discrete meander-belt trends, they left behind a sand-dominated sequence that today constitutes some of the thickest and richest oil pay zones in the entire deposit.
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