Comparative Analysis of the Petroleum Systems of Two Super-Giant Foreland Basin Accumulations: The Canadian Athabasca Tar Sands and the Venezuelan Orinoco Heavy Oil Belt
John R. Suter
ConocoPhillips, Houston, TX
The enormous accumulations of the Athabasca Tar Sands of Canada and the Faja Petrolifera del Orinoco, or Orinoco Heavy Oil Belt, of Venezuela dominate global resources of bitumen and heavy oil. The overall Athabasca resource contains some 1.7 trillion barrels of bitumen, about 935 billion barrels in the lower Cretaceous McMurray Formation. Around 1.4 trillion barrels of heavy and ultra-heavy oil occur in the Early Miocene Oficina Formation of Venezuela. Despite significant differences, these huge accumulations are remarkably similar petroleum systems. Each occurs on the flexural margin of a complex foreland basin, superimposed onto pre-existing passive margins. The petroleum of the respective deposits was generated from passive-margin, shallow marine source rocks, and emplaced by long-distance migration as conventional oils into broad, low-amplitude traps with significant stratigraphic components. Sediments were principally sourced from granitic shield terrains. Neither deposit has experienced substantially greater burial than present day; consequently their unconsolidated, dominantly quartz sands have exceptional reservoir quality. The shallow, relatively cool reservoirs had ideal conditions for biologic activity, resulting in severe biodegradation to produce heavy oils and tar sands. Sedimentologic, ichnologic, and biostratigraphic data from cores and outcrops, coupled with interpretations of extensive well log and 3D seismic datasets, reveal that both formations comprise non-marine to marginal marine sediments, mainly channelized fluvial, tidal-fluvial, and estuarine deposits. Imaging, predicting, understanding, and modeling reservoir and fluid heterogeneity and compartmentalization of these deposits are crucial to the ongoing and future success of projects within both the Faja Petrolifera del Orinoco and the Athabasca Tar Sands.