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ABSTRACT: Comparative Hydrocarbon Geology of Two Mesozoic Circum-Pacific

Author(s): H. M. Hawlader

The Surat basin in Queensland and New South Wales, Australia, is a foreland basin formed in response to a magmatic arc during Early Jurassic to mid-Cretaceous time. It has a maximum basin-fill of about 2.5 km of Jurassic and Lower Cretaceous sediments. The first commercial production of oil in Australia came from this basin in the early 1960s.

The Western Canada basin is a retro-arc foreland basin with up to 3.5 km of sediments deposited druing the Middle Jurassic to Late Cretaceous. The basin was developed on the cratonward side of an arc/cordillera by plate convergence. It is a composite basin with sediments ranging in age from Devonian to Tertiary and is one of the prolific petroliferous basins of the world. The famous Athabasca-Peace River-Lloydminister tar sands alone contain a reserve of about 3 × 1012 barrels of oil, which exceeds three times the recoverable reserves of the world's known oil.

The main sediment source was, in both basins, a rising arc/cordillera that shed a cratonward tapering clastic wedge into the flanking foreland basins. Sedimentation, in both cases, was episodic and the patterns of sedimentation in each present striking similarities. During the waxing phase of magmatism/orogeny in the arc/cordillera, the foreland subsided in response to flexural loading of the foreland fold-thrust belt and downward drag by the subducting plate. Continental synorogenic sediments were rapidly emplaced in mainly terrestrial environments into the subsiding foreland. These sediments are lithic-labile in nature and because of their physical and chemical reactivity are prone to be "tight" and thus of little hydrocarbon reservoir potential. During the waning phase of the arc/o ogen the foreland gently rose in response partly to the cessation of drag (decoupling) by the subducting plate and to isostatic rebound (tectonic relaxation). Supracrustal volcano-sedimentary cover in the arc/orogen was dissected to the plutonic-metamorphic core which supplied quartz-lithic debris to the foreland with additional contribution of quartzose sediments from the cratonic source. Reworking of the sediments of both sources in periods of relative tectonic quiescence made them mineralogically and texturally more mature. As a consequence they are prone to have good reservoir characteristics.

Hydrocarbons are generally associated with the characteristically more porous and permeable quartzose facies in contrast to the labile petrofacies which is relatively "tight." With a few exceptions the above generalizations hold true for both basins.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990