COMPARISON OF SELECTED NON-MARINE PETROLEUM-BEARING BASINS IN AUSTRALIA AND CHINA

Abstract

This paper summarises the geology and hydrocarbon potential of two Chinese and two Australian basins (Ordos, Northern Jiangsu, Eromanga, and Surat basins) in order to compare factors affecting the generation, migration, and entrapment of hydrocarbons. In all four basins, hydrocarbons are generated from nonmarine source rocks of lacustrine and fluvial-overbank origin. While the Chinese and Australian basins contain a similar range of sedimentary facies, from alluvial fan to lacustrine, the arrangement and relative thicknesses of these facies vary considerably as a result of different tectonic and palaeoclimatic settings.

During the Triassic, the Ordos Basin was dominated by retroarc foredeep subsidence and the development of deep, fresh-water lakes with anoxic bottom waters. This non-bioturbated substrate, with Type I and II kerogen precursors, provided an excellent oil source for adjacent fan-delta, deltaic, and fluvial reservoirs, and for the unconformably overlying Jurassic fluvial valley-fill sandstone reservoirs.

The Northern Jiangsu Basin was initiated by back-arc extension and underwent very rapid half-graben subsidence in the Eocene. Alluvial fan, shoreline, and fluvial facies aggraded in a relatively narrow zone along the active, faulted margin, and merged laterally into organic-rich shales which provided a local source for oil.

By comparison, the Eromanga/Surat basins developed in response to gentle downwarp and reactivation of older structural trends. Reservoirs are largely restricted to craton-derived quartzose facies such as in the Hutton, Precipice, and Namur sandstones. There is probably a dual source for oil, from the underlying Permian (which may be the dominant source in the Surat Basin), and from shales deposited in shallow, partly oxygenated lakes and overbank facies of Jurassic age (important in the Eromanga, and possibly subordinate in the Surat Basin). Deep lacustrine facies, typical of the Chinese basins, did not develop. The greater abundance of oil in the Chinese nonmarine basins is explained in terms of tectonic and palaeoclimatic factors which yielded thicker and better quality source rocks, more rapid maturation, and a better juxtaposition of source rocks and good-quality reservoirs, thus providing short, highly efficient migration routes.