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XU MIN, Curtin University of Technology, School of Applied Geology, Perth, Australia

ABSTRACT: Syn- and Post-Rift Petroleum Systems in the Songliao Basin, China

The non-marine, extension-related, Songliao basin is one of the most important oil and gas-producing basins in China. It consists of Cretaceous syn- and post-rift successions, with contrasting features of petroleum systems. The syn-rift successions in the Songliao area developed in 32 fault basins, all of which were overlain by a post-rift succession. The source rocks of syn-rift petroleum (gas) system developed in some coal-bearing sections in the fluvial and lacustrine sequences. Geochemical data indicates that these source rocks contain humic kerogen, which produced the main gas pools in the Songliao basin. The Late Cretaceous post-rift source rocks developed in the Qingshankou and Nenjiang Formations, representing the primary flooding surface and maximum flooding surface, respectively. They are dominated by lacustrine mudstones containing sapropelic kerogen which provide oil-favourable source beds as well as some oil-associated gas. The widely distributed post-rift petroleum system is the main hydrocarbon-producing systems in the Songliao basin. Geochemical studies on trace elements, dinosterane and C31 sterane of the post-rift successions suggest the existence of a temporary marine transgression in the Late Cretaceous, which resulted in deepening of basins and formation of mudstone rich in organic matter. This can explain the existence of the favorable petroleum system in the post-rift succession in the Songliao basin, rather than in the syn-rift succession, although the average heatflow, subsidence rate and thermal gradient are higher in syn-rift stage than that in post-rift stage.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid