NON‐MARINE SETTING OF PETROLEUM IN THE SUNGLIAO BASIN OF NORTHEASTERN CHINA

The Sungliao basin of NE China today is the most important petroleum‐producing basin in the People's Republic of China (PRC). The basin was formed during the Yenshan orogenic movement of Middle Jurassic time. Tectonically, the basin is a NNE‐SSW‐striking horst‐and‐graben complex, with three major types of structures—troughs, uplifts, and grabens. A stratigraphic sequence of mainly non‐marine deposits of Middle Jurassic through Quaternary ages is present in the basin. This sequence is interrupted by three major unconformities—one between the Jurassic and Cretaceous, one within the Cretaceous, and another between the Cretaceous and Cenozoic. The Jurassic formations consist mainly of swamp and lacustrine strata with volcanic and volcanic‐derived rocks. The Cretaceous, which is the best‐developed section, is a thick series of non‐marine clastic strata of possible deltaic facies associated with a lacustrine environment of deposition. In the middle of the Cretaceous section, shallow‐marine (littoral) conditions are developed. The Cenozoic consists principally of swamp, lacustrine, and aeolian deposits. The productive section is mainly Lower Cretaceous. During the Cretaceous, three overall climatic cycles are expressed in the sediments, and each complete cycle shows a period of dry climate and a period of wet climate. During dry times, the water level within the basin was low, and the depocenters were small isolated basins, low in organic matter (there were few living organisms), and high in coarse terrigenous clastic content. The coarse clastics subsequently became reservoir rocks in the Lower Cretaceous. During wet times, the water level was high and the entire basin was flooded. The high‐water deposits are rich in organic matter (living organisms were varied and abundant), and fine black sediments were deposited. These black rocks, mainly organic‐rich mudstones, became the source beds for the petroleum which accumulated in the coarse clastics deposited during the dry periods. Ten fossil groups are reported, of which two are the most important in oil exploration in the Sungliao basin—ostracods, and pollen and spores. They are useful not only for dating and correlation, but also for indicating ancient environments. Ye et al. (1976) established 12 Cretaceous ostracod assemblages. Among the various pollen types, the Aquilapollen, Wodehouseia and Betpakdalina, are used for the subdivision and correlation of the Upper Cretaceous units (Gao and Zhao, 1976). A littoral pelecypod fauna is found in one of the units (Ku, 1962). On the basis of these and other fossils, the Cretaceous has been divided into six formations. Paleontologists working in the basin have two divergent opinions regarding the position of the boundary between the Upper and Lower Cretaceous of the Sungliao basin. Ostracod paleontologists place the lower four formations in the Lower Cretaceous and the upper two in the Upper Cretaceous. The bases for this viewpoint are (1) a drastic change in ostracod assemblages and (2) a major unconformity between the lower four and upper two formation sequences. In contrast, palynologists, malacologists, and most crustacean experts place the lower three formations in the Lower Cretaceous and the upper three in the Upper Cretaceous. The bases for this opinion are (1) correlations of the index floras and faunas of these fossil groups with those in other parts of the world, (2) the changing characteristics of the fossil assemblages, and (3) the presence of a thin bed deposited in a littoral‐marine environment.