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Abstract

Fluvial sandstone bodies play an important role in forming hydrocarbon reservoirs, especially in lake basins. They possess more than onehalf of the oil in place in known oil fields in Chinese Mesozoic and Cenozoic lake basins. The strong reservoir heterogeneity and restricted lateral continuity of the fluvial sandstone bodies, however, result in some difficulties in secondary and tertiary oil recovery.

Almost all types of fluvial deposits documented from studies of modern river patterns have been identified in ancient Chinese lake basins. The identification of depositional facies models that relate to fluvial sandstone body geometry in the basins has been possible because of close well spacing and complete coring through pay zones. Six depositionalfacies models have been identified: (1) shortcoursed braided river, (2) longcoursed braided river, (3) highly sinuous meandering river, (4) low sinuosity meandering river, (5) straight distributary channel river, and (6) confinedvalley channel. Each depositional group is identified by its depositional character and respective heterogeneities. The depositional composition of the sandstone unit is a result of the depositional processes that took place during its formation in the stream. These processes and heterogeneities thus formed vary from one facies to another and depend on such depositional processes as lateral accretion in point bars, vertical accretion in braided bars, aggradation in confinedvalley channels, and channel fill in straight distributary channels.

Although there are variations in lateral continuity of sandstone bodies among fluvial types, all are rather discontinuous in lateral dimension. Only those sandstone bodies deposited during the transition from a period of tectonic uplift to one of subsidence, and during episodes of change from lacustrine regression to transgression, result in a wide areal distribution of the size of coalesced sandstone bodies. The vertical density of channel sandstone bodies of 50 percent and 30 percent may be empirically used to predict laterally extensive and isolated fluvial sandstone bodies, respectively, but much care should be exercised in the prediction of lateral continuity of sandstone units when the density of channel sandstone bodies ranges between 30 and 50 percent.

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