Sedimentary and geochemical characteristics of sandstones and mudstones deposited in surface rivers and subsurface rivers in Ordovician karsted carbonates, Tahe Oilfield

Key words:  karsted carbonate reservoir  surface-river  subsurface-river  sedimentary sandstone  physical property  Tahe Oilfield

Abstract:

Ordovician carbonates in Tahe Oilfield were typical karst reservoirs in the world. Fracture-caves were general storage spaces for oil and gas in the karst carbonates, but the spaces more than 70％ had been filled with river deposits, collapsed breccias and chemical materials, which resulted in heavy heterogeneity in the karst reservoirs. By investigating drilling cores and well-logging and by using rock-mineral and geochemical measurements on some core samples, the authors found that sedimentary sands and muds were over 60％ in the total fracture-cave fillings; the sediments were obviously in sequences of surface rivers and subsurface rivers which were developed in the carbonates during the karst on the Tabei uprise; and the sandstones deposited in the subsurface rivers were important reservoirs in caves of runoff karst zone. Channel bars and river banks are identified from the sediments deposited in either surface rivers or subsurface rivers. The sandstones deposited in surface rivers are usually in coarse-to-medium grain size, but in the subsurface rivers they are characterized as fine-sands or silt-sands. Composition and texture maturities of the subsurface-river sandstones are much higher than those of surface-river sandstones:the quarts contents in the subsurface-river sandstones is 40％ higher than those in surface-river sandstones. The surface-river sandstones were deposited in open oxidative water, and interbedded with slope wash breccias; in comparison the subsurface-river sandstones were deposited in weak-oxidative to weak reductive waters, and interbedded with collapsed breccias, which were in good physical condition for oil and gas. The subsurface-river sandstones are good reservoirs in runoff karst zones in Tahe Oilfield. Oil production from the sandstones is expected to increase the oil-recovery in the karst reservoirs.