Genetic mechanism of multiphase states of Ordovician oil and gas reservoirs in Fuman oilfield, Tarim Basin, China

Abstract

Abundant oil and gas are yielded from Ordovician reservoirs in the Fuman oilfield of Tarim basin and behave in multi-phase states at comparable burial depths, including light oil, volatile oil, and condensate gas. Oil and gas phase are distributed in the west and east, respectively, with heavier oil property in the north than that in the south. However the genetic mechanism of oil and gas phase states in the study area remains obscure. This has been examined based on fluid inclusions and geochemical parameters of light hydrocarbons and diamondoids in this paper. The results show that most Ordovician reservoirs were charged by oils in the late Caledonian and the late Hercynian, and the oil maturity charged in the same period decreased from south to north. In the Himalayan, dry gas and condensate oil-wet gas were respectively filled into the Yuke area in the east and local areas in the west. The multi-period differential charging of oil and gas with different properties is the main cause of the current oil and gas phase states. Two and three periods of oil charging are responsible for the formation of light and volatile oil, respectively, in western regions. Condensate gas reservoirs in Yuke area were formed by three periods of oil and gas charging, with the dominance of dry gas in the Himalayan. The phase states were transformed into volatile oils with the decrease in the charging intensity of dry gas to the west. The oil property becomes lighter from north to south is mainly related to the oil maturity charged during the same period, but also affected by the gradually decreasing cracking degree of oils from south to north. The current oil and gas phase states were ultimately produced by the genetic model of “multi-period differential charging and spatial differential cracking”.