The phases of Ordovician hydrocarbon and their origin in the Tabei uplift,Tarim Basin

Ordovician reservoirs of the northern Tarim Basin with abundant hydrocarbon resources are one of the most important exploration targets,and they have many complex hydrocarbon phases,such as condensates,normal oils,heavy oils and asphalts.Through analyses of the formation and conservation process of marine oil reservoirs and integrated with geochemical data of hydrocarbons and fluid inclusions,we showed that the complexity of hydrocarbon phases was controlled by multi-stage hydrocarbon charging and secondary alteration.Ordovician reservoirs in the eastern Tabei area experienced three stages of hydrocarbon charging,which occurred in Late Caledonian-Early Hercynian,Late Hercynian and Late Himalayan.The oil in these reservoirs was generated mainly from Middle-Upper Ordovician source rocks,while the gas was derived from cracking of liquid hydrocarbons associated with Cambrian source rocks.During Late Hercynian,Ordovician reservoirs experienced hydrocarbon charging in the central and western region of the Tabei uplift and there occurred three stages of hydrocarbon adjustment after the formation of Ordovician reservoirs.During Early Hercynian,Silurian-Devonian sequences were eroded due to tectonic uplift,which destroyed paleo-reservoirs of Cambrian hydrocarbons in the east region and formed asphalts.The tectonic movement of pre-Triassic sedimentation in Late Hercynian caused large-scale degradation and densification of Ordovician oils.During Late Himalayan,the gas from the Manjiaer sag charged these reservoirs from east to west,severely altering the early-formed reservoirs and forming secondary condensate gas reservoirs in the eastern uplift.However,Ordovician reservoirs in the central and western region were in a continuously subsiding process and were well preserved after the degradation and densification of the pre-Triassic sedimentation.They are unsaturated reservoirs characteristic of low light components,very low gas/oil ratios and heavy oils due to the early accumulation of hydrocarbons.The study concluded that the diversity of hydrocarbon phases was controlled mainly by two processes,one is the biodegradation due to the Late Hercynian tectonic uplift and the other is the large-scale reconstruction of hydrocarbon reservoirs from east to west by the gas derived from in the Late Himalayan tectonic movement.