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New Evidences for Natural Gas Source of Zhujiadun Gasfield

XU Zheng-long   

  1. Geological Research Institute, JOECO, SINOPEC, Jiangsu Yangzhou 225009, China
  • Received:2002-12-20 Revised:2002-12-20 Online:2002-12-20 Published:2002-12-20

Abstract: Zhujiadun gasfield is located in Yancheng depression,Jiangsu Province of China. Former researchers believed that gases deposited in the E1-K2t sandstone beds came from the Paleozoic source-rocks,but liquid hydrocarbons in condensate associated with natural gas mainly came from sapropelic kerogen in the Taizhou Formation (K2t). Recent studies on maturity correlation of gases and source-rocks, anatomizing oil-gas mixing, the Paleozoic source-rock outcrop of depression bottom have suggested that high iso-heptane value(5~8)is characteristic for over maturity of the natural gas.Experiments also indicate that the iso-heptane values from the Triassic and the Paleozoic source-rocks areas high as the natural gases,and that iso-heptane values from the Early Tertiary and the Late Cretaceous source-rocks are much louser than the natural gases.Therefore,it is provided again that natural gases originated in the Paleozoic source-rocks. Based on carbon isotopic comparison of ethane and propane in natural gas and pyrolysis gas from source-rocks of various age,it also is suggested that limestone of the Qixia Formation of the Permian is likely a source-rock of the natura1 gas.The distribution of norma1 chain hydrocarbon analyzied recently has“two peaks”structure,but that analyzed two years ago only has one peak for same condensate sample associated with natural gas.The two peaks appeared because frontal peak became louser as the dissipater of light hydrocarbon actually reflect mixing of two source component.The latter peak would be crude oil came from sapropelic kerogen in the Taizhou.Formation(K2t).It is important that a feculence part of condensate has lighter carbon isotopic composition(δ13C-30.5‰)and shows sapropelic feature,but the natural gas has heavier carbon isotopic composition(δ13C2-27‰,δ13C3-25.6‰ )and shows humic feature. According to ananlysis of some hole data,outcrop under the Yancheng depression bottom is southern half of a large anticline and from north to south the formation age gradually becomes yanger.Limestone of the Qixia Formation in the Permian locates southern side of Yancheng depression bottom . The natural gas from source-rocks passed fault No.2 up to pool,and/or sidespin into reservoir from southern side of fault,and fault No.3 plays good side envelop.