苏北朱家墩气田气源的新证据

针对盐城朱家墩气田的气源问题，在前人研究基础上重点补做了天然气和源岩的对比，油气混合及凹陷底部源岩展布等研究，研究再次证实天然气来自古生界，并明确主要是二叠系栖霞组灰岩，同时查明了尽管在数量上不足道，但属于上白垩统泰州组来源的凝析油的组成面貌，盐城册陷底部出露的海相古生界地层可能是一大型背斜的南翼，由北向南地层时代逐渐变新，栖霞组灰岩位于册陷南侧，古生界天然气主要从盐（2）断层通过垂向，侧向运移进入储层，在朱家墩构造聚集成藏，盐（3）断层起到了很好的侧向封堵作用。

New Evidences for Natural Gas Source of Zhujiadun Gasfield

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.