济阳拗陷高含CO_2气藏的同位素特征和成因探讨

THE STUDY ON THE ISOTOPIC CHARACTERS ANDTHE ORIGIN OF THE CO2RICH GAS DEPOSITSOF JIYANG DEPRESSION

济阳坳陷无机成因ＣＯ２气藏多沿断裂呈带状分布，且与新生代玄武岩在空间分布上关系密切．它具有高的ＣＯ２含量（６４．５５％～９９．９６％，平均为８８．１７％）和高的δ１３ＣＣＯ２值（－７．５４‰～－３．３５‰，平均为－５．０３‰），４０Ａｒ／３６Ａｒ（３１８～３０００，平均为１３５５）和３Ｈｅ／４Ｈｅ（４．６８×１０－６～３．５５×１０－６，平均为４．０１×１０－６）．这些特征表明，气藏中ＣＯ２为深部地幔来源ＣＯ２和地壳来源ＣＯ２按一定比例混合的产物．研究区内前第三系碳酸盐岩石的ＣＯ２含量及其δ１３Ｃ值的降低进一步表明，地壳来源ＣＯ２主要由碳酸盐岩石的脱碳反应形成．

Jiyang depression is a subelement of Bohai Gulf Cenozoic extensional basins, where inorganic gases widely distribute. These CO2rich gas deposits are mainly distributed near the faults. They are immediately related with Cenozoic basalt in spatial distribution. The gas deposits are high in CO2 contents (64.5599.96%, with average 88.18%), δ13C(7.543.35‰, with average 5.05‰),40 Ar/36Ar(3183000, with average 1355) and 3 He/4He(4.68×1063.55×106, with average 4.01`×106). These characteristics prove that CO2 of the gas deposits are the mixture of deep mantle derived CO2 and the crust derived CO2 in certain proportion. The mantle derived CO2 immediately related with the MesoCenozoic volcanic rocks. The relatively poor CO2 content and its values of δ13C of the preTertiary carbonate rocks also prove that the crust derived CO2 mainly came from the decarbonation of the preTertiary carbonate rocks.