塔北地区甲烷碳同位素特征与烃类运移方式

doi:10.11743/ogg20050409

摘要/Abstract

摘要：

不同的烃类运移方式产生了不同的甲烷碳同位素分馏效应。其中,以扩散型方式运移的烃类甲烷碳同位素具有随运移距离的增加而显示趋重的变化特征,例如塔北地区不同井中甲烷碳同位素由深层三叠系的-46.98‰～-55.98‰至浅层第四系为-31.47‰～-37.51‰。北海富提斯油田近地表沉积物甲烷碳同位素在油气田上方分布着较轻的甲烷碳同位素(δ¹³C₁值为-40‰),向外围,随着距离的增加甲烷碳同位素(δ¹³C₁值为-30‰)也呈明显趋重的变化分布。渗漏型方式运移的烃类甲烷碳同位素则不随运移距离而变化,如塔北地区阿克库木构造奥陶系烃源岩天然气甲烷碳同位素值-34.85‰～-34.98‰,地表化探甲烷碳同位素值稳定在-34.62‰～-36.56‰,与地下奥陶系天然气甲烷碳同位素值近似或等同,显示出地表甲烷碳同位素与地下甲烷碳同位素之间具有同源的关系。不同烃类的运移方式,甲烷碳同位素明显不同的规律性变化,主要是由于扩散型运移,甲烷中轻碳同位素分子因溶解而分馏,从而形成酸解烃甲烷碳同位素沿着地层剖面向上的趋重分馏,渗漏型运移由于天然气运移规模大、速度快,甲烷在水中的微量溶解难以改变大规模运移中的甲烷碳同位素特征。

关键词: 甲烷碳同位素, 运移方式, 扩散型, 渗漏型, 塔北地区

Abstract:

Various patterns of hydrocarbon migration would result in different fractionation effects of methane carbon isotopes.When hydrocarbons migrate through diffusion,methane carbon isotopes tend to get heavier along with increasing migration distance.For example,methane carbon isotopes in different wells in Tabei area change from-46.98‰—55.98‰ in the deep Triassic to-31.47‰—37.51‰ in the shallow Quaternary.In Forties oilfield in North Sea,the methane carbon isotopes are lighter in the near surface sediments over the oilfield(the value of δ¹³C₁ being-40‰),and they also tend to get heavier along with the increasing migration distance to the periphary(the value of δ¹³C₁ being-30‰).When hydrocarbons migrate through filtration,methane carbon isotopes do not change along with migration distances.For example,the methane carbon isotopes of natural gas in the Ordovician source rocks in Akekumu structure,Tabei area,is in the range of(-34.85‰)—34.98‰.The methane carbon isotopes measured through surface geochemical exploration is in the range of-34.62‰-36.56‰,which is near or equal to that in the subsurface Ordovician,indicating that the surface and subsurface methane carbon isotopes have cogenetic relationship.The reason that causes apparent and regular variation of methane carbon isotopes in various hydrocarbon migration patterns is that the methane molecules with light carbon isotopes would be dissolved during diffusion migration.As a result,the carbon isotopes detected with acidolitic hydrocarbon method would be fractionated,or become heavier upward along the stratigraphic section.While during filtration migration,negligible dissolution of methane in water would not change the methane carbon isotope features in extensive migration,due to the large scale and rapid migration of natural gas.

Key words: methane carbon isotope, migration pattern, diffusion type, filtration type

中图分类号:

TE112.3

朱怀平,程同锦,李武,等. 塔北地区甲烷碳同位素特征与烃类运移方式[J]. 石油与天然气地质, 2005, 26(4): 450-454,460. doi: 10.11743/ogg20050409 Zhu Huaiping,Cheng Tongjin,Li Wu,et al. Features of methane carbon isotope and patterns of hydrocarbon migration in Tabei area[J]. Oil & Gas Geology, 2005, 26(4): 450-454,460. doi: 10.11743/ogg20050409

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