鄂尔多斯盆地奥陶系马家沟组盐下天然气成因类型及来源

Genetic types and sources of the subsalt natural gas in the Ordovician Majiagou Formation,Ordos Basin

鄂尔多斯盆地中东部奥陶系天然气资源丰富,随着深层天然气勘探的进一步深入,在奥陶系盐下地层也相继发现了可观的天然气资源,但盐下天然气的成因类型及来源仍有较大争议。基于天然气的组分、稳定碳同位素等资料,结合实际地质背景,对鄂尔多斯盆地中东部奥陶系马家沟组盐下天然气成因类型及来源进行了系统的分析。研究表明:鄂尔多斯盆地中东部马家沟组盐下天然气以干气为主,非烃含量较低,甲烷碳同位素(δ^(13)C_(1))值主频分布在-40‰~-32‰之间,乙烷碳同位素(δ^(13)C_(2))值主频分布在-40‰~-28‰之间,主要表现为腐泥型气特征。综合天然气组分、碳同位素与平面分布特征,将奥陶系盐下天然气细分为油型气、煤型气和混合气,其中油型气分布范围较广,来源于奥陶系马家沟组盐下碳酸盐岩烃源岩,部分天然气发生TSR反应;煤型气和混合气分布范围小,集中在靠近盐下地层尖灭线边缘部位,主要来源于上古生界烃源岩,奥陶系烃源岩也有少量贡献。

The Ordovician natural gas resources are abundant in the middle and eastern of Ordos Basin.With the further exploration of deep natural gas,a mass of natural gas resources have been found in the Ordovician subsalt strata,but the genetic types and sources of subsalt natural gas are still controversial.Based on the data of natural gas composition and stable carbon isotope,combined with the actual geological background,this paper systematically analyzes the genetic types and sources of subsalt natural gas in Majiagou Formation,central and eastern Ordos Basin.The natural gas in the middle and east of Ordos Basin is mainly dry gas,and the gaseous non-hydrocarbon component content is less.The methane carbon isotopes composition(δ^(13)C_(1)) is chiefly between-40‰ and-32‰,and that of ethane(δ^(13)C_(2))is between -40‰ and-28‰,which is characterized by sapropelic origin.According to the composition,carbon isotope and plane distribution characteristics of natural gas,the Ordovician subsalt natural gas can be divided into oil type gas,coal type gas and mixed gas.Oil type gas is widely distributed and comes from the subsalt carbonate source rock of the Ordovician Majiagou Formation,and some natural gas has TSR reaction.The distribution range of coal type gas and mixed gas is small,and they are concentrated near the edge of subsalt formation pinch out line.They mainly come from the Upper Paleozoic source rocks,and the Ordovician source rocks also make a small contribution.