埃塞俄比亚欧加登盆地天然气成因及来源

在一个含油气盆地的勘探开发早期阶段,研究天然气的成因及来源非常重要,可以为油气勘探潜力评价和资源评价提供理论依据。欧加登盆地勘探程度低,迄今发现了3个气田,天然气成因及来源一直缺乏系统研究和认识,为此对欧加登盆地气田的天然气组分、同位素特征、轻烃组分及烃源岩等特征进行综合分析研究。结果表明:①欧加登盆地天然气组分含量以甲烷为主,主要为湿气;②天然气δ¹³C₁、δ¹³C₂和δ¹³C₃值分别介于-41.9‰~-36.9‰、-34.1‰~-29.4‰和-31.6‰~-26‰之间,符合δ¹³C₁<δ¹³C₂<δ¹³C₃的正碳序列;③轻烃组成相似,轻烃参数(K1)值基本一致,C7系列表现出支链异构体优势;④天然气成因类型图版判识为腐泥型有机质生成的油型气,有机质的热演化阶段从成熟阶段到过成熟阶段,对应的成熟度(R_o)范围在0.8%~1.5%之间,天然气来自湖相沉积,与下三叠统Bokh组湖相沉积的烃源岩成熟度相匹配。因此,认为天然气的气源为下三叠统Bokh组烃源岩。

Origin and source of natural gas in Ogaden Basin,Ethiopia

Making clear the origin and source of natural gas is of great significance in the early stage of exploration and development in a petroliferous basin,which can provide theoretical basis for evaluating hydrocarbon potential and estimating resources.In Ogaden Basin with low exploration degree in Ethiopia,even though there are three gasfields discovered,and the natural-gas origin and source have never been studied and understood systematically.So,the gas composition,isotope characteristics,light-hydrocarbon composition,and source-rock characteristics in these gasfields were comprehensively analyzed and studied.Results show that(1)the natural gas in this basin is mostly wet gas with methane as the main composition;(2)the values ofδ¹³C₁,δ¹³C₂ andδ¹³C₃ are in the range of-41.9‰--36.9‰,-34.1‰--29.4‰and-31.6‰--26‰,respectively,which is in line with the positive carbon sequence ofδ¹³C₁<δ¹³C₂<δ¹³C₃;(3)light hydrocarbon is similar in composition,its parameter K1 is basically accordant and the C7 series present the predominance of branched isomer;and(4)according to the chart of natural-gas origin type,the natural-gas in Ogaden Basin is classified as the oil-type gas derived from sapropelic organic matter.The thermal evolution of organic matter is in the mature to over-mature stage,and the corresponding maturity(Ro)ranges from 0.8%to 1.5%.The natural gas is sourced from lacustrine sediment,whose maturity is in line with that of the lacustrine source rocks in the Lower Triassic Bokh Formation.Therefore,it is concluded that the natural gas is sourced from the source rocks of Lower Triassic Bokh Formation.