摘要
北部湾盆地涠西南凹陷油气勘探活动长期以来集中在原油领域,随着时间推移,原油发现难度逐渐增大。天然气是油气增储上产的重要驱动力,但勘探研究程度低,其地球化学特征及来源尚不清楚,制约了下一步勘探进程。基于天然气组成、轻烃组成和稳定碳同位素等数据,对北部湾盆地涠西南凹陷天然气的地球化学特征、成因类型及来源进行探讨,结果表明:①天然气组成以烃类气体为主,其中甲烷含量最高,介于53.73%~95.80%之间,平均为74.80%;重烃(C_(2-5))含量相对较高,平均为19.80%,干燥系数普遍小于0.95,为典型的湿气;非烃类气体含量低,主要为N_(2)和CO_(2);②在C_(7)轻烃化合物中,正庚烷含量相对较高,平均为37.18%(21.74%~51.02%),其次为甲基环己烷,含量平均为34.46%,各种结构的二甲基环戊烷含量略低,平均为28.36%,反映其母质来源复杂,以偏腐泥型干酪根为主,部分为混合型,少量为腐殖型;③δ^(13)C_(1)值和δ^(13)C_(2)值分别为-54.8‰~-34.4‰和-35.2‰~-25.6‰,烷烃气碳同位素总体呈正序分布特征,仅部分井区天然气碳同位素发生一定程度倒转,可能与同源不同期气或同型不同源气的混合作用有关;④天然气主要为油型气,且与原油同源共生,为偏腐泥型干酪根在成熟—高成熟阶段裂解的产物,主要来源于流沙港组二段下层序油页岩,部分有流沙港组三段上层序泥页岩的贡献。研究成果明确了北部湾盆地涠西南凹陷天然气地球化学特征及来源,对下一步天然气勘探部署决策具有重要的指导意义。
In the past,crude oil was the main petroleum exploration target in the Weixi’nan Sag of Beibuwan Basin.However,it’s difficult to find oil over time.Now natural gas is dominant in increasing reserve and production of petroleum in this area.The degree of exploration and research of natural gas is very low.Especially,the geochemical characteristics and source of natural gas still remain unknown,which constrains the following exploration progress.Based on the natural gas components,light hydrocarbon compositions and stable carbon isotope data,this research investigates the geochemical characteristics,genetic types and sources of natural gas in the Weixi’nan Sag of Beibuwan Basin.Results show that:(1)Natural gas is mainly composed by hydrocarbons.Methane is dominated in the natural gas.The molar proportion of methane is between 53.73%and 95.80%(averages 74.80%).The content of heavy hydrocarbons(C_(2-5))is relatively high,with an average molar proportion of 19.80%.The natural gas dryness coefficient is generally less than 0.95,implying that it’s a typical wet gas.The non-hydrocarbon gases are mainly N_(2) and CO_(2),and their contents are relatively low.(2)Among the C_(7) light hydrocarbon compounds,n-heptane content is relatively high,and averages at 37.18%(ranges from 21.74%to 51.02%).The second is methylcyclohexane,with an average content of 34.46%.The dimethylcyclopentane content is 28.36%.These indicate that the source materials are complex.The source materials are mainly sapropelic kerogen,and part of them are mixed kerogen.Only a few of the source materials are humic kerogen.(3)Theδ^(13)C_(1) and δ^(13)C_(2) values of natural gas are from-54.8‰to-34.4‰and-35.2‰to-25.6‰,respectively.The carbon isotope of the gaseous hydrocarbons is generally positive sequence distribution.Only some gas samples display partial reversal of the carbon isotope,which is probably related to the mixed gases that originates from the same source rock at different thermal evolution stages,or the same type of gases originating from different source rocks.(4)The natural gas is mainly oil-type gas,and originates from the same source rock with crude oil.They were produced by the decomposition of sapropelic kerogen during the matured-highly matured evolution stages.It is inferred that the natural gas is mainly contributed by the oil shales of the lower sequence of the second member of Liushagang Formation.The upper sequence of the third member of Liushagang Formation contributes partly to the formation of natural gas.The research results reveal the geochemical characteristics and sources of natural gas in the Weixi’nan Sag of Beibuwan Basin,and provide important guidance for the following exploration and development of natural gas in this area.
作者
周刚
胡德胜
游君君
朱继田
杨相
李兴
Gang ZHOU;Desheng HU;Junjun YOU;Jitian ZHU;Xiang YANG;Xing LI(Zhanjiang Branch,CNOOC China Limited,Zhanjiang 524057,China)
出处
《天然气地球科学》
CAS
CSCD
北大核心
2024年第11期1923-1934,共12页
Natural Gas Geoscience
基金
国家重点研发计划项目(编号:2019YFA0708504)
中海石油(中国)有限公司科技项目(编号:QGYQZYPJ2022-3)
中国海洋石油有限公司前瞻基础研究项目(编号:KJQZ-2024-2003)联合资助。
关键词
北部湾盆地
涠西南凹陷
天然气
地球化学特征
气源对比
Beibuwan Basin
Weixi’nan Sag
Natural gas
Geochemical characteristics
Gas sources comparison
