摘要
天然气稀有气体同位素研究作为一个有别于烃类地球化学的新体系,在进入油气地质、地球化学研究领域仅20余年的时间里已取得了长足进展。元坝气田是继发现国内最大的海相气田——普光气田之后,在四川盆地内发现的又一个大型海相气田。元坝气田天然气烃源问题一直存在争议,主要原因是天然气地球化学数据揭示为源岩高—过演化阶段产物,而元坝气田能够供气的主要烃源层系均为高—过演化阶段,形成了对烃源认识的差异。研究共分析元坝气田天然气样品19个,组分分析结果表明干燥系数分布于0.98~1之间,甲烷和乙烷碳同位素组成分别为-35.6‰^-28.3‰、-34.6‰^-21.62‰;稀有气体同位素组成~3He/~4He值为(2.50~0.70)×10^(-8),^(40)Ar/^(36)Ar值为340.7~861.92,主频分布在340.7~500之间。天然气为高成熟—过成熟阶段的产物。研究认为,在元坝气田天然气有机地球化学数据展示多解而无法确定烃源层系的前提下,元坝气田天然气氩同位素特征所表征的源岩定年与发育的烃源岩不符,但又能揭示一些有意义的信息。元坝气藏烃源可能是震旦系或下寒武统筇竹寺组,燕山期早期热演化形成古油藏后,在地质历史变迁中遭到改造或破坏,于燕山中期又经历古油藏持续埋深,经历化学改造和流体调整,原油(或分散有机质)裂解生成天然气,再经过喜马拉雅期的调整改造在构造—岩性复合圈闭富集形成现今的天然气藏。原油裂解天然气中氩同位素特征表征的是古油藏裂解天然气的氩同位素特征和上三叠统须家河组高—过成熟的腐殖型烃源岩形成的天然气氩同位素特征的混源。
The noble gas isotope of natural gas is a new system,which is different from the conventional hydrocarbon geochemistry,and has made great progress in petroleum geology and geochemistry in not more than 20 years.Yuanba Gasfield is a very large marine facies gasfield in the Sichuan Basin,resembling the Puguang Gasfield.The geochemical data revealed that the gas was generated by the high-over mature source rocks,however,the major source rocks in the Yuanba Gasfield are at high-over mature stage.There has been a controversy that which sets of source rocks provided natural gas to Yuanba Gasreservoir.The chemical analysis of 19 gas examples from the Yuanba Gasfield suggests that the dry coefficient is from 0.98to1,δ^（13）C_1 is from-35.6‰to-28.3‰,δ^（13）C^2 is from-34.6‰to-21.62‰,~3He/~4He ratios is of（2.5-0.7）×10^（-8）,and ^（40）Ar/^（36）Ar ratio is from 340.7to 861.92 with the dominant frequency of 340.7-500.The maturity of natural gas is the same to the source rocks.Nevertheless,the Argon isotope age of natural gas is inconsistent with the eras of the source rocks.When the gas organic geochemical data of Yuanba Gasfield demonstrates multiple solutions and cannot determine hydrocarbon source rock,the Argon isotope characteristics may reveal some meaningful information.The source rock of Yuanba reservoir may be the Sinian or the Lower Cambrian Qiongzhusi Formation,which experienced thermal evolution and developed an ancient oil reservoir in the Early Yanshanian,then was transformed or destroyed in geological history.The ancient oil reservoir was buried and underwent the chemical and fluid modification in the Middle Yanshanian,where the crude oil（or dispersed organic matter）is cracked and generated natural gas.For the Himalayan tectonic movement,the natural gas was gathered in the structural-litho-traps.The Argon isotope characteristics of the Yuanba natural gas should be the mixture of the Argons,which came from the ancient oil cracking and the high-over maturity gas was generated in
出处
《天然气地球科学》
EI
CAS
CSCD
北大核心
2017年第7期1072-1077,共6页
Natural Gas Geoscience
基金
国家自然科学基金项目(编号:41402131)资助
关键词
天然气
稀有气体
氩同位素
元坝气藏
气源对比
Natural gas
Noble gases
Argon isotope
Yuanba reservoir
Gas-source correlation
