西湖凹陷天然气成因及来源分析

Analysis on the genesis and source of natural gas in Xihu sag, East China Sea basin

东海盆地西湖凹陷天然气资源丰富,但其成因与来源一直存在较大争议。综合分析天然气组分、碳同位素组成、轻烃指纹以及天然气伴生凝析油地球化学特征,对西湖凹陷天然气的成因、热演化程度与族群划分进行了探讨,在此基础上利用不同烃源岩热模拟产物碳同位素特征与伴生凝析油特征进行了气源分析。西湖凹陷天然气为煤型气,形成于中等成熟—高成熟热演化阶段,对应的Ro值约为1.16%~1.87%。西胡凹陷天然气可以划分为4个族群,其中A、B与C类天然气分别分布于平湖斜坡、黄岩构造带和宁波构造带,D类天然气零星分布于平湖斜坡。西湖凹陷A类天然气主要来源于平湖组煤,B类天然气主要来源于平湖组泥岩,C类天然气主要来源于高热演化阶段的平湖组煤,D类天然气中有3个样品来源于平湖组炭质泥岩,1个气样可能有中下始新统烃源岩贡献。本文研究结果可为西湖凹陷的天然气勘探提供理论依据和技术支持。

The Xihu sag in East China Sea basin is rich in natural gas resources,but the genesis and sources of the gas are still controversial. Through comprehensive analysis of natural gas composition,carbon isotope composition,light hydrocarbon fingerprints,geochemical characteristics of natural gas associated condensate,etc.,the genesis,thermal evolution degree and group classification of natural gas in Xihu sag have been studied. On this basis,gas source analysis has been carried out by utilizing the carbon isotope characteristics of thermal simulation products of different source rocks and the characteristics of associated condensates. The natural gas in Xihu sag is coal-type gas formed in the middle mature- high mature thermal evolution stages,and the corresponding R o values are about 1.16 %~1.87 %. The natural gas in Xihu sag can be divided into four groups,among which,Classes A,B and C natural gases are distributed in the PH slope,HY structural belt and NB tectonic belt respectively,and Class D natural gas is distributed in the PH slope sparsely. The Class A natural gas in Xihu sag is mainly originated from coal in the Pinghu Formation,Class B natural gas is mainly originated from mudstone in the Pinghu Formation,Class C natural gas is mainly originated from the Pinghu Formation coal in high thermal evolution stage,and 3 samples from Class D natural gas are sourced from carbonaceous mudstone in the Pinghu Formation,1 sample may be originated from the Middle-Lower Eocene source rocks. The research results in this paper can provide theoretical reference and technical supports for natural gas exploration in Xihu sag.