高石梯—磨溪地区灯影组、龙王庙组天然气气源分析

为弄清四川盆地高石梯—磨溪地区震旦系灯影组、下寒武统龙王庙组天然气的成因,在地质背景研究的基础上,通过对51个天然气样品的地球化学分析,认为该区灯影组和龙王庙组的天然气组分较之威远、资阳构造,具有甲烷含量略高、普遍含微量乙烷、非烃含量略低的特点,属于过成熟油和沥青裂解干气;灯影组天然气的甲烷碳同位素值(δ13 C1)介于-32.43‰~-34.59‰,乙烷碳同位素值(δ13 C2)介于-24.86‰~-28.02‰,龙王庙组天然气的δ13 C1介于-33.36‰~-36.72‰,δ13 C2介于-30.27‰~-32.51‰,二者天然气中的δ13 C1基本一致,但灯影组天然气的δ13 C2明显重于龙王庙组,这是过成熟原油裂解气与沥青裂解气混合所造成的结果。结合26块岩心样品的地球化学分析结果,确定了天然气的气源和成因:灯影组和龙王庙组天然气均为有机成因,其中龙王庙组天然气气源来自下伏下寒武统筇竹寺组页岩;灯影组天然气为混源气,其气源既来自筇竹寺组烃源岩和灯三段黑色泥岩,也有灯四段孔洞、裂缝中沥青裂解气的贡献。

Natural gas sources in the Dengying and Longwangmiao Fms in the Gaoshiti-Maoxi area,Sichuan Basin

In order to reveal the natural gas genesis in the Sinian Dengying and the Lower Cambrian Longwangmiao Fms in the Gaoshiti Moxi area, Sichuan Basin, we made a geochemical analysis of 51 gas samples based on the geologic research background. In comparison with the gas in Weiyuan and Ziyang areas, that in the Dengying and Longwangmiao Fms is characterized by a slightly higher content of methane, trace ethane and a relatively lower non hydrocarbon gas content. It is dry gas from the pyrolysis of overmature oil and bitumen. The gas in the Dengying Fm has a methane isotope value (δ¹³C₁) in the range of -32.43~-34.59‰, and an ethane isotope value (δ¹³C₂) in the range of -24.86~-28.02‰. In comparison, the gas in the Longwangmiao Fm has a methane isotope value (δ¹³C₁) in the range of -33.36~-36.72‰, and an ethane isotope value (δ¹³C₂) in the range of -30.27~-32.51‰. Theirδ¹³C₁ values are almost the same, but theδ¹³C₂ value of gas in the Dengying Fm is obviously greater than that in the Longwangmiao Fm, resulting from the mixing of gas generated through the pyrolysis of overmature oil and bitumen. The sources and genesis of gas were determined based on the geochemical analysis of 26 core samples. The gas in both the Dengying and Longwangmiao Fms is of organic origin. The gas in the Longwangmiao Fm was sourced from shale in the underlying Lower Cambrian Qiongzhusi Fm. In contrast, the gas in the Dengying Fm was sourced from both the shale in the Qiongzhusi Fm and the dark shale in the 3^rd member of the Dengying Fm, with some contribution from the pyrolysis of bitumen in the pores, caverns and fractures of the 4th member of the Dengying Fm.