西湖凹陷中央反转带天然气地球化学特征及其成因探讨

Geochemical Characteristics and Origin of Natural Gas in the Central Inversion Zone of Xihu Depression

西湖凹陷中央反转带发现的一批大中型油气田,是西湖凹陷重要天然气产区之一,该区含油气层系主要为渐新统花港组。地球化学特征研究表明,中央反转带天然气以甲烷为主,重烃含量较低,南部天台构造带、黄岩构造带的天然气主要为湿气,北部宁波构造带的天然气主要为干气。甲烷碳同位素δ13C1值主体介于-40. 11‰～-22. 6‰,整体上表现为南轻北重,烷烃气碳同位素系列在不同构造带均存在不同程度的倒转。天然气成因分析结果表明,天台构造带天然气主体为干酪根裂解气,混有部分原油裂解气,天然气处于成熟阶段;同时,黄岩构造带和宁波构造带天然气主要为不同期次干酪根裂解气混合气,其中宁波构造带天然气成熟度较高,处于高成熟—过成熟阶段。

A number of mid-large oil and gas fields have been found in the central inversion zone which is one of the most important natural gas producing areas in Xihu depression,and the Huagang Formation is the main oil-bearing series. The geochemical characteristics show that the natural gas in the central inversion zone is mainly composed of methane,while the heavy hydrocarbon gas content is relatively low. The natural gas in Tiantai tectonic belt and Huangyan tectonic belt is mainly wet gas,while it appears to be dry gas in Ningbo tectonic zone. The carbon isotope of methane δ13C1 is between-40. 11‰ ^-22. 6‰,so the overall performance is the north heavier than the south. In different tectonic belt,some gas samples display partial reversal of the carbon isotope. The identification of gas origin indicates that the natural gases in Tiantai tectonic belt are mainly composed of kerogen cracking gas and mixed with the oil cracking gas,at the mature stage. The natural gas in Huangyan tectonic belt and Ningbo tectonic belt is mainly composed by kerogen cracking gas mixture of different stages. The maturity of the natural gas in Ningbo structural belt is in high-over mature stage.