西湖凹陷北部气田天然气地球化学特征及气源探讨

东海盆地西湖凹陷具有丰富的天然气资源,但对于其天然气的来源却一直都存在着较大的争议,已有的研究成果多采用烃源岩成熟度进行气源对比,没有给出直接证据。为此,以近年来在该凹陷北部发现的大中型气田为研究对象,在分析区内天然气组分、碳同位素值、天然气轻烃组成的基础上,系统研究了该区天然气的地球化学特征及其成因类型,并结合烃源岩的热演化程度及干酪根碳同位素值,分析了天然气的来源、探讨了天然气的运移方式及通道。研究结果表明:①西湖凹陷北部渐新统花港组天然气主要为煤型气,由腐殖型干酪根初次裂解形成,其中中央反转构造带气田主要为干气,西次凹气田为湿气;②凹陷区烃源岩成熟度从南到北逐渐增加导致中央反转构造带天然气成熟度较高,西次凹相对较低;③西湖凹陷北部天然气主要来源于始新统平湖组煤系烃源岩,同时花港组烃源岩及平湖组下部烃源岩也有不同程度的贡献,其中中央反转构造带北部花港组煤系烃源岩成熟度较高,对该区天然气成藏的贡献较大;④凹陷区天然气具有垂向运移分馏的特征,龙井运动之后断裂再次活化,为平湖组及其下部天然气向上运移提供了有利的通道。

Geochemical characteristics and sources of natural gas in the northern Xihu Sag

The Xihu Sag in the East China Sea Basin is rich in natural gas resources, but the sources of its natural gas have always been controversial. The existing researches mostly adopt the source rock maturity for gas source correlation without any direct evidence. In this paper, the large- and medium-sized gas fields discovered in the northern Xihu sag in recent years were taken as the research objects. After compositions, carbon isotopes and light hydrocarbon compositions of the natural gas in this area were analyzed, the geochemical characteristics and genesis types of natural gas were studied systematically. Then, combined with the thermal evolution of source rocks and the carbon isotope of kerogen, the sources of natural gas were analyzed and its migration modes and pathways were discussed. And the following research results were obtained. First, the natural gas of Huagang Fm in the northern Xihu Sag is mainly coal-type gas, which is generated from humic kerogen via primary cracking. It is mainly dry gas in the central uplift and wet gas in the west subsag. Second, the maturity of the source rocks in this area gradually increases from the south to the north, resulting in a higher maturity of natural gas in the central uplift and a lower maturity of natural gas in the west subsag. Third, though the source rocks of the Huagang Fm and the lower Pinghu Fm have different contributions to the natural gas in the northern part of the Xihu Sag, the natural gas there is mainly derived from the coal-bearing source rocks of Pinghu Fm. Among them, the coal-bearing source rocks of the Huagang Fm in the central uplift are of higher maturity and contribute more to the natural gas in this area. Fourth, the natural gas in this area is characterized by vertical migration and fractionation. After the Longjing movement, the fault is reactivated and provides a favorable pathway for the upward migration of natural gas from Pinghu Fm and its lower formations.