四川盆地西部二叠系火山作用特征与天然气勘探潜力

过去一直都认为四川盆地西部(以下简称川西地区)火山岩不发育,因而制约了对该类型气藏的勘探。最近,中石化风险探井——YS1井在川西地区二叠系钻遇厚约300 m的火山岩,储层物性好、气显示强烈。为了评价该区火山岩气藏的勘探前景,基于对已钻探井的新认识,结合构造演化及其对火山活动控制作用的分析成果,对该区二叠系火山岩的形成发育特征、储层特征、天然气成因及成藏组合等开展了研究,确定了火山喷发机制、火山机构分布模式及火山岩气藏成藏模式,并评价了天然气勘探潜力。研究结果表明:①该区二叠系火山岩发育,具有与峨眉山玄武岩一致的构造环境,是峨眉山地幔热柱的产物;②火山岩受垂向断裂的控制,发育多个火山机构,存在着多个喷发旋回,火山岩纵向叠置、横向连片分布;③该区的火山岩岩性、岩相及储层特征与川西南地区存在着一定的差异,前者以爆发相凝灰岩为主,存在着部分溢流相玄武岩,火山岩储集空间类型以角砾之间的骨架孔隙、溶蚀孔隙及基质微孔隙为主。结论认为,围绕该区火山岩储层段发育有多套有效烃源岩,并且与前者构成了有效的天然气成藏组合,具有较好的天然气勘探前景,是一个值得重视的新领域。

Characteristics of Permian volcanism in the western Sichuan Basin and its natural gas exploration potential

It was previously deemed that volcanic rocks in the western Sichuan Basin were not developed, which restricted the exploration of volcanic gas reservoirs in this area. Recently, however, Sinopec's wildcat well, Well YS1 encounters Upper Permian volcanic rocks about 300 m thick in the western Sichuan Basin, the reservoir physical properties of which are good with strong gas shows. In order to evaluate the exploration prospect of volcanic gas reservoirs in western Sichuan Basin, we studied the Permian volcanic rocks in this area from the aspects of formation and development characteristics, reservoir characteristics, gas genesis and plays based on the new understandings on exploration wells, combined with the analysis on tectonic evolution and its controlling effect on volcanism. Then, the volcanic eruption mechanism, the distribution mode of volcanic edifice and the hydrocarbon accumulation pattern of volcanic gas reservoir were figured out. Finally, the exploration potential of natural gas was evaluated. And the following research results were obtained. First, the Permian volcanic rocks in western Sichuan Basin are developed and its tectonic setting is consistent with that of Emeishan basalt, which is the product of the Emeishan mantle plume. Second, under the control of vertical faults, a great number of volcanic edifices are developed and there are many eruption cycles. Volcanic rocks are superimposed vertically and connected laterally. Third, the volcanic rocks in western Sichuan Basin are different to some extent from those in southwestern Sichuan Basin in terms of lithology, lithofacies and reservoir characteristics. In the western Sichuan Basin, the tuff of explosive facies is dominant with some basalt of effusive facies, and the reservoir space of volcanic rocks is mainly acted by inter-breccia skeleton pores, dissolved pores and matrix micro-pores. In conclusion, multiple sets of effective source rocks are developed around the volcanic intervals in western Sichuan Basin, and together they constitute effective natural gas plays. Therefore, it has a better natural gas exploration prospect, and it is a new field worthy of attention.