四川盆地外复杂构造区页岩气地质条件及含气性特征:以湘西北五峰组—龙马溪组为例

Geological conditions and gas-bearing characteristics of shale gas in complex structure area out of Sichuan basin:a case of Wufeng—Longmaxi formation in Northwestern Hunan,China

以永页2井页岩气地质条件分析为基础,结合其他钻井及前人成果,对比国内外页岩气田地质特征,系统研究四川盆地外复杂构造区湘西北五峰组—龙马溪组页岩气地质特征及含气性,指出未来勘探方向。研究结果表明湘西北五峰组—龙马溪组与四川盆地涪陵地区的页岩气聚集条件相近,具有4个显著特征:1)深水陆棚相发育富有机质页岩厚度超过20 m,富含有机质、硅质、黄铁矿以及笔石化石;2)有机质属于Ⅰ型干酪根,有机碳质量分数w(TOC)普遍大于2.0%,自西向东逐渐减小,镜体反射率Ro主要在2.0%~3.0%之间,热演化达到过成熟阶段;3)矿物成分以石英为主,平均质量分数为47.9%,黏土矿物(主要是伊/蒙混层)次之,平均质量分数为26.9%,碳酸盐岩矿物质量分数低于10%,页岩普遍发育微孔隙(主要为有机质孔、晶间孔、溶蚀孔)和微裂缝,储层吸附能力强;4)五峰组—龙马溪组页岩含气量为1.0~3.5 m3/t,以解吸气为主,甲烷体积分数超过90%。深水陆棚相发育丰富有机质,奠定了五峰组—龙马溪组页岩气聚集基础,但含气性受控于盖层、埋深及断裂等保存条件。未来勘探方向应集中于桑植—石门复向斜内龙山—永顺一带向斜核部。

Based on the analysis of the geological conditions of shale gas in Yongye 2 Well,combined with other drilling wells and predecessors’results,and compared with successfully developed shale gas fields at home and abroad,shale gas geological characteristics and gas-bearing of Wufeng-Longmaxi formation in complex structure area out of Sichuan basin were comprehensively researched,and future exploration direction was also pointed out.The results show that shale gas accumulation conditions of Wufeng-Longmaxi formation in the northwest Hunan are similar to those in Fuling block of Sichuan Basin,and there are four apparent features.1)Under deep-water shelf facies,the thickness of shale is more than 20 m,which is rich in organic matter,siliceous,pyrite and graptolite.2)Organic matter belongs toⅠtype kerogens.The mass fraction of TOC is generally greater than 2.0%,and decreases from west to east gradually.Ro is mainly between 2.0%and 3.0%,and thermal evolution reaches over-mature stage.3)Mineral composition is mainly quartz,with the average mass fraction of 47.9%.Clay minerals(mainly I/S mixed layer)are secondary with average mass fraction of 26.9%,but carbonate minerals content is less than 10%.Micropores(mainly organic pores,intercrystalline pores,dissolved pores)and micro-cracks develop generally and adsorbed capacity is strong.4)Gas-bearing content of Wufeng-Longmaxi formation is 1.0?3.5 m3/t,adsorbed gas is dominant and methane volume fraction is more than 90%.Based on comprehensive study,deep-water shelf develops rich organic matter,which is the base of shale gas accumulation of Wufeng-Longmaxi formation.However,gas content depends on preservation conditions,such as capping layer,buried depth and fracture.The future exploration direction should be focused on the syncline core from Longshan to Yongshun of Sangzhi-Shimen Synclinorium.