川渝黔地区五峰组–龙马溪组页岩储层地质特征差异研究

Geological Characteristics Difference Study of Wufeng-Longmaxi Shale Formations from Sichuan, Chongqing and Guizhou Regions

上奥陶统五峰组–下志留统龙马溪组广泛分布于四川、重庆、湖北、贵州等中上扬子地区,是目前我国页岩气最主要的勘探开发地层。通过对比川渝黔地区五峰组–龙马溪组页岩储层地质背景、地球物理测井及岩心测试结果,分析了川渝黔地区五峰组–龙马溪组页岩地质特征、源岩品质、物性、测井响应及构造特征的差异及其原因,探索四川盆地盆内和盆外页岩气有效开发对策。结果表明:四川盆地五峰组–龙马溪组页岩厚度大,优质厚度达30米,而盆外地区页岩厚度逐渐减薄。页岩有机质含量以大于1%为主;涪陵地区有机碳含量最高,平均达2%以上。正安、涪陵、南川三个区块页岩孔隙度高频为2%~4%,但正安区块页岩的低频分布主要小于2%,而涪陵地区页岩孔隙度低频分布为5.5%~7.5%。涪陵地区页岩含气性最高,解析气分布主频约2 m3/t。正安、凤冈页岩的脆性指数高,达到60%以上,可能与陆源碎屑供给有关。地球物理测井响应方面,四川盆地内部页岩呈现高自然伽马、声波时差大、密度小,含气量高,盆外的正安、凤冈地区页岩则呈现电阻率高的特点,且含气量相对较低。四川盆地内部页岩地层普遍为“背斜”型正向构造,盆外则多为“向斜型”负向构造形态。盆外页岩地层产状陡,黔北地区龙马溪页岩地层倾角达20˚以上。通过对比分析,认为盆外页岩气勘探开发面临的主要问题是如何“效益开发”。“降本增效、提产提效”是四川盆地外部页岩气产业发展的攻关方向。

The Upper Ordovician Wufeng-Lower Silurian Longmaxi Formation, widely distributed in Sichuan, Chongqing, Hubei, Guizhou and other middle and upper Yangtze regions, is the main shale gas exploration and development strata in China. By comparing the geological background, geophysical and core testing results of Wufeng-Longmaxi shale formation from Sichuan, Chongqing, and Guizhou regions, the geological characteristics, source rock quality, physical properties, logging response, and tectonic characteristics of shales in Wufeng-Longmaxi Formation from these three regions are analyzed. Then the effective development strategies of shales from Sichuan Basin and out of Sichuan Basin are proposed. Results show that the thickness of high-quality Wufeng-Long- maxi Formation in Sichuan Basin is high and up to 30 meters, while the total thickness significantly decreases out of Sichuan Basin. The total organic carbon content of these shales is commonly higher than 1%. Shales in Fuling block have the highest organic carbon content with an average value of being more than 2%. Main porosity in Zheng’an, Fuling and Nanchuan blocks is between 2% and 4%. Many shales in Zheng’an block have porosity less than 2%, while some shales in Fuling block have porosity of about 5.5%~7.5%. Shales in Fuling block have the highest gas content, with the dominant gas content of about 2 m3/t. The brittleness index of Zheng’an and Fenggang shales is higher than 60%, which may be related to the supplement of terrigenous detrital. In terms of geophysical logging responses, shales in Sichuan Basin have high natural gamma ray, large acoustic time difference, low density and high gas content, while shales in Zheng’an and Fenggang areas, outside the basin, have high resistivity and relatively low gas content. Shale formations in the Sichuan Basin generally have positive anticlinal structure, while the structure of shale formations outside Sichuan basin is mostly negative synclinal. Longmaxi shale formation in northern Guizhou region has a dip angle of over 20˚. Through comprehensively comparative analysis, it is concluded that the main problem of shales outside Sichuan basin is how to develop shale gas with benefit. Reducing cost and improving production are the key direction for the development of shale gas industry outside of Sichuan Basin.