中扬子宜昌地区寒武系水井沱组页岩含气性及影响因素

摘要

The Yichang area in the Middle Yangtze region is a new exploration target for shale gas outside the Sichuan Basin. The shale of the Cambrian Shuijingtuo Formation has an enormous gas resource potential in view of gas shows in most wells. Gas contents vary in different wells. We investigated the gas-bearing characteristics and controls of shale in the Cambrian Shuijingtuo Formation in the Yichang area integrating geological and geochemical data. The black rock series of the Shuijingtuo Formation in this area is mainly carbonaceous shale and gray shale, about 50-140 m thick. They have a high organic carbon content and are mainly of typeⅠ, generally thermally over-mature, showing a good hydrocarbon generation potential. The on-site measured gas content is 0.32-5.48 m3/t, and the cumulative thickness of shale with gas content>2 m3/t is 44.05 m, which reflects a good overall gas content of shale in this area. The gas-bearing capacity of the Shuijingtuo Formation shale is affected by various factors such as sedimentary facies, organic carbon content, mineral composition, porosity, extent of fracture and formation pressure. The black carbonaceous shale deposited in the continental shelf facies has higher gas content than the gray calcareous shale of the slope facies, while the limited platform facies has the lowest gas content. The gas content has a positive correlation with the organic carbon and quartz mineral contents, a weak negative correlation with the carbonate mineral content, and a poor correlation with the clay mineral content. Organic carbon content has a more significant control on gas content. Shale porosity and pore distribution variations are important factors leading to the differences in gas content. In addition, shale gas content is also closely related to reservoir fracture development and formation pressure.%中扬子宜昌地区作为南方海相页岩气勘探的有利目标区之一,多口井于寒武系水井沱组页岩中见页岩气显示,但含气性差异较大.基于区域地质背景与页岩气钻井资料,结合现场解析含气量测试数据,描述了宜昌地区水井沱组页岩气纵向分布特征,探讨了页岩气含量的主要影响因素.该区水井沱组黑色岩系主要为碳质页岩和灰质页岩,厚50～140 m,有机碳含量高,以Ⅰ型为主,处于过成熟热演化阶段,具良好的生烃潜力;现场解析含气量为0.32～5.48 m3/t,含气量大于2 m3/t的页岩累计厚度达到44.05 m,反映该区页岩具较好的整体含气性.水井沱组页岩含气性受沉积相带、有机碳含量、矿物组分、孔隙度、裂缝发育程度、地层压力等多种因素影响.陆棚相沉积的黑色碳质页岩含气量高于斜坡相灰色钙质页岩,而局限台地相泥灰岩含气量最低;含气量与页岩中有机碳含量、石英矿物含量均呈较好的正相关性,与碳酸盐矿物含量呈弱的负相关性,与黏土矿物含量关系不大,而有机碳含量对含气量的控制作用更为显著;页岩孔隙度大小和孔隙分布差异是导致含气量差异的重要因素,此外页岩含气量还与储层裂缝发育程度和地层压力密切相关.