黔江区块龙马溪组页岩气保存条件研究

Shale Gas Preservation Condition of Longmaxi Formation in Qianjiang

黔江区块位于川东南主要页岩气富集区,整体勘探程度较低。通过地质-地球物理技术,结合成藏分析、构造特征、盖层条件、地层水特征及天然气组分等因素,对工区龙马溪组页岩气保存条件进行探讨。结果表明:后期的构造作用是页岩气散失的根本原因,体现在大、小尺度断裂的沟通性、直接盖层的致密性和区域盖层的年代性3个方面;区内构造主要以NE—NNW断裂为主,呈现"东西分异、南北分块"的分布模式,东南斜坡区相对宽缓,断层及裂缝发育较少,埋深适中,盖层条件相对较好;天然气组分及地层水特征表明,优良的盖层及构造条件可以有效减缓页岩气散失和地层水侵入。在上述研究的基础上,建立了页岩气保存条件评价指标,并对研究区保存条件相对有利区进行了优选,对盆外向斜区域的页岩气钻探具有一定借鉴意义。

Qianjiang block with a relatively low overall exploration level is considered as a major shale gas enrich- ment area in southeast Sichuan. The geology-geophysics technologies arc used to explore the shale gas preservation condition in the Longmaxi Formation by combining with hydrocarbon accumulation analysis, structure, cap-rock, formation water, natural gas composition, etc. Research indicates that the fundamental factor for the shale gas loss is the later tectonic movement, including the connection between faults with large and small scales, cap-rock seal- ing and regional eap-ruck age. The regional structure is dominated by the NE-NNW faults with a distribution pat- tern of ＂differentiation in east-west and block in north-south＂. The gentle southeastern slope area is characterized by less-developed fault and fractures, medium burial depth and high-quality cap-rock. Natural gas composition and formation water properties demonstrates that high-quality cap-rock and structural condition could effective reduce the shale gas loss and formation water invasion. A set of shale gas preservation indexes is established based on the above understanding to optimize the favorable preservation zones, which could provide certain reference for the shale gas drilling and exploration in the extra-basinal syncline zones.