Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China
Possible pore structure deformation effects on the shale gas enrichment: An example from the Lower Cambrian shales of the Eastern Upper Yangtze Platform, South China
复制标题
孔隙结构变形对页岩气富集的可能影响——以华南上扬子地台东部下寒武统页岩为例
DOI：
10.1016/j.coal.2019.103349
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发表时间：
2020-01-02
影响因子：
5.6
通讯作者：
Zhang, Cong
中科院分区：
文献类型：
--
作者：
Ma, Yong;Ardakani, Omid H.;Zhang, Cong
上扬子地台东部下寒武统富含有机质页岩的储集性能和储罐解吸实验表明，渝东北大巴山弧形褶皱冲断带(变形带)与冲东南微褶皱带(未变形带)的页岩气含量存在明显差异。利用扫描电子显微镜、低温氮气和二氧化碳吸附以及压汞毛细管压力等综合孔隙表征方法，对比研究了两个地区页岩气藏的孔隙大小变化，以及可能的微观孔隙结构对页岩气成藏的控制作用。下寒武统变形带(DZ)和未变形带(NDZ)的下寒武统页岩均沉积在深水陆架中，有机质丰度和热成熟度相似。在DZ样品中，有机质(OM)赋存的孔隙大多在纳米尺度范围内，在OM内部或在OM与矿物的界面上以微裂缝为主。相反，在NDZ样品中，OM寄主的中孔(2-50 nm)到大孔(>；50 nm)是主要的孔隙类型。在两个区域都有丰富的OM寄主微孔(<；2 nm)。氦离子显微镜观察进一步证实了在所研究的样品中存在以OM为主体的微孔。两个带的碳酸盐矿物中均含有丰富的赋矿孔隙，而NDZ样品中碳酸盐矿物周围的溶蚀边缘更为丰富，大巴山弧形褶皱冲断带发生在晚三叠世末，而下寒武统页岩已达到热成熟度高峰。DZ样品中OM寄主的中孔至大孔(>；50 nm)很可能在与构造挤压有关的构造变形中坍塌，而微孔则因其较小的尺寸而幸存于构造应力之下。OM赋存的微孔是DZ地区吸附天然气的主要储集空间。DZ中的微孔占主导地位，这些孔与基质孔之间缺乏联系，导致DZ样品中的气体含量较高。相反，NDZ中连通良好的OM赋存孔隙网络使天然气在岩石基质中更容易流动，最终导致该带在抬升和折返过程中大量泄漏天然气，并降低了天然气含量。研究结果表明，构造变形可能改变页岩的孔隙结构，进而改变页岩气含量，这对经历了复杂构造运动的中国南部地区的页岩气勘探开发具有重要意义。
Shale gas reservoir performance and canister desorption experiments of the Lower Cambrian organic-rich shales in the eastern Upper Yangtze Platform reveal a significant difference in shale gas content between the Dabashan arc-like fold-thrust belt in northeastern Chongqing (Deformed Zone) and the slightly folded area in southeastern Chong (Non-deformed Zone). Integrated pore characterization methods including scanning electron microscopy (SEM), low-temperature N-2 and CO2 adsorption, and mercury injection capillary pressure (MICP) analyses were comparatively conducted in both areas in order to examine shale gas reservoir pore size variations and thus the possible microscopic pore structure controls on shale gas enrichment.The Lower Cambrian shales in both Deformed Zone (DZ) and Non-deformed Zone (NDZ) were deposited in the deep-water shelf and show similar organic matter richness and thermal maturity. The majority of organic matter (OM)-hosted pores in DZ samples are in nanoscale size range with the dominance of micro-fractures within the OM or at the interface of OM and minerals. In contrast, OM-hosted meso-(2-50 nm) to macropores (> 50 nm) are the dominant pore types in the NDZ samples. OM-hosted micropores (< 2 nm) are abundant in both zones. Helium ion microscopy observations further confirm the presence of OM-hosted micropores in the studied samples. Mineral-hosted pores in carbonate minerals are abundant in both zones, while dissolution rims around carbonate minerals are more abundant in NDZ samples.The Dabashan arc-like fold-thrust belt took place by the end of the Late Triassic, while the Lower Cambrian shales have reached thermal maturity peak. OM-hosted meso-(2-50 nm) to macropores (> 50 nm) in DZ samples are most probably collapsed during structural deformation related to tectonic compression, while micropores due to their smaller size survived the tectonic stress. The OM-hosted micropores are the main storage space for adsorbed gas in the DZ area. The dominance of micro-pores in DZ and lack of connection between those pores and matrix pores led to higher gas content in DZ samples. On the contrary, the well-connected OM-hosted pore network in NDZ allows easier gas flow in the rock matrix that eventually led to significant gas leakage during uplift and exhumation and lower gas content in this zone. The results of this study suggest that structural deformation can potentially change the pore structure of shales and thus shale gas content which has major significance for shale gas exploration and development in south China where had experienced complex tectonic movements.