Characteristics and Main Controlling Factors of Natural Fractures in Marine Shale in Luzhou Area, Sichuan Basin
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摘要: 天然裂缝的特征是评价页岩气富集和保存规律中重要的地质指标.以四川盆地南部泸州地区上奥陶统五峰组-下志留统龙马溪组页岩为研究对象,基于地震、测井、岩心、薄片、扫描电镜及分析化验资料,深入开展了天然裂缝成因类型、发育特征和主控因素的研究.结果表明:泸州地区五峰组-龙马溪组页岩裂缝按照地质成因可分为构造裂缝、成岩裂缝和异常高压裂缝3种类型,构造裂缝按照裂缝的力学性质和与岩石力学层的关系细分为穿层剪切裂缝、顺层剪切裂缝和层内张开裂缝,成岩裂缝分为页理缝和收缩裂缝.泸州地区构造裂缝和页理缝大量发育,收缩裂缝和异常高压裂缝发育程度较低.构造裂缝的分布和发育程度受到断层、褶皱、岩石力学层和脆性的控制,页理缝的发育主要受脆性、有机质含量和纹层的控制.Abstract: The characteristics of natural fractures are important geological indicators in evaluating the law of shale gas enrichment and preservation. The research object is the Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation shale in the Luzhou area of southern Sichuan Basin. The characteristics and controlling factors of different types of natural fractures in the study area were studied by using seismic, logging, core, thin sections, scanning electron microscopy, and analytical laboratory data. The research results show that the Wufeng-Longmaxi shale fractures in the Luzhou area can be divided into three types: structural fractures, diagenetic fractures and abnormally high pressure fractures according to their geological origin. According to the mechanical properties of the fracture and the relationship with the rock mechanics layer, structural fractures are dominated by transformational shear fractures, intraformational open fractures, and bed-parallel shear fractures. Diagenetic fractures include lamellation fractures and shrinkage fractures. In the Luzhou area, a large number of structural fractures and bedding fractures are developed, while shrinkage fractures and abnormally high pressure fractures are relatively low. The distribution and development of structural fractures are controlled by faults, folds, rock mechanics layer and brittleness, and the development of lamellation fractures is mainly controlled by brittleness, organic matter content and lamina types.
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图 2 泸州地区五峰组‒龙马溪组岩心天然裂缝发育特征
a. 穿层剪切裂缝,未充填,LU221井,4 086.20 m;b. 多期次裂缝交切现象,早期裂缝被充填,晚期裂缝未充填,LU221井,4 079.17 m;c. 层内张开裂缝,方解石充填,等间距平行分布并被黄铁矿条带限制,ZU203井,4 108.85 m;d. 顺层剪切裂缝,镜面特征,方解石充填,LU211井,4 928.69 m;e.顺层剪切裂缝,方解石和黄铁矿充填,LU207井,3 434.11 m;f. 收缩裂缝,方解石充填,LU203H57-3井,3 752.28 m;g. 页理缝,平行纹层分布,底部可见纤维状方解石,LU211井,4 926.04 m;h. 异常高压裂缝,方解石和沥青充填,LU206井,4 043.21 m;i. 顺层脉状裂缝,纤维状方解石充填,可见垂直缝合线,ZI216井,3 984.75 m
Fig. 2. Characteristics of natural fractures in the core of Wufeng Formation-Longmaxi Formation in Luzhou area
图 3 泸州地区龙五峰组‒马溪组薄片天然裂缝发育特征
a. 收缩裂缝,方解石充填,裂缝开度变化大,LU207井,3 464.21 m;b. 页理缝,沿岩石力学薄弱面分布,顶部见大量硅质放射虫,LU205井,4 038.98 m;c. 页理缝,充填纤维状方解石和有机质,LU206井,4 039.92 m;d. 异常高压裂缝,延伸较短,沥青和方解石充填,LU206井,4 043.21 m;e. 顺层脉状裂缝,纤维状方解石充填,底部见大量硅质放射虫,沿岩石力学薄弱面破裂形成,LU207井,3 464.73 m;f. 构造裂缝,见硅质放射虫,方解石和石英充填,LU206井,4 045.42 m
Fig. 3. Characteristics of natural fractures in thin sections of Wufeng Formation-Longmaxi Formation in Luzhou area
表 1 LU206井五峰组‒龙一段Ⅰ亚段岩相划分及主要裂缝类型
Table 1. Lithofacies and main fracture types of Wufeng Formation-Long 1st Member Ⅰ in Well LU206
分层 岩石矿物组分质量分数(%) TOC(%) 岩性 主要裂缝类型 裂缝
密度石英+长石+黄铁矿 碳酸盐矿物 粘土矿物 五峰组 54.80 26.20 19.00 2.03 含有机质硅质页岩 构造裂缝、收缩裂缝 高 1小层 52.50 33.00 14.50 4.68 富有机质硅质页岩 构造裂缝、页理缝、异常高压裂缝 高 2小层 62.12 17.00 20.88 4.28 富有机质硅质页岩 构造裂缝、页理缝、异常高压裂缝 高 3小层 56.50 13.00 30.50 3.17 含有机质硅质页岩 构造裂缝、页理缝 中 4小层 45.61 17.47 36.91 2.22 含有机质粘土质硅质混合岩 构造裂缝 低 -
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