四川盆地南缘昭通页岩气示范区构造变形特征及页岩气保存条件

四川盆地南缘昭通国家级页岩气示范区地处我国南方海相构造复杂区,页岩变形改造强烈、甜点控制因素复杂,保存条件对页岩气富集具有重要的影响。为了支撑该区页岩气甜点区的优选评价工作、提高钻探成功率,通过对野外露头的地质调查,利用钻井、录井和物性测试资料,分析了该示范区构造变形样式及区域变形特征,研究了地层节理及裂缝发育特点、页岩围岩封盖能力、气藏展布特征,探讨了上奥陶统五峰组—下志留统龙马溪组页岩气的保存条件。研究结果表明:①该区由南向北依次发育隔槽式、等幅式与隔档式等3种构造变形样式,依次分布黔中隆起剪切变形区、滇黔北坳陷压扭变形区与蜀南坳陷挤压变形区等3大区域;②五峰组—龙马溪组及其上覆地层发育与地层走向呈高角度、中—低角度及顺层相交关系的3类节理与裂缝,其产状特征与3大变形区基本一致;③五峰组—龙马溪组页岩北厚南薄,具备源储一体、自身封盖成藏的保存条件,外加上覆地层与顶底板的封盖能力,保存条件总体较好;④该区页岩气组分自北向南可划分为甲烷、甲烷+氮气混合、氮气等3个带,页岩气保存条件总体呈北好南差的格局。结论认为,该区中—北部(挤压变形区及其南缘)页岩厚度大、改造与变形弱、节理及裂缝顺层发育、封盖性能好、保存条件优越,为最有利区;中部滇黔北坳陷压扭变形区保存条件中等,为较有利区;南部黔中隆起剪切变形区保存条件差,为远景区。

Structural deformation characteristics and shale gas preservation conditions in the Zhaotong National Shale Gas Demonstration Area along the southern margin of the Sichuan Basin

The Zhaotong National Shale Gas Demonstration Area along the southern margin of the Sichuan Basin is located in the complex marine tectonic area of South China, where shale deformation and reformation are intense and the factors controlling sweet spots are complex, so the preservation conditions have an important impact on the enrichment of shale gas. In order to support the selection and evaluation of shale gas sweet spots in this area and improve the success rate of drilling, this paper carried out a geological survey on field outcrops. Then, based on drilling, mud logging and physical property test data, the structural deformation pattern and the regional deformation characteristics of this demonstration area were analyzed, and the development characteristics of formation joints and fractures, the sealing capacity of shale surrounding rock and the distribution characteristics of gas reservoirs were studied. Finally, the preservation conditions of shale gas in the Wufeng Formation of Upper Ordovician and the Longmaxi Formation of Lower Silurian were discussed. And the following research results were obtained. First, in the Zhaotong National Shale Gas Demonstration Area, three structural deformation patterns are developed from south to north, including trough type, equal amplitude type and baffle type, which are distributed in three major deformation zones, respectively, i.e., the shear deformation zone of Central Guizhou Uplift, the compression–torsion deformation zone of Northern Yunnan–Guizhou Depression and the compression deformation zone of Southern Sichuan Depression. Second, three types of joints and fractures whose relationships with the direction of strata are high angle, middle–low angle and bedding intersection are developed in the Wufeng Formation–Longmaxi Formation and its overlying strata, and their occurrence characteristics are basically consistent with those of the three major deformation zones. Third, the shale of Wufeng Formation–Longmaxi Formation is thick in the north and thin in the south and possesses the preservation conditions of source–reservoir integration and self-sealing hydrocarbon accumulation. And combined with the sealing ability of the overburden strata and the roof and floor, its preservation conditions are overall better. Fourth, from the perspective of shale gas component, this area can be divided into three belts, i.e., methane, methane + nitrogen mixture and nitrogen from north to south. And the preservation conditions of shale gas are generally better in the north and worse in the south. In conclusion, the shale in the central–northern part of Zhaotong National Shale Gas Demonstration Area (compression deformation area and its southern margin) is the most favorable area because of its large shale thickness, weak reformation and deformation, bedding development of joints and fractures, good sealing performance and excellent preservation conditions. The compression–torsion deformation zone of Northern Yunnan–Guizhou Depression in the central part is moderate in preservation conditions, and it is the relatively favorable area. The shear deformation zone of Central Guizhou Uplift in the southern part has poor preservation conditions, and it is a prospective area.