Pressure evolution of shale gas reservoirs in Wufeng-Longmaxi formations, Lintanchang area, southeast Sichuan Basin and its geological significance
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摘要: 川东南林滩场地区的上奥陶统五峰组—下志留统龙马溪组具备较好的勘探开发条件,查明该层系古流体压力演化对揭示页岩气成藏及逸散过程具有重要意义。以林滩场地区五峰组—龙马溪组页岩构造裂缝和流体超压裂缝脉体为研究对象,综合阴极发光观察、包裹体显微测温、激光拉曼分析及盆地模拟等技术手段查明该地区的古流体压力演化过程及其控制作用。研究表明:(1)林滩场地区经历了常压、微超压—常压、超压、泄压4个阶段,其中超压主要来源于生烃作用,泄压主要由页岩气逸散导致,且使气藏泄压54%。(2)五峰组—龙马溪组黑色页岩层系底部的裂缝存在两期脉体充填。第一期形成于沉降埋深阶段,时间在131~118 Ma,形成温度介于178~205 ℃,甲烷包裹体捕获压力为105.6~119.8 MPa;第二期形成于构造抬升阶段,时间在25~18 Ma,形成温度介于150~175 ℃,甲烷包裹体具有相对较低的盐度,其捕获压力介于80.8~92.1 MPa,较低的压力系数(1.37~1.49)指示气藏已发生大量逸散。(3)晚期构造运动,特别是喜马拉雅晚期的快速抬升是造成气藏逸散和泄压的根本原因。有机孔的圆度减弱、孔径减小表明储层物性变差。由于气藏形成和保存时间长,林滩场地区五峰组—龙马溪组页岩仍具备较好的勘探潜力。该研究有助于深化林滩场常压页岩气藏富集规律的认识,可为该地区页岩气勘探有利区的优选提供一定的理论支撑。Abstract: The Upper Ordovician Wufeng Formation-Lower Silurian Longmaxi Formation in Lintanchang area of the southeastern Sichuan Basin has good exploration and development conditions, and clarifying the paleo-fluid pressure evolution is of great significance for revealing the process of shale gas accumulation and fugitive emission. Taking the shale tectonic fractures and fluid overpressure fracture veins of the Wufeng-Longmaxi formations in Lintanchang area as the research object, the paleo-fluid pressure evolution process in this area and its controlling impact were investigated using cathodoluminescence, inclusion temperature measurement, laser Raman analysis, and basin simulation. The study shows that: (1) Lintanchang area has experienced four stages: normal pressure, mild overpressure to normal pressure, overpressure, and pressure relief. Overpressure is mainly due to hydrocarbon generation. Pressure relief is mainly caused by shale gas fugitive emission. The gas reservoir pressure relief reaches 54% of the initial pressure during the pressure relief stage. (2) There are two stages of vein filling in the fractures at the bottom of the black shale in the Wufeng-Longmaxi formations. The first stage is formed in the sedimentation and burial stage at 131-118 Ma, the temperature is 178-205 ℃, and the trapping pressure of methane inclusions is 105.6-119.8 MPa. The second stage is formed in the tectonic uplift stage at 25-18 Ma, the temperature is 150-175 ℃, the methane inclusions have relatively low salinity and its trapping pressure is 80.8-92.1 MPa. The low pressure coefficient (1.37-1.49) indicates that mass fugitive emission has occurred. (3) The late tectonic movement, especially the rapid uplift in the late Himalayan period, is the root cause of the fugitive emission and pressure relief of the gas reservoirs. The decrease of the roundness and pore size of the organic pores indicates the deterioration of reservoir physical properties. However, due to the long formation and preservation time of the gas reservoirs, the shale in the Wufeng-Longmaxi formations in Lintanchang area still has good exploration potential. This study is helpful to deepen the understanding of the accumulation mechanism of the normal-pressure shale gas reservoirs in Lintanchang area and can provide theoretical guidance for optimal selection of favorable shale gas exploration areas.
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图 3 川东南林滩场地区LY3井页岩岩心裂缝脉体特征
a.水平层理缝充填块状方解石,五峰组,4 133.9 m;b.水平层理缝充填的方解石呈网状,五峰组,4 134.1 m;c.网状裂缝,岩心具揉皱破碎现象,龙马溪组一段,4 133.5 m;d.层间滑移缝充填方解石和黄铁矿,龙马溪组一段,4 074.7 m;e.高角度剪切缝充填方解石,龙马溪组一段,4 170.0 m;f.水平流体超压缝充填方解石,龙马溪组一段,4 129.8 m。
Figure 3. Characteristics of fracture veins of shale cores of well LY3 in Lintanchang area, southeastern Sichuan Basin
图 8 川东南林滩场地区LY3井五峰组—龙马溪组一段页岩有机孔形态扫描电镜照片
a.不规则角状、椭圆状的有机孔,4 132.34 m;b.狭缝状、扁平椭圆状有机孔,4 130.7 m;c.不规则角状有机孔,部分孔隙相互结合,连通性变差,4 125.91 m;d.孔隙网络提取结果,4 130.7 m。
Figure 8. FE-SEM observations of shale organic pore morphology in the Wufeng Formation - the 1st member of Longmaxi Formation of well LY3 in Lintanchang area, southeastern Sichuan Basin
表 1 川东南林滩场地区甲烷包裹体捕获压力计算结果
Table 1. Calculation results of trapping pressure of methane inclusions in Lintanchang area, southeastern Sichuan Basin
样品编号 测点数 ν1/cm-1 ρ/(g/cm3) 同期盐水包裹体均一温度/℃ 捕获压力/MPa 范围 平均 BG1 5 2 911.871~2 911.062 0.268~0.276 183~208 198 107.4~116.1 BG2 3 2 910.926~2 910.984 0.271~0.274 184~203 196 109.8~112.5 BG3 4 2 910.743~2 911.045 0.266~0.269 182~205 193 106.7~109.1 BG4 4 2 911.281~2 911.602 0.267~0.282 180~194 196 110.7~119.8 BG5 3 2 911.278~2 912.516 0.244~0.256 150~184 163 80.7~88.7 BG6 3 2 911.203~2 911.381 0.251~0.259 165~179 165 85.8~92.4 BG7 4 2 911.267~2 911.418 0.249~0.256 155~177 163 83.0~88.2 表 2 川东南林滩场地区LY3井聚焦离子束扫描电镜提取的孔隙参数
Table 2. Pore parameters extracted by FIB-SEM observations of well LY3 in Lintanchang area, southeastern Sichuan Basin
深度/m 有机质/% 孔隙度/% 有机质内孔隙度/% 连通孔隙度/% 孔隙连通率/% 4 132.34 65.3 1.06 3.72 2.09 56.2 4 130.70 41.8 2.87 4.76 2.08 43.7 4 125.91 53.4 1.43 7.05 5.28 74.9 -
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