Geological Characteristics, Sweet Spot Evaluation Issues and Technical Countermeasures of Deep Longmaxi Formation Shale Gas in South Sichuan Basin
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摘要: 针对川南龙马溪组深层页岩气开发工程问题频发、建产不及预期等难题，通过地质特征分析、开发问题梳理与成因探讨，探索深层页岩气甜点评价技术对策.研究表明，川南龙马溪组深层（埋深>3 500 m）页岩气储层具有沉积相变复杂、断层裂缝与微幅构造发育、地应力高且复杂、高温高流体压力、局部低电阻等地质特征，储层非均质性与工程复杂性远高于中深层.利用现有页岩气甜点评价技术优选出的深层甜点区，开发中出现局部资源品质差、水平井套管变形率高达53%、井间压窜普遍、单井产能与经济效益未达预期等问题，核心原因是现有评价体系未充分考虑深层地应力差大、天然裂缝发育、低电阻资源风险等深层特有的地质‒工程耦合问题.据此，提出建立地质‒工程一体化页岩气甜点高精度“透明地质体”、建立以“人造气藏”为核心的动态甜点风险评价体系、采用两次井网分步释放地应力的开发方式，通过资源、工程多要素精细刻画页岩储层特点，动态量化评估甜点开发风险，同时采用两次井网的开发方式，分步骤释放地应力以解决套变的问题.研究成果可为川南等深层页岩气甜点评价与开发风险防控提供技术对策参考.Abstract: To address the frequent engineering problems and lower-than-expected construction and production in the development of deep shale gas in the Longmaxi Formation of South Sichuan, this study explores technical countermeasures for deep shale gas sweet spot evaluation through analysis of geological characteristics, sorting out development challenges and discussing their genesis.Results show that deep shale gas reservoirs (buried depth>3 500 m) of the Longmaxi Formation in South Sichuan are characterized by complex sedimentary facies changes, well-developed faults, fractures and micro-amplitude structures, high and complex in-situ stress, high temperature and high fluid pressure, and local low resistivity.The reservoir heterogeneity and engineering complexity are much higher than those of mid-deep layers. Deep sweet spots optimized by existing shale gas sweet spot evaluation technologies have encountered problems in development, including poor local resource quality, horizontal well casing deformation rate as high as 53%, widespread inter-well fracturing communication, and unsatisfied single-well productivity and economic benefits.The core reason is that the current evaluation system insufficiently considers the unique geological-engineering coupling problems of deep reservoirs, such as large in-situ stress difference, well-developed natural fractures, and low-resistivity resource risks. Accordingly, in this paper it proposes to establish a high-precision "transparent geological body" for shale gas sweet spots through geology-engineering integration, build a dynamic sweet spot risk evaluation system centered on "artificial gas reservoirs", and adopt a development mode of two-step well pattern deployment to release in-situ stress step by step. The shale reservoirs are finely characterized by multiple factors of resources and engineering, and the risks of sweet spot development are dynamically and quantitatively evaluated. Meanwhile, the two-step well pattern development mode is adopted to release in-situ stress gradually to solve casing deformation.The research results can provide technical references for sweet spot evaluation and development risk prevention and control of deep shale gas in South Sichuan and other similar areas.
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图 1 川南地区龙马溪组沉积相分布（据施振生等（2020）有改动）
Fig. 1. Sedimentary facies distribution of the Longmaxi Formation in South Sichuan (modified from Shi et al., 2020)
图 2 泸州深层典型地震剖面及构造解释成果
Fig. 2. Typical seismic profile and structural interpretation of deep formations in Luzhou
图 3 断裂滑移和层理滑移套变模式(据金亦秋等（2024）)
Fig. 3. Schematic diagrams of casing deformation modes caused by fault slip and bedding slip (from Jin et al., 2024)
图 4 泸州深层泸203H62-1井水平井各段压裂施工压力与储层破裂压力关系对比
Fig. 4. Comparison between fracturing operation pressure and reservoir breakdown pressure in each horizontal section of Well Lu 203H62-1 in the deep Luzhou area
图 5 典型井龙马溪组底部页岩样品擦痕电阻率
Fig. 5. Scratch resistivity of shale samples from the bottom of the Longmaxi Formation in typical wells
图 6 电阻率与页岩含气性统计关系
Fig. 6. Statistical relationship between resistivity and gas-bearing property of shale
表 1 川南深层与中深层龙马溪组页岩气地质特征对比
Table 1. Comparison of geological characteristics of deep and mid-deep shale gas in Longmaxi Formation, South Sichuan basin
关键参数 深层页岩气田 中深层页岩气田 泸州 自贡 大足 长宁 威远 焦石坝 构造特征 川南低陡褶皱带，次级背斜/向斜构造 川南低缓褶皱带，次级向斜为主 渝西低缓褶皱带，蒲吕场向斜为主 川南低陡褶皱带，长宁背斜为主 川南低缓褶皱带，威远背斜为主 川东高陡褶皱带，焦石坝背斜为主 断裂特征 较发育，高角度正断层为主 不发育，局部小型平移断层 不发育，局部小型逆断层 较发育，高角度逆断层为主 较发育，高角度正断层为主 发育，高角度逆断层为主 地层埋深(m) 3 800~5 000 3 500~4 500 3 800~5 000 2 800~3 500 2 500~3 500 2 300~3 500 优质页岩厚度（TOC>3%, 即Ⅰ类储层厚度）（m） 10~18 7~14 2~10 12~21 5~16 18~27 TOC含量(%) 2.0~5.0（平均3.5） 1.5~4.0（平均2.8） 1.8~4.5（平均3.2） 2.2~5.5（平均3.8） 1.9~4.8（平均3.3） 2.5~6.0（平均4.2） 孔隙度(%) 3.0~6.0（平均4.5） 2.5~5.0（平均3.8） 2.8~5.5（平均4.2） 3.2~6.5（平均4.8） 3.0~6.0（平均4.5） 3.5~7.0（平均5.2） 含气饱和度(%) 60~80 55~75 58~78 62~82 60~80 65~85 含气量(m3/t) 3~10 3~8 2~8 2~9 2~10 2~10 地层压力（MPa） 40~70 35~65 38~72 32~62 34~64 38~75 地层温度(℃) 120~160 110~150 115~155 105~145 108~148 115~160 压力系数 1.8~2.2 1.8~2.0 1.8~1.9 1.3~2.0 1.2~2.0 1.55 储量丰度(108m3/km2) 6~10 6~8 5~7 6~10 6~8 6~10 杨氏模量 30~40 28~38 29~39 32~42 30~40 22~25 泊松比 0.16~0.22 0.19~0.27 0.20~0.31 0.20~0.24 0.19~0.27 0.24~0.27 脆性指数 50~70 45~65 48~68 55~75 52~72 58~78 水平最大主应力（MPa） 95~115 87~115 99~106 65~85 86~98 65~80 水平最小主应力（MPa） 83~99 78~99 79~87 45~60 73~83 52~68 垂向主应力（MPa） 90~108 92~111 99~103 52~60 81~90 67~75 主应力差（MPa） 15~22 10~17 18~20 9~15 12~15 5~12 地应力机制 走滑‒挤压型 走滑‒挤压型 挤压‒走滑型 挤压型 走滑‒挤压型 挤压型 
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表 2 四川盆地五峰组-龙马溪组页岩储层分类标准
Table 2. Classification criteria for shale reservoirs of the Wufeng-Longmaxi formations in the Sichuan basin
参数 Ⅰ类储层 Ⅱ类储层 Ⅲ类储层 TOC含量(%) ≥3 2~3 1~2 孔隙度(%) ≥5 3~5 1~3 脆性指数(%) ≥55 45~55 30~45 含气量(m³/t) ≥3 2~3 1~2 注：据马新华（2018）；张成林等（2019）.
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表 3 长宁—威远示范区页岩气选区评价指标体系
Table 3. Evaluation index system for shale gas target selection in the Changning-Weiyuan demonstration area
评价参数 一类区 二类区 三类区 沉积环境 沉积微相 深水硅质泥棚 深水‒半深水粘土质泥棚 半深水‒浅水陆棚 古地貌 深水洼陷区 深水‒半深水斜坡区 浅水区/古隆起 U/Th>1.25厚度（m） >4 4~2 <2 储层条件 Ro(%) 2.5~3.0 3.0~3.5 >3.5 Ⅰ类储层连续厚度（m） ≥10 10~5 <5 保存条件 构造保存条件 稳定区 较稳定区 改造区 气源类型 油裂解气为主 油裂解/干酪根裂解气 干酪根裂解气为主 压力系数 ≥1.2 1.2~1.0 <1.0 距剥蚀线距离（km） >10 10~8 <8 距I级断层距离(km) >1.5 1.5~0.7 <0.7 注：据张鉴等（2016）; 雍锐等（2020, 2024).
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