Typical coalbed methane (CBM) enrichment and production modes under the control of regional structure and evolution
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摘要: 煤层气勘探开发对改善我国能源结构、解决煤矿灾害与生态环境问题意义重大。结合我国区域构造与演化过程及煤层气地质条件,将煤层气富集产气模式划分为4种主要类型:构造简单裂隙系统、褶皱系统(较浅向斜轴部、褶皱翼部、次级构造高部位)、冲断构造系统(褶皱冲断带、高陡冲断构造)和构造叠加系统模式。其中,(1) 构造简单裂隙系统模式发育在构造相对稳定的地区,煤层气以深成热成因为主,也可受岩浆热接触作用影响,在裂隙中等发育区形成高产富集区。(2) 褶皱系统模式中,较浅向斜轴部挤压应力利于煤层气保存从而富集产气;褶皱翼部压力分布均匀,封闭性较好,其含气量与渗透率匹配适中形成富集高产带;次级构造高部位模式主要是在构造作用下形成的伴生构造(背斜、鼻隆构造、断块等)高部位形成构造圈闭,生成“气顶”。(3) 冲断构造系统模式中,褶皱冲断带模式中逆冲断层阻止了煤层气的逸散,在靠近逆冲断层的相对构造高点富集产气;高陡冲断构造模式发育在复杂断裂区,深部煤层气在一定温压作用下,解吸游离至上部地层,重新被吸附或部分仍呈游离状态而富集产气。(4) 构造叠加系统模式形成于受多期构造活动共同作用的煤储层中,不同的应力方向和机制引起的构造叠加使含气量和渗透率相匹配,煤层气富集且有一定产量。因此,区域构造特征控制着煤层气富集产气的每个模式,构造演化过程决定了富集产气的不同模式。这些富集产气模式对系统认识中国煤层气富集规律,指导“十四五”煤层气勘探开发具有重要意义。Abstract: Coalbed methane (CBM) exploration and development is of great significance to optimize China’s energy structure and cope with coal mine disasters and ecological environment problems. In this paper, combined with regional structure and evolution and CBM geological conditions, the enrichment and recovery modes of CBM are innovatively divided into four categories: simple structural fracture system, fold system (shallower synclinal shaft, fold limb, secondary structural high position), thrust system (fault-thrust-fold belt, high-steep thrust structure) and structural superposition system. Among them, simple structural fracture system mode is often encountered in the areas with relatively stable structure, where the CBM is mainly derived from deep thermogenesis, and can also be reformed by magma thermal contact, forming high-yield enrichment zones with moderately developed fracture. In the fold system mode, the extrusion stress at the shallower synclinal shaft is conducive to the preservation of CBM. The fold limbs with uniform pressure distribution and superior sealing performance are compatible with both gas content and permeability, thus forming high-yield enrichment zones. The secondary structural high position mode refers to the structural traps formed in the high part of the associated structures (anticlines, nose-shaped uplift, fault blocks, etc.) under compression, which generate “gas cup”. In the fault-thrust-fold belt mode, the thrust fault prevents the escape of CBM, and captures gas at the relative structural high position close to the thrust fault. The high-steep thrust structure mode is developed in the complex fault area. Subjected to the specific temperature and pressure fields, the desorbed deep CBM migrates to the upper strata, and is re-adsorbed or partially still in a free state and accumulated. The structural superposition system mode, in contrast to single main control mode, is formed in the coal reservoirs affected by multi-stage tectonic movements. The structural superposition caused by different stress directions and mechanisms superposition achieves optimal matching of gas content and permeability, which is also favorable for CBM enrichment. Therefore, regional structural characteristics control the mode of CBM enrichment and recovery, which is fundamentally determined by tectonic evolution. Conclusions drawn in this study are of great reference value to systematically understanding the law of CBM enrichment in China and guiding the exploration and exploitation of CBM during the 14th Five-Year Plan.
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表 1 煤层气富集产气模式及机理
Table 1 Mode and mechanism of CBM enrichment and production
煤层气富集产气模式 构造形态 典型区块 富集产气机理 构造简单裂隙
模式鄂尔多斯盆地东缘大宁−吉县区块 煤层含气性、渗透性、保存条件均适中。尽管裂隙发育,但上覆盖层封闭,含气量与渗透率相匹配 褶皱
模式较浅向斜轴部 沁水盆地南部潘庄、潘河区块;贵州西部六盘水矿区 仅适合煤层埋深较浅地区。压应力有利于煤层气吸附,两翼地下水补给形成水力封闭,富气的同时渗透率无明显减小 褶皱翼部 沁水盆地南部樊庄区块;鄂尔多斯盆地东缘延川南区块 受剪切应力作用,整体压力分布均匀,同时受到褶皱翼部至向斜轴部的水力封堵,封闭性较好,含气量和渗透率匹配较好 次级构造
高部位沁水盆地南部郑庄区块;鄂尔多斯盆地东缘延川南区块 局部应力拉张,裂隙发育使渗透率明显增加。储层压力下降,煤层气饱和度增高。部分吸附气转换为游离气向构造高点运移富集 冲断构造
模式褶皱冲断带 准噶尔盆地南缘头屯河−乌鲁木齐河区块 逆冲断层以煤层韧性变形为主,阻止了煤层气逸散并使煤层气吸附能力增强。深部的煤层气向浅部运移,在靠近断层的相对构造高点富集 高陡冲断带 准噶尔盆地南缘阜康区块 超深部、深部煤层气在一定温度作用下,吸附气转变为游离气,向中深部煤层运移。仰起端地层倾角大,逆断层同煤层顶底板及煤层火烧区滞流段的水力封堵作用共同抑制了煤层气继续向外界逸散,从而富集并产气 构造叠加模式 华北克拉通盆地南部淮南、淮北矿区 煤层早期受逆冲推覆向深部就位,生气的同时封闭性较好。后期岩浆岩侵入,生烃的同时改善了储层的渗透率。煤层早期韧性变形与后期脆性变形叠加,渗透率和含气量匹配较好 -
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