Caprock Type and Sealing Mechanism of Quaternary Ultra Shallow Large Gas Reservoir in Deep Water Area of Qiongdongnan Basin, China
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摘要: 盖层是决定油气能否成藏的重要因素，目前盖层研究多关注中深层固结岩石的封盖能力，而围绕浅层疏松未固结盖层的研究较少，对于深水浅埋地层中的盖层评价则更为少见.基于琼东南盆地深水区三维地震解释、三轴应力实验、测井评价、圆锥触探试验等研究成果，明确了L36区第四系乐东组浅埋气藏盖层的发育类型及封堵能力，总结了研究区盖层封盖机理.结果表明：（1）深水区乐东组发育深海泥、块体流、含水合物地层3类盖层，三者均可起到有效封盖作用；（2）综合地震‒地质解释及实验成果，明确了3类盖层封盖能力从小到大依次为深海泥、块体流、含水合物地层，其中深海泥在海底埋深200 m处可封盖59 m天然气，为研究区主要目的层盖层封堵下限.（3）深水浅埋盖层封堵机理占主导作用的是毛细管封闭、水力封闭，低温高压环境下饱和盐水封堵机制发挥了特色性增封作用.以上认识得到近两年钻探的验证，这不仅为琼东南盆地乐东组的后续勘探开发提供了理论依据，也可为其他深水浅埋地区的天然气勘探提供有益借鉴.Abstract: Caprock plays an important role in determining whether hydrocarbon accumulation is available. Previous studies of caprock mainly focus on the sealing ability of medium-deep consolidated rock, while less attention is paid to the shallow unconsolidated caprock, and the evaluation of caprock of shallow buried formations in deep water area is rare. Based on the research results of seismic interpretation, uniaxial stress test, logging evaluation, and cone penetration test in the deep water area of Qiongdongnan Basin, the development type and sealing ability of the caprock for the gas reservoir of the Ledong Formation in the L36 area are studied in this paper. The results show that: (1) There are three types of caprock (deep-sea mud, mass transport deposits, and hydrate-bearing strata) in the deep water area of the Ledong Formation, which could all have the effective sealing effect. (2) Based on the seismic and geological interpretation and experimental results, it is clear that the sealing capacity increases in the order of deep-sea mud, mass transport deposits, and hydrate-bearing strata; and the deep-sea mud can seal 59 m hydrocarbon at 200 m below seafloor, which is the threshold of sealing ability for the main target layer in the study area. (3) The capillary sealing and hydraulic sealing plays a dominant role in the sealing mechanism of shallow buried caprock in the deep water area, under low temperature and high pressure environment, the saturated salt water sealing mechanism plays a characteristic role in increasing sealing capacity. The above results have been verified by drilling in the past two years, which not only provides a theoretical basis for the follow-up exploration and development of the Ledong Formation in Qiongdongnan Basin, but also can provide a useful reference for the natural gas exploration in other shallow buried areas in the deep water area.
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Key words:
- Qiongdongnan Basin /
- deep water area /
- Quaternary /
- ultra shallow gas reservoir /
- caprock type /
- sealing mechanism /
- petroleum geology
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图 1 琼东南盆地构造区划（a）、地层综合柱状图（b）
Fig. 1. Structural division (a) and stratigraphic comprehensive histogram (b) of Qiongdongnan Basin
图 4 琼东南盆地西北方向块体流地震剖面及弧长属性
Fig. 4. Seismic profile and arc length attribute of MTDs from northwest direction in Qiongdongnan Basin
图 5 琼东南盆地西南方向块体流地震剖面（a）及振幅属性（b）
Fig. 5. Seismic profile (a) and amplitude attribute (b) of MTDs from southwest direction in Qiongdongnan Basin
图 6 L36-1a井乐三段泥岩盖层排替压力与盖层孔隙度、埋深、泥岩厚度的关系
Fig. 6. The relationship between the displacement pressure of mudstone caprock and the porosity, buried depth and mudstone thickness of the third member of Ledong Formation in Well L36-1a
图 7 基于CPT原位测试分析含水合物地层封盖能力
Fig. 7. Analysis of sealing ability of hydrate-bearing strata based on CPT in-situ test
图 8 毛细管失效（a）与水力破裂（b）模式
Fig. 8. Patterns of capillary failure (a) and hydraulic fracture (b)
图 9 琼东南地区深水浅埋盖层封盖机理
Fig. 9. Sealing mechanism of deep-water shallow-buried caprock in Qiongdongnan area
图 10 不同温度条件下盖层表面张力（a）、变温‒变压条件下天然气密度（b）
Fig. 10. Caprock surface tension (a) under different temperature conditions, natural gas density (b) under variable temperature and pressure conditions
图 11 深水井LS17-2（a）、浅水井DF-X（b）海底埋深30 m圆锥静力触探实验对比
Fig. 11. Deep water Well LS17-2 (a), shallow water Well DF-X (b) seabed buried depth of 30 m cone static cone penetration test comparison
表 1 L36-1a井盖层排替压力及其可封堵最大气柱高度
Table 1. The displacement pressure of Well L36-1a caprock and the maximum height of gas column that can be sealed
顶深(m) 底深(m) 厚度(m) 孔隙度（%） 排替压力（MPa） 可封最大气柱高度（m） 1 770 1 782 12 22 0.9 116 1 795 1 822 27 20 1.1 144 1 827 1 829 2 15 1.9 246 1 843 1 858 15 25 0.7 84 1 860 1 874 14 20.5 1.1 136 1 878 1 889 11 20 1.1 144 1 894 1 896 2 20 1.1 144 1 897 1 899 2 25 0.7 84 1 914 1 921 7 17 1.6 198 1 924 1 943 19 16 1.7 221 1 948 1 992 44 18 1.4 178 2 011 2 053 42 19 1.3 160 
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表 2 块体流与深海泥测井参数(Sun and Alves, 2020)
Table 2. Logging parameters of MTDs and deepsea mud (Sun and Alves, 2020)
测井参数 块体流 深海泥 体积密度(g/cm3) 1.8 1.7 孔隙度(%) 42 50 电阻率(Ω·m) 2 1.5 声波时差(µs/ft) 160 177 含水饱和度(wt%) 22 25
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