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摘要: 近年来在四川盆地三叠系飞仙关组鲕粒白云岩储层中发现的高含硫天然气三级储量已近1×1012 m3。鲕粒白云岩储层集中分布在含蒸发岩的层序中,储层中有残余石膏、硬石膏及它们的模孔、方解石铸模(假象)等,表明白云石化流体与蒸发海水有关。白云石化由层间古暴露面向下增强的成岩层序说明这种白云石化是成岩早期发生的,并可能有大气降水与蒸发海水混合的影响。白云岩样品各结构组分氧、碳同位素微区测试数据差别显著,这说明采用岩石混合样做研究白云石化成因的地球化学分析可能导致误解。飞仙关组的白云岩中多数保存了原岩结构幻影、结构残余或原岩的全部结构说明白云石化过程是等体积交代。白云岩储层中的孔隙是各种溶蚀孔而非白云石化等摩尔交代的体积收缩孔。飞仙关组高含硫气藏储层中沥青和溶蚀孔的关系表明深埋晚期溶蚀孔最发育。在溶液中碳酸的电离常数大大高于氢硫酸,飞仙关组高含硫气藏天然气组分中的CO2相对于H2S的质量亏损和储层中有富轻碳的高温方解石充填晚期溶孔、裂缝表明储层中的晚期溶解作用是由硫酸盐热还原过程(TSR)中生成的CO2引起的。Abstract: The proved reserves of sour gas pools were up to 1×1012 m3 from oolitic dolostone reservoirs of Feixianguan Formation in northeast Sichuan Basin,that are mainly distributed in the sedimentary sequence with evaporites. Remnant gypsum and anhydrite , their moidic pores or molds in the reservoirs indicate that the dolomitization was related to the evaporated seawater. Furthermore, in some diagenesis sequence of the reservoirs, the dolomitization extent increases with depth from ancient subaerial exposure surface downwards. This implies that the dolomitization of the oolitic limestone took place in the early stage of diagenesis,and was possibly influenced by mixing rain water and evaporated seawater.
For δ13C and δ18O values between structural constituents of the oolitic dolomite in the reservoirs, the difference is great, so the values of intermix sample of reservoir rock are not suited for dolomitization research.
In the reservoirs , the most oolitic dolostone preserved the remainder of oolitic fabric or original rock framework , the various pores of the dolostone are almost secondary dissolved pores and not shrinkage pores due to dolomite crystals shrink. The replacement process of dolomitization is a volume replacement for volume, not a mole for mole. The relationship between bitumen and secondary dissolved pores indicates that late burial dissolution pores are dominant in the reservoirs .
The ionization constant of H2CO3 is much higher than that of H2S in the solution ,whereas the mole of CO2 is much lower than that of H2S in the sour gas of Feixianguan Formation. The late period calcite cement ,filled in the dissolved pores and fractures of the oolitic dolomite reservoirs, is characterized by low δ13C (-18‰) and high homogenization temperature (130~180℃), so the CO2,related to thermochemical sulfate reduction in the reservoirs , causes mainly late burial dissolution.-
Key words:
- Sichuan Basin /
- Feixianguan Formation /
- dolomitization /
- secondary pores /
- TSR /
- C /
- O stable isotope /
- CO2
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