普光气田的天然气可能是无机成因的

普光气田天然气的成因由于地质储量和气源等认识有待深入而备受关注。普光气田的深部中地壳存在一个低速层，该低速层是一个塑性层，具流变性质，是充满气体及离子的热液流体。一方面，热液流体与橄榄岩等超基性岩发生蚀变反应，生成Fe²⁺、Mg²⁺及V、Ni等；另一方面，CO₂(或CO)与H₂在中地壳的恰当温度、压力下，在V、Ni等金属的催化下，发生费-托合成烃的反应。这时，携带K⁺、Na⁺、Cl^-、Fe²⁺、Mg²⁺的热液体上升到沉积地层，当遇到碳酸盐岩时，便发生Mg²⁺交代或Fe²⁺、Mg²⁺交代形成铁白云石及盐岩等，当遇到碎屑岩（硅酸盐）时，便发生绿泥石化（Mg²⁺交代）、伊利石化（K⁺交代）、蒙脱石化（Mg²⁺交代）作用并生成盐类矿物。费-托反应生成的烃可沿断裂系统上升到储层中（如白云岩及砂岩）而成藏。研究结果表明，普光气田天然气的成因及成藏模式可能就是通过上述途径实现的，即普光气田的天然气可能是无机成因的。川东地区还是一个地球强烈排气的区域，那些尚没有圈闭成藏的天然气影响了当地气候（如酷热、森林大火等）。结论认为：寻找普光气田型的天然气藏（生物礁-白云岩-膏盐组合）是21世纪四川盆地天然气勘探的主要目标

DISCUSSION ON INORGANIC ORIGIN OF GAS IN THE PUGUANG GAS FIELD

Origin of gas in the Puguang gas field is much concerned due to the limited knowledge in respects of the resource volume and origin of gas.A low velocity layer exists in deep middle crust of the Puguang gas field,and it is a ductile bed with rheological properties and contains hydrothermal fluid saturated with gas and ions.On the one hand,the hydrothermal fluid reacts with the ultrabasic rocks such as peridotite,generating Fe2 ,Mg2 ,V and Ni etc.On the other hand,hydrocarbons are generated through Fischer-Tropsch synthesis reaction of CO2(CO) with H2 at the appropriate temperature and pressure of the middle crust and under the catalysis of metals such as V and Ni.The hydrothermal fluid containing K ,Na ,Cl-,Fe2 and Mg2 flows upward.It generates ankerite and halite through Mg2 replacement or Fe2 and Mg2 replacement when encountering carbonates and generates chlorite(Mg2 replacement),illite(K replacement) and smectite(Mg2 replacement) as well as saline minerals when encountering clastic rocks(silicate).The hydrocarbons generated through Fischer-Tropsch synthesis reaction can migrate upward along fault systems into reservoirs such as dolomite and sandstone and get accumulated there.The study results show that the gas in the Puguang gas field might have been generated and accumulated in this way.That is to say,the gas in the Puguang gas field might be inorganic.Strong gas expulsion occurs in the eastern Sichuan basin and the gas that has not been entrapped and accumulated influences the local climate,such as extremely hot and forest fire.It is concluded that gas reservoirs of the Puguang gas field type(bioherm-dolomite-evaporate combination) are the major targets of gas exploration in the Sichuan basin in the 21st century.