富CO2深部流体对碳酸盐岩的溶蚀-充填作用的热力学分析

由于广泛而强烈的岩浆作用,我国东部的松辽、渤海湾、莺歌海以及西部的塔里木等盆地中都有富CO₂深部流体的活动。富CO₂深部流体与碳酸盐岩相互作用可用Duan and Li(2008)所建立的CO₂-H₂O-CaCO₃-NaCl体系的热力学模型来进行模拟计算。计算结果表明,富CO₂深部流体在自深部向浅部运移过程中对CaCO₃的溶解度会逐渐增加,到达一定深度后溶解度达到最大值,再向浅部溶解度开始逐渐降低; 也就是深部流体具有深部溶蚀碳酸盐岩-浅部沉淀碳酸盐矿物的规律。与浅部地层中的流体发生混合会使流体的CO₂含量和盐度降低,会导致CaCO₃的沉淀充填; 深部流体进入开启性断裂/裂缝体系中时,由于压力的降低,也会发生CaCO₃的沉淀充填。深部流体的CO₂含量、盐度、温度和压力的变化影响着实际的溶蚀-充填过程。塔中地区钻井也揭示了深部下奥陶统碳酸盐岩中发育有丰富的溶蚀孔隙,而在相对浅部的奥陶系灰岩和志留系砂岩中见有大量方解石的充填,这是富CO₂流体深部溶蚀-浅部充填的一个较好的实例。基于理论和实际分析,本文认为在岩浆火山作用广泛发育的塔里木等盆地中下古生界深部优质碳酸盐岩储层存在的可能性。

Thermodynamic analysis for carbonate dissolution-filling under influence of CO2-rich deep fluid

There are extensive activities of CO₂-rich deep fluids in Songliao,Bohai Bay and Yingge Sea basins in the eastern China and Tarim Basin in the western China because of extensive and intensive magmatic activities.The interaction between CO₂-rich fluid and carbonate can be simulated and calculated by a thermodynamic model for CO₂-H₂O-CaCO₃-NaCl system which is created by Duan and Li(2008).The simulating result indicates that the solubility of CaCO₃ in the CO₂-rich fluid rises gradually when the fluid migrates from deep to shallow stratum and then reaches to a peak value at some certain depth.However,the solubility decreases gradually when the fluid migrates upwards further.The solubility feature reveals a rule that the CO₂-rich deep fluid can dissolve carbonate in deep stratum and then fill pores by precipitating carbonate minerals in shallow stratum.Decrease of CO₂ content or decrease of salinity in the fluid due to mixing with fluid in shallow strata can cause precipitation of CaCO₃.CaCO₃ can also precipitate from the fluid when it migrates into open fault/fracture system because of decrease of pressure.The changes of CO₂ content,salinity and pressure affect actual characteristics of CaCO₃ dissolution-filling.A typical example for such rule can be gotten from the central area of the Tarim Basin.Drillings in the center of Tarim Basin indicate that there are numerous dissolution pores in the lower Ordovician carbonates,however there are large amount of calcite fillings in the relatively shallow Ordovician carbonates or Silurian sandstones.Both the rule and example indicate that the existence of well carbonate reservoir bed is possible in the deep lower Paleozoic strata in the Tarim Basin and the similar others where magma or volcano activities were extensive and intensive.