杭州湾地区浅层生物气资源量计算及其地质意义

杭州湾地区晚第四纪气源岩主要为河漫滩相和浅海相。经计算,河漫滩相源岩总生气量为1 032.52×10⁸m³,总吸附气量为274.66×10⁸m³,总溶解气量为693.10×10⁸m³,总扩散气量为4.29×10⁸m³,总游离气量为60.47×10⁸m³;而浅海相源岩相应为1 412.75×10⁸,556.68×10⁸,749.83×10⁸,10.28×10⁸和95.96×10⁸m³。这一计算结果显示了研究区浅层形成的生物气体首先受地层水的溶解和粘土的吸附,大部分溶解在粘土层水中呈水溶气相态发生运移,当甲烷气在地层水中溶解达到饱和后才会出现游离态。游离气主要赋存于沉积物颗粒间隙中,浅地层的游离气运移量远小于溶解气运移量,河漫滩相游离气量小于浅海相游离气量。浅海相沉积物总吸附气量为河漫滩相沉积物总吸附气量的2倍多,泥岩层的吸附气量是砂岩层吸附气量的18倍多,说明浅层沉积物颗粒大小与甲烷的保存有关,沉积物颗粒小、比表面积大有利于吸附甲烷。计算结果还表明,杭州湾地区浅层天然气具有广泛的勘探开发前景。

Estimates of shallow biogas resources in Hangzhou Bay area and its geological implications

Late Quaternary gas source rocks in Hangzhou Bay are mainly of flood-plain and shallow marine fa-cies.The total amount of gas generated from the source rocks in flood-plain facies is estimated to be 103.252 BCM,with the total adsorbed gas 27.466 BCM,total dissolved gas 69.31 BCM,total diffused gas 429 MCM,and total free gas 6.047 BCM.While those from the source rocks in shallow marine facies are 141.275BCM,55.668 BCM,74.983 BCM,1.028 BCM and 9.596 BCM,respectively.These estimations show that the biogas generated in shallow layers in the study area would have first been dissolved in formation water and adsorbed by clays,and most of the gas dissolved in water in clay layers would migrate in the form of dissolved gas.Free methane can appear only when it has been saturated in the formation water.Free gas mainly occurs in the inter-granular spaces of sediments.The amount of free gas migration in shallow layers is much smaller than that of dissolved gas.The amount of free gas in flood-plain facies is less than that in shallow marine facies.The total amount of adsorbed gas in shallow marine facies sediments is over twice as large as that in flood-plain facies sediments,while the amount of adsorbed gas in mudstone is over 18 times of that in sandstone.All these indicate that preservation of methane is related to the grain sizes of sediments in shallow layers,and small grain size and large specific surface area are favorable for adsorption of methane. These estimates also indicate that Hangzhou Bay has large potentials for exploration and development of shallow biogas.