鄂尔多斯盆地上古生界准连续型气藏天然气运移机制

基于鄂尔多斯盆地上古生界主要产气层段天然气地球化学特征和运移基本地质条件的分析,对上古生界准连续型致密砂岩大气田的天然气运移特征进行了研究。研究表明,鄂尔多斯盆地上古生界准连续型致密砂岩大气田天然气组分和碳同位素组成在平面上的变化主要受烃源岩热成熟度控制,纵向上受到烃源岩热成熟度和天然气运移分馏作用的共同影响。结合运移动力和输导条件进一步分析认为,上古生界致密砂岩大气田主要为初次运移直接成藏和短距离二次运移近源成藏。运移的动力主要为异常高压和气体分子浓度差产生的扩散作用力,浮力作用弱或无。运移方式主要为异常高压驱动下的幕式涌流和扩散作用引起的扩散流。其中,位于生烃体系内的太原组、山2段主要为异常高压驱动下的幕式涌流运移;生烃体系外的山1段、盒8段及以上层段扩散作用引起的扩散流运移对于天然气的成藏具有重要贡献,苏里格气田天然气组分和碳同位素纵向上的规律变化就是其直接反映。

Gas migration mechanism of quasi-continuous accumulation in the Upper Paleozoic of Ordos Basin

Based on the natural gas geochemical behaviors and basic geological conditions for gas migration,this paper studied the gas migration characteristics of the quasi-continuous tight sand gas field of the Upper Paleozoic in Ordos Basin.Laterally,the variations of gas components and carbon isotope composition are mainly controlled by thermal maturity of source rocks (R_o),while vertically,they are affected jointly by thermal maturity of source rocks and gas migration fractionation.Further analysis of the migration driving force and pathway conditions indicates that the large tight gas field of the Upper Paleozoic is characterized by near-source accumulation via primary migration and short-distance secondary migration.The principle driving force for migration are diffusion force generated by the gradient of gas molecule concentration and abnormal high pressure,while buoyancy force has little or no contributions to gas migration.There are two main migration styles:surge flow caused by overpressure and diffusion flow resulted from diffusion.The overpressure-dri-ven surge flow is predominant in the Taiyuan Formation and the 2^nd Member of Shaanxi Formation within source rock interval,while the diffusion-driven diffusion flow is predominant,in the 1^st Member of Shaanxi Formation and the 8^th Member of Xiashihezi Formation as well as reservoirs above them.The vertical regular variations of gas composition and carbon isotope are the direct responses to these gas migration patterns in Sulige gas field.