鄂尔多斯盆地靖边－绥德地区古生界天然气运移和聚集的数值模拟

Numerical Modelling of the Palaeozoic Natural Gas Migration and Accumulation in the Jingbian-Suide area, Ordos Basin

本文用二维盆地模拟方法研究鄂尔多斯盆地靖边－绥德地区古生界天然气的运移和聚集。在数值模型中，天然气的运移由渗流和扩散两种运移形式组成。模拟结果表明，研究区古生界天然气的运移以渗流运移为主、扩散运移为辅。以早白垩世末期燕山运动后幕为界，运移期大体上可分为两个阶段。由此，形成了研究区各井的三种不同的运移聚集史。

Primary migration and secondary migration of the natural gas have generally two forms, percolation and diffusion stream. The percolation migration is under the control of fluid potential, its direction is defined in such a way that gas percolation migrates from high to low gas potential. The diffusion stream is decided by concentration gradient, that gas moves from high to low concentration range. Based on the above considerations, the bidimensional computer model of gas migration and accumulation is set up and consists of diphasic fluid percolation model ( Ungerer, 1990) and diffusion stream model in this paper, and its application to the Jingbian-Suide area of Ordos Basin is also discussed in the present paper. The study area is situated in the Yishan slope of Ordos Basin. Carbonate rocks of Ordovician and coal-bearing series of Upper Palaeozoic possess good condition of gas generaton, and they are major source rocks and reservoir rocks of the area. Modelling results have demonstrated that percolation migration is the principal form of the Palaeozoic gas migration, and diffusion migration is the secondary in this area. The period of migration may be divided into two stages. The first one occurred before Early Cretaceous. After the end of Late Cretaeous the area was lifted by the Yanshan Crustal Movement and Xishan Crustal Movement, Lower Cretaceous and Jurassic sedimentary strata of different places were eroded in varying degrees, distribution of gas potentials was changed, from then on , the period of migration entered the second stage. There are three types of gas migration and accumulation history for wells of this area: (1) Gas accumulation was formed in the first stage of migration, and the gas saturation is still high at the present time. (2) Gas accumulation was also formed in the first stage of migration. Thereafter, due to the Yanshan and Xishan Crustal Movement, the accumulated gas had partially migrated outward, resulting in the decrease of the gas saturation. (3) Gas accumulation was mainly been formed in the second stage of migration.