鄂尔多斯盆地上古生界天然气运移聚集特征

鄂尔多斯盆地上古生界主要发育煤系气源岩和海相碳酸盐岩气源岩，在晚三叠世进入生气门限，早中侏罗世进入排气门限，晚侏罗世─早白垩世末达到生排气高峰期。上古生界最大埋深时古异常超压对天然气的运移富集起着重要作用，是微裂缝排气的主要动力。由于排气较晚，天然气的初次运移主要为扩散排气和微裂缝排气。二次运移期间，在上古生界石盒子组出现较大面积的异常高压，形成了阻止天然气向上穿层运移扩散的压力封闭；而部分地区上古生界内部的异常高压为上古天然气向下穿层运移进入奥陶系提供了动力。分析区域构造演化和古流体势分布对天然气运移的综合控制作用后认为，地史期紧邻生气中心、处于构造高部位上的相对低势区及低压区为天然气聚集成藏的最有利地区，这一认识已被勘探实践所证实。

Characteristics of natural gas migration and accumulation in the Upper Paleozoic of Ordos basin.

Upper Paleozoic gas source rock of Ordos basin includes mainly coal measure source rocks and marine Carbonate source rocks, which entered into gas-generating threshold in late Triassic, into gas-expulsing threshold in early to middle Jurassic, and reached gas generating and expulsing peak period in late Jurassic to early Cretaceous. The ancient abnormal overpressure of Upper Paleozoic at the maximal burial depth had played important role in natural gas migration and accumulation, and it was the main force of gas-expulsion through micro-fissure. Because the gas expulsion happened in relative late time, the primary gas migration was mainly in the way of disperse gas- expulsing and microfissure gas-expulsion. The abnormal high- pressure of great area appeared in Shihezi Formation of Upper Paleozoic during secondary gas migration period, Which became pressure-sealing to prevent gas from its upward-cross migration and dispersion, while in local area the abnormal high-pressure of Upper Paleozoic strata provided dynamic to its downward-cross migration. Through the analysis of general control of regional tectonic evolution and paleo-fluid potential distribution on natural gas migration, it has come to the following conclusion that the relative low-potential area and low abnormal pressure area, which are adjacent to gas-generation center in geologic time period and located at structural high position, ought to be the most favourable area for gas accumlation and gas pool formation. The results of this theory have been confirmed by natural gas prospecting practice in this basin.