鄂尔多斯盆地生烃过程与流体流动及铀矿化关系

摘要

The Ordos Basin is not only an important uranium mineralization province but also a major producer of oil,gas and coal in China.The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies,but it has not been well understood in terms of hydrodynamics of basin fluid flow.In a previous study we have demonstrated that the preferential localization of uranium mineralization in the upper part of the Jurassic strata may have been related to the interface of an upward flowing,reducing fluid and a downward flowing,oxidizing fluid,and that this interface may have been controlled by the interplay between the fluid overpressure which was related to disequilibrium sediment compaction and drove the upward flow and the topographic relief which drove the downward flow.In the present study,we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure,in addition to sediment compaction and heating.Our results indicate that when hydrocarbon generation was taken into account,fluid overpressure during the Cretaceous was more than doubled in comparison with the simulation when hydrocarbon generation was not considered.Furthermore,the fluid overpressure dissipation after the ceasing of sedimentation slowed down relative to the non-hydrocarbon generation case.These results suggest that hydrocarbon generation may have played an important role in uranium mineralization,not only in providing reducing agents required for the mineralization,but also in contributing to the driving force to maintain the upward flow against the pushing of topography-driven,downward flow,thus helping stabilize the interface between the two fluid system and the localization of uranium mineralization.%鄂尔多斯盆地是我国有机能源矿产重要产区,作为铀成矿省的重要性备受关注。鄂尔多斯盆地铀矿化与烃类的关联性不断被认知,但从盆地流体动力学角度开展的研究鲜见。先前的研究表明,铀矿化在侏罗系上部地层的优先定位机制与还原渗出流和氧化渗入流两种流体系统的混合界面密切相关,混合界面的位置受控于流体超压力与地势差的相互作用。流体超压力驱动渗出流,与不均衡沉积压实作用有关,而地势差控制渗入流。研究中除考虑沉积压实与地势差两个因素外,同时还考虑了盆地油气生成对流体超压力的影响。研究结果表明,考虑生烃的影响时,白垩纪盆地流体超压力是不考虑生烃时的两倍还高,且相对于没有生烃效应的情形,沉积停止之后流体超压的消失过程极为缓慢。因此,笔者认为生烃效应在鄂尔多斯盆地铀矿化中发挥重要作用。生烃不仅为铀矿化提供还原剂,而且驱动渗出流与地势差控制的渗入流混合有利于两种流体系统以及铀矿化层位在混合界面中保持稳定。