罗布泊第四纪卤水钾矿储层孔隙成因与储集机制研究

Formation of Pores and Brine Reserving Mechanism of the Aquifers in Quaternary Potash Deposits in Lop Nur Lake, Xinjiang, China

罗布泊第四纪盐层中储集丰富的卤水钾盐资源,储集层主要为钙芒硝岩。研究表明,其储卤孔隙可划分为:原生晶间孔隙、准原生晶间孔隙、次生孔隙;次生孔隙又细分为溶蚀孔隙、交代孔隙和晶洞孔隙。本文主要通过类比分析,确定钙芒硝岩在浅埋藏期间,在其原生沉积孔隙基础上,发育准原生晶间孔隙,即蜂窝状孔隙,并在其中储集了大量富钾卤水。研究揭示出钙芒硝岩孔隙的发育与该矿物的菱片—菱板状结构、点-面式接触的“杂乱分布”密切相关。由于钙芒硝岩具有相对较大的硬度,其中大多数孔隙在埋藏过程中得以保存,同时,钙芒硝岩在承压环境中受溶蚀作用还可产生晶洞孔隙,这两种孔隙均储集了大量的卤水钾盐资源。

Lop Nur Lake is located in the eastern part of the Tarim basin, Xinjiang, in the northwestern China. It is a sub-basin of the Tarim basin. Since the late Pliocene-early Pleistocene, it became a sink of surface water of the Tarim basin and its drainage area. Hence, plenty of potash materials have been poured into it from that time. AT the same time, lacustrine sedimentation was dominant in Lop Nur. During the Quaternary, the palaeoclimate was generally semi-arid to arid in the area. Impacted by neotectonic movements, the northern part of Lop Nur was uplifted and a few depressions were formed. The Luobei depression is the largest one. In 1995, a large-scale potash deposit was discovered in this depression. The potash resource is reserved in brine of the Quaternary salt strata. The average tenor of KCl in the brine is 1. 4%. It is interesting that the brine aquifers consist mainly of glauberite rocks, in which a great deal of 'honeycomb pores ' were developed. The porosity of the 'honeycomb pores' in the glauberite rocks is from 20% to 42%, averaging about 28%, in unconfined aquifers, while in the confined aquifers, the porosity of glauberite rocks is about 14% on average. Studies show that brine-holding pores can be divided into the following types; primary pores, meta-primary pores and secondary pores. The latter can be further divided into dissolved pores and replacement pores and miarolitic pores. Here, The emphasis of this study is focused on the origin of pores in glauberite rocks. During the shallow-buried stage, on the basis of primary pores, the meta-primary pores called ' honeycomb pores ' were developed, and plenty of potassium-rich brine was reserved in the pores. This study also shows that the formation of these pores has a close relationship with rhombohedral-slate structure and ' mixed and disorder distribution ' of glauberite minerals. Owing to large hardness of glauberite, majority of the meta-primary pores remained in glauberite rocks during a burying and confined condition. Meanwhile, miarolitic pores can be formed by dissolution in glauberite strata. These two types of pores together hold a great deal of brine and potash resources in glauberite rocks.