Application of nuclear magnetic resonance (NMR) technology to study the space distribution features of underground brine from playa lakes:A case study of Kunteyi Playa Lake

Abstract

昆特依干盐湖位于柴达木盆地西北部，为特大型综合盐类矿床.大盐滩是昆特依干盐湖内最大的盐滩，地下赋存有一定量的卤水矿床，但该矿床的水文地质条件差，主要卤水矿层含水性弱，开采难度大.核磁共振找水方法作为当今世界上唯一的直接找水地球物理新方法，具有高分辨力、高效率、信息量丰富和解的唯一性等优点，本文运用该方法对昆特依干盐滩地区地下卤水空间分布特征进行研究，通过对核磁共振数据进行处理与反演，结合已有的地质与钻井资料，对测点进行综合地质地球物理解释，获得以下认识：1）大盐滩0~130 m深度范围内，共存在3个卤水含水层，主要呈扁平状或漏斗状、近似层状展布，W₁为晶间潜卤水层，渗透系数较大，颗粒较粗，单位体积含水量为0.4%~2.7%，W₂和W₃为晶间承压卤水层，渗透系数较小，颗粒较粗，单位体积含水量分别为0.2%~1.1%和0.1%~0.8%；2）大盐滩地区存在两个卤水富集区，分别为研究区西南部沉积盆地中心的Ⅰ号富卤区和盆地东北部的Ⅱ号富卤区；3）根据区域内卤水富集分布以及构造情况，划定大盐滩向斜沉积中心、大盐滩北侧F1~F8及遥F6断裂发育区和冷湖构造带为区域内主要的找矿找水远景区；4） GMR核磁共振系统在干盐滩地区理论探测深度为130 m，该系统不仅可以有效地探测自由水，而且可以依据束缚水的分布解译地下各类含水盐类矿物和含水黏土矿物的存在与分布.;Kunteyi Playa Lake, located in the northwest of Qaidam Basin, is an extremely large comprehensive salt deposit. The Great Salt Beach is the largest salt beach in the Kunteyi Playa Lake with underground brine deposit, poor hydrogeological conditions, and extremely low water content in the main seam, so it is difficult to exploit. Nuclear Magnetic Resonance (NMR) is a effective geophysical method currently used to detect groundwater directly. Comparing with the traditional geophysical methods, the NMR method is of higher resolution, more efficiency, more information and unique interpretation for hydrogeological investigations. In this paper, NMR sounding is used to study the space distribution characteristics of the underground brine. The NMR data were processed with the inversion, combined with geological and drilling data, and integrated with geological and geophysical interpretation for each measuring point. We find that in Kunteyi Playa Lake there are three brine aquifers exists from 0 to 130 m below the surface. The distribution of three brine aquifers is like funnel-shaped or approximate layer. The W₁ is latent brine formations, large permeability and coarse grain, with per unit volume of water of 0.4%-2.7%. The W₂ and W₃ are pressurized brine, low permeability and coarse grain, with per unit volume of water of 0.2%-1.1% and 0.1%-0.8% respectively. There existed two brine enrichment regions in the study area, the southwest centre of the exploration basin and the northeast basin. According to the distribution of brine and the structure in this area, we delimitate prospecting area of underground brine, the depositional centre of Great Salt Beach Synclinal, fault F1 to F8 and remote sensing fault F6 development areas in the north of the Great Salt Beach, cold-lake Structural belt. In dry salt lake area, the reliable detection depth of GMR system is 130 m. GMR systems can detect not only free water effectively, but also distribution of bound water to interpret the distribution rule of water-bearing salt minerals and aquiferous clay minerals.