江汉盆地潜江凹陷地下卤水地球化学特征和分布规律

Geochemical characteristics and distribution patterns of subsurface brines in the Qianjiang Depression,Jianghan Basin

江汉盆地潜江凹陷潜江组赋存有丰富的地下卤水,因其有较高的经济价值,是江汉盆地油气资源之外的又一种重要的战略资源。然而,长期的油田注水开采也对地下卤水产生了一定的影响,为进一步评估这种影响并探讨地下卤水微量元素的分布情况,通过采集和分析潜江组第一至第四卤水组的地下卤水样品开展其地球化学特征和分布规律的研究。结果表明,潜江凹陷潜江组地下卤水受油田生产影响,虽然普遍发生了淡化,但仍然拥有较高的TDS含量,淡化影响有限。卤水储层从上至下,第一卤水组至第四卤水组,随着深度的增加,硫酸钠亚型卤水和碳酸盐型水逐渐变少,氯化物型水逐渐增多。尽管区内地下卤水普遍经受油田注水生产的影响,但是卤水中Li、B、Br等元素依然保持较高的含量,大部分样品达到或超过边界工业品位,尤其以第二、第三卤水Li、B、Br品位较高,第四卤水组次之,第一卤水组最低。潜江凹陷地下卤水样品的钠氯系数接近于1,具备溶滤卤水特征,属于非海相地层的卤水,氯溴系数分析揭示区内现存地下卤水是溶滤卤水和沉积卤水混合以后的结果,卤水的脱硫系数和钙镁系数反映卤水的储集空间封闭性相对较差,凹陷内从第一至第三下卤水储集体的空间的封闭性逐渐变好,第四卤水组略变差。平面上,Li、B、Br具有相似的分布特征,凹陷内,卤水的浓集中心从下至上具有明显的迁移特征,钟市、浩子口、王场、周矶所围限的区域是高浓度卤水分布中心地带,其分布明显受控于卤水储集体的分布。

Subsurface brines of the Qianjiang Formation,Qianjiang Depression,Jianghan Basin are one type of strategic resource besides oil and gas for their economic values.However,chronic flooding in oilfield has certain influence to subsurface brines.In order to further assess this influence and investigate the distribution of trace elements in subsurface brines,brine samples from the 1st-4th brine beds were analyzed to study their geochemical characteristics and distribution patterns.The subsurface brines of the Qianjiang Formation are apparently affected by chronic flooding in oilfield.Although desalination occurred in general,subsurface brines still have high TDS contents,and the influence of desalination is limited.In the first to the fourth brine bed,arranged from top to bottom of brine reservoir,carbonate content and sodium sulfate subtype brine gradually decreased while chloride content gradually increased with increasing depth.Despite the influence of flooding on subsurface brines,the Li,B and Br contents in brines are still high,mostly reaching or exceeding the industrial grade threshold,with the grades highest in the second and third brine beds and decreasing from the fourth to the first.The n_(Na+)/n_(Cl-) ratio of subsurface brines are close to 1,exhibiting the characteristics of lixiviation brine found in continental strata.The ρ_(Cl-)/ρ_(Br-) ratio of brines revealed that the local subsurface brines are the mixing of lixiviation and sedimentary brines.The desulfurization coefficient and calcium/magnesium coefficient of brines revealed that the reservoir space sealing capacity is rather poor,and it improves from the first to the third brine bed,then worsen slightly in the fourth.Horizontally,Li,B and Br showed similar distribution characteristics.In the depression,the brine concentration centre showed obvious bottom-to-top migration characteristics.The area enclosed by Zhongshi,Haozikou,Wangchang and Zhouji Districts is the centre of high-concentration brines,whose distribution is obviously controlled by the distribution of brine reservoir.