柴西地区古近系和新近系地层水化学特征及其成因

摘要

The chemical data collected from oilfield water of Paleogene and Neogene in the western Qaidam Basin were analyzed and summarized,and the characteristics and origin of formation water were discussed. The results show that the type of formation water is mainly CaCl2 composed of Na++K+;the salinity is high with the average of 166 g·L-1 ,and the lowest reaches 3.90 g·L-1;the formation water is highly concentrated,belonging to brine level;the main ions present increase-decrease alternately trends,and salinity firstly decreases and then increases with the increase of depth,which is significantly different from the eastern basins of China;various maj or ion ratios including Na-Cl coefficient, metamorphic coefficient and desulphidation coefficient all reflect that the formation of western Qaidam Basin overall are closed well;the Ca2+accumulation at Upper and Lower Youshashan Formations is caused by dolomitization,which results in the poor of Mg2+,followed by calcite dissolution;there are only a few data points at Upper and Lower Ganchaigou Formations showing Ca2+ accumulation,which is caused by calcite dissolution and chloritization,is the main reason for the lack of Mg2+;the precipitation of a lot of gypsum and the reduction of large amounts of sulfate reducing bacteria result in a loss of SO2-4 ;besides the HCO-3 produced by the reduction of SO2-4 ,CO2 and organic acids can be added to cause an increase of HCO-3 of formation water in the process of organic maturation.%在系统收集柴达木盆地西部地区各主要油田古近系和新近系油田地层水化学测试分析资料后，发现该地区地层水矿化度很高，均值为166 g·L－1，最低值达到3·90 g·L－1，具有高浓缩地层水特征，属卤水级，且主要是由Na＋＋K＋组成的CaCl2型水，反映封闭条件好。各主要离子在纵向上呈增加和减小交替变化的趋势，矿化度随深度的增加表现为先减小后增大的趋势，这与中国东部盆地明显不同。各离子比值(钠氯系数、变质系数和脱硫酸系数)均反映出该地区地层整体封闭性好。上、下油砂山组Ca2＋富集主要由白云石化作用导致，其次是方解石溶解，白云石化同时也使其Mg2＋贫乏。上、下干柴沟组仅有少数数据点表现出 Ca2＋富集，应是方解石溶解导致的，绿泥石化是其Mg2＋贫乏的主要原因。SO2－4亏损主要是由大量膏岩的沉淀和油藏中大量硫酸盐还原细菌的还原作用导致。除了SO2－4被还原会产生 HCO－3之外，有机质成熟过程中CO2及有机酸的加入也可造成地层水中 HCO－3质量浓度的增加。