塔中奥陶系地层水化学特征及其成因与演化

Geochemical behaviors of the Ordovician formation water in the Tazhong area and its origin and evolution

李鹏春 刘春晓 张渊 龙利平 张利红

Li Pengchun Liu Chunxiao Zhang Yuan Long Liping Zhang Lihong

塔中地区奥陶系地层水化学组成以(K+Na),Cl离子为主,大部分为CaCl型水,矿化度变化介于4.8～220.2g/L,r/r比值变化介于0.26～0.85之间,说明变质程度小,可能是海水发生蒸发浓缩作用之后被淡水所混合的产物。受断裂作用影响,地层水矿化度在平面上具有明显的分区性;受区域不整合影响,纵向上地层水离子浓度和矿化度存在明显的分带性。地层水中阴离子Cl和HCO的浓度与阳离子(Na+K)和Ca的浓度的关系表明,本区可能的成岩作用过程主要为氯化钠矿物和碳酸盐矿物的溶解作用,而方解石的(铁)白云石化及钠长石化作用导致了Ca离子的富集与Mg离子的亏损,形成了CaCl型流体。区内地层水可分为常压高矿化度水和异常高压低矿化度水两种类型。前者可能与大气淡水淋滤和埋藏过程中的水-岩相互作用有关,后者可能与其形成的封闭体系有关。油气聚集和保存条件较好的区域位于塔中Ⅰ号坡折带和Ⅱ号构造带之间。

Chemical compositions of the Ordovician formation water in Tazhong area are dominated by cations as K + Na and anions like Cl.The water is mostly of CaCl type with salinity ranging from 4.8 to 220.2 g/L and r/r ratio varing between 0.26 and 0.85,suggesting a small change of water property and a possible origin from mixing of fresh water with sea water after evaporation and inspissation.Influenced by fracturing effect, the plane distribution of formation water salinity is characterized by obvious regionalization ;while affected by regional unconformities,the vertical distribution of ion concentration and salinity is featured in zonation.The relationship between cation concentration of Cl and HCO and anion concentration of Na + K and Ca indicated that the likely diagenesis process in the study area was mainly dissolutions of NaC1 and carbonate,and that the dolomitization(ferrodolomite)and albitization of calcite resulted in excess of Ca and deficit of Mg, forming CaCl-type fluid.Formation water in the area can be classified into two groups:normal-pressure high-salinity water and abnormal high-pressure low-salinity water.The former may be related the leaching of atmospheric water and to the water-rock interaction during burial,while the latter may be connected with the closed system formed by the type of water itself.The area between TZ-Ⅰ slope break and TZ-Ⅱ structural zone is considered to have favorable conditions for accumulation and preserving of hydrocarbons.

10.11743/ogg20070615