杭锦旗地区上古生界地层水成因及其与油气的关系

THE GENESIS OF FORMATION WATER AND ITS RELATION TO OIL-GAS ACCUMULATION IN UPPER PALEOZOIC OF HANGJINQI AREA

鄂尔多斯盆地北部杭锦旗地区上古生界地层水总矿化度为13831mg／L～89445mg／L，属于盐水和卤水的范畴，pH值在4．37～7．3，在强酸性一弱碱性之间，地层水水型基本为CaCl2型水。根据对鄂尔多斯盆地杭锦旗地区上古生界地层水矿化度、PH值及离子含量等的分析发现，泊尔江海子断裂带及其以南、以北地层水特征差异明显：泊尔江海子断裂带地层水矿化度最高（47061mg／L-89445mg／L）、pH值最低（4．37～6）及Mg^2＋离子含量最高，最高达7024mg／L；断裂带以南地层水阴离子主要为Cl^-和HCO3^-，不含SO4^2-离子；断裂带以北含SO4^2-离子，而且往北SO4^2-离子增加的趋势非常明显。采用Davission地层水Nadeficit与Caexcess之间关系及其作用过程的地层水成因模式和地质特征进行综合研究，认为泊尔江海子断裂带的地层水成因为混合作用，主要是由断裂带热液活动导致深部偏酸性的高温、高Mg^2＋含量、高矿化度水与上古生界地层水混合所致。现令实测表明断裂带地温梯度2．72℃／hm～2．9℃／hm，为研究区最高；断裂带以北地温梯度只有2．27℃／hm～2．36℃／hm，与后期地表水的渗入降温有关。断裂带以外主要的流体一岩石相互反应是斜长石的钠长石化过程。地层水特征变化与油气特征的关系显示热液活动有利于烃源岩的成熟；SO4^2-离子往北逐渐增加表明受大气淡水影响，保存条件逐渐变差；热流影响区可能成为油气有利聚集区。

The total mineralization of formation water is 13 831 mg/L-89 445 mg/L in Upper Paleozoic of Hangjinqi area in north of Ordos basin, belong to the category of brine and bittern. Most of the formation water is CaCl2 with a pH value of 4.37-7.3, between strong acidity and alkalescence. According to the analysis of water salinity, pH value and ion concentration etc. , it was found that the characteristics of formation water have obvious difference in Boerjianghaizi fault zone and in the north and south area. In Boerjianghaizi fault zone, the water salinity is as high as 47 061 mg/L-89 445 mg/L, pH value is as low as 4.37-6, and Mg2＋ concentration reaches to 7 024 mg / L. In the south of fracture zone, formation water anion is mostly Cl^- and HCO3^- , without SO4^2-. In the north of fracture zone, formation water contains SO4^2- , and the SO4^2- concentration increased distinctly from south to north. The active mechanism and geologic feature were studied synthetically in reference to the formation water genetic model of Davission. It was thought that formation water in Boerjianghaizi fault zone has a mixing formation,mainly by hydrothermal fluid activity in fault zone resulted from deep part meta-acid water of high temperature,high Mg^2＋ and hypersalinity admixture with formation water in Upper Paleozoic. Present measurement indicated that the thermal gradient in fault zone is the highest in the region, ranging from 2.72 ℃/hm to 2.9 ℃/hm. In the north of fault zone, thermal gradient is only 2.27℃/hm ~2.36 ℃/hm,associated with infiltration and cooling of late surface water. Primary fluid-rock interaction outside fault zone is the albitization of plagioclase. The establishment of formation water fluid thread of Upper Paleozoic in basin provided evidence for the research of genesis of formation water. The relationship between variation of formation water and characteristics of hydrocarbon indicated that the activity of hydrothermal fluid was in favor of the maturity of hydrocarbon source rock. Increase of SO4^2- northward showed that it was affected by atmosphere fresh water, and the preservation condition was worsen gradually.