鄂尔多斯盆地下侏罗统地层水化学特征及成因分析

通过对鄂尔多斯盆地下侏罗统地层水研究发现，延安组、富县组地层水化学特征具有复杂性和规律性。复杂性表现在地层水矿化度、钠氯系数、变质系数等水化学参数，在纵向上和平面上变化显著，地层水类型多样；规律性表现在地层水化学特征的垂直分异性、东西分带性、地貌分区性。分析结果表明:1）不同水文旋回地层水的混合交替作用导致了地层水化学特征的垂直分异性；2）构造特征决定了水化学特征的东西分带性。在盆地东部和西缘地区，地层埋藏浅或出露地表，渗入水补给交替强烈；靠近天环坳陷轴部地区，地层埋藏深度大，且远离补给区，处于相对滞留环境；3）富县组、延10古地貌特征、水岩反应导致地层水化学特征地貌分区性；4）阶地、斜坡、河间丘地区，在渗入水交替活跃程度、水动力、岩性组合、低幅度圈闭发育条件等方面均具有成藏优势。该研究为侏罗系古地貌油藏有利目标预测提供重要依据。

Formation Hydrochemical Characteristics and Genesis of the Lower Jurassic,Ordos Basin

The formation water characteristics are closely related to hydrocarbon migration and accumulation. The main factors affecting differences of the formation water from the Yan'an and Fuxian Formations are analyzed， based on the vertical and plane distribution characteristics of water type and hydrochemical composition of the formation water. The relationship between formation water characteristics and reservoir distribution and formation is discussed， and the factors that are important to in the process of finding Jurassic reservoirs are clarified in this paper. Study of formation water of the lower Jurassic Yan'an and Fuxian Formations in the Ordos basin reveal the complexity and regularity of the hydrochemical characteristics. The complexity lies in the diversity of formation water and the hydrochemical parameters， such as water salinity， sodium chloride coefficient， metamorphism coefficient， etc.， which vary significantly vertically or laterally. The regularity is reflected in that the varying formation hydrochemistry forms zonation， vertically， east?westward， or geomorphologically. It is pointed out that: 1） The mixing and alternation of formation water in different hydrological cycles lead to the vertical differentiation of formation water chemical characteristics. From Fuxian to Y10 to Y9 strata， the salinity of the formation water gradually decreases， the proportion of CaCl₂ type water gradually decreases， and the proportion of NaHCO₃ type and Na₂SO₄ type formation water gradually increases. The coefficient of sodium chloride and desulfurization gradually increase， while the coefficient of metamorphism decreases. 2） Structural evolution determines the east?west zonation of the hydrochemical characteristics. The Jurassic strata at the east or west margin of the basin are buried shallowly or exposed where infiltration water supply alternates easily with a relative lower salinity， and the strata at or close to the axis of the Tianhuan depression are buried deeply far away from the supply area， imprisoned in a relatively stagnant environment， making the salinity relatively high. 3） Regionalization of the formation hydrochemical feature changes with geomorphology of the Fuxian Formation?Yan10 member and is strengthened by the water?rock reaction during the diagenesis process. In the river developed area， formation water salinity is low. In the direction of the slope， terrace， and highland， it increases gradually， while the coefficient of the sodium chloride and desulfurization decreases， and the coefficient of metamorphism increases by degrees. Formation water type changes from Na₂SO₄ or NaHCO₃ to CaCl₂. 4） The terraces， slopes， and channel bars have the advantage of reservoir formation in terms of the infiltration water alternating extent， hydrodynamics， lithologic association， and low?amplitude structural trap development conditions， and they are the favorable targets prediction method of Jurassic Palaeogeomorphic reservoir is summarized.