鄂尔多斯盆地延长组下组合地层水特征及其油气地质意义

Characteristics of formation water and its geological significance of lower combination of Yanchang Formation in Ordos Basin

鄂尔多斯盆地中生界延长组下组合长10—长8油层组油水分布关系复杂,地层水特征及其与油藏的关系缺少系统研究。为明确地层水性质及其与油藏的关系以及地层水矿化度对测井精细解释的影响,基于地层水样品筛选,通过对鄂尔多斯盆地延长组1万余个地层水水样测试数据分析,研究地层水性质及分布特征,分析地层水矿化度、特征参数等的油气地质意义。结果表明,鄂尔多斯盆地中生界延长组地层水性质具有区带性分布的特点,各油层组地层水矿化度自西缘冲断带、天环坳陷轴部向湖盆中部总体呈先增加后减小的趋势;延长组下组合地层水矿化度较低,平均为28.4 g/L,属盐水地层水类型,以CaCl2型为主。地层水的离子组成、矿化度、同位素、特征参数等较为相近,且明显不同于其他油层组地层水特征,属于同一流体系统;地层水钠氯系数、脱硫系数、变质系数等特征参数表明,盆地中生界延长组下组合油藏的封闭性整体较好,油藏及出油井点主要分布在地层水矿化度相对较高的区域,西缘冲断带部分油藏受构造作用影响进行了调整改造,油藏封闭性变差;湖盆中部高阻水层、天环坳陷轴部低阻油层发育,地层水矿化度是影响测井解释的重要因素。该研究成果为鄂尔多斯盆地中生界延长组流体系统研究以及复杂油水关系分布的测井精细识别提供了依据。

The distribution relationship between oil and water of the lower combination Chang10 to Chang 8 of Mesozoic Yanchang Formation in Ordos Basin is complex,and the characteristics of formation water and its relevance with reservoir are lack of systematic study.In order to clarify the formation water property and its relationship with reservoir,and the influence of formation water salinity on well logging interpretation,based on the selection of formation water samples,through the analysis of more than 10000 formation water samples from Yanchang Formation in Ordos Basin,the properties and distribution characteristics of formation water were studied,and the geological significance of formation water salinity and characteristic parameters were analyzed.The results show that the formation water of Yanchang Formation has the characteristics of zonal spreading distribution.The salinity of formation water of each oil layer group increases first and then decreases from the western margin thrust belt and the axis of Tianhuan depression to the center of the lacustrine basin.The formation water of the lower combination of Yanchang Formation has relatively low salinity,with an average value of 28.4 g/L,which belongs to saline formation water,mainly of CaCl2 type.The ion composition,salinity,isotope and characteristic parameters of formation water are similar,and obviously different from other formation water characteristics,belonging to the same fluid system.The feature parameters of Na+/Cl-coefficient,desulfuration coefficient and metamorphism coefficient of formation water show that reservoir sealing property of the lower combination of Yanchang Formation is good as a whole.The reservoirs and oil production wells are mainly distributed in the area with relatively high salinity of formation water.Some reservoirs in the western thrust belt have been adjusted and reformed due to the influence of tectonism,and the sealing property of the reservoir became poor.The water layer with high resistivity in the central lacustrine basin and the oil layer with low resistivity in the axis of Tianhuan Depression are developed,and formation water salinity is an important factor affecting well logging interpretation.The research results could provide a basis for the study of fluid system of Mesozoic Yanchang Formation and fine identification of complex oil-water relationship distribution in Ordos Basin.