鄂尔多斯盆地陇东地区长8致密砂岩油藏地层水化学特征成因及其油气指示意义

国内对地层水与油藏的关系研究多集中在断块坳陷内,在岩性油藏中此类研究相对较少。基于鄂尔多斯盆地陇东地区长8致密砂岩油藏大量的地层水化学分析及薄片和扫描电镜资料,对长8地层水化学特征及其成因进行了分析,并结合油藏地质条件,探讨其油气指示意义。长8致密砂岩岩性油藏地层水矿化度平均为33.6 g/L,地层水类型以Ca Cl2型为主,整体为封闭的水文地质环境,有利于油气保存。长8地层水中的离子特征与砂岩成岩具有良好的对应关系。Mg2+亏损、Na+轻微亏损及富集主要与绿泥石化、斜长石的钠长石化及溶解作用有关;Ca2+富集不仅与斜长石的钠长石化及溶解作用有关,还与泥岩中火山灰大量释放的Ca2+有关;SO42-与HCO3-含量则主要受脱硫酸作用和碳酸盐胶结作用控制。此外,由于长8砂岩成岩作用受烃类充注影响较大,研究区西部与中部及东部地层水化学特征存在差异,西部地层水中的钠氯系数整体小于中部及东部,SO42-平均含量及脱硫系数平均值大于中部和东部等特征都与长8油藏在平面上充注条件差异有着很大联系。

Hydrochemical Feature Genesis and Hydrocarbon Exploration Significance of Chang 8 Tight Sandstone Reservoir in Longdong Area,Ordos Basin

Domestic studies on the relationship between formation water and reservoirs are mostly concentrated in fault block depressions, and such studies are relatively rare in lithologic reservoirs. Based on a large data set of the formation water chemical compositions,thin-section petrography and scanning electron microscopy of the Chang 8 tight sandstone reservoir in Longdong area of Ordos Basin, the chemical characteristics and genesis of the Chang 8 formation water were analyzed and their hydrocarbon exploration implications were discussed. The study shows that the average salinity of formation water in the reservoir is 33.6 g/L. The formation water is mainly of CaCl₂ type, and the hydrogeological system is closed, conducive to oil preservation. Ionic characteristics of the formation water are closely linked with the Chang 8 sandstone diagenesis. The Mg²⁺ depletion and slight Na⁺ enrichment/depletion may have mainly related to chloritization, albitization and dissolution of plagioclase. The Ca²⁺ excess is unlikely only related to albitization and plagioclase dissolution, but also related to the Ca²⁺ release by volcanic ash in mudstone. The {{\mathrm{SO}}_4}^{2-} and {{\mathrm{HCO}}_3}^{-} contents may have been controlled mainly by desulfurization and carbonate cementation. Since the diagenesis of the Chang 8 sandstone was much affected by hydrocarbon charging, the water chemistry characteristics of the western/central/eastern sequences of the study area are different. The Na/Cl coefficient of the formation water in the western part is lower than that of the central and eastern parts, whereas the {{\mathrm{SO}}_4}^{2-} content and desulfurization coefficient of the formation water are higher. We suggested that this is closely related to the different filling conditions of the reservoir.