柴达木盆地致密油形成的地质条件及勘探潜力分析

柴达木盆地具备致密油形成的地质条件。柴达木盆地大面积分布的中下侏罗统（J₁₊₂）半深湖相泥岩、第三系下干柴沟组（E₃）、上干柴沟组（N₁）半深湖及深湖相烃源岩和与其互层或位于其附近的滨浅湖相砂体或碳酸盐岩构成了对致密油形成非常有利的源储共生关系。柴北缘侏罗系湖相烃源岩有机碳平均1.85%，有机质类型以Ⅰ-Ⅱ₂型为主，处于成熟-高成熟阶段，具有较好的生油潜力。柴西第三系烃源岩有机碳一般在0.4%~1.2%，有机质类型以Ⅰ-Ⅱ₁型为主，R_O分布在0.4%~1.2%范围内，处于生油窗内。与国内其它盆地相比，柴西第三系烃源岩具有烃转化率较高的特点。碎屑岩储集空间以残余粒间孔和溶蚀孔隙为主，孔隙度3.8%~10.2%，渗透率0.1~2.0×10-3 μm2，储层具有单层厚度较薄、纵向上多层叠加、累计厚度较大、平面上多油层复合叠置分布的特点。碳酸盐岩储集空间以溶蚀孔和层间收缩缝为主，孔隙度5%~7%，渗透率0.2~0.7×10-3 μm2，储层层数较多，单层厚度薄，多沉积相带控制，具有横向广覆连片分布的特点。古构造斜坡区和生油凹陷中心是致密油分布集中区。致密油可能的成藏组合包括源内包裹组合、源上广覆组合、源下依伏组合和源侧披覆组合等四种。柴达木盆地致密油资源丰富、潜力大，初步估算资源量在8.16~10.46×108 t。通过成藏条件分析，认为柴西扎哈泉-乌南、小梁山-南翼山、七个泉-跃进和柴北缘冷湖等四个有利勘探区带可作为近期致密油勘探的首选目标。

Exploration Potential and Geological Conditions of Tight Oil in the Qaidam Basin

There are geological conditions for the formation of tight oil in the Qaidam basin. The Middle and Lower Jurrasic semi-deep lacustrine facies mudstone and the Tertiary semi-deep lake facies to deep lake facies mudstone are widely distributed in the Qaidam basin, which constitutes a favorable relationship of source and reservoir coexistence for the formation of tight oil with bank to shallow lake facies sand body or carbonate rock which is interbeded with or situated near the source rocks. The Jurassic source rocks in the northern margin of the Qaidam basin have an average organic carbon value of 1.85, Ⅰ-Ⅱ₂organic types and a maturity range from mature stage to over mature stage with better oil generation potential. The Tertiary source rocks in the west of the Qaidam basin have a range of organic carbon from 0.4%to 1.2%Ⅰ-Ⅱ₁ organic types and a Ro value from 0.4% to 1.2% within an oil generation window. Compared to other Chinese basins, the degree of hydrocarbon inversion of the Tertiary source rocks is high. The clastic reservoir space is dominated by remained intergranular pores and corroded pores with porosity from 3.8% to 10.2% and permeability from 0.1×103 μm2 to 2×103 μm2. The reservoir rocks have characteristics of thinner single beds, vertical multilayer’s superimposition, thicker accumulated thickness and many horizontal  oil beds overlayed. The reservoir space of the carbonate rock is predominated by corroded pores and interbeded contracted cracks with porosity from 5% to 7% and permeability from 0.2×103 μm2  to 0.7 ×103 μm2. The reservoir rock is characterized by more single beds, thinner single beds, more sedimentary facies controlled and horizontal widely continuous oil beds. The groups of oil reservoir formation include four types, that is, inner source included group, upper source widely distributed model, lower source layered group, and beside source covered group, which are mainly controlled by source depressions. The tight oil in the Qaidam basin is enriched and has a great potential, being estimated the resource yield ranges from 8.16 to 10.46×108 Tons. After the analysis of reservoir forming conditions, we think that four favorable exploration areas, i.e., Zhahaquan-Wunan, Xiaoliangshan-Nanyishan and Qigequan-Yaojin in the western Qaidam basin and Lenghu in the northern Qaidam basin, can be served as current exploration targets.