渤海湾盆地歧口凹陷沙河街组一段下亚段湖相白云岩及其与海相白云岩的差异

渤海湾盆地歧口凹陷古近系沙河街组一段下亚段湖相白云岩主要为颗粒白云岩和泥晶白云岩两大类,常与泥质岩成薄互层或与其他碳酸盐岩互层产出。该湖相白云岩有序度较低,类似于海相准同生白云岩;Mg/Ca值偏离正常的化学计量值,具有富钙特征,说明其为快速交代的产物;碳氧同位素、Z值分析结果则反映了湖水的咸化程度较高。根据低海平面期蒸发浓缩盐水在湖底形成密度层并向灰质沉积层中回流渗透导致白云石化发生的机理,建立了歧口凹陷沙一下亚段湖相白云岩的成因模式。湖相白云岩的分布特征及其与海相白云岩的差异分析表明,湖相白云岩具有分布面积小、白云石化作用不彻底等特征,油气储集性能较为局限。

Feature of Oligocene Shahejie Lower-1st Member Lacustrine Dolostone in Qikou Depression, Bohaiwan Basin, and Difference of It from Marine Dolostone

Lacustrine dolomite that develops in the Lower-1st Member of Oligocene Shahehie Formation in Qikou Depression of Bohaiwan Basin consists of grain dolostone and micrite dolostone and commonly occurs in thin interlayers with pelyte or interbeds with other carbonate rock alternately. Analysis result shows that the lacustrine dolostone has a lower order degree, which is similar with that of penecontemporary marine dolostone, but it is rich in calcium as the Mg/Ca ratio of it is deviated from the usual value, which indicates that the dolostone formed under a condition of rapid replacement. In addition, the analysis result of stable carbon isotope values and "Z"values reveal that it has high lacustrine saltness. Based on the dolomitization mechanism,an original model of Shahehie Lower 1st Member lacustrine dolostone is proposed. Contrasted with the origin mode of marine dolostone, the lacustrine dolostone is characterized of smaller distribution area and unthorough dolomitization so as to limited hydrocarbon reservoir capability.