利用碳酸盐矿物成分研究沉积-成岩过程中流体盐度演化──以东营凹陷沙四段低熟油烃源岩为例

Study of Salinity Evolution of Geofluids during Syngenesis and Diagenesis Using Composition of Carbonate Minerals ──An Example of the Immature Source Rocks of Shasi Member in Dongying Depression

以东营凹陷沙四段上部烃源岩为研究对象，利用电子探针微区测试技术，对同生、成岩及成岩期后不同阶段碳酸盐矿物进行了系统分析，揭示出不同古地理位置碳酸盐成分的三种变化情况，即Mg／Ca比值①由高到低，②一直保持很高，成岩期后降低，和③由低到高，反映了不同层段从沉积到成岩直至成岩期后水体或孔隙流体盐度的三种变化历程，进而确定了该凹陷中心区域烃源岩成岩过程中处于高盐度介质环境中，对于分析该区油气生成条件具有重要意义。本文的研究思路和方法对于其它地区的类似工作也有一定借鉴意义。

Sedimentational/syngenetic (SS),diagenetic (DG) and post-diagenetic (PD) carbonate minerals in the immature oil source rocks of the upper part of the Shasi member in Dongying Depression were identified under microscope and analyzed by EPMA (electronic probe micro-analysis). Three composition change patterns were proposed as follows: (1)The Mg/Ca atomic ratio deceases from 0. 9 in the SS carbonate to 0. 01 in the DG one; (2)the Mg/Ca ratio keeps high values in the SS and DG ones, but it becomes very low in the PS ones; and (3 ) the ratio increase largely from the SS carbonate to the DG ones. According to the relationship between salinity of pore-fluids and the Mg/Ca ratio of carbonates,the above patterns were considered as the result of different environmental evolution histories from the SS to DG,then to PS Stages. The salinity of diagenetic pore-fluid was. proved very high in the center of the depression,which was believed to have been favorable to the generation of immature oil in the area. In addition,the research methods in this paper might be useful for similar studies in other areas.