珠江口盆地珠江组碳酸盐岩碳同位素组成负偏的成因意义

Genesis of Negative Carbon and Oxygen Isotopic Composition of Carbonate Rocks in Lower Miocene Zhujiang Formation,Pearl River Mouth Basin

在岩芯观察、铸体薄片以及阴极发光分析的基础上,测定了珠江口盆地珠江组碳酸盐岩的碳氧同位素组成,获取判别流体性质的地球化学信息,探究其成岩意义。研究结果表明:1珠江组碳酸盐岩全岩样品的δ^(13)C(PDB)值主体分布在0‰±2‰之间,与该时期底栖有孔虫的碳同位素值接近,反映了与早中新世同期海水碳源一致的特征;2珠江口盆地东沙隆起流花地区部分钻井中存在碳同位素明显偏负并且胶结物发育的样品,全岩δ^(13)C(PDB)值可低至-7.2‰,碳氧同位素之间以及碳同位素与岩石中胶结物的含量之间均显示出良好的正相关关系(相关系数在0.6以上),碳同位素偏负伴随着氧同位素变轻,表明碳同位素的偏负是由胶结物造成的;3成岩演化序列分析以及块状亮晶方解石单矿物的碳氧同位素分析进一步证实了是由晚期胶结物造成碳同位素的偏负,埋藏成岩过程中有机酸的作用可能是造成碳同位素偏负的主要原因。

Carbonate reservoir of Zhujiang Formation in Pearl River Mouth Basin is rich in oil and gas resources. Previous studies have focused on sedimentary environment and evolution, geological structure, sequence stratigraphy, and so on, with little reference to carbonate diagenesis. We made a detailed study of the petrologic and geochemical characteristics of carbonate rocks in Zhujiang Formation.Methods： Based on detailed petrographic observation, cathodoluminescence （CL） analysis and carbon and oxygen isotopic analysis, we have studied the carbon and oxygen isotopic compositions and diagenesis of carbonate rocks in Lower Miocene Zhujiang Formation, Pearl River Mouth Basin.Results： The results show that： ① δ13C（PDB） values of whole rock carbonate samples of Zhujiang Formation are mainly distributed between -2‰~2‰, consistent with that of benthic foraminifera in the same period, reflecting the same carbon source with Early Miocene seawater. ② Samples abundant equant and blocky calcite cements from Liuhua area are with significant more negative δ13C（PDB） as low as -7.2‰. The δ13C（PDB） and δ18O（PDB） of these samples show good positive correlation （coefficient greater than 0.6）, and good correlation also between the δ13C of whole rock and the content of cements, indicating that the negative excursion of carbon isotope is caused by cementation. ③ Diagenetic sequence analysis reveals that equant and blocky calcite cements are precipitated in middle—deep environments. The average δ13C（PDB） and δ18O（PDB） of 5 blocky calcite cements are -13‰ and -8‰, respectively, which confirm that the cements formed during burial condition result in the lower carbon isotope value, and the carbon source is possibly originated from organic acids.Conclusions： Organic acids sourced from burial diagenesis effect mainly contribute to the significant more negative δ13C and δ18O of Zhujiang carbonates in Liuhua area, providing some new information for understanding the relations between carbonate diagenesis and reservoir development and evolution.