四川盆地中南部寒武系白云岩特征及形成机制

Characteristics and Formation of the Cambrian Dolomite in Middle-South Sichuan Basin,China

四川盆地寒武系厚度达千米,岩性从下到上依次由泥页岩-粉砂质泥岩-灰质白云岩-纯白云岩组成,其优质储层主要为中—上寒武统冼象池组的白云岩。本文通过与川东北长兴组—飞仙关组、塔河油田奥陶系白云岩对比,应用微量元素、稀土元素、碳、氧、锶同位素等分析测试结果,对四川盆地中南部寒武系白云岩特征及形成机制进行了研究。研究结果表明:四川盆地中南部寒武系白云岩具有较高的铁、锰质量分数,平均值分别为6 502.9×10^(-6)和468.3×10^(-6),较低的锶质量分数,平均值为74.0×10^(-6),较低的碳氧同位素值,δ^(13)CVPDB和δ^(18)OVPDB平均值分别为-2.38‰和-7.74‰;四川盆地中南部寒武系碳酸盐岩经历了大气淡水淋滤作用,可能为准同生白云石化或混合水白云石化作用。

The Cambrian strata in Sichuan basin are thick up to 1 000 m, and composed, from bottom to top, of mud shale, silty mudstone, calcite dolomite and dolomite. The major favorable reservoir is the dolomite in the Middle-Upper Cambrian Xixiangchi Formation. On basis of comparison of the Cambrian dolomite in middle-south Sichuan basin with those in Feixianguan-Changxing Formation in NE Sichuan basin and Ordovician system in Tahe oilfield, the analysis results of trace element, rare earth element and isotope of C, O, Sr have been involved to discuss characteristics and formation mechanism of the Cambrian dolomite in middle-south Sichuan basin. The research shows that the Cambrian dolomite in study area has relative high Fe and Mn content, with average content of 6 502.9× 10^-6 and 468.3× 10^-6, respectively. It also has relative low Sr Content, δ^13C andδ^18O, with average of 74.0× 10^-6, -2.38‰（VPDB） and -7.74‰（VPDB）, respectively. It＇s concluded that the carbonate in middle-south Sichuan basin experienced the fresh water leaching. The diagenesis can be penecontemporaneous dolomization or mixing water dolomization.