羌塘盆地角木茶卡地区展金组白云岩岩石学及地球化学特征

Petrological and geochemical characteristics of dolomite of Zhanjin Formation in Jiaomuchaka area, Qiangtang Basin

目前学术界对于羌塘盆地的油气研究大多集中于侏罗系,针对古生界的相关研究较少。利用薄片分析、X射线衍射、阴极发光、元素地球化学和碳、氧同位素等资料,综合研究了羌塘盆地角木茶卡地区羌资5井二叠系展金组白云岩的岩石学和地球化学特征,并探讨了其成因机制。结果表明:细晶和粉晶白云岩主要发育在局限碳酸盐台地中的潮坪相带,以雾心亮边结构、生长环带结构和残余颗粒结构为典型组构特征,交代现象明显;δ13C为低—中正值(2.7‰~4.4‰),δ18O为低—高负值(-8.4‰^-2.8‰);成岩温度大部分低于44℃,属于低温白云岩;盐度为20.98‰~30.16‰,低于海水盐度;白云石有序度整体为中值,Mg/Ca值(0.44~0.47)及Fe,Mn和Sr含量相对较低,稀土元素配分模式显示具有埋藏白云石化特征。综合分析认为,羌塘盆地展金组白云岩主要形成于早期浅埋藏阶段,其白云岩形成温度较低,在成岩过程中可能受到大气降水的影响。

Current studies on oil-gas geology in Qiangtang Basin mostly focused on Jurassic strata, and only a few researches about the Paleozoic. Based on the data of thin section analysis, x-ray diffraction, cathodeluminescence, element geochemistry and C-O isotope, this paper studied the petrological and geochemical characteristics of dolomite of Zhanjin Formation in Qiangzi 5 well in Jiaomuchaka area, and discussed its genesis. The result shows that fine crystalline dolomite and crystal powder dolomite mainly developed in tidal fiats of limited carbonate platform. They are characterized by cloudy centers and clear rims, clitellum and residual-grain, and the metasomatism is obvious. They are characterized by low-middle δ13C positive value （2.7‰-4.4‰）, low-high δ18O negative value （-8.4‰-2.8‰）. Most diagenetic temperature are below 44℃, which indicates that the temperature degree of dolomitization is low. Salinity ranges from 20.98‰ to 30.16‰, below the salinity of sea. The middle value of the order degree of dolomite, together with the low content of Mg/Ca, Fe, Mn, Sr, and REE distribution pattern reveal a characteristic of burial dolomitization. All evidences indicate that the dolomite of Zhanjin Formation mainly formed in an early shallow hurial environment with low temperature, and there may be a fresh water injection during the diagenetic process.