二连盆地阿南凹陷下白垩统腾格尔组湖相云质岩成因

二连盆地阿南凹陷下白垩统腾格尔组发育一套湖相的富含致密油云质岩段，包括云质泥岩、云质沉凝灰岩和云质砂岩，但这类云质岩中的白云石岩石学特征及成因尚不明确.通过岩心、薄片、扫描电镜及碳氧同位素分析，将云质岩中的白云石分为两类:(1) 分布在纹层状和星点状云质泥岩或云质沉凝灰岩的非平直晶面、半自形微粉晶白云石；(2) 分布在块状云质沉凝灰岩或云质砂岩的非平直-平直晶面、自形粉细晶白云石.两类白云石的成因机制不同:(1) 半自形微粉晶白云石具偏高的δ13C_PDB值(平均为5.4‰)，主要受产甲烷菌的代谢活动影响，其δ18O_PDB值负偏移量较小(平均为-13.0‰)，计算形成温度约50~75 ℃，推测为成岩早期成因白云石，Mg2+离子主要来源于富火山灰的凝灰物质大量水解.(2) 自形粉细晶白云石晶体较大、晶形好，其δ13C_PDB值偏低(0.2‰~0.7‰)，受产甲烷菌的代谢活动和深层有机质热催化共同影响，其δ18O_PDB值较低(平均为-19.4‰)，计算形成温度约119 ℃，推测为成岩晚期埋藏成因白云石，Mg2+离子可能与粘土矿物水解及深部流体上涌有关. 

Origin of Lacustrine Dolomitic Rocks of the Lower Cretaceous Tengge'er Formation in Anan Sag,Erlian Basin

The Lower Cretaceous source rocks in Anan sag of Erlian basin develop a set of lacustrine fine-grained dolomitic rocks, composed of dolomitic mudstone, dolomitic tuffite, and dolomitic sandstone, which are rich in tight oil. But its petrological characteristics and origin of dolomite are still unclear.Analysis of core observation, thin section, Scanning Electron Microscope (SEM), and geochemical data show the dolomitic rocks mainly include: (1) The subhedral, micritic-powder crystalline dolomite distributing in laminated and star structure dolomitic mudstone and dolomitic tuffite; (2) The euhedral, powder-fine crystalline dolomite distributed massive dolomitic tuffite and dolomitic sandstone. Their mechanisms of dolomitization are different: (1) The subhedral, micritic-powder crystalline dolomite with higher δ13C_PDB values (av. 5.4‰), higher δ18C_PDB (av. -13‰), and lower fluid inclusion homogenization temperature (50-75 ℃), is dominated by early diagenetic dolomitization associated with methanogenesis. The source of Mg2+ is mainly from alteration of tuffaceous materials. (2) The euhedral, powder-fine crystalline dolomite is characteristic of bigger and finer crystals, lower δ13C_PDB values (0.2‰-0.7‰), lower δ18O_PDB (av. -19.4‰), and higher fluid inclusion homogenization temperature (about 119 ℃). It is dominated by burial dolomitizationand affected by the controls of methanogenesis anddegradation of organic matter.The source of Mg2+ is mainly from transformation of clay minerals and deep fluid.