二连盆地巴音都兰凹陷下白垩统湖相云质岩成因研究

巴音都兰凹陷下白垩统发育的云质岩是一套具有特殊成因的重要标志层，且近年来在其中发现大量油气显示，但目前对这套云质岩成因研究程度较低。为此，通过岩芯观察、薄片鉴定、全岩分析、微量元素和碳氧同位素测定等方法，综合研究了云质岩的岩石学特征、古水介质、成因类型等。研究认为，本区古水介质为淡水-微咸水，而白云石中铁含量高，是埋藏白云化作用的产物，发生白云化作用的母质是凝灰物质，凝灰物质蚀变作用形成大量的斜长石和镁、铁离子，并在二氧化碳参与下发生水解，形成铁白云石，其中二氧化碳主要来源于埋藏条件下的有机质甲烷化作用。

Genesis of Dolomitic Rock within the Lower Cretaceous Lacustrine Facies in Bayindulan Sag,Erlian Basin

The dolomite rocks of the Lower Cretaceous are formed under special geologic conditions, and are the key horizon that can be correlated with the whole basin. In recent years, abundant  hydrocarbon shows have been found in the dolomitic rocks, but there is little research on the dolomitization of this deposit. 
  The genesis of dolomite in the study area is related to hydrolysis of volcanic material. Bulk rock  and microscopic petrologic features analysis  indicate that the specific suite of rocks is rich in volcanic ash which contains a large amount of Ca2+ and Mg2+. Dolomites, which originated from the reaction  of  CO₂, Ca2+ and Mg2+, replace volcanic material  to fill pores  and  fractures. Crystal structures of dolomite observed under microscope,  can be classified as  porphyritic crystal,  micritic crystal and   fine-powder crystal. Dolomite with porphyritic crystal behaves dispersire state and shows irregular shape. Additionally, the size of porphyritic crystal is relatively large, with an average diameter of about 0.2mm. Dolomite with micritic crystal is characterized of  laminated and agglomerate feature. The difference between porphyritic and micritic structure is related to the liquidity of pore fluids. Dolomites with fine-powder crystal concentrate in reservoir pores or fractures as cement. 
  The problem about origin of the CO₂ is as important as that of Mg2+ to the specific dolomitization. Analysis   of carbon isotope show that CO₂ used to hydrolysis is related to the methanation of organic matters. The carbons within original organic material are transformed to methane and bicarbonate by methanation in the absence of oxygen. The enrichment of 12C in methane results in the concentration of  13C in bicarbonate and a slightly positive of δ13C in dolomite.