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

Key words: 
• dolomitic rock /
• tuffaceous composition /
• buried dolomitization /
• the Lower Cretaceous /
• Bayindulan sag /
• Erlian Basin

Abstract: 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.