Geochemical Characteristics and Significance of Permain Dolomite Oil Seepages in Qiangtang Basin

Key words: 
• Biomarkers /
• carbon isotopic compositions of n-alkanes and isopre /
• oil source correlation /
• Permain dolomite oil seepages /
• Qiangtang Basin /

Abstract: Dolomite oil seepages and hydrocarbon source rocks of Permian Zhanjin Formation were collected systematically from Central uplift belt area in Qiangtang Basin.Based on the analytical data of group component, biomarkers and carbon isotopic compositions of n-alkanes and isopre, the author analyzed the geochemical characteristics of Permian dolomite oil seepages,and the oil-source rock correlation was also studied in the paper. The results indicated that the Permian dolomite oil seepages are high fraction saturated hydrocarbons,followed by non hydrocarbon, aromatics and asphaltene fraction are low. The ratio of saturates/aromatics of dolomite oil seepages are more than 1.0. The alkanes of dolomite oil seepages are composed of nC₁₅ - nC₃₆,with nC₁₇~nC₂₀ as the main peaks,and characteristics of single peak distribution.The nC_21-/nC₂₂₊ ratios from 1.0 to 4.56,the regular sterans C_27-C_28-C₂₉ distributed with irregular “V”shape and predominance of C₂₉,homohopane were detected from C₃₁~C₃₅, indicating dolomite oil seepages of hydrocarbon material sources were in algae and other lower aquatic organisms. The Pr/Ph values range of dolomite oil seepages were between 0.91 to 1.30,and gammacerane/C₃₀hopane values were from 0.42 to 0.88, the content of dibenzothiophene were the highest (average 67.39%) and low dibenzofuran content (average 7.89%). 4-MDBT/DBT values of dolomite oil seepages were 0.97 to 4.26, (2+3)-MDBT/DBT values were 0.37 to 1.62, showing dolomite oil seepages hydrocarbon material sources were formed in marine shale with certain salinity reduction environment. The OEP values, C₂₉ααα20S/ααα (20S+ 20R) values,C₂₉αββ/(ααα+αββ) values and C₃₁22S/(22S+22R) values etc., closed to equilibrium values 1, aromatic maturity parameters MPI₁,F₁ and 4,6/1,4-DMDBT calculated from the equivalent vitrinite reflectance showed that dolomite oil seepages were mature crude oil. The biomarkers of oil seepages are closely correlated to the Zhanjin Formation black mudstone, reflecting that the oil seepages were mainly originated from the Zhanjin Formation. However, carbon isotopic compositions of n-alkanes and isopre exists some differences between oil seepages and Zhanjin Formation black mudstone, indicating a mixture of sources.