Maturity effect on carbazole distributions in source rocks from the saline lacustrine settings, the western Qaidam Basin, NW China

Abstract

Carbazole distributions and biomarker compositions have been investigated in a series of source rocks from the Tertiary strata in the western Qaidam Basin, NW China. Typical biomarker signatures, high gammacerane, high C 35 hopane and low Pr/Ph, indicate a saline lacustrine environment. Maturity parameter C 29 sterane ααα S/S + R ranges from 0.06 to 0.58, indicating immature (ca. <0.35) and mature (ca. >0.35) stages. Significant variations were observed for carbazole distributions with maturation. As for C 1-carbazoles, the relative amounts of 1-MC (methylcarbazole) show a decrease trend in the immature stage and an increase trend in the mature stage, while the other three isomers (2-, 3- and 4-MCs) show a generally opposite change, indicating a “two-stage” variation. As for C 2-carbazole isomers, complex variations were observed for their relative amounts, an overall increase for 1,7-DMC (dimethylcarbazole), a slight “two-stage” change for 1,8- and 1,3-DMCs and a marked “two-stage” variation for 2,4-, 2,5- and 2,6-DMCs. Comparison of the distributions of carbazole and benzocarbazoles revealed a preferential generation for benzocarbazoles at high maturity levels. Benzocarbazole ratio a/a + c varies only in a narrow range 0.52–0.61 in the entire maturity range, suggesting limited maturity dependence. The results here support the idea that carbazole distributions have strong maturity effect in a wide maturity range and the “two-stage” variations for some pyrrolic nitrogen isomers may indicate their different geochemical controls between the immature and mature stages.