Molecular characteristics of source rocks in Upper Triassic Xujiahe Formation, Western Sichuan Depression, Sichuan Basin

Abstract

The Upper Triassic Xujiahe Formation (T3x) is the most important terrigenous stratum in the Western Sichuan Depression; however, it is still insufficient for the understanding of molecular compositional characteristics of its source rocks. Based on the analysis of both saturated and aromatic fractions of extracts of source rocks from different members of the Xujiahe Formation, the indication is then discussed for sedimentary environment and thermal evolution of source rocks. The distribution of tricyclic terpanes (TTs) and dibenzothiophene/phenanthrene (DBT/P) ratio of the argillaceous source rocks in 2nd to 4th members of the Xujiahe Formation mainly display the signatures of typical lacustrine source rocks, and the relatively high gammacerane/C30 hopane (0.16-0.23), as well as 9-/1-methylphenanthrene (1.16-1.41) ratios suggest relatively high salinity of water body. The parameters such as methylphenanthrene index (MPI1), methylphenanthrene distribution fraction (F1), and methyldibenzothiophene distribution index (MDBI) are effective for the evaluation of thermal maturity, and they show positively and negatively linear correlation with maturity degree before and after Ro=1.35%. The distribution pattern of C27-C29 regular steranes, relative composition of three fluorine series, and methyldibenzothiophene/methyldibenzofuran ratios for high to over-maturity T3x source rocks are distorted due to the effect of thermal maturation.

Keywords

• molecular geochemistry
• biomarker
• maturity
• sedimentary environment
• xujiahe formation
• upper triassic
• western sichuan depression
• sichuan basin