Geochemistry of Palaeozoic marine petroleum from the Tarim Basin, NW China. Part 2: Maturity assessment

Abstract

The thermal maturity of forty three crude oil samples collected from the cratonic portion of the Tarim Basin, NW China has been assessed using a variety of parameters. While compound ratios of diamondoids, alkylated naphthalenes, methyl phenanthrenes and methyl dibenzothiophenes showed consistent trends with increasing maturity, a large discrepancy was observed for the maturity levels of the oils determined from these parameters. Equivalent vitrinite reflectance values calculated from methylphenanthrene index 1 (MPI1), methyldibenzothiophene ratio (MDR) and methyldiamontane index (MDI) were in the range of 0.66–0.97%, 1.11–1.64% and 0.89–1.74% Ro, respectively. While we caution that some of the discrepancy may be related to calibration problems, the difference observed from different compound classes could be real, reflecting the multiple charge to petroleum accumulations, as the diamondoid hydrocarbons were possibly generated at later maturation stages than most aromatic hydrocarbons. As diamondoids with methyl substitution on the bridgehead positions are more stable than those with substitution on other positions, several compound ratios based on ethyl admantanes, dimethyl admantanes, trimethyl admantanes and dimethyl diamantanes also correlated with the established maturity indicators such as MDR and MDI. While condensates with high maturity levels represent the late charge hydrocarbons, some of them with relatively low maturity levels may be affected by evaporative fractionation processes. The normal gravity oils cover a wide maturity range, consistent with multiple charges and in-reservoir mixing. While the waxy oils with high apparent maturity levels may be the products of wax precipitation due to a later gas injection into an oil column, heavy oils with low apparent maturity levels could be the residue of oil that has lost its light end.