Maturity assessment of severely biodegraded marine oils from the Halahatang Depression in Tarim Basin

Seventeen Ordovician oil samples from Halahatang Depression are investigated for their thermal maturation. On the basis of GC analysis of the alkane fraction as well as GC-MS analysis of the demethylated hopanes, it is assumed that the oils are mixtures of severely biodegraded and nonbiodegraded oils. Therefore, their maturation is assessed on the basis of the distribution and abundance of compounds strongly resistant to biodegradation. The P-1 together with the P-2 values of C-7 light hydrocarbons suggest that the late filled nonbiodegraded oils for Halahatang are super mature. Triterpane and sterane isomerization maturation parameters indicate they barely reached the peak oil generation stage, consistent with the results obtained from the equivalent vitrinite reflectance Rc(1) (0.80-0.95%), while the Rc(2) (1.28-1.60%) delineate a high maturity of Halahatang oils. However these conventional triterpane and sterane isomerization maturation parameters gave only general information and a comparatively narrow range of maturity. Correlation between the maturation parameters of Pr/nC(17) and Ph/nC(18), Ts/(Ts + Tm) and C29Ts/(C29Ts + C29H), MPI1 and MPI3, 4/1-MDBT and 4. 6-/1,4-DMDBT are rather good, confirming the validity to assess and differentiate the maturity of the severely biodegraded oils examined, with the exception of naphthalene maturation parameters(DNR1 and TNR2). According to the maturation classification assembled with selected effective maturity parameters, such as the MPI3 together with the corresponding [P], the thermal maturities of Halahatang oils are moderate, while Ts/(Ts + Tm) together with 4-/1-MDBT demonstrate a moderate-high maturity.