Maturity indication of 17α(H)-diahopane in expelled and retained oils from artificial maturation experiments of mud shale

With the comparable sedimentary organic phase, the relative abundance of rearranged hopanes can be correlated to maturity degree. By the approach of artificial maturation experiments of low-maturity (R_o=0.58%) and organic-rich (TOC=3.87%) mud shale collected from the Ordos Basin, 17α(H)-diahopanes in expelled and retained oils were examined by gas chromatography-mass spectrometry (GC-MS). The thermal evolution characteristics of parameters related to 17α(H)-diahopane were discussed, and the maturity range of these parameters as maturity indicators was proposed, combined with the random mean vitrinite reflectance (R_o) of maturated rock samples of each step of pyrolysis. Results show that the parameters of C₂₉*/(C₂₉*+C₂₉H) and C₃₀*/(C₃₀*+C₃₀H) have similar three-stage variations. As experimental temperature increased, the values of the two parameters decreased firstly, then increased and finally decreased again, indicating that they may be good indicators for oil-source correlation. Before 325 ℃, i.e. R_o < 1.01%, both parameters showed insignificant changes. However, they both showed an obvious upward trend with increasing temperature from 325 to 385 ℃, indicating that they were valid indicators for maturity from the moderate to the early stages of high maturity (i.e. 1.01% < R_o < 1.48%).