Response of 17α(H)-diahopanes and 18α(H)-neohopanes to maturity in lacustrine source rocks derived from the Songliao Basin, NE China

Abstract

A suite of 33 lacustrine source rocks (Pr/Ph ranging from 0.29 to 0.84) from the Songliao Basin with uniform organic matter input have been examined to explore the 17α(H)-diahopane and 18α(H)-neohopane series distribution. The absolute concentrations of 17α(H)-diahopane and 18α(H)-neohopane were determined. In addition, an immature lacustrine source rock was chosen to conduct a thermal simulation experiment to further study the relationship between these components and maturity. Analysis results of geological samples show that the absolute concentrations of 17α(H)-diahopanes have an excellent correlation with 18α(H)-neohopanes, whereas the relative abundance of two series of components exhibited a general correlation. Thermal simulation experiment results show that low absolute concentration and relative abundance of 17α(H)-diahopanes and 18α(H)-neohopanes are presented from low-maturity to high-maturity. However, high absolute concentration and relative abundance are merely distributed near the oil-generative peak. Moreover, with increasing of maturity, the absolute concentrations of both 17α(H)-diahopanes and 18α(H)-neohopanes display an initial increase and a following decrease. Interestingly, the relative abundances of 17α(H)-diahopanes and 18α(H)-neohopanes demonstrate an initial increase and a following decrease with maturity before the oil-generative peak, and then, the ratios display a finally gradual increase with increasing maturity. It suggested that the ratios of rearranged hopanes to hopanes could act as maturity parameter at the high maturity.