A biological marker investigation of petroleums and shales from the Shengli oilfield, The People's Republic of China

Abstract

The distributions of the alkanes and aromatic steroid hydrocarbons of eleven oils and six shales from the Shengli oilfield (Zhanhua depression), have been examined by gas chromatography and gas chromatography-mass spectrometry. Particular attention has been paid to the so-called biological marker compounds (steranes, triterpanes and aromatic steroid hydrocarbons). Where metalloporphyrins are present, their distributions have been examined by high-performance liquid chromatography and mass spectrometry. The shales show progressive increases in the extent of aromatisation of aromatic steroid hydrocarbons and the extent of configurational isomerisation in the triterpanes which correlate well with present burial depth. They are less mature than all of the oils when maturity assessment is based on the degree of sterane configurational isomerisation. One oil (Yi-18) is less mature than all the other oils from most of the measurements. A number of ratios and compound distributions thought to be indicative of either maturity or palaeoenvironment (or both) suggest the Upper Shahejie (Oligocene) as the most appropriate source for the Yi-18 oil. The same ratios and distributions rule out the Lower Shahejie (as represented by the sample examined here) as a possible source for any of the oils (even in a more mature state). The main source(s) for all the other oils is more mature (by extent of sterane isomerisation) than all the shales examined. Suitable sources for these oils have not been found in this study. There is no correlation between oil maturity and reservoir depth, suggesting that the present maturity of the oils relates mainly to maturation effects in the source rocks and not the reservoir. All oils pooled at depths of ⪝ 1500 m (∼70°C) show the effect of biodegradation ( n-alkanes removed) whilst those pooled at depths ⪞ 1500 m appear to be unaffected.