Identification of a Complete Series of Short Side Chain Steroids (C₂₀C₂₆) in Crude Oil and Souree Rock and Its Geochemical Significance

Abstract: A complete series of short side chain steroids (C₂₀-C₂₆) have first been identified in the crude oil and source rock of saline lake facies from Jinxian Sag of North China by GC / MS and the biological origin and geochemical significance of these compounds are dicussed. The short side chain steroid compounds identified include: (a) steranes, which have m/ z 217 or m/z218 base peak in its mass spectrum but the length of their side chain at C-20 position are different (fig.2). (b) 4-methyl and 4, 4-dimethyl pregnanes and homopregnanes (Fig.3). (c) sterenes, whichs molecular weight are 2 units less than the carbons number corresponding steranes and have m / z 281 base peak or characteristic peak on their mass spectrum.lt can be inferred that the position of double bond is on the D ring (Fig.4). There are some geochemical phenomena accompaning the occurrence of short side chain steroids. The content of n?alkanes is lower and the content of lower carbon number is much more than the higher (∑C₂₁-/ ∑C₂₂⁺= 3.66), and the major peak is C₁₆, there is a higher concentration of C₂₃, C₂₄and C₂₅isoprenoid hydrocarbons (peak K, L and M in Fig.5) Besides higher concentration of pristane and phytane. It is proposed that the high concentrations of isoprenoid hydrocarbons and low carbon number n-alkanes are the result of bacterial input. It is intresting that in our samples the content of short side chain steroids (c₂₀-C₂₆) is much higher than the C₂₇-C₂₉ steranes. the occurrence of a series of short side chain steroids is considered to be the products of some halophilic bacterial activities and, the high concentration of γ- cerane provides another evidence. Furthermore, it has been noticed that the common C₂₁-pregnanes and C₂₂-homopregnanes are dominated in the short side chain steriods.They should be the derivatives from some microorganisms especially from pregnant sterone and pregnant sterol in the halophilic bacteria. As for the origin of all short side chain steroids, however, bacteria (fat discomposition bacteria) degradation leading to side chain rupture of normal steranes could not be excluded. There is nearly not the long side chain hopanes above C₃₀but a high content of C₃₀hopane.It shows that bacterial degradation may result a vanishing of long side chain hopanes and relative richment of C₃₀opane, and indicates shows the existence of the bacterial degradation. In short, the input of organic matter are mainly derived from bacteria and algal, and the original organic matter had experienced a bacterial reconstruction.Therefore, the existence of short side chain steroids and some accompaning marker compounds reflects a hypersaline and component reducing depositional envi- ronment and bacterial biodegradation of some biological precursor molecules.