Short-chain A-ring methylated steroidal hydrocarbons

Abstract

The distribution of steroids bearing short side-chains on the D-ring, with particular emphasis on the A-ring methylated series, is investigated over the late diagenesis-early catagenesis maturity range, by GC–MS analysis of a range of oils and selected immature source rock bitumens. A number of C19 steranes are commonly observed in oils, including 5α,14β-androstane and a compound tentatively identified as 4α-methyl-5α,14β-androstane. C21 and C22 desmethyl steranes, along with minor amounts of C23–C25 homologues, are ubiquitous, among which, by oil-window maturity, the 5α,14β,17β (diginane) isomers dominate, based on high m/z 218/217 ratios. The previously identified and commonly occurring C21 and C22 13β,17α-diapregnanes are accompanied by lesser amounts of their later-eluting 13α,17β isomers. 3-Methyl steranes appear limited to 5α,14β,17β isomers, primarily the C22 and C23 members, with no detectable corresponding diasteranes. Although relatively minor components overall, they are the main, if not only, methyl steranes in pre-Mesozoic samples. These distributions are consistent with the previously proposed mostly catagenetic generation from sterols bound to kerogen via their 3-hydroxy group with concomitant methylation at C-3 (in competition with reduction yielding desmethyl species). 4-Methyl steranes seem to be confined to post-Paleozoic samples, although trace amounts in older samples cannot be excluded. C22 components dominate, comprising 4α-methyldiginane and a compound suggested to be 4β-methyl-13β,17α-diapregnane, the latter identification being supported by close covariance of its abundance with that of the corresponding desmethyl 13β,17α-diapregnane. A much smaller amount of what appears to be 4β-methyl-13α,17β-diapregnane is present; again, its relative abundance closely covaries with that of its desmethyl analogue. In addition, the relative abundances of these proposed 4-methyldiapregnanes correlate negatively with 4-methyl diasterene and positively with diapregnane contents in immature bitumen, further supporting the proposed identifications. The reason for the meagre amounts of C23 4-methyl steranes is unclear, some possibilities are considered. Unlike their saturated counterparts, triaromatic pregnoids consistently exhibit methylation at C-2, C-3, C-4 and C-6, regardless of geological age, with C-3 and C-4 being most abundant, as also observed among C27+ homologues. We suggest it reflects a degree of random, geochemical methylation/transmethylation of aromatic pregnoids, as recognized for methylphenanthrenes. It is most obvious in oils from pre-Mesozoic marine sources, but is concealed in oils from younger sources by abundant biological contributions of 4-methyl steroids, resulting in a broadly linear positive correlation of the 4-methyl/3-methyl ratios for saturated and triaromatic pregnoids.