Modification of the petroleum system concept: Origins of alkanes and isoprenoids in crude oils

Kerogen, the source material for petroleum, can have a long history of alteration and diagenesis before crude oil forms. A common assumption is that the bulk composition of the expressed oil reflects so much of these progressive alterations that most of the primary biological information of the original fixed carbon has been lost. Exceptions are trace constituents, the biomarkers that comprise only a fraction of the organic material in most pristine crude oils. Analysis of the alkane/acyclic isoprenoid fraction of a large number of crude oils and rock extracts from the Timan-Pechora basin (Russia) suggest that this fraction, the main constituent of most crude oils, is a direct product of liquefaction of biological debris that was preserved essentially unaltered to the point of oil generation. Therefore, the primary biological provenance of this fraction is preserved in the oil fraction. A set of gas-chromatographic analyses of 242 crude oils as well as 83 solvent extracts from upper to middle Paleozoic putative source rocks from the Timan-Pechora basin (Russia) were analyzed by a multivariate data-analytical procedure new to organic geochemistry. The distributions of n-alkanes and acyclic isoprenoids (24 in all) in the 325 samples could be reproduced by linear combinations of six end-member compositions attributed to distinct biological inputs. Four of the six are assigned to primary producers (waxes from higher plants, cyanobacteria, microalgae, and the microorganism Gloeocapsomorpha prisca). These end members account for most of the n-alkanes and acyclic isoprenoids in our samples. The other two represent the products of secondary bacterial alteration of primary organics during sedimentation and low-level bacterial alteration in the reservoir (biodegradation). Each end member is composed of a spectrum of analytes whose abundances are related to one another by fixed ratios. We surmise that each primary end member represents the breakdown of a resistant biopolymer that forms cell walls and partitions of a given biological group. The n-alkanes and acyclic isoprenoids in crude oils represent the weighted signatures of their various ancestors (i.e., their primary organic inputs). If the precursors of most oils are the products of a small set of chemically simple biopolymers, then many of our assumptions concerning the importance of total organic carbon and the nature of the oil window must be reexamined.