Organic Geochemistry in the Genetic Age

Bradley, Alexander S.¹, Roger Summons² (1) Massachusetts Institute of Technology, Cambridge, MA (2) Massachusetts Institute of Technology, Cambridge,

Geological hydrocarbon accumulations are molecular remains of ancient organisms and include biomarker structures traceable to specific functionalized precursors. These molecular fossils are key tools for understanding the source, age, maturity, and history of petroleum deposits.

Traditionally, organic geochemists have regarded biomarkers as having specific biological sources that carry information about petroleum source environments. The association of molecular structures with phylogenetic groups has been empirically determined by surveying membrane lipids extracted from modern organisms. This approach has been highly successful, but is intrinsically limited because not every organism can be surveyed. The possibility remains that some biomarker molecules may be more widespread across phylogeny than is currently recognized.

The advent of the genomic age is revolutionizing our approach to organic geochemistry by allowing us to view biomarkers as molecular records of suites of functional genes. This approach has several advantages: i) it is more accurate, because biomarker lipids are products of genetically encoded enzymes that may be shared across lineages or highly conserved, ii) ever-growing genetic databases can be quickly and accurately searched to determine if sequenced organisms contain the genes of interest, and iii) understanding enzymatic pathways may reveal additional information about the meaning of molecular fossils, e.g. a synthetic requirement for certain environmental conditions such as the presence of oxygen.

The genetic approach strengthens organic geochemistry as a tool for understanding petroleum deposits by rigorously testing associations of biomarkers with phylogeny, and by providing insight into the physiological role – and function in source environments – of many geostable compounds.